UPTON, NY—A team of researchers led by chemists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory has discovered that an electrolyte additive enables stable high-voltage cycling of nickel-rich layered cathodes. Their work could lead to improvements in the energy density of lithium batteries that power electric vehicles.
The results, published on May 9 in Nature Energy, offer a cure for the notorious degradation problems that arise for nickel-rich cathode materials, especially at high voltages. This research was conducted as part of the DOE sponsored program Battery500 Consortium, which is led by DOE’s Pacific Northwest National Laboratory (PNNL) and is working to dramatically increase the energy density of lithium batteries for electric vehicles.
Sha Tan, co-first author and Ph.D. candidate at Stony Brook University conducting research with Brookhaven Lab’s Electrochemical Energy Storage Group, originally studied how an additive, lithium difluorophosphate ( LiPO2F2), could be used to improve the low temperature performance of batteries. Out of curiosity, she tried using the additive for high voltage cycling at room temperature.
“I found that if I pushed the voltage up to 4.8 volts (V), this additive really provided great protection on the cathode and the battery got great cycling performance,” Tan said.
Battery electrode protection
Battery consist of two electrical terminals – electrodes called the cathode and the anode – which are separated by another component of the battery, the electrolyte. The electrons pass through an external circuit connecting the two electrodes and the ions pass through the electrolyte. Both shuttle between electrodes during charge-discharge cycles.
Nickel-rich layered cathode materials promise high energy density for next-generation batteries when paired with lithium metal anodes. But these materials are subject to capacity loss. One of the main problems is particles cracking during high voltage charge-discharge cycles. High voltage operation is important because the total energy stored in a battery, important for vehicle range, 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. It’s called “crosstalk” in the battery community, said Brookhaven chemist Enyuan Hu, who led the research. During high voltage charging, small amounts of transition metals in the cathode crystal lattice dissolve, then pass through the electrolyte and deposit on the anode side. When this happens, the cathode and anode degrade. The result: poor retention of battery capacity.
Researchers have found that introducing a small amount of additive into the electrolyte quells this crosstalk.
As the additive breaks down, it produces lithium phosphate (Li3PO4) and lithium fluoride (LiF) to form a highly protective cathode-electrolyte interphase, a thin solid layer that forms on the cathode of the battery during the cycle.
“By forming a very stable interphase on the cathode, this protective layer significantly suppresses transition metal loss on the cathode surface,” Hu said. “Reducing transition metal losses helps reduce the deposition of these transition metals on the anode. In this sense, the anode is also protected to some extent. We believe that the suppression of transition metal dissolution is one of the main contributors to the significant improvement in cycling performance.
The electrolyte additive allows a nickel-rich layered cathode to be cycled at high voltages to increase energy density while retaining 97% of its original capacity after 200 cycles, the researchers found.
Storage of a polycrystalline solution
But improved performance wasn’t the only exciting result for the researchers, Hu said.
The most common nickel-rich cathode is in the form of polycrystals – aggregates of many nanoscale crystals, also called primary particles, joined together to form a larger secondary particle. Although this promises a relatively easy synthetic route, the polycrystalline nature is commonly blamed for causing particle cracking and eventual capacitor discoloration.
Recent research has indicated that single crystal-based cathodes may be advantageous over their polycrystalline counterparts in suppressing particle crack formation. However, this study suggests that the use of additive engineering can also effectively solve the problem of cracking in polycrystalline materials.
“Our work indicates that polycrystalline materials cannot be excluded from consideration, not least because they are easier to fabricate, which may translate to lower cost.” Hu said.
Tan added, “Our strategy uses a very small amount of additive to achieve such an improvement in electrochemical performance. Concretely, this could be an inexpensive and easy-to-adopt solution. »
Looking ahead, the researchers want to test the additive under harsher conditions to determine if the cathode materials can withstand even more cycles for practical battery use.
To understand how the additive breaks down and protects the cathode surface, the researchers performed a series of synchrotron experiments, Tan said.
Four lines of light National synchrotron-II light source (NSLS-II), a user facility at the DOE Office of Science at Brookhaven that generates ultra-bright X-rays to study the properties of materials at the atomic scale, has played different roles in the research.
The scientists used the Fast absorption and scattering of X-rays (QAS) to understand the dissolution process of transition metals, i.e. how transition metals arrive at the anode side.
They used the Submicron resolution X-ray spectroscopy (SRX) to investigate the effectiveness of the new interphase in suppressing transition metal dissolution by mapping the amount of transition metal deposited on the anode surface. These experiments revealed that the cathode-electrolyte interphase significantly inhibited the migration of transition metals to the anode when the additive was in play.
The researchers also used the In situ and operando soft X-ray spectroscopy (IOS) to characterize the surface of the cathode when the additive is introduced and allows the formation of a robust interphase.
And they used the X-ray powder diffraction (XPD) to look at the crystal structure of the cathode to see if it has changed over several cycles.
Additionally, the team coordinated across time zones with beamline scientists at European synchrotron radiation facility in Grenoble, France. The collaborators used X-rays to examine the morphology and chemistry of thousands of electrode particles, allowing scientists to visualize defects and energy density.
To visualize the evolution of the surface structure of the cathode during the cycle and for computational analysis, the researchers turned to the capabilities of the Brookhaven laboratory. Functional Nanomaterials Center. These imaging and computational studies helped the team pinpoint the additive’s working mechanism, Hu said.
“This project required the perfect combination of advanced techniques and advanced analytics across all facilities to provide crucial insight into the impact of this additive at various levels, from particle to electrode,” Hu said. “Research analysis offers statistically reliable and compelling evidence that it works.”
In addition to Tan, Zulipiya Shadike of the Chemistry Division at Brookhaven Lab and Jizhou Li, a postdoctoral researcher at SLAC National Accelerator Laboratory, are also co-first authors of this research.
The researchers also collaborated with experts from the US Army Research Laboratory, PNNL, Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory and the University of Washington, Seattle.
“With the excellent platform provided by Battery 500, we have a lot of expertise to work with,” said Xiao-Qing Yang, head of the electrochemical storage research group at Brookhaven. “It really is an incredible effort with many other institutions within and outside of the Battery500 consortium.”
This study was supported by DOE’s Office of Energy Efficiency and Renewable Energy (EERE), Office of Vehicle Technologies, and DOE’s Office of Science. Operations at NSLS-II are supported by the Office of Science. The US Army research lab was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub program of the DOE Basic Energy Sciences (BES) program. SLAC’s contributions have been supported by laboratory-directed research and development funds.
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