N–Doped Porous Carbon Microspheres Derived from Yeast as Lithium Sulﬁde Hosts for Advanced Lithium-Ion Batteries

: Lithium sulﬁde (Li 2 S) is considered to be the best potential substitution for sulfur-based cathodes due to its high theoretical speciﬁc capacity (1166 mAh g − 1 ) and good compatibility with lithium metal-free anodes. However, the electrical insulation nature of Li 2 S and severe shuttling of lithium polysulﬁdes lead to poor rate capability and cycling stability. Conﬁning Li 2 S into polar conductive porous carbon is regarded as a promising strategy to solve these problems. In this work, N-doped porous carbon microspheres (NPCMs) derived from yeasts are designed and synthesized as a host to conﬁne Li 2 S. Nano Li 2 S is successfully entered into the NPCMs’ pores to form N-doped porous carbon microspheres–Li 2 S composite (NPCMs–Li 2 S) by a typical liquid inﬁltration–evaporation method. NPCMs–Li 2 S not only delivers a high initial discharge capacity of 1077 mAh g − 1 at 0.2 A g − 1 , but also displays good rate capability of 198 mAh g − 1 at 5.0 A g − 1 and long-term lifespan over 500 cycles. The improved cycling and high-rate performance of NPCMs– Li 2 S can be attributed to the NPCMs’ host, realizing the strong ﬁxation of LiPSs and enhancing the electron and charge conduction of Li 2 S in NPCMs–Li 2 S cathodes.


Introduction
With the ever-growing demand for lightweight electric vehicles with high mileages, there is an urgent need to develop new energy storage devices with higher energy density to replace the current intercalation-type lithium-ion batteries (LIBs) [1][2][3][4][5][6][7]. Lithium-sulfur (Li-S) batteries based on the multi-electron conversion reaction between S and Li 2 S are regarded as one of the most promising energy storage devices due to their high theoretical specific capacity (1675 mAh g −1 ) and energy density (2600 Wh kg −1 ) [8][9][10][11][12][13][14]. However, the actual electrochemical performance of Li-S batteries is limited by the low conductivity of S, the large volume expansion of S cathodes during the discharge process and the serious shuttle of intermediate products of lithium polysulfides (LiPSs) [15][16][17][18]. Moreover, we know that the S cathode is usually required in order to use lithium metal as the matching anode, which will greatly increase the safety hazards caused by lithium dendrite [19][20][21]. In this regard, replacing S with full lithiation-state Li 2 S is considered to be the most effective way to avoid the formation of lithium dendrite since Li 2 S has good compatibility with lithium metal-free anodes (e.g., carbonaceous material, siliceous material and metallic oxide) [22][23][24][25]. Li 2 S not only has high theoretical specific capacity (1166 mAh g −1 ), but also effectively avoids structural damage caused by volume expansion during discharge [26,27]. Nevertheless, Li 2 S is also accompanied by low electron conductivity and severe LiPSs shuttling, similar to S [28][29][30][31]. To improve the electrochemical performance of Li 2 S, various strategies are proposed, including adding conductive metals, combining sulfides or oxides and introducing carbon materials [32][33][34][35]. Among them, mixing Li 2 S with carbon materials, especially porous carbon, to form Li 2 S/C composites is considered to be the most effective approach because the abundant pore structure, high specific surface area and good conductivity of porous carbon are conducive to the improvement in the electrochemical performance of Li 2 S [26,32,36]. However, most porous carbons exhibit non-polar characteristics, leading to weak immobilization between polar LiPSs and porous carbons [37]. Therefore, exploring porous carbon frameworks with a strong polar surface as a Li 2 S host to improve the conductivity of Li 2 S and achieve strong chemical fixation with LiPSs is an important step in the development of Li 2 S cathodes. Nitrogen-doped (N-doped) carbon, as a common carbon material modification method, can improve the surface polarity and electronic conductivity of carbon material, and maintain the integrity of the microstructure [38,39]. The synthesis of N-doped porous carbon is mainly through high temperature pyrolysis of N-containing polymer materials (such as polyaniline, melamine, polypyrrole, and metal-organic framework) [37,40]. The high cost, severe toxicity and low reproducibility of N-containing polymers determine that such precursors are difficult to be scaled up to mass production. Consequently, it is urgent to develop a facile, green and low-cost way to prepare N-doped porous carbon for Li 2 S cathodes.
Preparation of N-doped porous carbon from biomass has become a recent research hotspot due to its abundant resources, low price, diverse structures and nontoxic characteristics. Yeast, as a microorganism, has a uniform submicron size structure (1-4 µm) and a rich nitrogen content (7.5-10 wt%) [41]. Herein, we used yeast as a precursor to synthesize N-doped porous carbon microspheres (NPCMs) through carbonization and etching. NPCMs exhibit abundant micropores and mesopores, ultra-high specific surface area (2005.6 m 2 g −1 ) and strong chemical polarity. When using NPCMs to confine Li 2 S, the obtained NPCMs-Li 2 S composites exhibit high discharge capacity, excellent rate capability and cycling stability.

Materials and Methods
Materials Synthesis: The yeast powder (Yichang Angel Yeast Co., Ltd., China), Li 2 S (99.9%, Alfa Aesar), CuCl 2 ·2H 2 O (99%, Macklin), formaldehyde solution (37%, Macklin), anhydrous ethanol (EtOH) (99.5%, <0.005% water, Sigma Aldrich), H 2 SO 4 (50%, Alfa Aesar) and H 2 O 2 (35%, Alfa Aesar) were used as raw materials. A total amount of 15 g yeast powder was immersed in 100 mL deionized water for 1 h to wake up the yeast cells. The woken yeast cells were mixed with 10% formaldehyde solution and stirred for 1 h to achieve cell morphology fixation. Subsequently, the above solution was transferred to Teflon lined autoclave for 10 h at 180 • C. After heat treatment, the dark brown product was filtered, washed and dried at 80 • C for 10 h. The obtained dark brown powder was mixed with CuCl 2 ·2H 2 O in a weight ratio of 3:50, and then calcined the mixture at 900 • C under argon for 2 h. The calcined product was immersed in a mixed solution of 0.5 M H 2 SO 4 -1.2 M H 2 O 2 to remove Cu-based impurities. Finally, NPCMs were obtained by filtering, washing and drying the black powder from above solution. The synthesis of N-doped carbon microspheres (NCMs) is similar to that of NPCMs, except that the CuCl 2 ·2H 2 O pore former is not added. NPCMs-Li 2 S and NCMs-Li 2 S were prepared via a typical liquid infiltration-evaporation method. Firstly, 0.24 g Li 2 S was added in 10 mL EtOH and stirred 6 h to synthesize Li 2 S solution. Secondly, the EtOH−Li 2 S solution was slowly and periodically dropped on 0.16 g NPCMs (or NCMs) to ensure that the Li 2 S can effectively enter into the pore channels. Finally, the above powder was dried at 360 • C for 1.5 h under vacuum to remove EtOH and obtain NPCMs-Li 2 S (or NCMs-Li 2 S) composite.
Characterizations: The X-ray diffraction (XRD) patterns were employed to study the crystal structural of samples. The morphologies of samples were observed by using scanning electron microscope (SEM, Hitachi SU8010). The microstructure and element composition of samples were investigated by transmission electron microscopy (TEM, Processes 2021, 9, 1822 3 of 12 JEM2100) attached with an energy dispersion X-ray spectroscopy (EDS) detector. The nitrogen adsorption analyzer (Micromeritics ASAP 2020 plus) was used to measure the surface area and porous characteristic. The pore size distribution was calculated by the density functional theory (DFT) method. The X-ray photoelectron spectroscopy (XPS) spectra analysis was conducted on ESCALAB 250XI spectrometer by using an Al-Ka radiation source. The ultraviolet-visible (UV-VIS) absorption spectra test was performed on a spectrophotometer (Agilent Technologies Cary 60).
Electrochemical Measurements: The electrochemical performances of NPCMs-Li 2 S and NCMs-Li 2 S were evaluated in the 2025 coin-type cell by using lithium foil as the reference electrode and Celgard 2400 membrane as the separator. For the working electrode, 80 wt% active material (NPCMs-Li 2 S or NCMs-Li 2 S), 10 wt% polyvinylidene fluoride (PVDF) binder and 10 wt% acetylene black (Super-p) were added to the Nmethyl-2-pyrrolidinone (NMP) and stirred to from a uniform slurry. Then, the above slurry was coated on aluminum foil and dried at 120 • C for 6 h under argon protection. The average mass loading is~1.2 mg cm −2 and 4.1 mg cm −2 for the thin and thick electrodes, respectively. The used electrolyte is composed of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, 1 wt% LiNO 3 , 50 vol% DOL and 50 vol% DME. Cyclic Voltammetry (CV) curves were measured on a CHI760E electrochemistry workstation (Shanghai Chenhua, China) with a scan rate of 0.1 mv s −1 . The scan voltage range is set at from open circuit voltage to 3.8 V and 1.5 V to 3.0 V for the first and subsequent scans, respectively. Galvanostatic charge-discharge tests were performed on the battery test system (Wuhan LAND, China). The first charge activation of electrode was realized by using high charging cut-off voltage (3.8 V) and low current density (0.05 A g −1 ). The subsequent charge-discharge test voltage range is adjusted to 1.8-2.8 V. Electrochemical impedance spectroscopy (EIS) measurements were tested by a CHI760E electrochemistry workstation with a frequency range of 10 −2 -10 5 Hz. Figure 1 shows the main synthesis process of the NPCMs-Li 2 S composite. In a typical process, the EtOH−Li 2 S solution was slowly and periodically dropped on the NPCMs to ensure that the Li 2 S can effectively infiltrate into the pore channels of NPCMs. As depicted in Figure 2a, several characteristic XRD peaks at 27.0, 31.2, 44.8, 53.1, 55.6, 65.2, 71.9 and 74.1 • were observed in both the NPCMs-Li 2 S and NCMs-Li 2 S composites, which correspond to the standard peaks of Li 2 S (PDF#26-1188). Compared with NCMs-Li 2 S, the NPCMs-Li 2 S displayed a weaker diffraction peak, signifying that most of the Li 2 S enters into the pore channels of NPCMs. It is worth noting that the characteristic peaks belonging to NPCMs and NCMs did not appear in the above two composites due to the weak XRD peaks of both NPCMs and NCMs ( Figure S1). analysis was conducted on ESCALAB 250XI spectrometer by using an Al-Ka radiation source. The ultraviolet-visible (UV-VIS) absorption spectra test was performed on a spectrophotometer (Agilent Technologies Cary 60).

Results
Electrochemical Measurements: The electrochemical performances of NPCMs-Li2S and NCMs-Li2S were evaluated in the 2025 coin-type cell by using lithium foil as the reference electrode and Celgard 2400 membrane as the separator. For the working electrode, 80 wt% active material (NPCMs-Li2S or NCMs-Li2S), 10 wt% polyvinylidene fluoride (PVDF) binder and 10 wt% acetylene black (Super-p) were added to the Nmethyl-2-pyrrolidinone (NMP) and stirred to from a uniform slurry. Then, the above slurry was coated on aluminum foil and dried at 120 °C for 6 h under argon protection. The average mass loading is ~1.2 mg cm −2 and 4.1 mg cm −2 for the thin and thick electrodes, respectively. The used electrolyte is composed of 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, 1 wt% LiNO3, 50 vol% DOL and 50 vol% DME. Cyclic Voltammetry (CV) curves were measured on a CHI760E electrochemistry workstation (Shanghai Chenhua, China) with a scan rate of 0.1 mv s -1 . The scan voltage range is set at from open circuit voltage to 3.8 V and 1.5 V to 3.0 V for the first and subsequent scans, respectively. Galvanostatic chargedischarge tests were performed on the battery test system (Wuhan LAND, China). The first charge activation of electrode was realized by using high charging cut-off voltage (3.8 V) and low current density (0.05 A g -1 ). The subsequent charge-discharge test voltage range is adjusted to 1.8-2.8 V. Electrochemical impedance spectroscopy (EIS) measurements were tested by a CHI760E electrochemistry workstation with a frequency range of 10 -2 -10 5 Hz. Figure 1 shows the main synthesis process of the NPCMs-Li2S composite. In a typical process, the EtOH−Li2S solution was slowly and periodically dropped on the NPCMs to ensure that the Li2S can effectively infiltrate into the pore channels of NPCMs. As depicted in Figure 2a, several characteristic XRD peaks at 27.0, 31.2, 44.8, 53.1, 55.6, 65.2, 71.9 and 74.1° were observed in both the NPCMs-Li2S and NCMs-Li2S composites, which correspond to the standard peaks of Li2S (PDF#26-1188). Compared with NCMs-Li2S, the NPCMs-Li2S displayed a weaker diffraction peak, signifying that most of the Li2S enters into the pore channels of NPCMs. It is worth noting that the characteristic peaks belonging to NPCMs and NCMs did not appear in the above two composites due to the weak XRD peaks of both NPCMs and NCMs ( Figure S1).  To further characterize the porosity and surface area of NCMs, NPCMs, NCMs-Li2S and NPCMs-Li2S, N2 adsorption-desorption isothermal measurement was carried out. A typical mixed type I and IV adsorption-desorption curve can be seen in the NPCMs, suggesting hierarchical porosity consisting of micropores and mesopores (Figure 2b). The micropores and mesopores displayed a continuous pore diameter distribution in the ranges of 0.8-1.9 nm and 2.1-2.8 nm, respectively ( Figure 2c). However, NCMs showed typical non-porous characteristics (Figure 2b,c). The specific surface area of NPCMs is as high as 2005.6 m 2 g −1 , which is ~32 times higher than that of NCMs (63.1 m 2 g −1 ). After impregnating Li2S into the NPCMs host, the NPCMs-Li2S exhibited a typical type I adsorption-desorption curve, indicating the micropores feature of NPCMs-Li2S (Figure 2b). The disappeared mesopores in the NPCMs-Li2S composite can be attributed to the filling of Li2S (Figure 2c). In addition, the specific surface area and total pore volume of NPCMs dramatically decreased from 2005.6 m 2 g −1 to 590.2 m 2 g −1 and 0.97 cm 3 g −1 to 0.44 cm 3 g −1 , To further characterize the porosity and surface area of NCMs, NPCMs, NCMs-Li 2 S and NPCMs-Li 2 S, N 2 adsorption-desorption isothermal measurement was carried out. A typical mixed type I and IV adsorption-desorption curve can be seen in the NPCMs, suggesting hierarchical porosity consisting of micropores and mesopores (Figure 2b). The micropores and mesopores displayed a continuous pore diameter distribution in the ranges of 0.8-1.9 nm and 2.1-2.8 nm, respectively ( Figure 2c). However, NCMs showed typical non-porous characteristics (Figure 2b,c). The specific surface area of NPCMs is as high as 2005.6 m 2 g −1 , which is~32 times higher than that of NCMs (63.1 m 2 g −1 ). After impregnating Li 2 S into the NPCMs host, the NPCMs-Li 2 S exhibited a typical type I adsorption-desorption curve, indicating the micropores feature of NPCMs-Li 2 S (Figure 2b). The disappeared mesopores in the NPCMs-Li 2 S composite can be attributed to the fill-ing of Li 2 S (Figure 2c). In addition, the specific surface area and total pore volume of NPCMs dramatically decreased from 2005.6 m 2 g −1 to 590.2 m 2 g −1 and 0.97 cm 3 g −1 to 0.44 cm 3 g −1 , respectively, for NPCMs-Li 2 S (Figure 2b). In contrast, NCMs-Li 2 S also shows typical non-porous characteristics (Figure 2b,c). Based on the above results, we can infer that most of Li 2 S may enter into the mesopore channels of NPCMs.

Results
The XPS was used to confirm the elemental composition and chemical state of NPCMs. Three peaks at 531.3, 401.3 and 285.3 eV can be clearly observed in the NPCMs surface, assigning to the O 1s, N 1s and C 1s, respectively ( Figure S2) [42,43]. For the N 1s spectrum, three different peaks located at 404.9, 400.9 and 398.4 eV can be fitted and divided, corresponding to the N-O, pyrrolic N and pyridinic N, respectively (Figure 2d) [44][45][46]. Meanwhile, the peaks related to the C-N (287.5 eV) and C=C (284.8 eV) bonds can be fitted and observed in the C 1s spectrum (Figure 2e) [44,47]. This high nitrogen content (7.2 wt%) endows NPCMs with a strong chemical polarity, resulting in powerful capture ability for LiPSs.
The ability of NPCMs to capture LiPSs is confirmed by the adsorption experiment combined with the UV-VIS absorption spectra test. In this case, the same amounts of NPCMs and NCMs were employed and added to 0.005 M Li 2 S 6 in DOL/DME (1:1, v/v) solution to measure the capture effect on LiPSs. Two obvious peaks centered at 265 and 280 nm are appeared in the original Li 2 S 6 solution, which can be ascribed to the S 6 2− species (Figure 2f) [48]. After NPCMs adding, the NPCMs containing Li 2 S 6 solution is gradually changed from brown to transparent and the peak intensity of S 6 2− is sharply decreased (Figures 2f and S3), suggesting the strong LiPSs capture ability. However, both S 6 2− peak intensity and color of NCMs contained Li 2 S 6 solution are no obvious change (Figures 2f and S3). This phenomenon can be attributed to the low specific surface area and absent pore structure of NCMs, which caused a significant reduction in nitrogen adsorption sites toward LiPSs, resulting in weak LiPSs capture ability.
The morphologies and microstructures of as-synthesized samples were investigated by using SEM and TEM. As shown in Figure 3a,c, the NPCMs display a typical microsphere structure with an average size of 2 µm and hierarchical porous structure. The low graphitization degree of NPCMs is further confirmed in HRTEM, in good agreement with the XRD result ( Figures S1 and S4). After Li 2 S was loaded into NPCMs host, NPCMs-Li 2 S still maintains a microsphere structure as NPCMs do, signifying that most of the Li 2 S enters into the mesopores of NPCMs (Figure 3b). Meanwhile, the disappearing porous structure of NPCMs-Li 2 S further proves that most of the pores in NPCMs are loaded with Li 2 S (Figure 3c,d). For comparison, NCMs-Li 2 S shows irregular morphology accompanied by serious particle agglomeration ( Figure S5). This phenomenon can be attributed to the fact that NCMs lack the pores and space to confine Li 2 S, resulting most of Li 2 S direct depositing and covering on the surface of NCMs (Figures 2c and S5). The particle size and state of Li 2 S in the NPCMs host are displayed in Figure 3e. The Li 2 S nanoparticles in the diameter range of 6-8 nm are coated with amorphous carbon, demonstrating that the Li 2 S is confined in the pores of NPCMs (Figure 3e). Moreover, the uniformly distribution signals of C, N and S in elemental mapping signify that the Li 2 S is homogeneous distributed in the NPCMs host (Figure 3f-i). Therefore, it can be believed that the NPCMs host improves the electrochemical performance of Li 2 S due to its abundant porous structure, ultra-high specific surface area and strong polar surface.  The electrochemical performance of NPCMs-Li2S and NCMs-Li2S composites were evaluated by 2025 coin-type cell and tested at room temperature. For Li2S-based cathodes, a high cutoff voltage should be adopted to overcome the kinetic barrier of phase nucleation from Li2S to LiPSs in the first charge, which is referred to the initial activation process of Li2S cathodes [25,43]. According to our previous study [25,43], 3.8 V was employed to activate Li2S in the initial charge. Three anodic peaks located at 2.39, 3.36 and 3.76 V can be observed in the first anodic scanning of NCMs-Li2S cathode, corresponding to the transition from Li2S to S and the initial charge kinetic barrier ( Figure S6) [25,43]. For NPCMs-Li2S cathode, three slightly lower anodic peaks at 2.38, 3.19 and 3.68 V are appeared in the first anodic scanning, signifying the slightly increased activation barrier (Figure 4a). After the initial charge activation, both NPCMs-Li2S and NCMs-Li2S are displayed in the typical CV curves of S cathodes. EIS was used to further study the electrochemical reaction kinetics of NPCMs-Li2S and NCMs-Li2S. As shown in Figure 4b, the EIS curves of both NPCMs-Li2S and NCMs-Li2S consist of a line in the low frequency region and a semicircle in high frequency range, which correspond to the Warburg diffusion process and the charge transfer resistance (Rct), respectively [49,50]. The Rct of NPCMs-Li2S (49.8 Ω) is smaller than that of NCMs-Li2S (122.2 Ω), representing faster electron and charge transfer The electrochemical performance of NPCMs-Li 2 S and NCMs-Li 2 S composites were evaluated by 2025 coin-type cell and tested at room temperature. For Li 2 S-based cathodes, a high cutoff voltage should be adopted to overcome the kinetic barrier of phase nucleation from Li 2 S to LiPSs in the first charge, which is referred to the initial activation process of Li 2 S cathodes [25,43]. According to our previous study [25,43], 3.8 V was employed to activate Li 2 S in the initial charge. Three anodic peaks located at 2.39, 3.36 and 3.76 V can be observed in the first anodic scanning of NCMs-Li 2 S cathode, corresponding to the transition from Li 2 S to S and the initial charge kinetic barrier ( Figure S6) [25,43]. For NPCMs-Li 2 S cathode, three slightly lower anodic peaks at 2.38, 3.19 and 3.68 V are appeared in the first anodic scanning, signifying the slightly increased activation barrier (Figure 4a). After the initial charge activation, both NPCMs-Li 2 S and NCMs-Li 2 S are displayed in the typical CV curves of S cathodes. EIS was used to further study the electrochemical reaction kinetics of NPCMs-Li 2 S and NCMs-Li 2 S. As shown in Figure 4b, the EIS curves of both NPCMs-Li 2 S and NCMs-Li 2 S consist of a line in the low frequency region and a semicircle in high frequency range, which correspond to the Warburg diffusion process and the charge transfer resistance (R ct ), respectively [49,50]. The R ct of NPCMs-Li 2 S (49.8 Ω) is smaller than that of NCMs-Li 2 S (122.2 Ω), representing faster electron and charge transfer rate of NPCMs-Li 2 S, which could be attributed to the abundant porous structure providing fast channesl for electron and charge transport. Figure 4c depicts the first three charge-discharge profiles of the NPCMs-Li2S cathode. Similar to CV test, a high cutoff voltage (3.8 V) was employed for the initial activation of Li2S. The voltage plateaus of the NPCMs-Li2S cathode are consistent with the CV result. The NPCMs-Li2S exhibits higher first discharge capacity of 1077 mAh g −1 at 0.2 A g −1 , which is greater than that of 639 mAh g −1 for NCMs-Li2S, indicating the high Li2S utilization rate in NPCMs-Li2S (Figures 4c and S7). In subsequent cycles, NPCMs-Li2S composite displays high discharge capacities of 872 and 762 mAh g −1 at the 2nd cycle and 60th cycle with a high average coulomb efficiency of ~98%. By contrast, both discharge capacity and coulomb efficiency of NCMs-Li2S are lower than those of NPCMs-Li2S (Figure 4d).
The rate capability of NPCMs-Li2S and NCMs-Li2S at current densities from 0.2 A g −1 to 5.0 A g −1 are shown in Figure 5a. The discharge capacities of NPCMs-Li2S at 0.2, 0.5, 1.0, 2.0 and 5.0 A g −1 are around 869, 663, 519, 362 and 198 mAh g −1 , respectively. After various rate cycles, the discharge capacity of NPCMs-Li2S recovers to 805 mAh g −1 when the current density is returned to 0.2 A g −1 . It should be noted that the rate capability of NPCMs-Li2S is higher than that of NCMs-Li2S and other reported Li2S cathodes (Figures  5a and S8). The excellent rate and cycle performance of NPCMs-Li2S composites can be attributed to the host of NPCMs, which can not only improve the electron and charge transfer rate of Li2S, but also realize the strong fixation of LiPSs. Figure 4c depicts the first three charge-discharge profiles of the NPCMs-Li 2 S cathode. Similar to CV test, a high cutoff voltage (3.8 V) was employed for the initial activation of Li 2 S. The voltage plateaus of the NPCMs-Li 2 S cathode are consistent with the CV result. The NPCMs-Li 2 S exhibits higher first discharge capacity of 1077 mAh g −1 at 0.2 A g −1 , which is greater than that of 639 mAh g −1 for NCMs-Li 2 S, indicating the high Li 2 S utilization rate in NPCMs-Li 2 S (Figures 4c and S7). In subsequent cycles, NPCMs-Li 2 S composite displays high discharge capacities of 872 and 762 mAh g −1 at the 2nd cycle and 60th cycle with a high average coulomb efficiency of~98%. By contrast, both discharge capacity and coulomb efficiency of NCMs-Li 2 S are lower than those of NPCMs-Li 2 S (Figure 4d).
The rate capability of NPCMs-Li 2 S and NCMs-Li 2 S at current densities from 0.2 A g −1 to 5.0 A g −1 are shown in Figure 5a. The discharge capacities of NPCMs-Li 2 S at 0.2, 0.5, 1.0, 2.0 and 5.0 A g −1 are around 869, 663, 519, 362 and 198 mAh g −1 , respectively. After various rate cycles, the discharge capacity of NPCMs-Li 2 S recovers to 805 mAh g −1 when the current density is returned to 0.2 A g −1 . It should be noted that the rate capability of NPCMs-Li 2 S is higher than that of NCMs-Li 2 S and other reported Li 2 S cathodes (Figures 5a and S8). The excellent rate and cycle performance of NPCMs-Li 2 S composites can be attributed to the host of NPCMs, which can not only improve the electron and charge transfer rate of Li 2 S, but also realize the strong fixation of LiPSs. Long-term cycling and high-rate performance are considered important factors for Li2S cathodes. As depicted in Figure 5c, NPCMs-Li2S delivers a high initial discharge capacity of 704 mAh g −1 at 1.0 A g −1 , and retains 465 and 354 mAh g −1 after 100 and 500 cycles, respectively. However, NCMs-Li2S only displays the discharge capacities of 396, 288 and 205 mAh g −1 for the 1st, 100th and 500th, respectively, which are much lower than that of NPCMs-Li2S. The capacity decay of NPCMs-Li2S and NCMs-Li2S cathodes during the long cycling can be attributed to the slow shuttle of lithium polysulfides and cumulative deposition of insulating Li2S2/Li2S on electrode surface [51,52]. The high mass loading properties are also as a crucial factor for the commercialization of Li-ion batteries. For the NPCMs-Li2S, the thick electrode with 4.1 mg cm −2 was employed to investigate the high mass loading performance. As shown in Figure 5b, the NPCMs-Li2S exhibits a discharge capacity of 670 mAh g −1 at 0.5 A g −1 , and still maintains a considerable discharge capacity of 421 mAh g −1 after 50 cycles. It is noted that this electrochemical performance of NPCMs-Li2S is superior to other Li2S-based electrodes (Table S1).
To further demonstrate the stability of the NPCMs host, SEM and XPS were adopted to measure the NPCMs-Li2S electrode after 500 cycles at 1.0 A g −1 (Figures 6 and S9). The NPCMs-Li2S electrode displays a relatively smooth surface, which can be attributed to the chemical interaction between electrolyte and electrode material during the long-term repeated de/intercalation of lithium ion (Figure 6a) [25,43]. After detached from the current collector, NPCMs-Li2S composite still retains a typical microsphere structure, suggesting the excellent electrochemical structural stability (Figure 6b). Three peaks at 168.9, 164.3 and 161.7 eV can be fitted and divided from the S 2p spectrum of NPCMs-Li2S, which are assigned to the S-O, S-C and Li-S bonds, respectively (Figure 6c) [53][54][55]. Meanwhile, two peaks related to Li-N (55.9 eV) and Li-S (55.3 eV) bonds can be fitted and divided into the Li 1s spectrum of NPCMs-Li2S (Figure 6d) [56]. According to the XPS results, we can further confirm that the NPCMs host has a strong capture ability for LiPSs. Based on the Long-term cycling and high-rate performance are considered important factors for Li 2 S cathodes. As depicted in Figure 5c, NPCMs-Li 2 S delivers a high initial discharge capacity of 704 mAh g −1 at 1.0 A g −1 , and retains 465 and 354 mAh g −1 after 100 and 500 cycles, respectively. However, NCMs-Li 2 S only displays the discharge capacities of 396, 288 and 205 mAh g −1 for the 1st, 100th and 500th, respectively, which are much lower than that of NPCMs-Li 2 S. The capacity decay of NPCMs-Li 2 S and NCMs-Li 2 S cathodes during the long cycling can be attributed to the slow shuttle of lithium polysulfides and cumulative deposition of insulating Li 2 S 2 /Li 2 S on electrode surface [51,52]. The high mass loading properties are also as a crucial factor for the commercialization of Li-ion batteries. For the NPCMs-Li 2 S, the thick electrode with 4.1 mg cm −2 was employed to investigate the high mass loading performance. As shown in Figure 5b, the NPCMs-Li 2 S exhibits a discharge capacity of 670 mAh g −1 at 0.5 A g −1 , and still maintains a considerable discharge capacity of 421 mAh g −1 after 50 cycles. It is noted that this electrochemical performance of NPCMs-Li 2 S is superior to other Li 2 S-based electrodes (Table S1).
To further demonstrate the stability of the NPCMs host, SEM and XPS were adopted to measure the NPCMs-Li 2 S electrode after 500 cycles at 1.0 A g −1 (Figures 6 and S9). The NPCMs-Li 2 S electrode displays a relatively smooth surface, which can be attributed to the chemical interaction between electrolyte and electrode material during the long-term repeated de/intercalation of lithium ion (Figure 6a) [25,43]. After detached from the current collector, NPCMs-Li 2 S composite still retains a typical microsphere structure, suggesting the excellent electrochemical structural stability (Figure 6b). Three peaks at 168.9, 164.3 and 161.7 eV can be fitted and divided from the S 2p spectrum of NPCMs-Li 2 S, which are assigned to the S-O, S-C and Li-S bonds, respectively (Figure 6c) [53][54][55]. Meanwhile, two peaks related to Li-N (55.9 eV) and Li-S (55.3 eV) bonds can be fitted and divided into the Li 1s spectrum of NPCMs-Li 2 S (Figure 6d) [56]. According to the XPS results, we can further confirm that the NPCMs host has a strong capture ability for LiPSs. Based on the above analysis and results, the good electrochemical performance of NPCMs-Li 2 S is Processes 2021, 9, 1822 9 of 12 mainly attributed to the NPCMs host, which can not only improve the electronic charge conductivity and structural stability, but also achieve strong capture and fixation for LiPSs. R PEER REVIEW 9 of 12 conductivity and structural stability, but also achieve strong capture and fixation for LiPSs.

Conclusions
In summary, submicron NPCMs with an abundant pore structure, ultra-high specific surface area and strong chemical polarity were successfully synthesized for Li2S cathodes. The abundant pore structure of NPCMs can not only offer sufficient space for Li2S storage, but also provides a high-speed channel for electron and charge conduction. The ultra-high specific surface area and strong chemical polarity of NPCMs can achieve strong chemical adsorption and immobilization of LiPSs, thereby inhibiting the shuttle of LiPSs. Moreover, the unique submicron microsphere structure of NPCMs host can effectively improve the electrochemical structural stability. Benefiting from the above advantages of the NPCMs host, NPCMs-Li2S displays a high discharge capacity and long-term lifespan. This research will provide a valuable reference for the application of biological carbon host in the alkaline metal-sulfur battery.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure  S1: XRD patterns of NPCMs and NCMs. Figure S2: XPS spectra of NPCMs. Figure S3: Digital images of Li2S6 adsorption at different time. Figure S4: HRTEM image of NPCMs. Figure S5: SEM images of (a) NCMs and (b) NCMs-Li2S. Figure S6: CV curves of NCMs-Li2S cathode. Figure S7: Charge−discharge curves of NCMs-Li2S cathode in the first three cycles. Figure S8: Comparison of the rate capabilities of NPCMs-Li2S and various Li2S cathodes. Figure S9: XPS survey spectra of NPCMs-−1 Figure 6. SEM images of NPCMs-Li 2 S electrode after 500 cycles at 1.0 A g −1 (a) attached on current collector and (b) detached from current collector. XPS spectra of NPCMs-Li 2 S electrode after 500 cycles at 1.0 A g −1 ; (c) S 2p spectrum and (d) Li 1s spectrum.

Conclusions
In summary, submicron NPCMs with an abundant pore structure, ultra-high specific surface area and strong chemical polarity were successfully synthesized for Li 2 S cathodes. The abundant pore structure of NPCMs can not only offer sufficient space for Li 2 S storage, but also provides a high-speed channel for electron and charge conduction. The ultra-high specific surface area and strong chemical polarity of NPCMs can achieve strong chemical adsorption and immobilization of LiPSs, thereby inhibiting the shuttle of LiPSs. Moreover, the unique submicron microsphere structure of NPCMs host can effectively improve the electrochemical structural stability. Benefiting from the above advantages of the NPCMs host, NPCMs-Li 2 S displays a high discharge capacity and long-term lifespan. This research will provide a valuable reference for the application of biological carbon host in the alkaline metal-sulfur battery.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/pr9101822/s1, Figure S1: XRD patterns of NPCMs and NCMs. Figure S2: XPS spectra of NPCMs. Figure S3: Digital images of Li 2 S 6 adsorption at different time. Figure S4: HRTEM image of NPCMs. Figure S5: SEM images of (a) NCMs and (b) NCMs-Li 2 S. Figure S6: CV curves of NCMs-Li 2 S cathode. Figure S7: Charge−discharge curves of NCMs-Li 2 S cathode in the first three cycles. Figure S8: Comparison of the rate capabilities of NPCMs-Li 2 S and various Li 2 S cathodes. Figure S9: XPS survey spectra of NPCMs-Li 2 S after 500 cycles at 1.0 A g −1 . Table S1: Electrochemical performance of various lithium sulfide−based cathodes.

Author Contributions:
The experimental work, original draft preparation, and modification, S.L. and J.C.; methodology for experiments, manuscript review and editing, X.H. and L.L.; conceptualization and data analysis, resources, project administration, funding acquisition, S.L., N.Z., T.Y., C.T. and C.L.; data analysis, L.H.; formal analysis, L.W. and D.L. All authors have read and agreed to the published version of the manuscript.