In situ x-ray absorption spectroscopic study of the Li[Ni1∕3Co1∕3Mn1∕3]O2 cathode material

Abstract

The layered ${\mathrm{LiNi}}_{1∕3}{\mathrm{Co}}_{1∕3}{\mathrm{Mn}}_{1∕3}{\mathrm{O}}_2$ system has recently drawn considerable interest for use as a cathode material for rechargeable lithium batteries. In order to investigate the charge-compensation mechanism and structural perturbations occurring in the system during cycling, in situ x-ray absorption spectroscopy (XAS) measurements were performed utilizing a novel electrochemical in situ cell specifically designed for long term x-ray experiments. The cell was cycled at a moderate rate through a typical Li-ion battery operating voltage range (2.9–4.7 V). The electrode contained 2.025 mg of ${\mathrm{LiNi}}_{1∕3}{\mathrm{Co}}_{1∕3}{\mathrm{Mn}}_{1∕3}{\mathrm{O}}_2$ on a $25\text{-}\mu \mathrm{m}$ Al foil and had an area of $0.79{\mathrm{cm}}^2$ . The x-ray absorption spectroscopy (XAS) measurements were performed at the Ni, Co, and the Mn edges at different states of charge (SOC) during cycling, revealing details about the response of the cathode to Li insertion and extraction processes. Changes of bond distance and coordination number of Ni, Co, and Mn absorbers as a function of the state of charge of the material were obtained from the extended x-ray-absorption fine structure (EXAFS) region of the spectra. The x-ray absorption near-edge structure (XANES) region was studied in order to characterize the oxidation states of the $3d$ transition metals during cycling (Li extraction/insertion). We found that oxidation states of transition metals in ${\mathrm{LiNi}}_{1∕3}{\mathrm{Co}}_{1∕3}{\mathrm{Mn}}_{1∕3}{\mathrm{O}}_2$ are ${\mathrm{Ni}}^{2+}$ , ${\mathrm{Co}}^{3+}$ , and ${\mathrm{Mn}}^{4+}$ , whereas during charging ${\mathrm{Ni}}^{2+}$ is oxidized to ${\mathrm{Ni}}^{4+}$ through an intermediate stage of ${\mathrm{Ni}}^{3+},{\mathrm{Co}}^{3+}$ is oxidized almost to ${\mathrm{Co}}^{4+}$ and, utilizing Faraday’s calculation and XAS results, the Co was found to be at ${\mathrm{Co}}^{3.92+}$ at the end of the charge, while Mn was found to be electrochemically inactive and remains as ${\mathrm{Mn}}^{4+}$ . The EXAFS data that were collected continuously during cycling revealed details about the response of the cathode to Li insertion and extraction. These measurements on the ${\mathrm{LiNi}}_{1∕3}{\mathrm{Co}}_{1∕3}{\mathrm{Mn}}_{1∕3}{\mathrm{O}}_2$ cathode confirmed that the material retains its symmetry and good structural short-range order leading to superior cycling.