High-nickel-content cathodes offer high capacity, a chemical property that could power electric vehicles over much longer distances than current batteries support. Unfortunately, the high nickel content also causes these cathode materials to degrade more quickly, creating cracks and stability issues as the battery cycles.
In search of solutions to these structural problems, scientists have synthesized materials made with a nickel concentration gradient, in which the concentration of nickel gradually changes from the surface of the material to its center, or the bulk. These materials have exhibited greatly enhanced stability, but scientists have not been able to determine if the concentration gradient alone was responsible for the improvements.
The concentration gradient has traditionally been inseparable from another effect called the valence gradient, or a gradual change in nickel’s oxidation state from the surface of the material to the bulk.
In a new study led by Brookhaven Lab, chemists at DOE’s Argonne National Laboratory synthesized a unique material that isolated the valence gradient from the concentration gradient. An open-access paper on the work is published in Nature Communications.
In this work, to pinpoint the surface stabilization mechanism(s) for high-nickel-content materials and to find approaches to optimize the material, we synthesize and investigate a model system that has a valence-gradient hierarchical secondary particle architecture. Different from concentration-gradient or core–shell-structured NMC materials where Mn/Ni concentration varies throughout the particle, our newly designed high-Ni-content cathode material has a uniform Ni:Mn:Co = 8:1:1 chemical composition throughout the secondary particle.
However, in this architecture, there is a nickel-valence-state gradient along the radial direction of the secondary particle. In this architecture, for the discharged state, a large amount of Ni ions at the surface and in the near-surface region (~ top 900-nm surface region) are at the Ni2+ state correlated with oxygen deficiency, while in the center bulk are at closer to the Ni3+ state. Using this model structure for our mechanistic study, we are able to separate the effects of the nickel-valence state from nickel concentration. Electrochemical testing shows that, with the help of Ni-valence gradience only, our valence-gradient LiNi0.8Mn0.1Co0.1O2 (VG-NMC811) shows improved capacity retention than the conventional NMC811 material.—Lin et al.
We used a very unique material that included a nickel valence gradient without a nickel concentration gradient. The concentration of all three transition metals in the cathode material was the same from the surface to the bulk, but the oxidation state of nickel changed. We obtained these properties by controlling the material’s atmosphere and calcination time during synthesis. With sufficient calcination time, the stronger bond strength between manganese and oxygen promotes the movement of oxygen into the material’s core while maintaining a Ni2+ oxidation state for nickel at the surface, forming the valence gradient.—Brookhaven chemist Ruoqian Lin, first author
Once the chemists successfully synthesized a material with an isolated valence gradient, the Brookhaven researchers then studied its performance using two DOE Office of Science user facilities at Brookhaven Lab—the National Synchrotron Light Source II (NSLS-II) and the Center for Functional Nanomaterials (CFN).
At NSLS-II, an ultrabright x-ray light source, the team leveraged two cutting-edge experimental stations, the Hard X-ray Nanoprobe (HXN) beamline and the Full Field X-ray Imaging (FXI) beamline. By combining the capabilities of both beamlines, the researchers were able to visualize the atomic-scale structure and chemical makeup of their sample in 3-D after the battery operated over multiple cycles.
At the CFN Electron Microscopy Facility, the researchers used an advanced transmission electron microscope (TEM) to visualize the sample with ultrahigh resolution. Compared to the x-ray studies, the TEM can only probe a much smaller area of the sample and is therefore less statistically reliable across the whole sample, but in turn, the data are far more detailed and visually intuitive.
By combining the data collected across all of the different facilities, the researchers were able to confirm the valence gradient played a critical role in battery performance. The valence gradient “hid” the more capacitive but less stable nickel regions in the center of the material, exposing only the more structurally sound nickel at the surface. This important arrangement suppressed the formation of cracks.
The researchers say this work highlights the positive impact concentration gradient materials can have on battery performance while offering a new, complementary approach to stabilize high-nickel-content cathode materials through the valence gradient.
This study was a collaborative effort among several universities and DOE laboratories, including research teams involved in DOE’s Battery500 Consortium, which aims to make lithium-metal battery cells with an energy density of 500 watt-hours per kilogram, more than double the energy density of today’s state-of-the-art batteries. The research was supported by DOE’s Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office and DOE’s Office of Science.
Additional x-ray experiments were carried out at the Advanced Light Source (ALS) and the Advanced Photon Source (APS), two DOE Office of Science user facilities that are located at DOE’s Lawrence Berkeley National Laboratory and Argonne National Laboratory, respectively. Operations at NSLS-II, CFN, ALS, and APS are supported by the Office of Science.
Lin, R., Bak, SM., Shin, Y. et al. (2021) “Hierarchical nickel valence gradient stabilizes high-nickel content layered cathode materials.” Nat Commun 12, 2350 doi: 10.1038/s41467-021-22635-w