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Long-Life, Ultrahigh-Nickel Cathodes with Excellent Air Storage Stability for High-Energy Density Lithium-Based Batteries
With increased energy density and cost advantage, ultrahigh-nickel layered oxides (LiNi x M 1-x O 2 , x = 0.9 - 1.0) are becoming front-runner as cathodes for next-generation lithium-based batteries, yet their commercialization is blocked both by severe capacity fade and exponentially aggravated air degradation. Thus, it is imperative to find effective solutions to address these issues simultaneously. Here, a significant enhancement in both the cycling and air storage stability of the ultrahigh-Ni cathode LiNi 0.94 Co 0.06 O 2 is achieved via a distinctive phosphoric acid treatment strategy. The modified cathode displays remarkably improved capacity retention (from 36 % to 80 % after 1,000 cycles) and rate capability (from 0 to 105 mA h g -1 at 30C rate) in pouch cells. Impressively, the modified cathode, after air storage for 450 days, maintains the morphology and 92% of the initial capacity of the fresh sample with excellent cyclability. Comprehensive interphase and structural analyses reveal that the enhanced electrochemical performance is due to a highly stabilized electrode/electrolyte interphase that suppresses electrode corrosion and lattice reconstruction. The excellent air stability results from an adsorption-buffering effect enabled by phosphoric acid to air attack. Finally, the study demonstrates an engineering pathway to improve the cycling and air stability, facilitating the practical viability of high-capacity, affordable, ultrahigh-Ni cathodes in lithium-based batteries.
A Mg-Doped High-Nickel Layered Oxide Cathode Enabling Safer, High-Energy-Density Li-Ion Batteries
High-nickel layered oxide cathodes with a Ni content of > 90% show substantial potential for next-generation lithium-ion batteries (LIBs) due to their high capacity and lower cost. However, they are plagued by rapid capacity decay and poor thermal stability, which hamper their practical viability. Here we present Li 0.98 Mg 0.02 Ni 0.94 Co 0.06 O 2 (NC-Mg) with 2% Mg doping, aiming to provide a strategic guideline for solving the issues. The Mg 2+ ions occupy the lithium layer and are pro-posed to act as pillar ions, which substantially enhances the structural reversibility and reduces the anisotropic lattice distortion upon cycling, thereby greatly improving the electrochemical and thermal stability of NC-Mg compared to the undoped LiNi 0.94 Co 0.06 O 2 (NC). Specifically, NC-Mg delivers 214 mA h g -1 with a capacity retention of 80.1% after 500 cycles in pouch-type full cells, much higher than the retention of NC (56.3%). A discharge capacity of 158 mA h g -1 at 10C rate demonstrates its remarkable rate capability. Additionally, the Mg doping significantly elevates the exothermic peak tem-perature of NC-Mg to 211 °C, in sharp contrast to 177 °C for NC, highlighting the improved thermal stability of NC-Mg. Collectively, the superior performance of NC-Mg demonstrates a feasible alternative strategy for developing safer, high-energy-density LIBs.
Pseudodiagonalization Method for Accelerating Nonlinear Subspace Diagonalization in Density Functional Theory
In density functional theory, each self-consistent field (SCF) nonlinear step updates the discretized Kohn-Sham orbitals by solving a linear eigenvalue problem. The concept of pseudodiagonalization is to solve this linear eigenvalue problem approximately, and specifically utilizing a method involving a small number of Jacobi rotations that takes advantage of the good initial guess to the solution given by the approximation to the orbitals from the previous SCF iteration. The approximate solution to the linear eigenvalue problem can be very rapid, particularly for those steps near SCF convergence. Here, we adapt pseudodiagonalization to finite-temperature and metallic systems, where partially-occupied orbitals must be individually resolved with some accuracy. We apply pseudodiagonalization to the subspace eigenvalue problem that arises in Chebyshev-filtered subspace iteration. In tests on metallic and other systems for a range of temperatures, we show that pseudodiagonalization achieves similar rates of SCF convergence to exact diagonalization.
Density Functional Theory Investigation of Oxidation Intermediates on Gold and Gold–Silver Surfaces
Gold and gold-silver alloys can be active and selective oxidation catalysts. Previous work has suggested that O 2 dissociation occurs at bimetallic step sites on gold-silver alloys, but the site responsible for the rest of the reaction steps has not been studied. As a first step in gaining insight into this issue, in this work we investigated the adsorption of oxygen and other oxidation intermediates on the (111) and (211) facets of gold-silver alloys using density functional theory. Oxygen and silver coverage effects were analyzed, and different model structures were compared. We also examined the energy barriers for the diffusion of atomic oxygen to gain insight into O migration and spillover. On (111) surfaces, O adsorption is much stronger at low O coverage (less than 0.22 ML), while on (211) surfaces O is strongly bound at both high and low O coverage. O diffusion across the step is faster than diffusion along the step. Ag stabilizes O, both when directly bound to it and when in an adjacent site. Ag also reduces repulsive O-O interactions at low O coverage. Our calculated reaction barriers for O-assisted CH 3 O dehydrogenation suggest that reaction is faster on steps than on terraces. Overall, our findings suggest that spillover of O from Ag-rich steps to Au-rich terraces does not occur, and that oxidation reactions on gold-silver alloys occurs on step sites. More specifically, oxidation likely occurs either on Ag-rich step sites or on Au-rich step sites that are adjacent to Ag-rich step sites.
Density Functional Theory Study of the Adsorption and Dissociation of Copper(I) Acetamidinates on Ni(110): The Effect of the Substrate
In order to assess the role of a metal substrates on the thermal chemistry of adsorbed acetamidinate metalorganic compounds, we have studied the surface chemistry of copper(I)–N,N'-dimethylacetamidinate on Ni(110) using density functional theory and contrasted it with similar calculations we previously carried out on Cu(110). At low coverages, it was found that, in its most stable configuration, the molecular adsorption of copper(I)–N,N'-dimethylacetamidinate dimers occurs with the Cu atoms occupying surface hollow sites. In this work, the ligands reorient away from those metal centers, and the N atoms develop new direct bonds with surface Ni atoms. However, this configuration is not stable and decomposes by losing both ligands to the surface. In the final state, the two ligands bind via their N atoms to Ni sites one lattice space away from the sites where the Cu atoms remain, with their molecular planes perpendicular to that of the surface. This is in contrast with what happens in the case of adsorption on Cu(110), where the Cu atoms from the metalorganic complex still occupy hollow sites but where only one of the ligands breaks away and binds directly to the surface; the other remains on top of the two Cu ions. In terms of the energetics of adsorption and decomposition, the reactions on Ni(110) are much more exothermic than on Cu(110). Further analysis of the distribution of charge within the adsorbates shows a minor reduction of the Cu atoms of the dimer upon interaction with the surface; full reduction to metallic copper is complete only when both ligands have fully migrated to their new Ni surface sites.
Adsorption Properties of Au−Ni Surface Alloys with a Nonstoichiometric Moiré Structure: A Density Functional Theory Study
Due to the large lattice mismatch between gold and nickel, gold–nickel surface alloys can form unique nonstoichiometric overlayer structures characterized by a moiré pattern and subsurface defects. For this work, we performed density functional theory (DFT) calculations to study the adsorption of molecular oxygen, atomic hydrogen, and atomic carbon on a gold–nickel(111) surface alloy with 0.46 monolayer gold randomly distributed in the surface layer. We observed six distinct adsorption structures for molecular oxygen characterized by intramolecular stretching frequencies of <700, 729, 795, 857, 929, and 1004 cm –1 , which describe well the experimentally observed high-resolution electron energy-loss spectra. Surface atomic hydrogen adsorption is associated with adsorbate–surface modes in the ∼1000 cm –1 range, while subsurface hydrogen can have features as low as ∼400 cm –1 . We observed a unique adsorption structure for atomic carbon inside the surface dislocation loop defect, which explains the experimentally observed low carbon-surface mode at ∼340 cm –1 . Our study sheds light on the unique adsorption properties of the gold–nickel surface alloys and helps with rationalizing vibrational frequency experimental studies for this system.
Enhanced Density-of-States Effective Mass and Strained Endotaxial Nanostructures in Sb-Doped Pb 0.97 Cd 0.03 Te Thermoelectric Alloys
Here we report that CdTe alloying and Sb doping increase the density-of-states effective mass and introduce endotaxial nanostructuring in n-type PbTe, resulting in enhanced thermoelectric performance. A prior theoretical prediction for the presence of resonance states in the conduction band of this system, however, could not be confirmed. An amount of 3 mol % CdTe alloying widens the band gap of PbTe by 50%, leading to enhanced carrier effective mass and Seebeck coefficient. This effect is even more pronounced at high temperatures where the solubility of CdTe increases. At 800 K, when the carrier concentration is the same (4 × 10 19 cm -3 ), the Seebeck coefficient of CdTe-alloyed PbTe is -195 μV K -1 , 16% higher than that of the Cd-free control sample (-168 μV K -1 ). Sb doping considerably increases the electron concentration of Pb 0.97 Cd 0.03 Te, giving rise to optimized power factors of ~17 μW cm -1 K -2 at 800 K. More importantly, Sb induces strained endotaxial nanostructures evenly distributed in the matrix. Furthermore, these Sb-rich nanostructures account for the ~40% reduction in the lattice thermal conductivity over the whole measured temperature range. As a result, a maximum ZT of 1.2 is attained at 750 K in 0.5 mol % Sb-doped Pb 0.97 Cd 0.03 Te alloys.
Virgin-like High-Density Polyethylene from Recycled Mixed Polyolefins
Reported herein is the compatibilization of mixed polyolefins containing a rheology modifier (M-PO RM ) derived from municipal solid waste (MSW) by using different types of compatibilizers. One compatibilizer was produced “in situ” during reactive extrusion (REX), and the other compatibilizers were commercial styrenic block copolymer and maleated polymer-based compatibilizers. Both “in situ” and commercial compatibilizers were first used in laboratory-controlled polyolefins containing a rheology modifier (denoted as L-PO RM ) and were investigated for their mechanical properties. The study was then extended to a rheology modifier containing recycled mixed polyolefins (M-PO RM ), where the effects of both compatibilizers were evaluated. Compared to the in situ and other commercial compatibilizers, the styrenic block copolymer offered superior performance, as it enabled the synthesis of virgin-like high-density polyethylene (HDPE) from the M-PO RM . The findings suggest that M-PO RM can be transformed into blends with virgin HDPE-like properties, thus offering an industrially relevant approach for the re- and upcycling of mixed polyolefins. Finally, the M-PO RM pellets with uniform MFI and good mechanical properties can be used as injection mold feedstocks for different applications such as in the construction and automotive sectors.
Accurate Thermochemistry of Complex Lignin Structures via Density Functional Theory, Group Additivity, and Machine Learning
A molecular-level understanding of lignin structures and bond dissociation energies could facilitate depolymerization technologies. Still, this information is currently limited due to the lack of databases and the simplification of surrogate models. Here, substitution effects on seven common linkages in lignin polymers are systematically investigated. An automated reaction network generator is employed to create a database of structures. A new group additivity (GA) model based on principal component analysis (PCA) descriptors is introduced and trained on gas-phase density functional theory data of 4100 species at the M06-2X/6-311++G(d,p) level. Hydrogen bonds, local steric, and nonaromatic ring contributions are also incorporated. Lastly, we improve the accuracy of the group additivity model to reach the G4 theory by computing a data set of 770 species at this level and using a data fusion approach.
Real-Time Time-Dependent Density Functional Theory for Simulating Nonequilibrium Electron Dynamics
The explicit real-time propagation approach for time-dependent density functional theory (RT-TDDFT) has increasingly become a popular first-principles computational method for modeling various time-dependent electronic properties of complex chemical systems. Here, in this Perspective, we provide a nontechnical discussion of how this first-principles simulation approach has been used to gain novel physical insights into nonequilibrium electron dynamics phenomena in recent years. Following a concise overview of the RT-TDDFT methodology from a practical standpoint, we discuss our recent studies on the electronic stopping of DNA in water and the Floquet topological phase as examples. Our discussion focuses on how RT-TDDFT simulations played a unique role in deriving new scientific understandings. We then discuss existing challenges and some new advances at the frontier of RT-TDDFT method development for studying increasingly complex dynamic phenomena and systems.
Probing charge density in materials with atomic resolution in real space
The charge distribution in materials at the nanoscale can often explain the origin of macroscopic properties such as localized conductivity or the plasmonic response and illuminate more fundamental changes in the microscopic structure such as changes in chemical bonding characteristics. Previously, direct visualization of the charge density with high spatial resolution was often a missing link in the formation of structure–property relationships, especially in heterogeneous materials systems. Furthermore, recent advancements in microscopy technology have enabled researchers to visualize the charge distribution in materials down to subatomic length scales. In this Technical Review, we discuss the developments in high-resolution real-space charge distribution imaging using diffraction techniques and electron microscopy, with a focus on the recent advancement of four-dimensional scanning transmission electron microscopy, electron holography, and applications to materials interfaces.
Incorporation of density scaling constraint in density functional design via contrastive representation learning
We demonstrate that contrastive representation learning is a computationally efficient and flexible method to incorporate physical constraints, especially those defined by equalities, in machine-learning-based density functional design.
A new toroidal x-ray crystal spectrometer for the diagnosis of high energy density plasmas at the National Ignition Facility
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