Evolution of LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) Cathodes for Li-ion Batteries: an In Situ Electron Paramagnetic Resonance Study

The rapid voltage and capacity fade of the otherwise promising Ni-rich layered LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode are the primary obstacles to its successful commercialization in lithium-ion batteries (LIBs). Here, in situ electrochemical electron paramagnetic resonance (EPR) spectroscopy is employed to gain insight into the cation redox behavior of the NMC811 cathode during the cell charge/discharge process. Different oxidation states of Ni ions are detected by variations in the signal of the EPR spectra. Ex situ studies of NMC811 at different SOC levels also confirm changes in the local Mn–Ni environment. A comparison of in situ studies on fresh and cycled NMC811 electrodes demonstrates that the fundamental redox processes remain unchanged upon cycling of the material. Finally, dissolved Mn and Co ions from the bulk are found using ex situ EPR characterization of the cycled cathode and separator. The dissolution of these metal ions can accelerate the degradation of the entire battery.