Electrolyte solutions function as ionic conductors in Li-ion batteries and inevitably induce concentration gradients during battery operation 1-3. 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 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 rate distribution inside the porous electrode and depend on both the open circuit voltage (OCV) curve (thermodynamics) and charge transfer reaction heterogeneity (kinetics), as shown in the figure below. Charge transfer reaction heterogeneity causes the nonuniform utilization of the porous electrode. The plateaus of OCV curve tend to make the electrode utilization even more nonuniform, while the sloping parts of OCV curve make the electrode utilization relatively uniform. The transitions from the plateaus to the sloping parts of OCV curve generate the waves of the reaction rate distribution, which further result in Li+ concentration waves in the electrolyte. A four-electrode device is used to experimentally validate the electrolyte concentration waves. The 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 sloping regions in the battery output voltage illustrates the dependency of Li+ concentration waves on the thermodynamics and reaction kinetics of the electrochemical charge transfer reaction.