Visualisation and Chemical Characterisation of the Cathode Electrolyte Interphase in High Voltage Li-ion Battery Material LiCoPO4 using He-ion Microscopy and in-situ Time-of-Flight Secondary Ion Mass Spectroscopy

The development of high voltage Li-ion battery materials has been hindered by undesired reactions at the electrolyte-electrode interface. Here, the process of cathode electrolyte interphase (CEI) formation in high voltage cathode material, LiCoPO4, has been investigated for the first time using Helium Ion Microscopy (HIM), and in-situ Time-of-Flight (ToF) Secondary Ion Mass Spectrometry (SIMS). The combination of HIM and Ne-ion ToF-SIMS has been used to correlate the morphology and chemistry of the CEI layer on LiCoPO4 with local cathode microstructure. Helium ion microscopy was found to be capable of imaging the CEI layer with higher resolution than scanning electron microscopy (SEM), and at large fields of view. The CEI layer was found to grow on LiCoPO4 when the cathode was charged, and undergo partial dissolution on discharge. CEI layer coverage on LiCoPO4 was inhomogenous, and typically thinner on larger agglomerates. Ne-ion SIMS characterisation identified the presence of oxyfluorophosphates from HF attack by the electrolyte, and the presence of uncycled Li and potential Co dissolution on the surface. The variable thickness of the CEI layer with inactive Li on the surface of the LiCoPO4 electrodes led to severe degradation over the course of 10 cycles.