Li⁺ concentration waves in a liquid electrolyte of Li-ion batteries with porous graphite-based electrodes

Electrolyte solutions function as ionic conductors in Li-ion batteries and inevitably induce concentration gradients during battery operation. It is shown that in addition to these concentration gradients, very specific Li⁺ concentration waves in the electrolyte are formed in graphite-based porous electrode/Li cells. This phenomenon has been investigated by both simulations and experiments. From the simulations, it has been concluded that the occurring Li⁺ concentration waves in the electrolyte vary with position and time. Such waves originate from the fluctuations of the reaction distribution inside the porous electrode and depend on both the thermodynamics (open-circuit voltage, OCV) and kinetics (charge transfer reaction heterogeneity). Li⁺ concentration waves occurring inside the separator region are directly related to the battery output voltage at low current applications. A four-electrode device is used to validate the electrolyte concentration waves experimentally. The electric potential differences between the reference electrodes and counter electrode show regular fluctuations, demonstrating the existence of concentration waves in the electrolyte. The simultaneous appearance of the fluctuations in the potential differences and the transitions from plateaus to slopes in the battery output voltage illustrates the dependency of Li⁺ concentration waves on the thermodynamics and kinetics of the electrochemical reactions.