| With portable electronic products and electric vehicles more and more universal,people have the more great requirements at the endurance and safety performance of battery.In order to satisfy the growing energy demands,it is urging to find a new type of high energy density battery.Because of the high specific energy(2600 Wh kg-1)(the specific energy is 3-5 times than that of lithium-ion battery)and the abundant reserve,low cost and environment-friendly of sulfur,lithium-sulfur battery system has been a hot spot in battery research recently.However,there are still many problems that need to be solved in lithium-sulfur battery before applying,such as the poor conductivity of sulfur and Li2S/Li2S2,volume expansion and shuttle effect,etc.Those can lead to the low electrochemical performance and the poor cycling stability of lithium-sulfur battery.So,this paper mainly aims to sulfur/metal compound composites and sulfur/carbon/metal compound composites to advance the integrated performance of lithium-sulfur battery.We select the high electrochemical activity,easy managed morphology of NiCo2S4 and the NiCo2S4-based composites as lithium-sulfur battery cathode materials to solve the problems mentioned above.We use the urchin and the flake of Ni-Co-O as the precursor.After the hydrothermal treatment,the urchin NiCo2S4 and the flaky NiCo2S4 that is formed with hollow tubular structure are prepared,and then are applied to the positive electrode of lithium-sulfur battery to explore the influence of different shapes of NiCo2S4 on the electrochemical performance of lithium-sulfur battery.Because NiCo2S4 can absorb polysulfide ions,it not only inhibits the dissolution of polysulfide ions into the electrolyte,but also enriches the polysulfide ions on the surface of the cathode material,those can improve the conversion of sulfide ion and utilization rate of active materials,the urchin and the flaky NiCo2S4/S electrode exhibit excellent electrochemical performance.On the other hand,the more abundant microporous,mesoporous and the hollow tubular structure of NiCo2S4 help the flaky NiCo2S4/S electrode to absorb more electrolysis,those make the active substance and electrolysis contact more sufficiently.Those unique structures can also protect and fix the active substance and lithium sulfide,which reduces the loss of active material and alleviates the volume change of sulfur during the charge/discharge process.So the flaky NiCo2S4/S electrode exhibits more excellent electrochemical performance than the urchin NiCo2S4/S electrode:at a current density of 4 C,the flaky NiCo2S4/S electrode has a pristine discharge specific capacity of 673 mAh g-1 and a coulombic efficiency of 97.00%,the specific capacity can remain at 526 mAh g-1 even after 130 cycles;at a current density of 5 C,the specific capacity of the first cycle is 593 mAh g-1,and the coulombic efficiency reaches to 96.60%,the specific capacity remains 417 mAh g-1 after 200 cycles with an average attenuation of 0.15%.In summary,the flaky NiCo2S4/S electrode exhibited excellent electrochemical performance under high rate charge and discharge.In addition,the highly conductive Ketjen black(KB)is mixed with Ni-Co-O to prepare the Ni-Co-O&KB precursor.After vulcanized by 0.5 M sodium sulfide,the NiCo2S4&KB(0.5 M)with hollow tubular structure of NiCo2S4 is formed and then is applied to the positive electrode of lithium-sulfur battery.The results show that Ketjen black improved the dispersion of NiCo2S4 nanotubes and the electrical conductivity of composite material in the NiCo2S4&KB(0.5 M)system;the tubular structure of NiCo2S4 can not only accommodate and protect active material,but also reduce the volume expansion during the process of charge/discharge.In summary,these reasons lead to NiCo2S4&KB(0.5 M)/S electrode to produce an excellent electrochemical performance in battery:at a current density of 0.2 C,the initial specific capacity of the NiCo2S4&KB(0.5 M)/S electrode reaches to 1278 mAh g-1,the first cycle coulombic efficiency is 94.20%,the specific capacity of 984 mAh g-1 is maintained after 100 cycles.Compared to other carbon materials,the electrode material exhibits excellent charge and discharge performance. |
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