Unraveling Lithium Storage and Defect-Modulated Transport in 2D Copper Boride: From First Principles to Machine-Learned Molecular Dynamics

In this work, we combine first-principles calculations, ab initio molecular dynamics (AIMD), and machine-learning-interatomic-potential molecular dynamics (MLIP-MD) to investigate lithium storage and transport in two-dimensional Cu${}_8$B${}_{14}$, including an experimentally identified line-defect configuration. Pristine Cu${}_8$B${}_{14}$ remains metallic upon lithiation and exhibits favourable Li adsorption, a single-surface theoretical capacity of 427~mAh~g${}^{-1}$, and an average open-circuit voltage of approximately 0.53~V. The lowest Li migration barrier on the pristine surface is 0.32~eV. The line-defected structure retains a capacity of approximately 400~mAh~g${}^{-1}$ while reducing the local migration barrier to 0.21~eV. To access Li dynamics beyond the picosecond time scale of AIMD, a pretrained MACE-MP-0 model was fine-tuned against system-specific DFT energies and forces and employed in nanosecond-scale simulations from 400 to 600~K. Both pristine and line-defected lithiated monolayers remain structurally stable over this temperature range. At 400~K, the line-defected system exhibits a higher in-plane Li tracer diffusivity ($3.97\times {10}^{-5}$~cm${}^2$~s${}^{-1}$) than the pristine monolayer ($2.50\times {10}^{-5}$~cm${}^2$~s${}^{-1}$), whereas the pristine system becomes more diffusive at 500 and 600~K. Arrhenius analysis yields effective activation energies of 0.147 and 0.225~eV for the line-defected and pristine systems, respectively. These results show that the line defect modifies the Li-transport landscape and its temperature dependence rather than uniformly enhancing long-range diffusion, while preserving competitive Li-storage capacity and structural stability.