Aqueous Rechargeable Zinc Batteries: from Cathode Materials to Flexible Devices

水系鋅基電池︰從正極材料到柔性器件

Student thesis: Doctoral Thesis

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Award date26 Nov 2020

Abstract

The rapid development of wearable electronics promotes a high demand for flexible power sources. Flexible rechargeable batteries, as the stars of flexible energy storage and conversion systems, possess simultaneously high flexibility, high energy density and dynamically stable output. Especially, aqueous rechargeable Zn batteries (ZBs) have garnered increasing attention as favorable energy storage devices for flexible electronics, owing to their high safety, high capacity, low cost, the potential of high volumetric energy density and eco-friendliness. Extensive efforts have been devoted to developing flexible ZBs (FZBs) in the last few years. Since the negative potential of Zn anode lies within the safe decomposition scale of water molecules, mild aqueous electrolyte is successfully applied and already received ideal outcomes. However, energy density often conflicts with flexibility, and the progress on high-energy ZBs is limited.

At the very beginning, the work on the energy density of flexible lithium-ion batteries (FLIBs), aqueous flexible sodium-ion batteries (FSIBs), and aqueous flexible zinc batteries (AFZBs) are reviewed and analyzed. Thus, several efficient strategies are summarized towards high energy density accompanied by high flexibility.

Subsequently, we applied these strategies to build flexible high-energy ZBs from cathode materials, hydrogel electrolytes to flexible devices.

Firstly, α-Bi2O3, as a cathode for ZB batteries, delivered an extra-flat discharge plateau [slope: ~0.1 V/(Ah·g–1)] and high specific capacity (323 mAh·g–1) based on a reversible single-order phase transition mechanism. Based on the phase-transition mechanism between the α-Bi2O3 and Bi, the battery system delivered a remarkable rate capability and a high areal energy density of 1.5 mWh·cm–2 at a power density of 4.4 mW·cm‒2, and the volumetric energy density was around 37.5 mWh·cm‒3 based on the volume of the total cathode.

Secondly, as a suitable cathode candidate for ZBs, δ-MnO2, with a layered stacking, failed to deliver its high specific capacity. Here, Na ion and water molecules were pre-intercalated to effectively activate the stable Zn ion storage of δ-MnO2. Our results revealed that the resulted Zn//pre-intercalated δ-MnO2 battery delivered an extraordinarily high-rate performance and a high energy density of 396 Wh kg−1 at 1 C. The capacity retention was as high as 98 % after charged-discharged up to 10000 cycles benefiting from the smooth Zn ion diffusion in the pre-intercalated structure.

Apart from the cathode materials, the attachment between the electrode and electrolyte is also a significant factor. How to enhance their tolerance to shear force which is inevitably applied on the batteries during stretching, bending and twisting is a key issue. Here, we designed a sewable Zn-MnO2 battery based on nanofibrillated cellulose (NFC)/polyacrylamide (PAM) hydrogel with relatively high mechanical strength, a large stretchability, and large pores as channels for electrolyte diffusion. Furthermore, the sewing effect was analyzed on enhancing the shear resistance of the solid batteries. The sewed Zn-MnO2 battery kept 88.5 % capacity after 120 stitches and withstood a large shear force of 43 N.

In summary, the determining factors on the energy density of flexible batteries were analyzed based on the development of high-energy FLIBs, aqueous FSIBs, and aqueous FZBs. The strategies towards high energy density while keeping high flexibility were elucidated. Among these three categories, FZB batteries stand out due to their high safety (hydrogel aqueous electrolyte), and potentially high energy density (thin separator and thin package). Thus, several materials, including α-Bi2O3, δ-MnO2, and CNT/α-MnO2 was researched as the high-capacity cathode for constructing high-energy FZBs. In addition, mechanical durability, especially shear-force tolerance was addressed based on an NFC/PAM hydrogel electrolyte and sewing method.