A facile approach to the surface modification of spinel LiNi0.5Mn1.5O4 (LNMO) cathode active materials for high-voltage lithium ion batteries is demonstrated. This strategy is based on nanoarchitectured polyimide (PI) gel polymer electrolyte (GPE) coating. The PI coating layer successfully wrapped a large area of the LNMO surface via thermal imidization of 4-component (pyromellitic dianhydride/biphenyl dianhydride/phenylenediamine/oxydianiline) polyamic acid. In comparison to conventional metal oxide-based coatings, distinctive features of the unusual PI wrapping layer are the highly continuous surface coverage with nanometre thickness (∼10 nm) and the provision of facile ion transport. The nanostructure-tuned PI wrapping layer served as an ion-conductive protection skin to suppress the undesired interfacial side reactions, effectively preventing the direct exposure of the LNMO surface to liquid electrolyte. As a result, the PI wrapping layer played a crucial role in improving the high-voltage cell performance and alleviating the interfacial exothermic reaction between charged LNMO and liquid electrolyte. Notably, the superior cycle performance (at 55 °C) of the PI-wrapped LNMO (PI-LNMO) was elucidated in great detail by quantitatively analyzing manganese (Mn) dissolution, cell impedance, and chemical composition (specifically, lithium fluoride (LiF)) of byproducts formed on the LNMO surface.
