First-principles Study On Modified TiO₂-B As The Anode Material Of Rechargeable Lithium-ion Batteries

With the increasing demand of anode materials implemented in advanced rechargeable Li-ions batteries, searching for substitute of traditional carbon anode materials has interested many research groups. Among various types of anode, metal oxides have always been the favorite species for most researchers. And among metal oxides, TiO₂-B has been viewed as the first choice of advanced rechargeable Li-ions batteries due to its high Li-ions capacity, high heat stability and unique insertion/extraction reaction mechanism. However, as the other polymorphs of titanium dioxide, TiO₂-B possesses a poor ion/econductivity which limits its application in practice. Given these, we have considered modifying the structure of Ti O2-B properly to improve its ion/econductivity. In this thesis, first-principles study on TiO₂-B and its modified materials was carried out by adopting multi-cell combination method to explore the insertion and diffusion properties of Li-ions and electronic structure of materials at the atomic level, and causes of ion/e- conductivity change of TiO₂-B and its modified materials were also analyzed. The concrete analytic processes and calculated results were discussed as follows:Chapter one, the primary structure and working process of rechargeable Li-ions batteries was introduced in detail. Main performance parameters and advantages/disadvantages of several well-studied positive/negative materials were analyzed, and the significance of the selected topic was elaborated in here.Chapter two, basic models, including the supercell model and slab model, and calculation methods, including Density Functional Theory and Transition State Theory, in the VASP calculation were introduced simply. Involved calculated parameters, such as Cutoff, convergence criteria, k-point grid and（U-J）, were tested to meet the required standard. Finally, the main analytical methods, such as the activation energy, density of states, Bader charge and the calculated method of insertion voltages, were defined in here.Chapter three, the perfect TiO₂-B model was built by MS Modeling Software. The concrete calculation contents were described as follows: 1. The stable insertion sites within the lattice system were studied. After the comparison of insertion voltages, the C site was viewed as the most stable site in low Li-ions concentration with the insertion voltage of 1.29 V; 2. The migration processes of Li-ions along three directions were calculated by CI-NEB method. The analyzed results suggested that the diffusion along b-axis was the best migration path for Li-ions with the activation energy of 0.51 eV; 3. The insertion voltages in high Li-ions concentration were calculated and the results showed that the voltage platform of perfect TiO₂-B was with 0.72⁰.94 V; 4. The density of states（DOS） of Ti O2-B was exported. It indicated that TiO₂-B was the semi-conductive material with the band gap of 3.0 eV, and the states of Ti-3d and O-2p were the main contributor of conduction band and valence band respectively.Chapter four, the oxygen-deficient TiO₂-B models was built based on the perfect TiO₂-B model. The concrete calculation contents were described as follows: 1. The site of oxygen vacancies was studied and it indicated that the O3f（1） and O4 f atoms were easy for removing; 2. The insertion sites and diffusion activation energies were calculated and it showed that C site still was the most stable insertion site for Li-ions in low concentration and the activation energies of defected models along b-axis were decreased and within 0.39⁰.49 eV; 3. The insertion voltages in high concentration were calculated and the voltage platform of model（2） was within 0.69⁰.89 V; 4. The DOS of defected models were calculated and it indicated that the band gaps of defected models were all decreased indeed and less than 2 eV. The analysis showed that the density of states of Ti-Ov-3d was the main contributor of the narrowing band gap.Chapter five, the passivated co-doping TiO₂-B was built by introducing co-doping atoms in perfect TiO₂-B model. The concrete calculation contents were described as follows: 1. The positions of N+V and C+Cr co-doping systems were confirmed to build the calculation models; 2. The insertion sites and diffusion activation energies were calculated and it indicated that C site also was the most stable insertion site for Li-ions in low concentration and the activation energies of co-doping models along b-axis were 0.47 and 0.42 e V respectively; 3. The insertion voltages in high concentration were calculated. The results showed that the N+V co-doping system with a voltage platform of 0.83⁰.97 V was more suitable for the application of anode materials comparing to the C+Cr co-doping system with a voltage platform of 1.05¹.27 V; 4. The DOS calculation of N+V and C+Cr co-doping systems showed the band gaps were reduced to 1.7 and 1.4 eV respectively. Analysis showed that the valence band structure was mainly modified by N and C doping atoms and the Conduction band structure was mainly modified by V and Cr doping atoms.Chapter six, the surface-bulk diffusion processes of Li-ions in three low-index surfaces of TiO₂-B were discussed in here. Calculation and analysis showed that [100] surface was not suitable for the surface-bulk diffusion process while [001] and [110] surfaces were proved to be the main diffusion paths duo to the existence of open c-axis and b-axis tunnels. The analysis of diffusion activation energies showed that the energy barrier mainly occured in the first layer of the surface-bulk diffusion process.In the end, all calculated results were concluded in the Conclusions and Suggestions, and there are three orientations for the future computing research, that is the diffusion process study in high Li-ions concentration, the research of surface modification for Ti O2-B surfaces and the search for suitable cathode materials of advanced rechargeable Li-ions batteries with TiO₂-B anode.