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Both the direct-current and alternating-current conductances can be enhanced by the line defects in β12-borophene, but the mechanisms are different.

Maintaining layer-like features and regulating V atoms will be important directions to improve the performance of NaxV2(PO4)2FO2.

Due to the low cost, high element abundance and intrinsic safety, potassium-ion batteries (KIBs) have attracted a surge of interest in recent years. Currently, the key challenge and obstacle to the development of KIBs is to find suitable anode materials with large capacity, high rate capability and small lattice changes during the charge/discharge process. MXenes with excellent energy storage properties are promising anode materials for KIBs and their energy performance largely depends on the surface termination. Here, two-dimensional O- and S-terminated V2C MXene anode materials are designed to model high performance potassium-ion batteries. Using first-principles calculations, the structural properties and potential battery performance in KIBs of V2CO2 and V2CS2 are systematically investigated. The inherent metallic nature, a small diffusion barrier, a low average open circuit voltage, and a high theoretical specific capacity (489.93 mA h g-1 of V2CO2 and 200.24 mA h g-1 of V2CS2) demonstrate that both of them are ideal anode materials for KIBs. Meanwhile, we also investigated the mechanism of the difference in energy performance between V2CO2 and V2CS2 at atomic and electronic levels, in other words, the energy performance difference introduced by surface O- and S-terminations.

The lack of high-performance anode materials has become a major obstacle to the development of Li- and Na-ion batteries. Recently, 2D transition metal borides (e.g. MBenes) have attracted much attention due to their excellent stability and electrical conductivity. Unfortunately, most of the reported MBene phases typically have an intrinsic metal-rich structure with metal atoms exposed on the surface, which harmfully affect the adsorption of Li/Na atoms. Here, through crystal structure prediction combined with the first-principles density functional theory, a novel TiB3 MBene has been determined by altering the proportion of non-metallic element boron to wrap metal atoms and weaken nearest-neighbor electrostatic repulsion. Electrostatic potential analysis visually shows a surface with low potential on the TiB3 monolayer implying high adsorption capacity, and also can be used to quickly screen out the Li/Na adsorption sites. Accurate half-cell battery simulation confirmably shows that the TiB3 monolayer possesses a theoretical specific capacity of 1335.04 and 667.52 mA h g-1 for Li and Na, respectively. The TiB3 monolayer can remain metallic after adsorbing Li/Na atoms, which ensures good conductivity during battery cycling. The ultra-low barrier energy (only 38 meV for Li) and suitable open-circuit voltage indicate excellent charging and discharging capabilities. These results suggest that the TiB3 monolayer could be a promising anode material for Li- and Na-ion batteries, and provide a simple design principle for exposing non-metallic atoms on the surface.

The development of lithium-sulfur (Li-S) batteries is hindered by capacity loss due to lithium polysulfide (LIPS) dissolution into electrolyte solutions (known as the "shuttle effect"). MXenes with excellent electrical conductivity, high mechanical strength and multiple possible active two-dimensional surface terminations are attracting much attention as anchoring materials of Li-S batteries. Here, the S-functionalized V2C (V2CS2) is designed and demonstrated to have not only dynamic and thermal stability, but also metallic character. Compared with bare V2C and V2CO2, V2CS2 exhibits a moderate adsorption effect to suppress the "shuttle effect" and can preserve the structure of LIPSs without any decomposition. Moreover, the metallic properties of V2CS2 are maintained after LIPSs are adsorbed, which can promote the electrochemical activity during the charge and discharge process. The low energy barriers of Li2S decomposition and Li diffusion on the V2CS2 surface promise the phase transformation of LIPSs and assist the electrochemical process. Based on these remarkable results, we can conclude that V2CS2 is a promising anchoring material for lithium-sulfur batteries. Our work may also inspire the exploration of other MXenes and new surface functionalization methods to improve the performance of MXenes as host materials for high performance Li-S batteries.