The rapid development of flexible and wearable electronics requires concomitant advances in high-performance flexible batteries. A prototype battery engineered at Columbia University combines these requisite features: remarkable flexibility, high energy density and stable voltage no matter how it is flexed or twisted.
Designers of the lithium-ion battery took a cue from the mammalian spine to impart flexibility. A thick, rigid segment to store energy through winding the electrodes corresponds to the vertebra of animals. The anode/separator/cathode/separator stacks were separated into long strips with multiple “branches” extending out 90 degrees from the “backbone.” A thin, unwound and flexible part acts as marrow to interconnect all vertebra-like stacks together, providing excellent flexibility for the whole battery. As the volume of the rigid electrode part is significantly larger than the flexible interconnection, the energy density of such a flexible battery can be over 85 percent of that in conventional packing.
The battery demonstrates stable capacity upon cycling and a stable voltage profile no matter how it is flexed or twisted. Disassembly and inspection after cycling revealed that the positive electrode was intact with no cracking or peeling from the aluminum foil, confirming the mechanical stability of the design. Neither continuous flexing nor twisting the battery during discharge interrupted the voltage curve. The battery in the flexed state was also cycled at higher current densities, and the capacity retention was quite high (84 percent at 3C, the charge in 1/3 of an hour).
A non-optimized flexible battery boasted an energy density of 242Wh L−1, including the package, nearly on par (over 86 percent) with that of conventional prismatic cells.