Structural Design And Study Of High-performance Lithium Metal Anodes

As the gradual exhaustion of traditional energy and the increasingly serious environmental pollution caused by fossil fuels,it is urgent to develop advanced energy conversion and storage devices,in which lithium batteries play an important part.Anode is significant in battery and can greatly affect its energy density.At present,the current commercial used anode material is graphite,which is stable during cycling and cheap in price.However,the theoretical specific capacity of graphite is low and gradually can't meet the demand for portable electronic devices and electric vehicles.Therefore,developing advanced high-capacity anode materials is in urgent need.Lithium metal,with the highest theoretical specific capacity(3860 mAh g-1)and relatively lower electrochemical potential(-3.04 V vs standard hydrogen electrode),is an ideal anode material.Lithium metal battery,which is based on lithium metal anode,possesses high volume energy density and mass energy density,and thus is considered as one of the most promising next-generation high-performance energy storage devices.However,before lithium metal anode can be employed in practice,a variety of challenges need to be overcome,including lithium dendrites formation,irreversible side reaction between metallic lithium and electrolyte and the relatively infinite volume change during lithium plating/stripping process,which will lead to low Coulombic efficiency and severe safety issues.To address the above problems and improve the electrochemical performance of lithium metal batteries,the following strategies have been put forward:adoption of suitable host materials,electrolyte composition regulation,design for artificial protective layer and separator modification.Among these methods,using three-dimensional skeletons as host materials can effectively reduce local current density,suppress dendrites formation and limit the huge volume change of electrode during cycle.A variety of 3D structures of copper have been applied to lithium anode and proved to be effective host materials,however,considering the high density of copper compared with lithium,the overall energy density of lithium anode will be greatly decreased if copper is used as the host.Carbon-based materials have high electrical conductivity,low density,good chemical and electrochemical stability in electrolyte,and low interfacial energy with lithium metal,therefore,carbon-based 3D skeletons are considered as suitable lightweight host materials.In this thesis,we have designed and fabricated two kinds of carbon-based structures used as hosts for lithium metal anodes.The main contents are as follows.(1)Graphene aerogel with 3D porous structure was prepared by a hydrothermal-annealing method.GA was proved to have ultra-high specific surface area and a large number of defects and functional groups,showing superior lithiophilicity and ability to regulate deposition behavior of lithium.In addition,its 3D framework can limit the large volume change of lithium anode during cycle and stabilize SEI film.When lithium deposits on GA electrode,it shows uniform morphology without appearance of lithium dendrites.And the obtained composite anode displays superior long-term cycling stability with high Coulombic efficiency.(2)A 3D porous carbon skeleton decorated with ultra-fine silver nanoparticles(Ag-NCNS)was synthesized by a viable salt-assisted polymer blowing-up method.During the plating process,lithium can preferentially deposit on Ag NPs due to the extremely low nucleation overpotential.Therefore,Ag NPs can act as nucleation sites to regulate the deposition behavior of lithium and effectively inhibit the formation of dendrites.In addition,Ag-NCNS has a relative higher specific surface area,so the uniform deposition of lithium can also be promoted by reducing the local current density.As a result,half cells,symmetric cells and full cells based on Ag-NCNS electrodes all exhibit superior cycling stability.