Preparation And Electrochemical Properties Of Thulium And Cerium Doped TiNb₂O₇ Anode Materials

In recent years,various types of titanium niobium oxides（TNO）are favorable for Li⁺intercalation and deintercalation because of their unique crystal structure stability.Meanwhile,5-electron transfer in redox couples of Ti⁴⁺/Ti³⁺,Nb⁵⁺/Nb⁴⁺,Nb⁴⁺/Nb³⁺makes a high theoretical specific capacity(387 m Ah g^-1)in the process of lithium deintercalation.In addition,its high voltage platform（～1.6 V vs.Li⁺/Li）can effectively avoid the formation of SEI membrane and lithium dendrite.These advantages make it an ideal anode material for fast-charge lithium-ion batteries.However,its wide inherent band gap leads to poor conductivity,which is close to the insulator,which greatly limits the application of TNO as the cathode of commercial lithium ion batteries.Therefore,doping engineering modify the TNO material and optimize its structure and properties as follows:（1）Rare earth element-doped titanium,niobium and oxygen material microspheres were synthesized by hydrothermal method,and the crystal structure changes before and after TNO material doping were studied by using the rietveld refinement XRD data.The proportion of pseudocapacitance in the total contribution capacity increases after doping.GITT test calculated that the diffusion coefficient of lithium ions increased significantly after doping.In addition,the doped electrode material has high cycle stability and excellent reversibility.In the half-cell test,the initial capacity is 289 m Ah g^-1at 1 C.After 500 cycles,the capacity is 244 m Ah g^-1,and the capacity retention rate is 84.4%.At a high current density of 50 C,the capacitance remains at 150 m Ah g^-1.In the full battery test,the high capacity retention rate was 77.5%after 200 cycles at the 1 C.The specific capacity can still reach 115 m Ah g^-1even at 5 C,and the rate performance is excellent.After density functional calculation,doping not only reduces the inherent band gap of the material,but also has a lower lithium ion diffusion energy barrier.（2）Highly oxidized state ions（Ce）are introduced into TNO material to regulate its electronic structure.The doping of Ce gives the material more oxygen defects,thus improving its electrochemical properties.Moreover,the doping of highly oxidized state ions increase the proportion of pseudocapitance contribution capacity.EIS and GITT tests show that the introduction of highly oxidized ions enhances the interfacial kinetics of Li⁺transfer.The half-cell test showed that the capacity remained at 227.2 m Ah g^-1at40 C.And the high capacity retention rate of 84.3%after 1000 cycles at the ultra-high current density of 20 C.The cyclic stability of the material was verified by ex-situ TEM and XRD tests.In the end,the density functional calculation verifies the oxygen vacancy formed by the strong bond energy between high oxidation state ions and oxygen.This results in the narrowing of band gap and the lowering of Li⁺diffusion barrier.To sum up,it has excellent electrochemical performance after modification.