Pan-Scale Research On Lithium Metal Anode

The energy density of traditional lithium-ion batteries represented by lithium cobalt oxide and graphite is close to the theoretical limit.Due to the extremely high theoretical specific capacity(3860 m Ah g^-1)and the most negative electrochemical potential?-3.04V vs.standard hydrogen electrodes?,lithium is considered to be the ultimate negative electrode material in lithium battery systems.However,the huge volume change during the charging and discharging of lithium metal anodes,uncontrolled Li⁺deposition and dissolution behavior?such as lithium dendrite growth?,high chemical reactivity and a series of electrode problems have hindered the commercial application of lithium metal batteries.Current understanding of lithium metal anode at different scales is not comprehensive and thorough enough to provide guidance for its commercial application.The deposition and dissolution behavior of lithium metal at the micro-nano scale is the most characteristic.In this paper,the electrode behavior at the micro-nano scale is studied from the electrode structure and interface of the lithium metal anode.Relevant background of lithium metal anodes is firstly discussed in this article,from the nature of lithium metal,the properties and problems of lithium metal anode,the electrode process of lithium metal,the deposition and dissolution morphology of metal lithium,the solid electrolyte interface phase?SEI?films,measurement/evaluation methods,history and prospects,experimental methods and characterization methods are introduced in detail.Then based on the understanding of the background of lithium metal anodes,the following research was carried out.The first part studies the influence of the three-dimensional ordered structure of the lithium metal surface on the deposition and dissolution behavior of lithium metal in the micro-scale liquid electrolyte system.First,using high-precision industrial X-ray computed tomography imaging detection system?Micro-CT?to non-destructively study three kinds of lithium metal monolithic soft battery systems at the micrometer scale.The study show that the huge volume expansion of the lithium metal andoe under the large surface capacity and large current density is an important reason of battery failure.Afterwards,a three-dimensional ordered cylindrical triangular close-packed hole structure was constructed on the surface of lithium metal through micro-fabrication experimental techniques.The deposition and dissolution behavior of lithium metal was studied under various variables,such as battery external stress and electrolyte,with and without structure,pore size,capacity matching,deposition and dissolution order,current density,specific surface capacity density.Studies have shown that the three-dimensional structure can induce the deposition in pores of lithium ion,inhibit the formation of lithium dendrites,and alleviate the huge volume expansion of the lithium metal electrode.So it exhibits more excellent cycling performance at different current densities.The second part studies the influence of the three-dimensional open ordered structure on the surface of the micrometer-scale inorganic solid electrolyte on the process of the lithium metal electrode.First,the in-situ scanning electron microscope was used to study the deposition and dissolution of metallic lithium in a lithium symmetric battery with an inorganic oxide electrolyte.The study show that lithium dendrite could puncture the NASICON&garnet type inorganic solid electrolyte,resulting in battery short-circuit.A garnet-type solid electrolyte was selected,a three-dimensional porous titanium electrode was constructed on the surface using micro-fabrication method,and the deposition behavior of lithium ion in the three-dimensional structure of the solid-state battery was studied using neutron depth spectroscopy?NDP?.The study exhibit that lithium metal will first form multi-site nucleus along the edge of the hole,and as the surface capacity increases,the lithium metal gradually merges and grows out of the structure.The interface between the titanium thin film and the solid electrolyte remains stable,without the penetration of lithium dendrites.The third part studies the effect of nanoscale interface on the deposition/dissolution behavior of lithium metal.Nano-scale copper nitride film was deposited on the copper foil surface by physical vapor deposition method to study its regulation on the lithium metal deposition/dissolution behavior.The cycle performance and electrochemical performance of the batteries were studied under different current densities in carbonate and ether electrolytes,Li|Cu and Li|Li Fe PO₄ battery systems,respectively.Morphological studies have shown that a large amount of lithium dendrites are generated in the carbonate electrolyte on the surface of the commercial copper foil,comparatively,lithium metal is deposited uniformly on the surface of the modified copper foil.The study found that copper foils modified with copper nitride all showed better capacity retention,higher and stable Coulomb efficiency,low polarization and battery impedance.Electronic conductivity distribution was measured by Peak Force Tunneling Atomic Force Microscope?PFTUNA?,and the results show that it is more uniform on the modified copper foil than the commercial copper foil.Combined with X-ray photoelectron spectroscopy,Raman spectroscopy,secondary ion mass spectrometry,and soft X-ray absorption spectroscopy to study the interface chemical state,a large amount of impurities such as copper oxide and basic copper carbonate existing on the surface of commercial copper foil is obtained,which causes the uneven distribution of electronic conductivity.The fourth part studies the electrolyte of the lithium metal battery and the SEI film on the sub-nanometer scale.In this section,the carbonate-based electrolyte of lithium difluorosulfonylimide?Li FSI?is used as the research object,focusing on the two variables of temperature and concentration.Cyclic performance and SEI film on lithium metal anode with this electrolyte are studied.Phase diagram of SEI chemical composition with temperature and concentration are charted.First,the intrinsic properties of the electrolyte such as variable temperature conductivity,lithium ion migration number,viscosity-temperature relationship,and electrochemical window were tested.The cyclic performance of Li|Li₄Ti₅O₁₂ in this electrolyte with different concentrations at 90?was compared.It has been found that the very low concentration electrolytes exhibit significantly superior cycle stability.Morphology of the lithium metal anode observed by Scanning electron microscopy show that the electrolyte with a high salt concentration would seriously corrode the lithium metal at high temperatures,making its deposition morphology island-like distribution.The electrolyte with very low concentration remained good.Chemical composition of the SEI film was measured by X-ray photoelectron spectroscopy shows that the higher the concentration and the higher the temperature,the greater the proportion of the SEI film due to lithium salt decomposition.In summary,this article focuses on the electrode behavior of lithium metal electrodes at the micro-nano scale.