Surface Structure Control and Charge/Discharge Characteristics of Bismuth Anode Materials By Electrodeposition for Magnesium-Ion Batteries

Abstract

The magnesium ion secondary battery (MIB) has attracted much attention because magnesium (Mg) metal has high theoretical capacity (3,755 mAh/cm3), high electrochemical stability, and high abundance in the earth. However, developing anode and cathode materials with superior capacity for conventional electrolytes has required considerable study. Bismuth (Bi) has high capacity as an anode and also resists oxidation in Mg-containing electrolyte, so is a promising candidate material. However, experimental studies suggest that the available capacity has not yet reached the theoretical limit, and that the cycle life is short. This is due to sluggish diffusion of Mg2+ into the anode material and the slow interfacial charge transfer. We aimed to improve the electrochemical properties by atomizing the internal and surface structure of the Bi anode and enhancing the diffusibility of Mg2+. In this study, we prepared Bi films by an electrodeposition method, in which the structure of the Bi electrodes could be controlled by changing some of the electrodeposition conditions. We also evaluated the electrochemical properties of Bi anode materials deposited with different deposition times and current densities, to elucidate the relationships between the structure of the Bi anode and its capacity and cycle life.We prepared an electrolytic bath by dissolving H2SO4, Na2SO4 and polyethylene glycol in pure water, and subsequently added H2SO4 to the electrolytic bath to control the pH at 1.2. Before electrodeposition, we removed organic species and native oxide from the Cu foil surface with NaOH and HCl. We employed a Cu plate with an exposed foil area of 2 × 2 cm2 and a carbon plate as the cathode and anode, respectively. The Bi electrodeposition was carried out, while changing the deposition time and current density (from 10 to 50 mA/cm2). The film thickness was controlled by the deposition time (from 1.5 to 3.0 μm), and was calculated based on the weight increase under the assumption that the film was uniform. The surface morphology and crystal structure of the Bi films were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. In addition, the characteristics were compared before and after discharge/charge. The cycle characteristics of the Bi anodes were studied in PhMgCl/THF electrolyte. The bipolar half-cells were assembled with Mg plate as the anode and Bi electrode as the cathode. We established representative galvanostatic charge/discharge profiles of the Bi electrodes, at a current density of 50 mA/g within 0–0.6 V, versus Mg2+/Mg. We evaluated the cyclic stability of the Bi electrodes under the same conditions until the 50th cycle. The elemental composition and chemical bonding state of the Bi films after charge/discharge were evaluated by energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS).Figure 1 shows the cycling performance of Bi films with thicknesses of 1.5, 2.0, and 3.0 μm until the 50th cycle. Each film showed a capacity of about 300 mAh/g at the early stage of cycling, which is the maximum capacity. The capacities were about 1.5 times those of the Bi anodes which Niu et al. prepared by powder metallurgy. The cycle performance differed among Bi films differing in thickness, and the 1.5-µm Bi film maintained the maximum capacity until the 25th cycle. Thus, film thickness had an impact on the cycle performance.Figure 2 shows the surface morphology of Bi films with different thicknesses before and after the charge/discharge reaction. The figure also shows the compositional ratio of Bi to Mg on the surface of the Bi films. As the film thickness increased, agglomerates involving a large number of particles, indicated by the circles marked by “A” in Fig. 2, tended to increase in diameter from 1 to 3 µm. After charging/discharging, Mg dendrites were observed on the surface of the Bi films with thicknesses of 2 and 3 µm (circles marked by “B”), which resulted in a low capacity maintenance rate. Thus, the localized formation of Mg dendrites on the Bi surface could explain the decreased cycle performance of the Bi films prepared in this study. Figure 1