Effects Of Crystal Structures On The Electrochemical Properties Of Nb/mo-based Oxides

The development of new energy storage systems is conducive to utilizing renewable green energy.Owing to high energy density and long cycle life,lithium-ion batteries（LIBs）have been extensively commercialized in common electronic products.Recently,sodium-ion batteries（SIBs）have been regarded as important supplements to LIBs,and both of them will together meet the needs of society for different energy storage devices in the future.However,the commercial graphite anode has low capacity and is inapplicable for Na⁺storage.Therefore,it is an urgent need to develop new generation anode materials.Specifically,transition metal oxides（TMOs）are the promising anode materials benefitting from abundant natural resources,high mass specific capacity,environmental friendliness,and high stability.However,TMOs still suffer from poor rate and cycle performance,due to their inherently low electron/ion transport and large volume expansion during electrochemical processes.In view of the abovementioned problems,this thesis focuses on modulating the crystal structures of TMOs via defect/phase engineering strategies to enhance their electron/ion transport kinetics and further improve their electrochemical Li⁺/Na⁺storage performance.The details are as follows.（1）The uniform Nb₂O₅ microflowers composed of nanosheets were prepared by a simple hydrothermal method,and oxygen vacancies were successfully introduced into the Nb₂O_5-x microflowers by thermal reduction treatment.The two-dimensional nanosheets have large contact area with the electrolyte,shortening the Li⁺transmission distance.Oxygen vacancies not only provide more active sites for lithium storage,but also act as additional electron donors to increase the number of charge carriers and thus enhance the electron/ion conductivity.As a consequence,Nb₂O_5-x exhibits superior electrochemical performance(320 m Ah g^-1 at 0.1 A g^-1).In addition,oxygen vacancies also help to alleviate the volume expansion of the electrode during Li⁺（de）intercalation processes,resulting in an excellent cycling stability with a capacity retention of 94%after 3000 cycles.（2）The construction of the polymorphic phase interface for Nb₂O₅ was realized by a facile heat treatment in air.Compared with the single crystal phase Nb₂O_5,the prepared polymorphic phase Nb₂O₅ exhibits better lithium storage performance.The specific capacity is as high as 397 m Ah g^-1 at 0.1 A g^-1,while the capacity of 243 m Ah g^-1 can still be maintained after 550 cycles at 1.0 A g^-1.Excellent rate capability,cycling stability,and higher solid-state diffusion of lithium ions can be mainly attributed to the migration of electrons at the interface of the polymorphic phase,which effectively reduces the interface resistance,accelerates the ion diffusion,and enhances the reaction kinetics.Moreover,the hetero-phase interface can alleviate the volume expansion of the electrode material to maintain its structural integrity during（de）lithiation processes.（3）A facile hydrothermal method was proposed to prepare Mo O₂ nanoclusters.The crystallinity modulation was achieved by readily varying the volume ratio of reaction medium.It was evident that the homogenous interface and structural defects were in-situ introduced in the amorphous/crystalline Mo O₂（a/c-Mo O₂）hetero-phase homojunction（HPHJ）,which can regulate its electronic structure and induce an unbalanced charge distribution,providing an inbuilt driving force to facilitate the electron/charge transport.The kinetics analyses revealed the a/c-Mo O₂ HPHJ exhibited faster Na⁺mobility,higher Na⁺absorbability and lower Na⁺diffusion barrier,compared with the pure amorphous（a-Mo O₂）or crystalline（c-Mo O₂）.Additionally,the structural defects can provide abundant voids for extra Na⁺uptake and also alleviate the internal stress variation as buffer zone during the（de）sodiation processes.Relying on these crystallographic merits,the a/c-Mo O₂ HPHJ nanoclusters achieved a higher capacity(307 m Ah g^-1 at 0.1 A g^-1)with better rate capability(132 m Ah g^-1 at 2.5 A g^-1)and cycling stability than either a-Mo O₂ or c-Mo O₂.Moreover,the combined in-situ XRD,ex-situ XPS,and electrochemical analyses revealed the‘adsorption–insertion–conversion’mechanism for the Na⁺storage of Mo O₂ with good structural reversibility.