| The growing demand for the utilization of renewable energy is a driving force behind the advancement of high-energy-density electrochemical energy storage technology.However,the energy and power density of lithium-ion batteries dominated by graphite anode are difficult to meet the huge demands in many emerging application fields,such as electric vehicles.Lithium metal has the lowest redox potential（-3.04 V vs SHE）and a higher theoretical specific capacity(3860 m A h g-1,2061 m A h cm-3).Nevertheless,the inhomogeneous deposition of lithium metal can readily induce the formation of lithium dendrites during cycling,subsequently resulting in interface instability,an accumulation of inactive lithium,and substantial volume expansion.In response to the challenges posed by lithium dendrites and their associated issues,this thesis is dedicated to a comprehensive exploration of several key areas:structural design,lithiophilic modification,and lithium-carbon composites based on carbon materials.The primary objectives encompass an in-depth investigation into the lithium plating/stripping mechanisms,the implementation of protection strategies for the lithium metal anode,and the effective suppression of dendritic growth,all aimed at establishing a stable lithium metal anode.Firstly,three distinct commercial graphite materials with varying micro-nano structures were thoughtfully selected as lithium ion/lithium metal hybrid anode to explore the lithium plating/stripping behaviors.Subsequently,innovative approaches involving lithiophilic onion-like carbon and interface-anchored carbon spheres were employed to create a dendrite-free lithium metal anode and to unravel the nucleation and growth mechanisms of lithium on carbon spheres.The study also addresses the challenge of electrochemical corrosion between traditional copper foil collectors and metallic lithium,which often leads to the detachment of active materials.To mitigate this,a self-supporting three-dimensional carbon paper host was chosen for lithiophilic modification,thereby enhancing lithium plating and stripping performance.Through the utilization of mechanical ball milling,a self-supporting lithium-carbon composite anode,characterized by a high dispersion of lithium metal,was prepared.We also propose a comprehensive elucidation of the structural characteristics and the underlying lithium plating/stripping mechanisms.Ultimately,the research resulted in the achievement of highly stable lithium plating and stripping processes and an extended cycle life for the lithium metal anode.The specific research tasks undertaken in this study are outlined as follows:（1）Three kinds of graphite were chosen as the lithium ion/lithium metal hybrid anode.The graphite with an onion-like layer confined structure inhibits the migration of lithium ions and readily leads to the formation of needle-like dendrites;Graphitized mesocarbon microspheres with a parallel layer structure tend to delaminate when exposed to ether-based electrolytes,resulting in the accumulation of non-active lithium and dendritic growth;The surface amorphous carbon-coated natural graphite（MSD）material enhances the wettability of the electrolyte and electrode surface,while its highly crystalline internal structure enhances graphite lithiation.Furthermore,our research uncovered that the deposition of metallic lithium on graphite impedes the intercalation behavior of lithium ions,both in traditional ether-based and ester-based electrolytes.In an ether-based electrolyte,the use of MSD as a lithium ion/lithium metal hybrid anode demonstrated the ability to cycle 260 cycles at0.5 m A cm-2,1 m A h cm-2.When used solely as a lithium metal host,it exhibited stable cycling for more than 600 cycles at 0.5 m A cm-2,2 m A h cm-2.This work provides valuable insights into the lithium ion/lithium metal hybrid process from graphite structure and electrolyte.The simple strategy of amorphous surface treatment on graphite offers a promising solution for stabilizing the lithium metal anode.（2）Using pyridine and silver nitrate as carbon sources and lithiophilic elements,onion-like carbon-coated silver microspheres（Ag@NCS）were synthesized in a one-step pyrolysis process.The abundant defects on the surface of Ag@NCS creates pathways for lithium ions to enter the carbon layer.The lithiophilic silver induces lithium to enter the carbon layer,while the nanogap structure under onion-like carbon layers offers substantial lithium storage capacity.The mutual synergistic mechanism effectively inhibits the growth of lithium dendrites.Therefore,the nucleation overpotential on Ag@NCS host is impressively low,measuring only about 1 m V（in comparison to 38 m V for copper foil and 24 m V for pure onion-like carbon）.Furthermore,Ag@NCS host exhibits stability,sustaining than 400 times at 0.5 m A cm-2,2 m A h cm-2.Even at higher current densities,such as 1 m A cm-2,4 m A h cm-2,it exhibits 150 stable cycles.To address the issue of silver nanoparticles（AgNPs）tending to agglomerate and disperse poorly within carbon materials,which can lead to uneven lithiophilic sites.A kind of anchored AgNPs sesame-like carbon microspheres（AgNPs@CS）was designed and prepared.Within this structure,30 nm diameter AgNPs are uniformly anchored,and a 10 nm thick carbon layer encapsulates the AgNPs,effectively preventing the detachment of silver sites.This structure promotes the rapid migration of lithium ions within the carbon spheres.Consequently,AgNPs@CS electrode exhibits exceptional stability,enduring 300 cycles at 0.5 m A cm-2,2 m A h cm-2.Even when the stripping voltage is increased to 1 V,the stability extends beyond 260 cycles,with negligible growth in hysteresis voltage at 1 m A cm-2,1 m A h cm-2.The introduction of silver through structural design enhances the lithiophilicity of carbon materials,offering an innovative approach to overcome the insufficiencies of traditional lithiophilic carbon materials.This method not only prevents the loss of metal sites but also averts the shortcomings associated with post-loading lithiophilic metals,thus providing valuable insights for the design and modification of lithiophilic carbon materials.（3）By employing AgNO3 as silver source and PVDF as fluorine source,fluorine-doped and uniformly AgNPs loaded amphiphilic carbon paper collector（CP@Ag-F）was prepared,with F content of 3.2 at%and Agcontent of 5.1 at%.The presence of AgNPs within the amorphous carbon layer structure significantly enhances the lithophilicity of the host.Furthermore,F-doping induces the formation of Li F-rich thick SEI,effectively mitigating the volume changes associated with lithium metal.These combined advantages work in tandem to ensure uniform and stable process for lithium plating/stripping.Consequently,the half-cell can stably for 120 cycles at 0.5 m A cm-2,2 m A h cm-2.In addition,the symmetric cell maintains a small polarization voltage of 8m V to work more than 1400 h at 1 m A cm-2,1 m A h cm-2.Even under the harsh conditions of 3 m A cm-2,1 m A h cm-2,the polarization voltage is maintained at approximately 38 m V,with CP@Ag-F electrode continuing to cycle reliably for more than 500 h.This research underscores the effectiveness of combining a lithophilic three-dimensional host with a stable SEI in enhancing the uniform of lithium plating/stripping.（4）A self-supporting composite anode,consisting of highly dispersed metallic lithium within a lithium carbon composite（GKBLi）material,was prepared by ball milling a mixture of micron-stabilized lithium metal powder and carbon nanomaterial（Graphitized Ketjen black）.Ball milling serves to reduce the size of lithium particles and ensures their high dispersion within the carbon nanopowders.Concurrently,the formation of lithium-graphite intercalation compounds under the action of mechanochemical forces improves the lithophilicity.The Li3N-rich surface contributes to high ionic conductivity and stability,consequently accelerating the diffusion rate of lithium ions.The highly dispersed structure between lithium metal and nanocarbon promotes the rapid reduction of lithium ions in various electrode spaces,thereby significantly restraining dendrites growth and facilitating uniform lithium plating/stripping under high current density.As a result,GKBLi electrode demonstrates outstanding long-cycle electrochemical performance.In symmetrical cells,GKBLi electrode has a stable low-voltage hysteresis cycling for more than 1200h at 1 m A cm-2,1 m A h cm-2.Compared to a bare lithium foil anode,the full cell employing GKBLi as the anode and lithium iron phosphate as the cathode showcases exceptional cycle stability.After 1000 cycles at a current density of2 C,the capacity retention rate remains at an impressive 90.9%.The unique structure and excellent electrochemical performance of GKBLi electrode hold promise for replacing traditional lithium foil anodes in the next generation of high-energy-density lithium metal batteries.This straightforward dry preparation method opens up a new insight to realize safe lithium metal batteries. |
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