Sodium-ion battery cathodes Na2FeP₂O₇ and Na2MnP₂O₇: Diffusion behaviour for high rate performance

Na-ion batteries are currently the focus of significant research activity due to the relative abundance of sodium and its consequent cost advantages. Recently, the pyrophosphate family of cathodes has attracted considerable attention, particularly Li₂FeP₂O₇ related to its high operating voltage and enhanced safety properties; in addition the sodium-based pyrophosphates Na₂FeP₂O₇ and NaMnPO are also generating interest. Herein, we present defect chemistry and ion migration results, determined via atomistic simulation techniques, for NaMPO (where M = Fe, Mn) as well as findings for Li2FeP₂O₇ for direct comparison. Within the pyrophosphate framework the most favourable intrinsic defect type is found to be the antisite defect, in which alkali-cations (Na/Li) and M ions exchange positions. Low activation energies are found for long-range diffusion in all crystallographic directions in Na MPO suggesting three-dimensional (3D) Na-ion diffusion. In contrast LiFePO supports 2D Li-ion diffusion. The 2D or 3D nature of the alkali-ion migration pathways within these pyrophosphate materials means that antisite defects are much less likely to impede their transport properties, and hence important for high rate performance.