The battery is generally exothermic during use, so the effect of temperature is very important. In addition, road conditions, usage, ambient temperature, etc. will have different effects.

The capacity loss of LiFePO4 power batteries during cycling is generally considered to be caused by the loss of active lithium ions. The research shows that the aging of LiFePO4 power battery during cycling is mainly through a complex growth process that consumes the active Li- ion SEI film. In this process, the loss of active lithium ions directly reduces the capacity retention rate of the battery; the continuous growth of the SEI film, on the one hand, increases the polarization resistance of the battery, and at the same time, the thickness of the SEI film is too thick, and the electrochemical performance of the graphite negative electrode is reduced. The activity is also partially inactivated.

During high temperature cycling, Fe2+ in LiFePO4 will dissolve to a certain extent. Although the amount of Fe2+ dissolved has no obvious effect on the capacity of the positive electrode, the dissolution of Fe2+ and the precipitation of Fe on the graphite negative electrode will play a catalytic role in the growth of the SEI film. . Quantitative analysis of where and in which step the active lithium ions are lost, it is found that most of the loss of active lithium ions occurs on the surface of the graphite negative electrode, especially during high-temperature cycling, that is, the loss of high-temperature cycling capacity is faster; and the destruction of the SEI film is summarized. There are three different mechanisms from repair: (1) electrons in the graphite anode pass through the SEI film to reduce lithium ions; (2) the dissolution and regeneration of some components of the SEI film; (3) due to the volume change of the graphite anode. SEI membrane rupture.

In addition to the loss of active lithium ions, both positive and negative electrode materials deteriorate during cycling. The appearance of cracks in LiFePO4 electrodes during cycling can lead to an increase in electrode polarization and a decrease in the conductivity between the active material and the conductive agent or current collector. The changes of LiFePO4 after aging were studied semi-quantitatively by scanning extended resistance microscopy (SSRM), and it was found that the coarsening of LiFePO4 nanoparticles and the surface deposits produced by some chemical reactions jointly led to the increase of LiFePO4 cathode impedance. In addition, the reduction of active surface and exfoliation of graphite electrodes caused by the loss of graphite active materials are also considered to be the reasons for battery aging. The instability of graphite negative electrodes will lead to the instability of SEI film, which will promote the consumption of active lithium ions. .

The large rate discharge of the battery can provide large power for the electric vehicle, that is, the better the rate performance of the power battery, the better the acceleration performance of the electric vehicle. The results show that the aging mechanisms of LiFePO4 cathode and graphite anode are different: with the increase of discharge rate, the capacity loss of cathode increases more than that of anode. The loss of battery capacity during low-rate cycling is mainly caused by the consumption of active lithium ions at the negative electrode, while the power loss of the battery during high-rate cycling is caused by the increase in the impedance of the positive electrode.

Although the depth of discharge in the use of the power battery does not affect the capacity loss, it will affect its power loss: the speed of power loss increases with the increase of the depth of discharge, which is related to the increase in the impedance of the SEI film and the increase in the impedance of the entire battery. directly related. Although the effect of the upper limit of charging voltage on battery failure is not obvious relative to the loss of active lithium ions, too low or too high upper limit of charging voltage will increase the interface impedance of LiFePO4 electrodes: the lower upper limit voltage cannot be very good. A passivation film is formed on the ground, and a too high upper voltage limit will lead to the oxidative decomposition of the electrolyte, resulting in the formation of products with low conductivity on the surface of the LiFePO4 electrode.

The discharge capacity of LiFePO4 power batteries decreases rapidly when the temperature decreases, mainly due to the decrease of ionic conductivity and the increase of interfacial impedance. By studying the LiFePO4 cathode and the graphite anode respectively, it was found that the main controlling factors limiting the low temperature performance of the cathode and anode are different. The decrease of ionic conductivity in the LiFePO4 cathode is dominant, while the increase in the interface impedance of the graphite anode is the main reason.

During use, the degradation of LiFePO4 electrode and graphite negative electrode and the continuous growth of SEI film cause battery failure to varying degrees; in addition, in addition to uncontrollable factors such as road conditions and ambient temperature, the normal use of the battery is also very important, including appropriate The charging voltage, suitable depth of discharge, etc.

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