Durable batteries are large, but cheap -- perfect for grid storage

You won't find this battery in your laptop or smartphone anytime soon, but it may help deliver steady power to those devices.

I. Wood Battery -- Inspired by Nature

As the world races forward towards intermittent alternative energy sources such as wind and solar power, a major looming power is how to prevent power shortages when the wind isn't blowing or the sun isn't shining.  A proposed solution is to use batteries to store up power during peak generation times, and discharge it during lulls (for example, at night, for a solar powered grid).  However, batteries tend to be expensive, hard to recycle, and often harmful to the environment if not properly disposed.

Researchers at the University of Maryland think they have found a fix for these issues -- make your batteries out of wood.

A team led by materials science Professor Liangbing Hu and mechanical engineering Professor Teng Li found that a thin sheet of wood-derived cellulose soaked in sodium ions and coated with a thin film tin (Sn) anode was more environmentally friendly, closer to the durability of, and most importantly drastically cheaper than lithium-ion batteries.
The Univ. of Maryland nanobattery is based on cellulose. [Image Source: Wikimedia Commons]

Professor Hu says the battery was inspired by observations of natural processes, stating, "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 but an active part of the battery."

II. Cheap, Durable -- Good for the Grid

The sheet used in the battery was thousands of times thinner than a sheet of paper.  It was composed of molecules of cellulose -- rigid sugar chains.  It offered 400 cycles of charging/discharging at an initial capacity of 339 mAh/g, making it among the most efficient nanobattery designs found to date, according to the researchers.  By contrast a lithium battery typically has a capacity of 3,860 mAh/g [source] and a similar cycle life of between 300 and 500 cycles [source].

The new battery's long life owes to a special property of the cellulose bundles that compose the ion storage layer.  When they lose their sodium they "wrinkle" assuming a flower-like cross section.  This wrinkling was shown in computer simulations to relieve stress, making the battery last longer.  Longevity is further improved due to the properties of tin -- a soft, pliable conductor.

Carbon process
The cellulose bundles can undergo many cycles soaking up sodium and releasing it without significant degradation. [Image Source: ACS]

That longevity, combined with low production costs make the new wood nanobattery an intriguing alternative to lithium for large storage, despite the fact that it takes more than 10 times as much material by weight to store an equivalent amount of energy to a lithium ion battery. 

Comments Hongli Zhu, a postdoctoral researcher who led the study, "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 team published [abstract] their work last month in the peer-reviewed ACS journal Nano Letters.  The study was funded by grants from the University of Maryland and the U.S. National Science Foundation [grant].

Sources: Nano Letters [abstract], University of Maryland [press release]

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