By Will Soutter
Researchers at the University of Maryland's Nanocenter have developed a new battery using novel materials - but not the usual suspects. Their battery uses wood fibers, coated with carbon nanotubes and tin, and could be perfectly suited for large-scale grid storage applications.
FE-SEM image of the coated wood fibers used to create the Na-ion battery at Maryland Nanocenter
Lithium ion batteries are the current best-performing battery for most applications - their energy storage density and long-term cycle stability have not been matched by any other commercial battery technology.
Lithium-based batteries are not without their issues, however. Lithium itself is not very common, and supply chians are limited, making it quite an expensive material to use. There can also be environmental issues if lithium ion batteries aren't disposed of properly.
Greener batteries with sodium
Sodium-ion batteries are known to solve many of these problems - sodium is an extremely common element which is economical to produce, and it has very few environmental problems associated with it.
However, sodium-based batteries to date have not been able to come close to the performance of lithium batteries. Electrolyte and electrode materials which have been proposed previously have suffered from low energy density, and degraded quite rapidly over a relatively small number of charge-discharge cycles.
This is due to the physical processes that take place when a sodium-ion battery is charged and used. The most common electrode material is tin, which is very good at absorbing sodium ions. However, as the tin electrode saturates, it can swell to over 400% of it's volume. This causes extreme physical strain on the battery itself, effectively destroying the electrodes after around 20 charge cycles.
Even advanced materials that have been suggested as possible electrodes for these batteries have not performed too well - even a tin oxide/graphene composite lacked cycling performance and processability for large scale use.
Naturally flexible fibers
Now, however, the research group at the Maryland Nanocenter, led by Hongli Zhu, has created a sodium ion battery based on wood fibers. Whilst this may sound conterintuitive, it actually works perfectly - the wood is naturally able to store electrolyte solutions, making it ideal for acting as a reservoir for sodium ions, and it is also mechanically flexible enough to withstand the strain caused by the expansion of the tin electrodes. Zhu explains:
"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."
Lead author Hongli Zhu explains the inspiration behind the project
The cellulose fibers in the battery are around 25 um in diameter. They have a mesoporous structure, which makes them perfect for storing high concentrations of sodium ions. The fibers are made conductive by coating them in a 10nm-thick layer of carbon nanotubes, which are applied dispersed in a liquid ink. The conductive fibers are then coated in a thin film tin electrode.
The study showed that after up to 400 charge cycles, when previous batteries had been destroyed by the expanding electrodes, the wood was wrinkled but intact and still effective, albeit with a lower capacity than in its fresh state - this is a common feature of batteries and well within expectations for an early-stage prototype such as this.
Grid storage applications
The performance of the battery still doesn't quite match Li-ion batteries on some scores - for example, the rate at which the battery can be charged or discharged is slower. Whilst this is a limitation for smartphone or laptop batteries, there are other applications which these properties are perfectly suited to.
The researchers envisage that these batteries could be used where large volume, medium- to long-term energy storage is required - for example, storing electricity generated by large solar plants until it is needed. If widely implemented, large-scale applications like this have the potential to revolutionize how we generate and use electricity at the grid level.
In these large-scale applications, Li-ion batteries are extremely expensive to install. The cost benefits of using durable, cheap materials like wood and tin, along with the inherent environmental benefits of sodium-ion batteries, mean that these batteries could make large-scale solar power generation, as well as many other "smart-grid" technologies, much more competitive.
Nanotechnology has been behind a huge number of innovations in battery technology in recent years. However, it is interesting to see researchers taking such a different approach to the technology, and exciting to see a nanotechnology-based product that will be much simpler to commercialize than most of the new technology we have heard about recently.