A team of researchers led by chemists from the Brookhaven National Laboratory of the US Department of Energy (DOE) found that the addition of electrolyte creates strong (Breakthrough in lithium battery performance) high-voltage bands of nickel-rich cathode layers. Their work can increase the energy density of lithium batteries that produce electric cars.
Findings (Breakthrough in lithium battery performance) published on May 9 at Nature Energy provide a solution to the unusual deposition problems that occur in nickel-rich cathode materials, especially at high voltages. The study was conducted by the DOE-sponsored Battery500 consortium, led by the Pacific Northwest National Laboratory DOE (PNNL), and is working to increase the energy efficiency of lithium batteries for electric vehicles.
Sha Tan, co-author, and Ph.D. A candidate at Stony Brook University who studies electrochemical energy storage at Brookhaven Lab originally studied how the additive, lithium difluoro phosphate (LiPO2F2), can be used to improve battery (Breakthrough in lithium battery performance) performance at low temperatures. Out of curiosity, he tried to use the additive for high-voltage tapes at room temperature.
“I found that when I compressed the voltage to 4.8 volts (V), this additive provided great cathodic protection and the battery achieved very good cyclic performance,” Tan said.
Battery electrode protection
The batteries consist of two electrical terminals – electrodes called cathodes and anodes – which are separated by another part of the battery, the electrolyte. Electrons pass through an external circuit that connects the two electrodes and ions pass through the electrolyte. Both move back and forth between the electrodes during charge and discharge cycles.
Rich nickel-plated cathode materials promise high energy densities for next-generation batteries in conjunction with lithium metal anodes. However, these materials are rapidly losing capacity. One of the main problems is the cracking of particles during high voltage charging and discharging cycles. High voltage operation is important because the total energy stored in the battery, important for the size of the car, increases with increasing useful operating voltage.
Another problem is the decay of the transition metal from the cathode and the subsequent deposition of the anode. According to Brookhaven chemist Enyuan Hua, who is leading the study, he is known in the battery community as “crosstalk.” During high-voltage charging, a small amount of transition metals melts in the crystal lattice of the cathode, which then passes through the electrolyte and settles on the anode part. If this happens, the cathode and anode will be damaged. The result: poor battery life. The researchers found that this prevented them from introducing a small amount of crosstalk with the addition of electrolyte.
Without the additive, it produces lithium phosphate (Li3PO4) and lithium fluoride (LiF), which form a more protective cathode-electrolyte interface – a solid thin film formed by the cathode of the battery during cycling. . .
“By creating a strong cathode interface, this protective layer significantly prevents the loss of metal moving along the cathode,” Hu said. “Reducing the loss of the metal transition will help reduce the location of the anode transition metals. In this sense, the anode is also partially protected. We believe that suppressing the transmission of metal decomposition is one of the most important factors that contribute to better driving habits.
The electrolyte additive allows the cathode to cycle the nickel layer at high voltage to increase energy density and still retain 97 percent of its original capacity after 200 cycles, the researchers found.
Store in polycrystalline solution
But improving performance is not the only exciting result for scientists, Hu said.
The most common nickel-rich cathode is in the form of polycrystalline aggregates of several nanometer-scale crystals, also called primary particles, which combine to form larger secondary particles. Although it promises a relatively fast route of synthesis, its polycrystalline nature is often blamed for particle cracking and possible capacity loss. Recent research has shown that a crystalline cathode can be more useful than polycrystalline counterparts in inhibiting particle cracking. However, this study suggests that the use of additive engineering can also effectively solve the cracking problem of polycrystalline materials.
“Our work says that polycrystalline materials are unimaginable, especially because they are easier to produce, which can be translated at a lower cost.” and Hu.
Tan added: “Our strategy uses a very small number of additives to achieve such a large improvement in electrochemical performance. In practice, this can be a cheap and easy solution.”
In the future, researchers want to test the additive under harsher conditions to see if cathode materials can have multiple cycles for practical battery use. Advanced analysis
To understand how the additive lacks and protect the cathode surface, the researchers conducted a series of synchrotron experiments, Tan said.
The four beams of the National Synchrotron Light Source-II (NSLS-II), a user installation of the DOE Office of Science in Brookhaven that produces ultra-bright X-rays to study the properties of materials at the atomic scale, have different roles in research.
Researchers are using the Quick X-ray Absorption and Scattering (QAS) beam to understand the process of decomposition of transition metals – how it works in transition metals in the anode phase.
They used submicron resolution (SRX) X-ray spectroscopy to study the effectiveness of the new interface in suppressing the transition metal solution by mapping how much transition metal is deposited on the anode surface. These experiments show that the cathode-electrolyte interface significantly prevents the metal from migrating to the anode when the additive is in play. The researchers also used In Situ and Operando Soft X-ray Spectroscopy (IOS) to identify the cathode surface as it introduced additives and could create a robust interface.
And they used an X-ray powder diffraction (XPD) beam to look at the crystal structure of the cathode to determine if it changed over several cycles.
In addition, the team coordinates scientists’ time zones at the European Synchrotron Radiation Facility in Grenoble, France. Staff there used X-rays to look at the morphology and chemistry of thousands of electrode particles, allowing scientists to visualize defects and energy densities.
To illustrate how the structure of the cathode surface develops during cycling and for computer analysis, the researchers looked at the possibilities of the Brookhaven Lab Functional Nanomaterials Center. These imaging and computational studies helped the team identify the mechanism by which the additive works, Hu said.
“This project requires a perfect combination of advanced techniques and advanced analysis on equipment in order to critically understand the impact of this additive at various levels, from particles to electrode,” Hu said. “Research analysis provides statistically reliable and convincing evidence of how it works.”
In addition to Tan, Zulipiya Shadike of Brookhaven Lab’s Chemistry Division and Jizhou Li, a postdoctoral fellow at SLAC National Accelerator Laboratory, also co-authored the study.
The researchers also collaborated with experts from U.S. Pat. Army Research Laboratory, PNNL, Stanford Synchrotron Radiation Light source, SLAC National Accelerator Laboratory, and the University of Washington, Seattle. “With the excellent platform provided by Battery500, we have a lot of skills at our disposal,” said Xiao-Qing Yang, head of the Brookhaven Electrochemical Storage Research Group. “This is a unique effort by many other institutions within and outside the Battery500 consortium.”
