Artistic representation of the plating and stripping reactions at a lithium-metal electrode. Image: Yamada & Kitada Lab., Department of Chemical System Engineering, The University of Tokyo.
Artistic representation of the plating and stripping reactions at a lithium-metal electrode. Image: Yamada & Kitada Lab., Department of Chemical System Engineering, The University of Tokyo.

A team of researchers has discovered a new mechanism for stabilizing both the negative electrode and the electrolyte in lithium-metal batteries. This new mechanism, which does not depend on the traditional kinetic approach, has potential for greatly enhancing the energy density – the amount of energy stored relative to the weight or volume – of batteries. The team reports its findings in a paper in Nature Energy.

Lithium-metal batteries are a promising technology with potential to meet the demands for high-energy-density storage systems. However, because the electrolyte in these batteries is prone to decomposing, their Coulombic efficiency is low. Also known as the current efficiency, the Coulombic efficiency describes the efficiency with which electrons are transferred in a battery – batteries with a high Coulombic efficiency have a longer battery cycle life.

“This is the first paper to propose electrode potential and related structural features as metrics for designing lithium-metal battery electrolytes, which are extracted by introducing data science combined with computational calculations,” said Atsuo Yamada, a professor in the Department of Chemical System Engineering at the University of Tokyo in Japan. “Based on our findings, several electrolytes, which enable high Coulombic efficiency, have been easily developed. The team’s work has the potential to provide new opportunities in the design of next-generation electrolytes for lithium-metal batteries.”

In lithium-ion batteries, lithium ions move from the positive electrode to the negative electrode, through the electrolyte, when the battery is charging, and then move back when it is discharging. By introducing high-energy-density electrodes, the battery’s energy density can be improved. In this context, many studies have been conducted over the past decades to replace the graphite-based negative electrode with one made of lithium metal. Unfortunately, lithium metal has a high reactivity, which causes the electrolyte to be reduced at the surface of the electrode. Because of this, the lithium-metal electrode displays a poor Coulombic efficiency.

To overcome this problem, scientists have developed functional electrolytes and electrolyte additives that form a surface protective film known as a solid electrolyte interphase (SEI), which has an impact on the safety and efficiency of lithium batteries. By preventing direct contact between the electrolyte and the lithium-metal electrode, the SEI kinetically slows the reduction of the electrolyte. Yet, until now, scientists had not fully understood the correlation between the SEI and the Coulombic efficiency.

Scientists know that if they improve the stability of the SEI they can slow the electrolyte decomposition, and thereby increase the battery’s Coulombic efficiency. But even with advanced technologies, scientists have found it difficult to analyze the SEI chemistry directly. Most of the studies on the SEI have been conducted with indirect methodologies, making it hard to develop an electrolyte-stabilizing lithium-metal electrode with a high Coulombic efficiency.

Yamada and his team determined that if they could shift the oxidation-reduction potential of the lithium-metal electrode in a specific electrolyte system, they could decrease the thermodynamic driving force that reduces the electrolyte, and thus achieve a higher Coulombic efficiency. This strategy has rarely been applied to lithium-metal batteries.

“The thermodynamic oxidation-reduction potential of lithium metal, which varies significantly depending on the electrolytes, is a simple yet overlooked factor that influences the lithium-metal battery performance,” said Yamada.

The researchers studied the oxidation-reduction potential of lithium metal in 74 types of electrolytes, introducing a compound called ferrocene into all the electrolytes as an internal standard for determining the electrode potentials. They found a correlation between the oxidation-reduction potential of lithium metal and the Coulombic efficiency, and obtained a high Coulombic efficiency by shifting the oxidation-reduction potential of the lithium-metal electrode.

Looking ahead to future work, the research team’s goal is to unveil the rational mechanism behind the oxidation-reduction potential shift in more detail.

“We will design the electrolyte guaranteeing a Coulombic efficiency of greater than 99.95%,” said Yamada. “The Coulombic efficiency of lithium metal is less than 99%, even with advanced electrolytes. However, at least 99.95% is required for the commercialization of lithium-metal-based batteries.”

This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.