Lithiation and Delithiation Dynamics of Different Li Sites in Li-Rich Battery Cathodes Studied by Operando Nuclear Magnetic Resonance

Li in Li-rich cathodes mostly resides at octahedral sites in both Li layers (LiLi) and transition metal layers (LiTM). Extraction and insertion of LiLi and LiTM are strongly influenced by surrounding transition metals. pjMATPASS and operando Li nuclear magnetic resonance are combined to achieve both high spectral and temporal resolution for quantitative real time monitoring of lithiation and delithiation at LiLi and LiTM sites in Li2MnO3, Li1.2Ni0.2Mn0.6O2, and Li1.2Ni0.13Mn0.54Co0.13O2 cathodes. The results have revealed that LiTM are preferentially extracted for the first 20% of charge and then LiLi and LiTM are removed at the same rate. No preferential insertion or extraction of LiLi and LiTM is observed beyond the first charge. Ni and Co promote faster and more complete removal of LiTM. The recovery of the removed Li is <60% for LiTM and >80% for LiLi upon first discharge. The study sheds light on the activity of LiLi and LiTM during electrochemical processes as well as their respective contributions to cathode capacity.