In-situ reconstruction of N-doped carbon nanoflower coating layer for enhancing high pseudo-capacitance in Bi-based fast-charging lithium-ion batteries

As one of the alloy-type lithium-ion electrodes, Bi has outstanding application prospects for large volume capacity (3800 mAh·cm⁻³) and high electronic conductivity (1.4 × 10⁷ S·m⁻¹). However, the fast-charging performance is hindered by significant volume expansion (> 218%) and a low rate of phase diffusion. To overcome these two problems, an N-doped carbon nanoflower coating layer was elaborately in-situ reconstructed on a multiple-wall Bi microsphere by hydrothermal methods and subsequent calcination in this study. The carbon nanoflowers greatly increase specific surface area (40.0 m²·g⁻¹) and alleviate the volume expansion (130%). In addition, the incorporation of N-doped carbon nanoflowers leads to a gradual enhancement in the Li adsorption energy of Bi during the process of lithium insertion and improves the electrical conductivity. Therefore, the contribution rate of pseudo-capacitance reached 87.5% at the scan rate of 0.8 mV·s⁻¹, and the Li-ion diffusion coefficient ( \(D_{\text{Li}^{+}}\) $D_{{\text{Li}}^{+}}$ ) was calculated in the range of 10⁻¹⁰ to 10⁻¹² cm²·s⁻¹. The Bi@CNFs anode provided a high specific volumetric capacity of 2117.0 mAh·cm⁻³ at 5C and a high capacity retention ratio of 93.2% after 800 cycles. The Bi@CNFs//LiFePO₄ full cell also displayed a stable capacity of 113.9 mAh·g⁻¹ and energy density of 296.1 Wh·kg⁻¹ after 100 cycles with a Coulombic efficiency of 97.6%. The mechanism of fast-charging lithium storage was verified by distribution of relaxation time analysis and density functional theory calculation. This paper provides a new strategy to increase the pseudo-capacitance and reduce the volume expansion for the preparation of alloy-type fast-charging electrodes.