Theoretical research of two-dimensional germanether in sodium-ion battery

Because sodium is more abundant in earth's reserves and the lower cost to produce, sodium-ion batteries (SIBs) have become the most popular energy storage system in research after lithium-ion batteries. However, the the lack of suitable anode materials is a major bottleneck for the commercialization of SIBs. Owing to their large specific surface area and high electron mobility, two-dimensional (2D) materials are considered as the promising anode materials. Some 2D materials have already demonstrated remarkable properties, such as 2D BP (1974 mAh center dot g(-1)) and BC7 (870.25 mAh center dot g(-1)). However, most of the predicted 2D materials are difficult to satisfy the various requirements for high-performance battery materials. Therefore, it is still necessary to find a new 2D material with excellent properties as electrode material. Recently, Ye et al. [Ye X J, Lan Z S, Liu C S 2021 J. Phys. condens. Mat. 33 315301] predicted a potential 2D material named germanether. The germanether exhibits high electron mobility, which is higher than that of phosphine and MoS2, indicating its great potential applications in Nano Electronics. Therefore, by first-principles calculations based on density functional theory (DFT), the electrochemical properties of germanether as an anode material for SIBs are fully investigated. The computation results reveal that Na atoms can be adsorbed on germanether without clustering, and the adsorbed energy of Na-ion on the germanether is -1.32 eV. Then the charge redistribution of the whole system is also investigated through Mulliken charge population. In the adsorption process, Na atom transfers 0.71e to germanether. Even at low intercalated Na concentration, the Na adsorbed germanether system demonstrates metallic characteristics, showing good electronic conductivity. Two possible diffusion paths of material are calculated: one is along the armchair direction and the other is along the zigzag direction. The diffusion barrier along the zigzag direction is 0.73 eV for the most likely diffusion path, which is slightly higher than the diffusion barrier of MoS2, but still lower than many electrode materials used today. Meanwhile, germanether has a suitable specific energy capacity (167.1 mAh center dot g-1) and open circuit voltage (1.12 V). The volume change rate is only 10.8 %, which is lower than that of phosphorene and graphite. Based on the above results, germanether can serve as a potential anode material for SIBs.