Optimization of lithium content in LiFePO(4) for superior electrochemical performance: the role of impurities

Carbon coated Li(x)FePO(4) 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(1.05)FePO(4) has the highest content (i.e., ∼91.5%) of the actual electroactive phase (viz., crystalline LiFePO(4)), followed by samples Li(1.02)FePO(4), Li(1.1)FePO(4) and LiFePO(4); with the remaining content being primarily Fe-containing impurities, including a conducting FeP phase in samples Li(1.02)FePO(4) and Li(1.05)FePO(4). Electrodes based on sample Li(1.05)FePO(4) 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(1.02)FePO(4) (∼140 mA h g(−1)), LiFePO(4) (∼120 mA h g(−1)) and Li(1.10)FePO(4) (∼115 mA h g(−1)). Furthermore, the electrodes based on sample Li(1.05)FePO(4) retain ∼107 mA h g(−1) even at a high current density of 5C. Impedance spectra indicate that electrodes based on sample Li(1.05)FePO(4) 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.