Calcium – Oxygen Batteries: Challenges and Possibilities

Abstract

The present lithium-ion battery technology has a hard time keeping pace with the ever-increasing demands and requirements of various applications and markets. To enable continuous and sustainable growth of society, we need to develop new less expensive and less environmentally unfriendly energy storage solutions, and preferably also with higher gravimetric and volumetric energy densities, high power densities, and long(er) cycle-life. In this context, rechargeable metal–oxygen batteries are often considered the “Holy Grail” – this as the cathode of conventional metal-ion batteries most often is the most expensive component and at the same time the most challenging from both environmental and resource perspectives. An oxygen/air cathode, especially when used with a multivalent metal anode, can thus be a path to secure reliable energy storage for the future with significantly fewer concerns.Herein we describe the efforts made in the framework of our NATO-funded project: “High-energy Ca–O2 batteries”. The overall objective is to overcome the challenges of both a reversible oxygen cathode and the passivation of calcium metal via a proper selection of Ca metal, electrolyte design, and oxygen reduction catalysts/oxygen evolution mediators. We do this by developing two compatible half-cells with full cell KPIs in mind.Initially, we have explored, mainly via cyclic voltammetry, standard style organic electrolytes based on either Ca(TFSI)2 or Ca(OTf)2 and a boron-based additive, showing the latter to significantly enhance the Ca plating/stripping at room temperature. For the Ca(OTf)2 salt we create electrolytes using a tailored mixture of organic solvents, which not only allows for higher salt concentrations but also a wide ranges of additives possible.At the cathode/electrolyte interface a novel bi-functional multi-component catalyst has been employed to create gas-diffusion electrode (GDE) assemblies based on transition metal (Co, Mn) oxides as interactive supports for Ir and Pt in few-monolayer sizes. These are first tested using model aqueous solutions before turning to the organic electrolytes. Preliminary tests indicate controlled kinetics of the hydrothermal oxide generation and the electrochemical deposition/galvanic replacement, which could result in adequate bi-functional activity. Figure 1