Multiscale understanding of high-energy cathodes in solid-state batteries: from atomic scale to macroscopic scale

Different cathode materials have distinct properties due to their different electronic and crystal structure, which leads to different electrochemical processes. However, there is one thing in common for all cathode materials that no electrode can escape the effect of surface/interface structure evolution at the atomic scale during battery cycling. In SSBs, it has been reported that many transition metals (TMs) at a high oxidation state have been found to be unstable against SEs [61–63]. The instability of surface phase and side reactions at CAM–SE interface could cause severe structure evolution, resulting in fast capacity fading. An example can be found in a high-voltage CAM LiNi_0.5Mn_1.5O₄ (LNMO), which undergoes the evolution of both electronic and atomic structures during the delithiated process owing to the migration of oxygen and TM ions [64]. Specifically, at the beginning of delithiation, Li ions were extracted by the electrochemical force. However, the varing contact condition between CAM and SEs results in different Li ions migration rates in composite cathodes, which causes different delithiation states (oxidation state) of LNMO lattices along the electric field [65, 66]. In particular, the area with high-level delithiation (under high voltage) has more TM ions migration than an area with low-level delithiation owing to the instable structure after the formation of lithium vacancy. In this case, oxygen easily flows from the unstable structure. According to density functional theory (DFT) calculations, the formation of oxygen vacancies will further contribute to the reduction of defect formation energy, indicating that oxygen vacancies and Li vacancy concentration (V_Li) are two synergetic factors for forming defects (figure 4(a)), thus accelerating the structural degradation [59, 67].

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Additionally, when a SE is not thermodynamically stable at a high voltage, it tends to react with CAMs and induces phase change at the SE–CAM interfaces (figure 4(b)) [54, 68–71]. DFT calculations based on the atomic structure of the β–Li₃PS₄ (LPS)(010)/LCO(110) interface has demonstrated that Li⁺ in the LPS side with high Li chemical potentials begins to transfer toward the anode side during the initial charging process, resulting in the growth of a Li⁺-depleted layer [72, 73]. Moreover, the energetically preferable formation of P–O bond indicates that the interfacial reaction tends to occur along with the interdiffusion of cations (Co and P) and anions (O and S), which further enhances this Li-ion depletion (figure 4(c)). As a result, the interfacial resistance is increasing and results in the fast capacity fading. Wang et al [74] found that the layered structure of LiNi_0.5Mn_0.3Co_0.2O₂ (NCM532) would be restructured to rock-salt phase after repetitive cycling and became fatigued, which is attributed to the CAM–SE reactions and surface oxygen loss at the high voltage (figure 4(d)). The wide electrochemical window (2.5–4.4 V vs. Li/Li⁺) beyond that of Li₁₀GeP₂S₁₂-based SEs (1.7–2.1 V vs. Li/Li⁺) would induce the generation of non-oxygen species such as elemental sulfur and polysulfides at the CAM–SE interface. The highly oxidized species, together with interfacial structural evolution (like layered-to-rock salt) substantially impede the interfacial charge transport kinetics, leading to the large interfacial resistance in composite cathodes.

In conventional batteries, the excellent wettability of liquid electrolytes enables Li ions to transport through the grains in active materials. Cathodes with different properties like crystallographic presentation, pore size, contact area, and morphology can still achieve high performance in LLIBs [74, 75]. However, in SSBs, different cathode properties may imply tortuous and/or blocked pathways for ionic/electronic transport [48, 76, 77]. With the good tunability on nanoscale geometries and dimensions, cathode properties could be regulated at atomic levels and unique phenomena could be observed. Significant achievements have been made like the manipulation of interfacial crystallographic orientation, which can improve ionic transport kinetics due to increased charge accessibilities and shortened diffusion lengths [78–80]. While desired transport kinetics can be achieved at the microscale, the shifting of this kinetics from microscale to macroscale is usually impeded originating from intrinsic complexity within composite cathodes, including the distribution of CAMs and SEs, the existance of pores, etc. Therefore, multiscale investigations should be carried out in order to augment comprehension of ionic transport in composite cathode. It is also imperative to break the space or time resolution limitation of multi-technique approaches for probing ionic and electronic transport at varying scales.

3.2.1. Microscopic scales.

Structural inhomogeneities inside the composite cathode ranging from nanometer to micrometer scales can often dominate the ionic conductivity [37]. The prime example is the influence of interface crystallographic orientation between SEs and CAMs [78, 80, 81]. In composite cathodes, CAM and SE often possess different crystal structures, which usually causes the mismatch of interfaces, further seriously blocking the transportation of Li⁺ at the interface, especially for CAMs with high specific surface area. To demonstrate the effect of CAM surface crystallography on SSBs performance, epitaxial growth of the cathode materials is used to control the crystallographic orientation. Such interfacial structure design can serve as a model electrode to demonstrate the properties of ionic transport at the CAMs–SEs interfaces [82, 83]. Manipulating the microstructure enables us to quantitatively study the impact of the crystallographic orientations on charge transport kinetics. For example, Nishio et al fabricated two types of NMC films with (001)- and (104)-oriented crystals (figure 5(a)) [84]. The batteries with (104)-oriented NCM thin film exhibited higher interface resistances than those of (001)-exhibited NCM thin film, suggesting the importance of the crystallographic presentation of SEs and CAMs on interfacial diffusion kinetics. To further prove the effect in larger format SSBs concepts, Zahiri et al prepared a series of highly crystallographically oriented, crystalline, thick, and dense alkali ion TM oxide cathodes contacted with various SEs to comprehensively study the role that crystallography and interface morphology plays in the performance of SSBs [81]. As expected, a direct effect of interfacial crystallography on SSBs performance is shown by the linear correlation between interfacial ion transport and capacity fade. This linear relationship also demonstrated that interfacial resistance is the dominant factor of the cycling performance and even a predictor for the future performances of SSBs.

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In addition, an interphase will form between CAM and SE in the most case due to their chemical incompatibility, which usually impeded the charge transfer at the interface. For example, oxide-based SEs are usually rigid and thus a co-sintering process at high temperature between CAMs and SEs is required to have intimate contact. Nevertheless, co-sintering requires a high temperature to obtain sufficient contact areas. High temperatures are generally favorable for severe side reactions, including, interfacial decomposition, interdiffusion of elements, and structural reorganization, thus resulting in high interfacial resistance in most cases. Furthermore, the electronic network is also important to boost the interfacial charge transport. However, Zhang et al recently found that conductive carbon could accelerate the electrochemical decomposition of LGPS by providing sufficient electronic pathways. Therefore, instabilities caused by electrolyte decomposition will eventually lead to increased interfacial resistance and degraded performance in SSBs.

3.2.2. Mesoscopic and macroscopic scale.

Apart from surface/interface structure evolution at the atomic scale and multiscale ion/electron transport encountered in the composite cathode, another critical challenge is mechanical degradation. In the case of materials level (microscale and mesoscale), most of CAM will experience volume shrinkage/expansion during (charge) delithiation and (discharge) lithiation processes. Considering the fact that SEs possess low plasticity and thus cannot infiltrate or flow crack caused by the volume evolution of CAMs during the electrochemical process, rigid contact of solid–solid materials in composite cathode will be prone to generating macroscale strains/stresses, which further affects the evolution of microstructure and performance of the battery [94–96]. Both fracture and delamination are catastrophic and dependent on the response of the CAMs matrix and CAM–SE interface to the developed stresses [97]. For example, the NCM underwent volume shrinkage during the delithiation process [98]. This volume contraction becomes more severe along with the increment of Ni content in NCM [99]. Owing to the rigid feature of solid materials in composite cathodes, even slight volumetric change of CAMs during electrochemical process causes serious stress problems. Notably, it will be worse for the polycrystalline cathode particles composed of densely packed primary grains with random orientations due to the anisotropic volumetric strains of these primary grains [100, 101]. The microstructural evolution of CAMs characterized by cross-sectional SEM–BSE measurements (as displayed in figure 6(a)) demonstrates that polycrystalline LiNi_0.88Co_0.11Al_0.01O₂ (NCA) particles show obvious internal cracks, stemming from severe internal stress [102].

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To facilitate the understanding of the mechanical degradation at materials level, Lou et al employed finite element modeling (FEM) to explore the stress evolution induced by chemomechanical impact within NCM polycrystalline particles (poly–NCM), as detailedly shown in figure 6(b) [48]. In consideration of the practical operation condition, theoretical models of poly–NCM in the SSBs were established by using aggregated primary grains with random orientations. Heterogeneous stress distribution can be clearly observed in the partially charged primary particles (50% SOC). Along the radial direction of the particles, stress was shown as a function of NCM particle radius. The anisotropic crystallographic orientation and Li⁺ transport are mainly attributed to the local stress along the grain boundaries. During delithiated process, core region with a higher Li content than outer shell induces an apparent gradient of Li concentration, resulting in compressive stress at the center and tensile stress near the surface for poly–NCM [103, 104]. Thus, the nonequilibrium in repeated delithiation/lithiation processes will destroy particle structure and accelerate the capacity decay of SSBs. Furthermore, the severe stress within CAMs will cause CAM–SE interface to form crack and delamination, and prevent Li⁺ from synchronously transporting across the solid–solid interface (figure 6(c)). In this case, mechanical stability is of particular importance and must be considered for designing and fabricating a high-performance composite cathode.

Meanwhile, for the electrode level, contact loss will not only form during the first charge process but also in the following cycles, which is also responsible for the continuing capacity fade in addition to side reactions and interphase formation at the interface [96, 105, 106]. Shi and Zhang have reported that many small voids are generated during the cold-pressing process [107]. However, after 50 cycles, the void morphology has changed substantially and the total void volume increased to 9.5% of the total volume, which is more than three times that in the pristine sample (figure 6(d)). These voids are also more connected and form large fake-like cracks near the cathode particles, resulting in a significant increase in interfacial resistance [108, 109]. Herein, surface adhesion as an important parameter, which means the binding state between CAM and SE in cathode, can be employed to evaluate the interface. It is a typical example of chemical–mechanical coupling that is necessary to understand the contributions from [110]: (a) mechanical strain deriving from the lattice mismatch between the contacted phase of CAMs and SEs, (b) the chemical interfacial energy originating from the difference in coordination and bonding at the interface in comparison to the bulk and (c) electrical attraction owing to interfacial charge reorganization. All togther, the unequilibrated charge distribution leads to the non-uniform stress field inside the CAMs, which further exacerbates CAM–SE interface, facilitating thegeneration of initiation, propagation of microcracks, contact loss, and high porosity within compsoite cathodes.

Coating as an important method is able to reconstruct the AM–SE interface structure while forming two new interfaces: (a) the coating–CAM interface and (b) the SE–coating interface. Since CAMs usually undergo substantial volume change during the delithiation/lithiation processes, an ideal coating layer can effectively confine strain by elastic deformation. However, conventionally physical coating enables the interface to have weak interaction with CAMs, which easily delaminates from the CAM upon the formation of crack or the contraction of CAM during the electrochemical process. Accordingly, the advanced coating technology based on interatomic interaction is a requirement toward mechanically 'plastic' and deformable interface structure. Wang et al established the chemical interaction between LCO and TiO₂, in-situ forming a continuous Li₂CoTi₃O₈ (LCTO) layer with relatively high lithium diffusion coefficient (8.22 × 10⁻⁷ cm² s⁻¹), low electronic conductivity (2.5 × 10⁻⁸ S cm⁻¹), and stable 3D network of spinel structure, as shown in figure 7(a)[111]. As a consequence, the pristine LCO–Li₁₀GP₂S₁₂ (LGPS) interface is replaced by two new interfaces LCTO–LGPS and LCO–LCTO. Particularly, LCTO coating has interatomic interaction with LCO and thus exhibits large work of adhesion for LCTO–LCO interface, suggesting a high chemical affinity between the two surfaces. In addition, LCTO–LGPS interface is not only electrochemically and thermodynamically more compatible but also has higher interfacial affinity than LCO/LGPS interface. Therefore, the ASSLB with a modified cathode shows significantly reduced interfacial impedance. It thus delivers a high capacity of 140 mAh g⁻¹ and shows excellent cycling stability which retains 83% capacity after 200 cycles at 0.1 C. In comparison, the ASSB with a pristine cathode exhibits a capacity of 98 mAh g⁻¹ and only delivers poor stability (22.4% capacity after 100 cycles at 0.1 C). Moreover, interposing coating layer is also an effective strategy to restrain the formation of space charge layer (SCL) to some extent, which is generally regarded as one of the reasons for the sluggish interfacial charge transport kinetics in SSBs [112, 113]. Particularly, chemical potential coupling strategy via coating dielectric materials, such as BaTiO₃ nanoparticles, can establish the built-in electric field, greatly suppressing the SCL effect [114, 115].

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Apart from introducing a coating layer with chemical interactions with CAMs, it is of particular importance to construct a seamless interface between CAMs and SEs. Although the fabrication of CAM–SE interface with atomic interaction may not be feasible in commercialization, understanding the fundamental mechanisms at atomic scale is indispensable to offer crucial insights into enabling fabrication of a high-performance composite cathode. For example, Li et al prepared an epitaxial interface between 0.54Li₂TiO₃–0.46LiTiO₂ (LLO) CAMs and perovskite SEs [116]. Such a seemingly impossible intimate contact between CAMs and SEs nearly surpasses the one-based solid-liquid contact (figure 7(b)). Therefore, with LLO embedded within the SEs matrix in such a way, the epitaxial composite electrode showed an outstanding rate capability comparable to the slurry-cast electrode composite in conventional LLIBs.

As described in section 3.3, polycrystalline CAMs with randomly oriented grains undergo conspicuous internal stress during repeated delithiation and lithiation processes, as illustrated in figure 8(a), which not only causes the disintegration of the particles within secondary particles but also induces the contact loss between CAMs and SEs. In contrast, by microstructural manipulation of polycrystalline LiNi_0.75Co_0.10Mn_0.15O₂ (FCG75), the FCG75 with radially oriented rod-shaped grains is able to withstand the internal stress and keep mechanical integrity [33]. In addition, by making the concentration gradient of Ni, the Ni content at the surface of FCG75 is lower than internal, remarkably prevents the occurrence of interfacial side reactions. As a consequence, the SSB with FCG75 cathode delivers a high initial Coulombic efficiency (84.9%) at 0.1 C and excellent cycling stability (79.1% retention at 0.5 C after 200 cycles).

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Although rationally designing microstructure of CAM is capable of keeping mechanical integrity, the certain stress/strain inside CAMs can have a cooperative effect and may lead to severe contact loss between CAMs and SEs. Thus, the most promising way is to design zero-strain CAMs to achieve excellent mechanical stability. For instance, Strauss et al prepared a quasi-zero-strain Co-rich NCMs, NCM271 (70% Co) and NCM361 (60% Co) [39]. As opposed to NCM811 and LCO, both NCM271 and NCM361 exhibit negligible volume change (<1%) during charging up to 4.5 V vs Li/Li⁺ (figure 8(b)). Pressure changes in operating SSBs with NCM271 and NCM361 are shown in figure 8(c), indicating no substantial changes in linear elastic stress during cycling processes, which is consistent with the volume changes. Consequently, the use of these quasi-zero-ztrain (active) CAMs can effectively prevent gap formation between the SE and CAM during SSBs operation.

As mentioned above, in LLIBs, cathode possessing a porous architecture is beneficial as Li ions can diffuse with liquid electrolytes to reach active materials. However, pores in SSBs are ionic blocking and thus a dense cathodic architecture is necessary to ensure continuous electronic and ionic pathways. Meanwhile, robust architectures of composite cathodes are able to accommodate the mechanical evolution associated with lithiation/delithiation of the CAMs [96, 105]. Therefore, optimizing composite cathode architecture from material level (microscale/mesoscale) and electrode level (macroscale) is important to achieve the goals of high energy density and power density for next-generation solid-state batteries. Especially, advanced manufacturing technology at electrode level is indispensible to optimize the architectures of the composite cathode. Therefore, a multi-objective optimization routine should be employed to manipulate the architecture of composite cathodes for maximizing CAMs loading, energy density, mechanical stability, and three-phase contact area, as well as minimizing void phases and tortuosity.

4.3.1. Material level.

To break through the dilemma of electrode architecture, 3D material configurations were proposed [20]. Such 3D structure effectively enhances the areal contact between the CAMs and SEs, enabling a robust electrode with interconnected ion transportation pathways. Thus, when the thickness of the electrode is increased, the mechanical integrity of 3D structure is not sacrificed [117]. For example, Yi et al prepared a composite cathode by infiltrating NCM622 into 3D porous Li₇La₃Zr₂O₁₂ (LLZO) scaffolds [118]. The unidirectional pores within this 3D scaffold promote the infiltration of components cathode and significantly shorten the Li-ion transport lengths, while the utilization of the soft ionically conductive materials within the scaffold guarantees a good mechanical stability among these components. With this unique design, the SSB achieves a reversible capacity of 125–135 mAh g⁻¹ at 0.1 C and can surpass the energy density of state-of-the-art LLIBs by 1.8–2.6 times if the thick composite cathode is used. Similarly, Zhang et al designed a new electrode with 3D interpenetrating structure containing a NCM811 cathode, a tri-layer SE, and the Li anode, as illustrated in figure 9(a) [110]. Therein, NCM811 CAMs were filled in porous Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) layer, enabling high-loading CAMs (∼13 mg cm⁻²) due to the stable mechanical support porous layer.

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Additionally, morphological control is also required in the CAMs themself. Commercial-grade NCM/NCA was susceptible to serious disintegration of the particles even at the first delithiation/lithiation process owing to the anisotropic volumetric strains from grains with random orientations. Consequently, emerging researchers are developing cracking-free single-crystalline NCM/NCA for practical SSBs [119, 120]. As shown in figure 9(b), no obvious change was observed for the composite cathode using single-NCA after the first delithiation. In contrast, severe internal cracks were clearly observed within poly-NCA particles, which originates from lattice shrinkage, associated with the detrimental H2–H3 phase transition at ⩾4.1 V vs. Li/Li⁺ [60, 102, 121–123]. Moreover, high energy and power density need appropriate electronic and ionic transport pathways within composite cathodes. Tuning the ratio of CAMs and SEs is an effective way to ensure the electronic and ionic networks inside the composite cathode. Park et al found that 80 wt% NCM within the composite cathode shows higher electron density than that of the 60 wt% NCM. However, in the composite cathode with 80 wt% NCM, the ionic pathways become more limited and localized than that of the 60 wt% NCM (figure 9(c)), significantly reducing the utilization of CAMs [124]. Thus, carefully designing the blending ratios of CAMs to SEs is of particular importance to ensure the high performance of SSBs.

In the case of particle size of CAMs, it also has a significant effect on the architecture of composite cathodes. For small particle sizes of CAMs, they possess better electronic contacts and short ionic pathways [125]. However, some studies have presented that the size of both the CAM and SE significantly influences the morphology of the pressed composite cathodes and full-cell performance [86, 125, 126]. An ideal morphology is able to guarantee good CAM–SE contact and minimal void space. Shi et al have found that the key to achieving high energy density is to increase the ratio of CAM to SE particle size (λ). While the λ is less than 1, ionic percolation always substantially worsens [82]. In any case, the CAM particle size should be kept larger than the SE particle size. By increasing the CAM particle size to 2–3 times larger than SE, the CAM utilization can be enhanced from 20% to 100% even at a high loading. As shown in figure 9(d), both decreasing the fraction of CAMs and increasing λ are able to consistently improve the ionic percolation in composite cathode. Consequently, a large value of λ is necessary to achieve high capacity with high mass loading. Nevertheless, the benefits of increasing λ to improve the CAM's utilization are strongly dependent on the fraction of CAMs: for 80% CAMs fraction in the composite cathode, enhancing λ from 1 to 3 can increase the utilization of CAMs from 30% to 60%; however, at 85% CAMs fraction, only slight increment of the utilization of CAMs from 16% to 20% is achieved even by controlling the λ from 1 to 8.

4.3.2. Electrode level.

Manufacturing technology determines the ionic/electronic, chemical, and mechanical properties of electrodes, and also decides their potential for scale-up. Advanced manufacturing technology can not only fabricate composite cathode at low cost but also establish advanced ionic/electronic networks and achieve stable mechanical stability in composite cathodes. Therefore, scalable manufacturing of composite cathode with advanced manufacturing technology is an essential part to further improve battery performances.

4.3.2.1. Solution infiltration for scalable manufacturing of composite cathodes.

For composite cathode in SSBs, large fractions of the SEs are typically necessary to ensure that all CAMs are homogeneously surrounded by SEs. However, the low fraction of CAMs severely limits the electrode-level energy density. In order to break the limitation of the conventional composite cathode, solution infiltration of SE into electrode was proposed. The SE solution with good wetting ability enables intimate contact with CAMs like in LLIBs and thus greatly enhances the electrochemical performance of SSBs [127–130]. Yao et al employed a solvent-mixing process to perform core–shell pyrene-4,5,9,10-tetraone (PTO)–Li₆PS₅Cl particles, which was demonstrated to be an effective way to manipulate the structure of electrode (figure 10(a)) [131]. Compared with the solvent-mixing method, dry mixing will form undesired architecture, substantially limiting the ionic diffusion kinetics. As a result, this solvent-assisted process increases the CAM fraction from 20% to 40 wt% in combination with the high utilization (97.6%). In addition, Jung et al prepared iodine-based Li-argyrodites in a new solution-processable way (figure 10(b)) [132]. Simultaneously, they applied solution-processed Li_6.5P_0.5Ge_0.5S₅I (LPGeSI) for the infiltration of LCO electrodes. It can be confirmed that LPGeSI makes intimate contact with LCO and generates negligible void spaces. Notably, the mass fraction of LPGeSI in the composite cathode was only 12 wt% and initial Coulombic efficiency is up to 88.0%. At the same time, the SE-infiltrated LCO electrodes show excellent cycling stability of 94.9% capacity retention at 0.2 °C after 100 cycles.

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4.3.2.2. Melt infiltration for scalable manufacturing of composite cathodes.

Infiltrating low-melting-point SEs into the composite cathode is another promising way to control the electrode structure. Moderately elevating temperatures will make low-melting-point SEs in a liquid state and infiltrate the electrode. Then, this composite cathode is solidified after cooling. This method imitates the low-cost manufacturing of commercial LLIBs where dense electrodes could be produced in the ambient environment before electrode drying and electrolyte filling. As such, almost all the commercial equipment could be used for the fabrication of electrodes and SSBs, which significantly decreases the obstacle for industry adoption and enables the potential for scalable fabrication of SSBs. Moreover, rapid filling of molten SEs into dense cathode is easy to generate a uniform and conformal CAM–SE interface without porosity remaining. Therefore, such an approach is beneficial to attaining electrodes with high densification and CAM–SE interfaces with low resistances, without stress concentration, resulting in high volumetric energy density, high power density, and outstanding cycling performance. Xiao et al proposed the melt-infiltration technology for the disruptive and scalable fabrication of inorganic SSBs [130]. They firstly prepared NCM111 as a cathode material and Li₄Ti₅O₁₂ as an anode, using commercial manufacturing of electrodes [130]. Subsequently, the low-melting-point Li_1.9OHCl_0.9 SE (∼300 °C) is quickly molten at 300 °C and rapidly infiltrates into the NCM111 and Li₄Ti₅O₁₂ via the capillary effect. As a result, this ASSLB delivered a high reversible capacity of 150 mAh g⁻¹ and over 80% capacity retention after 100 cycles at the current rate of 70 mA g⁻¹. Additionally, this method was also employed in the production of Li₂S–C cathodes where molten 3LiBH₄–1LiCl was infiltrated and achieved outstanding cycling stability (∼80% capacity retention after 1000 cycles) with conversion-type cathodes.

4.3.2.3. All-electrochem-active cathode design for scalable manufacturing of composite cathode.

The equivalent capacity (${\text{ESC}} = \frac{{\text{C}}}{{{m_{{\text{electrode }}}}}},$ ${m_{{\text{electrode}}}} = {m_{{\text{electrode}}}} + {m_{{\text{SEs - in - electrode}}}} + {m_{{\text{carbon addictives}}}} + {m_{{\text{binder}}}}$ and C represents the capacity of cathode) is defined to evaluate the electrode capacity. In conventional SSBs, they perform a low ${\text{ES}}{{\text{C}}_{{\text{electrode}}}}$ owing to involving many non-electrochemical active components within the composite cathode, containing SEs and conductive agents, which are regarded as irreducible and indispensable parts of the construction of electronic/ionic transport network (figure 10(c)). As a function of reported data, the weight fraction of CAMs in the composite cathode for SSBs is less than 80 wt% [133, 134]. In addition, the introduction of SEs would induce various interfacial issues between CAMs and SEs. Therefore, designing an all-electrochem-active (AEA) electrode with high enough ionic and electronic conductivity can increase the volume and weight percentages of CAMs to 100%. To realize this concept, Li et al designed the crystal TM sulfides, i.e. chevrel-phase Mo₆S₈ and layer-structured TiS₂, which not only possess high conductivity but also a very stable structure [135]. Specifically, both TiS₂ and Mo₆S₈ have high electronic conductivity comparable to the conductivity additive (super P) and such high electronic conductivity allows TiS₂ and Mo₆S₈ to abandon the use of conductive carbon in the cathode. Meanwhile, TiS₂ and Mo₆S₈ also possess a high Li-ion diffusion coefficient (8 × 10⁻⁹–9 × 10⁻¹⁰ and 1.8–9.8 × 10⁻⁸ cm² s⁻¹), which is several orders of magnitude higher than that of the conventional cathode materials (LiCoO₂: 10⁻¹¹–10⁻¹² cm² s⁻¹, NCM 2.8–8 × 10⁻¹¹ cm² s⁻¹, LiFePO₄: 6.8 × 10⁻¹⁶–1.8 × 10⁻¹⁴ cm² s⁻¹¹), and comparable to the SEs (Li_6.25Al_0.25La₃Zr₂O₁₂, 1–1.1 × 10⁻⁸ cm² s⁻¹¹, Li₁₀GeP₂S₁₂, 8.8–9 × 10⁻⁸ cm² s⁻¹¹) [136–139]. As a consequence, they can act as a SE rather than introducing excess SE in the cathode. Based on these physicochemical properties, the TiS₂-based AEA-SSBs can exhibit an initial discharge capacity of 213 mAh g⁻¹ and Mo₆S₈-based AEA–SSBs deliver a reversible capacity of 130 mAh g⁻¹. To further demonstrate the advantage of AEA concept, a hybrid S₈–Mo₆S₈ cathode was fabricated owing to the high theoretical capacity of S cathode. By matching S₈–Mo₆S₈–AEA cathode with the Li metal, the SSBs can deliver the capacity of 483 mAh g⁻¹, with volumetric and gravimetric energy densities of 2778 W h l⁻¹ and 905.5 W h kg⁻¹, respectively. Nagao et al also designed a novel Li-rich cathode materials Li₂Ru_0.8S_0.2O_3.2 [80Li₂RuO₃ · 20Li₂SO₄] through the amorphization of Li₂RuO₃ with Li₂SO₄, which enables the CAMs to have high ductility and conductivity for obtaining favorable interfaces, resulting in the stable operation of SSBs [27].

It is a long-term dream for researchers to observe the physicochemical evolution of composite cathode at an atomic scale. High-resolution electron microscopy techniques enable us to detect the structure evolution at the atomic or microscopic scale [5, 144, 145]. Meanwhile, in combination with EELS analysis, it is able to detect the structural, chemicaland morphological information of electrodes. Gong et al performed in situ aberration-corrected scanning transmission electron microscope to uncover the evolution of the electronic and atomic structure of the spinel LiNi_0.5Mn_1.5O₄ (LNMO) during the charging process in SSBs (figure 11(a)) [64]. It is found that the uneven extraction of Li⁺ induces localized migrations of TM ions and forms antiphase boundaries. Dislocations accelerate TM ions migration as well. By coupling EELS, Wang et al observed the charge transfer in SSBs (figure 11(b)) [146]. This microscale spectroscopic characterization demonstrated the severe ion diffusion and the formation of disordered phase between LiCoO₂ and LiPON, which results in performance decay and high ion-transport resistance. Additionally, it is also important to understand the spontaneous ion transport at microscale over the CAM–SE interface owing to the abundant interface within composite cathodes. The nuclear magnetic resonance (NMR) technique has the prominent ability to probe the ionic exchange between different phases [147–149]. Recently, 2D Li-ion exchange NMR was employed to quantitatively evaluate the transport rate over the interface between the Li₂S CAMs and Li₆PS₅Br SEs, offering new insight into the ion transport at the interface (figure 11(c)) [150]. These results demonstrate that preparing the composite cathode by ball milling and nanosizing CAMs can significantly speed up the spontaneous exchange of Li⁺ at the interface.

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Time-of-flight secondary-ion (SI) mass spectrometry (ToF-SIMS) is a semiquantitative technique to characterize the local enrichments of some fragments at interfaces [151, 152]. Moreover, it is feasible to reconstruct the depth profiles in 3D for the exhibition of the spatial fragment distribution (figure 12(a)). Walther et al demonstrated that NCM622–LPSCl composite cathode undergoes an interfacial phase change and elemental interdiffusion, which induces high interfacial resistance and deteriorates the cycle stability [153]. By 3D reconstruction of the cycled composite cathode, it was found that phosphates and sulfates play a significant role in the formation of an SEI within the composite cathode. Apart from interfacial chemical reactions, the distribution of CAMs and SEs, and mechanical behavior in the composite cathode are also crucial to the electrochemical performance of SSBs. SEM is a simple technique to observe the generation of contact loss and crack during electrochemical cycling [154]. However, it is a non-invasive characterization technique. To further insight the internal structural evolution at the 3D view, x-ray tomography was employed to show the 3D structure of the sample through nondestructive visualization. It allows the quantification and visualization of 3D morphological information, including spatial distribution, particle crackingand volume/contact area [155]. By visualizing various components of CAMs, SEs, pore space, and carbon black in the composite cathode, physical parameters of the real electrodes at different cycles are able to be quantified. Subsequently, the corresponding mathematical models are established and explain the observed degradation mechanism at meso- and macroscopic scales. In addition, with directly showing 3D morphological changes and geometrical properties by x-ray tomography, it can quantitatively map the distribution of Li-ion concentration and inhomogeneous composition in SSBs (figure 12(b)) [156, 157]. One disadvantage of x-ray tomography is the low contrast between the carbon and pores in electrode [108, 158]. Nonetheless, more accurate 3D information inside cathode could be obtained with the assistance of other techniques, such as the aforementioned ToF-SIMS.

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