Pilot-scale continuous synthesis of a vanadium-doped LiFePO4/C nanocomposite high-rate cathodes for lithium-ion batteries

Abstract

A high performance vanadium-doped LiFePO4 (LFP) electrode is synthesized using a continuous hydrothermal method at a rate of 6 kg per day. The supercritical water solvent rapidly generates core/shell nanoparticles with a thin, continuous carbon coating on the surface of LFP, which aids electron transport dynamics across the particle surface. Vanadium dopant concentration has a profound effect on the performance of LFP, where the composition LiFe0.95V0.05PO4 achieves a specific discharge capacity which is among the highest in the literature (119 mA h g-1 at a discharge rate of 1500 mA g-1). Additionally, a combination of Xray absorption spectroscopy analysis and hybrid-exchange density functional theory suggest that vanadium ions replace both phosphorous and iron in the structure, thereby facilitating Li + diffusion due to Li + vacancy generation and changes in the crystal structure.

Towards High Capacity Li-ion Batteries Based on Silicon-Graphene Composite Anodes and Sub-micron V-doped LiFePO4 Cathodes

“…This is consistent with V-LFP synthesised by the authors previously, where V was found to reside on both Fe and P sites with oxidation states of 3 + and 5 + respectively from EXAFS (Extended X-ray Absorption Fine Structure) and DFT (Density Functional Theory) analysis18. This gave a systematic distortion of the unit cell with V content, decreasing the b parameter and increasing the c parameter18. The lattice constants of the sample in this study deduced from LeBail analysis were: a  = 10.3278(2) Å, b  = 6.0044(1) Å, c  = 4.6964(1) Å (R wp  = 7.62).…”

“…Inspection of the XRD patterns from Figure S3 indicate that there are no impurities evident, with peaks characteristic for the expected orthorhombic pnma space group symmetry of olivine LFP. This is consistent with V-LFP synthesised by the authors previously, where V was found to reside on both Fe and P sites with oxidation states of 3 + and 5 + respectively from EXAFS (Extended X-ray Absorption Fine Structure) and DFT (Density Functional Theory) analysis18. This gave a systematic distortion of the unit cell with V content, decreasing the b parameter and increasing the c parameter18.…”

“…FE-SEM (Field Emission Scanning Electron Microscopy) images were obtained using a JEOL JSM-6700F microscope, indicating the particle size. Figure S3 shows a mixed morphology of platelet-like and rod-like particles – the authors have previously reported that vanadium doping can influence the formation of platelet-like LFP particles within the CHFS process18 Peak fitting was carried out using the LeBail method28. Inspection of the XRD patterns from Figure S3 indicate that there are no impurities evident, with peaks characteristic for the expected orthorhombic pnma space group symmetry of olivine LFP.…”

“…The CHFS process is scalable as reported by some authors2021, where production of ZnO has been successfully scaled up by a factor of 40 with little change observed in the crystallite size21. This process can be used to make crystalline nanomaterials rapidly with controlled particle properties compared with more common synthesis methods, such as traditional solid-state (polydispersed grain growth due to the high processing temperatures) or batch hydrothermal approaches182122. A major benefit of the solution route is that nucleation and growth phenomena can be controlled more easily to generate materials with desirable phase purity, grain size and morphology.…”

“…Continuous hydrothermal flow synthesis (CHFS) is a relatively underexplored method for the production of battery anodes151617 and cathodes18 including V-LFP nanomaterials. The process involves the reaction of a feed of supercritical water with a reagent stream in an engineered mixer, over a period of a few seconds or minutes.…”