Silicon Hysteresis and Voltage Relaxation Phenomena: Implications on the Characterization of Anode Overpotentials

Silicon anode active materials for lithium-ion batteries are becoming increasingly relevant for industrial applications. However, their material-intrinsic voltage hysteresis and voltage relaxation behavior is not fully understood and introduces challenges to the modeling of the material for battery management systems. Here, we characterize the hysteresis and voltage relaxation behavior of an electrode for which the capacity is almost exclusively attributed to the silicon active material, which is based on silicon micro-particles. By comparing it to a common graphite electrode, we highlight the unusual behavior of silicon at open-circuit voltage (OCV) conditions: the voltage relaxes significantly more strongly than what could be explained by the removal of ohmic, kinetic, and mass-transport overpotentials. Furthermore, it appears that during OCV conditions there is a reversible build-up of additional overpotentials which disappear after a subsequent (de)lithiation for a small increment of the overall capacity. By comparing the rate performance to impedance measurements and current-interrupt experiments, we find that relative differences of the potentials under load can be characterized similarly for silicon and graphite, but for experiments where the silicon OCV potential is compared to a potential under load, artefacts are introduced to the analysis that can lead to an overestimation of electrode resistances.