Dispersion-corrected DFT investigation on defect chemistry and potassium migration in potassium-graphite intercalation compounds for potassium ion batteries anode materials

The first-principles calculations based on the density function theory (DFT) with the dispersion-corrected DFT-D2 method were performed to investigate the structure, energetics, electronic structure, defects properties and cation migration of KC8n (n = 1, 2 and 3) and KC12n (n = 1 and 2). Compared with the standard DFT, DFT-D2 method offers good agreements with the previous experiments and calculations for c crystallographic parameters and interlayer distances. The inserted potassium atoms can significantly improve the electronic conductivity of graphite and K-GICs due to those contributions of extra electrons from the inserted potassium atoms making the Fermi levels shift to blue. Two types of potassium point defect (Schottky and Frenkel defect) were investigated and their calculated formation energies are 1.39-2.09 eV and 1.14-1.89 eV, respectively. Potassium ion migrations in the ab plane via the vacancy and Frenkel mechanism in K-GICs were researched. The calculated results of migration barrier show that potassium migration via the vacancy mechanism (0.11-1.58 eV) is absolutely prior to that via the Frenkel mechanism (2.42-7.92 eV). Besides, the occupying forms of potassium atoms have significantly influences on the defect properties and potassium migration. Compared with lithium migration in Li-GICs, potassium migration via the vacancy mechanism in K-GICs is more excellent. Thus, K-GICs are promising candidates for anode materials of potassium ion batteries. (C) 2016 Elsevier Ltd. All rights reserved.