Low Temperature and High Rate Performance of Graphite Anode Materials for High Power Li-Ion Batteries

摘要

A new experimental method is reported in this paper for evaluating the rate capability of graphite anode materials for high power lithium-ion batteries at different temperatures. Comparisons of AC impedances and pulse current polarization curves for three grades of high rate-capable graphite anode materials are presented to illustrate their differences in polarizations due to mass transfer and reaction kinetics. Both AC impedance and symmetric current pulse techniques were employed using a symmetric two-electrode cell in determining the rate-capability of electrodes for graphite anode materials. It was found that polarization of graphite anodes at low temperatures (＜ -5 °C) results predominately from resistance across the electrolyte/solid interface of active material particles due to reaction kinetics. Reducing particle size (or increasing reaction surface area) would be the most effective way to reduce the polarization at low temperatures. Three grades of graphite anode materials (A, B, and C) made using ConocoPhillips' proprietary processes were evaluated with the new method. As illustrated, the electrodes made with the very fine graphite powder ( ＜ 5 μm, C) exhibited good capacity, high initial coulombic efficiency, and significantly lower impedance or polarization than electrodes made with the other graphite powders that were also made for high power Li-ion batteries. The full experimental coin cells (LiCoO_2 vs. anode material sample B) yielded a power capability of 4.5 kW/kg at 90% of state of charge, and yet still have a round-trip energy efficiency of 91% under a 17 C discharge rate and one C recharge rate. It is expected that use of grade C anode material would enable designing even more powerful lithium ion batteries.