Freeze-drying enabled porous layered δ-MnO2/rGO composites from MnO2 polymorphs for high-performance AZIBs

Abstract

Compared with α-, β- and γ-MnO₂, δ-MnO₂ possesses a large interlayer spacing, abundant active sites and high compatibility with aqueous electrolytes. Due to these advantages, δ-MnO₂ is considered a potential candidate for efficient aqueous zinc-ion battery and high-safety energy storage systems. Nevertheless, its structural instability and suboptimal electrical conductivity hinder it from being an ideal electrode material. In this study, a 3D conductive framework was fabricated by uniformly integrating δ-MnO₂ and ultrathin rGO nanosheets through a simple freeze-drying process. The introduction of rGO endows the δ-MnO₂/rGO composites with a unique “conductive network–dispersion support” structure. The as-constructed 3D architecture effectively overcomes the intrinsically low conductivity of δ-MnO₂ and decreases the charge transfer resistance. Meanwhile, the porous framework enlarges the electrode–electrolyte contact area, accommodates the volume variation and lattice distortion during Zn²⁺ insertion/extraction, and thus maintains structural integrity upon long-term cycling. Consequently, the δ-MnO₂/rGO electrode exhibits excellent cycling stability, with a maximum specific discharge capacity of 127.6 mAh g⁻¹ at 1 A g⁻¹, which remains at 82.1 mAh g⁻¹ after 600 cycles. The specific capacity is obviously better than that of δ-MnO₂ (63.2 mAh g⁻¹ after 450 cycles).