Optimization of lithium content in LiFePO₄ for superior electrochemical performance: the role of impurities
2018
Halankar, Kruti K. | Maṇḍala, Vindeśvara Prasāda | Jangid, Manoj K. | Mukhopadhyay, A. | Meena, Sher Singh | Acharya, R. | Tyagi, A. K.
Carbon coated LiₓFePO₄ samples with systematically varying Li-content (x = 1, 1.02, 1.05, 1.10) have been synthesized via a sol–gel route. The Li : Fe ratios for the as-synthesized samples is found to vary from ∼0.96 : 1 to 1.16 : 1 as determined by the proton induced gamma emission (PIGE) technique (for Li) and ICP-OES (for Fe). According to Mössbauer spectroscopy, sample Li₁.₀₅FePO₄ has the highest content (i.e., ∼91.5%) of the actual electroactive phase (viz., crystalline LiFePO₄), followed by samples Li₁.₀₂FePO₄, Li₁.₁FePO₄ and LiFePO₄; with the remaining content being primarily Fe-containing impurities, including a conducting FeP phase in samples Li₁.₀₂FePO₄ and Li₁.₀₅FePO₄. Electrodes based on sample Li₁.₀₅FePO₄ show the best electrochemical performance in all aspects, retaining ∼150 mA h g⁻¹ after 100 charge/discharge cycles at C/2, followed by sample Li₁.₀₂FePO₄ (∼140 mA h g⁻¹), LiFePO₄ (∼120 mA h g⁻¹) and Li₁.₁₀FePO₄ (∼115 mA h g⁻¹). Furthermore, the electrodes based on sample Li₁.₀₅FePO₄ retain ∼107 mA h g⁻¹ even at a high current density of 5C. Impedance spectra indicate that electrodes based on sample Li₁.₀₅FePO₄ possess the least charge transfer resistance, plausibly having influence from the compositional aspects. This low charge transfer resistance is partially responsible for the superior electrochemical behavior of that specific composition.
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