Sb2S3@PPy Coaxial Nanorods: A Versatile and Robust Host Material for Reversible Storage of Alkali Metal Ions

Chalcogenides have attracted great attention as functional materials in optics, electronics, and energy-related applications due to their typical semiconductor properties. Among those chalcogenides, Sb2S3 holds great promise in energy storage field, especially as an anode material for alkali metal (Li, Na, and K) batteries. In this work, a one-dimensional coaxial Sb2S3@PPy is investigated as a versatile and robust anode in three kinds of alkali metal batteries for the first time, and the energy storage mechanism of these batteries is systematically discussed. As an anode material for sodium ion batteries (SIBs) and potassium ion batteries (KIBs), Sb2S3@PPy exhibits high reversible capacity and impressive cycle lifespan. Sb2S3@PPy anode demonstrates an adsorption behavior that has a significant influence on its sodium storage behavior, providing a universal model for studying the application of chalcogenide compounds.

As an important kind of chalcogenides, metal sulfide is one of the research hotspots in recent years. For example, Zhao et al. [7] proposed that n-type PbS achieved a high dimensionless figure of merit (ZT) value of 1.1 at 923 K. Efren et al. [8] reported two-dimensional (2D) superconductivity of atomically thin 2H-TaS 2 . In recent years, chalcogenides have received extensive attention in the energy storage field owing to the ultra-high theoretical specific capacity. For example, Hu et al. [9] first reported FeS 2 as an anode for sodium ion batteries (SIBs), and the assembled half cell showed excellent cycling performance (90% after 20,000 cycles) and rate performance (170 mAh g −1 at 20 A g −1 ).
Recently, Sb 2 S 3 emerged as a versatile and promising functional material widely applied in various fields. For example, Chang et al. [3] used Sb 2 S 3 as an absorbing semiconductor in solar cells, owing to its high absorption coefficient (1.8 × 10 5 cm −1 at 450 nm) and optical band gap (E g = 1.7 eV). Zhang et al. [10] incorporated Sb 2 S 3 onto WO 3 , and its photoelectroncatalytic activity under visible-light illumination was improved. Among them, the application in battery field is particularly attractive, as evidenced by an increasing number of research works [11][12][13], because the high theoretical capacity (946 mAh g −1 ) of Sb 2 S 3 is much higher than that of commercial graphite. For example, Xiong et al. [11] prepared S-doped graphene sheets (SGS)-supported Sb 2 S 3 as SIBs anodes, which delivered high capacity, good rate performance, and excellent cyclic stability. The calculation result of density functional theory (DFT) showed that the SGS had a stronger affinity for Sb 2 S 3 and intermediate products, demonstrating the more stable structure of the SGS supported Sb 2 S 3 , which remarkably strengthens its cyclic stability. With respect to the development of next-generation high-performance batteries, potassium ion batteries (KIBs) are gradually attracting much interest [14][15][16]. Compared with lithium, potassium resource is more abundant [17,18]. In addition, the redox potential of K/K + (−2.93 V versus standard hydrogen electrode) is lower than that of Na/Na + (−2.71 V), implying that KIBs have a high voltage plateau and high energy density. Liu et al. [19] firstly investigated Sb 2 S 3 for KIBs anode and synthesized a few-layered Sb 2 S 3 /carbon sheets composite. The synthesized composite showed a high reversible capacity (404 mAh g −1 after 200 cycles) and good rate capability. However, there are few systematic studies on the electrochemical behavior of Sb 2 S 3 in alkali metal based batteries, and differences between lithium, sodium, and potassium storage behavior are still unknown. Moreover, even though Sb 2 S 3 has a high theoretical capacity, its cyclability and rate performance still need to be improved.
Generally, nanostructured materials can shorten ion diffusion paths and improve the conductivity [20,21], and conductive coating can enhance its cycle stability [22]. Some work also demonstrates that polymer materials with abundant functional groups on the surface enhance the ion adsorption capability of the material, positively affecting its electrochemical performance in supercapacitor and battery applications [23]. Herein, we synthesized the Sb 2 S 3 nanorods through a facile hydrothermal method [24]. Then, in order to improve the cycle stability of Sb 2 S 3 , a low-cost polypyrrole (PPy) layer was coated on the surface of Sb 2 S 3 nanorods by a room temperature stirring method, and Sb 2 S 3 @PPy with a one-dimensional coaxial structure was obtained. Afterward, we systematically explored the electrochemical performances of Sb 2 S 3 @PPy nanorods as anode materials of alkali metal batteries, including lithium ion batteries (LIBs), SIBs, and KIBs. Through efficient coating, the Sb 2 S 3 @PPy nanorods as LIBs and SIBs anodes had excellent cycle stability and rate performance. The electrochemical performance of the synthesized Sb 2 S 3 @PPy coaxial nanorods was higher than those reported of sulfide-polymer anodes (detailed results are discussed in the following section). More importantly, it was found that the Sb 2 S 3 @PPy nanorods also showed excellent electrochemical performance in KIBs. A universal model based on alkali metal batteries was developed, showing wide applications in alkali metal ions storage. In the study of lithium and sodium storage, it was found that their energy storage behaviors were different, and the adsorption behavior had a significant influence during the sodium storage process. Our work provides a research model of versatile material for the future research of chalcogenides applied in alkali metal rechargeable batteries.

Synthesis of Sb 2 S 3 Nanorods
First, 1.92 g Na 2 S·9H 2 O (Alfa Aesar Chemicals Co., Ltd, Shanghai, China), 0.969 g C 3 H 7 NO 2 S (Alfa Aesar Chemicals Co., Ltd, Shanghai, China), and 0.912 g SbCl 3 (Alfa Aesar Chemicals Co., Ltd, Shanghai, China) were added into 80 mL distilled water sequentially. After stirring for 3 h, the mixed solution was transferred into a 100 mL Teflon-lined autoclave and heated at 180 • C for 12 h. After the solution was cooled down to the ambient temperature, the precipitate was obtained by centrifugation and washed with distilled water and alcohol several times. The Sb 2 S 3 powders were obtained after being dried under vacuum at 80 • C for 8 h [24].

Synthesis of Sb 2 S 3 @PPy Coaxial Nanorods
The synthesized Sb 2 S 3 nanorods were further coated by PPy through a solution reaction [24]. Then, 4 mg sodium dodecyl sulfate and 80 mg prepared Sb 2 S 3 nanorods were added into 40 mL deionized (DI) water followed by dispersion using sonication for 0.5 h. After mild stirring for 1 h, 21 µl pyrrole (Alfa Aesar Chemicals Co., Ltd, Shanghai, China) monomer was added into the solution, and after vigorously stirring for 1 h, 4 mL with 0.1 mol L −1 (NH 4 ) 2 S 2 O 8 (Alfa Aesar Chemicals Co., Ltd, Shanghai, China) as the oxidant was added to the above solution drop-wise. After stirring for another 4 h, the product was collected by centrifuging and washing, followed by a freeze-drying process overnight.

Materials Characterization
The crystalline structure of the samples was measured by a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) using Cu Kα radiation. Raman spectra of the samples were obtained on a Renishaw in Via Raman microscope (Horiba, Tokyo, Japan) with a 632.8 nm He-Ne laser. SEM images and energy dispersive spectroscopy (EDS) results were obtained using a JEOL7100F SEM/EDS microscope (JEOL, Tokyo, Japan). TEM and high resolution TEM (HRTEM) images were collected on a JEM-2100F scanning transmission electron microscopy (STEM)/EDS microscope (JEOL, Tokyo, Japan). FT-IR spectra were collected on a 60-SXB IR spectrometer (Nicolet, Madison, Wisconsin, USA).

Electrochemical Performance
The electrochemical behaviors of samples were characterized through assembly of 2016 coin cells in the argon-filled glove box. The pure Li, Na, or K discs were used as the counter electrode, and the working electrode consisted of Sb 2 S 3 @PPy nanorods, ketjen black, and carboxyl methyl cellulose with a weight ratio of 7:2:1 on a copper foil. The electrolyte was 1 M LiPF 6 in ethylene carbon (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1:1:1 by volume) for LIBs, 1 M NaClO 4 in EC/DMC (1:1 by volume) with 5% fluorinated ethylene carbonate for SIBs, and 0.8 M KPF 6 in EC/DEC (diethyl carbonate) with a volume ratio of 1:1 for KIBs. The galvanostatic charge-discharge tests were conducted through a multichannel battery testing system (LAND CT2001A), and cyclic voltammetry (CV) curves were collected using an electrochemical workstation (Autolab PGSTAT 302).

Materials Synthesis and Characterization
The Sb 2 S 3 @PPy coaxial nanorods were prepared by two steps. First, Sb 2 S 3 nanorods were prepared using a hydrothermal method. Surfactant, antimony, and sulphur sources were added into DI water. After stirring, the mixture was transferred into an autoclave for a hydrothermal reaction. Then, as the template, Sb 2 S 3 nanorods were coated with a homogeneous PPy layer through a solution reaction. Figure 1a displays the XRD patterns of bare Sb 2 S 3 and Sb 2 S 3 @PPy. All diffraction peaks of the two samples were well indexed to the Sb 2 S 3 (JCPDS (Joint Committee on Powder Diffraction Standards): 00-006-047), demonstrating the pure antimonite phase of Sb 2 S 3 and Sb 2 S 3 @PPy nanorods, suggesting PPy coating did not affect the crystal structure of Sb 2 S 3 . Figure 1b,c shows FT-IR spectra of Sb 2 S 3 and Sb 2 S 3 @PPy. Before coating, no distinctive FT-IR adsorption peak could be observed for the Sb 2 S 3 sample. With regard to Sb 2 S 3 @PPy, the peaks at 1554 and 1480 cm −1 corresponded to the typical ring vibrations of PPy. The peak at 3444 cm −1 corresponded to the vibration with the N-H bond. The C-N stretching vibrations of the benzoid was depicted by the peak at 1296 cm −1 . The peaks at 1205 and 1045 cm −1 corresponded to the breathing vibration of the PPy ring and the C-H deformation, respectively. The C-H wagging was proved by the band at 965 and 791 cm −1 [25]. All of these results confirmed the PPy was coated on the Sb 2 S 3 nanorods. Raman spectrum of Sb 2 S 3 indicated their carbon characteristic. The bands at 1365 and 1580 cm −1 corresponded to the D-band (disordered carbon) and G-band (graphitic carbon) [26], respectively. The I D /I G ratio of Sb 2 S 3 @PPy nanorods (≈0.98) represented a relatively high graphitization degree of the sample (Figure 1d). Morphology characteristics of the samples were demonstrated in SEM and TEM images. As shown in Figure 1e, the Sb 2 S 3 @PPy sample showed a uniform nanorod morphology. The length of the nanorods ranged from 3 to 15 µm, and the diameter ranged from 100 to 200 nm. Figure 1f, g revealed the nanostructure of the sample, and the Sb 2 S 3 nanorods were coated with a 19 nm thick PPy layer. Furthermore, the elemental mapping ( Figure 1h) results showed that Sb, S, C, and N elements were evenly distributed within the sample, further illustrating that PPy was uniformly coated on the Sb 2 S 3 nanorods. Figure 1g shows the moire fringe of the Sb 2 S 3 @PPy nanorods, indicating that the Sb 2 S 3 @PPy nanorods had good dispersion and were not agglomerated. Furthermore, the selected area electron diffraction (SAED) pattern of the Sb 2 S 3 @PPy nanorods indicated the single-crystal characteristic. All characterization results showed that the synthesized Sb 2 S 3 @PPy nanorods had uniform morphology and good dispersion. Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 12 from 3 to 15 μm, and the diameter ranged from 100 to 200 nm. Figure 1f, g revealed the nanostructure of the sample, and the Sb2S3 nanorods were coated with a 19 nm thick PPy layer. Furthermore, the elemental mapping ( Figure 1h) results showed that Sb, S, C, and N elements were evenly distributed within the sample, further illustrating that PPy was uniformly coated on the Sb2S3 nanorods. Figure  1g shows the moire fringe of the Sb2S3@PPy nanorods, indicating that the Sb2S3@PPy nanorods had good dispersion and were not agglomerated. Furthermore, the selected area electron diffraction (SAED) pattern of the Sb2S3@PPy nanorods indicated the single-crystal characteristic. All characterization results showed that the synthesized Sb2S3@PPy nanorods had uniform morphology and good dispersion.

Lithium Storage Performances
The lithium storage performances of Sb2S3 and Sb2S3@PPy nanorods were evaluated in half cells. Figure 2a exhibits the charge and discharge voltage profiles of Sb2S3@PPy at 0.1 A g −1 . The initial discharge and charge capacities of the working electrode were 1215.7 and 727.4 mAh g −1 , respectively. Additionally, the initial curve was characterized by three discharge platforms at 1.7, 1.3, and 0.7 V, respectively, consistent with reported results [27]. As shown in Figure 2b, Sb2S3@PPy nanorods exhibited a competitively high capacity and good stability after cycling. The capacity remained at 608 mAh g −1 after 40 cycles (95% capacity retention). The Coulombic efficiency of the initial cycle was 59.8%, which was a result of the formation of solid electrolyte interphase (SEI) film. Then, the Coulombic efficiency gradually increased in the following six cycles, and then it remained around 100%. This result was caused by the fact that intermediate polysulfides created during the

Lithium Storage Performances
The lithium storage performances of Sb 2 S 3 and Sb 2 S 3 @PPy nanorods were evaluated in half cells. Figure 2a exhibits the charge and discharge voltage profiles of Sb 2 S 3 @PPy at 0.1 A g −1 . The initial discharge and charge capacities of the working electrode were 1215.7 and 727.4 mAh g −1 , respectively. Additionally, the initial curve was characterized by three discharge platforms at 1.7, 1.3, and 0.7 V, respectively, consistent with reported results [27]. As shown in Figure 2b, Sb 2 S 3 @PPy nanorods exhibited a competitively high capacity and good stability after cycling. The capacity remained at 608 mAh g −1 after 40 cycles (95% capacity retention). The Coulombic efficiency of the initial cycle was 59.8%, which was a result of the formation of solid electrolyte interphase (SEI) film. Then, the Coulombic efficiency gradually increased in the following six cycles, and then it remained around 100%. This result was caused by the fact that intermediate polysulfides created during the electrochemical reactions tended to dissolve in the electrolyte according to Equation (1). Equation (2) displays the alloying process.
Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 12 electrochemical reactions tended to dissolve in the electrolyte according to Equation (1). Equation (2) displays the alloying process. Furthermore, the capacity of Sb2S3@PPy nanorods remained 597 mAh g −1 after 40 cycles (97.8% capacity retention). Figure 2c displays the rate performance of Sb2S3@PPy nanorods. With the current density increasing from 0.1 to 2 A g −1 , the capacities were 637.8, 560, 480, 410, and 310 mAh g −1 , respectively. When the current density reverted back to 0.1 A g −1 , the capacity quickly recovers to 610 mAh g −1 , verifying good structural stability and electrochemical reversibility of Sb2S3@PPy. Figure 2d displays the long cycling performance of the samples. The capacity of Sb2S3@PPy remained at 390 mAh g −1 at 2 A g −1 after 400 cycles, while the capacity of the Sb2S3 nanorods almost attenuated to 0 mAh g −1 after 150 cycles. It has been reported that under high current density, the decay in capacity is due to the decrease in crystallinity caused by the aggregation of the material [28]. However, in our case, due to the effective PPy coating, even though a moderate capacity decrease was observed in the beginning 100 cycles, Sb2S3@PPy still maintained a relatively high capacity. In contrast, the capacity of Sb2S3 without PPy coating faded to almost 0 mAh g −1 after 150 cycles, demonstrating that the cycling performance of Sb2S3 nanorods was enhanced obviously by PPy coating.

Sodium Storage Performance
The sodium storage behavior of Sb2S3@PPy was studied using the same method. Figure 3a displays the charge and discharge profiles of Sb2S3@PPy nanorods for first three cycles at 0.1 A g −1 . Its initial discharge and charge capacities were 1350 and 860 mAh g −1 , respectively. Compared with LIBs, the charge-discharge curve had no obvious voltage platform, indicating that it was more similar as a (c) Rate capability of Sb 2 S 3 @PPy from 0.1 to 2 A g −1 . (d) Cycling stability of Sb 2 S 3 @PPy and Sb 2 S 3 at 2 A g −1 , and corresponding Coulombic efficiency of Sb 2 S 3 @PPy at 2 A g −1 .
Furthermore, the capacity of Sb 2 S 3 @PPy nanorods remained 597 mAh g −1 after 40 cycles (97.8% capacity retention). Figure 2c displays the rate performance of Sb 2 S 3 @PPy nanorods. With the current density increasing from 0.1 to 2 A g −1 , the capacities were 637.8, 560, 480, 410, and 310 mAh g −1 , respectively. When the current density reverted back to 0.1 A g −1 , the capacity quickly recovers to 610 mAh g −1 , verifying good structural stability and electrochemical reversibility of Sb 2 S 3 @PPy. Figure 2d displays the long cycling performance of the samples. The capacity of Sb 2 S 3 @PPy remained at 390 mAh g −1 at 2 A g −1 after 400 cycles, while the capacity of the Sb 2 S 3 nanorods almost attenuated to 0 mAh g −1 after 150 cycles. It has been reported that under high current density, the decay in capacity is due to the decrease in crystallinity caused by the aggregation of the material [28]. However, in our case, due to the effective PPy coating, even though a moderate capacity decrease was observed in the beginning 100 cycles, Sb 2 S 3 @PPy still maintained a relatively high capacity. In contrast, the capacity of Sb 2 S 3 without PPy coating faded to almost 0 mAh g −1 after 150 cycles, demonstrating that the cycling performance of Sb 2 S 3 nanorods was enhanced obviously by PPy coating.

Sodium Storage Performance
The sodium storage behavior of Sb 2 S 3 @PPy was studied using the same method. Figure 3a displays the charge and discharge profiles of Sb 2 S 3 @PPy nanorods for first three cycles at 0.1 A g −1 .
Its initial discharge and charge capacities were 1350 and 860 mAh g −1 , respectively. Compared with LIBs, the charge-discharge curve had no obvious voltage platform, indicating that it was more similar as a capacitor during charge and discharge processes. Figure 3b displays the cycling performance of the samples. The Sb 2 S 3 @PPy nanorods had an initial reversible capacity of 580 mAh g −1 at 0.5 A g −1 , which remained at 632 mAh g −1 after 150 cycles, and the Coulombic efficiency was maintained at about 100%. Meanwhile, the Sb 2 S 3 nanorods had only 30% capacity retention after 30 cycles. During charge and discharge processes, there was a gradual increase in the capacity from the first cycle to the thirtieth cycle. The originally inserted Na ions were extracted after 30 cycles, contributing to the increase in capacity [29]. Figure 3c shows the rate capability of Sb 2 S 3 @PPy nanorods, and the average capacity of the initial six cycles was 940 mAh g −1 at 0.1 A g −1 . As the current densities increased to 0.2, 0.5, 1, and 2 A g −1 , the average capacities were 807, 690, 490, and 290 mAh g−1, respectively. When the current density reverted back to 0.1 A g −1 after 30 cycles, its capacity recovered to 920 mAh g −1 . Figure 3d shows the long-term cycling performance of Sb 2 S 3 @PPy at 2 A g −1 . During the cycling process, the capacity also suffered a slowly rising trend, which was similar as that at low current densities. After 150 cycles, the capacity retained at 275 mAh g −1 , equivalent to the initial reversible capacity. Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 12 remained at 632 mAh g −1 after 150 cycles, and the Coulombic efficiency was maintained at about 100%. Meanwhile, the Sb2S3 nanorods had only 30% capacity retention after 30 cycles. During charge and discharge processes, there was a gradual increase in the capacity from the first cycle to the thirtieth cycle. The originally inserted Na ions were extracted after 30 cycles, contributing to the increase in capacity [29]. Figure 3c shows the rate capability of Sb2S3@PPy nanorods, and the average capacity of the initial six cycles was 940 mAh g −1 at 0.1 A g −1 . As the current densities increased to 0.2, 0.5, 1, and 2 A g −1 , the average capacities were 807, 690, 490, and 290 mAh g−1, respectively. When the current density reverted back to 0.1 A g −1 after 30 cycles, its capacity recovered to 920 mAh g −1 . Figure 3d shows the long-term cycling performance of Sb2S3@PPy at 2 A g −1 . During the cycling process, the capacity also suffered a slowly rising trend, which was similar as that at low current densities. After 150 cycles, the capacity retained at 275 mAh g −1 , equivalent to the initial reversible capacity. The Sb2S3@PPy had a good sodium storage performance; specifically, the reversible capacity and the rate performance were higher than those reported of MWNTs@Sb2S3@PPy [30] and flowerlike Sb2S3@PPy microspheres [31]. Interestingly, the storage behavior was different from lithium storage. First, capacity suffered an increase during the first 30 cycles and then remained in a stable state. Second, the electrode did not present an obvious charge and discharge platform, which did not resemble the electrochemical behavior in LIBs, and its energy storage mechanism was more similar to a capacitor than to LIBs. The reason for these differences remains unclear. It was assumed to be related to the solvation of sodium ion electrolyte with the surface charge groups of electrode material. Figure 3. Electrochemical performances of Sb 2 S 3 @PPy or Sb 2 S 3 as an anode in sodium ion batteries (SIBs). (a) Galvanostatic charge and discharge curves of Sb 2 S 3 @PPy at 0.1 A g −1 for the first three cycles. (b) Cycling stability of Sb 2 S 3 @PPy and Sb 2 S 3 at 0.5 A g −1 , and corresponding Coulombic efficiency of Sb 2 S 3 @PPy at 0.5 A g −1 . (c) Rate capability of Sb 2 S 3 @PPy from 0.1 to 2 A g −1 . (d) Cycling stability and corresponding Coulombic efficiency of Sb 2 S 3 @PPy at 2 A g−1.

Potassium Storage Performance
The Sb 2 S 3 @PPy had a good sodium storage performance; specifically, the reversible capacity and the rate performance were higher than those reported of MWNTs@Sb 2 S 3 @PPy [30] and flowerlike Sb 2 S 3 @PPy microspheres [31]. Interestingly, the storage behavior was different from lithium storage. First, capacity suffered an increase during the first 30 cycles and then remained in a stable state. Second, the electrode did not present an obvious charge and discharge platform, which did not resemble the electrochemical behavior in LIBs, and its energy storage mechanism was more similar to a capacitor than to LIBs. The reason for these differences remains unclear. It was assumed to be related to the solvation of sodium ion electrolyte with the surface charge groups of electrode material.

Potassium Storage Performance
Until now, only few works had studied the potassium storage performance of Sb 2 S 3 [29]. In our work, KIBs were assembled through the same method. Sb 2 S 3 @PPy nanorods were paired with a metallic K foil.
The constant current charge and discharge curves of KIBs are shown in Figure 4a. During the discharge process, there was an inclined platform around 0.7 and 0.3 V, respectively, and in the charge process, there was also a platform at 0.6 and 1.3 V, respectively, corresponding to the conversion and alloying reaction [17]. The initial discharge and charge capacities were 986.2 and 628.1 mAh g −1 , respectively, and the corresponding Coulombic efficiency was 63.7%. Figure 4b delivers the cycle performance of Sb 2 S 3 @PPy nanorods. The reversible capacity remained at 487 mAh g −1 after 18 cycles, and a Coulombic efficiency near 100% during charge and discharge processes was obtained. Figure 4c shows the rate capacity of Sb 2 S 3 @PPy nanorods. The reversible capacity was 690 mAh g−1 at 0.1 A g −1 , which remained at 220 mAh g −1 when the current density increased to 2 A g −1 . The long cycle performance at 1 A g −1 is displayed in Figure 4d. The capacity of Sb 2 S 3 @PPy nanorods remained at 157 mAh g −1 after 50 cycles. The constant current charge and discharge curves of KIBs are shown in Figure 4a. During the discharge process, there was an inclined platform around 0.7 and 0.3 V, respectively, and in the charge process, there was also a platform at 0.6 and 1.3 V, respectively, corresponding to the conversion and alloying reaction [17]. The initial discharge and charge capacities were 986.2 and 628.1 mAh g −1 , respectively, and the corresponding Coulombic efficiency was 63.7%. Figure 4b delivers the cycle performance of Sb2S3@PPy nanorods. The reversible capacity remained at 487 mAh g −1 after 18 cycles, and a Coulombic efficiency near 100% during charge and discharge processes was obtained. Figure 4c shows the rate capacity of Sb2S3@PPy nanorods. The reversible capacity was 690 mAh g−1 at 0.1 A g −1 , which remained at 220 mAh g −1 when the current density increased to 2 A g −1 . The long cycle performance at 1 A g −1 is displayed in Figure 4d. The capacity of Sb2S3@PPy nanorods remained at 157 mAh g −1 after 50 cycles. Sb2S3@PPy coaxial nanorods had a great electrochemical performance when applied in KIBs. The energy storage mechanism was a conversion reaction and an alloying reaction [17]. Unlike the sodium storage process, there was a voltage platform in the charge and discharge curve, but it was not as obvious as that in LIBs. Its initial reversible capacity was up to 700 mAh g −1 at 0.1 A g −1 , higher than those of reported Sb2S3 materials.
At last, the performances of SIBs and KIBs in our work were compared with the reported works. As shown in Table 1, the synthesized Sb2S3@PPy coaxial nanorods had a competitive electrochemical performance in SIBs and KIBs. Especially in SIBs, its reversible capacity was much higher than those reported of the sulfide-polymer anodes. Sb 2 S 3 @PPy coaxial nanorods had a great electrochemical performance when applied in KIBs. The energy storage mechanism was a conversion reaction and an alloying reaction [17]. Unlike the sodium storage process, there was a voltage platform in the charge and discharge curve, but it was not as obvious as that in LIBs. Its initial reversible capacity was up to 700 mAh g −1 at 0.1 A g −1 , higher than those of reported Sb 2 S 3 materials.
At last, the performances of SIBs and KIBs in our work were compared with the reported works. As shown in Table 1, the synthesized Sb 2 S 3 @PPy coaxial nanorods had a competitive electrochemical performance in SIBs and KIBs. Especially in SIBs, its reversible capacity was much higher than those reported of the sulfide-polymer anodes.

Alkali Metal Ions Storage Mechanism
In order to investigate the charge storage mechanism, the CV curves at various scan rates of LIBs, SIBs, and KIBs were recorded. Figure 5a exhibits the CV curves of LIBs at various scan rates from 0.1 to 2 mV s −1 . In the CV curves, an obvious anodic peak at 1.24 V and two cathodic peaks at 0.75 and 1.26 V illustrated the lithiation and delithiation processes of Sb 2 S 3 @PPy nanorods, respectively. These peaks also corresponded to the voltage platform in the charge and discharge curve. When the peak current was plotted with respect to the square root of the scanning rate (v 1/2 ), a linear relationship was obtained (Figure 5b). The result obviously proved that the Li + storage in Sb 2 S 3 @PPy nanorods proceeded via a diffusion-limited faradaic mechanism instead of a surface-mediated capacitive mechanism [28]. The CV curves of SIBs are displayed in Figure 5c; at 0.1 mV s −1 , there was a significant anodic peak at 0.8 V. Moreover, as the scan rates increased, the CV curve approached a rectangular similar to the capacitor, which was consistent with the charge and discharge profiles. Figure 5d delivers the CV curves of KIBs at different scan rates. There was an obvious anodic peak at 1.3 V and an unobvious peak around 0.8 V in the CV curve, corresponding to the alloying reaction and the conversion process. Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 12

Alkali Metal Ions Storage Mechanism
In order to investigate the charge storage mechanism, the CV curves at various scan rates of LIBs, SIBs, and KIBs were recorded. Figure 5a exhibits the CV curves of LIBs at various scan rates from 0.1 to 2 mV s −1 . In the CV curves, an obvious anodic peak at 1.24 V and two cathodic peaks at 0.75 and 1.26 V illustrated the lithiation and delithiation processes of Sb2S3@PPy nanorods, respectively. These peaks also corresponded to the voltage platform in the charge and discharge curve. When the peak current was plotted with respect to the square root of the scanning rate (v 1/2 ), a linear relationship was obtained (Figure 5b). The result obviously proved that the Li + storage in Sb2S3@PPy nanorods proceeded via a diffusion-limited faradaic mechanism instead of a surface-mediated capacitive mechanism [28]. The CV curves of SIBs are displayed in Figure 5c; at 0.1 mV s −1 , there was a significant anodic peak at 0.8 V. Moreover, as the scan rates increased, the CV curve approached a rectangular similar to the capacitor, which was consistent with the charge and discharge profiles. Figure 5d delivers the CV curves of KIBs at different scan rates. There was an obvious anodic peak at 1.3 V and an unobvious peak around 0.8 V in the CV curve, corresponding to the alloying reaction and the conversion process. By comparing the CV curves of LIBs, SIBs, KIBs, it was found that all batteries had obvious redox peaks at low scan rates during charge and discharge. However, as the scan rates increased to 2 mV s −1 , SIBs and KIBs did not have obvious redox peaks. The storage behavior in SIBs and KIBs was similar to the capacitor under high scan rates. In order to further study the charge storage mechanisms in LIBs, SIBs, and KIBs, we calculated the surface diffusion coefficients of three ions (Li + , Na + , and K + ) in alkali metal batteries using the equation as follows [24]: Ip = 2.69 × 10 5 ACD 1/2 n 3/2 ν 1/2, in which Ip is the peak current, A means the surface area of active materials in anode, C is the ions concentration, D is the apparent alkali metal ions diffusion coefficient, n represents the number of electrons transferred per molecule during the electrochemical reaction, and v means scan rate. As shown in Table 2, the diffusion coefficient of Li + was higher than those of Na + and K + . The reasons for this result were explained as follows. First, the Li + diffusion coefficient was the highest, which was By comparing the CV curves of LIBs, SIBs, KIBs, it was found that all batteries had obvious redox peaks at low scan rates during charge and discharge. However, as the scan rates increased to 2 mV s −1 , SIBs and KIBs did not have obvious redox peaks. The storage behavior in SIBs and KIBs was similar to the capacitor under high scan rates. In order to further study the charge storage mechanisms in LIBs, SIBs, and KIBs, we calculated the surface diffusion coefficients of three ions (Li + , Na + , and K + ) in alkali metal batteries using the equation as follows [24]: I p = 2.69 × 10 5 ACD 1/2 n 3/2 ν 1/2, in which I p is the peak current, A means the surface area of active materials in anode, C is the ions concentration, D is the apparent alkali metal ions diffusion coefficient, n represents the number of electrons transferred per molecule during the electrochemical reaction, and v means scan rate. As shown in Table 2, the diffusion coefficient of Li + was higher than those of Na + and K + . The reasons for this result were explained as follows. First, the Li + diffusion coefficient was the highest, which was because of the larger radius of Na + and K + resulting in the most rapid Li + diffusion rate. Second, from the CV and galvanostatic current charge and discharge curves, it could be demonstrated that surface adsorption played an important role in the charge and discharge processes of SIBs (unlike LIBs and KIBs, which mainly involve a two-phase reaction), which may have led to the slower diffusion rate of Na + in the bulk phase. It should be noted that the calculation was based on the value of the redox peaks of CV curves, which reflected the two-phase reaction during charge and discharge processes. Thus, the calculated value only presented the diffusion rate corresponding to the two-phase reaction, rather than the true diffusion rate of Na + . Therefore, the calculated diffusion rate of K + was higher than that of Na + . Table 2. Diffusion coefficient of alkali metal ions of Sb 2 S 3 @PPy anode.

Conclusions
In summary, we synthesized Sb 2 S 3 @PPy coaxial nanorods as versatile anodes in LIBs, SIBs, and KIBs with excellent electrochemical performance. Specifically, it showed a reversible capacity of 940 mAh g −1 at 0.1 A g −1 (SIBs), higher than those of similar works. The Sb 2 S 3 @PPy coaxial nanorods also showed excellent rate capacity and cyclic stability in SIBs. Additionally, the reversible capacity of Sb 2 S 3 @PPy nanorods reached up to 700 mAh g −1 at 0.1 A g −1 (KIBs). A systematic study was carried out on the electrochemical behavior of Sb 2 S 3 @PPy nanorods in lithium, sodium, and potassium storage, including cycle performance, rate capacity, galvanostatic current charge and discharge, and CV curves under different scan rates. This study provides a universal model for the study of chalcogenide compounds as anodes for alkali metal ion batteries.