Direct Quantification of Anionic Redox over Long Cycling of Li-Rich NMC via Hard X-ray Photoemission Spectroscopy

Cumulative anionic/cationic bulk redox processes lead to the outstanding specific energy (1000 Wh kg(-1)) of Li-rich Mn-based layered oxides as lithium-ion battery cathodes. Previous attempts to quantify redox processes in these materials were either limited to initial cycles or relied solely on the transition metals. It thus remains unclear to what extent does oxygen redox persist over cycling. This study provides an answer via synchrotron-based bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES) by directly following the changes in the electronic state of lattice oxygen. We find that oxygen redox contribution stabilizes after initial cycles in Li1.2Ni0.13Mn0.54Co0.13O2 (Li-rich NMC), and even after 70 cycles, it accounts for more than one-third of the overall capacity. Consequently, we observe a gradual but limited growth of Mn3+/4+ redox, instead of a complete activation. Partial degradation of the Ni2+/3+/4+ redox is also detected. This fundamental study generates optimism for the concept of anionic redox in long-cycling batteries and also highlights the capability of HAXPES for understanding bulk versus surface effects in energy materials.