The Formation Mechanism of Fluorescent Metal Complexes at the LixNi_0.5Mn_1.5O_4-δ/Carbonate Ester Electrolyte Interface

Electrochemical oxidation of carbonate esters at the Li_xNi_0.5Mn_1.5O_4−δ/electrolyte interface results in Ni/Mn dissolution and surface film formation, which negatively affect the electrochemical performance of Li-ion batteries. Ex situ X-ray absorption (XRF/XANES), Raman, and fluorescence spectroscopy, along with imaging of Li_xNi_0.5Mn_1.5O_4−δ positive and graphite negative electrodes from tested Li-ion batteries, reveal the formation of a variety of Mn^II/III and Ni^II complexes with β-diketonate ligands. These metal complexes, which are generated upon anodic oxidation of ethyl and diethyl carbonates at Li_xNi_0.5Mn_1.5O_4−δ, form a surface film that partially dissolves in the electrolyte. The dissolved Mn^III complexes are reduced to their Mn^II analogues, which are incorporated into the solid electrolyte interphase surface layer at the graphite negative electrode. This work elucidates possible reaction pathways and evaluates their implications for Li⁺ transport kinetics in Li-ion batteries.