Operando Raman Spectroscopy Measurements of a High-Voltage Cathode Material for Lithium-Ion Batteries

As a high-voltage spinel, LiNi_0.5Mn_1.5O₄ (LNMO) is a promising candidate for high energy density cathodes in lithium-ion batteries (LiBs). The material has not yet achieved any commercial success, as there remain problems with capacity fade after extended charge and discharge cycling. In order to enable improvements, it is necessary to understand the fundamental underlying processes in the material. In this experimental study, we present operando Raman measurements to investigate the potential-resolved structural evolution of ordered LNMO as a cathode material during the charging and discharging process. Using the method of Raman spectroscopy, only two phases can be unequivocally distinguished in the case of ordered LNMO, namely, LiNi_0.5Mn_1.5O₄ and Ni_0.5Mn_1.5O₄ (NMO). The half-delithiated phase, Li_0.5Ni_0.5Mn_1.5O₄, cannot be discriminated by using this spectroscopic method. The dynamics of the phase changes between LiNi_0.5Mn_1.5O₄ and Ni_0.5Mn_1.5O₄ differ for lithiation and delithiation. Long-term operando Raman measurements of half-cells prove that a decomposition of the solvent takes place and that the conductive salt LiPF₆ is consumed, i.e., the concentration of PF₆^- is strongly decreasing. The solvent component ethylene carbonate (EC) is preferentially decomposed during the cycling process, and byproducts such as esters and alcohols can be detected.