Study On Synthesis And Electrochemical Properties Of Low-Valent Niobium-Based Anode Materials

Due to the advantages of high energy density,long cycle life,high working voltage,poor self-discharge and fast charging rate,lithium-ion batteries（LIBs）are widely used in digital products,electric vehicles and large-scale energy storage systems.The performance of anode and cathode materials plays a decisive role in battery performance.At present,the commercial anode materials are mainly graphite and lithium titanate(Li₄Ti₅O₁₂).The kinetic of graphite during charging/discharging is slow,resulting in lithium dendrites and potential safety problems.Although Li₄Ti₅O₁₂has high working voltage and good cycle stability,its theoretical specific capacity is too low.Compared to cathode materials,the single choice of anode electrode materials and slow performance improvement are important factors limiting the development of lithium-ion batteries.Transition metal oxide anode materials have been widely studied,and niobium-based anode materials have become candidates for new anode materials due to their high pseudocapacitance contribution,high specific capacity,high stability,and good safety.However,the low intrinsic electronic conductivity of niobium-based anode materials hinders their further development and application.To solve this problem,this paper synthesized niobium based negative electrode materials containing Nb⁴⁺from the perspective of the relationship between structure and performance.The existence of low valence Nb⁴⁺can regulate the energy band structure,crystal structure and narrow the band gap of materials,thus improving the intrinsic electronic conductivity and structural stability of materials.Firstly,in order to synthesize niobium-based materials containing low valence Nb⁴⁺,Nb₁₂O₂₉ was obtained in this the thesis by partial reduction of Nb₂O₅ using Se powder.The average valence state of Nb in Nb₁₂O₂₉ is 4.83.The presence of low-valence Nb⁴⁺contributes to the narrowing of the band gap width and increases the intrinsic electronic conductivity.In addition,the apparent electrical conductivity was further enhanced by using a ball-mill carbon-coating method to prepare the Nb₁₂O₂₉@C composite anode material.Electrochemical performance tests（voltage range 0.5～3.0 V vs.Li⁺/Li）showed that the discharge specific capacity of Nb₁₂O₂₉@C reached 300 m Ah·g^-1 at 0.1 C.The discharge specific capacity of Nb₁₂O₂₉@C was200 m Ah·g^-1 at 2 C,and the capacity was 109 m Ah·g^-1 after 500 cycles,with a retention rate of 54.5%.The ex-situ XRD tests demonstrated the reversible structural transformation and lithium-ion（de）intercalation of Nb₁₂O₂₉@C during the charge/discharge process.Secondly,the combined effects of morphology modulation and low-valent niobium on the electrochemical properties of Nb-based anode materials were also investigated in this thesis.The Nb₂O₅@C submicron particles were firstly prepared by electrostatic spinning,and then the Nb₂O₅@C-Se anode material with low valence Nb⁴⁺was synthesized by selenization treatment,and the electrochemical performance was enhanced by the synergistic effect of morphology modulation and low-valent niobium.The discharge specific capacity of Nb₂O₅@C-Se is 250 m Ah·g^-1 at 0.1 C（voltage range 0.5～3 V vs.Li⁺/Li）;Nb₂O₅@C-Se achieves 80%capacity retention after 1000 cycles at 2 C.Ex-situ XRD tests also demonstrated the reversible structural transformation and lithium-ion（de）intercalation of Nb₂O₅@C-Se during charge/discharge.Finally,a new anode material K₃Nb₆P₄O₂₆@C（KNPO@C）containing low valence Nb⁴⁺was designed and synthesized in this thesis.KNPO@C is a typical tetragonal cyanotungsten copper structure with an open skeleton structure,which can provide fast Li⁺migration channels and maintain good structural stability during lithium-ion（de）intercalation.Thanks to the above advantages,it exhibits good electrochemical performance.KNPO@C delivers a discharge specific capacity of 260m Ah·g^-1 at 0.1 C（0.5～3.0 V vs.Li⁺/Li）and retains 60.7%capacity after 1000 cycles at 2 C.Ex-situ XRD and XPS tests demonstrated the reversible structural transformation,lithium-ion（de）intercalation and Nb valence change during charge/discharge.In addtion,K₃Nb₆P₄O₂₆@C//Li Mn₂O₄ full cell also exhibited good electrochemical performance.This thesis provides new material synthesis methods,new alternative material systems,and new optimization ideas to enhance the electrical conductivity and electrochemical performance of niobium-based anode materials for LIBs.