AG10 battery last 'infinitely' long? Researchers introduce initial stress state to change material's structure

Batteries are widely used in everyday applications, such as powering electric vehicles, electronic devices, and are promising candidates for sustainable energy storage. However, as you may have noticed, their ability to store electricity degrades over time as we charge them every day. Eventually, we need to replace these batteries, which is not only expensive but also depletes the rare earth elements used to make them.

: Pexels, Mohamed Abdelghaffar

A key factor in the shortened life of batteries is the degradation of their structural integrity. To stop structural degradation, a team of researchers at the USC Viterbi School of Engineering wanted to introduce "stretch" into battery materials so they can be cycled repeatedly without structural fatigue. The research was led by researchers Ananya Renuka-Balakrishna, WiSE Gabilan Assistant Professor of Aerospace and Mechanical Engineering, Delin Zhang, a USC Viterbi doctoral student, and Brian Sheldon, a professor at Brown University. Their work was published in the Journal of the Mechanics and Physics of Solids.

Typical batteries work through repeated cycles of inserting and extracting lithium ions from electrodes. This insertion and extraction expands and compresses the electrode lattice. Over time, these volume changes produce microcracks, fractures and defects.

"These microcracks and fractures in the battery material will lead to structural degradation, which will eventually reduce the battery capacity," Zhang said. "Eventually, the battery will have to be replaced with a new one."

To stop this, Zhang, who studies intercalation materials (a class of materials used as electrodes in lithium-ion batteries), stretches these intercalation electrodes in advance. This change in the initial stress state regulates the phase transition voltage, making the electrode more resilient to fracture or amorphization (losing its crystalline properties).

Wider voltage, greater capacity

Phase transitions, when battery materials change physical form, are produced by the expansion and compression cycles that accompany daily charging and use.

"These phase transitions can make the electrode more susceptible to structural degradation, especially when the process is repeated so frequently," Zhang said.

The reversibility of the phase is key to allowing batteries to maintain efficient function over time.

"Reversibility is best enhanced by ensuring that the material remains in its crystalline form," Renuka-Balakrishna said. "At certain voltages, when materials transition from one phase to another, they become powdery, which is not ideal for efficient battery operation."

The researchers therefore asked themselves, "Is there a way to keep battery materials in crystalline form as they cycle back and forth between energy landscapes?" The answer: by introducing an initial stress state that changes the material's structure.

By stretching the electrodes before charging/discharging, the researchers altered the electrode's energy landscape from the charged state to the discharged state. This also allows the battery to operate over a wider voltage range, as shown in the figure at right. : Delin Zhang

"By stretching the electrodes before charging and discharging, we are altering the electrode's energy landscape from the charged state to the discharged state. This initial strain allows us to reduce the energy barriers for these transitions and prevent detrimental lattice deformations that lead to material failure," Zhang said. "This change in the energy landscape helps prevent microcracks and fractures, protecting the battery's sustainability and energy storage capabilities."

Another benefit is that by stretching the electrodes, the battery can also operate in a wider voltage window, thereby increasing its energy storage capabilities.

Challenges of Modern Energy Storage

One of the main focuses of the energy storage community has been getting rid of the flammable liquid electrolytes typically used in batteries and putting them in solid materials. This presents new challenges.

It is well known that solid objects deteriorate over time when repeatedly stressed. Once a crack is introduced, the two sides of the surface lose contact. In the case of batteries, it creates a simple mechanical problem; without that connection, it is difficult to transport ions in the material, Renuka-Balakrishna said.

The approach Zhang identified is to try to move toward safer, more sustainable batteries while addressing this mechanical challenge. The novelty of this approach, the researchers said, is that you can extend the life of existing materials by introducing basic mechanical concepts, rather than looking for new materials to extend battery life.

"Mechanics has not always been a component of developing batteries," Renuka-Balakrishna said, "but now engineers can use this theory/tool Zhang created to design battery materials for life."

Extending the life of batteries will benefit users of electronic devices and electric vehicles, extending the use of their devices and minimizing battery replacements. Given the cost of lithium-ion batteries, it could also save users a lot of money over time.

Not only that, sustainable energy storage is an important part of reducing harmful greenhouse gas emissions and minimizing battery waste, and we hope that our work will open up a new research line to improve the reversibility of materials.

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