A High-Performance Primary Nanosheet Heterojunction Cathode Composed of Na_0.44 MnO₂ Tunnels and Layered Na₂ Mn₃ O₇...

Owing to its large capacity and high average potential, the structure and reversible O-redox compensation mechanism of Na₂ Mn₃ O₇ have recently been analyzed. However, capacity fade and low coulombic efficiency over multiple cycles have also been found to be a problem, which result from oxygen evolution at high charge voltages. Herein, a Na_0.44 MnO₂ ⋅Na₂ Mn₃ O₇ heterojunction of primary nanosheets was prepared by a sol-gel-assisted high-temperature sintering method. In the nanodomain regions, the close contact of Na_0.44 MnO₂ not only supplies multidimensional channels to improve the rate performance of the composite, but also plays the role of pillars for enhancing the cycling stability and coulombic efficiency; this is accomplished by suppressing oxygen evolution, which is confirmed by high-resolution (HR)TEM, cyclic voltammetry, and charge/discharge curves. As the cathode of a Na-ion battery, at 200 mA g^-1 after 100 cycles, the Na_0.44 MnO₂ ⋅Na₂ Mn₃ O₇ heterojunction retains an 88 % capacity and the coulombic efficiency approaches 100 % during the cycles. At 1000 mA g^-1 , the Na_0.44 MnO₂ ⋅Na₂ Mn₃ O₇ heterojunction has a discharge capacity of 72 mAh g^-1 . In addition, the average potential is as high as 2.7 V in the range 1.5-4.6 V. The above good performances indicate that heterojunctions are an effective strategy for addressing oxygen evolution by disturbing the long-range order distribution of manganese vacancies in the Mn-O layer.