Investigation of charge compensation mechanism of LiNi_⅓Co_⅓Mn_⅓O₂ Positive Electrode during Initial Charge Process by Operando Soft X-ray XAFS Measurement

Abstract

The charge compensation mechanism of a LiNi_⅓Co_⅓Mn_⅓O₂ (NCM333) positive electrode, used in a commercial lithium ion battery, was investigated by operando soft X-ray absorption fine structure (SX-XAFS) spectroscopy. The SX-XAFS spectral analyses recorded with the partial fluorescence yield revealed followings. The pristine NCM333 consisted of Ni²⁺, Co³⁺, Mn⁴⁺ and O^2-. During charge, Ni²⁺ was oxidized to Ni⁴⁺, and Mn⁴⁺ did not change at all. O^2- was oxidized by creating O 2p holes during charge, which caused an appearance of a new pre-edge peak in the O K-edge XAFS spectra. Co L₃-edge peak shifted to higher energy, indicating that Co³⁺ is oxidized during charge. Hence nickel ions play the central role of the charge compensation in the NCM333 positive electrode, and Mn ions are not contribute to the charge compensation. O ions and Co ions are not as large as Ni ions, but they are also contribute to the charge compensation.