Researchers at the Nagoya Institute of Technology and the University of Tokyo have found a new way to keep lithium metal batteries’ electrolytes and electrodes stable. The energy density of batteries — the amount of energy stored in relation to the weight or volume — can be significantly improved by this new mechanism, which does not rely on the conventional kinetic approach.
“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. Based on our findings, several electrolytes, which enable high Coulombic efficiency, have been easily developed,”
said Atsuo Yamada, a Department of Chemical System Engineering professor at the University of Tokyo.
High Coulombic Efficiency
Lithium metal batteries are a promising technology that could help meet the growing need for high-energy-density storage systems. However, these batteries’ continuous electrolyte decomposition results in low Coulombic efficiency.
The Coulombic efficiency, which is also called the current efficiency, is a way to measure how well the battery moves electrons. This means that a battery with a high Coulombic efficiency has a longer cycle life.
During charging and discharging, lithium ions move through the electrolyte from the positive electrode to the negative electrode. The energy density of the battery can be increased by adding high-energy-density electrodes.
Numerous studies have been done in this context over the years to replace the graphite negative electrode with lithium metal. However, due to the lithium metal’s high reactivity, the electrolyte at its surface is reduced. As a result, the lithium metal electrode exhibits a low Coulombic efficiency.
Solid Electrolyte Interphases
To solve this problem, scientists have made functional electrolytes and electrolyte additives that form a protective film on the surface. The safety and effectiveness of lithium batteries are impacted by this solid electrolyte interphase.
The surface protective film slows down the electrolyte reduction by stopping the electrolyte from coming into direct contact with the lithium metal electrode. But until recently, it wasn’t clear what the relationship was between the solid electrolyte interphase and the Coulombic efficiency.
Scientists are aware that by increasing the solid electrolyte interphase’s stability, they can reduce the rate of electrolyte decomposition and raise the battery’s Coulombic efficiency. However, despite modern technology, it is challenging for researchers to directly examine the solid electrolyte interphase chemistry.
The majority of research on the solid electrolyte interphase has been done using indirect methods. Because these studies only show indirect evidence, it is hard to make the lithium metal that stabilizes the electrolyte and gives a high Coulombic efficiency.
The research team came to the conclusion that increasing the lithium metal’s oxidation-reduction potential in a certain electrolyte system would lower the thermodynamic driving force needed to reduce the electrolyte and lead to higher Coulombic efficiency. Rarely had this approach been used in the creation of 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 Atsuo Yamada.
The group looked at the ability of 74 different electrolytes to oxidize or reduce lithium metal. As an internal standard for electrode potentials that was recommended by the International Union of Pure and Applied Chemistry (IUPAC), the researchers added a substance called ferrocene to all of the electrolytes.
The researchers showed a connection between the Coulombic efficiency and the oxidation-reduction potential of lithium metal. To get the high Coulombic efficiency, they used the fact that the oxidation-reduction potential of lithium metal went up.
The research group’s long-term objective is to fully understand the rational mechanism underlying the oxidation-reduction potential shift.
“We will design the electrolyte guaranteeing a Coulombic efficiency of greater than 99.95%. 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,”
concluded Atsuo Yamada.
Electrode potential influences the reversibility of lithium metal anodes. Nature Energy, 27 Oct 2022, doi: 10.1038/s41560-022-01144-0
Cover image shows a conceptual rendering of the plating and stripping reaction of the lithium metal electrode. Credit: Yamada & Kitada Lab., Department of Chemical System Engineering, The University of Tokyo.
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