Polyimides: Stable cathode active material coatings at high voltage

With much consideration on the stability of Ni-rich LiNixCoyMn1−x−yO2 (x > 0.5) cathode materials at high voltage, we use polyimides (PIs), a kind of mechanical, thermal and electrochemical performance all outstanding polymers, to coat LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode materials via the self-growing method. The common cognition for the advantages of PI coatings is mainly about reduced interfacial side reactions, little transition metal dissolutions and a small amount of oxygen release, which prohibits the degradation of cathode materials and enhances electrochemical performance. However, the true state of PI coatings in LIBs at high voltage is not clear. Therefore, we resort to the electrochemically characterizing techniques combined with TEM, SEM, EDS, in-situ XRD and XPS to reveal the root cause of PI coatings. We find that the increment of electrochemical performance is the physical barrier interactions of PI coatings. Even at high voltage, PI coatings are stable without structural changes. These findings provide robust support for PI materials applied in LIBs, especially for coating high‑nickel materials to promote their commercialization.