Study of Capacity Loss Mechanism of Lithium Manganate Cathode during Electrochemical Cycles at High Temperature by in situ TXRF and XAFS Analyses

Abstract

Dissolution process of Mn from LiMn₂O₄ cathode during the cyclic use of an electrochemical cell was examined by newly developed in situ TXRF (Total Reflection X-ray fluorescence) technique. Mn concentration in the electrolyte solution during electrochemical cycling was successfully determined for the first time without disassembling the cell using ten micro liters of the sample solution taken by inserting a fused silica capillary. The Mn concentration versus XRF intensity calibration curve showed a good linearity from 3 to 500 ppm. The dissolution of Mn in the cell increased with increasing the cycle number at 50°C. The Mn concentration is the highest at the center of the electrodes. It was also found that dissolution of Mn from the cathode material took place during the change process. The chemical state of Mn in the electrolyte solution was examined by using fluorescence X-ray absorption fine structure (XAFS) technique utilizing a 19-element solid state detector and synchrotron radiation. Mn K-XANES (X-ray absorption near-edge structure) spectra of the dissolved Mn in the electrolyte solution suggested that Mn existed as Mn²⁺ state regardless of the temperature. The unknown material deposited on the Li anode was estimated to be a divalent manganese compound with fluorine ligand (Mn-F distance = 2.16 Å) by XAFS analysis.