Preparation of a NaFePO4 Cathode Material via Electrochemical Sodiation of FePO4 Layers on Al Substrates

In this research, a cyclic voltammetry (CV) method was applied to intercalate Na+ into an FePO4/Al substrate to produce NaFePO4/Al as a potential cathode material. The sodiation was conducted directly to FePO4 instead of applying delithiation to LiFePO4 followed by sodiation, as was done in previous research. CV was conducted within a potential window of 2.0–4.0 V using a scan rate of 0.05 mVs-1. The result was compared to LiFePO4/Al treated with a similar method. The various scan rate was then applied to understand its effect on the electrochemical activity recorded in the voltammogram and its impedance profile. The results show that the CV product of FePO4/Al (NFP(A)) was crystallized in an orthorhombic olivine NaFePO4, as a result of Le Bail refinement. Orthorhombic Na0.7FePO4, trigonal FePO4, and monoclinic FePO4 were presented as secondary phases. Meanwhile, the CV product of LiFePO4/Al (NFP(B)) was also crystallized in olivine NaFePO4 and possessed secondary phases similar to NFP(A) with an additional Fe2O3 phase. NFP(A) showed two significant peaks at 2.442 V and 3.534 V, confirming sodiation/de-sodiation and Fe3+/Fe2+ activity, respectively. Meanwhile, NFP(B) showed two peaks at 3.183 V and 3.04 V, corresponding to de-lithiation and sodiation, respectively. The Nyquist plots of both materials show a similar profile, with the impedance value of NFP(A) being lower than that of NFP(B). This confirms that the CV treatment of FePO4/Al is more facile than the treatment of the LiFePO4 layer, while also producing a cathode with higher electrical conductivity. Scan rate reduction to 0.04 mVs-1 produced a much lower impedance value, confirming higher electrical conductivity.