| Sodium-ion batteries(SIBs)are considered as the most promising new secondary battery due to abundant resources,low cost,environment-friendly and similar working principle to lithium-ion battery.The energy density and cycle life of batteries are closely related to the positive electrode material,so the cathode materials of SIBs are focused on the research.Among many cathode materials,P2-type Mn-based transition metal oxides have been widely applied in the design and development of high-performance SIBs due to abundant raw materials,simple synthesis method,high theoretical specific capacity and controllable components.However,Mn-based layered oxides are susceptible to structural instability caused by multiple phase transitions in the high-voltage range,and their anionic redox reactions induce issues such as ion migration and voltage hysteresis.In addition,Mn3+with Jahn-Teller(J-T)effect and Mn2+ easily dissolved in electrolyte will be generated during the cycling of Mn-based oxides.The former disturbs the structural stability,while the latter results in the loss of cation capacity,thereby greatly reducing the cycle life and rate performance of batteries.To address these issues,this study prepared three Mn-based layered oxides using strategies of anion doping,highvalence metal doping,and bimetal doping to improve and optimize the aforementioned drawbacks,which significantly enhanced the battery performance.Synchrotron radiation X-ray absorption spectroscopy technologies and quantitative fitting were employed to investigate the valence state changes and local structure evolution of the cathode material during sodium insertion/extraction,revealing the intrinsic correlation between charge compensation mechanisms and electrochemical performance.The main research findings are summarized as follows:Firstly,the reversible Mn2+/Mn4+ redox reaction was successfully introduced into P2-type Na0.6Mgo.3Mn0.7O2(NMMO)cathode material by the F anion doping strategy,which inhibited the dissolution of Mn2+ and effectively restrained the J-T effect,resulting in optimized electrochemical performance.Experimental results corroborate that when an appropriate amount of F-substituted O2-,the interlayer distance expands,accelerating the diffusion of Na+and enhancing the rate performance of the material.More importantly,after F enters the lattice,the strong interaction between Mn and F initially attenuates the J-T effect by inhibiting the dissolution of Mn2+ during discharge and promoting the reversible conversion of Mn2+ to Mn4+.Subsequently,it weakens the interaction between Mn and O,resulting in more O participating in the reversible redox process,ultimately efficiently enhancing the performance of the material.Secondly,Ru4+ is introduced into the transition metal layer of NMMO to weaken the ordered arrangement of Mg/Mn to achieve local construction,which effectively inhibits multiple phase transitions and ion migration,suppress J-T effect,and alleviate battery voltage hysteresis and capacity attenuation,comprehensively improving electrochemical performance.Specifically,Ru-doped Na0.6Mg0.3Mn0.6Ru0.1O2(NMMRO)cathode material possesses strong Ru-O covalent bond,which not only weakens the covalence between Mn and O,resulting in an enhancement of the redox ability of Mn and weakening the J-T effect,but also delocalizes electrons between Ru and O,weakening the O oxidation reaction,improving the voltage hysteresis and capacity decay caused by the excessive oxidation of O.In addition,P2-OP4 phase transition is inhibited by the strong Ru-O bond,while the interlayer migration of Mg2+ and intra-layer migration of Mn4+ is inhibited by local reconstruction,which improves the capacity retention and the rate performance.Thirdly,a multi-function Na0.67Fe0.2Mn0.65Cu0.12Mg0.03O2(NFMCM)cathode material is realized through the Cu/Mg doped strategy,which can simultaneously inhibit irreversible ion migration and activate the stable anion redox reaction,thus improving the electrochemical performance.Experimental results show that Cu/Mg co-doping can not only expand the layer spacing to accelerate ion diffusion,but also increase the proportion of Na+at the Naf site to improve the structural stability.More importantly,the introduction of Mg2+ activated more anions to participate in the redox reaction,while the introduction of Cu2+ further improved the stability of anion redox.Cu/Mg co-doping not only can improve the voltage hysteresis caused by anion redox,but also can improve the structural stability of the material by promoting the reversible migration of Fe ions,thus enhancing the voltage retention,cycle performance and rate performance of the cathode material. |
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