Graphite has long been an important part of lithium ion batteries. However, as our requirements for batteries have increased, graphite-based batteries have been unable to meet our requirements for battery performance. So the researchers turned their attention to silicon, the core material of the digital revolution.
Researchers from the Pacific Northwest National Laboratory (PNNL) in the United States have proposed a novel method to use this energy storage material that has potential applications but is still problematic. Silicon is used in computer chips and many other products. Because it has ten times the lithium storage capacity of graphite anodes, it is considered to be the ideal anode for next-generation lithium-ion batteries. However, silicon anodes involve huge volume changes during lithiation / delithiation, resulting in poor cycle stability and being too weak to withstand the pressure of electrode manufacturing, which restricts the practical application of silicon-based anodes.
To solve these problems, a team led by PNNL researchers Jiguang Zhang (Jason) and Xiaolin Li developed a unique nanostructure that uses carbon materials to limit silicon expansion while strengthening silicon. The research results were published in "Nature Communications", which provides new design ideas for other types of batteries, and ultimately helps to increase the energy capacity of lithium-ion batteries in electric vehicles, electronic devices, and other devices.
Eliminate the disadvantages of silicon
As a conductive and stable carbon, graphite is very suitable for packaging lithium ions into the battery's anode when the battery is charging. Silicon can absorb more lithium than graphite, but its volume expands by 300%, causing the anode to rupture. The researchers made porous silicon by agglomerating small silicon particles into microspheres about 8 microns in diameter-about the size of a red blood cell.
Graphite is a conductive and stable form of carbon, which is very suitable for filling lithium ions into the negative electrode of the battery when charging. Silicon can absorb more lithium than graphite, but its volume tends to expand by 300%, causing the anode to rupture. The researchers made silicon in a porous form by aggregating small silicon particles into microspheres about 8 microns in diameter-about the size of a red blood cell.
Zhang said: "For example, a solid material like stone will break if it expands too much." "What we create is more like a sponge, and there is space inside to absorb expansion."
The study found that the thickness of the electrode with a porous silicon structure changed less than 20%, and at the same time it contained twice the charge of a typical graphite anode. However, unlike the previous version of porous silicon, because the carbon nanotubes make the microspheres similar to yarn balls, the microspheres also show extraordinary mechanical strength.
Super Microsphere
The researchers prepared this structure in several steps: First, the carbon nanotubes were coated with silicon oxide. Next, put the nanotubes in an emulsion of oil and water. Then heat them to boiling.
Li said: "When the water evaporates, the coated carbon nanotubes will condense into a spherical shape." "Then, we use aluminum and higher heat to convert silicon oxide to silicon, and then immerse in water and acid to remove by-products. "The result is a powder consisting of silicon particles on the surface of carbon nanotubes.
Atomic force microscope probes were used to test the strength of porous silicon spheres. The author found that one of the nano-yarn balls "may yield slightly and lose some pores under very high compressive forces, but it will not break."
This heralds the development of commercialization, because the anode material must be able to withstand the high compression of the rollers during the manufacturing process. Zhang said the next step is to develop a more scalable and economical method of manufacturing silicon microspheres so that they can one day be applied to the next generation of high-performance lithium-ion batteries.