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Battery-boosting breakthrough grows on trees – literally

As the hoary baseball cliché puts it, 'Grab some pine, meat!'

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Battery technology has stubbornly resisted major breakthoughs, but a team of researchers at the University of Maryland has found help from a most unlikely source: pine trees.

In a paper published in the American Chemical Society's Nano News, the team describes an experiment they conducted which solves a number of the challenges that have prevented battery designers from substituting cheap, abundant sodium for more expensive, rarer lithium.

Lithium, as anyone who has been following battery technology for the past decade or so, is the basis for the lithium-ion, lithium-polymer, lithium-sulphide, and lithium ion phosphate batteries that power everything from smartphones to electric racing cars.

Sodium and lithium are both quite capable of becoming positively charged ions by dumping an electron, which makes them both prime candidates for battery usage, since a battery works by moving ions between an anode and a cathode.

One problem, however: a sodium atom is a big ol' sucker, having an atomic weight of 22.99 compared to lithium's svelte 6.94. When a sodium ion slams into those electrodes it can do some serious damage – well, serious when compared with a lithium ion.

This damage shortens the number of times a sodium-ion battery can be recharged, making it less economical – which is too bad, seeing as how there's so much cheap, easily available sodium in nature.

To the rescue come the researchers and their paper, "Tin Anode for Sodium-Ion Batteries Using Natural Wood Fiber as a Mechanical Buffer and Electrolyte Reservoir". Their brainstorm was to take cellulose fibers from a yellow pine tree, coat them first in carbon nanotubes to improve conductivity and then a tin film – tin being the anode material of choice for both lithium and sodium ion batteries.

Not only did the yellow-pine cellulose fiber absorb much of the shock caused by the heavy sodium ions slamming into the tin, it also served as a fine receptacle for the necessary electrolytes. Or, as the researchers put it in fine scientific prose:

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's woodworking skills resulted in a battery that performed reasonably well for a first-try prototype, dropping from its initial capacity of 339 milliamp hours per gram to 145 mAH/g after 400 rechargings. A little tuning, and it's well-nigh inevitable that its performance will increase.

But don't think that you'll soon be slipping slivers of pine into your smartphone or laptop. As the teams writes in the abstract to their paper, the goal of their research is to produce "low cost grid scale storage" – meaning batteries that can, for example, feed a power grid at night after photovoltaics have charged them during the day, or do the same when their electricity-producing wind generator is becalmed.

And don't worry about deforestation – a single yellow pine can provide enough cellulosic material for plenty of sodium-ion batteries. ®

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