Authors: Shuo Huang, Fang Wan, Songshan Bi, Jiacai Zhu, Zhiqiang Niu and Jun Chen
Journal: Angewandte Chemie International Edition
We are inseparable from our portable devices. We use mobile phones to take pictures, post on Instagram, tweet about our day; tablets to watch a movie on trains; or smartwatches to monitor our sleep patterns and heart-rate. Our constant use of these gadgets calls for long-lived and efficient batteries. Furthermore, with the development of new foldable technologies, we need batteries that are flexible and do not break easily.
A group from Nankai University in China developed a new, highly durable zinc ion battery (ZIB), by embedding zinc ionic species in a self-healing hydrogel. These hydrogels are made of networks of hydrophilic polymers that are bound to each other through a large quantity of non-covalent interactions. When these bonds are broken (e.g. when the battery is damaged), they can easily be re-formed.
More in detail, they use a hydrogel based on polyvinyl alcohol (PVA, Figure 1) – that self-heals due to the formation of new hydrogen bonds – and Zinc trifluoromethanesulfonate, Zn(CF3SO3)2 as the anode.
Figure 1: Hydrogen bonds network in PVA
They produce the PVA\Zn(CF3SO3)2 battery by storing an aqueous solution of hydrogel and Zn(CF3SO3)2 at – 18° for 15 hours and then letting it melt at room temperature. During the hydrogel crystallization, in the freezing stage, the hydrogen bonds stretch in a way that is similar to what happens to H-bonds during ice formation. This generates pores that can host Zn2+ and CF3SO2– ions. This new PVA\Zn(CF3SO3)2 hydrogel electrolyte shows good conductivity and self-healing properties.
The researchers showed the self-healing properties of
PVA\Zn(CF3SO3)2 hydrogel by working with two samples of different colours – one coloured with rhodamine B and the other transparent (no dyes). They cut both in half and connected opposite halves. Soon the cut healed, and the rhodamine migrated to the transparent side, as shown in Figure 2.
Figure 2: Demonstration of self-healing properties of PVA\Zn(CF3SO3)2 hydrogel. Adapted with permissions from John Wiley and Sons (A Self-Healing Integrated All-in-One Zinc-Ion Battery)
But a battery to work needs an anode, a separator and a cathode. Thus, the researchers integrated all these components into the hydrogel, chosing polyaniline (PANI) nanorods as a base for the cathode. The integrated battery was still able to self-heal and conduct electricity after many cycles of cutting and healing, as it is demonstrated in Figure 3. Here, the researchers connected the battery to a LED array and powered it (left). They cut the battery, breaking the electric circuit (centre) and thus turning the lights off. Upon healing, the LEDs turned back on (right).
Figure3: Self‐healing batteries powering an LED array before cutting (left), after cutting (centre), and after self‐healing (right). Adapted with permissions from John Wiley and Sons (A Self-Healing Integrated All-in-One Zinc-Ion Battery)
If the future is as technologically advanced as we expect, we will need far more powerful, longer-lived batteries and this seems a great step forward!