Research On Lithiophilic Biochar-based Materials Used As Current Collector For Lithium Metal Anode

The actual energy density of traditional lithium-ion batteries is gradually approaching its theoretical limit value,reaching the bottleneck period of development.The lithium metal anode,which has the two major advantages of higher specific capacity(3860 m A h g^-1)and lower reduction potential（-3.04 V vs.standard hydrogen electrode）,is the preferred anode material for the next generation of high-energy density lithium batteries.However,in practical applications,lithium metal batteries have problems such as side reactions with electrolytes,growth of lithium dendrites,and volume expansion.Therefore,the three-dimensional composite metal lithium anode electrode constructed by using a light-weight and high-conductivity carbon material as a three-dimensional framework combined with metal lithium can increase ion diffusion channels,improve the conductivity of the anode electrode,relieve volume expansion,and effectively improve the stability and electrochemistry of the electrode.performance.Among them,biochar materials with a wide range of sources and diverse structures have abundant pore structures and surface functional groups,which have great application prospects.However,in practical applications,non-polar carbon materials have a poor lithiophilicity for lithium,which is not conducive to the uniform deposition of metallic lithium.Studies have shown that the use of lithiophilic materials to modify carbon materials can provide more nucleation sites for lithium,thereby regulating the deposition behavior of lithium.This research work focuses on the structure design of the anode electrode.Biochar is selected as the framework material for lithium metal,and the lithiophilic coating is modified on its surface.The current collector is combined with lithium metal by electrochemical deposition method to obtain a composite lithium anode electrode with excellent electrochemical performance,which reduces the nucleation overpotential of metallic lithium and regulates the deposition behavior of lithium.The research content of this article mainly includes the following two aspects:（1）In this paper,several types of biomass with different morphologies and structures have been studied.A series of biochar materials have been prepared by carbonization or carbonization activation combined methods,including fungus residue carbon,silkworm excrement carbon,straw carbon and chestnut carbon.Through characterization and analysis such as SEM,silkworm excrement carbon（SC）has a unique three-dimensional hierarchical porous honeycomb structure and a large specific surface area of 2254.6 m² g^-1,which is an excellent host for lithium metal.Through further half cell coulombic efficiency tests,it was found that the SC/Li composite anode showed better electrochemical performance among the four biochars,and could maintain a coulombic efficiency of 97%after 60 cycles of stable cycling.Therefore,the subsequent further modification is based on this kind of silkworm excrement carbon.Aiming at the problem of poor lithophilicity of biochar-based materials,this work introduced transition metal nanoparticles（Co,Ni,Cu）on the surface of silkworm excrement carbon through wet chemistry and high-temperature reduction processes.A series of characterization and electrochemical tests proved that cobalt nanoparticles with a particle size of 50-70 nm provide more nucleation sites and achieve uniform deposition of lithium metal.The SC@Co/Li symmetrical battery can be cycled stably for 1000 hours,and the overpotential is about 25 m V.The full cell constructed with the Li Fe PO₄ cathode electrode has been stably cycled for more than 200 cycles at a current density of 2 C,and the capacity retention rate is 97.2%.The improvement of battery performance is due to the modification of transition metal nanoparticles,which makes lithium metal nucleate and deposit more uniformly on the current collector,effectively alleviating the growth and volume expansion of dendrites,thereby improving the capacity retention rate and cycle stability of the battery.（2）In order to solve the problem of nanoparticle modification being covered during lithium deposition,using silkworm excrement carbon as a substrate,the surface of the carbon material is modified by hydrothermal reaction and high temperature calcination to modify the cobalt nanosheet array（SC@Co₃O₄）.The oxidation-reduction reaction between Co₃O₄ and lithium provides nucleation sites,and the unique array structure provides more deposition space.The SC@Co₃O₄/Li symmetrical cell exhibits a polarization voltage of 40m V at a current density of 0.5 m A cm^-2.Even at a larger current density of 4 m A cm^-2,the polarization voltage is 150 m V.The test rate performance of a full cell matched with the Li Fe PO₄ cathode electrode has a specific capacity higher than 100 m A h g^-1 at a current density of 5 C.It has good rate performance and improves the cycle stability of the lithium metal anode electrode.