Researchers from Empa and ETH Zürich have used graphene, waste graphite and scrap metal to make low-cost batteries.
The researchers’ ambitious goal at Empa is to make a battery out of the most common elements in the Earth’s crust such as magnesium or aluminum. These metals offer a high degree of safety, even if the anode is made of pure metal. This also offers the opportunity to assemble the batteries in a very simple and inexpensive way and to rapidly upscale the production. To make such batteries work, the liquid electrolyte needs to consist of special ions that do not crystallize at room temperature. The researchers were looking for a suitable cathode material, and decided to turn the principle of the lithium ion battery upside down.
In conventional Li-ion batteries, the anode (the negative pole) is made of graphite, the layers of which (in a charged state) contain the lithium ions. In this new battery, however, the graphite is used as a cathode (the positive pole). The thick anions are deposited in-between the graphene layers and the anode is made of metal.
The teams made a remarkable discovery while searching for the right graphite: they found that waste graphite produced in steel production, referred to as kish graphite, makes for a great cathode material. Natural graphite also works equally well if it is supplied in coarse flakes and not ground too finely or into folded, non-flake shapes. The reason is that the graphite layers are open at the flakes’ edges and the thick anions are thus able to slip into the structure more easily. The fine-ground graphite normally used in lithium ion batteries, however, is ill-suited for this battery: by grinding the graphite particles, the layers become creased like crumpled-up paper. Only small lithium ions are able to penetrate this crumpled graphite, not the new battery’s thick anions.
The graphite cathode battery constructed from steel production kish graphite or raw, natural graphite flakes has the potential to become highly cost-effective. Initial experiments indicate that it is also long-lasting: for several months, a lab system survived thousands of charging and discharging cycles. The aluminum chloride graphite cathode battery could last decades in everyday household use, explains the team and adds similar demonstrations, but further increased battery voltages, without compromising capacities, and of even lighter elements are on the way and will offer further increase in energy densities from current 60 Wh kg-1 to above 150 Wh kg-1.
