Single-crystal β-MnO2 hollow bipyramids: synthesis and application in lithium ion batteries

Abstract

Single-crystal β-MnO₂ hollow bipyramids (HB-β-MnO₂) were synthesized via a template-free hydrothermal method. The as-synthesized hollow bipyramids with 100–300 nm pores along the axis direction of the β-MnO₂ bipyramids are formed through self-assembly and phase transformation processes from α-MnO₂ nanowires to β-MnO₂ bipyramids followed by chemical etching of their metastable crystal faces. Compared to the commercial bulk β-MnO₂ (c-β-MnO₂), the as-synthesized HB-β-MnO₂ exhibits a better electrochemical performance with an initial discharge capacity of 269 mA h g⁻¹, which equates to up to 0.87 Li⁺ intercalation per β-MnO₂ unit, while only 0.2 Li⁺ intercalation per β-MnO₂ unit is observed for the commercial c-β-MnO₂. The excellent electrochemical activity of the as-synthesized HB-β-MnO₂ can be attributed to its hollow structure and single crystal nature. The former can provide higher contact area with the electrolyte and is able to act as a buffer against volume change during the charge/discharge processes, while the latter contributes to good electronic conductivity and stable structural integrity. Moreover, the initial HB-β-MnO₂ bipyramids undergo an irreversible phase transformation to orthorhombic Li_xMnO₂ after the first discharging process, which shows good structural stability and capacity (up to 150 mA h g⁻¹) after 50 charge/discharge cycles.