Capacity Fading Of LiFePO₄/Graphite Batteries Cycled At Elevated Temperature

LiFePO4/graphite lithium ion battery has attracted considerable attention for large scale applications in the automotive, energy storage and national defense industry due to its well-known advantages of low cost, environmental benignity, long cycling life and excellent safety. However, gradual capacity fade of the cell at elevated temperature has been observed. Devices used in complicated circumstances constantly put forward new requests to power sources, and the problem is foreseen to seriously hamper their usage in above application.In this study, we present aging results on commercial18650-type LiFePO4/graphite cells. Our goal is to diagnose aging mechanism of cells, improving cycling performances of cells at elevated temperature and accelerating their usage in expansive fields. Charging/discharging performances, rate performances and cycling performances of the cells are analyzed at room temperature and elevated temperature. Diagnosis of changes in cells was performed with a combination of electrochemical and structural physical analyses, and the reason for capacity fading was proposed. Then, a film-forming additive and a new lithium salt were selected for improving cycling performances of LiFePO4/graphite cells. Effects of VC additive and LiPF6/LiBOB blend salt on stability of electrodes and cycling performances of cells were discussed in detail. The main results of the thesis are as follows.(1) At elevated temperature, the electrolyte conductivity increases and the electrode wetting characteristics are improved, resulting in a decreased polarization, and consequently, the rate performances were improved. Meanwhile, irreversible capacity was increased and charging/discharging efficiency was decreased at elevated temperature.(2) The cycling capacities fade slowly with cycle number at room temperature, and95%of the initial capacity was retained after600cycles. However, the cycling capacities fade more quickly with increasing the temperature and it keep70%and55%of the initial capacities when cycled at55℃and65℃. Cycling at high temperature increased polarization of cells, and rate performances of the cycled cells was worse than that of the cell cycled at room temperature.(3) The anode showed no capacity fading after cycling, and the cathode showed73%of its initial capacity at first charging process after cycling, but a similar value as the fresh electrode in the following cycles. It is proposed that Fe ions were dissolved from LiFePO4powder during cycling, and the dissolved Fe2+was reduced at the graphite surface; then the iron species played a catalytic role in formation and growth of the interfacial film at graphite surface, leading to the huge rise of the interfacial impedance of the negative electrode and excessive consumption of active lithium, although the content of dissolved Fe2+is too low to cause any change in LiFePO4electrode. The process could be accelerated by an elevated temperature. Graphitic carbon graphite experiences a great change in volume during battery cycling, especially at elevated temperature, and the SEI layer on the surface of anode is not elastic enough to accommodate the volume change, thus, the breakage of SEI layer exposes a fresh surface of carbon negative, which needs to be passivated with consumption of active lithium, and this will naturally lead to capacity decreasing. The capacity fade of the cells at elevated temperature was attributed to the formation of interfacial films that were produced on the graphite electrodes as a result of catalytic effects of the iron species particles and the damage to the graphite SEI layer due to the intrinsic volume changes occurring during the cycling of the negative.(4) Due to presence of VC, deposition on surface of LiFePO4cathode was reduced, with a decreased dissolution of Fe from LiFePO4material, and stability of cathode was improved; meanwhile, the additive can also reduce reduction of electrolyte on graphite surface, and stability of graphite was also improved by a optimized SEI layer; all these processes prevent excessive consumption of active lithium and subsequently, cycling performance of the LiFePO4/graphite cells at elevated temperature is enhanced. Presence of VC in electrolyte improves cycling stability of cells at elevated temperature;(5) Dissolution of Fe from LiFePO4was depressed in LiBOB based electrolyte, and columbic efficiency of LiFePO4/Li cell at elevated temperature was increased; LiBOB was reduced earlier on anode surface, forming compact SEI layer; and the capacity retention of LiFePO4/graphite cells at55℃increases with the LiBOB concentration due to protective depositions of LiBOB on cathode surface and the improved SEI layer on anode surface, while the larger impendence of depositions formed in LiBOB based electrolytes increased the impedance of the cell. LiPF6/LiBOB blend salt-based electrolyte combines the advantages of the different salts and maximizes the performance of cells. When electrolytes with LiPFg/LiBOB blend salt was used, the LiFePO4/graphite cells have excellent capacity retention at55℃, while the impedance was dramatically decreased.