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。
Layered Li_(1.2)Mn_(0.56)Ni_(0.16)Co_(0.08-x)Al_xO_2(0 ≤ x ≤(0.08)) cathode materials were successfully synthesized by a sol-gel method. X-ray diffraction and the refinement data indicate that all materials have typical α-NaFeO_2 structure with R-3m space group, and the a-axis has almost no change, but there is a slight decrease in the c lattice parameter as well as the cell volume. Scanning electron microscopy and high resolution transmission electron microscopy prove that all the samples have uniform particle size of about 200–300 nm and smooth surface. The energy-dispersive X-ray spectroscopy mapping shows that aluminum has been homogeneously doped in the Li_(1.2)Mn_(0.56)Ni_(0.16)Co_(0.08)O_2 cathode material. The cyclic voltammetry and electrochemical impedance spectroscopy reveal that appropriate Al-doping contributes to the reversible lithium-ion insertion and extraction, and then reduces the electrochemical polarization and charge transfer resistance. Li_(1.2)Mn_(0.56)Ni_(0.16)Co_(0.08-x)Al_xO_2(x = 0.05)shows the lowest charge transfer resistance and the highest lithium-ion diffusion coefficient among all the samples. The Li-rich electrodes with low-level Al doping shows a much higher discharge capacity than the pristine one, especially the Li_(1.2)Mn_(0.56)Ni_(0.16)Co_(0.08-x)AlxO_2(x = 0.05) sample, which exhibits greater rate capacity and better fast charge-discharge performance than the other samples. Li_(1.2)Mn_(0.56)Ni_(0.16)Co_(0.08-x)Al_xO_2(x = 0.05) also exhibits higher discharge capacity than the pristine one at each cycle at 55°C. These results clearly indicate that the high rate capacity together with a good high rate cycling performance and high-temperature performance of the low-Co Li_(1.2)Mn_(0.56)Ni_(0.16)Co_(0.08-x)Al_xO_2(x=0.05) is a promising alternative to next-generation lithium-ion batteries.
