The effects of cycling on ionic and electronic conductivities of Li –ion battery electrodes

Abstract

Li-ion battery performance is dependent on ionic and electronic transport, in turn dependent on electrode microstructure. Changes in microstructure during cell formation and cycling are poorly understood. In this work, the changes in effective ionic conductivity and diffusivity are quantified in terms of MacMullin number, a dimensionless ionic resistance that is related to tortuosity. Using an AC impedance technique, namely the blocking electrolyte method, the ionic resistance of electrode films at varying extents of cycling was determined. Variations in electronic resistivity were quantified by micro-scale measurements using a previously developed micro-four-line probe. Cycling effects on the ionic and electronic resistivity were investigated for a graphite anode and multiple cathode chemistries including LiCoO2 (LCO), LiNixCoyMnzO2 (NCM), and LiFePO4 (LFP). Clear evidence of changes in ionic and electronic resistivity were observed during cell formation and cycling. The magnitude of the changes depended on the chemistry of the electrodes and cycling conditions. The results indicate that, under normal cycling conditions, electronic resistivity increases while ionic resistivity is relatively stable and in some cases decreases, the latter being unexpected. However, under accelerated (high-temperature) cycling conditions, both electronic resistivity and ionic resistance were observed to increase.