Electrochemical memory in LiFePO4 batteries arises from diffusion-driven dynamical asymmetry

Voltage relaxation in lithium-ion batteries reflects internal electrochemical states but is generally assumed to follow a unique history-independent pathway toward equilibrium. From paired relaxation experiments on LiFePO 4 cells over broad states of health, we show that this assumption is incomplete. Relaxation exhibits "electrochemical memory," a measurable trajectory-level property of nonequilibrium battery relaxation dependent on the charge/discharge history. It shows a robust nonmonotonic dependence on state of charge (SOC), with reduced memory at ∼50%–70% SOC, and an increase toward low and high SOCs. After controlling for SOC, we show that electrochemical memory remains strongly associated with transport-related dynamical asymmetry, supporting a diffusion-mediated interpretation of the observed behavior. Reconstructed phase-space analyses reveal persistent organization of history-dependent relaxation trajectories, while trajectory recoverability demonstrates electrochemical memory encoded throughout relaxation. Overall, voltage relaxation is redefined as an electrochemical-history nonequilibrium dynamical process. Our trajectory-centric framework enables the investigation of electrochemical memory, nonequilibrium battery dynamics, and physics-informed battery diagnostics.