Capacity fading of 18650 Li-ion cells with cycling

The cycle performance and capacity fading mechanism of 18650 Li-ion batteries cycled at room temperature were studied.The charge-discharge cycle of 18650 Li-ion batteries was tested for 200 cycles by constant current-constant voltage(CC-CV) protocol.The electrochemistry properties of batteries after different cycles were studied by EIS.The capacities of cathode and anode pieces cut from the disassembled cell after different cycles were studied against fresh lithium.The morphology and structure of cathode,anode and separator of Li-ion batteries after different cycles were characterized by SEM,XRD and air-permeability test.The results show that the cell loses 15.6% of its initial capacity after 200 cycles,and the cathode and anode loses 6.6% and 4.3% of their initial capacities after 200 cycles respectively.The capacity fading of batteries can be attributed primarily to the loss of active Li+ and the losses of cathode and anode active materials.The loss of active Li+ is caused by the side reactions of electrolyte species on the electrode.More cation mixing and less ordered layer structure are observed from the cathode material.The degree of graphitization of anode material decreases and the surface film is developed on the anode.The porosity of separator decreases.These cause the increase of charge transfer impedance and the losses of capacity.