Electrochemically Synthesized Sn Nanodendrites As High-Performance Na-Ion Batteries Anodes
Authors: Kim Tae-Hee , Nam Do-Hwan , Hong Kyung-Sik , Sohn DongRak , Cho EunAe , Kwon HyukSang 
Published: February 2020

Because of their high energy density, Li-ion batteries have attracted attention as large storage systems over their traditional use for portable and electric vehicle applications. However, there are some concerns about the scarcity, uneven global distribution and high cost of lithium resourced. Compared with Li, Na has the advantages of natural abundance and low cost. Furthermore, Na and Li are in the same main group, exhibiting similar chemical properties, which translates to a resemblance between operational characteristics in Li-ion batteries and Na-ion batteries. Therefore, Na-ion batteries have been suggested for the alternatives to Li-ion batteries for large-scale systems. For the practical use of Na-ion batteries, studies for finding suitable electrode materials with high energy density, good cyclability and rate performance for Na-ion batteries have been actively conducted. Although several cathode materials have been suggested, the anode choices are severely limited because of the unique characteristic of Na. Na-ion cannot be stored much in commercial layered graphite because of its large radius. Si based materials are expected to be the most promising anode for Li-ion batteries, but it is electrochemically inactive with Na. On the contrary, Sn can store 3.75 Na/host-atom (Na15Sn4) resulting a maximum storage capacity of 847 mAh g-1 as an anode for Na-ion battery. Besides the high theoretical capacities, Sn also has advantages as an anode material for Na-ion battery such as low reaction potential and non-toxicity. However, pure Sn anode exhibits poor cyclability due to a loss of electrical contact with the current collector, resulting from a severe volume change (up to approximately 420%) during sodiation/desodiation. To improve the cyclability of Sn electrode, several strategies have been suggested such as introducing carbon or designing Sn-M intermetallic compounds for providing an active or inactive phase that acts as buffer matrix, and synthesizing a micro/nanostructured electrode for facilitating the relaxation of material stress. Among these strategies, synthesizing a nanostructured pure Sn, especially using electrodeposition, should be ideal solution. The material fracture is presumably due to the repeated non-uniform expansion and contraction during alloying and de-alloying. Because of a sluggish kinetic of sodium ion, the diffusion time is large compared to the charge/discharge time for the large sized particles which results uneven sodiation and desodiation states (different phases with different mechanical strengths) inside one particle. These variations in phase lead to stress, which can lead to crack propagation and pulverization. For this reason, nanostructured Sn could greatly reduce the stress induced during sodiation/desodiation by reducing the diffusion time of sodium ions in the electrode. Furthermore, using electrodeposition process, theoretical specific capacity of Sn can be fully used because the Sn anode are deposited directly on the current collector without mixing and drying processes for binder and conductive agent. Recently, we synthesized Sn nanodendrites with nano-sized branches. The Sn nanodendrites exhibited a high reversible capacities and excellent cycle performances. The maximum capacity was measured to be 783.88 mAh g-1 and the charge capacity at 100th cycle was measured to be 759.27 mAh g-1 that correspond to 96.86% of the maximum value. After cycling, the Sn nanodendrites exhibits no loss of active materials and it was attributed to the nano-sized branches greatly reducing the internal stress during sodiation/desodiation.