In general, the capacity loss that plagues most high-voltage lithium-ion batteries developed to date results from a tradeoff related to the electrolyte salt stability. A highly stable electrolyte salt has the advantage of suppressing the dissolution of the battery's transition metal electrode, but also has the disadvantage of accelerating the dissolution of the battery's aluminum current collector. An unstable electrolyte salt (for example, the widely used LiPF6) has the exact opposite effects. In either case, the dissolution results in severe capacity loss.
In the new study, the researchers have overcome this electrolyte tradeoff by mixing a stable lithium salt (LiN[SO2F]2 and related formulas) in a solvent at an extremely high concentration of 1:1.1, so that nearly half of the solution is lithium salt. Whereas a slightly less concentrated solution of 1:2 still suffers from unwanted aluminum dissolution and high capacity loss, the superconcentrated solution manages to overcome the electrolyte tradeoff due to its unusual 3D liquid structure.
"At present, the biggest challenge is materials cost, because the LiN(SO2F)2 salt is currently more expensive than currently used LiPF6 salt," Yamada said. "However, the mass production of the LiN(SO2F)2 salt has recently been initiated and is becoming increasingly available at much lower cost. Hence, we expect that the cost will not be a problem in the future. Moreover, the cost of electrolyte is only less than 7% of the total battery price."
