Application Of Modified Cobalt-based Sulfides In Lithium-sulfur Batteries

Lithium sulfur batteries have been considered as a new generation of promising high energy storage systems,owing to the advantages of high specific capacity,low cost,and environmental friendliness.However,its large-scale production and commercial application are hindered by many problems,including low conductivity of the active material,volume changes during charging and discharging progress,shuttle effect caused by soluble intermediate product.Cobalt-based sulfides can not only strongly adsorb soluble lithium polysulfides?LiPS?,but also effectively accelerate the progress of electrochemical redox reactions,thus they are widely used to solve the above problems.However,with the increase of sulfur loading,their limited adsorption sites and catalytic active sites are not enough to effectively suppress the shuttle effect of LiPS.This thesis further improves the electrochemical catalysis and adsorption capacity of cobalt-based sulfides through heterostructured design and heteroatom doping.The specific research contents are as follows:First of all,to address the issue of limited adsorption or catalytic sites of cobalt-based sulfides and their insufficient conductivity,we prepared a heterostructured Co₃S₄/MnS nanotube array using the self-sacrificing template method based on Kirkendall effect.The 3D array structure avoids the use of binders and builds a great 3D conductive network,facilitating the rapid transmission of electrons and the full exposure of active sites.Theoretical calculations and experimental results also indicate that the Co₃S₄/MnS heterostructure can improve the adsorption capacity and catalytic capacity of Co₃S₄ for soluble LiPS.Benefiting from the above advantages,the CC@CMS-NA based lithium sulfur batteries exhibit good cycle performance at a current density of 2.67 mA cm^-2 under a sulfur loading of 3.2 mg cm^-2?95.8%retention of initial capacity after 200 cycles?.In order to further improve the adsorption and catalytic capabilities of cobalt-based sulfides,3D spongy carbon sheets embedded P-doped Co₉S₈ nanoparticles were prepared by a two-step method of freeze drying and pyrolysis,which were used as electrochemical catalysts and adsorbents for lithium sulfur batteries.Co₉S₈ has mixed valence metal ions(Co³⁺and Co²⁺),which can provide good inherent catalytic activity for the electrochemical reaction of LiPS;nanometerization allows the active sites to be fully exposed;P atoms doping can further promote its adsorption and catalytic capabilities by adjusting the electronic structure of Co₉S₈.The prepared 3D sponge carbon embedded with P-doped Co₉S₈ nanoparticles is used to modify the separator of the lithium-sulfur battery,so that the batteries show high areal capacity and excellent cycle stability.They can provide an initial specific capacity of 961 mAh g^-1 at 1 C and53.3%capacity retention rate after 900 cycles?0.05%decay per cycle?.Even under the high sulfur loading of 5.6 mg cm^-2,lithium sulfur batteries still deliver a high specific capacity of783 mAh g^-1at 0.1 C(equivalent to a areal capacity of 4.38 mAh cm^-2).In summary,heterostructured design and heteroatom doping can promote the adsorption and catalytic ability of cobalt-based sulfides for LiPS,which can improve the cycle performance and sulfur utilization of lithium sulfur battery.This thesis provide a new avenue for the design of attractive LiPS catalysts.