Structural and electrochemical investigation of binary Na2Fe1-xZnxP2O7 (0 ≤ x ≤ 1) pyrophosphate cathodes for sodium-ion batteries

Transition metal pyrophosphate material forms a robust polyanionic cathode family for sodium-ion batteries. Here, binary Na-2(Fe1-yMny)P2O7 (0 <= y <= 1) system has been recently investigated by different groups, as Na2FeP2O7 is reported as a low-cost cathode with promising electrochemical performance and thermal stability. While the isostructural Na2FeP2O7 and Na2MnP2O7 assume triclinic P1 (#2) framework, pyrophosphate system shows structural diversity/polymorphism. Considering this, we have investigated the binary Na-2(Fe1-xZnx)P2O7 (0 <= x <= 1) pyrophosphate family with anisostructural end members Na2FeP2O7 (P1, #2) and Na2ZnP2O7 (P4(2)/n, #86). The current study reports solution combustion as well as solid-state preparation of novel Na-2(Fe1-xZnx)P2O7 (0 <= x <= 1) family of materials, their structural and electrochemical characterizations. The degree of solid-solution formation and effect of Zn on Fe-redox activity in Na-2(Fe1-xZnx)P2O7 (x = 0, 0.25) cathodes has been examined using electrochemical titration techniques such as galvanostatic intermittent titration (GITT) and potentiostatic intermittent titration (PITT) mode.