Next-generation batteries using Li negative electrodes have been attracting much attention recently. They still have many tough problems to be challenged, such as instability of surface chemistry and anomalous deposition of Li. Therefore, Si is regarded as a promising alternative for Li because of its relatively stable surface chemistry. Simple substitution of Si for Li induces two issues: one is large irreversible capacity of Si electrodes, and the other is lack of Li source in itself because some of cathode materials of next-generation batteries cannot supply. However, Li pre-doping enables us to straighten out these issues. We have already reported that the Li pre-doped capacities by Li-naphthalenide (Li-NTL) solution depended on the solvent of the Li-NTL solution [1]. In this study, we analyzed the effect of Li concentration in Li-NTL solution on Li doped capacities and equilibrium potentials by evaluating the solvation structures around Li⁺ ions and pairing structure of the solvated Li⁺ ions with [NTL]^・⁻ in Li-NTL solutions by IR and UV-vis spectra. The results allowed us to discuss on the mechanism of the lithiation of Si in Li-NTL pre-doping solution.

Si electrodes were formed on Cu foil substrate with a NMP slurry containing Si powder, Ketjen Black, and polyimide binder in a mass ratio of 80: 5: 15. Li-NTL pre-doping solution was prepared with 2-methyltetrahydrofuran (MeTHF) as solvent, naphthalene fixed at 2.5 mmol, and Li of 2.5, 5.0 mmol or “saturated Li” which meant to supply Li enough to leave a certain amount of it intact. Another Li-feeding method is continuous supply of Li by immersing Li foil in the solution during pre-doping, named “Li-supply.” The pre-doping time of the Si electrode was fixed at 24 h, and the pre-doped capacity was measured by a constant current discharge of 0.05 mA cm⁻² between OCV and 1.5 V at 30℃. We employed spectroscopic method to analyze the solvation structure of the pre-doping solutions by FT-IR and UV-vis spectra.

Figure 1 shows discharge voltage profiles for the Si electrodes pre-doped in Li-NTL solutions of different Li concentrations, which demonstrates clear dependency of pre-doped capacities on the Li concentrations. Li-supply delivered the largest pre-doped capacity among the four solutions. Figure 2 shows the UV-vis spectra of Li-NTL solutions with different Li concentrations. We focus two absorption bands at 320 nm and 550 nm in the spectra. The former is assigned to ion pair structure formed by radical anion of naphthalene and solvated Li⁺ ion [2], whose intensity decreased as Li concentration increased. The latter is assigned to the naphthalene dianion [3], whose intensity increased as the Li concentration increased. Thus, the results presented above evidently elucidate that the dianion plays a crucial role in Li pre-doping reactions.

On our presentation, we will discuss the Li pre-doping reaction mechanism by adding more data on the characterization of the pre-doping solutions.

[1] M. Saito et al., Abstracts of PRiME 2020, A02-0410 (2020).

[2] M. Szwarc et al., J. Am. Chem. Soc., 98, 5707 (1976).

[3] D. Vofsi et al., J. Polym. Sci. Polym. Chem. Ed., 20, 901 (1982).