Electrolyte Systems for Lithium Metal Anodes and Rechargeable Lithium Metal Batteries
Ever since the first introduction of commercial lithium-ion batteries (LIBs) in the 1990s, LIBs have held a monopoly on the portable electronic market as the major power source. However, LIBs can no longer meet the increasing demand for high-energy storage systems such as long-range electric vehicles due to their relatively low energy densities (ca. 350 Wh kg−1). Among the anode materials which have recently been studied as a replacement for carbonaceous materials, lithium (Li) metal is an ideal candidate, as it has the highest theoretical specific capacity (3,860 mAh g−1) and the lowest redox potential (ca. −3.040 V vs. standard hydrogen electrode). Unfortunately, even though lithium metal batteries (LMBs) have been studied since the 1970s, they have not been commercialized due to the presence of three major challenges: (1) uncontrolled growth of lithium dendrites, causing safety issues; (2) low coulombic efficiencies (CEs) with capacities fading during cycling, and (3) large volumetric and morphological changes occurring on the Li metal anode surface during cycling due to unstable SEI layers. As such, most strategies to improve battery lifespan and performance have focused on increasing the SEI layer stability by modifying its chemical composition. In general, the SEI layer undergoes repetitive cycles of formation and depletion. Because the absence of a stable SEI layer severely impedes Li stripping and requires excessive amounts of electrolyte for its repeated formation, the rapid consumption of both the electrolyte and Li metal are major concerns. Thus, the presence of a sound SEI layer can enhance CE and suppress lithium dendrites.
As the primary solvation sheath in the electrolyte is the precursor of solid electrolyte interphase, the interfacial chemistry on the electrodes is closely correlated to the solvation structure of electrolytes. In the case of conventional dilute electrolytes where the Li ions are usually solvated by strongly solvating polar solvents and most anions are excluded from the solvation sheath, the SEI layer is mainly composed of organic species originated from the solvent molecules. One major innovation of unconventional electrolytes is the concept of highly concentrated electrolytes (HCEs), with high salt concentration (> 3.0 M). In HCEs, anions inevitably appear in the primary solvation sheath of Li+ to form ion pairs or aggregates because of the scarcity of solvents and abundance of anions. Such solvation structures lead to an anion-derived SEI that enables high-rate and long-term cycling of Li metal electrodes.