리튬 함량 및 단위 셀 압력이 열전지용 리튬 음극의 방전 성능에 미치는 영향

Effect of Lithium Contents and Applied Pressure on Discharge Characteristics of Single Cell with Lithium Anode for Thermal Batteries

Abstract

Lithium anodes (13, 15, 17, and 20 wt% Li) were fabricated by mixing molten lithium and iron powder, which was used as a binder to hold the molten lithium, at about $500^{\circ}C$ (discharge temp.). In this study, the effect of applied pressure and lithium content on the discharge properties of a thermal battery's single cell was investigated. A single cell using a Li anode with a lithium content of less than 15 wt% presented reliable performance without any abrupt voltage drop resulting from molten lithium leakage under an applied pressure of less than $6kgf/cm^2$. Furthermore, it was confirmed that even when the solid electrolyte is thinner, the Li anode of the single cell normally discharges well without a deterioration in performance. The Li anode of the single cell presented a significantly improved open-circuit voltage of 2.06 V, compared to that of a Li-Si anode (1.93 V). The cut-off voltage and specific capacity were 1.83 V and $1,380As\;g^{-1}$ (Li anode), and 1.72 V and $1,364As\;g^{-1}$ (Li-Si anode). Additionally, the Li anode exhibited a stable and flat discharge curve until 1.83 V because of the absence of phase change phenomena of Li metal and a subsequent rapid voltage drop below 1.83 V due to the complete depletion of Li at the end state of discharge. On the other hand, the voltage of the Li-Si anode cell decreased in steps, $1.93V{\rightarrow}1.72V(Li_{13}Si_4{\rightarrow}Li_7Si_3){\rightarrow}1.65V(Li_7Si_3{\rightarrow}Li_{12}Si_7)$, according to the Li-Si phase changes during the discharge reaction. The energy density of the Li anode cell was $807.1Wh\;l^{-1}$, which was about 50% higher than that of the Li-Si cell ($522.2Wh\;l^{-1}$).

Fig. 1. The photography of Φ90 ㎜ Li anode.

Fig. 2. Single cell test assembly.

Fig. 3. SEM image (a) Fe powder (×1,000), (b) Fe particles (×10,000), and (c) Fe single particle (×10,000).

Fig. 4. SEM image of Li anode (a,b) Li 17 wt%, (c,d) Li 15 wt%, and (e,f) Li 13 wt% (bright area: Fe, dark area: lithium).

Fig. 5. Discharge performance of Li anode (Li 13 wt%) single cell by applied pressure.

Fig. 6. Discharge performance of Li anode (Li 15 wt%) single cell by applied pressure.

Fig. 7. Discharge performance of Li anode (Li 17 wt%) single cell by applied pressure.

Fig. 8. Li anode (Li 17 wt%) single cell after the discharge test by applied pressure (red circles: short spot).

Fig. 9. Discharge performance of Li anode (Li 20 wt%) single cell by applied pressure.

Fig. 10. Discharge performance of Li anode (Li 13 wt%) single cell with different thickness electrolytes.

Fig. 11. Discharge performance of single cell with Li anode (Li 13 wt%) and Li-Si anode at pulse current.

Fig. 12. SEM image and EDS analysis of 13 wt% Li anode after the discharge test.

Fig. 13. Total polarization of single cell with Li anode (Li 13 wt%) and Li-Si anode.

Table 1. The properties of as-fabricated Li anode.

Table 2. Electrochemical performance of Li anode (Li 13 wt%) single cell with different electrolytes.

Table 3. Discharge results of Li anode and Li-Si anode single cell at the first phase (cut-off).