CEI Optimization: Enable the High Capacity and Reversible Sodium‐Ion Batteries for Future Massive Energy Storage

Sodium‐ion batteries (SIBs) have attracted attention due to their potential applications for future energy storage devices. Despite significant attempts to improve the core electrode materials, only some work has been conducted on the chemistry of the interface between the electrolytes and essential electrode materials. Therefore, the different cathode–electrolyte interfaces (CEIs) that form between various cathode materials and liquid electrolytes for SIBs are briefly reviewed, and the requirements of CEI performance in low‐temperature SIBs. Modifying the cathode materials and electrolyte formulas offers an efficient strategy for CEI enhancement. The summary and analysis, given in this article, may serve as a reference for the development of better sodium‐ion batteries.


Introduction
With increasing raw material consumption, sodium-ion batteries (SIBs) have gained increasing global enthusiasm due to the limited global supply of lithium compounds and the significant cost of transition metal elements such as cobalt and nickel for Lithium-ion batteries (LIBs).Designing air-stable SIBs follows the so-called "Rocking Chair" mechanism, well known for LIBs, with a detailed charging process in which sodium cation extracts cathode material, diffuses into the electrolyte, migrates to the anode, and finally intercalates into the anode materials.A reversible process also occurs upon discharge (Figure 1a). [1]ignificant efforts have been made to promote critical electrochemical parameters for core SIB materials, including cathode materials, anode materials, and electrolytes.Various oxide compounds, polyanion compounds, Prussian blue analogs, and organic cathode materials have also been investigated for cathodes, with preferable anode materials consisting of hard carbon, intercalation compounds, and organic materials. [2]These electrode materials can provide sites for sodium storage, especially in their bulk phase.However, the expected energy density and cycling stability of a battery require robust interfaces, a solid-electrolyte interphase (SEI) between the anode and the electrolyte, and a cathode-electrolyte interphase (CEI) between the cathode material and the electrolyte (Figure 1b).Hence, the observed electrochemical outputs for rechargeable batteries not only depend on the reversibility of electrode reactions for active electrode materials, for which the voltage profile and maximum reversible capacity vary depending on the type of electrode material, but are also determined by the interfacial properties between the electrolyte and electrode.Notably, actual electrodes for secondary batteries often contain other compositions, including conducting agents (i.e., carbon black) and binding agents (i.e., PVDF or other polymers), contributing to some minor performance.
In most cases, the formation of CEI and SEI expands the electrochemical stability window of the electrolyte, favoring cell operation (Figure 1c).The anode acts as the reductant, while the cathode acts as the oxidant, and the energy separation E g of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the electrolyte dominate the intrinsic window of the electrolyte. [3]The formation of SEI compounds on the anode surface typically results in higher LUMO energy, with the CEI compounds also obtaining a lower HOMO energy, thus leading to more significant energy separation after the formation cycle.This favors long-term stability for the operation of the cathode and anode. [3]esearch on CEI for sodium-ion batteries is limited, based on existing published literature.In the 1960s, researchers started to recognize that a solid film formed on the surface of lithium metal, ensuring that LIBs could operate stably, but it was not until 1979 that Peled defined this solid film as SEI. [4]The first SEI structure appeared in 1997, in the form of a model proposed by Peled et al.This model was named according to its inorganic and organic components, which were distributed in a similar manner to a mosaic structure. [5]Around 2015, Hu et al. [6,7] and Nazar et al. [8,9] attempted to improve the structural reversibility of cathode materials by considering CEI in subsequent research studies.In 2017, Meng et al. initiated a study on SIBs using atomic layer deposition (ALD), and the XPS test results indicated that CEI on the electrodes contained fewer carbonate species and more inorganic species, which allowed for fast Na kinetics.This led to a significant increase in Coulombic efficiency and a decrease in cathode impedance. [10]The structure of CEI was shown to be similar to that of SEI, however, the exact structure remains unknown.Notably, however, CEIs in SIBs and LIBs were found to be fundamentally similar. [11]Researchers have also recently continued various attempts to develop better CEI through main electrolyte regulation using key salts, solvents, and additives, as well as through direct surface medication or indirect synthesis optimization for cathode materials.14][15][16][17][18][19][20][21][22][23][24] 2. Cathode Materials and Electrolytes for Sodium-Ion Batteries

Cathode Material for Sodium-Ion Batteries
The cathode materials of sodium-ion batteries mainly include transition metal oxide materials with typical layered and tunnel structures, polyanionic compound materials, Prussian blue compounds, and organic cathode materials. [2]By contrast, the optional cathode materials for lithium-ion batteries are generally considered only transition metal oxides, polyanions, and organic cathode materials.In certain solid oxide cathode materials, the energy storage activity may be significantly better than their lithium counterparts.For example, LiCrO 2 typically exhibits poor electrochemical lithium storage activity compared to NaCrO 2 , which can obtain a higher reversible sodium storage capacity. [25,26]This offers new possibilities for the research and development of related materials for sodium-ion batteries.
Transition metal oxides (TMO 2 ) mainly include TMO materials with layered and tunnel structures, with the classification of layered TMO 2 materials mainly following the structural classification method proposed by Delmas et al. [27] This method is based on the difference in stacking order of O, and generally divides such oxides into O3, P2, and P3 phases (O3: ABCABC stacking; P2: ABBA stacking; P3: ABBCCA stacking) (Figure 2a). [28]Sodium cations can also diffuse along the inter-layer spacing via the triangular (P phase) or octahedron (O phase) sites.Of note, the relatively poor cyclical performance of oxide cathode materials has been observed based on a single transition metal in the early literature, with severe fading observed within several weeks of cycling.[31] In addition, electrolyte purity remained low in the early years before 2010, with no Figure 1.a) Diagram of a newly-assembled full-cell for SIBs.Reproduced with permission. [1]Copyright 2011, Wiley-VCH.b) Diagram of CEI and SEI formation in SIBs.Adapted with permission. [1]Copyright 2011, Wiley-VCH.c) Schematic diagram of the formation and impact of CEI and SEI in SIBs.[14][15][16][17][18][19][20][21][22][23][24] optimized electrolyte formula, resulting in the inability to obtain stable CEI and SEI for experimental batteries. [32]ntroducing other metal elements such as Mg, Mn, and Fe in layered transition metal oxides can regulate the structure, thus, inhibiting phase transition and obtaining better electrochemical reversibility.By doping Mn elements into the α-NaFeO 2 frame structure, a P2-Na x Fe 0.5 Mn 0.5 O 2 (0.13 ≤ x ≤ 0.86) cathode material can be obtained, which has received widespread attention. [33]esearch by Komaba et al. [1] also showed that NaNi 0.5 Mn 0.5 O 2 exhibited a reversible capacity of 105-125 mAh g À1 at a current density of 4.8 mAh g À1 and a voltage range of 2.2-3.8V (vs.Na þ / Na).Moreover, O3-NaNi 0.33 Mn 0.33 Co 0.33 O 2 prepared by Sathiya et al. exhibited a reversible capacity of 120 mAh g À1 in the voltage range of 2-3.75 V (vs.Na þ /Na), accompanied by a series of O3!O1!P3!P1 phase transitions in the electrochemical cycle. [34]Subsequent layered materials with distinct compositions, such as O3-NaNi 0.33 Fe 0.33 Mn 0.33 O 2 and Na 0.67 Ni 0.33 Mn 0.67 O 2 have also been developed with enhanced electrochemical reversibility. [35,36]otably, although general layered TMOs based on polytransition metals or polymetallic cations exhibit good electrochemical sodium storage properties, they are highly unstable in H 2 O and CO 2 environments.The absorption of H 2 O into the crystal structure will trigger a secondary chemical reaction on the cathode surface, accelerating the evolutionary instability of the CEIs.Therefore, subsequent work has focused on improving stability in air and water. [37]The Guo Yuguo Research Group of the Institute of Chemistry of the Chinese Academy of Sciences selected O3-NaNi 0.5 Mn 0.5 O 2 as the research object.The researchers synthesized this material using the sol-gel method, which had a high specific capacity of 141 mAh g À1 and high-capacity retention of 90% after 100 cycles.This long cycle life and good multiplicity could help accelerate research on cathode materials for SIBs. [38]In a subsequent study, the O3-NaNi 0.45 Cu 0.05 Mn 0.4 Ti 0.1 O 2 material was further prepared by introducing copper ions to reduce the distance between the Na layers and increase the valence state of the transition metals, significantly improving the air stability of the O3-type cathode material and simultaneously maintaining excellent cycling performance. [39]An effective synergistic strategy of multiple metal ions can also be applied to P2-type layered transition oxides.Ti 4þ can provide high redox potential, Mg 2þ can stabilize the structure, and Li þ can smooth the electrochemical curve.Guo et al. utilized the synergistic effect of these three metal ions to convert P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 into P2-Na 0.7 Li 0.03 Mg 0.03 Ni 0.27 Mn 0.6 Ti 0.07 O 2 with a moderate "Z"intergrowth structure (Figure 3a,b), and this material exhibited excellent electrochemical performance, with a capacity retention rate of 82% after 200 cycles (Figure 3c). [40]a 0.44 MnO 2 has an orthogonal crystal system tunnel structure, with a Pbam space group (Figure 2b).In 2005, Sauvage et al. conducted a systematic study on the electrochemical properties of Na 0.44 MnO 2 in nonaqueous electrolytes. [41]The results showed that the electrode process underwent six two-phase transformations.By contrast, and despite its excellent cycle performance, the reversible capacity was only 35 mAh g À1 in the full battery system, because Na 0.44 MnO 2 in the full-cell could not provide sufficient sodium.In addition, some work explored tunneltype sodium storage materials with higher sodium content, such as Na 0.61 Mn 0.27 Fe 0.34 Ti 0.39 O 2 . [7]Oxide cathode materials based on other transition metals have mainly consisted of V oxides. [42,43]In 2023, Hou et al. conducted a more in-depth study of Na 0.44 MnO 2 and demonstrated a competitive insertion reaction between H þ and Na þ , indicating that the diffusion energy barrier of Na þ increased with increasing Na content, however, the opposite was true for protons.By decreasing the diffusion energy barrier of Na þ and increasing the diffusion energy barrier of protons, a high reversible capacity of 101 mAh g À1 of Na 0.44 MnO 2 was achieved for the first time in an aqueous electrolyte. [44]However, Huang et al. improved Na 0.44 MnO 2 by coating the material with added Na 2 TiO 3 , which not only stored Na þ but could also avoid electrolyte etching. [45]olyanionic compounds have an open-frame structure, strong inductive effects, and strong X-O covalent bonds (X = P, S, Si, B), with the advantages of fast ion diffusion, high operating voltage, and a stable structure in electrode reactions.Various anions can be used, such as (PO 4 ) 3À , (SO 4 ) 2À , (SiO 4 ) 4À , and (P 2 O 7 ) 4À , and the mixed adoption of these anions may be feasible.NASICON (Na super-ionic conductor) has a molecular formula of A x MM' (XO 4 ) 3 , and this compound contains a threedimensional mesh structure with connected MO 6 and XO 4 polyhedra.The typical Na 3 V 2 (PO 4 ) 3 material has a very flat charge and discharge curve, which has received widespread attention, and the cycle stability reported in the literature reached a high level (Figure 2c). [46]Peridot NaFePO 4 has a theoretical capacity of up to 154 mAh g À1 as a cathode material for sodium-ion batteries, which has attracted widespread attention (Figure 2d).
Unlike the thermodynamically stable phase of LiFePO 4 , the critical material of lithium-ion batteries, the olivine phase has been shown to be electrochemically active.The thermodynamically stable NaFePO 4 phase exhibits a phosphoferrite-sodium ore structure.Theoretical and experimental investigations have demonstrated that this phase is electrochemically inert when used as the positive electrode of a sodium-ion battery.Kim et al. [47] found that the FePO 4 electrode became amorphous during the first cycle.Further charge and discharge tests showed that its specific capacity could reach 142 mAh g À1 . [48]Mai et al. prepared a series of NaFePO 4 composite materials with different amorphous phase contents, verifying the relationship between amorphous phase content and sodium storage capacity.The optimized NaFePO 4 composite material exhibited excellent cycle stability, with a capacity of about 115 mAh g À1 at a magnification of 1 C, and a capacity retention rate of 91.3% after 800 cycles. [49]otably, one of the failure modes of traditional cathode materials involves amorphous or pulverized failure during long-term circulation.Therefore, the characteristics of this sodium cathode material require amorphous failure to release reversible specific capacity may be unfavorable in terms of mechanical properties.
Another polyanionic cathode material is sodium iron sulfate, which has received less attention.Reynaud et al. noted that Na 2 Fe (SO 4 ) 2 materials can be reversibly embedded and prolapse 0.7 Na on a %3.3 V platform. [50]Zhao et al. also used carbon nanotubes combined with sodium iron sulfate to improve electrical conductivity and obtain excellent electrochemical stability. [51]Polyanionic materials can be derived by combining phosphate with other anions with rich components and different structures.Among these, a fluorinated phosphate material with a particular sodium storage structure and high sodium storage potential has attracted the attention of researchers, and Na 2 FePO 4 and NaVPO 4 F in this material have been extensively studied with excellent kinetic properties.Ellis et al. [52] first studied the orthogonal crystal system Na 2 FePO 4 F electrode material under high voltage (%3.5 V vs. Li þ /Li) electrochemical activity.Subsequently, Kawabe et al. [33] [40] Copyright 2022, Wiley-VCH.
researched Na 2 FePO 4 F/C composite materials.By contrast, Barker et al. [53] first reported the electrochemical properties of NaVPO 4 F with a square structure as the cathode material of a sodium-ion battery.The prepared material exhibited an 82 mAh g À1 discharge capacity and average sodium storage potential of 3.7 V. Other cathode materials with mixed anions include Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 [54] and NaFe 2 PO 4 (SO 4 ) 2 . [55]Cao et al. prepared a Na 3 Fe 2 (PO 4 )P 2 O 7 /rGO based on a simple preparation process, which demonstrated excellent electrochemical stability. [56]russian blue (PBAs) has a molecular formula of KM II Fe III (CN) 6 (M = Mn, Fe, Co, Ni, and Zn), and this compound belongs to a cubic crystal system with a spatial group of Fm3m (Figure 2e). [57,58]This structure contains a large number of alkali ion channels, which facilitate the rapid decolonization/embedding of Na þ without structural distortion.In 2012, Lu et al. [59] first reported a study on KM II Fe III (CN) 6 as the cathode material of a nonaqueous sodium-ion battery.The results showed that KFe 2 (CN) 6 had a reversible capacity of about 100 mAh g À1 , and the two sodium storage potentials were 3.5 and 2.6 V, which corresponded to the changes in the high-spin state Fe 3þ /Fe 2þ electrical pair bonded to N and the low-spin state Fe 3þ /Fe 2þ electrical pair bonded to C. The presence of a certain amount of moisture was found to be conducive to the stability of the structure, but reversibility in the battery must consider the elimination of moisture to avoid adverse effects on interfacial film formation.The work of Song et al. showed that for general Na 2 MnFe(CN) 6 •zH 2 O cathode materials, the removal of intermittent water could change the charge and discharge curves, and significantly improve the reversible specific capacity and cycle stability.At 20 C, the half-battery capacity was found to be 120 mAh g À1 , while at 0.7 C, the capacity retention rate remained at 75% after 500 cycles. [60]Rudola et al. developed a nonflammable full-cell based on Na 2 Fe 2 (CN) 6 •2H 2 O, and graphite anode and Na 2 Ti 3 O 7 /C anode materials.The half battery was mainly based on carbonate electrolyte, and the whole battery was based on ether electrolyte. [61]Wang et al. believed eliminating trace amounts of moisture in Prussian blue cathode materials was necessary for developing its practical value.Although Na 2Àx FeFe(CN) 6 samples can undergo absorption after water removal, they still exhibit good electrochemical properties and can be stably cycled for more than 2000 cycles at a controlled charge cut-off voltage.The experimental results showed that the sample remained in the new triangular phase after dehydration, and the redox reaction of low-spin Fe 2þ /Fe 3þ was activated.In addition, after the sample was removed from the water, the high-temperature storage performance improved. [62]eneral cathode materials are based on inorganic elements, but rely on natural resource extraction, which is not environmentally friendly.By contrast, organic cathode materials based on CHO and other elements exhibit outstanding advantages of good sourcing and environmental friendliness.The organic molecular structure also provides a wealth of regulatory space, and compared to the structure of small molecules, organic materials have a more prominent theoretical reversible specific capacity.Carbonyl small molecules are the most widely used organic cathode materials in organic SIBs, and can be generally classified into four types, namely, ketones and quinones, anhydride compounds, imide compounds, and carboxylate compounds, which serve as typical n-type electrodes in organic SIBs. [63]Chihara, [64] Luo, [65] and Deng [66] reported on the electrochemical properties of disodium rose palmitate (Na 2 C 6 O 6 ), perylene tetra formate dianhydride (PTCDA), and perylene imide (PTCDI), respectively.The results showed high reversible capacity and excellent cycle performance, however, the operating voltage was low.Organic sodiumion batteries have also been constructed.For example, Wang et al. [67] built a full battery based on 2,5-dihydroxyterephthalic acid (Na 4 DHTPA; Na 4 C 8 H 2 O 6 ), which demonstrated an average operation voltage of 1.8 V and practical energy density of about 65 Wh kg À1 .Carbonyl polymers, [68][69][70] conjugated conductive polymers, [71,72] covalent organic frameworks, [73][74][75][76] organometallic compounds, [77,78] and organic radical polymers [79][80][81] can also serve as organic cathode materials for sodium-ion batteries, in the form of polymer cathode materials.Jiang et al. synthesized anthraquinone-based conjugated polymer cathodes, which served as a cathode material consisting of anthraquinone and benzene in different linking modes.The 1,2,4,5 linkage pattern on the benzene ring gave this material a high specific capacity and stable cycling ability, with a capacity retention of 95.8% after 1000 cycles at 0.05 A g À1 and 83.1% after 40 000 cycles at 3 A g À1 . [82]

Liquid Electrolytes for Sodium-Ion Batteries
A suitable electrolyte requires high ionic conductivity, with a wide electrochemical window, and good thermal and chemical stability.According to existing research, sodium-ion battery electrolytes mainly include three liquid electrolytes (organic electrolytes, ionic liquid electrolytes, and aqueous electrolytes), and two solid electrolytes (inorganic solid electrolytes and polymer solid electrolytes).Solid electrolytes also support the formation of effective CEIs, which predominantly affect the electrochemical performance of solid batteries.However, in this work, we discuss the CEI phenomenon between various cathode materials and liquid electrolytes.
Liquid electrolytes (LEs) are composed of free or paired cations, or similar anions.As a result, they can typically diffuse efficiently in the liquid phase, with the ionic conductivity of LEs generally exceeding 10 À3 S cm À1 .
In 2012, Ponrouch et al. prepared several 1 M nonaqueous liquid electrolytes by dissolving three different ionic salts, NaClO 4 , NaTFSI, and NaPF 6, in several single organic liquid solvents or binary solvent mixtures such as DMC, PC, EC:PC, and EC: DMC. [83]The results indicated that the mixed solvent-based electrolyte was superior to a single solvent, with EC:PC (1:1) as the optimal formula.The NaClO 4 /NaPF 6 -EC: PC (1:1) electrolyte system was found to perform well following evaluation using hard carbon as the electrode material.Despite these results, a following study showed that the EC 0.45 :PC 0.45 :DMC 0.1 electrolyte well supported the Na 3 V 2 (PO 4 ) 2 F 3 (NVPF) hard carbon full-cell, indicating a reversible capacity of 97 mAh g À1 and stable cycle performance.Moreover, good rate capability was observed with a capacity of 70 mAh g À1 retained at a rate of 5 C.These results verified the feasibility of the practical sodium-ion batteries, offering some early evidence for the competitivity of Na-ion technology vs. Li-ion technology. [86]ubsequently, Li et al. investigated the chemistry of NaClO 4and NaPF 6 -based electrolytes with an Na 0.67 Ni 0.15 Fe 0.2 Mn 0.65 O 2 cathode material.The NaPF 6 -based electrolyte was found to be more compatible with the cathode material, facilitating Na þ transference. [87]A study by Ma et al. showed that PC/EMC served as an optimized electrolyte formula.When combined with various additives, this optimized electrolyte could inhibit the chemical dissolution of transition metal ions in dehydrogenated (charged) NFM.The optimized 0.8 M NaPF 6 /PC-EMC electrolyte also supported the stable operation of a complete cell with an NFNMO cathode and a hard carbon anode at 1 C for more than 2500 cycles. [88]ther-based electrolytes have the advantage of compatibility with sodium metal anodes, but can barely tolerate high-voltage operation.91][92][93][94] Ionic liquids (ILs) or molten salts consisting of specific cations and anions serve as new soft liquid functional materials at room temperature, with a wide electrochemical window, nonflammability, and non-volatility compared to carbonate-based organic electrolytes.Yamaki et al. prepared an electrolyte with the formula of NaBF 4 /EMIBF 4 , offering enhanced cyclic stability for an Na 3 V 2 (PO 4 ) 3 (NVP) symmetric cell, which was superior to conventional carbonate electrolyte 1 M NaClO 4 /PC. [95]The study also observed that the layered NaCrO 2 materials could operate stably at 90 °C in the ionic liquid NaFSA-KFSA electrolyte, with coulombic efficiency and capacity retention after the 100 th cycle of 99.6% and 98.5%, respectively. [96]alducci attempted to use a protic ionic liquid NaTFSI/ PyrH 4 TFSI as the electrolyte and observed excellent performance of Na 3 V 2 (PO 4 ) 3 , as well as poor activity with Na 0.67 Mn 0.89 Mg 0.11 O 2 . [97]ifferent electrode materials will require different solvation structures to facilitate interfacial ion diffusion and charge transfer.Wu demonstrated rechargeable Na/Na 3 V 2 (PO 4 ) 3 cells with NaPF 6 -incorporated 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide BMITFSI IL as the electrolyte.The optimized Na/Na 3 V 2 (PO 4 ) 3 cell with the IL electrolyte exhibited a high initial discharge specific capacity of 107.2 mAh g À1 and good cycling stability.
Despite a high voltage supported by the wide electrochemical window for the nonaqueous electrolyte, the organic solvent exhibited defects of flammability and the potential for leaking, resulting in possible safety hazards.IL electrolytes have demonstrated good thermal and electrochemical stability, wide electrochemical windows, and limitations on high viscosity and cost.By contrast, aqueous electrolytes have unique advantages such as high electrical conductivity and excellent safety.
Whitacre et al. demonstrated Na 4 Mn 9 O 18 as a cathode material for aqueous electrolyte energy storage devices, with an activated carbon counter electrode using a 1 M Na 2 SO 4 aqueous electrolyte.The optimized Na 4 Mn 9 O 18 had a specific capacity of 45 mAh g À1 , and the appropriate mass ratio of positive to negative electrodes allowed the cell to be charged to 1.7 V without significant water electrolysis. [98]To construct a new configuration, Wu et al. considered using an Na 0.44 MnO 2 cathode material, with an NaTi 2 (PO 4 ) 3 anode material in 5 M NaClO 4 aqueous solution.102][103] Highly concentrated electrolytes have a tendency to crystallize near room temperature, resulting in cell failure.As a result, Reber et al. investigated a ternary sodium-ion battery electrolyte using NaTi 2 (PO 4 ) 3 as the anode and Na 3 (VOPO 4 ) 2 F as the cathode, inhibiting crystallization through asymmetric anions.The results showed that the capacity retention rate was 85% after 100 cycles at 0.2 C, and 77% after 500 cycles at 1 C.The electrolyte could operate normally when the temperature was as low as À10 °C. [104]Similarly, Kosuke et al. investigated the effect of concentrated electrolytes on aqueous sodium-ion batteries with an Na 2 MnFe(CN) 6 cathode and NaTi 2 (PO 4 ) 3 NASICON-type anode.The results showed that the electrochemical window of the diluted 1 M NaClO 4 aqueous electrolyte was only 1.9 V, while that of the concentrated 17 M NaClO 4 aqueous electrolyte expanded to 2.8 V, which could easily support the Na 2 MnFe(CN) 6 // NaTi 2 (PO 4 ) 3 aqueous sodium-ion cell. [105]Other research studies also assessed high-concentration electrolytes.Based on the sodium ion salt-in-water electrolyte (NaWiSE), a full battery with Na 0.66 [Mn 0.66 Ti 0.34 ]O 2 as the cathode and NaTi 2 (PO 4 ) 3 as the anode demonstrated good electrochemical properties at different magnifications.At 0.2 C, ultra-high Coulombic efficiency was achieved after more than 350 cycles, while after more than 1200 cycles at 1 C, the capacity loss was negligible (0.006% per cycle), indicating high stability. [106] CEI on Cathode for Sodium-Ion Batteries

CEI on the Surface of Transition Metal Oxide Cathode Material
A landmark study published by Komaba et al. in 2011 proposed that hard carbon materials could be passivated at the interface of PC-based electrolytes to obtain a high specific capacity and long cycle life, following the shortcomings of previous research on anode materials for sodium-ion batteries.The hard carbon anode could also be combined with the cathode material NaNi 0.5 Mn 0.5 O 2 to fabricate a full battery that could complete dozens of cycles. [1]However, the study did not specifically assess the interface between the positive electrode and the electrolyte.The chemical components formed by the electrolyte on the surface of the positive electrode of the oxide still need to be expressly confirmed.
General research work has used XPS to detect the components and elemental valence states on the surface of cathode materials, however, due to interactions between the material and the surface, XPS has limited detection depth for these materials.Therefore, Siham et al. mainly observed the elemental and chemical composition changes of the positive electrode surface and near-surface during the electrode process, by comparing hardwire XPS and XPS.Studies showed that between 4.5 and 2.0 V, Co, Ni, and Mn were all electrochemically active.All of these materials were in the þ4 state at the end of charging.When discharged, these materials would be reduced to Co 3þ , Ni 3þ , and Mn 3þ , and partially reversibly reduced to Co 2þ and Ni 2þ .Among these, phosphates mainly formed at the end of the charging cycle (4.5V), while fluorophosphates were produced at the time of discharge (2.0 V). [16] Since the primary CEI performance of oxide cathode materials is usually not sufficient, it is necessary to further regulate the composition of the CEI film, such as the introduction of a layer of inorganic substances on the surface of the cathode material, such as oxides or phosphates, as only some substances serve as good ion conductors of sodium ions.This approach can avoid adverse effects on the diffusion of sodium ions, such as surface inorganic layers with a nano-level thickness.Generally, liquid-assisted surface modification methods have been used.For example, in an attempt to stabilize high nickel cathode materials for lithium batteries, Jiang et al. coated high nickel cathode materials with a uniform layer of Al 2 O 3 . [107]nalogously, sodium batteries should follow a similar mechanism.Notably, aluminum trioxide serves as a good conductor of sodium ions, offering many possibilities, including surface enrichment and diffusion, providing gradient doping, and the coating of Ni-rich cathodes with enhanced kinetic and interfacial chemistry.
Subsequently, physical methods can be used to achieve atomic layer deposition on the active material surface.Alvarado compared the electrochemical activity of uncoated P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 electrodes to that of ALD-coated Al 2 O 3 P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 electrodes, and analyzed the CEI on ALD-coated electrodes. [10]ALD treatment resulted in fewer carbonate species and more inorganic species, which allowed for fast Na kinetics, resulting in a significant increase in Coulombic efficiency, a decrease in cathode impedance, and enhanced mechanical stability of the particle.The mechanism involved inhibiting the precipitation of oxygen at the positive electrode of the oxide at higher voltages, or the occurrence of side reactions at the interface.
In addition to the method by which CEI can form by introducing a layer of inorganic material on the surface of the cathode material, CEI can also be generated by using different liquid electrolytes. [3,108]Sun et al. conducted in-depth studies on the commonly used electrolyte systems of 1.0 M NaPF 6 /diglyme (NP-005), 1.0 M NaPF 6 /PC/5.0%FEC (NP-009), 1. 0 M NaClO 4 /EC-DMC/ 5.0% FEC (NC-019), and 1.0 M NaClO 4 /PC (NC-013).The results showed that the CEIs of different systems were also different.Specifically, the CEI of NP-005 not only contained more NaF, but was also thinner, resulting in less polarization and interfacial resistance, which led to faster Na þ diffusion (Figure 4).c) Impedance curves of electrodes under different electrolyte conditions.Adapted with permission. [109]Copyright 2022, Wiley-VCH.
EC/EMC, EC/DEC, and EC/PC as the solvent solvents.The results showed that the CEIs formed by the electrolyte with EC/DMC as the solvent were more stable, effectively facilitating reversible Na þ reversal in long-cycle cycle transport, resulting in 83.3% capacity retention of the battery after 300 cycles. [110]Li et al. used Na 0.67 Ni 0.15 Fe 0.2 Mn 0.65 O 2 as the cathode and NaClO 4 and NaPF 6 as the electrolyte salts to conduct a comparative study of the two formed CEIs.The study found that the stability of CEI, resulting from oxide cathode materials, was more affected by oxidizing perchlorate than other cathode materials, thus, some of the CEI that formed in NaClO 4 would detach from the cathode surface, accelerating the dissolution of the transition metal ions, and accelerating the decomposition of the electrolyte.This resulted in more organic compounds and fewer inorganic compounds, with a decrease in the stability of the CEI. [87]ocal high-concentration electrolytes have been extensively studied for lithium-ion batteries. [111,112]By contrast, relatively little research has been conducted on using local high-concentration electrolytes in sodium-ion batteries.Jin et al. reported a nonflammable localized high-concentration electrolyte for highly reversible NIBs, (formula of sodium bis(fluoro sulf onyl)imide-triethyl phosphate/1,1,2,2-tetrafluoroethylene-2,2,3,3-tetrafluoropropyl ether 1:1.5:2 in molar ratio)) (Figure 5a).The study found that this allowed for the formation of an ultrathin (3 nm) and robust interphase layer on the cathode surface, which in turn allowed the NaCu 1/9 Ni 2/9 Fe 1/3 Mn 1/3 O 2 cathode material to maintain a long and stable cycle (Figure 5b-d). [21]otably, a single electrolyte additive will often not be sufficient for sodium-ion batteries.Sun et al. investigated succinic anhydride (SA) as a synergistic filming additive to fluoroethylene carbonate (FEC).The study observed a significantly improved lifespan of an Na/Na 0.6 Li 0.15 Ni 0.15 Mn 0.55 Cu 0.15 O 2 (NLNMC) cell and a high capacity retention rate of 87.2% over 400 cycles at a rate of 1 C with dual-additive application.The addition of SA resulted in a homogeneous and stable interphase layer on the surface of the NLNMC material, due to more oxygen-rich organics and fewer NaF.After testing using online differential electrochemical mass spectrometry (OEMS), the results showed that the dual-addition cell produced less CO 2 in the first two cycles compared to the cell using only FECs.The chemical environments of C, O, and F were identified by deconvoluting the XPS spectra, and only CO 2 gas was detected during the charge/discharge process (Figure 6). [22]

CEI on the Surface of a Polyanionic Cathode Material
Compared to oxide cathode materials, phosphate cathode materials exhibit higher structural stability, however, the influence of CEI cannot be ignored when obtaining excellent electrochemical cycle life.Jang et al. investigated the electrochemical performance of 1 M NaClO 4 in EC:DEC (1:1, v/v) and EC:PC (1:1, v/v) using Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) as the cathode of a sodium battery.Using 13 CNMR spectroscopy, the study demonstrated that the electrolyte containing DEC was unstable when in contact with Na metal, and electrolyte additive FEC could solve this problem. [113] low-temperature sodium-ion full battery assembled with a three-dimensional selenium/graphene composite anode and Adapted with permission.[21] Copyright 2020, American Chemical Society.
high-pressure cathode (Na 3 V 2 (PO 4 ) 2 O 2 F) was developed by Wu et al.The capacity retention of this cell remained as high as 86.3% after 15 000 cycles at 1 A g À1 . [114]This indicated one of the essential characteristics of CEI, in which the low-temperature interfacial dynamics of the phosphate cathode were not destructive.
This cathode consisted of regular and uniform Na 3 V 2 (PO 4 ) 2 O 2 F 0.99 Cl 0.01 (NVPFCl) microcubes with stable high specific capacity cycling performance.The specific capacity was 128.2 mAh g À1 at 0.1 C and 79.8 mAh g À1 at 20 C (Figure 7a,b).Possible reasons for the stability and electrochemical activity of NVPFCl were revealed through in situ/ex-situ characterization and electrode kinetics, offering new ideas for developing all-climate and storage-stable SIB cathode materials. [115]Wang et al. adopted a series of modification strategies that rationally utilized the electrolyte concentration effect to integrate the solvation structure, and adjusted the composition, distribution, and properties of the electrode/electrolyte interface phase.The anion-tuned electrode/electrolyte interface phase with a more uniform thickness and more stable properties was successfully constructed on the positive electrode of the high-voltage vanadium-based sodium battery (Figure 7c). [116]ther electrolytes are an essential organic material but generally perform poorly above 4.0 V. From the perspective of CEI performance regulation, these properties can be improved.Ether electrolyte has also been widely adopted, though minimal information has been obtained regarding the cathode-ether electrolyte interphase (CEI ether) and its critical effects on battery performance.Ba et al. provided direct evidence for forming beneficial CEI ether on 4.3 V cut-off-voltage fluorophosphate cathode in ether electrolyte.The synergistic effect of the ether solvent and salt in the NaPF 6 -DIGLYME ether electrolyte was shown to be vital for the electrochemical stability window, as the generation of CEI ether and robust fluorine-rich inorganic-organic phase effectively improved the interface, facilitating ultra-fast charge transfer, and improving the stability of the high-voltage cathode by more than 10 000 cycles with the help of cathode engineering.As a result, Ba et al. investigated a full-cell with fluorophosphate 3D submicrospheres as the cathode and Na 2 Ti 2 O 5 Figure 6.a) XPS devolution spectra of C 1s, O 1s, and F 1s. b) OEMS test cycled at a rate of 0.2 C for two cycles.Adapted with permission. [22]Copyright 2021, Wiley-VCH.
nanosheets as the anode.This cell had a high initial coulombic efficiency of 90% and could stably cycle at a high multiplicity of 40 C with no degradation after 4000 cycles, which was shown to be impossible with ester electrolytes. [23]Stefano et al. also explored the formed CEI and its electrochemical properties using Na 3 V 2 (PO 4 ) 2 F 3 as cathode 1 M NaPF 6 -diglyme and 1 M NaPF 6 -PC/FEC as the electrolytes.The absence of Na 2 CO 3 in the CEI in the ether electrolyte facilitated the stabilization of the CEI, contributing to cyclability. [117]n addition to polyanionic phosphate cathode materials, the large band gap in polyanionic iron-based sulfates will hinder rapid Na þ transfer, oxidative decomposition of the electrolyte under high pressure, and produce thicker CEIs.In the previous figure, the blue flat column represents the positive active material, and the ball on the right represents the ions and solvent molecules in the electrolyte.120] To establish continuous Na þ transfer channels at all length scales, Chen et al. improved the ionic conductivity of Na 6 Fe(SO 4 ) 3 particulate bodies by introducing the Na þ migrationchannel-dense, low-barrier ion-conducting Na 6 Fe(SO 4 ) 4 phase, which improved the ionic dynamics inside the bulk phase of Na 2.26 Fe 1.87 (SO 4 ) 3 , resulting in a uniform and stable CEI.After initial cycling, the thickness of the CEI was 5-7 nm, which completely covered the surface of the cathode (Figure 8b).Lowtemperature transmission electron microscopy (cryo-TEM) images further demonstrated that the CEI thickness was 10-14 nm (Figure 8c), and capacity retention after 1300 cycles was as high as 80.69% (Figure 8d).This provided a good strategy for improving polyanionic iron-based sulfates. [121]

CEI on the Surface of Prussian Blue Cathode Materials
Prussian blue cathode materials can be prepared in an aqueous solution using a controlled precipitation method, with high water content, which is not conducive to constructing a stable CEI.Therefore, Prussian blue electrode material structure regulation, surface interface modification, and electrolyte optimization can be used to improve electrochemical properties.Zuo et al. proposed a novel strategy of in situ capturing for residual coordinated water during the sodiation/dissociation process, achieving a high cyclability by using the aluminum chloride Lewis acid as an electrolyte additive for an MnCoNico-doped Prussian blue analog.The MnCoNi co-doped composite delivered a capacity of 111 mAh g À1 even at 1 C and retained a capacity retention rate of 78.7% after 1500 cycles for sodium-ion batteries (Figure 9a). [122]ccording to the spontaneous redox reaction between the chemical preodiation solvent and polyvinylidene fluoride (PVDF) binder, a PB surface can be conformally coated with a uniform, thin, NaF-rich CEI to form CEI@PB.This consists of inorganic Na salts (NaF, Na 2 CO 3, and NaHCO 3 ) and organic Na salts (R-O-Na, R-CO-Na, and R-O 2 CO-Na), which can ensure rapid Naþ ion transport and effectively prevent the cycling stability of CEI@PB from organic solvent attack (Figure 9b).Studies have also shown that NaF can be used as a stable CEI, because NaF serves as an excellent electronic insulator and ionic conductor.The CEI@PB cathode has also demonstrated a high specific  [115] Copyright 2021, The Authors, published by Elsevier.b) Cycling stability of the NVPFCl materials at 0.5 C after 26 months of exposure to air.Reproduced under the Creative Commons CC BY-NC-ND license. [115]Copyright 2021, The Authors, published by Elsevier.c) A schematic showing the interfacial processes at the electrode/electrolyte interface in NVPF-HU and NVPF-UU systems.Adapted with permission. [116]Copyright 2022, Wiley-VCH.
capacity and stable long-term cycling, with an average capacity of 0.61 mAh cm À1 and a cycle life of up to 3000 cycles (Figure 9c). [123]n addition to the use of binder to generate stable CEI, citric acid (CA) (C 6 H 8 O 7 ) can be introduced as a carboxylic acid source to the surface of a PB material to inhibit the thick growth of CEI, due to the presence of high spin iron (Fe HS ) ions on the surface of PB compared to other cathode materials. [124]Copyright 2019, Elsevier.b) Schematic depiction of CEI growth for CEI@PB and PB cathodes after cycling.Adapted with permission. [123]Copyright 2021, Elsevier.c) Long-term cycling of the CEI@PB‖‖Na and PB‖‖Na cells.Adapted with permission. [123]Copyright 2021, Elsevier.material-generated CEI and electrochemical properties. [125]CA lost two H þ per molecule in a neutral aqueous solution to achieve acid-base equilibrium and stabilized in the chemical form of two -O-C=O ligands.The study found that CA molar ions (H 3 cit) were ionized to carboxylate ions (Hcit 2À ), and Hcit 2À was strongly bound to high-spin iron (Fe HS ) ions on the PB surface, thus affecting the formation of CEI.The inserted Na þ during discharge was more likely to be stored on the Hcit 2À activated surface sites, and Na þ was quasi-reversibly extracted during charging.In addition, the introduction of CA reduced the parasitic reaction of Na þ with the electrolyte to produce by-products (RNaCO 3 , Na 2 CO 3 ), inhibiting the increase in CEI thickness. [125] CEI at Low Temperature In practical industrial applications, SIBs must exhibit high energy and power densities, with low-temperature performance also critical.Under low-temperature conditions, the ionic conductivity of liquid electrolytes often demonstrates a significant decrease with increased viscosity, resulting in slow charge transfer at the electrochemical interface.
In liquid electrolytes, the Na þ /solvent structure in lowconcentration electrolytes will have a lower binding energy and may favor the Na þ desolvation process when participating in charge transfer reactions. [126]Wang et al. used Na 3 V 2 (PO 4 ) 2 F 3 as the cathode material and found that it had a thinner CEI and contained less NaF at low temperatures, which facilitated Na þ transfer.The material also showed excellent electrochemical performance at both room and low temperatures, with the retention rate reaching 93.4% after 1000 cycles at À25 °C (Figure 10a). [126]u et al. designed a carbonate electrolyte system consisting of 1 M NaFSI in EC/PC/DEC (1:1:4, v/v) for ultra-low temperature (À40 °C) sodium-metal batteries.In addition, an ethyl sulfate (ES) additive was employed to reduce the desolvation energy of Na þ , promote the formation of more inorganic species on the surface, along with ionic migration, and inhibit the growth of dendrites.The cycling stability of the Na||Na symmetric cell was 1500 h at À40 °C (Figure 10b), while the capacity retention of the Na||NVP cell reached 88.2% after 200 cycles (Figure 10c). [127]n the other side, considering the low conductivity of electrolytes at low temperatures, the cathode materials will require a larger contact area with the electrolytes to support sodium diffusion, which will also depend on a stable CEI for guaranteeing the reversibility of the SIBs.
The operation of batteries with different carbonate-based electrolytes varies at low temperatures.Ma et al. investigated NaPF 6 as the electrolyte salt, with PC-and EC-based electrolytes.The study found that the EC-based electrolytes contained more organic matter on the CEI and were less stable than the PC-based Figure 10.a) Long-term cycling performance of the NVPF cathodes at 1 C at À25 °C.Adapted with permission. [126]Copyright 2021, Elsevier.b) Cycling performance.Adapted with permission. [127]Copyright 2023, Wiley-VCH.c) Cyclability at 0.1 C. Adapted with permission. [127]Copyright 2023, Wiley-VCH.
electrolytes.Thus, the specific capacity of the batteries with PC-based electrolytes was lower at room temperature than at low temperatures, while those with EC-based electrolytes could barely operate at À20 °C. [88]Therefore, to improve the operation of organic electrolytes at low temperatures, Huang et al. added 1,1, 2,2,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) to the electrolyte. [128]The improved electrolyte consisted of 0.8 M NaPF 6 FEC/ EMC/ HFE, and the NVPOF cathode not only had a thinner (%5 nm) and highly homogeneous CEI, but also achieved a discharge capacity of 92.1 mAh g À1 at a low temperature of À30 °C.However, Jin et al. also prepared a Na 3 V 1.5 Al 0.5 (PO 4 ) 3 cathode material with a thick layer of conformal composite CEI, with a thickness of about 2.8 nm, which was doped with Al. [129] The conformal composite CEI not only isolated the cathode from the carbonate electrolyte but also allowed the material to exhibit excellent cycling stability at À20 °C, and the capacity retention rate reached 98.9% after 1000 cycles at 5 C. [129] By contrast, ether-based electrolytes exhibit low freezing points, low nucleophilicity, and weak interactions with sodium ions, along with low solvation/desolvation energies and excellent electrode wettability.Thus, the ether-derived CEI layer will be dominated by inorganic substances, which are more homogeneous and stable.Wang et al. found that the NVPF anode exhibited a more stable structural framework in diethylene glycol dimethyl ether-based electrolyte, which also formed an anionderived CEI, and this contributed to interfacial stability and fast Na þ transport in the interface.At À20 °C, the NVPF cathode material demonstrated good multiplicity performance and stable cycle life, with a capacity of 87.0 mAh g À1 at 10 C and a capacity retention rate of 95.58% after 100 cycles (Figure 11a).The material exhibited good multiplicity performance even at low temperatures of À40 and À60 °C (Figure 11c).In addition, the NVPF//Na 2 Ti 2 O 5 full-cell still had a high specific capacity of 62.4 mAh g À1 at À20 °C after 10 000 cycles of stable operation at 5 C multiplicity (Figure 11b). [130]u et al. proposed a novel ether-based electrolyte using tetrahydrofuran (THF) as the main solvent, and PC and ethoxy (pentafluoro) cyclophosphazene (PFPN) as the co-solvents. [131]he battery was assembled using Na 2/3 Mn 2/3 Ni 1/3 O 2 as the cathode and tested.The results indicated that compared to electrolytes using pure THF as the solvent, this new type of etherbased electrolyte not only improved the electrochemical stability of the material, but also formed a light, stable, and inorganic-rich CEI on the cathode material surface due to the presence of cosolvent PFPN.Even under low-temperature conditions of À40 °C, CEI could promote the electrochemical performance of the cathode, with a capacity retention rate of 94.1% after 100 cycles at 0.2 C.These research developments offer new research directions for the design of stable SIB operation at extremely low temperatures. [131]

Conclusion
In this review, we comprehensively summarized current cathode materials for sodium ion batteries and systematically analyzed their interfacial chemistry on electrochemical performance.Significant advances in sodium storage mechanisms may assist in designing advanced cathode materials, and the further optimization goals of cathodes include improving the specific capacity, ICE, rate performance, long-cycle stability, and safety.To advance commercial applications, the overall performance of various cathode materials must be optimized, rather than just improving one point at the expense of others.
Further suggestions are listed as follows: The evolution of CEI remains uncertain in the high-voltage region.Benefiting from continuous research over the past several years, significant progress has been made in the area of sodium storage performance for cathode materials, primarily determined by a more robust CEI.However, CEI evolution, especially in the  [130] Copyright 2022, Elsevier.
high-voltage plateau region, remains inconclusive and requires further exploration.However, cathode materials may undergo significant volume changes in the high-voltage region, accompanied by general CEI film rupture and regeneration process, making the problem more complicated.Thus, using a specific cathode material with less deformation is recommended to carry out related exploration work.
The heteroatom doping for various cathode materials will significantly impact electrochemical performance.Some heteroatom doping can promote the air stability of oxide cathode materials, however, the mechanism remains explored.The effect of doping on interphase chemistry is also unknown for sodiumion batteries.
ICE and other vital parameters serve as influencing factors.The ICE of cathode materials will be closely related to the electrode/electrolyte interface chemistry, which can be optimized by reducing the specific surface area and surface defects.Additionally, a surface layer can be added in the preparation stage of the cathode material, or a specific additive can be used in the electrolyte to regulate the inorganic and organic parts of the CEI.In certain cases, presodiation approaches are needed, and the effect of presodiation treatment on interphase stability must be carefully considered.
Notably, the observed electrochemical performance for any cell will rely on various parameters, and CEI itself cannot dominate it.In half-and full-cells, the CEI formed by the same electrolyte and the positive electrode may differ, mainly because the sodium metal anode used in the half-cell will significantly affect the evaluation of cyclic stability.By contrast, the full-cell generally uses hard carbon as the anode material, and the performance parameters of the cathode materials will be observed, including ICE, involving the phase stability of the positive electrode material.The feedback of the anode materials interface to the positive electrode interface by-products requires cut off of the positive and negative electrode interactions through a proven interface using more precise analysis, such as a three-electrode cell.Therefore, significant room for development remains in studying full-cell CEI.
The evolution of CEI contains corresponding electrolyte consumption issues.Supported by some stable CEI, the polyanion cathode materials have demonstrated durable stability in electrochemical cycling, and the oxide materials have shown merits in terms of tap density.The long-cycle stability of cathode material mainly depends on the nature of the CEI, but the formation/evolution process in practical cells remains unknown.The CEI film may change with voltage, and we must consider whether this change is within a controllable range.In general, significant changes in the CEI film must be repaired, and the corresponding functional components of the electrolyte should be considered in the battery cycle to provide reactants for repair.However, we also want this repair limited in terms of minimal electrolyte consumption, which may require additional work.

Figure 3 .
Figure 3. Schematic illustration showing a) the P2-O2 phase transition and b) the P2-Z phase transition during the Na insertion/extraction process.c) Cycling performance of five materials at 2 C. Adapted with permission.[40]Copyright 2022, Wiley-VCH.

Figure 4 .
Figure 4. a) Changes in the surface morphology of electrodes under different electrolytes.b) Cycling performance of different Electrolytes at 0.5 and 1 C.c) Impedance curves of electrodes under different electrolyte conditions.Adapted with permission.[109]Copyright 2022, Wiley-VCH.

Figure 5 .
Figure 5. a) Schematics showing the different reactions that will occur on both the cathode and anode with different electrolytes.b) Cryo-TEM characterization of the Na-CNFM cathode interphase.c,d) Cyclic performance and Coulombic efficiency of the Na||HC half-cell with different electrolytes.Adapted with permission.[21]Copyright 2020, American Chemical Society.

Figure 7 .
Figure 7. a) Rate capability.Reproduced under the Creative Commons CC BY-NC-ND license.[115]Copyright 2021, The Authors, published by Elsevier.b) Cycling stability of the NVPFCl materials at 0.5 C after 26 months of exposure to air.Reproduced under the Creative Commons CC BY-NC-ND license.[115]Copyright 2021, The Authors, published by Elsevier.c) A schematic showing the interfacial processes at the electrode/electrolyte interface in NVPF-HU and NVPF-UU systems.Adapted with permission.[116]Copyright 2022, Wiley-VCH.

Figure 8 .
Figure 8. a) Scheme showing the interfacial adsorption and CEI formation.b) HAADF-STEM image.c) Cryo-TEM images.d) Cycling performance and DC internal resistance.Creative Commons Attribution 4.0 International License.[121]Copyright 2023, The Authors, published by Springer Nature.