Research On Modified Cathode And Designed Structure For High Performance Lithium Sulfur Battery

Lithium-sulfur(Li-S)battery has high theoretical specific and high specific energy density.Besides,the element sulfur is naturally abundant,low-cost and environmentally friendly.Therefore,Li-S battery is considered as one of the most promising candidates for the next-generation energy storage systems.Nevertheless,the Li-S battery has its intrinsic problems:(1)poor electronic conductivity of sulfur and its discharged products;(2)large volume expansion during discharge and charge processes;(3)the dissolution and shuttle of lithium polysulfides(LiPSs);(4)the unstable lithium anode.Those issues result in low active materials utilization,poor rate performance,low coulombic efficiency and poor cycling stability,which seriously impede the practical commercialization of Li-S battery.To overcome the above mentioned problems,this thesis mainly focus on modifying sulfur cathode and designing cell structure to study their effects on the electrochemical reactions of Li-S battery.On the one hand,aiming at confining sulfur on the cathode,different sulfur hosts are designed,such as Prussian blue(PB),dual-doped hollow carbon,dual-doped carbon nanotube/carbon materials.On the other hand,aiming at adsorbing and recycling LiPSs,different cell structures are designed,such as constructing fibers on the cathode by electrospinning technology,inserting carbon nanotube papers(CNTp)on both cathode and anode.The specific research can be divided into the following 5 parts:(1)Based on surface chemistry,a new class of material is developed to trap LiPSs.Considering its polar Fe?-C-N-Fe? units,PB is chosen as a typical host for PB analogues to explore their potential application in Li-S battery.The results show that the addition of PB has a pronounced effect on improving the electrochemical reaction kinetics and facilitating the transfer of sulfur species.As a result,the dissolution of LiPSs in the electrolyte can be efficiently reduced.However,the PB nanoparticles lose the touch with sulfur particles after long cycling,leading to fast capacity fading.Therefore,only chemical bond is not enough to completely anchor sulfur on the cathode.(2)Confining sulfur with both physical confinement and chemical bonding.Based on the above research,nitrogen/oxygen dual-doped hollow carbon nanospheres(DHCSs)are designed as sulfur hosts.The results show that the prepared sulfur/DHCSs(S/DHCDs)cathode delivers enhanced active material utilization,rate performance and cycling performance.Further mechanism studies are investigated by the combination of experiment characterizations and theoretical calculation.It is found that the formation of S-O and S-C bonds during heating diffusion,assisting trapping sulfur species on the carbon matrix.Further calculation demonstrates the synergistic effect of dual-doped N/O,providing strong adsorbed energy with LIPSs.(3)In-situ confining sulfur with carbon materials.Based on the above result,the nitrogen/oxygen dual doped carbon nanotube/carbon(Li2S/CNT/C-N/O)composite is fabricated to reduce the diffusion of sulfur species from porous structure.It is found that the nano-sized Li2S is well imbedded in the CNT/C-N/O framework.As a result,the prepared Li2S/CNT/C-N/O cathode exhibits enhanced active material utilization,great cycling stability and rate capacity.In-situ electrochemical impedance spectra test is further used to get insightful understanding about the improvements.The result reveals that compared with Li2S cathode,Li2S/CNT/C-N/O composite exhibits decreased resistance and increased lithium-ion diffusion,especially at the initial charging stage of polysulfides nucleation.(4)Interlayer for the cathode.The above studies show that though modified sulfur cathode materials can effectively confine sulfur species,the sulfur loads on the cathode is limited.Therefore,based on the idea of adsorbing and recycling the solvent LiPSs,an ultralight interlayer composed of polyacrylonitrile(PAN)and nitrogen-doped carbon black(NC)fibers are directly constructed on the cathode by electrospinning technology.The results show that the PAN-NC coated sulfur cathode exhibits enhanced active material utilization,rate performance and cycling stability.A high initial discharge capacity of 1279 mAh g-1 is achieved at 200 mA g-1 with a reversible capacity of 1030 mAh g-1 remained after 100 cycles.Further investigation reveals that the PAN-NC interlayer possesses great structural integrity and polysulfides adsorption,which can efficiently trap the LiPSs and make them well reutilized.(5)A dual protection strategy for both electrodes.Based on the above research,a dual protection strategy is proposed by inserting two nanotube paper(CNTp)interlayers on both electrodes of Li-S cells.The results show that the Li-S cell with CNTp on both electrodes exhibits high active material utilization,rate performance and stable cycling periormance.Even at ultrahigh sulfur load of 12.1 mg cm-2,a high areal capacity of 12.6 mAh cm-2 is still achieved,which can maintain at 11.1 mAh cm-2 after 30 cycles.The effects of CNTp on both electrodes are further confirmed by the micro-zone current distribution and the COMSOL simulation.On the one hand,the CNTp can facilitate uniform current distribution on the sulfur cathode,contributing to enhance active material utilization.On the other hand,uniform Li-ion distribution and stable Li surface can be achieved after inserting CNTp.