The electrolyte coupler offers lithium battery performance improvement.

U.S.  A team of researchers led by chemists from the Department of Energy’s (DOE) Brookhaven National Laboratory has learned that the electrolyte coupler allows stable high-voltage cycling of nickel-rich layered cathodes.  Their work could lead to improvements in the energy density of lithium batteries powered by electric vehicles.

  

Co-first author Shah Tan, left and Brookhaven chemist Nguyen Hu at the IOS Beamline at NSLS-II, where the research was conducted.  Credit: Brookhaven National Laboratory

The findings, published in Nature Energy, address the notorious degradation problems of nickel-rich cathode materials, especially at high voltages.  The research was conducted as part of the DOE-sponsored Battery500 Consortium, which is headed by DOE’s Pacific Northwest National Laboratory (PNNL) and is working to significantly increase the energy density of lithium batteries for electric vehicles.

 Shah Tan, co-premiere author and Ph.D.  A candidate at Stony Brook University is researching with the Electrochemical Energy Storage Group at the Brookhaven Lab, originally studying how the composite lithium difluorophosphate (LiPO2F2) can be used to improve the low-temperature performance of batteries.  Out of curiosity, he tried to use the coupler for high voltage cycling at room temperature.

“If I push the voltage to 4.8 volts (V), I find that this coupler really provides better protection over the cathode and the battery achieves excellent cycling performance,” said Tan.

Protecting battery electrodes

Batteries consist of solobaric tricycle terminals — electrodes called cathode and anode — that are separated from another battery unit, electrolyte.  The electrons go through an external circuit that connects the two electrodes and the ions go through the electrolyte.  Both shuttle back and forth between electrodes during charge-discharge cycles.

Nickel-rich layered cathode materials promise higher power density for the next generation of batteries when paired with lithium metal anodes.  But those items are vulnerable to capacity loss.  Particle cracking is a major problem during high-voltage charge-discharge cycles.  High voltage operation is important because the total energy stored in the battery is important to the vehicle range, increasing as the useful operating voltage increases.

Another problem is the dissolution of the transition metal from the cathode and its subsequent deposition at the anode.  It is called “crosstalk” in the battery community, says Nguyen Hu, a research-led Brookhaven chemist.  During high-voltage charging, a small amount of transition metals dissolve in the cathode crystal mesh, and then travel through the electrolyte and deposit on the anode side.  When this happens, both the cathode and the anode degenerate.  Result: Poor battery capacity retention.

Researchers have found that introducing a small amount of additives to the electrolyte prevents it from passing through.

  As the condenser decomposes, it produces lithium phosphate (Li3PO4) and lithium fluoride (LiF) to form a highly protective cathode-electrolyte-interface — a solid thin layer that forms on the battery’s cathode during cycling.

“By forming a completely strong interface at the cathode, this protective layer can widely suppress the loss of transition steel on the cathode surface,” Hu said.  “Reduced transition metal loss helps reduce the storage of those transition metals in the anode. In that sense, the anode is protected to some extent. We believe that suppression of transition metal dissipation is one of the most important contributors to improved cycling performance.”

Researchers have found that the electrolyte coupler enables the nickel-rich layered cathode to rotate at higher voltages to increase energy density and maintain 97 percent of its initial capacity after 200 cycles.

Preserving the polycrystalline solution

But improved performance is not the only exciting outcome for researchers, Hu said.

The most common nickel-rich cathode is in the form of multiple crystals – aggregates of many nanometer-scale crystals, also called primary particles, that come together to form a large secondary particle.  Although this promises a relatively easy synthesis path, the polycrystalline nature usually causes particle cracks and eventually the capacity to fade.

Recent research has suggested that single-crystal-based cathodes may be advantageous over polycrystalline counterparts in suppressing particle crack formation.  However, this study suggests that the use of coupling engineering can effectively solve the crack problem in polycrystalline materials.

“Our paintings are telling us that polycrystalline materials are not excluded from consideration, especially since they are easier to manufacture, which can be translated into lower costs,” Hu said.

“Our technique uses a very small amount of additives to achieve such a great improvement in electrochemical performance. Practically speaking, it is a low-cost and easy-to-implement solution.”

Looking ahead, researchers would like to test the coupler under more challenging conditions to explore whether cathode materials can withstand even longer cycles for practical battery use.

  Advanced analysis

To understand how the additive decomposes and protects the cathode surface, the researchers conducted a series of synchrotron experiments, Tan said.

Four beamlines in the National Synchrotron Light Source-II (NSLS-II), the DOE Office of Science user facility in Brookhaven, which produces ultrabright x-rays to study material properties at the nuclear scale, have played different roles in research.

Scientists used the Quick X-Ray Absorption and Scattering (QAS) beamline to understand the process of transition metal dissolution – how transition metals make it to the anode side.

He studied the effectiveness of the new interface in suppressing the transition metal dissipation by mapping the submicron resolution X-ray spectroscopy (SRX) beamline to how much transition metal is deposited on the anode surface.  These experiments revealed that the cathode-electrolyte-interface coupler significantly inhibited the migration of transition metals to the anode while in play.

Researchers have also used the in situ and operando soft X-ray spectroscopy (iOS) beamline to characterise the cathode surface when the coupler is introduced and the robust interface structure is enabled.

And he used the X-ray powder diffraction (XPD) beamline to see the crystal structure of the cathode, to see if it changed over many cycles.

In addition, the team coordinated with Beamline scientists in time zones at the European Synchrotron Radiation Facility in Grenoble, France.  X-rays were used by colleagues there to look at the morphology and chemistry of thousands of electrode particles, allowing scientists to visualise defects and energy concentrations.

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To illustrate how the surface structure of the cathode evolved during cycling and for computational analysis, the researchers turned to the capabilities of Brookhaven Lab’s Functional Nanomaterials Centre.  Hu said these imaging and computational studies have helped the team identify the mechanism of how the additive works.

“This venture calls for accurate aggregation and superior evaluation of the top strategies across centres to give a key understanding of the impact of this additive at unique stages, from particle to electrode,” Hu said.  “The analysis in the research provides statistically reliable, convincing evidence on how it works.”

Further information: Shaw Tan et al., Composer Engineering for Robust Interfaces for Stabilising High-Ni Layered Structures at Ultra-High Voltage of 4.8 V, Nature Energy (2022).  DOI: 10.1038 / s41560-022-01020-x

Journal Information: Nature Energy

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