Charge/Discharge Properties of Li Pre-Doped Si Anodes in Glyme-Based Electrolytes for Li-Air Batteries
Authors: Matsudaira Masamichi , Kosaka Shinpei , Saito Morihiro 
Published: February 2020

In order to realize next-generation energy devices such as Li-air batteries, suppression of Li dendrite deposition at Li metal anode is one of the serious problems before the commercialization. Use of Li-Si alloy anode is a solution to prevent from the problem because the charge/discharge proceeds by Li-Si alloying/de-alloying reaction and the potential is relatively higher (ca. 0.02-0.6 V) than that of the Li dissolution/deposition one. However, to use the Si anode for the Li-air batteries the Li pre-doping is necessary because of lack of Li+ ion inside of Si. Also, the surface of Si anode is very active to decompose the electrolytes, which leads to a large initial irreversible capacity during 1st charging. In the previous study, we tried to address a Li pre-doping in 1 M LiBF4/PC with additives, i.e. VC and FEC, and vacuum pressure impregnation (VPI) treatment to a Si nanopowder (Φ = 50 nm) to charge Li+ ions and stabilize the surface of Si, and successfully demonstrated to be able to fabricate and operate a Li-ion capacitor using the Li pre-doped Si anodes, namely Si-CAP [1,2]. The energy density was > 110 mWh g-1 comparable to those of Li-ion batteries. In this study, as the next step, we tried to apply the Li pre-doping techniques in 0.2 M LiTFSI/diglyme(G2) and evaluate the half cells of Li pre-doped Si electrodes with a Li metal counter electrode to investigate the charge/discharge properties. Here, two types of binders, i.e. polyimide (PI) and carboxymethyl cellulose sodium salt (CMC)), were selected, and the effect of binders especially on the cycleability was examined in detail for the anode of Li-air batteries.
Fig. 1 shows the cycleability of the discharge capacities for the Li-pre-doped Si anodes with different binders and the contents at 30oC. For both binders the best cycleability was obtained at the content of 15 wt%. This indicates that the less content of binders caused to lose the contact between the Si-Si and Si-Cu current collector and the much higher content led to cover the surface of Si by the binder, resulting in preventing from smooth Li alloying/de-alloying reaction. Fig. 2 shows the cycleability of discharge capacities for the Li pre-doped Si anode at the capacity-controlled operation (ca. 1200 mAh g-1). The cycleability was much more stabilized by the capacity control, and consequently the 15 wt% PI binder exhibited a good cycleability over 70 cycles, while for the 15wt% CMC one it was below 50 cycles. Therefore, it was found that the Li-pre-doped Si was also successfully able to operate in glyme-based electrolyte by using some effective techniques such as additives, VIP treatment and capacity control. The experimental methods and results in more detail will be presented in the meeting.

This study was supported by JST “A Tenure-track Program” from MEXT, Japan.

References
[1] M. Saito et al., Abstract in 2014 ECS and SMEQ Joint International Meeting, (2014).
[2] M. Saito et al., Abstract in 4th International Symposium on Enhanced Electrochemical Capacitors (ISEE Cap 15) (2015).

Figure 1