Synthesis And Electrochemical Properties Of Electrode Materials Based On Na2O-MnO-P₂O₅ Ternary Phase Diagram For Na-ion And Li-ion Batteries

In this thesis,the research content contains the exploration,synthesis,structural characterization and electrochemical properties of Mn-based phosphate electrodes materials for sodium and lithium ions batteries based on Na₂O-MnO-P₂O₅ ternary system.Firstly,the subsolidus phase relation of Na₂O-MnO-P₂O₅ system has been successfully determined by the combination of solid state reaction and X-ray powder diffraction and,within these ternary system,the electrochemical properties of related compounds have been preliminary studied by bond valance sum?BVS?calculation and galvanostatic charge/discharge test.Secondly,we have systemically studied the cstructure and electrochemical properties of Fe-doped Na₂Mn_3-xFe_x?P₂O₇?₂ compound as cathode for sodium ion batteries.Lastly,the electrochemical properties and Li-storage mechanism of Na₂Mn₃?P₂O₇?₂ compound as anode for lithium ion batteries have been successfully studied.The details are as follows:Firstly,by combining the solid state reaction and X-ray powder diffraction methods,the subsolidus phase relation of Na₂O-MnO-P₂O₅ ternary system has been successfully determined.The result shows that,within this ternary system,there exist 7 binary compounds,7 ternary compounds and 22 three-phase regions.Two new phases:Na₂Mn₈?PO₄?₆ and A which has not been reported yet have been verified in this system.The XRD refinement results show that Na₂Mn₈?PO₄?₆ is isotypic with fillowite-type compounds.The indexing result of XRD pattern of the new phase A reveals that the new phase A forms the hexagonal lattice with the space group P2₁2₁2₁.By combining the BVS and galvanostatic charge/discharge method,we reveal that Na₂Mn₃?P₂O₇?₂ compound is electrochemical active and it might be a potential cathode material for sodium ion batteries.Secondly,we synthesize the carbon-coated solid solutions Na₂Mn_3-xFe_x?P₂O₇?₂?0?x?2?via sol-gel method and give the experimental results on structural information and electrochemical properties of the solid solutions as cathode material for sodium ion batteries.The result reveals that the Fe-substitution on the Mn-sites of Na₂Mn₃?P₂O₇?₂ compound will not change its crystal structure.The results of electrochemical properties test show that Fe-doped can vastly improve the electrochemical properties of Na₂Mn₃?P₂O₇?₂ compound.Besides,sodium ion diffusion coefficient of Na₂Mn₃?P₂O₇?₂ can be increased by about two orders of magnitude with the Fe-doping.Thirdly,Na₂Mn₃?P₂O₇?₂ compound is exploited for the first time as an anode material for lithium ion batteries.Furthermore,the Na₂Mn₃?P₂O₇?₂/C nanocomposite has been successfully synthesized via a simple sol-gel method followed by a facile mechanical ball-milling treatment,the electrochemical property of nanosized Na₂Mn₃?P₂O₇?₂/C electrode has been successfully improved compared to Na₂Mn₃?P₂O₇?₂ compound.Besides,the structure transformation and Li ions storage mechanism upon lithiation and delithiation process have been studied and discussed via ex-situ electrochemical impedance spectroscopy?EIS?,XRD and XPS.The result reveals that the Li ions storage mechanism of Na₂Mn₃?P₂O₇?₂ is conversion type and the formation/de-formation of solid electrolyte interface?SEI?layer is a reversible reaction which can be also attributed to the extra specific capacities.