Transmission electron microscopy of copper sulfide and iron-doped NASICON-based electrode materials for sodium ion full cell battery

Abstract

Recent surge of lithium ion battery demand and its unstable raw materials prices have suggested necessity of an alternative for lithium ion battery. To develop an alternative for lithium ion battery, several energy storage chemistries have been explored. One of the promising alternatives is sodium ion battery based on the similar chemical nature of sodium to lithium. However, commercialization of sodium ion battery has been suffered from a lack of suitable electrode materials in the aspects of performance and cost. Therefore, new electrode materials with high cyclic stability, high capacity, and low cost should be developed. In the case of anode materials, to achieve the high capacity, conversion or alloying reactions type are should be investigated. For cathode materials, materials with high voltage and high capacity should be investigated to compete with lithium ion battery. In this thesis, suitable anode and cathode materials for sodium storage are investigated. For anode material, exceptionally stable sodium storage mechanism of copper sulfide is investigated utilizing in-/ex-situ transmission electron microscopy in conjunction with electrochemical characterization. Sodiation pathway of copper sulfide is revealed in atomic scale for the first time. Furthermore, origin of the inherent pulverization-tolerance and capacity recovery during sodiation is also unveiled. These findings will contribute to designing of stable conversion reaction anode materials. For cathode part, iron-doping of sodium vanadium fluorophosphates/sodium vanadium phosphate composite is suggested to strengthen its sodium storage performance and economic competitiveness. Iron doping into the cathode enhances the performance with fully evolved V3+/V4+ redox pair and enhanced sodium diffusivity and lowered bandgap. Finally, a full cell configuration of copper sulfide and iron-doped sodium vanadium fluorophosphates is realized. The materialized full cell is economically reasonable and shows a competitive energy density and cyclic retention.