Pechini Synthesis of Na3V2(PO4)2F3/C Doped with Aluminum As Cathode for Lithium Ion Batteries
Authors: Pineda Aguilar Nayely , Torres González Luis C. , Sánchez Cervantes Eduardo M. , Gover Richard K.B. , Garza-Tovar Lorena L. 
Published: February 2020

The demand of rechargeable batteries had increased significantly every year during the last decade, driven for the needs associated with technological development (portability, high performance of electronic devices and vehicles). Lithium ion battery is a device of mayor consumption, and it is designed for energy storage and conversion based on intercalation electrodes. Nowadays the efforts are directed to the improvement and replacement of current battery components: anode, cathode (LiCoO2) electrolyte, with materials that have higher efficiency in terms of energy, power, cost, reliability, life time and safety. In recent years there has been significant interest in polyanion-based active materials as safe alternatives for the traditional oxide cathodes. For example, phosphate phases such as LiFePO4 [1], Li3V2(PO4)3, [2], Li2.5V2(PO4)3, [3], LiVOPO4, [4,5] and LiVP2O7[6] have all been proposed. Therefore, the search of new cathode materials is an important task for researchers in materials science. The possibility of using sodium directly in lithium ion cells allows the study of new compositions and structures.
In this research work a series of four compounds with formula Na3V2-xAlx(PO4)2F3 (x= 0, 0.02, 0.05, 0.1) were prepared, characterized and applied as cathode materials in lithium ion batteries. These materials were synthesized by sol-gel Pechini method. Aqueous solutions containing appropriate amounts of NH4VO3, NH4H2PO4 and NaF were poured into a mixture of citric acid and ethylene glycol solution. Mixture was then heat treated under reflux at 80°C until gel formation. Fresh samples were heated at 300°C under air to eliminate volatile matter. Resulting powders were grinded and formed into pellets for reaction between 300 to 650°C under nitrogen atmosphere. Thermal stability of materials was evaluated by simultaneous termogravimetric and differential analysis (TGA-DTA). Morphological and microstructural characterization were carried out with field emission scanning electron microscopy (FESEM), textural analysis by N2 physisorption with BET method; chemical composition and crystallographic parameters were determined with Induced coupled plasma – optic emission spectroscopy (ICP-OES), energy dispersive X-ray spectroscopy (EDXS) and X-ray powder diffraction (XRD); the application of materials as cathodes in lithium ion batteries was evaluated through electrochemical charge discharge experiments. Electrodes were prepared using a mixture of each synthesized materials, conductive carbon and PVDF binder. CR2032 coin cells were assembled inside a glove box under Ar atmosphere, using LiPF6electrolyte and Li° as anode. Experiments were performed using a MacPile II by Biologic.
Thermal analysis of sol-gel reaction products exhibited an exothermic even between 550 and 750°C attributed to the crystallization of the fluorophosphates. Results from XRD analysis showed that Al doped Na3V2(PO4)2F3 crystalline phase was formed at 650°C for 8h. According to cell parameters Na3V2(PO4)2F3 can incorporate aluminum content up to x=0.1, without the presence of secondary phases or structural transitions. Granular morphology and small particles size of about 40 to 100 nm were observed, this can be attributed to the effect of residual carbon within samples (8% by weight) since this inhibits particle grown and also allows contact among particles, improving electrical conductivity. Materials present average porous size of about 20nm, with surface area of 30m2/g. The sample with x=0.05 of Aluminum content, presents the best textural properties. This material also presents high specific charge/discharge capacity (123/101 mAh/g at a 4.4 V vs Li cell) and good capacity retention (82%), in comparison to the material without doping (128/63 mAh/g and 49% of capacity retention). Aluminum doping of Na3V2(PO4)2F3phase permitted the stabilization of the structure related to cycling processes. These results are promising for future application of the material in lithium and sodium ion batteries.
Keywords: Pechini, cathodes, phosphates, lithium ion batteries
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