writes with news on work toward flexible batteries good enough for Real World use (you have to power those flexible electrionics somehow). From the article: "LG Chem ... has devised a cable-type lithium-ion battery that's just a few millimeters in diameter, and is flexible enough to be tied in knots, worn as a bracelet, or woven into textiles. The underlying chemistry of the cable-type battery is the same as the lithium-ion battery in your smartphone or laptop — there's an anode, a lithium cobalt oxide (LCO) cathode, an electrolyte — but instead of being laminated together in layers, they're twisted into a hollow, flexible, spring-like helix. flexible batteries have been created before — but they've all just standard, flat, laminated batteries made from sub-optimum materials, such as polymers. As such, as they have very low energy density, and they're only bendy in the same way that a thin sheet of plastic is bendy. LG Chem's cable-type batteries have the same voltage and energy density as your smartphone battery — but they're thin and highly flexible to boot. LG Chem has already powered an iPod Shuffle for 10 hours using a knotted 25cm length of cable-type battery."
(Extreme Tech claims it is paywalled, but it looks like it's not). The hollow core seems to be the key: "Moreover, a nonhollow anode proved to have serious problems with penetration of the electrolyte into the essential cell components such as the separator and active materials ... However, we were able to overcome these drawbacks by devising a unique architecture comprising a skeleton frame surrounding an empty space, that is, a hollow-spiral anode with a multi-helix structure This design enables easy wetting of the battery components with the electrolyte and the hollow space allows the device to compensate for any external mechanical distortion while maintaining its structural integrity. In addition, this helical architecture possibly enables the battery to be more flexible, owing to its similarity to a spring-like structure."