Formation of ultra-small Mn₃O₄ nanoparticles trapped in nanochannels of hollow carbon spheres by nanoconfinement with excellent supercapacitor performance

Ultra-small Mn₃O₄ nanoparticles are formed in the shell of hollow carbon spheres (HCS-Mn₃O₄) via nanoconfinement growth. The HCS with special cylindrical channels (about 2 nm) provides the limited space for growth of Mn₃O₄ nanoparticles and serves as a conductive substrate in electrochemical applications. The unique hollow structure shortens the distance for electrolytes to access Mn₃O₄ during the charging/discharging process. In the hybrid HCS-Mn₃O₄ structure, the interaction between Mn₃O₄ nanoparticles and HCS was enhanced resulting in good conductivity and electrochemical properties. The two-electrode supercapacitor cell composed of HCS-Mn₃O₄ shows high capacitance and energy density. The highest specific capacitance of 430 F g⁻¹ is achieved at a scanning rate of 1 mV s⁻¹ and the largest energy density of 13.5 Wh kg⁻¹ is observed at a power density of 0.3 kW kg⁻¹ by HCS-Mn₃O₄5 with 93.15% retention after 10,000 cycles in 5 M LiCl. Symmetric HCS-Mn₃O₄-5//HCS-Mn₃O₄-5 shows higher energy density of 22.6 Wh kg⁻¹ at a power density of 0.4 kW kg⁻¹ and power density of 3.3 kW kg⁻¹ at an energy density of 16 Wh kg⁻¹ with 1 M Li₂SO₄. The technique to prepare the HCS-Mn₃O₄ hybrid materials is simple and readily scalable to satisfy industrial demand.