Degradation Behavior in Lithium Iron Phosphate Secondary Cells Under High Rate Pulsed Discharge

Abstract

LiFePO4 secondary cells cycled under a ultra-high rate pulsed discharge profile are dismantled to study the changes in surface chemistry in pulse power load systems. Three cells lost the ability to charge (0 Ahrs) in less than 50 cycles at 15C (40A) pulsed discharge/continuous recharge conditions. Electrode samples show varying levels of solid electrolyte interphase growth. However, x-ray absorption near edge structure analysis conducted on the cathode material shows minimal loss of active lithium. A cell cycled under the same conditions with a 60°C operating temperature limit showed more gradual capacity fade and a higher degree of shift in the photon energy of the XANES spectrum. XPS analysis shows thicker SEI formation in the 60°cells. This indicates that the additional thermal energy in the 75°C cells provide more favorable conditions for the evolution of a degradation mechanism that is more complex than lost active material. Increased concentration in oxygen at the anode surface indicates heavy reduction in the electrolyte salt forming LixPFy-Oz. The 0 Ahr cells showed higher concentration of Fluorine and an SEI that contains mostly LiF. Results suggests that high temperature, ultra-high rate pulsed cycling favors LiPF6 reduction into LiF and thus the concentration of Li2CO3 in the SEI is reduced creating a high impedance film.