Study On Electrochemical Lithium Storage Mechanism And Performance Optimization Of Fast Charging TiO₂（B）

TiO₂（B）,a fast charging lithium-ion anode electrode material,has the advanta ges of high safety,high rate performance,zero strain,nontoxicity,low cost and equivalent theoretical capacity to that of commercial graph ite electrode,making it a promising anode electrode material for lithium-ion batteries.However,the current pract ical application of TiO₂（B）is still mainly subject to the following bottlenecks:1)the electrochemical lithium storage mechanism of TiO ₂（B）is still unclear.The reported lithium storage mechanism of TiO₂（B）is still controversial with limited experimental research evidences,which hinders the optimal design of high-performance materials.2)It is difficult to synthesize high-purity TiO₂（B）materials due to its metastable nature.The traditional liquid phase preparation process will inevitably introduce a small amount of anatase phase impurities,which will deteriorate the electrochemical performance.It is particularly important to develop ne w methods for the preparation of high-purity TiO₂（B）materials and high-precision purity detection.3)Low electronic conductivity(～10^-12 S cm^-1).Ion doping strategy can fundamentally improve the intrinsic conductivity of materials.Among many doped ions,N is competitive because it can effectively reduce the band gap of TiO₂（B）and the diffusion barrier of Li⁺.However,there are few research work on N-doped TiO₂（B）materials,and the doping mechanism needs to be further studied.In view of the above bottleneck problems,this paper uses a variety of in-situ spectral characterizations and theoretical calculation to reveal the lithium storage mechanism and dynamic characteristics of TiO₂（B）,develops new methods for rapid detection of TiO₂（B）purity,and modifies the material to further improve the electrochemical characteristics of TiO₂（B）.The main conclusions of this paper are as follows:（1）The"C→A1→A2"lithium storage mechanism,interface formation mechanism and kinetic characteristics of TiO₂（B）with key influencing factors were revealed by the combination of multi in situ spectral characterization,electrochemical analysis and theoretical calculation.1)In-situ Raman spectroscopy showed that:In the Li⁺intercalation stage,Li⁺first enters the C site,followed by enters the A1 site,and finally enters the A2 site.In the Li⁺deintercalation stage,the opposite process occurred,indicating high reversibility.2)The in-situ infrared spectroscopy and XPS showed that the SEI film of TiO₂（B）electrode changes dynamically with the process of charge and discharge,which is mainly composed of ROCO₂Li,ROLi,Li₂CO₃,Li F and Li_xPF_yO_z,and the content decreases in turn.3)The in-situ EIS and GITT showed that:the four stages of Li⁺entering into or leaving from TiO₂（B）were deeply analyzed:（i）the diffusion of solvated lithium ion in electrolyte.The diffusion resistance of solvated lithium ion in electrolyte is small and stable;（ii）the desolvated lithium ion passes through the SEI film.The kinetic characteristics of this process are related to the formation process of SEI film.The analysis sho wed that the presence of SEI film,especially the organic salt component ROCO₂Li,is unfavorable to lithium ion diffusion;（iii）lithium ions enter into or leave from each lithium storage site.The kinetic characteristics of this process are related to the lithium storage site.The migration of Li⁺to C site and A2 site is favorable,whereas its migration to A1 site is blocked.On the contrary,the escape of lithium ion from C site and A2 site are blocked,and it is easier to escape from A1 site;（i v）diffusion of lithium ions in the lattice.Similar lithium diffusion coefficients during intercalation(1.46978E-11cm²·S^-1)and deintercalation(1.36414E-11 cm²·S^-1)have been observe,indicating excellent reversible kinetic characteristics.（2）A method for accurately determining the purity of TiO ₂（B）was established by combining Raman spectroscopy and electrochemical characterization with synchrotron radiation XRD.Based on the calibration of synchrotron radiation XRD data,the functional relationship between TiO ₂（B）phase purity and two-phase Raman spectrum/electrochemical eigenvalues（I_B/（I_A+I_B）and Q_B/（Q_A+Q_B））is established,which realized the accurate and rapid detection of TiO₂（B）purity with our method.（3）A new idea of improving the electronic conductivit y of TiO₂（B）by interstitial type N doping was proposed to significantly improve its high-rate performance.N-doped TiO₂（B）was prepared by calcining HTO precursor under ammonia.Spectroscopic and XPS characterizations showed that the doped N occupies the interstitial site,which forms O-Ti-N bonding with adjacent Ti and O atoms.Raman spectra showed that the vibration of Ti-O bond is not affected after N doping,so it is inferred that the doping position of N is near the most stable A2 lithium storage site.The interstitial N narrows the energy gap of TiO₂（B）,increases the carrier concentration and intrinsic conductivity.The impedance of TiO ₂（B）/Li battery decreases,together with increased specific capacity and excellent rate performance.