Study On Interfacial Construction And Electrochemical Properties Of "Solid-Solid" Reaction Of Sulfur Cathode

Lithium-sulfur（Li-S）batteries are considered to be one of the promising next-generation energy storage systems due to their high theoretical energy density(2600Wh kg^-1),as well as the cost-effectiveness of sulfur.However,sulfur cathodes usually react with lithium ions under a“solid-liquid-solid”mechanism in conventional ether-based electrolytes,and the solid phase sulfur would be transformed into a highly soluble lithium polysulfides.The dissolution of lithium polysulfides in electrolytes not only leads to the so-called shuttle effect,but also results in irreversible loss of active material and corrodes lithium anodes.Therefore,developing a“solid-solid”reaction mechanism that does not involve the generation of soluble lithium polysulfides is particularly important.This thesis mainly focused on the development of high-performance Li-S batteries.In this thesis,“solid-solid”reactive sulfur cathodes were successfully constructed,and the fading mechanism was revealed.Meanwhile,a high lifespan capacity of sulfur can be released by adjusting the operation procedure.The main contents and results are as follows:1.A cathode electrolyte interface（CEI）was in-situ formed on the surface of sulfur cathode during the first activation process by using a carbonate-based electrolyte with high-concentration dual-salt electrolyte.The CEI only allows the reversible insertion/de-insertion of lithium ions,thereby effectively inhibiting the dissolution of lithium polysulfides and eliminating the shuttle effect.The“solid-solid”reaction of the cathode with 65 wt%sulfur content is achieved,and the cellexhibits a specific capacity of871 m Ah g^-1 after 100 cycles at a rate of 0.1 C.2.To improve the ionic conductivity and reduce the cost of high-concentration electrolytes,diluent solvent of hydrofluoroether is adopted to prepare a localized high-concentration electrolyte.The introducing of diluent not only reduces the dosage of expensive of lithium salt and costs,but also increases ionic conductivity.It is found that a stable CEI is formed on cathode with this localized high-concentration electrolyte,which enables a“solid-solid”reaction of cathode with a sulfur content of 70 wt%.In addition,the growth of lithium dendrites can be suppressed.Therefore,the Li-S battery based on localized high-concentration electrolyte can deliver a stable cycling performance over 340cycles at a rate of 0.5 C.3.In order to further study the decay mechanism of sulfur under“solid-solid”reaction,the relationship between sulfur content and cycle life of cells was explored.When the volume of reduction products（Li₂S₂/Li₂S）is larger than the maximum volume of hosts（excessive sulfur content）,the as-formed CEI would fracture because of the volume variations.The repeated destruct and repair of CEI further lead to continuous consumption of electrolytes and active materials,thus accelerating the degradation of electrochemical performance.Hence,in order to achieve prolonged cycle stability under the“solid-solid”mechanism,the volume of reaction products and the interior space of hosts should be well matched.Based on the above understanding,by optimizing the sulfur content,the well-designed sulfur-based full cell shows an excellent cycling stability over2000 cycles.4.The optimization of working mode to achieve a high total capacity of Li-S batteries throughout the cycle life while maintaining long cycle life is of great importance.Herein,by investigating the relationship between cycle life and depth of discharge,it is found that the cycling life can be prolonged as the discharging depth decreases,and there exists a relationship similar to the normal function distribution between the total capacity released during the lifespan and the limited discharge specific capacity.When the limited discharge specific capacity is limited to 300 m Ah g^-1,the total capacity released throughout the lifespan is highest(289 Ah g^-1,based on sulfur).Besides,it is also proved that when CEI was stable,the main factor limiting the capacity degradation is the corrosion of lithium metal anodes.