Realizing the Performance of LiCoPO4 Cathodes by Fe Substitution with Off-Stoichiometry

Abstract

Fe substituted lithium rich cobalt phosphates (Li1+x[Co1−yFey]1−xPO4, 0 ≤ x ≤ 0.2, 0 ≤ y ≤ 0.2) were successfully synthesized by the solid-state reaction. Firstly, the optimization of appropriate temperature conditions (800, 850 and 900°C for 10 h) to yield the phase pure LiCoPO4 was conducted. Based on the cell performance of LiCoPO4 prepared aforementioned conditions, 800°C for 10 h calcinations condition was found better performance than rest. Lithium phases optimization was carried out and it was found that the cell performance obtained when the lithium content was 1.05 (real content is 1.02) was better than that of the stoichometric and other lithium rich compositions prepared. Similarly, the substitution of Fe into the Co sites was also conducted for the lithium rich cobalt phosphate (Li1.02[Co1−yFey]0.98PO4, 0 ≤ y ≤ 0.2) and excellent cycling performance was obtained when the Fe content was 0.1 in conventional solutions. The X-ray diffraction pattern and Rietveld analysis showed that Li1.02[Co0.9Fe0.1]0.98PO4 has a well-developed orthorhombic structure with Pnma space group. The lattice parameters of Li1.02[Fe0.1Co0.9]0.98PO4 are slightly larger than those of LiCoPO4 due to the lager ionic size of doped Fe2+ (0.78 Å) compared with Co2+ (0.745 Å). In conclusion, the Li/Li1.02[Co0.9Fe0.1]0.98PO4 cell delivered an initial discharge capacity of 130 mAh g−1 and retained 90% of its initial capacity (117 mAh g−1) at the end of the 15th cycle at 0.1 C rate. We concluded that the substituted Li and Fe ions play an important role in enhancing the battery performance of the LiCoPO4 material by improving the kinetics of the lithium insertion/extraction reaction on the electrode.