In commercial lithium-ion batteries, carbonate-based electrolytes are commonly used because of their high electrochemical stability with a range of electrode materials. In lithium-metal batteries (LMBs) — batteries in which the anode is metallic lithium — ether electrolytes appear to be the preferred choice because they are less corrosive than carbonates towards lithium metal, thereby offering potentially superior cyclability; nevertheless, ether electrolytes have poor compatibility with high-energy cathode materials. Intensive efforts are underway to develop LMBs with carbonate electrolytes, but their lifespan is far from satisfactory to meet the market requirements. Now, Chong Seung Yoon, Yang-Kook Sun, and colleagues at Hanyang University report a boehmite-coated separator for LMBs with a carbonate electrolyte, and by applying external compressive pressure during operation, they demonstrate exceptionally high cyclability.
The electrolyte chosen by the researchers consists of lithium hexafluorophosphate and lithium difluoro(oxalate)borate dissolved in a mixture of ethyl methyl carbonate and fluoroethylene carbonate. Although the electrolyte is known to enable the formation of a more stable anode–electrolyte interphase than conventional carbonate electrolytes, the lithium deposition during cycling quickly leads to dendrite growth, causing early failure of LMBs. It is also known that the application of compressive pressure during operation regulates lithium deposition morphology, suppressing dendrite growth, but excessive pressure could lead to a short circuit. By studying the effects of pressure on the morphology of deposited lithium, the researchers identify the optimal pressure to be 1,200 kPa. Working in tandem with the insulating boehmite coating, the external pressure enables high-performance LMBs with the carbonate electrolyte (500 cycles with over 80% capacity retention in full cells with high loadings of high-energy cathode materials).
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