尖晶石型LiMn2O4晶体结构及锂离子筛H+/Li+交换性质研究

采用密度泛函理论平面波超软赝势和广义梯度近似法对尖晶石型LiMn2O4及其锂离子筛HMn2O4的晶体结构和性质进行了从头计算。PW91泛函最为有效,Li+被H+取代后HMn2O4晶胞收缩,点阵常数从LiMn2O4的0.823 nm减小至0.799 nm,其XRD峰也相应向高角度方向明显位移。经同种格点原子的XRD分析表明,Mn、O两元素对XRD方式和强度起着决定作用。其中Li呈+1价完全离子化,可被H+彻底交换,H与周围O在等电子密度图中呈现电子云相互连接,只带有0.42个正电荷。价轨道分态密度表明,Mn-O之间强的共价键合主要归因于Mn-d和O-p在费米能级下-7.3~-1.6 eV间的轨道重叠,形成了有利于H+/Li+交换的骨架空穴隧道。阵点和空穴多面体的体积遵守如下顺序:V8a>V48f>V8b、V16c>V16d、V16c>V48f。Li+最易迁移至邻近的16c位置,碱金属离子的交换受到离子半径和作用能大小的限制。

Spinel LiMn2O4 Crystal Structure and Lithium Ion-Sieve Property of H+/Li+ Exchange

The crystal structures and properties of spinel-type LiMn₂O₄ and lithium ion-sieve HMn₂O₄ were investigated by using the plane-wave ultrasoft pseudopotential and the generalized gradient approximation based on density functional theory. The PW91 functional was the most beneficial. HMn₂O₄ unit cell contracted after Li⁺ was substituted by H⁺, the lattice constant decreased to 0.799 nm from 0.823 nm of LiMn₂O₄, and the XRD peaks also obviously shifted toward high angle correspondingly. According to the XRD analyses of homologous lattice atoms, we concluded that two elements Mn and O play a decisive role in the XRD mode and intensity. Among them, Li presents +1 valence and is completely ionized, and can be exchanged by H⁺ thoroughly; while H displays the mutual connections of electron clouds with ambient O in the isoelectronic density map, and only takes 0.42 positive charge. Atomic partial densities of states show that the strong covalent bonding between Mn-O is mainly attributed to the orbital overlap of Mn-d and O-p between -7.3~-1.6 eV below the Fermi level, and forms the hole tunnels of framework that are beneficial to ion exchange. The volumes of lattice point and hole polyhedrons comply with the following sequences: V_8a>V_48f>V_8b, V_16c>V_16d, V_16c>V_48f. Li⁺ is most apt to migrate to an adjacent 16c position, and alkali metal ion exchanges are subjected to the limitation of ionic radius and the size of acting energy.