Study On The Interface Protection Of Anode And Cathode Using Two-dimensional Carbon-based Materials In Lithium Metal Batteries

Due to the advantages of high specific energy,low self-discharge and long cycle life,lithium-ion batteries have important applications in many social fields such as 3C digital,electric vehicles and grid energy storage.However,the current commercial lithium-ion batteries using graphite anodes are approaching the limit of their theoretical energy density and cannot meet the rapidly growing demand for high-energy storage devices,while lithium metal anodes have become one of the most promising anode among all of the candidates because of its highest theoretical capacity and lowest electrode potential.However,lithium metal batteries also have various interface problems.Due to the strong reactivity of lithium metal and uneven deposition,it would cause the solid electrolyte interface（SEI）to be continuously generated during cycling,and at the same time produce lithium dendrites resulting in short circuit and other safety issues.Therefore,for the negative electrode interface,this subject has done the following research:（1）Ultrathin graphitic carbon nitride（g-C3N4,CN）nanosheet was prepared with a thickness of ¹ nm and formed CN thin layer over the lithium metal anode.The abundant nitrogen species within CN nanosheets can form transient Li-N bonds to powerfully stabilize the lithium-ion flux and enhance the affinity of electrodes with electrolytes.On top of that,the thin layer can act as an artificial solid electrolyte interface（SEI）to suppress lithium dendrite growth and enable stable Li plating/stripping over 300 cycles at a high current density of 5 m A cm-2 with low overpotential of about 50 m V.（2）Graphitic carbon nitride-poly（1,3-dioxlane）（CN-PDOL）composite layer was prepared via in-situ polymerization upon lithium metal anode.The adding of CN nanosheets could reduce the crystallinity of PDOL,increase ion conductivity and uniform the Li-ion flux.PDOL could offer mechanical strength to suppress lithium dendrite growth.Thus CN-PDOL composite later could enable stable Li plating/stripping over 850 cycles at a high current density of 3 m A cm-2with a low overpotential of about 70 m V.The solid polymer electrolyte has mechanical strength,which is expected to inhibit the growth of lithium dendrites to a certain extent,and at the same time can improve the current safety risks of organic electrolyte leakage and combustion.However,solid electrolytes also face some problems,such as high interface impedance and low ionic conductivity.Polypropylene carbonate（PPC）has high ionic conductivity and electrochemical window,but there are side reactions at the cathode interface.In the cathode side research,the PPC/NMC interfacial reaction mechanism was studied and graphene and graphene oxide were used to improve the interface.Research work includes:（1）Interfacial side reaction mechanism between PPC-SPE and NMC532 cathode was investigated by XPS and FTIR.Ni³⁺ and Co⁴⁺ species generated by electrochemical oxidization process can decompose poly（propylene carbonate）to aldehyde.To address this interface issue,graphene interlayer modified Li Ni_0.5Mn_0.3Co_0.2O₂ cathode（G@c-NMC）is synthesized via a facile method.After 50 cycles,the capacity retention of G@c-NMC is 97.9% at 0.3 C,as the result of graphene interlayer can slow the side reaction,facilitate interfacial charge-transfer process and stabilize the structure of cathode material.（2）Electrochemical and chemical interfacial reaction mechanisms between NMC532 and PPC-SPE were studied by XPS,FTIR and NMR.During the first charging process,the Ni²⁺and Co³⁺ species in the NMC positive electrode will be electrochemically oxidized to Ni³⁺ and Co⁴⁺.Ni³⁺ and Co⁴⁺ will catalyze the oxidation of the alcohol compounds produced when PPC contacts the lithium anode to produce ethers by dehydration condensation.To retard this side reaction,graphene oxide is applied to coat commercial NMC532 particles via a facile chemical approach.After 300 cycles,the capacity retention of GO@NMC is 69.2% at 0.3 C.This is due to the graphene layer facilitates the interfacial charge-transfer process and slows the cathode/electrolyte interfacial side reaction.This paper mainly studies the interface modification of the positive and negative electrodes of lithium metal batteries by two-dimensional carbon materials,which can effectively inhibit the growth of lithium dendrites on the negative electrode side and the side reactions of the positive electrode side,and is expected to play a reference role in the research on the interface protection of lithium metal batteries.