Preparation,Modification And Electrochemical Properties Of Titanium Niobium Oxide

As a secondary power supply with high energy density,lithium-ion battery has been widely used in portable electronic equipment,electric vehicles and many other fields.The current economic and social development needs,such as the rapid development of electric vehicles,have higher requirements for the performance of lithium-ion batteries,that is,higher energy density and power density,longer cycle life,lower cost and higher safety.Ti₂Nb₁₀O₂₉,with the advantages of high theoretical capacity(396 m Ah·g^-1),stable structure,moderate charge/discharge potential（～1.6 V）,no SEI film and lithium dendrite problems and high safety,is a very promising new anode material for lithium-ion batteries.However,the poor electronic conductivity and rate performance limit its application.In view of the shortcomings,the electrochemical performance of Ti₂Nb₁₀O₂₉ was improved by ion-doping modification and ion-doping/carbon composite synergistic modification in this paper,and the mechanism for performance improvement was also discussed.The main research contents and results of this paper are as follows:1.M_xTi_2-xNb₁₀O₂₉（M=Cr,Mo）materials were prepared by high temperature solid state method,and the effects of doping elements,doping amount and calcination atmosphere on the structure and properties of the as-prepared materials were investigated.The results show that appropriate amount of Cr and Mo doping can improve the charge/discharge performance of Ti₂Nb₁₀O₂₉ material.For the material prepared in air,Cr and Mo doping with x of 0.1can increase the lithium-ion diffusion channel,improve the conductivity,reduce the interface impedance and improve the charge/discharge performance.Among them,Mo_0.1Ti_1.9Nb₁₀O₂₉ shows the best electrochemical performance.Its capacity at 0.1C and 10C rate is297.5 m Ah·g^-1 and 150.2 m Ah·g^-1 respectively.After charge/discharge at0.5 C rate for 100 cycles,the capacity retention reaches 95.7%.However,when prepared in argon atmosphere,the material with Mo doping x of 0.05can present a capacity of 150.1 m Ah·g^-1 at 10 C rate.The theoretical calculation results based on the first principle show that Mo doping can reduce the band gap of Ti₂Nb₁₀O₂₉,move the Fermi level up into the conduction band region,and thus improve the conductivity.The oxygen vacancy defect produced in argon atmosphere can move the Fermi line to a higher energy level,further reduce the band gap,and then further improve the intrinsic electronic conductivity of the material.2.Mo_0.05Ti_1.95Nb₁₀O₂₉(M_0.05TNO)sample prepared by solid state high temperature annealing was selected to be modified by carbon coating,and the effects of carbon source（glucose,urea formaldehyde resin）and the amount on the structure and properties of the material were investigated.The results show that the carbon coating modification can further improve the charge/discharge performance of the based material.The material M_0.05TNO@5wt%C_GLC prepared with 5 wt%glucose as carbon source,has a uniformly coated carbon layer with a thickness of about 2 nm,significantly decreased impedance,and improved lithium-ion diffusion coefficient and charge/discharge performance.Its capacity at 0.1 C and 10C rate is 313.6 m Ah·g^-1 and 174.3 m Ah·g^-1 respectively.After charge/discharge at 0.5 C rate for 100 cycles,the capacity retention reaches97.6%.The material M_0.05TNO@4wt%C_UF prepared with 4 wt%urea formaldehyde resin as carbon source,has a uniformly coated carbon layer with a thickness of about 1 nm,also shows good charge/discharge performance.Its capacity at 0.1 C and 10 C rate is 324.9 m Ah·g^-1 and 157.3m Ah·g^-1 respectively.After charge/discharge at 0.5 C rate for 100 cycles,the capacity retention reaches 97.5%.3.Using urea formaldehyde resin as nitrogen source and carbon source,Mo_0.05Ti_1.95Nb₁₀O₂@C microsphere material was prepared by spray drying followed by high temperature calcination method,and the effect of the added urea formaldehyde resin amount on the structure and properties of the as-prepared materials was investigated.The results shows that the prepared materials were porous caged microsphere particles with a diameter of 2～3?m.Under the combined action of argon atmosphere and carbon source pyrolysis,nitrogen doped carbon coating is formed on the surface of the material.At the same time,a small amount of Ti³⁺、Nb⁴⁺、Mo⁵⁺are also generated,which improves the intrinsic electronic conductivity of the material.The SM_0.05TNO@4wt%C_UF microspheres prepared with 4 wt%urea formaldehyde resin exhibits a capacity of 331.9m Ah·g^-1 and 175 m Ah·g^-1 at 0.1 C and 10 C rate respectively.Compared with M_0.05TNO@4wt%C_UF material,the capacity of SM_0.05TNO@4wt%C_UF microspheres is increased by 17.7 m Ah·g^-1 at 10 C rate.This is mainly due to the porous cage structure with large electrolyte contact area and the nitrogen doped carbon coating which can improve the transmission rate of electrons and lithium ions.4.To further improve the properties of SM_0.05TNO@4wt%C_UFmicrospheres,double carbon composite Mo_0.05Ti_1.95Nb₁₀O₂@C_UF/C_CNTsmicrosphere materials were prepared with urea formaldehyde resin and carbon nanotubes as carbon sources.The effects of the amount of carbon nanotubes on the structure and properties of the materials were studied.The results reveal that when adding 4 wt%urea formaldehyde resin and 3 wt%carbon nanotube double as carbon source,the prepared SM_0.05TNO@C_UF/C_CNTs-3 microspheres show a remarkable improved charge/discharge performance.Its capacity at 0.1 C and 10 C rate is 370.6m Ah·g^-1 and 214.3 m Ah·g^-1 respectively,and its 10 C rate capacity is increased by 39.3 m Ah·g^-1 while compared with SM_0.05TNO@4wt%C_UFmicrosphere material.This is mainly due to the fact that the added carbon nanotubes bridge the carbon coated active material particles together to form a fast electron conduction network,which improves the utilization of active material particles,speeds up the electron conduction rate,and then reduces the polarization of charge transfer process and lithium-ion diffusion process.