稀土掺杂对LiFePO_(4)性能影响的第一性原理研究

First-principles study of properties of rare-earth-doped LiFePO_(4)

掺杂是提高LiFePO_(4)体相电子电导率,优化其电化学性能的重要方法之一.稀土元素因具有高的电子电荷、大的离子半径以及强的自极化能力,成为掺杂改性的重要选择.本文利用基于密度泛函理论的第一性原理方法研究了稀土元素(La,Ce,Pr)掺杂的锂离子电池正极材料LiFePO_(4)的性质.计算结果表明,稀土元素掺杂均不同程度地增加了LiFePO_(4)的晶格常数和晶胞体积.在脱锂过程中,稀土掺杂后材料体积变化率明显减小,材料的循环性能提升,但电池能量密度下降.稀土掺杂使LiFePO_(4)由原来的半导体特性转变为金属特性,增加了材料的电子电导率.力学特性的计算表明稀土显著增加了LiFePO_(4)材料的延展性.另外,La和Ce掺杂后的LiFePO_(4)在Li离子迁移过程中表现出复杂的能垒变化,在远离稀土离子处迁移势垒呈现出不同程度的减小,而在靠近稀土离子处迁移势垒起伏较大.与Ce掺杂相比,La掺杂造成的离子迁移势垒的变化程度更大,表明稀土离子掺杂对体系局域结构产生较大的影响.

Doping is one of the most important methods to improve the electronic conductivity and modify its electrochemical performance of LiFePO_(4).Rare earth elements have become an effective selection for doping modification due to their high electronic charges,large ion radii and strong self-polarization ability.In this work,we study the structural,electronic and ionic diffusion properties of LiFePO_(4) with rare earth(RE)doping(La,Ce,Pr)by using first-principles calculation based on density functional theory.The calculated results show that the lattice constant and cell volume of LiFePO_(4) increase to a different degree after RE doping.In the delithiation process,the volume change rate of the material after RE doping is significantly reduced,indicating the cycle performance of the material is improved,on the other hand,the energy density is reduced.The calculated density of states suggests that RE-doped LiFePO_(4) exhibits metallic characteristics,which is different from the undoped one with semiconductor characteristics.As a result,the RE-doping can increase the electronic conductivity of the material.The calculation of elastic modulus demonstrates the increase of ductility for RE-doped LiFePO_(4),and it can be predicted that the cycle performance and the rate performance of the RE-doped battery have great improvement.In addition,La and Ce doped LiFePO_(4) materials exhibit that the complex energy barrier can change during the Li ion migration,and the migration barriers vary considerably,depending on different paths,which is related to the variation of potential energy surface caused by the doping of rare-earth elements.The Li-ions are far from the RE ions,the migration barriers are obviously lower than the undoped one,while the Li-ions are closest to RE ions,the migration barriers increase essentially.Compared with Ce doping,the change of the Li-ion migration barrier caused by La doping is great,indicating that RE ion doping has a greater influence on the local structure of the system.