Liquid Metal / Molten Salt Battery
Current state of this technology
Liquid metal / molten salt batteries are now modular, and are beginning to be deployed
The original design proposal for a Liquid Metal/Molten Salt battery, which could store electrical energy at a grid scale was to use huge vats of two liquid metals separated by a layer of molten salt, which would form an immiscible electrolyte layer between the two liquid metals.
The technology, currently produced by a company called Ambri, has matured and become a little more user-friendly and is now packaged into self-contained modules the size of a standard 10-foot shipping container, providing up to one MWh of energy storage, and up to 250kW of power from each 10-foot, thermally insulated, container.
By stacking these containers and wiring them together in parallel or series you can increase the total energy storage and the peak power as required. A 250MWh system is planned for deployment alongside PV to power a data centre in Reno, Nevada.
They are designed for use in places which need high energy capacity, frequent cycling, long life and high efficiency.
Key features
Selling points for this technology
- Minimal fade – battery does not degrade over time or with any number of cycles
- Not temperature sensitive
- Rapid response – less than 0.5 second response to need to charge or discharge.
- Out-performs Lithium-ion
- Zero maintenance – containers fully set up and fully sealed
- DC efficiency exceeds 80%
- Not active when at ambient temperature, so inert and safe during transport and assembly
- System designed for places which need high energy capacity, frequent cycling, long life and high efficiency.
- Cost of components 1/3 of Lithium ion cells
- Do not need cooling or fire-suppression
Internally, there are individual cells each about the size of a shoe-box, assembled onto trays with ten or more trays all connected within a thermal 10-foot enclosure to form a MWh scale system. This is coupled to the grid using standard industrial AC/DC bi-directional inverters. The system is insulated and self-heating when operated, needing no external heating or cooling to stay at operating temperature. Multiple modules placed together in parallel enable unlimited upward scalability for large scale projects.
How does it work?
Overview
Each cell has stainless steel housing in a positively polarized case, with negative terminal protruding from centre of the lid. During transport, cells are at ambient temperature and are not active and have zero cell voltage and the active materials are not even conductive. They, and the 10-foot shipping containers they are arranged in, are thus completely safe to handle during assembly and transport.
Once the 10-foot containers are delivered and installed, heaters within the system bring them up to operating temperature which activates them and allows them to start storing electricity. They are expected to stay at operating temperature for lifetime of operation. However cells are designed to undergo dozens of thermal cycles from room temperature to 500oC without impacting cell performance if required. Cells are very tolerant of over-charging, over dis-charging and are not subject to thermal runaway, electrode decomposition or electrolyte off-gassing – all of which cause problems in many other battery cell chemistries.
During operation, the active materials reversibly alloy and de-alloy while charging and discharging. The electrolyte is thermodynamically stable with the electrodes, so there are no problems with film formation at the electrode/electrolyte interfaces. The negative electrode is fully consumed when discharged so the cell has just an alloy at the cathode. The two metals separate on each charging cycle, which results in a highly repeatable process with no memory effect.
This means that there are no degradation mechanisms which cause fade which is seen in other battery chemistries. The system is not relying on preserving any particular crystal structure or any other particular fixed structure which could deform over time. The cells do not deteriorate over the lifetime of the battery.
There are links to several YouTube videos about this technology on the Videos page
Chemistry
The Cell chemistry has calcium (Ca), antimony (Sb) and a calcium-chloride based molten-salt electrolyte. Once the cell is at operating temperature, the electrolyte melts, so that the battery can work.
During charging:
An external voltage pushes electrons into the anode and pulls electrons out of the cathode this causes the following to happen:
At the anode at the calcium / electrolyte interface incoming electrons join with calcium ions in the molten salt electrolyte:
At the cathode at the calcium and antimony alloy / electrolyte interface electrons are produced and the calcium ions escape into the molten-salt electrolyte:
Overall effect:
In its charged state the inside of the battery has
- Liquid metal calcium for negative electrode.
- Calcium chloride based molten salt electrolyte.
- Solid antimony particles for positive electrode.
During discharging:
At the Sb cathode / electrolyte interface:
At Ca anode / electrolyte interface:
Overall effect:
In its discharged state the calcium and antimony form an inter-metallic alloy.
At room temperature the cells are non-conductive. The active materials are solid metals with a solid electrolyte. Once heated to 500oC they operate at maximum performance, no matter what the external temperature. They do not need air-conditioning.
The cells generate their own heat during use, so there is no need for auxiliary power for temperature control other than for the initial heating phase. They work best with a full charge/discharge cycle at least every two days, but the more cycles the better. They do not degrade.
This operational cycle is ideal for converting the diurnal power generation from PV into the diurnal power demand cycle.
References / external websites:
News article about Liquid metal/molten salt battery usage in Nevada
Company making Liquid Metal & Molten salt batteries
Thermal Stability of Eutectic mixtures of salts for thermal energy storage
https://www.sciencedirect.com/science/article/pii/S1876610217309918