Taking inspiration from trees, scientists have developed a sliver of wood coated with tin that shows promise for becoming a tiny, long-lasting, efficient and environmentally friendly battery.

The report on the device, some 1,000 times thinner than a sheet of paper, has been published in the journal Nano Letters.

A team at the University of Maryland has demonstrated that a material consisting of a thin tin (Sn) film deposited on a hierarchical conductive wood fiber substrate is an effective anode for a sodium-ion (Na-ion) battery.  The goal is to address some of the limitations of other Na-ion anodes such as capacity fade due to cycling tearing the anode apart.

Using sodium instead of lithium, as many rechargeable batteries do, makes the battery environmentally benign. Sodium doesn’t store energy as efficiently as lithium, but its low cost and common material making it ideal to store huge amounts of energy at once, such as solar energy at a power plant.

Most all production batteries are created on stiff electrodes, which are too brittle to withstand the swelling and shrinking that happens as electrons are stored in and drawn out from the battery electrodes.

Liangbing Hu, Teng Li and their team found that wood fibers are supple enough to let their sodium-ion battery last more than 400 charging cycles, which puts it among the longest lasting nanobatteries.

Hu, an assistant professor of materials science explained, “The inspiration behind the idea comes from the trees. Wood fibers that make up a tree once held mineral-rich water, and so are ideal for storing liquid electrolytes, making them not only the base (component of the electrode) but an active part of the battery.”

Wood Fibers In a Nanobattery. Click image for more info.

Wood Fibers In a Nanobattery. Click image for more info.

The soft nature of wood fibers effectively releases the mechanical stresses associated with the sodiation process, and the mesoporous structure functions as an electrolyte reservoir that allows for ion transport through the outer and inner surface of the fiber. The team reported stable cycling performance of 400 cycles with an initial capacity of 339 mAh/g – a significant improvement over other reported Sn nanostructures.   The team suggests the soft and mesoporous wood fiber substrate can be utilized as a new platform for low cost Na-ion batteries.

Wood fibers, the authors note, are tracheids – hollow elongated cells that transport water and mineral salts. Pores in the fiber wall allow for intercellular fluid transportation. Natural wood fibers with diameters on the order of 25 µm serve as the substrate for the tin film.

The team initially coats the fibers with a thin layer (10 nm) of single-walled carbon nanotubes to provide electrical conductivity. Looking ahead the team suggests various other conductive materials, including graphene, metal nanowires, and conductive polymers could be deposited on wood fibers with similar solution-based processes.

Zhu and other team members noticed that after charging and discharging the battery hundreds of times, the wood ended up wrinkled but intact. Computer models showed that that the wrinkles effectively relax the stress in the battery during charging and recharging, so that the battery can survive many cycles.

Li, an associate professor of mechanical engineering explains, “Pushing sodium ions through tin anodes often weaken the tin’s connection to its base material. But the wood fibers are soft enough to serve as a mechanical buffer, and thus can accommodate tin’s changes. This is the key to our long-lasting sodium-ion batteries.”

The researchers noted the wood fiber has a high capacity for electrolyte absorption. Liquid electrolytes penetrate the porous structure of the fiber, allowing for sodium ion diffusion through the fiber cell walls in addition to diffusion at the tin film surface. This creates a dual ion transport path that effectively addresses the slow kinetics of tin anodes for Na-ion batteries.

The idea, and that it works is quite pleasing.  The Maryland team’s nature-inspired idea neatly solves a troublesome matter for the light metal class of batteries.  You can be just certain, with the promise of low cost production this research is going to be repeated across a huge array of cellulosic products, tree species and every other innovation that can be imagined.

The tin on wood anode is already suggested to be ideal for grid scale storage. The materials used are earth abundant and environmentally friendly, and electrodeposition and conductive fiber substrates are scalable for large throughput manufacturing.

This is just step one.


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