Abstract
High-voltage stability of LiBF4 - propylene carbonate solutions in intermediate concentration range is studied by means of cyclic voltammetry, galvanostatic cycling and X-ray photoelectron spectroscopy using LiCoPO4 and LiNi0.5Mn1.5O4 cathode materials. Coulombic efficiency improves with increasing salt-to-solvent molar ratio from 1:12 to 1:4 (∼0.8 m–2.5 m solutions), reaching 98% at 1C charge/discharge rate for 1:4 electrolyte upon cycling of LiNi0.5Mn1.5O4 up to 5 V vs. Li/Li+. The same positive trend is observed for discharge capacities, cycling stability and capacity scattering for both high-voltage cathode materials. X-ray photoelectron spectroscopy of the electrodes studied after cycling in solutions of different concentrations does not reveal any drastic difference in surface composition. Interface pre-formation experiment shows that the presence of the interface layer formed at semi-concentrated 1:4 electrolyte does not have a major impact on the electrochemical properties of the dilute solution. We assume that the enhanced oxidation stability of the concentrated solutions itself is a reason of improving the electrochemical performance rather than the cathode-electrolyte interface properties.
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