Periodic Reporting for period 2 - SOLSTICE (Sodium-Zinc molten salt batteries for low-cost stationary storage)
Reporting period: 2022-07-01 to 2023-12-31
An energy landscape shaped by renewable sources requires large amounts of stationary energy storage to balance supply and demand. Within Europe, storage options on various time scales and with a total capacity on the TWh scale will be necessary. None of the technologies available today fulfils all the requirements for an ideal stationary energy storage system. New technologies therefore need to be developed – with a focus on abundance and price of the raw materials as well as a circular economy.
The Horizon 2020 project SOLSTICE answers the quest for stationary energy storage with two Na-Zn molten salt battery concepts, which operate at elevated temperature. The first concept benefits from the existing and successful ZEBRA® (sodium-nickel chloride) technology. Replacing their Ni-electrode by cheap and abundant Zn will only minimally affect other system parts thereby ensuring fast commercialisation. The second approach, an all-liquid cell, will apply the same chemistry, but does not require a ceramic electrolyte thus reducing cost further.
The Na-Zn technology is exceptionally performant as it promises similar efficiency and depth of discharge as Li-ion cells, but under extreme current densities. Featuring molten electrodes, Na-Zn cells actually work better when being cycled, as operation keeps them warm; several cycles per day and a lifetime exceeding 10,000 cycles can be legitimately expected. Na-Zn storage is perfectly sustainable: the raw materials, table salt and Zn, are abundant in the EU, cheap and not harmful. The environmental impact of Zn-mining and battery production is expected to be minimal. Finally, recycling is greatly simplified due to the large, molten electrodes. The most valuable element, Zn, can simply be recovered as pure metal and reused after dismantling the cells.
The overall objective of the SOLSTICE project is to further develop both battery concepts to finally obtain sustainable molten salt batteries for use as large-scale energy storage. The project is divided into four large working areas. Firstly, the Na-Zn chemistry will be investigated to obtain batteries with high performance and long life-time. Secondly, the battery system itself – including materials, housing and manufacturing – will be further developed. Thirdly, the Na-Zn battery and its potential as large-scale storage will be assessed, e.g. by a cost and social impact assessment. Finally, its sustainability will be improved by working on life cycle assessment and on the details of the recycling strategies.
The Horizon 2020 project SOLSTICE answers the quest for stationary energy storage with two Na-Zn molten salt battery concepts, which operate at elevated temperature. The first concept benefits from the existing and successful ZEBRA® (sodium-nickel chloride) technology. Replacing their Ni-electrode by cheap and abundant Zn will only minimally affect other system parts thereby ensuring fast commercialisation. The second approach, an all-liquid cell, will apply the same chemistry, but does not require a ceramic electrolyte thus reducing cost further.
The Na-Zn technology is exceptionally performant as it promises similar efficiency and depth of discharge as Li-ion cells, but under extreme current densities. Featuring molten electrodes, Na-Zn cells actually work better when being cycled, as operation keeps them warm; several cycles per day and a lifetime exceeding 10,000 cycles can be legitimately expected. Na-Zn storage is perfectly sustainable: the raw materials, table salt and Zn, are abundant in the EU, cheap and not harmful. The environmental impact of Zn-mining and battery production is expected to be minimal. Finally, recycling is greatly simplified due to the large, molten electrodes. The most valuable element, Zn, can simply be recovered as pure metal and reused after dismantling the cells.
The overall objective of the SOLSTICE project is to further develop both battery concepts to finally obtain sustainable molten salt batteries for use as large-scale energy storage. The project is divided into four large working areas. Firstly, the Na-Zn chemistry will be investigated to obtain batteries with high performance and long life-time. Secondly, the battery system itself – including materials, housing and manufacturing – will be further developed. Thirdly, the Na-Zn battery and its potential as large-scale storage will be assessed, e.g. by a cost and social impact assessment. Finally, its sustainability will be improved by working on life cycle assessment and on the details of the recycling strategies.
Research within SOLSTICE is structured into 12 work packages, with two of them directly dedicated to the two battery concepts, while the others provide the necessary support.
Work on the solid-electrolyte concept benefited from the existing ZEBRA batteries available at FZSoNick and EMPA. For simplicity, the battery designs were reused with only minor modification – only Ni was replaced by Zn. During the last years, several large tubular cells of the commercial geometry and smaller planar cells have been operated for more than 40 (tubular) and 200 (planar) cycles successfully for up to 3 months. Further, the operating temperature range of the new battery has been identified as well as the aging mechanism. To optimize the cell’s efficiency, numerical simulations of the battery have been performed with the aim to obtain an optimal electrically conducting path between the granular Zn particles in the cathode. In order to understand the charge-discharge behaviour, the phase diagram of NaCl-ZnCl2-AlCl3 has been successfully measured and simulated. Neutron imaging, performed of planar Na-Zn batteries at PSI/Switzerland, has shown many unexpected phenomena occurring during the cell cycling. Further, the design of the tubular cells has been improved by replacing the housing material with a cheaper alternative and by developing new sealing techniques which require less energy in the production process. Finally, a detailed cost model for the solid-electrolyte battery has been developed and was published within a master thesis.
Starting at a lower TRL, the work on the all-liquid battery has focused more on basic research. For example, tungsten and graphite have been identified as suitable housing materials for molten zinc, while nickel, steel and molybdenum have not been sufficiently resistant. The requirements to, and possible ingredients of the molten salt electrolyte have carefully been analysed leading to two promising salt mixtures of NaCl-CaCl2-BaCl2 and NaCl-SrCl2-KCl. A new electrochemical model for a completely liquid molten salt battery has been developed and thoroughly validated. The very first results show that not only ohmic losses, but also concentration polarisation in the molten electrolyte might have an important effect on the cell efficiency. Finally, two battery housing concepts have been developed and approximately 5 large and 30 small cells have been operated for up to 6 weeks in ambient air.
Work on the solid-electrolyte concept benefited from the existing ZEBRA batteries available at FZSoNick and EMPA. For simplicity, the battery designs were reused with only minor modification – only Ni was replaced by Zn. During the last years, several large tubular cells of the commercial geometry and smaller planar cells have been operated for more than 40 (tubular) and 200 (planar) cycles successfully for up to 3 months. Further, the operating temperature range of the new battery has been identified as well as the aging mechanism. To optimize the cell’s efficiency, numerical simulations of the battery have been performed with the aim to obtain an optimal electrically conducting path between the granular Zn particles in the cathode. In order to understand the charge-discharge behaviour, the phase diagram of NaCl-ZnCl2-AlCl3 has been successfully measured and simulated. Neutron imaging, performed of planar Na-Zn batteries at PSI/Switzerland, has shown many unexpected phenomena occurring during the cell cycling. Further, the design of the tubular cells has been improved by replacing the housing material with a cheaper alternative and by developing new sealing techniques which require less energy in the production process. Finally, a detailed cost model for the solid-electrolyte battery has been developed and was published within a master thesis.
Starting at a lower TRL, the work on the all-liquid battery has focused more on basic research. For example, tungsten and graphite have been identified as suitable housing materials for molten zinc, while nickel, steel and molybdenum have not been sufficiently resistant. The requirements to, and possible ingredients of the molten salt electrolyte have carefully been analysed leading to two promising salt mixtures of NaCl-CaCl2-BaCl2 and NaCl-SrCl2-KCl. A new electrochemical model for a completely liquid molten salt battery has been developed and thoroughly validated. The very first results show that not only ohmic losses, but also concentration polarisation in the molten electrolyte might have an important effect on the cell efficiency. Finally, two battery housing concepts have been developed and approximately 5 large and 30 small cells have been operated for up to 6 weeks in ambient air.
Selected highlights obtained within SOLSTICE so far include:
- Na-Zn solid-electrolyte batteries of commercial size have been successfully operated for more than 200 cycles and 3 months
- Na-Zn all-liquid batteries of various sizes have been operated for up to 6 weeks
- Three cheap, low-melting glasses for sealing the solid-electrolyte cell have been developed
- A cost assessment has been performed and predicts a system cost of 200$/kWh for a solid-electrolyte battery and 53$/kWh for a future commercial all-liquid battery
Until the end of the project, it is planned to build demonstrators for both battery concepts. Further, the efficiency and lifetime of the batteries shall be improved as much as possible. The performance indicators for, e.g. price, efficiency, life-time, recycling or cell currents as well as the admissible operating range shall be specified better by operating these demonstrators. Finally, challenges and future research areas shall be identified and narrowed down in preparation of the future work and an envisaged commercialisation on the long term.
A successful commercialisation already of one of the Na-Zn battery concepts would provide a highly performant stationary energy storage with extreme current densities at a low cell resistance. The price of such storage could be very competitive considering the extremely low price of the active materials of only 4 €/kWh at a life-span presumably exceeding 10,000 cycles. This all together as well as the perfect recyclability and the usage of abundant and non-hazardous materials promise an ideal life-cycle for a circular economy. Such a storage system would help to avoid greenhouse gas emissions while damping electricity and energy costs. A production of such batteries in Europe would foster job creation and contribute to sustain the wealth and prosperity of European citizens.
- Na-Zn solid-electrolyte batteries of commercial size have been successfully operated for more than 200 cycles and 3 months
- Na-Zn all-liquid batteries of various sizes have been operated for up to 6 weeks
- Three cheap, low-melting glasses for sealing the solid-electrolyte cell have been developed
- A cost assessment has been performed and predicts a system cost of 200$/kWh for a solid-electrolyte battery and 53$/kWh for a future commercial all-liquid battery
Until the end of the project, it is planned to build demonstrators for both battery concepts. Further, the efficiency and lifetime of the batteries shall be improved as much as possible. The performance indicators for, e.g. price, efficiency, life-time, recycling or cell currents as well as the admissible operating range shall be specified better by operating these demonstrators. Finally, challenges and future research areas shall be identified and narrowed down in preparation of the future work and an envisaged commercialisation on the long term.
A successful commercialisation already of one of the Na-Zn battery concepts would provide a highly performant stationary energy storage with extreme current densities at a low cell resistance. The price of such storage could be very competitive considering the extremely low price of the active materials of only 4 €/kWh at a life-span presumably exceeding 10,000 cycles. This all together as well as the perfect recyclability and the usage of abundant and non-hazardous materials promise an ideal life-cycle for a circular economy. Such a storage system would help to avoid greenhouse gas emissions while damping electricity and energy costs. A production of such batteries in Europe would foster job creation and contribute to sustain the wealth and prosperity of European citizens.