(Invited) Electrolyte Design Strategies to High-Voltage and Safe Batteries

Abstract

Safety is becoming of prime importance for rechargeable batteries. The electrolyte for lithium-ion batteries is traditionally composed of mixed organic carbonates (e.g., ethylene carbonate (EC) and dimethyl carbonate (DMC)) because they can form a stable passivation film (solid electrolyte interphase, SEI) on graphite negative electrodes. However, organic carbonates are highly flammable and cause fire accidents. Previous efforts to introduce flame-retardant solvents (e.g., trimethyl phosphate, TMP) in electrolytes have generally resulted in compromised battery cycleability because those solvents do not suitably passivate the negative electrodes. Here we introduce two essential strategies regarding electrolytes toward safe lithium-ion batteries with high energy densities.The first strategy is to manipulate the ion-solvent coordination states via increasing salt concentrations (Fig. 1a).[1] Particularly, the extensive coordination of counter anion to Li+ results in the preferential reduction of the anion to form an anion-derived stable SEI,[2] which makes various solvents (including water) compatible with low-potential negative electrodes.[3,4] For example, we demonstrated that concentrated 5.3 mol dm-3 (M) LiN(SO2F)2 (LiFSI)/TMP electrolyte enabled the long-term cycling of graphite|Li half cells owing to the formation of FSI-derived SEI.[4] This high-concentration strategy enables an electrolyte to achieve both non-flammability and SEI-forming ability, but its high viscosity and high cost are problematic in its commercialization.[1] The second strategy is to design a multifunctional solvent molecule that has both intrinsic non-flammability and excellent SEI-forming ability.[5] We synthesized a fluorinated cyclic phosphate, 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEP, Figure 1b). The design rationale is that this solvent has a fused molecular structure of EC that forms stable SEI and organic phosphates that scavenges hydrogen radicals to prevent combustion. We formulated a non-flammable electrolyte composed of 0.95 M LiFSI dissolved in a mixture of TFEP and 2,2,2-trifluoroethyl methyl carbonate (FEMC) as a low-viscosity co-solvent (1:3 by volume). Owing to the SEI formation by TFEP, this electrolyte enabled the reversible reaction of graphite electrodes (Fig. 1c). Importantly, this electrolyte does not need high salt concentrations for such stable SEI formation, thus overcoming the issues of concentrated electrolytes.