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Hollow CuSbSy Coated by Nitrogen-Doped Carbon as Anode Electrode for High-Performance Potassium-Ion Storage

Hollow CuSbSy Coated by Nitrogen-Doped Carbon as Anode Electrode for High-Performance... batteries Article Hollow CuSbS Coated by Nitrogen-Doped Carbon as Anode Electrode for High-Performance Potassium-Ion Storage 1 , † 2 , † 1 1 1 2 1 2 , Ping Hu , Yulian Dong , Guowei Yang , Xin Chao , Shijiang He , Huaping Zhao , Qun Fu and Yong Lei * Institute of Nano Chemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China; pinghu1998@shu.edu.cn (P.H.); yangguowei@shu.edu.cn (G.Y.); xinchao@shu.edu.cn (X.C.); 21722731@shu.edu.cn (S.H.); ygfqyx@shu.edu.cn (Q.F.) Fachgebiet Angewandte Nanophysik, Institut für Physik & IMN MacroNano, Technische Universität Ilmenau, 98693 Ilmenau, Germany; yulian.dong@tu-ilmenau.de (Y.D.); huaping.zhao@tu-ilmenau.de (H.Z.) * Correspondence: yong.lei@tu-ilmenau.de † These authors contributed equally to this work. Abstract: As a potential anode material for potassium-ion batteries (PIBs), bimetallic sulfides are favored by researchers for their high specific capacity, low cost, and long cycle life. However, the non-ideal diffusion rate and poor cycle stability pose significant challenges in practical applications. In this work, bimetallic sulfide CuSbS @C with a yolk-shell structure was synthesized by in situ precipitation and carbonization. When CuSbS is applied in the anode of PIBs, it can provide the desired capacity and reduce the volume expansion of the compound through the synergistic effect between copper and antimony. At the same time, the existence of the nitrogen-doped carbon shell confines the material within the shell while improving its electrical conductivity, inhibiting its volume expansion and aggregation. Therefore, CuSbS @C exhibits a satisfactory capacity (438.8 mAh g 1 1 1 at 100 mA g after 60 cycles) and an excellent long cycle life (174.5 mAh g at 1000 mA g after 1000 cycles). Keywords: CuSbS @C; potassium-ion batteries; yolk shell structure; bimetallic sulfide Citation: Hu, P.; Dong, Y.; Yang, G.; Chao, X.; He, S.; Zhao, H.; Fu, Q.; Lei, Y. Hollow CuSbS Coated by 1. Introduction Nitrogen-Doped Carbon as Anode Electrode for High-Performance Because the typical reduction potential of potassium (2.93 V) is comparable to the Potassium-Ion Storage. Batteries 2023, reduction potential of lithium (3.04 V), potassium ion batteries (PIBs) are drawing more 9, 238. https://doi.org/10.3390/ attention as a useful alternative to lithium-ion batteries (LIBs) [1]. In addition, massive batteries9050238 energy storage systems can be equipped with enough potassium thanks to the abundance of resources that are kept in the earth’s crust. [2,3]. Despite these mentioned advantages, Academic Editor: Vilas Pol there are still several challenges to impede the practical applications of PIBs. For instance, Received: 15 March 2023 the slow kinetic process of potassium ions (K ) during charging/discharging and the Revised: 19 April 2023 poor rate performance are caused by the large ionic radius of K (1.38 Å) [4–6]. Besides, Accepted: 21 April 2023 + undesirable expansion of the material and K due to the reaction, known as the intercalation Published: 23 April 2023 + + of K or alloying with K , causes the destruction of the active material structure and an irreversible decrease of capacity [7–9]. Therefore, developing various anode materials with high capacity, high stability, and facilitated K kinetic for PIBs effectively pushes the further development of PIBs. Copyright: © 2023 by the authors. Anode materials research reveals promising storage systems for PIBs, such as carbon- Licensee MDPI, Basel, Switzerland. based materials [10,11], organic materials [12,13], metallic oxides [14,15], metallic sul- This article is an open access article fides [16,17], metallic selenides [18,19], metallic phosphides [20,21], etc. Among them, distributed under the terms and carbon-based materials could be more practical and convenient since they are easily acces- conditions of the Creative Commons sible from nature. However, they deliver a lower reversible capacity. Organic materials Attribution (CC BY) license (https:// with tunable chemical composition have expanded their applications as anode/cathode creativecommons.org/licenses/by/ electrode materials. But they are usually limited by poor electronic conductivity [10,12]. 4.0/). Batteries 2023, 9, 238. https://doi.org/10.3390/batteries9050238 https://www.mdpi.com/journal/batteries Batteries 2023, 9, 238 2 of 15 Metal sulfides with tantalizing capacities can promote the electrochemical performance of PIBs, Nevertheless, the bulk dilatation of these metallic sulfides and the limited cycle life of these electrodes would adversely affect their practical applications [22–24]. Therefore, much attention has been focused on pursuing effective strategies to realize the high perfor- mance of PIBs by nanostructure engineering, compositing with conductive materials [25], and heteroatoms doping in metal sulfides [26]. These strategies dramatically enhance the capacity and stability of the materials and provide more opportunities for the commercial application of bimetallic compounds in PIBs. For instance, Huang et al. embedded Cu S 9 5 into a porous carbon framework derivatized from Zn-based MOF by vulcanization and subsequent ion exchange [27]. Cu S anode exhibited a reversible capacity of 316 mA h g 9 5 1 1 1 after 200 cycles at 100 mA g and a better rate performance of 170 mAh g at 2.0 A g . Wang et al. obtained Bi S @SC by coating sulfur-doped carbon on the surface of Bi S [28]. 2 3 2 3 Bi S @SC anode achieved high reversible capacity and superior rate capacity. These strate- 2 3 gies are effective in mitigating the bulk changes of electrode materials, but the rate and cycle performance at a high current density of the monometallic sulfides has been limited to a certain extent. Therefore, we focus on bimetallic sulfides with synergistic effects that have higher capacity and stability than monometallic sulfides. Structural integrity and cyclic stability are maintained by reducing volume expansion and pulverization of the material. Since the redox potentials of the two metals are different, bimetallic sulfides take full advantage of their differences in electrochemical reactions and exhibit higher electrochemical performance. So far, some bimetallic sulfides have made some research progress in PIBs [29–33]. The reduced graphene oxide encapsulated flower-like spherical FeCoS obtained by Chen et al. was used as the anode for PIBs, it was capable of main- 1 1 taining a reversible capacity of 365.2 mAh g at 100 mA g after 150 cycles [16]. The CoS /ZnS@rGO prepared by Sikandar Iqbal et al. exhibited excellent cycling performance, 1 1 maintaining a high reversible capacity of 565 mAh g after 100 cycles at 100 mA g [17]. These studies show that bimetallic sulfides can achieve higher capacities in PIBs, but they are generally costly because of the expensive graphene used. Additionally, the electrode materials have shown average rate performance at high current densities. So far, there are still limitations in the study of bimetallic sulfides, and further investigation is needed. Therefore, in this work, the synthesis of yolk-shell structured CuSbS @C nanocompos- ites via in-situ precipitation and carbonization is simple and the precursor materials are cheap. The intrinsic properties of CuSbS delivered high capacity, benefiting from the ap- pealing electrochemical conversion and alloying mechanisms. The nitrogen-doped carbon shell is considered a buffer layer to limit the bulk expansion of CuSbS nanoparticles to a limited space while avoiding the aggregation of nanoparticles, and in addition, it provides more defects and active sites to store more K during the cycling process. At the same time, the existence of a carbon shell also increased the electroconductivity of CuSbS material. 1 1 Thus, CuSbS @C exhibited excellent reversible capacity (438.8 mA h g at 100 mA g 1 1 after 60 cycles) and long cycle life (174.5 mA h g at 1000 mA g after 1000 cycles) in PIBs. 2. Materials and Methods 2.1. Materials Sodium citrate (C H Na O , 99.5%), copper sulfate pentahydrate (CuSO 5H O, 6 5 3 7 4 2 ACS reagent grade, 99%), antimony chloride (SbCl , 99.95%), sodium hydroxide (NaOH), L-ascorbic acid, sodium sulfide nonahydrate (Na S9H O, 99.5%), muriatic acid 2 2 (HCl, 37%), tris (hydroxymethyl) aminomethane (Tris), thioacetamide (TAA), dopamine hydrochloride (PDA), potassium metal (K, chunks in mineral oil, 98%) was purchased from China National Medicines Corporation Ltd., (Beijing, China) potassium bis (fluorosulfonyl) imide (KFSI, 97%), ethylene carbonate (EC, 99%), and diethyl carbonate (DEC, 99%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Batteries 2023, 9, 238 3 of 15 2.2. Synthesis of Cu S and Sb S 9 5 2 3 First, 6 mmol of CuSO 5H O and 2 mmol of C H Na O were mixed into 100 mL of 4 2 6 5 3 7 distilled water and stirred for 30 min at a constant speed to form clarified solution A. A total of 0.1 mol NaOH was dispersed in 80 mL distilled water and the clarified solution B was achieved by magnetic stirring for 15 min. Slowly drop solution B into solution A under magnetic stirring. Next, 6 mmol of L-ascorbic acid was poured into the solution. After stirring for some time, the mixture was centrifuged to obtain Cu O. Then, the resulting Cu O was supersonically distributed in 100 mL distilled water for 30 min to form solution C. To make solution D, 24 mmol Na S9H O was poured into 40 mL of distilled water. After 2 2 pouring solution D into the burette, adding solution C in a uniform drop, and stirring for 1 h, the centrifuged solid was scattered in 100 mL of 2 M hydrochloric acid to wash out the superfluous Cu O. The obtained sediment was cleaned by centrifugation to a pH-neutral supernatant and dried for 12 h to yield Cu S. To acquire Sb S , 0.5 mmol of SbCl was 2 2 3 3 distributed in 40 mL of ethylene glycol and agitated for 4 h, subsequently, adding 2 mmol TAA and stirring for 5 min. The mixture was hydrothermally treated in a reaction vessel at 160 C for 12 h and then centrifuged and dried at 60 C. In a tube furnace, Sb S was 2 3 heated to 400 C at an annealing temperature of 3 C/min and then held for 2 h. 2.3. Synthesis of Cu S@C, Sb S @C 2 2 3 Firstly, 700 mg Cu S was ultrasonically distributed in 150 mL tris buffer (1 M) for 30 min. To the mixture, 700 mg of dopamine hydrochloride was added and stirred con- tinuously for 24 h. The resulting mixture was washed with deionized water to neutrality and then washed by centrifugation with anhydrous ethanol. Cu S@PDA was acquired by vacuum stoving the precipitate at 60 C. Then, 1 g Cu S@PDA was put in a tubular furnace, warming to 450 C under the protection of the Ar atmosphere, and kept for 3 h. The Cu S@C composite was removed after natural cooling. Sb S @C was produced using 2 2 3 the same method as Cu S@C except that Cu S was replaced by Sb S . 2 2 2 3 2.4. Synthesis of CuSbS @C and CuSbS y y Firstly, 500 mg of Cu S@C composite material was dissolved in 100 mL of anhydrous ethanol, and solution E was formed after 30 min of ultrasonic dispersion. Addition of 7 mmol of antimony trichloride (SbCl ) to 100 mL of anhydrous ethanol to fully dissolve, and after complete dissolution, solution F was formed. Solution F was slowly dripped into solution E under magnetic stirring, and after adding all drops, the solution was mixed with magnetic stirring for 24 h. CuSbS @C was obtained by centrifugally cleaning the obtained black sediment with anhydrous ethanol and drying it for 12 h at 60 C. A total of 500 mg CuSbS @C was held at 450 C for 1 h with a temperature rise rate of 3 C/min to improve crystallinity. The synthesis process of CuSbS was the same as that of CuSbS @C except y y that no carbon was added. 3. Results and Discussion 3.1. Material Characterization Figure 1 depicts the schematic method for the synthesis of the yolk-shell structure CuSbS @C. First of all, CuSO 5H O and C H Na O were dissolved in deionized water, 4 2 6 5 3 7 and Cu(OH) precipitate was formed after adding NaOH, and then L-ascorbic acid was added to form Cu O red precipitate. Then, Na S9H O was added and the excess Cu O 2 2 2 2 was washed away with HCl to obtain Cu S, which was subsequently coated with dopamine and carbonized to obtain Cu S@C. Finally, SbCl was added and carbonized again to form 2 3 the CuSbS @C composite with a yolk-shell structure. As shown in Figure 2a, the morphology of Cu S@C presented irregular and unevenly distributed nanoparticles. The Sb S @C consisted of a large number of stacked nanopar- 2 3 ticles with a diameter of approximately 180 nm (Figure 2b). Clearly, the CuSbS hollow nanoparticle has a uniform shell and a wall thickness of around 50 nm (Figure 2c). As seen in Figure 2d–f, the CuSbS @C exhibited a homogeneous micro-spherical yolk-shell struc- Batteries 2023, 9, 238 4 of 15 ture after carbon coating and annealing. The acquisition of this unique yolk-shell structure may be due to the volume of the internal metallic material first expanding the carbon layer after the expansion during the ion exchange reaction, and the internal material shrinkage during annealing, leading to the formation of large voids between the spherical shell and the internal nano microspheres. This unique structure improved the stability of the material compared to Cu S@C, Sb S @C, and CuSbS and effectively avoided agglomeration and 2 2 3 y escape of active substances. The presence of a yolk-shell structure obtained more uniform void distribution, more electrochemically active sites, larger nanomaterial/electrolyte con- tact area, shorter ion diffusion paths, accelerated ion transport, and adapted to volume changes during potassiation [34–36]. Figure 2e shows the transmission electron microscope (TEM) images of CuSbS @C. The part surrounded by the green circle was the pure material of CuSbS , and the position of the carbon layer is in the middle of the red and green lines. To confirm the precise morphology and distribution of the components of the substance, selecting elemental mapping (EDS) tests were performed on the active material in a defined area. The Cu, Sb, S, C, and N elemental distributions can be seen visually by EDS (Figure 2f). Batteries 2023, 9, x FOR PEER REVIEW 4 of 16 In addition, the homogeneous dispersion of carbon and nitrogen characteristics was seen, which also demonstrated that the surface of CuSbS was covered by a uniform layer of nitrogen-doped carbon. Figure 1. Schematic illustration of the synthesis process of CuSbSy@C. Figure 1. Schematic illustration of the synthesis process of CuSbS @C. X-ray diffraction (XRD) and Raman were used to characterize Cu S@C, Sb S @C, 2 2 3 As shown in Figure 2a, the morphology of Cu2S@C presented irregular and unevenly CuSbS , and CuSbS @C. Figure 3a shows the XRD spectra of CuSbS @C in agreement with y y y distributed nanoparticles. The Sb2S3@C consisted of a large number of stacked nanoparti- the standard card (Cu Sb S # PDF 24-1318, CuSbS # PDF 44-1417). The XRD patterns 12 4 13 2 cles with a diameter of approximately 180 nm (Figure 2b). Clearly, the CuSbSy hollow of Cu S @ C, Sb S @C, and CuSbS also corresponded to the standard cards (Figure S1). 2 2 3 y nanoparticle has a uniform shell and a wall thickness of around 50 nm (Figure 2c). As seen The peaks corresponding to the elements in CuSbS @C were found on the diffraction in Figure 2d–f, the CuSbSy@C exhibited a homogeneous micro-spherical yolk-shell struc- planes (013), (110), (111), (410), (020), and (301), which was confirmed the successful ture after carbon coating and annealing. The acquisition of this unique yolk-shell structure preparation of CuSbS @C [33,37–39]. To obtain the content of Cu, Sb, and S elements in the may be due to the volume of the internal metallic material first expanding the carbon layer compound more precisely, the compound was tested by Inductively coupled plasma-Mass Spectrometry (ICP-MS), and the results showed that the ratio of the three elements in after the expansion during the ion exchange reaction, and the internal material shrinkage the compound was 1:1.63:0.62 (Table S1). The compound was named CuSbS @C in the during annealing, leading to the formation of large voids between the spherical shell and text. As for Raman spectra of Cu S@C, an obvious characteristic peak of Cu S appears 2 2 the internal nano microspheres. This unique structure improved the stability of the mate- 1 1 at 290 cm , while the weak peak at 610 cm could be attributed to the signal of CuO rial compared to Cu2S@C, Sb2S3@C, and CuSbSy and effectively avoided agglomeration due to the oxidation of the material exposed to the air [40]. From the Raman spectra of and escape of active substances. The presence of a yolk-shell structure obtained more uni- Sb S @C, we can see that there are two continuous characteristic peaks at 284 and 311 cm , 2 3 form void distribution, more electrochemically active sites, larger nanomaterial/electro- lyte contact area, shorter ion diffusion paths, accelerated ion transport, and adapted to volume changes during potassiation [34–36]. Figure 2e shows the transmission electron microscope (TEM) images of CuSbSy@C. The part surrounded by the green circle was the pure material of CuSbSy, and the position of the carbon layer is in the middle of the red and green lines. To confirm the precise morphology and distribution of the components of the substance, selecting elemental mapping (EDS) tests were performed on the active material in a defined area. The Cu, Sb, S, C, and N elemental distributions can be seen visually by EDS (Figure 2f). In addition, the homogeneous dispersion of carbon and ni- trogen characteristics was seen, which also demonstrated that the surface of CuSbSy was covered by a uniform layer of nitrogen-doped carbon. Batteries 2023, 9, 238 5 of 15 corresponding to the vibration of the Sb-S bond [41]. The characteristic peak at 250 cm could be ascribed to the vibration of the Cu-S bond in the sample of CuSbS and CuSbS @C, y y while the characteristic peak at 468 cm is due to the characteristic phonon vibration of chalcopyrite CuSbS phase [42]. The weak peaks at 624 and 785 cm may be related to the oxidation of CuSbS to form oxides when exposed to air (Figure 3b). In addition, strong peaks of Cu S@C, Sb S @C, and CuSbS @C in the Raman spectra appear at 1563.2 and 2 2 3 y 1365.9 cm , correlating to the G-band of graphitic carbon and the D-band of amorphous carbon, respectively. The proportion of the two peaks was determined as the strength ratio (I /I ) of the material, and the ratio was 0.94, which was less than 1.0, indicating D G that there were many defects in the carbon in the composites [16,43]. According to the existing research, materials with a lot of defects largely provided sufficient K storage sites and showed excellent electrochemical performance in the application of PIBs [44,45]. To further characterize the carbon content of the material, the samples were subjected to thermogravimetric analysis (TGA) at temperatures ranging from 25 to 800 C. As shown in Figure 3c, the evaporation of residual moisture in the material caused a small mass loss (1.77%) of CuSbS @C in the temperature range of 25–250 C. The slow mass loss (11.7%) of CuSbS @C at 250–500 C would be explained in two ways. On one hand, the carbon in CuSbS @C was burned completely, resulting in mass loss. On the other hand, the quality was improved by forming oxides (CuO and Sb O ). As a result, the overall capacity loss 2 3 was relatively slow. The loss of mass was highest at about 750 C when the oxide was completely decomposed, and the mass loss reached 31.14% tremendously. According to the mass loss in each temperature range, the carbon content of CuSbS @C was 31.3–34.5%, Batteries 2023, 9, x FOR PEER REVIEW 5 of 16 and its low carbon content makes the compound maintain high capacity under the basis of stability. (b) (a) (c) (f) (d) (e) CuSbS Figure 2. (a–d) SEM images of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C. (e) Transmission electron Figure 2. (a–d) SEM images of Cu S@C, Sb S @C, CuSbS , and CuSbS @C. (e) Transmission electron y y 2 2 3 microscope images of CuSbSy@C. (f) Element mapping images of CuSbSy@C. microscope images of CuSbS @C. (f) Element mapping images of CuSbS @C. y y X-ray diffraction (XRD) and Raman were used to characterize Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C. Figure 3a shows the XRD spectra of CuSbSy@C in agreement with the standard card (Cu12Sb4S13 # PDF 24-1318, CuSbS2 # PDF 44-1417). The XRD patterns of Cu2S @ C, Sb2S3@C, and CuSbSy also corresponded to the standard cards (Figure S1). The peaks corresponding to the elements in CuSbSy@C were found on the diffraction planes (013), (110), (111), (410), (020), and (301), which was confirmed the successful preparation of CuSbSy@C [33,37–39]. To obtain the content of Cu, Sb, and S elements in the compound more precisely, the compound was tested by Inductively coupled plasma-Mass Spectrom- etry (ICP-MS), and the results showed that the ratio of the three elements in the compound was 1:1.63:0.62 (Table S1). The compound was named CuSbSy@C in the text. As for Raman −1 spectra of Cu2S@C, an obvious characteristic peak of Cu2S appears at 290 cm , while the −1 weak peak at 610 cm could be attributed to the signal of CuO due to the oxidation of the material exposed to the air [40]. From the Raman spectra of Sb2S3@C, we can see that there −1 are two continuous characteristic peaks at 284 and 311 cm , corresponding to the vibra- −1 tion of the Sb-S bond [41]. The characteristic peak at 250 cm could be ascribed to the vibration of the Cu-S bond in the sample of CuSbSy and CuSbSy@C, while the characteristic −1 peak at 468 cm is due to the characteristic phonon vibration of chalcopyrite CuSbS2 phase −1 [42]. The weak peaks at 624 and 785 cm may be related to the oxidation of CuSbS2 to form oxides when exposed to air (Figure 3b). In addition, strong peaks of Cu2S@C, Sb2S3@C, and –1 CuSbSy@C in the Raman spectra appear at 1563.2 and 1365.9 cm , correlating to the G- band of graphitic carbon and the D-band of amorphous carbon, respectively. The propor- tion of the two peaks was determined as the strength ratio (ID/IG) of the material, and the ratio was 0.94, which was less than 1.0, indicating that there were many defects in the carbon in the composites [16,43]. According to the existing research, materials with a lot of defects largely provided sufficient K storage sites and showed excellent electrochemi- Batteries 2023, 9, x FOR PEER REVIEW 7 of 16 Batteries 2023, 9, 238 6 of 15 (a) (b) (c) Figure 3. (a) XRD of CuSbS @C. (b) Raman spectra of Cu S@C, Sb S @C, CuSbS , and CuSbS @C. y 2 2 3 y y (c) TGA patterns of CuSbS @C. The chemical composition of CuSbS @C was further determined by X-ray Photoelec- tron Spectroscopy (XPS) analysis. The real presence of Cu, Sb, S, C, N, and O elements in CuSbS @C composites was confirmed by XPS spectra in the 0–1000 eV binding energy y Batteries 2023, 9, 238 7 of 15 range (Figure 4a). Figure 4b–f presents the fine spectrum of the different elemental orbitals in CuSbS @C. Figure 4b shows the fine spectrum of C 1s with C=O, C-N/C-S, and C=C bonds belonging to the peaks at 288.9 eV, 286.0 eV, and 284.8 eV, respectively [46]. Among them, the presence of C-S bonds indicated that the sample had been doped with some S atoms in addition to binding to the metal, whereas the appearance of C=O bonds was possibly caused by the sample being exposed to air. From the N 1s fine spectrum of the material, characteristic peaks were found at 398.7 eV, 400.4 eV, and 401.5 eV, corresponding to pyridine nitrogen (38.91%), pyrrole nitrogen (40.8%), and graphitized nitrogen (20.29%), respectively (Figure 4c) [27]. Four characteristic peaks appeared in the 2p orbitals (2p 3/2 + 2+ and 2p ) of Cu. Cu had two characteristic peaks at 932.1 eV and 952.0 eV, while Cu 1/2 had two peaks at 933.1 eV and 951.7 eV. (Figure 4d). The simultaneous presence of copper + 2+ elements in two valence states (Cu and Cu ) in CuSbS @C had been demonstrated. A pair of characteristic peaks belonging to Sb 3d and Sb 3d were observed in the XPS 5/2 3/2 spectra of Sb 3d at 531.3 eV and 539.7 eV [37,38]. Additionally, surface oxidation from air exposure produced an O 1s peak at 533.5 eV (Figure 4e). The XPS spectrum of S 2p in Figure 4f shows two pairs of peaks, one at 165.7 eV and 164.6 eV, and the other at 163.4 eV 2 2 and 161.3 eV corresponding to the spin orbitals of S 2p and S 2p for (S ) and (S ), 3/2 1/2 2 respectively [47]. 3.2. Electrochemical Performance The performance of the CuSbS @C anode in PIBs was investigated using cyclic voltam- metry (CV) and constant current cycle tests. Figure 5a depicts the CuSbS @C CV curve at a scan speed of 0.1 mV s (0.01–3 V). Compared with Cu S@C, Sb S @C, and CuSbS , 2 2 3 the anode of CuSbS @C had a larger CV curve area, indicating that it had a stronger potassium storage capacity (Figure S2). CuSbS @C exhibited a CV profile that differed significantly from that of its precursors after coating with carbon, because the coating and annealing of dopamine hydrochloride changed the structure and morphology of CuSbS . The formation of a solid electrolyte interface (SEI) layer between the CuSbS @C electrode and the electrolyte caused the revivification peak to appear at around 0.84 V in the first cycle of the curve [48]. The reason for another reduction peak (0.51 V) was explained as a further alloying reaction between Sb metal and K formed after the conversion reaction. The two characteristic peaks gradually disappeared during the subsequent charging and discharging process. There were three oxidation peaks in CuSbS @C during charging, and the corresponding voltages were 1.03 V, 1.79 V, and 1.99 V, respectively, indicating that K was a multi-step depotassiation process in the active material. The new CV curve was nearly identical after the first scan, demonstrating that the CuSbS @C electrode had positive cycle stability and electrochemical performance. The irreversible loss of specific capacity observed in charge/discharge curves for different cycle turns of CuSbS @C at 0.1 A g was explained by the development of the SEI layer and irreversible electrolyte decomposition. [49,50]. The charge/discharge curves of CuSbS @C almost overlapped each other and maintained some stability after the first cycle, matching well with the CV curves and delivering great cycle stability (Figure 5b). In comparison to Cu S@C, Sb S @C, and CuSbS , CuSbS @C exhibited the highest capacity and stability (Figure S3). y y 2 3 Figure 5c shows the cycling performance with Cu S@C, Sb S @C, CuSbS , and CuSbS @C 2 2 3 y y at 100 mA g . From the curves, we can not only see that the CuSbS @C had high capacity, but also verified that the carbon shell effectively improved the reliability of the CuSbS @C. Cu S@C, Sb S @C, and CuSbS had obvious capacity attenuation after nearly 20 cycles 2 2 3 y at 100 mA g , while the CuSbS @C could keep a stable charge/discharge specific capac- ity. In addition, the CuSbS @C delivered remarking initial charge/discharge capacity of 423.1/632.4 mAh g with an initial coulombic efficiency of 66.89%. After 60 cycles at 1 1 100 mA g , CuSbS @C reserved a high specific capacity of 438.8 mAh g . The cycling stability of CuSbS @C was performed at 500 mA g , and exhibited a high capacity of 244.2 mAh g after 1000 cycles (Figure S4). After 1000 cycles at a high current density 1 1 of 1000 mAh g , the CuSbS @C electrode reached a high capacity of 174.5 mAh g y Batteries 2023, 9, 238 8 of 15 and retained 73.2% of its capacity (Figure 5d). The steadily increasing number of cycles implied that the presence of the carbon shell effectively suppressed the bulk expansion of the CuSbS during the reaction and the unfavorable by-reactions between the elec- trolyte and the electrode [51,52]. As shown in Figure 5e, CuSbS @C achieved excellent rate performance in existing bimetallic sulfides (More detailed information was in Table S2). To ulteriorly explore the rate performance of CuSbS @C electrode for PIBs, the charge and discharge capacities were tested at different current densities in the voltage range of 0.01–3 V. (Figure 5f). CuSbS @C electrode exhibited high reversible specific capacities of Batteries 2023, 9, x FOR PEER REVIEW 1 8 of 16 410.8, 356.5, 318.6, 294.5, 267.3, 241.1, 213.4, and 173.6 mAh g at 0.05, 0.1, 0.2, 0.3, 0.5, 1, 1 1 2, 5 A g , respectively. The recovered capacity is maintained at 337.5 mAh g (82.2%) when the current density is restored to 0.05 A g , showing excellent rate performance. Figure 3. (a) XRD of CuSbSy@C. (b) Raman spectra of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C. (c) CuSbS @C utilized the bimetallic synergy and carbon cladding not only obtained capacity TGA patterns of CuSbSy@C. enhancement but also improved the rate performance and long cycle life during cycling, which exhibited strongly competitive in PIBs. (b) Raw date (a) C 1s Fitted date C-C 284.8 eV C-N/C-S 286.0 eV C=O 288.9 eV 0 200 400 600 800 1000 280 282 284 286 288 290 292 Binding Energy (eV) Binding Energy (eV) Raw date Raw date (c) (d) Cu 2p N 1s Fitted date Fitted date Pyrrolic N 400.4 eV Pyridinic N Cu 2p 3/2 398.7 eV Graphitic N Cu Cu 401.5 eV 932.1 eV Cu 2p 952.0 eV 1/2 2+ 2+ Cu Cu 933.1 eV 951.7 eV 930 935 940 945 950 955 960 965 394 396 398 400 402 404 406 408 410 Binding Energy (eV) Binding Energy (eV) (e) Raw date Sb 3d (f) O 1s Fitted date Sb 3d 5/2 O 1s 531.3 eV 533.5 eV Sb 3d 3/2 539.7 eV 524 526 528 530 532 534 536 538 540 542 544 Binding Energy (eV) Figure 4. (a) XPS full spectra of CuSbS @C. XPS fine spectra of (b) C 1s, (c) N 1s, (d) Cu 2p, (e) Sb 3d, Figure 4. (a) XPS full spectra of CuSbSy@C. XPS fine spectra of (b) C 1s, (c) N 1s, (d) Cu 2p, (e) Sb 3d, and (f) S 2p. and (f) S 2p. 3.2. Electrochemical Performance The performance of the CuSbSy@C anode in PIBs was investigated using cyclic volt- ammetry (CV) and constant current cycle tests. Figure 5a depicts the CuSbSy@C CV curve −1 at a scan speed of 0.1 mV s (0.01–3 V). Compared with Cu2S@C, Sb2S3@C, and CuSbSy, Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) S 2p C 1s N 1s O 1s Sb 3d Cu 2p3 Cu 2p3 Intensity (a.u.) Intensity (a.u.) Batteries 2023, 9, x FOR PEER REVIEW 10 of 16 Batteries 2023, 9, 238 9 of 15 3.0 0.15 −1 CuSbS @C 0.1A g (a) CuSbS @C (b) y 1st 1.99V 1.79V 1.03V 0.10 2nd 2.5 4th 0.05 8th 2.0 10th −0.00 1.5 −0.05 −0.10 1st 1.0 2nd 0.84V −0.15 4th 0.5 0.51V 8th −0.20 10th 0.0 −0.25 0 200 400 600 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Voltage (V vs. K /K) Capacity (mAh/g) (c) (d) Cu S@C 800 2 800 Cu S@C Sb S @C 2 2 3 Sb S @C CuSbSy 2 3 600 CuSbS CuSbSy@C 600 −1 CuSbS @C Current density:0.1 A g −1 Current density:1.0 A g 200 200 0 0 0 10 20 30 40 50 60 0 200 400 600 800 1000 Cycle number Cycle number (e) (f) NiCo S @N-HCNFs 2 4 (Bi,Sb) S 2 3 Co Sn -S/rGO 1 6.75 Cu S@C FeCoS @rGO Sb S @C 2 3 NiFeS@C CuSbS This work CuSbS @C 0.05 0.1 0.05 0.2 0.3 0.5 1.0 2.0 5.0 0 0 0 10 20 30 40 50 60 70 80 90 0 2000 4000 6000 8000 Cycle number Rate (mA/g) Figure 5. (a) CV curves of CuSbS @C electrode. (b) Charge/discharge curves of CuSbS @C. y y Figure 5. (a) CV curves of CuSbSy@C electrode. (b) Charge/discharge curves of CuSbSy@C. (c) Cy- −1 (c) Cycling performance of Cu S@C, Sb S @C, CuSbS and CuSbS @C electrodes at 0.1 A g and cling performance of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes at 0.1 A g and (d) 1.0 2 2 3 y, y −1 1 (d A g ) 1.0 . A (eg ) Rate . (e )capabilit Rate capability y of Cu of SbS CuSbS y@C and @C and previo previously usly reported reported mmaterials. aterials. (f () f)Rate Rate of of Cu Cu2S@C, S@C, −1 Sb2S3@C, CuSbSy, and CuSbSy@C from 0.05 to 5.0 A g . Sb S @C, CuSbS , and CuSbS @C from 0.05 to 5.0 A g . y y 2 3 To further investigate the kinetic process of potassium storage in CuSbS @C, more To further investigate the kinetic process of potassium storage in CuSbSy@C, more electrochemical test techniques were used for us to analyze the electron/ion migration electrochemical test techniques were used for us to analyze the electron/ion migration processes. A semicircle made up the high frequency (HF) portion of the Nyquist plot, and processes. A semicircle made up the high frequency (HF) portion of the Nyquist plot, and a sloping straight line made up the low frequency (LF) portion [53]. In the HF region a sloping straight line made up the low frequency (LF) portion [53]. In the HF region (100 (100 kHz–1 kHz), the resistance is mainly controlled by charge transfer, in the medium kHz–1 kHz), the resistance is mainly controlled by charge transfer, in the medium fre- frequency (MF) region (1 kHz–1 Hz), the resistance is mainly controlled by charge transfer quency (MF) region (1 kHz–1 Hz), the resistance is mainly controlled by charge transfer and diffusion, and in the LF region (<1 Hz), the resistance is mainly controlled by diffusion Current (mA) Capacity (mAh/g) Capacity (mAh/g) Coulombic efficiency (%) Voltage (V vs. K /K) Capacity (mAh/g) Capacity (mAh/g) Coulombic efficiency (%) Coulombic efficiency (%) Batteries 2023, 9, 238 10 of 15 (Figure S5). The charge transfer resistance (R ) between the interface of the electrodes ct corresponded to the semicircle, and a lower charge transfer resistance corresponded to a smaller circle radius [54]. The straight line corresponded to Warburg impedance (Z ), and the smaller the slope, the smaller the diffusion resistance of K . Comparing the solution resistance (R ) of four materials, the R of CuSbS @C was the smallest, and that of s s y CuSbS was the largest (Table S3). This was due to the large contact area of the CuSbS @C y y electrode with the electrolyte and the smallest contact area of the CuSbS electrode with the electrolyte [55]. The Bode plots showed that the materials have double time constants, and the impedance fit showed that the difference between the double time constants is not very large, so the diameter of the first semicircle is small and not very significant (Figure S6). The equivalent circuit diagram of double time constants in series was obtained from the impedance fitting analysis [56]. The specific values of the time constants were given in the following Table S4. Figure 6a shows the Nyquist plots of Cu S@C, Sb S @C, CuSbS , 2 2 3 y and CuSbS @C at 1.0 A g . It was obvious from the images that in the high-frequency region, CuSbS @C shows a high charge transfer resistance (R = 211 W) after the first cycle, y ct because there was no fast ion conductor membrane on the fresh electrode [16]. However, after 30 cycles, the resistance of R only increased to 315.2 W, much lower than the charge ct transfer resistance of CuSbS (R = 664.2 W). The presence of nitrogen-doped carbon layers y ct has also been shown to be effective in improving the electron/ion transfer capability of the CuSbS @C. After the first cycle of CuSbS @C, the slope of Z in the low-frequency y y w zone was the shortest. During the 30 cycles, the slope became smaller and smaller and finally remained unchanged, and the resistance increased steadily, which was caused by the generation of a stable SEI layer [57]. These results again demonstrated the importance of the presence of the nitrogen-doped carbon layer for improving the electrical conductivity and stability of CuSbS @C. The kinetics of the K diffusion behavior of Cu S@C, Sb S @C, 2 2 3 CuSbS , and CuSbS @C electrodes during electrochemical processes were compared by y y GITT. Figure 6b,c shows the galvanostatic intermittent titration technique (GITT) curves for 1 + Cu S@C, Sb S @C, CuSbS , and CuSbS @C at 0.1 A g and the K diffusion coefficient D 2 2 3 y y K in relation to voltage during the charging and discharging. Compared with other electrode materials, the CuSbS @C electrode had the smallest overpotential. Also, the D value of y K the CuSbS @C electrode was higher than that of Cu S@C, Sb S @C, and CuSbS electrode y 2 2 3 y during the charging. In summary, the CuSbS @C electrode had the fastest K diffusion kinetics during the electrochemical reaction. To reveal the reason for the excellent reversibility formation of CuSbS @C, CV curves in the 0.2–1.0 mV s range for various sweep speeds were measured. (Figure S7). As shown in Figure 6d, the outstanding reversibility of the material is demonstrated by the CV curves, which did not noticeably change as the peak current increased with the scan rate. Equation (1) give the scan rate (v) and peak current (i) of CuSbS @C during the CV test [58]: i = av (1) For calculation convenience, the formula is simplified to Equation (2): Log(i) = b log(v) + log(a) (2) The simplified equation shows that the b-value depends on the slope of the logarithmic relationship between log(i) and log(v). In general, the electrochemical reaction primarily displays pseudocapacitive behavior when the b-value is close to 1.0, and the ion diffusion behavior when the b-value is close to 0.5 [59]. The b-value calculated from the curves of Figure 6e,f for the different peaks are 0.85 and 0.93, b-values close to 1.0 show that the pseudocapacitive contribution accounts for the majority of the capacity. Subsequently, to quantify the percentage contribution of the pseudocapacitive, Equation (3) is used to calculate. 1/2 i = k v + k v (3) 1 2 Batteries 2023, 9, 238 11 of 15 In the formula, i represents the current value at a fixed voltage, the contribution of the capacitive process is represented by k v, and the contribution of the diffusion control process 1/ 2 is denoted by k v [60]. As shown in Figure 7a–e, the pseudocapacitive contribution of CuSbS @C increased from 72.2% to 85.2% as the scanning rate increased from 0.2 Batteries 2023, 9, x FOR PEER REVIEW 12 of 16 to 1.0 mV s . Compared with Cu S@C, Sb S @C, and CuSbS , the pseudocapacitive 2 2 3 y contribution of CuSbS @C was the highest (Figure 7f), which is probably better explains why the core-shell of CuSbS @C electrode material is more favorable for K storage. 3.0 (b) (a) Cu S@C Sb S @C 2 3 2.5 CuSbS Cu S@C 2 CuSbS @C Sb S @C 2.0 2 3 CuSbS CuSbS @C y 1.5 Cycle 30th Cycle 1st 1.0 200 0.5 0 200 400 600 800 1000 Z' (ohm) 0.0 0 200 400 600 800 1000 0 20 40 60 80 100 Z' (ohm) Time (h) Cu S@C (c) (d) CuSbS @C 0.8 Sb S @C 2 3 Peak 1 11E-10 E−1 CuSbS CuSbS @C 0.4 1 1E-11 E−11 0.0 0.2 mV/s 1 1E-12 E−1 − -0.4 0.4 0.4 mV/s Peak 2 0.6 mV/s 0.8 mV/s − -0.8 0.8 1 1E-13 E−1 1.0 mV/s Discharge Charge 3.0 2.0 1.0 0 1.0 2.0 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 + + Voltage (V vs. K /K) Voltage (V vs. K /K) 0.0 0.0 (e) (f) b=0.82 Peak 2 Peak 1 b=0.85 −-0.3 0.3 − -0.3 0.3 b=0.74 b=0.93 -0.6 − -0.6 0.6 −0.6 b=0.85 b=0.72 -0.9 − -0.9 0.9 −0.9 b=0.48 -1.2 − -1.2 1.2 −1.2 b=0.38 Cu S@C Cu S@C -1.5 − -1.5 1.5 −1.5 Sb S @C 2 3 Sb S @C 2 3 CuSbS CuSbS −-1.8 1.8 −-1.8 1.8 CuSbS @C CuSbS @C -0.6 -0.3 0.0 00.3 .3 0.0 0.3 −0.6 −0.3 0.0 −0.6 −0.3 -0.6 -0.3 0.0 0.3 log (v, scan rate) log (v, scan rate) Figure 6. (a) Nyquist plots of Cu S@C, Sb S @C, CuSbS , and CuSbS @C electrodes with different 2 2 3 y y Figure 6. (a) Nyquist plots of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes with different cycle numbers at 1.0 A g . (b) GITT curves of Cu S@C, Sb S @C, CuSbS , and CuSbS @C electrodes. y y −1 2 2 3 cycle numbers at 1.0 A g . (b) GITT curves of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes. (c) The K diffusion coefficients of Cu S@C, Sb S @C, CuSbS , and CuSbS @C electrodes during + y y 2 2 3 (c) The K diffusion coefficients of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes during charging and discharging. (d) CV curves of CuSbS @C at various rates. Log(i) versus log(v) for charging and discharging. (d) CV curves of CuSbSy@C at various rates. Log(i) versus log(v) for (e) (e) peak 1, and (f) peak 2. peak 1, and (f) peak 2. To reveal the reason for the excellent reversibility formation of CuSbSy@C, CV curves −1 in the 0.2–1.0 mV s range for various sweep speeds were measured. (Figure S7). As shown in Figure 6d, the outstanding reversibility of the material is demonstrated by the CV curves, which did not noticeably change as the peak current increased with the scan rate. Equation (1) give the scan rate (v) and peak current (i) of CuSbSy@C during the CV test [58]: (1) 𝑖 = 𝑎 𝑣 For calculation convenience, the formula is simplified to Equation (2): Diffusion coefficient Z" (ohm) log (I, current) Z" (ohm) Voltage (V vs. K /K) log (I, current) Current (mA) Batteries 2023, 9, x FOR PEER REVIEW 13 of 16 𝐿𝑜𝑔 𝑖 =𝑏 log 𝑣 log 𝑎 (2) The simplified equation shows that the b-value depends on the slope of the logarith- mic relationship between log(i) and log(v). In general, the electrochemical reaction pri- marily displays pseudocapacitive behavior when the b-value is close to 1.0, and the ion diffusion behavior when the b-value is close to 0.5 [59]. The b-value calculated from the curves of Figure 6e,f for the different peaks are 0.85 and 0.93, b-values close to 1.0 show that the pseudocapacitive contribution accounts for the majority of the capacity. Subse- quently, to quantify the percentage contribution of the pseudocapacitive, Equation (3) is used to calculate. 𝑖= 𝑘 𝑣 𝑘 𝑣 (3) In the formula, i represents the current value at a fixed voltage, the contribution of the capacitive process is represented by k1v, and the contribution of the diffusion control 1/2 process is denoted by k2v [60]. As shown in Figure 7a–e, the pseudocapacitive contribu- tion of CuSbSy@C increased from 72.2% to 85.2% as the scanning rate increased from 0.2 −1 to 1.0 mV s . Compared with Cu2S@C, Sb2S3@C, and CuSbSy, the pseudocapacitive contri- bution of CuSbSy@C was the highest (Figure 7f), which is probably better explains why Batteries 2023, 9, 238 12 of 15 the core-shell of CuSbSy@C electrode material is more favorable for K storage. −1 Figure 7. Pseudocapacitive contribution of CuSbS @C electrode at different rates of (a) 0.2 mV s , Figure 7. Pseudocapacitive contribution of CuSbSy@C electrode at y different rates of (a) 0.2 mV s , 1 1 1 1 −1 −1 −1 −1 (b (b ) 0. ) 0.4 4 mV s mV s , (c ,)( 0. c)6 mV s 0.6 mV , ( s d), 0. (d8 mV s ) 0.8 mV , and s , (and e) 1.0 mV (e) 1.0smV . (f) P s e.rcentage contribution (f) Percentage contribution of pseu- of docapacitive for Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes. pseudocapacitive for Cu S@C, Sb S @C, CuSbS , and CuSbS @C electrodes. 2 2 3 y y 4. Conclusions 4. Conclusions In summary, in situ precipitation and carbonization were used to create CuSbS @C In summary, in situ precipitation and carbonization were used to create CuSbSy@C nanospheres with a distinctive yolk-shell structure. The internal bimetallic sulfide CuSbS nanospheres with a distinctive yolk-shell structure. The internal bimetallic sulfide CuSbSy makes use of the synergistic interaction between Cu and Sb to boost overall specific capacity makes use of the synergistic interaction between Cu and Sb to boost overall specific ca- while also reducing volume expansion and enhancing structural stability. The external pacity while also reducing volume expansion and enhancing structural stability. The ex- nitrogen-doped carbon shell protects the material and confines the bimetallic compound to ternal nitrogen-doped carbon shell protects the material and confines the bimetallic com- a limited space, making it more stable and providing longer cycle life. Thus, the CuSbS @C pound to a limited space, making it more stable and providing longer cycle life. Thus, the electrode maintains an appreciable reversible capacity (438.8 mAh g after 60 cycles at −1 CuSbSy@C electrode maintains an appreciable reversible capacity (438.8 mAh g after 60 1 1 1 100 mA g ), a good rate capability (173.6 mAh g at 5.0 A g ), and superior long cycle −1 −1 −1 cycles at 100 mA g ), a good rate capability (173.6 mAh g at 5.0 A g ), and superior long 1 1 life (174.5 mAh g after 1000 cycles at 1000 mA g ). −1 −1 cycle life (174.5 mAh g after 1000 cycles at 1000 mA g ). Supplementary Materials: The following supporting information can be downloaded at: https:// Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/batteries9050238/s1, Instrumentation and Sample Analysis. Electro- www.mdpi.com/xxx/s1, Instrumentation and Sample Analysis. Electrochemical measurement. Fig- chemical measurement. Figure S1: XRD patterns of (a) Cu S@C, (b) Sb S @C, (c) CuSbS . Figure S2. 2 2 3 y ure S1: XRD patterns of (a) Cu2S@C, (b) Sb2S3@C, (c) CuSbSy.; Figure S2. Initial ten CV curves of (a) Initial ten CV curves of (a) Cu S@C, (b) Sb S @C, and (c) CuSbS electrode at a scan rate of 0.1 mV s 2 2 3 y in a potential range from 0.01–3 V. Figure S3. Charge/discharge profiles of (a) Cu S@C, (b) Sb S @C, 2 2 3 and (c) CuSbS electrodes within the potential of 0.01–3 V at a current density of 0.1 A g . Figure S4. Cycling performance of CuSbS @C electrodes at current densities of 0.5 A g . Figure S5. The posi- tions of Cu S@C, Sb S @C, CuSbS , and CuSbS @C at (a) 100 kHz, (b) 1 kHz, and (c) 1 Hz. Figure S6. y y 2 2 3 Bode plots of (a) Cu S@C, (b) Sb S @C, (c) CuSbS , and (d) CuSbS @C electrode. Figure S7. CV y y 2 2 3 curves of (a) Cu S@C, (b) Sb S @C, and (c) CuSbS electrode at various scan rates of 0.2–1.0 mV s . 2 2 3 Table S1. Content of Cu, Sb, and S elements in the compounds. Table S2. Performance comparison between this study and existing studies. Table S3. The comparison of R , R and R of Cu S@C, s ct SEI, 2 Sb S @C, CuSbS , and CuSbS @C. Table S4. The comparison of  , and  of Cu S@C, Sb S @C, y y 2 3 1 2 2 2 3 CuSbS , and CuSbS @C. y y Author Contributions: Conceptualization, P.H. and G.Y.; methodology, P.H. and Y.D.; software, P.H., X.C. and G.Y.; validation, P.H., X.C. and S.H.; formal analysis, P.H. and Y.D.; investigation, P.H.; resources, Y.L.; data curation, P.H.; writing—original draft preparation, P.H.; writing—review and editing, Y.D., H.Z. and Y.L.; visualization, Y.L.; supervision, Q.F. and Y.L.; project administration, Y.L.; funding acquisition, Q.F. and Y.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (22076116) and the Sino-German Center for Research Promotion (GZ1579). Institutional Review Board Statement: Not applicable. 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Hollow CuSbSy Coated by Nitrogen-Doped Carbon as Anode Electrode for High-Performance Potassium-Ion Storage

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batteries Article Hollow CuSbS Coated by Nitrogen-Doped Carbon as Anode Electrode for High-Performance Potassium-Ion Storage 1 , † 2 , † 1 1 1 2 1 2 , Ping Hu , Yulian Dong , Guowei Yang , Xin Chao , Shijiang He , Huaping Zhao , Qun Fu and Yong Lei * Institute of Nano Chemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China; pinghu1998@shu.edu.cn (P.H.); yangguowei@shu.edu.cn (G.Y.); xinchao@shu.edu.cn (X.C.); 21722731@shu.edu.cn (S.H.); ygfqyx@shu.edu.cn (Q.F.) Fachgebiet Angewandte Nanophysik, Institut für Physik & IMN MacroNano, Technische Universität Ilmenau, 98693 Ilmenau, Germany; yulian.dong@tu-ilmenau.de (Y.D.); huaping.zhao@tu-ilmenau.de (H.Z.) * Correspondence: yong.lei@tu-ilmenau.de † These authors contributed equally to this work. Abstract: As a potential anode material for potassium-ion batteries (PIBs), bimetallic sulfides are favored by researchers for their high specific capacity, low cost, and long cycle life. However, the non-ideal diffusion rate and poor cycle stability pose significant challenges in practical applications. In this work, bimetallic sulfide CuSbS @C with a yolk-shell structure was synthesized by in situ precipitation and carbonization. When CuSbS is applied in the anode of PIBs, it can provide the desired capacity and reduce the volume expansion of the compound through the synergistic effect between copper and antimony. At the same time, the existence of the nitrogen-doped carbon shell confines the material within the shell while improving its electrical conductivity, inhibiting its volume expansion and aggregation. Therefore, CuSbS @C exhibits a satisfactory capacity (438.8 mAh g 1 1 1 at 100 mA g after 60 cycles) and an excellent long cycle life (174.5 mAh g at 1000 mA g after 1000 cycles). Keywords: CuSbS @C; potassium-ion batteries; yolk shell structure; bimetallic sulfide Citation: Hu, P.; Dong, Y.; Yang, G.; Chao, X.; He, S.; Zhao, H.; Fu, Q.; Lei, Y. Hollow CuSbS Coated by 1. Introduction Nitrogen-Doped Carbon as Anode Electrode for High-Performance Because the typical reduction potential of potassium (2.93 V) is comparable to the Potassium-Ion Storage. Batteries 2023, reduction potential of lithium (3.04 V), potassium ion batteries (PIBs) are drawing more 9, 238. https://doi.org/10.3390/ attention as a useful alternative to lithium-ion batteries (LIBs) [1]. In addition, massive batteries9050238 energy storage systems can be equipped with enough potassium thanks to the abundance of resources that are kept in the earth’s crust. [2,3]. Despite these mentioned advantages, Academic Editor: Vilas Pol there are still several challenges to impede the practical applications of PIBs. For instance, Received: 15 March 2023 the slow kinetic process of potassium ions (K ) during charging/discharging and the Revised: 19 April 2023 poor rate performance are caused by the large ionic radius of K (1.38 Å) [4–6]. Besides, Accepted: 21 April 2023 + undesirable expansion of the material and K due to the reaction, known as the intercalation Published: 23 April 2023 + + of K or alloying with K , causes the destruction of the active material structure and an irreversible decrease of capacity [7–9]. Therefore, developing various anode materials with high capacity, high stability, and facilitated K kinetic for PIBs effectively pushes the further development of PIBs. Copyright: © 2023 by the authors. Anode materials research reveals promising storage systems for PIBs, such as carbon- Licensee MDPI, Basel, Switzerland. based materials [10,11], organic materials [12,13], metallic oxides [14,15], metallic sul- This article is an open access article fides [16,17], metallic selenides [18,19], metallic phosphides [20,21], etc. Among them, distributed under the terms and carbon-based materials could be more practical and convenient since they are easily acces- conditions of the Creative Commons sible from nature. However, they deliver a lower reversible capacity. Organic materials Attribution (CC BY) license (https:// with tunable chemical composition have expanded their applications as anode/cathode creativecommons.org/licenses/by/ electrode materials. But they are usually limited by poor electronic conductivity [10,12]. 4.0/). Batteries 2023, 9, 238. https://doi.org/10.3390/batteries9050238 https://www.mdpi.com/journal/batteries Batteries 2023, 9, 238 2 of 15 Metal sulfides with tantalizing capacities can promote the electrochemical performance of PIBs, Nevertheless, the bulk dilatation of these metallic sulfides and the limited cycle life of these electrodes would adversely affect their practical applications [22–24]. Therefore, much attention has been focused on pursuing effective strategies to realize the high perfor- mance of PIBs by nanostructure engineering, compositing with conductive materials [25], and heteroatoms doping in metal sulfides [26]. These strategies dramatically enhance the capacity and stability of the materials and provide more opportunities for the commercial application of bimetallic compounds in PIBs. For instance, Huang et al. embedded Cu S 9 5 into a porous carbon framework derivatized from Zn-based MOF by vulcanization and subsequent ion exchange [27]. Cu S anode exhibited a reversible capacity of 316 mA h g 9 5 1 1 1 after 200 cycles at 100 mA g and a better rate performance of 170 mAh g at 2.0 A g . Wang et al. obtained Bi S @SC by coating sulfur-doped carbon on the surface of Bi S [28]. 2 3 2 3 Bi S @SC anode achieved high reversible capacity and superior rate capacity. These strate- 2 3 gies are effective in mitigating the bulk changes of electrode materials, but the rate and cycle performance at a high current density of the monometallic sulfides has been limited to a certain extent. Therefore, we focus on bimetallic sulfides with synergistic effects that have higher capacity and stability than monometallic sulfides. Structural integrity and cyclic stability are maintained by reducing volume expansion and pulverization of the material. Since the redox potentials of the two metals are different, bimetallic sulfides take full advantage of their differences in electrochemical reactions and exhibit higher electrochemical performance. So far, some bimetallic sulfides have made some research progress in PIBs [29–33]. The reduced graphene oxide encapsulated flower-like spherical FeCoS obtained by Chen et al. was used as the anode for PIBs, it was capable of main- 1 1 taining a reversible capacity of 365.2 mAh g at 100 mA g after 150 cycles [16]. The CoS /ZnS@rGO prepared by Sikandar Iqbal et al. exhibited excellent cycling performance, 1 1 maintaining a high reversible capacity of 565 mAh g after 100 cycles at 100 mA g [17]. These studies show that bimetallic sulfides can achieve higher capacities in PIBs, but they are generally costly because of the expensive graphene used. Additionally, the electrode materials have shown average rate performance at high current densities. So far, there are still limitations in the study of bimetallic sulfides, and further investigation is needed. Therefore, in this work, the synthesis of yolk-shell structured CuSbS @C nanocompos- ites via in-situ precipitation and carbonization is simple and the precursor materials are cheap. The intrinsic properties of CuSbS delivered high capacity, benefiting from the ap- pealing electrochemical conversion and alloying mechanisms. The nitrogen-doped carbon shell is considered a buffer layer to limit the bulk expansion of CuSbS nanoparticles to a limited space while avoiding the aggregation of nanoparticles, and in addition, it provides more defects and active sites to store more K during the cycling process. At the same time, the existence of a carbon shell also increased the electroconductivity of CuSbS material. 1 1 Thus, CuSbS @C exhibited excellent reversible capacity (438.8 mA h g at 100 mA g 1 1 after 60 cycles) and long cycle life (174.5 mA h g at 1000 mA g after 1000 cycles) in PIBs. 2. Materials and Methods 2.1. Materials Sodium citrate (C H Na O , 99.5%), copper sulfate pentahydrate (CuSO 5H O, 6 5 3 7 4 2 ACS reagent grade, 99%), antimony chloride (SbCl , 99.95%), sodium hydroxide (NaOH), L-ascorbic acid, sodium sulfide nonahydrate (Na S9H O, 99.5%), muriatic acid 2 2 (HCl, 37%), tris (hydroxymethyl) aminomethane (Tris), thioacetamide (TAA), dopamine hydrochloride (PDA), potassium metal (K, chunks in mineral oil, 98%) was purchased from China National Medicines Corporation Ltd., (Beijing, China) potassium bis (fluorosulfonyl) imide (KFSI, 97%), ethylene carbonate (EC, 99%), and diethyl carbonate (DEC, 99%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Batteries 2023, 9, 238 3 of 15 2.2. Synthesis of Cu S and Sb S 9 5 2 3 First, 6 mmol of CuSO 5H O and 2 mmol of C H Na O were mixed into 100 mL of 4 2 6 5 3 7 distilled water and stirred for 30 min at a constant speed to form clarified solution A. A total of 0.1 mol NaOH was dispersed in 80 mL distilled water and the clarified solution B was achieved by magnetic stirring for 15 min. Slowly drop solution B into solution A under magnetic stirring. Next, 6 mmol of L-ascorbic acid was poured into the solution. After stirring for some time, the mixture was centrifuged to obtain Cu O. Then, the resulting Cu O was supersonically distributed in 100 mL distilled water for 30 min to form solution C. To make solution D, 24 mmol Na S9H O was poured into 40 mL of distilled water. After 2 2 pouring solution D into the burette, adding solution C in a uniform drop, and stirring for 1 h, the centrifuged solid was scattered in 100 mL of 2 M hydrochloric acid to wash out the superfluous Cu O. The obtained sediment was cleaned by centrifugation to a pH-neutral supernatant and dried for 12 h to yield Cu S. To acquire Sb S , 0.5 mmol of SbCl was 2 2 3 3 distributed in 40 mL of ethylene glycol and agitated for 4 h, subsequently, adding 2 mmol TAA and stirring for 5 min. The mixture was hydrothermally treated in a reaction vessel at 160 C for 12 h and then centrifuged and dried at 60 C. In a tube furnace, Sb S was 2 3 heated to 400 C at an annealing temperature of 3 C/min and then held for 2 h. 2.3. Synthesis of Cu S@C, Sb S @C 2 2 3 Firstly, 700 mg Cu S was ultrasonically distributed in 150 mL tris buffer (1 M) for 30 min. To the mixture, 700 mg of dopamine hydrochloride was added and stirred con- tinuously for 24 h. The resulting mixture was washed with deionized water to neutrality and then washed by centrifugation with anhydrous ethanol. Cu S@PDA was acquired by vacuum stoving the precipitate at 60 C. Then, 1 g Cu S@PDA was put in a tubular furnace, warming to 450 C under the protection of the Ar atmosphere, and kept for 3 h. The Cu S@C composite was removed after natural cooling. Sb S @C was produced using 2 2 3 the same method as Cu S@C except that Cu S was replaced by Sb S . 2 2 2 3 2.4. Synthesis of CuSbS @C and CuSbS y y Firstly, 500 mg of Cu S@C composite material was dissolved in 100 mL of anhydrous ethanol, and solution E was formed after 30 min of ultrasonic dispersion. Addition of 7 mmol of antimony trichloride (SbCl ) to 100 mL of anhydrous ethanol to fully dissolve, and after complete dissolution, solution F was formed. Solution F was slowly dripped into solution E under magnetic stirring, and after adding all drops, the solution was mixed with magnetic stirring for 24 h. CuSbS @C was obtained by centrifugally cleaning the obtained black sediment with anhydrous ethanol and drying it for 12 h at 60 C. A total of 500 mg CuSbS @C was held at 450 C for 1 h with a temperature rise rate of 3 C/min to improve crystallinity. The synthesis process of CuSbS was the same as that of CuSbS @C except y y that no carbon was added. 3. Results and Discussion 3.1. Material Characterization Figure 1 depicts the schematic method for the synthesis of the yolk-shell structure CuSbS @C. First of all, CuSO 5H O and C H Na O were dissolved in deionized water, 4 2 6 5 3 7 and Cu(OH) precipitate was formed after adding NaOH, and then L-ascorbic acid was added to form Cu O red precipitate. Then, Na S9H O was added and the excess Cu O 2 2 2 2 was washed away with HCl to obtain Cu S, which was subsequently coated with dopamine and carbonized to obtain Cu S@C. Finally, SbCl was added and carbonized again to form 2 3 the CuSbS @C composite with a yolk-shell structure. As shown in Figure 2a, the morphology of Cu S@C presented irregular and unevenly distributed nanoparticles. The Sb S @C consisted of a large number of stacked nanopar- 2 3 ticles with a diameter of approximately 180 nm (Figure 2b). Clearly, the CuSbS hollow nanoparticle has a uniform shell and a wall thickness of around 50 nm (Figure 2c). As seen in Figure 2d–f, the CuSbS @C exhibited a homogeneous micro-spherical yolk-shell struc- Batteries 2023, 9, 238 4 of 15 ture after carbon coating and annealing. The acquisition of this unique yolk-shell structure may be due to the volume of the internal metallic material first expanding the carbon layer after the expansion during the ion exchange reaction, and the internal material shrinkage during annealing, leading to the formation of large voids between the spherical shell and the internal nano microspheres. This unique structure improved the stability of the material compared to Cu S@C, Sb S @C, and CuSbS and effectively avoided agglomeration and 2 2 3 y escape of active substances. The presence of a yolk-shell structure obtained more uniform void distribution, more electrochemically active sites, larger nanomaterial/electrolyte con- tact area, shorter ion diffusion paths, accelerated ion transport, and adapted to volume changes during potassiation [34–36]. Figure 2e shows the transmission electron microscope (TEM) images of CuSbS @C. The part surrounded by the green circle was the pure material of CuSbS , and the position of the carbon layer is in the middle of the red and green lines. To confirm the precise morphology and distribution of the components of the substance, selecting elemental mapping (EDS) tests were performed on the active material in a defined area. The Cu, Sb, S, C, and N elemental distributions can be seen visually by EDS (Figure 2f). Batteries 2023, 9, x FOR PEER REVIEW 4 of 16 In addition, the homogeneous dispersion of carbon and nitrogen characteristics was seen, which also demonstrated that the surface of CuSbS was covered by a uniform layer of nitrogen-doped carbon. Figure 1. Schematic illustration of the synthesis process of CuSbSy@C. Figure 1. Schematic illustration of the synthesis process of CuSbS @C. X-ray diffraction (XRD) and Raman were used to characterize Cu S@C, Sb S @C, 2 2 3 As shown in Figure 2a, the morphology of Cu2S@C presented irregular and unevenly CuSbS , and CuSbS @C. Figure 3a shows the XRD spectra of CuSbS @C in agreement with y y y distributed nanoparticles. The Sb2S3@C consisted of a large number of stacked nanoparti- the standard card (Cu Sb S # PDF 24-1318, CuSbS # PDF 44-1417). The XRD patterns 12 4 13 2 cles with a diameter of approximately 180 nm (Figure 2b). Clearly, the CuSbSy hollow of Cu S @ C, Sb S @C, and CuSbS also corresponded to the standard cards (Figure S1). 2 2 3 y nanoparticle has a uniform shell and a wall thickness of around 50 nm (Figure 2c). As seen The peaks corresponding to the elements in CuSbS @C were found on the diffraction in Figure 2d–f, the CuSbSy@C exhibited a homogeneous micro-spherical yolk-shell struc- planes (013), (110), (111), (410), (020), and (301), which was confirmed the successful ture after carbon coating and annealing. The acquisition of this unique yolk-shell structure preparation of CuSbS @C [33,37–39]. To obtain the content of Cu, Sb, and S elements in the may be due to the volume of the internal metallic material first expanding the carbon layer compound more precisely, the compound was tested by Inductively coupled plasma-Mass Spectrometry (ICP-MS), and the results showed that the ratio of the three elements in after the expansion during the ion exchange reaction, and the internal material shrinkage the compound was 1:1.63:0.62 (Table S1). The compound was named CuSbS @C in the during annealing, leading to the formation of large voids between the spherical shell and text. As for Raman spectra of Cu S@C, an obvious characteristic peak of Cu S appears 2 2 the internal nano microspheres. This unique structure improved the stability of the mate- 1 1 at 290 cm , while the weak peak at 610 cm could be attributed to the signal of CuO rial compared to Cu2S@C, Sb2S3@C, and CuSbSy and effectively avoided agglomeration due to the oxidation of the material exposed to the air [40]. From the Raman spectra of and escape of active substances. The presence of a yolk-shell structure obtained more uni- Sb S @C, we can see that there are two continuous characteristic peaks at 284 and 311 cm , 2 3 form void distribution, more electrochemically active sites, larger nanomaterial/electro- lyte contact area, shorter ion diffusion paths, accelerated ion transport, and adapted to volume changes during potassiation [34–36]. Figure 2e shows the transmission electron microscope (TEM) images of CuSbSy@C. The part surrounded by the green circle was the pure material of CuSbSy, and the position of the carbon layer is in the middle of the red and green lines. To confirm the precise morphology and distribution of the components of the substance, selecting elemental mapping (EDS) tests were performed on the active material in a defined area. The Cu, Sb, S, C, and N elemental distributions can be seen visually by EDS (Figure 2f). In addition, the homogeneous dispersion of carbon and ni- trogen characteristics was seen, which also demonstrated that the surface of CuSbSy was covered by a uniform layer of nitrogen-doped carbon. Batteries 2023, 9, 238 5 of 15 corresponding to the vibration of the Sb-S bond [41]. The characteristic peak at 250 cm could be ascribed to the vibration of the Cu-S bond in the sample of CuSbS and CuSbS @C, y y while the characteristic peak at 468 cm is due to the characteristic phonon vibration of chalcopyrite CuSbS phase [42]. The weak peaks at 624 and 785 cm may be related to the oxidation of CuSbS to form oxides when exposed to air (Figure 3b). In addition, strong peaks of Cu S@C, Sb S @C, and CuSbS @C in the Raman spectra appear at 1563.2 and 2 2 3 y 1365.9 cm , correlating to the G-band of graphitic carbon and the D-band of amorphous carbon, respectively. The proportion of the two peaks was determined as the strength ratio (I /I ) of the material, and the ratio was 0.94, which was less than 1.0, indicating D G that there were many defects in the carbon in the composites [16,43]. According to the existing research, materials with a lot of defects largely provided sufficient K storage sites and showed excellent electrochemical performance in the application of PIBs [44,45]. To further characterize the carbon content of the material, the samples were subjected to thermogravimetric analysis (TGA) at temperatures ranging from 25 to 800 C. As shown in Figure 3c, the evaporation of residual moisture in the material caused a small mass loss (1.77%) of CuSbS @C in the temperature range of 25–250 C. The slow mass loss (11.7%) of CuSbS @C at 250–500 C would be explained in two ways. On one hand, the carbon in CuSbS @C was burned completely, resulting in mass loss. On the other hand, the quality was improved by forming oxides (CuO and Sb O ). As a result, the overall capacity loss 2 3 was relatively slow. The loss of mass was highest at about 750 C when the oxide was completely decomposed, and the mass loss reached 31.14% tremendously. According to the mass loss in each temperature range, the carbon content of CuSbS @C was 31.3–34.5%, Batteries 2023, 9, x FOR PEER REVIEW 5 of 16 and its low carbon content makes the compound maintain high capacity under the basis of stability. (b) (a) (c) (f) (d) (e) CuSbS Figure 2. (a–d) SEM images of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C. (e) Transmission electron Figure 2. (a–d) SEM images of Cu S@C, Sb S @C, CuSbS , and CuSbS @C. (e) Transmission electron y y 2 2 3 microscope images of CuSbSy@C. (f) Element mapping images of CuSbSy@C. microscope images of CuSbS @C. (f) Element mapping images of CuSbS @C. y y X-ray diffraction (XRD) and Raman were used to characterize Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C. Figure 3a shows the XRD spectra of CuSbSy@C in agreement with the standard card (Cu12Sb4S13 # PDF 24-1318, CuSbS2 # PDF 44-1417). The XRD patterns of Cu2S @ C, Sb2S3@C, and CuSbSy also corresponded to the standard cards (Figure S1). The peaks corresponding to the elements in CuSbSy@C were found on the diffraction planes (013), (110), (111), (410), (020), and (301), which was confirmed the successful preparation of CuSbSy@C [33,37–39]. To obtain the content of Cu, Sb, and S elements in the compound more precisely, the compound was tested by Inductively coupled plasma-Mass Spectrom- etry (ICP-MS), and the results showed that the ratio of the three elements in the compound was 1:1.63:0.62 (Table S1). The compound was named CuSbSy@C in the text. As for Raman −1 spectra of Cu2S@C, an obvious characteristic peak of Cu2S appears at 290 cm , while the −1 weak peak at 610 cm could be attributed to the signal of CuO due to the oxidation of the material exposed to the air [40]. From the Raman spectra of Sb2S3@C, we can see that there −1 are two continuous characteristic peaks at 284 and 311 cm , corresponding to the vibra- −1 tion of the Sb-S bond [41]. The characteristic peak at 250 cm could be ascribed to the vibration of the Cu-S bond in the sample of CuSbSy and CuSbSy@C, while the characteristic −1 peak at 468 cm is due to the characteristic phonon vibration of chalcopyrite CuSbS2 phase −1 [42]. The weak peaks at 624 and 785 cm may be related to the oxidation of CuSbS2 to form oxides when exposed to air (Figure 3b). In addition, strong peaks of Cu2S@C, Sb2S3@C, and –1 CuSbSy@C in the Raman spectra appear at 1563.2 and 1365.9 cm , correlating to the G- band of graphitic carbon and the D-band of amorphous carbon, respectively. The propor- tion of the two peaks was determined as the strength ratio (ID/IG) of the material, and the ratio was 0.94, which was less than 1.0, indicating that there were many defects in the carbon in the composites [16,43]. According to the existing research, materials with a lot of defects largely provided sufficient K storage sites and showed excellent electrochemi- Batteries 2023, 9, x FOR PEER REVIEW 7 of 16 Batteries 2023, 9, 238 6 of 15 (a) (b) (c) Figure 3. (a) XRD of CuSbS @C. (b) Raman spectra of Cu S@C, Sb S @C, CuSbS , and CuSbS @C. y 2 2 3 y y (c) TGA patterns of CuSbS @C. The chemical composition of CuSbS @C was further determined by X-ray Photoelec- tron Spectroscopy (XPS) analysis. The real presence of Cu, Sb, S, C, N, and O elements in CuSbS @C composites was confirmed by XPS spectra in the 0–1000 eV binding energy y Batteries 2023, 9, 238 7 of 15 range (Figure 4a). Figure 4b–f presents the fine spectrum of the different elemental orbitals in CuSbS @C. Figure 4b shows the fine spectrum of C 1s with C=O, C-N/C-S, and C=C bonds belonging to the peaks at 288.9 eV, 286.0 eV, and 284.8 eV, respectively [46]. Among them, the presence of C-S bonds indicated that the sample had been doped with some S atoms in addition to binding to the metal, whereas the appearance of C=O bonds was possibly caused by the sample being exposed to air. From the N 1s fine spectrum of the material, characteristic peaks were found at 398.7 eV, 400.4 eV, and 401.5 eV, corresponding to pyridine nitrogen (38.91%), pyrrole nitrogen (40.8%), and graphitized nitrogen (20.29%), respectively (Figure 4c) [27]. Four characteristic peaks appeared in the 2p orbitals (2p 3/2 + 2+ and 2p ) of Cu. Cu had two characteristic peaks at 932.1 eV and 952.0 eV, while Cu 1/2 had two peaks at 933.1 eV and 951.7 eV. (Figure 4d). The simultaneous presence of copper + 2+ elements in two valence states (Cu and Cu ) in CuSbS @C had been demonstrated. A pair of characteristic peaks belonging to Sb 3d and Sb 3d were observed in the XPS 5/2 3/2 spectra of Sb 3d at 531.3 eV and 539.7 eV [37,38]. Additionally, surface oxidation from air exposure produced an O 1s peak at 533.5 eV (Figure 4e). The XPS spectrum of S 2p in Figure 4f shows two pairs of peaks, one at 165.7 eV and 164.6 eV, and the other at 163.4 eV 2 2 and 161.3 eV corresponding to the spin orbitals of S 2p and S 2p for (S ) and (S ), 3/2 1/2 2 respectively [47]. 3.2. Electrochemical Performance The performance of the CuSbS @C anode in PIBs was investigated using cyclic voltam- metry (CV) and constant current cycle tests. Figure 5a depicts the CuSbS @C CV curve at a scan speed of 0.1 mV s (0.01–3 V). Compared with Cu S@C, Sb S @C, and CuSbS , 2 2 3 the anode of CuSbS @C had a larger CV curve area, indicating that it had a stronger potassium storage capacity (Figure S2). CuSbS @C exhibited a CV profile that differed significantly from that of its precursors after coating with carbon, because the coating and annealing of dopamine hydrochloride changed the structure and morphology of CuSbS . The formation of a solid electrolyte interface (SEI) layer between the CuSbS @C electrode and the electrolyte caused the revivification peak to appear at around 0.84 V in the first cycle of the curve [48]. The reason for another reduction peak (0.51 V) was explained as a further alloying reaction between Sb metal and K formed after the conversion reaction. The two characteristic peaks gradually disappeared during the subsequent charging and discharging process. There were three oxidation peaks in CuSbS @C during charging, and the corresponding voltages were 1.03 V, 1.79 V, and 1.99 V, respectively, indicating that K was a multi-step depotassiation process in the active material. The new CV curve was nearly identical after the first scan, demonstrating that the CuSbS @C electrode had positive cycle stability and electrochemical performance. The irreversible loss of specific capacity observed in charge/discharge curves for different cycle turns of CuSbS @C at 0.1 A g was explained by the development of the SEI layer and irreversible electrolyte decomposition. [49,50]. The charge/discharge curves of CuSbS @C almost overlapped each other and maintained some stability after the first cycle, matching well with the CV curves and delivering great cycle stability (Figure 5b). In comparison to Cu S@C, Sb S @C, and CuSbS , CuSbS @C exhibited the highest capacity and stability (Figure S3). y y 2 3 Figure 5c shows the cycling performance with Cu S@C, Sb S @C, CuSbS , and CuSbS @C 2 2 3 y y at 100 mA g . From the curves, we can not only see that the CuSbS @C had high capacity, but also verified that the carbon shell effectively improved the reliability of the CuSbS @C. Cu S@C, Sb S @C, and CuSbS had obvious capacity attenuation after nearly 20 cycles 2 2 3 y at 100 mA g , while the CuSbS @C could keep a stable charge/discharge specific capac- ity. In addition, the CuSbS @C delivered remarking initial charge/discharge capacity of 423.1/632.4 mAh g with an initial coulombic efficiency of 66.89%. After 60 cycles at 1 1 100 mA g , CuSbS @C reserved a high specific capacity of 438.8 mAh g . The cycling stability of CuSbS @C was performed at 500 mA g , and exhibited a high capacity of 244.2 mAh g after 1000 cycles (Figure S4). After 1000 cycles at a high current density 1 1 of 1000 mAh g , the CuSbS @C electrode reached a high capacity of 174.5 mAh g y Batteries 2023, 9, 238 8 of 15 and retained 73.2% of its capacity (Figure 5d). The steadily increasing number of cycles implied that the presence of the carbon shell effectively suppressed the bulk expansion of the CuSbS during the reaction and the unfavorable by-reactions between the elec- trolyte and the electrode [51,52]. As shown in Figure 5e, CuSbS @C achieved excellent rate performance in existing bimetallic sulfides (More detailed information was in Table S2). To ulteriorly explore the rate performance of CuSbS @C electrode for PIBs, the charge and discharge capacities were tested at different current densities in the voltage range of 0.01–3 V. (Figure 5f). CuSbS @C electrode exhibited high reversible specific capacities of Batteries 2023, 9, x FOR PEER REVIEW 1 8 of 16 410.8, 356.5, 318.6, 294.5, 267.3, 241.1, 213.4, and 173.6 mAh g at 0.05, 0.1, 0.2, 0.3, 0.5, 1, 1 1 2, 5 A g , respectively. The recovered capacity is maintained at 337.5 mAh g (82.2%) when the current density is restored to 0.05 A g , showing excellent rate performance. Figure 3. (a) XRD of CuSbSy@C. (b) Raman spectra of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C. (c) CuSbS @C utilized the bimetallic synergy and carbon cladding not only obtained capacity TGA patterns of CuSbSy@C. enhancement but also improved the rate performance and long cycle life during cycling, which exhibited strongly competitive in PIBs. (b) Raw date (a) C 1s Fitted date C-C 284.8 eV C-N/C-S 286.0 eV C=O 288.9 eV 0 200 400 600 800 1000 280 282 284 286 288 290 292 Binding Energy (eV) Binding Energy (eV) Raw date Raw date (c) (d) Cu 2p N 1s Fitted date Fitted date Pyrrolic N 400.4 eV Pyridinic N Cu 2p 3/2 398.7 eV Graphitic N Cu Cu 401.5 eV 932.1 eV Cu 2p 952.0 eV 1/2 2+ 2+ Cu Cu 933.1 eV 951.7 eV 930 935 940 945 950 955 960 965 394 396 398 400 402 404 406 408 410 Binding Energy (eV) Binding Energy (eV) (e) Raw date Sb 3d (f) O 1s Fitted date Sb 3d 5/2 O 1s 531.3 eV 533.5 eV Sb 3d 3/2 539.7 eV 524 526 528 530 532 534 536 538 540 542 544 Binding Energy (eV) Figure 4. (a) XPS full spectra of CuSbS @C. XPS fine spectra of (b) C 1s, (c) N 1s, (d) Cu 2p, (e) Sb 3d, Figure 4. (a) XPS full spectra of CuSbSy@C. XPS fine spectra of (b) C 1s, (c) N 1s, (d) Cu 2p, (e) Sb 3d, and (f) S 2p. and (f) S 2p. 3.2. Electrochemical Performance The performance of the CuSbSy@C anode in PIBs was investigated using cyclic volt- ammetry (CV) and constant current cycle tests. Figure 5a depicts the CuSbSy@C CV curve −1 at a scan speed of 0.1 mV s (0.01–3 V). Compared with Cu2S@C, Sb2S3@C, and CuSbSy, Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) S 2p C 1s N 1s O 1s Sb 3d Cu 2p3 Cu 2p3 Intensity (a.u.) Intensity (a.u.) Batteries 2023, 9, x FOR PEER REVIEW 10 of 16 Batteries 2023, 9, 238 9 of 15 3.0 0.15 −1 CuSbS @C 0.1A g (a) CuSbS @C (b) y 1st 1.99V 1.79V 1.03V 0.10 2nd 2.5 4th 0.05 8th 2.0 10th −0.00 1.5 −0.05 −0.10 1st 1.0 2nd 0.84V −0.15 4th 0.5 0.51V 8th −0.20 10th 0.0 −0.25 0 200 400 600 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Voltage (V vs. K /K) Capacity (mAh/g) (c) (d) Cu S@C 800 2 800 Cu S@C Sb S @C 2 2 3 Sb S @C CuSbSy 2 3 600 CuSbS CuSbSy@C 600 −1 CuSbS @C Current density:0.1 A g −1 Current density:1.0 A g 200 200 0 0 0 10 20 30 40 50 60 0 200 400 600 800 1000 Cycle number Cycle number (e) (f) NiCo S @N-HCNFs 2 4 (Bi,Sb) S 2 3 Co Sn -S/rGO 1 6.75 Cu S@C FeCoS @rGO Sb S @C 2 3 NiFeS@C CuSbS This work CuSbS @C 0.05 0.1 0.05 0.2 0.3 0.5 1.0 2.0 5.0 0 0 0 10 20 30 40 50 60 70 80 90 0 2000 4000 6000 8000 Cycle number Rate (mA/g) Figure 5. (a) CV curves of CuSbS @C electrode. (b) Charge/discharge curves of CuSbS @C. y y Figure 5. (a) CV curves of CuSbSy@C electrode. (b) Charge/discharge curves of CuSbSy@C. (c) Cy- −1 (c) Cycling performance of Cu S@C, Sb S @C, CuSbS and CuSbS @C electrodes at 0.1 A g and cling performance of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes at 0.1 A g and (d) 1.0 2 2 3 y, y −1 1 (d A g ) 1.0 . A (eg ) Rate . (e )capabilit Rate capability y of Cu of SbS CuSbS y@C and @C and previo previously usly reported reported mmaterials. aterials. (f () f)Rate Rate of of Cu Cu2S@C, S@C, −1 Sb2S3@C, CuSbSy, and CuSbSy@C from 0.05 to 5.0 A g . Sb S @C, CuSbS , and CuSbS @C from 0.05 to 5.0 A g . y y 2 3 To further investigate the kinetic process of potassium storage in CuSbS @C, more To further investigate the kinetic process of potassium storage in CuSbSy@C, more electrochemical test techniques were used for us to analyze the electron/ion migration electrochemical test techniques were used for us to analyze the electron/ion migration processes. A semicircle made up the high frequency (HF) portion of the Nyquist plot, and processes. A semicircle made up the high frequency (HF) portion of the Nyquist plot, and a sloping straight line made up the low frequency (LF) portion [53]. In the HF region a sloping straight line made up the low frequency (LF) portion [53]. In the HF region (100 (100 kHz–1 kHz), the resistance is mainly controlled by charge transfer, in the medium kHz–1 kHz), the resistance is mainly controlled by charge transfer, in the medium fre- frequency (MF) region (1 kHz–1 Hz), the resistance is mainly controlled by charge transfer quency (MF) region (1 kHz–1 Hz), the resistance is mainly controlled by charge transfer and diffusion, and in the LF region (<1 Hz), the resistance is mainly controlled by diffusion Current (mA) Capacity (mAh/g) Capacity (mAh/g) Coulombic efficiency (%) Voltage (V vs. K /K) Capacity (mAh/g) Capacity (mAh/g) Coulombic efficiency (%) Coulombic efficiency (%) Batteries 2023, 9, 238 10 of 15 (Figure S5). The charge transfer resistance (R ) between the interface of the electrodes ct corresponded to the semicircle, and a lower charge transfer resistance corresponded to a smaller circle radius [54]. The straight line corresponded to Warburg impedance (Z ), and the smaller the slope, the smaller the diffusion resistance of K . Comparing the solution resistance (R ) of four materials, the R of CuSbS @C was the smallest, and that of s s y CuSbS was the largest (Table S3). This was due to the large contact area of the CuSbS @C y y electrode with the electrolyte and the smallest contact area of the CuSbS electrode with the electrolyte [55]. The Bode plots showed that the materials have double time constants, and the impedance fit showed that the difference between the double time constants is not very large, so the diameter of the first semicircle is small and not very significant (Figure S6). The equivalent circuit diagram of double time constants in series was obtained from the impedance fitting analysis [56]. The specific values of the time constants were given in the following Table S4. Figure 6a shows the Nyquist plots of Cu S@C, Sb S @C, CuSbS , 2 2 3 y and CuSbS @C at 1.0 A g . It was obvious from the images that in the high-frequency region, CuSbS @C shows a high charge transfer resistance (R = 211 W) after the first cycle, y ct because there was no fast ion conductor membrane on the fresh electrode [16]. However, after 30 cycles, the resistance of R only increased to 315.2 W, much lower than the charge ct transfer resistance of CuSbS (R = 664.2 W). The presence of nitrogen-doped carbon layers y ct has also been shown to be effective in improving the electron/ion transfer capability of the CuSbS @C. After the first cycle of CuSbS @C, the slope of Z in the low-frequency y y w zone was the shortest. During the 30 cycles, the slope became smaller and smaller and finally remained unchanged, and the resistance increased steadily, which was caused by the generation of a stable SEI layer [57]. These results again demonstrated the importance of the presence of the nitrogen-doped carbon layer for improving the electrical conductivity and stability of CuSbS @C. The kinetics of the K diffusion behavior of Cu S@C, Sb S @C, 2 2 3 CuSbS , and CuSbS @C electrodes during electrochemical processes were compared by y y GITT. Figure 6b,c shows the galvanostatic intermittent titration technique (GITT) curves for 1 + Cu S@C, Sb S @C, CuSbS , and CuSbS @C at 0.1 A g and the K diffusion coefficient D 2 2 3 y y K in relation to voltage during the charging and discharging. Compared with other electrode materials, the CuSbS @C electrode had the smallest overpotential. Also, the D value of y K the CuSbS @C electrode was higher than that of Cu S@C, Sb S @C, and CuSbS electrode y 2 2 3 y during the charging. In summary, the CuSbS @C electrode had the fastest K diffusion kinetics during the electrochemical reaction. To reveal the reason for the excellent reversibility formation of CuSbS @C, CV curves in the 0.2–1.0 mV s range for various sweep speeds were measured. (Figure S7). As shown in Figure 6d, the outstanding reversibility of the material is demonstrated by the CV curves, which did not noticeably change as the peak current increased with the scan rate. Equation (1) give the scan rate (v) and peak current (i) of CuSbS @C during the CV test [58]: i = av (1) For calculation convenience, the formula is simplified to Equation (2): Log(i) = b log(v) + log(a) (2) The simplified equation shows that the b-value depends on the slope of the logarithmic relationship between log(i) and log(v). In general, the electrochemical reaction primarily displays pseudocapacitive behavior when the b-value is close to 1.0, and the ion diffusion behavior when the b-value is close to 0.5 [59]. The b-value calculated from the curves of Figure 6e,f for the different peaks are 0.85 and 0.93, b-values close to 1.0 show that the pseudocapacitive contribution accounts for the majority of the capacity. Subsequently, to quantify the percentage contribution of the pseudocapacitive, Equation (3) is used to calculate. 1/2 i = k v + k v (3) 1 2 Batteries 2023, 9, 238 11 of 15 In the formula, i represents the current value at a fixed voltage, the contribution of the capacitive process is represented by k v, and the contribution of the diffusion control process 1/ 2 is denoted by k v [60]. As shown in Figure 7a–e, the pseudocapacitive contribution of CuSbS @C increased from 72.2% to 85.2% as the scanning rate increased from 0.2 Batteries 2023, 9, x FOR PEER REVIEW 12 of 16 to 1.0 mV s . Compared with Cu S@C, Sb S @C, and CuSbS , the pseudocapacitive 2 2 3 y contribution of CuSbS @C was the highest (Figure 7f), which is probably better explains why the core-shell of CuSbS @C electrode material is more favorable for K storage. 3.0 (b) (a) Cu S@C Sb S @C 2 3 2.5 CuSbS Cu S@C 2 CuSbS @C Sb S @C 2.0 2 3 CuSbS CuSbS @C y 1.5 Cycle 30th Cycle 1st 1.0 200 0.5 0 200 400 600 800 1000 Z' (ohm) 0.0 0 200 400 600 800 1000 0 20 40 60 80 100 Z' (ohm) Time (h) Cu S@C (c) (d) CuSbS @C 0.8 Sb S @C 2 3 Peak 1 11E-10 E−1 CuSbS CuSbS @C 0.4 1 1E-11 E−11 0.0 0.2 mV/s 1 1E-12 E−1 − -0.4 0.4 0.4 mV/s Peak 2 0.6 mV/s 0.8 mV/s − -0.8 0.8 1 1E-13 E−1 1.0 mV/s Discharge Charge 3.0 2.0 1.0 0 1.0 2.0 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 + + Voltage (V vs. K /K) Voltage (V vs. K /K) 0.0 0.0 (e) (f) b=0.82 Peak 2 Peak 1 b=0.85 −-0.3 0.3 − -0.3 0.3 b=0.74 b=0.93 -0.6 − -0.6 0.6 −0.6 b=0.85 b=0.72 -0.9 − -0.9 0.9 −0.9 b=0.48 -1.2 − -1.2 1.2 −1.2 b=0.38 Cu S@C Cu S@C -1.5 − -1.5 1.5 −1.5 Sb S @C 2 3 Sb S @C 2 3 CuSbS CuSbS −-1.8 1.8 −-1.8 1.8 CuSbS @C CuSbS @C -0.6 -0.3 0.0 00.3 .3 0.0 0.3 −0.6 −0.3 0.0 −0.6 −0.3 -0.6 -0.3 0.0 0.3 log (v, scan rate) log (v, scan rate) Figure 6. (a) Nyquist plots of Cu S@C, Sb S @C, CuSbS , and CuSbS @C electrodes with different 2 2 3 y y Figure 6. (a) Nyquist plots of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes with different cycle numbers at 1.0 A g . (b) GITT curves of Cu S@C, Sb S @C, CuSbS , and CuSbS @C electrodes. y y −1 2 2 3 cycle numbers at 1.0 A g . (b) GITT curves of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes. (c) The K diffusion coefficients of Cu S@C, Sb S @C, CuSbS , and CuSbS @C electrodes during + y y 2 2 3 (c) The K diffusion coefficients of Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes during charging and discharging. (d) CV curves of CuSbS @C at various rates. Log(i) versus log(v) for charging and discharging. (d) CV curves of CuSbSy@C at various rates. Log(i) versus log(v) for (e) (e) peak 1, and (f) peak 2. peak 1, and (f) peak 2. To reveal the reason for the excellent reversibility formation of CuSbSy@C, CV curves −1 in the 0.2–1.0 mV s range for various sweep speeds were measured. (Figure S7). As shown in Figure 6d, the outstanding reversibility of the material is demonstrated by the CV curves, which did not noticeably change as the peak current increased with the scan rate. Equation (1) give the scan rate (v) and peak current (i) of CuSbSy@C during the CV test [58]: (1) 𝑖 = 𝑎 𝑣 For calculation convenience, the formula is simplified to Equation (2): Diffusion coefficient Z" (ohm) log (I, current) Z" (ohm) Voltage (V vs. K /K) log (I, current) Current (mA) Batteries 2023, 9, x FOR PEER REVIEW 13 of 16 𝐿𝑜𝑔 𝑖 =𝑏 log 𝑣 log 𝑎 (2) The simplified equation shows that the b-value depends on the slope of the logarith- mic relationship between log(i) and log(v). In general, the electrochemical reaction pri- marily displays pseudocapacitive behavior when the b-value is close to 1.0, and the ion diffusion behavior when the b-value is close to 0.5 [59]. The b-value calculated from the curves of Figure 6e,f for the different peaks are 0.85 and 0.93, b-values close to 1.0 show that the pseudocapacitive contribution accounts for the majority of the capacity. Subse- quently, to quantify the percentage contribution of the pseudocapacitive, Equation (3) is used to calculate. 𝑖= 𝑘 𝑣 𝑘 𝑣 (3) In the formula, i represents the current value at a fixed voltage, the contribution of the capacitive process is represented by k1v, and the contribution of the diffusion control 1/2 process is denoted by k2v [60]. As shown in Figure 7a–e, the pseudocapacitive contribu- tion of CuSbSy@C increased from 72.2% to 85.2% as the scanning rate increased from 0.2 −1 to 1.0 mV s . Compared with Cu2S@C, Sb2S3@C, and CuSbSy, the pseudocapacitive contri- bution of CuSbSy@C was the highest (Figure 7f), which is probably better explains why Batteries 2023, 9, 238 12 of 15 the core-shell of CuSbSy@C electrode material is more favorable for K storage. −1 Figure 7. Pseudocapacitive contribution of CuSbS @C electrode at different rates of (a) 0.2 mV s , Figure 7. Pseudocapacitive contribution of CuSbSy@C electrode at y different rates of (a) 0.2 mV s , 1 1 1 1 −1 −1 −1 −1 (b (b ) 0. ) 0.4 4 mV s mV s , (c ,)( 0. c)6 mV s 0.6 mV , ( s d), 0. (d8 mV s ) 0.8 mV , and s , (and e) 1.0 mV (e) 1.0smV . (f) P s e.rcentage contribution (f) Percentage contribution of pseu- of docapacitive for Cu2S@C, Sb2S3@C, CuSbSy, and CuSbSy@C electrodes. pseudocapacitive for Cu S@C, Sb S @C, CuSbS , and CuSbS @C electrodes. 2 2 3 y y 4. Conclusions 4. Conclusions In summary, in situ precipitation and carbonization were used to create CuSbS @C In summary, in situ precipitation and carbonization were used to create CuSbSy@C nanospheres with a distinctive yolk-shell structure. The internal bimetallic sulfide CuSbS nanospheres with a distinctive yolk-shell structure. The internal bimetallic sulfide CuSbSy makes use of the synergistic interaction between Cu and Sb to boost overall specific capacity makes use of the synergistic interaction between Cu and Sb to boost overall specific ca- while also reducing volume expansion and enhancing structural stability. The external pacity while also reducing volume expansion and enhancing structural stability. The ex- nitrogen-doped carbon shell protects the material and confines the bimetallic compound to ternal nitrogen-doped carbon shell protects the material and confines the bimetallic com- a limited space, making it more stable and providing longer cycle life. Thus, the CuSbS @C pound to a limited space, making it more stable and providing longer cycle life. Thus, the electrode maintains an appreciable reversible capacity (438.8 mAh g after 60 cycles at −1 CuSbSy@C electrode maintains an appreciable reversible capacity (438.8 mAh g after 60 1 1 1 100 mA g ), a good rate capability (173.6 mAh g at 5.0 A g ), and superior long cycle −1 −1 −1 cycles at 100 mA g ), a good rate capability (173.6 mAh g at 5.0 A g ), and superior long 1 1 life (174.5 mAh g after 1000 cycles at 1000 mA g ). −1 −1 cycle life (174.5 mAh g after 1000 cycles at 1000 mA g ). Supplementary Materials: The following supporting information can be downloaded at: https:// Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/batteries9050238/s1, Instrumentation and Sample Analysis. Electro- www.mdpi.com/xxx/s1, Instrumentation and Sample Analysis. Electrochemical measurement. Fig- chemical measurement. Figure S1: XRD patterns of (a) Cu S@C, (b) Sb S @C, (c) CuSbS . Figure S2. 2 2 3 y ure S1: XRD patterns of (a) Cu2S@C, (b) Sb2S3@C, (c) CuSbSy.; Figure S2. Initial ten CV curves of (a) Initial ten CV curves of (a) Cu S@C, (b) Sb S @C, and (c) CuSbS electrode at a scan rate of 0.1 mV s 2 2 3 y in a potential range from 0.01–3 V. Figure S3. Charge/discharge profiles of (a) Cu S@C, (b) Sb S @C, 2 2 3 and (c) CuSbS electrodes within the potential of 0.01–3 V at a current density of 0.1 A g . Figure S4. Cycling performance of CuSbS @C electrodes at current densities of 0.5 A g . Figure S5. The posi- tions of Cu S@C, Sb S @C, CuSbS , and CuSbS @C at (a) 100 kHz, (b) 1 kHz, and (c) 1 Hz. Figure S6. y y 2 2 3 Bode plots of (a) Cu S@C, (b) Sb S @C, (c) CuSbS , and (d) CuSbS @C electrode. Figure S7. CV y y 2 2 3 curves of (a) Cu S@C, (b) Sb S @C, and (c) CuSbS electrode at various scan rates of 0.2–1.0 mV s . 2 2 3 Table S1. Content of Cu, Sb, and S elements in the compounds. Table S2. Performance comparison between this study and existing studies. Table S3. The comparison of R , R and R of Cu S@C, s ct SEI, 2 Sb S @C, CuSbS , and CuSbS @C. Table S4. The comparison of  , and  of Cu S@C, Sb S @C, y y 2 3 1 2 2 2 3 CuSbS , and CuSbS @C. y y Author Contributions: Conceptualization, P.H. and G.Y.; methodology, P.H. and Y.D.; software, P.H., X.C. and G.Y.; validation, P.H., X.C. and S.H.; formal analysis, P.H. and Y.D.; investigation, P.H.; resources, Y.L.; data curation, P.H.; writing—original draft preparation, P.H.; writing—review and editing, Y.D., H.Z. and Y.L.; visualization, Y.L.; supervision, Q.F. and Y.L.; project administration, Y.L.; funding acquisition, Q.F. and Y.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (22076116) and the Sino-German Center for Research Promotion (GZ1579). Institutional Review Board Statement: Not applicable. 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Journal

Batteries – Multidisciplinary Digital Publishing Institute

Published: Apr 23, 2023

Keywords: CuSbSy@C; potassium-ion batteries; yolk shell structure; bimetallic sulfide

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