Calendar Aging Research Is Critical to Future of Silicon-Based Batteries

Sept. 22, 2021

A man in a lab holds up three vials.

Max Schulze studies vials containing silicon nanoparticles in the Nanoscale Systems Fabrication Lab at NREL’s Solar Energy Research Facility in support of Silicon Consortium Project work to eliminate barriers to implementing silicon-based anodes in lithium-ion cells. Photo by Dennis Schroeder, NREL

In the race against climate change, state-of-the-art energy storage solutions are paramount to leveraging power generation from clean energy sources, and National Renewable Energy Laboratory (NREL) researchers are exploring strategies to increase battery cell energy density while decreasing manufacturing costs.

Silicon anodes offer a promising improvement to existing lithium-ion (Li-ion) battery capabilities for electric vehicles and stationary storage. Replacing the graphite anode material typically used in Li-ion batteries with silicon anodes may pave the way to reducing battery pack size by 25%–30% and increase driving range by 30%–40%.

NREL researchers are at the forefront of silicon anode research, using cutting-edge facilities to explore the foundational science of organic electrolytes with silicon interfaces. However, recent findings highlight the importance of continued studies on the lifetime of silicon-based cells. In a new Nature Energy Perspective article, “Calendar aging of silicon-containing batteries,” battery scientists from the U.S. Department of Energy’s Vehicle Technologies Office Silicon Consortium Project emphasize the importance of ongoing research into the calendar life of this anode material.

“New research shows that calendar aging of silicon anodes is a bigger problem than we originally thought,” said Max Schulze, postdoctoral researcher at NREL and co-author of the perspective. “With this report, we hope to encourage researchers to consider the unique challenges these batteries face. Collaboration across laboratories and in industry to evaluate and address this issue now is essential to the success of silicon electrodes in batteries.”

Researchers Investigate New Uncertainties in Battery Chemistry

To understand the lifetime of a battery, researchers must consider the effects of both battery usage and time. New electrodes are typically evaluated by cycling the cells—evaluating the capacity degradation over a set number of charge/discharge cycles—to determine the capacity loss during usage. To determine the capacity loss over time, charged cells are simply allowed to sit unused with periodic measurements of their capacities.

These types of calendar aging experiments are time intensive, so many calendar life studies extrapolate data from short-term tests to forecast aging trends. While this approach has worked for traditional graphite-based anodes, researchers face new uncertainties when altering the electrode chemistry. As a result, comparatively little is known about the time-dependent degradation of silicon-based batteries.

“Silicon does not form the same stable interface with the liquid electrolyte that gives graphite electrodes their long lifetimes,” said Andrew Colclasure, NREL scientist and another co-author of the perspective. “Further research is crucial to better understanding how the different electrode chemistry affects battery lifetime.”

Recent studies outlined in the Nature Energy Perspective suggest that the addition of silicon to the battery anode may result in a higher susceptibility to calendar aging. When compared to graphite anodes alone, silicon-containing anodes are significantly more reactive. These cells are vulnerable to damaging chemical reactions, such as gas formation, electrolyte decomposition, or dissolution of the silicon electrolyte interface, stressors that can lead to degradation over time, regardless of whether the battery is used. As such, comprehensive calendar aging studies are vital to understanding the long-term stability of silicon in batteries.

NREL-Led Consortium Focuses on Lifetime Predictions

The Silicon Consortium Project led by NREL is investigating strategies to learn more about calendar aging of silicon in batteries. Researchers developed new test protocols to assess the progress of silicon modifications, cell designs, electrolytes, or additives faster and more efficiently. The current methods do not yet provide absolute calendar lifetime predictions; however, researchers can use this data to identify the most promising strategies for mitigating calendar aging in silicon electrodes. Ultimately, the Silicon Consortium Project aims to implement those strategies to develop and demonstrate long-lived batteries with silicon anodes.

Learn more about NREL’s energy storage research.