In situ synthesis of Fe_(1−x)Co_xF₃/MWCNT nanocomposites with excellent electrochemical performance for lithium-ion batteries

Due to their high theoretical capacities and high energy densities, metal fluorides have attracted significant attention as cathodes for lithium-ion batteries. However, thus far, their low conductivities have limited the performance of these materials. In this work, the Fe_(1−x)Co_xF₃/MWCNT (multi-walled carbon nanotube) nanocomposites (x = 0, 0.02, 0.04 and 0.06) are obtained by an in situ solvothermal method with Co-doping and wrapping of the MWCNTs. The results indicate that Co-doping can adjust the crystal structure, decrease the band gaps and enhance the Li⁺ diffusion coefficient of FeF₃. Additionally, the wrapped network of MWCNTs enhances the conductivity of the composites and improves their electrochemical performances. The Fe_0.96Co_0.04F₃/MWCNT nanocomposites exhibit a high initial discharge capacity of 217.0 mAh g⁻¹ at rate of 0.2 C within the potential range of 2.0–4.5 V, which is much higher than that of the FeF₃/MWCNT counterpart (192.1 mAh g⁻¹). The discharge capacities of these two samples remain at 187.9 and 160.7 mAh g⁻¹ even after 50 cycles. Meanwhile, the EIS results reveal that both the Li⁺ charge transfer resistance (R_ct = 31.25 Ω) and Li⁺ diffusion coefficient (1.40 × 10⁻¹¹ cm² s⁻¹) are satisfactory from Co-doping and the in situ wrapping of the MWCNTs.