Advances in inorganic, polymer and composite electrolytes: Mechanisms of Lithium-ion transport and pathways to enhanced performance

The growing demand for enhanced batteries with higher energy density and safety is pushing lithium-ion battery technology towards solid-state batteries. Replacing the liquid with a solid electrolyte significantly improves safety by removing the possibility of leaking flammable organic solvents. Solid electrolytes also enable the use of lithium metal as anode material to obtain battery cells with higher energy density. This review summarizes the classification of all three state-of-the-art solid electrolyte types (inorganic, polymer and composite solid electrolytes) and their governing lithium ion transport mechanisms. Nevertheless, to make solid-state batteries applicable, improvements in ionic conductivity of the solid electrolyte, low electrode-electrolyte interfacial resistance and high compatibility of the solid electrolyte with the electrodes are required. This review paper discusses improvement strategies for solid electrolytes to achieve high ionic conductivity, good flexibility, and high electrode compatibility. Enhanced ionic conductivity can be obtained by suppressing the polymer phase's crystallization (e.g., copolymerization, inorganic fillers, adjusting polymer matrix) and optimizing the physicochemical parameters and the surface of the inorganic phase. Interfacial stability can be improved by using multilayered electrolytes or applying coatings and passivation layers on electrolyte or electrode particles.