Preparation Of BCN-based Nanocomposites And Their Applications In Lithium-Sulfur Batteries

Lithium-sulfur(Li-S)battery stands out from the new generation of energy storage devices for its ultra-high theoretical specific capacity(1672 mAh g-1)and energy density(2600 Wh kg-1).In addition,as the cathode material of Li-S battery,the elemental sulfur has the advantages of abundant reserves,environmental friendliness and cost-effectiveness,which are more advantageous than the current widely used cathode material for Li-ion battery.Therefore,Li-S battery is regarded as one of the most competitive candidates for the next generation of rechargeable batteries.However,the sulfur cathode is still limited by the low conductivity of elemental sulfur,large volumetric expansion issue during cycling process,and the shuttle effect of lithium polysulfides(LiPSs),which result in low sulfur utilization and poor cycling performance of Li-S battery.All the problems mentioned above severely hamper the usage and the commercial development of Li-S battery.In order to solve the above problems in Li-S battery,based on the most widely used carbon-based material,this thesis focuses on the poor restriction of carbon-based material on shuttle effect of LiPSs.We have designed and prepared a series of BCN-based nanocomposites as the efficient sulfur carrier materials by optimizing and improving the BCN materials.Furthermore,their electrochemical properties were studied and the mechanisms of BCN-based nanocomposites in improving the performance of Li-S battery were explored.The main research contents are as follows:(1)BCN nanotubes(BCNNTs)materials were prepared by a simple solid-phase reaction.The electrochemical properties of BCNNTs as sulfur-carrying material in Li-S battery were studied,and the mechanism of action with LiPSs was analyzed.The results of XRD,Raman,SEM and TEM confirmed the successful preparation of BCNNTs with a tubular structure,which laid the foundation for the mass storage of sulfur and relieved the volumetric expansion during the cycling process.N2 isothermal adsorption and desorption test showed that the specific surface area of the material was 249.3 m2 g-1,which can provide more active sites for the adsorption of LiPSs.Theoretical calculations and XPS results proved that the shuttle effect was greatly inhibited by BCNNTs through strong chemical interaction with LiPSs.The Li-S batteries based on BCNNTs/S electrodes demonstrated excellent electrochemical performance.The initial specific capacity at 0.2 C was 1233.0 mAh g-1 and at rather large 1C rate it maintained 619.6 mAh g-1 after 1000 cycles,the average capacity attenuation per cycle was only 0.041%.(2)In order to further enhance the restriction on the shuttle effect of LiPSs,a Ga2O3/BCNNTs hybrid material was established by solid-phase synthesis on the basis of high-conductivity BCNNTs.It was served as the carrier material for sulfur in Li-S battery for the first time to study its electrochemical properties and the mechanism of LiPSs adsorption was explored.Morphology and structure characterizations proved the successful combination of Ga2O3 and BCNNTs and the structure of the nanotube was preserved.This unique structure could alleviate the volume changes during the cycling process.The LiPSs adsorption experiments and XPS results showed a strong chemisorption effect performed by Ga2O3/BCNNTs,effectively inhibited the dissolution of LiPSs in the electrolyte.The Ga2O3/BCNNTs/S composite electrode exhibited a high specific capacity and good cyclic stability by the synergy of Ga2O3 and BCNNTs.It delivered a high specific capacity of 914.1 mAh g-1 after 200 cycles with a capacity retention rate of 80%and a Coulomb efficiency of 99.5%.(3)The V2O3/BCNNTs composite material used as sulfur carrier was designed and synthesized by a simple solid-phase reaction process.Not only the charging/discharge performance,but the interaction between the V2O3/BCNNTs and LiPSs was analyzed as well.SEM and HRTEM results confirmed the tight interface between BCNNTs and V2O3 which ensured the large active space for LiPSs adsorption and transmission of Li+.EIS and CV test analysis results showed that the conductive network of the BCNNTs modified the disadvantage of the rather low conductivity of V2O3 and enhanced its electrochemistry dynamics.The galvanostatic charge/discharge test showed the V2O3/BCNNTs/S electrode had a very high active material usage and discharge capacity.After cycled at 0.2 C for 200 times,it still delivered a discharge specific capacity of 1018.3 mAh g-1 and a capacity retention rate of 74.5%and a Coulomb efficiency of 99.6%.Combined with the XPS,DFT and the symmetric battery test,it was found that V2O3/BCNNTs composite material could largely release the shuttle effect by the strong chemical adsorption and catalytic conversion of LiPSs.(4)To optimize the experimental procedure and further analyze the mechanism of BCN-based nanocomposites with LiPSs,VN/BCNNTs composite materials were directly synthesized by a simple one-step heat treatment using urea as the nitrogen source,and then its electrochemical performance as the sulfur carrier material was studied.This simple and low-cost method avoided the use of high-pressure ammonia gas in the conventional nitride synthesis process,and the safety in the material preparation is guaranteed.Morphology characterization showed the in-situ growth of VN possessed a rather small diameter which could provide much more active sites for the electrochemistry reaction.CV and GITT results showed that VN/BCNNTs/S electrode prepared based on VN/BCNNTs had an enhanced kinetic behavior.The Li-S battery with VN/BCNNTs/S electrode exhibited outstanding long cycling and rate performance.After 1000 cycles at a current density of 1.0 C,the discharge specific capacity could still reach 826.9 mAh g-1.The average capacity loss was just 0.021%per cycle,and the Coulomb efficiency more than 99.5%.Theoretical calculations and XPS results showed that VN exerted a strong chemical restriction on LiPSs.Simultaneously,symmetrical cell and Li2S nucleation deposition test indicated that highly conductive VN/BCNNTs greatly promoted the catalytic conversion of LiPSs.