Transition-metal redox evolution in LiNi_0.5Mn_0.3Co_0.2O₂ electrodes at high potentials

The mixed transition-metal layered compound, LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532), is a promising high-energy cathode material. However, the required high-voltage (>4.3 V) cycling is accompanied by a rapid capacity fade associated with a complex redox mechanism that has not been clarified. Here we report soft x-ray absorption spectroscopy of NMC532 electrodes, both pristine and those charged to 4.2, 4.35, or 4.5 V in graphite/NMC532 cells. A quantitative sXAS analysis shows that about 20% of the nickel exists as Ni⁴⁺ in the as-synthesized NMC532. The Ni redox reaction contributes only to the experimental capacity obtained below 4.2 V, while Co redox reactions take place throughout the entire electrochemical cycling up to 4.5 V. In contrast to the changing ratio of the well-defined Ni²⁺, Ni³⁺ and Ni⁴⁺ ions, Co always displays ill-defined intermediate valence states in the charged NMC532 electrodes. This indicates an itinerant electron system in NMC electrodes related to the improved rate performance through Co doping. Additionally, about 20% of Ni²⁺ is found on the electrode surface at the high potential, which suggests that the electrode surface has either gone through surface reconstruction or reacted with the electrolyte at high voltage.