Surface/Interface Engineering Of Cobalt-Based Materials For Lithium-Sulfur Batteries

Due to their high theoretical specific energy and environmental friendliness,lithiumsulfur(Li-S)batteries have shown great promise as one of the next generations of electrochemical energy storage systems.Nevertheless,there are some tough problems existing in Li-S batteries,such as the shuttle effect of lithium polysulfides,sluggish sulfur redox kinetics,lithium dendrites and so on,which result in obvious self-discharge,low capacities,poor rates and short cycle life,seriously hindering their commercialization.Hence,taken cobalt-based materials as the research object,this thesis focuses on the rational design on their surface/interfaces though doping and heterostructure engineering to build an efficient and smooth interface for regulating polysulfides,accelerating sulfur reaction kinetics,inhibiting the shuttle effect and lithium dendrites growth,and finally improving the electrochemical performances.Main results are listed as follows:(1)P-CoS2 nanoboxes are synthesized through anion doping and metal-organic frameworks(MOFs)self-sacrificial template methods,which are further mixed with carbon nanotubes to yield binder-free P-CoS2/CNTs@Celgard separators.On one hand,P doping can participate in building a Co-O-P speices-rich oxide layer on the CoS2 surface so as to effectively adsorb polysulfides;On the other hand,P doping can be favorable to dissociate the S-S bonds of polysulfides,and then propel their conversion reaction.The Li-S cell with P-CoS2/CNTs@Celgard shows a high discharge capacity(1643 mAh g-1)and a low capacity decay rate(0.066%per cycle).(2)A self-supported Co4S3/C@CC interlayer is in situ prepared by one-step sulfidation and heterostructure engineering.Thanks to the continuous electron/ion migration channels,strong Li+affinity,and smooth polysulfides-regulated interface,Co4S3/C@CC encourages the selective Li+conduction and guides the flower Li2S deposition.The Li-S cell with Co4S3/C@CC has a low capacity fading rate of 0.045%per cycle over 1400 cycles at 2.0 C,and its initial areal capacity is up to 4.83 mAh cm-2 at an areal mass loading of 4.6 mg cm-2.(3)A ZnSe@CoSe2/C@CC catalytic interlayer is in situ prepared using core-shell MOFs as the template and a heterostructure engineering strategy.This catalytic interlayer has both high-efficiency adsorption-diffusion abilities of polysulfides(ZnSe)and fast Li+diffusion behavior(CoSe2),constructing a smooth interface for the adsorption-diffusioncatalytic conversion of polysulfides,which effectively inhibits the shuttle effect of polysulfides.Experimental results and mathematical model fittings further reveal the catalytic mechanism that ZnSe@CoSe2/C@CC can accelerate the reaction kinetics of polysulfides and guide the three-dimensional growth of Li2S.For Li-S cells with ZnSe@CoSe2/C@CC,a high initial areal capacity of 4.2 mAh cm-2 is obtained,with the areal capacity remaining at 4.3 mAh cm-2 after 100 cycles.