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

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

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).