Self-assembled Graphene Fiber/Tungsten Disulfide Composites For Cathodes Of Lithium-sulfur Batteries

Lithium-sulfur batteries?LSBs?have been regarded as one of the most promising energy storage systems for the next generation because of their high energy density(2600 W h kg^-1),high theoretical specific capacity(1675 mA h g^-1),friendly environment and rich sulfur resources.However,compared with the traditional lithium-ion batteries,the electrochemical reaction processes of LSBs involve not only a small amount of intercalation/deintercalation of Li ions,but also a large amount of multi-step complex phase evolution processes?S?LiPSs?Li₂S_x,4?x?8??Li₂S/Li₂S₂?.Thus,there still exist some urgent issues before the real application of LSBs,mainly including the irreversible loss of sulfur active materials and the decreased cycle stability of LSBs caused by the shuttle effect of lithium polysulfide,and the slow electrochemical reaction kinetics and the decreased utilization efficiency of sulfur caused by the low conductivity of elemental sulfur materials.In view of the above problems,it is of great significance to select reasonable matrix materials and structures of sulfur hosts for effectively inhibiting the shuttle of lithium polysulfide,accelerating the electrochemical reaction processes,and improving the capacity and cycle life of LSBs.During the past two decades,because of their excellent mechanical property,high electrical conductivity,and high specific surface area,graphene materials have become a hot topic in field of energy,and also have great potential applications in lithium-sulfur batteries.On the other hand,two-dimensional transition metal sulfides have been widely used in fields of energy,environment and catalysis,due to their strong adsorption and catalysis.In this dissertation,we have first contructed a type of self-assembled graphene fibers under the action of the unbalanced electrostatic force of ions,and synthesized graphene-fiber@nano-sulfur particle composites at the same time,which can be direct used as cathodes for LSBs.Second,then,tungsten disulfide were incorporated onto surface of self-assembled graphene fibers,which can further accelerate the electrochemical reaction processes of LSBs and improve the cycle stability of LSBs.The main contents of this dissertation are as follows:?1?One-step synthesis of self-assembled reduced graphene oxide fiber@nano-sulfur?rGOF@nano-S?cathodesIt has been reported that the microwave-treated graphene oxide nansheets?GO NSs?will self-asseble into GOFs under the unbalanced electrostatic force of the ions.Inspired by this principle,sulfur-containing ions(S₂O₃^2-)were introduced in this work,and a type of GOF@nano-S composites were synthesized one-step under the unbalanced electrostatic force of S₂O₃^2-ions.Compared with the traditional sulfur loading via a two-step melt-diffusion method,this novel one-step sulfur loading,i.e.,the sulfur loading occurred at the same time as the assembly of GOFs,can not only avoid the surface of rGOFs coated with non-conductive elemental sulfur,which will affect the conductivity of the whole cathodes,but also effectively confine sulfur active materials in the nano-space constructed by rGO NSs,which can accordingly improve the reaction efficiency between sulfur and Li ions,and achieve physical confinement of sulfur.After treated by a reduction annealing,the prepared rGOF@nano-S composite cathodes exhibit a high reversible specific capacity(886 mA h g^-1,0.1 C),a good rate capability?61%at 2 C?,and an excellent cycle stability?a low decay rate of 0.04%per cycle over 800 cycles at 1 C?.Furthermore,compared with the reduced graphene oxide/nano-sulfur?rGO/nano-S?composite cathodes without microwave treatments,the rGOF@nano-S ones showed both higher specific capacity and better cycling stability.?2?Design of tungsten disulfide microflower/rGOF@S?WS₂/rGOF@S?cathodesTransition metal sulfides have been widely used in the field of electrochemistry because of their excellent adsorption and catalytic properties.In this work,taking tungsten disulfide as instance,we have explored its enhancement mechanism on the capacity and cycle life of LSBs.The prepared rGOFs were used as the substrate for the growth of tungsten disulfide microflowers by a hydrothermal method,which can enhance the chemisorption of lithium polysulfides?LiPSs?on graphene-based cathodes,and accordingly enhance the overall performance of LSBs.Moreover,the incorporated tungsten disulfide can also catalyze the redox reaction of LiPSs on the cathodes of LSBs,accelerate the transformation of the intermediate product of long-chain LiPSs to short-chain Li₂S,and accordingly effectively inhibit the dissolution and shuttle of LiPSs.Results showed that the prepared WS₂/rGOF@S composite cathodes exhibit a high spectic capacity of 1523.6 mA h g^-1 at current density of 0.1 C.And,at current density of 2.0 C,after 600 cycles,the capacity of the assembled LSBs kept almost unchanged.This reseach suggests that rational combination of physical confinement via carbon nanomaterials and chemisorption via polar transition metal sulfides is a very effective means to improve the performances of LSBs.