| As traditional fossil fuels are becoming increasingly depleted and new energy sources such as solar and wind power are volatile and uncontrollable,appropriate energy storage devices are crucial in balancing supply and demand in energy transition and energy conservation and emission reduction.Compared to other energy storage devices,sodium-ion batteries（SIBs）have the advantages of high energy density,abundant resources,and environmental friendliness.However,since the traditional anode materials for lithium-ion batteries may not be suitable for SIBs,it is necessary to explore suitable anode materials for SIBs.In this paper,three novel two-dimensional materials were designed through first-principles calculations,and they exhibited excellent performance as anode materials for sodium-ion batteries.The specific work performed is as follows:（1）Based on the unique geometry of[4.6.4.6]fenestrane skeleton,a two-dimensional（2D）carbon allotrope which possesses a planar tetra-coordinate carbon is theoretically designed,namely THFS-carbon.It is intrinsically metallic with excellent dynamical,thermal,and mechanical stabilities.The Young’s modulus along the x direction（311.37 N/m）is comparable to that of graphene.Intriguingly,THFS-carbon possesses an in-plane half-NPR distinct from most other 2D crystals.When uniaxial strain is applied along the x（or y）direction,THFS-carbon always expands along the other direction,whether it is under compressive or tensile strain.As a promising anode for sodium-ion batteries,THFS-carbon delivers an ultra-high theoretical storage capacity(2233 m A h g-1),a low diffusion energy barrier（0.03～0.05 e V）,a low open-circuit voltage（0.14～0.40 V）,and a good reversibility for Na insertion/extraction.Moreover,the commonly used electrolytes（ethylene carbonate,ethyl methyl carbonate,and dimethyl carbonate）can promote the adsorption of Na and maintain the rapid ion migration.（2）Janus graphene with asymmetric surface functionalization exhibits strong Na-ion adsorption capability.However,the introduction of functional groups often opens up the band gap of the material,resulting in poor conductivity.Herein,inspired by the asymmetric structural characteristics of triquinacene,a metallic two-dimensional Janus carbon allotrope is constructed,termed TQ-graphene.After confirming its stability,TQ-graphene is found to be an auxetic material with large in-plane negative Poisson’s ratios（-0.39）,deliviering a stronger auxeticity than penta-graphene（-0.068）,ographene（-0.11）,TAL-carbon（-0.060/-0.066）,and Be5C2（-0.16/-0.041）.Furthermore,it exhibits excellent performance as an anode material for SIBs with extremely high-capacity of 2436 m A h g-1,a small diffusion barrier（0.01～0.34 e V）,and a low average voltage（0.32 V）.The covalent bonding of bilayer TQ-graphene gives rise to a new carbon allotrope with semiconductor feature,which stands in stark contrast to the metallic nature of TQ-graphene.（3）Magnesium,one of the lightest metallic elements,possesses advantages such as abundance,low cost,environmental friendliness,and safety.In this work,a two-dimensional Mg material（named magnesene）is predicted as an excellent anode material for SIBs based on first-principles calculations.It is demonstrated to be stable in terms of the cohesive energy,phonon spectrum,ab initio molecular dynamics simulation,and elastic constants.The magnesene monolayer exhibits good SIBs performances,including a high storage capacity of 551.3 m A h g-1,low diffusion energy barrier（0.16～0.19 e V）,acceptable open-circuit voltage（0.71～0.82 V）,and small volume change（4.7%）.In practical applications,graphene or h-BN can be used as protective layers for materials to prevent their degradation in the atmosphere.Our calculations reveal that when magnesene monolayer is covered with graphene or h-BN,the resulting heterostructure can maintain the excellent properties of the original magnesene,such as metallic behavior,strong Na adsorption capability,and fast ion migration rate. |
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