Batteries are everyday objects people don't think about -- until they run out of juice. That's especially true the more ubiquitous an object is, such as laptop computers and cellphones that need to have their batteries charged seemingly every day.
But Missouri University of Science and Technology researchers are working to solve the problem of short-life of lithium-ion batteries like those used in laptops and cellphones, making them reliable and longer-lasting using a thin-film coating technique called atomic layer deposition (ALD). Their paper, titled "Employing Synergetic Effect of Doping and Thin-Film Coating to Boost the Performance of Lithium-Ion Battery Cathode Particles," is being published in Scientific Reports, a Nature publishing group journal.
Dr. Xinhua Liang, assistant professor of chemical and biochemical engineering at Missouri S&T, leads the study to dope and coat lithium magnesium nickel oxygen (LMNO) with iron oxide through ALD -- at the same time. Doping means adding an element or compound into the crystalline structure, or lattice, filling in the gaps in the LMNO. Coating is what it sounds like, putting ultra-thin layers of iron oxide around the whole compound. Rajankumar Patel, a Missouri S&T Ph.D. candidate in chemical engineering who will graduate next week, did the majority of the experimental work in the project.
The operating voltage window of LMNO makes it a potential candidate for use in hybrid electric vehicles (HEV). However, it has not gained commercial usability in HEV because of high-capacity fade during cycling at elevated temperatures and manganese(3+) dissolution by hydrogen fluorine.
"Unlike current research practice that either covers the particles' surface with insulating film or dopes the particles to improve the performance of the battery," Liang says, "this ALD process combines the coating and doping processes into one, and applying this technique makes rechargeable lithium-ion batteries last longer."
"This is the first report for a unique phenomenon of ionic iron entering the lattice structure of LMNO during the ALD coating process," Patel says.
More important than the manufacturing aspect is the batteries' ability to deliver a sustained performance over a long period of time, which is a big benefit to the consumer. The Missouri S&T process makes lithium-ion batteries that have 93 percent capacity retention after 1,000 cycles of charge and discharge at room temperature and 91 percent at elevated temperatures. That is equivalent to about three years of battery life with almost same performance as of a new battery, Liang says.
The work is done with an ALD reactor system that the S&T researchers built, and the coating process is carried out at 450 degrees Celsius under reduced pressure. The materials are placed inside a fluidized bed reactor, and vibrating motors shake the reactor to improve the mixing of particles and gaseous chemicals. Patel and his colleagues found that 30 to 40 cycles -- resulting in about 0.6-nanometer coatings -- produced the best results.
The ionic iron penetration into the lattice structure of LMNO was verified by cross-sectional scanning transmission electron microscope equipped with an electron energy dispersive spectroscope (STEM-EDS) of iron oxide coated samples, and the ultra-thin iron oxide films were directly observed by a transmission electron microscope.
Finally, they make coin-cell batteries, charging and discharging dozens at a time at various testing conditions to measure and ensure that the results hold up over time. ALD has the potential to prepare these ultra-thin electrochemically active films with optimal thickness and synergetic effect of conductive coating and element doping, providing the industry with novel-design electrodes that are durable as well as functional at high temperature and fast cycling rates.
With all their work, the exact phenomena behind the process that produces better lithium-ion battery characteristics is an ongoing study.
"That's still kind of a mystery for us," Liang says. "Our focus is to understand better how that process happens and to control it."
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