Molten salt electrolyte in Na-ZnCl2 solid-electrolyte battery for electricity storage


Molten salt electrolyte in Na-ZnCl2 solid-electrolyte battery for electricity storage

Kumar, S.; Ding, W.; Bonk, A.; Heinz, M. V. F.; Weber, N.; Bauer, T.

Increasing share of intermittent renewable energy resources such as PV and wind energy in the grid has led to increasing demand in grid-scale stationary storage batteries. The commercial ZEBRA (Na-NiCl2) batteries are one kind of low-cost stationary storage batteries, which have been developed in the past decades to increase their efficiency, safety and performance.

Replacing Ni with abundant and low-cost Zn (i.e., Na-ZnCl2 batteries) could make these batteries more cost-effective (cut down around 46 and 20% of the cell material and overall battery costs). Fig. 1 shows the material cost breakdown of Na–NiCl2 and Na-ZnCl2 batteries. Compared to ZEBRA battery (Ni: 63%), the Zn electrode in the Na-ZnCl2 battery has a much lower material cost share (Zn: 23%). Several studies have been done on these novel Na-ZnCl2 batteries, but there is still a lack of understanding for the electrolyte system (AlCl3-NaCl-ZnCl2) of the Na-ZnCl2 batteries in terms of melting temperature, phase changes and salt vapor pressures at various temperatures. These properties of the salt electrolyte are vital for the battery design and optimization.

In this work (as part of the SOLSTICE EU H2020 project for development of Na-ZnCl2 batteries), the simulation-assisted in-depth analysis on the AlCl3-NaCl-ZnCl2 salt electrolyte for the Na-ZnCl2 solid electrolyte batteries was performed via the thermodynamic software FactSage and thermo-analytical techniques like Differential Scanning Calorimetry (DSC) and OptiMeltTM. Moreover, an estimation model for the unit storage salt material cost (i.e., salt cost/storage capacity, $/kWh) was developed and used for the pre-optimization of this salt electrolyte.
The simulation and experimental results show that increasing the concentration of AlCl3 in the AlCl3-(NaCl)2-ZnCl2 salt mixture decreases the melting point significantly, which could enlarge the charge/discharge range of the battery. However, when containing more than 40 mol% AlCl3, the vapor pressure of the salt mixture could be above 1 atm at 300 °C. Thus, this factor is suggested to be considered in the electrolyte selection and battery operation for the battery safety. Moreover, the estimation results of the unit storage salt material cost in Fig. 2 indicate that as AlCl3 has a much higher cost than NaCl, increasing the concentration of AlCl3 in the AlCl3-(NaCl)2-ZnCl2 salt electrolyte from 5 to 50 mol% could increase the unit storage salt material cost from 0.2 to 2.8 $/kWh, when the salt electrolyte in the full discharged state and Zn metal are used as the starting cathode in the battery. However, when the salt contains a low concentration of AlCl3 (e.g., 5 mol%), the possibly low conductivity of the salt electrolyte due to its high melting temperature (small ratio of the liquid salt phase) could limit the battery performance. While the results of this study, such as the AlCl3-(NaCl)2-ZnCl2 phase diagramme, its vapour pressure and price will be very beneficial for preselecting possible salt compositions, the final selection will need to be made based on real battery tests and accounting for further issues, such as corrosion, as well. These experiments are currently under way.

  • Lecture (Conference)
    International Renewable Energy Storage Conference, 20.09.2022, Düsseldorf, Deutschland

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Publ.-Id: 34520