Electrochemical Activity of P2 Structured Na-M-O Cathode for Sodium Ion Batteries

A market of plug-in electric vehicles (PEV) or hybrid electric vehicles (HEV) using lithium ion battery is being expanded, then demand for large-scale energy storage devices is increasing gradually. However, percentage mass of lithium in the earth is only 0.006%. Indeed, the cost of lithium resource increased after commercialization in 1991, might be soared when EVs or energy storage systems (ESS) are used. Sodium resource, the element which is quite abundant (Clarke’s number is 2.64), can be an alternative for the expensive and rare lithium. For those reasons, development of sodium ion battery is essential for solving previous problems. Recently, P2-phase materials are widely attracted due to their large initial capacity. However, obvious capacity fading is shown in almost P2 type active materials. In this study, we report to synthesize P2 structured Na-M-O (where M is transition metal) compounds and investigate physical and electrochemical behaviors of the Na-M-O cathode for sodium-ion batteries.

The Na-M-O powders were synthesized by solid-state method. Stoichiometric mixture of Na₂CO₃ and precursor were mixed by ball-milling. Then the mixed powders were pelletized, and the pellets were calcined at high temperature. After calcination, heated pellets were quenched. All chemicals and products were handled in an Ar-filled gloved box to avoid air exposure. Phase identification of calcined products was done by X-ray diffraction (XRD) with Cu Kα radiation and the collected XRD data were analyzed by using the Rietveld refinement program, Fullprof. Electro-chemical test were carried out in a 2032 coin type sodium cell. Galvanostatic electrochemical charge and discharge tests in 0.5M NaPF₆ EC:DEC solutions were carried out at room temperature. Also, the products were characterized by SEM, TEM and so on.

The XRD patterns of Na-M-O compounds were obtained without impurities peak, respectively. The produced materials showed P2 phase even if adding other transition metal elements. Electrochemical tests were observed in the potential domain between 2.0 and 4.5 V at 20 mA g^-1 current density. Initial capacity of Na-M-O electrodes showed high capacity. And the capacity retention was significantly improved by adding other transition metal elements. Details will be discussed in the conference site.