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

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](center dot-)) 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](center dot-) radical ions.