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.