Separating electronic and ionic conductivity in mix-conducting layered lithium transition-metal oxides

Electronic and ionic conductivities of active materials are the fundamental properties in determining the dynamical behavior and rate performance of batteries, yet, they are typically hard to be distinguished due to the convolution of the two conduction processes, the complexity of the dynamic responses in composite electrodes, and other extrinsic factors such as porosity and interfacial effects. Herein, we report a simple and reliable method that combines the electrochemical impedance spectroscopy with direct-current polarization to accurately resolve the electronic and ionic conductivities of conventional layered cathode materials for Li-ion batteries, including LiCoO2 and LiNi1-x-yMnxCoyO2. A significant increase in the electronic conductivity was observed with the increasing Ni content, accompanied by a large decrease in the activation energy. Meanwhile, a similar variation trend in Li+ conductivity was observed, together with a large decrease in the Li+ hopping barrier. These can be attributed to the variations in electronic structure and defect chemistry, the apparent lattice expansion with incorporating Ni, especially along the c-axis, as well as a weakened Li+-transition metal interaction via decreasing the Mn4+ content.