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.