Simulating Electrochemical Aging in NCM111 Materials Through Controlled Chemical Delithiation

To meet the cycle life requirements and to guarantee the safe operation of lithium-ion batteries, the upper cutoff potential of cathode active materials based on mixed transition metal layered oxides like NCM (LiMO2, with M = Ni, Co, Mn) must be restricted, thereby limiting the available specific capacity. A significant degradation mechanism in NCM materials involves the harmful release of lattice oxygen at high degrees of delithiation (reached at high cathode potentials), forming an oxygen-depleted surface phase on the active material particles accompanied by electrolyte oxidation. To mimic the electrochemically-induced lattice oxygen release, NCM materials can be subjected to chemical delithiation and subsequent heat-treatment. We thus investigated the chemical delithiation of NCM111 (Li1.0(Ni1/3Co1/3Mn1/3)O2) with NO2BF4, followed by a heat-treatment. The resulting materials are characterized with regards to their electrochemical characteristics as well as by thermogravimetric analysis-mass spectrometry, X-ray diffraction, scanning electron microscopy, and gas adsorption analysis. We discovered that chemical delithiation initially produces a disordered layered phase that is electrochemically less active. Upon heat-treatment, this phase restructures into a fully-lithiated and well-ordered layered phase and an electrochemically inactive spinel phase. This study enhances our understanding of the phases that form when NCM materials undergo extensive delithiation.