Effect of Li Ratio in Li-Naphthalenide Solution on Li Pre-Doping to Si Negative Electrode for Next-Generation Batteries

Abstract

In order to realize a carbon-free society, there is a strong need to increase the energy density of secondary batteries such as Li-ion batteries (LIB). In recent years, study on next-generation batteries such as Li-air O2 (LAB) and Li-S has been actively conducted. Si is one of the most attractive materials for the negative electrode of these secondary batteries, with the high theoretical capacity of 3580 mAh g-1 based on Li3.75Si. When Si is used as the negative electrode of LAB and Li-S batteries, however, it is a critical issue to add Li resource to the electrode since the positive electrode also does not contain Li resource. One of the solutions is to conduct Li pre-doping to the Si electrodes. Various methods have been reported, including direct contact of Li metal foil with Si electrode, electrochemical Li pre-doping, and mixing Li metal and active materials method [1].We reported that Si electrodes pre-doped with Li by the solution method, in which a Si electrode is attached to a Li metal through a separator and immersed in Li-naphthalenide (Li-NTL) solutions with different ether solvents [2]. In particular, 2-methyltetrahydrofuran (MeTHF), a solvent of Li-NTL solution, showed a higher Li pre-doping capacity and Li3.75Si crystal phase was identified on a 24 h pre-doping electrode. In this way, deep Li doped level and relatively uniform Li doping was achieved with this method. DFT calculation showed that MeTHF has a higher unoccupied molecular orbital (LUMO) energy than the other ester, meaning that the Li-NTL solution using MeTHF solvent is difficult to be subjected a reductive decomposition. We exhibited that the Li-NTL solution having a low equilibrium potentials provided a deeper pre-doping capacity of Li. On the other hand, Yus et al. revealed that Li-NTL/THF solution contained the two types of NTL anions, i.e. radical anion (Li+NTL· -) and dianion (Li+ 2NTL2 -), and the ratio is changed by the Li concentration [3]. It is considered that the same phenomena should be occurred in the case of MeTHF solvent and probably affects it on Li-doped capacity.In this study, we investigated the effect of Li concentration of Li-NTL/MeTHF solution in order to improve the Li pre-doping capacity and to elucidate the Li pre-doping mechanism using the Li-NTL solution. Si electrodes with a weight ratio of Si : Ketjen black : polyimide binder = 80:5:15 were prepared and pre-doped with Li for 24 h using 0.5 M Li-NTL solution (Li : naphthalene = 0.5 : 1.0 or 1.0 : 1.0 by molar ratio). The pre-doping capacity was confirmed by the discharge capacity of the electrochemical delithiation reaction from open circuit voltage (OCV) to 1.5 V. Figure 1 shows the equilibrium potentials of Li-NTL solutions obtained in a two-electrode cells with a Li metal and a Ni mesh electrode immersed in 1.0 M LiPF6/EC+DMC (1:1 by vol.) electrolyte solution and Li-NTL solution, respectively, and two solutions were separated by a solid-state electrolyte Li7La3Zr2O12 (LLC). The equilibrium potential linearly decreased with increasing temperature, and drastically increased with the molar ratio of Li : naphthalene. Therefore, a higher Li concentration will provide a higher Li pre-doping capacity. The change in the Li pre-doping capacity, cycleablity, and pre-doping mechanism by using the Li-NTL dianion solution will be discussed in the meeting.This work was partially supported by NEDO RISING2 project (JPNP16001).