Heteroatom interface engineering for enhanced Na+ kinetics in Na2Fe1.5Mn1.5(PO4)3

Abstract

The growing demand for affordable and sustainable electrochemical energy storage has prompted increased research into sodium-ion batteries (SIBs) as an alternative to lithium-ion systems. However, the sluggish kinetics of Na+ and low conductivity hinder the advancement of effective cathode materials. Herein, alluaudite Na<inf>2</inf>Fe<inf>1.5</inf>Mn<inf>1.5</inf>(PO<inf>4</inf>)<inf>3</inf> (NFMP) and N-doped carbon-coated NFMP (NFMP@NC) are synthesized via a hydrothermal process followed by annealing. The phase purity, chemical composition, and crystallinity were investigated systematically. The higher surface area and mesoporous architecture of NFMP@NC facilitate rapid Na+ transport. The coexistence of Fe2+/Fe3+ redox couple and Mn2+, along with N functionalities in the carbon layer, collectively enhances ion transport. As a cathode, NFMP@NC, delivers a reversible capacity of 77.1 mAh/g at 0.1C with 91.99% and 82.62% retention at 1C and 2C after 100 and 150 cycles, respectively, outperforming NFMP and its previously reported derivatives. Cyclic voltammetry and impedance studies indicate that the introduction of N into the carbon layer promotes a predominantly diffusion-controlled mechanism, characterized by lower charge-transfer resistance and higher Na+ diffusion coefficients. This study provides the first systematic analysis of NFMP and its N-doped derivative as cathodes, highlighting the effectiveness of heteroatom-engineered carbon coatings in enhancing the electrochemical properties of phosphate-based frameworks for next-generation sustainable SIBs. © © 2025. Published by Elsevier Ltd.