Metal-Air Batteries: Metal Deposition and Alloying in Aprotic Electrolytes for Anode Materials

Abstract

Metal air batteries concerning the increase in storage capacity, increase the lifetime, and reduction of weight, size and costs leads to more and more interest in battery research. The alloying-type anodes (Sb, Sn, and Bi as host material) for magnesium and calcium batteries are good choices for rechargeable metal batteries because of their high electrochemical properties.<br /> The lithium deposition/dissolution are investigated in 1 M and 3 M LiTFSI/DMSO solution on Au and Pt electrode. The highest coulombic efficiency of Li deposition/dissolution is observed in 3 M solution on Au electrode. MACC/TG and Mg(BH₄)₂/TG electrolyte systems show high reversibility and high coulombic efficiency (99 %). However, the freshly prepared MACC/TG electrolyte needs to be conditioned until it shows high reversible magnesium plating and stripping and the Al and Cl co-deposition is observed. High concentration of Mg(BH₄)₂/TG (1.5 M) electrolytes results in a coulombic efficiency of 70 % without addition of MgCl₂. With an addition of 0.5 M MgCl₂ the coulombic efficiency increase to 98 %. No reversible Ca deposition or alloying were observed in most Ca²⁺ containing electrolytes except 1.5 M Ca(BH₄)₂/THF (up to 98 % coulombic efficiency and 100 mV overpotential). <br /> The cyclic voltammogram for alloying/de-alloying with Sb, Sn, and Bi modified electrode shows a positive shift of the onset potential of bulk deposition compared to that at bare Au electrode. The ratio of moles agrees with the stoichiometry of Li₂Sb and Li₃Sb for Li alloying; Mg₃Sb₂, Mg₂Sn, and Mg₃Bi₂ for Mg alloying; and CaSb₂, Ca₃Sn, and CaBi₂ for Ca alloying. Three completely different behaviours were observed. First, the metal alloying is controlled first by the nucleation and charge transfer after the double-layer charging and then by the diffusion; or first by the charge transfer after the double-layer charging and then by the diffusion. Second, the other behaviour was controlled simultaneously by both charge transfer and diffusion. The current drops directly after the double-layer charge. Third, the metal alloying is only diffusion-determined process. The current drops directly after the double-layer charge.<br /> The diffusion coefficients were estimated to be 4-7×10^-14 cm²/s for Mg alloying with Sb and 1-4×10^-14 cm²/s for Mg alloying with Bi, which are one magnitude less than the diffusion coefficient 4-8×10^-13 cm²/s for Mg alloying Sn. The diffusion coefficients of Ca alloying were estimated to be 0.9-1×10^-13 cm²/s for Ca alloying with Sb and 2-3×10^-13 cm²/s for Ca alloying with Bi, which is one magnitude higher than the diffusion coefficient of 3×10^-14 cm²/s for Ca alloying with Sn. Furthermore, the diffusion rate of divalent cations (Mg and Ca) is less than the diffusion rate of Li in the Sb adlayers (4-6×10^-12 cm²/s) The Bi bulk shows similar crystalline structure before (in aqueous solution) and after (in organic solution) transfer into the glovebox. After the Mg alloying/de-alloying the surface of Bi is smoother and the crystalline structure disappears.