Three-Dimensional Porous Sb/Cu2Sb Anode 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 [1], 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 [2]. 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. Thus, in recent years, rechargeable sodium-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.Among the various candidate anode materials, Sb has been considered to be very attractive anode material due to its high theoretical capacity of 660 mAh g-1. However, pure Sb electrode exhibits the poor cycleability because of their significant volumetric changes (up to approximately 390%) that occurs during alloying/dealloying with Na.In a recent communication, we reported that when the porous Cu foam was used as a substrate for the Sn anode in Li-ion batteries, the Cu foam accommodated the volumetric expansion of Sn and effectively inhibited the delamination of the active materials, and hence the cycle performance of the Sn anode was considerably improved [3]. Therefore, the objective of this work is to fabricate the Sb-based electrode using a highly porous Cu foam as a substrate and to examine the electrochemical properties of such electrode for SIBs anode. Experimental. A three-dimensional porous Cu foam was prepared by the electrodeposition at a constant current density of -3 A cm-2 for 4 s from a Cu sulfate electrolytic bath containing 0.2 M CuSO4, 0.7 M H2SO4, 1.2 M (NH4)2SO4and 0.4 mM BTA(benzotriazole). After the Cu foam was prepared, the Sb was electrodeposited on the foam substrate.The electrochemical properties of the Sb/Cu2Sb electrode were investigated using Swagelok-type cells assembled in an Ar-filled glove box. The cell is composed of a sheet of the sb electrode (with an area of 1 cm2) and a Na metal electrode (with an area of 1 cm2) without a separator. Electrolyte was anhydrous propylene carbonate (PC) containing 1 M NaClO4. The charge/discharge characteristics of the electrode were galvanostatically examined at a current density of 0.1 C (based on mass of Sb) between 0.01 V and 1.5 V (vs. Na/Na+). To examine the rate capabilities of the Sb/Cu2Sb electrode, the cell was cycled at different current densities for the following cycles in the voltage ranges between 0.01 and 1.5 V (vs. Na/Na+). Results and discussion. As shown in Fig. 1, the Sb electrodeposit layer was uniformly deposited along the surface of the Cu foam substrate; thus, the prepared electrode exhibits a three-dimensional porous structure with a large surface area. Whereas the cyclic stability of the pure Sb electrode deposited on the smooth Cu sheet is poor, the Sb/Cu2Sb electrode exhibits outstanding cycle stability and excellent rate capability; the charge capacity is sustained at 485.64 mAh g-1 over 120 cycles with a high coulombic efficiency of 97%, and the charge capacity retention is approximately 70% of the capacity at 0.1 C-rate, even at high 3 C-rate. The improvement in the electrochemical performance of the Sb/Cu2Sb electrode is attributed not only to the highly porous structure of the electrode but also to the improvement of bonding between substrate and active materials by the formation of Cu2Sb intermetallic. The sodiation/desodiation reactions of Sb/Cu2Sb with Na+ is proposed based on the ex-situ XRD analysis. After being fully desodiated, the electrode is restored to the crystalline Cu2Sb phase, confirming that the Cu2Sb phase reversibly reacts with Na+[4]. Fig. 1.(a) Surface morphology and (b) cycle performance of the Sb electrode electrodeposited onto the Cu foam substrate [4].