Electrodeposition of multilayered sn dendrites and their electrochemical properties for Na-ion batteries anodes

Abstract

Under non-equilibrium condition (a large undercooling in solidification or a large overpotential in electrodeposition), crystallites rapidly grow along energetically favorable crys-tallographic directions, resulting in a characteristic tree-like structure, termed dendrite. The den-drite is constituted by a main stem and primary, secondary, tertiary, and even high-order branch-es, providing exceptionally large surface area and extremely short diffusion pathway. Driven by the advantages, metallic dendrites have been studied for use in various electrochemical applica-tions. However, the dendritic structure has shown two limitations to applications for active materials of energy storage systems such as batteries and supercapacitors. Dendrites generally consist of needle-like deposits and have narrow contact areas at material/material interfaces. Ac-cordingly, they have a relatively weak mechanical strength and an undesirably high contact re-sistance compared with bulky structures. The low mechanical stability may cause a detachment of active material from substrate during electrochemical reaction, which are usually accompa-nied by the volume change of active material. Moreover, the high contact resistance give rise to high ohmic polarization which leads the loss of power density of the energy storage systems. Therefore, in order to use dendrite in batteries and supercapacitors, the structural modification is needed to have a high mechanical strength and a low contact resistance. In this study, multilayered 2D nanodendrites were formed by one-step, ultra-fast electrodeposition on a smooth Cu substrate from aqueous solution containing 0.1 M $SnSO_4$, 0.7 M $H_2SO_4$, and $0.2 g L^{-1}$ coumarin at high cathodic current density of $-2 A cm^{-2}$ for 2 s. The Sn den-drites showed clear distinctions with the morphology of Sn dendrites formed in solution without coumarin. Solely standing 3D dendrites with micron-sized branches, which has grown in ran-dom directions, were formed in coumarin-free solution. Contrarily, 2D dendrites with nano-sized branches, which has grown along the set of <220> direction, were formed in coumarin-containing solution. The 2D dendrites were lying along the substrate laterally and grown layer-by-layer, forming multilayered structure embedding high density of pore volumes between adja-cent layers. In the electrodeposition process, the coumarin performed three vital roles to form mul-tilayered 2D nanodendrites. First, the coumarin restricted the growth and aggregation of Sn crys-tallites, successfully limiting the Sn branches to ultra-small sizes. Second, the coumarin modified the Sn dendrites to grow two dimensionally by negating the effects of hydrogen bubbles, which were generated simultaneously with Sn electrodeposition at a high cathodic current density, on the morphology of Sn dendrites. Generally, the Sn dendrites are known to grow along the [110] direction, and have two dimensional structure. However, the hydrogen bubbles greatly change the growth tendency of Sn dendrites by causing local convention in electrodeposition solution and also by changing electric field distribution at the adjacent of substrate surface, forming fern-shaped 3D dendrites. In this electrodeposition process, coumarin greatly reduced the size of the hydrogen bubbles, and negated the effects of hydrogen bubbles on the crystallization of Sn, and hence lead to growth of the 2D Sn dendrites along the [110] direction. Third, the coumarin sig-nificantly reduces Sn crystallization rate by participating in reduction reaction itself, thereby causing Sn ion to accumulate in solution adjacent to the surface of electrode. The accumulated Sn ions near the surface of electrode cause the 2D Sn dendrite to grow laterally or parallel to the substrate. Once the 2D Sn dendrite has grown on the substrate, the minor branches along the c-axis developed on a main dendrite stem. Using the minor branches as initiation sites for the growth of dendrite, a new 2D dendrites progressively grow on the pre-grown dendrites progres-sively, eventually forming the multilayered 2D Sn dendrites. The main stem-connected multilayered 2D Sn dendrites with high density of Sn crystal-lites are expected to have a high mechanical strength in comparison with solely standing 3D Sn dendrites. Moreover, multilayered structure may offer the wide contact interfaces at materi-al/material interfaces, which can greatly relieve the electrical resistances that are always generat-ed in traditional 3D dendrites. Ultra-fine sub-branches composing the multilayered Sn dendrites may also offer a much higher surface area and shorter diffusion pathway than any other hierar-chical 3D structures. The Sn multilayer, when used as an anode for Na-ion batteries, delivered a high re-versible capacity and excellent cyclability. The maximum capacity was measured to be $783.88 mAh g^{-1}$ at 15th cycle. The charge capacity of the Sn multilayer anode after 100 cycles was measured to be $759.27 mAh g^{-1}$ that correspond to 96.86% of the maximum value. The rate ca-pability was also enhanced to a charge capacity of $412.84 mA g^{-1}$ at 5 C, and then fully recov-ered with good cycle stability when the current density returned to 0.1 C from 5 C. The superior electrochemical performance of the Sn multilayer was mainly attributed to their high mechanical and structural stability during sodiation/desodiation process, extremely high surface area, and large contact area between material/material interfaces, or the characteristics of the Sn multilayer.