Foldable and Rechargeable Zn-Air Batteries
Authors: Liu Zhaolin 
Published: February 2020

In this work, we fabricate a solid-state, foldable, and rechargeable Zn-air battery which has similar discharge/charge curve after 100 times of repeated folding. Furthermore, the battery exhibits stable cycling performance after 100 times of folding. The foldable air cathodes are based on the directly grown bifunctional catalyst on the graphene coated carbon cloth. Random coating of graphene on the pretreated carbon cloth provides additional surface area to grow the catalyst while maintaining the electrically and mechanically interconnected 3D network in the air cathode. The directly grown catalyst on air cathode improves adhesion at air cathode/catalyst interface, preventing the de-attachment of catalyst during the continuous mechanical deformations.
We grow manganese oxides (MnOx) on graphene coated carbon cloth as an active and low cost bifunctional catalyst for solid-state and rechargeable Zn-air batteries via simple immersion process. Solid-state electrolyte allows good flexibility and high safety without the need of costly sealing and the risk of electrolyte leakage. Cross-linked PAA-based electrolyte had given sufficient mechanical strength and conductivity (up to ≈0.46 S cm-1) to be used as solid-state electrolyte. Combining the advantage of MnOx grown on graphene coated carbon cloth and solid electrolyte result in the high performance all-solid-state, foldable, and rechargeable Zn-air batteries. The battery exhibits similar polarization curve and resistance at its fold and flat states. The fold battery is able to achieve to a power density of 32 mW cm-2, almost twice of the power density at its flat state. The cycling stability of the battery at its flat and fold state is better than the batteries with Pt/C on carbon cloth air cathode and the reported solid-state Zn-air batteries in the literature.
Polyacrylic acid (PAA)-based electrolytes showed enhanced conductivity as compared to polyvinylalcohol (PVA) or polyethylene oxide (PEO)-based electrolytes due to their low crystallinity and high water retention, resulting in the high conductivity while maintaining desirable mechanical robustness. The obtained PAA-KOH solid electrolyte is transparent, free-standing, and flexible. Unlike gel electrolyte, the solid nature of crosslinked polymer electrolyte is able to prevent short circuit during continuous mechanical deformations.
At the same applied current during polarization measurement, the folded battery has an almost similar discharge and charge voltage with the battery at its flat state. The polarization curve of the battery with MnOx-GCC air cathode is enhanced significantly as compared to the battery with GCC and MnOx-CC air cathodes, which only show slightly better performance than the battery with CC air cathode (i.e. without catalyst). At discharge voltage of 1 V, the batteries with GCC and MnOx-CC air cathode have low current (»2.0 and 2.2 mA), five times lower than the battery with MnOx-GCC air cathode. The small voltage gap (i.e. the difference between discharge and charge voltage) indicates an improved rechargeability of the battery with MnOx-GCC air cathode as compared to others batteries. Despite the similar discharge profile, the small footprint of the folded battery results in the high current per unit area. Thus, it has larger power density than the battery at its flat state. The folded battery delivers a power density as high as »32 mW cm-2, almost twice of power density at its flat state (»18 mW cm-2). Under the same device configuration, power density of battery with Pt/C-CC air cathode (»12 mW cm-2) is lower than the battery with MnOx-GCC air cathode, indicating the advantage of high loading density of thin MnOx layer on the dense and interconnected GCC.
The battery with MnOx-GCC air cathode has higher discharge voltage and lower charge voltage than the batteries with MnOx-CC air cathode, agreeable to the measured polarization curve. It maintains the same voltages (i.e. discharge voltage of 1.3 V and charge voltage of 1.9 V) for about 170 cycles of discharge/charge test. On the other hand, voltage profile of the battery with Pt-C/CC air cathode degrades gradually over the time. At the end of 70th cycle, its discharge voltage reduces from 1.3 to 0.9 V while its charge voltage increases from 2.0 to 2.4 V, suggesting its low re-chargeability. Battery with MnOx-CC air cathode exhibits stable performance for up to 140 cycles. However, it has poorer discharge and charge voltages (i.e. 1.0 and 2.3 V) than the battery with MnOx-GCC and Pt/C-CC air cathodes. The battery with MnOx-GCC air cathode shows stable performance for up to 110 of discharge/charge cycles, 60 cycles lesser than the same battery at its flat state.