Synthesis,Structure And Electrochemical Performance Of Modified Materials For Lithium-Sulfur Battery Separator

With the development of society,it is difficult for rechargeable lithium-ion(Li-ion)batteries to fully meet the increasing demand for energy storage devices with high energy density.Therefore,lithium–sulfur batteries(LSBs)have received numerous attentions in recent years because of their superior theoretical energy density(2600 Wh kg^-1)and capacity(1675 m Ah g^–1).Moreover,elemental sulfur(S₈)possesses remarkable economic and environmental benefits due to its natural resources and non-toxic properties,which is especially favorable for its large-scale application in the realm of energy storage.Despite all these merits,the practical implementation of LSBs is still hindered by several intractable technical challenges.First,the high electrical resistivity of sublimed sulfur and its discharge products of Li₂S/Li₂S₂ limits the utilization of active substance sulfur,which leads to the decrease in energy density and power density.Second,due to the great difference in the densities between sulfur and its discharge products,there exists a large volume variation(80%)during charging and discharging processes,resulting in an irreversible deterioration of cathode structure.Third,the notorious polysulfide(Li PS)shuttle leads to the poor cycle stability and low Coulombic efficiency of LSBs.Finally,the Li PS conversion reactions usually present slow kinetics process,which limits the discharge depth.In response,some researchers have started their investigations from the perspective of capture and conversion of Li PS.Along this line,fruitful achievements have been attained by designing the cathodes,modifying the separator or inserting an interlayer and optimizing the electrolytes.Of those many efforts,separator modification has been considered as one of the most effective strategies to optimize the electrochemical performance of LBSs thus far.In view of the selection of modified materials,it was initially proposed to coat the separator by using carbonaceous materials,which can accelerate the transfer of electrons.However,the weak interaction between non-polar carbon and polar Li PSs cannot effectively inhibit the shuttle effect,still causing irreversible loss of active material and capacity decay.Polar nonconducting materials can chemically adsorb Li PS,thereby benefiting to maintaining the cycling stability of batteries.However,the low electrical conductivity of these materials implies an inferior electrocatalytic activity,which is insufficient to the enhancement of battery performances.Especially under high current density,Li PSs tend to accumulate on the anode and separator and thus increase the battery resistance.The current investigations have corroborated that the use of modified separator materials with electrocatalytic activity can promote the mutual conversion of Li PS and thereby improve the battery performances.In this thesis,we have developed three novel nanomaterials with strong polarity and unique microscopic morphology to optimize the separator of LIBs and the composition,microstructure and electrochemical properties of the materials were investigated systematically.The main research content and results are as follows:1.A nitrogen-doped Fe/Fe₃C@GC composite material was synthesized through a simple and effective method of high-temperature annealing in a nitrogen atmosphere and used as a modified seperator material for lithium-sulfur batteries.The iron-based nanoparticles are wrapped by a partially graphitized carbon layer.The cross-linked carbon layer composite structure not only ensures rapid and continuous charge transfer,but also is advantageous to the integrity of the structure and the full penetration of the electrolyte,which can effectively promote the progress of electrochemical reactions.The N-doped porous carbon network favors the transport of Li⁺and electrons,while N atoms and iron-based substances can provide chemisorption sites to capture polysulfides and accelerate the redox reaction of polysulfides due their strong polarity.Therefore,the Li-S battery with N-Fe/Fe₃C@GC modified separator can provide a high discharge capacity of 861 m Ah g^-1 at 2 C and after 500 cycles,the capacity can still be maintained to 589 m Ah g^-1.2.The precursor Co(CO₃)_0.5(OH)·0.11H₂O was grown on PP by a hydrothermal method and subsequently vulcanizatized to obtain a hydrated Co SO₄nanomaterial for decorating the separator of LIBs,and the effect of the thickness,specific surface area and catalytic activity of modified layer on the shuttle effect of lithium-sulfur batteries was studied in detail.As the hydrothermal reaction time of the precursor increases,the thickness of the modified layer increases.The optimum hydrothermal time for precursor is 6 hours(CS/PP-6).The material for modifying the separator has abundant active sites of polar adsorption and catalytic conversion of polysulfide.As a result,the battery with CS/PP-6 separator exhibits the initial discharge specific capacity of 1410.3 m Ah g^-1 at0.1 C and the capacity of 873.7 m Ah g^-1 evenat a high rate of 2 C.In addtion,after 500cycles at 1 C,the capacity of the battery can still maintain 504.6 m Ah g^-1 and the decay rate per cycle is as low as 0.075%.3.An in-situ electrodeposition technique was developed to grow cobalt selenide Co_0.85Se(CS)nanoarrays on carbon nanotubes(CNT)and acetylene black(AB)decoragted separator of LBSs.The first three-dimensional conductive carbon layer containing carbon nanotubes and acetylene black introduced on the separator not only improves the electrochemical rate of the separator to lithium ions,but also provides attachment sites for electrodeposition.The electrodeposited strongly polar cobalt selenide can not only chemically anchor polysulfides,but also catalyze the conversion between polysulfides.Therefore,compared with a single-layer carbon material modified separator,the modified layer after electrodeposition of CS not only achieves physical blocking of polysulfides,but also promote chemical adsorption and catalytic conversion of polysulfides to enhance electrochemical performance of the battery.The sepatrator decorated by CNT/AB and cobalt selenide has a first-lap discharge specific capacity of1560.4 m Ah g^-1 at low current density(0.1 C),and a discharge specific capacity of1158.9 m Ah g^-1 at 5.0 C.And after 400 cycles,there is still a discharge specific capacity of 753.3 m Ah g^-1,and the capacity decay rate per cycle is only 0.08%.