Mixing Two Unsuitable Solvents Nearly Triples Lithium-Metal Battery Life [Reading Science]

UNIST, KAIST Maintain 80% Capacity After 150 High-Voltage Charge-Discharge Cycles
Identify "Cooperative Solvation" Principle That Weakens Lithium-Ion Binding

Mixing two solvents considered unsuitable for lithium-metal batteries instead nearly tripled battery life. When used separately, the solvents bound lithium ions so strongly that they interfered with charging and discharging. But when combined, they became more likely to release the lithium ions. A South Korean research team has extended the life of next-generation batteries using an approach that overturns conventional wisdom about electrolyte design.

Performance comparison of lithium metal batteries using a cooperative solvation electrolyte. Their charge-discharge cycle life was approximately three times longer than that of batteries with conventional electrolytes, and lithium was found to deposit evenly on the anode surface. Gas generation and electrolyte consumption were also greatly reduced, improving battery stability. Provided by the research team.

Ulsan National Institute of Science and Technology (UNIST) said on October 12 that a joint research team led by Professor Song Hyeon-gon of its Department of Energy and Chemical Engineering and Professor Seo Donghwa of the Department of Materials Science and Engineering at the Korea Advanced Institute of Science and Technology (KAIST) had developed an electrolyte that extends the charge-discharge life of lithium-metal batteries by about three times. The findings were published in the international journal ACS Nano on August 11.

Unlike the widely used lithium-ion batteries, lithium-metal batteries use lithium metal instead of graphite as the anode. They have attracted attention as next-generation, high-capacity batteries because they can store more electrical energy at the same weight. However, lithium metal reacts unnecessarily with the electrolyte during charging and discharging, causing capacity to decline rapidly. This problem becomes more severe when batteries are charged at high voltages to store more energy.

A reversal for two unsuitable solvents...reducing lithium loss

The research team mixed two solvents, ethylene carbonate (EC) and sulfolane (SL), in a 3:7 ratio, then added a lithium salt and a small amount of an additive to create a new electrolyte.

Both solvents were considered difficult to use in lithium-metal batteries because they bind strongly to lithium ions. During charging, lithium ions must shed the surrounding solvent molecules and deposit as metal on the anode surface. But when solvents hold the lithium ions tightly, this process does not proceed smoothly. Lithium then deposits unevenly, and side reactions with the electrolyte reduce battery capacity.

When the team mixed the two solvents, the results were contrary to expectations. As the solvent molecules attracted one another, their hold on the lithium ions actually weakened. This allowed the lithium ions to shed the solvent molecules more easily and deposit evenly on the anode surface.

The team named this effect "cooperative solvation." Solvation is the process in which solvent molecules surround lithium ions. The term describes how two types of solvents influence one another to regulate their bonds with lithium ions.

Experiments confirmed the performance improvement. The developed electrolyte recorded an initial Coulombic efficiency of 98.24% and an average Coulombic efficiency of 99.35%. Coulombic efficiency indicates how much of the lithium deposited during charging can be recovered during discharging. A higher value means less lithium is consumed during charge-discharge cycles.

Three times longer life even with high-voltage charging...further validation needed for commercialization

The research team tested the developed electrolyte in batteries with high-nickel cathodes and compared their performance. When charged to 4.5 V at 25°C, the batteries retained about 80% of their initial capacity even after approximately 150 charge-discharge cycles.

By contrast, batteries using a conventional carbonate-based electrolyte fell below 80% of their initial capacity after about 50 cycles. Under the same conditions, the charge-discharge life was therefore about three times longer. The team also confirmed reductions in gas generation caused by electrolyte decomposition and in the dissolution of nickel from the cathode.

UNIST researchers (from left): Professor Song Hyungon and researcher Hong Jingi. Photo courtesy of UNIST.

The study is significant because it improved battery performance using combinations of commonly used solvents, without relying on expensive fluorinated solvents or highly concentrated lithium salts. However, 150 charge-discharge cycles remain limited for applications in electric vehicles or energy storage systems (ESS). Further validation is needed to determine whether the performance can be maintained during long-term use, across a range of temperatures, and under fast-charging conditions.

First author and researcher Hong Jin-gi said, "Mixing two solvents with strong bonding forces to weaken their bonds with lithium ions is a new design principle that overturns conventional wisdom about electrolyte design." He added, "It could also be used to improve electrolyte performance in commercial lithium-ion batteries, in addition to lithium-metal batteries."

UNIST Professor Song Hyeon-gon said, "We improved electrolyte performance using commercial solvents without relying on expensive fluorinated solvents or highly concentrated salts." He added, "We expect this will help extend the service life of high-capacity lithium-metal batteries for energy storage systems (ESS) and electric vehicles."

This content was produced with the assistance of AI translation services.