High electrochemical performance of nanocrystallized carbon-coated LiFePO4 modified by tris(pentafluorophenyl) borane as a cathode material for lithium-ion batteries

Abstract

Tris(pentafluorophenyl) borane (C₁₈BF₁₅) was first adopted as a boron source, which clearly demonstrated its modification effects. XPS and EDX mapping proved that boron can be successfully doped into a carbon layer. The high number of defects in the carbon induced by boron was demonstrated via Raman spectroscopy and thus, the electric conductivity of LiFePO₄ was greatly enhanced. The boron-doped composite possessed a higher specific discharge capacity and rate capability than the undoped sample. For instance, the reversible specific capacity for the boron-doped cathode reached 165.8 mA h g^-1 at 0.5C, which was almost close to its theoretical capacity (166 mA h g^-1). Even at a high rate of 5C, it still possessed a high specific capacity of 124.8 mA h g^-1. This provides for the possibility that boron-doped carbon-coated LiFePO₄ cathodes may deliver high energy and power density for rechargeable lithium-ion batteries.

Fig. 1. (a) XRD patterns and (b) Raman spectra of the as-prepared LiFePO₄/C and various LiFePO₄/CB samples; XPS spectra of (c) C 1s and (d) B 1s for the LiFePO₄/CB_0.3 sample.

Fig. 2. SEM images of (a) LiFePO₄/C, (b) LiFePO₄/CB_0.1, (c) LiFePO₄/CB_0.3 and (d) LiFePO₄/CB_0.5 powders.

Fig. 3. (a) SEM image and (b–f) the corresponding EDX mapping images of LiFePO₄/CB_0.3.

Fig. 4. HRTEM images of (a) LiFePO₄/C, (b) LiFePO₄/CB_0.1, (c) LiFePO₄/CB_0.3 and (d) LiFePO₄/CB_0.5 samples; the inset shows the SAED patterns of the samples.

Fig. 5. Electrochemical behaviors of LiFePO₄/C and various LiFePO₄/CB electrodes: (a) initial specific capacities and (b) cycling performance combined with coulombic efficiency at 0.5C in the potential range of 2.0–4.3 V (Li⁺/Li). (c) EIS spectrum after 50 cycles at 0.5C. (d) The plot of Z_revs. the reciprocal root square of the lower angular frequencies (ω^−1/2) at different amounts of boron doping in LiFePO₄/C composites. (e) CV curves at a scan rate of 0.1 mV s⁻¹.

Fig. 6. The initial charge and discharge profiles of all the samples at rates from 0.1C to 5C at room temperature: (a) LiFePO₄/C, (b) LiFePO₄/CB_0.1, (c) LiFePO₄/CB_0.3 and (d) LiFePO₄/CB_0.5 samples.