Experimental and computational study of Zn doping in Li5+xLa3Nb2−xZrxO12 garnet solid state electrolytes

While garnet Li ion conductors are attracting considerable interest as potential solid state electrolytes for Li ion batteries, a key challenge is to improve the conductivity, which is associated with the Li content in the structure, and the density of the sintered electrolyte membranes. In this work we show that Zn can be doped on the 16a octahedral Nb site increasing the Li content, while also leading to substantially improved sintering in Li5+xLa3Nb2−xZrxO12. As a result of the enhanced sintering, and the increase in Li content, the conductivities were significantly enhanced on Zn doping, up to 2.1 × 10−4 S cm−1 at 25 °C for Li6.6La3ZrNb0.8Zn0.2O12. Computational modelling supports favourable doping of Zn on the Nb site with 3 Li interstitials as per experimental findings. Furthermore, it suggests Li ion diffusion via a knock-on mechanism, but crucially the saturation of sites closest to the Zn means that migration barriers are similar for doped and pure systems, with the increased Li ion conductivity attributed to larger pre-factors due to increased number of Li ions in the doped material. A challenge with these Zn doped garnet is the reduction of Zn in contact with Li metal. Nevertheless, surface fluorination or employing the Zn doped garnet as a buffer layer with an alternative garnet electrolyte is shown to be effective to inhibit dendrite growth, and stable cycling exceeding 250 hours is demonstrated.