In this study, the deterioration of lithium iron phosphate (LiFePO4) /graphite batteries during cycling at different discharge rates and temperatures is examined, and the degradation under high-rate discharge (10C) cycling is extensively investigated using full batteries combining with post-mortem analysis. The results show that high discharge current results in an instability of electrode/electrolyte interface and unstable solid electrolyte interphas. In this study, the deterioration of lithium iron phosphate (LiFePO4) /graphite batteries during cycling at different discharge rates and temperatures is examined, and the degradation under high-rate discharge (10C) cycling is extensively investigated using full batteries combining with post-mortem analysis. The results show that high discharge current results in an instability of electrode/electrolyte interface and unstable solid electrolyte interphase (SEI) layers are expected to form on the newly exposed graphite anode surface, which cause sustainable consumption of active lithium and further lead to the performance degradation of active materials. For LiFePO4 cathode, the initial capacity is largely recovered under low rate (0.1-0.2C), whereas a decline in the capability is observed at higher rates (0.5-3.0C). For graphite anode, half-cell study shows that considerable capacity loss occurs even at low rates. A small amount of Fe deposition is observed on graphite anode after cycling under 10C discharge at 55 °C. X-ray photoelectron spectroscopy (XPS) analysis confirms that a layer composed of lithium compounds is formed on the surface of anode, which can not participate in the reversible electrochemical reaction again. In addition, electrochemical impedance spectrum (EIS) measurements of half-cell indicate that the increased resistance of the positive electrode is suggested to be the root cause of power fading under high-rate discharge cycling, especially at high temperature.••Decay of battery during cycling under high discharge current is investigated.••A decline in the capability of LiFePO4 electrode is observed at higher rates.••The detailed degradation mechanism is proven by post-mortem analysis.••Increased resistance in the LiFePO4 cathode is suggested to be the root cause of power fading under high-rate discharge.Lithium iron phosphateGraphiteDeteriorationCycling performanceLithium iron phosphate (LiFePO4) is one of the most significant and promising cathode materials with high theoretical capacity (170 mAh·g−1), high thermal stability, low cost, environmental benignity and cycling stability,,,,. The graphite anode has a theoretical capacity of 372 mAh·g−1, good electrical conductivity, high crystallinity and stability of layered structure, which make it as a major commercial battery anode material,. The advantages of LiFePO4 cathode and graphite anode promote a wide application of LiFePO4/graphite batteries as power sources for hybrid electric vehicles (HEVs) and energy storage systems (ESSs). It is believed that the LiFePO4/graphite power batteries will play a significant role in the future low carbon life.Although LiFePO4 has a very stable structure, it has been indicated that the LiFePO4/graphite batteries still suffer from life and power decay during long time cycling,,,. Therefore, aging of this type of battery remains one of the biggest concerns of automakers and end customers. Understanding the degradation mechanism clearly is favorable to improve the battery performance. It is worth noting that the properties of batteries at high discharge rate become the most important factor for their applications in the HEVs and ESSs, which affects the output power of energy storage power system. ESSs used in HEVs are required to supply and store electric energy at comparably h. The 18650-type commercial LiFePO4/graphite batteries were used as the candidate for the investigation, which were obtained from Tianjin Lishen Battery Joint-Stock Co., Ltd. The designed nominal capacity of the batteries under 1C-rate is 1.06 Ah. The cycling performance of batteries were examined between 2.0 and 3.6 V under 1C charge rate and different discharge rates (1C, 10C) and temperatures (25, 55 °C). Two cells were tested at each of the conditions. Before the cycling tests, the capacities of all cells were determined by a 1C charge/discharge current at 25 °C. All the cells were firstly charged to 3.6 V and kept at 3.6 V until the current dropped to 0.02C (22 mA), then the cells were discharged to 2.0 V. During the cycling tests, the real capacity of each cell was verified every 200 cycles by using the procedures described above. The electrochemical working station (IM6eX, Germany) was used to measure the electrochemical impedance spectrum (EIS) of 18650 LiFePO4/graphite batteries and coin cells before and after cycling tests. The impedance was measured by applying a 5 mV of ac oscillation with the frequency ranging from 100 kHz to 0.01 Hz. The EIS measurement of 18650 LiFePO4/graphite batteries before and after cycling was carried out in the state of 60% SOC at 25 °C.The capacity and cycling performance tests of batteries were performed using Neware CT-4008 Battery Tester. The Espec Environmental Chamber was ado.
