Layered Ni‐rich lithium transition metal oxides are promising cathode materials for high‐energy‐density lithium‐ion batteries. These cathodes, however, suffer from rapid performance decay under high‐voltage operation. In this work, the electrochemical properties and structural evolution of the LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode upon high‐voltage cycling are investigated. The results show that the NMC811 cathode not only experiences surface evolution with the formation of Li‐deficient rock‐salt layers, but also suffers from drastic intragranular structural changes inside bulk grains after high‐voltage cycling. Direct evidence for the formation of transition‐metal/Li disordering domains with uneven Li content and lattice plane distortion at the internal grains of 4.6 V‐cycled NMC811 are provided with their atomic ordering and spatial distribution clearly resolved. The complex intragranular structural changes impede Li+ diffusion inside bulk material, resulting in kinetic limitation and capacity loss. The results demonstrate that the high‐voltage cycling would induce severe structural degradation at the grain interior of the cathode material beyond surface evolution, which contributes significantly to the rapid performance decay of the NMC811 cathode. The findings provide new insights for developing effective countermeasures to mitigate this degradation pathway.

Solid‐state lithium batteries are considered promising energy storage devices due to their superior safety and higher energy density than conventional liquid electrolyte‐based batteries. Lithium aluminum germanium phosphate (LAGP), with excellent stability in air and good ionic conductivity, has gained tremendous attention over the past decades. However, the poor interface compatibility with Li anode, slow Li‐ion conduction in thick pellets, and high‐temperature sintering procedure limit the further development of LAGP solid electrolytes in practical applications. This review comprehensively summarizes the crystal structure, Li‐ion conducting mechanism, and various synthesis methods, especially the latest thin‐film preparation approach. The underlying reason for Li/LAGP interfacial instability is identified, followed by several advanced interface engineering strategies, for example, introducing a functional interlayer. The integration design of LAGP‐based solid electrolytes and cathode is also highlighted to enable high‐loading cathodes. Additionally, recent progress of lithium‐oxygen and lithium‐sulfur batteries with LAGP‐based solid electrolytes is discussed. Moreover, the different Li‐ion migration pathways, preparation procedures, and electrochemical performance of polymer‐LAGP composite solid electrolytes in Li‐ion batteries are introduced. Lastly, the remaining challenges and opportunities are proposed to encourage more efforts in this field. This review aims to provide fundamental insights and promising directions toward practical LAGP‐based solid‐state batteries.

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