Effect of Na-ion doping on the improved electrochemical performances of olivine structured LiNi_1/3Mn_1/3Co_1/3PO₄ for Li-ion hybrid capacitors

Olivine structured materials (LiMPO₄; M = Fe, Mn, Co, Ni) are promising for energy storage applications due to their appreciable electrochemical performance, stability, safety, and cost-effectiveness. However, the poor electronic and Li-ion conductivity is the primary drawback of such materials. Therefore, in the present study, the monovalent larger Na⁺ ion (0.098 nm) is doped in the Li⁺ (0.076 nm) site of LiNi_1/3Mn_1/3Co_1/3PO₄ using the sol–gel method to facilitate the smooth flow of Li⁺ ions during intercalation/de-intercalation by increasing unit cell volume. The X-ray diffraction pattern reveals that Li_1-xNa_xNi_1/3Mn_1/3Co_1/3PO₄ (x = 0, 0.025, 0.05, 0.1, and 0.15) are orthorhombic structures with the Pnma space group. The slight peak shift of the (311) plane towards lower degrees infers the increased d-spacing. The high-resolution transmission electron microscopic image corroborates that the prepared Li_0.95Na_0.05Ni_1/3Mn_1/3Co_1/3PO₄ particles are in size range of 150–200 nm. The EDX mapping confirms the uniform distribution of transition metals throughout the system. The observed redox peaks in the cyclic voltammogram suggest the Faradaic behavior of the electrodes in 1 M LiOH electrolyte. The calculated specific capacity of Li_0.95Na_0.05Ni_1/3Mn_1/3Co_1/3PO₄ is 117 C g⁻¹ (32.4 mAh g⁻¹) at 5 A g⁻¹. The charge transfer resistance of Li_0.95Na_0.05Ni_1/3Mn_1/3Co_1/3PO₄ is substantially lower than the pristine LiNi_1/3Mn_1/3Co_1/3PO₄ electrode infers the higher ionic conductivity. The Li_0.95Na_0.05Ni_1/3Mn_1/3Co_1/3PO₄ || AC device delivered an energy density of 11.7 Wh kg⁻¹ with a power density of 2400 W kg⁻¹ @ 3 A g⁻¹ and has appreciable stability over 2000 cycles.