Preparation Of Gradient-modified Bamboo Carbon Material And Electrochemical Performance Of Lithium Metal Battery Anode

Currently,the actual energy density of lithium-ion batteries is very close to its theoretical value,and further construction of electrochemical energy storage systems with higher energy density has become a current research hotspot.Lithium metal has many advantages,such as high theoretical specific capacity(3860 m A h/g),low electrochemical potential(-3.04 V,vs.SHE)and low mass density(0.534 g/cm~3).It is considered to be a promising anode material.However,there are many problems in using bare lithium foil as anode directly,such as uncontrolled growth of lithium dendrites during cycling,unstable solid electrolyte interface,and infinite expansion of lithium metal anode volume,resulting in poor cycleability and serious safety risks,which greatly hinder the further commercial application of lithium metal batteries.Researchers have proposed a variety of strategies to address these challenges,including optimizing the electrolyte system,constructing a solid electrolyte,constructing an artificial solid electrolyte interface layer(SEI),and designing a negative electrode structure to improve the cycle stability and electrochemical performance of lithium metal anodes.In the design of anode structure strategy,the construction of three-dimensional current collector can effectively alleviate the volume expansion during lithium deposition/stripping and inhibit the growth of lithium dendrites.The three-dimensional carbon-based current collector has the advantages of light weight and good conductivity.The 3D pore results can be used as an effective deposition space to alleviate the volume expansion of lithium metal.Biomass materials has wide range of sources and rich pore structures.After a series of carbonization and activation,their intrinsic structure can be well maintained.Applying them to lithium metal anodes can homogenize the lithium ion current and improve the electrochemical performance.However,the different specific surface area of the current collector will affect the coulombic efficiency of the battery,which is an important index to be considered in the design of porous carbon-based current collector.In addition,the non-polarity of the carbon material leads to its poor lithium affinity.During the cycling process,lithium metal preferentially deposits at the anode/separator interface with high ion-flux,resulting in the formation of lithium dendrites and’top growth’behavior.Therefore,it is necessary to modify it to obtain a lithium-philic three-dimensional carbon-based current collector with excellent electrochemical performance.Based on this,the paper has carried out the following work:(1)Three kinds of biomass carbon materials with different specific surface areas were prepared according to the properties of biomass in herbaceous and woody plants.Through structural analysis,it has a stable three-dimensional vertical pore structure and a low specific surface area.By analyzing the electrochemical data,it was found that the bamboo carbon material exhibited better electrochemical performance.Bamboo carbon current collector can be stably cycled for 100 cycles at a current density of 1 m A/cm~2,and has a higher average coulombic efficiency(85.3%).The full cell with lithium iron phosphate(LFP)cathode,at 0.5 C current density,the capacity retention rate is still 93.8%after 100 cycles.(2)In order to further improve the lithium affinity of carbon-based current collectors and enhance the electrochemical performance of bamboo carbon materials,we doped nitrogen on the surface of bamboo carbon three-dimensional pores to increase the nucleation sites.At the same time,the metal silver evaporation modification is carried out on the side far away from the diaphragm,so that the resistance of the side is lower than that of the side near the diaphragm,so that the lithium ion conductivity gradient inside the electrode is increased,so as to obtain a three-dimensional current collector that can gradiently adjust the lithium metal deposition behavior.Compared with the unmodified bamboo carbon current collector(BC),the gradient-adjustable deposition current collector bamboo carbon(BNAC)can effectively induce the deposition of lithium metal on the side away from the separator during the cycle,achieving“bottom-up”deposition and inhibiting the growth of lithium dendrites.At the same time,the three-dimensional current collector carbon skeleton plays a limiting role in the deposition of lithium metal,alleviating the volume expansion of lithium metal during the deposition process and prolonging the service life of lithium metal anode.The average coulombic efficiency can reach98.6%under the test conditions of the three-dimensional composite lithium anode deposition of 10 m A h/cm~2.The LFP full battery has 140 m A h/g at a current density of 1 C,showing good rate cycle performance.The capacity retention rate of BNAC/Li||NCM full battery is 95.0%after 100 cycles at 0.2 C current density.