Enhancing the electrochemical properties of Na2Mn2P3O9N by partial substitution at Mn site: A first-principles study

Abstract

Nitridophosphate materials have recently emerged as promising cathode materials for Na-ion batteries due to their excellent ionic conductivity, but their low electronic conductivity has limited their practical applications. In this study, the electrochemical properties of Na2Mn2P3O9N, a nitridophosphate material containing manganese was examined for the first time and the prevalent issue of low electronic conductivity in nitridophosphate-based materials was addressed through partial substitution of transition metal (TM) at the Mn site. The present work aims at improving the electronic conductivity of this material while maintaining desirable properties such as ionic conductivity, voltage, and structural stability. The study employed density functional theory (DFT) calculations to explore the role of TM substitutions in electronic conductivity and electrochemical properties. From the partial substitution of different electrochemical active and inactive elements at the Mn site, we found that the Cu/Ag substitutions improve the electronic conductivity while retaining other desirable electrochemical properties. The electronic and electrochemical properties of these materials were further studied in detail to reveal the redox mechanisms. Our DFT-assisted studies prove that the parent and Ag-substituted materials exhibit a stable O network even under a high state of charge. Furthermore, our nudged elastic band calculations revealed that Na2Mn2P3O9N, along with the Cu/Ag substituted variants, exhibit favourable Na-ion diffusion. The parent and Ag-substituted materials demonstrate exceptional ionic conductivities comparable to existing commercial cathodes used in Li-ion batteries. These findings suggest that partial substitution of copper/silver at the manganese site could be a promising strategy for improving the electronic conductivity of nitridophosphate materials without compromising their beneficial properties to use in higher energy density Na-ion batteries.