Reaction kinetics inside pore spaces in lithium-ion battery porous electrodes: Coupling of equivalent-circuit models and electrochemical reactions

Improving the performance of secondary batteries is indispensable for promoting the proliferation of electric vehicles, and many researchers are working toward further sophistication of lithium-ion batteries (LIBs). In general, the electrode of a battery has a three-dimensional structure, and a reaction distribution occurs inside the electrode because of changes in the load current. In particular, because LIBs require high output, a thin porous material with a thickness of several tens of micrometers is used as an electrode material; the electrolyte permeates into the spaces of the pores constituting this electrode, and the electrochemical reaction proceeds. From the perspective of an electric circuit, this pore space can be modeled by a transmission line model using a distributed-constant circuit as a frame. Here, this equivalent circuit was analyzed using a circuit simulator, which is an electrical engineering tool. We then developed a new method to analyze the distribution of the electrode reaction at each position by dividing the pore space of the positive electrode into N points. That is, the transmission line model of the current, potential, and resistance distribution in the pore space was analyzed by the circuit simulator and a new algorithm coupled with electrochemical reaction analysis (Butler–Volmer equation/diffusion equation) was constructed. In addition, to characterize the LiCoO2 positive electrode itself, we measured the discharge performance under a constant current from 1C to 10C using the three-electrode cell method and reproduced the discharge characteristics using the aforementioned simulation method. On the basis of the results, the distribution of electrochemical reactions occurring in the electrolyte-filled pore space of the porous electrode (positive electrode) was widely analyzed.