锂离子电池LiTi_(0.25)Fe_(0.75)SO_4F正极材料的电子结构

采用密度泛函理论平面波赝势的方法,计算了LiFeSO_4F和LiTi_(0.25)Fe_(0.75)SO_4F正极材料的电子结构。计算结果表明:当锂嵌入材料后,S、O和F的原子布居变化较小,电子主要填充在过渡金属的3d轨道,导致过渡金属被还原,成为电化学反应的活性中心。在嵌锂态中,锂和氧(氟)之间形成了离子键,而过渡金属(Ti和Fe)与氧(氟)之间则形成了共价键,S-O键的共价性最强。态密度的计算结果则表明:Ti和Fe均保持高自旋排列结构;LiFeSO_4F的两个自旋通道的带隙分别为2.88和2.29 e V,其导电性很差;Ti掺杂使体系的带隙消失,显著地提高了正极材料的导电性;LiTi_(0.25)Fe_(0.75)SO_4F系统中Ti-O和Ti-F键均比纯相中的Fe-O和Fe-F键的共价性更强,因此Ti掺杂材料具有更好的结构稳定性。

Electronic Structure of LiTi0.25Fe0.75SO4F Positive-Electrode Material for Lithium-Ion Battery

By using density functional theory plane-wave pseudo-potential technique (DFT-PW-PS), the electronic structures of LiFeSO₄F and LiTi_0.25Fe_0.75SO₄F cathode materials were calculated. The computational results indicated that after lithium was intercalated into the materials, the atomic populations of S, O and F atoms are changed slightly, and that the electrons are mainly filled on the 3d orbits of the transition metals, leading to the reduction of the transition metals and making the transition metals being as redox centers. In the intercalation states, ionic bonds were formed between lithium and oxygen (fluorine), while covalent bonds were formed between the transition metals (Ti and Fe) and oxygen (fluorine),and the covalency of the S-O bonds is the strongest. The results from the density of states (DOSs) suggested that both titanium and iron take a high spin arrangement; the band gaps for the two spin channels of LiFeSO₄F are 2.88 and 2.29 eV, implying a rather poor electric conductivity; Ti doing will result in the disappearance of the band gap, enhancing significantly the electric conductivity of the cathode materials; As Ti-O and Ti-F bonds in LiTi_0.25Fe_0.75SO₄F system are stronger than Fe-O and Fe-F bonds in pure phase, Ti doping cathode materials would have a much better structural stability.