Multi-scale analysis for accelerated degradation mechanisms of cylindrical LiFePO4/graphite batteries under distinct positions of jelly roll

Abstract

Lithium iron phosphate (LFP) batteries are widely used in the electric vehicle due to the great chemical stability and cost advantages. Fluctuations in the environment and driving conditions lead to a deterioration in battery performance that is inevitable. In addition, due to the winding manufacturing characteristics of cylindrical battery electrodes, the uneven distribution of current and temperature accelerates the aging process. In this paper, the cross-comparison aging test of cylindrical LFP batteries is designed, in which three kinds of constant current and two different dynamic working conditions are used for accelerated aging test at high temperature. The aging behavior and mechanism are investigated quantitatively by combination of in-situ electrochemical test and ex-situ Post-Mortem methods. During the long-term cycle of the battery, external characteristic parameters such as capacity, internal resistance, and open circuit voltage are periodically checked up. Until the capacity retention falls below 80%, the batteries are disassembled to sample the electrodes at the specified positions from the jelly roll. The material morphology changes of electrodes are observed by scanning electron microscopy. The half-cell tests were produced to obtain material-level aging information based on performance tests. This multi-scale analytical method enables the detailed process of performance loss, which guides performance prediction and safety diagnostics.