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.

Key words: Zinc-ion battery, PPy@MnO₂, Carbon nanotube, Flexible battery, Long life