Research On Matrix Modification And Interface Protection Of Lithium Metal Anode For Lithium Secondary Batteries

Li metal is considered as one of the most promising anode materials for high specific energy batteries due to its extremely high theoretical specific capacity(3860 m Ah·g^-1)and extremely low electrode potential（-3.04 V vs.standard hydrogen electrode）.However,there are still many challenges to realize the practical application of Li metal anode,such as the Li dendrites growth and the generation of“dead Li”because of the uneven deposition and dissolution of Li,the continuous consumption of active Li and electrolyte owing to the severe side reactions between Li anode and electrolyte,and the pulverization of Li anode as a result of the repeated volume changes.These problems lead to poor cycle stability,short lifespan,and potential safety hazards of lithium metal batteries,which seriously hinder the practical application of lithium anodes.In order to improve the electrochemical performance of lithium anode,the specific research works about the modification of the deposition matrix and the protection layer of electrode/electrolyte interface were carried out as follows:The effect of pure phase lithium fluoride（Li F）protective layer on the morphology of lithium deposition and the electrochemical performance of lithium anode was investigated.A pure phase Li F protective layer was constructed on the surface of the lithium anode by vacuum evaporation method.The lithium-ion conductivity of different position in the Li F layer is consistent due to the uniform composition of the protective layer.Therefore,the lithium-ion flux evenly distributed on the anode surface,which promotes the uniform deposition of lithium and inhibits the growth of lithium dendrites and the volume change of the anode.In addition,the Li F protective layer can effectively suppress the side reactions between the lithium anode and the electrolyte.The introduction of the pure-phase Li F protective layer significantly improves the electrochemical performance of the lithium anode.The lifespan of Li|Li Fe PO₄ battery with Li anode protected by the pure-phase Li F layer increased to 1000 cycles from 400cycles of battery with untreated Li anode.And the capacity retention of the Li|Li Fe PO₄ battery with pure-phase Li F protective layer is 86%after 1000 cycles at 1C.The effect of lithiophilic In₃Li₁₃ substrate on the Li deposition behavior and electrochemical performance of Li anode was investigated.A thin layer of In metal is prepared on the surface of copper foil by vacuum evaporation method.And then the In₃Li₁₃ intermetallic compound is formed through electrochemical alloying reaction,which is used as substrate for Li deposition.By analyzing the kinetic parameters of the lithium deposition reaction on Cu,Li and In₃Li₁₃ substrates and combining with SEM characterization,the deposition behavior of lithium on the Cu,Li,In₃Li₁₃ substrates was studied.In the same system,the current density of reduction reaction of Li⁺on the In₃Li₁₃ substrate is higher when the electrode potentials of Cu,Li and In₃Li₁₃ are the same.After the initial deposition of a small amount of lithium,the subsequent Li⁺tends to deposit on the exposed In₃Li₁₃surface,which facilitates the formation of uniform deposited lithium.Using In₃Li₁₃ as the substrate of Li deposition can reduce the nucleation overpotential of Li⁺and inhibit the growth of lithium dendrites,thus improving the cycle stability of Li anode.Cu@In₃Li₁₃-Li anodes were prepared by depositing metallic lithium of 2 m Ah cm^-2on Cu@In₃Li₁₃ substrates.The symmetric cell with Cu@In₃Li₁₃-Li anodes can stably cycle for 260 h at a current of 0.5 m A cm^-2 and a capacity of 1 m Ah cm^-2.An integrated Li-In composite anode containing three-dimensional lithiophilic framework and metallic lithium was prepared,and a Ni F₂ protective film was introduced to stabilize the interface between Li-In composite anode and electrolyte.The Li-In composite anode was prepared in one step by a simple co-melting method.Compared with the complicated process of“preparation of three-dimensional current collectors,introduction of lithiophilic sites or lithiophilic layer,pre-storage of lithium metal”,this strategy greatly simplifies the preparation process.The lithophilicity of the In₃Li₁₃ framework can induce uniform Li nucleation.The framework structure with high specific surface area can reduce the electrode current density and provide sufficient internal space to accommodate the deposited lithium metal.Therefore,the growth of lithium dendrites and the volume change of the anode were effectively suppressed.Replacing pure Li anode with Li-In composite anode,the lifespan of Li|Li Co O₂ batteries with high cathode capacity loading of 2.5 m Ah cm^-2 and low N/P ratio of about 2.7 was increased to80 cycles from 20 cycles.Furthermore,a Ni F₂ protective film is constructed on the surface of the Li-In composite anode,which further slows down the side reaction between Li-In composite anode and electrolyte by blocking the direct large-area contact.By pre-dissolving part of the metal lithium in Li-In composite anode,Li can deposit in the three-dimensional In₃Li₁₃ framework under the Ni F₂protective film when large amount of lithium was deposited into the anode.The Ni F₂ protective film can work continuously to block the direct contact between Li-In composite anode and electrolyte.With the same high cathode capacity loading of 2.5 m Ah cm^-2 and low N/P ratio of about 2.7,the lifespan of Li|Li CoO₂battery was improved to 136 cycles.