Lithium-sulfur batteries have attracted a great interest in electrochemical energy conversion and storage, but their discharge mechanism remains not well understood up to now. Here, we report density functional theory (DFT) calculation study of the discharge mechanism for lithium-sulfur batteries which are based on the structure of S8 and Li2S x (1≤x≤8) clusters. The results show that for Li2S x (1≤x≤8) clusters, the most stable geometry is chainlike when x = 1 and 6, while the minimal-energy structure is found to be cyclic when x = 2-5, 7,8. The stability of Li2S x (1≤x≤8) clusters increases with the decreasing x value, indicating a favorable thermodynamic tendency of transition from S8 to Li2S. A three-step reaction route has been proposed during the discharge process, that is, S8 →Li2S4 at about 2.30 V, Li2S4 →Li2S2 at around 2.22 V, and Li2S2 → Li2S at 2.18 V. Furthermore, the effect of the electrolyte on the potential platform has been also investigated. The discharge potential is found to increase with the decrease of dielectric constant of the electrolyte. The computational results could provide insights into further understanding the discharge mechanism of lithium-sulfur batteries.
We report on the preparation of spinel Li4Ti5O12 submicrospheres and their application as anode materials of rechargeable lithium-ion batteries. The spinel Li4Ti5O12 submicrospheres are synthesized with three steps of the hydrolysis of TiCl4 to form rutile TiO2, the hydrothermal treatment of rutile TiO2 with LiOH to prepare an intermediate phase of LiTi2O4+δ, and the calcinations of LiTi2O4+δ to obtain spinel Li4Ti5O12. The as-prepared products are investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The diameters of Li4Ti5O12 submicrospheres with novel hierarchical microstructures are about 200–300 nm with the assembly of 20–30 nm nanoparticles. The electrochemical properties of Li4Ti5O12 submicrospheres are measured by galvanostatical discharge/charge test and cyclic voltammetry (CV). The as-prepared Li4Ti5O12 display excellent discharge/charge rate and cycling capability. A high discharge capacity of 174.3 mAh/g is obtained in the first discharge at 1 C rate. Meanwhile, there is only tiny capacity fading with nearly 100% columbic efficiency in the sequential 5–50 cycles. Moreover, lithium-ion diffusion coefficient in Li4Ti5O12 is calculated to be 1.03 × 10-7 cm2/s. The present results indicate that the as-prepared Li4Ti5O12 submicrospheres are promising anode candidates of rechargeable Li-ion batteries for high-power applications.
ZHANG Ai, ZHENG ZongMin, CHENG FangYi, TAO ZhanLiang & CHEN Jun Key Laboratory of Advanced Energy Materials Chemistry, Ministry of Education
