Liquid metal batteries are being developed primarily for battery energy storage system (BESS) systems but may find future applications in electric vehicles (EVs) and wearables and portable devices. The first commercial BESS using a liquid metal battery is expected to become operational soon, but the longer-term outlook for large-scale market penetration remains cloudy. The first and foremost advantage of liquid metal batteries is that they may be about one-third the cost of Li-ion batteries. So, how do they work and what are their disadvantages?
This FAQ will review the operation, advantages, and disadvantages of current high-temperature liquid metal batteries, look at how the development of fusible alloy technologies could expand the use of liquid metal batteries, and consider what technological advances will be needed to enable the production of room-temperature liquid batteries.
Current liquid metal batteries are designed to operate at several hundred °C for use in megawatt (MW) scale BESS installations. They are not intended for use in EVs.
They don’t require extensive heating or cooling. They’re well insulated and once heated to operating temperature, can maintain their temperature with periodic charge/discharge cycling. In addition, they don’t require the fire suppression or other safety systems found in Li-ion-based BESS and their operating lives are anticipated to be 20 years. Current liquid metal battery designs have received UL 1973 certification.
A liquid metal battery consists of calcium (Ca) as the liquid metal negative electrode, a calcium-chloride CaCl2-based salt electrolyte, and solid antinomy (Sb) particles as the positive electrode. When charged, the Ca and Sb are separated. When discharging, the Ca and Sb combine into CaSbx compounds plus electrons. When fully discharged, all the Ca and Sb completely combine to form an intermetallic alloy. During charging, the intermetallic alloy is decomposed into Ca and Sb (Figure 1).
Figure 1. A high-temperature liquid metal battery with a Ca negative electrode and Sb particles as the positive electrode is nearing commercial production (Image: Ambri).
The completed BESS module based on liquid metal battery technology includes shelves of cells, plus thermal and battery management systems in a weatherproof enclosure like a shipping container. The system also includes a bidirectional inverter for grid connectivity. The battery shelves inside the container are insulated and ‘self-heating’ if they are cycled daily. In that case, they require no external heat source to maintain the proper operating temperature. Multiple BESS containers can be directly connected in parallel on-site to produce larger systems without the need for trenching or additional cabling.
While they are very low cost, have long operating lives, and can be assembled into MW systems, liquid metal batteries have some drawbacks including:
- An efficiency of about 80% compared to 90% for Li-ion.
- A specific energy density of about 200 Wh/kg compared to 260 Wh/kw for Li-ion.
- Its cell voltages are below 1 V compared with 3.6 V for Li-ion.
Fusible alloy technology
Fusible alloys are being investigated as a possible technology for producing lower-temperature liquid metal batteries. A fusible alloy is a metal alloy that can be easily fused and is easily meltable at relatively low temperatures. “Fusible alloy” is sometimes used to describe alloys with a melting point below 183 °C. Those fusible alloys are used to produce materials like solder. Fusible alloys being investigated for liquid metal batteries are eutectic systems. A eutectic system is a homogeneous mixture with a melting point lower than those of the constituent elements. The lowest melting point over all the mixing ratios of the elements is the eutectic temperature (Figure 2).
Figure 2. Fusible alloys made using eutectic systems are expected to support low-temperature liquid metal batteries (Image: ACS central science).
There’s a wide range of possible eutectic systems that can be used to develop fusible alloys for liquid metal batteries. The melting temperature of the metal electrodes determines the operating temperature of the battery, but it’s not the only important consideration. Factors related to cost and safety can be used to further narrow the field to practical possibilities. For example, mercury (Hg) has a low melting temperature but is toxic and can cause long-term environmental damage. So, Hg is not a good choice.
Alkali metals like rubidium (Rb) and cesium (Cs) are possibilities, but they are too costly. In the case of Rb, it’s the production cost that causes the trouble. Rb is more abundant than Li, but it has a much higher price since it’s highly dispersed in the Earth’s crust and not easily obtained in large quantities. Additionally, highly reactive elements require more costly preparation due to safety considerations. For example, many are highly corrosive due to their high chemical reactivity. In the future, technologies may be developed that reduce the cost of obtaining and using Rb, but for now, it’s too costly and challenging to use in liquid batteries.
Other alkali metals like sodium (Na) and potassium (K) are possible candidates for fusible alloys, and both are more abundant than Li and are highly reactive. In general alkali metals are easily oxidized and can be flammable in the air. Gallium (Ga), Indium (In), and tin (Sn) are relatively stable in air and water and don’t present serious health or safety concerns. Ga-based alloys are safer and relatively less harmful. Some of these fusible alloy combinations can also have voltages approaching 1 V, giving them superior performance compared with other liquid metal battery technologies. Therefore, many recent efforts have been dedicated to using these elements in electrodes for liquid batteries. Combinations like NaK-GaIn can have very low operating temperatures approaching room temperature. (Figure 3).
Figure 3. All metal liquid batteries have been researched for many decades with recent efforts focused on new combinations of alkali metals (Image: ACS central science).
Getting to room temperature
Ga-based room-temperature liquid metals (GBRTLMs) offer inherent liquidity, biocompatibility, and safety with metallicity. They can be used to produce flexible batteries that could power a wide range of future applications including deformable devices, epidermal electronics, sensors, soft robotics, and portable devices like cell phones. With different packaging, they can be adapted to high-power applications like EVs and BESS. GBRTLMs can be used for the main reacting electrodes, for auxiliary electrodes that function as a cathode whenever the working electrode is operating as an anode and vice versa, and for interconnecting electrodes in applications like photovoltaic cells (Figure 4).
Figure 4. Potential applications and characteristics of GBRTLMs in liquid batteries (Image: Frontiers in Energy).
The potential for scalability is one of the primary attractions for room-temperature liquid batteries using GBRTLMs. Low cost is a primary consideration for EV and BESS applications while miniaturization and adaptability are critical for wearables and other small devices. The use of GBRTLMs can reduce the needed investment in thermal management, hermeticity, and corrosion protection.
The primary weakness of GBRTLMs is the use of Ga which is currently very costly. Two possible solutions exist for the high cost. One is to reduce the cost of mining Ga. That may be possible in the longer term. The abundance of Ga is greater than Li and comparable with copper (Cu) and Ni and larger than cobalt (Co), indicating the possibility of lower costs for Ga in the future. In the near term, research is underway to develop lower-cost Ga alloys like Ga-tin (Sn) that have comparable performance to pure Ga electrodes but with significantly lower costs.
Figure 5. This prototype liquid metal battery remains liquefied at a temperature of 20 °C (Image: University of Texas, Austin).
In one case, a prototype battery with an anode using a NaK alloy, a Ga-based alloy for the cathode, and an organic electrolyte have been fabricated. The prototype battery remains liquefied at a temperature of 20 °C (68 °F), the lowest operating temperature recorded for a liquid-metal battery at the time of this writing (Figure 5). The researchers are now focused on addressing two challenges. One related to cost is reducing (or eliminating) the use of Ga. Second is the need for an electrolyte with better conductivity at room temperature that can support higher power capabilities. Due to the lack of conductivity of the electrolyte, the current room-temperature design can’t compete with high-temperature liquid-metal batteries.
Summary
High-temperature liquid metal batteries suited for use in MW-scale stationary energy storage systems are nearing commercial production. Low-temperature liquid metal batteries based on GBRTLMs are still in the development stage. If those development efforts are successful, low-cost GBRTLM-based liquid metal batteries may prove suitable for a range of applications from flexible electronics and wearables to EVs and BESS.
References
Ambri Battery Platform, Ambri
New Room-Temperature Liquid-Metal Battery Could Be the Path to Powering the Future, University of Texas, Austin
Next-Generation Liquid Metal Batteries Based on the Chemistry of Fusible Alloys, ACS central science
Perspective on gallium-based room temperature liquid metal batteries, Frontiers in Energy