Novel Alkali Metal Anodes Based On Three-dimensional Carbon Nano-materials

Alkali metal anodes(AMAs),Li and Na for instance,are promising candidates for future energy storage facilities,owing to their high gravimetric capacity(Li:3860 mA h g-1,Na:1166 mA h g-1),low negative electrochemical potential(Li:-3.080 V vs.SHE,Na:-2.714 V vs.SHE)and capability to match with high capacity cathodes such as Air,Sulfur and layered compounds.However,practical use of AMAs is humbled by the dendrite problems,rooting from the uneven growth behavior of plated metal and uncontrollable volume expansion.According to previous works,composite anodes fabricated by 3D nano materials paired with alkali metal show much improved performance than bare metal anodes,since the 3D networks are able to help distribute charges in the composite anodes and relieve the large deformation strength from the plated alkali metal.To match with these property requirements,the network should be light weight,highly electronic conductive,ideal affinitive to alkali and mechanically stable.Carbon nano-networks,with diverse macro-features,ideal structure stability,good conductivity and easily modification,are perfect candidates to pare with AMAs.Depending on the great advantages of the carbon nano-networks,we designed and fabricated series of carbon nano-networks and composite materials based on these networks.Composite AMAs consist of the novel networks are able to withstand extremely high current density and areal capacity with much prolonged cyclic life.The accomplishments are displayed as follows:1.We employed a highly robust,flexible and expandable CNT network as host for Li metal anodes.The CNT appears an intact,continuous objective structure,and possesses high electron conductivity of 1.1E5 S m-1,tensile strength of 27 M Pa and areal density of 0.68 mg cm-2.Most importantly,although the pristine thickness of the CNT is 12 μm(3.2 μm after compacted),it was able to repeatedly expand to 115 μm and contract to 40 μm synchronously with the plated/stripped Li.Owing to this character,the plated Li kept tight contact with the CNT fibers and maintained its recyclability.The plated Li was totally preserved in the CNT network while no visible Li spot appeared on the surface of the anode.Owing to these advantages,the composite anode proved able to withstand more than 1000 rounds of cycling at a high current density of 10 mA cm-2 with a high cyclic areal capacity of 10 mA h cm-2.As the CNT network is flexible,robust and light-weight,such anode demonstrated a high cyclic mass capacity of 2870 mA h g-1.2.Given that the CNT was totally sodiophobic,we decorated one side of the CNT with sodiophilic Ag nanoparticles(ANPs).This 5 pm thick gradient film showed good contact with the surface of Na metal,so as to be a gradient buffer layer for Na metal anode(SMA).The plated Na was guided by the ANPs to deposit and grow at the interface between the buffer layer and Na anode.As a result,Dendrite or mossy Na structure rooting from the top-growth behavior was eliminated.The film shows high tensile strength of 73 M Pa,as to withstand the high deformation stress caused by the plated Na.Void of the buffer layer could accommodate the inhomogeneous growth of Na.SMA coated by such buffer layer could handle repeated plating/stripping of Na for more than 1000 rounds at a high current density of 8 mA cm-2 with a high cyclic areal capacity of 8 mA h cm-2.3.By step-by-step electrospinning method,we successfully established a novel 3D carbon current collector with both gradient electronic conductivity and gradient affinity to alkali metal.The raw material,with pure polyacrylonitrile(PAN)at one side and PAN with Co and Ag at the other side,was heated and graphitized at 800℃.The doped Co helped to improve the graphitization degree of the doped side,while the doped Ag improved the affinity to Na of the same side.As a combination of the advantages,such gradient network could evidently restrict the top growth behavior of plated Na when applied as the host for Na metal anode.The final product of Na composite anode could handle more than 300 h of Na plating/stripping at a high current density of 4 mA cm-2 with a high cyclic areal capacity of 8 mA h cm-2.