Electrodeposition of Crystalline Si Using a Liquid Zn Electrode in Molten KF–KCl–K₂SiF₆

Abstract

We propose a novel Si electrodeposition method using a liquid Zn electrode in molten KF–KCl. Electrochemical measurements and electrolysis were conducted in a KF–KCl–K2SiF6 melt at 923 K. Cyclic voltammograms at a liquid Zn electrode revealed that the reduction currents at 0.75–1.0 V vs. K+/K were attributed to the formation of Si–Zn liquid alloy. Additionally, Si was deposited through potentiostatic electrolysis at 0.75 V using liquid Zn in a boron nitride (BN) crucible as an electrode. Cross-sectional scanning electron microscopy and energy-dispersive X-ray spectroscopy showed that deposited Si was located at the bottom and side of the interface between Zn and the BN crucible instead of at the interface between Zn and the molten salt, indicating the electrodeposition of Si attributed to Si–Zn liquid alloy formation. The obtained Si was confirmed to be the crystalline form by X-ray diffractometry, and the maximum grain size was approximately 2 mm. Galvanostatic electrolysis at –20 mA cm–2 with varying electrical charges showed that the Si grain size increased with increasing charge, confirming the growth of crystalline Si. Finally, the mechanism of Si electrodeposition on a Zn electrode through Si–Zn alloying was discussed.

Electrodeposition of Crystalline Si Using a Liquid Ga Electrode in Molten KF–KCl–K2SiF6

“…5. 31 The mechanism for the growth of solid Si is expected to be similar in this study using liquid Ga electrodes; thus, the growth of crystalline Si is considered to have occurred mainly during electrolysis and only slightly during the cooling process. Hereafter, Zn and Ga are referred to as M. First, as shown in Fig.…”

“…The crucibles were composed of boron nitride (BN) or alumina. The detailed structure of the liquid Ga electrode using a BN crucible is described in our previous study, 31 whereas that using an alumina crucible is shown in Fig. 2b.…”

“…Third, many impurity elements have small distribution coefficients between the solid Si and liquid Ga, which prevent the impurity elements from mixing into solid Si. As a result of the initial study using a liquid Ga electrode in molten KF-KCl-K 2 SiF 6 , large millimeter-sized Si grains were obtained via electrolysis at a relatively low temperature of 923 K. 33 We also studied Si electrodeposition using a liquid Zn electrode in molten KF-KCl-K 2 SiF 6 at 923 K. 31 In this study, large millimeter-sized Si grains were also obtained. We then examined the change in the grain size distribution of Si by varying the charge on the electrode and proposed a growth mechanism for Si crystals.…”

“…2 Si electrodeposition using high-temperature molten salts yields highly crystalline Si. Studies on Si electrodeposition using fluoride, [3][4][5][6][7][8][9][10][11] chloride, [12][13][14][15][16][17][18] and fluoride-chloride mixed molten salts [19][20][21][22][23][24][25][26][27][28][29][30][31] have been reported. We focused on using KF-KCl molten salt.…”

“…We focused on using KF-KCl molten salt. [25][26][27][28][29][30][31] Fluoride-based molten salt is a reasonable choice for obtaining compact and smooth Si films, as the high fluoride ion concentration enables SiF 6 2− ions to exist stably in the melt. 2,25 Using single KF molten salt is difficult due to the high melting point, but electrolysis at lower temperatures can be conducted with a KF-KCl binary system, which also simplifies the salt-removing step due to the high solubilities of KF and KCl in water.…”