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Article

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

1
School of Energy, Materials and Chemical Engineering, Hefei University, Hefei 230601, China
2
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China
3
School of Chemistry and Civil Engineering, Shaoguan University, Shaoguan 512005, China
*
Authors to whom correspondence should be addressed.
These authors contributed equally to this work.
Processes 2021, 9(10), 1822; https://doi.org/10.3390/pr9101822
Submission received: 24 September 2021 / Revised: 9 October 2021 / Accepted: 11 October 2021 / Published: 14 October 2021
(This article belongs to the Special Issue State of the Art of Energy Storage and Conversion Materials)

Abstract

:
Lithium sulfide (Li2S) 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 Li2S and severe shuttling of lithium polysulfides lead to poor rate capability and cycling stability. Confining Li2S 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 Li2S. Nano Li2S is successfully entered into the NPCMs’ pores to form N-doped porous carbon microspheres–Li2S composite (NPCMs–Li2S) by a typical liquid infiltration–evaporation method. NPCMs–Li2S 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–Li2S can be attributed to the NPCMs’ host, realizing the strong fixation of LiPSs and enhancing the electron and charge conduction of Li2S in NPCMs–Li2S cathodes.

1. 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 Li2S 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 Li2S is considered to be the most effective way to avoid the formation of lithium dendrite since Li2S has good compatibility with lithium metal-free anodes (e.g., carbonaceous material, siliceous material and metallic oxide) [22,23,24,25]. Li2S 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, Li2S is also accompanied by low electron conductivity and severe LiPSs shuttling, similar to S [28,29,30,31].
To improve the electrochemical performance of Li2S, various strategies are proposed, including adding conductive metals, combining sulfides or oxides and introducing carbon materials [32,33,34,35]. Among them, mixing Li2S with carbon materials, especially porous carbon, to form Li2S/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 Li2S [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 Li2S host to improve the conductivity of Li2S and achieve strong chemical fixation with LiPSs is an important step in the development of Li2S 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 Li2S 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 m2 g−1) and strong chemical polarity. When using NPCMs to confine Li2S, the obtained NPCMs–Li2S composites exhibit high discharge capacity, excellent rate capability and cycling stability.

2. Materials and Methods

Materials Synthesis: The yeast powder (Yichang Angel Yeast Co., Ltd., China), Li2S (99.9%, Alfa Aesar), CuCl2·2H2O (99%, Macklin), formaldehyde solution (37%, Macklin), anhydrous ethanol (EtOH) (99.5%, <0.005% water, Sigma Aldrich), H2SO4 (50%, Alfa Aesar) and H2O2 (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 CuCl2·2H2O 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 H2SO4–1.2 M H2O2 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 CuCl2·2H2O pore former is not added. NPCMs–Li2S and NCMs–Li2S were prepared via a typical liquid infiltration–evaporation method. Firstly, 0.24 g Li2S was added in 10 mL EtOH and stirred 6 h to synthesize Li2S solution. Secondly, the EtOH−Li2S solution was slowly and periodically dropped on 0.16 g NPCMs (or NCMs) to ensure that the Li2S 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–Li2S (or NCMs–Li2S) 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, 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–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 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 10−2–105 Hz.

3. Results

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 m2 g−1, which is ~32 times higher than that of NCMs (63.1 m2 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 m2 g−1 to 590.2 m2 g−1 and 0.97 cm3 g−1 to 0.44 cm3 g−1, respectively, for NPCMs–Li2S (Figure 2b). In contrast, NCMs–Li2S also shows typical non-porous characteristics (Figure 2b,c). Based on the above results, we can infer that most of Li2S may enter into the mesopore channels of NPCMs.
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 Li2S6 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 Li2S6 solution, which can be ascribed to the S62− species (Figure 2f) [48]. After NPCMs adding, the NPCMs containing Li2S6 solution is gradually changed from brown to transparent and the peak intensity of S62− is sharply decreased (Figure 2f and Figure S3), suggesting the strong LiPSs capture ability. However, both S62− peak intensity and color of NCMs contained Li2S6 solution are no obvious change (Figure 2f and Figure 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 Li2S was loaded into NPCMs host, NPCMs–Li2S still maintains a microsphere structure as NPCMs do, signifying that most of the Li2S enters into the mesopores of NPCMs (Figure 3b). Meanwhile, the disappearing porous structure of NPCMs–Li2S further proves that most of the pores in NPCMs are loaded with Li2S (Figure 3c,d). For comparison, NCMs–Li2S 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 Li2S, resulting most of Li2S direct depositing and covering on the surface of NCMs (Figure 2c and Figure S5). The particle size and state of Li2S in the NPCMs host are displayed in Figure 3e. The Li2S nanoparticles in the diameter range of 6–8 nm are coated with amorphous carbon, demonstrating that the Li2S 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 Li2S is homogeneous distributed in the NPCMs host (Figure 3f–i). Therefore, it can be believed that the NPCMs host improves the electrochemical performance of Li2S 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 rate of NPCMs–Li2S, 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 (Figure 4c and Figure 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 (Figure 5a and Figure 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.
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 (Figure 6 and Figure 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 above analysis and results, the good electrochemical performance of NPCMs–Li2S is 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.

4. 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 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 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–Li2S 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.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (51902079, 52072342), Anhui Provincial Natural Science Foundation (2008085QE271, 2008085QE277), Talent Scientific Research Foundation of Hefei University (18–19RC21, 18–19RC22), Research Development Foundation of Hefei University (19ZR12ZDA), Guangdong Basic and Applied Basic Research Foundation (No.2021A1515010671,2020A1515011221).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Illustration of synthesis process of NPCMs–Li2S composite.
Figure 1. Illustration of synthesis process of NPCMs–Li2S composite.
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Figure 2. (a) XRD patterns of NCMs–Li2S and NPCMs–Li2S composites; (b) N2 adsorption–desorption isotherms and (c) corresponding pore size distributions of NCMs, NPCMs NCMs–Li2S and NPCMs–Li2S; XPS spectra of NPCMs (d) N 1s spectrum and (e) C 1s spectrum; (f) UV-VIS absorption spectra of NCMs and NPCMs in Li2S6 solution.
Figure 2. (a) XRD patterns of NCMs–Li2S and NPCMs–Li2S composites; (b) N2 adsorption–desorption isotherms and (c) corresponding pore size distributions of NCMs, NPCMs NCMs–Li2S and NPCMs–Li2S; XPS spectra of NPCMs (d) N 1s spectrum and (e) C 1s spectrum; (f) UV-VIS absorption spectra of NCMs and NPCMs in Li2S6 solution.
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Figure 3. SEM images of (a) NPCMs and (b) NPCMs–Li2S; TEM images of (c) NPCMs and (d) NPCMs–Li2S; (e) HRTEM image of NPCMs–Li2S; (f) STEM image of NPCMs–Li2S and corresponding elemental distributions of (g) C, (h) N and (i) S.
Figure 3. SEM images of (a) NPCMs and (b) NPCMs–Li2S; TEM images of (c) NPCMs and (d) NPCMs–Li2S; (e) HRTEM image of NPCMs–Li2S; (f) STEM image of NPCMs–Li2S and corresponding elemental distributions of (g) C, (h) N and (i) S.
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Figure 4. (a) CV curves of NPCMs–Li2S cathode; (b) EIS spectra of NPCMs–Li2S and NCMs–Li2S electrodes in the fresh state; (c) charge–discharge profiles of the NPCMs–Li2S cathode in the first three cycles; (d) cycling performance of NPCMs–Li2S and NCMs–Li2S electrodes at 0.2 A g−1.
Figure 4. (a) CV curves of NPCMs–Li2S cathode; (b) EIS spectra of NPCMs–Li2S and NCMs–Li2S electrodes in the fresh state; (c) charge–discharge profiles of the NPCMs–Li2S cathode in the first three cycles; (d) cycling performance of NPCMs–Li2S and NCMs–Li2S electrodes at 0.2 A g−1.
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Figure 5. (a) Rate performance of NPCMs–Li2S and NCMs–Li2S electrodes; (b) cycling performance of the NPCMs–Li2S cathode at 0.5 A g−1 with a mass loading of 4.1 mg cm−2; (c) long-term cycle life of NPCMs–Li2S and NCMs–Li2S electrodes at 1.0 A g−1.
Figure 5. (a) Rate performance of NPCMs–Li2S and NCMs–Li2S electrodes; (b) cycling performance of the NPCMs–Li2S cathode at 0.5 A g−1 with a mass loading of 4.1 mg cm−2; (c) long-term cycle life of NPCMs–Li2S and NCMs–Li2S electrodes at 1.0 A g−1.
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Figure 6. SEM images of NPCMs–Li2S 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–Li2S electrode after 500 cycles at 1.0 A g−1; (c) S 2p spectrum and (d) Li 1s spectrum.
Figure 6. SEM images of NPCMs–Li2S 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–Li2S electrode after 500 cycles at 1.0 A g−1; (c) S 2p spectrum and (d) Li 1s spectrum.
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Liang, S.; Chen, J.; He, X.; Liu, L.; Zhou, N.; Hu, L.; Wang, L.; Liang, D.; Yu, T.; Tian, C.; et al. N–Doped Porous Carbon Microspheres Derived from Yeast as Lithium Sulfide Hosts for Advanced Lithium-Ion Batteries. Processes 2021, 9, 1822. https://doi.org/10.3390/pr9101822

AMA Style

Liang S, Chen J, He X, Liu L, Zhou N, Hu L, Wang L, Liang D, Yu T, Tian C, et al. N–Doped Porous Carbon Microspheres Derived from Yeast as Lithium Sulfide Hosts for Advanced Lithium-Ion Batteries. Processes. 2021; 9(10):1822. https://doi.org/10.3390/pr9101822

Chicago/Turabian Style

Liang, Sheng, Jie Chen, Xuehua He, Lingli Liu, Ningning Zhou, Lei Hu, Lili Wang, Dewei Liang, Tingting Yu, Changan Tian, and et al. 2021. "N–Doped Porous Carbon Microspheres Derived from Yeast as Lithium Sulfide Hosts for Advanced Lithium-Ion Batteries" Processes 9, no. 10: 1822. https://doi.org/10.3390/pr9101822

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