Electrolytic calcium hexaboride for high capacity anode of aqueous primary batteries

Abstract

The use of lightweight elements/compounds and the employment of multi-electron reaction (MER) chemistry are two approaches used to develope high energy density materials for batteries. In this work, nanoscaled calcium hexaboride (CaB₆) was prepared in one-step by the electro-reduction of solid calcium borate (CaB₂O₄) in molten CaCl₂–NaCl. CaB₂O₄ was synthesized by a simple co-precipitation method. It was revealed that the electrolytic CaB₆ delivers a specific capacity of 2400 mA h g⁻¹ in 30% KOH solution though a multi-electron reaction mechanism. Its practical gravimetric energy is three times that of Zn. Decrease of particle size substantially improves both the electrochemical activity and discharge capacity of CaB₆. The electrolysis of CaB₂O₄ in molten salt provides a straightforward and sustainable way to prepare high-capacity CaB₆ using low-cost and environmentally friendly boron and calcium resources, which can be recycled from the used CaB₆ primary batteries.