Can batteries of the future do without metal?
Dhe worldwide market for batteries is growing rapidly: By the end of the decade, the battery capacities required for the electrification of road traffic could increase tenfold to more than 2000 gigawatt hours. The demand for materials for the electrodes of lithium-ion batteries, such as lithium, nickel, cobalt and graphite, is growing accordingly. In order to avert a shortage of raw materials and to improve the ecological balance of battery production, scientists are looking for cheap and readily available substitutes for these substances. Several research groups have been trying for some time to use the biopolymer lignin, which is produced during paper production, in the negative electrodes (anodes). And with success.
In particular, it could replace graphite. Battery manufacturers have been using this mineral as an anode material since the 1990s. The dark grey, soft carbon is heat-resistant, conductive and capable of absorbing and releasing large amounts of lithium ions over many charge and discharge cycles without falling apart. On average, a single one contains around 75 kilograms of graphite battery an e-car, industry is expected to need around 900,000 tons per year in 2025.
The disadvantage of using graphite is its ecological footprint. Natural graphite occurs in metamorphic rock, so it needs to be mined and processed. Synthetic graphite, in turn, is produced by the thermal treatment of calcined petroleum coke and coal tar at temperatures of over 2800 degrees Celsius. China currently dominates the graphite market, which mainly uses coal as an energy source in its production. For this reason, a report by the British management consultancy Minviro concludes that the carbon footprint of Chinese graphite is significantly higher than previously assumed. Therefore, many battery developers consider lignin to be a suitable substitute.
A waste as organic waste
The polymer accounts for up to 35 percent of the mass of hardwood. As a component of the plant cell wall, it welds the cellulose fibers together and gives them the necessary rigidity. Lignin also contains carbon, but not in its pure form as in graphite, but in the form of aromatic benzene ring structures. When cellulose is obtained in the paper industry, it accumulates as biowaste and is usually incinerated. A waste – that’s what I thought too the Finnish forest company Stora Enso, which, as one of the world’s largest paper manufacturers, produces around 50,000 tons of lignin per year. His engineers have succeeded in chemically converting the brown, brittle substance into hard carbon powder and using it to produce thin carbon foils. These form the starting material for anodes. In a pilot plant in Sunila Mill, Finland, Stora Enso is further developing the technology to be able to produce negative electrodes on an industrial scale.
Ten years ago, researchers at Oak Ridge National Laboratory in Tennessee already recognized the potential of lignin-based carbon anodes. Lithium-ion batteries equipped with this achieve similarly high energy densities as classic lithium batteries with graphite electrodes. The prototypes were stable for up to 70 charge cycles. IIn 2017, Swedish scientists led by Göran Lindbergh from the Royal Institute of Technology in Stockholm electrodes from pulp production waste. For this, the lignin was first heat treated under vacuum to reduce the volatile content and then carbonized at 1000 to 1700 degrees. The specific capacity of the anode turned out to be inversely proportional to the temperature. At 1000 degrees it was 335 milliampere hours per gram and was almost as high as with conventional graphite anodes. In another test, it was possible to supply a remote-controlled model car with electricity using four such batteries.