A Biodegradable and Stretchable Battery

Next generation wearables will interface intimately with the human body. They will be used for health monitoring, soft robotics, and futuristic consumer electronics such as e-skin and e-textiles. For seamless integration with the human body, mechanical properties such as stretchability, softness and bendability need to be optimized. Extensive research has already been done on improving the mechanical properties of various components, such as transistors, conductors, diodes, sensors etc., to complement next-generation wearables. Batteries give autonomy to these wearables, by providing a portable power source. However, as the complexity of wearable devices increase, so would the energy demand and form factor. Capacity is often sacrificed increasing stretchability and reduction of size, since lowering the loading of active material reduces the capacity. Additionally, a growing issue is the accumulation of hazardous e-waste and mining of finite resources. Since the battery is often the most toxic and unsustainable part of electric devices, due to the use of harmful organic electrolytes and critical raw materials. Therefore, the development of batteries that are environment and human friendly is urgently needed.<br/><br/>Zn-ion batteries show great potential as a sustainable alternative to current Li-ion battery technology. Zn has a high theoretical capacity, is biodegradable and abundant, and can operate in safe and sustainable aqueous electrolytes. Today, most biodegradable batteries lack the mechanical properties needed (i.e. stretchability) for next-generation wearables. Only a handful biodegradable and stretchable batteries have been reported. However, their electrochemical performance is limited. The aim of this work is to address the abovementioned challenges by designing the first biodegradable and stretchable Zn-ion battery that can also deliver a high capacity.<br/><br/>Here, we studied the use of biomass-derived polymers in all components of the biodegradable and stretchable battery. For the active electrode, we developed a new biodegradable elastomeric binder by copolymerizing a polyester, poly(glycerol sebacate) (PGS), with a protein (gelatine). The gelatine improved the electrolyte uptake and the stretchability of the active electrode which resulted in improved capacity. A gelatine/cellulose-based composite was used in the electrolyte. It enhanced the adhesion to the electrodes which is critical for stretchability and increased the electrochemical stability by supressing Zn dendrite and passivation layer formation. PGS was also used in the encapsulation which permitted the biodegradation of the full cell. Lastly, at the battery’s end-of life the components are degraded into non-toxic products, many of which are today used in skincare or dietary supplements.