Controllable Construction And Characteristics Of Flurinated Solid Electrolyte Interface For Lithium-metal Batteries

Currently,electrochemical storage systems with high energy density and safety are urgently needed to meet the increasing energy demand of electric vehicles and consumer electronics.Lithium metal,which is abundant in nature and inexpensive,has high theoretical capacity(3860 m Ah g^-1)and low electrochemical potential（-3.04 V,vs SHE）,and is considered as a promising anode material for rechargeable batteries.However,there is no nucleation site on the surface of Li metal,resulting in uneven flux of Li ions and random nucleation and deposition of Li,thus forming mossy or dendrite Li dendrites.Secondly,Li metal has active chemical properties,when contacting with liquid electrolyte,the electrolyte components are highly susceptible to reductive decomposition on its surface,thus forming a fragile solid electrolyte interface（SEI）.During the Li plating/stripping process,the SEI film continuously breaks/reorganizes,consuming a large amount of electrolyte and causing the uncontrolled growth of Li dendrites,which can pierce the diaphragm and cause direct contact between the cathode and anode materials,thus causing a series of safety problems such as battery fire and explosion.To solve these problems,firstly,this project constructed an artificial mixed ion alloy layer on the surface of Li metal,and explored its guiding effect on uniform deposition of Li.Secondly,gel polymer electrolyte was prepared by in-situ polymerization to explore its inhibition of Li dendrite growth and improvement of electrochemical performance.In addition,fluorine-containing gel polymer electrolyte was obtained by free radical polymerization to explore its guiding effect on Li deposition behavior and improve battery cycle performance.The main research contents and results of this paper are as follows:（1）Taking the interfacial modulation of the Li metal anode as an entry point,we pretreated the Li anode surface by immersing the Li metal in a solution of ethylene glycol dimethyl ether/fluoroethylene carbonate（DME/FEC）dissolved with magnesium fluoride（Mg F₂）to form an artificial mixed ionic alloy layer of SEI composed of Li F and Mg F₂.Starting from the content of Mg F₂,we found that the 35μm thick interfacial layer（M-3@Li）formed on the Li metal surface using 3%Mg F₂ solution was the most effective for the protection of the Li metal anode,which could effectively isolate the Li metal from the electrolyte and inhibit its interfacial side reactions.The electrochemical tests show that the Li||Li cell with M-3@Li as the working electrode has the best electrochemical performance with a stable cycle of 1500 h at 0.5 m A cm^-2 and a capacity of 1 m Ah cm^-2 compared to bare Li,when paired with lithium iron phosphate cathode（Li Fe PO₄）,M-3@Li as the working electrode can be stably cycled for 200 cycles with a capacity retention of 93.75%and a coulombic efficiency of 99.8%.In the Li||Li Fe PO₄ cell test,the specific capacity of 119.06 m Ah g^-1 was maintained at 5 C,with a coulombic efficiency of 98.9%.（2）To improve the stability of the Li metal surface interface,reduce the problem of unstable SEI formation due to the chemical decomposition of liquid electrolyte on its surface,and solve the problem of its flammable omission.We obtained a quasi-solid gel polymer electrolyte（GPE）by in situ polymerization,while the incorporated FEC can form a Li F-rich interfacial protective layer on the Li metal surface,which is used to inhibit Li dendrites to protect the Li metal electrode.The morphological characterization of SEM reveals that GPE@FEC effectively inhibits the growth of Li dendrites,where the addition of FEC plays a key role in the construction of dense and uniform SEI and fast interfacial charge transport.Further electrochemical performance characterization revealed that GPE@FEC exhibited high oxidation stability up to 5.02 V（vs Li/Li⁺）and ionic conductivity up to 7.81×10^-4 S cm^-1 at room temperature,in addition to Li⁺mobility number up to0.71.The Li||Cu cell with GPE@FEC as the electrolyte can be stably cycled for 400 cycles at 0.5m A cm^-2 and a capacity of 1 m Ah cm^-2,when the Li||Li Fe PO₄ cell is assembled by pairing GPE@FEC as the electrolyte with Li Fe PO₄,the Li||Li Fe PO₄ cell can be used at a high current 2 C(1 C=170 m A g^-1),after 1000 cycles,the discharge specific capacity is as high as 102 m Ah g^-1,and a capacity decay rate of only 0.02%per cycle.（3）To investigate the safety and electrochemical properties of other types of gel polymer electrolytes,we use the polyethylene glycol monomethyl ether acrylate（PEGDA）as a monomer and dimethyl ester carbonate（DMC）/1,2-dimethylethane（DME）as a co-solvent,which was thermally initiated by AIBN to generate a gel polymer electrolyte（DMC/DME/PEGDA）.Starting from the content of PEGDA,and the optimal component ratio was determined.Based on this,a fluorinated gel polymer electrolyte（DMC/DME/PEGDA+10%FEC）was obtained by adding FEC,which plays a key role in the construction of dense and uniform SEI and fast interfacial charge transport.After electrochemical performance characterization,its electrochemical window can be as high as 5.2 V（vs Li/Li⁺）with a room temperature ionic conductivity of 2.44×10^-4 S cm^-1.By further morphological characterization of the material,the electrolyte was found to be effective in inducing the uniform deposition of Li and thus inhibiting the growth of Li dendrites.When Li||Li cells using DMC/DME/PEGDA+10%FEC as the electrolyte possess an ultra-low overvoltage of45 m V at 0.1 m A cm^-2.As the current density increases to 0.5 m A cm^-2,the cell can be stably cycled for more than 1300h.