Fabrication Of Hierarchical Porous Copper-based Shape Memory Alloy Composite Electrode And Its Performance Characterization

Sn anode is considered to be one of the most ideal anode materials for the next-generation lithium ion batteries because of its high theoretical capacity,good conductivity and moderate working voltage.However,Sn anode suffers from huge volume expansion during charge and discharge process,which would cause pulverization of Sn particles and continuous formation of solid-electrolyte interface(SEI),leading to the sharply decreased capacity.Nanocrystallization,multi-phase composite and constructing porous current collector are thought to be three efficient ways to improve the cycling performance of Sn anodes.However,the huge volume expansion of Sn anode cannot be totally relieved by singly using one of the three methods mentioned above.Thus,in this article,we fabricated a micro-nano hierarchical porous Cu-based shape memory alloys(SMAs)current collector by combining the merits of the three methods to better relieve the volume expansion of Sn anode.The fabrication of the hierarchical porous Cu-based SMAs current collector was studied by adjusting the pore structure and phase constitution,and we further studied the influence of pore structure and phase constitution of the porous current collectors to the cycling performance of Sn anode.The following conclusions can be obtained from the results of the experiments:Firstly,we fabricated a macro-nano hierarchical porous Cu/?-CuAlMn SMAs composite current collector with nano-pores in size of 100-500 nm distributed on large-scale pores in size of 700-800 ?m through a molten-metal infiltration and dealloying method.From the results,it can been found that the size of nano-pores,and the mechanical properties decreased with the increasing of dealloying time.We can conclude from the experimental results that the sample dealloyed for 30 min with nano-pores in size of 100-150 nm possesses the best mechanical properties.And after electroless plating of Sn,the Sn electrode with the hierarchical porous Cu/?-CuAlMn SMAs current collector dealloyed for 30 min shows the best cycling performance,with a capacity retention of 35.1% after 100 cycles,which is much better than the Sn electrode with planar Cu foil.However,the large-scale pores are so largethat the SMAs matrix cannot have sufficient effect on accommodating the volume expansion of Sn anode distributed in the nanoporous Cu layer.Thus,the pore structure need to be further optimized.Then,we fabricated a micro-nano hierarchical porous Cu/?/? composite current collector by one-step dealloying of Cu-34Zn-6Al(wt.%)ribbons and heat-treatment.The large-scale pores of the composite current collector are in size of 1-2 ?m with nano-pores in size of 40-50 nm.After 3h heat treatment at 850? under Ar gas protection,the diffusion of Zn and Al elements can be clearly observed,and the hardness and recovery ability of the hierarchical porous current collector can be improved by the formation of ? phase layer with super-elasticity and ? phase layer with high hardness.After loading with Sn,the micro-nano hierarchical porous Cu/?/? composite current collector with higher hardness and recovery ability have greatly improved the cycling performance of Sn anode,with a capacity retention of 64.1% after 100 cycles,in comparison with the micro-nano hierarchical porous Cu current collector without heat-treatment and planar Cu foil.However,only small amount of ? phase can be formed during the heat treatment under Ar gas protection,and the effect of super-elasticity is not obvious.Thus,we need to further optimize the phase constitution to form a single ? phase structure.Finally,a micro-nano hierarchical porous ?-CuZnAl SMAs current collector was fabricated by optimizing the heat-treatment process.It is found that only the sample dealloyed for 90 min and after 850? vacuum heat treatment can form a hierarchical porous structure with single ? phase.The nano-indentation results show this hierarchical porous SMAs current collector possesses excellent super-elasticity,with a high recovery rate of 88.6% under the condition of 5 mN loading force.After loading with Sn,the micro-nano hierarchical porous?-CuZnAl SMAs current collector with higher recovery ability has further improved the cycling performance of Sn anode,in comparison with the current collectors mentioned above.