Over time, the loss of trace amounts of oxygen from within the lithium-ion electrode can weaken battery performance.
Enhancing lithium-ion battery performance often requires careful and extremely accurate measurement of changes in material properties. It is well known that as lithium ions flow during charging and discharging, the voltage of the battery decreases, and over time, the accumulation of these losses can easily result in a 10-15% reduction in the battery's energy storage.
Oxygen vacancy
This ultra-slow process has been measured in great detail by researchers at the SLAC National Acceleration Laboratory (a national laboratory under the U.S. Department of Energy and operated and managed by Stanford University), and the results show how 'the loss of cavities or vacancies in oxygen atoms changes the electrode structure and chemistry, and how it gradually reduces energy storage. and how the energy storage is gradually reduced.'
The study, described in a SLAC news release, was described by Peter Csernica, Ph.D., of Stanford University, who said, "In experiments with associate professor Will Chueh, we were able to measure extremely small amounts of oxygen outflow over hundreds of cycles at a rate that was so slow that it was difficult to detect. "
Lithium-ion batteries temporarily store charge by moving lithium ions back and forth between the cathode (positive) and anode (negative). The researchers found that at some point, as the lithium moves back and forth, oxygen atoms are lost from the billions of particles that make up each electrode. Measuring these oxygen losses is difficult, Csernica says, "The total amount of oxygen lost reaches 6 percent over more than 500 charge/discharge cycles. That's not a small number, but if you try to measure the amount of oxygen lost in each charge/discharge cycle, it's about 1 percent."
At the temperatures at which batteries operate, oxygen atoms have a hard time moving through solid materials, and the conventional wisdom is that oxygen loss comes only from the surface of the nanoparticles. the SLAC researchers are studying how oxygen loss changes the chemistry and structure of the particles, rather than trying to measure oxygen loss directly. They have conducted separate trajectories of oxygen movement over several lengths, from the smallest nanoparticles to clumped nanoparticles to the entire thickness of the electrode.
Through their investigations, the Lawrence Berkeley National Laboratory's Advanced Light Source (ALS) research team disassembled the cells and examined the electrode nanoparticle slices for detailed analysis, depending on how long the cells were cycled The electrode nanoparticles were examined in detail. The samples were scanned using a specialized X-ray microscope to generate high-resolution images, carefully observing each tiny chemical component to demonstrate nanoscale details, measured in parts per billion meters.
The test results were compared to a computer model to observe the potential for oxygen loss. The team concluded that the initial oxygen loss starts at the surface and then slowly flows out from the inside. As the nanoparticles coalesced into larger groups, those near the center lost less oxygen than the surface.
What's left behind?
Chueh points out that another very important question is how the loss of oxygen atoms affects the material left behind, Chueh says. Imagine nanoparticles in which the atoms are like tightly packed spheres. If you keep taking out oxygen atoms, then everything will collapse and densify because of the change in structure."
The team said, "Since this aspect of the electrode structure could not be directly imaged, the scientists once again compared other types of experimental observations with computer models of various oxygen loss scenarios. The results show that cavities do exist and that the material does not cause structural collapse and densification, thus illustrating how oxygen loss can lead to a gradual decrease in battery power."
Chueh said, "When oxygen leaks out, the surrounding manganese, nickel and cobalt atoms move with it. All the atoms are out of their ideal positions. This reorganization of metal ions with the loss of oxygen causes chemical changes that reduce the battery voltage and efficiency over time. We have known about these phenomena for a long time, but we didn't know the real cause."
Now we have a scientific, shallow to deep understanding of an important source of battery degradation that could lead to new ways to reduce oxygen loss and its damaging effects.
Shown in the inset: SLAC and Stanford scientists have made detailed measurements of how oxygen leaks out of the billions of nanoparticles that make up the electrodes of a lithium-ion battery, reducing the battery's voltage and energy efficiency over time. In the figure, the red spheres are leached oxygen atoms and the purple spheres are metal ions. This new insight may minimize the problem and improve battery performance.
