Design,Synthesis And Characterization Of Novel Electrolytes For Magnesium/Sulfur Battery

Magnesium(Mg)is a promising metal anode for multivalent batteries.It can afford nearly twice the volumetric capacity than that of lithium(3833 mA h cm-3 vs.2062 mA h cm-2)and possesses several unique characteristics.For instance,Mg is a low-cost(?$2700/ton vs.Li?$64000/ton)and abundant material.The electrochemical plating of Mg is a much safer process that is not troubled by the dendrite growth.Therefore,it significantly improves the operation safety compared with lithium anode.The combination of Mg and conversion-based cathode,for instance,sulfur,has been expected to yield promising battery systems that utilize nature abundant resources and potentially realizes lightweight and nontoxic electrochemical energy storage.The merits of Mg/S battery also include a high theoretical volumetric energy density(3260 Wh L-1)superior to that of Li/S battery(2800 Wh L-1).Unfortunately,the development of Mg/S battery is still in its infancy due to a lack of a proper electrolyte which has the advantages of compatibility with Mg and S,wide electrochemical window,high ionic conductivity,simple synthesis process and low cost,simultaneously.Therefore,finding strategies to improve the performance of present electrolytes or developing novel electrolytes for Mg/S batteries is critical.Based on the application requirements of the electrolytes for Mg/S battery,we have mainly carried out the following studies:(1)LiCl additive was proposed to improve the performance of Mg anode.In contrast to organometallic electrolytes,all-inorganic electrolytes based on MgC12-AIC13 complexes are more cost-effective and hold better stability to air and moisture.A recently developed electrolyte that contains tetrahydrofuran(THF)solvated divalent Mg cation,[Mg·6THF][AlC14]2,has exhibited decent compatibility with the sulfur cathode,however,it suffers a large overpotential and short cycle life,which hinders its applications in Mg/S batteries.We successfully realize the efficient plating/stripping of Mg by using LiCl additive in[Mg·6THF][AlCl4]2-based electrolyte.Both experiments and DFT calculations reveal that the LiCl assisted solubilization of MgCl2 facilitates the exposure of fresh surface on Mg anode.Utilizing such a LiCl-activation strategy,we successfully achieve Mg/S full batteries with a significantly extended cycle life of over 500 cycles as well as a coulombic efficiency close to 100%.(2)Synthesis of a low-cost and highly efficient electrolyte MgC12-LiCl/THF.The development of low-cost and highly efficient non-nucleophilic electrolytes plays a significant role in the progress of rechargeable Mg/S batteries.A new-type all-inorganic electrolyte by dissolving MgCl2 and LiCl salts in THF solvent has been firstly developed.The MgC12-LiCl electrolyte features a large electrochemical window up to 3.3 V(vs.Mg/Mg2+)and a remarkably reduced overpotential during Mg plating/stripping process(±40 mV at 0.5 1A cm-2).The chemical interplay between MgC12 and LiCl was analyzed by Raman and nuclear magnetic resonance spectroscopy.By taking advantage of this novel electrolyte,Mg/S full cell exhibits a superior discharge specific capacity over 1100 mA h g-1 for over 500 cycles at 0.1 C(1 C=1675 mA g-1).It also demonstrates a high rate performance with a specific capacity of 800 mA h g-1 at 2 C.(3)Synthesis of a conditioning-free,low-cost and highly efficient electrolyte[Mg2C13(THF)6][HMDSLiCl](HMDS=hexamethyldisilazide).The conditioning-free and low-cost non-nucleophilic electrolytes are urgently needed for rechargeable Mg/S batteries.The all-inorganic electrolytes based on MgC12-AIC13 complexes is the most cost-effective,while only after a long period of electrochemical activation,the Coulomb efficiency of Mg deposition/dissolution in this electrolyte can reach 100%.Using a reaction between MgC12 and HMDSLi,a conditioning-free,low-cost and highly efficient electrolyte[Mg2Cl3(THF)6][HMDSLiCl]was synthesized.This electrolyte shows a high Coulombic efficiency of nearly 100%at the first cycle of Mg plating/stripping.By taking advantage of this novel electrolyte,Mg/S full cell exhibits a superior discharge specific capacity over 1000 mA h g-1 for over 40 cycles.(4)Synthesis of a gel-type polymer electrolyte[magnesium cholride-(fluorinated tetraethylene glycolic)borate](MgCl-FTGB)for Mg/S battery.To further improve the safety and flexibility of Mg/S battery,developing a non-liquid Mg electrolyte is critical.Using a cross-linking reaction of magnesium chloride borohydride(MgClBH4)and perfluoro tetraethylene glycol(FTG),we synthesized a non-nucleophilic gel-type electrolyte.This gel electrolyte shows a high magnesium ion transfer number(0.733)at room temperature,a wide electrochemical window(up to 4.8 V vs.Mg/Mgr,),and a remarkable cycling stability with Mg anode.Mg/S battery assembled with this electrolyte can cycle 50 times with a high specific discharge capacity retention of over 1100 mA h g-1.