In situ XAFS of the Li{sub x}Ni{sub 0.8}Co{sub 0.2} cathode for lithium-ion batteries

The layered LiNi{sub 0.8}Co{sub 0.2}O{sub 2} system is being considered as a new cathode material for the lithium-ion battery. Compared with LiCoO{sub 2}, the standard cathode formulation, it possesses improved electrochemical performance at a projected lower cost. In situ x-ray absorption fine-structure spectroscopy (XAFS) measurements were conducted on a cell cycled at a moderate rate and normal Li-ion operating voltages (3.0--4.1 V). The XAFS data collected at the Ni and Co edges approximately every 30 min. revealed details about the response of the cathode to Li insertion and extraction. These measurements on the Li{sub x}Ni{sub 0.8}Co{sub 0.2}O{sub 2} cathode (0.29 < x < 0.78) demonstrated the excellent reversibility of the cathode's short-range structure. However, the Co and Ni atoms behaved differently in response to Li insertion. This study corroborates previous work that explains the XAFS of the Ni atoms in terms of a Ni{sup 3+} Jahn-Teller ion. An analysis of the metal-metal distances suggests, contrary to a qualitative analysis of the x-ray absorption near-edge structure (XANES), that the Co{sup 3+} is oxidized to the maximum extent possible (within the Li content range of this experiment) at x = 0.47 {+-} 0.04, and further oxidation occurs at the Ni site.