Unconventional Designs for Functional Sodium‐Sulfur Batteries

Sodium‐sulfur (Na–S) batteries that utilize earth‐abundant materials of Na and S have been one of the hottest topics in battery research. The low cost and high energy density make them promising candidates for next‐generation storage technologies as required in the grid and renewable energy. In recent years, extensive efforts have been devoted to the diversity and functionalities of Na–S batteries, aiming to extend their potential applications across multiple temporal and spatial dimensions. Here, we summarize the unconventional designs for the functionalities of Na–S batteries such as flexible batteries, solid‐state cells, flame resistance, and operation at extreme temperatures. By highlighting these design strategies that help to realize the functionalities, we hope this review offers a pathway to foster the bright future of Na–S batteries in diverse applications.


Introduction
Despite their dominant position in powering small electronics, the deployment of Li-ion batteries in vehicles and renewable energy storage has been restrained by the issues of cost and energy density. [1,2] Mitigation of these problems relies on the exploration and development of lithium-beyond battery technologies such as Na-S batteries. [3,4] The operation of Na−S batteries relies on the use of an S cathode and Na anode. [5] During the discharge process, S 8 molecules are reduced by Na to generate several soluble long-chain polysulfide intermediates of Na 2 S n (4 ≤ n ≤ 8), which further transform insoluble short-chain polysulfides of Na 2 S n (1 ≤ n ≤ 3). Upon the reversal charge, Na 2 S n is oxidized to S through a series of intermediate phases (Equation 1). This conversion reaction of S leads to a large theoretical capacity of 1672 mAh g −1 , and the stripping and plating of Na metal give a high capacity of 1166 mAh g −1 . [6,7] The combination of these two highcapacity electrode materials leads to a high specific energy of 1274 Wh kg −1 (based on the product of Na 2 S), which is much greater than that of Li-ion batteries. Current Li-ion batteries in electronics need to recharge at least once each day. However, with Na−S batteries, it would be possible to recharge once 3-4 days.
2Na þ n=8S 8 $ Na 2 S n 1 ≤ n ≤ 8 ð Þ Historically, Na-S batteries were implemented at a temperature of~300°C because the high temperature is required to keep the cathode and anode materials in a molten state for battery operation. [8] High-temperature Na-S batteries have already been commercialized. However, the energy efficiency and safety hazards associated with high-temperature Na-S batteries pose a challenge to reducing the operation temperature. To this target, operation of Na-S batteries at room temperature has been demonstrated, although they exhibit limited cycling stability in the early days. [9] The working principle of Na-S batteries, at either high temperature or room temperature, is quite analogous. [10] However, the reduction of operating temperature brings significant challenges in the performance of Na-S batteries, mostly involving the S cathodes. [11] First, S and Na 2 S at ambient temperature are both poorly conductive to efficiently transport electrons and ions during the reaction, leading to sluggish charge transfer kinetics. The kinetic barriers related to the S conversion cannot be completely overcome, and the electrochemical reversibility of roomtemperature Na-S batteries is much inferior to that at high temperatures. In addition, soluble sodium polysulfides (NaPSs) formed on the cathode side will diffuse to the anode, leading to a so-called shuttle effect. This phenomenon corrodes metallic Na to produce a lessconductive passivation layer on the anode, resulting in low Coulombic efficiency and continuous capacity decay of batteries. The shuttle effect has been considered the most notorious issue affecting the lifespan of S-based batteries. Moreover, the S electrode expands (~170%) and contracts substantially during electrochemical cycling, leading to continuous electrode evolution and finally pulverization, causing rapid capacity decay. [12] The profit of using abundant and cost-effective S and Na electrode materials prompts the rapid development of Na-S batteries. In particular, recent knowledge accumulated for the Li-S technology can be well leveraged for Na-S batteries. As a result, the Na-S system has captured a great deal of attention in the battery community. Great efforts have been devoted to the selection of materials and optimization of systems. Several reviews are available, focusing on the choice and design of S cathode, Na anode, separator, and electrolytes. [13][14][15][16][17][18] On the other hand, the prospect of Na-S batteries fosters their bright future in diverse applications across multiple temporal and spatial dimensions. For instance, flexible Na-S batteries have been developed to meet the high-energy demands of wearable devices. Non-flammable electrolytes have been introduced to impart flame-resistance functions, while solidstate designs help to realize high safety. In this sense, it is essential to explore and develop their multiple functionalities to adapt for a specific purpose. To this end, we summarize the unconventional designs for the functionalities of Na-S batteries such as flexible batteries, solid-state cells, flame resistance, and operation at extreme temperatures (Scheme 1). We highlight the design principles of how these functionalities can be recognized in Na-S batteries. The challenges and research directions to foster their future potential are also discussed at the end of this review.

Flexible Batteries
Flexible batteries, which are different from conventional batteries with rigid and fixed shapes, can be changed according to the shape required by the external environment. [19,20] These batteries can resist external forces and bend to a certain angle without the formation and propagation of cracks. [21,22] Therefore, flexible batteries expand the practical application of batteries in wearable devices, folding electronics, and bendable gadgets. At present, flexible lithium-ion batteries are booming toward multiple applications. However, their limited energy density hardly meets the energy needs of flexible devices. In contrast, highenergy Na−S batteries make them one of the most promising competitors for powering wearable devices.
To assemble flexible Na−S batteries, researchers usually adopted flexible substrates to host the electrodes of sodium and sulfur ( Figure 1). Carbon-based materials, such as one-dimensional (1D) carbon fiber, two-dimensional (2D) carbon cloth, and three-dimensional (3D) porous carbon, have been broadly applied in flexible Na-S batteries, owing to their lightweight, mechanical robustness, and high conduction. [23,24] Moreover, they are chemically and electrochemically stable, thus avoiding possible structural and performance degradation.
Because of the low melting point of sodium, it is general to combine the molten sodium with carbon substrate. [25] The electrode obtained has both flexibility and good mechanical properties. As non-polar carbon materials have a relatively weak interaction with polar Na, it is necessary to modify the carbon matrix to strengthen their interaction. For example, Yu's team used a 2D carbon cloth loaded with RuO 2 as a host of molten sodium, [26] and Li et al. [27] introduced SnO 2 arrays with high sodiphilicity onto the carbon cloth. Alternatively, Fang et al. [28] coated carbon cloth with MXene to load Na metal. By utilizing the sodiphilicity of MXene, molten liquid sodium can evenly penetrate the substrate, which alleviates the poor wetting of molten sodium on the surface of carbon cloth. [29] In addition, 3D carbon substrates such as carbon felt can simultaneously improve the mechanical and electrochemical properties of flexible electrodes through the interwoven network structure. [30] The flexible Na electrode can operate at a high current density of 5 mA cm −1 for many cycles.
Similarly, flexible S electrodes can be obtained by loading S directly onto carbon substrates. In general, a mild heating of the S powder and the substrate will lead to the deposition of S on the flexible substrate. By this means, both flexible and binder-free features can be realized simultaneously, thus greatly improving the energy density of batteries. For instance, Yao et al. [31] directly loaded S 0.6 Se 0.4 onto 1D carbon fibers. Due to the existence of a strong C-S covalent bond, S 0.6 Se 0.4 can be firmly attached to the flexible substrate. In another case, nitrogen-doped porous carbon nanofibers embedded by Co nanoparticles were used as the sulfur host. [32] Carbon nanofibers support the electrochemically active materials, while the embedded Co particles can anchor polysulfides, accelerate their conversion, and reduce their dissolution loss. In addition, 2D carbon substrates such as 2D graphene have been widely used in flexible Na-S batteries. [33] Their cross-linked network structure not only alleviates the stress caused by the volume change but also improves the mechanical strength of the electrode. Du et al. [34] combined the flexibility of rGO with the polarity of VO 2 to chemically anchor the polysulfide species, showing favorable cycle stability. Qin et al. [35] loaded S into hierarchical fluorinatedcarbon nanotube encapsulated 3D frameworks; this electrode features robust conversion kinetics of NaPS and minimal shuttle effects. Sulfurized polyacrylonitrile nanofibers (SPANs), which are the heating product of S and polyacrylonitrile nanofibers, have the features of high flexibility and high conductivity. These SPAN webs are ideally flexible S electrodes. Kim et al. [36] produced an SPAN electrode by electrospinning and following heat treatment, which shows paper-like flexibility and can be folded by 180°. Flexible electrodes with hollow tubular nanofibers (H-SPAN), fabricated by coaxial electrospinning and heat treatment, can enhance the loading of S without compromising the flexibility of the electrode. [37] H-SPAN delivers a considerably high capacity (1250 mA h g −1 ) with no capacity loss even after 300 cycles, thus demonstrating their potential in practical applications.

Solid-State Cells
Room-temperature Na−S batteries have recently been widely studied with organic electrolytes. Although liquid electrolytes show high ionic conductivity, several obstacles hinder their further applications. One is the dissolution of transient NaPS species, which generally leads to poor Coulomb efficiency and continuous capacity decay during repeated charge and discharge. [12,38] In addition, liquid organic electrolytes face the issues of leakage and the potential formation of Na dendrites, which will cause significant safe concern in energy storage. [39] These stimulate the adoption of solid-state electrolytes to develop solid-state Na−S cells.

All-Solid-State Cells
The common inorganic solid electrolytes include three types: ceramic, glass-ceramic, and glass. NASICON-(Na superionic conductor-) type oxides and β″-alumina are two representative ceramic electrolytes. While β″-alumina only operates at high temperatures (∼300°C), NASICON-type oxides exhibit considerable conductivity at room temperature. [3,40] Generally, solid electrolytes could prevent the growth of Na dendrites due to their high mechanical moduli. However, Hu's group recently observed the propagation of Na dendrites through the NASICON electrolyte. [41] Therefore, additional material strategies should be employed to mitigate this issue. Also, solid electrolytes show much lower ionic conductivity than the liquid ones, often leading to sluggish Na-ion transport across the electrode/electrolyte interphase. The ionic conductivity and working temperature of common solid-state electrolytes are summarized in Figure 2. Note that surface coating, element substitution, and doping have proven capable of enhancing their ionic conductivity. For instance, Manthiram and Yu coated Na 3 Zr 2 Si 2 PO 12 with a polymer buffer layer with intrinsic nanoporosity (Figure 3a,b). [42] Such a coating design allows for an adequate ionic conducting interface and increases the robustness of Na 3 Zr 2 Si 2 PO 12 pellets. The resultant Na−S batteries demonstrate desirable performance with greatly enhanced cycling stability.
In addition, glass-ceramic electrolytes are considered promising candidates owing to their high mechanical moduli over 200 GPa and high ionic conductivity. [43,44] The sulfide glass electrolyte of Na 3 PS 4 has been explored. [45,46] It was reported that the powder-compressed pellet of Na 3 PS 4 shows a high sodium ion conductivity over 10 −4 S cm −1 at room temperature. [47] Solid-state Na-S cells with this kind of electrolyte afford a large reversible capacity of over 1000 mAh (g-sulfur) −1 by utilizing the full redox reaction of S → Na 2 S. [48] Impressively, the utilization value of S is about twice of the cells that operated at high temperatures, which has been understood based on the structural evolution and electrochemical analyses. To further improve the performance of Na 3 PS 4 , An et al. reported a material and interfacial  modification using an ionic liquid of N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide. [49] Enhanced interfacial stability was confirmed by an overpotential of 0.55 V after 900 h of battery operation. However, the interaction between glass-ceramic electrolytes and Na metal would produce an unstable solid-electrolyte interphase (SEI). Alloys such as Na-Sn and Na-Sb are often used to replace the Na anode, at the expense of low energy density of the battery. [50,51] Recently, oxysulfide glass solid electrolytes (Na 3 PS 4−x O x , 0 < x ≤ 0.60) have been reported for Na-S cells with Na metal anode. [52] By forming bridging oxygen units, the oxygen-doped sulfide solid electrolytes possess a fully homogeneous glass microstructure with robust mechanical strength at ambient temperature (Figure 3c,d). Compared with other sulfide-based solid electrolytes, Na 3 PS 4−x O x exhibits the highest critical current density and outstanding electrochemical stability through the generation of a self-passivating SEI. Hence, oxysulfides would hold great potential for the development of all-solid-state Na-S batteries.
In addition to electrolytes, electrode materials are also the key to improving the energy storage performance of Na−S batteries. [53,54] Sodium sulfide (Na 2 S) has been extensively employed as a cathode material because of its large theoretical capacity and low cost. [55,56] However, its poor electrical and ion conductivities limit the potentiality. A strategy of mixing Na 2 S with NaI was proposed to obtain a Na 2 S-NaI composite. [57] The composite material shows a consierable ionic conductivity of~10 −7 S cm −1 at 25°C, five orders of magnitude higher than Na 2 S. A high capacity of 670 mAh g −1 was achieved for the cells using the Na 2 S-NaI composite, equal to 94% of the theoretical value for Na 2 S.

Quasi-Solid-State Cells
Another research direction is to develop quasisolid-state Na-S cells with non-aqueous polymer electrolytes. Compared with inorganic electrolytes, polymers demonstrate superior mechanical strength for flexible batteries and can restrain polysulfide dissolution. [58][59][60] Unfortunately, batteries with polymer electrolytes also face the issues of large polarization and poor cycling, caused by the limited ionic conductivity and unstable electrode/electrolyte interphase. These obstacles must be well addressed before these cells can hit the market.
Wang group assembled quasi-solid-state Na-S batteries using a (PETEA-tris[2-(acryloyloxy)ethyl] isocyanurate (THEICTA))based gel electrolyte and poly(S-pentaerythritol tetraacrylate (PETEA))-based cathode. [61] As depicted in Figure 4, the in situ formed polymer electrolyte not only displays a high ionic conductivity but also enables a stable interphase free of dendrites. In addition, the strong chemical binding between the electrolyte and S-rich copolymer cathode effectively confines NaPSs and suppresses the shuttle effect. Such a design leads to a high capacity of 877 mAh g −1 and improved cycling stability.
To elevate the ionic conductivity and mechanical robustness of polymer electrolytes, addition of inorganic components has drawn research interest. [62,63] For example, a composite solid electrolyte of polyether and Na 3 Zr 2 Si 2 PO 12 was prepared to utilize both merits. The composite displayed lower apparent activation energy and interfacial resistance relative to the Na 3 Zr 2 Si 2 PO 12 -free electrolyte. In another study, a reactivity-guided strategy was used to synthesize composite electrolytes with optimized compositions of polyethylene oxide, polyester, and ceramic NASICON for Na-S cells. [64,65] Compared with the conventional polyethylene oxide electrolyte, the designed composite shows 10 times lower interfacial impedance and 25 times higher ion conductivity.

Flame Resistance
Rechargeable batteries are important power sources for modern electronic devices and electrified vehicles. Their safety becomes a topic of concern when triggering accidents. One major drawback of Na-S batteries is the flammability of adopted organic electrolytes. [66,67] Therefore, exploiting non-flammable electrolytes is of great significance to enhance cell safety. In this section, we will discuss the recent progress on non-flammable liquid electrolytes for Na−S batteries.

Flame Resistance in Organic Electrolytes
The common organic electrolytes with carbonate ester or ether solvents are flammable. An effective approach to address the flammability concern is introducing the flame retardants such as organic phosphates and fluorinated phosphates. [13,68,69] A free radical capturing mechanism has been proposed to terminate the combustion process, where the free radical [P] Á generated by additives can capture the radicals of HO Á and H Á released by combustion chain reactions. [70] Among diverse flame retardants, trimethyl phosphate (TMP) draws extensive interest because of its high dielectric constant (21.6), low viscosity (2.3 mPa s), and wide temperature range as a liquid phase (46-197°C). [71,72] Typically, a nonflammable electrolyte can be developed by introducing TMP into common carbonate electrolytes for Na-S batteries. [73] With 15 wt% TMP, the self-extinguishing time of the electrolyte can be reduced from 108.3 to 1.1 s g −1 .
Apart from its usage as an additive, TMP can also function as a main component for flame-resistance electrolytes. Nevertheless, TMP would decompose on the Na anode surface, unfavorable for a stable SEI film. [74] To remove this obstacle, Kim's group reported a nonflammable electrolyte of sodium trifluoromethanesulfonimide in a mixture of fluoroethylene carbonate (FEC) and TMP. [75] It shows a high ionic conductivity of~6.0 mS cm −1 and allows for highly reversible Na plating and stripping because FEC helps form a NaF-rich SEI layer with reduced interfacial resistance. The crucial functions of FEC have been further confirmed. Zhu et al. reported a non-flammable electrolyte of FEC and 1,3,2-dioxathiolane 2,2-dioxide for Na metal batteries (Figure 5a). [76] It generates a stable SEI film to eliminate the dendrite growth and side reactions for Na anode, resulting in an average Coulombic efficiency of 93.4%. Very recently, an all-fluorinated electrolyte consisting of 2,2,2-trifluoro-N,N-dimethylacetamide solvent, 1,1,2,2tetrafluoroethyl methyl ether anti-solvent and FEC additive has been developed for non-flammable Na-S batteries (Figure 5b-f). [77] It enables a "quasi-solid-phase" Na−S conversion through controllable nucleophilic reactions.

Flame Resistance in Ionic Liquid Electrolytes
Room-temperature ionic liquids are composed entirely of ions and feature a wide electrochemical window, high thermal stability, and low volatility for non-flammable Na−S batteries. [78,79] Wei et al. [80] reported an ionic liquid electrolyte comprising the 1-methyl-3propylimidazolium-chlorate tethered to silica nanoparticles for Na−S batteries. The tethered ionic liquid can form a chemically stable and mechanically robust SEI layer on the Na anode surface, which prevents contact and side reactions from the electrolyte. The cell delivers a reversible capacity of ∼860 mAh g −1 at 0.1 C and distinctive cyclic stability with almost 100% of Coulombic efficiency.
Ionic liquid has also been used to optimize the electrode/electrolyte interphase. For instance, N-butyl-Nmethylpyrrolidinium bis (fluorosulfonyl) imide can form a stable interface between NaSn anode and Na 3 PS 4 electrolyte. [81] It is worth noting that ionic liquids have two serious drawbacks of high viscosity and cost, which largely limit the research of flame-resistance ionic liquid electrolytes in room-temperature Na-S cells.

Operation at Extreme Temperatures
Operating temperature significantly affects the performance of energy storage devices. In addition to room-temperature Na−S batteries, cells capable of working under extreme temperatures are also important for a specific application. In this regard, high-temperature Na−S cells that have received extensive attention since the 1970s will be introduced and discussed in this part.

High Temperatures
Conventional Na−S cells that operate at high temperatures of 300-350°C have been commercialized for utility-scale stationary energy storage. As depicted in Figure 6a, a high-temperature battery is composed of molten Na anode and liquid S cathode, which are separated by β-alumina solid electrolyte. [82] Such a battery can be designed as Na (l)|β″-Al 2 O 3 |Na 2 S x (l) + S(l). Upon operation, the high ionic conductivity of the ceramic electrolyte and its fine contact with two electrodes minimize the Ohmic resistance of the cell. A representative voltage profile during discharge is offered in Figure 6b. [83] Two voltage plateaus can be identified. One is located at 2.075 V representing the reaction between liquid sulfur and molten Na 2 S 5 ; the other is located at 1.74 V reflecting the transition of Na 2 S 2 into Na 2 S. Since the solid precipitates of Na 2 S 2 and Na 2 S are electrochemically poor active, the capacity at 1.74 V is not recommended for high-temperature Na−S batteries. Consequently, the maximum capacity decreases from 1672 mAh g −1 (S to Na 2 S) to 557 mAh g −1 (S to Na 2 S 3 ).
The low utilization of S limits the available energy of hightemperature Na−S batteries. This issue could be alleviated through an in-depth understanding of the electrochemistry of NaPSs, but remains challenging. Recently, Li et al. [84] reported the direct investigation of electrochemical reactions of NaPSs in cells using in situ transmission electron microscopy (TEM) implemented with a microelectromechanical system. As can be seen in Figure 6c-l, Na ions transport quickly and react violently with S in the carbon nanotubule, producing massive nanowires-like species. The discharge products of high-temperature Na−S batteries are dominated by Na 2 S 2 and Na 2 S. During charging, these Na 2 S 2 and Na 2 S rapidly decompose to generate meniscus-like molten Na 2 S x . It is suggested that constructing highways of electrons and ions can facilitate the complete conversion of S, which might serve as a vital clue to raising the capacity and energy of hightemperature Na-S batteries.

Intermediate Temperatures
High operation temperatures increase manufacturing costs and trigger severe safety concerns for the Na−S system. Reducing the working temperature would be a great benefit to raise cell safety and decrease the maintenance cost. [85] For this reason, extensive efforts have been devoted to the intermediate-temperature (100-250°C) Na−S batteries. However, lowering the temperature would cause the possible precipitation of intermediate NaPSs and even S solid. These solid products have weak reversibility, leading to low cathode utilization. [86,87] Carbonaceous materials are usually employed to enhance the cathode performance of Na−S cells worked at intermediate temperatures, as either current collectors or additives. [31,[88][89][90][91][92] The functions of carbon Figure 5. a) Schematic illustration of Na metal cell with conventional organic electrolyte and non-flammable electrolyte. Reproduced with permission. [76] Copyright 2021, Elsevier. b) Snapshots obtained from MD simulation of 1 M NaTFSI-FDMA:MTFE+1 wt% FEC electrolyte. c) Combustion tests of 1 M NaTFSI-TEGDME and 1 M NaTFSI-FDMA:MTFE+1 wt% FEC electrolytes. d) In situ Raman spectra of a S@C electrode. The corresponding charge-discharge-charge profiles are shown on the left. e) Voltage profiles of symmetric Na metal cells using 1 M NaTFSI-TEGDME and 1 M NaTFSI-FDMA:MTFE+1 wt% FEC electrolytes at 0.5 mA cm −2 . f) Comparison of specific capacities and cycling performance with reported Na-S batteries. Reproduced with permission. [77] Copyright 2022, Wiley-VCH.
Energy Environ. Mater. 2023, 6, e12589 6 of 11 materials on cell performance are still under controversy. [93] Hence, it is necessary to understand the working mechanism of carbon materials during battery operation by advanced characterization techniques.
In the intermediate-temperature system, molten Na shows poor wettability of β″-Al 2 O 3 solid electrolyte. An effective method is to design Na alloy anodes, which can reduce the melting point of the anode. Diverse alkali metals including K, Rb, and Cs have been introduced to Na to form alloys. [94] Cells with Na-Cs alloy anodes demonstrate improved cycling life in the temperatures ranging from 95 to 175°C. Other alloy anodes such as Na-In, Na-Bi, and Na-Sn have also been studied. [95] The contact angle tests suggested that the Bi-and Sncontaining alloys can enhance wettability. Especially, the Na-Sn alloy afforded a lower contact angle of~105°at 200°C, relative to~133°f or pure Na. These results confirm that β″-Al 2 O 3 solid electrolyte can be operated at lower temperatures.
The issues of ceramic β″-Al 2 O 3 electrolytes in Na-S cells mainly include its low ionic conductivity and poor Na wettability. [86] Several methods are proposed to increase ionic conductivity. 1) Raising the purity of β″-Al 2 O 3 . This is because β″-Al 2 O 3 features a much higher conductivity than β-Al 2 O 3 . [96,97] 2) Decreasing the thickness of electrolyte films. The resistance can be significantly reduced when the film Figure 6. a) Schematic illustration of a high-temperature Na-S cell and the crystal structure of ceramic β-alumina solid electrolyte. Reproduced with permission. [13] Copyright 2018, Wiley-VCH. b) Voltage profile together with the formed polysulfides in high-temperature Na-S batteries. Reproduced with permission. [83] Copyright 2013, Royal Society of Chemistry. c-h) Time lapse TEM images showing the cycling of a Na-S nanobattery at 300°C. i, j) Magnified TEM images from e) and g), respectively. k, l) Electron diffraction patterns acquired from e) and g), respectively. Reproduced with permission. [ Figure  7a). [98] Accordingly, a dual electrolyte with an ionic liquid was used to improve the Na-S performance at 150°C. As for wettability, one strategy is to coat a thin layer on the anode side of the electrolyte. [99][100][101][102] Li's group reported a heat treatment of β″-Al 2 O 3 electrolyte with lead acetate trihydrate and achieved a superior Na anode wettability. [103,104] The Na-S cells showed a capacity of 520.2 mAh g −1 at 120°C and excellent cyclic performance over 1000 cycles. Similarly, Deng et al. [105] demonstrated a yttria-stabilized zirconia (YSZ)-enhanced β″-Al 2 O 3 electrolyte, between which and Na the interphase impedance was reduced to 3.6 Ω cm 2 at 80°C. Figure 7b presents the wettability of molten Na on different electrolytes at 200 and 250°C, conforming the superiority of YSZ@β″-Al 2 O 3 at both temperatures.
In addition, other inorganic electrolytes such as NASICON-type and sulfide-based compounds have also been examined for intermediate-temperature Na-S batteries. [106,107] Their high conductivity of 0.15-0.2 S cm −1 at 200°C is fully comparable with that of β″-Al 2 O 3 . [108] Recently, Gross et al. [109] developed an NaI-GaCl 3 molten salt that can reduce the working temperature of molten Na from 300 to 110°C (Figure 7c,d). This study supplies a feasible method for tailoring the performance of intermediate-temperature systems.

Conclusion and Outlook
In summary, Na−S batteries at either high temperature or room temperature have become a rising star in energy storage technologies because of their high energy, low cost, and abundant resources. In particular, the reduction of operating temperature to ambient temperature renders a bright future in the diverse fields across multiple temporal and spatial dimensions. To meet various demands, multiple functionalities have been enthusiastically exploited. In this review, we concentrate on various functionalities such as flexibility, solid state, flame resistance, and extreme temperatures. We highlight the unconventional designs that help to realize these functionalities. As research in these areas is still in its infancy stage, critical issues and challenges remain and thus call for great efforts and  [98] Copyright 2021, Wiley-VCH. b) Wetting behaviors of molten Na on the surfaces of β″-Al 2 O 3 (left), YSZ (middle) and YSZ@β″-Al 2 O 3 (right) at 200 and 250°C. Reproduced with permission. [105] Copyright 2022, Nature Publishing Group. Efficiency profile c) and capacity profile d) of cell at 5 mA cm −2 at 110°C. Reproduced with permission. [109] Copyright 2021, Cell Press. devotion. To lay a foundation in the multiple functionalities, here we offer some personal insights ( Figure 8).
First, it is of significance to deepening the fundamental understanding of the kinetic barriers in the S cathode. Conversion of S in the Na−S system is much slower than in the Li−S counterpart. As a result, proper catalysts are generally required. However, the current selection of catalysts often follows the path of trial and error because of the lack of precise knowledge on the conversion kinetics of S. Of course, in situ characterization techniques and theoretical simulation and modeling have played a crucial role in this understanding. [110,111] These studies could offer a vital understanding of issues facing the current Na−S system.
Second, a stable Na metal anode would be the focus of future roomtemperature Na−S batteries. By no means could the importance of Na metal be exaggerated, because the Na anode equally, if not, more importantly, determines the battery performance and safety. Metallic Na is more active than Li, which indicates that the interphase of Na/electrolyte would continuously evolve. This evolution would consume Na and trigger side reactions, leading to potential safety concerns. Albeit with ongoing progress in metal anodes, their practical deployment still arises huge concerns. One strategy to bypass this is to construct metalfree cells in the discharge state. However, the use of Na 2 S cathode has to overcome the large kinetic barrier in the initial cycle and has to add additional Na to supplement metal loss upon cycling.
Third, more efforts need to be devoted to the electrolyte components as they ensure functionalities while maintaining battery performance. Highly conductive, chemically and electrochemically stable, and structurally robust electrolyte systems are extremely desired. Although solid-state electrolytes are of necessity in high-temperature systems, room-temperature Na−S batteries prefer liquid electrolytes to retain reasonable reaction kinetics. In this sense, the shuttle effect of NaPSs and the unstable Na metal in a non-aqueous solution should also be considered. Previous studies have confirmed the effectiveness of the proper design of electrolyte systems and additives. It is believed that electrolytes will play a pivotal role in regulating the electrochemical behavior of the S cathode and Na anode.
Fourth, more challenges such as cell packing and function elevating should be met before these functionalities can be deployed in practice. This has largely been ignored in lab research but presents a critical component in the practice. In particular, it is a necessity to construct microscale Na−S batteries adopted in microelectronics and the internet of things. Miniaturization of power sources is a critical solution to these devices. [112,113] Conventional lithium batteries of Li//LiCoO 2 and Li// V 2 O 5 cannot afford enough energy to power wireless electronics. Microbatteries having high areal energy have not been demonstrated over the last decades. [114,115] In this regard, Na−S batteries with higher energy are perfectly adapted to power them. Efforts in this area are demanded to stimulate related research and foster wireless infrastructure across the world. Moreover, broadening the temperature window of Na−S batteries to adapt to all climates is significant. To date, lowtemperature Na−S batteries have almost not been reported and would be one direction for future research. Furthermore, designs of selfhealing and integration would be profitable and deserve further efforts.