A group of researchers of the Institute of Materials Science of Barcelona (ICMAB – CSIC) has made a discovery that could entail the replacement in the future of the currently omnipresent Lithium–ion batteries. From mobiles to laptops, drills to hedge trimmers, or electric bicycles and cars, these batteries are used in countless devices, small and large.
Now, building on top of their previous discoveries in this field, the research group of M. Rosa Palacín has proven the feasibility of electrochemically extracting calcium from a calcium containing oxide. This opens the possibility for developing cathodes and giving proof-of-concept for calcium-based batteries, something previously considered unfeasible. These batteries may outperform Li-ion batteries in terms of energy density in the future.
Calcium and Lithium
Calcium, in its many forms, has applications in industries ranging from the construction and the manufacturing industry to the chemical, pharmaceutical or the food industry. With the discovery by ICMAB scientists, a new application could enlarge the repertoire: the design and construction of rechargeable batteries using calcium-based electrodes.
Are modern Lithium-ion batteries perhaps unsatisfactory? Certainly a common complaint in (intensive) mobile phone users is about the duration of their battery. Battery duration is related to its energy density, and is a key characteristic in any hand-held battery-powered device. Owing to the divalent nature of calcium ions (Ca2+), if compared to the monovalent lithium ions (Li+), batteries based on the first could be designed to have considerably higher energy. Result: batteries could last considerably longer.
In addition to this, Lithium ranks as the 33th most abundant element on the Earth crust, while calcium is the fifth. Being close to two-thousand fold more abundant than Lithium, the economic impact of switching to Calcium-based batteries could be enormous. Moreover, lithium is relatively scarce and potentially subject to future demand and supply issues.
The sources or processes to obtain Lithium and Calcium share similarities. Both can be obtained from ore materials, or also by means of brine mining. Regarding the first method, the most abundant mineral source of calcium is limestone, a sedimentary rock naturally rich in calcium carbonate. This mineral is the direct or indirect source of diverse calcium salts of industrial application. Remarkably, a process invented in the 1860s in Belgium, the notorious “Solvay process” for soda ash production, still significantly contributes today to global calcium chloride production.
In addition to its extraction from ore, calcium can also be obtained industrially by the more cost- and energy-effective brine mining method. In this process, dissolved salts are precipitated by evaporation. The precipitated salts may contain different proportions of ions of interest, components that can be purified later on. Whether from ore or brine, finally a stable calcium product is obtained, from which downstream uses will stem. The abundance of calcium will, in the long run, ensure that its price and supply remain stable, to the benefit of present and future industries relying on it.
Batteries, a simple view
One can consider, simplifiedly, that a battery has two main components: the electrolyte and the electrodes (the cathode and the anode). These electrodes correspond to the positive, and negative poles, respectively. Each electrode is host to a chemical reaction: the one releasing electrons; the other one, taking them up. The flow of electrons through the circuit from the anode towards the cathode is, ultimately, what generates the electric current our devices can use.
Another component is necessary, as the course of the reduction and oxidation reactions taking place at the electrodes would generate a charge imbalance blocking the operation of the battery. The charge is compensated by the use of a “salt bridge”. This, in practice, is a semipermeable membrane or separator disk, soaked in the electrolytic solution, allowing ion transport, in turn allowing the battery to avoid that charge imbalance, and keep operating.
If the battery needs to be rechargeable, the reactions in the cathode and anode have to be reversible. This allows that, upon application of an opposing external voltage, the electron current runs in the opposite direction. This allows to restore the electrodes to their initial state. If during operation or recharge some end-product products form that cannot be transformed back into the original elements of the battery, the battery will not be decreasingly rechargeable. The maximization of the reversibility of reactions inside the battery is a key: it ensures that it can be re-charged numerous times and extends its operation life.
The discovery: a new calcium-based cathode
Two years ago, unlike previously thought, the group of Prof. Palacín proved that metallic calcium could be used as an anode for batteries. The study was published in Nature Materials in 2016. Now, their discovery of a potential calcium cathode adds to this previous research, even if reversibility needs to be improved. Once this is achieved, a full calcium-based rechargeable battery will be feasible.
In the recent study, published in the journal Dalton Transactions, the electrochemical extraction of calcium from a metallic oxide has been achieved, proving that this oxide could be used as a cathode for calcium rechargeable batteries if reversibility was possible. M. Rosa Palacín, the ICMAB researcher leading the study, further explains that, in this case, a calcium-cobalt oxide was used.
Calcium ions (Ca2+) are divalent cations, meaning they have a double positive charge. Lithium ions (Li+), by comparison, have only one. Divalent charge carriers, as Ca2+, need only half the number of ions to achieve a certain electrochemical capacity if compared with lithium. The direct consequence of this is the possibility for the design of higher energy density batteries.
The experiments by the Palacín team were performed in conventional organic electrolytes and moderate operating conditions. The electrolytes used were similar to those used in the Li-ion battery technology and thus enable high cell potential. These findings are the first step to solve one of the main problems facing calcium-based batteries: to find cathodes able to incorporate and release calcium ions in a reversible way. This enables not only battery operation, but also re-charge and re-use.
Similar developments have been proposed using other divalent metal ions, as magnesium (Mg2+). However, amongst divalent electropositive ions, calcium ion (Ca2+) remains especially attractive due to its abundance. Another advantage in the context of the design of rechargeable batteries is that it has a standard reduction potential very close to that of lithium, which again enables high cell voltage. Using calcium, cells with voltages of up to 4 or 4.5 volt could be attained (similar to that of Lithium), making them suited for use in power-hungry devices and applications. Using magnesium, on the other hand, would allow at most voltages of only around 3 volt.
In addition to the previous, the higher charge to radius ratio of magnesium ions suggests that the reaction kinetics (the speed or reactions, hence the responsiveness of the battery) would be higher in the case of a calcium battery. In other words: calcium batteries would have a higher performance than their equivalent magnesium counterparts.
These results represent a crucial step towards the development of enhanced alternatives to the current Li-ion batteries. The development has resulted in a patent together with Toyota Motor Europe, the company partnering in the research. In order to bring rechargeable calcium batteries to the market, however, some hurdles have to be overcome. Firstly, a working prototype of rechargeable calcium battery needs to be developed. In addition, the reversibility of the cathode must be achieved, and all the components of the battery, optimized.
The story of calcium batteries is still to be continued. To support in the challenges ahead, the acquired experience with lithium batteries will be of inestimable help, as lithium batteries have been on the market for nearly 30 years. The calcium relief to the already veteran lithium-ion batteries is under way.