While the Li-ion battery is initially built from a conventional anode/separator/cathode/ separator stack, the spine-inspired design involves cutting strips that extends from a common backbone, then wrapping those strips around their connecting backbone, forming thicker stacks flexibly connected to one another.
The result described in a paper titled “Bioinspired, Spine-Like, Flexible, Rechargeable Lithium-Ion Batteries with High Energy Density” published in the Advanced Materials journal shares some similarities with a spinal column. The thick stacks (with a high energy density) are akin to the vertebra in the spine, while the thin unwound backbone connecting all the stacks could be compared with the soft marrow of the spinal cord, the whole assembly remaining flexible.
In their paper, the researchers describe a LiCoO2/graphite flexible cell, which, although non-optimized, boasted an energy density of 242Wh L−1 including the package, nearly on par (over 86%) with that of conventional prismatic cells, but with the added flexibility. Under various mechanical tests (flexing and twisting at 90º), the fabricated samples exhibited stable charge/discharge cycles, even under continuous dynamic load.
Their capacity retention was over 94.3% after 100 cycles even with flexing to a diameter of 20mm and twisting at an angle of 90°, from an initial discharge capacity of 151 mA h g−1. The samples barely 80mm long and 2.37mm thick at their wrapped nodes were used to continuously power a LED and then a smartwatch, while being subjected to mechanical deformations, from resting flat to being flexed or twisted.
The researchers noted that because in the spine-like design, the strain concentrates at the boundary of marrow and vertebra (where the stack is very thin and flexible), strain is limited to about 0.04% as it is alleviated by the flexible part. When submitted to similar flexural tests, conventional prismatic cell designs saw the formation of cracks.
The beauty of this bio-inspired design is that these spine-like batteries are easily scalable, both in size and energy density, as both the rigid segments (the wound electrodes) and the flexible backbone can be cut to any desired length.