Conduction and Surface Effects in Cathode Materials: Li₈ZrO₆ and Doped Li₈ZrO₆

Doped Li₈ZrO₆ (LZO) is a pseudolayered material under consideration for lithium-ion battery cathodes and solid electrolyte coatings. The effects of doping LZO with Ce, Ti, Mg, Nb, and Y on structure, band gaps, conductivity, and activation energy for ion migration are investigated both experimentally and by quantum mechanical calculations. Optical band gaps decrease for all doped materials compared to undoped LZO. While all dopants reduce the electronic conductivity at room temperature slightly, doping with Mg or Nb increases ionic conductivity by an order of magnitude. Introducing a high loading of Nb into LZO decreases the activation energy for Li-ion diffusion in the 22-120 °C range. Calculations on lithium-ion diffusion in LZO show that it occurs by a polaron-vacancy complex mechanism. The energy barrier is lowest for the lithium hopping in a zigzag fashion between tetrahedral voids within adjacent layers. The diffusion barrier is reduced as the number of Li vacancies increases during battery charging. We calculated surface energies for 10 surfaces, and we find that the most stable surface is the (001) surface with the tetrahedral Li layer being exposed. The delithiation energy on the (001) surface was found to be slightly higher than that in the bulk. The Li-ion diffusion barriers from the surface to the bulk were also calculated on the (001) surface, and the diffusion energy barrier across the (001) surface was found to be smaller than the energy barrier along the (001) direction in the bulk, and also lower than the barrier for the lowest-energy path in the bulk (which is a hop between tetrahedral voids in adjacent layers as shown in the related graphic). These characterizations of surface and doping effects will assist future materials design.