Electrolyte additive leads to a protective surface layer for nickel-rich cathodes, improving battery performance at high voltages
Courtesy of Brookhaven National Laboratory.
Written by Kelly Zegers
UPTON, NY – A team of researchers led by chemists at the National Laboratory in Brookhaven of the US Department of Energy (DOE) has learned that the electrolyte additive enables stable high-voltage circulation of nickel-rich layered cathodes. Their work could lead to an improvement in the energy density of lithium batteries powered by electric vehicles.
Findings, published on May 9 The energy of nature, offer a cure for the notorious degradation problems that occur for nickel – rich cathode materials, especially at high voltages. This research was conducted as part of the DOE-sponsored Battery500 consortium, led by DOE’s Pacific Northwest National Laboratory (PNNL) and working to significantly increase the energy density of lithium batteries for electric vehicles.
Sha Tan, first author and others. A candidate at Stony Brook University, who conducted research with the electrochemical energy storage group at the Brookhaven Laboratory, originally studied how an additive, lithium difluorophosphate (LiPO2F2), could be used to improve battery performance at low temperatures. Out of curiosity, she tried to use an additive for high-voltage cycling at room temperature.
“I found that if I increased the voltage to 4.8 volts (V), this additive really provides excellent protection over the cathode, and the battery has achieved excellent cycle performance,” said Tan.
Battery protective electrodes
Batteries consist of two electrical terminals – electrodes called cathodes and anodes – which are separated by another component of the battery, the electrolyte. Electrons pass through the outer circuit that connects the two electrodes, and ions pass through the electrolyte. Both move back and forth between the electrodes during the charge-discharge cycle.
Nickel-rich layered cathode materials promise high energy density for next-generation batteries when paired with lithium metal anodes. But these materials are prone to loss of capacity. One of the main problems is the cracking of particles during high-voltage charge-discharge cycles. Working at high voltage is important because the total energy stored in the battery, important for the range of the vehicle, increases as the useful operating voltage increases.
Another problem is the dissolution of the transition metal from the cathode and its subsequent deposition on the anode. This is known as “listening” to the battery community, said Brookhaven chemist Enyuan Hu, who led the research. During high-voltage charging, small amounts of transition metals in the crystal lattice of the cathode dissolve, then travel through the electrolyte and deposit on the anode side. When this happens, both the cathode and the anode degrade. The result: poor retention of battery capacity.
Researchers have found that introducing a small amount of an additive into an electrolyte suppresses overheating.
As the additive decomposes, it produces lithium phosphate (Li3PO4) and lithium fluoride (LiF) to form a highly protective cathode-electrolyte-interphase – a solid thin layer that forms on the cathode of the battery during the cycle.
“By forming a very stable intermediate phase on the cathode, this protective layer significantly suppresses the loss of transition metal on the cathode surface,” Hu said. “Reduced loss of transition metals helps to reduce the deposition of these transition metals on the anodes. In this sense, the anode is to some extent protected. We believe that combating the dissolution of transition metals is one of the key factors leading to significantly improved cycle performance. ”
The addition of electrolyte allows the layered nickel-rich cathode to rotate at high voltages to increase energy density and maintain 97 percent of its initial capacity after 200 cycles, the researchers found.
Preservation of polycrystalline solution
But improved performance was not the only exciting result for the researchers, Hu said.
The most common nickel-rich cathode is in the form of polycrystals – aggregates of many nanometer crystals, also known as primary particles, joined together to form a larger secondary particle. Although this promises a relatively easy synthesis pathway, polycrystalline nature is usually blamed for causing particle cracks and ultimate fading capacities.
Recent research has shown that single-crystal cathodes may have an advantage over polycrystalline counterparts in suppressing the formation of particle cracks. However, this study suggests that the use of additive engineering can also effectively solve the problem of cracking in polycrystalline materials.
“Our work shows that polycrystalline materials cannot be excluded from consideration, especially because they are easier to make, which can be translated into a lower price,” Hu said.
Tan added: “Our strategy uses a very small amount of additives to achieve such a large improvement in electrochemical performance. Practically speaking, this could be a cheap solution that is easy to adopt. ”
Looking ahead, researchers want to test the additive in more demanding conditions to investigate whether cathode materials can withstand even more cycles for practical battery use.
To understand how the additive degrades and protects the cathode surface, researchers conducted a series of synchrotron experiments, Tan said.
Four beams in the National Synchrotron Light Source-II (NSLS-II), a user facility of the DOE Office of Science in Brookhaven that generates ultra-bright X-rays to study the properties of materials on the atomic scale, played different roles in the research.
Scientists used the line of rapid absorption and scattering of X-rays (QAS) to understand the process of dissolving transition metals – how transition metals reach the anode side.
They used the submicron X-ray spectroscopy (SRX) line to study the efficiency of the new intermediate phase in suppressing the dissolution of the transition metal by mapping how much transition metal is deposited on the anode surface. These experiments found that the cathode-electrolyte-interphase significantly prevented transition metals from migrating to the anode when the additive was in play.
The researchers also used the In Situ beam line and Operando Soft X-ray Spectroscopy (IOS) to characterize the cathode surface when the additive is introduced and allow the formation of a robust intermediate phase.
And they used the X-ray powder diffraction (XPD) line to look at the crystal structure of the cathode to see if it had changed in multiple cycles.
In addition, the team coordinated in different time zones with beamline scientists at the European Synchrotron Radiation Plant in Grenoble, France. Collaborators there used X-rays to look at the morphology and chemistry of thousands of electrode particles, allowing scientists to visualize defects and energy density.
To record how the surface structure of the cathode evolved during the cycle and for computer analysis, the researchers turned to the possibilities at the Brookhaven Lab Center for Functional Nanomaterials. These imaging and computer studies helped the team identify the mechanism by which the additive works, Hu said.
“This project required a perfect combination of advanced techniques and advanced analysis in all plants to provide a key insight into the impact of this additive at different levels, from particles to electrodes,” Hu said. “Analysis in research offers statistically reliable, convincing evidence of how it works.”
In addition to Tan, Zulipiya Shadike, of the Department of Chemistry at Brookhaven Lab, and Jizhou Li, a postdoctoral fellow at SLAC National Accelerator Laboratory, are also co-authors of this study.
The researchers also collaborated with experts from the U.S. Army Research Laboratory, PNNL, Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, and the University of Washington, Seattle.
“With the great platform that the Battery 500 provides, we have a lot of expertise to work with,” said Xiao-Qing Yang, leader of the Electrochemical Storage Research Group in Brookhaven. “This is a truly amazing effort with many other institutions inside and outside the Battery500 consortium.”
This study was supported by the Office of Energy Efficiency and Renewable Energy DOE (EERE), the Office of Vehicle Technology and the Office of Science DOE. Work in NSLS-II is supported by the Office of Science. The U.S. Army Research Laboratory is supported by the Joint Energy Storage Research Center, DOE Basic Energy Sciences (BES) Energy Innovation Hub. SLAC contributions are supported by laboratory research and development funds.
The Brookhaven National Laboratory is supported by the U.S. Department of Energy’s Office of Science. The Office of Science is the largest advocate of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.
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