In Situ Construct Polymer/Inorganic Interface Layer For Lithium Metal Anode

Lithium（Li）metal is considered as one of the most potential anode materials for rechargeable batteries with high energy density due to its ultra-high theoretical specific capacity(3860 m Ah g^-1)and low redox potential.However,the uncontrollable side reactions and dendrite growth of Li metal electrode limit its cycle stability and arise the safety issue of battery.The construction of a stable Solid Electrolyte Interphase（SEI）on the surface of Li anode to achieve rapid and uniform lithium-ion transmission is considered to be an effective means to inhibit dendrite growth and side reactions,and improve the cycle stability and safety of Li metal.However,SEI generated spontaneously from Li metal and electrolyte components is difficult to accommodate to the huge volume deformation in the process of Li deposition and stripping,which leads to its continuous rupture and repair,resulting in electrolyte consumption,active lithium loss,and the increase of polarization voltage,which ultimately limits the application of Li anode.In this paper,the polymer/inorganic SEI layer（PVDF/Li NO₃/Li F）is constructed in situ on the surface of Li metal,which significantly improves the cycling stability of Li anode.SEI with double-layer characteristics can inhibit the interfacial side reactions of Li anode and alleviate the volume expansion and dendrite growth in the process of Li plating and stripping.Furthermore,it is found that by adding lithium salt to PVDF polymer solution and using the gelling reaction of the polymer,the above-mentioned bilayer film can be directly in-situ constructed on the surface of Li metal as a gel polymer electrolyte,which can be applied to solid Li metal batteries.The gel polymer electrolyte can avoid the drying and oxidation decomposition of the electrolyte caused by the poor of volatility and oxidation resistance of the traditional ether electrolyte,and show excellent electrochemical stability and cycling stability in a wide temperature range.The specific research content and results are as follows:（1）To solve the problem of dendrite growth and side reactions caused by the uneven distribution of SEI chemical components on Li anode,we have constructed a polymer/inorganic double-layer SEI structure（Polyvinylidene Fluoride（PVDF）/Lithium Fluoride/Lithium Nitrate Film,referred to as PLL Film）on Li metal surface in situ.The results show that Li NO₃ in PLL film is released slowly in ether（or ester）electrolyte during the battery cycling process,forming a local Li NO₃ concentration differential electrolyte structure at the early stage of the cycle,and ultimately forming SEI layer containing relatively high concentration of Li F/Li₂O/Li₃N on the surface of Li metal,which contributes to the rapid transmission of lithium ions.In addition,Li F can significantly improve the uniformity and mechanical strength of PLL film,which is conducive to slowing down the volume expansion and Li dendrite growth.In the process of constructing the PLL film in-situ,we have used ethylene glycol dimethyl ether（DME）as the solvent of PVDF,which avoids the side reactions between conventional polar solvents and Li metal.On this basis,the lithium symmetric battery with a double-layer PLL film has a cycle life of more than 6000 hours when the current density 1 m A cm^-2and the limited capacity is 1 m Ah cm^-2,which is 6 times that of the Li metal-symmetric battery without PLL film containing the same amount of Li NO₃（0.46 M）electrolyte,highlighting the importance of Li NO₃ concentration gradient electrolyte formed by PLL layer for the stability of Li anode.Similarly,the Li|Li Fe PO₄ battery,Li|NCM811 battery,and pouch cell with PLL film show excellent cycling stability and high capacity retention rate.The above results indicate that in situ PLL film can form a SEI layer with high ion transmission,avoid the formation of Li dendrite,and significantly improve the cycling stability of Li anode.（2）Based on the above work,aiming at the stability and safety of ether liquid electrolytes,lithium difluorosulfonimide（Li FSI）has been introduced into the DME solution of PVDF in the above study,and we have constructed it into a gel polymer electrolyte（DPLL electrolyte）by gelling reaction.Because polymers are intertwined with each other under certain conditions,the gel-like polymer electrolyte has a highly ordered network structure,which avoids the leakage of liquid electrolytes and can improve the safety of Li metal batteries.To construct a stable inorganic SEI in this system,we have added inorganic substances Li NO₃ and Li F into the gel polymer electrolyte,and constructed a SEI layer rich in nitride and fluoride in the cycle to improve the stability of Li anode interface.In addition,in-situ gel polymer electrolyte has a high redox voltage window（4.7 V）,also improves lithium-ion transmission and optimizes the interface contact of the gel electrolyte.When the current density is 0.5 m A cm^-2 and the area capacity is 0.5 m Ah cm^-2,the cycle life of the Li symmetric battery with DPLL electrolyte can reaches 3000 hours.At the same time,the DPLL electrolyte achieves stable cycling at high and low temperatures,and with pouch cell still has excellent stability after folding and cutting.In this paper,to solve the problems of poor interfacial stability and dendrite growth of Li metal batteries,an artificial PVDF/Li F/Li NO₃ film（PLL film）of polymer/inorganic material has been constructed in-situ on the surface of Li anode by ether solvent,thus generating a SEI layer with high and uniform ion flux in the cycling process,which significantly extends the cycle life of Li anode.At the same time,we have utilized the gelling reaction to prepare the gel polymer electrolyte in situ,realizing the stable cycle of Li metal batteries in high voltage and wide temperature range.