Electrochemical Performance and Structural Interpretation of Sodium Metal Pyrophosphates as a Cathode for Sodium-Ion Batteries

Abstract

Recently, sodium-ion batteries (SIBs) become promising alternatives for lithium-ion batteries (LIBs) due to abundance and cheap cost of sodium resources compared with lithium sources. Like cathode materials for LIBs, various compounds of layered oxides and polyanion materials are synthesized through several methods. However, conventional active materials have difficulties for satisfying both kinetics and safety issues. Here, new types of polyanion material, sodium metal pyrophosphate, are synthesized having composition of Na3.12M2.44(P2O7)2 (M = Fe, Fe0.5Mn0.5, Mn, Co). Among them, sodium iron pyrophosphates are made into two ways: stoichiometric and off-stoichiometric ratio. This compounds have triclinic structure having three-dimensional tunnels, with sub-micron- or micron-sized particles. Indeed, the off-stoichiometric sodium iron pyrophosphate, involving continuous solid solution system, shows 85 mA h g-1 of reversible capacity with excellent cycleability and kinetics. Inspired by that, compounds containing Mn and Co, which are expected higher redox potentials, are tried. They are also investigated in electrochemical and structural ways.