Direct relations between ion diffusion constants and ionic conductivity for lithium electrolyte solutions

Abstract

Lithium cation and anion diffusion constants (DLi and DAnion) are plotted versus ionic conductivity (σ) (σ–D relation) for lithium organic electrolyte solutions which are important to investigate Li secondary batteries. Lithium salts studied are LiPF6, LiBF4, LiN(SO2CF3)2 (LiTFSA), LiN(SO2C2F5)2 (LiBETI), LiC4BO8 (LiBOB), LiSO3CF3 and Li2B12F12. Main organic solvents are propylene carbonate (PC), ethylene carbonate (EC), γ-butyrolactone (GBL), diethyl carbonate (DEC), four glymes and polyethylene glycol (PEG) dimethyl ethers of average molecular weights of 400 and 1,000. Mixing effects of two solvents are shown for EC-DEC-LiPF6 systems. In total, about 20 organic electrolyte solutions were studied. In addition, for six binary ionic liquid systems, σ–D relations are plotted. Values for the degree of apparent ion dissociation (α) were calculated from the DLi, DAnion and σ using the Nernst-Einstein (NE) relation and are plotted against σ (σ–α relation). The number of charge-carrying ions (Ncarrier) was estimated from the α and salt concentration, and plotted versus σ (σ-Ncarrier relation). To estimate the velocity of ions, the σ–Dsolvent relations are plotted. Also, the apparent lithium transference number (tLi) obtained simply from the DLi and Danion values are plotted versus σ (σ −tLi relation). While we have already reported all the data in our previous papers, we did not plot directly the ion diffusion constants and other parameters versus ionic conductivity. In this paper, the importance of the direct relations of σ–D, σ–α, and σ-Ncarrier is demonstrated to understand functions of lithium electrolyte solutions.The numerical data are freely available in Supplementary Information (SI), covering broad aspects of the fundamental and practical lithium electrolyte solution systems. We believe that they will contribute to the research and development for next generation Li battery systems.