Investigation of alkali-metal clusters with pseudopotential multireference double-excitation configuration interaction method

2D phosphorus carbide as promising anode materials for Na/K-ion batteries from first-principles study

First-principles calculations based on density functional theory were used to investigate the electrochemical performance of monolayer γ-PC for Na- and K-ion batteries. Molecular dynamics simulations indicate that the monolayer γ-PC have the thermal and dynamic stability. A substantial charge transfer from the Na/K atoms to the γ-PC sheet enhances the electrical conductivity of γ-PC. The results show that the adsorption energies of Na and K are 1.53 eV and 2.04 eV, respectively, which are much higher than Na/K bulk cohesive energy and sufficiently ensure stability and safety. Additionally, the low diffusion barriers on γ-PC monolayer are 0.034 eV for Na and 0.027 eV for K, indicating excellent rate performance. The γ-PC sheet has a high theoretical capacity for both Na (519.9 mAh/g) and K (326.6 mAh/g) ion batteries, which can satisfy the requirement of energy storage devices to anode materials. Our results strongly suggest that 2D γ-PC monolayer is an exceedingly promising anode material for both NIBs and KIBs with high adsorption energies, high capacity, and low diffusion barriers.

Production and ionization energies of K(n)F (n = 2-6) clusters by thermal ionization mass spectrometry

RATIONALE

The very small clusters of the type K(n)F are of particular importance since their first ionization energies (IEs) are lower than those of the alkali metal atoms. Theoretical calculation has demonstrated that this kind of cluster represents a potential 'building block' for cluster-assembly materials with unique structural, electronic, optical, magnetic, and thermodynamic properties. To date, however, there have been no experimental results on the IEs of K(n)F (n >2) clusters.

METHOD

K(n)F (n = 2-6) clusters were produced by the evaporation of a solid potassium fluoride salt using a modified thermal ionization source of modified design, and mass selected by a magnetic sector mass spectrometer where their IEs were determined.

RESULTS

Clusters K(n)F (n = 3-6) were detected for the first time. The order of the ion intensities was K(2)F(+)> > K(4)F(+)> > K(3)F(+)K(6)F(+)> K(5)F(+). The determined IEs were 3.99 ± 0.20 eV for K(2)F, 4.16 ± 0.20 eV for K(3)F, 4.27 ± 0.20 eV for K(4)F, 4.22 ± 0.20 eV for K(5)F, and 4.31 ± 0.20 eV for K(6)F. The IEs of K(n)F increase slightly with the increase in potassium atom number from 2 to 6. We also observed that the presence of a fluorine atom leads to increasing ionization energy of bare metal potassium clusters.

CONCLUSIONS

The modified thermal ionization source provides an efficient way of obtaining the fluorine-doped potassium clusters. These results also present experimental proof that K(n)F (n = 2-6) clusters belong to the group of 'superalkali' species.