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

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.

• The paper demonstrates that weak ion-substrate coupling, not just physical spacing, limits sodium capacity in graphite.
• Using DFT with VASP and the PBE functional, it quantifies binding energies, revealing a trend where Na shows notably weaker binding.
• The study suggests that increasing graphite interlayer spacing to around 4.3 Å may enhance battery performance.

Analyzing the Weak Binding Energetics of Sodium and Magnesium in Battery Applications

The paper under consideration provides a comprehensive analysis of the binding energetics of alkali and alkaline-earth metals, with a focus on sodium (Na) and magnesium (Mg) in their interactions with various substrates. Leveraging quantum mechanical calculations, the authors seek to elucidate the challenges associated with using sodium in graphite-based anodes for battery applications, a topic of considerable interest due to the need for alternative cation batteries beyond lithium.

Key Findings and Methodology

Utilizing Density Functional Theory (DFT) with the Vienna Ab-initio Simulation Package (VASP), the study investigates the formation energies and binding characteristics of alkali metals with graphite and other substrates. The methodology also incorporates various pseudopotentials and functionals, such as the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, to deliver accurate predictions of molecular energetics. The paper identifies a consistent trend where Na and Mg exhibit the weakest binding energies across a range of substrates, including graphene, MoS2, and metallic surfaces. This insight stands in contrast to a previous hypothesis that attributes Na's low capacity in graphite to unfavorable physical spacing and strain energies.

The central hypothesis proposed by the authors attributes this weak binding to an interplay between metal ionization energy and ion-substrate coupling. Specifically, they observe a non-monotonic variation along the column of the periodic table. For sodium, while the ionization energy decreases—as is expected for elements descending the periodic table—the coupling strength also declines, resulting in a pronounced increase in binding energy, unique to Na among the alkali metals.

Numerical Results and Implications

The study presents numerical evidence supporting the claim of weakened sodium binding. This is observed not only in graphite but also in a variety of other materials. The results show that the binding energy trend for Na follows a trajectory of Na > Li > K > Rb > Cs, which corresponds well with the experimental reality of battery performance. Similarly, Mg reflects comparable behavior among alkaline-earth metals, explaining its similarly low capacity in graphite substrates.

Practically, the recognition of Na's weak binding is informative for battery design, suggesting that expanding graphite interlayer distance could optimize Na storage. Computational models suggest a target interlayer distance of approximately 4.3 Å to enhance capacity, potentially achievable through material modifications or pre-straining strategies.

Theoretical Implications and Future Directions

Theoretically, the insights presented in this paper advance the understanding of binding energetics for alkali and alkaline-earth metals, providing a foundational framework for future research. The work extends its significance by linking the weak binding of Na and Mg to broader principles of ionization energy and bonding interaction dynamics, applicable across several materials systems.

Future research could explore the practical applications of these findings, examining novel material substrates or hybrid systems that better accommodate elements with weak binding properties. Furthermore, exploring alternative computational methods or experimental techniques to overcome the binding energy limitations could prove valuable for developing high-performance, cost-effective battery technologies.

In conclusion, this paper delineates a significant contribution to understanding the binding properties of Na and Mg in energy storage contexts. The findings have critical implications for the development of advanced battery systems, facilitating an informed approach to tackling the inherent challenges of using sodium and magnesium in energy storage technologies.