Synthesis Of Transition Metal Sulfides（MoS₂,CoS₂） And Investigation Of Its Electrochemical Performance

Lithium-ion battery as a green energy has been widely used in various devicespower, but improving the specific capacity and cycle life of lithium-ion battery is stillthe main issue in the process of development of the lithium-ion battery. The cathodematerial is one of the key factors restricting the overall performance. Due to thecapacity limit of traditional cathode material such as lithium transition metal oxide,there is an urgent need to develop a high specific capacity and high energy density.The lithium-sulfur secondary battery based on metal lithium as anode materials andpristine sulfur or containing sulfur composites as cathode materials is currentlyconsidered one of the most attractive study secondary battery systems. The theoreticalenergy density of the battery was2600Wh/kg, the actual energy density can bereached of300Wh/kg, most likely within the next few years, up to about600Wh/kg.In this thesis, we explore the feasibility of sulfur cathodes by the reaction oftransition metal sulfides （MoS₂, CoS₂）.The MoS₂/S composite material comprises include mesoporous MoS₂/Scomposites and tube MoS₂/S. The sulfur content of MoS₂/S composites was about50%.When tested at100mA/g between1.0-3.0V, mesoporous MoS₂/S showed aninitial discharge capacity of650mAh/g, only350mAh/g was maintained after50cycles （specific capacity the MoS₂/S was based on the overall quality）. When tubeMoS₂/S composite materials were tested under the same condition, the initialdischarge capacity was630mAh/g and only450mAh/g was maintained after50cycles. The results showed that on one hand the aperture of MoS₂can effectivelyinhibit dissolution of the elemental sulfur and the discharge products of lithiumpolysulfides in the electrolyte, on the other hand, MoS₂can contribute a portion of thecapacity between1.0and3.0V, which improves the reversible specific capacity ofcomposites. It also provide a new methode for the preparation of lithium-sulfurelectrode materials.Different sizes of CoS₂were synthesized by hydrothermal methods. The resultsshow that CoS₂of about100nm can be synthesized with the existence of grapheneoxide （GO）.What’s more, we evaluate the electrochemical performance of differentparticle sizes CoS₂. CoS₂/RGO （reduced graphene oxide） shows an improved cyclingstability of640mAh/g at0.1C. The electrode was charged at1C after the initial10 cycles,600mAh/g was maintained when charged at0.1C.However, CoS₂of3umshows1250mA/g at the first cycle, after50cycles only200mAh/g was kept at0.1C.CoS₂of4um and200nm yield an initial charge capacity of1350mAh/g, after50cycles the electrode retain a charge capacity of300mAh/g at0.1C.In this paper,we also investigate intermediate electrochemical reaction process through non-situXRD and non-situ EXAFS analysis. The electrode is cycled in voltage range from0.02-3.0V in the first10cycles and then changed the voltage range from1.0-3.0V.The results showed that nanosized Co and Li2S were generated after the initial deepdischarge and CoS₂was reformed during the charge process. Compared with MoS₂,the sulfur content in CoS₂was high, it did not to increase the sulfur content. What’smore, the catalytic ability of Co was strong, it can catalyze sulfur-sulfur bondcleavage and bonding easily and improve the electrochemical activity of thecomposites. So this work provides a new method for the development oflithium-sulfur batteries.