Revealing the Role of Electrolyte Salt Decomposition in the Structural Breakdown of LiNi0.5Mn1.5O4

Spinel LiNi0.5Mn1.5O4 (LNMO) is one of the most promising cobalt-free high-performance electrodes for boosting energy density in future Li-ion batteries and microbatteries. However, its high operating potential of over 4.7 V vs Li+/Li leads to undesired secondary reactions derived from the oxidative electrolyte decomposition that cause active material dissolution and structural degradation. Even though great effort has been put into understanding the decomposition products in the liquid electrolyte, the effects that the electrolyte decomposition has on the electrode integrity are often overlooked. Sputtered thin film electrodes with simplified geometries and composition are ideal model systems to deconvolute the effects and isolate the influencing factors. The influence of electrolyte salt on the electrochemical performance and chemical stability is analyzed here on thin film LNMO employing spectroscopic and microscopic tools. The negative effects of electrolyte decomposition on cyclability and electrode composition are noticeable when using LiPF6 salt, but the degradation with an LiClO4-based electrolyte was found to be more severe. LiClO4 decomposing in the operating potential range is associated with the generation of Cl compounds that etch the spinel oxide dissolving transition metal, diminishing the available intercalating active material and triggering the electrode breakdown. Acidification of the electrolyte occurs during repeated cycling, and the proton source is attributed to the hydrogen abstraction from the solvent oxidation induced by the high operating voltages. Our results suggest that electrolyte decomposition could trigger significant proton intercalation and surface reconstruction in the electrode, ultimately causing its electrochemical and structural breakdown.