Characterization of the mechanical responses of a LiFePO4 battery under different operating conditions

The mechanical responses of a 10-Ah LiFePO4 battery were investigated under different operating conditions. The swelling of the cell was measured at different C-rates and temperatures under constraint-free conditions, and the force was measured at different initial pressures and C-rates under constrained conditions. These experiments yielded information about the mechanical characteristics of LiFePO4 batteries, which is crucial for their design and control. Remarkably, the observed mechanical behavior is unique: the LiFePO4 battery swells at medium state of charges (SOCs) and shrinks at low and high SOCs during discharge. This mechanical characteristic could be affected by the co-existence of Li-poor LieFePO(4) and Li-rich Li1-delta FePO4 phases in the cathode. Differential analyses of the swelling and force evolution with change in SOC validate the direct correlation between the mechanical characteristics and cell chemistry. Consequently, our analysis allows the identification of the electrode phase and transition stages at different C-rates, temperatures, and preloads. As a result, the coexistence of two coherent LiFePO4 phases and a phase/staging diagram for graphite have been established. Finally, the evolution of the equivalent stiffness and that of the equivalent modulus of elasticity were estimated for the first time by synchronizing the swelling and force measurements with respect to the SOC. This analysis revealed that the mechanical characteristics can be separated into three regions over the SOC and are strongly influenced by the co-existence of Li-poor Li epsilon FePO4 and Li-rich Li1-delta FePO4 phases in the cathode.