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

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 Mncontent 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+/Fe(3+ )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+beta Fe0.8Mn0.2PO4 is identified as a thermodynamically stable intermediate with charge order, the Na-poor phase Na0.2+gamma Fe0.8Mn0.2PO4 is here shown to be kinetically induced. Such reaction mechanism allows improving the electrochemical performance of triphylite Fe-based Na-ion cathode materials.

Automated summary

This study explores the effect of Mn substitution in NaFe1-yMnyPO4 on the electrochemical performance. The results show that increasing the Mn content improves the average discharge voltage but reduces the capacity. However, an optimal composition of y = 0.2 is identified for energy density and efficiency of the Fe2+/Fe(3+) reaction. The reaction mechanism of NaxFe0.8Mn0.2PO4 involves two intermediate phases with extended solubility limits, which can buffer the volume mismatch of the system. The Na-rich phase Na0.73+beta Fe0.8Mn0.2PO4 is thermodynamically stable with charge order, while the Na-poor phase Na0.2+gamma Fe0.8Mn0.2PO4 is kinetically induced.