Importance of High-Concentration Electrolytes for Lithium-Based Batteries

Each battery cell consists of three main components: the anode, the cathode, and the separator soaked with liquid electrolyte, the medium in the battery that allows charged ions to move between the two electrodes. Besides a wide electrochemical stability window and good compatibility with both electrodes, the electrolyte should also be safe, thermally stable and environmentally benign, showing a high ionic conductivity of the charge-carrying Li ions and finally a low price. This unique combination of properties is impossible to achieve with a simple salt–solvent mixture and usually requires a combination of different electrolyte components, i.e., several liquid solvents and additives and one or more conducting salt(s). For lithium-based batteries, which are the most common electrochemical energy storage devices today, a solution based on lithium hexafluorophosphate (LiPF6) in a mixture of organic carbonates as the solvent is used. Usually, the conducting salt concentrations used for lithium-based electrolytes are in the range of ≈1 to 1.2 M, but recently, electrolytes with much higher conducting salt concentrations of 5 M and even over 10 M have been investigated as they offer several benefits ranging from increased safety to a broadened electrochemical stability window, thus enabling cheap and safe solvents, even water.