Study Of Alloying Modification And Surface Protection Of Lithium Metal Anode For Lithium Secondary Batteries

Due to the limited specific capacity,lithium ion batteries can't meet the application demand of high specific energy density in the future.Recently,lithium metal secondary batteries with ultrahigh theoretical specific capacity have become a hot research area.However,the potential safety hazard and poor cyclic performance caused by the inherent lithium dendrite growth block the commercial application of lithium metal batteries.The repeated Li deposition/dissolution behavior during cycling process is the critical factor to determine the growth morphology of lithium metal and the stability of electrode-electrolyte interface.Guiding homogeneous Li deposition can improve the cycling stability and safety of lithium metal batteries.Therefore,this dissertation focuses on alloying modification and surface protection of lithium metal anode for the purpose of optimizing the interface of lithium metal deposition and promote the battery cycle life.The research includes the following four parts.(1)A simple,repeatable,environmentally friendly and large-scale in-situ method was developed to grow a uniform and dense layer of Ag nanoparticles on Cu foil.The nano-metal protective layer obtained by the in-situ method has a strong binding force with Cu substrate and will not fall off from the base during cycling process.Moreover,Ag has a solubility zone inside Li metal,which can greatly reduce the nucleation overpotential of lithium metal.The lithiophilic of Li-Ag alloy can guide the uniform nucleation and deposition of lithium metal on the Cu current collector.The initial deposition process is conducive to the Li deposition/dissolution in the subsequent cycling process,contributing to a stable interface and eventually enhancing the cyclic stability of batteries.(2)A simple physical method is used to prepare the Ni-Al-Ni composite structure,which is applied as the alloy-type anode after pre-lithiated process.The threedimensional skeleton structure of the composite anode can reduce the effective current density of the batteries,and the two-layer porous conductive layers can not only provide buffer space for the volume expansion of Li Al alloy,but also maintain good electrical contact activity during the cycling process,which is conducive to the dynamic process of charge transfer.In addition,the Li Al alloy in the composite anode has good lithiophilic properties,which can lower the nucleation overpotential of lithium metal and promote the formation of uniform Li deposition process.The active lithium stored in Li Al alloy can also partly compensate the irreversible capacity loss and thus improve the coulombic efficiency of the batteries.(3)?-Cyclodextrin cross linked citric acid(?CD-CA)polymer was prepared by the polycondensation reactions between the hydroxyl groups of ?CD and the carboxyl groups of CA,and used as the protective coating layer for the lithium metal anode.The resultant ?CD-CA polymer has abundant functional groups such as hydroxyl groups and carbonyl groups and hydrogen bond interaction,which are evenly distributed on the surface of lithium metal anode.The strong binding energy can induce the uniform deposition of lithium and optimize the growth morphology.In addition,?CD-CA polymer is chemically stable to ester electrolyte,which can reduce the excessive consumption between lithium metal and electrolyte and further improve the cyclic stability of the batteries.(4)Three kinds of cyclodextrin-based polymers were prepared by esterification and silylanization methods respectively.By comparing the ionic conductivity and electrochemical cycling performance of the three polymers,the interfacial protection of cyclodextrin-based polymers for lithium metal anode was studied.The results showed that ?CD-MTMS polymer demonstrate the optimal ionic conductivity with lithium salt and binder owing to the anion characteristics of the silicate tetrahedron network and the ordered ion channel structures.The ?CD-MTMS can use as solid polymer electrolyte for lithium metal secondary batteries and can obtain considerable electrochemical cycling performance at 50 ? in both Li/Li symmetric cell and LFP/Li full cell.