Optimization of lithium content in LiFePO₄ for superior electrochemical performance: the role of impurities

Carbon coated Li FePO₄ 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 Li_1.05FePO₄ has the highest content (i.e., ∼91.5%) of the actual electroactive phase (viz., crystalline LiFePO₄), followed by samples Li_1.02FePO₄, Li_1.1FePO₄ and LiFePO₄; with the remaining content being primarily Fe-containing impurities, including a conducting FeP phase in samples Li_1.02FePO₄ and Li_1.05FePO₄. Electrodes based on sample Li_1.05FePO₄ 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 Li_1.02FePO₄ (∼140 mA h g^-1), LiFePO₄ (∼120 mA h g^-1) and Li_1.10FePO₄ (∼115 mA h g^-1). Furthermore, the electrodes based on sample Li_1.05FePO₄ retain ∼107 mA h g^-1 even at a high current density of 5C. Impedance spectra indicate that electrodes based on sample Li_1.05FePO₄ 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.