Towards stable electrode–electrolyte interphases: Regulating solvation structures in electrolytes for rechargeable batteries

Rechargeable batteries are highly in demand to power various electronic devices and future smart electric grid energy storage. The electrode–electrolyte interphases play a crucial role in influencing the electrochemical performance of batteries, with the solvation chemistries of the electrolyte being particularly significant in regulating these interfacial reactions. However, the reaction mechanisms of electrolyte solvation and their specific functions in batteries are not yet fully understood. In this review, we embark on an exploration of the fundamental principles governing solvation and present a comprehensive overview of how solvation structures impact interfacial reactions at the electrode–electrolyte interface. We underscore the significance of interactions among cations, anions, and solvents in shaping electrolyte solvation structures. The primary strategies for controlling solvation structures are also discussed, including the optimization of salt concentrations, solvent interactions, and the introduction of functional cosolvents. Furthermore, we elucidate the oxidation/reduction reaction mechanisms of electrolyte components in different solvation structures and the new understanding of electrolyte additives in modulating interfacial chemistries in batteries. Additionally, we emphasize the importance of incorporating new characterization techniques and theoretical simulations to attain a deeper understanding of the intricate processes taking place within batteries. This review provides an in‐depth understanding in solvations and interphasial properties and new ideas for designing advanced functional electrolytes for rechargeable batteries.


| INTRODUCTION
The electrolyte is an essential component for all electrochemical storage and conversion devices, such as batteries, in which electrolytes closely interact with other components in devices.[3][4] The SEI model in batteries was first proposed by Peled for metal deposition-dissolution in nonaqueous battery systems in 1979, for which a passive layer was formed on the surface of alkali electrodes and acted as the interphase layer between electrode and electrolyte. [5]Peled first considered that proper control of the properties of the SEI, including its morphology (porosity and crystal size), thickness, lattice defects, electron conductivity, and cation/anion transfer number, is crucial for the functioning of nonaqueous batteries. [6,7]It can be said that Peled's work has provided the widely accepted basic understanding for SEI layers even until nowadays. [8,9]Later, Thomas et al. reported a passivation film formed on the LiCoO 2 cathode, [10] which is now referred to as cathode electrolyte interphase (CEI) by researchers to distinguish it from the SEI formed on the anode side.Both the CEI and SEI, derived from interfacial reactions, are collectively known as electrode-electrolyte interphases (EEI).Over the past few decades, it has been widely recognized that the EEI film formed on the electrode surface significantly impacts battery performance.For example, the EEI formed by the decomposition of the ethylene carbonate (EC) enables reversible Li + insertion and extraction into/from the graphite anode, thus promoting the commercialization of lithium-ion batteries (LIBs). [11]n 2010, Goodenough and Kim reported preliminary considerations for the schematic of the relative energies of the electrodes and electrolytes in a thermodynamically stable battery, as shown in Figure 1A. [12]The energy diagram in a battery is widely employed to design more stable electrolytes to understand and guide the formation of EEI layers on electrodes. [13,14]In the energy diagram, the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels of electrolyte govern the thermodynamic stability of the electrolyte and the battery.EEI formed on electrode-electrolyte interfaces should be ideally electronic insulating but good ionic conducting, which effectively inhibits the thermodynamic electron transfer between electrodes and electrolytes, thus kinetically broadening the voltage windows for the stable operation of the battery.
As the formation of EEI consumes electrolytes and active ions, batteries thus may lose achievable energy and/ or power performance.Ideally, EEI should form only once in the initial cycles and then exist as a stable passivation layer throughout the battery's lifespan.However, the formation of EEI actually includes complex multistages of electrochemical/chemical oxidation-reduction processes, and the composition, structure, and functions of the EEI in a battery also undergo dynamic changes upon cycling. [17,18]Under practical cycling conditions, certain EEI components may dissolve, decompose, or reform, resulting in the continuous degradation of electrolytes to cope with the dynamic evolution of EEI throughout the lifetime of a battery. [19]heoretically, the electrolyte at the electrode interface serves as the main precursor for the EEI layer.22] The decomposition kinetics of the same electrolyte components may even differ from one battery chemistry to another. [22,23]To date, various SEI models that include different compositions and specific distributions have been reported based on systematic electrochemical analysis and advanced characterization techniques in different battery chemistries. [8,24]Although LIBs were commercialized in the 1990s, the underlying correlation between the fundamental structures/compositions, functionality, formation mechanisms of the EEI layer, and electrolyte structures and cell performance remains a great challenge in the field of rechargeable batteries.
27] High-concentration electrolytes (HCEs), [27,28] localized high-concentration electrolytes (LHCEs), [25,29] diluted concentration electrolytes, [30] and the uses of various types of unconventional solvents and salts [31,32] have generated new electrolyte structures with tunable local and global solvation networks and unique EEI layers.Surging interest in the study of solvation structures of electrolytes and publications in this area have been rapidly increasing in the past decade (Figure 1B).Besides, the development of advanced characterization methods such as cryoelectron microscopy, [33] synchrotron radiation techniques, [34] and various in situ spectroscopy methods [7,35] have greatly benefited the understanding of the formation of EEI layers and the origin of their functions in batteries.This work will provide a timely review of the latest developments in achieving effective EEI layers through the regulation of electrolyte solvation structures.Starting from the fundamental characteristics of solvation structures in electrolyte, this work elaborates on the precise construction of stable interphasial chemistry and structure by regulating solvation structures through different strategies involving salt concentration, solvent regulation, and additives.Finally, based on the latest characterization techniques, the paper summarizes the structure and chemical composition of EEI layers and proposes further research directions energy diagram of a battery.Φ A and Φ C are the anode and cathode work functions.E g is the window of the electrolyte for thermodynamic stability.Reproduced with permission. [11]Copyright 2010, American Chemical Society.(B) Publication distribution in the field of solvation structure and electrolyte structure in the last 10 years.(Note: publication statistics data from Web of Science, last updated in September 2023.)(C) The HOMO and LUMO energy levels and corresponding optimized geometrical structures of pure EC and pure DME.Reproduced with permission. [15]Copyright 2021, Wiley.Summary of HOMO (D) and LUMO (E) energy level changes of the ion-solvent complexes compared with pure solvents.Reproduced with permission. [16]Copyright 2020, American Chemical Society.DME, 1,2-dimethoxyethane; EC, ethylene carbonate; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; SEI, solid electrolyte interphase.
in understanding the relationship between electrode and interfacial electrolyte structures.

| DISCOVER THE IMPORTANCE OF THE ELECTROLYTE SOLVATION STRUCTURE
From a thermodynamic point of view, electrolyte components with lower LUMO values and higher HOMO energy levels are more prone to decompose on the electrode, leading to the formation of SEI and CEI layers on the anode and cathode, respectively.[38] The LUMO and HOMO energy levels of two representative solvent molecules, namely, carbonate esters (e.g., EC) and ethers (e.g., 1,2dimethoxyethane [DME]), are compared under different chemical environments in Figure 1C.This can be employed to predict the redox potential of the electrolyte and understand the mechanisms of interfacial chemistry.As shown in Figure 1C-E, pure EC exhibits lower LUMO and HOMO energy levels, which are suitable for reduction and the formation of the SEI layer on the anode while remaining relatively stable on the cathode.On the other hand, pure DME with higher LUMO and HOMO energy levels presents better reduction stability but poor oxidation stability on the cathode side.However, all ion-solvent complexes exhibit decreased LUMO and HOMO energy levels compared with the corresponding free solvents, [16,39] leading to a variable voltage window and redox chemistry of the solvents in different electrolytes, as shown in Figure 1D,E.
Therefore, solely comparing the frontier molecular orbitals of isolated molecule components in the electrolyte is usually insufficient for understanding the electrochemical stability of the electrolytes.For instance, high concentration or locally high concentration can enhance the electrochemical operation window of ether-based electrolytes compared with dilute ether electrolytes. [15,40]Furthermore, electrolytes with the same components but different concentrations usually exhibit noticeable differences in the structure and composition of the formed EEI layers. [22,25]These phenomena are highly correlated to the interactions between solvent and solutes, resulting in the reorganizing of the solvent and solute molecules into different solvation complexes in the electrolytes, usually called solvation structures.The solvation structure of electrolytes arises from the solvent-solute interactions, while the nature and strength of the interactions influence the properties of the electrolytes.The earliest research on solvation theory can be traced back to 1981 when Miertuš et al. first proposed a continuum model based on electrostatic interactions to describe solvation behavior in solutions. [41]Taking nonaqueous electrolytes as an example, the electrolyte salt dissolves in the polar solvent, in which the cation and anion of the salt are dissociated by the nonaqueous solvent and spontaneously interact with the solvent to form a stable solvation structure.The solvation structure reflects the interactions among the cations, anions, and solvent molecules, such as the Coulombic interactions, hydrogen bonding, van der Waals, and dipole-dipole interaction between ions and solvents, as illustrated in Figure 2. [16] The competition among these interactions directly influences the form of the solvation complexes, thereby affecting the bulk and interfacial properties of the electrolytes.On the basis of the number of coordinating cations around an anion in the solvation shell, the solvation structure can be roughly classified into three different states (Figure 2): (1) solventseparated ion pairs (one anion with no cation coordination), (2) contact ion pairs (CIPs; one anion with one cation coordination), and (3) ion aggregates (AGGs; one anion with two or more cation coordination).The form of the solvation complexes can thus affect the properties of EEI, which affects the cycle life, rate capacity, energy density, and safety of the battery.

| SOLVATION STRUCTURES OF ELECTROLYTES AND INTERFACIAL CHEMISTRIES
Coordination between solvents and ions affects their redox stability by reorganizing the electron cloud and energies (entropy) in the solvation complex, based on the Marcus theory. [42]In recent years, more and more researchers have paid attention to the underlying correlation between specific types of solvation structures and the electrochemical oxidation/reduction stability and kinetics of electrolyte components.These correlations could be the key to achieving the rational design of electrolytes and interfacial chemistry in batteries.Although fundamental understanding is still lacking, recent studies have attempted to uncover the underlying relationship between solvation structures in the electrolyte and their physicochemical properties. [16,43]On the one hand, several studies have shown that solvent oxidation in electrolytes primarily occurs through H-transfer reactions and have proposed the oxidation sequence of solvent molecules with different surrounding components in electrolytes. [44,45]herefore, strategies that can suppress H-transfer could enhance the oxidation stability of solvent molecules in electrolytes. [46,47]On the other hand, the reduction stability of electrolyte solvents and salts likely correlates with their capability and/or extent of donating electrons to their surroundings. [16]Compared with free solvents and salt anions, coordinated solvent molecules and anions in the solvation sheath of cations can donate electrons to the cations, thereby reducing the electron cloud and the LUMO energy levels of solvent molecules and anions.This makes them more prone to getting reduced on the anode. [20,48]Therefore, by rationally designing and optimizing the solvation structure of the electrolyte, it is possible to regulate the electrochemical stability of the electrolyte and construct a more robust EEI layer on the electrode surface.
The enhancement of EEI properties through electrolyte optimization can be categorized into three primary strategies: (1) enhancing the interaction between cations and anions and promoting the participation of anions in the solvation structure by optimizing electrolyte composition (i.e., salt concentration and solvent types), promoting the formation of an inorganic-based EEI layer on the electrode surface; (2) optimizing the solvation configuration of the electrolyte by regulating the competitive interactions among solvents, cations, and anions with unique solvents and salts, thereby promoting the electrochemical oxidation/reduction stability and kinetics; (3) employing film-forming additives in electrolytes.

| Salt concentration regulation
In conventionally 1 M concentration electrolytes, the prevalent solvent molecules dominate the solvation sheath of cations, leading to the preferential decomposition of solvent molecules at the electrode interface and subsequent formation of organic-rich EEI layers. [20]owever, the organic-rich EEI layers derived from reactive solvent-cation complexes exhibit unstable structures, resulting in repeated dissolution and regeneration during subsequent cycling processes and degraded electrochemical performance of batteries, especially for new battery chemistries, such as LMBs and NIBs. [18,40]o achieve an inorganic-rich interfacial chemistry, a common strategy is to increase the proportion of anions to solvents.One example is a class of HCEs, typically >3 M for nonaqueous electrolytes.In HCEs, the interaction between anions and cations is enhanced because the limited number of solvent molecules is unable to meet the typical coordination number of cations.This leads to the formation of solvation structures containing CIP and AGG.Increasing the participation of anions in the coordination shell of cations shifts the LUMO energy of the electrolyte from solvents to salt anions.As a result, the reduction-decomposition of salt anions occurs before solvents, leading to the formation of SEI layers enriched in inorganic components.Li/Na salts with fluorine, sulfur, oxygen, and nitrogen-rich anions (e.g., FSI − and TFSI − ) are typically used in HCEs due to their higher solubility. [20,40,49]Consequently, SEI layers with fluorides, nitrides, and sulfides/sulfates are widely reported in HCEs in different battery chemistries (Figure 3A).These inorganic-rich interphasial passivation layers significantly enhance the reduction stability of electrolytes and the electrochemical performance of batteries.For instance, functional electrolytes using solvents beyond the conventional carbonates, such as nitriles, [50] phosphate, [20,22] or ether [51] are successfully designed based on the concept of HCEs for new battery chemistries such as LMBs and NIBs, resulting in improved lifetime, safety and Coulombic efficiency (Figure 3B-D).
However, it is worth noting that researchers have also designed HCEs with nonfilm-forming salts, such as LiClO 4 , which have demonstrated improved reduction stability of electrolytes.This indicates an improvement in the stability of the electrolyte itself rather than solely relying on the formation of inorganic-rich SEI. [52]oreover, HCEs also exhibit significantly improved oxidative stability and enable the operation of highvoltage cells. [40,53]Therefore, the correlation between the HCE and the improved electrochemical stability of electrolytes and electrodes and their interface could largely correlates to each specific battery chemistries and operation conditions.

| Solvating solvents
Solvent is an important electrolyte component that not only provides the media for salt dissolution and transport but also affects the fundamental charge transport mechanisms, and electrochemical and thermal stability. [26,32]The physical properties of solvents in the mixture solution of electrolytes usually differ from free solvent molecules due to specific interactions and configurations among cation, anion, and solvent.
Recently, there has been increasing attention on understanding the configuration of electrolyte solvents in relation to their electrochemical stability.Studies have indicated that the oxidation of solvents in electrolytes can be attributed to the H-transfer reaction under high voltage.Borodin et al. conducted a systematic study on the oxidative stability and initial decomposition of different types of electrolytes based on carbonates, sulfones, and phosphates as solvents using quantum chemistry calculations.They elucidated that the presence of anions such as PF 6 − , FSI − , and BF 4 − around the solvent molecules significantly reduces the oxidation stability of many solvents due to the spontaneous or low barrier H-and F-abstraction reactions. [47,54]Additionally, solvent dimers have been observed to induce a low barrier H-transfer, resulting in a significant decrease in the oxidation potential.However, it has been found that the oxidation potential of cation-polarized solvents, such as EC n /LiBF 4 and EC n /LiPF 6 complexes, is 0.3-0.6V higher compared with EC/BF 4 − and EC/PF 6 − (Figure 4A). [47]H-transfer reactions between electrolyte components have a critical influence on the electrolyte reaction mechanism at the cathode.By regulating solvents, it is possible to directly adjust the composition and configuration of the solvation structure, thereby inhibiting unfavorable Htransfer and achieving intrinsic electrolyte stability.Fan et al. recently discussed the design principles for electrolytes and proposed that the oxidation potentials of different types of solvation clusters follow the order: solvent-Li + > solvent-solvent > solvent-Li + -anion > solvent-anion-Li + > solvent-anion (Figure 4B). [45]By introducing dimethyldimethoxysilane with a Si─O bond-rich structure as a solvent, the most easily oxidizable solvent-anion clusters in the electrolyte can be eliminated, thereby improving the compatibility between the electrolyte and high-voltage cathodes Reproduced with permission. [20]Copyright 2018, Nature.(D) Coulombic efficiency of Li plating/stripping with different electrolytes.Reproduced with permission. [25]Copyright 2018, Elsevier.DEC, diethyl carbonate; EC, ethylene carbonate; HCE, high-concentration electrolyte; NaFSI, sodium bis(fluorosulfonyl)imide; NEDC, sodium double alkyl carbonate; PVDF, polyvinylidene fluoride; TMP, trimethyl phosphate; XPS, X-ray photoelectron spectroscopy.
(Figure 4C).Previous studies have shown that the Htransfer reaction between solvent and anion has the lowest oxidation potential, [45,55] possessing a prominent role during the CEI formation process.Ren et al.
found that the adjustment of the ─CH 2 ─ units in the ether structure shows an apparent influence on their interaction with the FSI − and the H-transfer reaction between solvent and anion, as shown in Figure 4D. [56]I G U R E 4 (A) Effect of solvent dimer, solvent-anions complex, and solvent-anion-cation complex (take EC solvent as an example) on H-transfer reaction.Reproduced with permission. [47]Copyright 2013, American Chemical Society.(B) Schematic illustrations of five solventcentric clusters: (i) solvent dimers in free solvent domains, (ii) solvent only, in a separated solvated Li + sheath, (iii) solvent in Li + sheath partially solvated by anions, (iv) solvent near anions in the Li + sheath, and (v) solvent contacting free anions.(C) The proportion and oxidation potentials of the five solvent clusters in different electrolytes.The minimum of the oxidation potentials of the existent clusters in the electrolytes is framed by a red dashed line.Reproduced with permission. [45]Copyright 2022, Royal Society of Chemistry.(D) Electrolyte species and their roles at the cathode surface and the snapshots of anion-solvent complexes with H-transfer reactions.Reproduced with permission. [56]Copyright 2023, Wiley.
(E) The visual LUMOs and energy level of solvents and ion-solvent-coordinated structures.Reproduced with permission. [52]Copyright 2021, Royal Society of Chemistry.DMB, The most negative H-transfer reaction of 1,3dimethoxypropane (DMP) can lead to the lowest oxidation potential of DMP-FSI − complex.This favorable H-transfer reaction between DMP and FSI − can induce the decomposition of FSI − into LiF-rich CEI film on the cathode surface.Therefore, DMP-based electrolytes significantly improve the cycling performance of nickel-rich cathodes at 4.7 V.
For the reduction stability of solvents, Shen et al. reported the reduction mechanism of organic electrolytes and observed a "V-shaped" change in the LUMO energy level of organic solvents when increasing the salt-tosolvent ratios based on ab initio molecular dynamics (AIMD) (Figure 4E). [52]They demonstrated that the reduction stability of solvent molecules can be greatly improved as anions enter the coordination shell of cations when the salt-to-solvent molar ratios increase beyond a certain threshold.

| Nonsolvating and weak solvating solvents
The regulation of the solvation structure of solvents is fundamentally driven by energy/entropy.The strength of interactions between solvents and salts is influenced by the solvating capability of the solvent and the dissociation strength of salts in the electrolyte solution.In the previous section on salt regulation, highly solvating solvents, such as DME, [40] trimethyl phosphate, [20] sulfolane, [23] and so forth, are typically employed in HCEs to promote the dissolution of a high concentration of salt and the formation of AGG and CIP complexes.In contrast to the conventional concept of using polar solvents, recent work has proposed the use of nonsolvating and/or weak solvating solvents as diluent solvents to adjust the macroscopic properties of the electrolyte, such as viscosity and conductivity, without disrupting the local solvation structures of salts and polar solvents (Figure 5).One successful design is the concept of LHCEs (0.75LiF-SI:1TEP:3BTFE) reported by Chen et al. [25] Various types of LHCEs, obtained by combining different fluorinated ethers with HCEs, have been reported for various battery chemistries, especially for metal anodes and high-voltage cells. [57,58][64] Due to the reduced charge density on the ligand atoms of low-polarity solvent molecules, the interaction between them and cations (Li + or Na + ) is weakened, allowing anions to competitively coordinate with cations.For example, Shi et al. reported an amphiphilic weakly solvating molecule that, by engaging in the solvation structure of Li + , induced the self-assembly of the electrolyte, forming a unique "core-shell" solvation structure, thus introducing more AGGs (Figure 6A). [65]owever, the introduction of such solvents may narrow the electrochemical window. [66]This challenge can be addressed by manipulating solvent interactions.Recently, Ma et al. have developed a high-performance flame-retardant phosphate-based electrolyte with an ultralow concentration (0.16-0.85 M). [67] By optimizing the ratio of strong and weak solvents, they achieved optimal intermolecular interactions, inducing a stable "solvation cluster" reorganization structure (Figure 6B).This leads to a strengthened solvation network and enhances the electrochemical stability window of the electrolytes on both the negative and positive voltage sides (Figure 6C).Besides, the appropriate weak solvent in the electrolyte structure provides important roles in lowering the activation energy for the Na + transport.An enhanced charge transfer number of Na + of 0.51 can be observed in such electrolytes.

| THE ROLES OF ELECTROLYTE ADDITIVES FOR SEI FORMATION
Another important component in electrolytes that affects the electrolyte-electrode interphase is electrolyte additives. [68,69]The content of additives in the electrolyte is typically lower than 5%. [70]However, even trace amounts of additives can remarkably influence the functionality of the electrolyte and battery performance. [71]Electrolyte F I G U R E 5 Schematic diagram of solvation structure of (A) HCE and (B) LHCE.Reproduced with permission. [21]Copyright 2023, American Chemical Society.AGG, aggregate; CIP, contact ion pair; HCE, high-concentration electrolyte; LHCE, localized high-concentration electrolytes.
additives have been the most economical and efficient approach for enhancing battery performance since the commercialization of LIBs in the 1990s. [72]he fundamental concept of electrolyte additives is using appropriately lower LUMO or higher HOMO energy levels than the electrolyte components, which can preferentially decompose and form EEI between electrodes and electrolytes. [73]Most additives contain functional groups that compromise early decomposition, such as unsaturated bonds, fluorine substitution, and cyclic structure compounds. [74,75]A high-quality EEI behaviors, the solid electrolyte with high ionic conductivity but low electronic conductivity, thus can effectively suppress continuous side reactions at the electrode-electrolyte interface and allow fast ion transport through the EEI layers. [76]Electrolyte additives typically present unique and/or synergic roles with the electrolyte to tackle certain issues of batteries by introducing specific components in EEI layers. [77]I G U R E 6 (A) Amphiphilic solvent design strategy for LMB electrolyte.Being markedly different from the diluted electrolyte and LHCE system, the amphiphilic solvent (i.e., TFMP) can induce the self-assembly of 1 M LiFSI−TFMP/DME electrolyte to form a peculiar core-shell-like solvation structure that further reduces DME-Li + coordination and boosts FSI − -Li + pairing.Reproduced with permission. [65]Copyright 2023, Wiley.(B, C) Solvent reorganization induces a strengthened solvation network and enhances the electrochemical stability window of the electrolytes.Reproduced with permission. [67]Copyright 2022, American Chemical Society.CEI, cathode electrolyte interphase; DME, dimethyl ether; HC, hard carbon; LHCE, localized high-concentration electrolytes; LiFSI, lithium bis (fluorosulfonyl)imide; LMB, Li-metal battery; NaFSI, sodium bis(fluorosulfonyl)imide; SEI, solid electrolyte interphase; VC, vinylene carbonate.
Therefore, traditional additives for batteries are typically sorted by their roles.Previous reviews [71,74,[77][78][79][80][81] have systematically summarized different types of conventional additives for rechargeable batteries, such as fluoroethylene carbonate. [73]vinylene carbonate, [75,82] lithium bis(oxalate) borate, [83] 1,3-propane sultone, [84] adiponitrile, [85] and tris (trimethylsilyl)phosphite. [86] So far, various types of electrolyte additives are designed to solve the specific issues in nonaqueous batteries during the practical application, such as transition metal dissolution, high interphasial resistance, gas generation, and so forth. [87,88]In most cases, a combination of different additives is required for the special application of batteries. [69]Of course, it should be noted that the roles of electrolyte additives could differ from one battery chemistry to another due to special electrode materials and surface properties.In this section, we will not rediscuss the conventional electrolyte additives.Instead, we will focus on discussing the recent new understanding of electrolyte additives for batteries.
In recent years, the influence of certain additives on tuning the solvation structures in electrolyte have attracted researchers' attention.Several recent studies have started to rediscover the roles of electrolyte additives in batteries. [89,90]It is considered that though the additive of working cations provides new research directions to understand how additives affect interfacial structure and composition.The solvation sheath of the working ion in the electrolyte refers to the material located near the Helmholtz layer at the electrode surface before any charge transfer occurs. [91]herefore, targeted modification of the solvation sheath structure of the working ion is crucial for regulating the chemical processes of the SEI.Ming et al. introduced highly polar additives such as LiNO 3 / DTD into unstable ether-based electrolytes and studied the influence of these additives on solvation structures. [92,93](Figure 7A,B) They reported that the polar additives entered the first solvation sheath of Li + and weakened the interaction strength between Li + and solvents, thus suppressing Li + -solvent cointercalation-induced graphite exfoliation.They further reassembled the cycled graphite@SEI electrode with PC electrolyte without LiNO 3 /DTD additive and found that the graphite@SEI electrode could no longer inhibit Li + -PC co-intercalation. [93]Therefore, they proposed a new understanding that the electrolyte additive could be beyond the conventional role of filmforming agent in electrolytes, they could rather fundamentally tune the electrochemical stability of electrolytes by intensive participation in the solvation structures. [94,95]esides, it was recently reported that additives could increase the concentration of electrolyte anions in the solvation sheath, and indirectly promote the decomposition of anion at the anode for forming an inorganic-rich SEI, rather than a direct film-formation process on the anode. [98,99]For example, the additive of 4-acetylpyridine (4-APD) with an electron-withdrawing acetyl group can induce the formation of an anion-rich solvation structure of Na + , thereby the 4-APD facilitates the formation of a NaFenrich SEI and effectively suppresses the growth of Na dendrites [96] (Figure 7C,D).Additionally, introducing a superior anion additive into the solvation sheath is also a viable strategy.Su et al. proposed a fine-tuning of carbonatebased electrolytes with 1-ethyl-3-methylimidazolium diethyl phosphate, which possesses significant steric hindrance and strong binding affinity with Li + [97] (Figure 7E-G).By generating a DEP anion-rich solvation sheath, the solvation energy of Li + is reduced, leading to the formation of an SEI rich in inorganic species and achieving high stability for the Li metal anode.Different types of polar additives, such as succinic anhydride [100] and pentafluorophenylboronic acid, [101] have also been reported to change the solvation structure of Li + through their strong complexation effect.This effect weakens the interaction between the solvents and Li + , thus promoting the desolvation process of Li + instead of participating in SEI layer formation processes.

| ADVANCED CHARACTERIZATION AND ANALYSIS TECHNIQUES FOR SOLUTION STRUCTURES AND INTERPHASIAL LAYERS
Effective characterization, analysis, and computational simulation of electrolyte solvation structure are essential for understanding and designing electrolytes. [102]Currently, the characterization of electrolyte solvation structures mainly relies on different spectroscopic and scattering methods. [103]For interphasial properties, since Peled first introduced the concept of SEI on Li metal anode, the research history of SEI has been accompanied by the development of battery technologies in the past five decades. [35,104,105]Conventional methods, such as Xray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), Raman, nuclear magnetic resonance (NMR), high-resolution transmission electron microscopy (HRTEM), and so forth, have successfully provided primary information of ingredients, structures, and morphologies of EEI layers.However, the understanding of EEI is still insufficient in batteries.Several factors contribute to the challenges associated with research on EEI due to the complex and dynamic interphasial reactions in the operation of batteries. [106]Therefore, developing advanced characterization and analysis methods that can provide advanced time, special, and energy resolution for solvation structures and interphasial layers could be crucially important.

| Characterizations and simulations of solution structure
Spectroscopic methods such as FTIR and Raman are widely employed to characterize solvation structure and differentiate the free or coordinated solvent molecules or anions by detecting cloud density or bond vibration of organic molecules (ions).Yamada et al. [50] used Raman spectroscopy to analyze the solvent and anion states in the high-salt concentration LiTFSI-acetonitrile electrolyte and analyzed the solvation structure (Figure 8A).NMR can also be applied to characterize the chemical environment in electrolytes.Valuable information about the interactions between different functional groups/molecules can be provided by NMR, especially when employing two-dimensional NMR (2D-NMR). [107,108]Twodimensional NMR collects signals in two frequency dimensions, usually referred to as F1 and F2.Wahyudi et al. [62] used 2D-NMR to elucidate the weak long-range interactions between solvent-anion and solvent-solvent in electrolytes in Figure 8B.These interactions arise from hydrogen bonds formed between solvent molecules and other solvent molecules or negative-charge anions.Scattering methods are another significant characteristic method of solution structures, which involve measuring the scattering patterns and intensities of particles or beams (such as X-rays or neutrons) in a material to obtain structural information. [110]Scattering techniques are important for understanding the structure of materials as they can directly provide information about the real distances between atomic pairs and the number of interactions, which are difficult to measure directly using other methods.Efaw et al. [109] utilized small-angle and wide-angle X-ray scattering (SAXS-WAXS) to reveal the micelle-like solution structure in LHCE (Figure 8C-F).Additionally, SAXS-WAXS analysis was performed to investigate the structural changes in the solution above and below the critical micelle concentration (CMC).The SAXS-WAXS results revealed the correlation between the micelle structure and ion conductivity, showing an increase in ionic conductivity with increasing ion concentration in a uniformly dispersed solution.Above the CMC, the ion conductivity is primarily determined by the formation and connectivity of the micellar structure.
In recent years, theoretical simulations of electrolytes have presented a molecular understanding of electrolyte structures and properties based on classical molecular dynamics [111,112] and AIMD simulations. [113,114]Combining the simulation and experimental results could provide an overall picture of solvation structures in F I G U R E 8 Solution structure characterization methods and results.(A) Schematic diagram of the solution structure and Raman spectroscopy of diluted solution and high-concentration solution.Reproduced with permission. [50]Copyright 2014, American Chemical Science.(B) NMR characterization and schematic diagram of solvent-anion pairs and solvent-solvent pairs in the LiTFSI-LiNO 3 -DME-DOL electrolyte.Reproduced with permission. [62]Copyright 2022, Wiley.(C, D) Schematic diagram of the solvent structure in LHCE and molecular dynamics (MD) simulation results.(E, F) Corresponding ionic conductivity and SAXS-WAXS results of LHCE crossing the critical micelle concentration (CMC).Reproduced with permission. [109]Copyright 2023, Nature Springer.AGG, aggregate; AN, acetonitrile; CIP, contact ion pair; DME, dimethyl ether; DOL, dioxolane; LHCE, localized high-concentration electrolytes; LiFSI, lithium bis (fluorosulfonyl)imide; NaFSI, sodium bis(fluorosulfonyl)imide; NMR, nuclear magnetic resonance; SAXS-WAXS, small-angle and wideangle X-ray scattering; TFSA, bis (trifluoromethylsulfonyl) amide.
electrolytes and their correlation to the macroscopic ion properties and electrochemical properties of electrolytes.

| Electrochemical techniques for interphasial characterization
Developing advanced characterization and analysis methods that can provide advanced time, special, and energy resolution for interphasial layers is crucially important.New characterization tools and analysis methods present useful information to uncover the chemical, structural and mechanical formation, and origin of its functions of EEI layers, and thus guide the rational design of functional electrolytes and artificial EEI layers. [115,116]Electrochemical techniques are widely employed in traditional methods to study and analyze the formation and evolution of EEI layers.Electrochemical methods are convenient and feasible in situ analysis, which allow rapid and accurate acquisition of electrochemical information related to the formation and involvement of EEI during battery operation. [117]Among different electrochemical methods, electrochemical impedance spectroscopy (EIS) provides an informative understanding of interface reactions, including systemic resistance, double-layer capacitance, charge transfer resistance, SEI and diffusion processes, and so forth.
Peled first employed the equivalent circuit model of the EIS spectrum and correlated it to the mosaic structure of the SEI layer in 1997. [8]Aurbach et al. later illustrated the formation processes of SEI layer combing EIS and other techniques in 1999. [118]In recent years, EIS fitting and equivalent circuit methods have been widely employed for the analysis of electrode interfacial reactions.However, it should be noted that multiple equivalent circuits can be employed when analyzing EIS results, leading to confusion in the analysis of EIS results. [119]Therefore, it is essential to combine results from different methods and understand the underlying physiochemical processes at the electrode interface.While the distribution of relaxation times (DRTs) obtained from measured spectra can be a complementary tool for EIS analysis. [120,121]DRT allows for the separation, visualization, and quantification of different electrochemical processes within a system, providing valuable information (Figure 9A). [122]To achieve a more comprehensive interpretation of EIS results, Lu et al. employed the DRTs method to analyze the kinetic transition of Li-In from alloying to metal deposition (Figure 9B). [123]The results demonstrate that rapid diffusion of lithium atoms and fast charge transfer are key factors in ensuring a stable alloying process, while a decrease in charge transfer triggers the transition from alloying to Li metal deposition reaction.
F I G U R E 9 (A) Scheme of the analysis process for DRT.Reproduced with permission. [122]Copyright 2022, Elsevier.(B) DRT analysis of the kinetic transition from Li insertion to metal deposition in Li-In alloy.Reproduced with permission. [123]Copyright 2022, Elsevier.DRT, distribution of relaxation time; EIS, electrochemical impedance spectroscopy.

| Advanced interphasial component characterization methods
Characterizing the composition of the EEI is crucial for understanding its complexity and variability.Advanced analytical techniques, such as XPS, [124,125] Raman spectroscopy, [126] secondary ion mass spectrometry (SIMS), [127,128] and so forth, are commonly employed to investigate the chemical composition and distribution of the SEI.Additionally, solid-state nuclear magnetic resonance (ss-NMR) has been used to study the interfacial composition, particularly the coordination structures. [129,130]Recently, synchrotron-based technique using a high-quality light source provides a powerful tool for precise identification of SEI composition (Figure 10A). [131]Hu et al. identified and differentiated two elusive SEI components, LiH and LiF, and unraveled the dynamic interphasial reactions on Li metal anodes, combining synchrotron X-ray diffraction and pair distribution function (PDF) analysis. [34]They revealed the abundance existence of LiH in the SEI layer and observed the form of nanocrystalline LiF (<3 nm) rather than bulk LiF in the SEI layer (Figure 10B).PDF analysis demonstrated the dominant anion reduction process for SEI formation in HCEs (Figure 10C).Mao et al. employed in situ deep-sensitive surface-enhanced Raman spectroscopy (DS-PERS) to study the nanostructure and chemical composition changes of the SEI on Li metal under different states and tried to correlate the electrolyte structure of SEI formation (Figure 10D). [131]DS-PERS experiment revealed the interplay between the SEI and Li + desolvation and Li deposition at the molecular level.In addition to the surface characterization methods, SIMS can provide the special resolution of different components.To address the lack of in situ techniques with spatial resolution for monitoring SEI composition, Zhu et al. designed in situ liquid-SIMS to study the composition of the SEI in the presence of electrolyte and revealed a double-layer structure of SEI (Figure 10E). [128]The inner layer is dense and thin and rich in inorganic compounds, and the outer layer is rich in organic species with high permeability to the electrolyte.

| Advanced interphasial structure and morphology characterization methods
The structure and morphology of EEI are informative to unravel the formation and function of interphasial layers.Scanning electron microscopy (SEM), [132,133] HRTEM, [134,135] and atomic force microscopy (AFM) are widely used for observing surface features and morphology at the meso-to nanoscale. [136,137]SEM is primarily used to acquire surface morphology information of the EEI.When combined with focused ion beam technology, SEM can also provide crosssectional information. [138]In comparison, transmission electron microscopy (TEM) offers higher resolution visualization of EEI film.It should be noted that the electron beam energy used in SEM and TEM may impact the structure and composition of the EEI, leading to distorted EEI information. [139]Apart from SEM and TEM, AFM provides a method for nondestructive characterization of the EEI layer. [140,141]Zheng et al. provide in situ morphological and mechanical imaging of silicon SEI layers under various electrolyte media (Figure 11A). [142]Kelvin probe force microscopy (KPFM) is a powerful tool based on AFM, which has been applied to investigate the charge and potential distribution at the electrode-electrolyte interface in recent years, which is significant for understanding the charge mechanisms of the electrolyte/electrode interface.Masuda et al. [143] directly visualized the change in the potential distribution of a cathode composite electrode during charging processes with KPFM.A local difference in electrical potential indicated the importance of the local structures of the composite electrodes in controlling Li ion behavior (Figure 11B).
Traditional ex situ characterization methods could bring an ambiguous understanding of EEI due to the dynamic and sensitive features of EEI after exposure to high-energy beams. [139]In recent years, cryogenic electron microscopy (cryo-EM) has been introduced for studying the chemical composition and structure of the EEI.Cryo-STEM was initially designed for preserving hydrated biological specimens and has unique advantages in characterizing radiation-sensitive materials, [145][146][147] therefore, cryo-TEM/STEM provides the opportunity to observe the structure/component of EEI layer at the atomic scale.Li et al. first applied cryo-TEM to identify the SEI on Li metal anodes in different electrolytes. [33]They observed crystalline Li dendrites growing along certain preferred directions in carbonatebased electrolytes (Figure 11C,D).Fang et al. further revealed the evolving nanostructure of Li deposition at various transition states in the nucleation and growth process of Li, and observed the disorder-order phase transition of Li as a function of current density and deposition time (Figure 11E). [144][150][151][152][153] Wang et al. summarized the progress and challenges of cryo-EM in battery and interface research. [146]Cryo-EM offers a way to preserve the native state and image the battery materials at the nano/ atomic scale and has been employed to explore materials containing reactive elements such as lithium or lightweight elements (C, O, F, and S), capturing and visualizing the distribution of certain reaction intermediates in the electrolyte, such as lithium superoxide, lithium polysulfides, and soluble redox mediators.This technique greatly aids researchers in gaining a deep understanding of the dynamic structures and reaction mechanisms of battery materials.The development of in situ cryo-EM experiments for investigating the nucleation and growth of Li metal anode is particularly (C) PDF analysis of amorphous components in SEI.(The lower part is calculated PDF data of possible components in the SEI.) Reproduced with permission. [34]Copyright 2020, Springer Nature.(D) Schematics of the DS-PERS that enable detection of the signals from different depths in the SEI. [126](E) The scheme illustration of liquid-SIMS analysis of solid-liquid interface.Reproduced with permission. [128]Copyright 2020, Springer Nature.DMC, dimethyl carbonate; DME, dimethyl ether; DS-PERS, deep-sensitive surface-enhanced Raman spectroscopy; LiFSI, lithium bis(fluorosulfonyl)imide; LMC, lithium methyl carbonate; LPDC, dilithium propylene dicarbonate; PC, propylene carbonate; PDF, pair distribution function; SEI, solid electrolyte interphase; SIMS, secondary ion mass spectrometry; XRD, X-ray diffraction.beneficial for exploring its growth mechanism.However, it should be noted that materials undergoing phase transitions during the cooling process are not suitable for cryo-EM.Additionally, cryo-EM remains a technique sensitive to local regions of crystalline materials, and it requires the integration of other relevant techniques to obtain a more comprehensive understanding of sample information.
In summary, intensive separative information on the composition, structure, and morphology of the EEI has been revealed using advanced characterization methods in the past five decades.However, the overall physiochemical picture of the EEI layer regarding their formation, evolution, and functions is not well understood yet.Combining different nondestructive online characterization methods with high energy, space, and time resolution could be crucial to effectively uncover the dynamic EEI processes in future studies.Characterization methods with functions of rapid mapping or 3D configuration are also required to get the macroscopic picture of EEI and their evolution.

| CONCLUSION AND OUTLOOK
In this work, we summarize recent progress in investigating solvation structures and their correlations with the electrochemical stability of electrolytes and interphases in rechargeable batteries.We discuss various strategies for optimizing solvation structures, including the modulation of salt concentrations and solvent interactions, the introduction of cosolvents with low-solvation capabilities, and the combination of electrolyte additives, among others.Additionally, we provide a detailed elucidation of the influence of solvation structures on the electrochemical stability of electrolytes and the redox mechanisms at the electrode interface.Recent developments in advanced characterization methods for interfacial layers in batteries F I G U R E 11 (A) Three-dimensional plot of modulus and structures of the silicon SEI detected by AFM.Reproduced with permission. [142]Copyright 2014, Royal Science Chemistry.(B) The visualization of the change in the potential distribution of a cathode composite electrode during charging processes observed with KPFM. [143]Cryo-TEM first observation of SEI on Li metal anodes: (C) schematic of the sample freezing process, (D) Cryo-TEM images of mosaic-type and multilayer model SEI, along with their corresponding schematic diagrams.Reproduced with permission. [33]Copyright 2017, AAAS.(E) Pressure-tailored Li deposition under pressure condition: Cryo-TEM images of Li anode SEI under 350 kPa pressure, along with schematics of the deposition behavior.Reproduced with permission. [144]Copyright 2021, Springer Nature.AFM, atomic force microscopy; KPFM, Kelvin probe force microscopy; SEI, solid electrolyte interphase; TEM, transmission electron microscopy.
are also briefly reviewed.Finally, we present a few thoughts on the rational design of advanced electrolytes and the regulation of interfacial layers in batteries for future research.We believe that this timely review work will provide helpful insights into the development of new electrolytes for rechargeable batteries.
(1) Appropriate control of the coordination degree of anions.Constructing a solvation structure dominated by anions is an effective approach to obtaining inorganic-rich SEI layers and improving the electrochemical performance of batteries.However, increasing the coordination of anions in the solvation shells may reduce the ionic conductivity of electrolytes and increase the energy barrier for desolvation in batteries.Furthermore, a better understanding of the reduction/ oxidation pathway, sequence, and kinetics of different electrolyte species in solvation structures is necessary to achieve precise control of interfacial layers in batteries.Besides, enhancing the macroscopic ion properties such as ionic conducting and electrolyte wettability towards electrodes and separators is also important to enhancing the battery performance.( 2 U R E 2 Correlations between solvation structures and interphasial properties and battery performance.AGGs, aggregates; CIP, contact ion pair; EEI, electrode-electrolyte interphase; SSIP, solvent-separated ion pair.

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I G U R E 3 (A) XPS spectra of the cycled hard carbon (HC) electrodes and the schematic illustrations of passivation films derived from EC solvent in a conventional dilute electrolyte and from NaFSI salt in the concentrated electrolyte.(B) Cycling performance of the HC||Na half-cells using concentrated 3.3 M NaFSI/TMP electrolyte and conventional 1.0 M NaPF 6 /EC:DEC (1:1 by vol) electrolyte.(C) Flame tests of 3.3 M NaFSI/TMP electrolyte and conventional 1.0 M NaPF 6 /EC:DEC (1:1 by vol) electrolyte.

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I G U R E 10 (A) A scheme of synchrotron-based characterizations.(B) Refined synchrotron-based SEI XRD data of 5 M LiFSI in PC.
structures in the design and application of additives/electrolytes, thereby shortening the development period of additives/electrolytes. (4) Advanced in situ and nondamage characterization methods for EEI layers in batteries are still required to reach an accurate and deep understanding of physicochemical processes at the electrode-electrolyte interface in batteries.A deep understanding of the EEI process is also important to the development of fundamental theories of interphasial models in the field of electrochemistry.
) On the basis of the fundamental concept of regulating electrolyte structures, numerous types of electrolytes and unique interfacial properties have been recently introduced.Considering the essential requirements of electrolytes for practical use (such as reasonable conductivity, good wettability, low flammability, low toxicity, low cost, etc.), standard evaluation methods of new types of electrolytes for different battery chemistries can be proposed for effective comparison and boosts the practical use of new types of electrolytes.Meanwhile, with the rapid development of LIBs and NIBs in different application fields of electric vehicle and grid energy storage, increasing demands on battery technologies with features of extremely fast charge and wide temperature operation range are emerging.Future developments of functional electrolytes that enable fast charge and wide temperature ranges (without sacrificing the existing properties of electrolytes) are important research areas.(3)Theunderstanding of electrolyte additives has been broadened beyond solely being film-forming agents.A comprehensive study of additives is beneficial for understanding and improving battery performance.Additionally, experimental investigations on electrolyte additives can be combined with fundamental mechanism studies, big databases, or artificial intelligence to derive models and mechanisms of electrolytes/additives.This combination will enable a more precise selection of additives or additive