Physics of band-filling correction in defect calculations of solid-state materials.

In solid-state physics/chemistry, a precise understanding of defect formation and its impact on the electronic properties of wide-bandgap insulators is a cornerstone of modern semiconductor technology. However, complexities arise in the electronic structure theory of defect formation when the latter triggers partial occupation of the conduction/valence band, necessitating accurate post-process correction to the energy calculations. Herein, we dissect these complexities, focusing specifically on the post-process band-filling corrections, a crucial element that often demands thorough treatment in defect formation studies. We recognize the importance of these corrections in maintaining the accuracy of electronic properties predictions in wide-bandgap insulators and their role in reinforcing the importance of a reliable common reference state for defect formation energy calculations. We explored solutions such as aligning deep states and electrostatic potentials, both of which have been used in previous works, showing the effect of band alignment on defect formation energy. Our findings demonstrate that the impact of defect formation on electronic structure (even deep states) can be significantly dependent on the supercell size. We also show that within band-filling calculations, one needs to account for the possible change of electronic structure induced by defect formation, which requires sufficient convergence of electronic structure with supercell size. Thus, this work emphasizes the critical steps to accurately predict defect formation energy and paves the way for future research to overcome these challenges and advance the field with more efficient and reliable predictive models.

New Emerging Fast Charging Microscale Electrode Materials.

Fast charging lithium (Li)-ion batteries are intensively pursued for next-generation energy storage devices, whose electrochemical performance is largely determined by their constituent electrode materials. While nanosizing of electrode materials enhances high-rate capability in academic research, it presents practical limitations like volumetric packing density and high synthetic cost. As an alternative to nanosizing, microscale electrode materials cannot only effectively overcome the limitations of the nanosizing strategy but also satisfy the requirement of fast-charging batteries. Therefore, this review summarizes the new emerging microscale electrode materials for fast charging from the commercialization perspective. First, the fundamental theory of electronic/ionic motion in both individual active particles and the whole electrode is proposed. Then, based on these theories, the corresponding optimization strategies are summarized toward fast-charging microscale electrode materials. In addition, advanced functional design to tackle the mechanical degradation problems related to next generation high capacity alloy- and conversion-type electrode materials (Li, S, Si et al.) for achieving fast charging and stable cycling batteries. Finally, general conclusions and the future perspective on the potential research directions of microscale electrode materials are proposed. It is anticipated that this review will provide the basic guidelines for both fundamental research and practical applications of fast-charging batteries.

Noble Gas Functional Defect with Unusual Relaxation Pattern in Solids.

The conventional understanding has always been that noble gases are chemically inert and do not affect materials properties. This belief has led to their use as a standard reference in various experimental applications through noble gas implantation. However, in our research, using first-principles calculations, we delve into the effects of noble gas defects on the properties of several functional oxides, thereby questioning this long-held assumption. We provide evidence that noble gases can indeed serve as functional defects. They have the potential to decentralize the localized defect states and prompt a shift of electrons from the localized state to the conduction band. Our investigation unveils that noble gas defects can indeed significantly alter the material properties. Thus, we underscore the importance of factoring in such defects when assessing material properties.

Spontaneous off-stoichiometry as the knob to control the dielectric properties of gapped metals.

Using first-principles calculations and La3Te4 as an example of an n-type gapped metal, we demonstrate that gapped metals can develop spontaneous defect formation resulting in off-stoichiometric compounds. Importantly, these compounds have different free carrier concentrations and can be realized by optimizing the synthesis conditions. The ability to tune the free carrier concentration allows the tailoring of the intraband and interband transitions, thus controlling the optoelectronic properties of materials in general. Specifically, by realizing different off-stoichiometric La3-xTe4 compounds, it is possible to reach specific crossings of the real part of the dielectric function with the zero line, reduce the plasma frequency contribution to the absorption spectra, or, more generally, induce metal-to-insulator transition. This is particularly important in the context of optoelectronic, plasmonic, and epsilon-near-zero materials, as it enables materials design with a target functionality. While this work is limited to the specific gapped metal, we demonstrate that the fundamental physics is transferable to other gapped metals and can be generally used to design a wide class of new optoelectronic/plasmonic materials.

Effect of pressure on the electronic structure of antiferromagnetic and paramagnetic YNiO3: the role of disproportionation.

The dependence of electronic properties of quantum materials on external controls (e.g., pressure and temperature) is one of the fundamentals of neuromorphic computing, sensors, etc. Until recently, it has been believed that the theoretical description of such compounds cannot be accomplished using "traditional" density functional theory, and more advanced methods like dynamic mean-field theory should be utilized instead. Focusing here on the example of long-range ordered antiferromagnetic and paramagnetic YNiO3 phases, we show the interplay between spin and structural motifs under pressure and their impact on electronic properties. We successfully describe the insulating nature of both YNiO3 phases and the role of symmetry-breaking motifs in the band gap opening. Moreover, by analyzing the pressure-dependent distribution of local motifs, we show that external pressure can significantly reduce the band gap energy of both phases, originating from the reduction of structural and magnetic disproportionation - change in the distribution of local motifs. These results thus demonstrate that some of the experimental observations in quantum materials (e.g., YNiO3 compounds) can be fully understood without accounting for dynamic correlation.

Fermi-Level Instability as a Way to Tailor the Properties of La3Te4.

Traditionally, the formation of off-stoichiometric compounds is believed to be the growth effect rather than the intrinsic tendency of the system. However, here, using the example of La3Te4, we demonstrate that in n-type gapped metals having a large internal gap between principal band edges and the Fermi level inside of the principal conduction band, Fermi-level instability can develop, resulting in a decrease in the formation energy for acceptor defects. Specifically, La vacancies in La3Te4 form spontaneously to produce the acceptor states and remove a fraction of free carriers from the principal conduction band via electron-hole recombination. Such a unique self-doping mechanism allows stabilization of a range of off-stoichiometric La3-xTe4 compounds, which have different electronic properties. Moreover, we thus show how controlling synthesis conditions can be used as a knob to reach the target functionality, including the controllable metal-to-insulator transition.

Chemistry of Oxygen Ionosorption on SnO2 Surfaces.

Ionosorbed oxygen is the key player in reactions on metal-oxide surfaces. This is particularly evident for chemiresistive gas sensors, which operate by modulating the conductivity of active materials through the formation/removal of surface O-related acceptors. Strikingly though, the exact type of species behind the sensing response remains obscure even for the most common material systems. The paradigm for ab initio modeling to date has been centered around charge-neutral surface species, ignoring the fact that molecular adsorbates are required to ionize to induce the sensing response. Herein, we resolve this inconsistency by carrying out a careful analysis of all charged O-related species on three naturally occurring surfaces of SnO2. We reveal that two types of surface acceptors can form spontaneously upon the adsorption of atmospheric oxygen: (i) superoxide O2- on the (110) and the (101) surfaces and (ii) doubly ionized O2- on the (100) facet, with the previous experimental evidence pointing to the latter as the source of sensing response. This species has a unique geometry involving a large displacement of surface Sn, forcing it to attain the coordination resembling that of Sn2+ in SnO, which seems necessary to stabilize O2- and activate metal-oxide surfaces for gas sensing.

Deep Cycling for High-Capacity Li-Ion Batteries.

As the practical capacity of conventional Li-ion batteries (LIBs) approaches the theoretical limit, which is determined by the rocking-chair cycling architecture, a new cycling architecture with higher capacity is highly demanded for future development and electronic applications. Here, a deep-cycling architecture intrinsically with a higher theoretical capacity limit than conventional rocking-chair cycling architecture is developed, by introducing a follow-up cycling process to contribute more capacity. The deep-cycling architecture makes full use of movable ions in both of the electrolyte and electrodes for energy storage, rather than in either the electrolyte or the electrodes. Taking LiMn2 O4 -mesocarbon microbeads (MCMB)/Li cells as a proof-of-concept, 57.7% more capacity is obtained. Moreover, the capacity retention is as high as 84.4% after 2000 charging/discharging cycles. The deep-cycling architecture offers opportunities to break the theoretical capacity limit of conventional LIBs and makes high demands for new-type of cathode materials, which will promote the development of next-generation energy storage devices.

Direct coherent multi-ink printing of fabric supercapacitors.

Coaxial fiber-shaped supercapacitors with short charge carrier diffusion paths are highly desirable as high-performance energy storage devices for wearable electronics. However, the traditional approaches based on the multistep fabrication processes for constructing the fiber-shaped energy device still encounter persistent restrictions in fabrication procedure, scalability, and mechanical durability. To overcome this critical challenge, an all-in-one coaxial fiber-shaped asymmetric supercapacitor (FASC) device is realized by a direct coherent multi-ink writing three-dimensional printing technology via designing the internal structure of the coaxial needles and regulating the rheological property and the feed rates of the multi-ink. Benefitting from the compact coaxial structure, the FASC device delivers a superior areal energy/power density at a high mass loading, and outstanding mechanical stability. As a conceptual exhibition for system integration, the FASC device is integrated with mechanical units and pressure sensor to realize high-performance self-powered mechanical devices and monitoring systems, respectively.

Understanding Doping of Quantum Materials.

Doping mobile carriers into ordinary semiconductors such as Si, GaAs, and ZnO was the enabling step in the electronic and optoelectronic revolutions. The recent emergence of a class of "quantum materials", where uniquely quantum interactions between the components produce specific behaviors such as topological insulation, unusual magnetism, superconductivity, spin-orbit-induced and magnetically induced spin splitting, polaron formation, and transparency of electrical conductors, pointed attention to a range of doping-related phenomena associated with chemical classes that differ from the traditional semiconductors. These include wide-gap oxides, compounds containing open-shell d electrons, and compounds made of heavy elements yet having significant band gaps. The atomistic electronic structure theory of doping that has been developed over the past two decades in the subfield of semiconductor physics has recently been extended and applied to quantum materials. The present review focuses on explaining the main concepts needed for a basic understanding of the doping phenomenology and indeed peculiarities in quantum materials from the perspective of condensed matter theory, with the hope of forging bridges to the chemists that have enabled the synthesis of some of the most interesting compounds in this field.

Mechanically Reinforced Localized Structure Design to Stabilize Solid-Electrolyte Interface of the Composited Electrode of Si Nanoparticles and TiO2 Nanotubes.

Silicon anode with extremely high theoretical specific capacity (≈4200 mAh g-1 ), experiences huge volume changes during Li-ion insertion and extraction, causing mechanical fracture of Si particles and the growth of a solid-electrolyte interface (SEI), which results in a rapid capacity fading of Si electrodes. Herein, a mechanically reinforced localized structure is designed for carbon-coated Si nanoparticles (C@Si) via elongated TiO2 nanotubes networks toward stabilizing Si electrode via alleviating mechanical strain and stabilizing the SEI layer. Benefited from the rational localized structure design, the carbon-coated Si nanoparticles/TiO2 nanotubes composited electrode (C@Si/TiNT) exhibits an ideal electrode thickness swelling, which is lower than 1% after the first cycle and increases to about 6.6% even after 1600 cycles. While for traditional C@Si/carbon nanotube composited electrode, the initial swelling ratio is about 16.7% and reaches ≈190% after 1600 cycles. As a result, the C@Si/TiNT electrode exhibits an outstanding capacity of 1510 mAh g-1 at 0.1 A g-1 with high rate capability and long-time cycling performance with 95% capacity retention after 1600 cycles. The rational design on mechanically reinforced localized structure for silicon electrode will provide a versatile platform to solve the current bottlenecks for other alloyed-type electrode materials with large volume expansion toward practical applications.

Elementary models of the "flux driven anti-ripening" during nanobelt growth.

A stirring solution hydrothermal approach is widely used to rationally grow elongated oxide nanostructures with controllable aspect ratios. Depending on the synthesis conditions, the following are observed: (i) no nanostructure formation (the system exists as a pure liquid), (ii) formation of nanostructure starting from a critical powder/initial volume of the liquid solution, and (iii) monotonic increase in the nanostructure's aspect ratio (towards asymptotic value) with stirring rate. Despite these experimental observations, the theoretical understanding of the process is limited. Herein, using an athermal ballistic atomic jump model, we develop a phenomenological theory of nanostructure growth under different stirring rates, demonstrating the conditions necessary for breaking the equilibrium Wulff shape, the formation of elongated one-dimensional structures, and explaining regimes (i-iii) reported experimentally. Moreover, the comparison of the phenomenological models without and with the account of ripening effects in the open ensemble of nanowires under stirring provides the theoretical guidance for the controllable growth of elongated nanostructures by the stirring solution hydrothermal approach.

Energy, Phonon, and Dynamic Stability Criteria of Two-Dimensional Materials.

First-principles calculations have become a powerful tool to exclude the Edisonian approach in search of novel two-dimensional (2D) materials. However, no universal first-principles criteria to examine the realizability of hypothetical 2D materials have been established in the literature yet. Because of this, and as the calculations are always performed in an artificial simulation environment, one can unintentionally study compounds that do not exist in experiments. Although investigations of physics and chemistry of unrealizable materials can provide some fundamental knowledge, the discussion of their applications can mislead experimentalists for years and increase the gap between experimental and theoretical research. By analyzing energy convex hull, phonon spectra, and structure evolution during ab initio molecular dynamics simulations for a range of synthesized and recently proposed 2D materials, we construct energy, phonon, and dynamic stability filters that need to be satisfied before proposing novel 2D compounds. We demonstrate the power of the suggested filters for several selected 2D systems, revealing that some of them cannot be ever realized experimentally.

Identifying the Origin and Contribution of Surface Storage in TiO2 (B) Nanotube Electrode by In Situ Dynamic Valence State Monitoring.

Fundamental insight into the surface charging mechanism of TiO2 (B) nanomaterials is limited due to the complicated nature of lithiation behavior, as well as the limitations of available characterization tools that can directly probe surface charging process. Here, an in situ approach is reported to monitor the dynamic valence state of TiO2 (B) nanotube electrodes, which utilizes in situ X-ray absorption spectroscopy (XAS) to identify the origin and contribution of surface storage. A real-time correlation is elucidated between the rate-dependent electrode performance and dynamic Ti valence-state change. A continuous Ti valence state change is directly observed through the whole charging/discharging process regardless of charging rates, which proves that along with the well-known non-faradaic reaction, the surface charging process also originates from a faradaic reaction. The quantification of these two surface storage contributions at different charging rates is further realized through in situ dynamic valence state monitoring combined with traditional cyclic voltammetry measurement. The methodology reported here can also be applied to other electrode materials for the real-time probing of valence state change during electrochemical reactions, the quantification of the faradaic and non-faradaic reactions, and the eventual elucidation of electrochemical surface charging mechanisms.

Suppression of surfaces states at cubic perovskite (001) surfaces by CO2 adsorption.

By using first-principles approach, the interaction of CO2 with (001) surfaces of six cubic ABO3 perovskites (A = Ba, Sr and B = Ti, Zr, Hf) is studied in detail. We show that CO2 adsorption results in the formation of highly stable CO3-like complexes with similar geometries for all investigated compounds. This reaction leads to the suppression of the surfaces states, opening the band gaps of the slab systems up to the corresponding bulk energy limits. For most AO-terminated ABO3(001) perovskite surfaces, a CO2 coverage of 0.25 was found to be sufficient to fully suppress the surface states, whereas the same effect can only be achieved at 0.50 CO2 coverage for the BO2-terminated surfaces. The largest band gap modulation among the AO-terminated surfaces was found for SrHfO3(001) and BaHfO3(001), whereas the most profound effect among the BO2-terminated surfaces was identified for SrTiO3(001) and BaTiO3(001). Based on these results and considering practical difficulties associated with measuring conductivity of highly resistive materials, TiO2-terminated SrTiO3(001) and BaTiO3(001) were identified as the most prospective candidates for chemiresistive CO2 sensing applications.

Distance-Dependent Sign Reversal in the Casimir-Lifshitz Torque.

The Casimir-Lifshitz torque between two biaxially polarizable anisotropic planar slabs is shown to exhibit a nontrivial sign reversal in its rotational sense. The critical distance a_{c} between the slabs that marks this reversal is characterized by the frequency ω_{c}∼c/2a_{c} at which the in-planar polarizabilities along the two principal axes are equal. The two materials seek to align their principal axes of polarizabilities in one direction below a_{c}, while above a_{c} their axes try to align rotated perpendicular relative to their previous minimum energy orientation. The sign reversal disappears in the nonretarded limit. Our perturbative result, derived for the case when the differences in the relative polarizabilities are small, matches excellently with the exact theory for uniaxial materials. We illustrate our results for black phosphorus and phosphorene.

Tailoring electronic properties of multilayer phosphorene by siliconization.

Controlling the thickness dependence of electronic properties for two-dimensional (2d) materials is among the primary goals for their large-scale applications. Herein, employing a first-principles computational approach, we predict that Si interaction with multilayer phosphorene (2d-P) can result in the formation of highly stable 2d-SiP and 2d-SiP2 compounds with a weak interlayer interaction. Our analysis demonstrates that these systems are semiconductors with band gap energies that can be governed by varying the thicknesses and stacking arrangements. Specifically, the siliconization of phosphorene allows designing 2d-SiPx materials with a significantly weaker thickness dependence of electronic properties than that in 2d-P and to develop ways for their tailoring. We also reveal the spatial dependence of electronic properties for 2d-SiPx highlighting the difference in the effective band gaps for different layers. Particularly, our results show that the central layers in the multilayer 2d systems determine their overall electronic properties, while the role of the outermost layers is noticeably smaller.

Reducing the Charge Carrier Transport Barrier in Functionally Layer-Graded Electrodes.

Lithium-ion batteries (LIBs) are primary energy storage devices to power consumer electronics and electric vehicles, but their capacity is dramatically decreased at ultrahigh charging/discharging rates. This mainly originates from a high Li-ion/electron transport barrier within a traditional electrode, resulting in reaction polarization issues. To address this limitation, a functionally layer-graded electrode was designed and fabricated to decrease the charge carrier transport barrier within the electrode. As a proof-of-concept, functionally layer-graded electrodes composing of TiO2 (B) and reduced graphene oxide (RGO) exhibit a remarkable capacity of 128 mAh g-1 at a high charging/discharging rate of 20 C (6.7 A g-1 ), which is much higher than that of a traditionally homogeneous electrode (74 mAh g-1 ) with the same composition. This is evidenced by the improvement of effective Li ion diffusivity as well as electronic conductivity in the functionally layer-graded electrodes.

Water-Soluble Sericin Protein Enabling Stable Solid-Electrolyte Interphase for Fast Charging High Voltage Battery Electrode.

Spinel LiNi0.5 Mn1.5 O4 (LNMO) is the most promising cathode material for achieving high energy density lithium-ion batteries attributed to its high operating voltage (≈4.75 V). However, at such high voltage, the commonly used battery electrolyte is suffered from severe oxidation, forming unstable solid-electrolyte interphase (SEI) layers. This would induce capacity fading, self-discharge, as well as inferior rate capabilities for the electrode during cycling. This work first time discovers that the electrolyte oxidation is effectively negated by introducing an electrochemically stable silk sericin protein, which is capable to stabilize the SEI layer and suppress the self-discharge behavior for LNMO. In addition, robust mechanical support of sericin coating maintains the structural integrity during the fast charging/discharging process. Benefited from these merits, the sericin-based LNMO electrode possesses a much lower Li-ion diffusion energy barrier (26.1 kJ mol-1 ) for than that of polyvinylidene fluoride-based LNMO electrode (37.5 kJ mol-1 ), delivering a remarkable high-rate performance. This work heralds a new paradigm for manipulating interfacial chemistry of electrode to solve the key obstacle for LNMO commercialization, opening a powerful avenue for unlocking the current challenges for a wide family of high operating voltage cathode materials (>4.5 V) toward practical applications.

Band gap modulation of SrTiO3 upon CO2 adsorption.

Herein, CO2 chemisorption on SrTiO3(001) surfaces is studied using ab initio calculations to establish new chemical sensing mechanisms. It was found that CO2 adsorption opens the band gap of the material. However, the mechanisms are different: the CO2 adsorption on the TiO2-terminated surface neutralizes the surface states at the valence band (VB) maximum, whereas for the SrO-terminated surface it suppresses the conduction band (CB) minimum. For the TiO2-terminated surface, the effect is explained by the passivation of dangling bonds, whereas for the SrO-terminated surface, the suppression is caused by surface relaxation. Modulation of the VB states implies a more direct change in charge distribution, and thus, the induced change in the band gap is more prominent at the TiO2 termination. Further, it has been shown that both CO2 adsorption energy and surface band gap are strongly dependent on CO2 coverage, suggesting that the observed effect can be utilized in sensing applications for a wide range of CO2 concentrations.