Crystal structure, morphology, and electrical properties of aluminum-doped LFP materials

The effect of doping with aluminum compounds on the crystal structure, morphology, and electrochemical properties of LiFePO4 has been investigated with aluminum stearate, alumina, aluminum sulfate, and aluminum phosphate as dopants. The contraction of unit cell observed by XRD analysis and reduced lattice spacing determined by HRTEM of the doped crystals indicate that Al3+ ions, which occupy smaller space than lithium ions, are successfully doped into the lattice of LiFePO4. Lattice doping of aluminum ions enlarges Li+ transport channels; 1%-AlP-LFP has the slowest attenuation of discharge specific capacity. After 30 cycles of charge and discharge curve test at 0.5C, the retention rate of the sample is 97.43%. Owing to the substitution of S for O sites, and SO4 2- has a breaking effect on the carbon layer, this accelerates the capacity decay of 1%-AlS-LFP. The discharge capacity of 1%-AlS-LFP is 132.9 mAh/g, which is lower than 139.8mAh/g of LFP. The electrochemical impedance spectroscopy (EIS) results show that the resistance of 1%-AlP-LFP is 147.1 omega, the resistance of LFP is 138.9 omega, and the resistance of LFP is 183.9 omega. The Li+ diffusion coefficient of 1%-AlP-LFP is partially increased; the double substitution of Al3+ and S2- slows the migration rate of Li+.