Figure  1.  Understanding the composite cathodes in solid-state batteries from the atomic scale to macroscopic scale. The properties of interfacial atoms and ions, such as ionic interdiffusion and vacancy, determine the interfacial chemical/electrochemical stability. Situated at a larger scale, the crystal structure of cathode materials involving surface structure and crystallographic orientations affect the interfacial charge transport kinetics and stability. A further step towards the macroscopic scale including cathode materials and electrode design requires extensive engineering aimed at establishing continuous electronic and ionic networks, tuning materials’ morphology, designing advanced electrode architecture, and avoiding mechanical issues like crack and delamination. Note that to design a high-performance composite cathode, considering these issues comprehensively is of particular importance.

Figure  2.  Understanding the composite cathodes in liquid-state batteries from the atomic scale to macroscopic scale.

Figure  3.  Performances of four typical cathode materials in solid-state batteries. Radar plots of different high-energy cathode materials: LiCoO₂ (LCO), LiNi_xCo_yMn_zO₂ (NMC), Li[Li_xTM_1-x]O₂ (0 x 0.33) (LRLO), and Li-free cathode materials FeF₃ (in ascending order of specific capacity).

Figure  4.  Surface/interface structure evolutions in SSBs. (a) Theoretical calculations of the energy difference between bulk and antiphase boundary with/without considering oxygen vacancies at a high delithiation state. Reproduced with permission [64]. Copyright 2018, Springer Nature, CC BY 4.0. (b) The voltage profile of LGPS during the electrochemical process according to the first-principles calculation. Reproduced with permission [71]. John Wiley & Sons. [© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (c) Ionic interdiffusion at the heterogeneous AM-SE interface in an SSBs. Reproduced with permission [72]. Copyright (2019) American Chemical Society. (d) Interfacial structure evolution of NMC532 cathode at a high voltage in SSBs based on Li₁₀GeP₂S₁₂ SEs. Reproduced with permission [42]. John Wiley & Sons. [© 2021 Wiley-VCH GmbH].

Figure  5.  Multiscale charge transport in composite cathodes. (a) NMC (104)-Li₃PO₄ and NMC (001)-Li₃PO₄ interface with an antiphase inversion grain boundary. Red arrows represent Li-ion transfer pathways. Reproduced with permission [84]. Copyright 2020, American Chemical Society. (b) Schematic illustration of disconnected Li⁺ percolating network in unfavorable microstructure that leads to an electrochemically inaccessible PTO. Reproduced with permission [131]. Copyright 2021, Elsevier. (c) Schematic illustration of a pore hindering the ionic transport. Reproduced with permission [87]. © The Author(s) 2019. Published by ECS. CC BY 4.0.

Figure  6.  Mechanical properties of composite cathodes. (a) Cross-sectional SEM images of NCA electrodes after first charge/discharge process. Reproduced with permission [102]. John Wiley & Sons. [© 2021 Wiley-VCH GmbH]. (b) The equivalent stress inside the NCM particles after delithiation of cathode. (c) Illustration of solid-solid interface models and kinetics. Reproduced with permission [48]. Copyright 2020, Springer Nature, CC BY 4.0. (d) Reconstructed 3D structures of composite cathode before cycling and after 50 cycles. Reproduced with permission [107]. Copyright 2020, Royal Society of Chemistry.

Figure  7.  Reconstructing AM-SE interface structures. (a) Schematic illustrating the in situ formation of the LCTO coating layer at the surface of LCO core at high temperatures. Reproduced with permission [111]. Copyright 2021, Royal Society of Chemistry. (b) Embedding AM particles within the grains of SE to establish seamless solid-solid electrode-electrolyte interface. Reproduced with permission [116]. Copyright 2019, Elsevier.

Figure  8.  Regulating the internal stress/strain of composite cathodes. (a) Schematic representation of the different microstructural and interfacial evolutions after structural manipulation in all-solid-state batteries. Reproduced with permission [33]. John Wiley & Sons. [© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (b) Relative volume change in unit cell versus the molar ratio of Co/(Ni + Co) for different layered cathode active materials. (c) Relative volume changes in unit cell during the electrochemical process for different layered cathode active materials. Reproduced with permission [39]. Copyright 2019, American Chemical Society.

Figure  9.  Tailoring composite cathode architectures from materials level. (a) Schematic illustration of the 3D interpenetrating structure of electrode with high mass loading of NCM811 cathode. Reproduced with permission [159]. Copyright 2020, Elsevier. (b) Cross sectional SEM images of single- (b₁-b₂) and poly-crystalline (b₃-b₄) NCM composite cathode before and after electrochemical cycling. Reproduced with permission [60]. John Wiley & Sons. [© 2021 Wiley-VCH GmbH]. (c) Comparative simulation results of the Li⁺ ion density in cathode structure with NCM 60% (top) and NCM 80 wt% (bottom). Reproduced with permission [124]. John Wiley & Sons. [© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim]. (d) Cathode utilization according to both particle size and CAM volume loading. First-cycle voltage curves of SSBs using different-sized SE particles in the composite cathode with fixed MCM size (5 m) and the ratio of CAMs (60 wt%). Reproduced with permission [91]. Copyright 2020, Wiley-VCH, CC BY 4.0.

Figure  10.  Tailoring composite cathode architectures from electrode level. (a) The cathodes prepared by two steps for dry- and solvent-mixed. (i) Mixing PTO AMs and Li₆PS₅Cl SEs in a dry or solvent-assisted methods. (ii) Powder compaction via uniaxial pressing. Reproduced with permission [131]. Copyright 2021, Elsevier. (b) Schematic of the infiltration of slurry-cast LCO cathode materials with LPGeSI-EtOH solutions and corresponding cross-sectional FESEM image and EDXS elemental maps. Reproduced with permission [132]. Copyright (2020) American Chemical Society. (c) The concept of all-electrochem-active’ (AEA) electrodes: conventional SSBs (80 wt% AMs, anode: Li metal) (left); the proposed SSBs based AEA cathode (100 wt% AEA cathode, anode: Li metal) (right). (d) Li-ion diffusion coefficients of AEA electrode detected by the potentiostatic intermittent titration technique method compared with the typical SEs and available traditional cathodes. Reproduced with permission [135]. John Wiley & Sons. [© 2021 Wiley-VCH GmbH].

Figure  11.  Atomic and microscopic scale characterizatin techniques for cathodes in SSBs. (a) Configuration of the all-solid-state battery fabricated by FIB and the atomic structure of LNMO at four different zone axes. Reproduced with permission [64]. Copyright 2018, Springer Nature, CC BY 4.0. (b) The in situ TEM for the characterization of STEM and EELS. Reproduced with permission [146]. Copyright 2016, American Chemical Society. (c) Solid state NMR techniques in the research of interfacial morphology and charge transport evolution and corresponding 2D-Exchange spectroscopy. Reproduced with permission [150]. Copyright 2018, Springer Nature, CC BY 4.0.

Figure  12.  Meso- and macroscopic scale characterizatin techniques for cathode in SSBs. (a) Three-dimensional reconstruction of the depth profile via time-of-flight secondary-ion mass spectrometry for the composite cathode. Reproduced with permission [153]. Copyright 2019, American Chemical Society. (b) 3D characterisation based on x-ray CT for the NMC cathode data: reconstructed volume of the cathode with different components represented by greyscale values (black: pore; dark grey: CBD; white: NMC AMs;); Simulated lithiation of the reconstructed composite cathode at 1.25 and 5 C. Reproduced with permission [156]. Copyright 2020, Springer Nature, CC BY 4.0.