Size Effects on Li+/Electron Conductivity in TiO2 Nanoparticles

TiO2 nanoparticles are the important components of nanostructured electrodes for Li-ion batteries. High Li-ion conductivity of the nanoparticles is the key design requirement for these materials. Using multiscale theoretical modeling, we study the influence of nanoparticle size on its Li+ conductivity and reveal the fundamental mechanism for the dramatic increase in conductivity with the decrease in the size of the nanoparticles. We show that the competition between Li+ and electron accumulation at the nanoparticle boundaries competes with the steady ion and electron fluxes. For nanoparticles smaller that 20 nm, the balance is shifted toward the steady charge transport, hence high conductivity, while for larger nanoparticles charge separation prevails. Size effects are also manifested in the change in the nature of charge transport from dual ionic/electronic for small nanoparticles to predominately ionic for larger ones.