Tuning the phase stability of sodium metal pyrophosphates for synthesis of high voltage cathode materials

Abstract

Properties of the electrode materials are strongly influenced by their crystal structures, yet there is still a lack of design principles to control the polymorphism, showing multiple structures for a given composition with varying battery performance. Here, the underlying mechanism that governs the phase stability of Na2CoP2O7, which has two polymorphs with different electrochemical properties, and a strategy to control it via transition metal substitution are investigated. It is found that the relative stability between the triclinic and orthorhombic polymorphs of Na2MP2O7 (M = transition metals) is determined by two factors, the ionic size and crystal field stabilization energy. On the basis of this understanding, a computational strategy is devised for selecting the optimal substituents to produce a desired polymorph, from which the introduction of Ca, Ni, or Mn into Na2CoP2O7 is identified to stabilize the preferred triclinic phase that has a higher voltage than the orthorhombic counterpart. This prediction of selective synthesis of a particular polymorph for improved battery performance is successfully verified by experimental syntheses, characterization, and electrochemical measurements. We expect that the current strategy can be generalized for other materials synthesis in which the functionalities of materials are sensitively dependent on the crystal polymorphs.