Abstract
We have prepared nano-structured In-doped (1 mol %) LiFePO4/C samples by sol–gel method followed by a selective high temperature (600 and 700 °C) annealing in a reducing environment of flowing Ar/H2 atmosphere. The crystal structure, particle size, morphology, and magnetic properties of nano-composites were characterized by X-ray diffraction (XRD), scanning electron microsopy (SEM), transmission electron microscopy (TEM), and 57Fe Mössbauer spectroscopy. The Rietveld refinement of XRD patterns of the nano-composites were indexed to the olivine crystal structure of LiFePO4 with space group Pnma, showing minor impurities of Fe2P and Li3PO4 due to decomposition of LiFePO4. We found that the doping of In in LiFePO4/C nanocomposites affects the amount of decomposed products, when compared to the un-doped ones treated under similar conditions. An optimum amount of Fe2P present in the In-doped samples enhances the electronic conductivity to achieve a much improved electrochemical performance. The galvanostatic charge/discharge curves show a significant improvement in the electrochemical performance of 700 °C annealed In-doped-LiFePO4/C sample with a discharge capacity of 142 mAh·g−1 at 1 C rate, better rate capability (~128 mAh·g−1 at 10 C rate, ~75% of the theoretical capacity) and excellent cyclic stability (96% retention after 250 cycles) compared to other samples. This enhancement in electrochemical performance is consistent with the results of our electrochemical impedance spectroscopy measurements showing decreased charge-transfer resistance and high exchange current density.
Highlights
LiFePO4 has become one of the most viable commercial cathode materials after the ground breaking work of Padhi et al [1]
We find that the In-doped-LiFePO4 /C sample annealed at 700 ◦ C which consists of 11 wt % of Fe2 P showed an improved discharge capacity
The electronic conductivity for the LFP-600, In-LFP-600, LFP-700 and In-LFP-700 are 2 × 10−3, 8 × 10−3, 8 × 10−2 and 1 × 10−2 S·cm−1, respectively. These results indicate that electronic conductivity of un-doped samples increases with the annealing temperature which is attributed to the formation of conductive Fe2 P phase at higher temperatures
Summary
LiFePO4 has become one of the most viable commercial cathode materials after the ground breaking work of Padhi et al [1]. This material has received an extensive attention due to its high thermal and electrochemical safety, lower cost compared to mixed oxide cathode materials, low toxicity, Inorganics 2017, 5, 67; doi:10.3390/inorganics5040067 www.mdpi.com/journal/inorganics. LiFePO4 decomposes above 700 ◦ C leading to in-situ formation of conductive iron phosphides (Fe2 P, FeP, Fe3 P), and compounds with superior lithium-ion diffusion coefficients, such as, Li3 PO4 and. Initial formation of conductive iron phosphides at the grain boundaries improves electrochemical performance, these phases are not electrochemically active and excessive decomposition of LiFePO4 reduces the active material leading to reduced specific capacity of the sample
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