| As one of the most prospective candidates for next-generation high-energy storage devices,lithium-sulfur batteries(LSBs)have emerged based on some attractive features,which can afford a high theoretical specific capacity of 1675 mA h g-1 and energy density of 2600 W h kg-1.Despite showing great promise,there are several limitations of the sulfur cathodes due to the insulating nature of sulfur and Li2S,the "shuttle effect" caused by the intermediate lithium polysulfides(Li2Sx,4≤x≤8)dissolved and migrated in the electrolyte and serious side effects,resulting in low utilization of the active material,poor cycling stability,rate performances,and Coulombic Efficiencies.Based on this,this research prepared new MoO2-carbon based porous hybrids by developing a facile and straightforward strategy.And we investigated the relationships of porous structure and composites for restraining the diffusion of lithium polysulfides in the electrolyte.The main contents and results of this research includes the following aspects:(1)Molybdenum-polydopamine spheres with different cavity structures were prepared based on the coordination ability of molybdate ion and dopamine.In this work,a self-assembly mechanism of the molybdenum-polydopamine hollow precursors was explored in detail and a determined theory is proposed in a water/oil(W/O)system.After 60 wt.%sulfur is loaded into the MoO2/N/C hollow structures,the cathode showed excellent cycling stability with a high reversible specific capacity of 642 mA h g-1 and an average degradation rate per cycle of 0.07%when cycling to 200 times at a current density of 1 C.(2)A facile route of assembling SiO2 particles with different sizes into superstructures utilizing molybdenum-polydopamine complex as the binder and curing agent was established.After heat-treatment,the MoO2/N/C superstructures with hierarchical pores was obtained with no significant deformation due to the good structural stability of the complex framework.The specific surface area and pore volume of the composite are 654.2 m2 g-1 and 2.2 cm3 g-1,which provide sufficient space for the distribution of sulfur and can effectively adsorb lithium polysulfides.With a sulfur loading of 2.8 mg cm-2,the cathode with 80 wt.%sulfur delivered a high initial capacity 1069 mA h g-1 and a capacity retention ratio of 60.8%after 150 cycles at 0.2 C.(3)MOF-derived(HKUST-1)porous C-NCs and MoO2/N/C-NCs octahedral materials were prepared and their electrochemical performances in LSBs were explored.Combined with the adsorption test and the density functional theory(DFT)calculations,the massive soluble lithium polysulfides was confirmed to be chemically adsorbed and conversed on the surface of the polar host,resulting in fast redox kinetics and promoted rate capabilities.As a result,the fabricated LSBs displayed a high initial specific capacity of 1317 mA h g-1 at 0.2 C,a promising capacity of 602 mA h g-1 and a capacity retention of 65.6%at 1 C when proceeding to 500 cycles.(4)Two-dimensional MSX/C(M:Cr,Mo,W)nanosheets were synthesized by the coordination ability of metal ions with dopamine.Electrochemical tests showed that the catalytic effect of MS.nanoparticles is bidirectional,which can effectively accelerate the oxidation of lithium polysulfides to sulfur and reduction to Li2S2/Li2S.The Gibbs free energy of the lithiation process of S8 on MS.was simulated by theoretical calculation,which is traced to the fact that the fast polysulfide conversion reactions have more negative Gibbs free energies on the edge sites.Consequently,the sulfur cathode with Cr3S4/C flake had superior performances in terms of rate capability and cycle stability,capable of retaining the capacities of 618 mA h g-1 with a capacity decay of 0.098%per cycle(over 400 cycles)at the 1 C rate.The results of this study provide valuable proof on designing ideal carriers to promote polysulfide conversion in LSBs. |
