The global market for batteries is growing rapidly: by the end of the decade, the battery capacities needed for the electrification of road transport could increase tenfold to more than 2000 gigawatt hours. Accordingly, the demand for materials for the electrodes of lithium-ion batteries, such as lithium, nickel, cobalt and graphite, is growing. In order to avert a shortage of raw materials and 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. Since the nineties, battery manufacturers have been using this mineral as an anode material. The dark grey, soft carbon is heat-resistant, conductive and capable of absorbing and releasing a large amount of lithium ions over many charge and discharge cycles without falling apart. On average, around 75 kilograms of graphite are contained in a single battery of an electric car, and the industry is expected to need around 900,000 tons annually in 2025.

The disadvantage of using graphite is its ecological footprint. Natural graphite occurs in metamorphic rock, so it must be extracted 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. Currently, China dominates the graphite market, which mainly uses coal's power generation 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 replacement.

As organic waste a waste

The polymer makes 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 pure form as with graphite, but in the form of aromatic benzene ring structures. When cellulose is extracted in the paper industry, it is produced as biowaste and is usually incinerated. A waste – so thought the Finnish forestry company Stora Enso, one of the world's largest paper manufacturers, produces around 50,000 tons of lignin per year. Its engineers have succeeded in chemically transforming the brown, brittle substance into hard carbon powder and producing thin carbon films from it. These form the starting material for anodes. In a pilot plant in Sunila Mill, Finland, Stora Enso is further developing the technology to produce negative electrodes on an industrial scale.

Ten years ago, researchers at Oak Ridge National Laboratory in Tennessee recognized the potential of lignin-based carbon anodes. Lithium-ion batteries equipped with it achieved similarly high energy densities as classic lithium batteries with graphite electrodes. The prototypes were stable for up to 70 charging cycles. In 2017, Swedish scientists led by Göran Lindbergh of the Royal Institute of Technology in Stockholm produced electrodes from waste from pulp production. For this purpose, the lignin was first heat-treated under vacuum to reduce the content of volatile components, and then charred at 1000 to 1700 degrees. The specific capacitance of the anode proved to be inversely proportional to the temperature. At 1000 degrees, it was 335 milliampere hours per gram and was almost as high as conventional graphite anodes. In another test, it was possible to supply a remote-controlled model car with power with four such batteries.