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Battery technology is a common feature of everyday life, such as powering electric cars and electronic gadgets. They are also promising candidates for energy storage. As you have likely seen, the functionality of your batteries decreases with time. These batteries will eventually need to be replaced. This is expensive and depletes the rare earth element used in their manufacture.
The degradation of a battery’s structural integrity is a key factor in reducing battery life. A team of USC Viterbi School of Engineering researchers are trying to prevent structural degradation by introducing "stretch” into battery materials that can be cycled repeatedly and without experiencing structural fatigue. Ananya Renuka Balakrishna is the WiSE Gabilan Assistant Professor in Aerospace and Mechanical Engineering at USC Viterbi. She also collaborated with Brown University researchers from Professor Brian Sheldon’s group. Their research was published in The Journal of Mechanics and Physics of Solids.
Zhang explained that a typical battery is made up of a repeating cycle of inserting and removing Li-ions from electrodes. The electrode lattices are compressed and expanded by the insertion and removal. These volume shifts can cause microcracks, fractures, and defects over time.
Zhang stated that "these microcracks or fractures in the battery materials will lead to structural decay, which will eventually reduce battery capacity." "Eventually, the battery will need to be replaced by a new one."
Zhang, who studies intercalation materiala class of materials that are used as electrodes for lithium-ion battery, prevents this from happening. This is because electrodes are more resistant to fracture and amorphization, which can result in their losing their crystalline properties.
Broader voltage, greater capacity
The cycle of expansion/compression that is part of daily charging and usage can cause phase transformations in battery materials. These are when they change their physical form. Zhang said that these phase transformations can make electrodes more vulnerable to structural degradation, particularly if they are repeated so often.
The key to ensuring that batteries can continue to function efficiently over time is the ability for phases to be reversed. Renuka Balakrishna said, "Reversibility can be most enhanced by ensuring that the material remains in its crystalline state." When materials are passed from one phase to the next at certain voltages, they can become powdery which is bad for battery efficiency.
Researchers asked the question: "Is it possible to preserve battery materials' crystalline structure while they switch between energy landscapes?" By introducing an initial stress condition, you can change the material's structure.
By stretching the electrodes prior to charging/discharging, the researchers changed the energy landscape across which an electrode goes from the charged to the discharged state. The graph to the right shows that the battery can operate at a greater voltage range. Credit: DELIN ZHANG
Zhang said, "By stretching electrodes before charging and discharging them, we change the energy landscape through which an electrode goes from the charged state to the discharged. This initial strain reduces the energy barrier and prevents material failure-causing lattice deformations. This shift in the energy landscape prevents microcracks and fractures. It also protects the battery's sustainability, energy storage and capacity.
Renuka Balakrishna added that the battery has an additional benefit by stretching its electrodes. This allows it to operate in a wider voltage range, which makes it more efficient in energy storage.
Modern energy storage: Challenges
Renuka Balakrishna stated that one of the main concerns for the energy storage community is the transition away from the flammable liquid electrolytes commonly used in batteries to solid materials. She said, "This presents new challenges."
As we all know, solid objects can become brittle over time if they are repeatedly stressed. The contact between the surfaces will be lost if a crack is created. Renuka Balakrishna stated that the crack in the battery creates a simple mechanical problem. Without the connection, it is difficult to transport ions across this material.
Zhang's approach is one way to improve the safety and sustainability of batteries, while also addressing this mechanical problem. This approach is different than finding a new material that will increase battery life. Instead of looking for a material that can be used to make batteries last longer, it's possible to extend the lifespan of an existing material by using fundamental mechanics concepts to increase their lifespan.
Renuka-Balakrishna stated that mechanics was not always an integral part in developing batteries. Engineers can now use the theory/tool Zhang created to design battery materials with a longer lifespan.
Zhang stated that extending the battery life would be a benefit to users of electric devices and vehicles. This would allow for longer usage and reduce the need for replacement. It could save users a lot of money, considering the high cost of lithium-ion batteries.
Zhang stated that sustainable energy storage is important in reducing harmful greenhouse gases emissions and reducing waste battery. We hope our research will open up new avenues for material reversibility research.
Learn more Speed and absorption are key factors in optimizing a rechargeable battery.
More information: Delin Zhang et. al., Film strains increase the reversible cycle of intercalation electrodes. Journal of the Mechanics and Physics of Solids (2021). Delin Zhang and colleagues, Film strains increase the reversible cycle of intercalation electrodes. (2021). DOI: 10.1016/j.jmps.2021.104551
