Review of Separator Modification Strategies: Targeting Undesired Anion Transport in Room Temperature Sodium–Sulfur/Selenium/Iodine Batteries

Rechargeable sodium–sulfur/selenium/iodine (Na–S/Se/I2) batteries are regarded as promising candidates for large‐scale energy storage systems, with the advantages of high energy density, low cost, and environmental friendliness. However, the electrochemical performances of Na–S/Se/I2 batteries are still restricted by several inherent issues, including the “shuttle effect” of polysulfides/polyselenides/polyiodides (PSs/PSes/PIs), sluggish kinetics of the conversion reactions at the cathodes, and Na dendrite growth at the anodes. Among these challenges, uncontrolled “shuttle effect” of PSs/PSes/PIs is a major contributing factor for the irreversible loss of active cathode materials and severe side reactions on Na metal anodes, leading to rapid failure of the batteries. Separator modification has been demonstrated to be an effective strategy to suppress the shuttling of PSs/PSes/PIs. Herein, the latest achievement in modifying separators for high‐performance Na–S/Se/I2 batteries is comprehensively reviewed. The reaction mechanisms of each battery system are first discussed. Then, strategies of separator modification based on the different functions for regulating the transportation of PSs/PSes/PIs are summarized, including applying electrostatic repulsive interaction, introducing conductive layers, improving sieving effects, enhancing chemisorption capability, and adding efficient electrocatalysts. Finally, future perspectives on the practical application of modified separators in high‐energy rechargeable batteries are provided.


Introduction
Rechargeable battery technologies are considered outstanding candidates for renewable energy storage due to their low geographical requirements, high flexibility, scalable manufacturing, and easy installation. [1]The booming demands for energy storage applications accelerate the development of cost-effective and high energy/power density rechargeable batteries. [2]Currently, the state of the art lithium-ion (Li-ion) batteries based on intercalation mechanisms have almost reached their theoretical energy density. [3]To further increase the energy density of rechargeable batteries and reduce the cost of manufacturing, exploring next-generation rechargeable batteries based on new electrochemistry and new materials beyond the horizon of Li-ion batteries is a formidable challenge.
The cathode materials based on the conversion reactions, such as sulfur (S), selenium (Se) and iodine (I 2 ), have attracted significant interests due to the following advantages. [4]First, the redox reactions of S/Se/I 2 cathodes are based on multi-electron conversion reactions, which provide high theoretical specific capacities.Second, S/Se/I 2 is abundant material in the Earth's crust, which reduces the production cost of battery manufacturing.Third, S/Se/I 2 cathodes are compatible with alkali metal anodes and show appropriate discharge voltage plateaus.Forth, the S/Se/I 2 cathodes show high chemical stability and good compatibility with organic electrolytes.Therefore, the S/Se/I 2 cathodes can be coupled with different metal anodes to fabricate various rechargeable batteries for sustainable energy storage.Among them, lithium/sodium metal sulfur/selenium/iodine (Li/Na-S/Se/I 2 ) batteries have attracted significant attention in both academia and industry.Li-based battery systems have been more comprehensively investigated so far.Some mechanism studies and materials design in Na-based systems were inspired by the research outcomes from Li-based systems. [5]However, the dissimilarities in microscopic characteristics determine that Na-based batteries are not the direct analog of Li-based batteries, especially in Na-S/Se/I systems.First, the preferred electrolyte systems are different between Na-S and Li-S batteries. [6]The high-order lithium polysulfides (LiPSs) produced in carbonates during the electrochemical process would not transfer to low-order LiPSs, but is consumed by side reactions with the solvents, which leads to the direct loss of active S species. [7]However, NaPSs present low solubility in carbonate solvents, which could reduce the side reactions between NaPSs and carbonate solvents. [8]Second, for Na-Se and Li-Se batteries, the first-principles density functional theory calculations showed that the electrical and ionic conductivities of all the Na-Se intermediate phases are much better than that of the Li-Se intermediate phases, which means a faster electrochemical reaction kinetic for Na-Se systems. [9]Third, compared with Li-I 2 system, more obvious hysteresis within the charge/discharge curves and lower capacities are detected for the Na-I 2 system, indicating the inferior kinetics of the Na-I 2 system. [10]However, the scarcity (0.0017 wt% in the Earth's crust) and the high cost (cost of carbonate: 6500 $ ton −1 , cost of industrial grade mater: 100 000 $ ton −1 ) of Li, make it unsuitable for large-scale energy storage applications. [11]In sharp contrast, Na is the fourth most abundant element on Earth and is widely distributed in the crust (2.7 wt% in the Earth's crust, cost of carbonate: 200 $ ton −1 , cost of industrial grade mater: 3000 $ ton −1 ) and scattered all over the world, as well as they can be prepared industrially in various ways. [12]Therefore, developing Na-S/Se/I batteries is more available for solving long-term energy bottlenecks in terms of technoeconomic. [13]n fact, the physical and electrochemical properties of S, Se, and I 2 cathodes are significantly different, as summarized in Table S1 (Supporting Information).Although the final reduced forms of S and Se are recognized to be similar in stoichiometry and equilibrium structure (face-centered cubic Na 2 S and Na 2 Se), the intermediate sodiation-desodiation processes are quite complicated and appear to be different between the Se cathode and the S cathode.[14] The insulating nature of solid S and its discharge products shows disadvantages to the transfer of electrons and ions during the reaction process, resulting in sluggish reaction kinetics, especially in the solid-solid reaction stage (Na 2 S 2 → Na 2 S), while the semiconducting Se obviously has faster electrochemical reaction kinetics.[15] Moreover, owing to the high electronic conductivities (I 2 = 1 × 10 −4 S m −1 ) and soluble intermediates, the Na-I 2 batteries exhibit remarkably fast reaction kinetics and good reversibility.[4e] However, I 2 is apt to sublime spontaneously due to its thermodynamic instability and is difficult to remain in the ambient environment.[16] Although S/Se/I 2 cathodes show different advantages in Nabased batteries, they all face similar issues.[10,17] First, the low electronic conductivities of the pristine cathode materials and the discharge products induce unsatisfied rate performances.Secondly, the significant volume fluctuation of the S/Se electrodes during cycling always causes the exfoliation of the active materials from the current collectors.Last but not least, the intermediate products (e.g., polysulfides (PSs), polyselenides (PSes), and polyiodides (PIs)) generated during the charge and discharge process are soluble in the organic electrolyte.The "shuttle effect" caused by those soluble intermediates results in the loss of active cathode materials, severe side reactions on Na metal anodes, and quick capacity fading upon cycling, which is considered as the most critical issue that hampers the future commercialization of Na-S/Se/I 2 batteries.[18] Various efforts have been made to improve the performance of Na-S/Se/I 2 batteries.On the cathode side, design multifunctional porous hosts for S/Se/I 2 cathode materials can improve electronic conductivity and provide buffer space for the volume changes of the cathodes.[19] Meanwhile, novel electrolytes are developed to alleviate the dissolution of PSs/PSes/PIs, such as optimizing electrolyte composition (e.g., Na salts, solvents additives) and replacing liquid electrolytes with solid-state electrolytes.[20] In addition, modified separators or interlayers are also an effective method to block PSs/PSes/PIs diffusion.[21] Furthermore, employing protective layers on the surface of Na metal anodes significantly reduce side reactions with PSs/PSes/PIs and facilitate uniform Na deposition.[22] Among those strategies, separator modification has been considered the most effective method to inhibit the "shuttle effect" of soluble PS/PSe/PI anions (Figure 1).In Na-S/Se/I 2 batteries, the separator, located between the S/Se/I 2 cathode and the Na metal anode, is the only pathway for the diffusion of soluble PSs/PSes/PIs anions from the cathode side to the Na metal anode side.For instance, the glass fiber (GF) separators commonly used in Na-S/Se/I 2 cells have micron-scale pores with the pore size distribution around 1.2 to 2.7 μm. [23] As a result, both Na ions and PSs/PSes/PIs are easily transported through the separator towards the anode, which leads to severe side reactions at the Na metal anode and rapid capacity decay during cycling.[24] Separator modification and introducing functional interlayers are effective strategies to prevent uncontrollable anion transport.These strategies play an important role in the mass transport at the interface between cathodes and Na anodes and simultaneously prohibit the "shuttle effect" of soluble PSs/PSes/PIs and side reactions of Na metal anodes.
17a,c,d,25] Herein, we comprehensively review the recent achievement in separator modifications for Na-based batteries, covering the room-temperature Na-S and the emerging Na-Se and Na-I 2 systems.Firstly, the fundamental reaction mechanisms of Na-S/Se/I 2 batteries are comprehensively summarized, which reveals the existing challenges in Na-S/Se/I 2 battery systems.Then, we summarize the latest reports on modified separators and interlayers, which are classified into five categories according to the mechanisms for restricting the diffusion of PSs/PSes/PIs, including creating electrostatic repulsion, optimizing the sieving effect, enhancing chemisorption capability, introducing conductive layer, and catalyzing redox reactions (Figure 2).Finally, we point out the challenges from both research and commercialization aspects of modified separators in Na-S/Se/I 2 batteries and provide recommendations for further development of advanced and practical separators in Na-S/Se/I 2 batteries, aiming to offer an indication for the future study of high-energy and low-cost energy storage systems.

Reaction Mechanisms in Ether-Based Electrolytes
Figure 3a clearly illustrates the discharge behavior of the S cathode in ether-based electrolytes. [6]From the perspective of phasetransition process, the discharge curve can be divided into the following four regions: Region I: A high-voltage-plateau region of approximately 2.20 V corresponds to the solid-liquid conversion: Region II: A sloping region in voltage range of 2.20-1.65 V corresponds to liquid-liquid conversion: (2) In this process, the following reactions may be involved: Region III: A low-voltage-plateau region of about 1.65 V corresponds to the liquid-solid conversion: Region IV: A second sloping region in voltage range of 1.65-1.2V corresponds to solid-solid conversion: Normally, the pristine sulfur material is a ring-like S 8 .During the initial discharge process, the solid-phase S 8 rings open and transform into a series of soluble high-order sodium polysulfides (Na 2 S n , 4 ≤ n ≤ 8) (Region I and Region II). [30]These .Reproduced with permission. [6]Copyright 2014, Wiley-VCH.b) The XRD patterns of a S electrode in a Na-S battery during initial cycling.Reproduced with permission. [32]Copyright 2021, Elsevier.c) Charge and discharge voltage profiles of Na-S cells with carbonate electrolyte (1 m NaClO 4 in propylene carbonate (PC)/ethylene carbonate (EC).19c] Copyright 2013, Wiley-VCH.d) In situ XRD pattern of Na-S battery during initial charging and discharging process.Reproduced with permission. [33]Copyright 2018, Springer Nature.e) The cycling profiles of Na-S cells with the carbonate electrolyte of 1 M NaClO 4 in EC/diethylene carbonate (DEC).Reproduced with permission. [36]Copyright 2016, Springer Nature.f) First-order derivative curves correspond to in situ UV-vis spectra for Na-S battery at different discharge states.Reproduced with permission. [37]opyright 2020, Wiley-VCH.
liquid-phase higher-order Na 2 S n species are further reduced to insoluble lower-order Na 2 S n (1 ≤ n ≤ 3) (Region III and Region IV). [31]Accordingly, there are two distinct plateaus (2.20 and 1.65 V) in the discharge curve of Na-S batteries (Figure 3a).During the following charging process, the low-order Na 2 S n is converted into high-order Na 2 S n in two steps and finally into S 8 . [23]he X-ray diffraction (XRD) pattern in Figure 3b clearly elucidates the transformation of polysulfides during the initial charging and discharging process. [32]

Reaction Mechanisms in Carbonate-Based Electrolytes
The electrochemical principle of Na-S batteries based on carbonate electrolyte has not been clearly revealed.In terms of the present research advances, it is divided into two working mechanisms: the solid-liquid-solid (S-NaPSs-Na 2 S) pathway and the direct solid-state conversion pathway. [31]The solid-liquid-solid pathway undergoes a complex activation process during initial discharge, which is similar to but different from the solid-liquidsolid pathway in ether-based electrolytes.19c] The discharge products identified by in situ XRD from Figure 3d are as follows: [33] S 8 → Na 2 S n → Na 2 S 4 → Na 2 S 2 → Na 2 S, 4 ≤ n ≤ 8 (10)   Compared with ether solvents, carbonate solvent molecules are more prone to undergo violent nucleophilic addition or substitution reactions with free long-chain PSs anions. [31]This parasitic reaction causes an ultra-high discharge capacity in the initial cycle, even exceeding the theoretical capacity of sulfur.Then, the Na-S battery experiences a rapid capacity decay due to the uncontrolled dissolution of the polysulfide. [34]Fortunately, the larger ionic radius of Na ion weakens the reactivity between Na ion and polysulfides compared to Li-polysulfide ion pairs, which prevents their dissociation in carbonate solvents. [8]Therefore, compared with Li-polysulfide, NaPSs have relatively lower solubility and less side reactions in carbonate-based electrolyte.
It has been reported that solid sulfur could be directly converted to solid Na 2 S 2 /Na 2 S in carbonate-based electrolytes in the form of small molecules or interacts strongly with the conductive host, thus avoiding the "shuttle effect." [35]As shown in Figure 3e, the discharge curve presents a long and continuous downward slope with a discharge plateau lower than 1.6 V in the first cycle.The following discharge plateaus range from 1.6 to 1 V in the subsequent cycles. [36]The first-order derivative curves from the in situ ultraviolet-visible spectroscopy (UV-vis) spectra under different states of discharge confirmed that Na 2 S is the only discharge product (Figure 3f). [37]The increased irreversible capacity observed in the discharge curve is mainly due to the decomposition of the electrolyte to form the solid electrolyte interface (SEI) on the Na metal anode and the cathode electrolyte interphase (CEI) on the S cathode. [31]In the subsequent cycles, the sulfur particles in the cathode under the protection of CEI layers undergo a quasi-solid-phase transformation (i.e., S 8 ↔ Na 2 S 2 and Na 2 S), reducing the formation of soluble S n 2− intermediates in the organic electrolyte. [38]Generally, the addition of fluoroethylene carbonate (FEC) additive to the carbonate-base electrolytes is conducive to the formation of protective SEI to prevent excessive side reactions. [22]Overall, Na-S cells exhibit improved cycling stability in carbonate electrolytes because of the stable interface phase and the low solubility of NaPSs.

Reaction Mechanisms of Na-Se Batteries
For Na-Se batteries, the Se atoms have similar electronegativity and ionic radius to that of S atoms, which has been considered as one of the promising cathode materials for Na metal batteries with a theoretical specific capacity of 675 mAh g −1 . [39]The high electronic conductivity of Se (1 × 10 −3 S m −1 ) also facilitates active material utilization and fast electrochemical reaction kinetics, contributing to the high-rate performance of Na-Se batteries. [40]n addition, due to the relatively high weight density of Se, the theoretical volumetric capacity of Se cathode is 3253 mAh cm −3 , which is comparable to that of S cathode (3467 mAh cm −3 ). [41]n Na-Se batteries, Se cathode materials are present in several molecular forms (e.g., ring-like Se 8 , chain-like Se n and small Se molecules) in different porous hosts.In general, the overall electrochemical reaction of the Se cathode is a two-electron conversion reaction between Se and Na ions, i.e., Se + 2 Na + + 2 e − ↔ Na 2 Se.However, the detailed reduction process of the Se cathode is a complex multi-step reaction that depends on the molecular forms of Se.

Reaction Mechanisms of Ring-Like Se 8 Cathode
The electrochemical reactions of the Na-Se cell in the electrolyte of 1 m NaClO 4 in EC/DEC can be expressed as follows: [42] n Se + 2 Na + + 2 e − → Na 2 Se n , 4 ≤ n ≤ 8 (11) Na 2 Se n + 2 (n − 1) Na + + 2 (n − 1) e − → n Na 2 Se (12) First, the long-chain PSes are formed at the first discharge plateau around 1.7 V, corresponding to the conversion of the solid Se 8 to the liquid sodium polyselenides (Na 2 Se n , 4 ≤ n ≤ 8) (Figure 4a).Then, with the further reduction of the active material, short-chain PSes are formed at the second discharge plateau around 1.3 V, which is still soluble in the liquid electrolyte when further discharging to 0.5 V, Se is completely converted to solid Na 2 Se, as demonstrated by Raman spectra (Figure 4b).

Reaction Mechanisms of Small Molecule Se
The typical charge-discharge voltage profiles of Na-Se cells with the carbonized polyacrylonitrile/Se composite electrodes are displayed in Figure 4c, in which the Se material is embedded within the matrix in the form of small molecules. [43]In addition, there is only one pair of reversible redox peaks in the cyclic voltammetry (CV) curves of Na-Se batteries (Figure 4d), suggesting that the Se composite electrode experiences a single-phase transition reaction from small-molecules Se to insoluble Na 2 Se without the formation of soluble intermediate species (e.g., Na 2 Se n , n ≥ 4).
Se + 2Na + + 2e − ↔ Na 2 Se (13) In addition, the Na-Se cell shows a lower redox potential, which is due to the additional energy required for the Se and Na reaction to dissociate Se from the covalent bond formed between Se and polyacrylonitrile.

Reaction Mechanisms of Chain-Like Se n
The chain-like Se n was prepared by Goodenough et al. through permeating molten Se in microporous carbon nanotubes. [44]During the cooling process, the molten Se in the pores of carbon nanotubes does not crystallize and maintains as isolated Se n chains, due to the space confinement effect from the micropores.The electrochemical reaction of the Na-Se cell with the electrolyte of 1m NaClO 4 in EC/PC and the chain-like Se n cathode can be expressed as: During the initial redox process, the chain-like Se n were gradually reduced to Na 2 Se and then oxidized to small Se molecules (e.g., Se 4 , Se 3 and Se 2 ), which shows multiple peaks in the CV curves (Figure 4e).Afterwards, these small Se molecules are further condensed to Se 2 (such as, 4 Na + Se 4 → 2 Na 2 Se + Se 2 ).After 5 cycles, Se 4 is almost completely decomposed into Se 2 , as well as the electrochemical reaction mainly occurred between Se 2 and Na 2 Se: A different reaction mechanism was proposed by Dravid et al., who investigated the reaction mechanism of Na-ion batteries using Se nanotubes with spiral chain structure as the cathode and Na 2 O as the solid electrolyte at the nanoscale. [9]The results show that Se alloyed with Na in a three-step reaction.The Se single crystal sequential reduction to amorphous Na 0.5 Se (58% volume expansion), polycrystalline Na 2 Se 2 , and Na 2 Se (about 336 vol%), as shown in Figure 4f-h.
The intermediate Na-Se alloy phase has a higher electronic conductivity, and a higher ionic conductivity than that of Li-Se alloy phase (3.3 × 10 −7 cm 2 s −1 for Na 0.5 Se, vs 2.2 × 10 −9 cm 2 s −1 for Li 2 Se), which imply a faster electrochemical reaction kinetic for Na-Se systems.
Similar to Na-S batteries, the discharge intermediate species (i.e., PSes) have relatively low solubility in carbonate electrolyte compared to ether-based electrolyte.However, the "shuttle effect" still exists, and physical or chemical restriction of the PSes is critical to prolong cycling performance of Na-Se batteries.Raman spectra of Se/C composite electrodes after discharged.Reproduced with permission. [42]Copyright 2019, Elsevier.c) The discharge-charge

Reaction Mechanisms of Na-I 2 Batteries
For Na-I 2 batteries, the iodine cathode material received widespread attention because of its high theoretical specific capacity (211 mAh g −1 ), high volumetric capacity (1040 mAh cm −3 ) and high average operating voltage of up to 2.9 V. [4e] The nonaqueous rechargeable Na-I 2 battery, where Na + /Na redox pair shows a potential of −2.71 V versus standard hydrogen electrode (SHE), enabled a high energy density of 473 Wh kg −1 . [10,45]inally, iodine has the advantages of high natural abundance (55 μg iodine L ocean −1 ), low cost, and environmentally friendly etc. [46] On the cathode side, the charge storage process involves excellent reversible reactions of redox couples I 2 /I 3 − and I 3 − /I − , which makes iodine a potential cathode material for high-rate rechargeable batteries. [47]Among them, the formation of soluble I 3 − intermediates bridges the solid-liquid reaction and ensures rapid reaction kinetics. [48]The typical charge/discharge voltage profiles of Na-I 2 cells with an iodine-carbon cathode are shown in Figure 5a. [49]The Na-I 2 cell with the electrolyte of 1 M NaClO 4 in EC/DEC undergoes a reversible reaction during cycling with a two-step redox process: During discharging, iodine ions are reduced to I 3 − by forming NaI 3 and then further reduced to NaI (I − ). [50]During charging, NaI is oxidized to NaI 3 at 2.75 V, followed by further oxidation to I 2 at 3.16 V (Figure 5b).The redox process has been demonstrated by Raman spectra as shown in Figure 5c.

Challenges
Based on the above-described electrochemical reaction mechanisms, the major challenges to develop high-performance Na-S/Se/I 2 batteries are summarized below.

Poor Electronic Conductivity of Cathode Materials
At room temperature, solid S and its discharge product are intrinsically electron insulators, which do not favor the electron transfer during the conversion reaction, resulting in large voltage polarization and low active materials utilization. [15]Although the electronic conductivities of Se and I 2 are much higher than that of S (Se , the full utilization of the active material is still limited at high current densities.

Significant Volume Changes of Cathodes During Cycling
The S/Se electrode is prone to drastic volume fluctuation due to the variation of the mass density of active materials during sodiation and desodiation (Se→Na 2 Se: 336 vol%, S→Na 2 S: 157 vol%). [51]2a]

Shuttle Effect of PSs/PSes/PIs
24b] The deposition of inactive Na 2 S 2 /Na 2 Se 2 and Na 2 S/Na 2 Se layers on the surface of the Na metal anode results in electrode passivation and increased polarization.Furthermore, the some shortchain Na 2 S n /Na 2 Se n species formed by reduction reaction can diffuse back to the cathode side and then be oxidized into longchain Na 2 S n /Na 2 Se n in the following cycle. [29]This repeated diffusion process between cathode and anode is called "shuttle effect," which causes the self-discharge phenomenon of the batteries and results in the loss of active materials and capacity attenuation.In Na-I 2 batteries, the microsoluble I 2 can combine with I − and spontaneously convert to highly soluble I 3 ), which results in strong "shuttle effect" and leads to loss of active cathode materials and undesired self-discharge phenomenon. [52]

Sluggish Reaction Kinetics of S/Se/I 2 Cathodes
Na ion has a larger ionic radius and high redox potential than that of Li ion (−2.71V for Na/Na + and −3.04 V for Li/Li + vs SHE), which indicates more sluggish reaction kinetics between Na ions and cathode materials (i.e., S, Se and I 2 ) under the same operating temperature. [29,53]The sluggish reaction kinetics result in insufficient reduction of active materials during discharge and the accumulation of PSs/PSes/PIs, which reduces the specific discharge capacities and rate performances of Na-S/Se/I 2 cells. [54]n addition, the low electronic conductivity of S/Se/I 2 materials and the complex multi-electron conversion processes also reduce the reaction kinetics of S/Se/I 2 cathodes. [55]

Degradation of Na Metal Anodes
The Na metal anode is highly reactive in the organic electrolyte and easily reacts with the soluble intermediates diffusing voltage profiles and d) CV curves of the Na-Se cell with carbonized polyacrylonitrile/Se cathode in the electrolyte of 1 m NaClO 4 in EC/DEC.Reproduced with permission. [43]Copyright 2014, The Royal Society of Chemistry.e) The CV figure of the initial cycle of the Na-Se cell with the chain-like Se cathode in the electrolyte of 1 m NaClO 4 in EC/PC.Reproduced with permission. [44]Copyright 2016, American Chemical Society.f) Schematic diagram of the sodiation process and g) discharge voltage profiles of sodiation and lithiation processes in Na/Li-Se cell with Se nanotubes as the cathode and Na 2 O as the solid-state electrolyte.h) Illustration of the atomic structures of Se, Na 0.5 Se, Na 2 Se 2 , and Na 2 Se phases appeared in the sodiation process.The yellow balls represent Na atoms, green balls represent Se atoms.A space group of P 3 1 2 1 means spiral chain structure.Reproduced with permission. [9]Copyright 2017, Springer Nature.
20b] The unstable SEI induces uneven deposition of Na metal and dendrite formation during cycling due to the inhomogeneous Na-ion conductivity of the SEI.In addition, the breakage of SEI during cycling exposes fresh Na metal to the electrolyte, which causes continuous side reactions that consume the limited Na metal and electrolytes. [56]How to protect Na metal anodes from soluble intermediates and facilitate homogeneous Na metal plating/stripping are essential to achieve long-term cycling stability of Na-S/Se/I 2 batteries.

Separator Modification Strategies
To date, surface modifications of the separators or introduction of the interlayers have been demonstrated to be effective strategies to solve the above-mentioned challenges simultaneously in Na metal batteries.In term of functions, these strategies can be divided into three categories: 1) regulating the transportation of PSs/PSes/PIs across the separator via electrostatic repulsion interaction, sieving effects, and chemisorption; 2) promoting the interfacial charge and mass transfer of the redox reaction by introducing conductive layer to reactivate PSs/PSes/PIs; 3) accelerating redox kinetics of PSs/PSes/PIs by electrochemical catalysis on separators.

Repelling PSs/PSes/PIs Migration by Electrostatic Repulsive Interaction
Generally, highly soluble PSs/PSes/PIs are negatively charged in organic electrolytes.24b] According to the principle of electrostatic repulsion theory between two identical charges, the migration of PSs from the sulfur cathode side to the Na metal anode side can be prohibited by introducing a negatively charged modified separator. [57]For example, Nafion films as separators with sulfonic acid groups (-SO 3 H) are prone to generate electrostatic repulsion with PSs anions (S n 2− ) to effectively block the shuttle of dissolved PSs to the anode. [58]In addition, on the Na metal anode side, the electrostatic interaction of the negatively charged modified separator enables the uniform distribution of the Na ion flux, thus reducing the dendrite growth on Na metal anodes. [59]n 2014, Kaskel et al. first used the sodiated Nafion coating on a porous polypropylene (PP) membrane as a modified separator for Na-S batteries. [60]The negatively charged sulfonic acid groups on the separator confined negatively charged polysulfides facing the cathode side by electrostatic repulsion and prevented them from passing through the separator, thus improving sulfur utilization.After initial cycling, compared with the bare PP separator, the capacity of Na-S cell increased from 200 mAh g −1 to 350 mAh g −1 .24b] They systematically studied the interaction mechanism between the Nafion membrane and the dissolved PSs.The Nafion membrane contains a hydrophobic region consisting of a poly (tetrafluoroethylene) main chain and a hydrophilic region consisting of ion clusters, which connect by a hydrophilic channel.The ─SO 3 − located on the surface of the hydrophilic pore of the Nafion membrane provides a negatively charged environment that significantly obstacles the migration of polysulfides.Moreover, the electrostatic interaction between the anionic groups of the Nafion membrane and the Na  and f) nanosheets containing Ti defects.Reproduced with permission. [61]Copyright 2021, Springer Nature.
ions promotes rapid diffusion and uniform distribution of Na-ion flux, thus facilitating homogenous Na deposition and suppressing dendrite growth.However, due to the poor conductivity of the short-chain NaPSs and Na 2 S, the capacity of the Na-S cells continued to decline upon cycling.
To address both the uneven transport of alkali metal ions (e.g., Li ions or Na ions) and the "shuttle effect" of PSs/PSes anions through the separators, Wang et al. coated PP membranes with negatively charged Ti 0.87 O 2 nanosheets. [61]The Ti 0.87 O 2 showed much higher electrostatic repulsion energy than that of TiO 2 or graphene oxide (GO) towards all PSs and PSes species, effectively inhibiting the "shuttle effect" (Figure 6a).In addition, Raman spectra clearly showed that PSs easily passed through the PP separator from the cathode side and reached the anode (Figure 6c).In contrast, the Ti 0.87 O 2 /PP separator effectively restricts the diffusion of PSs through the separator.Much-weakened signals of PSs were detected from Raman spectra at the anode side (Figure 6d).In addition, the charge density distribution in Figure 6b showed that the charge density around Ti vacancy significantly increased the charge attraction for Li ions, thus reducing the diffusion barrier of Li ions through the separator.In the restacked thin layer of traditional TiO 2 nanosheets, Li ions were transported through gaps between adjacent nanosheets, resulting in an uneven distribution of Li ions (Figure 6e).On the contrary, in the restacked thin layer of Ti 0.87 O 2 nanosheets, Li ions migrated not only between the gaps of the nanosheets but also through the Ti vacancies within individual nanosheets, thus providing homogeneous Li-ion flux distribution (Figure 6f).This Ti 0.87 O 2 /PP separator showed a similar effect to the Naion flux distribution.The Na-Se cell with Ti 0.87 O 2 /PP separator achieved a specific capacity of about 450 mAh g −1 at 0.2 C after 250 cycles, which was better than the cell with the bare PP separator.
Currently, there are no reports on the use of electrostatic repulsion strategies to modify the separator of Na-I 2 cells, but it has been reported in other metal-iodine batteries.For example, Wang et al. placed poly(3,4-ethylenedioxythiophene): polystyrene sulfonate thin film on both sides of a GF separator as an interlayer for Zn-I 2 cells, where the interlayer with negatively charged groups (─SO 3 − ) could effectively prevent the diffusion of PIs anion by electrostatic repulsion. [62]Thus, based on the above studies, the suppression of the "shuttle effect" by electrostatic repulsion strategy can be applied to further improve the electrochemical performances of Na-I 2 cells.
Unlike conventional chemisorption, electrostatic repulsion induced by the negatively charged separator merely confines the PSs/PSes/PIs at the cathode side, effectively reducing the loss of active species.However, we must acknowledge that the PSs/PSes/PIs exclusion strategy has inherent limitations.For instance, confining PSs/PSes/PIs at the cathode side increases the regional concentration of PSs/PSes/PIs in the electrolyte, which tends to form inert layers on the cathode surface, especially cycling at high current densities. [63]The inert layer obstructs the diffusion of Na ions and causes the increase in charge transfer resistance, resulting in a large irreversible capacity.In addition, the weak repulsive interaction will lead to insignificant shielding effects toward PSs/PSes/PIs.However, if the charge density of the separator is too strong to form ionic bonds with positively charged Na ions, the diffusion of Na ions through the separator will be restricted during cycling. [64]Therefore, it is important to optimize the charge density of the separator to effectively suppress the "shuttle effect" of PSs/PSes/PIs without forming ionic bonds with Na ions.

Regulating PSs/PSes/PIs Migration via Sieving Effects
The van der Waals diameters of Na 2 S n (4 ≤ n ≤ 8) are 10.99-19.19Å, calculated from the length of S−S bond (2.05 Å), the length of the Na-S bond (1.43 Å), and the ionic radius of S 2− (1.84 Å). [24b] The diameters of Na 2 Se 6 and I 3 − are 11.998Å and 5.144 Å, respectively, which are much larger than that of Na ions (diameter: 2.04 Å). [24a,65] In organic electrolytes, the solvated PSs/PSes/PIs have even larger diameters.For instance, the diameter of pristine Na 2 Se 6 is 11.998 Å, which increases to 15.483 Å in EC solvent and 16.982 Å in dimethyl carbonate (DMC) solvent.The solvated PSs/PSes/PIs showed different molecular structures in both bond length and bond angle, and some of them may form clusters (Figure 7a-d).The commonly used commercial separators have microporous structures.For instance, the pore size distribution of PP separators is around 50 to100 nm, and the pore size distribution of GF separators is around 1.2 to 2.7 μm, which allows the transport of both Na ions and PSs/PSes/PIs, resulting in severe "shuttle effect." [66]Therefore, one of the most effective strategies to inhibit the "shuttle effect" of PSs/PSes/PIs is applying sieving effects to construct a thin coating layer with nanopores or subnanopores on the commercial separators to physically inhibit the shuttle phenomenon of PSs/PSes/PIs and still allow the Na-ion migration.Due to the differences in diameters, the large PS/PSe/PI anions can be isolated on the cathode side of the separators by the sieving effect.Meanwhile, the small Na ions can still freely transport through the separators.
It is well known that an inhomogeneous pore size distribution of the separator results in uneven ion flux.The resultant uneven localized electric field leads to non-uniform Na deposition, and eventually the formation of dendrites. [67]Zhao et al. designed a polybenzimidazole (PBI) interlayer containing heterocyclic groups between PP separator and Na metal anode with an average pore size of a few nanometers. [68]Due to the high tensile modules of PBI film (about 82 MPa), the Na dendrite growth was effectively inhibited.In addition, the uniform structure of the PBI layer allowed the homogeneous distribution of Na ions and promoted smooth Na deposition.As a result, the symmetric cell with a PBI layer achieved excellent cycling performance, which stably cycled 100 times without voltage fluctuation.On the contrary, the cell without the PBI layer showed a considerable increase in the overpotential due to the inhomogeneous Na deposition.Moreover, the abundant nitrogen-containing functional groups (−N = and −NH) facilitated to limit NaPSs migration.As a result, the Na-S cell with the PBI interlayer experienced only a slight capacity loss over 160 cycles.In contrast, the cell with commercial PP separator decay sharply.
Among various coating materials, covalent organic frames (COFs) have attracted significant interest due to their unique crystal structure with regular nanoscale open channels that provide rapid ion diffusion. [69]Wang et al. prepared an azobenzenebranched COF (Azo-TbTh) coating layer on the GF separator via interfacial polymerization reaction.The Azo-TbTh layer showed a narrow pore size distribution of about 9.1 Å (Figure 7e). [70]ue to excellent sieving effects, the CV curves showed only one pair of redox peaks, indicating the direct conversion reactions between S and Na 2 S. The formation of hydrazone linkages through the polymerization process increased the negative charge of Azo-TbTh separator (zeta potential of −36 mV) (Figure 7g).The permeation experiments demonstrated that the modified separator effectively prevented the NaPS penetration, which was attributed to the sieving effect from the nanopores and the electrostatic repulsion of the negatively charged azo groups (Figure 7h).Moreover, the beneficial interactions between metal ions and azo groups promoted the fast diffusion of Na ions through the modified separator.Specifically, with the help of electrons provided by dissolved NaPSs, the modified separator displayed a reduction potential of 1.1 V versus Na/Na + for the reduced N-N fragments (Figure 7f).The Na-S battery using this modified separator showed significantly enhanced cycling performance with lean electrolyte (Figure 7i).
Lu et al. developed a free-standing multifunctional hydroxy naphthol blue (HB)/CNT@COF separator with a pore size of 7.2 Å and a negative charge. [21]The separator with angstrom-scale pores was a natural physical barrier to NaPSs.In addition, the negatively charged HB (−SO 3 − ) in the separator enhanced the inhibition of "shuttle effect."HB@COF was used as the separator to simulate the distribution of Na ions and PSs anions on both sides of the membrane during the charging process.The results showed that the proportion of Na ions in HB@COF separator nanochannels was significantly higher than that of PSs anions, and PSs anions gathered outside the pores under the synergistic effect of electrostatic and sieving, which further indicated that the HB@COF separator effectively inhibited the transmission of PSs anions to the cathode (Figure 7j-m).In addition, the conductive network in HB/CNT@COF separator effectively promoted the conversion of sulfur species.It is worth mentioning that the thickness of the separator was 4.8-5.6 μm, which helped to reduce the amount of electrolyte and shorten ion transport distance.With the synergistic effect of multiple functions, the Na-S cell using the HB/CNT@COF separator still maintained a high capacity of 733.4 mAh g −1 after 400 cycles at 4 C, far exceeding the cell with GF separator (260.8 mAh g −1 ).
At present, there are no reports on the modification of separators or introduction of interlayers through a sieving strategy The purple and yellow balls are Na and S atoms, respectively.Reproduced with permission. [68]Copyright 2018, Elsevier.b) Structural diagram of the monomers Na 2 S 6 , Na 2 S 8 and the dimer Na 2 S 4 , with the yellow and red balls in Na-Se/I 2 systems.24a,71] In Li-Se systems, Cai et al. used a modified separator with an average pore size of 5.6 Å to effectively constrain the migration of PSes. [72]These successful cases suggested functional separators with the sieving effect could be used for Na-Se/I 2 cells.
We cannot deny that the "harmful" anions in the electrolytes can indeed be effectively sieved by modifying separators with highly ordered pore structures and optimized pore sizes.However, there are inevitably gaps between the packed particles of the modified materials that are much larger than the pore size of the particles themselves. [23]Thus, the dissolved PSs/PSes/PIs are likely to pass through the separator along these gaps rather than being blocked by the sieving materials.Therefore, it is also essential to incorporate other functional materials that have stronger interactions with PSs/PSes/PIs to modify the separators.

Immobilizing PSs/PSes/PIs by Chemisorption
Compared with physical restriction strategies (e.g., electrostatic repulsion, sieving effect), immobilizing PSs/PSes/PIs by chemical adsorption mainly relies on the chemical bonding generated by electron transfer between PSs/PSes/PIs and coating materials on the modified separator, which shows a stronger adsorption capability and high selectivity. [53,73]Chemisorption effects can be divided into two types: polar-polar interaction and Lewis acidbase interaction. [74]n polar molecules, there are independent centers of positive and negative charges, and the charge distribution throughout the molecule is inhomogeneous. [75]In the process of forming the covalent bonds, different atoms show different capabilities to attract electrons because the shared electrons tend to favor the atoms with stronger attraction.These covalent bonds are called polar covalent bonds or polar bonds for short.Thus, the polar coating materials on the modified separators tend to form polar-polar bonds with the polar PSs/PSes/PIs, which confines them at the cathode side. [23]olymeric materials containing functional groups with highly electronegative elements (N, O, S) have been reported as effective PS anchoring sites. [76]Among them, the N-rich poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer matrix has been widely used because of its high porosity, high electrolyte absorption rate, good thermal stability, and structural integrity. [77]n addition, the introduction of inorganic fillers in the polymer matrix can further improve ionic conductivity.Sundara et al. added boron nitride (BN) to PVDF-HFP/poly(butyl methacrylate) (PBMA) polymer membrane as a NaPSs shield between the cathode and separator for Na-S batteries. [78]The electronegative N favored the formation of N-S covalent bond with S. Furthermore, the N═B and N-B in the interlayer caused the polarization of B atoms leading to charge attraction between B atoms and NaPSs anions, which helped to capture NaPSs.The Na-S cells using the interlayer showed excellent cycling stability and significant reduction of self-discharge.The Electrochemical impedance spectroscopy (EIS) spectra demonstrated that the dissolution of NaPSs was effectively suppressed (Figure 8a,b).
74b] Therefore, the combination of conductive carbon materials with polar materials is a feasible approach.Sundara et al. used Teflon-lined carbon substrate as an interlayer between the cathode and the separator in the Na-S battery. [79]The presence of electronegative F in the interlayer provided strong anchoring of NaPSs by forming covalent bonds with NaPSs, which was consistent with the results of UV-vis spectroscopy (Figure 8c).There was no NaPSs deposition on the surface of the interlayer (side toward the anode) in the scanning electron microscope (SEM) images, illustrating the enhanced chemisorption between the interlayer and NaPSs (Figure 8d).In addition, the good conductivity of the interlayer promoted the conversion of NaPSs.As a result, a Na-S cell with the interlayer exhibited a specific capacity of 650 mAh g −1 after 100 cycles at 0.1 C. In contrast, the battery without the interlayer only delivered a lower specific capacity of 60 mAh g −1 .
Introducing heteroatoms (N, O, S) on carbon-based materials can provide rich sites for the absorption of PSs and effectively prevent the migration of PSs through enhanced chemical interactions with PSs. [76]Jiang et al. recently coated N and S co-doped carbon nanofiber/carbon black composites onto the cathode side of GF separator (N, S-CNF/CB+GF). [80]Through density functional theory (DFT) calculation, the adsorption energy values of Na 2 S 6 on N,S co-doped graphene with different N doping sites were higher than those of graphene.The graphene and the pyrrolic-N are shown in Figure 8e,f, respectively.This indicated that N, S co-doping in the modified separator enhanced the anchoring of NaPSs and promoted the rapid transition of long-chain NaPSs into short-chain NaPSs.Thus, the N, S-CNF/CB+GF separator exhibited stable performance, with a specific capacity of 527 mAh g −1 after 900 cycles at 0.5 C.
Metal oxides are commonly used as polar materials for NaPSs adsorbent in Na-S batteries.Demir-Cakan et al. mixed Al 2 O 3 powder with Nafion solution and then coated the mixture on the surface of GF separator for Na-S battery to inhibit the diffusion of PSs. [81]The XPS spectra demonstrated no peaks associated being S and Na atoms, respectively.24b] Copyright 2016, American Chemical Society.c) The molecular structures and sizes of Se 8 , EC, DMC, Na 2 Se 6 , Na 2 Se 6 -DMC, Na 2 Se 6 -EC.Gray, white, brown, purple, and red balls represent C, H, Se, Na, and O atoms, respectively.Reproduced with permission. [65]Copyright 2017, Wiley-VCH.d) The molecular structure and size of I 3 − .24a] Copyright 2022, American Chemical Society.e) Schematic diagram of synthesis of the Azo-TbTh separator via interfacial polymerization.f) Schematic diagram of rapid Na ions migration in Azo-TbTh separator.g) Zeta potentials of the Azo-TbTh separator and the bare GF separator.h) NaPSs permeates through GF separator and Azo-TbTh separator.i) Cycling performance of Na-S batteries using Azo-TbTh separator with different amount of electrolyte at 1 C. Reproduced with permission. [70]Copyright 2022, American Chemical Societ.j,i) Numerical simulation of the distribution of j) Na ions and l) PSs anions in HB@COF separator nanochannels (width of 0.72 nm and a length of 5000 nm), with NaPSs at 0.5-2 m concentrations flowed into anode across HB@COF separator during charging.Schematic diagram of k) Na ions, m) PSs anions transport in the pores of HB@COF separator.Reproduced with permission. [21]Copyright 2023, Wiley-VCH.) with interlayer after certain cycles.Reproduced with permission. [78]Copyright 2019, Wiley-VCH.c) UV-vis spectra of the Na 2 S 6 solution before and after adsorption using Teflon-lined carbon substrate interlayer.d) SEM image of the interlayer facing cathode (left), and separator (right) after discharge.Reproduced with permission. [79]Copyright 2019, Wiley-VCH.Structure and adsorption energy of Na 2 S 6 on e) graphene and f) pyrrolic-N, S co-doped graphene.Reproduced with permission. [80]Copyright 2022, Elsevier.g) XPS spectra of modified with electrolyte decomposition or NaPSs on the surface of the Na metal anode, indicating that the modified separator effectively inhibited the shuttling of NaPSs.In addition, the authors demonstrated the synergistic effect of electrostatic repulsion (─SO 3 H group in Nafion) and chemisorption (Al 2 O 3 ) on the inhibition of NaPSs shuttling by the designed diffusion experiments and selfdischarge tests.
The polar bonds between metal cations and sulfur anions in metal sulfides can provide enough active polar sites to anchor PSs.In addition, metal sulfides usually exhibit high electrical conductivity, which facilitates the electrochemical conversion of PSs. [82]Therefore, many metal sulfides have been applied in metal-sulfur batteries.Xu et al. uniformly coated 10 μm hollow carbon spheres/MoS 2 (HCS/MoS 2 ) on GF separator for Na-S cells (Figure 8h). [83]It was observed that the electrons were transferred from Na 2 S 6 to Mo and N atoms, and correspondingly the XPS spectra of Mo 3d and N 1s were moved towards lower binding energies, indicating a strong interaction between the separator and Na 2 S 6 (Figure 8g).In addition, Na ions were embedded into the MoS 2 layer during discharge to form a sodiated MoS 2 (Na x MoS 2 , x < 2), which could also adsorb the polar NaPSs.Therefore, the discharge capacity of the Na-S cells with modified separator increased from 633 mAh g −1 to 1090 mAh g −1 at 1 C, demonstrating the improved utilization of sulfur.In another report, Zhang et al. designed a FeS 2 @C/CNT hybrid film coating on GF separator, which effectively anchored the NaPSs by enhanced chemisorption to prevent them from entering the anode (Figure 8k). [32]Under the sulfur loadings of 7.6 mg cm −2 , the Na-S cell with the modified separator and FeS 2 @C composite sulfur cathode exhibited a low attenuation rate of 0.024% per cycle during 1000 cycles at 2C (Figure 8l).In contrast, the reference cell showed a rapid capacity attenuation during the cycles, reaching a high attenuation rate of 0.072% per cycle after 1000 cycles.
Lewis acid-base interaction refers to the formation of a coordination covalent bond between Lewis acid and Lewis base.The PSs/PSes/PIs act as a Lewis base in the electrolytes.Therefore, the addition of coating materials with Lewis acid properties facilitates the formation of Lewis acid-base interactions that can anchor PSs/PSes/PIs at the cathode side. [57,84]Chelates composed of metal ions and organic ligands showed strong adsorption of NaPSs through Lewis acid-base interaction.Jiang et al. reported a functional separator for Na-S cells by mixing Fe 3+ /polyacrylamide nanospheres with graphene (FPNs-G) and then coating the mixture on a GF separator (Figure 8i). [85]The FPNs chelate is a Lewis base with strong adsorption for NaPSs.The Fourier transform infrared spectroscopy (FTIR) spectra of FPNs after adsorption of Na 2 S 6 showed that Na-O/Na-N and Fe-S bonds were formed between FPNs and soluble NaPSs, exhibiting enhanced chemical adsorption of NaPSs (Figure 8j).The Na-S cell assembled with a modified separator exhibited a stable specific capacity of 435 mAh g −1 after 400 cycles at 0.5 C, versus 288 mAh g −1 with the pristine separator.
Wang et al. used cetrimonium bromide (CTAB)/CNT/Ti 3 C 2 T x modified PP (CCNT/MXene/PP) separators to achieve a highperformance Na-Se cell (Figure 9a). [86]The ultrathin coating layer was about 270 nm with a mass loading of 0.09 mg cm −2 .Owing to the strong Lewis acid-base interactions between Ti sites and PSes (Ti-Se bonds), as well as the electrostatic attraction between the positively charged CTA + and negatively charged PSes anions, the Na-S cells with modified separators showed strong PSes chemisorption capability (Figure 9b-d), achieving an excellent cycling performance (554 mAh g −1 after 100 cycles at 0.1 C).The Na-Se cell with pristine PP separator exhibited rapid capacity degradation due to severe PSes shuttling (137 mAh g −1 after 100 cycles at 0.1 C).
Se x S y is a cathode material with different ratios of Se-S, combining both the high conductivity of Se and the high specific capacity of S, exhibiting well electrochemical properties. [87]t is likewise challenged by the shuttle of soluble long-chain PSs/PSes (Na 2 S n , Na 2 Se n , 4 ≤ n ≤ 8).Xu et al. modified GF separator for Na-SeS 2 cells with Ti 3 C 2 T x sheets to effectively avoid the "shuttle effect" of PSs/PSes via Lewis acidbase interactions (Figure 9e). [88]Cells with modified separators exhibited excellent rate performance (479 mAh g −1 at 1 C) compared with the cells with pristine separators, where capacity dropped to almost 0 mAh g −1 at 1.0 C as shown in (Figure 9f) (1 C = 1345 mA g -1 ).
Although there are no reports on the separator modification for Na-I 2 batteries by chemisorption strategies, this strategy has been successfully applied in the cathode modification.For example, Feng et al. used fully conjugated polyphthalocyanine copper MOFs in the cathode of Na-I 2 cells with strong polarization of PIs, effectively inhibiting the dissolution of PIs.The Na-I 2 cell exhibited excellent cycling stability of up to 3200 cycles at 1.5 A g -1 . [46]In addition, the immobilization of PIs by chemical interaction has been widely used to modify the separator or act as an interlayer on the cathode side for metal-iodine batteries.For example, functional groups (such as ─F, ─O and ─OH) on the surface of MXene play a crucial role in iodine fixation.4d,89] The above discussion could provide some ideas for the future modification of the separator in Na-I 2 batteries.
The proper adsorption strength between the modified materials of separators and dissolved PSs/PSes/PIs is essential.If the adsorption interaction is too weak, the PSs/PSes/PIs still diffuse through the separator to the Na metal anode side.However, extremely strong adsorption will make it difficult for PSs/PSes/PIs to separate from the separators, and thus accumulate on the surface of separators, which not only increases the internal resistance of the batteries but also leads to permanent retention of PSs/PSes/PIs, resulting in irreversible loss of active materials. [90]Therefore, the chemisorption sites on the modified separator should have appropriate binding energies for PSs/PSes/PIs absorption to inhibit "shuttle effect." separator with and without Na 2 S 6 .h) Schematic diagram of NaPSs adsorption by modified separator.Reproduced with permission. [83]Copyright 2019, Wiley-VCH.i) Schematic diagram of synthesis of modified separator and composite cathode.j) FTIR spectra before and after adsorbing NaPSs by FPNs.Reproduced with permission. [85]Copyright 2020, Wiley-VCH.k) Schematic diagram of RT-Na-S cells using a modified separator and optimized cathode (dual-defending).l) Long-term cycling performance of RT Na-S cells with a modified/bare GF separator at 2 C (mass loads of 7.6 mg cm −2 ).Reproduced with permission. [32]Copyright, 2021 Elsevier.The UVvis spectrum of electrolyte after discharging.Reproduced with permission. [86]Copyright 2020, Wiley-VCH.e) Cycling performances with modified/bare separator.f) The rate performance of RT Na-Se cells.Reproduced with permission. [88]Copyright 2023, Youke Publishing Co.,Ltd, publisher Springer Nature.
Meanwhile, the diffusion energy barriers of Na ions should also be minimized to ensure efficient ion transport.

Facilitating Interfacial Charge Transfer by Introducing Conductive Layer
It has been calculated that sulfur loading over 5.0 mg cm -2 is a key parameter to achieve high-energy-density Na-S cells. [91]owever, batteries with high mass loading of active materials require thick electrodes, which severely hinder mass transfer and cause low utilization of active materials. [92]In addition, the large volume expansion/contraction of S/Se electrodes (S to Na 2 S is 157 vol%; Se to Na 2 Se is 336 vol%) during charge and discharge processes significantly affects the rate performance of Na-S/Se batteries.During cycling, the repeated large volume changes always cause the collapse of the S/Se cathodes and the exfoliation of active materials from the conductive substrates. [93]Furthermore, due to the loss of electrical contact, these isolated S/Se species forms "dead S/Se" that cannot be further utilized in the subsequent cycles, resulting permanent capacity attenuation. [94]ore seriously, the large volume expansion causes local extrusion, deformation or fracture at the separator/electrode interface, exacerbating the formation of "dead S/Se." [95] Meanwhile, the traditional separators are insulated and cannot reactivate the "dead S/Se".To solve the above-mentioned issues, introducing a highly conductive coating layer or interlayer on the cathode side of the separator not only reduces the charge transfer resistance of the cathode but also helps to reactivate the absorbed PSs/PSes/PIs species.Due to the presence of the conductive coating layer or interlayer, electrons can be transferred smoothly from the cathode to the coating layer or interlayer. [96]The coating layer works as a secondary current collector to facilitate the redox reactions of the absorbed S/Se/I species, thus improving the utilization of active cathode materials during cycling.Furthermore, the interlayer can effectively mitigate the corrosion of the anode caused by soluble polysulfide intermediates, because the polysulfides are preferred to deposit on the interlayer during cycling, thus preventing the polysulfides from passing through the separator to the anode. [97]n the other hand, it has been reported that the infinite volume change and dendrite formation during the Na stripping/plating on the anode surface can be solved using interlayers on the anode side.The interlayer should have sufficient ionic conductivity and excellent mechanical strength. [98]However, the electronic conductive coating layer or interlayer on the anode side of the separator may accelerate the "shuttle effect" because it can act as an electronic extension of the anode to facilitate side reactions. [99]herefore, introducing the conductive coating layer or interlayer on the separator will likely be limited on the cathode side.
In 2014, Yu and Manthiram pioneered the work of modifying the separator by placing carbon nanofibers (CNFs), carbon nanotubes (CNTs), and commercially available carbon foams (CCFs) films on the cathode side of the separators as the conductive interlayers (Figure 10a). [6]Among them, CCF layer performed the best in Na-S cells with an initial discharge capacity exceeding 1000 mAh g -1 .In contrast, the Na-S cell with the pristine separator only delivered a discharge capacity of 400 mAh g -1 .In addition, the authors also indicated that the sharp capacity drop in the first few cycles is closely associated with the formation of NaPS, which is difficult to be oxidized during the later charging process.To resolve this problem, this group further adjusted the discharge cutoff voltage from 1.2 to 1.8 V, which significantly improved the cycling stability of the Na-S cell with the same configuration, particularly in the first few cycles. [100]oating a thin layer of conductive material on the separator directly has been demonstrated to be a more effective strategy for separator modification than placing a freestanding conductive interlayer on the separator.It significantly shortens the transport pathway for Na ions diffusion through the thin coating layer.Manthiram et al. used polysulfide catholyte as the starting cathode material and non-porous sodiated Nafion membrane as the separator. [101]The ─SO 3 -located on the surface of the Nafion membrane provided a negatively charged environment that significantly obstacles the migration of NaPSs (Figure 10b).However, due to the poor conductivity of short-chain NaPSs and/or Na 2 S, the cell showed a continuous capacity degradation after ten cycles at 0.2 C.They then introduced a nanostructured carbonbased interlayer on Nafion membrane facing the cathode side.However, the Na-S battery still exhibited relatively low discharge capacities and poor cycling stability.The authors suggested that this phenomenon may be due to the poor diffusion coefficient of Na ions in the untreated CNF layer.24b] Although the Na-S battery with activated CNF (AC-CNF) coating still requires several cycles (<5 cycles) to achieve its best cycling condition, the cell with the AC-CNF coated separator delivered a much improved specific capacity of 550 mAh g -1 after 100 cycles at 0.2 C, compared with the Na-S battery with a commercial PP separator (200 mAh g -1 after 55 cycles).
Due to the aggregation of selenium on the surface of CNF, the CNF/Se cathode showed poor electrochemical performance.Therefore, Manthiram et al. coated carbon black on the surface of GF separator facing the CNF/Se cathode side, aiming to reactivate the aggregated selenium species and mitigate the migration of PSes towards the anode. [65]As a result, Na-Se cells with modified separator showed improved capacity retention (a highly reversible capacity of 512 mAh g -1 after 120 cycles at 0.1C), effectively suppressed self-discharge behavior, and stable long-term cycling performances (Figure 10d-g).
Braun et al. introduced a CNT interlayer between the CNT/NaI cathode and the separator, significantly inhibiting the shuttling of PIs. [102]The authors showed that the NaI electrode had the same electrochemical reaction as the I 2 electrode, i.e., I -↔I 3 -↔I 2 , and this was confirmed by the ex situ X-ray photoelectron spectroscopy (XPS) and Raman results (Figure 10h,i).The ex situ Raman test result of the electrolyte after cycling showed that the cell containing the CNT interlayer had a much lower intensity of the I 3 -peak, suggesting that the CNT interlayer effectively trapped the dissolved PIs.The initial discharge capacity of the Na-I 2 battery with a CNT interlayer is 170 mAh g -1 at 0.56 C (1 C = 178.8mA g −1 ), and the capacity is still maintained at 156 mAh g -1 after 100 cycles.
In summary, introducing conductive layers to modify the separators shows several advantages.First, the porous structure and tortuous interspace of the conductive nanomaterials can physically confine PSs/PSes/PIs from migrating to the anode.Second, during cycling, the conductive layer acts as a secondary current collector to promote the electron transfer and the conversion of absorbed PSs/PSes/PIs.Third, the modified layer can also facilitate the release of severe stress from volume expansion.However, it must be pointed out that the conductive layers, especially carbon-based modifying layers, are limited in improving the utilization of active materials.The carbon-based materials without polar functional groups show a weak affinity to polar PSs/PSes/PIs (i.e., weak van der Waals interaction), and the adsorbed active materials are easy to escape during charged and discharged, which cannot effectively inhibit "shuttle effect."In general, introducing heteroatom dopants (e.g., N, O, S, P and B) or surface groups (e.g., ─NH 2 , ─OH and ─COOH) on the surface of carbon materials can adjust the physicochemical properties of carbon materials, including strengthening the affinity between PSs/PSes/PIs and the carbon surface, enhancing charge transfer and regulating the electronic structure of carbon. [103]In fact, the conductive layers were often combined with other materials to enhance their capability to adsorb PSs/PSes/PIs and facilitate conversion reactions.

Accelerating Redox Kinetics of PSs/PSes/PIs by Electrochemical Catalysis on Separators
Developing a promising strategy to effectively suppress the "shuttle effect" of PSs/PSes/PIs and improve the redox kinetics of the accumulated PSs/PSes/PIs near the surface of the separators are essential to improve the utilization of S/Se/I 2 in the Na-S/Se/I 2 cells during long-term cycling.Therefore, using catalytic materials to modify the separators has been used to improve the redox kinetics of PSs/PSes/PIs recently. [104]Specifically, the catalytic materials can promote the PSs/PSes redox kinetics in the liquid phase (S 8 2-/S 4 2-, Se 8 2-/Se 4 2-) and the transformation between soluble PSs/PSes and solid-phase (Na 2 S 2 /Na 2 S, Na 2 Se 2 /Na 2 Se).Meanwhile, the activation energies of the catalytic materials in converting insoluble Na 2 S 2 /Na 2 S/Na 2 Se 2 /Na 2 Se into soluble PSs/PSes in the charging process should be high enough to ensure fast reaction kinetics and high reversibility.As for Na-I 2 cells, the complete conversion of iodine to final I -also promotes the utilization of active cathode material.The catalytic materials with high activation energy include conductive metal carbides, nitrides, phosphates, sulfides, selenides, and so on, which can effectively improve the redox reaction kinetics and the conversion rates. [18,53,105]In general, the catalytic process occurs after physical/chemical adsorption of PSs/PSes/PIs on the active sites of the modified separators.In addition, combining catalytic materials with other conductive materials can be beneficial to further improve the reaction kinetics since the fast transfer of ions and electrons are both essential to achieve rapid conversion of PSs/PSes/PIs. [106]unctional separators with high weight density (>0.4 mg cm -2 ) will reduce the overall energy density of the battery.To avoid sacrificing the overall energy density of the Na-S battery, it is preferable to modify the separator with lightweight materials. [107]To this end, Peng et al. coated a lightweight 3D porous N, S codoped cellulose nanofiber-derived carbon aerogel (NSCA) onto a GF separator (area load 0.2 mg cm -2 , about 3 μm thick) and used it as a multifunctional separator. [108]In the symmetric cells with Na 2 S 6 as the active material, the NSCA electrodes provided the maximum current response in the CV tests, indicating the fastest reaction kinetics for the conversion of soluble polysulfides, which was in good agreement with the EIS results (Figure 11a,b).At the same time, the modified membrane promoted the formation of the Na-N bond and weakened the Na-S bond, thus reducing the decomposition barrier and catalyzing the conversion of NaPS (Figure 11c).As a result, the Na-S cell assembled with a NSCA modified separator achieved a very high reversible capacity of 788.8 mAh g −1 at 0.1 C after 100 cycles and superior cycling stability with only 0.059% capacity decay per cycle over 1000 cycles at 1 C.
Metal oxide materials with polar surfaces have shown stronger binding energies to anchor PSs than carbon materials (e.g., vanadium oxides, titanium oxides, and manganese oxides).103b,109] Therefore, combining carbon materials with metal oxides can achieve good electrical conductivity, strong absorptions and catalytic effects toward PSs.Ahn et al. prepared V 2 O 3 @CNF film and used it as an interlayer on the cathode side of the separator. [110]The V 2 O 3 nanoparticles on the interlayer acted as active centers to effectively anchor soluble high-order NaPSs and catalyze their conversion into insoluble lower-order NaPSs by forming S 2 O 3 2-groups (Figure 11d,e).Therefore, the Na-S cell with V 2 O 3 @CNF interlayer displayed a low decay rate of 0.076% per cycle over 1000 cycles at 2 C.
Metal selenides exhibit strong polarity, high conductivity and significant catalytic activity for sulfur conversion. [111]Jiang et al. designed a separator by coating 2H-MoSe 2 /nitrogen-doped hollow carbon spheres/graphene oxide (2H-MoSe 2 /H-HCS/GO) on GF membrane. [112]The 2H-MoSe 2 effectively activated Na 2 S, as could be seen from the charge density difference diagram of the Na 2 S/MoSe 2 system.There is a significant charge transfer between the S atom and Mo atom, and part of Na charge was transferred to the adjacent Se atoms in the MoSe 2 (Figure 11f,g).More importantly, according to the DFT results, Na 2 S is easily converted to NaPSs on the modified separator due to the low diffusion energy barrier (0.21 eV) on the MoSe 2 surface, thus accelerating the redox kinetics (Figure 11h,i).In addition, the relatively negative adsorption energy of short-chain Na 2 S n (1 < n < 2) in MoSe 2 indicated the rapid transformation from long-chain NaPSs to short-chain NaPSs.The discharge capacity of the Na-S cell with the modified separator reached 787 mAh g −1 after 100 cycles at 0.1 C, which was much higher than that of the pristine separator (103 mAh g −1 ).
For metal anodes, dendrite growth and unstable SEI growth are significant challenges. [113]Traditional separators aggravate these issues due to poor electrolyte wettability and uneven pore size distribution resulting in inhomogeneous ionic flux. [114]mong them, polypropylene separators show limitations in Na-S batteries due to their poor wettability toward cyclic carbonate electrolytes. [115]In order to solve these dilemmas, Wang et al. designed a Janus separator, where a single ion conducting polymer (PMTFSINa) was grafted onto a PP separator to improve wettability in circulating carbonate electrolytes. [116]In addition, the cathode side of the graft separator was coated with a defectrich and nitrogenous MXene (DN-MXene), as an electrocatalyst to promote the transformation kinetics of NaPSs (Figure 12a).The CV curves of the Na 2 S 6 symmetrical cell with a Janus separator showed increased current response and more pronounced peaks, indicating enhanced redox kinetics (Figure 12b).The free energy calculation of the transformation reaction from S to Na 2 S was shown as an inset in Figure 12c, in which the free energy values from Na 2 S 2 to Na 2 S on graphitized carbon and DN-MXene were 0.32 eV and 0.25 eV, respectively.It was proved that the thermodynamics conversion of NaPSs on DN-MXene was more favorable than that on graphitized carbon.In addition, the introduction of DN-MXene greatly enhanced the interaction between the separator and NaPSs, and Na 2 S 6 molecules tended Figure 10.a) Schematic diagram of a Na-S battery with a conductive interlayer.Reproduced with permission. [6]Copyright 2014, American Chemical Society.b) Schematic diagram of the electrostatic repulsion effect between the negatively charged Nafion film and NaPSs.c) Schematic diagram of a Na-S cell using sodiated Nafion (Na-Nafion) membrane with CNF coating.24b] Copyright 2016, American Chemical Society.d) Capacity retention of the battery at different resting times.e) The cycling curves of the cells with a modified/pure separator.f,g) Cell impedance tests with a f) modified separator and g) pure GF separator.Reproduced with permission. [65]Copyright 2017, Wiley-VCH.h) The XPS of I 3d before and after cycling from CNT/NaI electrodes with interlayer.i) Ex situ Raman of separators with and without interlayer during cycling.Reproduced with permission. [102]Copyright 2018, Wiley-VCH.c) Differences in the optimal adsorption configuration and charge density of Na 2 S 6 on different substrates.Gray, blue, yellow, and pink balls represent C, N, S, and Na atoms, respectively.Yellow and blue areas represent charge repulsion and charge accumulation on the iso-surfaces, respectively.Reproduced with permission. [108]Copyright 2022, Wiley-VCH.d, e) The XPS spectrum of d) V 2p, e) S 2p for V 2 O 3 @CNF interlayer before and after 1000 cycles at 2 C. Reproduced with permission. [110]Copyright 2021, Elsevier.f) Optimized geometric configuration and adsorption energy of NaPSs on the MoSe 2 surface.g) Charge density difference of Na 2 S on MoSe 2 surface.Red (blue) corresponds to charge accumulation (depletion).h) Adsorption energy of NaPSs on different substrates.i) Diffusion energy barrier of Na 2 S on the MoSe 2 surface.Reproduced with permission. [112]Copyright 2021, The Royal Society of Chemistry.The CV curves with/without Na 2 S 6 symmetric cells at a scan rate of 1 mV s -1 .c) Energy distribution of NaPSs reduction on graphitized carbon and DN-MXene.Reproduced with permission. [116]Copyright 2020, Wiley-VCH.d) Schematic diagram of the preparation of MXene@C/PP/MXene@C separator.e) Static contact angles on the bare (left)/modified (right) separator.Reproduced with permission. [34]Copyright 2022, Wiley-VCH.
to adhere to DN-MXene on the separator rather than dissolve in PC-based electrolytes by DFT calculations, which was consistent with the UV-vis test results.The strong capture and rapid conversion of NaPSs were expected to achieve excellent cycling performance and high sulfur utilization in Na-S cells.Similarly, Fang et al. modified both sides of PP separator for Na-S battery by vacuum filtration with nitrogen-doped porous carbon-coated MXene nanosheets (Figure 12d). [34]MXene materials, as a class of conductive polar materials, not only chemically anchored the NaPSs but also accelerated the redox reaction.To understand the catalysis effect of a modified separator on NaPSs conversion, the authors performed CV and XPS tests on symmetric Na 2 S 6 cells.The result showed that the modified separator had strong interaction with NaPSs than that of the pristine GF separator and PP separator, and the sulfate species was significantly increased, illustrating the modified separator effectively improved the NaPSs conversion.Benefiting from the high hydrophilic of the MXene, the static contact angles of the electrolyte on a PP separator and a modified separator were 56.2°and 21.8°, respectively (1 m NaClO 4 in EC/DEC, 1:1 by volume, with 5 wt% FEC).More impressively, the modified separator improved electrolyte wettability with the static contact angles being reduced from 93.7°to 13.4°(1 m NaClO 4 in EC/PC, 1:1 by volume) (Figure 12e).The superior wettability was beneficial to the cycling stability of Na metal anode as well.Thus, the Na-S cell with modified separator exhibited an initial discharge capacity of 968 mAh g -1 at 0.5C, and the capacity was maintained at 928 mAh g -1 after 650 cycles.
19b,46,117] In addition, Zhao et al. used single-walled carbon nanotubes decorated with Fe nanoparticles as a modified layer on the cathode side of the separator for Zn-I 2 battery, effectively preventing the diffusion of PIs to the anode by anchoring and catalyzing the redox kinetics of iodine species. [118]Lan et al. used the single-atom Zn and MOF as Janus separators to catalyze the rapid conversion of PSes in Li-Se cells. [119]Therefore, we believe that the introduction of catalytic materials on the separator for Na-Se/I 2 cell is an effective strategy to improve the utilization of Se/I species and inhibit the shuttling of PSes/PIs.
A variety of metal compounds have strong chemical polarity intrinsically, which makes them favorable to adsorb PSs/PSes/PIs intermediates, effectively solving the "shuttle effect." [120]Some of the metal compounds are also able to electrochemically catalyze the redox reaction of PSs/PSes/PIs.The catalyst has always been regarded as the most effective method to solve the problem of slow PSs/PSes/PIs reaction kinetics, thus effectively suppressing the "shuttle effect."In general, adsorption and catalysis are a progressive relationship, with catalysis occurring after adsorption.Therefore, exploring materials with dual adsorptioncatalysis effects as modification layers of separator can lower the chemical reaction energy barrier, reduce the accumulation of long-chain PSs/PSes/PIs and accelerate their conversion, which is conducive to the construction of high-performance Na-S/Se/I 2 batteries. [40]Secondly, the abundant and uniform active sites on the separator can maximize the adsorption and catalytic effects, thus maximizing the utilization of active materials.Finally, the conduction of electrons and ions directly affects the cycling per-formance of the cell, especially in Na-S/Se/I 2 cells with high mass loading.Therefore, the functionalized separator with both good electrical conductivity and adsorption-catalysis capabilities can effectively improve the utilization of active materials.

Conclusions and Perspectives
After decades of research and development, Na-S/Se/I 2 batteries have become the most promising candidates for the nextgeneration low-cost and high-energy-density rechargeable batteries.However, based on their complex multi-step conversion reaction mechanisms, the "shuttle effect" and the slow redox kinetics of PSs/PSes/PIs intermediates pose serious challenges to the cycling life required for commercial applications.In this regard, modification of separators to suppress the uncontrolled migration of PSs/PSes/PIs to Na metal anode side is considered to be an extremely effective strategy.
According to the interaction with PSs/PSes/PIs, the strategies of separator modifications can be classified into five categories, including repelling PSs/PSes/PIs migration by electrostatic repulsive interaction, introducing conductive layer, regulating PSs/PSes/PIs migration via sieving effect, immobilizing by PSs/PSes/PIs chemisorption and accelearting redox kinetcs of PS/PSe/PI conversion by electrocatalysts.Detailed information of Na-S/Se/I 2 batteries prepared with modified separators have been summarized in Table 1.Currently, the sulfur loading and cycling performance of Na-S batteries are contradictory. [121]ost studies were evaluated under ideal conditions with low sulfur loading cathodes, flooded electrolytes and thick Na metal anodes.With the increase of the active material loading in the cathode and the decrease of the electrolyte/active material ratios, Na-S/Se/I 2 batteries show significantly reduced specific capacity and shortened cycle life. [122]However, achieving stable cathodes with high areal specific capacities and low electrolyte/sulfur ratios are crucial for Na-S batteries.Although some improvements have been achieved in the past few years, it is still in an early stage for the development of high-performance Na-Se/I 2 cells.Several key points in the design of high-efficient separators for Na-S/Se/I 2 batteries should be considered: (a) The electrostatic interaction between the separator and the ions is a strategy to achieve high-efficiency ion selectivity.Compared to neutral separators, the modified separators with negatively charged groups only allow Na ions to pass through, showing good PS/PSe/PI repulsion effect.In order to achieve strong charge repulsion to PSs/PSes/PIs, the separators need to be equipped with appropriate charge density and favorable chemical/electrochemical stability.
(b) The highly soluble PSs/PSes/PIs in electrolytes and their low electron conductivity result in low conversion reaction kinetics.Therefore, the modified layer with good electronic conductivity on the cathode side can provide better electrical connectivity and additional reaction sites for active species, promoting the redox reaction of the captured PSs/PSes/PIs.Moreover, appropriate pore space and interconnected porous structure in the conductive coating layer or interlayer can optimally accommodate and confine PSs/PSes/PIs on the cathode side.(c) The sieving effect of nanopores or subnanopores separator on PSs/PSes/PIs species has been widely demonstrated in different metal-sulfur batteries.Regulating the pore size distribution of the separator to limit PSs/PSes/PIs diffusion is essential to improve battery performance.Hampered by the sieving effect, the PSs/PSes/PIs anions remain dissolved in the electrolyte on the cathode side and are more likely to be captured and utilized efficiently by the cathode host or modified separator during cycle.In addition, the pores within functional separator should improve the uniformity of Na ions fluxes, which could promote homogeneous nucleation/growth of the Na metal and reduce the tendency to form dendrites. [123] (d) In general, chemical anchoring mechanism is always necessary to effectively control the "shuttle effect" of PSs/PSes/PIs.The modified layer with high surface area can provide more active sites for anchoring PSs/PSes/PIs.In addition, the balance between adsorption and desorption effects should be carefully controlled to avoid being too strong or too poor.
(e) The catalytic active sites on the separator surface can effectively reduce the reaction barrier and greatly improve the reaction kinetics of PS/PSe/PI conversion.The overall activity and selectivity of a given electrocatalyst are closely related to the number and intrinsic activity of surface active sites. [124]n general, reducing the size of catalyst particles and increasing their surface area to expose more active sites can greatly improve electrocatalytic activity.Furthermore, the ideal catalyst medium should also have an appropriate affinity for PSs/PSes/PIs and excellent conductivity, thus improving the redox kinetics of the active materials. [125] summary, separator modification using a single strategy is not sufficient for the practical application of Na-S/Se/I 2 batteries.Optimized designs using different strategies with synergistic effects for multifunctional separators will be needed in the future to suppress the uncontrollable PSs/PSes/PIs transport.The multifunctional separators should have good structural adjustability, appropriate porosity, well electrolytes wettability, high electrocatalytic performance and excellent ion-selective property for realizing high-performance of Na-S/Se/I 2 batteries. [126]Some practical guidelines should be noted: [127] (a) The thickness and mass loading of the functional layer on the separator should be controlled reasonably to reduce the internal resistance and maintain energy density.It is generally believed that thick modified layer is an additional barrier for ions transfer (including both Na ions and PSs/PSes/PIs anions) due to the reduced porosity and increased tortuosity of the separator, which will increase the internal resistance of Na-S/Se/I 2 batteries, while the thin modified layer cannot effectively prevent the migration of PSs/PSes/PIs.Therefore, the tradeoff between the thickness of the functional layer and the ability to trap and utilize PSs/PSes/PIs is crucial to increase the service life and maintain the high energy density of Na-S/Se/I 2 batteries.An ideal functional layer should be as thin as possible to maximize Na ions transport without affecting the ability to inhibit PSs/PSes/PIs shuttling.
(b) The future separators should provide additional functions.For example, embedding thermally stable and flameretardant materials into the separators shows great promise to guarantee the safe operation of batteries under high temperatures.The release of flame-retardant materials could extinguish the fire when batteries overheat and catch fire.
(c) The physical properties of the separator, such as porosity, pore structure, and surface properties, should be given more attention.They have significant effects on the mechanical strength of the separators.Any damage or rupture of the separator during the battery assembly process may result in a short circuit.In addition, a certain degree of mechanical stiffness can help to endure volume changes of the electrodes, thereby producing a stable interface and avoiding safety issues caused by dendrite penetration.
Therefore, how to achieve high capacity and long cycling Na-S/Se/I 2 batteries in practical applications remains a challenging goal.According to current research results, rational-designed multifunctional separators with low cost and high efficiency features pave a practical pathway of achieving high-performance Na-S/Se/I 2 batteries.

Figure 2 .
Figure 2. Multifunctional separators to inhibit uncontrollable anion transport in Na-S/Se/I 2 batteries.

Figure 3 .
Figure 3. a) Discharge voltage profile of a Na-S cell with ether-based electrolyte (1.5 m NaClO 4 and 0.3 m NaNO 3 in tetraethylene glycol dimethyl ether (TEGDME)).Reproduced with permission.[6]Copyright 2014, Wiley-VCH.b) The XRD patterns of a S electrode in a Na-S battery during initial cycling.Reproduced with permission.[32]Copyright 2021, Elsevier.c) Charge and discharge voltage profiles of Na-S cells with carbonate electrolyte (1 m NaClO 4 in propylene carbonate (PC)/ethylene carbonate (EC).Reproduced with permission.[19c]Copyright 2013, Wiley-VCH.d) In situ XRD pattern of Na-S battery during initial charging and discharging process.Reproduced with permission.[33]Copyright 2018, Springer Nature.e) The cycling profiles of Na-S cells with the carbonate electrolyte of 1 M NaClO 4 in EC/diethylene carbonate (DEC).Reproduced with permission.[36]Copyright 2016, Springer Nature.f) First-order derivative curves correspond to in situ UV-vis spectra for Na-S battery at different discharge states.Reproduced with permission.[37]Copyright 2020, Wiley-VCH.

Figure 4 .
Figure 4. a) The discharge-charge voltage profile of Na-Se cell with Se/C cathodes in the electrolyte of 1 M NaClO 4 in EC/DEC at 0.2 C. b) Ex situRaman spectra of Se/C composite electrodes after discharged.Reproduced with permission.[42]Copyright 2019, Elsevier.c) The discharge-charge

Figure 5 .
Figure 5. a) The charge-discharge voltage profiles at different current densities of the Na-I 2 battery.b) The CV curve of the Na-I 2 battery at 0.1 mV s −1 .c) In situ Raman spectra of Na-I 2 battery in 1 M NaClO 4 in EC/DEC.Reproduced with permission.[49]Copyright 2017, Springer Nature.

Figure 6 .
Figure 6.a) Repulsive energy of different S n 2− on anatase TiO 2 , GO, and Ti 0.87 O 2 .b) Charge density of Ti 0.87 O 2 containing Ti defects.c,d) Raman spectra analysis during discharge and charge, c) PP separator and d) Ti 0.87 O 2 /PP separator.Distribution of Li ions in e) conventional TiO 2 nanosheetsand f) nanosheets containing Ti defects.Reproduced with permission.[61]Copyright 2021, Springer Nature.

Figure 7 .
Figure7.a) Crystal structures of Na 2 S, Na 2 S 2 , Na 2 S 4 and S 8 .The purple and yellow balls are Na and S atoms, respectively.Reproduced with permission.[68]Copyright 2018, Elsevier.b) Structural diagram of the monomers Na 2 S 6 , Na 2 S 8 and the dimer Na 2 S 4 , with the yellow and red balls

Figure 8 .
Figure 8. EIS spectra of RT Na-S cells a) without and b) with interlayer after certain cycles.Reproduced with permission.[78]Copyright 2019, Wiley-VCH.c) UV-vis spectra of the Na 2 S 6 solution before and after adsorption using Teflon-lined carbon substrate interlayer.d) SEM image of the interlayer facing cathode (left), and separator (right) after discharge.Reproduced with permission.[79]Copyright 2019, Wiley-VCH.Structure and adsorption energy of Na 2 S 6 on e) graphene and f) pyrrolic-N, S co-doped graphene.Reproduced with permission.[80]Copyright 2022, Elsevier.g) XPS spectra of modified

Figure 9 .
Figure 9. a) Schematic preparation of modified separator.The XPS spectra of b) MXene/PP, c) CCNT/MXene/PP separator after the first cycle.d)The UVvis spectrum of electrolyte after discharging.Reproduced with permission.[86]Copyright 2020, Wiley-VCH.e) Cycling performances with modified/bare separator.f) The rate performance of RT Na-Se cells.Reproduced with permission.[88]Copyright 2023, Youke Publishing Co.,Ltd, publisher Springer Nature.

Figure 11 .
Figure 11.a) The CV curve and b) EIS diagram of Na 2 S 6 symmetrical battery.NSCA, NCA, or CA composites were dispersed on carbon paper (CP).c)Differences in the optimal adsorption configuration and charge density of Na 2 S 6 on different substrates.Gray, blue, yellow, and pink balls represent C, N, S, and Na atoms, respectively.Yellow and blue areas represent charge repulsion and charge accumulation on the iso-surfaces, respectively.Reproduced with permission.[108]Copyright 2022, Wiley-VCH.d, e) The XPS spectrum of d) V 2p, e) S 2p for V 2 O 3 @CNF interlayer before and after 1000 cycles at 2 C. Reproduced with permission.[110]Copyright 2021, Elsevier.f) Optimized geometric configuration and adsorption energy of NaPSs on the MoSe 2 surface.g) Charge density difference of Na 2 S on MoSe 2 surface.Red (blue) corresponds to charge accumulation (depletion).h) Adsorption energy of NaPSs on different substrates.i) Diffusion energy barrier of Na 2 S on the MoSe 2 surface.Reproduced with permission.[112]Copyright 2021, The Royal Society of Chemistry.

Figure 12 .
Figure 12. a) Synthesis of Janus separator for RT Na-S cells.b)The CV curves with/without Na 2 S 6 symmetric cells at a scan rate of 1 mV s -1 .c) Energy distribution of NaPSs reduction on graphitized carbon and DN-MXene.Reproduced with permission.[116]Copyright 2020, Wiley-VCH.d) Schematic diagram of the preparation of MXene@C/PP/MXene@C separator.e) Static contact angles on the bare (left)/modified (right) separator.Reproduced with permission.[34]Copyright 2022, Wiley-VCH.

Table 1 .
Summary of multifunctional separators used in RT Na-S/Se/I 2 batteries and their performance characteristics.