Calcium Chemistry as A New Member of Post-Lithium Battery Family: What Can We Learn from Sodium and Magnesium Systems

The development of next-generation battery technologies needs to consider their environmental impact throughout the whole cycle life, which has brought new chemistries based on earth-abundant elements into the spotlight. Rechargeable calcium batteries are such an emerging technology, which shows the potential to provide high cell voltage and high energy density close to lithium-ion batteries. Additionally, the use of Ca²⁺ as a charge carrier renders significant sustainable values. Although pioneering work on the electrochemistry of Ca has been carried out for more than half a century, demonstration of reversible Ca⁰/Ca²⁺ redox chemistry in non-aqueous media was only achieved within the past decade. In this review, we will present recent development of rechargeable calcium batteries, focusing on mainly the similarities but also differences between Ca chemistry and other post-lithium chemistry. According to the periodic nature of elements, magnesium (an alkaline earth element as Ca) and sodium (a diagonally adjacent element to Ca) have similar chemical properties to Ca in various aspects. We shall elaborate on how the solution chemistry, metal behaviors and transport mechanisms of Ca-ions can be better understood in light of the established principles in the respective Mg/Na systems. We hope the discussion will inspire synergetic development between Ca batteries and other post-lithium systems.