One-Step Pyro-Synthetic Strategy of LiFePO4-Li3V2(PO4)3 Nanocomposite with a High Energy Density for Lithium-Ion Batteries

Abstract

In commercially available Li-ion cells, cobalt-based oxide is utilized as the cathode material, but its high cost and low thermal stability prohibit the large-scale use such as Energy Storage Systems (ESS) and Electric Vehicles (EV). Lithium iron phosphate (LiFePO4) and lithium vanadium phosphate (Li3V2(PO4)3) exhibit good thermal stability and high operating voltage, which make them the most promising cathode candidates so far. Despite of their advantages, LiFePO4 and Li3V2(PO4)3 also have some drawbacks. For instance, LiFePO4 has a stable voltage platform and its cyclic attenuation rate is almost negligible, but its relatively low capacity and low voltage limit its energy density and consequently limits its use. Li3V2(PO4)3 has a higher voltage, however, it suffers from its stepped voltage platforms and severe cycle attenuation. In order to combine the advantages from both materials, many research groups have attempted by designing hybrid materials since the doping approach can only improve the main material without combining the advantages of two different active materials. In this work, we focus on obtaining nanostructured materials as well as hybrid materials by using pyro-synthetic method which is very effective for production of highly crystalline carbon coated nanomaterials under very short reaction times in open-air conditions. Further, the pyro-synthetic route using polyol medium will be beneficial for producing nanomaterials in terms of time and cost effectiveness because the combustion strategy is also applied to this strategy. Moreover, poly alcohols or polyols such as ethylene glycol and di/tri/tetra ethylene glycol can play multi-roles of a solvent, a reducing agent and a carbon source. We have successfully synthesized 0.66LiFePO4•0.33Li3V2(PO4)3 nanocomposites with high crystallinity by one-step pyro-synthetic strategy using the polyol at low temperature. The as-prepared sample without any additional heat treatment showed average particle size of about 30 – 60 nm and spherical shape as observed by SEM studies. After moderate heat-treatment was applied, the particle sizes of 0.66LFP•0.33LVP increased due to crystal growth of monoclinic phase of LVP accompanied by particle agglomeration. When tested for lithium-ion cell, the nanoparticles composite electrode demonstrated impressive electrochemical properties with enhanced energy density. Furthermore, it showed high reversible capacity of 145.65 mAh g-1 at 0.1C and exhibited remarkable capacity of 119 mAh g-1 at 6.4C. After cycled at 6.4C, the electrode can recover to 85% of its initial capacity when the current density returned back to 0.1C indicating the excellent rate capabilities of the present electrode.