A simple facile synthesis for phase transforming of δ-MnO₂ into α-MnO₂ and thereby enhancing Na-ion supercapacitive performance

This study presents a phase transformation of MnO₂ from δ to α-phase through a simple facile and efficient method without any surfactant. Different hierarchical architectures of MnO₂ were achieved with the successful conversion of the crystal structure. Growth of 3D dandelion flowers is realized when MnO₂ is in the δ-phase whereas, with varying reaction time, it gets converted to α-MnO₂ with hollow 3D urchin-like architecture composed of 1D nanorods tightly arranged in a dense alignment. Initially, this phase transformation was realized from structural and morphological characterizations. The electrodes of δ and α-phase MnO₂ architectures were fabricated on Ni foam. These electrodes were then systematically tested for supercapacitive features in sodium ions, revealing that α-MnO₂ has superior Na-ion supercapacitive behavior compared to the δ-phase counterpart. Electrochemical performance reflects that the 3D urchin-like structure has the highest capacitive features with 173.6 F g⁻¹ at 3 mA cm⁻² with ultralong capacitive retention performance of 134.8% over 10,000 charge–discharge cycles when tested at a higher current density of 20 mA cm⁻². The synergistic impact of a 3D hollow structure and 1D nanorods allows enhanced ion diffusion, making α-MnO₂ favorable for energy storage electrode material. The study aims to provide valuable insights into a facile synthetic route for tailoring manganese dioxide phases, paving the way for developing advanced materials with enhanced energy storage capabilities for Na-ion supercapacitors.