Structural Regulation And Performance Research Of Lithium Titanate Anode Materials For Lithium Ion Batteries

Lithium-ion batteries（LIBs）are widely used in transportation,medical treatment,and energy storage due to their advantages such as high efficiency,low self-discharge,no memory effect,and environmental protection.They have been hot spots for new energy research and development for a long time.Lithium titanate(Li₄Ti₅O₁₂,LTO)anode material has high and stable platform voltage and"zero strain"characteristics for lithium-ion batteries,which can meet the safety and stability requirements of high-power electrical equipment,and has great development potential.Unfortunately,low theoretical capacity,intrinsic electronic conductivity and lithium ion diffusion lead to poor rate performance and poor volume energy density,which hinders its large-scale application.This paper systematically studied the effects of non-metallic element doping,second phase formation and morphology adjustment on the structure and electrochemical performance of Li₄Ti₅O₁₂anode materials to improve its rate performance and energy density.Combined with the first-principles calculations,this paper discussed the influence mechanism of different modification methods on the electrochemical reaction of Li₄Ti₅O₁₂anode materials in depth.It mainly includes the following research contents:First,this paper studied the effect of non-metal element doping on the structure and electrochemical performance of Li₄Ti₅O₁₂anode materials.F,Cl,Br,I,C,N were selected to dope with Li₄Ti₅O₁₂anode material.It was found that N,C or Br,doping has a large effect on the electrochemical performance of lithium titanate.All of the three elements can produce Ti³⁺active sites with varying degrees,and promote electron transfer in the lithium titanate cell.The first-principles calculation results of the electronic structure changes of doped lithium titanate found that the doping of C,N,Br elements can increase the electronic modification of n-type Li₁₀Ti₁₄O₃₂in the entire Li₄Ti₅O₁₂.Thus,it can effectively improve the electronic conductivity of Li₄Ti₅O₁₂and make the non-metal doped Li₄Ti₅O₁₂anode material exhibit excellent rate performance.When the rate was increased from 0.1 C to 20 C,the capacity retention rates of pure and C,Br or N doped Li₄Ti₅O₁₂anode materials increased from50.4%to 67.1%,70.3%,and 72.1%,respectively.In addition,the doping of C element makes the particles of Li₄Ti₅O₁₂smaller and more uniform,which shortens the transmission path of lithium ions.The lithium ion diffusion coefficient increased about 2.5 times after C doping.The doping of Br element make the volume of the unit cell increased,which promoted the lithium ion transfer in the unit cell.The capacity of lithium storage is increased,and the capacity improvement effect at a low rate is better.At a rate of 0.1 C and 0.5 C,the capacities are 174.5 m A?h?g^-1and 172.3m A?h?g^-1,which are close to the theoretical capacity of 175 m A?h?g^-1.N element doping not only produces oxygen vacancies,but also increases the"ion-hole concentration"while forming a metallic Ti-N structure,which can simultaneously promote the rapid transport of lithium ions and electrons,showing the best rate performance and highest cycle stability.At 20 C rates,the capacity of N-doped Li₄Ti₅O₁₂increased to 41.8%,compared with that non-doped Li₄Ti₅O₁₂.The capacity retention rate after 100 cycles at 10 C rate was increased from 78.6%to 93.8%.Therefore,non-metal doping is very effective in improving the capacity and rate performance of lithium titanate.It can be modified in accordance with the characteristics of the non-metal elements（such as radius,charge,and control of morphology）to maximize the improvement electrochemical performance of lithium titanate.Combining the advantages of C,N,and Br doping,this paper studied the synergistic effect of coating and co-doping on the structure and electrochemical performance of Li₄Ti₅O₁₂anode material.Using cetyltrimethylammonium bromide（CTAB）as the C-N-Br co-doping source and surfactant to obtain Li₂TiO₃coated and C-N-Br co-doped nano-scale Li₄Ti₅O₁₂anode material.The Li₂TiO₃layer formed in situ is uniformly and firmly fixed at the interface of Li₄Ti₅O₁₂,which can reduce the agglomeration of nano-sized lithium titanate,and plays a important role in improving the cycle stability of lithium titanate.After 500 cycles at a rate of 20 C,the capacity retention rate was 95.5%,which was 8 times higher than that of Li₄Ti₅O₁₂before modification.The cubic structure of Li₂TiO₃also provides a three-dimensional ion transmission channel,which is conducive to the transfer of lithium ions at the interface and greatly improves the lithium ion conduction rate.The lithium ion diffusion coefficient is increased by nearly an order of magnitude compared with that before unmodified.C-N-Br co-doping provides more Ti³⁺electrochemical active sites,the proportion of n-type Li₁₀Ti₁₄O₃₂,metallic Ti N structure and ion-hole concentration,which are conducive to electron conduction.The charge transfer resistance was reduced from 63.4Ωto 17.9Ωafter modification.Efficient electron conduction and ion conduction can make electrons and lithium ions reach the electrochemical active site at the same time in a short time,showing excellent rate performance.At a rate of0.1 C and 20 C,their capacities are 177.3 m A?h?g^-1and 153.9 m A?h?g^-1,respectively.Within this rate test range,the capacity retention rate is increased from 51%to 86%.Therefore,a stable in-situ layer can improve the cycle stability of nano-sized lithium titanate.The use of a coating layer with a lithium ion transmission channel and an increase in the concentration of doping defects can improve the efficiency of lithium ion and electron transport at the interface,both of which,is critical to improve the electrochemical performancerate performance of lithium titanate.Based on the successful preparation of co-doping and second-phase coating of Li₄Ti₅O₁₂anode material with high rate performance,the morphology control was used to further improve electrochemical performance of Li₄Ti₅O₁₂anode materials.Carboxyl-grafted nano-carbon powder（CC）and bromohexadecylpyridine（CPB）were used as C-N-Br co-doped sources and structure directing agents of TiO₂crystal plane.By controlling the precursor morphology,C-N-Br co-doped lithium titanate submicrospheres with hierarchical particle/pore size structureprepare hierarchical particle size/pores and C-N-Br co-doped Li₄Ti₅O₁₂submicrospheres was prepared.According to the selection of structure-directing agent,two other morphologies（diamond,truncated diamond）were also obtained.The submicrospheres with hierarchical particle/pore size structure show the most excellent electrochemical performance.The hierarchical particle size distribution reduces the surface energy of nano-sized lithium titanate and the stress during cycling,and significantly improves the cycling stability of the lithium titanate anode material.The modified Li₄Ti₅O₁₂anode material has a decay of only 0.008%per cycle after 1000 cycles at a rate of 20C,which is only 1/10 of the single attenuation before modification.This morphology has a higher tap density,which improves the volume energy density of lithium titanate.The hierarchical pores are composed of uniform micro mesopores,which can quickly wet the electrolyte,promote uniform contact between the electrolyte and the active material,and improve the diffusion rate of lithium ions at the interface.The lithium ion diffusion coefficient of the modified Li₄Ti₅O₁₂was 52 times than unmodified.Under the reduction of CC,the C-N-Br doping concentration and oxygen vacancy concentration increase,and the resulting Ti³⁺,metallic Ti N structure and ion-hole concentration further increase,which is more conducive to electron conduction.First-principles further clarified that doping has a greater effect on the electronic structure of n-type Li₁₁Ti₁₃O₃₂,the formation of oxygen vacancies has a significant effect on the electronic structure of p-type Li₁₁Ti₁₃O₃₂,and an increase in the concentration of oxygen vacancies can achieve the change.Structural defects caused by doping and oxygen vacancies greatly increase the electronic modification of n-type Li₁₀Ti₁₄O₃₂in Li₄Ti₅O₁₂,which significantly enhances the conductivity of lithium titanate.The electronic conductivity of the modified Li₄Ti₅O₁₂anode material was improved by nearly three orders of magnitude before modification.The charge transfer resistance was also reduced from 87.9Ωbefore modification to 11.1Ω.Under the combined effect of morphology and structural defects,the material shows excellent rate performance.At 0.1 C and 20 C rates,the capacity is 185.9 m A?h?g^-1and 157.7 m A?h?g^-1,respectively,the capacity retention rate is increased from 45%to 86%.which greatly improves the volume energy density of lithium titanate.Thus,it can synergistically improve the electrochemical performance of lithium titanate by adjusting the morphology and defect concentration,In this paper,the effects of morphology and structural defects on the electrochemical performance of lithium titanate are systematically studied through modification methods such as non-metal doping,forming a second-phase and hierarchical particle size/pore structure.This research design idea is expected to produce high rate performance and high energy density of lithium titanate batteries and then promote the large-scale application of lithium titanate batteries.