High Catalytic Performance of M₂B₂ MBenes in Aqueous Li-N₂ Battery Cathodes: A Theoretical Study

Aqueous Li-N₂ batteries are promising electrochemical energy storage devices, but their reaction mechanisms remain controversial. This study employed density functional theory to investigate the catalytic mechanism of M₂B₂ MBenes (M = Ti, Zr, Hf, Cr, Mo, and W) as cathodes for aqueous Li-N₂ batteries. M₂B₂ MBenes exhibit high conductivity due to strong d-electron states crossing the Fermi level. IVB-group MBenes (Ti₂B₂, Zr₂B₂, and Hf₂B₂) preferentially adsorb N₂ in side-on modes at hcp sites, while VIB-group MBenes (Cr₂B₂, Mo₂B₂, and W₂B₂) favor face-centered cubic sites, with adsorption strength inversely correlated to metal atomic number. The reaction cycle involves N₂ adsorption, Li₃N formation via discharge, ammonia synthesis through Li₃N hydrolysis, and LiOH decomposition during charging. The higher discharging overpotential compared to charging suggests that Li-N₂ batteries operate in a discharge-controlled manner. Both discharge and charge overpotentials follow Hf₂B₂ > Zr₂B₂ > Ti₂B₂ and W₂B₂ > Mo₂B₂ > Cr₂B₂. The difference in catalytic activity between the IVB-group and VIB-group MBenes arises from distinct adsorption sites and configurations. Notably, Cr₂B₂ MBene demonstrates exceptional catalytic performance (0.69 V discharge/0.16 V charge overpotentials) attributed to its high d-band center enhancing N₂ adsorption/activation and the distinctive Li₂NN* intermediate configuration where two Li atoms concentrate at one N terminus facilitating subsequent lithiation. This study establishes a theoretical foundation for designing high-performance Li-N₂ batteries by utilizing tunable electronic properties of MBenes.