Electrochemical Properties of Electrodeposited Sn Anodes for Na-Ion Batteries

Abstract

Introduction Over the past few decades, Li-ion batteries (LIBs) have been widely used as power sources for mobile electronic products and power tools, and nowadays a much effort has been focused on the use of these batteries in large-scale applications such as electric vehicles, hybrid electric vehicles and electrical energy storage (EES) devices. However, because the amount of the Li resources would not be sufficient to meet industrial needs in the long term, the expansion of the LIBs market takes concern on the sustainable supply of Li and the raising of the Li prices. Accordingly, in recent years, rechargeable Na-ion batteries (SIBs) have received great attention as a possible alternative to replace LIBs. Owing to the abundant resource, low cost and a relatively low redox potential (0.3 V above that of Li/Li+), NIBs are expected to be a near-term alternative for large-scale systems such as grid storages [1].Despite the high specific capacity, pure Na metal is inappropriate as an anode material for practical applications of NIBs because the dendiritc deposition of sodium during charging can cause the severe safety problems as well as the reduced capacity and increasing electrode impedance. To overcome these problems, the research for finding suitable electrode materials which have high specific capacity, low irreversible loss, high coulombic efficiency and long cycle life for SIBs have been extensively conducted. Among the various candidate materials, Sn is one of the most attractive anode materials because of its high theoretical capacity and low reaction potential. When assuming complete sodiation of Sn into Na15Sn4, the capacity of Sn is approximately 847 mAh g-1[1], which is substantially higher than the reversible capacity of carbonaceous materials (approximately 250 mAh g-1) [2]. In this study, the electrochemical performance and the sodiation/desodiation mechanisms of electrodeposited Sn were investigated based on previous studies. Herein, two Sn electrodes that have completely different morphologies and crystal structures were prepared by electrodeposition, and their electrochemical properties for Na-ion battery applications were examined, with an emphasis on the effects of the morphology and the phase structure of Sn on the cyclability of the electrodes. Experimental. Two different Sn electrodes were prepared by electrodeposition from different electrolytes. The electrodeposition of Sn was performed in a two-electrode cell that consists of a smooth Cu sheet as the cathode and a Sn plate as the anode. A constant current density of -10 mAcm-2 was applied for 4 min at room temperature. The electrochemical properties of the Sn electrodes were investigated using Swagelok-type cells assembled in an Ar-filled glove box. The cell is composed of a sheet of the Sn electrode (with an area of 1 cm2) and a Na metal film (with an area of 1 cm2) without using a separator. The electrolyte was anhydrous propylene carbonate (PC) containing 1 M NaClO4 and 0.5 vol.% fluoroethylene carbonate (FEC). The charge/discharge characteristics of the electrode were galvanostatically examined at a current density of 50 mA g-1 between 0.001 and 0.65 V (vs. Na/Na+). Results and discussion. Two Sn electrodes that have completely different morphologies and crystal structures were successfully prepared by electrodeposition. One consists of coarse and isolated particles, but the other exhibits a compact thin layer composed of fine grains. Both electrodes showed excellent reversibility via three conjugated sodiation/desodiation reactions. Nevertheless, the particle-type Sn electrode had the poor cycle performance due to the electrical isolation of the active materials. In contrast, the layer-type Sn electrode showed a stable cycle performance in which the charge capacity was maintained at 607.51 mAh g-1after 40 cycles, which corresponds to 98.21% of its initial charge capacity. The difference in the cyclability of the electrodes was attributed to the morphology of the Sn electrodeposits and to the bond between Sn and the Cu substrate.