Solid-state vs. liquid-metal batteries

A new room-temperature liquid-metal battery of the University of Texas may provide more power than lithium-ion batteries while competing with solid-state batteries for their chance under the hood of the electric vehicle of the future.

A report published in the journal Advanced Materials, describes a design which combines the strengths of both solid-state and liquid-state batteries while circumventing several of their disadvantages. This new battery has increased energy capacity, stability, flexibility, and more.

"[W]e demonstrate the first-of-its-kind room-temperature liquid metal battery by employing fusible alloys as both the positive and negative electrode, whereby the operating temperature is expected to be decreased below melting point of water [...] suggesting the potential applications in harsh environments. In the meantime, the extra cost on thermal management, special maintenance, seals, and corrosion in high-temperature liquid metal batteries can all be reduced remarkably. The liquid metal battery system is also analyzed in terms of cost," the researchers wrote.

The report also noted the battery's overall reduced cost comparison by using more commonly available minerals over lithium. With lithium alternatives few and far between, this is a breakthrough worth keeping an eye on.

Current state of EV batteries

The key challenge in EV battery design is to reach or surpass the mileage of current internal combustion engines, which requires that the next generation of batteries reach higher capacity than they currently do.

A solid-state battery has higher energy density than a lithium-ion battery, which uses a liquid electrolyte as the medium that enables movement of ions between a solid cathode and anode. Solid-state, as the name implies, has no liquid elements. It doesn't pose a risk of explosion or fire from damage involving the flammable liquid chemicals, reducing the need for safety and thermal management components, thus saving space to increase capacity by adding more batteries.

A solid-state battery also has increased energy density per unit, allowing for added volume by way of smaller, equally powerful batteries. For that reason, solid-state battery designs are being considered the final word for EV battery systems.

Why pursue liquid?

Solid-state batteries' inflexibility under pressure makes them prone to degradation over time. This technical challenge, along with manufacturing costs, and has limited their commercial EV applications thus far.

Conventional liquid-metal batteries are more robust; with a self-healing nature, maintaining an organic separation of their elements while charging and discharging without deterioration. However, their high energy demands are inefficient, requiring additional power to maintain temperatures above 464 degrees Fahrenheit (240 degrees Celsius) to maintain electrodes in a molten state.

Liquid metal batteries were initially considered to support intermittent renewable energy sources due to their ultrafast charge transfer and resistance to degradation. And without needing to be portable, size wasn't an issue.

Even when stationary, sustaining high temperatures to maintain a battery's conductivity of liquid electrodes and electrolytes creates its own challenges around maintaining hermetic sealing, thermal management, and the dangers of corrosive components and chemicals. Intermediate- and room-temperature liquid metal models have been testing alternate electrodes, electrolyte design concepts, and interfaces.

Cooling down

A room-temperature all-liquid metal battery combines the benefits of liquid and solid-state options. Enter the researchers at Texas U with a sodium-potassium alloy as the anode and a gallium-based alloy as the cathode, where both can remain liquefied at a temperature of 68 F (20 C).

"With facile cell fabrication, simplified battery structures, high safety, and low maintenance costs, room‐temperature liquid metal batteries not only show great prospects for widespread applications but also offer a pathway toward developing innovative energy‐storage devices beyond conventional solid‐state batteries or high‐temperature batteries," said Yu Ding, co-author of the report.

Room-temperature liquid metals inherently possess higher densities than conventional electrode materials, offering greater capacity. Without the bulk and energy consumption of auxiliary heating systems, these batteries may finally realize the promise of wider applications beyond bulky stationary energy storage – a flexibility allowing the next generation of liquid metal batteries to power smaller and smaller devices.

"We are excited to see that liquid metal could provide a promising alternative to replace conventional electrodes," explained co-author Guihua Yu. "Given the high energy and power density demonstrated, this innovative cell could be potentially implemented for both smart grid and wearable electronics."

With the exception of gallium itself, many of the elements of this new battery chemistry are more abundant, making them potentially easier and less expensive to scale. Gallium alloys are also nontoxic compared with conventional lead- and mercury-based liquid metal electrodes. Gallium, while rare, is available as a secondary product of aluminum and zinc refining, and can be recycled.

With continued materials research, it may be possible to create a battery with even lower melting points. The next step in liquid-metal battery power will come by improving the electrolyte chemistry for even better conductivity; with advanced simulations, modeling and AI-enhanced data analysis, more possible systems could be explored and properties modeled more accurately.