Influence of the POuN4−u structural units on the formation energies and transport properties of lithium phosphorus oxynitride: a DFT study

The potential of mobile applications for digital networking is constantly increasing. A key challenge is to ensure a reliable and long-term power supply. One possible solution is the use of all-solid-state thin-film lithium batteries which use amorphous lithium phosphorus oxynitride (LIPON) as solid electrolyte. It is well known that the electrochemical properties of this material are related to the amorphous state, which correlates with the nitrogen content. Due to the difficulty of calculating amorphous structures using first principles methods, three different LIPON structure models are considered in this study and the influence of the anion POuN4−u sublattice on the Li vacancy and Li interstitial formation as well as on the lithium ion transport is highlighted. While for all three model systems the migration energies of the energetically preferred Li vacancies increase with increasing complexity of the anion POuN4−u sublattice only slightly from 0.38 eV to 0.55 eV, the migration energies for the energetically preferred Li interstitials decrease with increasing complexity of the anion POuN4−u sublattice from 0.68 eV to 0.38 eV. Thus, it was found that the energetically preferred lithium ion (Li vacancy and Li interstitial ion) transport mechanism in LIPON can be explained on the basis of the present POuN4−u structural units. In the presence of isolated PON3x− tetrahedra or periodic PO2N2 chains, the lithium vacancy diffusion dominates, whereas in the presence of periodic POuN4−u planes, the lithium interstitial diffusion becomes dominant.