Preparation And Electrochemical Performance Study Of Cathode Materials For Lithium(Copper) Sulfur Batteries

Rechargeable lithium-ion batteries have been the most readily available source for portable electronics and electric vehicles over the past few decades.However,its limited theoretical capacity(<300 Wh kg^-1)cannot meet the growing demand for higher energy density for large-scale electric vehicles and smart grid applications.In comparison,sulfur electrode has many advantages,such as rich resources,low price,high theoretical specific capacity,etc.,and has received extensive attention in the field of metal-sulfur battery research.Metal-sulfur battery can choose different metal materials（Li,Na,K,Ca,Mg,etc.）as the negative electrode,which also endows various metal-sulfur batteries with their own advantages in cost,cycle life and energy density.Although metal-sulfur batteries have many advantages,their commercial application still faces many inevitable technical challenges.First of all,the dissolution and shuttling of polysulfides lead to the loss of cathode active materials and the attenuation of battery capacity.Secondly,soluble polysulfides will also react with various metal anodes,leading to corrosion,passivation,and continuous consumption of electrolyte.In addition,different radii and valence states of metal ions also lead to significantly different reaction paths and kinetic rates of sulfur electrodes.Generally,to solve the above problems,through appropriate structural design,the introduction of different types of catalytic materials,including metal-based nanomaterials,metal compounds and heterostructures,to achieve high cycle stability and high chemical reaction rate of sulfur cathode and metal anode.However,due to the existence of some performance defects,a single electrode material can not fully meet the electrode performance requirements,so researchers attempt to composite multiple electrode materials to fully leverage the synergistic effects between different materials and methods,and complement the advantages among the materials to improve the overall performance of metal-sulfur batteries.Based on the above background,in view of the main problems existing in metal-sulfur batteries,this paper focuses on solving the problems of polysulfide dissolution and diffusion,electrode passivation of sulfur cathode and metal anodes,and slow electrochemical kinetics by designing three unique functional carrier materials.These carrier materials were combined with sulfur by melting-diffusion method,the structure/phase characterization and electrochemical performance analysis of the obtained electrode were carried out to deeply understand the synergy/complementarity between the components.The main contents are summarized as follows:1.In order to improve the utilization rate of sulfur,in the work of this chapter,we designed and constructed a lightweight and porous cellulose nanofiber-based carbon aerogel as a multi-functional cathode carrier material for lithium sulfur（Li-S）batteries.Due to the porous structure and good electronic conductivity of the three-dimensional interconnected graphene network,as well as the co-doping of N and S,the cellulose nanofiber-based cathode exhibits strong polysulfide anchoring capability and fast reaction kinetics of polysulfide conversion.Moreover,the hetero‐doped N/S sites are believed to strengthen polysulfide anchoring capability via chemisorption and accelerate the redox kinetics of polysulfide conversion.Benefiting from the above qualities,Li-S cells using NSCA@S cathode exhibit a high reversible capacity(788.8m A h g^-1 after 100 cycles at 0.1 C)and superior cycling stability（only 0.059%capacity decay per cycle over 1000 cycles at 1 C）.2.Through the research in the first chapter,it is found that mediating the conversion kinetics and suppressing the shuttle effect are of vital importance for the pursue of high-capacity and long-lifetime lithium-sulfur（Li-S）batteries.Herein,a chessboard like electrode is proposed to tackle these problems,where MXene nanosheets function as“chessboards”to provide a conductive skeleton for the rapid conversion of polysulfides to Li₂S and the sulfur-loaded mesoporous carbon nanospheres（S@MCS-Si O₂）work as“chesspieces”to carry polysulfides through physical confinement and chemical adsorption.A uniform Li₂S precipitation with a high capacity and a series of highly reversible solid-liquid/liquid-solid phase reactions are demonstrated by experimental results.Under this regard,the proposed S@MCS-Si O₂/MXene electrode displays excellent electrochemical performance,with an initial discharge specific capacity of 1303.6 m A h g^-1 at 0.1C and a small attenuation rate per cycle of 0.046%in 1000 cycles at 1C,which is more superior than that of S@MCS-Si O₂ and S@MXene contrast eletrodes.3.In the previous two works,it is found that whether through material design or structural modification,the dissolution and shuttling of polysulfides is still an unavoidable problem.To circumvent this issue,we propose an simple and effective pre-copper strategy to realize a high-durability anode-free Cu-S battery.The pre-copper strategy can effectively promote a stable metal dissolution/deposition,compensate for charge carriers and facilitate reaction kinetics during the subsequent process.As a result,the aqueous anode-free Cu-S battery when coupled with S-decorated porous Ti₃C₂（S-d-Ti₃C₂）exhibits excellent electrochemical performance,delivering a highly reversible capacity of 1805.4 m A h g^-1 in the initial cycle at 0.8 A g^-1,impressive cycling stability with 90.2%capacity retention over 800 cycles and ultralow polarization～0.08 V even at a high current density of 3.1 A g^-1.The findings obtained in this work could pave the way for the design of high-performance anode-free batteries,which fills the vacancy of the necessary metal anode,delivering merits in both cost and cycle life.In summary,this thesis has designed and synthesized three kinds of functional carrier materials with different structures and characteristics by using different methods.The feasibility of those sulfur host materials is fully explored,which provides new research insights for broadening our understanding of lithium-sulfur batteries and further promoting the development of other metal-sulfur batteries.