A structural study on a specific Li-ion ordered complex in dimethyl carbonate-based dual-cation electrolytes

• Published in: Physical chemistry chemical physics : PCCP 2024-01, Vol.26 (5), p.392-3926
• Main Authors: Chikaoka, Yu, Tashiro, Tomoya, Sawayama, Saki, Kobayashi, Ayana, Matsumoto, Ayuna, Iwama, Etsuro, Naoi, Katsuhiko, Fujii, Kenta
• Format: Article
• Language: English
• Subjects: Cations; Distribution functions; Electrolytes; Function analysis; Ion currents; Ion pairs; Liquid phases; Lithium-ion batteries; Molecular dynamics; Phase separation; Quaternary ammonium salts; Radial distribution; Rechargeable batteries; Solvation; Solvents
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d3cp05526d

Dimethyl carbonate (DMC) is a linear carbonate solvent commonly used as an electrolyte for electric double-layer capacitors (EDLCs) and Li-ion batteries. However, there are serious problems with the use of DMC as an electrolyte solvent: (1) low ionic conductivity when using Li salts ( e.g. LiBF 4 ) and (2) liquid-liquid phase separation when using spiro-type quaternary ammonium salts ( e.g. SBPBF 4 ). Dual-cation electrolytes, i.e. , bi-salt (SBPBF 4 and LiBF 4 ) in DMC, are promising candidates to avoid the phase separation issue and to enhance the total and Li + conductivities. Herein, we reported a specific Li-ion structure in DMC-based dual-cation electrolytes by combining high-energy X-ray total scattering (HEXTS) and all-atom molecular dynamics (MD) simulations. Quantitative radial distribution function analysis based on experimental and simulation results revealed that the phase-separated SBPBF 4 /DMC ( i.e. , the bottom phase of 1 M SBPBF 4 /DMC) forms long-range ion ordering based on the structured SBP + -BF 4 − ion pairs. When adding LiBF 4 salt into SBPBF 4 /DMC ( i.e. , dual-cation electrolyte), the ordered SBP + -BF 4 − structure disappeared owing to the formation of Li-ion solvation complexes. We found that in the dual-cation electrolyte Li ions form multiple Li + -Li + ordered complexes in spite of relatively low Li-salt concentration (1 M), being a promising Li + -conducting medium with reduced Li salt usage and low viscosity. Li-ion ordered complexes formed in DMC-based dual-cation electrolytes.