Scientists at the U.S. Department of Energy's Brookhaven National Laboratory have made the first 3D observations of how the structure of a lithium-ion battery anode evolves at the nanoscale in a real battery cell as it discharges and recharges. The details of this research could point to new ways to engineer battery materials to increase the capacity and lifetime of rechargeable batteries.
Scientists have long known that repeated charging/discharging (lithiation and delithiation) introduces microstructural changes in the electrode material, particularly in some high-capacity silicon and tin-based anode materials. These microstructural changes reduce the battery's capacity—the energy the battery can store—and its cycle life—how many times the battery can be recharged over its lifetime. Understanding in detail how and when in the process the damage occurs could point to ways to avoid or minimize it.
"It has been very challenging to directly visualize the microstructural evolution and chemical composition distribution changes in 3D within electrodes when a real battery cell is going through charge and discharge," said Wang.
"For the first time," said Wang, "we have captured the microstructural details of an operating battery anode in 3D with nanoscale resolution, using a new in-situ micro-battery-cell we developed for synchrotron x-ray nano-tomography—an invaluable tool for reaching this goal." This advance provides a powerful new source of insight into microstructural degradation.
Developing a working micro battery cell for nanoscale x-ray 3D imaging was very challenging. Common coin-cell batteries aren't small enough, plus they block the x-ray beam when it is rotated.
"The whole micro cell has to be less than one millimeter in size but with all battery components—the electrode being studied, a liquid electrolyte, and the counter electrode—supported by relatively transparent materials to allow transmission of the x-rays, and properly sealed to ensure that the cell can work normally and be stable for repeated cycling," Wang said. The paper explains in detail how Wang's team built a fully functioning battery cell with all three battery components contained within a quartz capillary measuring one millimeter in diameter.
By placing the cell in the path of high-intensity x-ray beams generated at beamline X8C of Brookhaven's National Synchrotron Light Source (NSLS), the scientists produced more than 1400 two-dimensional x-ray images of the anode material with a resolution of approximately 30 nanometers. These 2D images were later reconstructed into 3D images, much like a medical CT scan but with nanometer-scale clarity. Because the x-rays pass through the material without destroying it, the scientists were able to capture and reconstruct how the material changed over time as the cell discharged and recharged, cycle after cycle.
Using this method, the scientists revealed that, "severe microstructural changes occur during the first delithiation and subsequent second lithiation, after which the particles reach structural equilibrium with no further significant morphological changes."
Specifically, the particles making up the tin-based anode developed significant curvatures during the early charge/discharge cycles leading to high stress. "We propose that this high stress led to fracture and pulverization of the anode material during the first delithiation," Wang said. Additional concave features after the first delithiation further induced structural instability in the second lithiation, but no significant changes developed after that point.
"After these initial two cycles, the tin anode shows a stable discharge capacity and reversibility," Wang said.
"Our results suggest that the substantial microstructural changes in the electrodes during the initial electrochemical cycle—called forming in the energy storage industry—are a critical factor affecting how a battery retains much of its current capacity after it is formed," she said. "Typically a battery loses a substantial portion of its capacity during this initial forming process. Our study will improve understanding of how this happens and help us develop better controls of the forming process with the goal of improving the performance of energy storage devices."
This story is reprinted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.