ULSAN — Korean researchers have developed an electrolyte technology that can triple the cycle life of next-generation lithium metal batteries by harnessing interactions between inexpensive commercial solvents. The approach addresses the rapid degradation that occurs during high-voltage charging without relying on costly specialty solvents or high-concentration salts.
A joint research team led by Professor Song Hyun-kon of the Department of Energy and Chemical Engineering at the Ulsan National Institute of Science and Technology (UNIST) and Professor Seo Dong-hwa of the Department of Materials Science and Engineering at the Korea Advanced Institute of Science and Technology (KAIST) said on the 12th that it had developed an electrolyte that extends lithium metal battery life by exploiting the attraction between two commercial solvents.
Lithium metal batteries are regarded as a next-generation high-energy-density option because they can deliver far greater capacity than commercial lithium-ion batteries that use graphite anodes. But their cycle life falls off sharply because lithium metal triggers unwanted side reactions with the electrolyte during charging and discharging, and the capacity loss becomes more severe under high-voltage charging conditions.
The team solved the problem by mixing two solvents, ethylene carbonate (EC) and sulfolane (SL), at a weight ratio of 3 to 7 and adding a small amount of an additive that stabilizes the protective film. Both solvents had previously been excluded from lithium metal battery design because they bind lithium ions too strongly, hindering deposition on the electrode surface.
When the two were combined, however, strong intermolecular attractions including hydrogen bonding formed between the hydrogen in EC and the oxygen in SL, which instead weakened the solvents' grip on lithium ions. The team identified this phenomenon — in which the two solvents influence each other to regulate lithium-ion binding and the interfacial accessibility of anions — as "cooperative solvation."
In performance testing, the new electrolyte recorded an initial Coulombic efficiency of 98.24% and an average Coulombic efficiency of 99.35%. A battery with a high-nickel (NCM811) cathode retained about 80% of its initial capacity after 150 charge-discharge cycles in a 4.5-volt high-voltage environment. That is three times the cycle life of conventional carbonate-based electrolytes, which fell below 80% after about 50 cycles. Gas generation from electrolyte decomposition and nickel leaching from the cathode material were also suppressed.
"We improved electrolyte performance using only commercial solvents, without depending on expensive fluorinated solvents or high-concentration salts," Song said. "This should help extend the service life of high-capacity lithium metal batteries for use in energy storage systems (ESS) and electric vehicles."
Hong Jin-ki, the researcher who is first author of the paper, said the work represents "a new design principle in which solvents with strong binding forces are mixed to weaken their bonding with lithium ions." He added that it "can be extended not only to lithium metal batteries but also to commercial lithium-ion batteries and next-generation battery systems."
The research was supported by the Korea Evaluation Institute of Industrial Technology (KEIT) under the Ministry of Trade, Industry and Energy and by the National Research Council of Science and Technology (NST), and was published in ACS Nano, an international journal in the field of nanoscience.