Three-dimensional lamination-like P2-Na2/3Ni1/3Mn2/3O2 assembled with two-dimensional ultrathin nanosheets as the cathode material of an aqueous capacitor battery

• 3D lamination-like P2-Na2/3Ni1/3Mn2/3O2 assembled with 2D ultrathin nanosheets is synthesized via a facile co-precipitation reaction.
• The electrochemical performance of P2-Na2/3Ni1/3Mn2/3O2 electrode in 1 mol L−1 Na2SO4 aqueous electrolyte is first investigated.
• P2-Na2/3Ni1/3Mn2/3O2 is capable for Na ions insertion/extraction in Na2SO4 aqueous electrolyte at different rates.
• The insertion/extraction behavior of Na ions in 1 mol L−1 aqueous electrolyte is described.

Three-dimensional (3D) lamination-like transition metal oxide Na2/3Ni1/3Mn2/3O2, assembled with two-dimensional (2D) ultrathin nanosheets, is successfully synthesized using a combined co-precipitation and thermal treatment method. The material was investigated, for the first time, as the positive electrode material of an aqueous Na-ion capacitor battery. The structure and morphology of the as-prepared material are systematically characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry and X-ray photoelectron spectroscopy. Its performance for Na-ion intercalation/deintercalation in a 1 mol L−1 Na2SO4 aquesous electrolyte is evaluated by cyclic voltammetry, galvanostatic cycling test and electrochemical impedance spectroscopy. The Na2/3Ni1/3Mn2/3O2 electrode is charged between -0.8∼1.0 V (vs. SCE) with no water decomposed. The reversible capacity of the electrode reached 157 mAh g−1 at 0.05 C and retained to be 84 mAh g−1 and 51 mAh g−1 at 0.20 C and 0.50 C, respectively. After 80 cycles at 0.20 C, the specific capacity of Na2/3Ni1/3Mn2/3O2 remains to be 42 mAh g−1. The wide charge/discharge potential range and the high capacity reveal that the 3D lamination-like P2-Na2/3Ni1/3Mn2/3O2 assembled with 2D ultrathin nanosheets can be a promising cathode material for low-cost aqueous Na-ion capacitor battery.