(Phys.org)—Tannins may be best known for their presence in red wine and tea, but in a new study researchers have demonstrated for the first time that tannins from tree bark can also serve as battery cathode materials. As tree bark is approximately 15% tannins by weight, tannins are naturally abundant, which is one factor that makes them a promising material for designing sustainable, low-cost, metal-free, high-performance batteries.