Conductivity Enhancement And Performance Of TiNb₂O₇ Anode For Li-Ion Battery

As one of the most important energy storage technologies,lithium-ion batteries（LIBs）have been receiving notable attention ever since their commercialization.Among a variety of anode candidates,titanium niobium oxide（TiNb₂O₇）stands out as the leading contender,as it offers good safety and high power density for LIBs that are suitable serve as automobile start-stop batteries,et al.However,the application of TiNb₂O₇ is hindered by its capacity degradation during the charge and discharge processes.While significant efforts have been dedicated to improve its performance,there still lacks an in-depth study to understand and therefore address such an issue.In this thesis,the low electronic conductivity(～10^-9 S cm^-1)and ion conductivity(Li⁺ionic diffusion coefficient of～10^-17 cm² s^-1)are accounted to be responsible for the performance degeneration of TiNb₂O₇.Based on these understandings,two strategies including composition/morphology regulation and material modification are carried out to tailor these problems,which lead to improved electrochemical performance.The fundamental physiochemical properties are also deeply studied.The Li⁺embedding positions are identified by first-principle calculations.In addition,the volume expansion is calculated to be 5.63%when fully lithiated to the form of Li₅TiNb₂O₇.These results indicate that TiNb₂O₇ with the open structure（i.e.,Re O₃）potentially offers large capability to host Li⁺,benefiting the structural stability during the charge and discharge processes.Moreover,it is found that the poor electron conduction and ionic diffusion limit the performance of TiNb₂O₇,which can cause incomplete delithiation and high overpotentials,as well as fast capacity decays during cycling.The self-doping of Nb is studied to improve the electronic conductivity of TiNb₂O₇.First,the feasibility of self-doping is predicted via first-principle calculations.Then,the Nb content is adjusted in Ti_1-xNb_2+xO₇ based on the calculation results.It is found that a small change of the Nb content does not affect the morphology and structure of Ti_1-xNb_2+xO₇,while the electronic conductivity can be enhanced dramatically.Two orders improvement of electronic conductivity is achieved when 2%of Ti is replaced by Nb,consistent with its narrowest band gap（0.768 e V）from the first-principle calculations.Furthermore,18 times improved of Li⁺diffusion coefficient realized for Ti_0.98Nb_2.02O₇.Through this modification method,a reversible capacity retention of 111.9 m Ah g^-1 can be achieved after 1000cycles for Ti_0.98Nb_2.02O₇ at 10 C.TiNb₂O₇ with a nano-micro hierarchical flake structure is prepared via process optimization,including screening of soluble transition metal salts and solvents,as well as the modification of the hydrolysis and solvothermal conditions.The transmission path of Li⁺is shortened in the nanoscale primary flaky particles（thickness of 20-30 nm）effectively.Meanwhile,the electrochemical reaction is further accelerated.In addition,the stability of the nanomaterial is improved with reduced side reactions owing to the self-assembly of micron-sized secondary particles.TiNb₂O₇ with the nano-micro hierarchical flake structure demonstrates a good capacity retention of 81.5%after 100 cycles at 0.1 C.By coating and pyrolysis of dihydrodiamine（DCDA）as the carbon and nitrogen sources,effective carbon coating is achieved for TiNb₂O₇,where the problems of carbon loss and structure degeneration can be mitigated.Surface nitridization and partial Nb⁵⁺reduction are also conducted simultaneously with the carbon coating during the pyrolysis of the coated DCDA.These three processes work together to improve the electronic conductivity two orders,which enables an outstanding performance for N-TiNb₂O₇@C.The prepared N-TiNb₂O₇@C demonstrates a capacity retention of 86.4%after 100 cycles at 0.1 C,and a reversible specific capacity of 135 m Ah g^-1 at 50 C.Based on a liquid-state approach and with the use of sodium lauryl sulfate,carbon nanotubes（CNTs）are bonded onto the surface of TiNb₂O₇ particles.A TiNb₂O₇@CNT composite is obtained by uniformly dispersing TiNb₂O₇ particles in the CNTs conductive network.It is found that the TiNb₂O₇@CNT composite with～6%CNT shows the most obvious improvement of electronic conductivity without affecting the specific capacity.The obtained TiNb₂O₇@6%CNT composite demonstrates a capacity retention of 87.4%after 60 cycles at 0.1 C,and a capacity higher than TiNb₂O₇ by 45 m Ah g^-1 at 20 C.