Scientists solve a long-standing puzzle for Na-ion battery leading to a general conceptual rule
The lithium-ion battery is a very popular energy storage device that provides power for our laptops, cellphones, hover boards, etc. However, there are serious safety problems with lithium-ion batteries, including serious fires caused by dendrite growth during charging. There are also cost issues. To overcome these problems, research has turned toward alternative batteries using other alkali metals such as sodium or alkaline-earth metals like magnesium as the active cation. Among these, sodium-ion batteries are most extensively studied.
However, a longstanding puzzle is why sodium leads to a lower storage capacity in graphite than lithium.
A paper published on Proceedings of the National Academy of Science reports the solution to this mystery by Caltech scientists: Dr. Yuanyue Liu, Dr. Boris Merinov and Professor William A. Goddard III of the Materials and Process Simulation Center at Caltech.
"We used quantum-mechanical (QM) calculations (solving the Schrodinger Equation) to show that the sodium capacity in graphite is, indeed, smaller than for lithium, but we discovered that it is also weaker than for potassium, rubidium, and caesium," said Liu. Previously, it was suggested that the sodium has too large a radius to intercalate into graphite. However, Liu says this couldn't be the correct explanation since potassium, rubidium and caesium have a larger radius than sodium, but also lead to higher battery capacity.
"Normally, there are periodic trends leading to monotonic behavior going down a column of the periodic table. More interestingly, we considered many other possible substrates with structures and chemistry that differ from graphite and find that sodium has a weaker binding than lithium, potassium, rubidium and caesium for all cases," said Merinov. "Moreover, we predict that magnesium also has the weakest binding compared with other elements in the same column of the periodic table (beryllium, caesium, strontium, barium)."
This raises the important question: Why do sodium and magnesium have the weakest binding? The QM results showed that it is because of the competition between trends in the ionization energy and the ion-substrate bonding, down the columns of the periodic table. "As one moves down a column of the periodic table, it becomes easier to lose an electron, which enhances the binding; but the ion also becomes larger, forcing it to be farther from the substrate, which weakens the ion-substrate binding. This competition gives rise to the weakest binding at sodium and magnesium," said Liu.
"In addition to identifying the origin of weak sodium capacity in graphite," said Professor Goddard, "we now have a conceptual a basis for analyzing the binding of alkali and alkaline-earth metal atoms over a broad range of systems. These concepts can now be used by experimentalists as they consider new materials for designing a better battery."
More information: Yuanyue Liu et al. Origin of low sodium capacity in graphite and generally weak substrate binding of Na and Mg among alkali and alkaline earth metals, Proceedings of the National Academy of Sciences (2016).
Journal information: Proceedings of the National Academy of Sciences
