Mechanism Analysis of Solution Li Pre-Doping Technique Toward Si-Based Anodes for Next-Generation Batteries
Authors: Saito Morihiro , Himata Yusuke , Enomoto Hikaru , Ozawa Fumisato , Nomura Akihiro 
Published: December 2024

In recent years, Li metal anode has attracted much attention to increase the energy density for next-generation batteries. However, there are still some problems to be solved such as extra electrolyte decomposition and Li dendrite deposition. Therefore, Si is expected as a promising alternative to Li especially for suppression of the Li dendrite. However, Li pre-doping toward Si anode is necessary because some of cathode materials for next-generation batteries such as Li-S and Li-O2 do not contain Li. Therefore, the Li pre-doping technique is a quite important for Si-based anodes. We have also researched a Li pre-doping technique using Li-naphthalenide (Li-NTL) solutions and reported that the pre-doped capacities depended on the kinds of solvents [1] and Li+ concentration of the Li-NTL solution. In this study, we investigated the mechanism from the viewpoint from kinds of the generated NTL anions by the solvents and discussed on the relationship among them, equilibrium potential of the Li-NTL solutions and the solvation structure including NTL anions and Li+.
Si electrode was prepared by coating the slurry of Si powder (f 40-50 nm), Ketjen Black conductive agent, and polyimide binder in a mass ratio of 80: 5: 15. Li-NTL pre-doping solutions were prepared with 5.0 mL of 2-methyltetrahydrofuran (MeTHF), monoglyme (G1), diglyme (G2) or triglyme (G3) solvents, 2.5 mmol naphthalene, and “saturated” Li. Moreover, extra Li foil immersing in the Li-NTL solution was also used during the Li pre-doping toward the Si anode. The equilibrium potentials of the Li-NTL solutions were measured up to 12 h. The amount of generated radical monoanion NTL·
- and dianion NTL2
-was evaluated by UV-Vis spectra for the Li-NTL solutions and the solvation structures was also analyzed by a DFT calculation.

Fig. 1 shows a change in the equilibrium potentials for the Li-NTL solutions with different solvents. The order of magnitude by the solvents was MeTHF < G1< G3 < G2, which is a good agreement with the pre-doped capacities toward the Si anode expect for the influence of some reduction decomposition of G1 and G2 on the surface of Li alloyed Si anode. Fig. 2 shows the UV-Vis spectra of Li-NTL solutions using different solvents. For the MeTHF solvent, a spectrum derived from NTL2- dianion was clearly appeared at 540 nm, which leads to lowering the equilibrium potential of the Li-NTL solution. From this point, G1 also exhibited the NTL2- dianion signal together with radical monoanion NTL·
- at 320 nm. The other glyme solvents exhibited only smaller monoanion signal than that for the G1. DFT calculation also supported the ease to form the dianion for the MeTHF solvent from the solvation structure including NTL2- dianion and Li+. The detail mechanism and the effect of Li+ concentration in the Li-NTL solutions on the relationship between the solvation structure and equilibrium potential will be also discussed in the meeting.
References
[1] M. Fukunishi, M. Saito et al., J. Electrochem. Soc., 169, 060534 (2022).
Acknowledgement
This study was supported by NIMS Joint Research Hub Program, Japan.

Figure 1