Construction Of Multifunctional Composite Artificial Sei Layer And Electrochemical Performance Of Lithium Metal Anode

The high theoretical specific capacity(3860 m A h g^-1)and the lowest electrode potential（-3.04 V vs.standard hydrogen electrode）of lithium metal make lithium metal batteries（LMBs）have the potential to become new high-energy-density energy storage devices.However,issues such as reduced reversibility and coulombic efficiency of batteries caused by the side reaction between the lithium metal anode and the electrolyte,short circuits and thermal runaway caused by dendrite growth,as well as dead lithium accumulation and volume change,urgently need to be addressed.This article mainly aims to regulate the lithium deposition process and enhance lithium-ion transportation by constructing a multifunctional composite artificial solid electrolyte interface（SEI）.Firstly,a bilayer structure of a lithiophilic matrix and ion-conductive polymer layer is designed using ex-situ methods,which achieves regulating the lithium deposition morphology,stabilizing the electrode/electrolyte interface,avoiding dendrite growth and severe side reactions;Secondly,an heterogeneous artificial SEI layer with single-ion conductivity,multiple lithium-ion transportation channels,and lithiophilic metals is prepared by in-situ method,which significantly improves the lithium-ion flux,achieves uniform lithium deposition morphology,and achieves good electrochemical performance of lithium metal batteries;Finally,an artificial SEI layer without binder is prepared by automatic transfer method,which promotes the process of lithium-ion desolvation,alleviates volume change,regulates lithium-ion transportation and deposition,and further improves the dynamic and electrochemical performance of lithium metal batteries.The concrete content of this research are as below:（1）A multifunctional bilayer structure combined of sponge-like lithiophilic silver layer and PVDF/Li F ion-conductor layer is constructed based on ex-situ method,achieving dendrite-free and high-performance lithium metal anode.To solve the problem of dendrite growth caused by uneven lithium deposition on lithium metal anodes and the serious side reactions caused by exposure to the electrolyte,a bilayer structure consisting of a sponge-like lithiophilic silver layer as the lithium deposition matrix and a PVDF/Li F layer as the physical barrier and ion-conductor is designed.The sponge-like porous lithiophilic silver layer provides abundant lithium nucleation sites and deposition space for the uniform growth of lithium,thus endowing the lithium metal anode with a uniform and dense deposition morphology,avoiding excessive growth of lithium dendrites.The PVDF/Li F layer with ion-conductivity and electronic insulation improves lithium-ion transportation paths and ensures interface stability by suppressing side reactions between lithium metal and liquid electrolyte.The deposition of lithium in the lithiophilic sponge-like silver layer is similar to"sponge water absorption"phenomenon and can fill the pores of the silver layer.The PVDF/Li F layer,as a physical barrier layer,can integrate with the silver layer during the deposition process.Even if the silver layer is"swollen"by lithium metal,the PVDF/Li F layer can avoid short circuits caused by the contact between lithium dendrites and the cathode.Compared with Cu||Li and Ag@Cu||Li half cells,The average coulombic efficiency of PVDF/Li F-Ag@Cu||Li is more stable.PVDF/Li F-Ag@Li||PVDF/Li F-Ag@Li symmetric cell exhibits longer cycle time and maintains a low polarization of～35 m V for at least 700 hours at a current density of 1.0 m A cm^-2.PVDF/Li F-Ag@Li||Li Fe PO₄ full batteries also have longer cycle lives,higher capacity retentions,and excellent rate performances.（2）A heterogeneous artificial SEI layer is constructed on the surface of lithium metal anode using in-situ chemical conversion method,achieving high lithium-ion flux and dendrite-free lithium deposition.In order to further enhance the lithium-ion flux and regulate the morphology of lithium deposition on the surface of lithium metal anode,a lithiophilic and lithium-ion conductive layer is constructed by one-step method,achieving uniform lithium deposition and high lithium-ion flux.A dense artificial SEI film is constructed by in-situ reaction of layered zinc silicate nanosheets and lithium metal anode.The lithiophilic metal Zn/Li_xZn_y is uniformly but nonconsecutively dispersed in a continuous lithium-ion conductor composed of three components:Li_xSi O_y,Li₂O,and Li OH.Continuous lithium-ion conductors not only promote the desolvation process of solvated lithium ions,but also regulate the transportation process of lithium ions.The special layered nanosheet structure can also provides multiple lithium-ion transportation paths;As a single-ion conductor,it can also effectively reduce the lithium-ion concentration gradient on the surface of lithium metal anode,increase the lithium ion concentration on the lithium metal surface,and alleviate the severe dendrite growth caused by space charge effects.nonconsecutively lithiophilic metals can be polarized by an internal electric field,promoting the transfer of lithium ions in the protective layer,and reducing the nucleation barrier of lithium deposition process on the lithium metal anode.Simultaneously,lithiophilic particles and a continuous ion-conductor layer can serve as a physical barrier to prevent lithium dendrites from piercing the separator and causing short circuits.Therefore,the protected Li||Li symmetrical cell achieves low polarization of～50 m V within 750 hours at a current density of2.0 m A cm^-2;Under the high area loading of lithium iron phosphate cathode,the full cell achieves higher capacity retention.（3）The artificial SEI layer is constructed by the automatic transfer method from the separator to the anode,which accelerates the kinetics and realizes the uniform lithium deposition.In order to further enhance the regulation of the lithium-ion transportation behavior,realize the construction of a binder-free protective layer,and explore the potential of the artificial SEI layer in practical applications,in this section,an artificial SEI layer is constructed by coating a zinc silicate/reduced graphite oxide（RGO）compound on the PP separator and automatically transferring the coating from the separator to the lithium metal anode after the battery assembly,which promotes the desolvation process,enhances lithium ion transportation,and promotes uniform lithium deposition.Due to the in-situ reaction of zinc silicate with lithium metal,the prepared artificial SEI layer has the advantages of an artificial SEI layer derived from zinc silicate.The nonconsecutively dispersed lithiophilic metal Zn/Li_xZn_y nano-particles and single-ion conductors of Li_xSi O_y,Li₂O,and Li OH reduces the lithium deposition barrier and provides high lithium-ion flux;At the same time,the electronegativity sp²electrons of RGO have electrostatic attraction with lithium ions,which further promote the desolvation process of lithium ions on the layer surface,regulate the transportation of lithium ions through the RGO sheets,and provide more lithium ion transportation paths.Besides,the uniformly distributed zinc silicates on RGO ensures that the zinc silicate nanosheets derived layer are evenly distributed on the lithium metal anode.RGO can also act as a membrane-support which helps to construct a binder-free protective layer.The flexible RGO can also alleviate volume change and further improve the cycling stability of lithium metal batteries.The automatic transfer strategy does not need to be prepared in an inert gas environment,and the process of coating on the separator is easy to achieve industrialization.After test of electrochemical performance,the anode with the prepared artificial SEI layer by automatic transfer method achieves a coulombic efficiency of 97.3%within350 cycles at a current density of 2.0 m A cm^-2 in a half cell,and can stabilize the polarization voltage at～40 m V within 800 hours at a current density of 2.0m A cm^-2 in a symmetrical cell.In full cell,at the area loading of 5 mg cm^-2 of lithium iron phosphate cathode,a higher capacity retention rate of 91.7%is maintained after 1000 cycles at 1C.