EC-Free Superconcentrated Electrolytes for Advanced Lithium-Ion Batteries

The development of a stable, functional electrolyte is urgently required for fast-charging and high-voltage lithium-ion batteries as well as next-generation lithium-oxygen batteries. Although there are numerous organic solvents with diverse characteristics used in the area of chemistry, lithium-ion batteries have exclusively employed an ethylene carbonate (EC) based electrolyte to ensure the reversibility of the graphite negative electrode reaction. Because of the limitation of electrolyte compositions, there has been no remarkable progress in commercial lithium-ion batteries despite active researches on positive electrode materials. Recently, we have presented a “salt-superconcentrating” strategy as a simple and effective method of expanding a graphite negative electrode reaction in a wide variety of organic solvents.¹ This finding can break the battery-performance limitation based on an EC-based electrolyte to open up a way to advanced lithium-ion batteries. For example, acetonitrile (AN) solutions, having remarkably high chemical and oxidative stability as well as excellent ion transport property, are one of the most promising electrolytes to realize high-voltage and fast-charging lithium-ion batteries. Herein we report unusual reductive stability of a superconcentrated AN electrolyte and its origin based on spectroscopic analyses and first-principle density functional theory based molecular dynamics (DFT-MD) simulations.²

Figure 1 shows charge-discharge curves of a natural graphite/lithium metal coin cell with superconcentrated 4.2 mol dm^-3 LiN(SO₂CF₃)₂ (LiTFSA)/AN electrolyte. Several voltage plateaus were observed in 0.05 – 0.25 V, which are characteristic of sequential formation of several stage structures of lithium-graphite intercalation compounds. The obtained reversible capacity was ca. 350 mAh g^-1, which is close to the theoretical capacity (372 mAh g^-1) based on fully lithiated LiC₆. This is the first to observe reversible operation of a graphite electrode in reduction-vulnerable AN solvent and clearly indicates enhanced reductive stability of superconcentrated electrolytes. X-ray photoelectron spectroscopy reveals the presence of a TFSA-based surface film on the cycled graphite electrode.

To elucidate the origin of the unusual reductive stability with a TFSA-based surface film, DFT-MD was applied to dilute and superconcentrated LiTFSA/AN solutions. The superconcentrated solution has a unique networking structure of Li⁺ cations and TFSA^- anions, which modifies frontier orbital characters in the solution. The projected density of states (PDOS) (Fig. 2) shows that the lowest unoccupied molecular orbitals (LUMOs), which dominate the behavior of its reduction reaction, shift from AN solvents to TFSA^- anions. Hence, the TFSA^- anions, instead of AN solvents, are predominantly reduced to form a TFSA-based surface film on a graphite electrode during first cycle, which kinetically suppresses further electrolyte decompositions. The modified surface film character, arising from the peculiar frontier orbitals, is the origin of the enhanced reductive stability of superconcentrated solutions that allow for reversible operation of a graphite negative electrode without EC. The salt-superconcentrating strategy, expanding the graphite electrode reaction for wide variety of organic solvents other than EC, will contribute to the development of advanced lithium-ion batteries with high-voltage and fast-charging characters based on new EC-free functional electrolytes.

References

1. Y. Yamada et al., ACS Appl. Mater. Interfaces, DOI: 10.1021/am5001163 (2014).

2. Y. Yamada et al., J. Am. Chem. Soc., DOI: 10.1021/ja412807w (2014).