Theoretical Study Of Beryllium-based Two-dimensional Materials As The Anode Of Magnesium-ion Batteries

Today,high specific capacity and fast charge/discharge rate are important indicators for the development of next-generation ion batteries.Compared with conventional monovalent ion batteries,multivalent ion batteries like magnesium ion batteries（MIBs）have attracted extensive attentions owing to their high energy densities.The large specific surface area,good mechanical stability,and excellent electronic properties of two-dimensional（2D）materials make them as promising candidate materials for MIBs.Using 2D Be-based materials as anodes of metal ion batteries could obtain high theoretical capacity due to their low molar masses,which is a broad application in metal-ion batteries.Here,we systematically explore the potential applications of 2D Be-based materials as anodes of MIBs by the first-principle calculations based on density functional theory,and the main results are as follows:（1）The performance of theα-beryllene monolayer as the anode of MIBs is systematically estimated by the first-principle calculations.The Mg ion(Mg²⁺)can be stably adsorbed on theα-beryllene monolayer with the adsorption energy of-0.24 e V,and the metallicity is well maintained after the adsorption process,which is beneficial to improvement of electrode conductivity.The diffusion barriers of Mg²⁺on the surface ofα-beryllene monolayer is about 0.1 e V,suggesting the fast Mg²⁺migration and charge/discharge rate.Moreover,α-beryllene monolayer exhibits ultra-high theoretical specific capacity of 5956 m A h g^-1for Mg,low average open-circuit voltage of 0.24 V,and a tiny volume change of-1.08%.Finally,the constructed h-BN/α-beryllene heterostructure shows that h-BN can serve as a protective cover to preserve pristineα-beryllene in respect of metallicity,Mg adsorption capability,and fast ionic mobility.The above-mentioned outstanding results makeα-beryllene a promising anode material for MIBs.（2）Based on first-principles calculations,we predict a novel 2D material named Be₂B.The structural stability of Be₂B monolayer is confirmed by superior cohesive energy,positive phonon modes,excellent thermal stability,and strong mechanical stability.Afterwards,we explore the performance of Be₂B monolayer as the anode material for MIBs.It exhibits stable Mg atom adsorption with the energy of-0.7 e V,low diffusion barrier（0.1 e V）,ultra-high specific capacity(7436 m A h g^-1),tiny lattice expansion（0.3%）,and low average open-circuit voltage（0.29 V）.Thereby,the above-mentioned intriguing findings suggest that Be₂B monolayer can act as a promising anode material for high performance MIBs.（3）The performance of 2D BeB₂as the anode of MIBs is evaluated by the first-principles calculations.Firstly,the result of phonon spectrum of BeB₂shows no imaginary phonon modes in the whole Brillouin zone,which confirms the dynamic stability of the BeB₂monolayer.The band structure exhibits an intrinsic metallic property of BeB₂,which is essential for MIBs applications.Comparing to the 2Dα-beryllene and Be₂B,the BeB₂monolayer has more stable adsorbability of Mg²⁺with the adsorption energy of-2.67 e V.Meanwhile,BeB₂possess low diffusion barrier,indicating the fast rate capability.By adsorbing multiple layers of Mg²⁺,the maximum theoretical capacity can reach up to 5250 m A h g^-1.The average open circuit voltage is 0.33 V,which is comparable to monolayerα-beryllene and Be₂B.Therefore,2D BeB₂is expected to be an excellent anode material for MIBs.