Lithium Sulfur Batterieswith NiCo₂S₄-Based Carriers

Owing to the limited availability of traditional fossil fuel and the environmental pollution as well as the rapid development of various electronic devices and electric vehicles,the development of environmentally friendly energy storage systems with high energy density and power density has been becoming a hot research topic.Lithium sulfur battery has been attracted more interests as one of the novel potential energy storage systems due to its high theoretical specific capacity(1675 m Ah g^-1)and energy density(2600 Wh kg^-1)and the cathodic material sulfur with abundant resources,non-toxicity and low cost.However,lithium-sulfur battery exists the following fatal defects:（1）The intrinsic insulation of sulfur and its product Li ₂S are not conducive to timely electron conduction,slowing down the reaction kinetics of the sulfur and thus decreasing significantly the sulfur utilization and;（2）The relatively large density difference between sulfur and its product Li₂S results in volumetric expansion during charging and discharging process,significantly affecting the stability and cyclability of the electrodes;（3）The intermediate products lithium polysulfide（Li PSs,Li₂Sn 4≤n≤8）are easily dissolved in the electrolyte to form the so-called shuttle effect,resulting in fast capacity decay,poor rate capability and low coulombic efficiency.Although much research has been carried out on lithiu m sulfur battery,it is still not enough to completely solve those defects and thus to promote the commercialization of lithium sulfur battery.In recent years,metal sulfides such as cobalt sulfide and nickel sulfide have received much attention due to their good electrical conductivity and catalytic effect on polysulfide conversion,especially the spinel structural nickel cobalt sulfide（NiCo ₂S₄）acting as a sulfur host for cathode material due to its rich mixed valence and redox reaction sites,is potential in lithium-sulfur batteries.Therefore,in this thesis,it aims to ameliorate the insulation,volumetric expansion and shuttle effect of lithium sulfur battery by tuning the morphology and structure of NiCo ₂S₄ as sulfur host.The four aspects of work is described in detail as following:（1）NiCo₂S₄ with different morphologies,including nano spherical NiCo ₂S₄particles,single-walled tubular NiCo₂S₄ and double-walled tubular NiCo₂S₄,were synthesized hydrothermally and solvothermally from different Ni,Co,S sources,and subsequently NiCo₂S₄@S composites were prepared by the traditional melt-permeation method and investigated electrochemically as cathode for lithium sulfur battery.The results show that NiCo ₂S₄ improves significantly the electrochemical performance of lithium sulfur batteries compared with bare elemental sulfur,which might be ascribed mainly to better electrical conductivity of NiCo₂S₄ and its strong interaction with Li PSs.Comparing the effect of different morphological NiCo₂S₄ on the electrochemical performance of lithium-sulfur battery,tubular NiCo₂S₄ have more remarkable effect than granular NiCo ₂S₄,especially double-walled tubular NiCo₂S₄ when used as sulfur host for lithium sulfur battery,which has a specific capacity of 400 m Ah g^-1 after 200 cycles at a current density of 0.1 C and also could maintain a specific capacity of 390 m Ah g^-1at a current density of 2 C.This might be attributed mainly the fact that tubular NiCo₂S₄ could ameliorate the conductivity of elemental sulfur a nd the shuttle effect of lithium-sulfur batteries,except for accommodating the volumetric expansion during charging and discharging process of lithium-sulfur battery.（2）NiCo₂S₄@C@S composite with the similar structure of methylimidazole cobalt salt（ZIF-67）was prepared by one-step method via direct co-heating with elemental sulfur during NiCo₂S₄ synthesis by the precursor ZIF-NiCo,which was prepared solvothermally using ZIF-67 as both template and Co source.The as-obtained NiCo₂S₄@C@S composite was also investigated electrochemically as cathode for lithium sulfur battery.The results show that NiCo ₂S₄@C@S composite deliver a stable specific capacity of as much as 572.4 m Ah g^-1 at a current density of 0.1C after 100 cycles,which might be due to the uni form distribution of sulfur in NiCo₂S₄@C@S composite,facilitating sulfur conductivity,sulfur utilization and the interaction of NiCo₂S₄ with the produced Li PSs,and thus mitigating the shuttle effect of lithium sulfur battery.More importantly,the speci fic capacity of the NiCo₂S₄@C@S composite still remains stable at 330 m Ah g^-1 at a large discharge current density of 2 C,indicating the typical catalytic effects of Ni,Co in NiCo₂S₄@C@S composite on the conversion kinetics of Li PSs and thus a potential in high-power characteristics.（3）The mesoporous NiCo₂S₄ was synthesized hydrothermally using cetyltrimethylammonium bromide（CTAB）as a template and the mesoporous NiCo₂S₄@r GO composite was also synthesized hydrothermally under the same condition except for with a dispersed reduced graphene oxide（r GO）.The mesoporous NiCo₂S₄@S and NiCo₂S₄@r GO@S composites were prepared by the sulfur introduction via the traditional melt permeation method and investigated electrochemically as cathode material for lithium sulfur battery.The results show that mesoporous NiCo₂S₄@r GO@S composite retains a specific capacity of 682.3m Ah g^-1 at a current density of 0.1C after 100 cycles,which is much larger than NiCo₂S₄@S composite,and also exhibits good cycling performance a nd rate capability,which might be mainly ascribed to the even distribution of elemental sulfur due to the much larger specific surface area and pore volume of mesoporous NiCo₂S₄@r GO composite than those of mesoporous NiCo ₂S₄.The conductivity and the interaction of NiCo₂S₄ with the intermediate products（Li PSs）are also greater in the presence of r GO,which significantly ameliorate the conductivity of the elemental sulfur and the shuttle effect of lithium-sulfur batteries.（4）Ellipsoidal NiCo₂S₄@Co₉S₈ composite with heterojunction structure was synthesized solvothermally by a facile two-step method,and thus the NiCo₂S₄@Co₉S₈@S composite was obtained by the introduction of elemental sulfur via the traditional melt permeation method and investigated electrochemically as cathode for lithium sulfur battery.The results show that after50 cycles at a current density of 0.1C,NiCo ₂S₄@Co₉S₈@S composite delivers a specific capacity of 559.1 m Ah g^-1 at 0.1 C after 50 cycles,and maintain at 533.5m Ah g^-1after 200 cycles,indicating a 95.4%retention of specific capacity and thus a good cycling performance.Meanwhile,NiCo ₂S₄@Co₉S₈@S composite is cycled at different current densities and finally returned to the initial current density of 0.1 C,showing the 90%retention of specific capacity and thus a good rate capability.This might mainly attributed to the significantly reduced Faraday charge transfer resistance of the electrochemical reaction of elemental sulfur due to the striking electrical conductivity of NiCo ₂S₄@Co₉S₈ with a heterojunction structure,the relatively strong interaction between NiCo ₂S₄@Co₉S₈ and Li PSs,and the catalytic effect of Ni,Co in NiCo ₂S₄@Co₉S₈ on the conversion kinetics of Li PSs,indicating the potential of the heterojunction structure of NiCo ₂S₄ in application of lithium sulfur battery.