Study On The Design And Performance Of Cathode For Alkali Metal-Sulfur/Selenium Batteries

The development of science and technology,as well as human life cannot be separated from the consumption of energy.However,fossil fuels cause global warming,environmental pollution and other problems.In order to reduce the dependence on fossil energy,it is imperative to develop green and clean energy.Clean energy sources such as solar and wind power are intermittent and random in nature,which require the development of sustainable energy storage systems to collect and store them.As an effective energy storage and conversion device,secondary batteries play an important role in the development of energy storage systems.However,traditional cathode materials for secondary batteries are limited by the capacity,which makes it difficult to meet the increasing demand for high specific energy materials.The exploitation of cathode materials for secondary batteries with high specific capacity has become a focus in the field of next-generation energy storage.Among many emerging energy storage materials,sulfur(S)is favored due to its high theoretical gravimetric capacity(1675 m Ah g-1).Meanwhile,sulfur also possesses the advantages of high natural abundance,environmental friendliness and non-toxicity.The selenium(Se)and sulfur are elements of the same family,and similar in nature.The energy storage mechanism of them is a conversion reaction involving multiple electrons.Although the theoretical gravimetric capacity of selenium(675 m Ah g-1)is lower than that of sulfur,selenium still possesses the advantages of high theoretical volumetric capacity(3254 m Ah cm-3)and considerable electronic conductivity,which makes it comparable with sulfur.Despite the obvious capacity advantages,the development of sulfur and selenium cathodes still faces many challenges.The low electronic conductivity of the activeVmaterials and their discharge products during electrochemical processes leads to low utilization of the active materials.The intermediate product polysulfide/polyselenide during the electrochemical reaction is soluble in the ether-based electrolyte and shuttling between the cathodes and anodes of the batteries,resulting in the loss of active materials.Moreover,the drastic volume change during charging and discharging processes leads to cracking and pulverization of the electrode.In this thesis,to address the problems in lithium-sulfur batteries,Ptnanoclusters confined in hollow porous carbon spheres are prepared as catalyst to boost the conversion reaction in this dissertation.The design of conductive cross-linking binder enables the preparation of sulfur cathode free from the conductive additives,and high sulfur loading and volumetric capacity output are realized.To address the problems in selenium cathodes,different carbon materials are prepared for loading selenium to limit the domain of the active substance selenium and enhance the conductivity of the electrodes through the pores.Additionally,ionic cross-linking binder is designed for stabilizing the electrode structure of selenium cathodes.The main findings of this thesis are as follows:(1)Study on application of platinum metal nanocluster catalyst in sulfur cathode.Small-sized platinum nanoclusters(PtNC)are prepared and further confined to pores in hollow carbon spheres(HCP)to form PtNC@HCP as the cathode host for lithium-sulfur batteries.The hollow internal structure of HCP can be used for sulfur loading,and the small-sized platinum nanoclusters possess excellent catalytic ability to accelerate the polysulfide sulfide conversion and inhibit the shuttle effect.In addition,the hollow carbon spheres wrapped around the sulfur surface can support a good electronic conductive network and reaction path for Li+ ions.As a result,the excellent cycling performance and rate performance of lithium-sulfur batteries are obtained.(2)Study on Conductive cross-linking binder for the construction of sulfur cathode with high volumetric capacity.Sulfur and polyacrylonitrile are compounded to prepare sulfur polyacrylonitrile(SPAN)with low porosity as cathode materials for lithium-sulfur batteries and sodium-sulfur batteries.The SA@Ti3C2Tx conductive cross-linking binder is prepared by cross-linking the highly conductive two-dimensional layered Ti3C2 Tx MXene material with sodium alginate(SA)through hydrogen bonding,and the cross-linking effect brought excellent mechanical properties to the electrode,realizing the fabrication of robust electrodes with high S loading,and the excellent electronic conductivity of the binder enabled the electrode fabricating without the low-density conductive additives,which ultimately realizes the excellent areal capacity and volumetric capacity performance in lithium-sulfur and sodium-sulfur batteries.(3)Study on the construction of high areal loading selenium cathode by biomass-derived porous carbon synergized with ionic cross-linking binder.Biomass-derived porous carbon(BPC)is synthesized for loading selenium by potassium hydroxide etching method.BPC/Se is prepared as the cathode material by melt-diffusion method.Fe@SA cross-linking binder is prepared by ionic cross-linking reaction between iron ions(Fe3+)and sodium alginate(SA)for coating BPC/Se as electrodes for lithium-selenium batteries.The Fe@SA cross-linking binder can bring a stable structure to the electrodes,avoiding electrode rupture caused by the volume change of the active material during the charging and discharging process.A reversible areal capacity output of 4.9 m Ah cm-2 is realized under an ultra-high Se loading of 12 mg cm-2.(4)Study on application of graphitized graded porous carbon in selenium cathode.Graphitized hierarchical porous carbon(HPC)is synthesized by reducing carbon dioxide using magnesium-thermal reaction,and the resultant HPC/Se is used as the cathode material for sodium/potassium-selenium batteries.HPC possesses a graphitized structure with high conductivity,which effectively reduces the charge transfer resistance of the cathode.In addition,the hierarchical porous structure provides sufficient space to buffer volume expansion after loading selenium,which stabilizes the electrode structure.As a result,reversible capacity of up to 579 m Ah g-1for sodium-selenium batteries is realized.Meanwhile,a reversible capacity of 450 m Ah g-1 is obtained in potassium-selenium batteries.In this thesis,two methods,namely,cathode host material preparation and binder design,are utilized to enhance the electrochemical performance of the batteries in response to the problems in cathodes for sulfur/selenium batteries.In addition,key parameters such as areal capacity,volumetric capacity are tested and characterized,which fully elucidates the positive impact of these methods in enhancing the material performance.It has good reference value and significance for the further development of high-capacity cathodes for sulfur/selenium batteries with conversion reaction.