Carbon-coated FePO4 nanoparticles as stable cathode for Na-ion batteries: A promising full cell with a Na15Pb4 anode

The cheap and non-toxic FePO4, which can be obtained from LiFePO4 recovered from spent Li-ion batteries, provides a reversible capacity of 103 mAh/g in coin cells, corresponding to 76% of theoretical capacity, with very good rate capability and stability. The study of the electrochemical mechanism during sodiation carried out by in situ X-ray diffraction (XRD) reveals a pure biphasic transition during Na+ (de)insertion. The chemical diffusion coefficient (DNa) determined from galvanostatic intermittent titration technique and electrochemical impedance spectroscopy varies in the range 10−16–10−10 cm2/s at room temperature depending on the degree of sodiation. As a proof of concept, a full cell fabricated using a carbon-coated FePO4@C cathode and a Na15Pb4 anode obtained by electrochemical sodiation of recyclable Pb, maintained 76% of the initial capacity over 100 cycles at constant C/10 rate. This outstanding electrochemical performance, based on the combination of the low-cost FePO4@C cathode with a recyclable lead-based anode, makes this new technology a promising real-world alternative for future commercial Na-ion systems.