Key to the development is distributing many charge-retaining cells in a grid within a silicone elastomer sheet, inter-connected by extraordinarily flexible conductors.
Capacity is 1.1mAh/cm2, dropping to 0.8 after 20 cycles, said the researchers in a paper in Nature Communications.
Each cell is circular and around 2mm across, and 100 of them are arranged in a square matrix on a pitch of approximately 4mm to form the battery. Different diameters and pitches have been tried to vary the capacity/elasticity trade-off.
A cell consists of a stack:
Copper top electrode
Li4Ti5O12 anode disc
LiCoO2 cathode disc,
Aluminium bottom electrode
All cells are connected in parallel, with fine (50nm wide) ribbons of the 0.6µm-thick metal current collectors extending to the four adjacent cells.
To allow for stretching, these wires are far longer than the 2mm inter-cell distance. They are formed as a serpentined serpentine pattern, snaking through 24 S-bends between cells, in a pattern that itself is bent up into one larger S-bend – see photo.
1.2µm polyimide layers on each side of both metal ribbons (four polyimide layers in total), patterned the same as the ribbons, keep bending close to the neutral mechanical plane, said the researchers.
Finite element analysis of the wires agreed closely with physical experiments – both showing the ribbons-like wires buckling at the same points as they are pulled. Analysis also indicated that third order serpentining was not necessary for the required elasticity.
On either side of this structure, 250µm layers of silicone elastomer support and protect the whole thing.
It takes over 300% strain in the finished structure to impart 1% strain in the metal interconnect – reckoned by the researchers to be the failure point of copper. At up to 150% elongation, the copper can return completely to its pre-stretched state.
Positive metal/polyamide and negative metal/polyamide structures are made by lithography on the surface of separate silicon wafers.
These are transfer-printed onto the silicone sheets that will later be the outsides of the battery, and the metal discs over-printed with appropriate slurries of the electrode material.
To finish the structure, the silicon sheets are bonded face to face with the metal and electrodes between them, and electrolyte gel is injected into the middle.
One last subtlety is that, to avoid electrodes shorting when the cell is squashed, anodes and cathodes are not positioned opposite each other, but in between opposing electrodes. So, each anode actually forms four part-cells with four opposing cathodes, and vice versa.
Output is 2.3V from around 70Ω, and self-discharge is somewhere between 1 and 10µA. Output power is around 5mW even when stretched in both directions at once.
On its first cycle, the battery delivered 60% of charge. This improved to 90% on cycle three, followed by a gradual reduction of capacity – a decline thought to be time rather than cycle-dependent, and due to the corrosive effects of residual moisture, as well as the electrodes crumbling.
Contributing institutions are: University of Illinois at Urbana-Champaign, Tsinghua University (Beijing) , Northwestern University (Illinois) and Hanyang University (Seoul).