Improved conductivity and ionic mobility in nanostructured thin films via aliovalent doping for ultra-high rate energy storage

Abstract

A high-rate lithium ion battery electrode consisting of nanostructured copper-doped TiO₂ films, synthesized using a single-step, template-free aerosol chemical vapor deposition technique, is reported herein. A narrowing of the band gap of the copper-doped films from 2.92 to 1.93 eV corresponds to a large increase in electronic conductivity, overcoming a major drawback of pristine TiO₂ in electronic applications. Lithium-ion batteries using copper-doped films as the negative electrode exhibit improved charge retention at ultra-high charge rates, up to 50C. Additionally, over 2000 charge–discharge cycles at a rate of 10C, the copper-doped TiO₂ electrodes display higher stable cycling capacities. Cyclic voltammetry (CV) and a galvanostatic intermittent titration technique (GITT) provide insight into the chemical diffusion of Li⁺ in the TiO₂ matrix, with copper-doped TiO₂ electrodes exhibiting an order of magnitude higher value in CV measurements over pristine TiO₂. GITT provided the state-of-charge (SoC) resolved chemical diffusion coefficient of Li⁺ and suggests that a minimum value occurs at a moderate SoC of 60%, with values near the extremes being over two orders of magnitude higher. Both techniques indicate increased Li⁺ mobility due to copper-doping, supporting improved electrochemical performance in ultra-high rate battery testing.