Effect of Nickel Doping on the Electrochemical Performances of Carbon-Coated Na₃V₂(PO₄)₃ Cathodes for Hybrid Lithium-Ion Batteries

Sol–gel synthesized nickel (Ni)-doped Na3V2(PO4)3@C (NVPNix@C, x = 0, 0.03, 0.05, and 0.07) compounds have been first studied as superior cathodes for hybrid Li-ion batteries (HLIBs). X-ray diffraction (XRD) Rietveld refinement results indicate that the unit cell volume is decreased with increasing Ni doping, which may stabilize the structure and facilitate the cycling and rate performance of the doped samples. Through bond structure analysis, it has been demonstrated that Ni doping reduces the sizes of VO6 octahedrons and PO4 tetrahedrons and thus broadens the pathway of Na+/Li+-ion intercalation/deintercalation in the NVP structure. In addition, the increased electrochemical reversibility and ionic mobility in Ni-doped materials are confirmed by the study of kinetic properties. It has been shown that the ion-transport mechanism in HLIBs involves multiple types of alkali ions (Li+ and Na+), resulting in a synergic effect. However, optimized Ni-doped and in situ carbon-coated NVP (NVPNi0.05@C) offers the best electrochemical performance among the tested cathodes. Compared to undoped NVP@C, the optimized NVPNi0.05@C cathode in HLIB yields superior discharge capacity (106 vs 87 mA h g–1 at 100 mA g–1) and capacity retention (97 vs 78% after 100 cycles). Furthermore, NVPNi0.05@C also exhibits impressive rate performance and delivers a discharge capacity of 79 mA h g–1 even after cycling at 700 mA g–1. In a wide voltage regime (1.0–4.2 V), NVPNi0.05@C offers an initial discharge capacity of 230 mA g–1, which is much higher than that of NVP@C (180 mA h g–1). The preliminary electrochemical results suggest that HLIBs with the structure of Li∥1 M LiPF6 in ethylene carbonate (EC) + diethyl carbonate (DEC)∥NVPNix@C, x = 0–0.07 have a lot of potential for developing low-cost, high-performance advanced energy devices.