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Inorganic Solid Electrolytes for the Lithium-Ion Batteries

Jiasheng Lu , Jiamiao Chen , Tianxian He , Jingwei Zhao , Jun Liu , Yanping Huo

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1344 -1361.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1344-1361 DOI: 10.7536/PC200772
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Inorganic Solid Electrolytes for the Lithium-Ion Batteries
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Abstract

Liquid lithium-ion batteries have fatal safety problems such as flammability, explosion, and short-circuit, as well as technical problems such as cruising range anxiety. The development of lithium-ion batteries with good safety performance and high energy density is an urgent need for industrial development. Compared with traditional liquid lithium-ion batteries, all-solid-state batteries have the advantages of safe use and high theoretical specific capacity, so they have been extensively studied and are known as the mainstream technology of next-generation batteries. Among them, the inorganic solid electrolyte plays an important role in the all-solid-state battery, and scientific researchers at home and abroad have conducted a lot of research work on this. This article introduces the latest developments in different types of inorganic solid electrolytes, including oxide solid electrolytes, sulfide solid electrolytes, and halide solid electrolytes. The researches on interface problems, crystal structure, preparation methods, and doping modification of inorganic solid electrolytes are also elaborated. Finally, the problems to be solved in inorganic solid electrolytes in recent years are discussed, and the future research directions are also given.

Contents

1 Introduction

2 Solid oxide electrolytes

2.1 Garnet type solid electrolytes

2.2 Perovskite solid electrolytes

2.3 NASICON type solid electrolytes

2.4 LiPON thin films electrolytes

3 Solid sulfide electrolytes

4 Solid halide electrolytes

5 Conclusion and outlook

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Keywords

lithium-ion batteries / inorganic solid electrolytes / interface compatibility / crystal structure / electrochemical performance

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Jiasheng Lu, Jiamiao Chen, Tianxian He, Jingwei Zhao, Jun Liu, Yanping Huo. Inorganic Solid Electrolytes for the Lithium-Ion Batteries. Progress in Chemistry, 2021, 33(8): 1344-1361 DOI:10.7536/PC200772

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1 Introduction

Environment and energy issues have become a huge challenge for sustainable development in the 21st century. The extensive use of traditional fossil energy brings convenience to our lives, but also brings serious pollution to the environment. Therefore, it is imperative to develop new green energy. At present, lithium-ion batteries (LIBs) have been widely used in electronic devices (mobile phones, cameras, laptops, etc.), vehicles (automobiles, airplanes, etc.) And other fields. Lithium-ion batteries have been widely used in various fields because of their high energy density, flexible scale and low maintenance cost[1]. However, the development of traditional liquid lithium-ion batteries is also limited by the inherent problems of existing battery materials, such as the difficulty of improving energy density in a short period of time, the rapid growth of lithium dendrites, and the low coulombic efficiency[2]. In particular, the problem of rapid growth of lithium dendrite: the formation and rapid growth of lithium dendrite will pierce the diaphragm and cause internal short circuit of the lithium ion battery, resulting in thermal runaway of the battery and causing combustion and explosion, which brings serious security risks[3]. On the contrary, All-solid-state batteries (ASSBs) have the advantages of safe use, high thermal stability and good chemical stability[4]. Therefore, replacing flammable organic liquid electrolytes with Solid state electrolytes (Solid state electrolytes, SSE) with excellent flame retardancy to assemble all-solid-state batteries is expected to fundamentally solve these problems.
Compared with commercial liquid lithium-ion batteries, solid-state lithium-ion batteries are still in the initial stages of development and optimization. For solid electrolytes, desirable physicochemical properties include: (1) high ionic conductivity; (2) high lithium ion transference number; (3) high chemical stability and electrochemical stability; (4) wide electrochemical window; (5) low electronic conductivity; (6) no side reaction with the electrode material, etc. At present, increasing energy density and charge-discharge speed, improving the safety of solid-state batteries and prolonging the cycle life have become an urgent need for new materials in the field of batteries. Therefore, the research focus of solid-state batteries has gradually shifted from how to improve the ionic conductivity of solid-state electrolytes to how to improve the energy density and stability of solid-state batteries and how to solve the interface problems.
Solid state electrolytes can be divided into three main categories: polymer solid state electrolytes, inorganic solid state electrolytes, and organic-inorganic composite solid state electrolytes[5]. The advantages of polymer solid state electrolyte are that it can well inhibit the growth of lithium dendrite in the battery, has good contact with the positive (negative) electrode interface, and has excellent flexibility[6]. However, their ionic conductivity is generally lower than that of inorganic solid electrolytes. Relatively speaking, inorganic solid electrolytes have a wider electrochemical window and good thermal stability, and their energy density is also higher[7]. Therefore, inorganic solid-state electrolytes would be a more suitable candidate for the preparation of all-solid-state batteries.
Inorganic solid electrolytes can be divided into oxide solid electrolytes, sulfide solid electrolytes and halide solid electrolytes, and their advantages and disadvantages are shown in Table 1. Based on the latest research progress of inorganic solid electrolytes at home and abroad (ionic conductivity and activation energy are shown in Table 2) and the related work of our group, this paper reviews the research work of inorganic solid electrolytes and puts forward the prospects for future development[8⇓~10].

2 Oxide solid state electrolytes

Oxide solid state electrolytes can be divided into crystalline and amorphous electrolytes according to their morphology[11]. The crystalline oxide electrolyte includes garnet type, perovskite and NASICON-type solid electrolytes[12]. And that crystalline oxide electrolyte has high air permeability and thermal stability, is low in manufacture cost, and is easy to realize large-scale production. At present, the widely studied crystalline solid electrolytes include garnet-type Li7La3Zr2O12 (LLZO) electrolyte, perovskite-type La2/3-xLi3xTiO3 (LLTO) electrolyte, NASICON-type Li1+xAlxGe2-x(PO4)3 (LAGP) and Li1+xAlxTi2-x(PO4)3 (LATP) electrolyte. And that amorphous solid electrolyte is mainly a LiPON type solid electrolyte. Although the ionic conductivity of amorphous oxide electrolyte is not high, it is stable to lithium metal and has a relatively wide electrochemical window[13]. And that amorphous oxide electrolyte can be easily prepare into an all-solid-state thin film battery, so that the amorphous oxide electrolyte can be widely applied to electronic device with low capacity requirements (such as smart cards, sensors, microelectronic device and the like).

2.1 Garnet type solid state electrolytes

Typical garnet structures have A3B3C2O12(A=Ca,Mg,Y,La or rare earth metals; B = Si, Ge, Al; C = Al, Fe, Ga, Ge, Mn, Ni or V) in which A, B and C are 8, 4 and 6 oxygen-coordinated cation sites, respectively, and they crystallize in a face-centered cubic structure[14,15]. Fig. 1 is a schematic diagram of the structure of a typical garnet, in which the octahedral structure represents the CO6 unit, and the black and gray spheres represent the cations of A and B, respectively[14]. Garnet solid state electrolytes can be divided into Li3Ln3Te2O12(Ln=Y,Pr,Nd,Sm-Lu), Li5La3M2O12(M=Nb,Ta,Sb), Li6ALa2M2O12(A=Mg,Ca,Sr,Ba;M=Nb,Ta) and Li7La3M2O12(M=Zr,Sn) according to the content of lithium in the compound[16⇓⇓⇓~20].
At present, LLZO solid state electrolyte is widely used in garnet solid state electrolyte. Murugan et al. Reported the synthesis of the first LLZO:Li7La3Zr2O12 in 2007[20]. The Li7La3Zr2O12 has high Li+ conductivity (the total conductivity can reach more than 3×10-4S/cm at room temperature), good thermal stability and chemical stability. Since the appearance of Li7La3Zr2O12(LLZO in 2007, it has attracted extensive attention from scholars at home and abroad, which has also led to a lot of research on the modification of LLZO garnet oxide solid electrolyte materials.
The research on the modification of LLZO is mainly based on cation doping, and the Li+ sites of LLZO can be partially replaced by Fe3+, Ga3+ and Al3+. Studies have shown that the electrochemical stability of Fe3+ may not be as good as that of other stable cations (such as Ga3+ and Al3+)[21,22]. For example, Wu et al. Reported a Ga-doped LLZO solid state electrolyte Li7-3xGaxLa3Zr2O12, which has a maximum conductivity of 1.46×10-3S/cm and a minimum activation energy of only 0.25 eV[23]. Most of the Li+ in the Li7-3xGaxLa3Zr2O12 is located at the octahedral 96h position, away from the central octahedral 48g position, while the remaining Li+ is located at the tetrahedral 24d position as shown in fig. 2. This Li+ distribution results in a high lithium ion mobility of the material.
In addition, doping cations at both the La and Zr sites of LLZO can produce a synergistic effect on LLZO, so the Li+ conductivity can be improved by double doping[24]. Gai et al. Prepared Li7La3ZrNb0.5Y0.5O12 garnet electrolyte by solid state reaction method, and the conductivity of the material can reach 8.29×10-4S/cm at 30 ℃[25]. More surprisingly, the conductivity of the material remained at the 6.91×10-4S/cm after 1.5 months of exposure to air, showing good stability.
At present, one of the great challenges facing garnet LLZO solid state electrolytes is the excessive resistance of the electrolyte/electrode interface and the rapid growth of lithium dendrites at the grain boundary[26]. As shown in fig. 3, in the garnet electrolyte, the Li2CO3 layer on the surface impedes the transport of Li+ at the garnet electrolyte/electrode interface. Due to the uneven distribution of current density, lithium dendrites grow rapidly, which eventually leads to the decrease of specific capacity and the shortening of cycle life of LLZO solid-state batteries[26][27]. Therefore, how to eliminate the Li2CO3 on the surface of LLZO or inhibit the formation of Li2CO3 at grain boundaries is one of the keys to improve the electrochemical performance of LLZO solid electrolyte.
In order to solve the interface problem and eliminate the Li2CO3 on the electrolyte surface, Li et al. Recently removed the Li2CO3 and protons in the garnet framework by a simple method of reacting Li6.5La3Zr1.5Ta0.5O12(LLZT) with carbon (C) at 700 ℃[26]. As shown in fig. 3, in the carbon treated LLZT (LLZT-C), the interfacial resistance of Li/garnet, garnet/electrolyte, and garnet/cathode is reduced due to the removal of the Li+ barrier layer on the surface of the garnet particles. They assembled Li/LLZT-C/Li, Li/LLZT-C/LiFeO4 and hybrid Li-S batteries with LLZT-C. At 65 ° C, the Li/LLZT- C, LLZT-C/LiFePO4 and LLZT- C/organic electrolyte interfaces showed low interfacial resistances of 28, 92 and 45Ω·cm2, respectively.
Wang et al. developed a heat pulse technique for rapid treatment of ceramic surfaces[28]. The technique can rapidly remove the Li2CO3 on the surface of LLZO electrolyte and at the grain boundary in less than 2 s, while the slight evaporation of the Li2O forms oxygen vacancies. The formation of oxygen vacancies can promote the migration of Li+, which is one of the reasons why the ionic conductivity of garnet LLZO solid state electrolyte increases from about 1.6×10-4S/cm to 3.2×10-4S/cm. After heat pulse treatment, the critical cycling current density in the LLZO symmetric cell can be increased from the 100μA·cm-2 to the 500μA·cm-2.
Similarly, Huo et al. Recently proposed a generally effective rapid acid treatment strategy: treating the electrolyte with HCl (1 mol/L) for 30 s is sufficient to remove the Li2CO3 from the surface of the Li6.4La3Zr1.4Ta0.6O12(LLZT) without significant damage to LLZT[29]. After acid treatment, the interfacial resistance of Li/LLZT was significantly reduced (from 940Ω·cm2 to 26Ω·cm2). Therefore, eliminating the Li2CO3 on the electrolyte surface can effectively reduce the resistance at the electrode/electrolyte interface and solve the interface problem of solid electrolyte, which is also a new strategy to further improve the electrochemical performance of solid state batteries.
In addition, in order to inhibit the formation of Li2CO3 at grain boundaries and improve the electrochemical performance of garnet electrolyte, Wu et al. Prepared Li6.25Ga0.25La3Zr2O12(Ga-LLZO) by high temperature calcination at 900 ℃, and formed Li defect shielding layer in situ[30]. In the assembled Li/Ga-LLZO/Li battery, an ultra-low interfacial resistance of only 35Ω·cm2 was generated during the charge/discharge process, and the current distribution was uniform. It is worth noting that when the working temperature is increased to 60 ℃, the stable lithium extraction/deposition current density in Ga-LLZO can reach 900μA·cm-2, which is one of the highest values for ceramic solid electrolytes. The low interface resistance and high Li extraction/deposition current density are due to the formation of a Li deficient barrier layer at the Li/Ga-LLZO interface, which effectively limits the formation of Li2CO3 at the Li/Ga-LLZO interface. In addition, the electrolyte is well wetted by Li, which enhances the ability of the electrolyte to inhibit the growth of lithium dendrites.
Studies have shown that garnet electrolytes can show good compatibility with metal lithium electrodes, and the Li/garnet electrolyte interface has low impedance and good stability[31,32]. Therefore, the modification of the lithium electrode side is also a very important method to solve the electrolyte/electrode interface problem.
Recently, Li et al. Have effectively reduced the interfacial impedance of Li/garnet electrolyte by eliminating the influence of lithium metal surface oxides on lithium metal[33]. Using first-principles calculations, it is confirmed that lithium can wet the garnet electrolyte well. They completely wetted the surface of LLZT garnet electrolyte with molten lithium metal and cooled it to make Li/LLZT/Li batteries. The LLZT/Li interface resistance in the Li symmetric cell is only 6.95Ω·cm2 at room temperature. In addition, the battery can be charged and discharged at room temperature for 950 H without short circuit, and its critical current density is as high as an astonishing 13.3 mA·cm-2, as shown in Figure 4 f. The above results show that the use of molten lithium metal to prepare the positive and negative electrodes of LLZT batteries can make the metal lithium in close contact with LLZT, and they use the first principles to explain this phenomenon. As shown in figs. 4a-d, the interface formation energies of LLZO/Li and LLZT/Li are as low as -2.52 and -6.14 J·m-2, respectively, which means that the garnet electrolyte has an inherent lithium affinity, and the wettability between the garnet electrolyte and lithium metal can be improved by doping Ta element. On the contrary, the interface formation energy of the Li2O/Li is 0.23 J·m-2, which indicates that the Li2O can not be well wetted by lithium metal, so the Li2O on the surface of lithium metal may be lithiophobic, which affects the wetting of electrolyte by lithium metal and leads to poor contact at the Li/electrolyte interface. In contrast, due to the inherent lipophilicity of garnet electrolyte, the contact between garnet electrolyte and molten lithium forms LLZT/Li interface in the form of regional contact, which can reduce the local current density and inhibit the formation of lithium dendrites.
Zhang et al. Integrated lithium foil with polydimethylsiloxane (PDMS) to construct a stress-adaptive Li/garnet electrolyte interface[34]. The assembled Li (PDMS)/LLZT/Li (PDMS) battery can be stably charged and discharged for more than 5000 times at 0.2 mA·cm-2 and high current density. Due to the inherent superelasticity of PDMS, part of the compressive stress can be counteracted during the lithium removal/deposition process. Therefore, the Li/LLZT interface can maintain local contact during the process of Li removal/deposition, which inhibits the growth of lithium dendrites.
Similarly, Huo et al. improved the LLZO/Li interface through the conversion reaction between the Cu3N and the lithium metal, and a Li3N/Cu ion/electron mixed conducting layer (MCL) was formed at the LLZO/Li interface[35]. MCL can not only increase the compatibility between metal lithium and LLZO, but also reduce the attack of electrons on garnet electrolyte, thus enhancing the stability of LLZO/Li interface. As a result, the assembled Li/MCL/LLZO/MCL/Li battery can be stably cycled for more than 1 000 H at 0.1 mA·cm-2, and its interfacial impedance can be reduced from 1138.4Ω·cm2 to 83.4Ω·cm2.
The above work emphasizes the importance of obtaining a stable Li/electrolyte interface by treating one side of the interface between lithium metal and electrolyte, which plays an important role in inhibiting the growth of lithium dendrites and improving the stability of the interface, and provides experience and ideas for the manufacturing process of high energy density solid-state batteries.

2.2 Perovskite type solid state electrolytes

The general chemical formula of perovskite is ATiO3(A=Ca, Sr, Ba), and the general formula can be written as ABO3[36]. The more common perovskite-type solid electrolyte is the La2/3-xLi3xTiO3(LLTO), and its crystal structure is shown in fig. 5. LLTO belongs to the cubic crystal system, and a large number of TiO6 octahedra are connected by a common vertex to form the basic framework of LLTO crystal, and lithium ions and lanthanide ions are located in the interstices of these octahedra. The migration process of Li+ in LLTO is dominated by the ion-vacancy mechanism. The vacancy at La site is a necessary condition for lithium ion conduction, and the size of cation A and the concentration of vacancy will affect the ionic conductivity of perovskite electrolyte[37⇓~39].
Perovskite-type electrolytes have attracted much attention due to their excellent electrochemical stability, high elastic modulus and low fabrication cost. Recently, Jiang et al. prepared a free-standing ceramic electrolyte membrane of perovskite-type Li0.34La0.56TiO3(LLTO) by tape casting (as shown in fig. 6e)[40]; Compared with the thick (> 200 μm) LLZO electrolyte obtained by cold pressing, the overall lithium ion conductivity of the LLZO membrane can be improved from 9.6×10-6S/cm to 2.0×10-5S/cm. They assembled an all-solid-state Li/LFP battery with the LLTO film (LLTO-41) of 41 μm thickness, and its electrochemical performance is shown in Figure 6a ~ d. It can be seen that the battery retains a capacity of 125 mAh/G after 50 cycles at a current density of 0.1 C, which is 86.2% of the initial capacity (fig. 6 B). In addition, as shown in Figure 6 C and d, 120 mAh/G and 55 mAh/G can also be achieved at current densities of 0.2 C and 0.5 C, respectively. These results show that the LLTO films have good ionic conductivity and good electrochemical properties.
Some studies have shown that too few La-rich layer vacancies and too high lattice stress will directly lead to the decline of the conductivity of perovskite-type LLTO electrolyte. By doping, the lattice constant of LLTO can be increased or the lattice stress can be reduced, and the electrochemical performance of the perovskite LLTO can be improved. Zhang et al. Synthesized Sr-doped perovskite-type electrolyte Li0.33+xLa0.56-xSrxTiO3(LLSTO) by sol-gel method, which has a low activation energy of 0.30 eV and an ionic conductivity (1.95×10-3S/cm) nearly twice as high as that of undoped LLTO[41]. The ionic radius of the Sr2+ is larger than that of the La3+, and the lattice constant can be increased by partially replacing the La site with the Sr2+, so that the vacancy of the La-rich layer is expanded, the longitudinal migration of the Li+ in the crystal is increased, and the diffusion of the Li+ is promoted. Similarly, Lee et al synthesized Y-doped perovskite-type electrolyte Li0.33La0.56-xYxTiO3(LLYT) by sol-gel method, and its conductivity can reach up to 9.51×10-4S/cm[42]. They believe that the higher conductivity of the material is due to the fact that the Y3+ can reduce the lattice stress of the crystal, inhibit/prevent the lattice distortion of the α-LLTO phase and increase the volume fraction of the α-LLTO phase.
High grain boundary resistance and instability to low potentials are major challenges for LLTO solid state electrolytes, and these obstacles lead to a continuous decrease in energy density and power with charge-discharge cycles. Xu et al. Revealed the interphase formation process of perovskite electrolyte (LLTO) and spinel cathode LiMn2O4(LMO) by comparing spark plasma sintering (SPS) with conventional sintering[43]. They found that a high resistance interlayer was formed on the LLTO/LMO formed by SPS co-firing due to the interdiffusion between titanium ions and manganese ions, as shown in fig. 7 (d). In figs. 7 (a-c), this intermediate layer results in an interface resistance of 4.66×105Ω·cm2, which is as much as 40 times that of a conventional LLTO. Therefore, in order to improve the performance of solid-state batteries, the emphasis should be placed on reducing the interdiffusion between phases and reasonably designing the phases to enhance the ionic conductivity.
Kwon et al. studied the relationship between the microstructure of LLTO perovskite at different sintering temperatures and the conductivity of the Li+[44]. As shown in fig. 8, the LLTO sintered at lower temperature has smaller grain size and higher grain boundary resistance; On the contrary, the higher the sintering temperature, the larger the grain size and the lower the grain boundary resistance. Therefore, LLTO electrolytes with lower grain resistance and higher conductivity can be obtained by adding excess lithium during high temperature sintering. Therefore, more attention should be paid to the control of microstructure factors in the development of solid-state batteries with high conductivity and high energy density.
In addition, the humid environment easily affects the stability of perovskite electrolytes: when they are exposed to the humid environment, an insulating layer will be formed at the anode/electrolyte interface due to the erosion of moisture.Thereby reducing the mobility of the Li+, and the insulating layer also prevents the lithium metal from wetting the solid electrolyte, and finally the interface resistance is continuously increased with the charge and discharge cycles. In order to solve the instability of perovskite solid state electrolyte in humid environment, Li et al. Synthesized perovskite solid state electrolyte Li0.38Sr0.44Ta0.7Hf0.3O2.95F0.05(LSTHF5) by partially replacing the O2- on LLTO with F-[45]. LSTHF5 is a perovskite-type solid electrolyte that does not react with water under the condition of 3 < pH < 14. As shown in figs. 9a and 9b, the Raman spectrum and XRD results of LSTHF5 after water treatment at 3 < pH < 14 are approximately the same as those of the original one. As shown in Fig. 9c and 9d, the material is assembled into a symmetric C/LSTHF5/C supercapacitor, and the charge-discharge curve of the supercapacitor after being treated in water with 3 < pH < 14 presents a symmetric triangle, and the specific capacitance is about 70 to 80 F/G; After 1000 charge-discharge cycles, the specific capacitances of the samples treated in water with pH of 3, 7 and 14 remained above 95%, 93% and 92. 5% of the initial values, respectively. The above results show that LSTHF5 has relatively stable chemical and electrochemical properties, and the fluorination technology provides a certain reference for the moisture-proof treatment of LLTO solid state electrolyte.

2.3 NASICON-type solid electrolytes

NASICON-type crystals were first discovered by Hagma et al. In 1968; Later, Mr. Goodenough, one of the Nobel Prize winners in chemistry, found that these compounds have high conductivity and high stability[46,47]. The general chemical formula of NASICON is AMM'(PO4)3, where A is the transport cation, usually an alkali metal ion (Li+,Na+,K+, etc.), or an alkaline earth metal ion (Mg2+,Ca2+, etc.), or a lanthanide ion, or empty without any ion[48]. As shown in fig. 10 (that green octahedron represent the MO6 or the M'O6 octahedron; The purplish red tetrahedron represents the PO4 tetrahedron), and the M(M')O6 octahedron and the PO4 tetrahedron are connected by a common vertex to form the three-dimensional skeleton of NASICON; M and M 'provide a gap site for the conduction of the Li+ so that the Li+ can migrate between M and M'[49,50].
According to the composition of M site elements, NASICON-type solid electrolytes can be divided into three main categories: :LiZr2(PO4)3(LZP), LiTi2(PO4)3(LTP) and LiGe2(PO4)3(LGP). Studies have shown that the partial replacement of Ti4+ (or Ge4+) by Al3+ with smaller ionic radii can reduce the size of crystal units and make the electrolyte more dense, thus improving the concentration and mobility of mobile Li+[51,52]. Due to the high lithium ion conductivity and good electrochemical stability of NASICON-type electrolytes doped with Al3+ in air, coupled with the low cost and easy preparation of materials, Li1+xAl3Ge2-x(PO4)3(LAGP) and Li1+xAlxTi2-x(PO4)3(LATP) have become popular research objects in recent years.
The ionic conductivity of LATP is relatively high, theoretically up to the order of 10-4~10-3[53]. He et al. Prepared NASICON-type Li1.3Al0.3Ti1.7(PO4)3 electrolyte materials by hydrothermal-assisted solid-state reaction method, and proved that the total conductivity of the materials could reach 4.6×10-4S/cm[54].
Compared with LATP, LAGP has a more stable structure, so LAGP can be compounded with organic electrolyte materials such as polyethylene oxide (PEO) to obtain better stability and higher conductivity. Ou et al. reported a PEO/LAGP composite electrolyte with the addition of 1-ethyl-3-methylimidazolium bis- (trifluoromethylsulfonyl) -imide (EMITFSI): PEO-LAGP-EMITFSI (PLE)[55]. The results show that the introduction of EMITFSI reduces the crystallinity of PEO and improves the ionic contact at the electrode/electrolyte interface, which promotes the migration of Li+ in the composite electrolyte. The ionic conductivity of the electrolyte was 8.85×10-4S/cm at 60 ℃, and the specific capacity of 114.7 mAh/G was still maintained after 120 cycles in the assembled LiFePO4/PLE/Li battery.
When lithium metal is used as an electrode to form a solid-state battery with LATP or LAGP electrolyte, the following side reactions are more likely to occur at the interface: :LiTi2(PO4)3+Li→Li2Ti2(PO4)3 and 2LiGe2(PO4)3+4Li→3 GeO2+6LiPO3+Ge[56]. This leads to a decrease in the concentration of the flowing Li+ and to a large interfacial resistance due to the deposition of side reaction products, thereby causing a decrease in cell performance. In order to reduce the interfacial reaction and increase the interfacial stability of LATP/Li and LAGP/Li, researchers have made in-depth studies.
For LATP type electrolyte, it is more effective to add a protective layer on its surface. By using the protective layer, the Ti4+ can be prevented from being reduced to low-valence Ti ions, and the contact mode between the electrode and the electrolyte can be changed from point-to-point contact to plane-plane embedded contact, so that the Li+ at the electrode/electrolyte interface can migrate more uniformly[57]. Therefore, the solid-state battery using the protective layer has good capacity retention and cycleability.
Lin et al. introduced polyethylene glycol (diol) diacrylate/N-propyl-N-methylpyrrolidinium bis (trifluoromethanesulfonyl) acyl (PEGDA/Pyr13TFSI for short, an ionic gel) into the interface of NASICON-type LATP solid electrolyte and electrode through simple dripping and in-situ polymerization methods (Figure 11), which improved the incompatibility and poor interface contact between solid electrolyte and electrode[58]. As shown in the inset of fig. 11, the ionic gel protective layer prevents the occurrence of side reactions, and the ionic gel easily penetrates into the voids in the cathode, facilitating the transport of Li+ at the electrolyte/electrode interface, resulting in higher active material utilization. The interface impedance of the assembled Li/ionic gel/LATP/ionic gel/LFP battery is only 80Ω·cm2. In addition, the all-solid-state battery has a capacity of 152.5 mAh/G at a current rate of 0.5 C and retains 91.5% of the capacity after 200 charge-discharge cycles. This work provides a way to solve the interface problem between SSE and electrode, and is expected to be extended to applications other than LATP.
Similarly, Cheng et al. Deposited boron nitride (BN) nanofilms on LATP particles by chemical vapor deposition and successfully prevented the reduction of LATP by lithium metal[59]. The BN thin film layer acts as a barrier layer, but it still allows the penetration of the Li+. The Li/LATP/Li symmetrical battery assembled by the lithium ion battery can be charged and discharged stably for a long time. In addition, LFP/PEO/LATP/BN/PEO/Li all-solid-state battery was assembled. The capacity of the battery hardly decays in 500 charge-discharge cycles. These results show that the chemically stable BN coating is a promising protective layer to prevent side reactions without reducing the energy density of the battery, which also provides a possibility to solve the problem of side reactions between other solid electrolytes and lithium metal anodes.
Similar to LATP, coating a thin film on the surface of LAGP particles can also effectively prevent side reactions at the LAGP/Li interface. For example, in order to solve the instability of LAGP in contact with Li electrode, He et al. Coated a layer of poly (acrylic acid-maleic acid) lithium [P (AA-co-MA) Li] on LAGP particles and added a proper amount of LiCl[60]. The Li/LAGP/Li battery assembled by the material shows excellent stability; As shown in figs. 12A and 12b, the battery was cycled stably at different current densities. In Fig. 12c, there are low overpotentials of 0.036, 0.105 and 0.257 V after 100 charge-discharge cycles at current densities of 15, 30 and 70μA·cm-2, respectively, and the overpotentials are linear with the applied current density, proving that the diffusion of Li+ is less restricted at various current densities. These data indicate that the P (AA-co-MA) Li coating effectively limits the side reactions at the LAGP/Li interface and improves the migration of Li+ at the interface due to the addition of LiCl.
In addition, it is also a good way to improve the interfacial stability of LATP or LAGP by using organic compounds to compound with them. Because the organic compound has good compatibility with the electrode and has excellent Li+ coordination efficiency, the growth of lithium dendrite can be inhibited to a certain extent[61]. Siyal et al. Prepared a composite gel polymer electrolyte PEO-Bp-LATP composed of polyethylene oxide (PEO), benzophenone (Bp) and LATP particles by efficient ultraviolet (UV) technology[62]. The electrolyte exhibits a high ionic conductivity of 3.3×10-3S/cm and a wide electrochemical window above 5 V at room temperature. In addition, Wang et al. Used propylene carbonate (PPC) and LAGP to form LAGP/PPC composite electrolyte, which showed excellent performance when LiFePO4 and Li were used as electrodes, with a high capacity of 151 mAh/G at 0.05 C and a capacity of 92.3% after 100 charge-discharge cycles[63]. The above studies have confirmed the protective function of organic compounds on the interface of LATP/LAGP solid electrolyte.

2.4 LiPON type thin film electrolytes

LiPON is the abbreviation of lithium phosphorus oxynitride. Unlike garnet, perovskite and NASICON-type solid electrolytes, it is an amorphous lithium ion solid electrolyte[64]. The ionic conductivity of LiPON-based solid state electrolyte is relatively low, but it has high chemical stability and wide electrochemical window. At present, the preparation of thin film electrolyte is the main research direction of LiPON. For example, Yamamoto et al. Recently reported an inverted-stack lithium-free thin-film all-solid-state battery :SUS/Pt/LiPON/LiCoO2/Au[65]. The highly crystallized LiCoO2 cathode film was deposited on the amorphous LiPON film by the vapor phase sol technique, and the lithium metal cathode film was electrochemically grown between SUS/Pt and LiPON. The battery can operate at 4 V and remain stable after more than 100 charge-discharge cycles.
The mechanical strength of LiPON is sufficient to suppress lithium dendrite growth, and it has considerable Li+ conductivity[66⇓⇓~69]. In addition, the LiPON film can separate the solid electrolyte from the Li electrode, thereby inhibiting the side reaction at the interface and improving the interface compatibility[70]. Wang et al. Prepared dense and uniform LiPON coating on Li anode by nitrogen plasma assisted deposition reactive evaporation method[71]. Utilizing the mechanical stability of the LiPON coating and its ability to promote uniform plating/stripping of the Li+, the Li-S battery prepared from the LiPON-coated Li metal anode has a high energy density of 300 Wh/kg and a good coulombic efficiency of 91%. The method of coating lithium metal with LiPON will contribute to the application of high energy density lithium ion batteries.
Similarly, Lv et al. Prepared amorphous LiPON films with a thickness of only 15 nm on LiNi0.5Mn1.5O4(LNMO) films by magnetron sputtering[72]. The LNMO/LiPON electrode still maintained a high voltage of 4. 9 V after 150 charge-discharge cycles, and its reversible capacity was 105 mAh/G. The LiPON film uniformly distributed on the surface of LNMO cathode can avoid the direct contact between LNMO and electrolyte, inhibit the side reaction of Mn dissolution, and provide a stable interface for ion transport. Therefore, using LiPON as an artificial cathode-electrolyte interface to cover the positive electrode can also make the battery have higher discharge capacity, longer cycle life and better rate capability.
Although oxynitrides have long cycle stability, little research has been done on the modification of LiPON. We believe that oxynitride solid state electrolyte will be an important direction of solid state electrolyte research in the future. In this regard, we also first proposed the development strategy of LiSiAlON solid state electrolyte[73]. The current research results show that LiSiAlON has similar electrochemical properties to LiPON, but its ionic conductivity is improved by more than one order of magnitude. Quantum chemical calculations show that the ionic conductivity of LiSiAlON solid state electrolyte can be up to 10-3S/cm.

3 Ulfide solid state electrolytes

Sulfide solid state electrolytes have higher conductivity and lower grain boundary resistance than oxide solid state electrolytes. The electronegativity of sulfur is smaller than that of oxygen, so the binding force of sulfur to Li+ is small, which is conducive to the migration of Li+, and the radius of sulfur is larger than that of oxygen, which provides a larger transmission channel for Li+, so the sulfide solid electrolyte has higher ionic conductivity. In particular, Li2S-GeS2-P2S5(LGPS) series electrolytes have ionic conductivity exceeding that of commercial electrolytes at room temperature[74,75]. At present, the key challenges of sulfide solid state electrolytes are how to improve the interfacial compatibility and electrochemical performance of electrodes and electrolytes, how to inhibit the rapid decay of capacity and how to reduce the "shuttle effect" of sulfur. Therefore, in recent years, researchers at home and abroad have focused on this.
Sulfide solid state electrolytes can be divided into amorphous sulfide solid state electrolytes and crystalline sulfide solid state electrolytes. The commonly used amorphous sulfide solid electrolytes are Li2S-P2S5, Li2S-GeS2, Li2S-B2S3 and Li2S-SiS2[76]. The conductivity of amorphous sulfide electrolyte is about 10-4S/cm, which has a wide electrochemical window. The common crystalline sulfide solid state electrolytes can be divided into four series: Li2S-GeS2-P2S5, Li2S-SiS2-P2S5, Li2S-SnS2-P2S5 and Li2S-Al2S3-P2S5[77].
At present, the sulfide solid electrolyte of Li2S-GeS2-P2S5 series has attracted much attention. The first solid electrolyte with room temperature ionic conductivity exceeding that of liquid electrolyte is Li10GeP2S12 reported by Kamaya et al. In 2011[78]. The room temperature lithium ion conductivity of the sulfide solid electrolyte material Li10GeP2S12 can reach 1.2×10-2S/cm,The crystal framework is composed of a long chain formed by alternately connecting (Ge0.5P0.5)S4 tetrahedra and LiS6 octahedra on a c-axis by sharing edges, and a netlike structure formed by connecting PS4 tetrahedra and LiS6 octahedra by sharing tops. The high conductivity of the Li10GeP2S12 is attributed to the highly anisotropic thermal vibration of the Li+ on the crystal. The Li2S-SiS2-P2S5(LSPS) series solid state electrolyte, which uses Si with higher cost performance to replace the expensive Ge in LGPS type solid state electrolyte, is considered as a promising candidate material for the development of a new generation of solid state electrolytes[77].
Substituting Ge in LGPS or modifying it is an expanding direction of LGPS research, which can be divided into cation doping and anion doping. The Ge4+ in the LGPS may be partially replaced by a + 3 valent or + 4 valent metal cation of the same main group. Cation doping can reduce the activation energy of LGPS and make the Li+ more active in the electrolyte. However, the mobility of the Li+ is more anion dependent. The anion with larger radius can expand the transport channel of the Li+ and help the Li+ to escape from the anion sublattice.
Therefore, Kato et al. Reported a Li9.54Si1.74P1.44S11.7Cl0.3(LSPSCl) of LGPS type sulfide solid electrolyte with ionic conductivity twice that of the Li10GeP2S12 at room temperature (2.5×10-2S/cm)[79]. This material is the first LGPS type sulfide solid electrolyte that can conduct in large area 3D. The results show that LSPSCl has a 3D lithium ion conduction mechanism: 1D conduction along the C axis and 2D conduction along the ab plane. The LiCoO2/LSPSCl/C and LiCoO2/LSPSCl/Li4Ti5O12 cells assembled with LSPSCl showed excellent cycle performance, with 75% of the initial capacity retained after 500 cycles, and the coulombic efficiency was as high as 100%. Therefore, they believe that LGPSCl has very good electrochemical stability and has potential application value. Recently, Li9.54Si1.74P1.44S11.7X0.3(X=F,Cl,Br,I) series of sulfide solid state electrolytes have been prepared by controlling the particle size of silicon and different halogen doping using silicon, phosphorus and sulfur as raw materials[80]. The results show that the Li9.54Si1.74P1.44S11.7I0.3 prepared with 3 nm Si particles have 1.35×10-3S/cm ionic conductivity at room temperature and a surprisingly wide electrochemical window of 9 V.
In recent years, the research on Li-P-S series sulfide electrolytes is also increasing. In many reports, the Li6PS5X(X=Cl,Br,I) electrolyte has been deeply studied by researchers because of its excellent conductivity and mechanical properties[81]. The crystal structure of Li6PS5X(X=Cl,Br,I) is shown in fig. 13. In the tetrahedral structure, the Li+ are randomly distributed in the interstice of the tetrahedron, and the P atom mainly occupies the 4B site of the tetrahedral interstice, while the S atom completely occupies the 16e site. It is worth noting that the Li+ occupies the 24 G site in the Li6PS5Cl lattice, while it is on the 24 G and 48 H sites in the Li6PS5Br lattice. These sites are interconnected by interstitial sites around the X- and the S2-, respectively, and the Li+ diffuses through these partially occupied sites. It is reported that the electrochemical window of the Li6PS5X(X=Cl,Br,I) is wide, ranging from 0 to 7 V, and the ionic conductivity at room temperature can reach up to 10-2S/cm[82,83]. In addition, the mechanical properties of the Li6PS5X(X=Cl,Br,I) are excellent. For example, Doux et al. Recently studied the effect of superimposed pressure on the performance of Li/Li6PS5Cl/LiNi0.80Co0.15Al0.05O2 cells[84]. The results showed that the battery could deposit/desorb lithium stably for more than 1 000 H at a low superimposed pressure of 5 MPa, and the capacity of the battery was still 81% of the initial charge-discharge capacity after 100 cycles. However, the electrolyte surface can be modified under the high superimposed pressure of 25 MPa, and the Li+ can enter the pores of the electrolyte along the electrode/electrolyte interface, so that the impedance of the battery is reduced to 32 Ω. In addition, Kasemchainan et al. Used Li/Li6PS5Cl/Li cells to study the process of lithium deposition/desorption at the interface of Li/Li6PS5Cl[85]. When they applied 7 MPa pressure to the Li symmetrical battery, its critical current density could reach 1.0 mA·cm-2. When the current density of delithiation exceeds the rate of lithium deposition, voids will form on the surface of Li metal electrode, which will accumulate in the subsequent lithium deposition process, eventually reducing the contact area at the Li/Li6PS5Cl interface, increasing the local current density and leading to the growth of lithium dendrites. The applied pressure can increase the critical current density, thereby inhibiting the growth of lithium dendrites at the interface and preventing the formation of lithium dendrites below the critical current density.
At present, the reported Li-P-S series sulfide solid state electrolytes include Li2P2S6,Li7P3S11, Li3PS4, Li7PS6, Li4P2S6, etc[86]. Among them, the ionic conductivity of Li7P3S11 electrolyte can reach 3.2×10-3S/cm at room temperature due to the existence of PSO43- tetrahedrons in the crystal[87]. The Li3PS4 electrolyte is a polymorphic electrolyte having three structures: an alpha phase formed under high temperature conditions, a beta phase formed under intermediate temperature conditions, and a gamma phase formed under low temperature conditions. In the γ phase Li3PS4, the PS43- tetrahedra are stacked along the same direction. When the γ phase transforms to the β phase, the distance of Li-S bond increases, and the sites occupied by Li increase, so the Li+ in the β phase Li3PS4 is easier to conduct[88].
Sulfide solid electrolytes are commonly used in lithium-sulfur batteries. At present, the bottleneck of lithium-sulfur batteries is mainly due to the high insulation of sulfur and the poor compatibility of electrode/electrolyte interface, which limits the use of active materials[89]. These obstacles lead to the rapid increase of interface resistance and the rapid decline of electrochemical performance of lithium-sulfur batteries during repeated charge-discharge processes. Therefore, the key to improve the electrode/electrolyte interface compatibility of lithium-sulfur batteries is to prevent the occurrence of side reactions at the electrode/electrolyte interface and inhibit the growth of high-impedance Solid electrolyte interphase (SEI).
Controlling and modifying the microstructure of sulfide electrolyte is a concrete measure to adjust and improve its stability. Wu et al improved the voltage stability of LGPS and LSPS electrolytes by controlling the synthesis process and preparing the corresponding core-shell structure[90]. The Li9.54Si1.74P1.44S11.7Cl0.3(LSPS-Cl) obtained after annealing at 450, 460, 480 and 500 ℃ all have core-shell structure. They used seven different LSPS-Cl materials to make Li4Ti5O12+LSPS-Cl+C/LSPS-Cl/ glass fiber/Li batteries. The results are shown in Figure 14 a ~ C. LSPS-Cl 480, 490 and 500 batteries could not be charged above 4 V due to severe decomposition. The LSPS-Cl450, 460 and 470 batteries can be cycled smoothly between 1 and 5 V. In addition, as shown in fig. 14d, the LSPS-Cl460 battery was able to suppress the capacity decay relatively well during the cycle. As a comparison, they used LSPS460 to make the battery, and they also got better cycle performance.
Recent studies have shown that anions with high ionic strength (Br-, NO3-, etc.) Can effectively reduce the resistance of Li-S batteries during cycling by transforming dense Li2S into porous three-dimensional sheet Li2S[91]. Zhang et al. Doped LiBr into Li10GeP2S12 electrolyte and obtained a new type of LiBr-Li10GeP2S12(LiBr-LGPS) composite electrolyte after adjusting its composition[92]. XRD and Raman spectra showed that the composite electrolyte was composed of Li6PS5Br and Li10GeP2S12. They assembled a L-NCA/LiBr-LGPS/L-NCA symmetric battery using a LiNi0.80Co0.15Al0.05O2(L-NCA) with a LiNbO3 coating as the electrode. After several times of charging and discharging, the interface resistance of the battery was only 71Ω·cm2. The Li6PS5Br in the composite electrolyte can form a passivation protective layer in situ, inhibit the growth of insulating Li2S and reduce the loss of Li+ at the L-NCA/LiBr-LGPS interface, so that the interfacial conductivity and chemical compatibility between the cathode or anode and the LiBr-LGPS electrolyte are obviously improved.
[HJ50x] Also, SEI can also act as a protective layer to prevent Li from side reacting with the sulfide electrolyte. Therefore, the interface compatibility of the lithium-sulfur battery can also be effectively improved by constructing the SEI with high lithium ion conductivity and low resistance. For example, Umeshbabu et al. significantly improved the Li/LGPS interfacial stability by using the ionic liquid lithium bis (trifluoromethanesulfonyl) imide (LiTFSI)/N-methyl-N-propylpyrrolidine bis (trifluoromethanesulfonyl) imide (PYR13TFSI) as an interfacial modifier[93]. PYR13TFSI can form in-situ SEI at the Li/LGPS interface, which is compared to the appearance of LGPS electrolyte without the addition of LiTFSI/PYR13TFSI, as shown in figs. 15A, B. In the Li/LGPS/Li symmetric cell, the LiTFSI/PYR13TFSI was very effective in reducing the interfacial resistance from 2021Ω·cm2 to 142Ω·cm2 and maintained a constant low interfacial resistance for 15 d (figs. 15C, D).

4 Halide electrolytes

There are few reports about halide electrolytes, such as LiMBr4(M=Al,In,Ga), Li3InBr6-xClx, etc[94,95,105]. The lithium ion conductivity of halide solid state electrolyte is usually in the range of 10-3~10-7S/cm, and there are unstable conductive phases and electrochemical instability at room temperature, which is usually considered as a solid state electrolyte material system with low ionic conductivity and low stability[96]. The general chemical formula of the halide solid state electrolyte is LiaMXb(X=F,Cl,Br,I), and the halide solid state electrolyte can be divided into four types according to the types of M elements: (1) halide solid state electrolytes in which M is a group 3 metal element (M = Sc, Y, La-Lu)[97]; (2) a halide solid electrolyte In which M is a group 13 metal element (M = Al, Ga, In); (3) a halide solid electrolyte in which M is a divalent metal element (M = Ti, V, Cr, Mn, Fe, etc.); (4) M is a halide solid electrolyte of a non-metal element (M = N, O, S).
For the halide solid electrolyte in which M is a group 3 metal element (M = Sc, Y, La-Lu), the typical electrolyte includes a Li3YCl6 of a hexagonal close-packed structure (hcp) and a Li3YBr6 of a cubic close-packed structure (ccp). For the hcp type electrolyte, the electrolyte with high ionic conductivity can be obtained directly by ball milling, while for the ccp type electrolyte, the electrolyte with high ionic conductivity needs to be obtained by sintering[96].
Recent studies have shown that halide solid electrolytes can also have high conductivity and stability. Asano et al. Reported two new lithium halide solid electrolyte materials, Li3YCl6(LYC) and Li3YBr6(LYB), and described the crystal structure and Li+ transport path of the two electrolytes[96]. The room temperature ionic conductivity of LYB in cold pressed powder state can reach the 1.7×10-3S/cm without additional intergranular resistance or grain boundary resistance. LYC and LYB all-solid-state batteries were assembled and tested with LiCoO2 and Li as positive and negative electrodes. The results show that the grain boundary resistance after the first charge is low, which is 16. 8 and 6.6Ω·cm2, respectively. The coulombic efficiency of the first cycle is higher than 94%, and the capacity can still maintain 98% after 100 cycles, showing a very stable cycle performance.
The crystal structures of LYC and LYB are shown in fig. 16, in which the LYC crystal is composed of hexagonal close-packed anion arrangement (hcp),Li+ migrates through the tetrahedral interstice along the Oct1- (Tet1 or Tet2) -Oct2 path on the ab plane, and jumps between octahedra along the Oct1-Oct3 path on the C axis; LYB is different from LYC in that its crystal is composed of cubic close-packed anions and can migrate in 3D in three directions, shuttling through tetrahedral interstices via Oct1- (Tet1 or Tet2) -Oct2 and Oct1-Tet3-Oct3 pathways, respectively. It can be seen from this that the anion sublattice of halide electrolytes is quite different from that of oxide and sulfide electrolytes in the body-centered cubic (bbc) arrangement. In most of the previous reports, the migration of Li+ in highly conductive electrolytes is achieved by covalent bonds, while in halide electrolytes, the migration of Li+ is achieved by ionic bonds with close anionic stacking layers (ccp or hcp).
Other studies have shown that it is easier to form cpp structure by using halide electrolytes with larger radius anions or smaller radius metal (Sc, Y, Er, etc.) Cations[97⇓~99]. For example, most of the Li3MBr6(M=Sm-Lu) are the same as LYB, and the sublattice arrangement of the anions is ccp[99]. Halides with an anionic sublattice arrangement of ccp promote faster migration of the Li+ than hcp[100]. Therefore, a halide electrolyte with a ccp structure, which has both high ionic conductivity and a wide chemical window, can be constructed using suitable metal cations. Inspired by this, Liang et al. Recently reported a halide electrolyte :Li3ScCl6 with good chemical and electrochemical stability and high ionic conductivity at room temperature[101]. The electrolyte has a wide electrochemical window from 0.9 V to 4.3 V, and its ionic conductivity can reach up to 3.02×10-3S/cm. In addition, the Li3ScCl6still has high oxidation stability above 4. 2 V, which greatly improves its compatibility with the electrode.
Similarly, in early 2020, Park et al. Reported a new mixed metal halide electrolyte Li3-xM1-xZrxCl6(M=Y,Er) with ionic conductivity higher than that of 1×10-3S/cm at room temperature[102]. Due to the unique structure of the Li3-xM1-xZrxCl6, its conductivity at room temperature is as high as 1.4×10-3S/cm, which is one of the highest values reported for halide electrolytes. The Li+ transport of Li3-xM1-xZrxCl6 in the ab plane is shown in Figure 17. The zigzag migration path of Li3-Oct.-Li3-Li2 is the most favorable one-dimensional ion transport path, and this migration path intersects with the migration path of Li2-Li3-Li1 in the ac plane, forming a 3D Li+ transport network. The electrolyte has high electrochemical oxidation stability, can be used as a cathode material without a protective coating, and can stably work in an environment of more than 4 V.
The halide solid electrolytes with group 13 metal M (M = Al, Ga, In) mainly include Li-Al-X, Li-Ga-X and Li-In-X (X = F, Cl, Br, I). Among them, the halide solid electrolytes of Li-In-X system have been studied In the early stage[95,103⇓~105]. Recently, Sun Xueliang and others have made a series of important research results in the Li3InCl6 system. In 2019, they reported a Li3InCl6 halide electrolyte synthesized in water, as shown in Figure 18[106]. Its ionic conductivity at room temperature can reach 2.04×10-3S/cm. In addition, the electrolyte can be completely recycled after dissolution in water. Solid-state lithium batteries assembled with LiNi0.8Co0.1Mn0.1O2 positive electrodes show amazing cycle stability. Although the Li3InCl6 halide electrolyte exhibits high ionic conductivity and good compatibility with the electrode, it is unstable to air. Therefore, they also tracked the decomposition process of Li3InCl6 exposed to air by optical microscopy, synchrotron X-ray powder diffraction, in-situ X-ray and other characterization methods[107]. For the first time, they found that Li3InCl6 is hydrophilic, Li3InCl6 will absorb water in the air, some Li3InCl6 will react with water in the air to form In2O3, LiCl, HCl, and the rest of the electrolyte will absorb water and form Li3InCl6·xH2O. This decomposition process leads to a decrease in the conductivity of the Li3InCl6, so in practical applications, the Li3InCl6 can show better stability in a low humidity environment, and is expected to become a promising solid electrolyte in commercial lithium ion battery applications.
The halide solid electrolytes with M as a divalent metal element (M = Ti, V, Cr, Mn, Fe, etc.) Can be divided into four types according to their crystal structures: spinel structure, olivine structure, distorted structure and Suzuki structure (Li6MX8)[97]. The room temperature ionic conductivity of these solid state electrolytes is generally not high, and the highest one is only 10-5S/cm at present. However, the ionic conductivity of the halide solid state electrolyte can be improved by adjusting the vacancy or Li+ in the structure[108,109].
The above three types of halide electrolytes are all composed of crystals with metal elements as the center, and when the center of the halide crystal is a non-metal element such as N, O, or S, they can also show high room temperature ionic conductivity, for example, the room temperature ionic conductivity of Li3OX(X=F,Cl,Br,I), Li2OHX(X=F,Cl,Br,I), and Li1-xOHxCl, which belong to the anti-perovskite crystal structure, can reach 10-3S/cm[110⇓~112]. Taking the Li3OX as a reference, Fang et al. Further explored the anti-perovskite electrolyte based on ionic clusters through theoretical calculations[113]. They predicted that the lithium ion conductivity of Li3SBF4 with similar structure to Li3OX can reach 10-2S/cm and have a low activation energy of 0.21 eV. In addition, in the case of appropriate ionic radius and internal charge distribution, ionic clusters can enhance the anti-perovskite structure with larger ion channels, thus providing more space for the migration of Li+. Recently, Yin et al. Enhanced the ionic conductivity of halide electrolytes by fluorination techniques to produce more lithium vacancies in perovskite-resistant electrolytes[114]. By fluorination, the ionic conductivity of the perovskite-resistant halide electrolyte is steadily increased from the 1.1×10-7S/cm to the 2.9×10-4S/cm, and the activation energy for Li+ migration is as low as 0.62 eV. In addition, the assembled LiFePO4/LiOHBrF/Li cell shows good interfacial compatibility, which proves the possibility of LiOHBrF electrolyte being applied to all-solid-state electrolyte. By further optimizing the composition of the electrode, it is expected to develop fluorinated lithium-rich anti-perovskite electrolyte (F-LiRAP) with higher ionic conductivity and more stable chemical properties.
The performance of early halide solid state electrolyte is not ideal, but the latest research results show that halide can also be a candidate material for solid state electrolyte with excellent performance and development potential. In addition, Kahle et al. used the Automated Interactive Infrastructure and Computational Science Database (AiiDA) Materials Information Platform to retrieve all CIF files of Li-containing structures from the Inorganic Crystal Structure Database (ICSD) and the Crystallography Open Database (COD), and found five materials with fast ion diffusion (Figure 19):Li10GeP2S12, Li5Cl3O, Li2CsI3, LiGaI4/LiGaBr3, and Li7TaO6[115]. According to the calculation results, three of them are halide solid electrolytes, which further predicts that halide is a promising solid electrolyte and may become a new research direction.

5 Conclusion and prospect

All-solid-state batteries provide a feasible way to further improve the energy density and safety of lithium-ion batteries, and it is believed that they will enter our lives thoroughly in the near future. Therefore, the research, development and application of solid state electrolyte will become more and more popular. Compare with that liquid electrolyte, the solid electrolyte has the advantages of high thermal stability, chemical stability and the like; As an important part of solid electrolyte, inorganic solid electrolyte has also been widely concerned by researchers at home and abroad. In this paper, the latest development of inorganic solid state electrolytes is reviewed, with emphasis on oxide electrolytes, sulfide electrolytes and halide electrolytes, and the advantages and problems to be solved in the development are discussed.
The oxide solid electrolyte has considerable room temperature ionic conductivity, wide electrochemical window and good thermal stability, and its high elastic modulus and high hardness inhibit the formation and growth of lithium dendrites in the battery to a certain extent. However, the problem of interface resistance has been hindering its wide application. How to reduce the interface resistance is one of the key factors to enhance the performance of oxide solid electrolyte. At present, some methods and strategies, such as the reaction of elemental carbon with oxide solid electrolyte, thermal pulse technology for rapid treatment of ceramic surface and rapid acid treatment, can effectively eliminate the Li2CO3 on the electrolyte surface or inhibit the formation of Li2CO3 at grain boundaries, thus improving the electrochemical performance of oxide solid electrolyte. In addition, the addition of a protective layer on the surface of the oxide solid electrolyte can also inhibit the occurrence of side reactions at the interface, thereby improving the problems of incompatibility between the solid electrolyte and the electrode and poor interface contact, and improving the cycle performance of the oxide solid electrolyte. The method is also applicable to other types of solid electrolytes, and provides an idea for solving the interface problem between the solid electrolyte and the electrode.
Sulfide solid electrolytes have higher conductivity than liquid electrolytes, but their texture is relatively soft, and lithium dendrites are easy to grow in the pores and grain boundaries of the electrolyte, which easily leads to poor compatibility of the electrode/electrolyte interface. These obstacles lead to a rapid increase in interfacial resistance and a rapid decline in electrochemical performance during repeated charge-discharge processes. Therefore, the key to improve the electrochemical performance of sulfide solid state electrolytes is to reduce the occurrence of side reactions at the interface, inhibit the formation of high impedance SEI and improve the interface compatibility. At present, the following methods can effectively improve the interfacial compatibility between sulfide electrolytes and electrodes: (1) controlling and modifying the microstructure of sulfide electrolytes to inhibit the occurrence of side reactions and thus inhibit the attenuation of capacitance; (2) introducing anions with high ionic strength (Br-, NO3-, etc.) To convert the dense Li2S into a porous three-dimensional sheet Li2S to effectively reduce the resistance of the battery during cycling; And (3) the battery interface compatibility is effectively improved by constructing the SEI with high lithium ion conductivity and low resistance.
As for the halide solid state electrolyte, the latest research results show that it has the characteristics of high conductivity, stable electrochemical performance, and can be stably cycled at a voltage of more than 4 V. It is a very potential electrolyte material and is expected to become an ideal solid state electrolyte in the future.
Based on the above research, it can be seen that inorganic solid electrolytes are also facing several challenges: (1) a lot of research is still needed to fully understand the basic mechanism of non-uniform deposition of Li in solid electrolytes; (2) There are also incompatibilities at the electrode/electrolyte interface, including the occurrence of side reactions and the formation of space charge layers; (3) Other interface problems with large deviation caused by the difference in the composition and structure of the interface between the solid electrolyte and the electrode material from that of the host material.
Therefore, while pursuing the dream of all-solid-state batteries with high safety and high energy density, the future research directions of inorganic solid-state electrolytes include but are not limited to: (1) improving the stability of the electrolyte/electrode interface; (2) reducing the electronic conductivity of the inorganic solid electrolyte, thereby reducing the formation of decomposition by-products due to electronic conduction; (3) designing a thinner and denser solid electrolyte that maintains satisfactory mechanical properties; (4) The exploration of oxynitride solid state electrolyte and halide solid state electrolyte; (5) Develop more advanced characterization and simulation techniques to monitor the interface reactions and the evolution of lithium dendrites in solid-state batteries.
There is still a long way to go to commercialize all-solid-state batteries, so we need to clarify the design ideas of solid-state electrolytes with first-principles thinking and make good use of current inorganic solid-state electrolyte materials. It is believed that inorganic solid electrolytes with high ionic conductivity, low interface resistance, good interface compatibility and high safety performance will play a huge potential in the next wave of green new energy development and help the application of all-solid-state batteries succeed as soon as possible.

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Funding

National Natural Science Foundation of China(21975055)

National Natural Science Foundation of China(61671162)

National Natural Science Foundation of China(21975053)

Guangdong Province Key Field R&D Program Project(2020B0101030005)

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