Thermal Analysis of Positive Electrodes of Lithium- ion Batteries

Development of lithium-ion battery with long calendar life beyond 10 years is required for hybrid electric vehicle (HEV), plug-in HEV (PHEV) and electric vehicle (EV). It is important to understand degradation mechanism of lithium-ion battery to develop suppression techniques of degradation. In our recent researches by using various analyzing methods, some variations were detected at the surface region of the active material in the positive electrode of lithium-ion batteries after power fading [1-5]. Some surface materials such as lithium carbonate, alkyl lithium carbonate, and phosphate were found on the electrode, which were thought to be formed by decomposition of components of the electrolyte. In addition, disordering structure (cubic phase with lithium deficient) was also observed at the surface of the active material. Unfortunately, from the limitation of quantitativity of analyzing methods, the main factor of power fade has not been still clear among these variations. In this study, we attempted quantitative analysis of surface materials of positive electrodes by using thermal analysis. Cylindrical lithium-ion batteries (18650-type) were prepared. For the positive electrode, 86 wt % of LiNi0.8Co0.15Al0.05O2 (NCA), 7 wt % of polyvinylidene fluoride (PVdF) binder, and 7 wt % of acetylene black as a conductive agent had been mixed and pasted on a 20 µm thick Al foil as a current collector. The electrolyte was mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) containing 1 mol dm-3 LiPF6 where volume ratio of EC and DMC was 1:2. After setting th