Constructing And Regulating The Micro Reaction Interface Of CNT/S Composite As The Cathode Materials Of Li/S Batteries

Under the situation of new energy,electric vehicles,based on the lithium ion power batteries,have already been widely regarded as one of the development direction for next generation of automobile industry.Currently,lithium ion batteries which always use LiCoO₂ or LiFePO₄ as the cathode material,are facing the technical bottleneck of low practical energy density,causing an extremely complicated battery management and an insufficient miles of range.Therefore,it is necessary to develop a new power battery system with the characteristics of higher energy density,longer cyclic performance,superior rate performance and lower cost.Among numerous battery system,lithium sulfur battery?Li/S?is extensively considered as one of the most promising candidate for next generation of power batteries,because of its high theoretical specific capacity?1675mAh/g?as well as high theoretical energy density?2600 Wh/kg?.However,there are several technical challenges of sulfur cathode restrict the practical application of Li/S battery.The“shuttle effect”caused by severe dissolution of the polysulfides,which is the intermediate products during the discharging/charging process,and the huge volume expansion will result in a poor cyclic performance and an unstable electrode structure.Though,introducing porous carbon materials can improve the poor electrical conductivity,the problem of low utilization of active materials under the high rate discharging still exists.Therefore,how to maintain the advantage of high energy density of Li/S batteries,meanwhile,realizing superior rate performance and long cycle life,is one of the research focus on the field of electrochemical energy storage.Encapsulating sulfur into the inner space of carbon materials can stablize the cathodestructure,as to lower down the dissolution probability of polysifides,it is necessary to reduce the contact interface between polysifides and electrolyte.While,to improve the utilization of active material,both excellent electron conductivity and thoroughly exposed sulfur interface to the electrolyte are indispensable.In consequence,the crux of obtaining Li/S batteries with long cyclic and superior rate performance,is constructing rational electrochemical reaction interface of carbon sulfur cathode materials,as well as regulating the reaction behavior of active material and dissolution behavior of polysulfides on the reaction interface.According to this consideration,this thesis selected carbon nanotubes?CNT?to encapsulate sulfur,by using the 1D tube to put up a stable cathode structure.Then,regulating the electrochemical reaction interface via constructing and optimizing the pore textures on the wall of CNT to prepared a series of CNT/S composite which are suitable for high rate discharging process.Furthermore,we designed a series of new binders which used polyvinyl alcohol?PVA?as the substrate,via esterification crosslinking reaction we synthesized polyvinyl alcohol-citric acid?PVA-CA?and polyvinyl alcohol-nitrilotrimethylene triphosphonic acid?PVA-ATMP?to enhance the cyclic performance of CNT/S cathode.The major contents and progress are as follows:1.We developed the“radical oxidation oriented-sub nanometer targeted etching”method to construct and regulate the electrochemical reaction interface and prepared CNT/S cathode composite material with good consistency which is suitable for discharging under high rate.Firstly,we systematically compared the etching effects on CNT by KOH,radical advanced oxidation and Co₃O₄ catalytic oxidation separately.For KOH etching,650?mainly introduced micropores and sub-mesopores,while 850?would enlarge the existed pores even caused severe fractures on the CNT walls.The dosage of KOH is the key to control the quantity of pores.Under the conditions of KOH:CNT=2:1,850?and activating for 1.5 h,the obtained ECNT-21-850/S composite exhibited a high initial capacity of¹100 mAh/g at 5C.Fenton oxidation process hardly modified the texture of CNT,while the SO₄^-·originated from potassium peroxomonosulfate?PMS?was able to generate large amount of carbonyl and parts of epoxy groups on the CNT.By using the Density Functional Theory?DFT?computational simulation,we found that,SO₄^-·facilitates the conversion of epoxy to carbonyl and breaking of nearby C-C bonds,which resulted in a mass of defects as well as a poor electronic conductivity,consequently,would not improve the rate performance of CNT/S.As for utilizing Co₃O₄ catalytic oxidation to etch the original and purified CNT under 350?and air conditions,diameter of the pores on the CNT wall was in accordance with the particle size of Co₃O₄.However,due to there were no uniformly distributed loading sites for catalyst on the two kinds of CNT substrate,it is difficult to coordinate quantity,distribution and diameter of the pores on tube walls.Thus,the obtained CNT/S could only enhance 200 mAh/g under the rate of 5C.On the basis of the results above,this thesis put forward by using the kind of CNT as substrate which has been pre-treated via PMS/Co²⁺in a short time,utilizing the introduced oxygen groups,which are a few but homogeneous,to oriented the adsorption of Co²⁺.During the calcination process,Co₃O₄ particles with the size of sub-nanometer were produced,meanwhile,Co₃O₄ would targeted catalyze the reaction between O₂ and CNT and generate uniformly pores on the tube walls,i.e.“radical oxidation oriented-sub nanometer targeted etching method”.By adjusting the dosage of PMS during the preparation to optimize the distribution and quantitiy of oxygen groups,to regulate the size and distribution of Co₃O₄:under this conditions(2.40 mmol PMS,5 min pre-treatment,adsorption of Co²⁺and calcined on 350?for 6 h),synthesized CNT/S composite presented an initial discharge capacity of¹000 mAh/g under 5C.This method has got the advantages of lower weight loss of CNT?¹0 wt.%?and would prepared CNT/S with consistent electrochemical performance under mild conditions.However,the pores on tube walls would certainly increased contact interface between polysulfides and electrolyte,leading to sever dissolution of polysulfides and capacity decay.2.According to the established criteria for estimating function of anchoring polysulfides by calculation simulation,this thesis screened out and prepared esterification cross-linked binders with C=O and P=O separately named PVA-CA and PVA-ATMP,which further improved the cyclic performance of CNT/S composite.Firstly,the dissolution probability of polysulfides in ether-based electrolytes was evaluated by a defined solvation potential?P_S-C,which confirmed that Li₂S₈ is the most soluble species;the competition of Li₂S₈ anchoring strength on different substrates and its solvation energy was described by a stabilizing potential?P_S-A,which can be used to verify if a certain substrate can effectively stabilize polysulfides in cathodes.By fast screening of a series of polymers,we found that polyethylene glycol?PEG?,polypyrrole?PPy?,polyvinyl alcohol?PVA?and polyaniline?PANI?could not absorb polysulfides,only polyvinyl pyrrolidone?PVP?and phosphorylated chitosan?PCS?were predicted to be a promising third-party substrate to preserve polysulfides in cathodes and prevent them from being dissolved.According to the calculated results,the type of groups on the substrates is crucial factor for anchoring polysulfides,moreover,C=O and P=O could render strong affinity to polysulfides.Based on the theoretical prediction above,we selected PVA as the substrate,by esterifying with CA and ATMP to prepare the cross-linked network binder with C=O and P=O groups,separately.Using ATMP as esterifying agent,the binding effect of synthesized PVA-ATMP could not reach to that of PVA.Under the rate of 0.5C,CNT/S cathode,which use PVA-ATMP as binder,had an initial discharging capacity of 996.6mAh/g,lower than the cathode of PVA,but the capacity retention were largely enhanced from 25.6%to 76.1%.As for regulating the temperature and reagent ratio of esterification reaction,120?rendered the highest esterification degree;the more dosage of CA,the more tightly combined of CNT/S particles and larger of the intervals between the particles.The obtained PVA-CA sample in PVA:CA=3:2 or 9:1 showed similar binding effect with PVA.Under the rate of 0.5C,PVA-CA?9V1?120 showed an initial performance of 1151.2mAh/g and the capacity retention was 74.4%.Particularly,when increased the coating weight to 3.0 mg,CNT/S could also present 987.5 mAh/g at 0.5C and maintain the retention of 74.5%.These results represent that,the dual function of“physical resisting”and“chemical anchoring”of cross-linked binder could render CNT/S a stable cyclic performance with high coating weight.The compact degree and types of groups of this cross-linked binders,were the key factors to impact the cyclic performance.However,the discharging performance under high rate was not satisfactory,controlling the compact degree of the cathode via using polyacid with different length of carbon chain is the next step to obtain a sulfur cathode with superior cyclic life no matter how the discharging rate varies.