Imagine your life in a world where the lithium ion battery was never invented. For starters, you probably wouldn’t even be reading this because you wouldn’t have a smartphone/tablet/laptop for which this platform was created. There is no doubt that the lithium ion battery has revolutionized the world of portable electronics, and is on its way to changing the transportation sector with its use in modern electric vehicles. Lithium battery research and development is similarly booming, with thousands of scientific articles published every year on the subject. But not everyone is convinced that lithium is the ‘holy grail’. Though we certainly have a lot of it, lithium is not one of the more abundant elements on earth, and much of it is trapped in geographically or politically sensitive areas. In fact, there is a whole world of research related to battery constructs based not on lithium but on similar metals such as sodium, magnesium, calcium and even potassium. For those of you who don’t have a periodic table handy these elements are all close to lithium in terms of size, weight, and importantly, charge. Magnesium and calcium are notably different as both carry a 2+ ionic charge in the framework of the battery, meaning that for every single ion that moves, so do two electrons. This is in contrast to the single electron released with the movement of a lithium ion across a battery. What this means is that although these elements may be heavier than lithium, they carry twice as much charge, making them an attractive area of study for researchers.
Calcium ion batteries are particularly interesting as calcium is one of the most abundant elements on earth and is characterized by a reduction potential similar to lithium, making it more attractive than it’s divalent cousin Magnesium. This high potential, which translates to high power density, combined with that two-electron charge transfer ability means that calcium ion batteries could be the future of energy storage.
A calcium battery operates in a nearly analogous manner to that of lithium ion battery, wherein Ca2+ ions are shuttled across an electrolyte (usually a liquid, salt-containing medium) between a positive electrode material and a calcium metal anode. In theory, the construction of a successful calcium ion battery would mean a significant breakthrough in terms of energy density capabilities compared to commercial batteries today. This means longer battery life for your portable electronics, and more importantly, increased driving range for electric vehicles.
Despite these promises, calcium ion batteries are essentially in their infancy compared to their lithium counterparts, and therefore they are plagued by issues that make success in their current state absolutely impossible. For starters, development of a suitable electrolyte that allows for the use of a Ca-metal anode is not complete. Current electrolyte compositions do not allow for efficient deposition and removal of Ca ions at the surface of the anode, rendering energy storage nonviable. Furthermore, finding appropriate solid electrode materials through which Ca2+ can diffuse readily has proven a challenge, likely due in part to the increased charge of the calcium ion hindering movement due to electrostatic interactions in the material.
But it’s not all bad. Every day researchers are working to identify appropriate materials via theoretical and modelling techniques, whilst materials scientists work towards actually making them a reality. In just the past couple of years, some electrolyte candidates have been reported that, although they don’t provide ideal performance, are actually able to create a functioning calcium cell. Fortunately for us, there are many talented scientists for whom calcium batteries are the future, and as was the case with lithium 40 years ago, materials development is only just beginning to determine what is even possible.
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