Wear and Wash: Stretchable Zn-MnO₂ Rechargeable Cells

Wearable devices require power. There are opportunities to store energy in watch straps, belts, waistbands, suspenders, bra straps, shoes, hats, jackets and in custom fabrics. Typically, however, batteries have a hard metal shell, and are not suited for washing. Here we present a cell that is entirely stretchable and washable. It need not be removed from a garment prior to washing, and it conforms to its surroundings. The approach makes use of one of the lowest permeability elastomers– poly(styrene – isobutylene – styrene) block co-polymer or SIBS – as the encapsulation layer. This allows it to operate over the course of more than a year and a half without drying out. The active materials are zinc and manganese dioxide, both in powder form, combined with a near neutral aqueous electrolyte, making for safe operation, low cost, and possible recycling. The current collectors, active layers, and separator all contain SIBS, which helps to bind the device together, and allows the device to stretch to more than double its length. Carbon black provides electrical conduction, combined with carbon nanofibers to reduce resistance (27 ohms per square) and keep resistance stable over time. The cell has a reversible capacity of 160 mAh/g based on cathode active mass, shows 75% capacity retention after 500 cycles, and underwent 39 washing cycles before it began to leak. The cell is shown to operate effectively after mechanical cycling, runs a low power digital watch from a custom strap, and can briefly drive low energy Bluetooth.<br/>In the process of applying the cell we have found that it is relatively stiff. The stiffness can be reduced by making the cell thinner or combining SIBS with a softer polymer. The separator is created using evaporation induced phase separation to create pores in SIBS. The hydrophobicity of the bare SIBS separator could be improved by incorporating surfactant into the electrolyte or embedding hydrophilic groups onto the separator surface.. The cell would also benefit from lower resistance current collectors, as these appear to be limiting power output. Applications such as wireless communications are difficult to run continuously as a result of the relatively high resistance. Finally, the chemistry can be improved – and in particular cycle life and stability in the discharged state. Some approaches to further improving the cell are discussed, along with prospects for application.