Electrochemical performance of (a) Na|NaOH-NaI|Bi-Pb and (b) Na|NaOH-NaI|Bi LMB cell. Electrochemical performance of Li|LiCl-LiF|Bi LMB cell. a) shows change of charge and discharge capacity and coulombic efficiency during cycling and (b) is the OCV after 30 minutes after charge and discharge. Change of charge and discharge capacity and coulombic efficiency of Li|LiCl-LiI|Bi-Pb LMB cell during cycling.
Introduction
Electrical energy storage(EES) systems
Schematic representation of the fluctuation of the power generated by power plants and the demand for electrical energy, in relation to the time and role of electrical energy storage devices as load leveling.
Electrochemical energy storage devices (EESDs)
Such conventional batteries are probably not suitable for fast charging and discharging at megawatt level, such as frequency converter21, due to their low current rate in practice. Solid components thereof, such as LiCoO2 (ionic conductivity: 5.5 x 10-4 s cm-1) and graphite for LIBs, beta-alumina (Na, β”-Al2O3, ionic conductivity: 1.0 x 10-1 s cm-1 -1) NaS batteries etc. have lower ionic conductivity than liquid components because mobile ions do not move easily in solid components.22, 23 While conventional batteries have these weaknesses, liquid metal batteries (LMBs), which are composed of all liquid states, such as liquid metal as anode and cathode and molten salt as electrolyte, overcome low flow rate due to fast ionic conductivity of molten salt (e.g. ionic conductivity of LiF-LiCl-NaF: 3.44 s cm-1). 25 For example, in a previously reported paper, the current density of VRFB is 40 mA/cm2 during charging and discharging.15, 16 On the contrary, that of LMBs is generally 200 mA/cm2 during processing. Therefore, it is probably more suitable for fast charging and discharging applications.
Liquid metal batteries (LMBs)
- Origin
- Research background
During discharge, the negative electrode, liquid metal A is electrochemically oxidized (A → 𝐴𝑧++ z𝑒−) and then it migrates to the positive electrode, liquid metal B, through the electrolyte. Finally, liquid metal A-B alloy will be formed (A in B), while releasing electrons in an external circuit. The reverse reactions occur during charging(𝐴𝑧++ 𝑧𝑒− → A).5 In accordance with the Nernst equation, the difference in the chemical potentials of pure liquid metal A and liquid metal A in liquid metal B can cause a cell voltage in the next equation, .
Research proposal
- Objective of study
- Design and fabrication process for standard cell
- Electrochemical measurement and characterization
Basic cell components are composed of positive current collector, negative current collector and insulator, except for anode, cathode and electrolyte as shown in Figure 12. Material for positive current collector must have electrical conductivity and also serve as a container for anode, cathode and electrolyte. In addition, considering the operating temperature of LMBs, a positive current collector without degradation, which originates from high temperature and side reaction with internal material, should be designed.
Previous research has investigated the mutual interaction between a positive current collector, using steel and stainless steel, and a Sb-Pb alloy.33 A cell based on stainless steel forms stable intermetallic compounds at a constant temperature (450 oC ) under an Ar atmosphere for 500 hours. Therefore, SUS304 is selected as the starting material for positive current collector and also for negative current collector. Isolator is used to prevent cell short circuit caused by direct contact between positive current collector and negative current collector.
For easy connection to the electrochemical measurement device, the negative current collector was designed in the form of a rod and for smooth contact with the anode material. Adhesive and sealant are required because using Al2O3 tubing alone is not enough to protect the inflow of the outside atmosphere and hold the negative current collector. The cell cover, which acts as a positive current collector, was isolated from the negative current collector due to the Al2O3 tube and then fixed with a ceramic sealant according to the following procedure.
After curing, the cell lid was moved into the glovebox filled with argon (Ar) gas, and then Na metal (ACS reagent, Sigma-Aldrich) was attached to the top of the negative current collector.
According to Gibbs' phase rule, the discharge voltage plateau comes from Na-Sn liquid and Na7Sn12 solid phase, but contrary to expectations, solid phase is generated before introducing 20 mol% liquid Na into liquid Sn. The partial molar concentration of liquid Na between Na, β”-Al2O3 and liquid Sn exceeded 20 mol%, therefore, as shown in Figure 22, a solid phase (brown line) is formed. When 20 mA was applied to the Na||Sn cell, it was broken, as shown in Figure 23 of the post-mortem cell.
This is because it could not withstand the high current applied to the small contact area between the liquid Na and it.45 In addition, the Na contained in it was almost depleted after death, probably due to the evaporation of the liquid Na in operating temperature. Galanostatic charge-discharge cycling at 1 mA, 300 oC. a) Time-matched voltage profiles and (b) time step. Solid phase Na7Sn12 (brown line) between liquid Na, β”-Al2O3 and Sn through the expanded cross section of the Na||Sn cell.
Summary
Figure 24(a) shows the expected cross-section of the Mg|MgCl2-NaCl-KCl|Sb composite cell, and the experimental images of inserting the graphite crucible and Al2O3 tube into the cell cup are shown in Figure 24(b). In order to inspect the inside of the cell, after lowering the temperature, we opened the cell lid and the picture of the inside is shown in Figure 26. As shown in Figure 26(b), the Mg was not placed exactly at the top position, but it looks like a small sink in the salt.
As shown in Figure 29, via Ellingham's diagram and Gibbs free energy change, aluminum (Al) and magnesium oxide (MgO) are generated. Fabrication and Electrochemical Performance of Na|NaOH-NaI|Bi-Pb Cell Na metal was attached to the negative current collector in Figure 17. It was prepared while the Na metal was attached to the negative current collector in Figure 17.
It was prepared using the same process, but the tip of the negative current collector was combined with the negative current collector using a thread and a nut, as shown in Figure 30. The NaOH-NaI salt was made using a cell cap without a hole, a cell cup, and a graphite crucible as shown in Figure 31(a). After that, a current of 3 mA was applied during charge and discharge at 280 oC, as shown in Fig. 34(b).
When a current of 100 mA is applied, the charge and discharge profile results are shown in Figure 37(b).
Summary
The Ni foam was fixed in a negative current collector as shown in Fig. 40(a), and then it was placed in full liquid Li that was melted in a SUS304 crucible at 480 oC as shown in Fig. 40(b ). After increasing the temperature, as shown in Fig. 41(a) , the OCV of the Li|LiCl-LiF|Bi cell almost stabilized at about 1.03 V, although it gradually decreased. In Fig. 43(a) , the OCV of the second Li||Bi cell was measured according to the elevated temperature.
As shown in Figure 46(b), it was suitably cut and then combined with the negative current collector strip using a nut. Through DSC and TGA measurement as shown in Figure 48, the melting point of LiCl-LiI was confirmed. During heating of the cell up to 410 oC, the OCV of a Li||Bi-Pb cell was measured as shown in Figure 49(a) and decreased continuously but was almost saturated at 0.72 V.
As shown in Figure 50, from the 1st cycle to approx. 40th cycle, the capacity of charging and discharging decreased to approx. 0.8 Ah, but the coulombic efficiency was almost 99. When operating the cell, the capacity and the coulombic efficiency had almost the same value as shown in Figure 50. Figure 51(a). As shown in Figure 53(a), the capacity of the Li||Bi cell decreased when the Ar gas flow was stopped.
When the measurement of the OCV ended, there was also a decrease after several cycles as shown in Figure 53(b).
Summary
Conclusions
To solve this problem, the Na||Bi-Pb cell was introduced due to the certain density difference and lack of reaction between Na and the electrolyte. The galvanostatic cycle test of this was operated at 300 oC while current of 1 mA was applied to it during charging and discharging for 10 hours, respectively. The result showed a relatively stable voltage profile, but when applying a current of 3 mA, the overpotential increased continuously because liquid Na was easily oxidized.
Liquid Li, which has a lower degree of oxidation than liquid Na, was used as the anode material for the Li||Bi cell. By applying a current of 50 mA and 100 mA to it at 540 oC under a shutdown condition of 10 hours, its galvanostatic cycle test was measured. Long-term cycling performance (>65 cycles) was achieved at 100 mA, but capacity fade likely occurred due to electrolyte contamination, which increases the solubility value.
Relatively higher speed capabilities and long-term cycling performance were achieved via the second designed cell. Using a third designed cell, galvanostatic cycling tests of the Li|LiCl-LiF|Bi and Li|LiCl-LiI|Bi-Pb were carried out at 560 oC and 410 oC respectively, while a current of 100 mA and 200 mA was applied to the Li||Bi cell. Both cells showed relatively stable cycling performance than previous cells using a second designed cell.
In particular, the Li||Bi-Pb cell operated at a lower temperature than others using a liquid Li anode.
Appendix A
Appendix B
To obtain the results as shown in Figure 34, gavanostatic charge and discharge tests were experimented using Na as anode and NaOH-NaI as electrolyte. Judging from the fact that the bulk Bi and Pb were not mixed, powder Bi and Pb were used to make alloy. The result, which is galvanostatic charge and discharge as shown in Figure B-2(b), was also difficult to interpret.
In response to these results, tin (Sn) was used as a cathode to create a stable liquid-liquid interface between electrolyte and cathode. Powder Sn was melted after being placed in the cell cup, as shown in Figure B-3(a). The problem arising from the use of bulk Bi and Pb was solved by the use of a single metal, as shown in Figure B-3(b).
However, as shown in Figure B-3(c), the result of the galvanostatic charge and discharge test is difficult to interpret as before. a) Bi and Pb in the cell cup after heat treatment and (b) galvanostatic charge and discharge cycling of the Na|NaOH-NaI|Bi-Pb cell at 390 oC. a). Sn powder in cell cup, (b) molten and cooled Sn, and (c) gavanostatic charge and discharge cycle of Na|NaOH-NaI|Sn cell at 320 oC.
Appendix C
M; Menictas, C.; Skyllas-Kazacos, M., Overview of materials research and development for vanadium redox flow battery applications. 25] Fujiwaraa, S.; Inaba, M.; Tasaka, A., Novel molten salt systems for high-temperature molten salt batteries: ternary and quaternary molten salt systems based on LiF–LiCl, LiF–LiBr and LiCl–LiBr. 41] Xiao, H.; Reitz, T., Anode-assisted solid oxide fuel cells with thin film electrolyte for use at lower temperatures.
L.; Liu, J., Liquid metal electrode to enable ultra-low temperature sodium-beta aluminum batteries for renewable energy storage. 52] Li, H.; Wang, K.; Cheng, S.; Jiang, K., Environmentally friendly antimony-tin positive electrode high-performance liquid metal battery.
Acknowledgements
