Liwu Huang

MgH2–TiH2 mixture as an anode for lithium-ion batteries: synergic enhancement of the conversion electrode electrochemical performance

A 0.7MgH2 + 0.3TiH2 mixture was prepared by reactive grinding of Mg and Ti powders under hydrogen and tested as a conversion electrode for lithium-ion batteries. This composite presents superior electrochemical properties compared to MgH2 or TiH2 based electrodes, either in terms of reversible capacities or polarization versus applied current rate. A substantial reversible capacity of 1540 mA h g−1 is measured at a suitable potential of 0.52 V vs. Li+/Li0 (current rate 0.1Li h−1). For the same current rate, electrode polarization is limited to 0.2 V and stabilizes at 0.46 V for 1.0Li h−1 while a continuous polarization increase is observed for MgH2 based electrodes. In situ XRD analyses of this composite demonstrate that Mg and Ti hydrides are both electrochemically active and contribute together to the capacity. TiH2 improves MgH2 conversion process kinetics while MgH2 enables a reversible conversion reaction for TiH2. The three consecutive reactions are: (1) MgH2 + 2Li+ + 2e− ⇌ Mg + 2LiH; (2) δ-TiH2 + xLi+ + xe− ⇌ δ-TiH2−x + xLiH (x ≤ 0.5) and (3) δ-TiH2−x + (2 − x)Li+ + (2 − x)e− ⇌ α-Ti + (2 − x)LiH (x = 0.5). This is, to our knowledge, the first example of a reversible lithium conversion process with TiH2 powder.

Improved rate ability of low cost sulfur cathodes by using ultrathin graphite sheets with self-wrapped function as cheap conductive agent

Rechargeable lithium sulfur batteries are considered as one of the most promising batteries of the future, but they still suffer from poor rate performance due to the insulating nature of sulfur and Li2S, especially in low cost and simple sulfur cathodes. Till date, most of the studies have focused on the design of the state of art S-containing composites or complex electrode architectures. However, few studies have aimed to improve the low cost sulfur cathodes using commercial sulfur powder because of the increased difficulty; moreover, few studies have investigated the effect of conductive agents on the rate ability of lithium sulfur batteries. To improve the rate ability of low cost sulfur cathodes, we propose a cheap conductive agent, namely ultrathin graphite sheet (UGS), prepared by a simple ultrasonic dispersion method to replace commercial carbon blacks. Surprisingly, we find that sulfur particles can be easily wrapped by the small UGS in the electrodes without the assistance of any surfactants or other special methods. This spontaneous wrapping can effectively improve the electrical contact between the micro-sized particles and the UGS. Benefiting from its intrinsic high electronic conductivity and ultrathin sheet structure, a more effective 3D electrical conductive network is formed, which causes the electrode to exhibit lower interphase contact resistance and charge transfer resistance. Thus, the discharge ability and the charge ability of the UGS/sulfur cathode at 1 C or higher rates are much better than that of the nanoscale carbon black/sulfur cathode. Better performance at high rates and a low cost make UGS more suitable for the low cost and high power sulfur cathode.