A research team discovered a new mechanism to stabilize lithium metal electrodes and electrolytes in lithium metal battery . This new mechanism does not rely on traditional dynamics methods. It has the potential to significantly increase the energy density of a battery - the amount of energy stored relative to weight or volume.
The team published their discoveries on Nature Energy on October 27 local time.
Lithium metal battery is a promising technology that has the potential to meet the needs of high energy density storage systems. However, because the electrolytes in these batteries are constantly decomposing, their Coulomb efficiency is very low. Coulomb efficiency, also known as current efficiency, describes the efficiency of electrons transporting in a battery. Therefore, high-coulomb-efficient batteries have longer battery cycle life.
"This is the first paper to propose electrode potentials and related structural characteristics as indicators for designing electrolytes in lithium metal batteries, extracted by the introduction of data science and computing," said Atsuo Yamada, professor in the Department of Chemical Systems Engineering at the University of Tokyo. "Based on our findings, several electrolytes that can achieve high Coulombic efficiency have been easily developed. The team's work has the potential to provide new opportunities for the design of next-generation electrolytes for lithium metal batteries.
In lithium ion battery , lithium ion moves from the positive electrode to the negative electrode through the electrolyte when charging, and returns when discharged. By introducing high energy density electrodes, the energy density of the battery can be improved. In this case, many studies have been conducted over the past few decades to change the graphite anode to metal lithium. However, metal lithium has a high reactivity, which reduces the electrolyte on its surface. Because of this, the lithium metal electrode shows poor Coulomb efficiency .
To overcome this problem, scientists have developed functional electrolytes and electrolyte additives to form a surface protective film. This solid electrolyte has an impact on the safety and efficiency of lithium batteries. The surface protective film prevents direct contact between the electrolyte and the metal lithium electrode, thereby slowing down the reduction of electrolyte dynamically. Until now, however, scientists have not fully understood the correlation between phases and Coulomb efficiency between solid electrolytes.
Scientists know that if they improve the stability of the interphase between solid electrolytes, they can slow down the decomposition of the electrolytes and the battery's Coulomb efficiency will be improved. But even with advanced technology, scientists have found it difficult to directly analyze the chemical properties of solid electrolyte interphase. Most studies on the interphase of solid electrolytes are carried out by indirect methods. These studies provide indirect evidence, so it is difficult to develop lithium metals that lead to high Coulombic efficiency.
research team determined that if they can increase the oxidation-reduction potential of lithium metal in a specific electrolyte system, they can reduce the thermodynamic driving force of the reduced electrolyte to achieve higher Coulomb efficiency. This strategy is rarely used in the development of lithium metal batteries. "The thermodynamic oxidation-reduction potential of metal lithium varies greatly due to electrolytes. It is a simple but overlooked factor that affects the performance of metal lithium batteries," said Atsuo Yamada. The team studied the oxidation-reduction potential of metal lithium in 74 electrolytes. The researchers introduced a compound called ferrocene into all electrolytes as an internal standard for electrode potential recommended by IUPACh (International Alliance for Pure and Applied Chemistry). The research team demonstrated that there is a correlation between the oxidation-reduction potential of metal lithium and Coulomb efficiency. They achieved high Coulombic efficiency with the redox potential of metal lithium shifting upwards. Looking forward to future work, the research team's goal is to uncover the rational mechanisms behind oxidation-reduction potential transfer in more detail. "We will design electrolytes that ensure that Coulomb efficiency is greater than 99.95%. Even with advanced electrolytes, the Coulomb efficiency of metal lithium is less than 99%. However, at least 99.95% is a requirement for commercialization of metal lithium-based batteries," said Atsuo Yamada.
