Preparation,lithium Storage Performances,and Scale-up Of Modified TiNb₂O7 Anode Materials

TiNb₂O₇（TNO）is an attractive anode material for high-safety,high-capacity and fast-charging lithium-ion batteries（LIBs）because of its medium working potential（1.6V vs.Li⁺/Li）and competitive capacity(～280 m Ah g^-1).The main challenges associated with TNO anodes are ultralow electronic conductivity(～10^-9 S cm^-1),large grain size caused by high phase formation temperature,and poor mass charge transport,leading to poor performances.To solve the above problems,modified TNO composites with porous structures are prepared using amorphous carbon and 2D transition metal carbides/nitrides（MXene）as conductive coating materials.Additionally,the feasibility of large-scale preparation using spray drying technology is explored.The main research contents and results of this paper are as follows:（1）TiO₂ and Nb₂O₅are used as raw materials to obtain bulk TNO by spray drying and calcination.To improve the electrochemical performance of TNO,carbon-coated TNO and nitrogen-doped carbon-coated TNO（TNO/C and TNO/C-N）are obtained after second spray drying-calcination,usingα-lactose and ammonium citrate as carbon sources,respectively.And the optimal carbon content is～5 wt%for both.It is shown that electronic conductivity of TNO/C-5%(7.90×10^-3 S cm^-1)and TNO/C-N-5%(8.03×10^-3 S cm^-1)is about two orders of magnitude higher than bulk TNO(2.14×10^-5S cm^-1).In half-cells,the rate capability of TNO/C-5%and TNO/C-N-5%is up to111.0 m Ah g^-1 and 128.0 m Ah g^-1 at 20C,respectively,which is significantly better than that of TNO(43 m Ah g^-1).After 1000 cycles at 1C,the capacity retention of TNO/C-5%and TNO/C-N-5%is 60%and 62%,respectively,which is better than that of TNO（30.2%）.This method is suitable for the large-scale preparation of TNO/carbon composite anode materials because of the easy availability of raw materials and simple process.（2）Porous TNO spheres are first prepared by hydrothermal method,and then MXene wrapped porous TNO spheres with oxygen vacancies(TNO_-x@MXene)are obtained by compounding with MXene and subsequent annealing treatment.It is found that introducing MXene and oxygen vacancies into TNO anode can result in electronic conductivity enhancement about four orders of magnitude while building a porous structure allows Li⁺（de-）intercalation reactions to dominantly occur at the material near-surface,boosting interfacial electron/Li⁺transfer kinetics.Meanwhile,3D conductive networks constructed by flexible MXene nanoflakes also help retain electrical connectivity.In half cells,the TNO_-x@MXene material demonstrates high reversible capacity(264 m Ah g^-1 at 0.1C),superior cycling stability（0.03%capacity fading per cycle during 1000 cycles at 4C）,extraordinary rate capability(78.0 m Ah g^-1at 100C),and high initial Coulombic efficiency（92%）.With Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）cathode,the assembled liquid-state full cells exhibit excellent capacity retention of 92%after 1000 cycles at 5C.Importantly,for the first time,we explore the potential of as-made TNO composite as the anode of all-solid-state LIBs with an inorganic sulfide electrolyte（Li₆PS₅Cl）.The solid-state TNO_-x@MXene//NCM811batteries also demonstrate stable cycling ability（98%capacity retention after 1000cycles at 2C）.Based on this fact,TNO-S/MXene composites are obtained by compounding TNO-S prepared by solid-phase method with MXene by spray drying.The electronic conductivity of TNO-S/MXene(1.18×10^-2 S cm^-1)is about three orders of magnitude higher than TNO-S.In half-cells,TNO-S/MXene exhibits better performance(93 m Ah g^-1 at 20C,and 40%capacity retention after 1000 cycles at 1C).