Synthesis,Modification And Electrochemical Performance Of Niobium-Based Oxide Anode Materials For Lithium/Sodium Ion Batteries

To cope with our society’s continued high energy consumption and push toward renewable energy to achieve the carbon peaking and carbon neutrality goals,establishing advanced electrochemical energy storage（EES）devices has received intense attention.The lithium-ion batteries（LIBs）,as one of the leading EES techniques,play a key role in the ongoing transition from fossil fuels to renewable energy owing to the advantages of high energy density and long cycling life.Meanwhile,the emerging sodium-ion batteries（SIBs）have also ignited interest because of abundant natural resources and the low cost.However,the conventional graphite anode of LIBs cannot meet the ever-increasing requirements for the EES systems,which results from its poor safety and slow diffusion.Among various candidates,niobium-based oxides materials are trigging considerable attention due to the relatively high capacity(200-300 m Ah g^-1),fast Li⁺/Na⁺ion diffusion kinetics and safe operating voltage.Niobium-based oxides materials can be classified into two structuralcategories:the Bronze structure represented by the T-phase（orthorhombic）Nb₂O₅ and Wadsley–Roth crystallographic shear structure represented by H-phase（monoclinic）Nb₂O₅.T-Nb₂O₅ and H-Nb₂O₅,as two typical materials of niobium-based oxides,possess individual unique structure,which endows them with potential ability to store Li⁺/Na⁺ion.Nevertheless,the performance of niobium-based oxides electrode is still restricted by the inherent low electrical conductivity(10^-11-10^-6 S cm^-1),which will lead to inferior rate performance and poor cycling stability.Focusing on above-mentioned problems,this work firstly designed and synthesized carbon-encapsulated Nb₁₂O_29-x by using in-situ carbon modification and oxygen defect engineering.Meanwhile,the work further developed oxygen defect engineering to synthesized Se doped T-Nb₂O₅.The electrochemical performance,key factors that influence materials electrochemical properties,and energy storage mechanisms were investigated in detail.Finally,the niobium-based oxide anode materials with high reversible capacity,excellent rate capability,and long cycling life were obtained,which paves a path for the research and development of niobium-based oxide materials.The main works are summarized as follows:（1）Aiming at the terrible bulk electrical conductivity of the niobium-based oxides electrode,the precursors were firstly synthesized through a facile wet-chemical method based on Nb Cl₅ as niobium source and 2-methylimidazole as ligand and carbon source.Then,the carbon-encapsulated Nb₁₂O_29-x(Nb₁₂O_29-x@C)was prepared through a two-step process,including in-situ carbonization process and subsequent annealed process in a H₂/Ar atmosphere.The introduction of carbon modification and oxygen defect engineering dramatically enhanced the electrical conductivity of Nb₁₂O_29-x@C(12.6 S cm^-1)compared with the H-Nb₂O₅(4.78×10^-5 S cm^-1).As a result,the as-prepared Nb₁₂O_29-x@C exhibits high reversible capacity(226 m Ah g^-1 at 0.1 A g^-1),excellent high-rate performance(83 m Ah g^-1 at 5.0 A g^-1)and excellent cycling stability(98.1%capacity retention at 1.0 A g^-1 after 3000 cycles).In situ XRD,Ex situ XPS and Ex situ Raman results revealed the intercalation-type lithium storage mechanism of Nb₁₂O_29-x@C.（2）To improve the electrical conductivity of T-Nb₂O₅,a facile oxygen vacancy-assisted Se doping protocol was developed to prepare T-Nb₂O_5-x-Nb Se₂@C for sodium storage.The introduction of oxygen vacancy and Se doping can not only accelerate the transport of electrons of T-Nb₂O_5-x-Nb Se₂@C(3.69 S cm^-1),but also optimize the crystal structure and Na⁺ion tansport.Therefore,as an anode material for SIBs,the T-Nb₂O_5-x-Nb Se₂@C shows high reversible capacity(249 m Ah g^-1 at 0.1 A g^-1),good rate capability(63 m Ah g^-1 at 5.0 A g^-1),and long cycling performance(107m Ah g^-1 after 500 cycles at 0.5 A g^-1).In situ XRD results revealed the intercalation-type sodium storage mechanism of Nb₂O_5-x-Nb Se₂@C,which paves a way forward to the development of niobium-based oxide anode materials for SIBs.