The Capacity Fading Mechanism Of Lithium Iron Phosphate Battery At Elevated Temperature And The Solution

The cycle performance of lithium ion battery determines its’ service life. The capacity fading is accelerated by the high temperature for the lithium iron phosphate battery, so the the cycle life is shortened. To extend cells’cycling life at elevated temperature, studies on the capacity fading mechanism and solution are of great significance.In this paper, constant current charge-discharge test was employed to study the electrochemical performance of LiFePO4/Li, Li/AG, LiFePO4/AG cells at different temperatures, materials and electrochemical methods were applied to study the capacity fading mechanism of cells. Based on the capacity fading mechanism, constant current charge-discharge test, Cyclic Voltammetry (CV), Electrochemical Impedancespectra (EIS), Raman Spectroscopy, Fourier Transform Infrared Spectrometer (FTIR) were employed to get insight into the influence of iron ions in electrolyte which are dissolved from the cathode on the performance of graphite anode. As far as binder is concerned, polyacrylic acid binder was applied to improve the poor performance of graphite anode in electrolyte containing iron ions which are dissolved from the cathode. The results are as following:1. Compared to LiFePO4/AG cells, the high temperature has little influence on the cycle performance of LiFePO4/Li, Li/AG cells. The main reasons of fast capacity fading of LiFePO4/AG cells were the loss of active lithium and the increase of impedance.2. The graphite anode exhibits fast capacity fading in Fe-containing electrolyte. The deterioration rates are21.5%and26%for700and1800ppm Fe-containing electrolyte respectively. The charge-discharge test shows that the iron deposition occurs at2.4V. On the one hand, the deposits accelerated decomposition of electrolyte, resulting in thicker solid-electrolyte interface SEI layer. On the other hand, the deposits hindered intercalation of Li ions into graphite. Both effects caused capacity fading upon cycling.3. The graphite using PAA as the binder shows almost no capacity fading in Fe-containing electrolyte after150cycles. The SEM and XPS analysis show that the the binding state with PAA is different from tha with PVDF, the graphite particles are coated with PAA binder, ther reduce the contact with the iron ions in the electrolyte and block the electroreduction of iron ions. Due to the reducng of iron deposits, so tha the cycle performances of graphite electrodes are improved...