Structural, magnetic and electrochemical investigation of novel binary Na2 − x(Fe1 − yMny)P2O7 (0 ≤ y ≤ 1) pyrophosphate compounds for rechargeable sodium-ion batteries

• First synthesis of Na2FeP2O7–Na2MnP2O7 solid-solution family
• Structural and magnetic analyses of Na2(Fe1 − yMny)P2O7 family
• Demonstration of preferential site occupancy by Fe and Mn
• Effect of Mn on the Fe3 +/Fe2 + redox activity

Pyrophosphate cathodes have been recently reported as a competent family of insertion compounds for sodium-ion batteries. In the current study, we have investigated the binary Na2(Fe1 − yMny)P2O7 (0 ≤ y ≤ 1) pyrophosphate family, synthesized by the classical solid-state method. They form a continuous solid solution maintaining triclinic P-1 (#2) symmetry. The local structural coordination differs mainly by different degrees of Na site occupancy and preferential occupation of the Fe2 site by Mn. The structural and magnetic properties of these mixed-metal pyrophosphate phases have been studied. In each case, complete Fe3 +/Fe2 + redox activity has been obtained centered at 3 V vs. Na. The Fe3 +/Fe2 + redox process involves multiple steps between 2.5 and 3 V owing to Na-cation ordering during electrochemical cycling, which merge to form a broad single Fe3 +/Fe2 + redox peak upon progressive Mn-doping.