Li Pre-Doping into Si Negative Electrodes Using Li-Naphthalenide Solutions with Various Ether Solvents For Next-Generation Batteries

Abstract

Four ethers were compared as solvents of lithium naphthalenide (Li-NTL) solutions to pre-dope Li into Si electrodes. The solvents of the Li-NTL solutions affected the stability and equilibrium potential (V eq). X-ray diffraction, thermodynamic characterization and ultraviolet-visible (UV–vis) spectroscopy were used to clarify the effects of the solvation structure, the lowest unoccupied molecular orbital (LUMO) energy of the solvent molecule and the ion pair structure between Li+ ions and naphthalenide radical anions ([NTL]·−) on doping capacity. A Li-NTL solution having a low V eq and sufficient stability under potentials as low as that of Li metal was found to provide the highest pre-doping capacity. In particular, a 2-methyltetrahydrofuran (MeTHF) solution exhibiting the lowest V eq showed a pre-doping capacity as high as 3250 mAh g−1 after 24 h. UV–vis spectra and Walden plots indicated that a Li-NTL solution using MeTHF provided less dissociation than a tetrahydrofuran (THF) solution. The doping capacity is evidently determined by the V eq of the Li-NTL solution as a consequence of the dissociation equilibrium of the ion pair of the solvated Li+ ion and [NTL]·− radical ions.