| dc.description.abstract | Thermodynamic properties of magnesium make it an attractive anode material in secondary battery. As compared to lithium and lead, magnesium has advantages of low cost, environmental friendliness, high safety and easy handling. But there are still two major challenges in developing magnesium batteries. The first one is the high chemical activity of magnesium, leading to the formation of passive layer on Mg surface (obstructing Mg ions transportation) and limiting the selection of a suitable electrolyte. The other one is the selection of the cathode materials because magnesium ions are difficult to intercalate/deintercalate into the structure in most of cathode materials. Up to now, Mo6S8, reported in 2000, is one of a few cathode materials that can accommodate magnesium ions, but still faces the drawback of the magnesium ions.
In this study, Mo6S8 is chosen as the first active material for establishing the Mg battery system. Mo6S8 is fabricated by solid state reaction synthesis of Chevrel-phase Cu2Mo6S8, followed by removing copper ions in 6 M HCl. At room temperature, the capacity of Mg//Mo6S8 cell in APC electrolyte is about 60 mAh/g. In contrast, the cell capacity significantly increases to 117 mAh/g at elevated temperature of 60 C. Besides, adding lithium salt into APC electrolyte improves the cell capacity to 127 mAh/g even at room temperature. The changes of working ion and ion mobility are believed to play important roles in electrochemical and energy storage performance.
After showing the remarkable benefits of lithium salt addition in Mg//Mo6S8 cell, it is worth expanding this concept to other cathode materials, which are not capable of being intercalated/deintercalated by magnesium ions before. Among them, we introduce another active material MoS2, which magnesium ions cannot intercalate/deintercalate into the structure in APC electrolyte. After adding the lithium ion in the APC electrolyte, the Mg//MoS2 cell starts storing energy and its electrochemical behavior is very similar to MoS2 in lithium-ion system. The cell delivers impressive capacities of 159 mAh/g (at 25 mA/g) and 110 mAh/g at higher current rate of 500 mA/g, respectively.
To further improve the conductivity of MoS2, 10 wt% carbon nanotubes or graphene is incorporated into MoS2 via a ball milling process. The capacity of Mg//MoS2 cell is thus significantly advanced. Specifically, 190, and 210 mAh/g discharge capacities are obtained at 25 mA/g for the cells consisting of MoS2 cathode with carbon nanotubes and with graphene, respectively. At a higher current rate of 500 mA/g, the cells with carbon nanotubes and with graphene still deliver 134 and 151 mAh/g, respectively, outperforming 110 mAh/g obtained for the cell without carbon addition.
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