Translated Abstract
In an endeavor to reduce CO2 emission, renewable energy technologies, such as wind and solar energies, are being integrated with the base load supply of the electric grid. However, to ensure continuous supply of electricity from the grid, the power generated from these renewable energy sources, which is quite intermittent, needs to be first stored in grid-scale energy storage devices. Amongst different large-scale stationary electrical energy storage devices (SEES), batteries provide very high spacial and temporal flexibility in transmitting electrical energy. An electrical power grid that integrates them with renewable energy sources will be highly reliable and efficient. However, despite these advantages, most of the existing batteries are expensive and their extensive use in large scale power grids is still uncommon due to economic factors. Therefore, it is of great significance to develop new grid-scale stationary electrical energy storage technologies with acceptable costs in the commercial market.
Molten salt batteries have apparent advantages in cost and performance when applied in the grid-scale energy storage market. The cost of molten salt electrolytes is far below that of common non-aqueous electrolytes, like ionic liquid and organic electrolyte. Then, compared with aqueous electrolytes, molten salts can provide a wider potential window and be used in the batteries with higher voltages. Also, the molten salt batteries usually have excellent shock-resistibility endowed by the very fast electrode reaction and ion transmission rate at high operating temperature, which meets the demands of grid-scale energy storage market very well.
Therefore, beginning with the liquid metal battery systems, this paper studied four kinds of molten salt batteries. The improvement directions of the four different molten salt batteries are promoting voltage, decreasing operating temperature and finally reducing cost. The main contents are as follows.
1. Based on the mature Li‖Bi liquid metal battery system, this research involves the antimony with lower cost and higher voltage into the bismuth cathode, so as to reducing the cathode cost and promoting the cell voltage. In the context of electrodes should maintain liquid at fully charged state, the 60:40 mol% Bi-Sb alloy has the highest discharge voltage at the current density of 50 mA∙cm-2. And the Li‖Bi-Sb cell using this 60:40 mol% Bi-Sb alloy cathode has been successfully cycled for more than 160 times with a coulombic efficiency of 99% and an energy efficiency of 89%. The energy efficiency of Li‖Bi-Sb cell is higher than the currently reported system and finally this system has a low electrode cost of 68 $∙kWh-1.
2. In order to further enhance the cell’s voltage, the pure solid antimony cathode is adopted in the Li‖Sb cell. Experiments shows that the Sb electrode has very good rate performance, which would rival that of a liquid electrode. Then the Li‖Sb cell has been cycled for 27 cycles with a coulombic efficiency of 99%, an energy efficiency of 88% and an electrodes’ cost of 55 $∙kWh-1. In addation, based on the Li-Sb phase diagram, a melting-like discharge mechanism of Sb cathode is proposed: it is the Li-Sb liquid layer produced during discharging that enhances the Li+ diffusion process. Also, in accordance with the solid-liquid-solid phase transition process, the concept of semi-liquid metal battery is proposed.
3. For purpose of breaking through the limitition of cell voltage and lowering the operating temperature of liquid metal battery, this research designed and successfully operated the intermediate temperature Al-NiCl2 replacement bimetallic battery, which is based on the replacement reaction between Al and NiCl2. The NaAlCl4 electrolyte with a melting temeprature of 154 oC is adopted to sharply reduce the operating temperature to 170 oC. The cell is cycled for 130 times and has a equilibium voltage of about 0.92 V. However, the subsequent study shows that the deposition process of Al and the low solubility of active cathode material NiCl2 have an conflict about the acid-base property of electrolyte. Also, the cell capacity fluctuate obviously during cycling for the lack of capacity controlling factor.
4. To solve the problems emerged in the Al-NiCl2 system, this paper developed a new Fe｜FeCl2｜NaAlCl4｜Graphite battery with a cycling life of more than 10000 times and an equilibium voltage of 1.4 V, and thoroughly solved the problems of Al-NiCl2 battery. The NaCl saturated NaAlCl4 electrolyte is used to ensure the low solubility of FeCl2 in it. And the low FeCl2 solubility renders the cell a solid to solid transition mechanism between FeCl2 and Fe at cell charging and discharging. Since the solid to solid transition mechanism in anode replace the supposed Fe2+ deposition process, the dendrite growth risk is competely eradicated. Then the mechanism on the cathode side is analysed and the intercalation process of AlCl4- into graphite during cell charging is proved. Besides, this paper studied the overcharging mechanisms of two different capacity design strategies, namely anode excess and cathode excess. It is found that the cell with a excessive cathode capacity could avoid the risk of Cl2 prodution during overcharging and the excessive part of cathode capacity would serve as the reserve capacity, which works at overcharging. Finally, considering the high voltage (~1.4V ) and low operating temperature (170 oC) and the most important low cost merits (80.2 $∙kWh-1，0.008 $∙kWh-1∙clc-1), the FeCl2-Graphite battery is expected to have strong market competitiveness and application potential in the commercial energy storage market.
Translated Keyword
[Aluminum alloy, Corrosion, Corrosion inhibition film, Electroless plating, Superhydrophobic surface]
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