Defect Chemistry Research Articles

Defect chemistry and ion transport in low-dimensional-networked Li-rich anti-perovskites as solid electrolytes for solid-state batteries

This work reveals the relationships between defects, Li-ion dynamics and dimensionality in anti-perovskite solid electrolytes. Significant increases in defect concentrations and ion transport are observed with decreasing dimensionality from 3D to 0D.

Conversion of La2Ti2O7 to LaTiO2N via ammonolysis: a first-principles investigation.

Perovskite oxynitrides are, due to their reduced band gap compared to oxides, promising materials for photocatalytic applications. They are most commonly synthesized from {110} layered Carpy-Galy (A2B2O7) perovskites via thermal ammonolysis, i.e. the exposure to a flow of ammonia at elevated temperature. The conversion of the layered oxide to the non-layered oxynitride must involve a complex combination of nitrogen incorporation, oxygen removal and ultimately structural transition by elimination of the interlayer shear plane. Despite the process being commonly used, little is known about the microscopic mechanisms and hence factors that could ease the conversion. Here we aim to derive such insights via density functional theory calculations of the defect chemistry of the oxide and the oxynitride as well as the oxide's surface chemistry. Our results point to the crucial role of surface oxygen vacancies in forming clusters of NH3 decomposition products and in incorporating N, most favorably substitutionally at the anion site. N then spontaneously diffuses away from the surface, more easily parallel to the surface and in interlayer regions, while diffusion perpendicular to the interlayer plane is somewhat slower. Once incorporation and diffusion lead to a local N concentration of about 70% of the stoichiometric oxynitride composition, the nitridated oxide spontaneously transforms to a nitrogen-deficient oxynitride. Since anion vacancies are crucial for the nitrogen incorporation and diffusion as well as the transformation process, their concentration in the precursor oxide is a relevant tuning parameter to optimize the oxynitride's synthesis and properties.

Cooperativity of silanol defect chemistry in zeolites.

Deboronation treatment of zeolite B-SSZ-55 can generate vacancy defects consisting of four silanol groups (silanol nests). However, 1H solid-state NMR spectroscopy indicates the prevalence of two silanol groups (silanol dyads) instead of four silanol groups. Such silanol dyads must be formed by the silanol condensation of two silanol groups at the silanol nests. Yet, the exact mechanism of this condensation and detailed structure of the silanol defect are not known. Here, the structure and formation mechanism of silanol dyads in the SSZ-55 zeolite have been investigated by both cluster and periodic density functional theory calculations. The calculated 1H NMR chemical shifts agree with the experimental values, showing that the silanol dyads are indeed commonly present at the vacancies and the vacancy density plays a role in the relaxation of the zeolite framework. The nature (size) of the silanol clusters influences their acidity.

Emerging disorder in Gd2(Ti1−xZrx)2O7 pyrochlores matrices for radioactive waste disposal: symmetry lowering versus defect clustering

The structure of Gd2(Ti1−xZrx)2O7 pyrochlores was investigated by means of synchrotron radiation powder diffraction, including pair distribution function analysis, Raman spectroscopy and DFT calculations to shed light on their rich defect chemistry.

Cation disorder dominates the defect chemistry of high-voltage LiMn1.5Ni0.5O4 (LMNO) spinel cathodes

Using first-principles calculations, a comprehensive intrinsic defect study on high-voltage spinel LiMn1.5Ni0.5O4 (LMNO) cathode shows that the defect chemistry of this material is dominated by cation antisites and their complexes.

Defect engineering in antimony selenide thin film solar cells

The complicated and unconventional defect chemistry of Sb2Se3 largely dictates photovoltaic device performance. This comprehensive review aims to increase understanding of defect engineering, which is essential for further efficiency improvements.

Complex defect chemistry of hydrothermally-synthesized Nb-substituted β′-LiVOPO4

Nb substitution via hydrothermal synthesis led to a new β′-LiVOPO4 phase with complex defect chemistries. The samples showed improved high-voltage rate capabilities and an enlarged voltage hysteresis due to a partial V4+/V3+ redox reaction.

Analysis of defect mechanisms in nonstoichiometric ceria–zirconia by the microwave cavity perturbation method

AbstractIn this microwave study, the defect chemistry of ceria–zirconia solid solutions (CZO, Ce1−yZryO2−δ) was investigated at high temperatures by a resonant microwave method. Specifically, the effects of temperature and Zr content on the dielectric properties and defect chemistry mechanisms in CZO were analyzed. Experiments were performed on a series of different CZO powders (y = 0.2, 0.33, 0.50, 0.67). Measurements at 600°C and different oxygen partial pressures (pO2 = 10−26–0.2 bar) confirm a dominant n‐type conduction of small‐polarons in CZO due to the preferred formation of oxygen vacancies, which is also supported by a multimodal analysis. Polarization losses were found to be negligible in the GHz range. Furthermore, an increased relative permittivity was observed in CZO, which correlates with the concentration of oxygen vacancies in CZO. Our microwave study is the first to provide a comprehensive data set for the dielectric properties of CZO powder sample in a wide range of different conditions. In addition, the connection of dielectric properties to CZO defect chemistry mechanisms is presented. The results are in good agreement with findings in the literature and may contribute to a better understanding of microwave‐based state diagnosis of CZO‐based materials, as it discussed for three‐way catalysts.

Defect Chemistry in Zn3V4(PO4)6

Zinc-ion batteries have attracted great interest for their low cost, safety, and high energy density. Recently, Zn3V4(PO4)6 has been reported to be a promising cathode material for zinc-ion batteries. The defect chemistry, diffusion of Zn-ions, and solution of dopants are examined by advanced simulation techniques. The simulation results show that the most favorable intrinsic defect is the Zn-V anti-site. A zig-zag pattern of long-range Zn2+ diffusion is observed and the activation energy of 1.88 eV indicates that the ionic conductivity of this material is low. The most promising isovalent dopants on the Zn site are Ca2+ and Fe2+. Although the solution of Ga3+, Sc3+, In3+, Y3+, Gd3+, and La3+ on the V site is exoergic, the most promising is In3+. Different reaction routes for the formation of Zn3V4(PO4)6 are considered and the most thermodynamically favorable reaction consists of binary oxides (ZnO, V2O3, and P2O5) as reactants.

Toward a Consistent Prediction of Defect Chemistry in CeO2.

Polarizable shell-model potentials are widely used for atomic-scale modeling of charged defects in solids using the Mott-Littleton approach and hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) embedded-cluster techniques. However, at the pure MM level of theory, the calculated defect energetics may not satisfy the requirement of quantitative predictions and are limited to only certain charged states. Here, we proposed a novel interatomic potential development scheme that unifies the predictions of all relevant charged defects in CeO2 based on the Mott-Littleton approach and QM/MM electronic-structure calculations. The predicted formation energies of oxygen vacancies accompanied by different excess electron localization patterns at the MM level of theory reach the accuracy of density functional theory (DFT) calculations using hybrid functionals. The new potential also accurately reproduces a wide range of physical properties of CeO2, showing excellent agreement with experimental and other computational studies. These findings provide opportunities for accurate large-scale modeling of the partial reduction and nonstoichiometry in CeO2, as well as a prototype for developing robust interatomic potentials for other defective crystals.

Intrinsic Defects and Their Role in the Phase Transition of Na-Ion Anode Na2Ti3O7.

The development of high-power anode materials for Na-ion batteries is one of the primary obstacles due to the growing demands for their use in the smart grid. Despite the appealingly low cost and non-toxicity, Na2Ti3O7 suffers from low electrical conductivity and poor structural stability, which restricts its use in high-power applications. Viable approaches for overcoming these drawbacks reported to date are aliovalent doping and hydrogenation/hydrothermal treatments, both of which are closely intertwined with native defects. There is still a lack of knowledge, however, of the intrinsic defect chemistry of Na2Ti3O7, which impairs the rational design of high-power titanate anodes. Here, we report hybrid density functional theory calculations of the native defect chemistry of Na2Ti3O7. The defect calculations show that the insulating properties of Na2Ti3O7 arise from the Na and O Schottky disorder that act as major charge compensators. Under high-temperature hydrogenation treatment, these Schottky pairs of Na and O vacancies become dominant defects in Na2Ti3O7, triggering the spontaneous partial phase transition to Na2Ti6O13 and improving the electrical conductivity of the composite anode. Our findings provide an explanation on the interplay between intrinsic defects, structural phase transitions, and electrical conductivity, which can aid understanding of the properties of composite materials obtained from phase transitions.

Effects of Selenium Doping in Zinc Telluride from First Principles

Point defects and compensation in zinc telluride have been subjects of study for several decades due to the compound’s scientific/technological applications. Despite this, there remains some disagreement in the literature as to the fine details of its defect chemistry. Recently, it has also come under renewed interest due to the emerging Cd(1–x)ZnxTe(1–y)Sey material family, used in nuclear radiation detector applications. In this article, we use state-of-the-art computational tools to re-examine the native defect chemistry of ZnTe and explore how it changes upon addition of selenium using density functional theory with screened hybrid exchange–correlation functionals, thermochemical models of the equilibration environment, and statistical mechanics models of the point defect ensemble.

Crystallization and Defect Chemistry Dual Engineering for MAPbI3 Perovskite Solar Cells with Efficiency Approaching 22%

Methylammonium lead triiodide (MAPI) has been the most commonly used light absorber layer for perovskite solar cells and also shows potentially high theoretical efficiency. However, the achievable efficiencies so far still lag behind those of their formamidinium- or mixed cation-based counterparts. In this work, the dual additives 2-mercaptopyridine (2-MP) and 1,2-dichlorobenzene (DCB) are applied to regulate the crystal orientation, yielding a high-crystal quality MAPI film and passivating the trap states of the film. Encouragingly, we are able to distinguish the efficacy of the two additives. DCB effectively reduced the size of colloidal clusters in the perovskite precursor, thus helping the wet film reach the critical concentration to initialize rapid nucleation. The resulting MAPI film exhibited extended grain size with a stronger grain orientation and smoother surface. With the subsequent introduction of 2-MP, the high crystal quality was maintained, and 2-MP interacted with uncoordinated Pb defects and passivated the related traps. The dual additive-processed devices exhibited the champion efficiency of 21.83% with a simultaneously high Jsc of 24.01 mA/cm2 and Voc of 1.15 V. This work provides a means to engineer the crystallization quality and defect chemistry toward efficient MAPbI3 perovskite solar cells.

Ferroelectric domain percolation in polycrystals

Ferroelectric materials possess the ability to switch between polarization states on application of external fields. This switching is facilitated via movement of domains walls and is a critical factor in the performance metrics of these materials. Hence, the interaction of domain walls with microstructural features such as grain boundaries can influence their performance. Experimentally, domain continuity has only been observed over a few grains with no information of the domain percolation length in larger polycrystals. For domain continuity to be energetically feasible, conditions of ferroelectric polarization continuity and domain wall plane matching need to be satisfied at the grain boundary. In this work, we have studied the extent of continuity of domains within modelled polycrystals. It is shown that under tighter conditions of plane matching and uncompensated polarization charge, favorable grain boundary character can have considerable impact on the domain percolation. However, when the conditions of domain continuity are relaxed, due to higher charge and geometric tolerance, domain percolation throughout the polycrystal is a likely phenomenon. It is shown that percolation of domains through a polycrystalline ferroelectric could be tailored by microstructural control of the grain boundary types and defect chemistry control of the charge compensation mechanisms. The ability to manipulate the length-scale of correlated domain wall motion may lead to new opportunities in ferroelectric functional device design. Additionally, these results can be extended to study the effect of grain boundary character on other material phenomena that involve planar interactions at grain boundaries, including ferroelastic twin boundaries and slip plane continuity.

Nonstoichiometry, Defect Chemistry and Oxygen Transport in Fe-Doped Layered Double Perovskite Cobaltite PrBaCo2-xFexO6-δ (x = 0-0.6) Membrane Materials.

Mixed conducting cobaltites PrBaCo2-xFexO6-δ (x = 0-0.6) with a double perovskite structure are promising materials for ceramic semi-permeable membranes for oxygen separation and purification due to their fast oxygen exchange and diffusion capability. Here, we report the results of the detailed study of an interplay between the defect chemistry, oxygen nonstoichiometry and oxygen transport in these materials as a function of iron doping. We show that doping leads to a systematic variation of both the thermodynamics of defect formation reactions and oxygen transport properties. Thus, iron doping can be used to optimize the performance of mixed conducting oxygen-permeable double perovskite membrane materials.

Intrinsic properties and dopability effects on the thermoelectric performance of binary Sn chalcogenides from first principles

High-performance thermoelectric (TE) materials rely on semiconductors with suitable intrinsic properties for which carrier concentrations can be controlled and optimized. To demonstrate the insights that can be gained in computational analysis when both intrinsic properties and dopability are considered in tandem, we combine the prediction of TE quality factor (intrinsic properties) with first-principles simulations of native defects and carrier concentrations for the binary Sn chalcogenides SnS, SnSe, and SnTe. The computational predictions are compared to a comprehensive data set of previously reported TE figures-of-merit for each material, for both p-type and n-type carriers. The combined analysis reveals that dopability limits constrain the TE performance of each Sn chalcogenide in a distinct way. In SnS, TE performance for both p-type and n-type carriers is hindered by low carrier concentrations, and improved performance is possible only if higher carrier concentrations can be achieved by suitable extrinsic dopants. For SnSe, the p-type performance of the Cmcm phase appears to have reached its theoretical potential, while improvements in n-type performance may be possible through tuning of electron carrier concentrations in the Pnma phase. Meanwhile, assessment of the defect chemistry of SnTe reveals that p-type TE performance is limited by, and n-type performance is not possible due to, the material’s degenerate p-type nature. This analysis highlights the benefits of accounting for both intrinsic and extrinsic properties in a computation-guided search, an approach that can be applied across diverse sets of semiconductor materials for TE applications.

Millisecond-induced defect chemistry realizes high-rate fiber-shaped zinc-ion battery as a magnetically soft robot

Developing a reliable method for tailoring the defect chemistry of electrode at the atomic scale is of importance for improving battery performance, which has been demonstrated to be rather challenging. Herein, we report a sample and facial quenching method to simultaneously realize metal ions doping and oxygen vacancy generation in reconfiguring the desired defect of electrode materials, by quenching V2O5 nanowires in cold NiCl2 aqueous solution (Ni-V2O5 NWs) as an example. Through detailed characterization studies, we confirm that the millisecond-induced defect-engineering modifies the local electronic structures and V coordination environmental of the quenched Ni-V2O5 NWs, thus promoting fast charge transfer and providing more storage/adsorption sites for Zn2+. Consequently, the quenched Ni-V2O5 NWs@CNT fiber (CNT = carbon nanotube) cathode exhibits a high-rate capability (71.2% retention after a 500-times increase in current density), and an ultrahigh stack volumetric energy density of 66.5 mWh cm−3, when assembled into a quasi-solid-state fiber-shaped Zn-ion batteries together with Zn NSs@CNT fiber (Zn nanosheets electrodeposited on CNT fiber) as the anode. Finally, we integrate the flexible QFZIB into a magnetically fibrous soft robot to demonstrate its promising potential for providing both energy storage and load-bearing capacities.

Mass and Charge Transport in Li1-δCoO2 Thin Films-A Complete Set of Properties and Its Defect Chemical Interpretation.

Lithium insertion materials are an essential class of mixed ionic and electronic conductors, and their electrochemical properties depend on the resistive and capacitive interplay of ions and electrons. However, complete sets of the corresponding elementary material parameters, that is, composition-dependent ionic and electronic conductivity, chemical capacitance, and charge-transfer resistance, are rarely reported for lithium-ion battery electrode materials. Moreover, the interpretation of these properties from a defect chemical point of view is not very common. In this work, the impedance of sputtered Li1-δCoO2 thin films is analyzed to extract the fundamental electrochemical properties as a function of state-of-charge (SOC). Within the accessible SOC range, the charge transfer resistance and ionic conductivity vary by more than 1 order of magnitude. The chemical capacitance determined from impedance spectra agrees excellently with the differential capacitance from charge/discharge curves, and, in the dilute regime, even matches the absolute values predicted by defect thermodynamics. The evolution of lithium diffusivity along the charge curve is deconvoluted into the separate contributions of ionic conductivity and chemical capacitance. Finally, we apply the principles of defect chemistry to evaluate the observed trends in terms of lithium activity and point defect concentrations and provide a tentative defect model that is consistent with our results. The consistency of impedance measurements, cycling data, and thermodynamic theory highlights the key role of the chemical capacitance as a powerful material descriptor and emphasizes the relevance of defect chemical concepts for all lithium insertion electrode materials.

Pre‐Deoxidation of Layered Ni‐Rich Cathodes to Construct a Stable Interface with Electrolyte for Long Cycling Life

AbstractNi‐rich layered oxides as the cathode materials of high‐energy‐density lithium‐ion batteries (LIBs) suffer from capacity decay and structural instability owing to oxygen loss during cycling. It is a huge challenge to prevent the oxygen loss of Ni‐rich cathode materials during long cycling. Here, a pre‐deoxidation of LiNi0.8Co0.1Mn0.1O2 (NCM811) single crystal materials is achieved by heat treatment at elevated temperatures in argon condition to form a stable surface with rock salt structure. The stable surface structure with oxygen vacancy defects successfully suppresses the harmful phase transitions of NCM811 and effectively improves the stability of the NCM811/electrolyte interface during cycling at a high cut‐off voltage. In addition, the intragranular structural evolution and cation mixing degree is inhibited to effectively suppress the intergranular cracking and particle pulverization of cathode during long cycling. The pre‐deoxidation of NCM811 exhibits 70.6% capacity retention after 1000 cycles at the current density of 0.5 C between 2.8 and 4.3 V, which is much larger than that of pristine NCM811 capacity retention of 27.3%. The strategy of pre‐deoxidation of Ni‐rich layered structure cathode to regulate the defect chemistry and surface structure provides a facile and effective way to achieve long cycling life high‐energy density LIBs.

Defect chemistry of electrocatalysts for CO2 reduction.

Electrocatalytic CO2 reduction is a promising strategy for converting the greenhouse gas CO2 into high value-added products and achieving carbon neutrality. The rational design of electrocatalysts for CO2 reduction is of great significance. Defect chemistry is an important category for enhancing the intrinsic catalytic performance of electrocatalysts. Defect engineering breaks the catalytic inertia inherent in perfect structures by imparting unique electronic structures and physicochemical properties to electrocatalysts, thereby improving catalytic activity. Recently, various defective nanomaterials have been studied and show great potential in electrocatalytic CO2 reduction. There is an urgent need to gain insight into the effect of defects on catalytic performance. Here, we summarized the recent research advances on the design of various types of defects, including carbon-based materials (intrinsic defects, heteroatom doping and single-metal-atom sites) and metal compounds (vacancies, grain boundaries, and lattice defects). The major challenges and prospects of defect chemistry in electrocatalytic CO2 reduction are also proposed. This review is expected to be instructive in the development of defect engineering for CO2 reduction catalysts.