The Lithium ion battery is widely used for portable devices and electric automobiles, and it is considered to be the most promising power source to replace fossil fuels.
The capacity fade of lithium ion batteries is strongly related to the use conditions, such as the charge and discharge rate, the cut-off voltage,the depth of discharge (DOD),the state of charge (SOC) and the ambient temperature, which all have influences on the performance and the lifetime of lithium ion batteries.
The influence of different discharge rates on the aging of LiFePO4/graphite full cells is investigated. The decay rate of full cell capacity is accelerated by increasing the discharge rate.However, when the discharge rate is higher than 3.0C, the degradation mechanism of full cell is changed.
Through disassembling the fresh and the aged full cells, it is concluded that the intrinsic capacity of graphite anode is mainly related to the SEI film on its surface.
The irreversible consumption of active lithium due to the formation and development of SEI film is the primary reason for the capacity fade of full cell aged at relatively low discharge rate. When the discharge rates are 4.0C and 5.0C,the capacity fade rate maintains a comparatively high level, for the SEI film on the anode is apt to be broken and unstable after the rapid extraction of lithium ions at high rates. Besides, the decay rate of capacity increases in the later period of cycling, which is attributed to the degradation of active materials. The performance degradation of LiFePO4 electrode and the unstable SEI film indicate that the degradation mechanism is changed when the full cells are aged at high discharge rates. Furthermore, the high discharge rate has adverse effects on the performance of graphite anodes and also can increase the internal resistance of full cell, which is detrimental to the high rate discharge ability.
In conclusion, the high discharge rates can result in the rapid fade of full cell capacity and the changed degradation mechanism. So, when the discharge rate is greater than or equal to 4.0C, it is not suitable to be used for accelerating the aging of the LiFePO4/graphite full cells.
