Origin of low sodium capacity in graphite and generally weak substrate binding of Na and Mg among alkali and alkaline earth metals

TL;DR

This study uses quantum mechanics calculations to explain why sodium and magnesium exhibit weak substrate binding and low capacity in graphite and other materials, impacting their performance in alternative cation batteries.

Contribution

It reveals a general phenomenon that Na and Mg have the weakest binding among alkali and alkaline earth metals due to ionization energy and ion-substrate interactions, across various substrates.

Findings

Na and Mg have the weakest chemical binding among their groups.

The low binding strength explains the low capacity in graphite for Na.

The cathodic voltage for Na and Mg is predicted to be lower.

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.