Computational insights of hydrogen vacancies mediating interaction of lithium and sodium adatoms with an electrode graphane monolayer

Abstract

Density functional theory calculations were performed to enhance interaction of Li/Na atoms with a graphane monolayer for LIBs and NIBs. The energetic stability, electronic and electrochemical properties of Li/Na atoms on a pristine graphane monolayer as well as on a graphane monolayer with H vacancies (VH) following a Line pathway and zigzag pathway were evaluated. Firstly, different Li adsorption sites and Na adsorption sites on a large supercell of 5×5×1 pristine graphane monolayer were considered, to establish the most energetically favourable site, based on binding energy calculations. All the adsorption sites of Li/Na atom on pristine graphane are energetically unfavourable (endothermic reaction). This is an indication that Li/Na atoms interacts weakly with graphane monolayer and could result with formation of unwanted Li/Na clusters. As a strategy to improve Li/Na atom interaction with graphane monolayer, a H vacancy (VH) was created. Different Li/Na adsorption sites at the vicinity of H vacancy were considered to establish the most energetically favourable site. All the Li/Na configurations relaxes towards the VH site. The Li/Na binding energies are higher than the cohesive energies of metallic bulk counterparts suggesting that clustering will not be reached easily. An increase in H vacancies along the line pathway from one VH1(L)to five VH5(L) leaves behind localized electrons ready to interact with the Li/Na atom resulting in high binding energies. The creation of H vacancies along the zigzag pathway from one VH1(Z) to ten VH10(Z) leavesbehind the dangling electrons on the nearest neighbour C atoms that pair and repel Li/Na atom away, yielding undesired low binding energies which become a setback for LIBs and NIBs. On the density of states analysis, adsorption of Li/Na atoms introduced new electronic states crossing the Fermi level. An observed transition from insulator to metallic behaviour will enhance the electron transmission in the graphane monolayer during battery operations. Li/Na storage capacities plots indicate that an increase of the Li/Na content result in an increase in the storage capacities. The average voltage obtained for the adsorption of five Li/Na atoms is 1.55 V/1.04 V. At this concentration, undesired clustering is unexpected until the voltages approaches zero. This implies that more adsorption of Li/Na atoms can still be considered to achieve a maximum concentration.