The triphylite NaFe1-yMnyPO4 solid solution (0 ≤ y ≤ 1): Kinetic strain accommodation in NaxFe0.8Mn0.2PO4

Abstract

Mn-substitution in triphylite NaFe1-yMnyPO4 is here explored with the aim to enhance the electrochemical performance of NaFePO4. Our results show that, similarly to the LiFe1-yMnyPO4 system, increasing the Mn-content raises the average discharge voltage but this comes at the expense of limiting the capacity. However, y = 0.2 is identified as an optimal composition in terms of energy density and efficiency of the Fe2+/Fe3+ reaction. The reaction mechanism of NaxFe0.8Mn0.2PO4, studied from operando XRD experiments, involves two intermediate phases with extended solubility limits that enable to buffer the volume mismatch of the system. While the Na-rich phase Na0.73+βFe0.8Mn0.2PO4 is identified as a thermodynamically stable intermediate with charge order, the Na-poor phase Na0.2+γFe0.8Mn0.2PO4 is here shown to be kinetically induced. Such reaction mechanism allows improving the electrochemical performance of triphylite Fe-based Na-ion cathode materials.