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Will the lithium iron phosphate battery fail if left unused?

by:dcfpower     2021-03-18

In the service life of lithium iron phosphate batteries, most of the time is in a state of shelving, generally after a long period of shelving, the battery performance will decrease, generally showing an increase in internal resistance, a decrease in voltage and discharge capacity Drop and so on. There are many factors that cause the degradation of battery performance, of which temperature, state of charge and time are the most obvious factors.

The aging mechanism of lithium iron phosphate batteries under different storage conditions is mainly the side reaction of the positive and negative electrodes and the electrolyte (as opposed to the side reaction of the positive electrode, the graphite negative electrode The reaction is more serious, mainly due to the decomposition of the solvent and the growth of the SEl film), which consumes active lithium ions, and the overall impedance of the battery increases. The loss of active lithium ions leads to the aging of the battery when it is left; and the capacity loss of the LiFePO4 power battery increases with storage. The temperature rise is severely increased. In contrast, as the storage state of charge increases, the capacity loss is smaller.

Storage temperature has a greater impact on the aging of lithium iron phosphate batteries, followed by storage state of charge. The capacity loss of lithium iron phosphate batteries is predicted based on factors related to storage time (temperature and state of charge). In a certain SOC state, as the shelf time increases, the lithium in the graphite will diffuse to the edge, forming a complex compound with the electrolyte and electrons, resulting in an increase in the proportion of irreversible lithium ions. Fourth, the SEl becomes thicker and conductive The decrease in performance (inorganic components increase, and some have a chance to re-dissolve) caused by the increase in impedance and the decrease in the activity of the electrode surface together cause the battery to age.

Regardless of the charging state or the discharging state, in the temperature range from room temperature to 85 ℃, differential scanning calorimetry (DSC) did not find LiFePO4 and different electrolytes (the electrolyte is LiBF2, LiAsF). Or LiPF) has any reaction. But LiFePO4 is immersed in LiPF for a long time. It still exhibits a certain degree of reactivity in the electrolyte: because the reaction to form the interface is very slow, there is still no film on the surface of LiFePO4 after soaking for one month to prevent further reaction with the electrolyte.

In the shelving state, poor storage conditions (high temperature and high state of charge) will increase the degree of self-discharge of the LiFePO4 power battery, making the battery aging more obvious.

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