Halide‐based solid electrolytes: The history, progress, and challenges

Lithium metal solid‐state batteries (LMSBs) have attracted extensive attention over the past decades, due to their fascinating advantages of safety and potential for high energy density. Solid‐state electrolytes (SEs) with fast ionic transport and excellent stability are indispensable components in LMSBs. Heretofore, a series of inorganic SEs have been extensively explored, such as sulfide‐ and oxide‐based electrolytes. Unfortunately, they both have difficulty in achieving a satisfactory balance of conductivity and stability, and oxides suffer from a high impedance of grain boundaries, while sulfides encounter poor stability. Halide‐based solid electrolytes are gradually emerging as one of the most promising candidates for LMSBs due to their advantages of decent room temperature ionic conductivity (>10−3 S cm−1), good compatibility with oxide cathode materials, good chemical stability, and scalability. Herein, research and development of the widely studied metal halide SEs including fluorides, chlorides, bromides, and iodides are reviewed, mainly focusing on the structures and ionic conductivities as well as preparation methods and electrochemical/chemical stabilities. And then, based on typical metal halide solid electrolytes, we emphasize the interface issues (grain boundaries, cathode−electrolyte and electrolyte–anode interfaces) that exist in the corresponding LMSBs and summarize the related work on understanding and engineering these interfaces. Furthermore, the typical (or in situ) characterization tools widely used for solid‐state interfaces are reviewed. Finally, a perspective on the future direction for developing high‐performance LMSBs based on the halide electrolyte family is put out.


| INTRODUCTION
Lithium-ion batteries (LIBs) have been generally recognized as ideal renewable and clean energy sources that can substitute fossil fuels to enable the sustainable development of our economy and society. Since the first commercialized LIB was proposed by Sony company in 1991, it has quickly dominated the mainstream market of portable electronic devices and became an inseparable part of modern society. [1,2] However, with the increasing demand and deployment of batteries, traditional LIBs are facing two main issues. On the one hand, the intrinsic flammability and easy leakage of presently commercial liquid carbonate electrolytes have brought potential safety hazards. On the other hand, there is an ever-increasing demand for batteries with even higher energy density as well as high power density. Unfortunately, conventional LIBs based on intercalation electrodes, such as graphite anodes and layered oxide cathodes, have almost reached their limit. [3] Therefore, it is urgent and necessary to develop new energy storage systems with higher safety and energy density.
Lithium (Li) metals exhibit extremely high theoretical specific capacity (3860 mA h g −1 ), low density (0.59 g cm −3 ), and the lowest negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode [SHE]). [4] Therefore, the use of Li metals as anodes to realize Limetal batteries (LMBs) is compulsory to achieve the high energy density (up to 500 Wh kg −1 ), which much exceeds that of the prevailing LIBs based on carbon anodes and organic liquid electrolytes. [5] However, in the process of battery operation, the growth of Li dendrites and low coulombic efficiency due to the continual formation and fragmentation of the solid electrolyte interphase (SEI) seriously hinder the development of LMBs.
Compared with the traditional organic liquid electrolytes, solid-state electrolytes (SEs) are expected in the LMB system due to their inherent advantages of low flammability, high thermal stability, no leakage, and low explosion risk. More importantly, because of their excellent mechanical strength, the SEs can effectively inhibit the growth of Li dendrites. By using the SEs, the parasitic reactions at the interface between the Li metal anode and electrolyte are expected to be suppressed. Moreover, the SEs can alleviate the dissolution of transition-metal ions and oxygen evolution, tolerancing the application of high-voltage cathodes. [6,7] Obviously, Li metal solid-state batteries (LMSBs) are promising to achieve double improvements in safety and energy density, and it is the research hotspot of nextgeneration electrochemical energy storage systems.
SEs with fast ionic transport and excellent stability are indispensable components in LMSBs. Heretofore, a series of SEs have been extensively explored. In general, they are classified into polymeric and inorganic solid electrolytes.
Polymer solid electrolytes have the advantages of good processibility and flexibility, but they are still far from satisfying practical applications due to their low Li-ion conductivity, poor mechanical strength, and high voltage instability. [6] Inorganic solid electrolytes widely researched can be categorized into oxide-, sulfide-, and halide-based series. Ceramic oxide SEs based on garnet, [8] perovskite, [9] and NASICON [10] structures generally exhibit high thermal stability, high bulk Li + conductivity (10 −3 -10 −5 S cm −1 at 25°C), and high Young's moduli (>150 GPa). [11] Nevertheless, they are always difficult to be integrated into allsolid-state batteries due to their high mechanical rigidity. Moreover, their high bulk electronic conductivity (10 −8 −10 −7 S cm −1 ) might instead encourage the formation of Li dendrites at the Li/SE interface and growth/penetration of dendrites along the grain boundaries. [12,13] Though sulfide-based electrolytes (e.g., Li 2 S-P 2 S 5 and Li 10 GeP 2 S 12 ) show both high total Li + conductivity and excellent mechanical flexibility, their chemical and electrochemical stability are poor, and they are sensitive to moisture. Thus, they are often accompanied by detrimental hydrolysis reactions and the release of toxic gas H 2 S when exposed to the air. [14,15] In brief, both oxide-and sulfide-based electrolytes are facing thorny interface issues.
Halide-based electrolytes have been developed as ionic conductors for decades. However, they did not attract enough attention because of their relatively low ionic conductivity at room temperature (RT). For instance, the spinel LiAlCl 4 developed in 1976 only displayed an RT ionic conductivity as small as 1.2 × 10 −6 S cm −1 . [16] Until 2018, Asano et al. successfully developed the trigonal Li 3 YCl 6 and monoclinic Li 3 YBr 6 , which exhibited excellent RT ionic conductivities of 5.1 × 10 −4 and 1.7 × 10 −3 S cm −1 , respectively. [17] Since then, the development of halide-based electrolytes has regained much attention. It is worth noting that the metal halide SEs (Li-M-X, where M is a metal element and X is a halogen) are emerging as new candidates with several appealing properties, including a wider electrochemical stability window (up to 6.71 V vs. Li/Li + ), improved scalability, and better chemical stability toward high voltage cathode materials than other SEs. [18] Despite the many potential benefits of metal halide SEs, some significant issues still need to be resolved before commercialization. Among the metal halide SEs that have been extensively explored so far, chlorides and bromides with high ionic conductivity generally exhibit serious moisture sensitivity, poor electrochemical reduction stability, and limited high-voltage compatibility. [19,20] On the contrary, the fluoride-based SEs with the widest electrochemical window and excellent air stability suffer from disappointing RT ionic conductivity. Thus, further research could lead to the identification of fluorides with improved ionic conductivities, which are expected to couple with the cathodes with high voltage of more than 5 V. [21] Moreover, according to some current research results, the grain boundary resistance may not be negligible. In a word, various interfaces (grain boundaries, cathode-electrolyte interfaces, and electrolyte-anode interfaces) that are present in the metal halide SEs and/or the corresponding batteries would hinder their practical application. Thus, future research is required to pursue a more in-depth understanding of interface issues for metal halide SEs. [22] In this review, the structures and ionic conductivities for widely studied metal halide SEs, as well as their preparation methods and electrochemical/chemical stabilities, are summarized, and these SEs are classified into fluorides, chlorides, bromides, and iodides ( Figure 1). Based on typical metal halide solid electrolytes, we then emphasize the interface issues (grain boundaries, cathode-electrolyte and electrolyte-anode interfaces) that exist in the corresponding LMSBs and summarize the related works on understanding and engineering them. Furthermore, the typical characterization tools widely used for solid-state interfaces and some in situ experimental characterizations are reviewed. Finally, a perspective on the future direction for developing highperformance LMSBs based on halide SEs will be put out. We hope that this review can provide guidance not only for the material design of halide-based solid electrolytes but also for their interface construction in corresponding LMSBs with improved electrochemical performance for practical applications.

| History of halide-based solid electrolytes
The history of halides reported as ionic conductors can be dated back to the 1930s, and the reported lithium F I G U R E 1 Overview of research progress on metal halide solid electrolytes and batteries. All the figures cited in Figure 1 are authorized by the publishers. [17,[19][20][21][22] halides LiX (X = F, Cl, Br, I) show a relatively low RT ionic conductivity, which is no more than 10 −7 S cm −1 . [23] Later, the halide LiI was first developed as an electrolyte and applied in battery systems, but it is prone to cause increasing polarization, which limits its further application. [24] After that, Huggins et al. demonstrated that LiAlCl 4 with an RT ionic conductivity of 1.2 × 10 −6 S cm −1 can be applied in battery systems. [16] It is worth noting that the molten LiCl − AlCl 3 can exhibit a higher ionic conductivity of 0.35 S cm −1 at 174°C. [25] In 1992, the rechargeable Li x TiS 2 / LiAlCl 4 / Li 1−x CoO 2 solid-state battery was successfully assembled by Plichta et al., [26] and it exhibited an excellent discharge characteristic at a current density of up to 0.1 mA cm −2 and there is only a slight loss of capacity over 100 charge-discharge cycles when operated at 100°C. This LiAlCl 4 electrolyte was found to be electrochemically stable in a battery voltage range of 0-2.3 V as verified from the potential measurement versus an aluminum reference electrode. Interestingly, LiAlF 4 with an ionic conductivity of 1.0 × 10 −6 S cm −1 at 25°C was reported nearly during the same period. [27,28] The severe hygroscopic property in LiAlCl 4 could be overcome by fluorine substitution. However, this LiAlF 4 is only in the form of amorphous thin films. It is still amorphous even after annealing at 600°C. LiF has been long disregarded as an ionic conductor in view of its insulating property, and its high crystallinity limits the creation of mobile defects and charge carriers. In 2011-2012, Li et al. proposed local and global disordering strategies to gradually improve the ionic conductivity of LiF. [29][30][31][32] The interfaces of LiF-oxides enable the space charge accumulation of Li vacancies as dominant charge carriers, but the conductivity enhancement effect is still limited in these interface zones. If these interface zones are overlapped and penetrated, the ionic conductivity of LiF can reach the record level of 6 × 10 −6 S cm −1 at 50°C. The defect and structure modulations pave the way to develop the high-conducvity halide SEs. Since then, various Li-M-X (M = metal element, X = F, Cl, Br, I) phases have been developed.
According to the level of ionic conductivity of these metal halide SEs, they can be roughly divided into two development stages. The first stage is before 2018, and the halide SEs developed in this stage do not perform well in terms of room-temperature ionic conductivity, but they are important for understanding the Li + diffusion in halides and for enriching the structural prototypes of halides. The metal halides based on divalent metal elements (Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, etc.) with spinel structures and trivalent metal elements (Al, Ga, In, Sc, Y, and La-Lu) had been extensively investigated at this stage. However, except for several halides with Li-In-Br phases, the rest all exhibit an RT ionic conductivity below 10 −5 S cm −1 . Yamada et al. have conducted a systematic study of Li-In-Br phases, and they found that Li 3 InBr 6 synthesized by solid-state reaction at 200°C shows a good Li-ion conductivity, which can reach 4 × 10 −3 S cm −1 at 330 K (57°C). However, this material suffers from a phase transition at 314 K (41°C). [33][34][35] Later, they synthesized Li 3 InBr 3 Cl 3 by substituting Br in Li 3 InBr 6 with Cl, which shows a higher ionic conductivity (1.2 × 10 −4 S cm −1 at 300 K) and a lower phase transition temperature (285 K). [36] Nevertheless, these high-temperature (HT) structures would suffer severe degradation and cause a significant reduction of ionic conductivity when reaching specific temperatures, and these factors make them not suitable for practical application. In addition, water-free conditions, such as a glovebox under a nitrogen atmosphere, are required during the preparation processes of chlorides and bromides due to their strong hygroscopic property. The high requirement for preparation conditions greatly increases the production cost and hinders the practical application of chlorides and bromides. On the contrary, the majority of fluorides exhibit high stability against moisture. Among them, the most studied fluoride-based SE is β-Li 3 AlF 6 , which was first reported as a potential Li-ion conductor by Esaka et al. in 1989. However, the ionic conductivity of β-Li 3 AlF 6 is only 6.0 × 10 −5 S cm −1 at 300°C. Additionally, the Si-or Ti-doped β-Li 3 AlF 6 shows a higher ionic conductivity of 4.0 × 10 −4 S cm −1 at 300°C. [37] Since the inspiring discovery of Li 3 YCl 6 and Li 3 YBr 6 by Asano et al. in 2018, [17] their excellent RT ionic conductivity (~10 −3 S cm −1 ) and electrochemical performance have set off a new wave of research related to halide-based SEs. Since then, a series of excellent performance metal halide SEs have emerged, such as Li 3 MCl 6 (M = In, Er, Sc, Yb, etc.), [38][39][40][41][42][43] Li 2 ZrCl 6 , [44] spinel Li 2 Sc 2/3 Cl 4 , [45] and Li 3 HoBr 6 . [46] In addition, aliovalent and isovalent substitutions have been proven to be effective strategies to further enhance the ionic conductivities of metal halide SEs. These substitutions can directly affect the defect concentration (vacancy or interstitial concentration) and migration barrier of the mobile ions, as well as the electronic conductivity in solid electrolytes. [47] Li 3−x M 1−x Zr x Cl 6 (M = Y, Er, Yb, In), [48][49][50] Li 3−x Yb 1−x M x Cl 6 (M = Hf 4+ , Zr 4+ ), [43] Li 2 In x Sc 0.666−x Cl 4 , [20] Li 3 Y(Br 3 Cl 3 ), [51] Li 3 InCl 6−x F x , [52] Li 3 YBr 6−x F x , [53] and Li 2 Zr x F 6−x Cl x [54] have been developed successively, and all of them show decent RT ionic conductivity (≈10 −4 -10 −3 S cm −1 ). It is essential to mention that Li  3000 cycles with a capacity retention of 80% and a high areal capacity of 4 mAh cm −2 for 500 cycles, which is expected to meet the requirement of practical application. [20] Furthermore, based on the investigation on Cl −substituted Li 2 ZrF 6 , the Li + conductivity of Li 2 ZrF 5 Cl is improved by ∼5 orders of magnitude. [54] We can conclude that isovalent anion substitution could be an effective strategy to enhance the ionic conductivity of fluorides, and it is critical for fluorides to be used as electrolytes for LMSBs. More attention should be given in terms of improving the Li + conductivity of fluoride-based SEs. The immense progress in discovering and exploring halide-based solid electrolytes in the last few decades is outlined in Figure 2.

| Structures of metal halide solid electrolytes
By sorting out the development history of halide-based solid electrolytes in the previous section, it is not difficult to find that the majority of these metal halide SEs with high ionic conductivity are chlorides and bromides. Though fluoride-based SEs exhibit the widest electrochemical stability window, [55,56] which is crucial for the compatibility with the high-voltage cathodes with cut-off voltages higher than 4.8 V versus Li/Li + and also for maintaining the stability with the Li metal anode, they still suffer from relatively lower RT ionic conductivity. Due to the uniqueness of the crystal configuration of fluoride-based SEs, their conduction mechanism and detailed structure-property relationship need further exploration. Thus, we decide to discuss fluoride-based SEs in more detail in a separate subsection, and this section will only provide an overview of the crystal structures and ion transport properties of several other types of halide SEs. According to the law of ionic packing, the structure is only stable when the cation and anion are in close contact. There are two types of halogen anion sublattice structures for Li-M-X ternary halides: cubic close packing (ccp, Figure 3A) and hexagonal close packing (hcp, Figure 3B). Each of them can be further subdivided into two types of crystal structures, that is, the cubic Fd3 ̅ m symmetry ( Figure 3C) and monoclinic C2/m symmetry ( Figure 3D) with ccp anion sublattices, the trigonal P3 ̅ m1 symmetry ( Figure 3E) and orthorhombic Pnma symmetry ( Figure 3F) with hcp anion sublattices. [57] Many research results have shown that the halogen anion sublattice greatly influences the coordination environment and ion diffusion pathway. [47,56,58,59] For example, Wang et al. have demonstrated that the different anionic frameworks of ccp Li 3 YBr 6 ( Figure 3G) and hcp Li 3 YCl 6 ( Figure 3J) result in different Li + diffusion mechanisms and pathways by ab initio molecular dynamics (AIMD) simulations. [56] In the case of ccp anion lattices, Li + ion diffusion occurs through an octahedral-tetrahedral-octahedral (oct-tet-oct) three-dimension (3D) isotropic network, where Li + ions hop between neighboring octahedral sites through a tetrahedral interstitial site ( Figure 3H,I). In the case of hcp anion lattices, Li + ions migrate through an octahedral-octahedral-tetrahedral-octahedral (oct-oct-tetoct) pathway, and the diffusion behavior is anisotropic with fast one-dimension (1D) diffusion channels along the c-axis. Therein, Li ions hop among adjacent face-sharing octahedral sites, and these 1D c-axis channels are connected through tetrahedral interstitial sites in ab-planes with slower diffusivity, leading to an anisotropic 3D diffusion network ( Figure 3K,L).
F I G U R E 2 Historical development of halide-based solid electrolytes with the denotation of ionic conductivities at room temperature if no special notation.  , and all these compounds crystallize in the spinel structure with an inverse distribution of metal ions (i.e., inverse spinel structure). They all possess a framework structure based on a ccp manner of halogen anions and exhibit two types of Li + sites. Taking Li 2 MgCl 4 as the representative, [60] it is constructed by edge-sharing (Mg 1 /Li 2 )Cl 6 octahedra and Li 1 fills the corner-sharing tetrahedral site, that is, Li + is located in both tetrahedral and octahedral sites. The distribution of Li + and M 2+ cations in octahedral sites is disordered. In contrast, in normal spinels, all the Li + locate in octahedral sites, such as low-temperature Li 2 ZnCl 4 . [61] Unfortunately, none of them show the ideal ionic conductivity at room temperature. The highest RT ionic conductivities are obtained from deficient chlorospinels Li 1.6 Fe 1.2 Cl 4 and Li 1.52 Mn 1.24 Cl 4 , and they are 1.3 × 10 −5 and 1.5 × 10 −5 S cm −1 , respectively. [62][63][64] It has been demonstrated that the higher ionic conductivities, when compared to the stoichiometric chlorospinels Li 2 MCl 4 , are originating from the extra vacancies that are induced by M 2+ cations, which are located in the tetrahedral sites within the structure. Obviously, the spinel halides with ionic conductivity of no more than 10 −5 S cm −1 are not competitive to serve as electrolytes in LMSBs. Until 2020, Nazar et al. reported a new superionic conductor Li 2 Sc 2/3 Cl 4 with a disordered spinel structure, which exhibits a decent RT Li + conductivity of 1.5 × 10 −3 S cm −1 as well as a low activation energy barrier of 0.34 eV for Li + ion diffusion. [45] The framework of Li 2 Sc 2/3 Cl 4 is somewhat similar to that of Li 2 MgCl 4 , and the significant difference is that the distribution of Li + in the lattices of Li 2 Sc 2/3 Cl 4 shows a higher disorder. It exhibits four types of Li + sites, Li 1 , Li 2 , Li 3 (occupying  [57] copyright 2020, the Royal Society of Chemistry. (D) Monoclinic crystal structure (C2/m) with a ccp anion arrangement. (E) Trigonal crystal structure (P3 ̅ m1) with an hcp anion arrangement. (F) Orthorhombic crystal structure (Pnma) with an hcp anion arrangement. Reproduced with permission, [70] copyright 2020, the American Chemical Society. Crystal structures of (G) Li 3 YBr 6 with ccplike anion lattices and (J) Li 3 YCl 6 with hcp-like anion lattices, superimposed with Li + probability density marked by yellow isosurfaces from AIMD simulation. (H) and (I) Schemes of Li-ion migration pathways in Li 3 YBr 6 . (K) and (L) Schemes of Li-ion migration pathways in Li 3 YCl 6 . Reproduced with permission, [56] copyright 2019, Wiley. the face-sharing octahedral and tetrahedral sites), and Li 4 (in the edge-sharing (Sc 1 /Li 4 )Cl 6 octahedra). Recently, the authors adopted the isovalent M-site substitution strategy and successfully prepared a mixed-metal chlorospinel Li 2 In 1/3 Sc 1/3 Cl 4 , [20] which exhibits a higher RT ionic conductivity of 2.0 × 10 −3 S cm −1 and a slightly lower activation energy of 0.33 eV when compared to Li 2 Sc 2/3 Cl 4 . The powder neutron diffraction result demonstrates that the structure of Li 2 In 1/3 Sc 1/3 Cl 4 is similar to that of Li 2 Sc 2/3 Cl 4 , and it also exhibits four types of Li + sites, containing a rigid framework formed by edge-sharing (Sc 1 / In 1 /Li 4 )Cl 6 octahedral. The other Li 1 , Li 2 , and Li 3 sites are spread over the other available octahedral and tetrahedral. There is a 3D Li + ion diffusion pathway formed by facesharing Li 2 octahedra and Li 1 or Li 3 tetrahedra with low occupancies (~0.2-0.3). [20] As for bromides, unlike spinel chlorides with the formula Li 2 MCl 4 , there is only the defect spinel HT-LiInBr 4 that has been reported so far, and the superionic phase of LiInBr 4 was found above ca 315 K, with In 3+ occupying half of the octahedral 16D sites randomly. [35] In summary, the general trend of ionic conductivity of spinel-type halides is σ deficient inverse (or σ disordered inverse ) > σ inverse > σ normal . Within the spinel structure, the Li + conducting paths are connected via the face-sharing octahedra and tetrahedra. The trend of ionic conductivity variation indicates that the Li + on the tetrahedral sites plays a predominant role in the achievement of high ionic conductivity of those halides with inverse spinel structure, while the normal spinel without Li + located in tetrahedral sites would suffer from a serious coulombic repulsion by M cations located in tetrahedral sites during the migration of Li + ions. [17] 2.2.2 | Metal halide SEs with monoclinic C2/m symmetry At present, limited works on the crystal compounds of Li-M-Cl with a monoclinic crystal structure have been reported, mainly containing Li 3 InCl 6 (up to 2.04 × 10 −3 S cm −1 ), [38,39] Li 3 ScCl 6 (3.0 × 10 −3 S cm −1 ), [42] and β-Li 2 ZrCl 6 (3.2 × 10 −5 S cm −1 ). [65] Furthermore, all known Li 3 MBr 6 (M = In, Y, Sm-Lu) bromides display the monoclinic structure with ccp anion sublattices. [17,46,66,67] For iodides, several Li 3 MI 6 compounds with monoclinic structure (space group of C2/c) have been predicted to show fast Li + migration, such as Li 3 ScI 6 (~10 −5 S cm −1 ), Li 3 YI 6 (~10 −4 S cm −1 ), and Li 3 LaI 6 (~10 −3 S cm −1 ). [58] However, only Li 3 ErI 6 with an ionic conductivity of 6.5 × 10 −4 S cm −1 was successfully synthesized by Schlem et al. in 2020. [68] Liang et al. have summarized the relationship between the stacking structure of the Li-M-X ionic crystal and its ionic radius.
They concluded that the crystal structures of metal halides are determined by the radius ratio of cation to anion t M+/X− . [18,69] When the t M+/X− value ranges from 0.414 to 0.599, the halide SEs always tend to crystallize with ccp monoclinic structure, which may explain why all known Li 3 MBr 6 bromides (M = In, Y, Sm-Lu) and Li 3 ErI 6 display the monoclinic structure. These monoclinic phase electrolytes with ccp anion sublattices generally exhibit higher ionic conductivity among metal halide SEs, which is consistent with the results of theoretical calculations. [56,70,71] Park et al. have systematically investigated the Li + ion transport mechanism of 17 Li 3 MCl 6 (M = Al, Bi, Dy, Er, Ga, Ho, In, La, Lu, Nd, Sb, Sc, Sm, Tb, Tl, Tm, and Y) by bond valence site energy (BVSE) and AIMD calculations. [71] The results disclose that the high ionic conductivity of Li 3 MCl 6 (M = In, Sc, etc.) originates from the lower migration energy barrier in the monoclinic structure, whereas the orthorhombic and trigonal structures exhibit higher energy barriers due to the sluggish diffusion along the twodimensional (2D) paths. Furthermore, the successful application of the substitution strategy has already allowed the further improvement of the ionic conductivity of monoclinic Li 3 MX 6 . The aliovalent M-site substitution, such as Zr-substituted Li 3−x In 1−x Zr x Cl 6 , improves the ionic conductivity by increasing Li-ion vacancy concentration and enlarging interplanar spacing. [72] Recently, Liu et al. proposed the isovalent X-site-substituted Li 3 Y(Br 3 Cl 3 ), which shows an excellent ionic conductivity of 7.2 × 10 −3 S cm −1 . [51] To the best of our knowledge, it is the highest ionic conductivity record for metal halide SEs. As we have mentioned above, Li + ion diffusion occurs through an oct-tet-oct pathway in the monoclinic structure, in which Li + ions hop between neighboring octahedral sites through a tetrahedral interstitial site. The high ionic conductivity of Li 3 Y(Br 3 Cl 3 ) exactly benefits from the high population of Li at the tetrahedral sites and the consequent weakening of the cation-blocking effect. [51] In addition, a drastically enhanced grain boundary contact resulting from hot-pressing is another significant contributor to the high ionic conductivity of Li 3 Y(Br 3 Cl 3 ). Li 3 MCl 6 (M = Y, Tb-Tm), [17,40,41,49] and α-Li 2 ZrCl 6 [65] have been demonstrated to be the trigonal phases with the P3 ̅ m1 space group structure. Disappointingly, all of them exhibit relatively low RT ionic conductivity, which is no more than 10 −3 S cm −1 . The highest ionic conductivity among them comes from α-Li 2 ZrCl 6 , and it is 8.08 × 10 −4 S cm −1 at room temperature. We can obtain a reasonable explanation for conductivity discrepancy by analyzing the difference in their crystal structures. The most studied metal halide SE is Li 3 YCl 6 , and the Y 3+ cations, Li + ions, and vacancies occupy the sixcoordinated octahedral sites that are surrounded by halogen anions. Li + ion diffusion is anisotropic in the trigonal phase with the hcp anion sublattices. Li + ions hop among the nearby face-sharing octahedral sites along the fast c-axis 1D diffusion channels, and an anisotropic 3D diffusion network is formed by connecting these 1D c-axis channels with the tetrahedral interstitial sites through ab-planes of slower diffusivity. [56] Similar to LiFePO 4 , the 1D diffusion channel in Li 3 YCl 6 is anticipated to be vulnerable to channelblocking defects, such as antisite defects, impurities, different microstructures, and grain boundaries, which could account for why the calculation value of RT ionic conductivity (1.4 × 10 −2 S cm −1 ) of Li 3 YCl 6 is one to two orders of magnitude higher than the experimentally reported value (5.1 × 10 −4 S cm −1 ). Moreover, Wang et al. found that the mechanochemically synthesized Li 2 ZrCl 6 (as-milled LZC) exhibits a drastically higher ionic conductivity than that annealed at elevated temperature. [65] To further identify the origination of the high ionic conductivity of α-Li 2 ZrCl 6 , they intentionally annealed the as-milled LZC at 215°C for 5 h for comparison. Such a heat treatment enables the preservation of the crystal structure but substantially suppresses the metastable nonperiodic characteristics as shown by the sharper and stronger X-ray diffraction (XRD) peaks. Therefore, the enrichment of nonperiodic domains induced by intense ball milling should be responsible for the high ionic conductivity of α-Li 2 ZrCl 6 .

| Metal halide SEs with orthorhombic Pnma symmetry
The orthorhombic metal halide SEs with the Pnma space group structure also generally show relatively lower ionic conductivities than those with monoclinic structures, but approximately one order of magnitude higher than that of the trigonal phase. [22,69] Combined with the aforementioned structure characteristics with hcp anion sublattices, the ion transport in the c-axis is most likely the rate-determining step in the anisotropic ion transport network. Thus, facilitating the ion transport along the c-axis is an effective ideology to improve the ionic conductivity of metal halide SEs. Taking the Li-Ho-Cl components as an example, both the orthorhombic and trigonal Li-Ho-Cl structures have the octahedral of six Cl − with central Ho 3+ , Li + , or vacancy sites. The only difference between them is the arrangement of cations (including Li + and In 3+ ). The Ho 3+ ions fully occupy the 4c sites in the orthorhombic Pnma structure, while there are three different Ho 3+ sites (one is fully occupied and the other two are partially occupied) in the trigonal structure, as shown in Figure 4A. [22] The different distributions of Ho 3+ sites yield different Li and vacancy sites and therefore different diffusion pathways. The results of ab initio simulations show that the diffusion of Li + ions in the c-axis of orthorhombic Li-Ho-Cl is more facile than that in the trigonal structure, resulting in higher diffusivity and lower activation energy for the former ( Figure 4B,C). [22] Therefore, the phase transition from trigonal to orthorhombic one is always accompanied by an increase in ionic conductivity. Moreover, the ionic conductivity can be further enhanced by aliovalent substitution. Park et al. have investigated the metal ion Zr 4+ (r = 72 pm) as a substituent for either Er 3+ (r = 89 pm) or Y 3+ (r = 90 pm), as shown in Figure 4D. A corresponding structural transition occurs from a trigonal (P3 ̅ m1, Phase I) structure, through a previously known orthorhombic structure (Pnma, Phase II), [69] to a new orthorhombic structure (Pnma, Phase III). [71] The XRD patterns of Li 3−x Er 1−x Zr x Cl 6 (0 ≤ x ≤ 0.6) indicate that the trigonal Li 3 ErCl 6 (x = 0) transforms into the orthorhombic structure (Phase II), which is isostructural to Li 3 LuCl 6 when the ratio of substituted-Zr 4+ reaches to 0.2. [69] Interestingly, the average radius of transitionmetal ions in Li 2.8 Zr 0.2 Er 0.8 Cl 6 (r = 85.6 pm) is comparable to that of Lu 3+ (r = 86.1 pm). As more Er 3+ is substituted by Zr 4+ (x = 0.367-0.6), a new orthorhombic structure (Phase III) is observed ( Figure 4E). Moreover, in terms of the ionic conductivities and activation energies of Li 3−x Er 1−x Zr x Cl 6 as shown in Figure 4G, the pure Phase III exhibits the highest ionic conductivity of 1.1 × 10 −3 S cm −1 (x = 0.367). Similar structural evolution is also observed in Li 3−x Y 1−x Zr x Cl 6 (0 ≤ x ≤ 0.6), and the authors concluded that the average size of transitionmetal ions is most likely a key factor in determining the crystal structure of these materials ( Figure 4F,H). [43,48] In summary, these research results reflect that the factors affecting ionic conductivity are extremely complex, including anion sublattice framework, cationblocking effect, metal ion distribution, vacancy concentration, local structural distortions, and so on. More systematic investigations are required to reveal the ion transport kinetics of metal halide SEs.

| Crystal structures of fluorinebased electrolytes
Among the metal halides SEs (fluoride, chloride, bromide, and iodide), fluorides exhibit the widest electrochemical stability window as well as the higher oxidative stability due to the highest electronegativity, largest formation potential (+2.87 V vs. SHE), and smaller ionic radius of fluorine. [28,55] At the same time, the low hydrophilicity of fluorides also allows them to exhibit better air stability than other halides. Currently, the main problems facing fluoride electrolytes are the low ionic conductivity at room temperature and the lack of mature synthetic solutions. Thus, the research on fluorine-based electrolytes is still in its infancy, and more attention must be paid to developing novel fluoride electrolytes with higher ionic conductivity. It is well known that the ionic conductivity of Li-M-X is highly dependent on the crystal structure, and the rational design of new fast ionic conductors is not possible without a comprehensive understanding of crystal structures and ion transport mechanisms. In this section, we summarize the theoretical and experimental insights on the crystal structures of fluorine-based electrolytes gained in recent years.
According to current research, there is a strong correlation between the arrangement of Li-ion sublattices and the energy barrier of Li-ion migration. Low migration barriers are observed in the compounds with interstitial Li-ions or strong Li-Li interactions, indicating that the fast ionic diffusion in superionic conductors does not occur through isolated ion hopping (as previously thought to be typical in solids), but instead proceeds through concerted migrations of multiple ions with low-energy barriers ( Figure 5A). [73,74] Zhang et al. conducted the highthroughput screening for fluorides by Li-ion radial distribution functions (RDFs), where the nearest neighbor Li-Li distance (nR Li-Li ) for these compounds is around 2.7 Å (highlighted with an arrow in Figure 5B), which is within the optimal range of nR Li-Li . They elucidated that the small Li-Li distance and disordered LiF x structural units in the fluoride electrolytes are the origins of their fast ionic diffusion. [75] In addition, the authors noted that Li x MF 6 (M is the metal cation) may be a new class of superionic conductors ( Figure 5B), as the predicted nR Li-Li of Li x MF 6 is on par with state-of-the-art Li-ion conducting electrolytes, such as Li 10 GeP 2 S 12 (LGPS), [15] NASICON-type Li 1.3 Al 0.3 -Ti 1.7 (PO 4 ) 3 (LATP), [76] and cubic Li 7 La 3 Zr 2 O 12 (c-LLZO). [77] There are three distinct crystal structures featuring the fast ionic diffusion channels for Li-ions in Li x MF 6 , Reproduced with permission, [22] copyright 2022, Wiley-VCH GmbH. (D) Phase evolution of Li 3−x M 1−x Zr x Cl 6 (M = Er, Y) with different Zr substitution amounts. XRD patterns of (E) Li 3−x Er 1−x Zr x Cl 6 and (F) Li 3−x Y 1−x Zr x Cl 6 (0 ≤ x ≤ 0.6). Ionic conductivities and corresponding activation energies of (G) Li 3−x Er 1−x Zr x Cl 6 and (H) Li 3−x Y 1−x Zr x Cl 6 . Reproduced with permission, [48] copyright 2020, American Chemical Society. SE, solid-state electrolyte. that is, the monoclinic structure of Li 3 GaF 6 and β-Li 3 AlF 6 (space group of C12/c1), the monoclinic structure of Li 2 ZrF 6 (space group of C2/c), and the trigonal structure of Li 3 ScF 6 (space group of P3 ̅ c1). β-Li 3 AlF 6 and Li 3 GaF 6 are both open-frame ionic conductors, and their crystal structures consist of an immobile framework formed by isolated GaF 6 (or AlF 6 ) octahedra that neither share edges nor corners. Therefore, a potential 3D diffusion channel is formed by interconnected LiF x polyhedrons. There are five different forms of Li 3 AlF 6 (α, β, γ, δ, and ε) that can be synthesized by adopting various synthesis conditions. [78] As early as 1989, Esaka et al. had already synthesized the orthorhombic β-Li 3 AlF 6 by the wet chemical method; however, the as-synthesized β-Li 3 AlF 6 only shows a disappointing ionic conductivity of 1.0 × 10 −6 S cm −1 at 200°C. [37] Later, Miyazaki et al. successfully synthesized the monoclinic β-Li 3 AlF 6 through mechanical milling, which shows a 1000-fold increase in conductivity compared with the orthorhombic β-Li 3 AlF 6 . [79] Nevertheless, it still does not exhibit the same superior ionic conductivity as predicted by theory. Up until 2018, Li et al. prepared the monoclinic β-Li 3 AlF 6 through a lowtemperature ionic liquid fluorination method, and the nanostructured Li 3 AlF 6 exhibits a decent RT ionic conductivity of 2.04 × 10 −5 S cm −1 ( Figure 5D). [80] Moreover, the Li 3 GaF 6 was successfully synthesized by a similar method, exhibiting a higher RT ionic conductivity close to 10 −4 S cm −1 (i.e., 8.8 × 10 −5 S cm −1 , Figure 5C). [21] The improved performance is due to the nanostructured effect and grain boundary modification caused by the ionic liquid-based dissolution-precipitation method. Based on the above results, we can easily conclude that apart from the bulk ion-channel modulation, the conductivity property of solid electrolytes is also closely associated with the synthetic method. Furthermore, for open-frame ionic conductors, such as β-Li 3 AlF 6 and Li 3 GaF 6 , interfacial modulation may be an effective strategy to further improve their electrical conductivity. We will discuss the interface issues in more detail in Section 6.
For the monoclinic Li 2 ZrF 6 with the C2/c space group structure, the crystal configuration is a rigid structural network formed by edge-sharing ZrF 8 polyhedrons, while the edge-sharing LiF 6 octahedrons are sandwiched between the layers of ZrF 8 polyhedrons ( Figure 5E). So, Li ions in the layered Li 2 ZrF 6 can diffuse from one octahedral site to the other along 2D channels in the F I G U R E 5 Computation and experimental studies for fast ion conduction in metal fluorides. (A) Illustration for lowered ion migration barriers through a multi-ion concerted migration mechanism. Empty circles represent the vacant sites for ion transport, while red circles stand for the mobile ion(s) during single-or multi-ion migration. Reproduced with permission, [74] copyright 2017, Springer Nature. (B) Li-Li RDFs for Li-ion sublattices of Li x MF 6 . Reproduced with permission, [75] copyright 2021, Elsevier. (C) Crystal structure of β-Li 3 GaF 6 viewed along [111] , and the corresponding impedance spectrum as SE measured at room temperature. Reproduced with permission, [21] copyright 2020, Elsevier. (D) Crystal structure of β-Li 3 AlF 6 with discrete Al-F polyhedral, and the corresponding impedance spectrum as SE measured at room temperature. Reproduced with permission, [80] copyright 2018, American Chemical Society. (E) Crystal structure of trigonal Li 2 ZrF 6 , and corresponding Nyquist plots from RT to 60°C. Reproduced with permission, [54] copyright 2022, American Chemical Society. bc-plane. To identify the phase stability of these fluoride compounds, the grand canonical linear programming method (GCLP) was used to explore the phase diagram of Li-M-F compounds. [81] The order of decomposition energy is Li 3 GaF 6 (52 meV/atom) > Li 3 AlF 6 (38 meV/ atom) > Li 2 ZrF 6 (13 meV/atom) > Li 3 ScF 6 (1 meV/atom). By comparison, Li 3 InCl 6 with potential air stability has a decomposition energy of 14 meV/atom, [38] which is comparable to that of Li 2 ZrF 6 . Thus, the authors inferred that Li 2 ZrF 6 is thermodynamically stable at room temperature. However, the results of AIMD simulations with a canonical ensemble at 900 K show that the framework of Li 2 ZrF 6 is unstable, and the migration of Zr 4+ eventually results in phase structure degradation. [75] On the other hand, the trigonal Li 2 ZrF 6 (space group P3 ̅ 1m) with hexagonal close-packed anion arrangement was synthesized by high-energy mechanical milling and subsequent annealing. [54] The Cl-substituted Li 2 ZrF 6−x Cl x has achieved a great improvement in ionic conductivity when compared to the pure trigonal Li 2 ZrF 6 . For example, the Li + conductivity for Li 2 ZrF 5 Cl is improved by ∼5 orders of magnitude. Moreover, Li 2 ZrF 5 Cl exhibits excellent Li plating/stripping stability over 180 h under 38 μA cm −2 , demonstrating the significant advantage of fluorides in constructing a stable interface with Li metal anodes. [54] The trigonal Li 3 ScF 6 was predicted to enable the fast Li + migration, and it consists of a framework of isolated ScF 6 octahedral units that are neither edge-sharing nor corner-sharing. [75] Therein, Li ions are placed in twisted octahedra that are face-sharing connected and alternate along the conduction channels to form a 3D Li + diffusion network throughout the crystal structure. In terms of the experiment, the trigonal Li 3 ScF 6 with a space group of P3 ̅ c1 could be prepared by reacting the binary components of LiF and ScF 3 at 820°C for 16 h in an argon atmosphere. [82] However, the property of ionic conduction of Li 3 ScF 6 has not yet been reported. The lack of structural recognition and corresponding synthesis strategy could be crucial reasons.

| SYNTHESIS APPROACHES FOR METAL HALIDE SES
Several methods have been used for the synthesis of halide electrolytes. The most common methods are mechanical ball-milling, solid-state sintering, and liquid-phase synthesis. In addition, the vapor deposition method was also developed for the synthesis of fluoridebased SEs in the form of thin films, such as thermal evaporation and atomic layer deposition. It is worth noting that the conventional solid-state reaction methods, such as annealing and mechanical milling, appear to fail to push fluoride electrolytes into the competitive candidate for LMSBs. [21,28] Obviously, the lack of structure recognition and the corresponding synthesis strategy is one of the most important reasons why fluoride electrolytes are significantly less reported than chlorides and bromides, though fluorides always exhibit a wider electrochemical stability window as well as the higher air stability. [56] 3.1 | RT mechanical ball-milling method Similar to the synthesis of oxide and sulfide electrolytes, mechanical ball-milling has been commonly used to prepare halide solid electrolytes due to its relatively low cost and simple operation. But there is something special about the synthesis of halide electrolytes, that is, most of the procedures have to be performed in a dry Ar-filled glovebox due to the severe hygroscopic properties of chlorides and bromides. Mechanical ball-milling always is performed at room temperature; however, heat is still generated during the high-energy ball-milling process, which would promote a chemical reaction between the halide starting materials rather than simply mixing them evenly. As a result, the structure of halide-based SEs is always different from its starting materials. Mechanical ball milling commonly tends to generate metastable phases, which may be amorphous and/or nanocrystalline. Therefore, mechanical ball milling shows some irreplaceable advantages compared with other synthetic methods. More and more halide-based SEs with a high RT conductivity have been developed by the mechanical ball-milling method. Schlem et al. prepared the highly crystalline Li 3 ErCl 6 by conventional solid-state synthesis at high temperatures; however, its ionic conductivity is only 1.71 × 10 −5 S cm −1 , which is much lower than that obtained by mechanical ball milling (3.15 × 10 −4 S cm −1 at RT). [41] The much higher ionic conductivity mainly originates from the distorted structure (cation site disorder) and more defects induced. The single-crystal diffraction data show that the structures of Li 3 MCl 6 (M = Y, Er; space group P3 ̅ m1) can be described as MCl 6 3− octahedra forming the trigonal lattices, where the Wyckoff 1a position is occupied by M 1 , and in the highly ordered structure, the Wyckoff 2d positions located in the (002) plane are fully occupied by M 2 . [41] However, there seems to exist a site disorder, in which some previous M 2 cations are located in another M 3 position. This new M 3 site can be regarded as the M 2 -equivalent position in the (001) plane. By monitoring the structural differences via Rietveld refinement of XRD data and exploring the structural differences via distribution function analysis, the authors concluded that a large degree of disordering occurs during the mechanochemical synthesis, that is, an almost complete site inversion from the M 2 to M 3 site. This in turn affects the ionic transport, significantly enhancing the transport properties by expanding the bottlenecks for Li diffusion and by a possible reordering of Li sublattices (corresponding to the M 2 -M 3 disorder). Note that the M 2 -M 3 site disordering would weaken along with the increase of crystallinity, which might account for the lowered ionic conductivity of Li 3 ErCl 6 synthesized by a HT solid-state reaction. The M 2 -M 3 site disordering was also observed in the hcp trigonal Li 2 ZrCl 6 and its Na + analogue Na 2 ZrCl 6 prepared by mechanical ball milling. [44,83] In addition, the posttreatment of products after ball milling was proven to be an effective strategy to further increase their ionic conductivity, such as annealing and hot-pressing. For example, the RT ionic conductivity of monoclinic Li 3 InCl 6 synthesized by mechanical ball milling increases from 8.37 × 10 −4 to 1.49 × 10 −3 S cm −1 after annealing, [38] and the RT ionic conductivity of ball-milled Li 3 YBr 3 Cl 3 increases from 1.6 × 10 −3 to 7.2 × 10 −3 S cm −1 after hotpressing at 170°C. [51] The improved conductivity originates from the enhanced grain boundary contact and the lower grain boundary resistance after the hot-pressing process. Nevertheless, it is necessary to consider the effect of the milling period, and its importance has been proved in the synthesis of sulfide electrolytes. Tatsumisago's group compared the effect of the milling period on the ionic conductivity of the Li 2 S-SiS 2 system. [84] As the milling time increases, the grains drastically decrease and homogeneous fine particles are obtained. The corresponding ionic conductivity can range from 10 −8 S cm −1 to that higher than 10 −4 S cm −1 .

| HT solid-state reaction method
During the early days of halide-based SE research, a series of thermodynamically stable halide-based SEs have been developed by HT solid-state reactions. The precursors are usually demanded to be sealed in an ampoule tube, as the majority of them are hygroscopic and unstable in moist conditions. As early as 1997, Bohnsack et al. successfully developed the trigonal Li 3 LnCl 6 (Ln = Tb, Dy, Ho, Er, Tm), orthorhombic Li 3 YbCl 6 , and monoclinic Li 3 ScCl 6 by the reaction of LiCl and MCl 3 (M = Tb-Yb, Sc) at 650°C. [69] Similarly, Li 3 MBr 6 (M = Sm-Lu, Y) was obtained by heating the corresponding binary bromides at 400°C for 2 weeks. [66] According to electrochemical impedance and 7 Li-nuclear magnetic resonance (NMR) measurements, the Li-ions exhibit high mobility. It is not hard to discover that HT solidstate reaction usually needs a relatively long period, due to the sluggish reaction kinetics between solid compounds. Uniformly mixing the starting materials is proven to be an effective strategy to decrease the reaction period drastically, for example, by introducing a ballmilling step before a HT solid-state reaction. For instance, the time of the solid-state reaction can be decreased to 1 h for preparing Li 3 YCl 6 and Li 3 ErCl 6 . Furthermore, it is worth noting that many binary halide mixtures show relatively low eutectic temperatures, such as the eutectic temperatures of LiCl-YCl 3 and LiCl-ErCl 3 are 679.92 and 752.07 K, respectively. [85,86]

| Liquid-phase synthesis method
Compared to solid-phase synthesis, liquid-phase synthesis has the advantages of the morphology and size control of solid electrolytes as well as easy scale-up. Currently, the liquid-phase synthesis methods used for preparing halide-based electrolytes can be mainly divided into three categories, that is, water-mediated synthesis, ammonium-assisted wet chemistry, and low-temperature ionic liquid fluorination method. In 2019, the Li 3 InCl 6 electrolyte was synthesized from a H 2 O-mediated route ( Figure 6A). [39] LiCl and InCl 3 precursors with a molar ratio of 3:1 were dissolved in deionized water, and then, the highly crystalline Li 3  intermediates ( Figure 6B). [87] The authors successfully synthesized Li 3 YCl 6 , Li 3 ScCl 6 , Li 3 YBr 6 , and Li 3 ErCl 6 with the RT ionic conductivities of 0.345, 1.25, 1.09, and 0.407 mS cm −1 , respectively. It is worth noting that the Li 3 YCl 6 shows a higher ionic conductivity than that synthesized by the comelting method (0.058 mS cm −1 ). [69] The microstrain and crystallite size of Li 3 YCl 6 were calculated by the Williamson-Hall method. The results show that the crystallize size of Li 3 YCl 6 synthesized by the ammonium-assisted wet chemistry method is only 159.11 nm, which is significantly smaller than the micrometer size of Li 3 YCl 6 synthesized by the mechanochemical or comelting method. The small size effect would induce the localized microstrain in the material because of the localized disturbance in lattices or defect formation, and it is found to be beneficial for Li + transport and conductivity. As mentioned above, traditional synthesis methods that are widely used to prepare the chloride, bromide, and iodide-based SEs appear to fail to push fluoride electrolytes into the competitive candidate for LMSBs. Therefore, developing novel synthesis methods is an urgent task for fluoride-based electrolytes to achieve high ionic conductivity as well as excellent electrochemical performance. A low-temperature ionic liquid fluorination method used to synthesize nanostructured Li-rich fluoride solid electrolytes has been proposed by our group. In 2018, we used Li 2 CO 3 as the Li source, Al(NO 3 ) 3 ·9H 2 O as the Al precursor, and the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF 4 ) as a solvent to synthesize the β-phase Li 3 AlF 6 ( Figure 6C). [80] The as-synthesized β-Li 3 AlF 6 exhibits an RT ionic conductivity of 2.04 × 10 −5 S cm −1 , which is much higher than that of β-Li 3 AlF 6 obtained from the HT solidstate reaction of LiF and AlF 3 (6 × 10 −5 S cm −1 at 300°C). [37] Subsequently, the nanostructured Li 3 GaF 6 SE was also synthesized in a similar way, and it shows the highest record of ionic conductivity (8.8 × 10 −5 S cm −1 at RT) in fluoride-based electrolytes that have been reported so far. [21] Furthermore, a solid-state Li/Li 3 GaF 6 /LiFePO 4 cell was successfully driven for at least 150 cycles at 1 C ( Figure 6D). The performance upgrade benefits from its nanostructured effect and solidified ionic liquid decoration in grain boundaries. In our work, we have demonstrated that grain F I G U R E 6 Advanced synthesis methods of metal halide SEs. (A) Illustration of the water-mediated synthesis method for preparing Li 3 InCl 6 and reversible conversion between hydrated Li 3 InCl 6 ·xH 2 O and dehydrated Li 3 InCl 6 . Reproduced with permission, [39] copyright 2019, Wiley. (B) Illustration of the ammonium-assisted synthesis method for halide SEs. Reproduced with permission, [123] copyright 2022, American Chemical Society. (C) Illustration of Li 3 AlF 6 synthesis from the BmimBF 4 -based fluorination method. Reproduced with permission, [80] copyright 2018, American Chemical Society. (D) Scanning electron microscopy image, RT ionic conductivity, crystal structure of Li 3 GaF 6 , and solid-state battery configuration and its electrochemical performance based on Li 3 GaF 6 SE. The cycling performance of solid-state Li/LiFePO 4 cells based on Li 3 GaF 6 is displayed at 0.5 and 1 C at 60°C. Reproduced with permission, [21] copyright 2020, Elsevier. RT, room temperature; SE, Solid-state electrolytes. boundary modification is an effective strategy to greatly improve the ionic conductivity of open framework fluorinated electrolytes.

| CHEMICAL STABILITY WITH HUMID AIR
Other than high ionic conductivity, the chemical stability of solid electrolytes is another important factor to influence the electrochemical performance of LMSBs. The humid air tolerance of halide-based SEs has recently gained significant interest. Halide-based SEs have been proven to exhibit excellent oxidative stability, while sulfide SEs are always observed to have severe decomposition with the release of toxic gas H 2 S. Asano et al. evaluated the stabilities of Li 3 YCl 6 , Li 3 YBr 6 , and glass-ceramic Li 2 S-P 2 S 5 by differential scanning calorimetry (DSC) measurements in both the inert and oxygen atmospheres. [17] Neither exothermic nor endothermic peak is observed from the DSC results of Li 3 YCl 6 and Li 3 YBr 6 , indicating no thermal decomposition, phase transition, or oxidation reaction occurring within the measured temperature range, and it is a clear indication of high thermal and oxidation stabilities of Li 3 YCl 6 and Li 3 YBr 6 . On the contrary, the DSC result of Li 2 S-P 2 S 5 in an O 2 atmosphere shows the obvious exothermic peaks that are not observed in an N 2 atmosphere, indicating the occurrence of oxidation reactions for Li 2 S-P 2 S 5 ( Figure 7A). Halide-based SEs exhibit better chemical oxidation stability compared to sulfide SEs. The intrinsic chemical features of halogen anions compared to the chalcogen counterparts could be the reason. Halide anions have higher electrochemical redox potentials than those of oxide and sulfide anions; for example, fluorine has a much higher formation potential (+2.87 V vs. SHE) even compared to the oxygen evolution potential (+1.23 V vs. SHE), which is the origin of electrochemical stability in fluoride-based electrolytes ( Figure 7B). [88] Unfortunately, most halide-based SEs suffer from irreversible chemical degradation when exposed to a humid atmosphere. As a result, the Li ionic conductivity sharply decreases. Until now, only Li 3 InCl 6 has demonstrated reversible ionic conductivity after being exposed to humidity and then undergoing a reheating process. Sun et al. have explored the origin of moisture stability of halide-based SEs, and they tracked the degradation process of Li 3 InCl 6 exposed to air by in situ and operando synchrotron X-ray analytical techniques. [70,89] The result shows that a part of Li 3 InCl 6 decomposes to nonionically conductive In 2 O 3 and LiCl along with the formation of HCl, while the majority evolves into the corresponding hydrate, Li 3 InCl 6 ·xH 2 O. Compared to other halide-based SEs, Li 3 InCl 6 possesses higher resistance against humidity as a consequence of conversion to the hydrated intermediates (Li 3 InCl 6 ·xH 2 O), which have much better resistance to the detrimental hydrolysis reaction, whereas other halide-based SEs are decomposed to form MCl 3 ·xH 2 O and LiCl·xH 2 O. And MCl 3 ·xH 2 O is further converted into M 2 O 3 and HCl during the heating dehydration process. This is consistent with the results of thermodynamic analysis based on a first-principles computation database. Zhu et al. systematically investigated the hydrolysis and reduction reactions in Licontaining sulfides and chlorides ( Figure 7C). [90] Their theoretical analysis indicates that the In 3+ cation exhibits the best humidity stability. Most recently, Ma et al. reported a humidity-tolerant chloride solid electrolyte, Li 2 ZrCl 6 , which demonstrates a humidity tolerance with no sign of moisture uptake or conductivity degradation after exposure to an atmosphere with 5% relative humidity. [65] This electrolyte is also cost-effective in view of the use of more abundant and cheaper elements such as zirconium by replacing resource-limited and costly metals such as indium or rare earth. On the contrary, the humidity tolerance of Li 3 InCl 6 originates from its recoverability after absorbing moisture.
It should be noted that the degradation products of halide-based SEs are highly corrosive and can erode the Al current collector. Therefore, the moisture sensitivity of halide-based SEs is one of the most important issues that have to be conquered. Based on the primary understanding and analysis, the most recent proposed strategies to reduce the moisture sensitivity of halide electrolytes are summarized. In addition to developing intrinsically air-stable halide-based SEs, like Li 2 ZrCl 6 mentioned above, structural modulation has been also proven to be an effective strategy. For instance, Sun et al. reported that the humidity tolerance is highly improved when introducing In 3+ into the Li 3 Y 1x In x Cl 6 SEs, and it can be mainly ascribed to the gradual structural conversion from the hcp to ccp anion arrangement ( Figure 7D). [70] Moreover, surface coating has also been proposed for tackling this issue. To improve the air stability of Li 3 InCl 6 , Li et al. proposed to protect its surface by coating an Al 2 O 3 layer via powder atomic layer deposition (named Li 3 InCl 6 @Al 2 O 3 ). The result shows that the water absorption rate in Li 3 InCl 6 @Al 2 O 3 in air reduces to 1/4 for initial Li 3 InCl 6 , and the corresponding liquefaction time in air increases by seven times ( Figure 7E). [91] Since most starting materials for metal fluorides are more hydrophobic than those for chlorides, the corresponding metal fluoride SEs have not been reported to have significant hygroscopicity. For example, opposite to LiCl and AlCl 3 , LiF and AlF 3 are nonhygroscopic, thus LiAlF 4 is considered to be a substitution of the severe hygroscopic LiAlCl 4 . [27] Furthermore, the formation of a fluoride-based hydrophobic SEI on a metallic Li anode has been proposed to maintain the stability of the Li-metal anode in air. [92] Along with the strategies mentioned above, more efforts are needed to improve the air stability of halidebased SEs. Developing hydrophobic metal fluoride SEs appears to be one of the most preferred choices.

| INTERFACES IN HALIDE-BASED LMSBS
Compared to the liquid electrolyte, the solid electrolyte not only undertakes the task of ion conduction but also plays the role of a separator. The solid electrolyte layer is sandwiched between the compressed powder cathode layer and Li metal anode to fabricate the bulk-type LMSBs. Li ions diffuse from electrodes (positive and negative electrodes) to solid electrolytes by migration across the corresponding interfaces. Therefore, the interface becomes one of the most important factors for the electrochemical activity of LMSBs.
Halide-based SEs bring together the combined advantages of sulfide and oxide SEs, namely, excellent chemical and electrochemical stability like ceramic electrolytes and exceptional deformability like sulfidebased electrolytes, and are thus among the best candidates for achieving high-energy LMSBs with large-scale manufacturing. Unfortunately, like other solid electrolytes, halide electrolytes also confront several severe interface issues that are waiting for solving urgently, as shown in Figure 8A, including (1) extra grain boundary impedance inside halide-based SEs, which may be the main reason for only mediocre ionic conductivities for fluorine-based electrolytes, (2) poor electrochemical reduction stability of halide-based SEs that prevents their direct contact with Li metal anode, and (3) high voltage compatibility and deformability necessary for a stable intimate contact interface between halide-based SEs and the cathode.

| Influence of grain boundaries on Li-ion diffusion and conductivity
Because of the soft nature of sulfides, their grain boundary resistance is generally negligible. Nonetheless, the grain boundary contribution has been shown to decrease the total ionic conductivity of most reported superionic conductors. [93] Similarly, the grain boundary impedance of metal halide SEs should not be naturally neglected. For example, the RT ionic conductivity of ballmilled Li 3 YBr 3 Cl 3 is 1.6 × 10 −3 S cm −1 and then greatly increases to 7.2 × 10 −3 S cm −1 by hot-pressing at 170°C. Scanning electron microscopy (SEM) images of ballmilled Li 3 YBr 3 Cl 3 samples after cold-and hot-pressing are shown in Figure 8B,C, respectively. According to these images, not only the larger particle size is achieved by hot-pressing, but also the particles appear to be fused/ sintered together to form a denser block. [51] The authors speculated that partial melting at the grain boundaries has taken place in the hot-pressing process. The lower grain boundary resistance then contributes to enhanced ionic conductivity. Furthermore, some results from our group show that grain boundaries may have a serious impact on the Li + ion transport in fluoride-based SEs. The Li-rich phases Li 3 AlF 6 and Li 3 GaF 6 have been synthesized by our group via the ionic liquid-based dissolution-precipitation method. Both the highresolution transmission electron microscopy (TEM) and SEM images show that the ionic liquid is residual and solidified not only on the surface of fluoride grains but also at the grain boundaries, resulting in the ionic conductivity improvement and cell performance upgrade F I G U R E 8 (A) Schematic illustration of interfaces in LMSBs with halide-based SEs. Scanning electron microscopy images of (B) cold-pressed ball-milled Li 3 YBr 3 Cl 3 and (C) hot-pressed Li 3 YBr 3 Cl 3 . Reproduced with permission, [51]  ( Figure 6D). To boost the practical applications of metal halide SEs, further investigations are needed to reveal the accurate impact of grain boundaries on their ionic conductivities.

| Interfaces between halide-based SEs and cathodes
Metal chloride and fluoride SEs exhibit excellent compatibility with high-voltage cathodes, due to the high oxidation potential of halogen anions (F − and Cl − ). Recent research efforts have demonstrated that halide electrolytes enable the operation of 4-V-class bulk-type all-solid-state batteries. In the pioneering work by Asano et al., Li 3 YCl 6 successfully matches with the LiCoO 2 cathode without any additional coating, and the corresponding batteries exhibit a high initial Coulombic efficiency of 94%, while that of the reference batteries using the sulfide electrolyte Li 3 PS 4 is only 84.0%. [17] Moreover, the electrochemical impedance spectra show that the interfacial resistance of the Li 3 YCl 6 /cathode (16.8 Ω cm 2 ) is almost one-eighth of that of the Li 3 PS 4 / cathode (128.4 Ω cm 2 ), which originates from the higher electrochemical stability of chlorine ions than sulfur ions. This is consistent with the first-principles calculation of thermodynamic electrochemical windows of common SE materials. [55] In addition, LiNi 0.6 Mn 0.2 -Co 0.2 O 2 , [45] LiNi 0.8 Mn 0.1 Co 0.1 O 2 , [39] LiNi 0.83 Co 0.12 Mn 0.05 O 2 , [94] LiNi 0.88 Co 0.11 Al 0.01 O 2 , [43,44,95,96] and Co-free LiNi 0.5 Mn 1.5 O 4 [97] cathodes have also been reported one after another, and they show excellent compatibility with halide-based SEs. It is worth noting that the excellent electrochemical performance was demonstrated for the bulk-type ASSBs with Li 2 Figure 8D). [20] These cells show long cycle life, with a capacity retention of 80% at a high current density of 3.36 mA cm −2 (3 C rate) for more than 3000 cycles ( Figure 8E). The authors highlighted the contribution of low electronic conductivity to the stability of electrolytes at high voltages. Li 2 In 1/3 Sc 1/3 Cl 4 exhibits a very low σ e /σ i ratio of~10 −7 , and its oxidation at high voltages is strongly limited by poor electron diffusion, leading to a higher electrochemical stability window than that predicted by thermodynamics.
Besides halogen anions and intrinsic electronic conductivity, the central metals were also demonstrated to play another critical role in determining the electrochemical stability windows of halide-based SEs. Sun et al. investigated and compared the electrochemical performance of a series of Li-M-Cl materials. [94] In terms of the atomic orbital theory, electron structures are stable when atomic orbitals are fully full, half full, or empty according to the special cases of Hund's rule. The valence electron structure of Yb is 4f 14 6s 2 , and both the outermost 6s and 4f orbitals are in the full state, while all the unused orbitals are in the empty state. Therefore, all the orbital structures are stable, suggesting the stable chemistry of Yb and the higher electrochemical oxidation stability above 4.5 V (vs. Li/Li + ) for Li-Yb-Cl, compared to those with Y (4d 1 5s 2 ) and In (5s 2 5p 1 ). Nazar et al. also investigated the decisive effect of the central metal M on the performance of ASSBs upon cycling to the cutoff voltages of 4.3 and 4.6 V. [98] They investigated the oxidative stability of halide-based SEs using a combination of cyclic voltammetry and ultraviolet photoelectron spectroscopy measurements, as well as evaluated the products formed at the interface at various upper cutoff potentials and degrees of delithiation by the DFT calculations and time-of-flight secondary ion mass spectrometry. They demonstrated that while Li 5 [99] Recently, Han et al. reported that the FeF 2 cathode demonstrates superior electrochemical performance in the Li 3 YCl 6 -based system, compared with the conventional liquid electrolyte system. This effect is ascribed to the restricted and reversible decomposition of Li 3 YCl 6 SEs, prevention of transition-metal dissolution, mechanical confinement of active materials, and improved kinetics. Besides, they highlighted the significant effect of SE composition on the redox behavior of the FeF 2 cathode, and Li 3 YCl 6 is demonstrated to enable a complete conversion and reconversion of FeF 2 that cannot be achieved with sulfide-based 75Li 2 S-25P 2 S 5 SEs. [99] It is well known that one of the most straightforward ways to enhance the energy density of LMSBs is to employ a high-voltage cathode. [100] Therefore, it is of vital importance to develop electrolytes with wider electrochemical stability windows that allow LMSBs to be cycled stably at a higher cutoff voltage (>4.6 V). Similar to the fluorination strategies used in sulfide/ liquid electrolytes, [101] fluorination has also been demonstrated as an effective strategy to further increase the electrochemical oxidation limit of halide-based SEs. Li 3 InCl 4.8 F 1.2 , for instance, can be directly matched with the bare high-voltage LiCoO 2 , allowing the corresponding LMSBs to be operated at room temperature with a cut-off voltage of 4.8 V (vs. Li/Li + ). [52] According to the experimental and theoretical data, the higher oxidation stability of Li 3 InCl 4.8 F 1.2 and its stabilization on the surface of the cathode at high cut-off voltages are both attributed to the in situ generation of the F-containing passivating interphase. Considering the characteristics of excellent stability against high-voltage cathodes, the halide electrolytes, especially the fluorides with the highest oxidation potential of F − , have demonstrated their great potential as a stable cathode coating layer. For example, the Li 1.17 Ni 0.17 Co 0.1 Mn 0.56 O 2 cathode coated by the Li-ion conductive Li 2 TiF 6 was proved to enable the reduction of electrode-surface resistance and the improvement of surface stability, leading to the enhanced cycle performance and rate capability. Furthermore, the coating effect of Li 2 ZrF 6 on a higher-voltage LiNi 0.5 Mn 1.5 O 4 cathode has also been investigated. This protective cathode-electrolyte interphase can inhibit the oxidation decomposition of the electrolyte and suppress the dissolution of the cathode material, also resulting in improved electrochemical performance.

| Interfaces between Li-metal anodes and halide-based SEs
Though the halide-based SEs always show high anodic limits due to their excellent oxidation stability, they are suffering from the cathodic limits that are caused by the reduction reaction of central metal cations at lower potentials. As shown in Figure 8F, there are three distinct forms of interface formation when the solid electrolyte contacts with the Li metal. In the first case, the solid electrolyte is thermodynamically stable with the Li metal, thus a sharp 2D interface is formed ( Figure 8F-a), such as Li 7 La 3 Zr 2 O 12 . [102] In the second case, the solid electrolyte is thermodynamically unstable with the Li metal and the formed reaction layer is an ionic conductor that is electronically nonconductive. It is a stable interface and comparable to SEI as known in commercial LIBs ( Figure 8F-b), such as Li 6 PS 5 Cl. [19] In the third case, the solid electrolyte is thermodynamically unstable with the Li metal and the formed reaction layer is an electronic and ionic mixed conducting interphase ( Figure 8F-c), which is prone to continue to thicken and finally leads to the short-circuit of a battery. [103,104] The former two cases are generally considered to have formed the stable interfaces; unfortunately, the metal halide electrolytes always cause the interface belonging to the third case, that is, the central metal M x+ is reduced to M 0 after contacting with the Li metal with the formation of an electrically conductive reaction layer. Riegger et al. have studied the formation of the reaction layer between Li 3 InCl 6 (or Li 3 YCl 6 ) and the Li metal by in situ X-ray photoelectron spectroscopy (XPS) as well as impedance spectroscopy. [19] They concluded that the interface is thermodynamically unstable and results in a continuously growing interphase resistance.
To improve the stability of metal halide SEs with the Li metal, the introduction of an additional electrolyte interlayer is one of the most effective strategies. Some previous reports have demonstrated sulfide electrolytes, such as Li 6 PS 5 Cl and Li 10 GeP 2 S 12 , as options for electrolyte interlayers. Li 6 PS 5 Cl is known to enable a stable ionic conductive interface primarily composed of Li 2 S, Li 3 P, and P 2 S 5 when in contact with the Li metal, and thus Li 6 PS 5 Cl is thought to be an ideal electrolyte layer. [87,105] Recent reports also indicate that the interface resistance between sulfide and halide electrolytes can be almost negligible. [19] Generally, the interface between halide electrolytes and additional sulfide electrolytes is unstable, in view of the potential anion intermigration between halides and sulfides, especially under the driving of the anion concentration gradient and electric field. This effect likely causes the interface passivation between both the solid electrolytes. However, such an intermigration is not always detrimental as mentioned above, and the coexistence of halogen and sulfur anions may promote the interface conductivity and stability (e.g., in the case of Li 6 PS 5 Cl).
Besides, β-Li 3 N and Li 3 PO 4 have also been proven to be suitable interface modification layers that enable the improved stability of interfaces between metal halide SEs and Li metal anodes. [106,107] However, the aforementioned methods would complicate the cell preparation process and are not conducive to large-scale applications. Therefore, a new fluorination strategy has been proposed by Yu et al., and they successfully synthesized the fluorinated Li 3 YBr 5.7 F 0.3 with a high ionic conductivity of 1.8 × 10 −3 S cm −1 . [53] Moreover, due to the high stability of the interface in view of the in situ formation of the F-rich component in the Li plating and stripping process, the as-synthesized Li 3 YBr 5.7 F 0.3 can directly work with the Li metal anode. Li symmetric cells with Li 3 YBr 5.7 F 0.3 as the electrolyte can cycle stably over 1000 h at 0.75 mA cm −2 with an areal capacity of 0.75 mAh cm −2 . [53] Therefore, more attention should be paid to developing metal halide SEs with intrinsic stability toward the Li metal. Inspired by the fluorination strategies to realize a stable interface between the electrolyte and Li metal, fluorine-based electrolytes are promising to achieve both the advantages of the wider electrochemical stability window and stable ionic conductive layer at the Li/SE interface. Recently, Umeshbabu et al. have developed a fluoride-based SE Li 2 ZrF 5 Cl. [54] Unlike chloride and bromide SEs, when Li 2 ZrF 5 Cl contacts with the Li metal, it can cause the formation of a stable interface layer that is primarily composed of LiF. This F-containing passivated interface blocks the electron pathway while preserving the Li + pathway, thus preventing the further decomposition of SEs. Therefore, researching metal fluoride SEs may be a viable area of study, aside from the F-substitution in chloride SEs, and fluorides show enhanced electrochemical stabilities for both the cathode and anode.

| ADVANCED CHARACTERIZATION TECHNIQUES
A prerequisite for engineering the interfaces of LMSBs is to fully clarify and understand the formation of interfaces. Therefore, the flexible application of diverse advanced characterization techniques is indispensable. Note that unlike liquid Li batteries, the tightly embedded interfaces in LMSBs are difficult to be fully exposed without causing damage to the interface condition. [108] Hence, in parallel to traditional characterization tools, some in situ experimental characterizations must be developed. In this section, we will summarize the typical characterization tools widely used for solid-state interfaces and then introduce several potentially useful techniques, particularly in situ methods.

| Electrochemical characterization
There are two types of typical and indispensable electrochemical characterization techniques widely used for entire batteries, that is, cyclic voltammetry and electrochemical impedance spectroscopy. The former displays a current-voltage graph that provides information about chemical reactions and cycling reversibility. The electrochemical stability window of solid electrolytes is typically obtained by a cyclic voltammetry test on the Li/SE/ inert semiblocking metal electrode. However, Han et al. found that the value of the electrochemical stability window obtained from the above method is overestimated due to the limited contact area between the solid electrolyte and the inert metal. [109] They demonstrated that the electrolyte-carbon mixture could be a substitution for the inert metal electrode. The measured electrochemical stability window by using a Li/electrolyte/electrolyte-carbon cell is corresponding to the result by the first-principles calculations.
Electrochemical impedance spectroscopy is a powerful technique for investigating the charge transfer processes occurring in electrochemical systems. Therein, the high-frequency region (usually >1 MHz) corresponds to the resistance contribution from the bulk electrolyte, the middle-frequency region (usually 1 kHz-100 Hz) represents the charge transfer at the interface, while the low-frequency region (usually ∼1 Hz) reflects the diffusion impedance. [110,111] Riegger et al. have studied the SEI growth of Li 3 InCl 6 -based Li symmetric cells by time-resolved impedance spectroscopy. The collected impedance spectra (15 min intervals at the beginning, then 30 min, and later 60 min intervals) are shown in Figure 9A, [19] and they are fitted with an equivalent circuit consisting of three parallel RQ elements in series (R: resistance, Q: constant phase element [CPE]). The resistance assigned to the interphase contribution is continuously growing with time, indicating an SEI with ionic conductivity lower than that of the bulk electrolyte.

| Microscopy observation
Combined with imaging, diffraction, and spectroscopy, electron microscopy (EM) enables the direct visualization of interfaces at the micro-and even nanoscale, thus providing significant information about composition, morphology, elemental distribution, structure, and so on. [112,113] As the intact interfaces in LMSBs are difficult to isolate, the preparation of cross-section samples is required. Diverse EM techniques have been extensively used for solid-state battery interface analysis, such as cross-section SEM, TEM, energydispersed X-ray analysis, electron energy loss spectroscopy (EELS), and atomic force microscopy. [114][115][116] Moreover, to unravel the unique interfacial phenomena related to Li-ion transport and its corresponding charge transfer, Wang et al. proposed a new approach to conduct the in situ scanning transmission electron microscopy (STEM) coupled with EELS ( Figure 9B). [113] The result indicates that the interfacial impedance at the LiCoO 2 /LiPON interface originates from the chemical change between the electrolyte and electrode rather than the space charge effect. A similar approach is also applicable to halide SE systems.

| Chemical composition analysis
The thermodynamic instability of the interface between the electrolyte and the electrode would result in a continuously growing interphase resistance. Chemical composition analysis is an effective means to verify the chemical stability of the electrolyte-electrode interface, including XRD, XPS, X-ray absorption spectroscopy (XAS), Raman spectroscopy, Fourier transform infrared spectroscopy, NMR, and others.
XRD is a standard method to identify the crystal phase composition and lattice parameters. [117] However, XRD is not suitable for the detection of amorphous materials without a complete crystal structure. This issue can be well addressed by XAS and XPS, which are known for their sensitivity to the electronic structure. Depending on the difference in detectable depth, XAS is used as a bulk technique, while XPS is regarded as a surface technique. Moreover, in situ XPS experiments have been reported to verify the decomposition of metal halide SEs. For example, Riegger et al. have investigated the formation of a reaction layer between Li 3 InCl 6 (or Li 3 YCl 6 ) and the Li metal. [19] They first deposited the Li metal on the Li 3 InCl 6 (or Li 3 YCl 6 ) SE pellet and then analyzed the reaction products by in situ XPS. The results show that both Li 3 InCl 6 and Li 3 YCl 6 exhibit fast decomposition when contacted with the Li metal anode ( Figure 9C). A mixed ion/electron conducting interphase forms as In 3+ is reduced by Li to form In 0 , and it results in a continuous decomposition of SE and ultimately a short circuit of corresponding LMSBs. F I G U R E 9 Advanced characterization techniques for interfaces of LMSBs. (A) Temporal evolution of impedance responses of the symmetric Li|Li 3 InCl 6 |Li cell. (B) Schematic of the experimental setup of a nanobattery mounted on a TEM grid. The cathode is electrically connected to the grid and a piezo-controlled STM tip makes contact with the anode current collector. Reproduced with permission, [113] copyright 2016, American Chemical Society. (C) Schematic illustration of Li metal deposition on SE with an argon sputter gun and subsequent XPS measurement, showing the In-3d (left) and , and Auger In-MNN (center) spectra during/after Li deposition on Li 3 InCl 6 . Reproduced with permission, [19] copyright 2021, Wiley-VCH. LMSBs, lithium metal solid-state battery. SEM, scanning electron microscopy; TEM, transmission electron microscopy.

| SUMMARY AND PERSPECTIVE
After years of silence, halide-based SEs are back in the spotlight with their unique advantages. In this review, we first exhibit the structural features and ionic conductivities of the halide-based SEs by focusing on their current fundamental understanding, challenges, and opportunities of them. According to the results of first-principles calculation by Wang et al., [56] halides allow for more facile Li-ion conduction than their oxide and sulfide counterparts owing to the hcp and ccp anion framework nature of halogen anions in halide-based SEs, indicating a high potential for achieving high ionic conductivity based on halogen chemistry. Then, various synthesis methods are summarized, including mechanical ball-milling, solid-state sintering, and liquid-phase synthesis, which are widely used for preparing halide-based SEs. It is worth noting that novelty methods, such as the low-temperature ionic liquid fluorination method, are urgently required for preparing highperformance halide-based SEs. Furthermore, the chemical and electrochemical stabilities of halide-based SEs are evaluated, and the main issues that hinder the practical applications of halide-based SEs are moisture sensitivity and interface instability between SEs and electrodes.
Despite the significant progress so far in halide-based SEs, many challenges remain and more efforts are still needed: (1) Development of fluoride-based SEs with higher ionic conductivity. Computational studies have demonstrated that fluorides exhibit the highest electrochemical stability windows among halide-based SEs, which are promising to enable the high voltage cathodes with cut-off voltages higher than 4.8 V versus Li/Li + while maintaining the stability with Li metal anodes. In terms of the development of fluoride-based SEs with higher ionic conductivity, mature synthetic solutions and deeper mechanistic understanding are crucial. (2) Development of more applicable synthesis methods. Green, economical, and scalable synthesis methods are indispensable for the practical application of halidebased SEs. Compared to mechanical ball-milling and solid-state sintering, liquid-phase synthesis is more attractive due to the lower energy consumption, more homogeneous particle size, shorter preparation period, larger-scale mass production, and so on. Nevertheless, the halide-based SEs prepared from liquid-phase synthesis are limited so far. (3) Improving the air stability of halide-based SEs. Moisture sensitivity is almost the generic disease in halide-based SEs except for fluorides. Although several strategies have been shown to somewhat mitigate the moisture sensitivity of halide-based SEs, developing intrinsically hydrophobic SEs is still the optimum option. (4) Improving the electrochemical stability of halide-based SEs. Cathodic limits that are caused by the reduction reaction of central metal cations at lower potentials seriously hinder the application of Li metal anodes. Constructing a stable artificial SEI would be an effective strategy. Meanwhile, note that F-substitution in chloride-based SEs enables to improve the electrochemical stabilities both for the sides of cathodes and anodes. We speculate that fluoride-based SEs are the answer that research workers have been struggling to find. [118] In addition, more efforts should also be dedicated to stabilizing the anode-electrolyte interface by using novel Li metal-based anode structures like Li-C composite and magnetic Li, [119,120] as well as to improving the cathode electrolyte interphase by adopting functional salt and polymer layers, for example, lithium difluoro-(oxalato)borate and oxidation-resistance poly (acrylonitrile). [121,122] (5) Breakthrough in all-solid-state pouch cells. All-solid-state pouch cells have exhibited satisfactory electrochemical performance and high feasibility for commercialization. However, inorganics are far more difficult to integrate into pouch cells than organic solid polymer electrolytes. More efforts are needed to focus on the all-solid-state pouch cells based on halidebased SEs. Overall, it is highly expected that more efficient and excellent solid-state energy storage systems could come into and further renovate the way we live in the future, and we believe that halide-based SEs will play an integral role in advancing the practical application of LMSBs.