Improved electrochemical properties of BiF3/C cathode via adding amorphous AlPO4 for lithium-ion batteries

• The BiF3/C/AlPO4 composites with different amorphous AlPO4 content are successfully prepared by solid state method.
• We initially prove the formation of SEI film on the surface of nanometal Bi via EIS measurement.
• FT-IR and XPS measurements show that amorphous AlPO4 indeed provides a different interface on BiF3 free from electrolyte reduction.
• The correlation between the amount of amorphous AlPO4 and the formation of SEI film is discussed.
• We present a discussion that the amorphous AlPO4 likes to interact with the BiF3 based on surface energy.

High voltage BiF3 electrode based on conversion reaction for lithium ion batteries is not immune to cyclic carbonate-based solid electrolyte interface (SEI) formation by a cathodic reaction even when discharge voltage exceeds 2 V. The SEI formation has considerable negative effect on the electrochemical properties of BiF3. To restrict this side-reaction effect, the BiF3/C/AlPO4 composites with different amount of amorphous AlPO4 have been successfully prepared by solid state milling process. The structure and morphology of BiF3/C/AlPO4 composites are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of BiF3/C/AlPO4 composites has been studied by galvanostatic charge/discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The results show that the addition of amorphous AlPO4 does not change the bulk structure of BiF3, and the BiF3/C/AlPO4 composites still keep face-centered cubic structure of BiF3. Besides, the results further confirm that SEI film will be formed on the Bi nanometal surfaces, and the amount of amorphous AlPO4 is closely relevant with the SEI formation. With the increase of the amount, the SEI formation vanishes gradually. When increasing the amount of amorphous AlPO4 to 15.0 wt.%, the SEI formation is restricted effectively. Particularly, the BiF3/C/AlPO4 composite with 15.0 wt.% AlPO4 exhibits the highest initial discharge capacity of 263.6 mAh g−1, the lowest irreversible capacity of 45.1 mAh g−1, the best rate capability and cycling stability.