Boosting electrochemical kinetics of S cathodes for room temperature Na/S batteries

Abstract

Room temperature (RT) Na/S batteries have attracted more and more attention due to their inexpensive raw materials and high theoretical energy density. However, their commercialization has been hampered by the shuttle effect caused by polysulfides and the low conversion efficiency of S species. Recently, the strategy of boosting electrochemical kinetics was adopted to resolve these issues, and relevant studies have been frequently reported. In this review, after introducing of working principles of RT-Na/S batteries, we provide a comprehensive overview of the field, with emphasis on the roles of carbon-based hosts, covalent S composites, catalytic metal-modified hosts, and other functional components. Finally, the key points to boost the electrochemical kinetics and practical applications of RT-Na/S batteries are also identified and discussed to provide guidelines for the development of RT-Na/S batteries and other energy-storage systems. Due to the low cost and high energy density, the room temperature (RT) Na/S battery system is deemed as one of the best potential energy-storage systems. However, various issues still severely hinder its practical applications, especially the shuttle effect caused by soluble sodium polysulfides (NaPSs) and the low conversion efficiency of S8 and Na2S2/Na2S in S cathodes. Recently, various strategies were adopted into RT-Na/S batteries to boost the electrochemical kinetics of S cathodes, which are capable of significantly accelerating (or altering) the redox process and mitigating the shuttle effect of S cathodes. Herein, the representative works have been systematically reviewed, with emphasis on the roles of carbon-based hosts, covalent S composites, catalytic metal-modified hosts, and other functional components. Furthermore, the future challenges and research directions for the S cathodes have also been prospected in detail to provide guideline for the development of RT-Na/S batteries. Due to the low cost and high energy density, the room temperature (RT) Na/S battery system is deemed as one of the best potential energy-storage systems. However, various issues still severely hinder its practical applications, especially the shuttle effect caused by soluble sodium polysulfides (NaPSs) and the low conversion efficiency of S8 and Na2S2/Na2S in S cathodes. Recently, various strategies were adopted into RT-Na/S batteries to boost the electrochemical kinetics of S cathodes, which are capable of significantly accelerating (or altering) the redox process and mitigating the shuttle effect of S cathodes. Herein, the representative works have been systematically reviewed, with emphasis on the roles of carbon-based hosts, covalent S composites, catalytic metal-modified hosts, and other functional components. Furthermore, the future challenges and research directions for the S cathodes have also been prospected in detail to provide guideline for the development of RT-Na/S batteries. Lithium-ion batteries have been widely studied and applied to various fields due to their high energy density.1Goodenough J.B. Kim Y. Challenges for rechargeable Li batteries.Chem. Mater. 2010; 22: 587-603Google Scholar, 2Goodenough J.B. Park K.-S. The Li-ion rechargeable battery: a perspective.J.Am. Chem. Soc. 2013; 135: 1167-1176Google Scholar, 3Wang F. Wang B. Ruan T. Gao T. Song R. Jin F. Zhou Y. Wang D. Liu H. Dou S. 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Kanchan D.K. Room temperature sodium-sulfur batteries as emerging energy source.J. Energy Storage. 2018; 18: 133-148Google Scholar Na is regarded as an ideal substitute for lithium because of its similar physical-chemical properties.16Li F. Wei Z. Manthiram A. Feng Y. Ma J. Mai L. Sodium-based batteries: from critical materials to battery systems.J. Mater. Chem. A. 2019; 7: 9406-9431Google Scholar,17Jiang Y. Wang B. Liu P. Wang B. Zhou Y. Wang D. Liu H. Dou S. Modified solid-electrolyte interphase toward stable Li metal anode.Nano Energy. 2020; 77: 105308Google Scholar More importantly, compared with lithium, Na has abundant reserves and is widely distributed on the earth.18Bao C. Wang B. Liu P. Wu H. Zhou Y. Wang D. Liu H. Dou S. Solid electrolyte interphases on sodium metal anodes.Adv. Funct. Mater. 2020; 30: 2004891Google Scholar Moreover, S is also an inexpensive material for its high abundance and yield around the world.19Medenbach L. Adelhelm P. Cell concepts of metal-sulfur batteries (metal = Li, Na, K, Mg): strategies for using sulfur in energy storage applications.Top. Curr. Chem. 2019; 375: 81Google Scholar In fact, high-temperature Na/S batteries have been commercialized for a long time.20Hueso K.B. Palomares V. Armand M. Rojo T. Challenges and perspectives on high and intermediate-temperature sodium batteries.Nano Res. 2017; 10: 4082-4114Google Scholar This system has the advantages of low cost, high efficiency, and stable cycling, therefore it could well meet the requirements of grid-scale application.21Hueso K.B. Armand M. Rojo T. High temperature sodium batteries: status, challenges and future trends.Energy Environ. Sci. 2013; 6: 734-749Google Scholar Unfortunately, the high operating temperature (270°C –350°C) not only requires expensive running costs but also is a severe challenge for safety. Consequently, it is important to realize smooth operation of Na/S batteries at room temperature (RT). In addition to high safety and low operating costs, RT-Na/S batteries could deliver a great specific theoretical capacity of 1,672 mAh g−1 and a large energy density of 1,274 Wh kg−1 (based on Na2S).22Wang J. Yang J. Nuli Y. Holze R. Room temperature Na/S batteries with sulfur composite cathode materials.Electrochem. Commun. 2007; 9: 31-34Google Scholar,23Park C.W. Ahn J.H. Ryu H.S. Kim K.W. Ahn H.J. Room-temperature solid-state sodium/sulfur battery.Electrochem. Solid State Lett. 2006; 9: A123Google Scholar However, the RT-Na/S battery system has numerous problems to be solved, especially for S cathodes.24Xu X. Lin K. Zhou D. Liu Q. Qin X. Wang S. He S. Kang F. Li B. Wang G. Quasi-solid-state dual-ion sodium metal batteries for low-cost energy storage.Chem. 2020; 6: 902-918Google Scholar,25Ye C. Chao D. Shan J. Li H. Davey K. Qiao S.-Z. Unveiling the advances of 2D materials for Li/Na-S batteries experimentally and theoretically.Matter. 2020; 2: 323-344Google Scholar Firstly, the volume of the S cathode will expand ~170% after being completely transferred to Na2S, and the volume will resume after charging.26Manthiram A. Yu X. Ambient temperature sodium-sulfur batteries.Small. 2015; 11: 2108-2114Google Scholar These repeated volume changes during the cycling process would cause the electrode material to be pulverized, resulting in rapid capacity decay. Secondly, at ambient temperature, solid S and Na2S have unpromising conductivities,27Wang Y.X. Zhang B. Lai W. Xu Y. Chou S.L. Liu H.K. Dou S.X. Room-temperature sodium-sulfur batteries: a comprehensive review on research progress and cell chemistry.Adv. Energy Mater. 2017; 7: 1602829Google Scholar which are not conducive to the transport of electrons and ions during the reaction processes, leading to slow electrochemical kinetics and inferior electrochemical performance of the batteries. In addition, similar to LSBs, RT-Na/S batteries also suffer from shuttle effect in ether-based electrolytes.28Wang Y.X. Lai W.H. Chou S.L. Liu H.K. Dou S.X. Remedies for polysulfide dissolution in room-temperature sodium-sulfur batteries.Adv. Mater. 2020; 32: 1903952Google Scholar,29Zhao M. Li B.-Q. Zhang X.-Q. Huang J.-Q. Zhang Q. A perspective toward practical lithium–sulfur batteries.ACS Cent. Sci. 2020; 6: 1095-1104Google Scholar The sodium polysulfides (NaPSs) produced in charge/discharge processes of S cathodes are easy to dissolve and migrate in ether electrolytes, some of which would reach the anode and react with highly reactive Na metal, resulting in low coulombic efficiency and loss of active S.30Yu X. Manthiram A. Capacity enhancement and discharge mechanisms of room-temperature sodium-sulfur batteries.ChemElectroChem. 2014; 1: 1275-1280Google Scholar,31Kim I. Park J.Y. Kim C. Park J.W. Ahn J.P. Ahn J.H. Kim K.W. Ahn H.J. Sodium polysulfides during charge/discharge of the room-temperature Na/S battery using TEGDME electrolyte.J. Electrochem. Soc. 2016; 163: A611-A616Google Scholar In order to deal with these problems, various efforts have been expanded. Inspired by LSBs, boosting the electrochemical kinetics of S cathodes, as an important strategy, was also introduced into the RT-Na/S batteries.32Liu D. Li Z. Li X. Cheng Z. Yuan L. Huang Y. Recent advances in cathode materials for room-temperature sodium-sulfur batteries.Chemphyschem. 2019; 20: 3164-3176Google Scholar,33Liu D. Zhang C. Zhou G. Lv W. Ling G. Zhi L. Yang Q.H. Catalytic effects in lithium-sulfur batteries: promoted sulfur transformation and reduced shuttle effect.Adv. Sci. 2018; 5: 1700270Google Scholar The effective methods, including improving ionic/electron transport and accelerating/adjusting the conversion of S species, can reduce the polarization of electrochemical reactions, accelerate the reaction kinetics, inhibit the shuttle effect, and greatly improve the utilization rate of active S. At present, the research on boosting redox kinetics of S in RT-Na/S batteries focus on several aspects. Carbon-based hosts could reduce electrochemical polarization by increasing the conductivity. The covalent S composites could realize the direct solid-state conversion due to the strong interaction between S and the matrix. More importantly, metal-modified hosts have the ability to accelerate the conversion of NaPSs through the catalytic effect of metals and compounds. Besides, other functional components, including electrolytes, current collectors, and additional layers, also play an important role in improving S redox kinetics (Figure 1). Herein, the principles of RT-Na/S batteries and recent advances on boosting electrochemical kinetics of S cathodes are systematically discussed and reviewed in this work. Furthermore, future challenges and prospects for boosting electrochemical kinetics are outlined. It is vital to deepen the fundamental understanding of reaction mechanism of S cathodes for the future development of RT-Na/S batteries. As shown in Figure 2A, the combination of solid S and Na metal at room temperature makes RT-Na/S batteries more competitive than other battery systems. A representative RT-Na/S cell typically consists of S-containing materials (such as S/C composite) as cathode, metallic Na as anode, and organic solution containing Na ions as electrolyte (such as NaClO4 in tetraethylene glycol dimethyl ether solvents). During the discharge process, Na anode is oxidized to Na+ and enters the electrolyte. In contrast, S cathode is reduced and theoretically combines with the Na+ to form Na2S. These reactions occurring in anode and cathode regions can be expressed as:Anode: 2 Na ↔ 2 Na+ + 2 e−(Equation 1) Cathode: S + 2 Na+ + 2e− ↔ Na2S(Equation 2) Overall reaction: 2 Na + S ↔ Na2S(Equation 3) In fact, the multistep conversion processes between S and Na2S are quite complex (especially in the common ether-based electrolytes and carbonate-based electrolytes), involving various disproportionation and coupling reactions,43Kumar A. Ghosh A. Forsyth M. MacFarlane D.R. Mitra S. Free-radical catalysis and enhancement of the redox kinetics for room-temperature sodium–sulfur batteries.ACS Energy Lett. 2020; 5: 2112-2121Google Scholar which could be affected by both the cathode materials (e.g., structure, morphology, catalytic component) and/or the electrolytes (e.g., composition, ionic conductivity, Na-ion transference number).28Wang Y.X. Lai W.H. Chou S.L. Liu H.K. Dou S.X. Remedies for polysulfide dissolution in room-temperature sodium-sulfur batteries.Adv. Mater. 2020; 32: 1903952Google Scholar,44Syali M.S. Kumar D. Mishra K. Kanchan D.K. Recent advances in electrolytes for room-temperature sodium-sulfur batteries: a review.Energy Stor. Mater. 2020; 31: 352-372Google Scholar To gain insight into the detailed information of these complex reactions, various research techniques with delicate modifications have been applied, including in situ or ex situ characterizations (i.e., X-ray diffraction [XRD], X-ray photoelectron spectroscopy, Raman spectra, transmission electron microscopy, etc.), theoretical calculations and molecular dynamic simulations.36Zhang B.W. Sheng T. Liu Y.D. Wang Y.X. Zhang L. Lai W.H. Wang L. Yang J. Gu Q.F. Chou S.L. et al.Atomic cobalt as an efficient electrocatalyst in sulfur cathodes for superior room-temperature sodium-sulfur batteries.Nat. Commun. 2018; 9: 4082Google Scholar,38Yang T. Guo B. Du W. Aslam M.K. Tao M. Zhong W. Chen Y. Bao S.-J. Zhang X. Xu M. Design and construction of sodium polysulfides defense system for room-temperature Na-S battery.Adv. Sci. 2019; 6: 1901557Google Scholar,45Wang Y. Hao Y. Xu L.-C. Yang Z. Di M.-Y. Liu R. Li X. Insight into the discharge products and mechanism of room-temperature sodium–sulfur batteries: a first-principles study.J. Phys. Chem. C. 2019; 123: 3988-3995Google Scholar,46Schaefer S. Vudata S.P. Bhattacharyya D. Turton R. Transient modeling and simulation of a nonisothermal sodium–sulfur cell.J. Power Sources. 2020; 453: 227849Google Scholar Up to now, although some significant progress has been achieved recently in understanding the detailed conversion processes of S cathodes in RT-Na/S batteries, there is still no consensus and further endeavors are still imperative. Generally speaking, these reactions mainly occur in the ether-based electrolytes, and the carbonate-based electrolytes could be understood as follows.22Wang J. Yang J. Nuli Y. Holze R. Room temperature Na/S batteries with sulfur composite cathode materials.Electrochem. Commun. 2007; 9: 31-34Google Scholar,44Syali M.S. Kumar D. Mishra K. Kanchan D.K. Recent advances in electrolytes for room-temperature sodium-sulfur batteries: a review.Energy Stor. Mater. 2020; 31: 352-372Google Scholar,47Ryu H. Kim T. Kim K. Ahn J.-H. Nam T. Wang G. Ahn H.-J. Discharge reaction mechanism of room-temperature sodium–sulfur battery with tetra ethylene glycol dimethyl ether liquid electrolyte.J. Power Sources. 2011; 196: 5186-5190Google Scholar In the ether-based electrolyte, this transition usually occurs through a solid-liquid-solid path. The reaction processes of RT-Na/S batteries undergo two voltage plateaus at around 2.20 V and 1.65 V and two voltage slopes in the ranges of 2.20–1.65 V and 1.60–1.20 V (Figure 2B).30Yu X. Manthiram A. Capacity enhancement and discharge mechanisms of room-temperature sodium-sulfur batteries.ChemElectroChem. 2014; 1: 1275-1280Google Scholar Specifically, the reaction mechanisms of S cathodes can be divided into four regions. Region I is the higher-voltage plateau at about 2.20 V, which is related to a solid-liquid transition from solid S8 to dissolved long-chain Na2S8:S8 + 2 Na+ + 2 e− → Na2S8(Equation 4) Region II represents a sloping region between 2.20 V and 1.65 V, which corresponds to a liquid-liquid reaction from Na2S8 to Na2S4:Na2S8 + 2 Na+ + 2 e− → 2 Na2S4(Equation 5) It could be a combination of the following three subtle reactions:Na2S8 + 2/3 Na+ + 2/3 e− → 4/3 Na2S6(Equation 6) Na2S6 + 2/5 Na+ + 2/5 e− → 6/5 Na2S5(Equation 7) Na2S5 + 1/2 Na+ + 1/2 e− → 5/4 Na2S4(Equation 8) Region III is the lower-voltage plateau at around 1.65 V, which is relevant to a liquid-solid transition from the dissolved Na2S4 to insoluble Na2S3, Na2S2 or Na2S:Na2S4 + 2/3 Na+ + 2/3 e− → 4/3 Na2S3(Equation 9) Na2S4 + 2 Na+ + 2 e− → 2 Na2S2(Equation 10) Na2S4 + 6 Na+ + 6 e− → 4 Na2S(Equation 11) Region IV shows the second sloping region between 1.65 V and 1.20 V, which is connected with a solid-solid reaction from Na2S2 to Na2S:Na2S2 + 2 Na+ + 2 e− → 2 Na2S(Equation 12) According to the above equations, it is obvious that:(1)Region I involves transition from S8 to dissolved NaPSs, which is usually thought to be easy to perform. Due to the lack of ions and electron transport path of S bulk, this dissolution process provides the possibility for the interior of S bulk to participate in the reaction and could be a necessary condition to realize a solid-liquid path.(2)Region II is supposed to be the most complicated one owing to the various NaPSs in the solution. Those NaPSs can dissolve and diffuse in ether-based electrolyte,47Ryu H. Kim T. Kim K. Ahn J.-H. Nam T. Wang G. Ahn H.-J. Discharge reaction mechanism of room-temperature sodium–sulfur battery with tetra ethylene glycol dimethyl ether liquid electrolyte.J. Power Sources. 2011; 196: 5186-5190Google Scholar and thus cause the notorious shuttle effect. However, they would react with solvent molecules through substitution or nucleophilic addition reactions in carbonate-based electrolyte.48Gao J. Lowe M.A. Kiya Y. Abruña H.D. Effects of liquid electrolytes on the charge–discharge performance of rechargeable lithium/sulfur batteries: electrochemical and in-situ X-ray absorption spectroscopic studies.J. Phys. Chem. C. 2011; 115: 25132-25137Google Scholar These negative actions occurring in this region will directly affect the cycle stability and availability of active S species. Accelerating the conversion of NaPSs to reduce their presence time or changing the reaction process to avoid the presence of NaPSs are effective strategies to the inhibition of shuttle effect and side reactions.(3)Region III is relevant to a liquid-solid transition. The reaction implies the elimination of large amounts of NaPSs. The amount of conversion of soluble NaPSs is closely related to the interaction between host materials and NaPS or Na2S products. A rapid conversion rate in this region can also reduce the existence time and concentration gradient of NaPSs to effectively inhibit the shuttle effect.(4)Region IV is likely to be kinetically slow due to the sluggish conversion of solid Na2S2 and Na2S. However, this region contains the greatest capacity of S. Reducing electrochemical polarization of this reaction is conducive to full capacity to release. Furthermore, S3⋅− radical monoanions were also detected in Na2S6@ activated carbon cloth (ACC) cathode through an in situ Raman study.43Kumar A. Ghosh A. Forsyth M. MacFarlane D.R. Mitra S. Free-radical catalysis and enhancement of the redox kinetics for room-temperature sodium–sulfur batteries.ACS Energy Lett. 2020; 5: 2112-2121Google Scholar They could be generated from S62− by a homolytic bond dissociation in the aprotic tetraethylene glycol dimethyl ether (TEGDME) solvents with a high donor number:Na2S6 ↔ 2 NaS3⋅ + 4 Na+ + 4 e− → 3 Na2S2(Equation 13) This species was anchored on the surface of ACC to form ACC-S3Na, which facilitated electron transfer from the substrate and led to the further reduction to Na2S2 and Na2S. As for the carbonate-based electrolytes system, no uniform mechanism has been proposed. As shown in Tables S1–S4, even if in the same electrolytes, different charge/discharge curves were presented by the S cathodes. The causes are still unclear and quite complex. In addition to the conversion of S, the electrochemical processes also involve other reactions, including the side reactions between S species and solvents48Gao J. Lowe M.A. Kiya Y. Abruña H.D. Effects of liquid electrolytes on the charge–discharge performance of rechargeable lithium/sulfur batteries: electrochemical and in-situ X-ray absorption spectroscopic studies.J. Phys. Chem. C. 2011; 115: 25132-25137Google Scholar and the formation of cathode electrolyte interphase (CEI).49Zhao X.M. Zhu Q. Xu S.D. Chen L. Zuo Z.J. Wang X.M. Liu S.B. Zhang D. Fluoroethylene carbonate as an additive in a carbonates-based electrolyte for enhancing the specific capacity of room-temperature sodium-sulfur cell.J. Electroanal. Chem. 2019; 832: 392-398Google Scholar,50Markevich E. Salitra G. Rosenman A. Talyosef Y. Chesneau F. Aurbach D. Fluoroethylene carbonate as an important component in organic carbonate electrolyte solutions for lithium sulfur batteries.Electrochem. Commun. 2015; 60: 42-46Google Scholar. The existing states of S in composites also could affect its electrochemical behavior. Moreover, fluoroethylene carbonate (FEC), as solvent or additive of carbonate-based electrolyte, conduce to form CEI on the surface of electrodes during the first cycle.49Zhao X.M. Zhu Q. Xu S.D. Chen L. Zuo Z.J. Wang X.M. Liu S.B. Zhang D. Fluoroethylene carbonate as an additive in a carbonates-based electrolyte for enhancing the specific capacity of room-temperature sodium-sulfur cell.J. Electroanal. Chem. 2019; 832: 392-398Google Scholar This protective film plays an important role in the conversions of S in carbonate-based electrolytes because it could prevent S from contacting electrolyte solvent molecules to prevent the excessive side reactions. At present, the working mechanisms of S cathodes in carbonate-based electrolytes can be mainly divided into two types: the S-NaPSs-Na2Ss path34Wang Y.X. Yang J. Lai W. Chou S.L. Gu Q.F. Liu H.K. Zhao D. Dou S.X. Achieving high-performance room-temperature sodium-sulfur batteries with [email protected] mesoporous carbon hollow nanospheres.J. Am. Chem. Soc. 2016; 138: 16576-16579Google Scholar,51Zhang L. Zhang B. Dou Y. Wang Y. Al-Mamun M. Hu X. Liu H. Self-assembling hollow carbon nanobeads into double-shell microspheres as a hierarchical sulfur host for sustainable room-temperature sodium-sulfur batteries.ACS Appl. Mater. Interfaces. 2018; 10: 20422-20428Google Scholar and direct solid-state conversion42Xin S. Yin Y.X. Guo Y.G. Wan L.J. A high-energy room-temperature sodium-sulfur battery.Adv. Mater. 2014; 26: 1261-1265Google Scholar (Figure 2C). The former path is similar to the solid-liquid-solid path in the ether-based electrolytes. Due to the large radius of Na+, NaPSs has a lower dissociation in polar solvents than lithium polysulfides (LiPSs), leading to a low solubility and side reaction activity in carbonate-based electrolytes.31Kim I. Park J.Y. Kim C. Park J.W. Ahn J.P. Ahn J.H. Kim K.W. Ahn H.J. Sodium polysulfides during charge/discharge of the room-temperature Na/S battery using TEGDME electrolyte.J. Electrochem. Soc. 2016; 163: A611-A616Google Scholar Therefore, the NaPSs could be observed by in situ XRD or Raman results during the cycling process.34Wang Y.X. Yang J. Lai W. Chou S.L. Gu Q.F. Liu H.K. Zhao D. Dou S.X. Achieving high-performance room-temperature sodium-sulfur batteries with [email protected] mesoporous carbon hollow nanospheres.J. Am. Chem. Soc. 2016; 138: 16576-16579Google Scholar However, the final product, Na2S, is generally hard to obtained because of the sluggish reaction kinetics. In these cases, metals and compounds with catalytic effects are usually introduced, which are necessary and important to enhance the S reaction kinetics.36Zhang B.W. Sheng T. Liu Y.D. Wang Y.X. Zhang L. Lai W.H. Wang L. Yang J. Gu Q.F. Chou S.L. et al.Atomic cobalt as an efficient electrocatalyst in sulfur cathodes for superior room-temperature sodium-sulfur batteries.Nat. Commun. 2018; 9: 4082Google Scholar The direct solid-state conversion has also been reported in carbonate-based electrolytes. According to the literature, in this case, S exists in the form of small molecules or has strong interaction with hosts.52Xin S. Gu L. Zhao N.H. Yin Y.X. Zhou L.J. Guo Y.G. Wan L.J. Smaller sulfur molecules promise better lithium-sulfur batteries.J. Am. Chem. Soc. 2012; 134: 18510-18513Google Scholar, 53Wei S. Xu S. Agrawral A. Choudhury S. Lu Y. Tu Z. Ma L. Archer L.A. A stable room-temperature sodium-sulfur battery.Nat. Commun. 2016; 7: 11722Google Scholar, 54Ma S. Zuo P. Zhang H. Yu Z. Cui C. He M. Yin G. Iodine-doped sulfurized polyacrylonitrile with enhanced electrochemical performance for room-temperature sodium/potassium sulfur batteries.Chem. Commun. 2019; 55: 5267-5270Google Scholar Therefore, the solid S could be converted into Na2S2/Na2S without the appearance of soluble NaPSs intermediates, and, thus, avoiding the shuttle effect. However, the solid reaction is inherently sluggish, and the accelerating of electrochemical reaction kinetics through improving the ionic and electronic conductivities of bulk phase is the main task.54Ma S. Zuo P. Zhang H. Yu Z. Cui C. He M. Yin G. Iodine-doped sulfurized polyacrylonitrile with enhanced electrochemical performance for room-temperature sodium/potassium sulfur batteries.Chem. Commun. 2019; 55: 5267-5270Google Scholar Therefore, Na-ion diffusion coefficient of S cathode during charge and discharge process becomes an important parameter, which could be calculated using the Randles-Sevick equation based on cyclic voltammograms at various scan rates or galvanostatic intermittent titration (GITT) technique at different discharge and charge states.53Wei S. Xu S. Agrawral A. Choudhury S. Lu Y. Tu Z. Ma L. Archer L.A. A stable room-temperature sodium-sulfur battery.Nat. Commun. 2016; 7: 11722Google Scholar,55Li S. Zeng Z. Yang J. Han Z. Hu W. Wang L. Ma J. Shan B. Xie J. High performance room temperature sodium–sulfur battery by eutectic acceleration in tellurium-doped sulfurized polyacrylonitrile.ACS Appl. Energy Mater. 2019; 2: 2956-2964Google Scholar Also, the Na-ion diffusion barrier of the S cathodes could be simulated by density functional theory.56Wang L. Chen X. Li S. Yang J. Sun Y. Peng L. Shan B. Xie J. Effect of eutectic accelerator in selenium-doped sulfurized polyacrylonitrile for high performance room temperature sodium–sulfur batteries.J. Mater. Chem. A. 2019; 7: 12732-12739Google Scholar,57Yan Z. Liang Y. Xiao J. Lai W. Wang W. Xia Q. Wang Y. Gu Q. Lu H. Chou S.L. et al.A high-kinetics sulfur cathode with a highly efficient mechanism for superior room-temperature Na-S batteries.Adv. Mater. 2020; 32: 1906700Google Scholar Structure design and doping modification of the S composites are the common adopted strategies to improve the Na+ diffusion process. In general, the S-containing composites using the hosts with large specific surface area exhibit stable cycling in the carbonate-based electrolytes. In order to realize the direct solid-state conversion process, the S hosts are required to be abundant in micropores, ensuring the strong interaction between the S and the host, and the S composites are usually amorphous without obvious XRD patterns. As to the S-NaPSs-Na2Ss path, the S host often have micro-mesoporous structures with the XRD diffraction peaks of S in the final S composites and the potential appearance of NaPSs in the electrochemical process (Tables S1, S3, and S4). From the above discussions, it is not hard to see that all of the conversion processes face a general concern; i.e., the sluggish reaction kinetics. Therefore, boosting the reaction kinetics of S cathodes is of great significance for improving the electrochemical performance. The “positive” method could not only improve charge transfer and ion transport but also accelerate or chan