| Sodium metal is regarded as one of the most prospective next-generation anodes material owing to its high theoretical capacity,low redox potential,low cost,and natural abundance.However,its most notable problem is the dendrite growth during Na plating/striping,which causes not only the generation of inactive Na(“dead Na”)but also the safety concern.In order to solve the problem of sodium dendrites formation in sodium metal batteries during the cycle,alloy-type current collectors(Bi(?)CNs@Cu and Bi2S3@Cu)was designed and prepared and the behavior of Na plating/striping was studied.The influences of inductive nucleation and uniform deposition of sodium on the current collector has been deeply studied by means of electrochemical performance analysis and morphology characterization.The mechanism for the excellent performance of Bi(?)CNs@Cu current collectors and Bi2S3@Cu current collectors as the nucleation layer of sodium metal battery are systematically analyzed.Firstly,two-dimensional carbon nanosheets with uniformly dispersed bismuth nanoparticles(Bi(?)CNs)were prepared and used as the nucleation layer of the sodium current collector to guide the uniform deposition of sodium metal.As a result,the metallic Na on the Bi(?)CNs nucleation layer is repeatedly dendrite-free plated/stripped for nearly 7700 h(1287 cycles)at 3 m Ah cm-2 with an average CE of 99.92%.Moreover,the Na||Na symmetric cells with the Bi(?)CNs buffer layer are stably plated/stripped for 4000 h at 1 m A cm-2 and 1 m Ah cm-2.Through electrochemical impedance spectroscopy(EIS),it is found that the first-turn interface resistance of the half-cell assembled by commercial copper foil is about 50?,and the interface resistance gradually increases as the number of cycles increases.The full battery test found that the discharge specific capacity of the full battery using only commercial copper foil dropped suddenly after 78 cycles,while the full battery using Bi(?)CNs@Cu could cycle 290 cycles,and the capacity retention rate was as high as 88%.The study show that the sodium deposition on the Bi(?)CNs@Cu current collector was very smooth,which further proved the uniform deposition of sodium;and cracked metallic sodium was produced on the copper foil,indicating that the sodium was not uniformly deposited on the copper foil,and cracks in the shape is easy to produce sodium dendrites,which pierce the diaphragm.The cross-sectional view also shows that the sodium deposition is very smooth and dense,with almost no dendrites,while the cross-section of the copper foil deposited with sodium is rugged and the sodium deposited on the top looks relatively fluffy.Secondly,for the purpose of reducing the difficulty of the nanostructure design and synthesis of the nucleation layer on the surface of the current collector,in this paper,Bi2S3 bulk was synthesized by a simple hydrothermal method and annealing process,then coated on the surface of the current collector.The battery using Bi2S3@Cu current collector exhibits excellent coulombic efficiency performance and voltage cycle stability.Through electrochemical impedance spectroscopy(EIS),it is show that the first-turn interface resistance of the half-cell assembled by commercial copper foil is about 100?;the first-turn interface resistance of the battery using Bi2S3@Cu collector is relatively small,only 1.5?.And in the 100th cycle,the interface resistance did not increase significantly.The nucleation overpotential of current collector found that the nucleation overpotential on the commercial copper foil is 19.5 m V,while the nucleation overpotential on the Bi2S3@Cu collector is only 3.1 m V.The full battery test show that the discharge specific capacity of the full battery using only commercial copper foil drops suddenly after 25 cycles,while the full battery using Bi2S3@Cu,it can circulate200 times.Both Bi(?)CNs and Bi2S3 have a positive effect on guiding the uniform deposition of sodium metal and maintaining the stability of the interface during the electroplating/stripping cycle. |
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