A phenomenological qualitative model is proposed to understand the decrease of leakage current in Na doped BiFeO ₃ nanoparticles.

The lack of high-performance cathode materials is a great challenge for the development of large-scale energy storage sodium ion batteries. Here, a new resource-rich P2-type Na_0.44Mn_0.6Ni_0.4−xCu_xO₂ (0 ≤ x ≤ 0.3) sodium-ion battery (NIBs) cathode material was designed and synthesized by a sol–gel method. Na ions occupy the Na_f sites and the Na_e sites in a proportion of 1 : 1, which maintains the high symmetry and stability of the P2-type structure. The charging/discharging tests show that Na_0.44Mn_0.6Ni_0.3Cu_0.1O₂ has a high initial capacity of 149 mA h g⁻¹ and 80% capacity retention after 50 cycles at 0.1C, showing a high energy density of 469 W h kg⁻¹. The addition of inactive copper enhances the lattice spacing, obviously reducing the irreversible reaction resistance (R_f) during the intercalation and deintercalation of Na⁺, thereby reducing the internal resistance of the battery and improving the cycle performance. In order to maintain the P2-type structure, the voltage is controlled at 1.5–4.0 V during charging and discharging, which inhibits the phase transition of P2-O2, leading to the improvement of cycling performance. Therefore, the Cu-substituted Na_0.44Mn_0.6Ni_0.3Cu_0.1O₂ possibly serves as a promising high capacity, high energy density and stable cathode for sodium ion battery applications.

Hybrid improper ferroelectric Ca₃Ti₂O₇and Ca₃Ti₁₉Ru_0.1O₇ceramics were successfully synthesized by conventional solid-state reaction method. Two strongest diffraction peaks located around 2θ= 33° shifted towards the lower angle region with Ru substitution, reflecting structure variation. Grain growth and higher oxygen vacancy concentration after doping resulted in a reduction in the coercive field about 20 kV/cm. Optical bandgap estimated by UV-vis diffuse reflectance (DR) spectrum and X-ray photoelectron spectroscopy (XPS) valence band spectra showed a decreasing trend due to the existence of impurity energy level upon Ru doping, which was consistent with the results of first-principles calculations. The origin of the unexpected induced magnetic moments in Ru-dope Ca₃Ti₂O₇is also discussed.

Ruddlesden–Popper (RP) structural Li _2x Ca _3−2x Mn ₂ O ₇ ( x = 0–0.05) particles were successfully synthesized via a citrate sol–gel method. Li doping was investigated to have a positive influence on enhancing the magnetism of the system.