| Lithium–sulfur(Li-S) battery is one of the most promising candidates for energy storage devices, as sulfur cathode possesses a high theoretical capacity of 1675 m Ah g-1. However, its commercialization is still thwarted by the high solubility of lithium polysulfide and serious safety concerns resulted from dendrite formation. Sulfur transforms into soluble lithium polysulfides in liquid electrolyte during the charge discharge process, resulting in a decrease of active material and capacity decay. In addition, the dendrite formation on the surface of lithium metal may lead to short circuiting and even catastrophic failure.In order to solve the above problems, physical constraint and chemical adsorption for lithium polysulfide molecule through graphene, PVP molecules and Ti O2 were studied. Si composites are also investigated as possible anode materials for Li-ion-S batteries.(1) Bulk sulfur electrode model was fabricated via a new synthetic method which is based on anodic deposition. The electrochemical deposition was successfully applied to control the morphology and structure of sulfur hybrid electrodes. Bulk electrode with sulfur packing on carbon fibers was obtained at a deposition voltage of 1.25 V. GS laminated hybrid electrode was obtained through alternate deposition of sulfur and graphene. Under the barrier effect produced by graphene, the dissolution and diffusion of polysulfides were limited. Discharge capacity of GS laminated electrode remained a discharge capacity of 700 m Ah·g-1 after 50 charge-discharge cycles at a current density of 200 m A·g-1.(2) The sulfur electrodes were coated with graphene via a novel electrochemical deposition process. Graphene with abundant functional groups was applied to modify sulfur cathodes. Lithium polysulfide molecules were immobilized due to the strong adsorption effects derived from functional groups on the surface of the graphene. Discharge capacity of GNS bulk electrode was up to 963 m Ah·g-1 after cycling for 150 times at a current density of 2.0 A·g-1, exhibiting a capacity retention of 72 %.(3) PVP connected Sulfur composites was designed and tested as cathode material. The PVP chains have provided a strong affinity to suppress the shuttle of lithium polysulfide in electrolyte. And PPy coating could facilitate the diffusion of lithium ions and suppress the dissolution of polysulfides. Synergistic effect from functional PVP chains and PPy shell was shown to improve the electrochemical performance. Controllable syntheses of S-PVP composite was realized by regulating SDS concentration and reaction time. The composite showed a high reversible capacity of 763 m Ah·g–1 at a current density of 200 m A·g–1 after 100 cycles.(4) H-Ti O2 was prepared to recapture lithium polysulfide from electrolyte. Hierarchical Ti O2 micron spheres assembled by nano-plates were prepared through a facile hydrothermal route. Chemical tuning of the Ti O2 through hydrogen reduction(H-Ti O2) is shown to increase oxygen-vacancy density and thereby modifies the electronic properties. Under the restricting and recapturing effect, the sulfur cathode could deliver a high reversible capacity of 928.1 m Ah·g-1 after 50 charge-discharge cycles at a current density of 200 m A·g-1.(5)Modified nano Si composite was studied as anode materials in Li-ion-S battery. Mediated by sodium dodecyl sulfate(SDS) micelles, nanoparticles were coated with a uniform layer of nickel by electroless plating method. Nano Si composite were also successfully wrapped by GO sheets. As anode material for Li-ion-S battery, Si@Ni and Si@G composite showed discharge capacity of 700 m Ah·g-1 and 580 m Ah·g-1, which may offer new possibilities to solve the safety problem of Li-S batteries. |
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