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

Abstract

Spinel LiNi0.5Mn1.5O4 (LNMO) isone of the most promising cobalt-free high-performance electrodesfor 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 oxidativeelectrolyte decomposition that cause active material dissolution andstructural degradation. Even though great effort has been put intounderstanding the decomposition products in the liquid electrolyte,the effects that the electrolyte decomposition has on the electrodeintegrity are often overlooked. Sputtered thin film electrodes withsimplified geometries and composition are ideal model systems to deconvolutethe effects and isolate the influencing factors. The influence ofelectrolyte salt on the electrochemical performance and chemical stabilityis analyzed here on thin film LNMO employing spectroscopic and microscopictools. The negative effects of electrolyte decomposition on cyclabilityand electrode composition are noticeable when using LiPF6 salt, but the degradation with an LiClO4-based electrolytewas found to be more severe. LiClO4 decomposing in theoperating potential range is associated with the generation of Clcompounds that etch the spinel oxide dissolving transition metal,diminishing the available intercalating active material and triggeringthe electrode breakdown. Acidification of the electrolyte occurs duringrepeated cycling, and the proton source is attributed to the hydrogenabstraction from the solvent oxidation induced by the high operatingvoltages. Our results suggest that electrolyte decomposition couldtrigger significant proton intercalation and surface reconstructionin the electrode, ultimately causing its electrochemical and structuralbreakdown.