Degradation Behavior And Aging Mechanism Of Lithium Iron Phosphate/Graphite Power Batteries

The LiFePO4/graphite power batteries pocesses the advantages of low cost, good cycling performance and high safety and nowadays they have been widely used in a wide range of applications ranging from Electric Vehicles to Energy Storage Systems, which is considered to be the most promising emerging green energy. However, with the development of power battery, lifespan is becoming a serious problem. The battery life is greatly shortened especially under working conditions such as high/low temperature or fast charging, which have a bad effect on the large-scale commercial applications. In this paper, the fading mechanisms of batteries under various conditions were systematic and comprehensive researched from meterial aspect, which has important theoretical significance to the development of electric vehicles’ battery.The non-destinctive method combining with post-mortem analysis were used in the paper, and product battery and button cell were used as the candidate for the investigation. The LiFePO4/graphite power batteries under various working conditions such as high-rate, storage, low temperature and over-discharge were tracking tested and investigated systematically using electrochemical impedance spectrum (EIS), X-ray diffraction (XRD), scanning electron micro-scope(SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS). The mechanism of high-rate discharge on the positive and negative electrodes was proposed for the first time. Effects of state of charge (SOC) and temperature on the degradation of batteries during storage process were analyzed. The cycling degradation mechanisms of LiFePO4/graphite power batteries under low temperature and over-discharge were further investigated for the first time.In the study of high-rate discharge cycling, the deterioration of LiFePO4/graphite batteries during cycling at different discharge rates and temperatures was examined firstly, and the degradation under high-rate discharge (10C) cycling was extensively investigated using full batteries combining with pot-mortem analysis. Results show that for LiFePO4 cathodes, a decline in the capability is observed at higher rates (0.5-3.OC) after cycling with 10C rate and a significant decreased can be found under high temperature. The FePO4 phase is firstly observed from the high-resolution TEM (HRTEM) images of the cycled LiFeP04 cathodes. A small amount of Fe deposition is observed on graphite anode after cycling under high rate and temperature (dissolution of LiFePO4).In the study of aging mechanism during storage process, the effects of SOC and temperature on the degradation mechanism of batteries was studied by pot-mortem analysis and nondestructive method, respectively. Firstly, the aging mechanism of batteries under different SOC and 55℃ were systematically investigated using pot-mortem analysis. The results show that more capacity fading is experienced for the batteries stored at higher temperatures and SOCs. The capacity reduces more seriously at various SOC and 55℃, while the rate capability is not changed during the storage process. A bigger amount of Fe deposition is observed on graphite anode after storage at 100% SOC and 55℃ compared with cycling at 10C and 55℃ (dissolution of LiFePO4). Futhermore, the aging process during storage at different temperature and 100% SOC were studied using batteries with a capacity of 12 Ah. Results show that the capacity loss under high temperature of 55℃ mainly caused by active lithium ion loss. The internal resistance increases greatly with increasing in storage times under high temperature of 55℃. Low temperature storage has no significant effect on the capacity and impedance of the battery.The capacity degradation during low temperature cycling under different charge/discharge rate and charging SOCs was researched, and the cycling degradation of batteries under various charge rate (1/10C,1/3C,1/2C, and 1C) at-10℃ were systematically investigated. Results show that the capacity loss under low temperature results from the lithium deposition on the anode surface, and the cell aging is significantly affected by temperature and charging SOC. The low temperature charging under relatively high charge rates of 1/3C,1/2C, and 1C results in severe lithium plating which leads to extremely serious capacity loss. In contrast, no lithium plating occurred under low charge rate of 1/10C. SEM analysis indicates that a layer composed of rod-like lithium is formed on the anode surface and the thinkness of lithium layer increases with the increase in cyling numbers.The LiFePO4/graphite power batteries were prepared and the capacity fading mechanism under different over-discharge voltage (1.5,1.0,0.50.0 V) was investigated using full batteries combining with post-mortem analysis. The results show that the batteries over-discharging with lower cut-off voltage loss its capacity more quickly. Over-discharging at 0.5 and 0.0 V experienced a serious capacity loss and further caused the poor cycling performance. A serious loss of active lithium and loss of anode material occur at 0.0 V during both over-discharging and the normal cycling stage. There is some swelling and it seems that a gas generates on the cycled batteries at 0.5 and 0.0 V and gas composition analysis reveals that the gas mainly contains H2, CH4 and C2H6. No capacity loss can be found on the LiFePO4 cathodes while a serious irreversible capacity loss can be found for graphite anode after cycling, especially at 0.0V.In conclusion, the capacity of LiFePO4 cathodes keep stable under various working conditions. The graphite anodes pocesses poor performance stability compared with LiFePO4 cathodes. An obvious capacity loss can be found under at over-discharge and high-rate discharge cycling. A small amount of Fe deposition is observed on graphite anode after storage and cycling under high temperature. The rod-like lithium deposition can be found on graphite anode during high-rate charge cycling under low temperature. The swelling can be found on the batteries over-discharging below 1.0 V and surface morphology of graphite anode is destroyed under this condition.