The Research Institute for Ubiquitous Energy Devices (RIUED) of the National Institute of Advanced Industrial Science and Technology (AIST) has developed ionic liquid electrolyte consisting of asymmetric cyclic quaternary ammonium-imide salt, as flame-resistant electrolyte for ensuring safety of lithium metal secondary battery. which promises a drastic increase in energy density.
While the lithium metal secondary battery is expected to provide twice or more as high energy density as existing lithium ion secondary battery, its practical application has been hampered by two impeding factors: (1) lithium anode forms dendritic deposits in organic electrolyte solution to cause inside short-circuit, and (2) coexistence of chemically active lithium metal with inflammable organic solvent makes it difficult to secure safety in commercial use. For ensuring safety of the lithium metal secondary battery, flame-resistant electrolyte is needed such as ionic liquid, but no ionic liquid, which is stable in the voltage region of lithium battery, has been available.
The newly developed ionic liquid electrolyte made of cyclic quaternary ammonium-imide salt is stable in wide voltage range of the lithium battery, and provides charge-discharge efficiency of lithium metal electrode which is equivalent to that in organic liquid electrolyte. Lithium deposition on a lithium metal substrate after deposition/dissolution cycles in a test cell did not form the dendritic structures but in flat and smooth form.
This ionic liquid is thermally stable, flame-resistant, non-volatile and has high lithium charge-discharge efficiency, opening the way to the practical use of lithium metal secondary battery. Furthermore, the use of ionic liquid is expected to make portable electronic devices lighter and their service life longer.
Further R&D efforts will be focused on the optimisation of purity and composition of the ionic liquid and upgrading of lithium metal electrode efficiency.
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