Lithium-Ion Batteries: Thermal Reactions of Electrolyte with the Surface of Metal Oxide Cathode Particles

Abstract

Thermal reactions between in 1:1:1 ethylene carbonate/dimethyl carbonate/diethyl carbonate and metal-oxide cathode particles were investigated by analyzing both the liquid electrolyte and solid cathode particles through the combined use of nuclear magnetic resonance spectroscopy, gas chromatography with mass selective detection, scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The reactions between the electrolyte and cathode particles inhibit the thermal decomposition of the electrolyte and modify the surface of the cathode particles. The on the surface of the metal oxides is removed and replaced by a complex mixture including poly(ethylene oxide), polycarbonate, , , and . Higher surface concentration of on allows a fragile temperature-dependent equilibrium to be established, accounting for the thermal stability of the electrolyte. Higher temperature leads to more and less on the surface. With , no equilibrium is established due to lower surface concentration of . Consequently, thermal reactions of the electrolyte with generate decomposition products, including and cobalt fluorides on the surface and bulk electrolyte decomposition. Independent addition of enhances the thermal stability of the -based electrolyte, confirming the thermal-stabilizing properties of surface films. The addition of a carbonate solution of to generates in the solution and on the surface.