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

Thermal reactions between 1.0 M LiPF 6 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 Li 2 CO 3 on the surface of the metal oxides is removed and replaced by a complex mixture including poly(ethylene oxide), polycarbonate, ROCO 2 Li, LiF, and Li x PO y F z . Higher surface concentration of Li 2 CO 3 on LiNi 0.8 Co 0.2 O 2 allows a fragile temperature-dependent equilibrium to be established, accounting for the thermal stability of the electrolyte. Higher temperature leads to more Li x PO y F z and less Li 2 CO 3 on the surface. With LiCoO 2 , no equilibrium is established due to lower surface concentration of Li 2 CO 3 . Consequently, thermal reactions of the electrolyte with LiCoO 2 generate decomposition products, including Li x PO y F z and cobalt fluorides on the surface and bulk electrolyte decomposition. Independent addition of Li 2 CO 3 enhances the thermal stability of the LiPF 6 -based electrolyte, confirming the thermal-stabilizing properties of Li 2 CO 3 surface films. The addition of a carbonate solution of PF 5 to LiNi 0.8 Co 0.2 O 2 generates LiPF 6 in the solution and Li x PO y F z on the surface.