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We have reported the synthesis and electrochemical characteristics of carbon-coated sodium vanadium phosphate/activated carbon (NVP@C/AC) bi-material electrodes. We have reported that bi-material type NVP@C/AC cathodes offer superior rate performance as compared to either NVP@C or AC electrodes. Through a detailed impedance spectroscopy analyses, we demonstrated that the synergic effect observed in the bi-material electrodes correlates well with the lowering of their charge transfer resistances (R CT ) and the increase of Li + diffusion coefficient (D Li+ ) with the increase of activated carbon content from 0 to 0.60 weight fraction. Through a detailed cyclic voltammogram analyses we have delineated the faradaic and capacitive contribution towards the overall capacities of the bi-material electrodes at various rate conditions. For both these bi-materials, irrespective of the AC content and rate, capacitive contribution dominates the overall capacity. NVP@C/AC40 yields discharge capacities of 67 and 40 mAh g −1 with capacity retention of more than 93% and 67% after 500 cycles measured at 50 and 1000 mA g −1 , respectively. These bi-materials are demonstrated to be excellent material candidates for high power density lithium titanium oxide (LTO)//NVP@C/AC hybrid battery-capacitor (bat-cap) energy storage devices.

In the present work, we have demonstrated that nanopetal-assembled hierarchical carbon-coated Na3V2(PO4)3 (nNVP@C) microflowers, synthesized via a microwave-assisted hydrothermal route, play an important role for yielding superior electrochemical characteristics of a Li4Ti5O12 (LTO)//nNVP@C full cell. Thus, the full cell yields superior power density with decent discharge capacity after extended cycling and good rate performance. The nanosize petals help Li+ to diffuse faster in NVP particles, and the inner mesoporous morphology of microflowers allows the electrolyte to easily penetrate into the embedded NVP@C nanocrystals. Furthermore, the homogeneous carbon coating provides an elastic buffer to mitigate the strain developed during Na+ extraction and subsequent Li+ insertion and extraction. The LTO//nNVP@C full cell is claimed to be suitable for power applications, where relatively thinner electrodes would be flooded with a sufficient amount of the lithium salt-containing organic electrolyte. To improve the cycleability characteristics, one requires to match carefully the Li+ activity in the organic electrolyte with electrode capacity. This would ensure stoichiometric lithium-ion insertion in the LTO electrode together with predominant lithium-ion insertion in the nNVP@C cathode.

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Sol−gel synthesized nickel (Ni)-doped Na 3 V 2 (PO 4 ) 3 @C (NVPNi x @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 VO 6 octahedrons and PO 4 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 iontransport 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 (NVPNi 0.05 @C) offers the best electrochemical performance among the tested cathodes. Compared to undoped NVP@C, the optimized NVPNi 0.05 @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, NVPNi 0.05 @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), NVPNi 0.05 @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 LiPF 6 in ethylene carbonate (EC) + diethyl carbonate (DEC)∥NVPNi x @C, x = 0−0.07 have a lot of potential for developing low-cost, high-performance advanced energy devices. KEYWORDS: sol−gel synthesis, Ni-doped Na 3 V 2 (PO 4 ) 3 , Rietveld refinement, hybrid Li-ion battery, cyclability, rate