Correlations between Li-Ion Concentration, Solvation Structure, and Equilibrium Potential of Li–Naphthalenide Solution for Li Alloying of Si Negative Electrode of Next-Generation Batteries

Lithium (Li) is a beneficial metal for use in the negative electrode (NE) of next-generation batteries, such as Li–S and Li–O2 batteries, because it can greatly increase the energy density compared to those of conventional Li-ion batteries. However, Li dendrite growth is a serious problem for the practical use of the Li NE because of cell short circuits. Using a Si NE can solve this problem, but the Si NE does not include a source of Li as the carrier ion. Therefore, Li predoping of the Si NE will become important for next-generation batteries. In this study, we prepared Li–naphthalenide (Li–NTL) solutions using Li foil, naphthalene, and 2-methyltetrahydrofuran as the solvent and investigated the effect of the Li concentration on the Li alloying behavior. In particular, the changes in the solvation structure and equilibrium potential of the Li–NTL solution and the resulting Li alloying depth of the Si NE were evaluated to clarify the Li alloying mechanism of the Li–NTL solution. Higher Li concentrations in the Li–NTL solution generated a larger amount of the [NTL]2– dianion than the [NTL]•– monoanion radical, and the solution exhibited a lower equilibrium potential. This led to deeper Li alloying of the Si NE, corresponding to a high predoping capacity of ∼3000 mAh g–1 for 24 h treatment. Furthermore, the initial irreversible capacity of a Li half-cell constructed using the Li-predoped Si NE was reduced compared with that of a Li half-cell constructed using the pristine Si NE, and the Li half-cell maintained a high capacity. It was found that the Li concentration has a significant role in controlling the amount of dianions in the Li–NTL solution and determines the depth of Li alloying in the Si NE.