Deciphering the Nature of an Overlooked Rate‐Limiting Interphase in High‐Voltage LiNi0.5Mn1.5O4 Cathodes: A Combined Electrochemical Impedance, Scanning Electron Microscopy and Secondary Ion Mass Spectrometry Study

Abstract High‐voltage cathode active materials, such as LiNi 0.5 Mn 1.5 O 4 (LNMO), are of major interest for the development of high‐energy lithium‐ion batteries. However, it has been reported that composite cathodes based on high‐voltage active materials suffer from high impedances and low rate capabilities. The origin of the high impedances has not yet been clarified. Here, we use a combination of electrochemical impedance spectroscopy (EIS), focused ion beam/scanning electron microscopy/energy‐dispersive X‐ray spectroscopy (FIB/SEM/EDX) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) for showing that in the case of LNMO‐based cathodes, a major part of the cathode impedance is related to the formation of a passivating interphase on the Al current collector. Remarkably, the impedance of this interphase can be mitigated by the targeted formation of a distinct passivating interphase, namely on the surface of the LNMO particles. The interplay between these interphases is discussed.