摘要/Abstract
采用第一性原理密度泛函方法优化了LiMn2O4尖晶石结构, 构建并计算了其低指数表面性质. 结果表明, 广义梯度近似(GGA)和自旋极化广义梯度近似(GGA+U)计算的LiMn2O4晶体体相结构中, Mn的d轨道选取有效U值时晶格参数会变大. 但两种计算结果都没有显示出电荷有序和Jahn-Teller畸变的情况. LiMn2O4尖晶石结构缺Li条件下, (001)、(010)和(100)表面Li终端与其他终端相比表面能更低; (110)表面Mn/O终端表面能较Li/Mn/O终端更低. 在所涉及的低指数表面中(111)表面能最低, 表面重构后(111)表面能低至0.270 J/m2, 是尖晶石结构中最稳定的切面. 关于反铁磁研究, (110)表面Mn/O终端表面能较Li/Mn/O终端更低. [↑↑↓↓]自旋排列下Mn/O终端表面能为1.050 J/m2, [↑↓↑↓]自旋排列下Mn/O终端表面能为1.061 J/m2, 即(110)-反铁磁型表面在[↑↑↓↓]自旋组态比[↑↓↑↓]的磁性顺序下更加稳定. 通过对(111)表面重构的研究, 发现该表面欠配位的锰离子会与完全配位的锂离子通过位置交换, 从而更加稳定. 重构表面的平均锰氧化态降低, 会减少Jahn-Teller效应的产生. 除(111)表面外, 其它低指数表面在铁磁和反铁磁下的表面能相似. 其中, (001)T3, (100)T1, (110)T1和(111)T2的表面结构在各自不同表面终端中具有最小的表面能. 本研究为理解LiMn2O4材料容量衰减问题和实验提供理论计算参考, 有助于推动高性能锂电池材料的研究.
关键词: LiMn2O4尖晶石氧化物, 第一性原理计算, Jahn-Teller效应, 低指数表面, 表面重构
The first principles density functional calculation method was used to calculate the index of low surface properties of LiMn2O4 spinel. Comparing with the bulk phase structures of LiMn2O4 crystals calculated by generalized gradient approximation (GGA) and generalized gradient approximation+on-site coulombic (GGA+U), it is found that the lattice parameters are obviously larger when the effective U value for the d orbital of Mn. However, the structural parameters of charge order and Jahn-Teller distortion are not found in own calculation. The results show that the Li terminal surface energy is lower when the (001), (010) and (100) surfaces of LiMn2O4 spinel in the state of lacking Li. The surface energy of (110) Mn/O terminal is lower than that of Li/Mn/O terminal in the state of lacking Li. The surface energy of (111) is the lowest in the low index surfaces. The surface energy of (111) after surface reconstruction is as low as 0.270 J/m2, which is the most stable section of spinel structure in this work. Considering antiferromagnetism, the surface energies of Mn/O terminal on (110) surface are lower than those of Li/Mn/O terminal, which Mn/O terminal surface energies are 1.050 J/m2 in the [↑↑↓↓] magnetic order and 1.061 J/m2 in [↑↓↑↓] magnetic order, respectively. The antiferromagnetic (110) surface is more stable in the spin configuration than in the magnetic order. We found that the structure of undercoordinated manganese ions on the surface are more stable by position exchange with the fully coordinated lithium ions by the observation of the surface reconstruction of (111). The Jahn-Teller effect is reduced when the average manganese oxidation state of the reconstructed surface is reduced. Except for (111) surface, the surface energies of other surfaces in the ferromagnetic state are similar to those in the antiferromagnetic state. Among them, the surface structures of (001)T3, (100)T1, (110)T1 and (111)T2 have the least surface energy among their own different surface terminals. This study provides theoretical calculation reference for understanding the capacity attenuation problem of LiMn2O4 materials and related experiments. It is also helpful to promote the research of high-performance lithium battery materials.
Key words: LiMn2O4 spinel oxide, first principle, Jahn-Teller effect, low exponent surface, surface reconstruction
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