Realistic models were constructed for various types of isolated defect in crystalline Li3PO4, involving O vacancies and N and Si dopants, and first-principles methods were used to study their effects upon the production and migration of mobile Li ions. It was found that mobile Li-ion vacancies were stabilized by removing oxygen from the lattice. This, in turn, caused re-bonding of nearby phosphate groups to form O3P-O-PO3 structures embedded in the crystal. In the presence of nitrogen in the system, the O3P-O-PO3 structures could be further stabilized by replacing the bridging oxygen with nitrogen. An examination was made of the electronic and geometrical structures of these stable O3P-O-PO3 and O3P-N-PO3 defects which were prototypes of the chain structures found in thin-film lithium phosphorus oxynitride (LiPON) electrolytes. It was also found that mobile interstitial Li ions were stabilized by N or Si dopants substituting for O or P host ions and forming PO3N or SiO4 defects, respectively. In all of these cases, the defect not only stabilized extrinsic mobile ions but also provided traps for the ions to escape into the bulk regions of the crystal: by as much as 1.5eV for the vacancies and 0.9eV for the interstitials. On the other hand, the migration barriers to diffusion steps near to the defects were as small as 0.4 to 0.6eV for the vacancies and 0.2 to 0.3eV for the interstitials. Upon extrapolating the results to crystals with appreciable concentrations of defects, it was found that the results compared favourably with published experimental migration energies.

Effects of O Vacancies and N or Si Substitutions on Li+ Migration in Li3PO4 Electrolytes from First Principles. Y.A.Du, N.A.W.Holzwarth: Physical Review B, 2008, 78[17], 174301