The promise of tree-sourced batteries

Scientists are looking for materials to make sustainable batteries as the demand for electric vehicles grows. Lignin, the substance that gives trees their woody appearance, is emerging as a strong contender.

A major Finnish paper producer realised the world was changing about eight years ago. Paper had been steadily declining due to the rise of digital media, a decline in office printing, and the dwindling popularity of sending things by post, among other factors.

Stora Enso describes itself as “one of the world’s largest private forest owners” in Finland. As a result, it has a large number of trees, which it uses to produce wood products, paper, and packaging, among other things. It now wants to make batteries, specifically electric vehicle batteries that can charge in as little as eight minutes.

Engineers were hired by the company to investigate the use of lignin, a polymer found in trees. Depending on the species, lignin accounts for about 30% of a tree’s weight; the remainder is mostly cellulose.

“Lignin is the glue in the trees that kind of glues the cellulose fibres together and also makes the trees very stiff,” says Lauri Lehtonen, director of Stora Enso’s lignin-based battery solution, Lignode.

Carbon is found in lignin, a polymer. Carbon is also an excellent material for the anode, a critical component in batteries. The lithium ion battery in your phone almost certainly has a graphite anode – graphite is a layered form of carbon.

Engineers at Stora Enso determined that they could extract lignin from waste pulp produced at some of their facilities and process that lignin to make a carbon material for battery anodes. The company is collaborating with the Swedish company Northvolt and intends to manufacture batteries as early as 2025.

With more people purchasing electric vehicles and storing energy at home, the global demand for batteries is expected to rise sharply in the coming years. “The demand is just mind-boggling,” says Lehtonen.

In 2015, a few hundred additional gigawatt hours (GWh) were required annually across the world’s battery stocks; however, this will increase to a few thousand additional GWh required annually by 2030 as the world transitions away from fossil fuels. according to the management consulting firm McKinsey. The issue is that the lithium ion batteries we use today rely heavily on environmentally hazardous industrial processes and mining. Furthermore, some of the materials used to make these batteries are toxic and difficult to recycle. Many are also sourced from countries with a poor record of human rights.

To make synthetic graphite, for example, carbon is heated to temperatures of up to 3,000 degrees Celsius (5,432 degrees Fahrenheit) for weeks at a time. According to consultancy Wood Mackenzie, the energy for this is frequently derived from coal-fired power plants in China.

The search for more widely available sustainable battery materials is on. Some say they can be found in trees.

All batteries, in general, require a cathode and anode – the positive and negative electrodes, respectively, through which charged particles known as ions flow. When a battery is charged, lithium or sodium ions, for example, move from the cathode to the anode, where they settle like cars in a multi-story parking garage, explains Jill Pestana, a battery scientist and engineer based in California who works as an independent consultant.

“The main property that you want in this parking structure of a material is that it can easily take in the lithium or sodium and let it go without crumbling apart,” she says.

When the battery is discharged to power an electric vehicle, the ions return to the cathode after releasing electrons – In an electrical circuit, electrons move through the wire, transferring energy to the vehicle.

Pestana describes graphite as a “spectacular” material because it functions so well as a reliable anode, allowing such reactions to occur. Alternatives such as lignin-derived carbon structures will have to fight to prove that they are capable.

However, several companies are investigating lignin’s potential in battery development, such as Bright Day Graphene in Sweden, which produces graphene – another form of carbon – from lignin.

Lehtonen extols the virtues of his company’s carbon anode material, Lignode, which Stora Enso has named. He won’t say how the company converts lignin into a hard carbon structure, or what that structure is, except to say that it involves heating the lignin – but not to temperatures as high as those required for synthetic graphite production.

According to Lehtonen, one important feature of the resulting carbon structure is that it is “amorphous,” or irregular: “It actually allows a lot more mobility of the ions in and out.”

According to Stora Enso, this will aid in the development of a lithium ion or sodium ion battery that can be charged in as little as eight minutes. Fast charging is a key goal for electric vehicle battery developers.

Separate research into lignin-derived carbon anodes by Magda Titirici and colleagues at Imperial College London in the United Kingdom suggests that conductive mats containing intricate, irregular carbon structures with many oxygen-rich defects are possible. According to Titirici, these defects appear to increase the anode’s reactivity with ions transferred from the cathode in sodium ion batteries, which shortens charging times: “This conductive mat is fantastic for batteries.”

Wyatt Tenhaeff of the University of Rochester in New York State has also created lignin-derived anodes in the lab. Lignin is “really cool,” he says, because it is a byproduct with many potential applications. In experiments, he and his colleagues discovered that they could use lignin to create an anode with a self-supporting structure that didn’t require glue or a copper-based current collector, both of which are common components in lithium ion batteries. Despite the fact that this could bring lignin-derived carbon anodes down in price, he is sceptical that they can compete commercially with graphite anodes.

“I just don’t think it’ll be a significant enough step change in terms of cost or performance to replace the entrenched graphite,” he says.