Prospects of polymer coatings for all solid-state and emerging Li-ion batteries

Polymers possess processing flexibility as they can be coated on cathode particles before/after electrode fabrication and on the solid-state electrolyte surface in all-solid-state batteries (ASSBs). Their narrow electrochemical stability window limits the use of polymers directly as an electrolyte against high voltage cathodes. However, when a polymer is coated directly on battery cathodes and cycled with conventional liquid electrolytes, they exhibit superior battery performance in comparison to uncoated ones. A deeper insight was not sought in the literature. There might be a great possibility of in situ formation of an ultra-thin protective layer in-between the polymer and cathode interface at the coating development stage or in the formation cycle of the electrochemical cell. The current ASSBs demand flexible, easily scalable coating materials, which can accommodate the volume expansion-contraction during cycling and can minimize the lattice stress. However, a much better fundamental understanding is needed on polymer/ceramic interfaces. This focused review is concentrated on flexible polymers with high ionic and electronic conductivities that can be used for coating cathode particles and Li anodes. Overall, this article has analyzed and validated the application of various types of polymers in lithium-ion batteries and ASSBs comprehensively with an emphasis on the effect of coating morphologies and thickness on performance. Finally, this review gives a brief discussion on the prospects and suitability of polymers as coating layers.