Research On Structure Regulation And Performance Optimization Of Electrode Interface Layer In Lithium Metal-NMC811 Batteries

Although lithium metal batteries can release extremely high theoretical energy density,their positive electrodes and lithium metal negative electrodes are plagued by their own problems.The inherent instability of lithium metal will lead to severe lithium dendrite growth,huge volume change,and even safety hazards.On the cathode side,high-nickel ternary cathode material(Li Ni_0.8Mn_0.1Co_0.1O₂,NMC811)is taken as an example.The macroscopic morphology of NMC811 is agglomerated secondary particle spheres,which are easily broken during electrochemical cycling,forming internal micro-cracks,resulting in the crushing and pulverization of the spherical structure,causing interfacial side reactions,resulting in rapid decay of cathode capacity.Therefore,it is of great significance to develop a suitable modification strategy of the positive and negative interface layers to solve the challenges faced by the positive and negative electrodes of lithium metal secondary batteries.Taking the lithium metal/NMC811 battery as the research object,this paper focuses on designing and constructing suitable solid electrolyte interphase on anode and cathode,and discusses the application of different solid and liquid materials as interface modifier on the lithium metal anode side and even on cathode side,in order to suppress the lithium dendrite growth,maintain the stability of cathodes,and finally improving the electrochemical performance of lithium metal batteries.The main work includes:（1）Based on interface layer construction,this work innovatively designed an in-situ grown ultrathin and nitrogen-defective carbonitride(C₃N_0.5)on copper foil as the electrode interface layer by reactive thermal evaporation method.The traditional carbonitride（g-C₃N₄）has advantages like lithiophilic polar functional groups,high mechanical modulus and good thermochemical stability.However,its band gap is really large which is harmful for the interface electron conduction,thereby impairing the electrode interface kinetics and increasing the lithium-ion nucleation overpotential.Compared with it,C₃N_0.5 has an extremely low bandgap of 0.63 e V,which exhibits enhanced interfacial electronic conductivity.This defect structure with enhanced space charge effect is beneficial to the interfacial charge transfer,not only significantly reducing the nucleation overpotential,cell polarization,and interfacial resistance,but also effectively regulating the deposition behavior of lithium ions with suppressed lithium dendrites.Benefiting from the unique spatial structure and improved electrochemical performance of the C₃N_0.5interfacial layer,the deposition morphology of Li ions exhibits a hierarchical porous network structure,which reduces the generation of dead Li,and improves the utilization rate of Li ions.And the corresponding Li metal/NMC811 also shows greatly improved capacity retention ability.However,although the defect-free carbonitride g-C₃N₄ can also inhibit the growth of Li dendrites,the Li-ion nucleation overpotential and cycling polarizations of it are significantly higher than those of C₃N_0.5,in turn proving the advantages of improved electronic conductivity of interface layer.（2）This work proposed a concept of non-consumable and flowable liquid fluorinated interface layer constructed by drop-coating small amount of liquid fluorine-rich material（perfluoropolyether,PFPE）on the lithium metal interface,in order to regulate the lithium ions nucleation behavior and effectively inhibit the growth of lithium dendrites.There is no obvious dissolution between the PFPE liquid interface layer and the electrolyte.And PFPE does not react with lithium metal.This non-consumable property enables it to hinder the side reactions between the electrolyte and Li metal and maintain the stability of the electrode interface.The electrochemically activated PFPE interfacial layer will promote the interface diffusion behavior of Li ions,reduce the interfacial resistance and corresponding activation energy.The fragment of PFPE marginal structure（C-F）could be catalyzed under the effect of electric field,making them lithiated to form robust interphase components Li F and Li C₆,so that the deposition morphology of lithium ions evolves into densely interconnected and dendrite-free network structure.（3）This work constructed an ion/electronic dual conductive Li₂CN₂/C interface layer in situ formed on the surface of lithium metal via solution reaction method in order to promote the conductivity of both ions and electrons.The formed Li₂CN₂/C interface layer have very intimate contact with Li metal substrate.Highly lithiophilic Li₂CN₂ is a Li-ion conductor that exhibits multiple Li-ion adsorption sites,forming three-dimensional Li-ion diffusion channels.The higher anti-reduction stability and longer critical length for Li dendrite endow the lithiophilic Li₂CN₂ with excellent dendrite suppressing ability.In addition,the existence of mixed carbon structure forms a three-dimensional conductive network,beneficial for homogenizing the current density on Li metal interphase,reducing the local current density,and promoting the uniform deposition of Li ions.The Li₂CN₂/C also inhibits the side reaction between the lithium metal and the electrolyte and increases the content of Li F,Li₂CO₃ and other stable components in the solid interface layer,further enhancing the kinetics and stability of the lithium anode.（4）Considering that there are no related reports about the interface design on the NMC811 primary particles,for the first time,this work used a low-cost method to successfully prepare the primary particles of NMC811 combined with interface tuning strategy,to optimize the electrochemical performance of high-nickel ternary materials.Based on the"melt-infiltration-evaporation"mechanism of P₃N₃Cl₆（PNCL）,the NMC811 spherical secondary particles are transformed into the basic constituent unit,namely primary particles.The contact between the primary particles and the surrounding auxiliary cathode components such as conductive carbon is very close,which promotes the integrity and stability of the electrode.NMC811 primary particles no longer suffers from the microcracks and structural collapse that often occurs in conventional polycrystalline NMC811 spherical secondary particles.And PNCL can play a role in interfacial regulation,which can provide N,P and Cl elements to participate the forming of robust surface structure by lithiated Li₃N,Li Cl and Li_xPO_yF_z.The stability and kinetic performance of the cathode interface layer are effectively improved and the interface side reactions caused by the increase of the specific surface area are suppressed by the PNCL tuning.Benefiting from the primary granulation combined with the interface regulation strategy,the Lithium metal/NMC811 battery exhibits excellent capacity retention ability with small capacity decay rate of only0.043%per cycle and ultra-long stable cycle life up to 1100 cycles at 1 C rate.