Aqueous lithium-ion battery

The prototype for the aqueous rechargeable battery was first proposed by Jeff Dahn in 1994, who used lithium manganese oxide as the positive electrode and bronze-phase vanadium dioxide as the negative electrode.[4] In 2014, a team of researchers led by Chunsheung Wang from UMD and Kang Xu from ARL created a new class of aqueous electrolytes called water-in-salt electrolytes (WiSE), which operated under the principle that a high concentration of a specific type of lithium salt resulted in the formation of a protective solid-electrolyte interphase (SEI) in between the electrode surfaces and electrolyte in water-based batteries. Previously, it was thought that this phenomenon could only occur in non-aqueous batteries.[2][3] Using this approach to create SEI, Wang and Xu dissolved extremely high concentrations of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in water (molality > 20 m) to create a WiSE that expanded the voltage window from 1.5V to around 3.0V.[5][6] The resulting aqueous Li-ion batteries was also capable of cycling up to 1000 times with almost 100% coulombic efficiency.[3]

In 2017, Wang and Xu's research team developed an "inhomogeneous additive" to coat the graphite electrode in their aqueous Li-ion battery, which allowed the battery to reach a 4V threshold and operate up to 70 cycles at that level or higher.[7][8] The coating, created using an extremely hydrophobic and highly fluorinated ether (HFE), 1,1,2,2-Tetrafluoroethyl-2′,2′,2′-trifluoroethyl ether, expelled water molecules from the electrode surface.[1][8] This minimizes competing water decomposition and creates a favorable environment for SEI to form. This version of the battery also demonstrated resilience against extreme levels of abuse due to the slow-reacting nature of the SEI[3] When subjected to cutting, external puncture, exposure to salt water, and ballistic testing, the battery did not produce any smoke or fire and continued to operate even with severe external damages.[6]

Aqueous Li-ion batteries have a relatively short battery cycle life, ranging from 50 to 100 cycles. As of 2018, research is being conducted to increase the number of cycles to 500 to 1000 cycles, allowing them to feasibly compete against other types of batteries that have a higher energy density. In addition, issues relating to the manufacturing of the protective HFE coating would need to be resolved before the batteries can be scaled up in production for commercial use.[8]