In many ways, one could say that batteries are among the least environmentally friendly technologies we work with on a daily basis, due to the chemistry involved. There is a great deal of effort to make modern batteries less toxic, but some researchers decided to address the issue from a different starting point. Researchers at the University of Maryland have built a battery from wood and tin, and instead of lithium it uses sodium, making it environmentally benign.
The ideal battery will have a high surface area for the electrodes, to enable fast charging and discharging. As wood is composed of extremely small fibers all tangled together, it has a high surface area, so the researchers decided to coat those fibers in tin. As wood fibers are supple and use to hold mineral-rich water, they are able to withstand the expansion and contraction involved with charging and discharging. Cycling the battery did wrinkle the wood, but the wrinkles appear to relieve some of the stress, and thus increase the battery's lifespan.
The researchers were able to put their wood battery through more than 400 charge/discharge cycles without damage, making it one of the longest lasting nanobatteries. Unfortunately you cannot expect this to be powering your devices anytime soon. Instead the research believe it will be employed as a low cost energy storage system possibly for power grid use.
Source: University of Maryland