A novel ion-conductive protection skin based on polyimide gel polymer electrolyte: Application to nanoscale coating layer of high voltage LiNi _1/3Co _1/3Mn _1/3O ₂ cathode materials for lithium-ion batteries

A new and facile approach for the surface modification of high-voltage LiNi _1/3Co _1/3Mn _1/3O ₂ cathode active materials is demonstrated. This strategy is based on polyimide (PI) gel polymer electrolyte (GPE)-directed nanoscale wrapping. The PI coating layer successfully wraps a large area of the LiNi _1/3Co _1/3Mn _1/3O ₂ surface via thermal imidization of (pyromellitic dianhydride/oxydianiline) polyamic acid. Salient features of the PI wrapping layer are the highly continuous surface coverage with nanometre thickness (∼10 nm) and the facile ion transport through the nanoscale layer. Based on a sound understanding of the nanoarchitectured PI wrapping layer, its influence on the cell performance and thermal stability of high-voltage LiNi _1/3Co _1/3Mn _1/3O ₂ is investigated as a function of charge cut-off voltage (herein, 4.6 and 4.8 V). The anomalous PI wrapping layer substantially improves the high-voltage cycling performance and alleviates the interfacial exothermic reaction between delithiated LiNi _1/3Co _1/3Mn _1/3O ₂ and liquid electrolyte. These results demonstrate that the PI wrapping layer effectively prevents the direct exposure of the LiNi _1/3Co _1/3Mn _1/3O ₂ surface to liquid electrolytes that are highly vulnerable to electrochemical decomposition at high charge voltage conditions, thus behaving as a novel ion-conductive protection skin that mitigates the unwanted interfacial side reactions.