Revealing the Rate-Limiting Li-Ion Diffusion Pathway in Ultrathick Electrodes for Li-Ion Batteries

Increasing the loading of active materials by thickening the battery electrode coating can enhance the energy density of a Li-ion cell, but the trade-off is the much reduced Li⁺ transport kinetics. To reach the optimum energy and power density for thick electrodes, the effective chemical diffusion coefficient of Li⁺ (D_Li) must be maximized. However, the diffusion of Li⁺ inside an electrode is a complex process involving both microscopic and macroscopic processes. Fundamental understandings are needed on the rate-limiting process that governs the diffusion kinetics of Li⁺ to minimize the negative impact of the large electrode thickness on their electrochemical performance. In this work, lithium Ni-Mn-Co oxide (NMC) cathodes of various thicknesses ranging from 100 to 300 μm were used as a model system to study the rate-limiting diffusion process during charge/discharge. The rate-limiting diffusion coefficient of Li⁺ was investigated and quantified, which was correlated to the electrochemical performance degradation of thick electrodes. It is revealed here that the under-utilization of the active material was caused by the limited diffusion of Li⁺ inside the porous electrode, leading to a critical electrode thickness, beyond which the specific capacity was significantly reduced.