Al,Fe, Mg掺杂Li2MnSiO4的电子结构和电化学性能的第一性原理研究

硅酸锰锂作为锂离子电池正极材料因具有高的理论电容量而一直备受关注, 但其较低的导电率和较差的循环性能阻碍了进一步的发展. 采用第一性原理广义梯度近似GGA+U的方法, 研究了Al, Fe, Mg掺杂Li₂MnSiO₄的电子结构、 脱嵌锂电压和导电性. 研究发现, Al 掺杂的Li₂Mn_0.5Al_0.5SiO₄结构中载流子的数目增加, 电子自旋向上和向下的态密度均穿过费米能级, 呈现金属特性, 提高了体系的导电率. 脱锂Li_xMnSiO₄ (x=1, 0)结构中, 通过计算一次脱锂相结构的形成能得到Al掺杂的一次脱锂结构最稳定, 并且Al掺杂的脱锂相结构体积变化小, 有利于材料循环性能的提高, 同时第一个锂离子脱嵌电压与未掺杂时(4.2 V)相比降低到2.7 V. Fe掺杂降低了Li₂MnSiO₄的带隙, 第一个锂离子脱嵌电压降低到3.7 V. 研究表明, Al的掺杂效果优于Fe和Mg, 更利于硅酸锰锂电化学性质的提高.

First-principles study of electronic structures and electrochemical properties for Al,Fe and Mg doped Li2MnSiO4

Li₂MnSiO₄ is one of the potential cathode materials for lithium batteries due to its high capacities, but the poor conductivity hinders its further development. The cycling performance and electrochemical property of Li₂MnSiO₄ cathode material can be improved by doping metal cation. Twelve structures Li_xMn_1-yM_ySiO₄ (x=2, 1, 0; y=0.5, 1; M= Al, Fe, Mg) by doping Al, Fe and Mg are constructed in this paper, and their structures, electronic properties and delithiation process are studied by using the density functional theory of first principles within the GGA+U scheme. The best doping site and delithiated structure are found by comparing their energies. The results show that Al-doping is the best way to improve the conductivity and cyclability of the cathode material Li₂MnSiO₄. The pure Li₂MnSiO₄ has a low conductivity because of its large band gap (3.41 eV), while Al-doping Li₂MnSiO₄ crystal has metallic characteristics due to its electron densities of state with up-spin and down-spin cross through the Fermi level. The band gap is also reduced when it is Fe-doped, which improves the conductivity of Li₂MnSiO₄. Among the delithiated structures Li_xMnSiO₄ (x=1, 0), Al-doping enhances the structural stability because of the lowest formation energy and its cyclability is improved by reducing the volume change. Within the lithium ion extraction from the Li₂MnSiO₄ and Li₂Mn_0.5M_0.5SiO₄ (M=Al, Fe, Mg), the Mn-O and M-O bonding have much more ionic features, while the covalent bonding feature between Si and O is almost unchanged. And the fully delithiated MnSiO₄ and Mn_0.5M_0.5SiO₄ show semic-metallic properties depending on the density of states of configuration. The delithiated voltages for the first Li extraction process decrease when Al and Fe are doped. Therefore the Al-doping in the Li₂MnSiO₄ is expected to be an effective way to improve the cycling performance and electrochemical property for Li-ion battery cathode material.