X-ray absorption study on LiNi0.8Co0.15Al0.05O2 cathode material for lithium-ion batteries

LiNi0.8Co0.15Al0.05O2, being one of the promising cathode materials for lithium-ion batteries, shows distinct capacity fades after charge/discharge cycling and/or storage at high temperatures. The origin of the capacity fade has been explored by investigating the electronic and structural changes of the cathode material using X-ray absorption spectroscopy (XAS). Ni K-edge XAS measurements were performed in two different modes: surface-sensitive conversion electron yield (CEY) mode and bulk-sensitive transmission mode. Ni K-edge XANES data revealed that, after the cycling and aging tests, the bulk-averaged Ni valences were reduced, implying the existence of divalent Ni atoms. Further reductions of Ni atoms were observed at the surface of the cathode material particles, and the ranges of the Ni valence change upon charging became narrower, indicating the existence of the Ni atoms that did not oxidize. These changes which occur prominently at the surface are probably the main causes of the capacity fade.