However, as you may have noticed, the energy storage capacity of batteries decreases over time as you charge them daily. Eventually, we'll need to replace these batteries, which is not only expensive but also depletes the rare earth elements used to manufacture them.
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A key factor contributing to shortened battery life is the degradation of battery structural integrity. To prevent this degradation, a team of researchers at the USC Viterbi School of Engineering hopes to introduce “stretching” into battery materials so they can cycle repeatedly without structural fatigue. This research was led by Ananya Renuka-Balakrishna, Assistant Professor of Aerospace and Mechanical Engineering at WiSE Gabilan, USC Viterbi PhD student Delin Zhang, and Brown University Professor Brian Sheldon. Their work was published in the *Journal of Solid Mechanics and Physics*.
A typical battery operates by repeatedly cycling through the insertion and extraction of lithium ions from the electrodes. This insertion and extraction expands and compresses the electrode lattice. Over time, these volumetric changes generate microcracks, fractures, and defects.
“These microcracks and fractures in the battery materials will lead to structural degradation, which will eventually reduce the battery capacity,” Zhang said. “Ultimately, the battery will have to be replaced with a new one.”
To prevent this, the study of intercalated materials (a class of materials used as electrodes in lithium-ion batteries) pre-stretches these intercalated electrodes. This change in the initial stress state modulates the phase transition voltage, making the electrode more resilient to fracture or amorphization (loss of its crystalline properties).
Wider voltage range, larger capacity
Phase transitions occur when battery materials change their physical form, resulting from the expansion and compression cycles that accompany daily charging and use.
Zhang said, "These phase transitions make the electrodes more susceptible to structural degradation, especially when the process is repeated so frequently."
Phase reversibility is key to allowing batteries to maintain high efficiency over time.
Renuka-Balakrishna said, “Reversibility can be maximized by ensuring that the material retains its crystalline form. At certain voltages, when the material transfers from one phase to another, it becomes powdery, which is not ideal for the efficient operation of the battery.”
Researchers thus asked themselves, "Is there a way to keep battery materials in crystalline form as they cycle back and forth between energy landscapes?" The answer is: by introducing an initial stress state to alter the material's structure.
By stretching the electrodes before charging/discharging, researchers altered the energy spectrum of the electrodes 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 on the right. (Zhang Delin)
Zhang said, “By stretching the electrodes before charging and discharging, we are changing the energy profile of the electrodes from the charging state to the discharging state. This initial strain allows us to reduce the energy barrier of these transitions and prevent harmful lattice deformation that leads to material failure. This change in the energy pattern helps prevent microcracks and fractures, protecting the battery’s sustainability and energy storage capacity.”
Another benefit is that by stretching the electrodes, the battery can also operate in a wider voltage window, thereby improving its energy storage capacity.
Challenges of Modern Energy Storage
One of the main concerns of the energy storage community is moving away from the flammable liquid electrolytes typically used in batteries and encapsulating them in solid materials. This presents new challenges.
It is well known that solid objects deteriorate over time when subjected to repeated pressure. Once a crack is introduced, the two sides of the surface lose contact. In the case of a battery, this presents a simple mechanical problem; Renuka-Balakrishna says that without this connection, it is difficult to transport ions within the material.
Zhang's approach aims to address this mechanical challenge while simultaneously moving towards safer and more sustainable batteries. Researchers say the novelty of this method lies in extending the lifespan of existing materials by introducing fundamental mechanical concepts, rather than searching for new materials.
“Mechanics isn’t always a part of battery development,” Renuka-Balakrishna said, “but now engineers can use this theory/tool created by Zhang to design the lifespan of battery materials.”
Extending battery life will benefit users of electronic devices and electric vehicles, thereby extending device usage time and minimizing battery replacements. Considering the cost of lithium-ion batteries, it can also save users significant money over time.
Moreover, sustainable energy storage is an important part of reducing harmful greenhouse gas emissions and battery waste, and we hope to open up a new research route through our work to improve the reversibility of materials.
