The Microwave-assisted Synthesis Of Metal Sulfide-graphene Composites For The Anode Of Sodium-ion Batteries

Lithium-ion batteries (LIBs) are currently widely used in portable electric devices such as mobile phones, etc. However, the damand of lithium increases dramatically due to the fast spreading of electric vehicles and the application of state grid energy storage in recent years. Correspondingly, the limited lithium resources in the earth crust and their high cost become the main concerns. Due to the low cost, highly natural abundance and wide distribution in the earth crust of sodium resources as well as the similarity between sodium and lithium, sodium-ion batteries (SIBs) are thus considered as one of the promising potential alternatives over LIBs and have attracted much research attention recently. At present, seeking for high performance electrode materials of SIBs is still highly desirable since the electrochemical performances of SIBs are mainly determined by the electrode materials.In this thesis, we investigated the electrochemical performances of a variety of metal sulfides as anode materials of SIBs systematically, mainly focusing on the improvment of the cycling stability. It’s found the the cycling stability is determined by several factors including graphene encapsulation of active materials, thermal treatment, binder used in fabricating electrode, electrolyte additive, electrolyte and cutting-off voltage etc. The major content of the thesis is summarized as following:1. MoS2 and MoS2-reduced graphene oxide (RGO) composites with different RGO contents were synthesized via a simple microwave-assisted method and applied as anode materials of SIBs. It’s found that the cycling performances can be improved significantly when the samples were thermally treated at 800 ℃ in N2/H2 atomsphere for 2 h, the cutting-off voltage was optimized at 0.005-2.5 V and 5 wt.% fluoroethylene carbonate (FEC) additive was added into the conventionnally used ester-based electrolyte. At the optimized condition, the composite electrodes exhibit a maximum reversible capacity of 305 mAh g-1 at a specific current of 100 mA g-1 after 50 galvanostatic cycles and excellent rate performance.2. Nickel sulfide and nickel sulfide-RGO composites with different RGO contents were synthesized via microwave-assisted method and applied as anode materials of SIBs. The effects of thermal treatment, binder, FEC additive in electrolyte and RGO contents in the composites over the electrochemical performance of SIBs were discussed. The optimized conditions are as follows:600℃ thermally treated for 2 h in N2/H2, carboxyl methyl cellulose binder rather than polyvinyldifluoride,5 wt.% FEC in electrolyte and 35 wt.% RGO content in the composite. Under such conditions, the electrode exhibits a maximum reversible capacity of 391.6 mAh g-1 at a specific current of 100 mA g"1 in the voltage range of 0.005-3.0 V after 50 galvanostatic cycles.3. ZnS and ZnS-RGO composites with different RGO contents were synthesized via microwave-assisted method and applied as anode materials of SIBs for the first time. It’s found that when the RGO content in the composite is 31 wt.%, a maximum reversible capacity of 481 mA h g"1 can be achieved at a specific current of 100 mA g"1 in the voltage range of 0.005-3.0 V after 50 galvanostatic cycles.4. CuS and CuS-RGO composites with different RGO contents were synthesized via microwave-assisted method and applied as anode materials of SIBs for the first time. To improve the overall cycling stability, the effects of two types of electrolytes (conventionally used ester-based and ether-based), cutting-off voltages and RGO contents in the composites were investigated. It’s found that the optimized conditions are as follows:NaFS/DGM electrolyte,0.4-2.6 V cutting-off voltage,26.7 wt.% RGO content. Under such conditions, the best sodium storage performance can be achieved with a maximum reversible capacity of 392.9 mAh g-1 at a specific current of 100 mA g-1 after 50 galvanostatic cycles and excellent long run stability, namely 345.7 mAh g-1 can be sustained at a high specific current of 1 A g-1 after 450 galvanostatic cycles.