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

Abstract

As a high-voltage spinel, LiNi_0.5Mn_1.5O₄ (LNMO) is a promising candidate\nfor high energy density cathodes\nin lithium-ion batteries (LiBs). The material has not yet achieved\nany commercial success, as there remain problems with capacity fade\nafter extended charge and discharge cycling. In order to enable improvements,\nit is necessary to understand the fundamental underlying processes\nin the material. In this experimental study, we present operando Raman\nmeasurements to investigate the potential-resolved structural evolution\nof ordered LNMO as a cathode material during the charging and discharging\nprocess. Using the method of Raman spectroscopy, only two phases can\nbe unequivocally distinguished in the case of ordered LNMO, namely,\nLiNi_0.5Mn_1.5O₄ and Ni_0.5Mn_1.5O₄ (NMO). The half-delithiated phase,\nLi_0.5Ni_0.5Mn_1.5O₄, cannot\nbe discriminated by using this spectroscopic method. The dynamics\nof the phase changes between LiNi_0.5Mn_1.5O₄ and Ni_0.5Mn_1.5O₄ differ\nfor lithiation and delithiation. Long-term operando Raman measurements\nof half-cells prove that a decomposition of the solvent takes place\nand that the conductive salt LiPF₆ is consumed, i.e., the\nconcentration of PF₆^– is strongly decreasing.\nThe solvent component ethylene carbonate (EC) is preferentially decomposed\nduring the cycling process, and byproducts such as esters and alcohols\ncan be detected.