Science in search of the next super battery
The entire economy is greedy for batteries. Electric cars depend on them, as do laptops, smartphones and power tools. Soon, another area will be added that needs rechargeable batteries on a large scale: the storage of surplus renewable energy that cannot be used immediately. The demand for low-cost stationary storage batteries will therefore rise sharply. If possible, these batteries should be made of environmentally friendly materials so as not to further strain the world’s reserves of lithium, cobalt and other expensive metals – these substances are contained in lithium-ion batteries. Swiss Federal Laboratories for Materials Testing and Research (Empa) and ETH Zurich researchers Kostiantyn Kravchyk and Maksym Kovalenko took a closer look at alternatives to lithium-ion batteries.
One of the simplest ideas would be to replace lithium with sodium. This substance is available everywhere. But the disadvantages outweigh the benefits: Because a sodium ion is about 50 percent larger than a lithium ion, the materials at the cathode are electrochemically less stable. For this reason, sodium cobalt oxide (which corresponds to the lithium cobalt oxide in a standard lithium-ion battery) can withstand much fewer charging cycles. This would eliminate the cost advantage. There are also problems on the opposite side of the battery, the anode material. Graphite (as in the lithium-ion battery) is useless for sodium batteries, because it stores too few sodium ions. Experiments with cheap tin, antimony or phosphorus showed good results in storing electric charges, but at charging the anode inflates to three times its original volume. This affects the battery’s mechanical stability. There is an even more serious problem with phosphorus anodes: at recharging, sodium phosphide (Na3P7) is formed in the anode, which, together with water, produces monophosphane, an extremely toxic gas. Hardly anyone would want to have such a battery, fully charged with solar power, in their cellar.
Sodium is followed by magnesium in the chemical periodic table. It is a small, light atom and can transfer two electrons at once. Magnesium is cheap and non-toxic. Could one make batteries from it? On the anode side of the battery, magnesium does indeed have advantages: You don’t need graphite like in lithium-ion batteries. You can use metallic magnesium directly as the anode. But the small, double-charged magnesium ion has disadvantages on the cathode side. The high electrical charge on a small diameter leads to high electrical attraction forces. For example, the ion only slips into a lattice of cobalt oxide with a great deal of force, and if it is stuck there, it is difficult to get it out again. Anyone who tries to do so with force – i.e. with higher voltages – runs the risk of triggering oxidation and reduction processes in the chemical components of the battery, thereby destroying it. Such batteries are therefore not suited fast-charging and can only be used in a small voltage range if they are to last for a long time. They are also very inefficient because they need particularly high charging currents.
If you go one step further in the periodic table, you end up with aluminium. This metal is also available in large quantities, non-toxic and inexpensive. It can transfer three electrons. Similar to the magnesium battery, the anode is easy to build; an aluminium sheet is sufficient. However, the rest of an aluminium battery works fundamentally different from a lithium-ion battery: lithium-ion batteries are following the so-called “rocking chair principle”: When discharging, the lithium ions migrate from the anode to the cathode; when charging, they migrate back. In an aluminium-graphite battery, in contrast, the aluminium ions do not migrate back and forth between the anode and cathode in a direct path. Instead, components of the electrolyte liquid are “consumed” by both electrodes during charging: the electrolyte supplies aluminium on one side, which is deposited on the anode in the form of metal. On the other side of the battery, AlCl4 ions are removed from the electrolyte liquid and deposited in the graphite anode. The available quantity of electrolyte is therefore decisive for the capacity of the battery. Thanks to this chemical functional principle, an aluminum-graphite battery will always be about five times heavier than a comparable lithium-ion battery. In addition, there is another problem: the graphite cathode expands to more than twice its original volume during each charging process and shrinks again during discharge. This means that in any case, such batteries need flexible outer shells and protective housings with sufficient space to “breathe”. Inflating and shrinking has a negative effect on vibration resistance and long-term stability. New design solutions are required at this point.
An additional challenge is the charging algorithm for all such non-lithium-ion batteries. The research group led by Kravchyk and Kovalenko found that the performance of an aluminum graphite electrode could be increased by up to 25 percent by means of a clever step-by-step approach charging process. An international research group from Taiwan, China, the USA and Germany discovered that such electrodes are significantly more efficient when cooled to -10 degrees Celsius. These results make clear that a completely new battery management system, i.e. new sensors, chargers and algorithms, must be developed for the chemically completely different batteries.
It is still unclear which of the battery technologies described here will prevail and can replace lithium-ion batteries in some areas. In their analysis, the researchers also stress that none of the technologies presented can compete with lithium-ion batteries in terms of energy density. The researchers are convinced that hat this will very likely remain the case in the future. These alternative batteries are therefore only conceivable for applications in which electricity is to be stored as cheaply as possible and the focus is on the environmentally friendly production of the batteries.
In total, it becomes clear that there remains a lot to be done by research groups worldwide before alternative batteries can achieve a breakthrough. Kostiantyn Kravchyk and Maksym Kovalenko wish for a more holistic approach. “In the research world, an experiment often only proves the feasibility of an idea – the cost of all necessary components and the estimated total weight of the complete battery system, on the other hand, is often neglected,” says Kravchyk. But it is precisely these parameters that are decisive for commercialization. “They should therefore be given greater consideration in research work than has been the case to date.” Despite their somewhat sobering study, Kostiantyn Kravchyk will continue to research alternative storage batteries in the future. “Systems using graphite as a cathode will remain very interesting. We have already been able to show that the problem of swelling and shrinking of the cathode material can be overcome.
Together with his colleagues, he is now researching “semi-solid” graphite electrodes that last a long time and can also transmit electricity well.
More information: https://www.empa.ch/technologietransfer
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