Significance
The growing demand for energy storage urges the development of alternative cation batteries, which calls for a systematic understanding of binding energetics. We discover a general phenomenon for binding of alkali and alkaline earth metal atoms with substrates, which is explained in a unified picture of chemical bonding. This allows us to solve the long-standing puzzle of low Na capacity in graphite and predict the trends of battery voltages, and also forms a basis for analyzing the binding of alkali and alkaline earth metal atoms over a broad range of systems.

Abstract
It is well known that graphite has a low capacity for Na but a high capacity for other alkali metals. The growing interest in alternative cation batteries beyond Li makes it particularly important to elucidate the origin of this behavior, which is not well understood. In examining this question, we find a quite general phenomenon: among the alkali and alkaline earth metals, Na and Mg generally have the weakest chemical binding to a given substrate, compared with the other elements in the same column of the periodic table. We demonstrate this with quantum mechanics calculations for a wide range of substrate materials (not limited to C) covering a variety of structures and chemical compositions. The phenomenon arises from the competition between trends in the ionization energy and the ion–substrate coupling, down the columns of the periodic table. Consequently, the cathodic voltage for Na and Mg is expected to be lower than those for other metals in the same column. This generality provides a basis for analyzing the binding of alkali and alkaline earth metal atoms over a broad range of systems.

http://www.pnas.org/content/113/14/3735.abstract