We conduct a comprehensive density functional theory investigation using r2SCAN-rVV10 functional on the structural stability and electrochemical properties of Boridenes for their use as the anode materials in the rechargeable...

Employing first-principles calculations, the energy storage properties and ion diffusion dynamics of Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Al3+ on bare (Mo2/3Sc1/3)2C and surface-functionalized (Mo2/3Sc1/3)2CT2 (T = −O, −OH, and −F) i-MXenes are predicted. The investigated i-MXenes show weak adsorption ability to the Zn2+ ion regardless of the surface terminations, excluding their use as anodes for Zn ion batteries. The first-principles molecular dynamics simulations indicate that the adsorption of alkaline (earth) metal ions and Al3+ on (Mo2/3Sc1/3)2C­(OH)2 and (Mo2/3Sc1/3)2CF2 causes the surface reaction between metal ions and surface terminations, leading to the formation of metal hydride or fluorite overlayers covering the underneath remaining i-MXenes. We find that the bare (Mo2/3Sc1/3)2C and (Mo2/3Sc1/3)2CO2 are suitable candidates for use as anodes for alkaline (earth) metal and Al ion batteries. Specifically, both (Mo2/3Sc1/3)2C and (Mo2/3Sc1/3)2CO2 i-MXenes show good ion storage capacities, ideal open circuit voltages, and fast ion diffusion dynamics for alkaline (earth) metal ions. Notably, the predicted theoretical capacities of (Mo2/3Sc1/3)2C ((Mo2/3Sc1/3)2CO2) for Li+, Mg2+, and Al3+ are 291 mAh g–1 (254 mAh g–1), 490 mAh g–1 (436 mAh g–1), and 886 mAh g–1 (640 mAh g–1), respectively. In addition, the calculated open circuit voltage profiles of Li+, Mg2+, and Al3+ exhibit the small on-set voltage and the board plateau region. The climbing image-nudged elastic band predicts that the diffusion energies of Li+, Na+, K+, and Ca2+ ions on bare (Mo2/3Sc1/3)2C are extremely small (<0.05 eV), and the O-terminated surface show higher diffusion energies for various metal ions. Overall, (Mo2/3Sc1/3)2C and (Mo2/3Sc1/3)2CO2 are good electrode materials for fast charging alkaline (earth) metal ion batteries and supercapacitors.

Surface terminations greatly affect the MXene performance. On the basis of the first-principles thermodynamic calculations, the surface termination preference of 33 different out-of-plane ordered double-transition metal MXenes when prepared by etching the o-MAX phases with HF acid is revealed. By calculating the reaction enthalpy and reaction Gibbs free energy at finite temperature, it is concluded that when the outermost transition metal is a 3d-orbital transition metal, the surface termination is dominated by -F, and when the outermost transition metal belongs to 4d or 5d-orbital transition metal, the surface termination is mainly -O. And this preference law may be related to whether the outermost transition metal can donate enough electrons to surface groups.

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