The Energy Storage Performances Of Lithium Sulfur Battery Using Lean Electrolyte

Electrochemical energy storage is one of the most efficient forms of energy storage in terms of energy conversion.Comparing the theoretical values of of various devices,it can be found that lithium-sulfur battery（LSB）is attractive with their theoretical specific capacity of 1675 m Ah g^-1 and energy density of up to 2600 Wh kg^-1.However,the operational specific capacity and energy density of LSB,currently,do not meet the theoretical values.Futhermore,most reports that present relatively good operational performance on energy storage are based on the LSB with flooded electrolyte(i.e.,the ratio of liquid electrolyte to active sulfur（E/S）>10μL mg^-1),in which,electrolyte accounts for～70 wt.%of the total mass.Electrolyte is not the active substance that directly contributes to capacity,so it is crucial to conduct LSB with lean electrolyte or even solid-state electrolyte to optimize the overall capacity and energy density.Decrease of the liquid electrolyte dosage to lean or even to zero（solid state）renders the sharp reduction of the macroscopic electrolyte-wetted interfaces,thus resulting in reaction delay and ions diffusion retardation.Given such problems,this thesis puts forwards the means of the construction and functional regulation of the interfaces in the limited and wetted space,aiming to increase the effective reaction area.Then,based on the three strategies,including the design of rich defect and highly active reaction sites,the construction of micro-reaction units,and the gradient distribution of functional components,lean electrolyte even solid-state LSBs are developed.Such LSBs specilize in the reaction enhancement of lithium PS conversion and diffusion reinforcement of Li⁺/polysulfide（PS）transfer.As a result,the electrochemical performances are improved.（1）Enhancing lithium PS conversion by defect-rich and highly reactive reaction sitesThe Mo P/Mo S₂@C heterojunction was synthesized and modified on the cathode side of the PP separator.The study found that the heterojunction（I）improves the wetting ability of the ether electrolyte into the modified separator;（II）forms numerous chemical adsorption sites with lithium PS via the chemical bonds of P-Li,PS,Mo-S,etc.;（III）catalyzes lithium PS conversion by its abundant Lewis/Bronst acid and S-defect sites.Mo P/Mo S₂@C heterojunction with both“lithophilic”and“sulfurophilic”zones enables it to exert high catalytic activity even under the lean-electrolyte condition(E/S=7μL mg^-1,only 1/3 of the conventional electrolyte dosage in coin cells).Due to the accelerated lithium PS reduction,lean-electrolyte LSBs run well.（2）Enhanceing mass transfer to facilitate short-chain Li₂S_n（n≤4）conversionThe short-chain Li PAA is implanted into the sulfur cathode to construct abundant micro-reaction units.The study found that the Li⁺cation of Li PAA plays the role of in-situ Li replenishment.The abundant C-O/C=O groups in the anion PAA^-have affinity for electrolyte.Besides,such groups guide PS targeting transfer to the wetted interfaces,promoting mass transfer in lean electrolyte.Li PAA containing 0.3 wt.%Li⁺and 2.8 wt.%O owns the ability to shorten the ion transfer path（from handredsμm to severalμm level）and to inhibit the lithium PS shuttling.Studies have confirmed that the enhanced mass transfer by Li PAA and the enhanced reaction by Mo P/Mo S₂@C have a synergistic effect,which markedly reduces the charge-transfer resistance of short-chain Li₂S_n（n≤4）and increases the rate,as well as reversibility,of the corresponding electrochemical reaction（reduction/oxidation）.So,the electrochemical performance of the lean electrolyte LSB has been significantly improved.（3）Accelerating energy storage of solid-state LSB by polymer solid electrolyte with gradient distribution of functional componentsRegarding to the different reaction mechanism between solid-state lithium-sulfur batteries（SSLSB）and liquid LSB,polymer solid electrolyte with a gradient distribution of functional components（Grad-SPE）p-PPm CMo Li is reasonable designed and controllable constructed due to the further deterioration of mass transfer in SSLSB.Microdomains near the cathode/Grad-SPE interfaces are rich in Li TFSI and Mo P/Mo S₂@C heterojunctions.These two components provide the catalytic active sites and reactants（Li⁺）required for the LSB reaction to store energy.Colsed to the anode/Grad-SPE interfaces,the high content of PMMA induces the uniform deposition of Li⁺.Super-P is distributed in the PAN-polymer electrolyte to enhance the flexibility of the base membrane.Thanks to the gradient distribution of functional components,the Grad-SPE electrolyte achieves a 300%enrichment of sulfur species within the cathode-side microdomains.Li⁺in Li TFSI exists in a free state,which makes up for the Li lack due to the vicious consumption of Li⁺in the redox reaction.Therefore,Grad-SPE promotes the improvement of reaction kinetics in the ways of increasing the active site and reactant concentration and taking mass transfer into account.The cycling life of SSLSB is prolonged（in 450 cycles,the decay rate is 0.079%/cycle）.