Commercial electrolytes typically contain a Li salt dissolved in an organic solvent and are often composed of two components: one for the dissolution of the salt and another that assists in the formation of a protective layer on the anode to prevent continuous electrolyte-reduction and self-discharge, e. g. ethylene carbonate. These electrolyte systems, being non-aqueous and highly air-sensitive, tend to be flam­mable and can turn from liquid to gas at elevated temperatures (Fig. 7.15). The electrolyte also determines the cathode and anode materials that can be used by limiting the applicable voltage range which is associated with the HOMO of the cathode and LUMO of the anode [4]. The key factors that determine a good electrolyte are ionic conductivity, flammability and chemical stability, and appli­cable voltage windows.

To overcome the safety and long-term reliability issues of using organic elec­trolytes, research has been directed to aqueous electrolyte systems with Li salts. Unfortunately, voltage limitations have hampered significant development of aqueous electrolytes, but these safe electrolyte-systems have found niche use in medical applications. In addition to aqueous electrolytes, liquid electrolytes based on ionic liquids have attracted significant attention.

Fig. 7.15 An example of a LiFePOdlgraphite battery containing 1:1 mol. % ethylene carbonate:dimethyl carbonate heated to 90°C where the dimethyl carbonate (organic solvent) has boiled, expanding the casing of the battery

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Fig. 7.16 A collection of conductivity data of pertinent electrolytes used for commercial and research-scale Li-ion batteries. Reprinted with permission from (N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A. Mitsui, Nat. Mater. 10, 682 (2011)) [150]. Nature Publishing Group

Apart from electrolytes in the liquid state, semi-solid or solid-state electrolytes such as gel and solid polymer electrolytes continue to be a preferred option in overcoming safety and leakage issues. Neutron scattering work has been undertaken on ceramic and glass-ceramic solid state Li-ion conducting electrolytes. Some of these electrolytes feature Li-ion conductivities that can be as good as commercial organic electrolytes as elegantly demonstrated for Li10GeP2S12 [150]. This is the first solid-state electrolyte that shows conductivity that matches that of commer — cially-available liquid electrolytes (Fig. 7.16).