Na_0.44MnO₂ with very fast sodium diffusion and stable cycling synthesized via polyvinylpyrrolidone-combustion method

Na_0.44MnO₂ is a very promising cathode material in sodium-ion batteries for large-scale application. Na_0.44MnO₂ with very fast sodium diffusion and stable cycling is prepared by polyvinylpyrrolidone (PVP)-combustion method. X-ray diffraction, scanning electron microscope and high-resolution transmission electron microscopy are conducted to explore the morphology and structure. Electrochemical performance of the samples is examined in coin cells with sodium foil anode and nonaqueous electrolyte. The Na_0.44MnO₂ sample synthesized at 900 °C (NMO-900) discharges the highest capacity of 122.9 mAh g⁻¹ at C/5. A fast-rate-test technique developed by Newman et al. is adopted for quick determination of the rate capability. All the samples exhibit good rate capability while the NMO-900 shows the best. Normal rate test result supports the reliability of the fast rate test. Even at 20C charge and discharge rate, the NMO-900 delivers 99 mAh g⁻¹ capacity. The chemical diffusion coefficient of sodium is measured to be around 3 × 10⁻¹² cm² s⁻¹ by potential intermittent titration technique (PITT). The cycling stability is also very good. The capacity retention after 100 cycles at 1C is 87.9% and the capacity still remains 82.9% even after 700 cycles at 10C. During cycling the coulombic efficiency keeps near 99.8%.