J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

/pdf/ultrawide-bandgap-semiconductors-research-opportunities-and-49ev1zu2ym.pdf

Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

The surface coating of cathodes using insulator films has proven to be a promising method for high-voltage cathode stabilization in Li-ion batteries, but there is still substantial uncertainty about how these films function. More specifically, there is limited knowledge of lithium solubility and transport through the films, which is important for coating design and development. This study uses first-principles calculations based on density functional theory to examine the diffusivity of interstitial lithium in the crystals of α-AlF3, α-Al2O3, m-ZrO2, c-MgO, and α-quartz SiO2, which provide benchmark cases for further understanding of insulator coatings in general. In addition, we propose an ohmic electrolyte model to predict resistivities and overpotential contributions under battery operating conditions. For the crystalline materials considered we predict that Li+ diffuses quite slowly, with a migration barrier larger than 0.9 eV in all crystalline materials except α-quartz SiO2, which is predicted to have a migration barrier of 0.276 eV along 〈001〉. These results suggest that the stable crystalline forms of these insulator materials, except for oriented α-quartz SiO2, are not practical for conformal cathode coatings. Amorphous Al2O3 and AlF3 have higher Li+ diffusivities than their crystalline counterparts. Our predicted amorphous Al2O3 resistivity (1789 MΩ m) is close to the top of the range of the fitted resistivities extracted from previous experiments on nominal Al2O3 coatings (7.8 to 913 MΩ m) while our predicted amorphous AlF3 resistivity (114 MΩ m) is very close to the middle of the range. These comparisons support our framework for modeling and understanding the impact on overpotential of conformal coatings in terms of their fundamental thermodynamic and kinetic properties, and support that these materials can provide practical conformal coatings in their amorphous form.

/pdf/lithium-transport-through-lithium-ion-battery-cathode-2zln4hb9da.pdf

Lithium transport through lithium-ion battery cathode coatings

The atomic layer deposition (ALD) of AlF3 was demonstrated using trimethylaluminum (TMA) and hydrogen fluoride (HF). The HF source was HF-pyridine. In situ quartz crystal microbalance (QCM), quadrupole mass spectrometer (QMS), and Fourier transform infrared (FTIR) spectroscopy measurements were used to study AlF3 ALD. The AlF3 ALD film growth was examined at temperatures from 75 to 300 °C. Both the TMA and HF reactions displayed self-limiting behavior. The maximum mass gain per cycle (MGPC) of 44 ng/(cm2 cycle) for AlF3 ALD occurred at 100 °C. The MGPC values decreased at higher temperatures. The MGPC values were negative at T > 250 °C when TMA and HF were able to etch the AlF3 films. Film thicknesses were also determined using ex situ X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE) measurements. The AlF3 ALD growth rate determined by the ex situ analysis was 1.43 A/cycle at 100 °C. These ex situ measurements were in excellent agreement with the in situ QCM measurements. FTIR analysis monitor...

Atomic Layer Deposition of AlF3 Using Trimethylaluminum and Hydrogen Fluoride

The surface coating of cathodes using insulator films has proven to be a promising method for high-voltage cathode stabilization in Li-ion batteries. However, there is still substantial uncertainty about how these films function, specifically with regard to important coating design principles such as lithium solubility and transport through the films. This study uses Density Functional Theory to examine the diffusivity of interstitial lithium in crystalline {\alpha}-$AlF_3$, {\alpha}-$Al_2O_3$, m-$ZrO_2$, c-MgO, and {\alpha}-quartz $SiO_2$, which provide benchmark cases for further understanding of insulator coatings in general. In addition, we propose an Ohmic electrolyte model to predict resistivities and overpotential contributions under battery operating conditions. For the crystalline materials considered we predict that Li+ diffuses quite slowly, with a migration barrier larger than 0.9 eV in all crystalline materials except {\alpha}-quartz $SiO_2$, which is predicted to have a migration barrier of 0.276 eV along . These results suggest that the stable crystalline forms of these insulator materials, except for oriented {\alpha}-quartz $SiO_2$, are not practical for conformal cathode coatings. Amorphous $Al_2O_3$ and $AlF_3$ have higher Li+ diffusivities than their crystalline counterparts. Our predicted amorphous $Al_2O_3$ resistivity (1789 M{\Omega}m) is near the top of the range of fitted resistivities extracted from previous experiments on nominal $Al_2O_3$ coatings (7.8 to 913 M{\Omega}m) while our predicted amorphous $AlF_3$ resistivity (114 M{\Omega}m) is close to the middle of the range. These comparisons support our framework for modeling and understanding the impact on overpotential of conformal coatings in terms of their fundamental thermodynamic and kinetic properties, and support that these materials can provide practical conformal coatings in their amorphous form.

/pdf/lithium-transport-through-lithium-ion-battery-cathode-3awdyjiw9j.pdf

Lithium transport through Lithium-ion battery cathode coatings

Freestanding LiCoO2/multiwall carbon nanotube/nanocellulose fibril (LCO-MWCNT-NCF) electrodes are fabricated by a vacuum filtration technique. The electrode has a high LCO loading of 20 mg/cm2 with excellent flexibility, uniform material distribution, and low surface resistivity. When coated with 2 ALD cycles of AlF3, LCO-MWCNT-NCF has a high specific capacity of 216 mAh/g at 4.7 V. The freestanding AlF3-coated electrode preserves 75.7% of its initial capacity after 100 cycles and 70% after 160 cycles of charge discharge. In contrast, electrodes coated with 2 ALD cycles of Al2O3 cannot be cycled above 4.5 V. By elimination of the unnecessary weight of current collector, and increasing in the working voltage simultaneously, this freestanding LCO-MWCNT-NCF electrode can significantly improve the gravimetric and volumetric energy density of lithium ion batteries.

Coating Solution for High-Voltage Cathode: AlF3 Atomic Layer Deposition for Freestanding LiCoO2 Electrodes with High Energy Density and Excellent Flexibility.

The material system aluminum fluoride-silicon oxide-silicon (AlF3∕SiO2∕Si) can localize a high density of fixed negative charges in fluorine (F) vacancies within AlF3, serving as a source for a strong drift field. This assignment is corroborated by a determination of the stoichiometry with Rutherford backscattering techniques and the transient response of the negatively charged defects with deep level transient spectroscopy. From the density-functional calculations of stoichiometric and understoichiometric AlF3, we deduce a microscopic picture where the energetic eigenstates of the the singly and doubly occupied F vacancies surround the Fermi energy. The lineup of the band edges of the system AlF3∕SiO2∕Si is determined by self-consistent electrostatic calculations including local charges, resulting in excellent agreement with the energetics deduced from the transient response of the charged defects.

/pdf/band-diagram-of-the-alf3-sio2-si-system-4vdnnjl6jg.pdf

Band diagram of the AlF3/SiO2/Si system

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Band diagram of the AlF3/SiO2/Si system