Probing the Structure Evolution of Na-Cu-Mn-O Based Layered Oxide Cathode Materials in Sodium Ion Batteries

Abstract

Sodium-ion batteries (SIBs) are considered as potential energy storage devices for large-scale energy storage system and smart power grids applications because of the low cost and abundant distribution of sodium in the earth's crust and ocean [1]. The development of highly efficient cathode materials for superior sodium storage is crucial for the development of SIBs. Among all the cathode materials, sodium transition-metal layered oxides, especially P2 and O3-typed layered oxides are of more interest due to their high theoretical capacity and easy synthesis [2,3]. From a structural point of view, P2 phase has an open pathway from one prismatic site to adjacent sites for more facile Na ion diffusion than O3 homologue. Therefore, the P2 phase shows not only better kinetic properties but improved cyclic stability thanks to its high structural stability.However, the pristine Na2/3MnO2 in P2 phase usually suffer from poor reversibility and dramatic capacity and voltage decay upon cycling, which origins form below issues: 1) the Na+/vacancy order-disorder transition, 2) the phase transition at high charge voltages due to MnO6 layers gliding 3) the Jahn-Teller distortion derived from Mn4+/Mn3+ redox couple, 4) the migration and dissolution of transition metals.[4] Up to now, a variety of improvement methods (doping and coating) have been utilized to stimulate the application of P2-type Na2/3MnO2 material. In particular, copper, as a low cost and environmental friendliness element, has also been introduced to synthesize multiple layered transition metal materials, like Na0.68Cu0.34Mn0.66O2, Na0.68Cu0.34Mn0.50Ti0.16O2, Na2/3Mn0.72Cu0.22Mg0.06O2 and so on. Furthermore, various redox behavior and structural evolution have also been explored. However, the process of structural transition during calcinating at different temperatures have been ignored, which leads to the unexpected neglect of some unique features of metastable phases in sintering materials.In our current work, Na2/3Cu1/3Mn2/3O2 in various phases have been successfully synthesized by sol-gel method and subsequently calcinated at different temperatures. XRD results show that Na2/3Cu1/3Mn2/3O2 calcinated at 600 °C (NCMO-600) is the triclinic phase, with the increasing calcination temperatures, the Na2/3Cu1/3Mn2/3O2 subsequently transfers to the hexagonal phase at 800 °C (NCMO-800). Subsequent electrochemical measurements show that NCMO-800 exhibit the better cycle stability and almost no voltage decay in the long term cycling, while the NCMO-600 sample exhibited rapid voltage attenuation at about 30 cycles. Besides, in situ XRD patterns and XANES spectra for both cathodes indicate slight different structural transition and electronic structure changes during charge and discharge. In a word, the NCMO-600 material exhibit the larger electron transfer at low current densities while the NCMO-800 material show the better cycle stability and rate performance.