Ambri, an American energy storage tech startup founded in 2010, produces liquid metal batteries to store renewable energy from wind and solar power systems for a long time. The company’s battery is made from antimony (Sb) and calcium (Ca), and it doesn’t need to be cooled or use expensive materials like lithium. This battery is cheaper, safer, and lasts longer than lithium-ion batteries. The liquid metal batteries have minimal degradation and can last for more than 20 years. Ambri aims to speed up the decarbonization of grids and let consumers get all the energy they need from renewable resources.
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Challenges: energy storage
More and more solar cells and wind turbines are being utilized to generate clean electricity. Ideally, the electricity they intermittently generate is fully used. The excess electricity should be stored. The ability to store their electrical energy would significantly increase the efficiency and reliability of these intermittent renewable energy technologies as base load supply.
Batteries are strong candidate solutions owing to their small spatial footprint, mechanical simplicity and flexibility in siting. Lithium-ion batteries are a mature technology. They have been used to store energy on the grid that is integrated with intermittent solar and wind energy. However, the high cost of lithium-ion batteries prevents their widespread use to store energy on the grid. Low-cost technologies, such as pumped hydroelectric, are preferred despite their low energy and power densities and geographical limitations. Additionally, grid energy store batteries must be safe. It is reported that several dozens of lithium-ion energy storage systems have resulted in explosions or fires.
Therefore, there is a need for low-cost and safe battery technology for grid-scale energy storage.
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Ambri provides low-cost and safe grid-scale energy storage systems based on the technology of UL 1973-certificated liquid metal batteries. The certification indicates that the battery is not only high performance but also safe. In addition, Ambri’s batteries are competitive because they use earth-abundant raw materials of calcium and antimony.
Ambri liquid metal battery
The following diagram illustrates the composition of the Ambri liquid metal battery (Ca‖Sb).
The image below depicts a cross section of a liquid metal battery at room temperature.
The negative electrode of the battery is composed of calcium (Ca) held within a porous metal current collector that is connected with the negative current lead.
The positive electrode is composed of antimony (Sb) particles ( less than one centimeter) held in place by a permeable metal separator which also serves as the positive current collector. The antimony particles are surrounded by the molten electrolyte.
The gap between the negative and positive electrodes does not exceed 10 millimeters.
The molten electrolyte consists of calcium chloride (CaCl2) and other salts. The electrolyte conducts ions of the negative electrode. The electrolyte has a broad chemical window, which prevents performance degradation even when the batteries are overcharged. Furthermore, the electrolyte is non-flammable, so there is no risk of ignition or catching fire.
A cell housing contains the negative electrode, positive electrode, and molten electrolyte. The cell housing welded with the permeable metal separator serves as the positive current collector. A negative current lead extends into the cell housing through a hoe in the cell housing.
To prevent short circuit, the hole in the cell housing is hermetically sealed by a robust aluminum nitride insulator disposed between the negative current lead and the cell housing. The seal also prevents air from entering the cell, which degrades performance.
An empty headspace is formed above the negative electrode.
Ambri’s liquid metal battery can have horizontal and vertical configurations, as depicted in the diagram below.
The vertical configuration has several advantages over the horizontal configuration.
- Permit shorter and lighter cell-to-cell interconnections as well as increased packing efficiency within trays and racks. Energy density within a system is increased by densely packed cells;
- Permit larger cells and reduce the complexity of the system; and
- Less sensitive to tilt and vibration.
How Ambri liquid metal battery works
The battery has zero voltage and is non-conductive at room temperature, because both electrode materials and electrolyte are solid.
Upon heating to an operating temperature of 500 ºC, the molten electrolyte and the negative electrode become a liquid state while the positive electrode remains solid (pure antimony has a melting point of about 630 ºC.). Using liquid metal negative electrode has several benefits:
- Increase the electron-transfer kinetics of the electrode;
- Avoid the formation of dendrites when plating liquid calcium during charging. The dendrites could cause a battery shorting; and
- Wick the liquid calcium into the porous metallic negative current collector.
As depicted in the diagram below, when charging the battery, antimony in the solid CaSbx alloy of the positive electrode is oxidized and calcium ions (Ca2+) are released: CaSbx (alloy) → xSb (solid) + Ca2+ + 2e−. Calcium ions dissolve in the molten electrolyte and diffuse to the negative electrode, where calcium ions are reduced to liquid calcium metal: Ca2+ + 2e− → Ca (liquid). The two cell-charging half-reactions may combine into a full reaction: CaSbx (alloy) → Ca (liquid) + xSb (solid).
The discharge of the battery completely consumes the liquid calcium metal by oxidizing it into calcium ions that dissolves in the electrolyte: Ca (liquid) → Ca2+ + 2e−. The electrons travel through an external circuit, where they perform electrical work. At the positive electrode, antimony metal is reduced to negative antimony ions, which combine with calcium ions from the molten salt to form CaSbx: xSb + Ca2+ + 2e− → CaSbx (alloy). The two cell-discharging half-reactions combine into a full reaction: Ca (liquid) + xSb (solid) → CaSbx (alloy). The reaction is driven by the relative activity of calcium between the negative electrode (close to 1) and the positive electrode ( between 3×10−11 and 3×10−13).
During high temperature operation, the antimony in the positive electrode reacts with the stainless steel of the cell housing, forming a stainless steel-antimony alloy on the surface of the cell housing, porous metal separator, and antimony particles, which reduces the electrochemical and structural stability of the battery. To prevent antimony from reacting with the components of the cell housing, the positive electrode is pre-alloyed with stainless steel.
The Ca‖Sb battery has an open circuit voltage between 0.9 volts (V) and 1 V. Compared to batteries with a higher voltage, such as lithium-ion batteries, the liquid metal batteries with the voltage within this range reduce the risk of thermal runaway, allow the production of larger cells, and simplify the battery management system.
At 0.85 V operation voltage and 90% electrode utilization, the calcium negative electrode has a specific energy of approximately 1023 watt-hours per kilogram (Wh/kg) and an energy density of approximately 1,659 watt-hour per liter (Wh/L), whereas the antimony positive electrode has a specific energy of about 505 Wh/kg and an energy density of about 3,385 Wh/L.
To increase electrode utilization, it is advantageous to reduce the thickness of the negative and/or particle size of the positive electrode, reduce the thickness of the electrolyte disposed between the electrodes, use electrodes with a large surface area (e.g., greater than or equal to about 10 cm2), operate at a charge rate of C/4 (4 hours of charging), and/or operate at a slower constant current rate.
The Ca‖Sb battery chemistry has demonstrated robust cycling performance, including low capacity degradation under full depth of discharge cycling, which is projected to last for decades of operation. The battery loses less than 0.5% of its capacity after 500 cycles at a cycling rate of C/3 (3 hours of discharging) and 90% positive electrode utilization.
How to make Ambri liquid metal battery
Particles of the negative electrode material (calcium) and the positive electrode material (Sb and stainless steel) are filled into the porous stainless steel bays welded to the interior surface of cell housing. The molten salt electrolyte is then delivered to the cell by pulling a vacuum on the cell connected to a molten salt bath via a hollow tube. A volume of molten electrolyte is filled in the cell housing so that the empty headspace above the reactive materials of the electrochemical cell is no more than about 2.5 centimeters.
The negative current lead is inserted through the aperture in the cell housing and into the electrolyte within the cell housing. An aluminum nitride insulator surrounds the conductor and seals the cell housing around it.
To build the energy storage system, multiple cells (about 1 kWh per cell) are first assembled and closely arranged on trays to prevent thermal runaway. The tray has a cell management system. The trays are stacked within racks to form cell towers. The cell towers are placed within a thermally insulated container (10 feet x 10 feet x 8 feet per container). The containers are shipped at ambient temperature. They have zero cell voltage and are unable to pass current, which provides a high level of safety during transport and assembly.
Once the energy storage system has been installed, energy is provided to initially bring the batteries to their operating temperature. Once the batteries are heated and operational, the charge and discharge process can generate heat and keep the temperature stable. Therefore, when batteries are in use, they “self-heat”, requiring no external heating to maintain their operating temperature.
The system alternates between full charging and full discharging in less than 500 milliseconds (ms), allowing it to respond quickly to the needs of grid operators and/or industrial customers.
The container is coupled to a computer system that regulates the charging and/or discharging, temperature, and battery management system.
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Ambri Liquid Metal Battery Application
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The carbon footprint of data centers is affected by electricity consumption, cooling, and location. Data centers require electricity to power their servers, and the location of the data center affects the amount of emissions generated by the data center’s electricity consumption. Data centers can reduce their carbon footprint by implementing energy-efficient cooling practices and utilizing renewable energy sources.
TerraScale and its data center development partners integrate a 250 MWh Ambri energy storage system for the TerraScale Energos Reno project.
Ambri’s Liquid Metal™ battery has received UL 1973 certification, the Standard for Batteries for Use in Stationary and Motive Auxiliary Power Applications. This means that the Liquid Metal™ battery is not only efficient and high-performing, but also extremely safe.
Ambri plans to manufacture cells and entire systems. Its pilot manufacturing facility in Massachusetts will house a high-speed cell production line and system assembly and testing. These battery systems will be used for trial deployments and then early commercial products in 2023. Starting in 2024, Ambri’s high volume production facility will produce multiple gigawatt-hours of systems in the USA .
Ambri has raised a total of $211.4M in funding over 7 rounds:
- a Series A round
- a Series B round
- s Series C round
- a Grant round
- two Venture-Series Unknown rounds
- a Series E round
Their latest funding was raised on Aug 24, 2021 from a Series E round.
Ambri is funded by 12 investors:
- Bill Gates
- TotalEnergies Ventures
- Khosla Ventures
- Goehring & Rozencwajg Associates
- Paulson & Co.
- Reliance Industries
- Building Insurance Bern
- KLP Enterprises
- Massachusetts Clean Energy Center
- Japan Energy Fund
- Elemental Excelerator
Bill Gates and Goehring & Rozencwajg Associates are the most recent investors.
Dan Leff is CEO.
Ambri Board Member and Advisor
Charles Bryceland is board observer.
Ben Hemani is advisor.