Constructing Low‐Impedance Li7La3Zr2O12‐Based Composite Cathode Interface for All‐Solid‐State Lithium Batteries

Garnet‐type solid‐state electrolytes (SEs) represented by Li7La3Zr2O12 (LLZO) are considered ideal ion‐conducting materials for oxide all‐solid‐state lithium batteries (ASSLBs) due to their high ionic conductivity and wide electrochemical window. However, there are still many problems at the interface of the composite cathode side with LLZO‐based SEs as ion conductors as LLZO has poor interfacial contact with other particles and shows air instability when exposed to air because of the spontaneous reaction with water and CO2. Among them, the high impedance at the interface is a key issue that severely limits the actual energy density and cycle life of ASSLBs. With the optimized modification of LLZO‐based SEs and the in‐depth study of the interfacial mechanism, some breakthroughs have been made in the electrochemical performance of LLZO‐based ASSLBs. Hence, this review first describes the factors causing high interfacial impedance inside LLZO‐based composite cathode, such as poor interparticle contact, stress disruption and elemental diffusion at the interface, and discontinuous ion/electron percolation paths and then summarizes the solution strategies, for example, microstructure design, component optimization, and internal interface modification. Finally, the development prospect of LLZO‐based ASSLBs composite cathode is prospected to provide a useful reference for exploring the practical application of LLZO‐based ASSLBs.

conductivity, electrochemical stability, and physical stability. [7] However, the interfacial issues between SEs and electrodes are vital issues that severely limit the practical application of ASSLBs.
The main interfacial challenges in ASSLBs involve SE/Li metal interface and SE/cathode interface. [8] A great deal of effort has been devoted to the SE/cathode interface since the cathode is essential to improve the energy density of ASSLBs. [9] To address the interfacial issues on the cathode side, highly conductive materials such as SE powder are usually added to the cathode as ionic conductors to build the conductive network. [10] Therefore, the cathode of ASSLBs is usually a composite cathode composed of cathode active material (CAM), SE, conductive carbon (CC), and binder. However, as an important parameter that affects the energy density of SSBs, the composite cathode has not yet achieved promising electrochemical performance. [3,6b] The high impedance of its internal interface is an important reason that limits the practical application of composite cathodes. An indepth understanding of the lithium-ion (Li þ ) transport mechanism at the internal interface of the composite cathodes, the source of high interfacial impedance, and the influencing factors is crucial for solving the internal interfacial issues and constructing a composite cathode with low interfacial impedance. In recent years, the reviews of LLZO-based SSBs have focused on the anode/SE interface, cathode/SE interface, and LLZObased SE. In contrast, the reviews of the internal interface on the cathode side are related to sulfide-based composite cathodes. There is no relevant systematic evaluation of the interface study inside the oxide-based composite cathode represented by LLZO. A comprehensive understanding of the internal structure and interfacial issues of LLZO-based composite cathodes is important to promote the large-scale application of oxide-based SSBs. Therefore, it is necessary to make a comprehensive summary of the relevant studies on LLZO-based composite cathodes for SSBs. In this review, we present the first comprehensive summary of the internal interface issues of LLZO-based composite cathodes for all-SSBs, detailing the internal Li þ transport mechanism and the source of the high interfacial impedance of LLZO-based composite cathodes. Some strategies for interfacial optimization of composite cathodes from the perspective of increasing the contact area, maintaining the stability of the interface, and constructing the ion/electron percolation path are mainly analyzed. Finally, our perspectives on the remaining challenges and the prospect of LLZO-based composite cathodes are discussed, with aspirations to provide useful guidance for their practical applications.

Li þ Transport Mechanism at the Composite Cathodes
The ASSLBs consist of three main components, cathode, anode, and SEs. The essence of the reaction is a Li þ concentration cell, and the working principle is the same as that of the traditional LIBs ( Figure 1a). [11] The difference with LIBs is that the SEs replace the diaphragm and organic electrolytes to conduct ions and prevent direct contact between the cathode and anode of the battery. The LLZO-based composite cathodes usually consist of CAM, LLZO-based SE, CC, and binder. The LLZO-based SE plays the role of ion transport because of its high ion transport properties. [12] Driven by chemical potential gradients and electric field gradients, Li þ in either CAMs as mixed-ion conductors or SEs as pure-ion conductors can be transported in solids along surfaces or across bulk and interfaces. Therefore, during the charging and discharging process of SSBs, the transport of Li þ in the composite cathodes includes the CAM particles, SE Figure 1. a) Schematic Illustration of ASSLBs with composite cathodes. Reproduced with permission. [92] Copyright 2018, Elsevier. b) Crystal structure of cubic LLZO and the 3D network structure of the Li þ migration path in cubic LLZO. Reproduced with permission. [19] Copyright 2014, American Chemical Society. c) Migration energy barrier in LLZO for concerted migration of multiple Li þ hopping and single Li þ along the migration channel into the next sites along the diffusion channel. Insets show the Li þ path (green spheres) and O 2À (yellow spheres). d) Energy schematic for single-ion migration (upper insets) and concerted migration of multiple ions (lower insets). Reproduced with permission. [93] Copyright 2017, Springer Nature. particles, and the solid-solid interface formed by the internal particles inside the composite cathodes.
The diffusion mechanism of Li þ in crystals mainly includes the Schottky-type vacancy transport mechanism and Frenkel-type interstitial transport mechanism. In addition, there may be a multi-ion cotransport mechanism in which several atoms move at the same time. The ion diffusion mechanism within the lattice in SSBs includes interstitial mechanism, vacancy mechanism, interstitial-substitutional exchange mechanism, collective mechanism, and interstitialcy mechanism. [13] The ion transport of the interstitial mechanism is mainly achieved through interstitial sites between ions. Solute atoms are considerably smaller than the host atoms, and atoms are incorporated into interstitial sites of the host lattice to form an interstitial solid solution. The interstitial mechanism called the direct interstitial diffusion mechanism is the simplest diffusion mechanism. The vacancy mechanism can be divided into single-vacancy and double-vacancy mechanism, and its ion transport is mainly carried out through its adjacent vacancies. The ions that have jumped to the vacancy will leave a new void structure in their original position, which will cycle in turn to achieve the purpose of ion transport. This is currently considered the most adopted mode of ion transport. Furthermore, when interstitial atoms occupy both interstitial sites and lattice sites, ions can be transported through interstitial-lattice exchange. Furthermore, when interstitial atoms occupy both interstitial sites and lattice sites, ions can be transported in the form of interstitial-lattice site exchange. For this interstitial-substitutional exchange mechanism, the diffusion coefficient of the interstitial mode is usually much higher than that of the substitutional mode, but the concentration of "solute" atoms in the interstitial position is smaller than that of the "substitutional" atoms. In addition to the three main mechanisms described earlier, there may also be a collective transport mechanism (a mechanism in which several atoms move at the same time), which applies to amorphous systems, where the atoms move collectively in a chain-like or crawler-like manner. Moreover, interstitialcy mechanisms and self-interstitials are also collective transport mechanisms, because the ion transition process requires more than one atom to move at the same time. The ultrahigh ionic conductivity in fast ionic conductors often originates from cotransport, which has been widely recognized and used as the basis for the judgment and design of fast ionic conductors.
The ionic conductivity can be represented by the Arrhenius expression [14] σ where the E A is the activation energy, T is the temperature, k B is the Boltzmann constant, and σ 0 is the conductivity prefactor. Different forms of grain boundaries are produced by the contact of various grains with different structures or orientations, which can be studied by impedance spectroscopy and equivalent circuits. The actual grain boundaries may either favor Li þ transport or block ion transport due to the presence of impurity aggregation segregation and different forms of defects.
Different polycrystalline powder CAMs containing defects and small single-crystal particles have different ion migration channels. According to the current research results, it is believed that the Li þ diffusion channel inside the olivine-structured LiFePO 4 is 1D, and the stable crystal plane (010) is the fastest channel for Li þ diffusion. The layered LiCoO 2 has 2D Li þ diffusion channels, and Li þ diffusion follows the double-vacancy mechanism. A 3D Li þ diffusion channel is established inside the spinel-structured LiMnO 2 with high rate capability. However, the concentration of Li þ in the lattice changes continuously and even causes phase transitions during the charging and discharging process of the battery, resulting in the change of the Li þ transport characteristics in the CAMs with the amount of Li intercalation.
The LLZO-based SEs contain two crystal structures: a tetragonal phase with space group I41/acd and a cubic phase with space group Ia-3d. Awaka et al. [15] proved that both the tetragonal and cubic LLZO crystal structure frameworks are composed of dodecahedral LaO 8 and octahedral ZrO 6 , and Li ions are filled in the framework voids utilizing single-crystal diffraction. Li atoms occupy three sites in the tetragonal LLZO crystal structure, which are tetrahedral Li1 vacancy (8a site), octahedral eccentric Li2 vacancy (32g site), and octahedral central Li3 vacancy (16f site). The distance between different Li atoms in the crystal structure is longer, and the Li þ sites and vacancies are arranged in an orderly manner. In contrast, in the cubic phase LLZO (Figure 1b), Li atoms occupy only two types of crystal sites (tetrahedral Li1 sites (24d) and twisted octahedral Li2 sites (96h)) which have shorter inter-Li atoms distances, and Li þ sites and vacancies are arranged in a disordered distribution. [17] Theoretical calculation results show that the structural framework with body-centered-cubic (bcc) anion packing yields the flattest energy landscape with the lowest Li þ migration barrier, whereas nonbcc structural frameworks such as face-centeredcubic or hexagonal-close-packed exhibit significantly higherenergy barriers (Figure 1c). [18] So the cubic-phase LLZO has higher ion conductivity, which is applied in ASSLBs as an ionic conductor, and the ionic conductivity between the two differs by 2-3 orders of magnitude. [19] A large number of vacancies in the cubic-phase LLZO and the disorder of the Li þ sublattice increase the chance of concerted ion motion, which have a much lower energy cost than single-ion transitions in complete synchronization at the same time( Figure 1d). What's more, the LLZO-based SEs present a semidisordered structure in the Li þ sublattice. The disorder of the crystal structure is improved by doping with elements such as Ta and Al or introducing more vacancies which enhance ionic conductivity. [15a,20] The ionic conductivity test which differs from the Arrhenius behavior of ordered crystals is fit by the Vogel-Fulcher-Tammann (VFT) equation or the Williams-Landel-Ferry (WLF) equation where the E is the activation energy, T is the temperature, k B is the Boltzmann constant, and T 0 is the temperature at which free volume disappears.

The Factors That Influence the Interface Impedance of Composite Cathodes
In the LIBs, the gaps between the individual materials are filled and wet by the electrolyte, which can effectively reduce the interfacial resistance, but the solid particles are mainly in physical contact with each other in ASSLBs. Moreover, there will be a variety of composite interface problems in the composite cathodes, including contact interfaces between the CAM particles of the cathode, between the CAM particles and the ion-/electron-conducting particles, and contact damage in the composite cathodes caused by the volume change of the active particles during charging and discharging, because the active material is mixed with the ion conductor/electron conductor. In addition, the ion and electron paths inside the composite cathodes should be balanced, and excessive SE particles and CAM particles will limit the electron and ion transport. Generally, the factors affecting the interfacial impedance inside the composite cathodes can be summarized as follows: 1) the contact interface between particles, 2) the stability of the interface, and 3) the internal percolation path of ions/electrons.

Interparticle Contact Interface
It is widely believed that the main obstacle to Li þ transport at the interface side of the cathode is the inhomogeneous physical contact between the CAM particles and the SE particles [8,21] The rigid contact between the LLZO-based SE particles and the CAM or CC particles also hinders the migration of Li þ . Unlike conventional LIBs where the organic electrolytes easily diffuse to the interface and porous electrode, it is difficult for the SEs to enter the void of the CAMs. The diffusion of Li þ inside the composite cathodes is slowed down by this disconnected connection, which leads to an increase in the interfacial impedance between the particles. The contact interfaces inside the composite cathode mainly include SE-CAM, SE-binder, SE-CC, CAM-binder, and CAM-CC ( Figure 2a). [ performed a 3D reconstruction of the composite cathode using focused ion-beam milling and computational image analysis and derived the two-phase boundary contact area quantitatively that is effective for either ion or electron conductivity. When the electrolyte volume ratio is close to 15%, a large number of micropores still exist in the composite cathode, and the effective contact interface between particles only accounts for 23% of the total interface, which makes a large contact resistance at the interface and can hinder Li þ transport. Reproduced with permission. [23] Copyright 2019, Elsevier. b) Li 2 CO 3 impurities on the LLZO surface. Reproduced with permission. [55] Copyright 2020, American Chemical Society.
In addition, layered oxide primary particles are generally selected as CAMs to obtain high-energy-density SSBs. The aggregated state of such particles is prone to cracking under extrusion, which intensifies the generation of cavities and reduces the contact area between the particles. This causes the Li þ transport channel to be blocked and the SSB exhibits poor electrochemical performance. [16] In addition, the instability of LLZO-based SEs in air is also an important cause of high impedance at the internal interface of composite cathodes. As the main carrier for Li þ transport inside the composite cathodes, LLZO-based ion conductors play an important role in the capacity performance of the CAMs and ion transport in SSBs. [5] However, LLZO-based SEs spontaneously react with moisture and CO 2 in the air to form harmful Li 2 CO 3 on its surface and grain boundaries. [23] Various mechanisms have been proposed to explain the reactions between LLZO-based SEs and air. Among the numerous explanations, the two-step reaction path mechanism is widely accepted. First, LLZO reacts with H 2 O in the air through Li þ /H þ ion exchange, and the LiOH subsequently reacts with CO 2 to further form Li 2 CO 3 on the surface of the LLZO (Figure 2b). The low ionic conductivity of the Li 2 CO 3 insulation layer severely affects the effective contact between the active particles and the LLZO inside the composite cathode, resulting in high internal impedance. Therefore, the issue of LLZO instability in the air cannot be ignored, which profoundly affects the internal interfacial wettability of composite cathodes.
In summary, the nonuniform physical contact between heterogeneous particles in the composite cathodes is an important source of internal interfacial impedance, which must be taken into account when designing the composite cathodes.

Stress-Strain
The interfacial stability of composite cathodes is severely affected by uneven Li þ diffusion and stress distribution caused by internal discontinuous physical contacts and inhomogeneous interfacial chemistry. [24] The submicrometer primary particles are usually densely packed together to form rough-surfaced CAM particles, which leads to the evolution of physical and chemical reactions at the interface along the ion transport path and causes high stress-strain at the interface ( Figure 3a). [25] According to recent studies, the surface layer of the material particles shrinks during Li þ detachment. [9b] To resist the volume change due to the surface layer shrinkage, tensile stresses are generated on the surface of the material particles and compressive stresses are generated inside. The stress distribution during Li embedding is the opposite of this. During the cycling process, the stresses generated by mutual extrusion between primary particles will destroy the surface crystal structure of primary particles and produce crystal defects such as dislocations. [26] When the stress accumulation reaches the yield limit of the material particles, the secondary particles will crack along the grain boundaries between the primary particles and produce plastic deformation. A large amount of stress accumulation leads to the material exhibiting fatigue and cracking (Figure 3b), mechanical deformation at the internal interface of the cathode, internal fracture of the CAMs, and inevitable physical contact failure ( Figure 3c). [27] The SEs are immobile and it is difficult to contact the CAMs again after contact failure, which greatly reduce the utilization of the CAMs and eventually lead to degradation of the solid composite cathodes and hinder its practical application ( Figure 3d). The generation of cracks also reduces the ionic and electronic conductivity of the material, leading to an increase in material impedance.
Computational studies have shown that the mechanical response of electrodes during lithiation and delithiation is different. The electrochemical stiffness of LiMn 2 O 4 was investigated by monitoring in situ strain and stress in composite LiMn 2 O 4 electrodes. Diffusion-induced stress models indicate that the variation of Young's modulus with lithium concentration during lithiation and delithiation results in asymmetric stress distribution. Microcracks are generated near the particle surface and in the center of the particles during the delithiation process. Yu et al. [26] applied the smoothed boundary method (SBM) to solve the mechanical equilibrium equation to evaluate the dilatational strain and the residual stress distribution arising from thermal contraction and lattice size change due to lithiation and delithiation. Besli et al. [28] investigated the morphology and state of charge (SOC) inhomogeneity of the active material inside the composite cathode during cycling, using secondary particles of LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) material as an example. The result shows that structural integrity within the secondary particles is lost after only 20 cycles due to significant intergranular cracking. The formation of mesoscale cracks results in the loss of ionic and electrical contacts within the cathode particles, triggering a local impedance increase and rearrangement of charge carrier transport paths.
Apparently, the key factor for stress accumulation and crack generation inside the composite cathodes is the volume change of the CAMs. Therefore, the selection of elemental materials matching the SEs is crucial for the construction of composite cathodes.

Interfacial Elemental Diffusion/Side Reactions
It is well known that high-temperature heat treatment is widely used in the process of composite cathode preparation. [9c,29] However, high-temperature treatment will inevitably cause elemental interdiffusion and side reactions at the SE/CAM interface, leading to the destruction of the material structure and the generation of intermediate reaction layers. It has been demonstrated that the intermediate reactive layer tends to inhibit ion/electron transport, increasing the internal interfacial impedance inside the composite cathode. For instance, Miara et al. [30] investigated the cosintering reaction of spinel-type CAMs and Li 6.6 La 3 Zr 1.6 Ta 0.4 O 12 and predicted the decomposition reaction process using density functional theory (DFT) calculations ( Figure 4b). The starting decomposition temperature of the CAMs/Li 6.6 La 3 Zr 1.6 Ta 0.4 O 12 (LLZTO) mixture in the composite cathode is 600°C, which is much lower than the decomposition temperature of each component. The spinel-based system CAMs and LLZTO can easily exchange Li and oxygen at high temperatures to form a highly stable cathode Li 2 MnO 3 (Figure 4c), which then decomposes to produce insulating byproducts and increase the interfacial impedance of the composite cathodes. Furthermore, they investigated the thermodynamic stability of the LLZO|cathode interphase over the voltage range seen in Li metal battery operation. [31] The results demonstrated that the formation of the LLZO|cathode intermediate phase is related to the operating voltage ( Figure 4e,f ).
It is worth mentioning that the interface compatibility of CAMs/LLZO-based SEs is variable. Kim et al. [32] characterized the interface layer between LLZO and LiCoO 2 by transmission electron microscope (TEM) ( Figure 4a) and energy dispersive spectroscopy (EDS). They demonstrated the existence of element diffusion at the LLZO/LiCoO 2 interface, which generated an elemental diffusion layer with a thickness of about 50 nm, the main component of which is La 2 CoO 4 ( Figure 4d). Similarly, Park et al. [6b] demonstrated that the elemental crossdiffusion occurs at the interface of LLZO and LiCoO 2 . In addition, the interfacial chemical compatibility of the layered cathode Li 1Àx Ni 0.5 Co 0.2 Mn 0.3 O 2 (NCM523) with Li 6.25 Ga 0.25 La 3 Zr 2 O 12 (LLZO-Ga) was investigated. [33] The results showed that LLZO-Ga tends to decompose to form La 2 Zr 2 O 7 impurity phase in O 2 atmosphere due to Li transition metal exchange at the interface between NCM523 and LLZO-Ga. In addition, it was found that the cubic-phase LLZO-Ga tends to convert to the tetragonal phase with low ionic conductivity due to the fact that Li diffuses from NCM523 to LLZO-Ga in N 2 atmosphere. Therefore, interfacial element diffusion and side reactions are important factors leading to high interfacial impedance inside the composite cathodes. Suppression of elemental diffusion at high-temperature heat treatment and interface phase formation induced by operating voltage is important for the development of advanced ASSLBs.

Internal Structure of Electrode
The ion/electron diffusion paths, reaction kinetics, and properties of the active particle/electrolyte interface inside the composite cathode are highly dependent on the internal structure of the composite cathode. The modulation of the internal structure of the electrode depends on the design of electrode www.advancedsciencenews.com www.small-structures.com parameters, including porosity, [34] particle size, [9d,35] and electrode distribution. [35a,36] It is well known that the residual voids in the composite cathode will disrupt the conduction paths of ions and electrons, resulting in increased interfacial impedance. The tortuosity of ion transport is a key parameter used to characterize ion transport efficiency. Hlushkou et al. [34a] determined the tortuosity of a typical composite cathode by impedance spectroscopy and electrode structure reconstruction to explore the influence of residual voids in the composite cathode on the tortuosity of ion transport ( Figure 5b). As a result, the residual voids in the composite cathode caused tortuous paths, which hindered ion transport. Bielefeld et al. [34b] modeled the 3D microstructure of composite cathode ( Figure 5a) and investigated the effect of macroscopic parameters of composite cathode on ion/electron conductivity using percolation theory and finally obtained a similar conclusion in terms of porosity. Therefore, internal porosity is an important factor that must be considered for the construction of composite cathodes. In addition to porosity, particle size is also an important parameter affecting composite cathode. When it comes to particle size, a small particle size of the CAM is beneficial for increasing the effective contact area and improving the electron conductivity due to the small size that increases the surface area of the active particles. [35b] However, the high specific surface area of active particles can lead to chemical degradation, which is detrimental to the performance of SSBs. In addition, a positive correlation between particle size of the CAM and mass fraction in electrode design is observed. [37] For SE, a small particle size facilitates a uniform charge/discharge reaction inside the electrode (Figure 5c), and the cell exhibits better electrochemical performance and lower interfacial impedance (Figure 5d). [35a,38] More importantly, a high and suitable cathode/solid electrolyte particle size ratio is the key to achieve high energy density and high utilization for ASSLMBs (Figure 5e). [39] Therefore, the trade-off of particle size is also important for the construction of composite cathodes, which affects electron conduction, ion conduction, and electrode distribution.
In summary, the structural distribution of composite cathodes directly affects their internal interfacial ion/electron percolation behavior, and we must consider the importance of parameters such as porosity, particle size, and electrode distribution when constructing composite cathodes. Alternatively, the research  [32] Copyright 2010, Elsevier. b) XRD patterns of LCMO þ LLZO: Ta mixtures that were heat treated at 800, 600, and 400°C and as mixed powder. c) XRD patterns of LNMO þ LLZO: Ta mixtures that were heat treated at 800, 600, and 400°C. Reproduced with permission. [30] Copyright 2016, American Chemical Society. d) Al distribution at the LCO/LLZO interface. Reproduced with permission. [6b] Copyright 2016, American Chemical Society. e) Driving force for interphase formation between LLZO and a cathode with voltages varying from 0 to 5 V versus Li þ / Li. f ) The pseudobinary phase diagram at LLZO/LCO interface at 3 V. Reproduced with permission. [31] Copyright 2015, American Chemical Society. on the internal structure distribution of oxide-based composite cathodes is more about sulfide-based composite cathodes, while the role of LLZO-based composite cathodes has just attracted attention.

Electrode Active Material Loading
At present, the loading of CAMs is far below the requirements of high energy density in the composite cathode of SSBs. [10] The thick electrode design can greatly increase the loading of active materials by reducing the proportion of inactive components, which can improve the energy density of the battery and reduce the cost. [40] Therefore, it is imperative to increase the thickness of the composite cathode to improve the energy density of solid LLZO-based SSBs. However, an increase in the loading of CAMs will lead to an increase in ion/electron diffusion distance and interface impedance inside the composite cathode, which will reduce the multiplicity characteristics and cycling stability of SSBs. Jia et al. [41] investigated the relationship between the loading of CAMs in the composite cathode and the energy density of SSBs based on a bipolar cell model. The results show that the loading of the composite cathode of SSB is proportional to the mass energy density, while the difference in the density of the SE determines the thickness required to achieve high energy density. Increasing the loading of CAM in the composite cathode is vital for improving the high energy density of SSBs while maintaining excellent electrochemical performance. As the electrode thickness increases, the charge transport kinetics will deteriorate due to the longer ion/electron transport distance. In addition, solid electrolytes are difficult to establish a continuous ion/electron transport path through solid-solid contact compared with liquid electrolytes, which reduces the utilization rate and capacity output of active materials.
[40b] Noh et al. [36a,42] found by analyzing the electrochemical impedance that the ion/electron conduction pathway was the limiting factor for energy density when having a high CAM loading, and the specific capacity of the cell reached a maximum in the 60%-80% loading range. Electron/ion transport channels or the ability to supply electrons and ions to all regions of the cathode is a crucial condition in the preparation of high-performance composite cathodes. Furthermore, both energy density and power density characteristics need to be satisfied in the practical application of SSBs.
[40c] Therefore, we need to construct ion-electron transport paths with 3D structures while maximizing the active Reproduced with permission. [38] Copyright 2020, The Royal Society of Chemistry. e) The schematic diagram of the microstructure of composite cathodes. Reproduced with permission. [37] Copyright 2021, Wiley-VCH GmbH.

Strategies for Constructing Low-Interface-Impedance Composite Cathodes
There are many complex small interfaces (interfaces between CAM particles and SE particles, ionic conductors, binders, conductive additives, etc.) in LLZO-based composite cathode, which lead to problems such as high internal interface impedance and weak ion transport inside the composite cathode of SSBs. To construct effective ion and electron transport channels inside the composite cathode and maintain the stability of the internal interface of the composite cathode, we summarize three optimization strategies to improve the internal interface of the composite cathode and reduce the interface impedance: 1) microstructure design, 2) component optimization, and 3) smallinterface modification.

Microstructure Design
The design of the composite cathode electrode structure mainly includes the regulation and optimization of active particles, ionic conductors, conductive agents, and binders. The composite cathodes prepared by different methods have different internal microstructures. [9d,43] Therefore, optimizing the preparation method to improve the particle size and distribution of the internal components is the key to reducing the interfacial impedance of the composite cathode. [43] It can not only increase the effective contact area between particles inside the composite cathode but also build a continuous ion/electron transport network inside the composite cathode, which is an important basis for realizing the high energy density and high power density in SSBs.

Internal Structure of Composite Cathodes
The reported methods of composite cathode preparation include coating method, [21b,44] high-temperature cosintering method, [6b,29a,b,45] template method, [46] and infiltration method. [47] Similar to the traditional electrode preparation method, the process steps of preparing the composite cathode by the coating method are very simple ( Figure. 6a). First, the cathode slurry containing CAM, LLZO-based SEs powder, conductive agent, binder, and other raw materials is mixed uniformly, and then it is coated on the current collector and dried to obtain the composite cathode. [48] The internal particles of the composite cathode are not uniform, because the internal structure and morphology of the composite cathode prepared by this method cannot be well controlled. This deficiency can be corrected to some extent using premixing. [44a] The composite cathode prepared by hightemperature sintering is to press the uniform mixture of SE particles and CAMs into sheets directly and produce high-density composite cathode blocks by high-temperature heat treatment. [45] The high-temperature sintering method can effectively increase the particle contact area inside the composite cathode, showing the advantages of a simple process and low cost. However, there is interdiffusion of elements at the interface. [6b,29a,b] Still, the preparation of composite cathodes by template method is to introduce the CAM precursor and SE precursor into the template established by the polymer, and then the polymer template is removed by calcination and the CAM and SE are crystallized in situ. Such a composite cathode possesses high CAM loadings and 3D ion transport channels in the interior. This is expected to realize the rational design of the internal structure of the composite cathode ( Figure 6b). Hiroaki used the self-assembled BCP structure as a template to synthesize LCO/LLZO composite cathode with 3D bicontinuous nanostructure conductive paths. [46,49] The composite cathode exhibits a high discharge specific capacity of 135 mAh g À1 and a cathode utilization rate of 98%, which are significantly superior to other reported composite cathodes for SSBs, benefiting from the optimization of the electrode structure. The percolation method is used to construct a composite cathode to increase the effective contact area between the particles within the composite cathode. Generally, there are two reliable approaches in the preparation of the composite cathode by the percolation method (Figure 6c). [47e] One is to pour the prepared SE slurry directly on the surface of the cathode layer, [47a,c] and the slurry gradually penetrates the interior of the cathode layer and occupies the internal pore position to construct the composite cathode structure. [47d,e] The other method is to coat the prepared cathode slurry directly on the surface of the SE to construct the composite cathode. The advantages of the percolation method are as follows: 1) The slurry fills the internal pores of the composite cathode, which can increase the interfacial wettability and effective contact area between the CAM particles and SE particles.
2) The interfacial adhesion between the CAM particles and SE particles is enhanced, and the physical contact rigid interface is enhanced. 3) This avoids the adverse reaction caused by hightemperature sintering and reduces the interfacial impedance.
[47e,f] At present, the composite electrolyte of polymer and inorganic filler is generally used in the composite cathode prepared by the percolation method, which can almost perfectly solve the contact problem between the CAM particles and SE particles in the composite cathode. It can form a continuous 3D ion transport network, which is an effective method to construct a low-interface-impedance composite cathode. Therefore, the percolation method is identified as the most promising strategy to comprehensively improve the internal interface problems of the composite cathode. In addition, the design of flexible composite cathode [50] and array structures [51] can be used to optimize internal ion transport paths and build interconnected 3D ion-conducting frameworks while adapting to cyclic volume changes as a way to improve the overall performance of SSBs.

Particle Size and Distribution
Filling the pore space inside the composite cathode through microstructural design is beneficial for reducing the interfacial impedance. Adjusting the composition, ratio, and size of solid particles in the composite cathode can increase the effective contact between particles and build a complete and efficient ion/electron transport path.
[9d] The reduction in the particle size of the particles in the composite cathode resulted in more uniform particle distribution, less aggregation of CAM particles, and improved contact between CAM particles and SE particles. [22b] It will enhance the reaction kinetics and shorten the Li þ diffusion path, which can significantly improve the cycling stability and rate performance of SSBs. [52] The CAM in composite cathodes is usually a mixed conductor of ions and electrons. The high loading of CAM eliminates the need for large amounts of carbon additives due to insufficient electronic conductivity. [12] When the active material fraction in the composite cathode is greater than 53 vol%, the specific capacity of the SSB decreases significantly with the increase of current density. In addition, the CAM utilization in the composite cathode is controlled by percolation. When the particle size ratio of the active material to the SE is larger, the CAM loading is higher and the ion transport network is more perfect. [39] The particle size ratios of CAM and SE ranged from 0.3 to 2.4, with the lowest interfacial impedance at a content of 1.0, which proved that the optimum particle size ratio in the composite cathode is the key to the design of the electrode interface. (Figure 7a). [43] The discrete powder has a higher packing density than uniform powder, which increases the effective contact area between the SEs and CAMs and reduces the interfacial impedance inside the composite cathode. Park et al. [53] used bimodal-sized solid-state electrolyte (SE) mixtures to prepare a composite cathode with increased packing density, which improved the rate capability of ASSLBs (Figure 7b). Similarly, the in situ-polymerized gel electrolyte was introduced into the composite cathode and filled the voids between the internal particles, which established a fast conduction path for Li þ and exhibited excellent cycling performance (Figure 7c). [54] Overall, the effective contact between particles at the interface is highly dependent on its particle morphology and distribution inside the composite cathode. The performance of SSBs is determined by the type of SE, the compatibility of the SE with the CAM, and the distribution of particles in the composite cathode. The smaller the proportion of SE, the higher the CAM loading, and the higher the energy density of the SSB. Therefore, it is necessary to carefully design the microstructure of the composite cathodes. The coordination and optimization of the components inside the composite cathode is an important strategy for the construction of a low-impedance interface inside the composite cathode. The construction of different composite cathode systems should consider different parameter designing.

LLZO-Based Ion Conductors' Surface Cleaning
As mentioned in the previous sections, LLZO-based SEs tend to form LiOH/Li 2 CO 3 contamination layers at the surface and grain boundaries during synthesis and storage, which is an important reason for the high interfacial impedance inside the composite cathode. To enhance the interfacial wettability and increase the effective contact area between particles inside the composite cathode, various strategies have been proposed to cope with interfacial contaminants at the surface of LLZO-based SEs. The methods can be divided into two categories according to the different stages of Li 2 CO 3 formation: 1) inhibiting the formation of surface Li 2 CO 3 and 2) removing the formed Li 2 CO 3 layer. Specifically, hydrophobic additives such as LiF [53] and Li 3 PO 4 [55] can effectively inhibit the formation of Li 2 CO 3 (Figure 8a). The in situ removal strategies mainly include post-treatment ( Figure 8b) and in situ conversion of the Li 2 CO 3 layer at the LLZO surface (Figure 8c). The main post-treatment methods that have been reported include physical polishing, heat treatment, [56] and acid treatment. [57] Physical polishing can remove the surface Li 2 CO 3 layer easily, but it cannot remove Li 2 CO 3 at the electrolyte grain boundaries. Acid treatment can effectively remove the surface contamination layer, but the acid type and etching time of acid treatment have a great impact on the SE structure. High-temperature thermal pulse technique can clean the surface contamination layer of LLZO-based SEs quickly and effectively, but it requires a more severe treatment environment and high precision equipment. In addition, in situ conversion is also an effective strategy to remove Li 2 CO 3 . For example, by adding Co 3 O 4 to the LLZO-based composite cathode system, Zhang's group used the reaction between LLZTO@Li 2 CO 3 and Co 3 O 4 to convert Li 2 CO 3 on the surface of LLZTO into LiCoO 2 active material in situ, which ensured the close contact between the LLZO and LiCoO 2 active inside the composite cathode. [58] The ASSLB with LLZTO@ LiCoO 2 composite cathode exhibited a capacity retention rate of 81% after 180 cycles at 0.1 C. Similarly, the addition of additives such as NH 4 F [59] and SiO 2 [60] to the composite cathode can also convert Li 2 CO 3 on the surface of the LLZO-based SEs into a fast ionic conductor, thus improving the wettability of the interface.
In conclusion, the elimination of Li 2 CO 3 at the surface and grain boundaries of LLZO-based SEs is an effective strategy to improve ion transport efficiency and reduce the interfacial impedance inside the composite cathode.

Electronic Conductor
The primary role of the conductive additive is to increase electronic conductivity. A certain amount of conductive agent is usually added to ensure that the composite cathode has a good charge and discharge performance. It acts as a microcurrent collector between the CAM and the collector fluid to reduce the contact resistance of the electrode and accelerate the rate of electron movement (Figure 9a). In addition, conductive additives can also improve the processability of the electrode and effectively reduce the polarization, thus improving the charging and discharging efficiency of the electrode and the service life of the LIBs. There are mainly granular conductive agents (such as acetylene black and carbon black), fibrous conductive agents (such as metal fibers and carbon nanotubes [CNTs]), and new graphene and its hybrid conductive pastes as conductive additives.
In ASSLBs, the composite cathode is mainly composed of CAM, SE, and CC. To obtain high energy density, the number of conductive additives used inside the composite cathode is limited, which plays a key role in building the electron-conducting network inside the electrode. Conductive additives need to be filled in the CAM particle interface and gap region because both the SE particles and the electron conductor must be in close contact with the active particle surface to obtain high discharge capacity. [42,44a] Therefore, the order and spatial location of the conductive additives added during the preparation of the composite cathode directly affect the charge transfer and multiplicative performance during the charging and discharging of the SSBs. Currently, the most effective way to incorporate conductive additives in the composite cathode is to mix the SE particles and Figure 8. Schematics of novel strategies of surface engineering to eliminate Li 2 CO 3 contaminants by a) hydrophobic additives to inhibit the initial formation of Li 2 CO 3 on the surface. Reproduced with permission. [55] Copyright 2020, American Chemical Society. b) Post-treatment. Reproduced with permission. [56,57] Copyright 2019, Elsevier. c) In situ conversion. Reproduced with permission. [58] Copyright 2020, Springer Nature, Reproduced with permission. [59] Copyright 2020, Wiley-VCH GmbH, Reproduced with permission. [60] Copyright 2020, Elsevier. only half of the CC to form a premixed conductor, which is subsequently mixed with the CAM particles to provide interfacial contact. Eventually, it is mixed with the other half of the retained CC to provide higher-pathway conductivity in the composite cathode gap region and obtain higher initial discharge capacity and specific capacity. [42] Theoretical calculations indicate that carbon additives may react with LLZO-based ionic conductors at high pressure. [61] However, CNTs are considered ideal additive materials for improving the electronic conductivity of LLZO-based composite cathode due to their high intercalation capability and high energy density, power density, and multiplicity capability. [62] Compared with Ketjen Black (KB) carbons and Super P (SP), vapor-grown carbon fibers (VGCF) carbons are more stable when used as composite cathode conductive additives and can effectively suppress interfacial side reactions (Figure 9d). [63] As a result, SSBs exhibit better cycling performance (Figure 9e). In addition, the addition of the composite conductive additive (SWCNTsþSP) significantly improves the electronic conductivity in the composite cathode (Figure 9c). [64] Its dense conductive network and structural stability can effectively reduce polarization and exhibit excellent electrochemical performance (Figure 9b). The above indicates that conductive additives are a key factor that must be considered when constructing a highly conductive and sustainable composite cathode internal interface. Obviously, the selection of suitable conductive additives and the optimal modification of conductive additives are necessary for the development of high-power ASSLBs. Its dense conductive network and structural stability can effectively reduce polarization, resulting in excellent electrochemical performance.

Binder Optimization
As an important auxiliary functional material inside the composite cathode of ASSLBs, the binder is a polymer compound that adheres to other composite cathode components, for example, active materials, to enhance the contact between particles of CAMs and conductive agents and stabilize the structure, which is an additional material with high technical content in battery materials. [65] Research shows that although the amount of binder in the electrode sheet is relatively small, the performance of the binder directly affects the capacity, life, and safety of ASSLBs. [66] As mentioned before, the electrode material will cause stress and strain due to volume change during charging and discharging, and the accumulation of stress and strain will lead to contact failure of the active particles and rupture of the active particles of the electrode, which will eventually manifest as rapid decay of capacity. The binder inside the composite cathode can effectively buffer the stress and strain caused by the volume change of the electrode during the charging and discharging process and maintain the integrity of the electrode structure. [67] In addition, the binder can be bonded and adsorbed to the surface of the active particles through its functional groups, forming an effective conductive network between the active particles and the binder and thus promoting the electrical contact between the active particles and the collector fluid (Figure 10a). For the composite cathode, the physicochemical properties of the internal binder and the content of the binder have important influences on the electrochemical performance of the composite cathode. [68] The small conductive nanoparticles are more concentrated on the top surface, and the bottom surface has more active particles in the composite cathode when conventional PVDF is used as the binder. Due to the inhomogeneity of particle accumulation, the composite cathode will have an inefficient and unreasonable gradient porous structure for Li þ transport, which reduces the conductive efficiency and increases the impedance. Compared with the PVDF binder, the new nanocoating poly(lactic acid)(PLLA) biobinder has stronger bonding and encapsulation ability for active particles and collector fluid (Figure 10b,c). The formation of a functional interface with uniformity and robustness inside the composite cathode contributes to the overall electrochemical performance of the electrode (Figure 10d). [65] Similarly, Cho et al. [69] proposed an ionic-conducting polymer Pyr14TFSI-LiTFSI (LCBIM) binder dissolved in the nonpolar Figure 10. a) Illustration of the roles of the polymer binder in controlling the assembling process and structures of composite electrodes. b) Schematic of the transformation of PLLA nanocoating into the ion-conductive poly-CEI possibly by electrochemical activation. c) Overall advantages of the functional PLLA biobinder over the conventional nonfunctional PVDF binder. d) Nyquist plots of the two kinds of fresh LFP composite electrodes (solid points) and the electrodes after C-rate testing (hollow points). Reproduced with permission. [65] Copyright 2020, American Chemical Society. e) Nyquist plots of sheettyped LCBIM-cathode and NBR-cathode with pellet-type NCM-based composite cathode: Current density is 0.05 C. Reproduced with permission. [69] Copyright 2020, Wiley-VCH GmbH. xylene solvent and compared it with the electrodes prepared by the conventional binder NBR. This binder has an interfacial enhancement and the SSB assembled from its composite cathode exhibits an extremely excellent reversible capacity at room temperature ( Figure 10e).
In conclusion, the mechanical and electrochemical stability of the composite cathode is directly determined by the binding quality of the binder with other particles as well as the collector fluid. Considering the rigid contact between particles inside LLZO-based composite cathode, the development and functionalized modification of new systems of binders is an important part of the development to achieve high-energy-density SSBs.

Cathode Active Material
The CAM is the main component in the LLZO-based composite cathode, which directly affects the electrochemical performance of SSBs. The CAMs used in SSBs are still Li-containing transition metal oxides similar to LIBs, such as layered LiCoO 2 , olivine-structured LiFePO 4 , spinel-structured LiMn 2 O 4 , and LiNi 1-x-y Co x Mn y O 2 (NCM) ternary cathode materials. [47e,70] Generally, it is effective to reduce the ion transport impedance of the CAM as a mixed-ion conductor that reduces the size of the material and shortens the diffusion path of Li þ from the cathode particles to the SE. [71] However, increasing the particle size of the CAM can reduce the number of grain boundaries that hinder the diffusion of Li þ . Therefore, the CAM must find an optimum particle size to construct composite cathodes with the low-impedance interface (Figure 11a,b). [39] Compared with polycrystal-like NCM particles, single-crystal NCM particles show relatively good cycle ability in LIBs. The single-crystal NCM particles exhibit stronger stability during mixing and densification due to the direct contact between primary particles and SE particles with a better Li þ transport interface and fewer grain boundaries to hinder Li þ migration (Figure 11d). [35b,72] In addition, the shape and crystal plane design of the CAM also greatly affect the solid-solid contact interface and Li þ migration associated with the CAMs. Typically, CAMs with higher aspect ratios tend to agglomerate, which results in reduced contact between the CAM particles and SE particles. For instance, NCM cathodes with low aspect ratios have higher rate capabilities, such as octahedral and platelet-like particles. This is mainly due to its exposed facet (012) which provides good lithium diffusion channels, and the unique planar contact with SE particles minimizes porosity (Figure 11e). [73] For polycrystalline NCM, the secondary particles formed by the agglomeration of primary particles have intergranular cracks during processing, breaking the direct contact with the SE and increasing the interfacial impedance of Li þ transport. [74] In addition, polycrystalline NCM particles developed cracks due to anisotropic volume expansion during cycling, exhibiting inferior electrochemical performance to single-crystalline NCM particles. [75] Therefore, optimizing the particle structure design of CAM (e.g., primary size, secondary porosity, morphology) will be the key to realizing composite cathode with excellent performance.
Furthermore, the better electrochemical performance is contributed by the enhancement of electronic conductivity and Li þ conductivity of the cathode material. [8] Particle cracking and disruption of physical contact between particles and SEs due to lattice distortion can be avoided by improving the bulk structural stability of the CAMs. The stable surface of the CAM can prevent the CAM particles from reacting with SE particles and reduce the blockage of the ion/electron transport Figure 11. a) The example microstructure model before and after compression of the composite cathode and the schematic diagram of lithium ionic percolating paths for two typical CAM particles. b) Comparison of the experimental capacity with the model-predicted capacity of the first-cycle voltage curves of SSB using differently sized NMC particles in the composite. Reproduced with permission. [39] Copyright 2019, Wiley-VCH GmbH. c) Schematic illustration of constructing the composite cathode with agglomerates and microsized crystalline grains for ASSLBs. Reproduced with permission. [35b] Copyright 2022, The Royal Society of Chemistry. d) Schematic illustration of solid-solid contact and Li-ion diffusion in the composite cathode for five single-crystal NCM particle shapes (octahedra, plates, rods, spherical single crystals, and spherical polycrystals) and exposed facets. Reproduced with permission. [73] Copyright 2022, Elsevier. channels inside the composite cathode due to the increase of the interface impedance. [29b] The structural stability of the CAM and the stability of the interface with the SE are improved by bulk doping, surface coating, and gradient material design of the CAM. [23,76] The particle deformation can be suppressed, and the transport of Li þ on the particle surface is facilitated at the same time. At present, there are relatively concentrated reports on coating an ultrathin SE layer or forming a SE layer in situ on the surface of cathode particles. [8,24a,77] The transport of ions in mixed-ionic conductors is affected by the transport properties of electrons because of the interaction between ions and electrons. A typical example is carbon coating on LiFePO 4 to promote electrochemical performance. Simultaneously, the long-range, continuous, stable electron and ion transport network constructed in the composite cathode determines the performance and lifetime of SSBs. For example, Wang et al. [47f] constructed a 3D continuous electron transport network within an LCO/LZO composite cathode utilizing rGO and CNTs. The SSB based on this composite cathode exhibits good cycling stability. However, the interfacial problems associated with CAM are complex and diverse. It is necessary to select methods for constructing stable interfaces according to the characteristics of different CAMs.

Internal Interface Modification
The interfacial impedance of LLZO-based composite cathodes mainly arises from the poor physical contact between the CAM particles and LLZO-based ionic conductors in the composite cathodes, the diffusion/side reactions of interfacial elements generated during the sintering process, and the grain boundary rupture due to the volume change of the active material during cycling. This causes a greater obstacle to Li þ transmission. [78] Several interfacial modification strategies have been used to modulate the interfacial contact between the CAM particles and LLZO ionic conductors and inhibit elemental diffusion at the interface, such as constructing surface coatings and artificial interfacial layers.
The artificial interface layer is a transition layer (e.g. Li 3 BO 3 , [79] Nb, [80] [83] ) introduced between the cathode and electrolyte interface, which effectively isolates the direct contact between the cathode and SE layers and forms two new interfaces for ion conduction. The interfacial element diffusion can be effectively suppressed and the stress-strain diffusion due to the change of CAM particles volume can be buffered, which leads to reduced resistances and improved structural stability of the interface, facilitating the rapid movement of Li þ at the interface. Kato et al. [80] introduced a %10 nm Nb layer at the LCO/LLZO interface by magnetron sputtering pulsed laser deposition and formed a Li-Nb-O amorphous interfacial layer with high Li þ conductivity after heat treatment, which effectively increased the first discharge specific capacity (140 mAh g À1 ) of the SSB. The introduction of the interface layer provides a stress buffer for the interface between the CAMs and LLZO-based ionic conductors and improves the compatibility of the interface. It can inhibit elemental interdiffusion at the interface and improve the Li þ diffusion rate at the interface layer, exhibiting a low-impedance interface. However, the selection of the composition and structure of the interfacial layer and the compatibility of the interfacial layer with the cathode and electrolyte layer are yet to be understood and studied systematically.
At present, the surface coating method is mainly applied to composite cathodes prepared by high-temperature sintering and coating methods. A conformal coating wrapped around the surface of the CAM particles and LLZO-based ionic conductors is formed in situ after melting during the annealing of the composite cathodes by adding sintering additives with a low melting point. [9c] The ideal coating requires the following properties: good phase stability, electrochemical stability, chemical stability, and reasonable ion mobility. Ohta in Japan first used Li 3 BO 3 (LBO) with ionic conductivity as a surface coating additive to improve the interfacial compatibility in composite cathodes. [5,79,84] [86] 0.44LiBO 2 ·0.56LiF, [87] and so on (Figure 12a). Feng et al. [88] used Li 2.985 B 0.005 OCl as the sintering solder to in situ coat on LiCoO 2 and LLZO surfaces and densified CE and CAMs by hot-pressing technology because high-temperature sintering is limited by the melting point of sintering additives (Figure 12b). This approach constructs a composite cathode with low interfacial impedance and close contact, which exhibits good overall electrochemical performance (Figure 12c,d). However, the ionic conductivity of the reported sintering additives is generally lower than that of the SEs, and there is an urgent need to develop additives with low melting point and high ionic conductivity to facilitate the sintering process of composite cathode.

Summary and Outlook
In recent years, to solve the issue of the SE/cathode interface, researchers have proposed to construct composite cathodes by combining SEs as ionic conductors with CAM particles. Among them, LLZO-based ASSLBs have attracted the attention of researchers due to their excellent properties in many aspects. However, there are many complex interfacial problems inside the LLZO-based composite cathodes due to the solid-solid contact between the particles. Therefore, understanding the internal interface problems of composite cathode and their optimization strategies is crucial for the construction of high-energy-density ASSLBs.
This review systematically analyzes the main causes and influencing factors of the interfacial problems inside LLZO-based composite cathodes and summarizes the effective strategies for the construction of low-impedance interfacial composite cathodes. First, the causes of the formation of interfacial problems are discussed, and it is proposed that interfacial wettability, interfacial stability, and interfacial ion/electron percolation paths are the three main aspects leading to interfacial problems. According to the causes of the interface problems, a systematic summary of the interface optimization strategies is presented with the starting point of increasing the contact area, maintaining the interfacial stability and constructing the ion/electron percolation network. Among them, strategies such as microstructure design, component optimization, and internal interface modification have been developed to address the composite cathode interface problems. Ultimately, the trade-off and combination of these strategies may be the key to achieving advanced ASSLBs. Furthermore, it is crucial to investigate the ion/electron transport mechanisms, the microscopic mechanisms at the solid-solid interfaces, and the chemical and electrochemical reaction mechanisms inside the LLZO-based composite cathodes. In situ characterization techniques can help us to understand the evolution of the solid-solid interfaces, especially to obtain critical information during cell operation. Typical in situ characterization techniques mainly include in situ microscopy, in situ X-Ray techniques, in situ neutron techniques, and in situ spectroscopy. The combination of the above in situ detection techniques will be more helpful to obtain effective information. For instance, in-operando synchrotron X-Ray spectroscopic microscopy and X-Ray nanotomography can monitor chemical evolution during chemical/electrochemical reactions inside the electrode. [89] In situ X-Ray absorption near-edge structure (XANES) combined with in-operando transmission X-Ray microscopy (TXM) allows real-time monitoring of structural and chemical composition changes in electrode materials during charge and discharge. [90] Using the in situ 2D/3D TXM-XANES imaging technique can monitor the charge state of the material to assess the potential ion transport behavior. [91] Although different strategies have been used to optimize the composite cathode interface problem, the practical application of the composite cathode still faces great challenges. To further improve the comprehensive performance of composite cathode,  the following research recommendations and perspectives are proposed: 1) To further investigate the mechanism of interfacial ion transport and the mechanism of interfacial stability, it is necessary to combine advanced in situ characterization techniques (in suit XRD, XANES, and TXM) to trace the origin and reveal the dynamic evolution of the solid-solid interface inside the LLZObased composite cathode, and more advanced in situ characterization techniques need to be vigorously developed. 2) Using various models combined with DFT calculations to optimize the electrode composition and structure of different systems, screening of SEs, CC, and binders, and matching different CAMs to guide the experimental design will be the key focus in the future.
3) The behavior of internal particles such as their self-properties and cracking, phase transition, deactivation, and chemical-mechanical degradation during cell cycling is discussed. In addition to the joint behavior between heterogeneous particle interfaces, including contact between particles with different properties, elemental diffusion, side reactions, and the inhomogeneity of electrode composition caused by them are discussed. All of the aforementioned will greatly affect the performance of LLZO-based composite cathodes and are of great importance for the development of new model systems for SSB composite cathodes, thus requiring an urgent understanding of these behaviors. 4) A variety of different interfacial layers are used for composite cathode interface optimization, but the intrinsic mechanism of action of interfacial layers is not clear, and an in-depth understanding of the beneficial properties of interfacial layers is important for rapid screening of interfacial layers.