Thermodynamic calculation of the ionic conductivity of LiPON glasses and solid electrolytes

Abstract

LiPON films are one of the most relevant solid electrolytes in Li-ion micro-batteries for low power applications, and the relationships between composition, processing parameters and ionic conductivity have challenged the materials community for the last three decades. Although films have almost exclusively been processed by deposition of Li₃PO₄ polycrystalline pellets, the amorphous nature of LiPON has permitted the study of compositions with glass-forming ability where the effect of improving the conductivity by nitrogen doping is similar. The aim of this work is to form a general model that allows us to calculate the ionic conductivity of LiPON solid electrolytes for whichever composition. The modelling is based on the application of the fundamental laws of chemical equilibrium to elucidate the different chemical species present for each given system composition. For this purpose, a program in the Matlab software was written, capable of solving simultaneous and competitive chemical equilibria. The method, known as the associated solutions model, was applied to the study of the Li₂O–P₂O₅ system, which is the starting point of the present work. The model assumes that the properties of a given glass or amorphous solid, such as the simulated LiPON films, can be obtained as a function of the weighted contributions from each stable compound in the respective composition diagram. Here, Li₂PO₂N oxynitride is used as the main phase and, to validate the computed chemical equilibria, nitridation of crystalline Li₄P₂O₇ has been performed by thermal ammonolysis. The combination of the calculations of chemical equilibria together with the structural characterization by XRD and NMR of the products of nitridation from Li₄P₂O₇ allowed us to predict both the formation of Li₂PO₂N and the existence of a second oxynitride stoichiometric compound with formula LiPO_1.5N that would be obtained by nitridation of LiPO₃. The solution of the system of equilibria was made possible on development of the Gibbs free energy functions of formation of both oxynitride phases, and the calculation of the activation energy and ionic conductivity in either LiPON thin films or oxynitride glasses, offering comparable values to those found in the literature.