In-Depth Study On Alloy-Type Interphases Modified Lithium Metal Anodes And Its Electrochemical Mechanism

As the global electrification process accelerates,high-energy density energy storage devices are emerging as the stars of this era of revolution.Metallic lithium has been considered as the ideal anode material for next generation high energy density battery systems due to its extremely high theoretical specific capacity(3860 m Ah g^-1)and the most negative electrode potential（-3.04 V vs standard hydrogen electrode）.However,the unstable electrode-electrolyte interface derived from the excessive reducing activity and huge volume changes of the Li metal anode can cause a series of problems,like:（1）The fragile solid electrolyte interface（SEI）layer on the lithium anode is incapable of preventing continuous corrosion of the lithium metal by the electrolyte,resulting in poor coulombic efficiency and premature failure of the lithium metal batteries（LMB）;（2）the continuous breakdown/regeneration of the SEI severely disrupt the charge distribution on the surface of the lithium metal anode,leading to uneven lithium deposition/dissolution,uncontrolled dendrite growth and“dead Li”accumulation,which greatly reduce the safety and cycling life of the LMB.In the last decade,impressive progress has been made on taming these extremely unstable electrode-electrolyte interfaces.One of the outstanding strategies is the designment of artificial SEI with excellent Li⁺conductivity,mechanical strength and electrochemical stability to inhibit the growth of lithium dendrites and the electrolyte corrosion.But the big mechanical mismatch existing between the artificial SEI and the electrode will lead to poor contact at the solid/solid interface and seriously interfacial instable problems,which will disrupt the uneven distribution of the Li⁺flux during deposition/dissolution and trigger uncontrolled dendrite growth,causing premature failure of the LMBs.Therefore,this thesis focuses on fixing the poor interface contact and the instability of the artificial SEI through interfacial engineering modification to optimize the components and structures of the interface,and then enhance the electrochemistry performance of the lithium metal anode in high-energy density LMBs.The highlights are summarized as follows:（1）A certain amount of the solution of Sn F₂ dissolved in DME is dropped to the surface of the polished lithium metal foil.After the reduction and alloying of Sn by Li,an artificial interphase layer containing lithium fluoride（Li F）,tin-lithium（Sn-Li）alloy and tin（Sn）is formed on the surface of the Li metal anode.The Sn-Li alloy not only acts as a fast Li⁺conductor to promote uniform Li deposition/dissolution,but also enhances the interfacial compatibility between the artificial SEI and the Li metal anode.The Li metal anode protected by the fluorinated Sn-Li interphase layer shows lower voltage polarization and longer cycling life in symmetric cells compared to the bare Li metal anode.The protected lithium anode after cycled exhibit a much more uniform and intact morphology,demonstrating that both Li dendrite and irreversible side reactions are effectively suppressed.（2）An artificial SEI containing Li F,Sb and Sb-Li alloy is fabricated on the surface of the Li metal anode through the reduction and alloying process.The ex-situ SEM study shows that the Li deposits exhibit a homogeneous and dense morphology underneath the layer in the fluorinated Sb-Li interphase protected lithium.In order to investigate the working mechanism of this mixed conductive interphase layer,the Li ion conductivity and the electronic conductivity of the interphase layer are tested to make a comparison.It is found that the Li⁺conductivity is much higher than the electronic conductivity in this fluorinated Sb-Li alloy interphase layer.Furthermore,molecular dynamics simulations reveal that the electronic insulated Li F can hinder the upward migration of the electrons under the interphase,creating an electric field across the interphase to drive the migration of the Li⁺to the sublayer,and the Sb-Li,which is a fast ion conductor,can promote a uniform Li+flux and achieve dense Li deposition morphology underneath the layer.As a result,this fluorinated Sb-Li interphase layer can effectively suppress the dendrite growth and the electrolyte corrosion,thus promote the cyclic stability of reversible deposition/dissolution of the Li metal anodes.（3）A chemical plating method is designed for the in-situ construction of interphase layer containing Li F and Sn Sb alloy on the surface of Li metal anode.The Li metal anode protected by this Sn Sb interphase layer enables continuous lithium deposition even at a high capacity of 5 m Ah cm^-2.In-situ optical observation studies show that the Sn Sb interphase layer is more capable of maintaining a stable interfacial structure than the Sn interphase layer to accommodate the Li sub-layer deposition process.In large current density and high-capacity symmetric cells,the Sn Sb interphase protected lithium exhibits long-term cycling stability and low voltage polarization.In-situ electrochemical transmission electron microscopy studies further demonstrate the microstructural morphology evolution of the Sn Sb（or Sn）interphase layer during the cycling.The results show that the Sn Sb interphase undergo grain refinement during the cycling which can effectively relieve the internal stress,thus exhibiting lower volume changes than Sn interphase.The Sn Sb interphase protected lithium metal anode achieves up to 82%capacity retention after 100 cycles at 4 C in a full cell matched with high-loading NCM811 cathode(10 mg cm^-2).（4）A novel Sn-Li/HFP-PVDF composite interphase layer is designed on the surface of the lithium by a simple method of surface chemical reduction.The Sn-Li alloy filler can increase the Li diffusion across the interphase,while HFP-PVDF polymers are used as the framework to improve the flexibility and electrochemical stability of the interphase.Furthermore,three kinds of Sn-Li/HFP-PVDF interphase layers with different interfacial distribution structures（impenetrating distribution,uniform distribution and double-layered distribution）are constructed to investigate the effect of the interphase structure on the stability of the electrode-electrolyte interface.Combining electrochemical,mechanical and in-situ observational analysis,the unique infiltration distribution of Sn-Li in the interphase layer can bring the interphase layer both rigid and flexible mechanical properties,and more importantly,eliminate the poor connection existing in Li metal/artificial SEI and Sn-Li filler/HFP-PVDF interfaces.The resulted symmetric cells exhibit superior cycling stability for over1000 hours at an ultra-high current density of 20 m A cm^-2.It also shows excellent cycling performance and capacity retention in full cells matched with both high-loading LFP(15 mg cm^-2)and NCM811(10 mg cm^-2)cathodes.