| Room-temperature sodium-ion batteries(SIBs)have attracted great attention particularly in large-scale electric energy storage applications for renewable energy and electric vehicle because of the huge abundant sodium resources,low cost,environmental friendliness and promising energy density in the near furture.Being one important electrode for SIBs,cathode materials play decisive roles in determinming the cost and energy density of SIBs.Among the amazing cathode candidates,layered NaxTMO2(TM referring to transition metals)has been well recognized as one large family of the most promising cathodes due to possessing structural simplicity,tunable stoichiometry,and excellent electrochemical performance.However,layered transition-metal oxides exposed in atmosphere,face a thorny problem originating from the spontaneous extraction of sodium,oxidation of transition metals,which are problematic during the handling materials and practical battery assembly.In addition,the size effect deriving from the large sodium ion radius further leads to both complex phase transitions and slow sodium-ion transport kinetics during the sodiation/desodiation.All the issues mentioned above severely limit the commercialization process of SIBs.Therefore,understanding the structure-activity relationship between layered structure and sodium storage performance,and designing stable cathode materials through reasonable strategies to solve the above problems are of crucial to advancing SIBs towards commercial application.To alleviate the aforementioned problems,this thesis mainly foucuses on the following two aspects.(1)The phase transition and electrochemical behaviour of air-stable and high-voltage P3-type cathode upon Na+ extraction/insertion.A low-temperature stable P3-Na2/3Ni1/4Mg1/12Mn2/3O2 cathode for advanced sodium-ion full battery was designed by constructing the superlattice in the layered structure in order to solve the problems of P3-type oxides unstable in air and high-voltage,and prepared successfully using specific sol-gel method.The structural characterization by X-ray diffraction(XRD)Rietveld refinement,scanning electron microscope and electrochemical testing show that the cathode maintains its structure commendably even when soaked in water for 12 h.Futhermore,the ex-situ XRD patterns show a reversible single-phase process that avoids any unfavourable phase transition of P3-O’3 during the sodium insertion/extraction,even when increasing the charge voltage of cell to 4.4 V.As a result,the cathode exhibited a remarkable long-cycling stability with a good capacity retention of 78%after 100 cycles at 1C.In particular,the assembled P3-Na2/3Ni1/4Mg1/22Mn2/3O2//hard carbon full battery exhibited a competitively high voltage of 3.45 V and an attractive energy density up to 412.2 Wh kg-1 based on cathode.(2)Structure designing and sodium-ion stroage mechanism of P2/P3 intergrowth cathode.A P2/P3-Na0.7Li0.06Mg0.06Ni0.22Mn0.67O2 composite layered cathode for SIBs has been designed and prepared by monitoring the phase transition via tuning the calcination temperature.The cathode integrates layered P3 and P2 phases,both of which synergistically improve the electrode performance by combining their advantages of single phase.Consequently,the as-synthesized P2/P3-NLMNM cathode material indeed exhibited a highly reversible capacity of 119 mA h g-1 and a high operating voltage of 3.53 V(versus Na+/Na)resulting from the Ni2+/Ni4+ redox couple,as well as a capacity retention of 97.2%after 50 cycles.The complementary results of ex situ X-ray absorption spectroscopy(XAS)and in situ XRD provide the convincing rationales of the only electrochemically active Ni species,and the Mn4+ species maintaining the structural stability,as well as the reversible P2/P3-OP4/P3 phase transition,all of which underpin excellent electrochemical performances.In particular,a Na-ion full cell of P2/P3-NLMNM cathode//hard carbon anode further delivered the impressive electrochemical performance with an energy density up to 218 W h kg-1.The energy density achieved comparably compares or outperforms those of the sodium-ion full batteries reported recently. |
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