Mass transfer analysis of boron-doped carbon nanotube cathodes for dual-electrolyte lithium-air batteries

Dual-electrolyte Li-air batteries (LABs) have the advantages of high specific energy density and low overpotential, but the mass transfer mechanism is still unclear. Mass transfer is essential to battery performance. To improve the mass transfer capability of the positive electrode, this research studied the effect of boron (B)-doped carbon nanotubes (BC₃NTs) with different amounts of defects as positive electrodes. Deep discharge results showed that the discharge voltage platform of BC₃NTs was higher than that of CNTs, and the more defects, the higher the discharge voltage platform. The electrochemical impedance spectroscopy (EIS) results also showed that the more defects, the smaller the charge transfer resistance. As the discharge current density increased, the discharge voltage platform decreased gradually. Traditional molecular dynamics calculation results showed that the topological defects caused by BC₃NTs reduced the Li⁺ and O₂ diffusion energy barriers from the sidewalls from 4.5 eV to 1.65 eV and 4.94 eV to 0.11 eV, respectively. The diffusion capabilities of Li⁺ and O₂ in BC₃NTs were improved, and at the same concentration, the diffusion coefficients of Li⁺ and O₂ increase to 3.0 × 10^-9 m² s^-1 and 0.415 × 10^-9 m² s^-1, respectively. The radial distribution function results showed that the larger the topological defect ring was, the greater the probability of finding Li⁺ and O₂ near the electrode. In summary, the defects produced by BC₃NTs were conducive to mass transfer, which could reduce polarization and impedance, and improve battery performance.