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

• Optimal electrochemical performance in Na x Fe 1-y Mn y PO 4 is obtained for y = 0.2. • Two intermediate phases with extended solubility limits enable to buffer the cell volume mismatch during electrochemical operation. • The thermodynamically stable Na-rich intermediate phase exhibits superstructure peaks arising from charge and/or Na/vacancy ordering. • The Na-poor intermediate phase is kinetically induced and it decreases the energy barrier for phase separation. • The sophisticated reaction mechanism of Na x Fe 0.8 Mn 0.2 PO 4 evolves to a full single-phase reaction after the first cycle. Mn-substitution in triphylite NaFe 1-y Mn y PO 4 is here explored with the aim to enhance the electrochemical performance of NaFePO 4 . Our results show that, similarly to the LiFe 1-y Mn y PO 4 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 Fe 2+ /Fe 3+ reaction. The reaction mechanism of Na x Fe 0.8 Mn 0.2 PO 4 , 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 Na 0.73+β Fe 0.8 Mn 0.2 PO 4 is identified as a thermodynamically stable intermediate with charge order, the Na-poor phase Na 0.2+γ Fe 0.8 Mn 0.2 PO 4 is here shown to be kinetically induced. Such reaction mechanism allows improving the electrochemical performance of triphylite Fe-based Na-ion cathode materials. .