Fabrication And Electrochemical Performance Of Novel Electrode Materials And Separator-modified Materials For Alkali-metal Batteries

With the rapid development of the global economy,the shortage of fossil fuels and the deteriorating environmental pollution have become a threat to mankind.Thus,to exploring a clean,safe and sustainable green energy sources is extremely urgent.Wind,solar and tide power are considered as promising candidates to replace traditional fossil fuels.However,the practical application of them is severely restricted by their dependent on weather feature and low storage efficiency.Satisfactorily,electrochemical energy storage provides a feasible way for the storage of these energy sources.Among the various energy storage systems(ESSs),lithium-ion batteries(LIBs)and room-temperature sodium-sulfur(RT-Na/S)batteries are receiving massive interest.Although the tremendous development,LIBs and RT-Na/S batteries still have some problems to be solved.The practical application of RT-Na/S batteries is plagued by several intrinsic challenges:1)The insulating nature of sulfur and its discharged products;2)The huge volume variation of element sulfur;3)The shuttling behavior of the soluble sodium polysulfides(SPSs).In order to meet the demand of increasingly updated ESSs,it is necessary to develop new electrode materials on the basis of commercial LIBs.On the other hand,to develop and design RT-Na/S batteries with low cost,high energy density and good cycle stability is in more and more urgent demand.This paper mainly focuses on the development of new anode materials for LIBs as well as the cathode materials and functional separator of RT-Na/S batteries,including the following aspects:(1)Transition metal oxides show great promise as high-energy anodes for lithium-ion batteries(LIBs),thanks to appealing combination of high theoretical specific capacity and low-cost.However,they still undergo dramatic volumetric expansion and low electronic/ionic conductivities,which leads to numerous problems,for instance,rapid capacity degradation and electrode pulverization,and thus severely hindering their practical applications.In this paper,a designed hydrangea-like microstructure consisting of MnO₂ nanosheets and ZnFe₂O₄ microspheres is achieved by a hydrothermal route.When evaluated as an anode material for LIBs,ZnFe₂O₄@MnO₂ electrode displays a high specific capacity of 2707 m A h g^-1 after100 cycles at 0.2 A g^-1.Even at a higher current density of 2 A g^-1,the electrode has a long lifespan with a specific capacity of 1458 m A h g^-1 after 800 cycles,and outperforms the previously reported zinc ferrite composite electrodes.These excellent electrochemical properties are ascribed to the hydrangea-like structure,which buffers the volume variation of ZnFe₂O₄ during charging/discharging process,and decreases the internal resistance significantly due to excellent contact between ZnFe₂O₄microparticles and MnO₂ nanosheets.Consequently,the facile synthesis strategy and superior Li-storage performance make the hydrangea-like ZnFe₂O₄@MnO₂microspheres the promising candidate for next-generation high-performance LIBs in the future.(2)Room-temperature sodium/sulfur(RT-Na/S)batteries are of considerable interest for next-generation energy storage systems because of the earth-abundant electrode materials,low cost,and high energy density.However,the widespread application of RT-Na/S batteries is severely hindered by dissolution,shuttling,and sluggish kinetics conversion of sodium polysulfides(SPSs)during the electrochemical processes.Herein,we prepare a powerful functional separator to suppress shuttling effect by directly coating 2H-MoSe₂/nitrogen-doped hollow carbon spheres/graphene oxide(2H-MoSe₂/N-HCS/GO)onto standard glass fiber separator.The rational design of the functional separator effectively suppresses the migration of SPSs and promotes conversion reactions of SPSs.Meanwhile,the N-HCS decorated by the few-layer 2H-MoSe₂ nanoflakes are used as sulfur host.As verified by a series of the density functional theory calculations and experimental characterization,a RT-Na/S battery with a functional separator and high sulfur content of 71.4 wt%delivers a high discharge capacity of 787 mAhg^-1 at 0.1 C after 100 cycles,and exhibits the outstanding long-term cycling stability(484 mAhg^-1 at 0.5 C after 500cycles),with a low capacity fading rate of 0.077%per cycle.This study demonstrates an effective strategy to develop the functional separator of shuttle suppression and provide a promising path for high-performance RT-Na/S batteries.(3)The 5.8 trillion cigarettes are smoked each year all over the world,which will producing more than 800,000 metric tons of discarded cigarette butts.Discarded cigarette filters is non-biodegradable,thus it will produce a mass of waste disposal and cause environmental pollution issue.For the purpose of transforming waste into wealth and reducing environmental pollution,nitrogen and sulfur co-doped carbon nanofiber/carbon black(N,S-CNF/CB)composites derived from discarded cigarette filter is employed to modify glass fiber(GF)membrane for the first time in this study.Meanwhile,carbon black/sulfur(CB/S)composites are used as cathode of RT-Na/S battery.Non-polar carbon black can limit the dissolution of soluble intermediate sodium polysulfides in the liquid electrolyte by physical adsorption to some extent.N,S-CNF can improve the binding ability of polysulfide by chemical adsorption.This kind of functional glass fiber separator(N,S-CNF/CB+GF)can effectively inhibit the dissolution and shuttling of negatively charged polysulfide anions without affecting the migration of sodium ions.The experiment and DFT calculation results demonstrate that RT-Na/S cells fabricated with N,S-CNF/CB+GF separator and CB/S cathode exhibit enhanced electrochemical performance.After 100 cycles at 0.1C(1C=1672 m A g^-1),the N,S-CNF/CB+GF cell with a sulfur mass fraction of 71 wt%delivers a high reversible specific capacity of 702.5 mAhg^-1.In addition,a high capacity of 527.4 mAhg^-1 can still be maintained at a high current density of 0.5 C after 900 cycles with a very low capacity fading rate of 0.035%per cycle,which demonstrates excellent cycling stability and reversibility.