Laminated Thin Li-Ion Batteries Using LiNi_0.8−yCo_0.2Al_yO₂ Cathode Materials

Abstract

Thermal stability, surface characteristics, and electrode performance for LiNi_1−x−yCo_xAl_yO₂-based materials in a 1.5 M LiBF₄ ethylene carbonate/γ-butyrolactone (1: 2) electrolyte have been investigated in order to develop laminated thin Li-ion batteries using LiNi_0.8−yCo_0.2Al_yO₂ cathodes. Using a LiNi_0.74Co_0.2Al_0.06O₂ cathode with low basicity, the laminated thin Li-ion batteries provided high energy density, long cycle life, and very low swelling. It was demonstrated that the prototype laminated thin Li-ion battery with a thickness of 3.8 mm achieved the energy densities of 200 Wh/kg and 407 Wh/l. The value of pH (definition is in experimental section) for LiNi_0.74Co_0.2Al_0.06O₂ should be less than 11 for the practical application because of a long cycle life of 500 cycles and very low swelling at an even high temperature of 85°C. Native alkaline impurities in LiNi_0.8−yCo_0.2Al_yO₂ led to CO₂ gas evolution and cycle degradation. The results of impedance and x-ray photoelectron spectroscopy measurements for LiNi_0.74Co_0.2Al_0.06O₂ and LiCoO₂ indicated that the surface film formation on LiNi_0.74Co_0.2Al_0.06O₂ cathode is more inactive and thermally stable than that of LiCoO₂ cathode. The films formation on LiNi_0.8−yCo_0.2Al_yO₂ cathode suppressed further oxidation of the electrolyte and the gas evolution at high-temperature condition.