Factors responsible for impedance rise in high power lithium ion batteries

High-power, 18,650 lithium-ion cells have been designed and fabricated in o rder to understand the factors Limiting the calendar life of the Lithium-io n system. Each cell consisted of a LiNi0.8Co0.2O2 positive electrode, a ble nd of MCMB-6 and SFG-6 carbon negative electrode, and a LiPF6 in EC:DEC (1: 1) electrolyte. These cells, which initially meet the power requirement set by the partnership for a new generation of vehicles (PNGV), were subjected to accelerated calendar life and cycle life testing. After testing at elev ated temperatures, the cells experienced a significant impedance rise and l oss of power. The fade rate of power in these cells was dependent of the st ate of charge and the temperature of testing. Micro-reference electrode and ac-impedance studies on symmetrical cells have confirmed that the interfac ial resistance at the positive electrode was the main reason behind the imp edance rise in the high power cell. (C) 2001 Elsevier Science B.V. All righ ts reserved.