Mechanism for capacity fading of 18650 cylindrical lithium ion batteries

The mechanism for capacity fading of 18650 lithium ion full cells under room-temperature (RT) is discussed systematically. The capacity loss of 18650 cells is about 12.91% after 500 cycles. The cells after cycles are analyzed by XRD, SEM, EIS and CV. Impedance measurement shows an overall increase in the cell resistance upon cycling. Moreover, it also presents an increased charge-transfer resistance (R-ct) for the cell cycled at RT. CV test shows that the reversibility of lithium ion insertion/extraction reaction is reduced. The capacity fading for the cells cycled can be explained by taking into account the repeated film formation over the surface of anode and the side reactions. The products of side reactions deposited on separator are able to reduce the porosity of separator. As a result, the migration resistance of lithium ion between the cathode and anode would be increased, leading the fading of capacity and potential.