Cycle life at ambient temperatures is an important indicator of power battery applications. With a stable cathode and a simple electrolyte, the analysis of the capacity fading mechanism in lithium iron phosphate (LFP) power batteries is of great significance for a comprehensive understanding of capacity fading in these power batteries and for improving electrochemical performance. This study discusses the capacity fading mechanism in ambient cycling based on commercial lithium iron phosphate power batteries at different states of health (SOH). Electrochemical differential capacity analysis is applied to batteries cycled at ambient temperature to determine the polarization alteration. The area charge of peaks on a differential capacity curve is used to analyze the source of the capacity loss. Capacity loss is mainly derived from the reaction of graphite on the third plateau, not from the result of polarization upon cycling. Charge transfer resistance of the anode is found to increase significantly in electrochemical impedance spectroscopy collected on tri-electrode cells. No evident capacity losses of positive and negative electrodes are observed on coin cells whose electrodes were collected from LFP batteries of different SOH, indicating no deterioration in cathode and anode materials. The investigation shows that the capacity fading at ambient temperature cycling is mainly caused by the active lithium loss from side reactions and kinetic fading of the anode. The kinetic fading of the anode is commonly exhibited during the cycles by the thickening of the SEI and stress on the batteries.
Keywords： lithium iron phosphate ; cycle at ambient temperature ; capacity fading mechanism ; active lithium loss ; kinetic