Better capacity in high voltage lithium-ion batteries

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Researchers from the University of Tokyo, Kyoto University, JST PRESTO, and the National Institute for Materials Science, claim to have found a way to overcome capacity loss in lithium-ion batteries for electric vehicles.

One way of improving driving range in electric cars running on lithium-ion batteries is by increasing the battery voltage from the present 4V to 5V. However, the higher voltage is accompanied by a capacity loss of more than 50% after only 100 charge/discharge cycles.

The researchers have developed a 5V lithium-ion battery that can maintain more than 90% of its capacity over 100 cycles by using a superconcentrated lithium based electrolyte.

"The 5V battery using the superconcentrated electrolyte has much higher energy density than current 4V batteries while achieving a comparable power density," said Atsuo Yamada, professor at the University of Tokyo.

He explained that the 5V battery's smaller size and weight also contribute to a longer driving range in electric vehicles.

The capacity loss in most high voltage lithium-ion batteries is related to electrolyte salt stability. A stable electrolyte salt suppresses the dissolution of the battery's transition metal electrode, but simultaneously has the disadvantage of accelerating the dissolution of the battery's aluminium current collector. An unstable electrolyte salt has the exact opposite effects. In either case, the dissolution results in capacity loss.

The researchers have overcome this by mixing a stable lithium salt (LiN[SO2F]2) in a solvent at a high concentration so that close to half of the solution is lithium salt. The superconcentrated solution overcomes the electrolyte trade off due to its 3D liquid structure. The new electrolyte is also safer due to its high thermal stability. The better stability is due to its lower content of organic solvents.

"At present, the biggest challenge is materials cost, because the LiN(SO2F)2 salt is 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 less than 7% of the total battery price."