Cell degradation in commercial LiFePO4 cells with high-power and high-energy designs

A quantitative analysis is presented to determine the degradation modes attributing to capacity fade in commercial LiFePO4 cells with high-power (HP) and high-energy (HE) designs. The capacity fade in the HP cell is predominantly due to the loss of lithium inventory. The fade in the HE cell is much more complicated as a function of rate. Using techniques including rest-cell-voltage measurements to track state-of-charge, dQ/dV analysis to trace peak area variations, and mechanistic model simulations (by the 'Alawa toolbox), the capacity fades in the initial 120 cycles and subsequent aging are analyzed and degradation modes identified. Detailed 'Alawa simulation with careful experimental validation explains the complexity of degradation in the HE cell. Peculiar rate-dependent initial capacity increases at rates higher than C/5 was likely attributed to electrochemical milling, resulting in active surface area increases and reduced polarization resistance (as the actual current density in the positive electrode was reduced). The mechanistic model and simulation capability illustrates the merits of this unique diagnostic approach with unprecedented holistic quantitative resolution for complicated cell degradation that seems hardly resolvable by other diagnostic methods.