1. Key Laboratory of Multifunctional Nanomaterials and Smart Systems, Advanced Materials Division, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, Jiangsu Province, China;2. Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, Jiangsu Province, China;3. School of Nano-Tech and Nano-Bionics, University of Science and Technology of China, Hefei 230026, China
Abstract:
Rechargeable aqueous Zinc-ion batteries (ZIBs) have emerged as potential energy storage devices due to their high energy density, low cost, and safety. To date, numerous cathodes based on manganese dioxide, vanadium dioxide, and polyanionic compounds have been reported. Among them, MnO2 cathodes are particularly desirable candidates for commercialization owing to their tunnel structure and affordability. In particular, the parasitic reaction of Mn-based cathodes in alkaline batteries can be suppressed in mild aqueous electrolytes, resulting in enthusiasm for the development of rechargeable Zn||MnO2 batteries. Even though various MnO2 phases have been reported as hosts for Zn2+/H+ insertion, MnO2 crystal structures undergo significant, irreversible transformations during cycling, which is a major challenge in Zn||MnO2 batteries. In addition, the tunnel structure can collapse under the insertion of the hydrated cation resulting in Mn2+ dissolution into the electrolyte and significant loss in capacity over long cycling periods. The MnO2 cathode also has low intrinsic electronic conductivity due to the large charge transfer resistance, which limits the diffusivity of divalent ions. Despite the achievements made in the field of ZIBs so far, designing active materials and ZIBs systems to meet commercial requirements is a significant challenge. In this study, we report the preparation of polypyrrole-wrapped MnO2/carbon nanotubes (PPy@MnO2/CNT) as composite cathodes for aqueous ZIBs. A combination of design strategies was used to increase structural stability and improve electronic conductivity, including increased electrode/electrolyte interaction by using nano-sized structures, shortened diffusion pathways through multistage composites, and enhanced electrical conductivity with conductive composites. The three-dimensional (3D) structured PPy/CNT network can facilitate mass and charge transport during the charge and discharge processes. The structure of MnO2 wrapped by polypyrrole effectively prevents the dissolution of MnO2. Thus, the assembled Zn||MnO2 batteries, using PPy@MnO2/CNT composite cathodes, exhibit a high capacity of 210 mAh·g-1 at 1 A·g-1, and achieve 85.7% capacity retention after 1000 charge/discharge cycles. Moreover, a high specific capacity of 100 mAh·g-1 could be maintained at 2 A·g-1, exhibiting excellent kinetic performance. The assembled quasi-solid Zn//MnO2 battery, benefiting from the xanthan gum electrolyte and flexible CNT film, possesses intrinsic safety, bending resistance, and high potential in wearable applications.
