Porous LiFePO4/PVDF composites for large scale redox targeting flow battery

Porous composites with 50 wt% LiFePO4 (LFP) and a poly(vinylidene fluoride) binder (PVDF) have been prepared by an industrially-scalable melt-extrusion technique. A third water-soluble component, i.e. poly(ethylene oxide) (PEO), was used to create open porous structures by water leaching. PEO was fully and selectively removed during leaching due to the formation of co-continuous morphologies in extruded blends. The porosity of LFP-PEO composites was precisely tuned up to 67 % by the initial PEO content and, beyond this limit, granules disintegrated during leaching due to phase inversion. Importantly, no LFP loss was detected during leaching indicating that LFP was fully trapped by PVDF. This selective localization of LFP could be linked to its the carbon coating. As expected, porous LFP-PVDF granules were found to be more electrochemically reactive (LiFePO4 → FePO4) than non-porous ones when reacted with Fe(CN)63− aqueous solutions. The extent and kinetics of this oxidation reaction were found to be improved when 1) the Fe(CN)63−/LFP ratio is increased, 2) when the porosity of the blend is raised, and 3) when flow is applied to the solution. Finally, a full LFP oxidation was observed for 65 % porous granules under flow mode within 5 h and with a 3–5 fold molar excess of Fe(CN)63−.