High Performance Cathode Materials For Rechargeable Sodium Ion Batteries

Sodium ion batteries have outstanding advantages like low cost,so they are promising to be alternative candidates to lithium rechargeable batteries in the market and become next-generation commercial energy storage devices.The energy densities of batteries are mainly decided by negative and positive electrodes materials,and the capacities of negative electrodes materials are higher than negative ones,therefore the performance of batteries is mainly up to positive electrodes materials.Typically reported positive electrodes materials,including polyanionic compounds,organic compounds,Prussian Blue Analogues and layered transitional metal oxides,are under investigated.Thus,it is urgent to explore high performance positive electrodes materials for sodium ion batteries.This dissertation focuses on developing high performance Prussian Blue Analogues and transitional metal oxides cathode materials.Prussian Blue Analogues exhibit high theoretical specific capacities but poor rate capability.Herein,a selective edge-etching approach was adopted to prepare porous structures,the strategy increases sodium ions diffusion coefficient and improves rate capability of Prussian Blue Analogues.Prussian Blue Analogues cubes were synthesized by hydrothermal method,which were then eroded in hydrochloric acid solutions to obtain edge-etched Prussian Blue Analogues.The selective edge-etching is because defects concentration（[Fe（CN）6]4-）on the edges is denser than faces,and surface with high defects concentration is firstly eroded.Due to the high sodium ions diffusion coefficient and abundant active sites,the edge-etched Prussian Blue Analogues display a capacity of 167 m Ah g^-1 at a current density of 5 m A g^-1 and a capacity retention of 82.7 % when the current density was increased from 5 m A g^-1 to 40 m A g^-1.Thus,selectively edge-etching is an effective strategy to improve rate capability of Prussian Blue Analogues.In the meanwhile,this dissertation employs structural modulation strategy to stabilize crystal structures with both cationic and anionic redox reactions.K-doping was adopted to tune two-dimensional Na_1.3Mn_0.7O₂ to three-dimensional K_0.2Na_1.3Mn_0.5O₂,stabilizing its the oxygen redox and crystal structure.X-ray diffraction patterns indicate that Na_1.3Mn_0.7O₂ and K_0.2Na_1.3Mn_0.5O₂ crystallize in two-dimensional and three-dimensional structures,respectively.In situ Raman spectra imply that Na_1.3Mn_0.7O₂ and K_0.2Na_1.3Mn_0.5O₂ exhibit both Mn and O redox reactions.In situ X-ray diffraction patterns reveal that Na_1.3Mn_0.7O₂ goes through irreversible phase evolutions while K_0.2Na_1.3Mn_0.5O₂ experiences reversible phase changes.K_0.2Na_1.3Mn_0.5O₂ and Na_1.3Mn_0.7O₂ deliver discharge capacities of 190 m Ah g^-1 and 71.4 m Ah g^-1 under a current of 5 m A g^-1 within 2.0-4.5 V and display cycle numbers of 50 cycles and 5 cycles at 50 m A g^-1,respectively.Our results demonstrate evidences that intercalation oxides via K doping with tridimensional structure are promising in developing high performance positive electrodes materials for next-generation sodium ion batteries.In this dissertation,we demonstrate a selectively edge-etching strategy,based on the solubility differences of hydrochloric acid towards different areas of Prussian Blue Analogues,to synthesize electrodes materials with high sodium ions diffusion coefficient,achieving high-rate capability.In the meanwhile,K doping is demonstrated to tune layered structures to tridimensional ones,stabilizing the the oxygen redox reactions.Our evidence is expected to pave the way towards the development of designing and modifying high-performance positive electrodes materials for next-generation sodium ion batteries.