The effect of gold plasma coating on the impregnation of molten lithium in the FeCrAl foam is presented in Figure 3. As shown in Figure 3(a), lithium was insufficiently impregnated into the as-received FeCrAl foam, which is ascribed to its lithiophobic properties. In addition, even when the as-received FeCrAl foam without a gold coating was dipped into molten lithium for 40 min at 350°C, no lithium impregnation was observed owing to the poor wettability of FeCrAl foam to molten lithium. In contrast, the gold-coated FeCrAl foam (Figure 3(b)) shows good lithium impregnation, with a shiny lithium surface on the foam becoming visible within 1 min. The gold coating enhances the impregnation of molten lithium into the porous medium by modifying the surface of the FeCrAl metal foam. As shown by the cross-sectional SEM image of the gold-coated FeCrAl after lithium impregnation in Figure S3, the impregnated lithium is evenly distributed inside the foam. Hence, the application of a gold coating on the FeCrAl foam is a useful method for improving the interfacial affinity between molten lithium and the surface of the lithiophobic FeCrAl foam. The lithium content in LIMFA FeCrAl, determined by measuring the mass of the sample before and after lithium impregnation, was 18 wt.%, which is 2 wt.% lower than that in the LAN (20 wt.%). A major advantage is that no cup, as reported in previous research [16], is required with the LIMFA FeCrAl to prevent molten lithium leakage during discharge. Thus, the real specific capacity of LIMFA FeCrAl is expected to be higher than that of LAN.
The discharge performance of Li(Si) and LIMFA FeCrAl cells are depicted in Figure 4(a). The open-circuit voltage of the LIMFA FeCrAl unit cell (2.06 V) was substantially higher than that of the Li(Si) unit cell (1.95 V), which is in accordance with a previous report [9]. These plots present the change in voltage according to the applied pulse current profile (4 A, 1 s → 2 A, 4 s → 0 A, 1 s). As a reference electrode for the anode, a FeS2 cathode with an almost three-fold excess electrochemical equivalent mass was intended to be used against Li(Si), as described in a previous report [10]. The fabrication of such an electrode, however, resulted in cracking during pressing. Thus, the voltage of the LIMFA FeCrAl unit cell does not present the typical cell voltage variation curve resulting from the electromotive force (emf) changes of the cathode and anode. Instead, the cell pulse discharge results exhibit three voltage plateaus that originate from the phase changes of FeS2 [3, 4]:
First Plateau (Plateau 1): FeS2 + 3/2 Li+ + 3/2 e- → 1/2 Li3Fe2S4 (“Z-phase”)
Second Plateau (Plateau 2): Li3Fe2S4 → Li2+xFe1-xS2 (x ~ 0.2) + Fe1-yS
Third Plateau (Plateau 3): Li2FeS2 (“X-phase”) + 2 e- → Fe + Li2S + S2-
The voltage change of the LIMFA FeCrAl cell is solely due to lithium depletion in the FeCrAl foam. Therefore, the voltage decreases dramatically at the end of the discharge. After the full discharge of LIMFA FeCrAl, the cathode (FeS2) was disassembled and an SEM analysis was conducted. Interestingly, as shown in Figure S4, the cathode (FeS2) of the LIMFA FeCrAl cell was completely changed to Fe and Li2S (sulfur (S) rich area), corresponding to the end of FeS2 discharge in the third plateau (Figure 4). In addition, SEM observations of the LIMFA FeCrAl after the discharge test showed that lithium was almost completely extracted and therefore is involved in the discharge reaction, as observed previously for LIMFA Ni foam [16]. After the discharge test, the appearance of the foam was similar to that before lithium impregnation. This LIMFA FeCrAl discharge behavior needs further research. However, the third step in the voltage change of the Li(Si) cell is in accordance with the phase change of Li(Si) and FeS2 during discharge reported by Guidotti et al. [3, 5]. As shown in Figure 4, the LIMFA FeCrAl has a specific capacity of 2,627 As·g-1, whereas the Li(Si) anode shows a relatively lower specific capacity of 982 As·g-1. The specific capacity of Li(Si) observed in this study is similar to that reported previously (1,050 As·g-1) [16]. In practice, thermal batteries commonly only use the first plateau for safety reasons, as well as strict voltage range regulations for such devices [1, 2]. Consequently, the specific capacity of LIMFA FeCrAl cell is 2.67 times higher than that of the Li(Si) cell.
The specific capacity of the LIMFA FeCrAl was compared with those of various state-of-the-art lithium anodes, as shown in Figure 5. When considering that a cup was applied, the practical specific capacity of LAN is 1,946 As·g-1 and that of the LIMFA Ni foam is 2,106 As·g-1. However, the performance of LIMFA FeCrAl is superior, with a specific capacity of 2,627 As·g-1, which, to the best of our knowledge, is the highest value that has been achieved without applying a cup. This improvement was attributed to the excellent impregnation of lithium into the high-porosity FeCrAl foam, which greatly increased the lithium content, as well as the high mechanical stability and robustness of the FeCrAl foam.
The total polarization was calculated according to the method of Fujiwara et al. [17, 18].
The FeCrAl foam has good mechanical robustness as well as a highly electroconductive 3D structure that can enhance ion conductivity and reduce contact resistance inside the anode. Therefore, as shown in Figure 6, the total polarization of the LIMFA FeCrAl is reduced owing to the highly electroconductive FeCrAl foam substrate as well as the gold coating on the surface of the FeCrAl foam. The total polarization of the LIMFA FeCrAl is lower than that of Li(Si). Specifically, the total polarization of the LIMFA FeCrAl is significantly lower than Li(Si) below 1,500 As·g-1. As shown in Figure 4, Li(Si) vs. FeS2 typically shows three discharge steps, but the first plateau can become dominant, as shown in Figure S5. Bernardi and Newman reported that when the lithium ratio (β) is increased to 2.51 (β = molar ratio of Li(Si)/FeS2) from 1.08, the % utilization of FeS2 in the first plateau is extended from 22.1–37.5% (Figure S5) [19]. In addition, Masset et al. reported that the conductivity of FeS2 decreases drastically from 80–100 S·cm-1 to ~6.3 S·cm-1 according to the phase change from Li3Fe2S4 (Z-phase) to Li2FeS2 (X-phase) [5]. The decrease in the total polarization of the LIMFA FeCrAl is attributed to the unique phase change of the extended first plateau of the LIMFA FeCrAl due to its high lithium ratio, which exceeds 2.0 (Li/FeS2 = 0.63 g/0.076 g). Therefore, the LIMFA FeCrAl shows higher conductivity owing to the extended first plateau of FeS2 and the delayed X-phase transformation of FeS2 in addition to the high electric conductivity of the 3D FeCrAl foam skeleton.
Without a cup on the LIMFA FeCrAl, no lithium leakage was observed and the foam frame was maintained following discharge. Therefore, the LIMFA FeCrAl developed in this study represents a significant advance for thermal battery technology.