【摘要】 本文采用基于密度泛函理论的第一性原理，使用CASTEP模块研究了Mg-Ti共掺杂对锂离子电池正极材料LiFePO₄的电子结构、弹性性质以及锂离子迁移能垒的影响。对掺杂前后材料的能带、态密度、电荷密度、差分电荷密度、布居值分别进行计算发现：掺杂前LiFePO₄的带隙为0.663 eV,Mg-Ti共掺杂材料的带隙降低至0.474 eV，相较于掺杂前降低了0.189 eV，同时Mg-O和Ti-O之间的相互作用比Fe-O更强，表明Mg-Ti共掺杂有助于材料电子电导率和结构稳定性的提高，进而改善其电化学性能。弹性性质计算结果表明，Mg-Ti共掺杂后材料的体积模量、剪切模量以及杨氏模量均有不同程度的下降。掺杂后Li_7/8Mg_1/8Fe_7/8Ti_1/8PO₄的平均剪切模量和平均体积模量比值G_H/B_H为0.69，低于掺杂前的比值0.71，表明掺杂后材料的塑性和延展性增强，有助于缓解电化学反应过程中材料的体积变化。最后，通过计算材料的锂离子迁移势垒发现：Mg-Ti共掺杂使得材料的锂离子迁移势垒从0.75 eV降低至0.52 eV，表明Mg-Ti共掺杂有助于提高材料的充放电过程中锂离子的扩散速率，进而提升其倍率性能。通过实验测量LiFePO₄和Li_7/8Mg_1/8Fe_7/8Ti_1/8PO₄的电导率和电化学阻抗谱（EIS），结果表明Li_7/8Mg_1/8Fe_7/8Ti_1/8PO₄有着更好的性能。更多还原

【Abstract】 LiFePO₄ has advantages such as structural stability,good safety,low cost,and environmental friendliness.Numerous studies have shown that ion doping not only helps to improve the electronic and ionic conductivity of LiFePO₄,but also helps to alleviate volume changes during charging and discharging processes,improve material ductility,and promote the improvement of its electrochemical performance.Taking into account both doping elements and doping positions,this paper utilized the first principles of density functional theory （DFT） and CASTEP module for calculations.The perfectly matched bases energies （PBE） exchange functional using the generalized gradient approximation （GGA） under CASTEP was employed,with the pseudopotential using ultra soft pseudopotential and the optimization algorithm using BFGS （broydenflechergoldfarbshanno）.Li site doping was carried out using the same family element Mg,and Fe site doping was carried out using the same period element Ti.The electronic structure （band,density of states,charge density,differential charge density,population） and elastic properties （bulk modulus,shear modulus,Young’s modulus） of Mg-Ti codoped LiFePO₄ cathode materials was systematically analyzed through first principles calculations,and further explored the mutual influence between electronic structure,elastic properties,and electrochemical performance.The calculations showed that the band gap of undoped LiFePO₄ was 0.663 eV.After single doping,the overall band structure of the sample shifted towards lower energy levels,but the band gap did not show significant changes.The band gaps of Li_7/8Mg_1/8FePO₄ and LiFe_7/8Ti_1/8PO₄ were 0.654 and 0.643 eV,respectively,indicating that the modification effect of Mg and Ti single element doping on the electronic conductivity of LiFePO₄ was not good.After Mg-Ti co-doping,the band gap value of Li_7/8Mg_1/8Fe_7/8Ti_1/8PO₄ could be reduced to 0.474 eV,indicating that Mg-Ti co-doping was beneficial for exciting electrons from the valence band to the conduction band,thereby improving the electronic conductivity of LiFePO₄.After Mg doping,new peaks appeared in the range of-50～-45 eV,mainly contributed by s state of Mg.After Ti doping,it could be found that the peaks at-70,-60,-50～-45 and-30 eV were all contributed by newly doped Mg and Ti elements.After Mg-Ti co-doping,the peak intensity near Fermi level increased.This was because s and d orbitals of Mg and Ti,together with s and d orbitals of Fe,provided electrons,forming valence and conduction bands,which increased the peak intensity near Fermi level.This would be beneficial for improving the conductivity of LiFePO₄.At the same time,the interaction between Mg-O and Ti-O was stronger than Fe-O,indicating that Mg-Ti co-doping helped to improve the electronic conductivity and structural stability of the material,thereby improving its electrochemical performance.The calculation results of elastic properties indicated that the bulk modulus,shear modulus,and Young’s modulus of Mg-Ti co-doped material all decreased to varying degrees.The average shear modulus and average bulk modulus ratio G_H/B_H of doped Li_7/8Mg_1/8Fe_7/8Ti_1/8PO₄ was 0.69,which was lower than the ratio before doping of 0.71.This indicated that the plasticity and ductility of the doped material were enhanced,which helped to alleviate the volume change of the material during the electrochemical reaction process.Finally,by calculating the lithium ion migration barrier of the material,it was found that Mg-Ti co-doping reduced the lithium ion migration barrier from 0.75 to 0.52 eV,indicating that Mg-Ti co-doping helped to improve the diffusion rate of lithium ions during the charging and discharging process of the material,thereby enhancing its rate performance.更多还原