Libis(fluorosulfonyl)Imide-Based Low EC-Content Electrolyte with Specific Coordination/Solvation Process

In the organic electrolyte for Li-ion batteries, mixed solvents containing a high-permittivity solvent such as ethylene carbonate (EC) and fluoroethylene carbonate (FEC) are essential to form a favorable solid-electrolyte interface (SEI) layer. The mixed solvents should also contain 30 – 50 vol.% high-permittivity solvents in order to obtain high ionic conductivity by sufficient dissociation of Li salts such as LiPF₆ and LiBF₄. On the other hand, lithium bis(fluorosulfonyl)imide (LiFSI) dissolves easily in a low-permittivity solvent such as dimethyl carbonate (DMC) because LiFSI has very weak interaction between Li⁺ and FSI^-. In the FSI-based systems, therefore, the role of high-permittivity solvents becomes SEI formation only. In this study, we prepared LiFSI-based organic electrolytes with low EC content and found that the Li-ion cells assembled with our electrolytes exhibit excellent charge and discharge rate performance by reduction of charge-transfer resistance associated with a solvation state of EC to Li⁺.

  1.0 mol dm^-3 LiFSI dissolved in mixed solvents of EC with DMC (1:9 v/v) was prepared and used as the electrolyte with low EC content (denoted by LiFSI/EC:DMC = 1:9), while 1.0 mol dm^-3 LiFSI or LiPF₆ dissolved in mixed solvents of EC and DMC (1:1 v/v) were used as reference electrolyte (denoted by LiFSI/EC:DMC = 1:1 and LiPF₆/EC:DMC = 1:1, respectively). Graphite or LiNi_1/3Mn_1/3Co_1/3O₂ composites containing conductive additive and binder were used as negative and positive electrodes, respectively. The respective mass of active materials in the negative and positive electrodes were ca. 4.5 and 9.5 mg cm^-2. We assembled Li/LiNi_1/3Mn_1/3Co_1/3O₂, Li/graphite and graphite/LiNi_1/3Mn_1/3Co_1/3O₂ cells with each electrolyte and evaluated their battery performances.

  Table 1 shows the ionic conductivities and viscosities of each electrolyte. LiFSI/EC:DMC = 1:1 shows higher ionic conductivity and slightly lower viscosity than those of the LiPF₆/EC:DMC = 1:1. LiFSI/EC:DMC = 1:9 exhibits the lowest ionic conductivity among  the tested electrolytes probably because EC content is extremely small in this electrolyte. Nevertheless, the conductivity is still sufficiently high. These characteristics should be ascribed from excellent dissociation ability of LiFSI in spite of low-permittivity conditions.

  Figure 1 shows the charge-discharge rate performances of Li/graphite cells assembled with each electrolyte. From the ionic conductivities of each electrolyte, it can be expected that the Li/graphite cell containing LiFSI/EC:DMC = 1:1 shows the highest rate performance. In spite of that speculation, however, it is surprising that the Li/graphite cell containing LiFSI/EC:DMC = 1:9 which has the lowest ionic conductivity shows the highest rate performance. In the graphite/LiNi_1/3Mn_1/3Co_1/3O₂ cell systems, LiFSI/EC:DMC = 1:9 shows the highest rate performance , which is similar to the Li/graphite cell system. In order to explain these unexpected results, we carried out AC impedance measurement of Li/graphite cells containing each electrolyte. As a result, the Li/graphite cell containing LiFSI/EC:DMC = 1:9 has significantly smaller charge-transfer resistance of the electrode/electrolyte interface when compared to cells containing the other electrolytes. Furthermore, we found that the reduction of charge-transfer resistance is attributed to a specific solvation state of EC to Li⁺. Raman spectra of each electrolyte show that the solvation number of EC on Li⁺ reduces significantly in LiFSI/EC:DMC = 1:9. From these results, we consider that the rate performances of the cells containing LiFSI/EC:DMC = 1:9 are improved  by the low-energy desolvation process at the Li⁺ insertion into the electrode by the reduction of the solvation number of EC on Li⁺.