Cycling performance of LiFePO4/graphite batteries and their degradation mechanism analysis via electrochemical and microscopic techniques

In this work, cycling-induced aging occurring in 18650-type LiFePO4/graphite full cells at different C-rates is studied extensively. The mechanism of performance degradation is investigated using a combination of electrochemical and microstructural analyses. Half-cell studies are carried out after dismantling the full cells, using fresh and cycled LiFePO4 cathode and graphite anode to independently study them. The results show that the capacity of LiFePO4 electrodes is significantly recovered. The rate of capacity fading in the discharge state considered as irreversible capacity in the graphite is higher than LiFePO4 half cells, indicating a greater degradation in the performance of this electrode. At relatively high current rates, this phenomenon is mainly attributed to the instability of the electrode/electrolyte interface and the solid electrolyte interphase (SEI) layer, causing the formation of active lithium ion-impermeable covering layer on the anode surface that strongly influences the cyclic aging. As a result, significant consumption of inventory active lithium ions occurred at relatively high current rates measured by half-cell studies. Forming thick covering layer and subsequently separation between active materials, which lead to the loss of electrical contact among them, result in electrode deactivation. To confirm this claim, various morphological, structural, and electrochemical analyses are employed.