Revealing the rate-limiting electrode of lithium batteries at high rates and mass loadings

Abstract

Lithium-ion batteries with superior capacities and rate performance are needed due to the soaring demands for higher energy and power device requirements. However, the main hurdle on achieving this predominately results from the poor rate performance of electrode, which is related to thermodynamic limitations and slow kinetics. To determine the rate-limiting electrode in NMC622 vs graphite cells, a methodology based upon the galvanic intermittent titration technique, for investigating the diffusion and reaction kinetics from the observed overpotential at each electrode has been developed. Variable current densities have been used to simultaneously extract the thermodynamic and kinetic properties of each electrode with increasing mass loading. Graphite is observed to reach its thermodynamic limits quicker than NMC, due to the flat plateaus and overpotentials observed from the charge transfer kinetics and mass transport. At high rates and high mass loadings, the graphite electrode is responsible for limiting both Li+ diffusion and reaction rates in full cells. Slow diffusion kinetics are caused by the transport of the electrolyte in the porous electrode, which limits the availability of Li+ for reaction at the surface of graphite. This methodology is proposed as a fast single technique for comprehensively parameterizing the rate limitations observed in a full cell configuration.