Polymer Electrolyte/Anode Interface in Solid-State Lithium Battery

Long Chen; Shaobo Huang; Jingyi Qiu; Hao Zhang; Gaoping Cao

doi:10.7536/PC200734

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1378-1389 DOI: 10.7536/PC200734

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Polymer Electrolyte/Anode Interface in Solid-State Lithium Battery

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Abstract

The energy density and safety of lithium secondary batteries are urgently required to be improved. Research on high-energy-density solid-state lithium batteries is of great significance to development of the new energy industries. Compared with the traditional organic electrolyte lithium-ion battery, the solid-state lithium battery with polymer solid electrolyte not only has significantly improved security, but also can match with high-capacity electrode materials to effectively improve the energy density. The polymer-based solid-state lithium battery is one of the most promising lithium secondary batteries. However, there are still some problems between polymer solid electrolyte and lithium anode, such as interface side reaction and lithium dendrites. In recent years, various methods have been used to improve the performance of solid-state lithium battery, including electrolyte composition regulation, electrolyte mechanical properties improvement, electrolyte/lithium anode interface regulation and matching three-dimensional lithium anode. Here, the common polymer solid electrolyte and its interface challenges with lithium anode are firstly introduced. Simultaneously, the recent research progress on improving the interface stability of polymer electrolyte/lithium anode is summarized and discussed in detail, including: adding inorganic fillers, using high-strength substrate film, building hierarchical layered structure, constructing interfacial buffer layer, designing and developing electrolyte with cross-linking network structure and fabricating protected solid-state lithium anode. Finally, the research and development trend of polymer solid electrolyte/lithium anode interface compatibility are prospected.

Contents

1 Introduction

2 Polymer solid electrolytes

3 The challenges in polymer electrolyte/lithium anode interface

4 Modification strategy of polymer electrolyte/lithium anode interface

4.1 Inorganic fillers

4.2 High strength substrate membranes

4.3 Design of hierarchical layered structure

4.4 Interfacial buffer layer

4.5 Design of structural cross-link network

4.6 Li-protection strategy in solid-state battery

5 Conclusions and outlook

Graphical abstract

Keywords

polymer electrolyte / lithium anode / interface / dendrite / solid-state lithium battery

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Long Chen, Shaobo Huang, Jingyi Qiu, Hao Zhang, Gaoping Cao. Polymer Electrolyte/Anode Interface in Solid-State Lithium Battery. Progress in Chemistry, 2021, 33(8): 1378-1389 DOI:10.7536/PC200734

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1 Introduction

With the increasing demand for electric vehicles and smart grids, the demand for lithium-ion batteries in the field of new energy will increase year by year in the future. Traditional lithium-ion batteries use organic liquid electrolyte, which is prone to side reactions between electrodes and electrolyte during charging and discharging, and irreversible capacity fading. At the same time, during the long-term use of lithium-ion batteries, the liquid electrolyte will volatilize, which will further affect the life of batteries^[1,2]. In addition, lithium-ion batteries have serious safety risks. Under abnormal conditions such as overcharge and internal short circuit, batteries are prone to heat, which can easily lead to fire, explosion and other safety accidents^[3,4]. Solid-state lithium batteries with solid electrolyte instead of organic electrolyte have obvious advantages, such as non-flammability, non-leakage, no sustained interface side reactions, easy packaging, wide working temperature range and so on, which are expected to completely solve the safety problems of batteries^[5]^[6]. At present, the most studied solid electrolytes mainly include oxides, sulfides and polymer solid electrolytes, among which polymer-based solid electrolytes are the closest to practical use. In addition, the solidification of lithium batteries also makes it possible to use high-capacity metal lithium negative electrodes. Lithium metal has a high theoretical specific capacity (3860 mAh·g^-1), a low oxidation potential (-3.040 V vs.H⁺/H₂), and a low bulk density (0.534 g·cm^-3)^[7]. Solid-state batteries with lithium metal as the negative electrode, including solid-state lithium batteries, solid-state lithium-sulfur batteries, and solid-state lithium-air batteries, all show significantly improved energy density.

However, it is found that when polymer-based solid electrolyte is matched with lithium anode to assemble solid-state lithium batteries, serious interfacial reactions between electrolyte and lithium anode and lithium dendrite growth still exist under long-term service conditions.The unstable interface between the electrolyte and the lithium negative electrode makes the battery cycle decay rapidly, and the lithium dendrite can pierce the organic matrix in the solid electrolyte, making the negative electrode contact with the positive electrode, resulting in a short circuit of the battery (Fig. 1)^[8,9]. The effective ways to improve the stability of electrolyte/lithium anode interface and inhibit the growth of lithium dendrite in solid-state lithium batteries are to adjust the composition of electrolyte, improve the mechanical properties of electrolyte, adjust the electrolyte/lithium anode interface and match the three-dimensional lithium anode with electrolyte^{[10⇓⇓~13]}. In this paper, the common polymer electrolytes and the interface challenges between electrolytes and lithium anodes are introduced, and the latest research progress in obtaining efficient and excellent interface stability of solid electrolytes/lithium anodes is reviewed.

2 Polymer solid electrolytes

Polymer solid electrolyte is composed of highly polar polymer and metal lithium salt. It is considered as a powerful candidate for the next generation of high energy density storage devices because of its low reactivity with electrode materials, light weight, easy processing into film and low production cost^[14]. Polyethylene oxide (PEO) -based electrolyte is the most studied system at present. The Li⁺ transport of this electrolyte is mainly based on the strong complexation-decomplexation effect between the Li⁺ and the ether oxygen groups on the PEO chain, and the Li⁺ migration is realized through the chain segment movement of the polymer matrix^[15]. However, the polymer matrix is prone to crystallization, resulting in low room temperature ionic conductivity (10^-8~10^-6S·cm^-1) of the electrolyte. The PEO matrix was modified by blending, copolymerization and crosslinking to inhibit the crystallization of the matrix and enhance the movement of its chain segments, thus improving the ionic conductivity of the electrolyte^[16,17]^[18,19]^[20,21]. In addition, the system is prone to oxidative decomposition when matched with high energy density oxide cathode materials (LiCoO₂, ternary materials, etc.), and the electrochemical stability of the electrolyte is poor^[22,23]. The surface coating of the positive electrode can inhibit the side reaction between the oxide positive electrode and the PEO matrix and meet the use requirements of the high-voltage positive electrode^[24,25]. Compared with PEO system, poly (ethylene carbonate), poly (propylene carbonate), poly (vinylidene fluoride), polyacrylonitrile, P (vinylidene fluoride-hexafluoropropylene) and poly (methyl methacrylate) have higher electrochemical stability.However, the room temperature ionic conductivity of polymer electrolytes composed of polymer and lithium salt is still low, only 10^-8~10^-6S·cm^-1, and has a large temperature dependence, which is far from meeting the actual needs^[26]^[27]^[28]^[29]^[30]^[31]. The composite electrolyte system obtained by adding inorganic filler, liquid electrolyte, nitrile and ionic liquid is an effective way to improve its ionic conductivity^[32,33]^[34,35]^[36,37]^[38,39].

3 Polymer electrolyte/lithium anode interface challenge

Similar to the lithium dendrite growth of lithium metal in liquid electrolyte lithium ion batteries, lithium metal can not completely inhibit lithium dendrite growth when matched with polymer solid electrolytes, mainly due to the low ion migration ability, poor self-supporting and mechanical properties of polymer electrolytes^[40,41]. When the polymer electrolyte is matched with the lithium negative electrode, the lithium dendrite can penetrate the flexible polymer matrix and quickly reach the positive electrode, causing a series of safety problems such as short circuit, thermal runaway, fire and explosion^[42,43]. Balsara et al. Suggested that lithium dendrite growth is closely related to impurities at the solid electrolyte/lithium anode interface^[44]. Due to the insulating nature of the impurity, it is unable to conduct electrons, and the deposition and stripping of lithium metal can only occur away from the impurity. It is difficult for lithium metal to deposit uniformly on the surface of the negative electrode, resulting in the increase of local current. In addition, the continuous uneven deposition will lead to the formation of holes in the upper part of the impurities, resulting in the formation of gaps between lithium metal and solid electrolyte, which will further accelerate the dendrite growth of lithium anode. Lascaud et al. Have studied the dendrite growth process in polymer lithium batteries and found that the dendrite growth of lithium depends heavily on the local current density on the surface of the negative electrode^[45]. At high current density, due to the lack of anions at the electrolyte/anode interface, the polarization of the battery will produce a space charge region, which will lead to the uncontrolled reduction of lithium ions and the rapid deposition of lithium metal at low-dimensional sites such as dendrites. At low current density, lithium dendrite growth is mainly affected by the local non-uniform current distribution at the anode/electrolyte interface. Tarascon et al. Observed the growth process of lithium dendrite in PEO-based polymer electrolyte by in-situ scanning electron microscopy^[46]. The results show that with the increase of current density, the morphology of lithium dendrite gradually changes from moss-like to dendritic. Under the 0.22 mA·cm^-2 current density, the growth is mainly moss-like, and under the 0.5 mA·cm^-2 current density, the growth is mainly dendritic. Further studies show that the lithium dendrite growth sites are mainly located between the polymer and lithium metal, and the dendrite growth occurs simultaneously in the axial and radial directions. Severe dendrite growth leads to the separation of the electrolyte from the substrate, which affects the cycle performance of the battery.

In addition, there is an obvious interfacial reaction between polymer/lithium salt system and lithium anode, which makes it difficult to form a stable solid electrolyte interface layer (SEI). Even in the static state of the battery, the interfacial impedance between polymer electrolyte and lithium metal generally shows an upward trend with the extension of time^[47,48]. Teeters et al studied the interfacial reaction process between PEO-lithium trifluoromethanesulfonate (LiCF₃SO₃) polymer electrolyte and lithium anode by infrared spectroscopy and atomic force microscopy.It is believed that the LiCF₃SO₃ lithium salt first reacts with lithium metal to form CF₃ free radicals, which can quickly take away the hydrogen atoms from the polymer backbone, and then the lithium atoms will destroy the C-O bonds on the polymer backbone to form Li-O-R compounds^[49]. Aurbach et al. used small molecule dimethyl ether (DME) solution to simulate the interfacial reaction process between PEO-based electrolyte and lithium metal, and proposed a two-step two-electron reduction mechanism of DME. First, electrons on lithium metal were transferred to DME to form radical anions, and then Li⁺ cations combined with radical anions and decomposed into alcohol oxides (CH₃OLi) and methoxyethyl radicals^[50]. Further, the methoxyethyl radical undergoes a single electron transfer reaction to produce more lithium methoxide and ethylene.

4 Polymer electrolyte/lithium anode interface modification strategies

In order to solve the interface problem of polymer electrolyte/lithium anode, not only the formation of stable SEI layer should be paid attention to, but also the high mechanical properties of composite electrolyte should be ensured. Only when the shear modulus of electrolyte material is 1.8 times that of lithium metal, can the electrolyte effectively inhibit the dendrite growth on the surface of lithium anode^[51]. Aiming at the key interface problems between polymer electrolyte and lithium anode, researchers mainly use the methods of adding fillers to construct organic/inorganic composite electrolyte, constructing high mechanical property electrolyte system, designing hierarchical structure of electrolyte, constructing interface buffer layer and matching lithium composite anode.O as to obtain the electrolyte/lithium negative electrode interface with efficient lithium ion transmission, obviously reduced interface resistance, and effectively inhibited volume expansion and surface dendritic growth of the lithium negative electrode^{[13,52⇓⇓~55]}.

4.1 Inorganic Filler

Adding inorganic filler to polymer matrix to obtain composite electrolyte is an effective way to obtain good solid electrolyte/inorganic interface. The flexible polymer can increase the interface contact between the electrolyte and the lithium anode, and the inorganic filler can not only improve the mechanical properties of the electrolyte, but also help to form a stable solid electrolyte interface layer (SEI) on the surface of the anode, thereby inhibiting the dendrite growth on the surface of the lithium anode and improving the interface stability of the electrolyte/lithium anode^[56⇓~58]. There are many kinds of typical inorganic fillers.Inert fillers (Montmorillonite, Palygorskite, Halloysite,Al₂O₃, SiO₂, TiO₂) and active filler (Li₇La₃Zr₂O₁₂(LLZO),Li₁₊_xM_xTi_2-_x(PO₄)₃(LATP), Li₁₊_xAl_xGe_2-_x(PO₄)₃(LAGP) and Li₁₀GeP₂S₁₂)^{[13,52,59⇓⇓ ~62]}^{[63⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~76]}. Cui et al. Prepared a SiO₂ based aerogel network, which can significantly improve the room temperature ionic conductivity and mechanical properties of the composite electrolyte as an inorganic filler. Its highly rigid characteristics increase the elastic modulus of the electrolyte by more than 10 times, about 0.43 GPa, thus effectively inhibiting the mechanical growth of lithium dendrites^[67]. The solid state LiFePO₄|Li lithium battery assembled by the electrolyte has good cycle performance and rate capability, and the maximum area capacity of the battery is up to 2.1 mAh·cm^-2. Compared with inert fillers, active fillers are more widely studied in inhibiting lithium dendrite. Fan et al. Developed a series of PEO-Li_6.4La₃Zr_1.4Ta_0.6O₁₂(LLZTO) composite solid electrolytes with excellent flexibility and mechanical properties by solvent-free hot-pressing method^[68]. With the increase of ceramic content, the transition from "ceramic-in-polymer" to "polymer-in-ceramic" is realized. At the same time, the electrolyte realizes the transition from single polymer lithium conduction to two-phase lithium conduction of polymer and inorganic ceramic electrolyte (Fig. 2a). Both types of solid electrolytes show good interfacial compatibility with lithium metal anodes, especially the "polymer-in-ceramic" electrolyte with high ceramic content has significantly enhanced dendrite inhibition ability. The symmetrical lithium battery has no short circuit after 680 hours of cycling at 0.5 mA·cm^-2 current density, showing a good ability to inhibit dendrite (Figure 2b). The solid state lithium battery assembled by the composite solid electrolyte shows excellent cycle and rate performance at 55 deg C. Solid-state batteries can still work stably in the case of bending and missing corners, showing good safety (Figure 2 C).

Zhang et al. explored the mechanism of PEO-LLZTO composite electrolyte in inhibiting lithium dendrite, and believed that LLZTO particles contributed to the formation of stable SEI on the surface of negative electrode, and more importantly, LLZTO was a single ion conductor.As a result, the ion transference number of the PEO/LiTFSI/LLZTO (PLL) electrolyte system was significantly increased, and a stable Li⁺ deposition/desorption electric field was obtained on the surface of the lithium anode, thus realizing the uniform deposition of Li⁺^[69]. Compared with the traditional liquid electrolyte, the PLL electrolyte effectively inhibits the growth of lithium dendrites (Figure 3). LLZTO single ionic conductor can not only be used as PEO-based electrolyte filler, but also be widely used in polymer-based electrolyte systems such as PEC, PPC, PVDF, PAN and P (VDF-HFP). The introduction of LLZTO makes the composite electrolyte show improved ionic conductivity, mechanical properties and excellent electrolyte/anode interface stability^{[26⇓⇓⇓~30]}.

In addition to LLZTO particles as fillers, LLZTO nanofibers are also excellent lithium dendrite inhibiting fillers^[70]. Hu et al. prepared LLZTO nanofiber membrane with three-dimensional conductive network by electrospinning technology (Fig. 4A), and the PEO-based composite solid electrolyte developed with this as filler has high room temperature ionic conductivity and excellent mechanical properties^[71]. The single ion conductor fiber provides a fast ion transmission channel, and the fiber network structure realizes high mechanical strength.

Symmetric lithium batteries assembled with the LLZTO nanofiber reinforced PEO electrolyte can be stably cycled for 1000 H at 25 ° C and 0.5 mA·cm^-2 (Figure 4 B), demonstrating excellent dendrite inhibition. A similar method has also been used in La_2/3-_xLi₃_xTiO₃(LLTO) nanofibers. Nan et al. Used the same electrospinning technology to prepare LLTO nano-three-dimensional network skeleton, and then prepared LLTO network reinforced PEO-based composite electrolyte by pouring PEO and hot pressing (Fig. 4C)^[72⇓~74]^[74]. The electrolyte has high mechanical strength and room-temperature ionic conductivity (1.8×10^-4S·cm^-1), and the symmetric lithium battery can be stably cycled for 800 H at 0.1 mA·cm^-2 current density, showing excellent lithium dendrite inhibition effect and interface stability.

However, compared with garnet-type LLZO, perovskite-type LLTO and NASICON-type LATP, the interface resistance between electrolyte and lithium anode increases with time due to the obvious reduction reaction between Ti⁴⁺ and lithium metal^[75]. It is an effective strategy to isolate the direct contact between the ceramic powder and the lithium anode by wrapping the ceramic powder. Fan et al. Wrapped the LATP ceramic powder inside the PAN fiber through a simple spinning method, which can effectively avoid the direct contact between the LATP ceramic powder and lithium metal, and improve the stability of the interface between the electrolyte and the lithium anode^[76]. At the same time, the PAN fiber has excellent mechanical properties and can inhibit the dendrite growth on the surface of the lithium anode. The PEO-based electrolyte was further cast on the above PAN-LATP fiber membrane to obtain a composite solid electrolyte having a bicontinuous three-dimensional conductive network structure (fig. 4D). The electrolyte has ultra-high mechanical strength (10. 72 MPa) and the ability to inhibit lithium dendrite growth. The assembled LiFePO₄|Li solid state lithium battery showed excellent performance, with a capacity retention of 96.7% after 100 cycles at 0.2 C.

4.2 High-strength basement membrane

It is the most direct and efficient way to inhibit the dendrite growth and improve the interface stability of lithium anode by casting the polymer electrolyte directly on the polymer film with high mechanical strength (such as polyimide (PI), cellulose acetate, etc.)^[80⇓~82]. Cui et al. Made PI/PEO/lithium bis (trifluoromethanesulfonimide) (LiTFSI) composite electrolyte by casting PEO-based polymer electrolyte on a PI film of 8.6 μm as a substrate^[80]. The PI film has excellent combustion resistance and high mechanical strength, and the assembled symmetric lithium battery has no short circuit after 1000 H cycling, which indicates that the lithium dendrite is effectively inhibited. In addition, the membrane also has a typical vertical ion transport channel, which can improve the ionic conductivity of the electrolyte. The solid state LiFePO₄|Li battery assembled with the PI/PEO/LiTFSI electrolyte exhibited excellent cycle stability and safety, and could be stably cycled for 200 times at 60 ℃ and 0. 5 C rate, which indicated that the application of PI film hindered the formation of lithium dendrite and avoided the safety problems such as short circuit. Similarly, Fan et al. poured the PVDF/LLZTO/LiTFSI composite system directly onto a high-porosity PI substrate to obtain a three-dimensional composite electrolyte system with a porous host^[81]. Compared with the PVDF/LLZTO/LiTFSI electrolyte without PI film, the PI/PVDF/LLZTO/LiTFSI three-dimensional composite solid electrolyte has significantly improved mechanical properties (11. 5 MPa) and lithium dendrite inhibition ability. The symmetrical lithium battery can work stably for 1 000 H at room temperature after 0.2 mA·cm^-2 current density cycling. Moreover, the system is also matched with a ternary LiNi_0.5Co_0.2Mn_0.3O₂ positive electrode with high voltage, and the capacity retention rate of the assembled solid-state soft-pack battery reaches 94. 9% after 80 cycles at 0. 1 C rate. All these indicate that the addition of PI substrate film with high mechanical strength is an effective means to stabilize the solid electrolyte/lithium anode interface.

in addition, a composite solid electrolyte with excellent lithium conductivity and mechanical properties was prepared by introducing the organic rigid cellulose acetate membrane skeleton into the Polymer-in-salt double grafted polysiloxane electrolyte by solution casting technology^[82]. The tensile strength (6. 8 MPa) of the electrolyte was improved by the introduction of the acetate fiber skeleton. The high-content lithium salt (LiTFSI) of the Polymer-in-salt type electrolyte is easy to react with the lithium negative electrode to form products such as LiF or Li₂S, thereby building a stable SEI layer. The interface impedance between solid electrolyte and lithium anode stabilized at 902 Ω after 240 H. The combined effect of the acetate fiber film with high mechanical properties and the stable SEI layer effectively inhibits the dendrite growth on the surface of the lithium anode. Solid-state lithium-sulfur batteries with MWCNT @ S as positive electrode have excellent cycle performance and rate capability. Cellulose acetate film can not only inhibit the dendrite growth of lithium negative electrode, but also prevent the dissolution and shuttling of polysulfide in lithium-sulfur batteries.

4.3 Hierarchical layered structure design

There are two kinds of interfaces in solid state lithium batteries, one is the interface between positive electrode and electrolyte, the other is the interface between electrolyte and negative electrode. The positive electrode-electrolyte interface needs to maintain a good ion transport channel, and the flexible interface design can provide a stable interface contact between the positive electrode and the electrolyte, which can alleviate the volume expansion of the electrode material and the interface during the charge-discharge cycle of the battery, and maintain a stable lithium ion transport channel^[83]^[84]. On the contrary, lithium anode has a serious problem of lithium dendrite growth during cycling, and the use of electrolytes with excellent mechanical properties can effectively inhibit the dendrite growth of lithium anode^[51]. Therefore, polymer-based electrolytes with good flexibility and good physical contact are used on the positive electrode side, and ceramic-based electrolytes with good interface stability, strong rigidity and ability to inhibit lithium dendrite growth are preferred on the negative electrode side.It is an effective strategy to solve the interface problem of solid-state lithium batteries by hierarchical differentiation design to obtain composite electrolytes with heterogeneous multilayer structure.

Sun et al. developed the hierarchical PEO-LLZTO electrolyte by using the polymer-in-ceramic electrolyte obtained by compounding LLZTO and PEO with particle size of 5 μm as the interlayer of sandwich structure, and adding the "ceramic-in-polymer" electrolyte obtained by compounding 200 nm inorganic LLZTO and PEO to coat both sides of the interlayer^[85]. By using the micron-sized ceramic particles, the electrolyte has more excellent mechanical properties and capability of inhibiting the growth of lithium dendrites; Due to the adoption of nanoscale ceramic particles, the composite electrolyte has better flexibility, and the contact stability between the composite electrolyte and the electrode material is improved. Therefore, the solid electrolyte with the sandwich structure not only has excellent interface contact performance with the electrode, but also can effectively suppress the direct growth of the surface of the lithium negative electrode (fig. 5A). The symmetrical lithium battery assembled with the sandwich electrolyte still has stable Li⁺ for more than 400 H under the conditions of 30 ℃ and 0.2 mA·cm^-2. Goodenough et al. designed a hetero-bilayer structure of polymer-based electrolyte (Fig. 5B), using poly N-methylmalonamide (PMA) -based electrolyte with high pressure oxidation resistance on the positive side and PEO-based electrolyte with stable interface with lithium metal on the negative side^[55]. In order to study the interface stability between the double-layer structure electrolyte and lithium anode, a layer of PEO-based electrolyte was also constructed on the surface of the positive PMA of the double-layer electrolyte to inhibit the side reaction between PMA and lithium metal, thus obtaining a sandwich structure PEO/PMA/PEO composite electrolyte. The symmetric lithium battery assembled with the PEO/PMA/PEO electrolyte exhibited excellent lithium dendrite inhibition after 14 days of stable cycling at 0.1 mA·cm^-2 current density at 65 ℃. Wang et al. Constructed a layered composite solid electrolyte with a hierarchical gradient structure (Fig. 5C) by compounding Polymer ion conductors PEO and succinonitrile (SN) with high salt concentration and PVDF membrane skeleton with high electrochemical stability to obtain a polymer-in-Separator, and modifying a high-density LLZTO inorganic nanoparticle layer (Garnet-at-Interface) on the lithium anode side^[86]. The PVDF framework with high dielectric constant and the LLZTO layer with high ionic conductivity/low electronic conductivity can uniformly stabilize the electric field at the interface between the electrolyte and the lithium anode, which is conducive to the uniform deposition of lithium ions on the surface of the anode. In addition, the presence of the LLZTO layer also contributes to the formation of a thin and dense composite SEI film on the surface of the lithium anode (fig. 5d). At room temperature, the symmetric lithium battery assembled with the hierarchical layered structure electrolyte can be stably cycled for 4800 H at 0.5 mA·cm^-2 current density, and lithium ions are uniformly and stably exfoliated and deposited on the surface of the negative electrode.

4.4 Interface buffer layer

The introduction of an interfacial buffer layer between the polymer-based electrolyte and the lithium anode is also an effective way to stabilize the electrolyte/anode interface. Yamauchi et al. introduced a 50 nm thick Al₂O₃ atomic layer into PEO-LiTFSI electrolyte by atomic layer deposition method (Fig. 6a, B). The existence of inorganic Al₂O₃ helps to improve the mechanical properties of the electrolyte and limit the ability of lithium salt anion movement, thus improving the Li⁺ transference number of the electrolyte^[87]. The symmetric lithium battery assembled with PEO polymer electrolyte modified by Al₂O₃ atomic layer can be stably cycled for 500 H at 70 ℃ and 0.25 mA·cm^-2 current density, while the corresponding pure PEO-LiTFSI electrolyte can be short-circuited after 250 H. In addition, symmetric lithium batteries assembled with PEO polymer electrolytes modified with Al₂O₃ can still operate at higher current densities (0.6 mA·cm^-2) (Fig. 6C), indicating that the rigid ceramic layer coated on the surface of polymer electrolytes can well inhibit the growth of lithium dendrites.

Using the design idea of heterogeneous multilayer electrolyte for reference, the composite solid electrolyte with asymmetric interface structure can be obtained by designing the interface with different functions on both sides of the solid electrolyte, which can not only simplify the preparation method, but also solve the interface contact problem between the solid electrolyte and the positive and negative electrodes, and effectively inhibit the growth of lithium dendrites^[88,89]. A highly rigid ceramic coating on the anode side of the polymer electrolyte is an effective way to inhibit the dendrite growth of lithium anode. Guo et al. Coated the negative side of Celgard separator with 5.7 μm thick rigid LLZO ceramic powder, and further cast the crosslinked poly (ethylene glycol) methyl ether acrylate electrolyte on the above asymmetric membrane to prepare a composite solid electrolyte with asymmetric structure^[90]. The electrolyte has excellent interface contact performance with a positive electrode and a negative electrode, and a stable SEI layer is formed on the surface of the negative electrode due to the existence of the rigid ceramic powder, so that the dendrite on the surface of the lithium negative electrode is effectively inhibited. Under the condition of 55 ℃ and 0.1 mA·cm^-2, the symmetrical lithium battery assembled with poly (ethylene glycol) methyl ether acrylate electrolyte and Celgard separator could only work stably for 1200 H, and then the voltage of the battery instantaneously changed to 0 V, and the battery was short-circuited. In contrast, the symmetric battery assembled with asymmetric separator can work stably for more than 3000 H under the same conditions, which indicates that the existence of ceramic powder can effectively inhibit the growth of lithium dendrite.

4.5 Cross-linked network structure design

The cross-linked polymer electrolyte system obtained by in-situ photo-thermal initiation not only has excellent ionic conductivity and mechanical properties, but also has good interfacial wetting and contact properties between the electrolyte and the electrode material, and is easy to realize stable interfacial ionic transport characteristics^[91,92]. In addition, the high mechanical properties also enable this type of electrolyte to effectively inhibit the growth of lithium anode dendrites, even at 90 ℃, the electrolyte can still prevent dendrites from passing through, and the lithium metal battery shows excellent high temperature performance. Li et al. Reported a cross-linked network polymer with polyhedral oligomeric silsesquioxane as cross-linker and amino-terminated poly (ethylene glycol) as monomer^[92]. The solid electrolyte developed with the cross-linked polymer still has a storage modulus of 33.6 MPa at 90 ℃, and the assembled symmetric lithium battery can operate stably for 2 600 H, 1 212 H and 441 H at current densities of 0.3, 0.5 and 1 mA·cm^-2, respectively. This shows that the crosslinked electrolyte system with excellent mechanical properties can still inhibit the growth of lithium dendrites in extreme high temperature environment, showing a significant improvement in battery safety and environmental adaptability. Yu et al. Obtained a cross-linked polymer electrolyte by using two liquid components of modified silicon-terminated polyether monomer (MSTP), LiTFSI and tetraethylene glycol dimethyl ether (TEGDME) through catalytic polymerization with di-n-butyltin bis (2,4-glutaric acid)^[93]. The Li⁺ transference number of the electrolyte reaches 0.68 at room temperature, and the assembled symmetric lithium battery can be stably cycled for 800 H at 0.1 mA·cm^-2. The solid state lithium battery assembled with the electrolyte matched with the LiFePO₄ positive electrode can still be stably cycled for 600 times at the rate of 3 C, and the coulombic efficiency is close to 99.7%. The excellent performance shows that the cross-linked system electrolyte can inhibit the growth of lithium dendrite and maintain good compatibility with electrode materials.

Guo et al. synthesized a novel, bifunctional cross-linked polymer electrolyte by encapsulating PEO electrolyte in the cross-linked structure constructed by ethoxylated trimethylolpropane triacrylate through photo-initiated in-situ polymerization (Fig. 7A)^[94]. This electrolyte not only exhibits excellent lithium deintercalation behavior (fig. 7B), but also has a room-temperature ionic conductivity of 2.2×10^-4S·cm^-1 and a tensile modulus of 12 GPa (fig. 7C). Compared with the traditional liquid electrolyte, the Li | Cu battery assembled with the electrolyte can stably deposit 2 mAh·cm^-2 at 0.5 mA·cm^-2 current density, a stable SEI layer is formed on the surface of the lithium negative electrode, and stable and uniform deintercalation of Li⁺ is achieved (Fig. 7d). The assembled LiFePO₄|Li battery was stable for 200 cycles at 1 C rate, and the LiNi_0.5Co_0.2Mn_0.3O₂|Li battery also had excellent cycle stability at 0. 5 C rate. Although the interpenetrating network crosslinking system realizes the intercrossing of two polymer matrixes, which is helpful to inhibit the crystallization of PEO matrix and improve the ionic conductivity, its lithium conduction channel is still a single conductive network. On this basis, they prepared a polymer electrolyte with a double conductive network through the structure. Through an in-situ crosslinking polymerization method, two polymer monomers of polyethylene glycol diglycidyl ether and polyethylene glycol diacrylate are reacted to obtain the in-situ plasticized double conductive network polymer electrolyte^[95]. The electrolyte material with excellent ionic conductivity, high mechanical property and capability of inhibiting the dendrite of the lithium negative electrode is obtained by regulating the chain length of the polymer monomer. The LiFePO₄|Li solid state lithium battery assembled by the double conductive network electrolyte has excellent cycle and charge-discharge performance at 55 deg C and 0.2 C. After cycling, the surface of the lithium sheet is smooth and no obvious lithium dendrite appears, which indicates that the double-network solid electrolyte has a good ability to inhibit dendrite.

4.6 Solid-state lithium anode protection strategy

There are few lithium anode protection technologies developed for existing solid electrolytes. At present, the surface modification of lithium negative electrode and the construction of three-dimensional lithium composite negative electrode are mainly used to improve the interface contact between electrolyte and lithium negative electrode and regulate the deposition sites of Li⁺ on the surface of negative electrode to inhibit the growth of lithium negative dendrite^[96⇓~98]. It is a simple and efficient strategy to obtain a highly stable inorganic nano-modified layer by in-situ modification of the surface of lithium anode^[99]. The method not only can avoid the interface side reaction between the electrolyte and the lithium negative electrode, but also can inhibit the growth of the dendrite of the lithium negative electrode. Fan et al. First adhered LLZTO directly to the nylon fiber membrane by a simple solvent-free rolling method, and then soaked in succinonitrile-based LiTFSI (SN-LiTFSI) to obtain polytetrafluoroethylene (PTFE) -LLZTO-SN composite solid electrolyte (Fig. 8A)^[100]. Afterward, lithium metal is directly soaked in fluoroethylene carbonate (FEC) by use an in-situ self-modification method, and a LiF-rich layer is formed on that surface of the metal in situ after the reaction of the lithium metal and the FEC, so that the lay can not only isolate chemical reaction with a polymer matrix, but also obtain a stable SEI film, and Li⁺ can be uniformly deposited on the surface of a negative electrode to prevent lithium dendrite from growing. A LiF-modified lithium anode was matched with a solvent-free PTFE-LLZTO-SN electrolyte system to assemble a Li symmetric battery and a Li-FEC symmetric battery, respectively (Fig. 8 B). Compared with the symmetrical lithium battery without FEC modification, the interface impedance of Li-FEC symmetrical battery is significantly reduced by the presence of FEC, and does not fluctuate significantly with time (Figure 8 C). In addition, the Li-FEC symmetric battery can be stably cycled for 5000 H at 25 ℃ and 0.1 mA·cm^-2, and the polarization voltage is stable at 50 mV (Fig. 8 d), showing excellent electrolyte/anode interface contact performance and ability to inhibit lithium dendrite growth.

Drawing on the experience of liquid lithium ion batteries, it is also an effective means to inhibit the dendrite growth of solid-state lithium batteries by constructing three-dimensional lithium-friendly current collector composite anode to match solid electrolyte. Three-dimensional current collector can reduce the current density and nucleation overpotential of electrode materials, effectively inhibit lithium dendrite and alleviate the volume expansion of negative electrode^{[101⇓⇓~104]}. A vertical graphene/copper mesh (VGCM) current collector (fig. 9a) was prepared by chemical deposition method, which was used as a three-dimensional lipophilic current collector in solid-state lithium batteries to improve the cycle stability of batteries^[105]. The three-dimensional structure not only has a unique confined space to achieve uniform deposition of Li⁺, but also can form a stable SEI layer (fig. 9B). And that unique confinement space and the stable SEI lay promote the improvement of the cycle stability and the coulombic efficiency of the battery. The VGCM @ Li three-dimensional composite negative electrode realizes obviously improved cycle stability in a liquid lithium ion battery and can effectively inhibit the dendrite growth of the lithium negative electrode. The solid-state lithium battery assembled with LLZO/PVDF/LITFS solid electrolyte and LiNi_0.5Co_0.2Mn_0.3O₂ positive electrode matched with lithium negative electrode shows less polarization (Fig. 9c) and good cycle stability (Fig. 9d) compared with Li metal negative electrode.

5 Conclusions and outlook

Solid-state lithium batteries with solid electrolytes can solve the problems of low energy density and poor safety of liquid lithium ion batteries. Polymer electrolytes are the most promising electrolyte systems for commercialization due to their good flexibility and film-forming properties. However, this type of electrolyte has the problems of low ionic conductivity at room temperature and narrow voltage window, which can be solved by adding inorganic single ion conductor powder and a small amount of plasticizer to the polymer-based system. However, when polymer electrolytes are matched with positive and negative electrode materials, the problems of large interface impedance, poor interface contact performance and easy lithium dendrite growth are the key factors limiting the development of solid-state lithium batteries. Therefore, how to further improve the interfacial compatibility between solid/solid electrolyte/electrode materials is a major problem to be solved urgently. Future research will focus on the following aspects: (1) To optimize the design of electrolyte components, develop electrolyte systems with self-healing characteristics, and improve the physical contact performance of the solid/solid interface. (2) To prepare three-dimensional current collector composite lithium anode with reasonable structure and understand the mechanism of composite anode inhibiting lithium dendrite growth. And (3) developing an in-situ self-modifying technology to obtain an in-situ, deformable and stable SEI layer. And (4) design and developing solid electrolyte material by using that differential interface, and designing and optimizing the solid/solid interface aim at the different characteristics of the positive-electrode electrolyte interface and the electrolyte-negative electrode interface. In a word, in order to obtain solid state lithium batteries with excellent performance and realize their popularization and application, we should conduct in-depth research on the basic scientific issues in the development of solid state lithium batteries, clarify the relationship between "solid electrolyte-electrode material-interface" and the new roll-to-roll packaging process of solid state batteries.

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