Formation Of Antisite Defects Research Articles

Li2SnO3 as a Cathode Material for Lithium-ion Batteries: Defects, Lithium Ion Diffusion and Dopants

Tin-based oxide Li2SnO3 has attracted considerable interest as a promising cathode material for potential use in rechargeable lithium batteries due to its high- capacity. Static atomistic scale simulations are employed to provide insights into the defect chemistry, doping behaviour and lithium diffusion paths in Li2SnO3. The most favourable intrinsic defect type is Li Frenkel (0.75 eV/defect). The formation of anti-site defect, in which Li and Sn ions exchange their positions is 0.78 eV/defect, very close to the Li Frenkel. The present calculations confirm the cation intermixing found experimentally in Li2SnO3. Long range lithium diffusion paths via vacancy mechanisms were examined and it is confirmed that the lowest activation energy migration path is along the c-axis plane with the overall activation energy of 0.61 eV. Subvalent doping by Al on the Sn site is energetically favourable and is proposed to be an efficient way to increase the Li content in Li2SnO3. The electronic structure calculations show that the introduction of Al will not introduce levels in the band gap.

Substitution of Zn in Earth‐Abundant Cu2ZnSn(S,Se)4 based thin film solar cells – A status review

Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) are the most promising quaternary earth abundant photo-absorber materials for thin film solar cells, with reported power conversion efficiencies (PCE) of more than 12%. In CZTS and CZTSSe based solar cells, similar atomic sizes of Cu and Zn is thought to be the main reason for CuZn and ZnCu antisite defect formation, resulted in severe potential fluctuations and tail states that can be inhibited by an incorporation of alternative bigger atoms such as Cd and Mn. In addition, the large open-circuit voltage deficit observed in CZTS and CZTSe device because of existence of Cu and Zn cation disorder in kesterite crystal. Recently, in the development of CZTS based thin film solar cells the reduction of antisite defects by cation substitution has received considerable attention. This also opens up the possibility to explore materials with similar crystal structures and band gaps as CZTS/CZTSSe such as Cu2XSnS4 (where X = Mn, Cd, Fe, Co, Ni and Ba). In this review, the effect of substituting other metals for Zn is discussed, providing a way to alter defects and tail states in the absorber material of CZTS/CZTSSe thin film solar cells.

Defects, Dopants and Lithium Mobility in Li9V3(P2O7)3(PO4)2

Layered Li9V3(P2O7)3(PO4)2 has attracted considerable interest as a novel cathode material for potential use in rechargeable lithium batteries. The defect chemistry, doping behavior and lithium diffusion paths in Li9V3(P2O7)3(PO4)2 are investigated using atomistic scale simulations. Here we show that the activation energy for Li migration via the vacancy mechanism is 0.72 eV along the c-axis. Additionally, the most favourable intrinsic defect type is Li Frenkel (0.44 eV/defect) ensuring the formation of Li vacancies that are required for Li diffusion via the vacancy mechanism. The only other intrinsic defect mechanism that is close in energy is the formation of anti-site defect, in which Li and V ions exchange their positions (1.02 eV/defect) and this can play a role at higher temperatures. Considering the solution of tetravalent dopants it is calculated that they require considerable solution energies, however, the solution of GeO2 will reduce the activation energy of migration to 0.66 eV.

Revealing the Mechanism of Sodium Diffusion in NaxFePO4 Using an Improved Force Field

Olivine NaFePO4 is a promising cathode material for Na-ion batteries. Intermediate phases such as Na0.66FePO4 govern phase stability during intercalation-deintercalation processes, yet little is known about Na+ diffusion in NaxFePO4 (0 < x < 1). Here we use an advanced simulation technique, Randomized Shell Mass Generalized Shadow Hybrid Monte Carlo Method (RSM-GSHMC) in combination with a specifically developed force field for describing NaxFePO4 over the whole range of sodium compositions, to thoroughly examine Na+ diffusion in this material. We reveal a novel mechanism through which Na+/Fe2+ antisite defect formation halts transport of Na+ in the main diffusion direction [010], while simultaneously activating diffusion in the [001] channels. A similar mechanism was reported for Li+ in LiFePO4, suggesting that a transition from one- to two-dimensional diffusion prompted by antisite defect formation is common to olivine structures, in general.

Local structure and defects in ion irradiated KTaO3

The modification of the local structure in cubic perovskite KTaO3 irradiated with 3 MeV and 1.1 GeV Au ions is studied by Raman and x-ray absorption spectroscopy, complemented by density functional theory (DFT) calculations. In the case of irradiation with 3 MeV Au ions where displacement cascade processes are dominant, the Ta L3-edge x-ray absorption measurements suggest that a peak corresponding to the Ta–O bonds in the TaO6 octahedra splits, which is attributed to the formation of TaK antisite defects that are coupled with oxygen vacancies, VO. This finding is consistent with the DFT calculations. Under irradiation with 1.1 GeV ions, the intense ionization and electronic energy deposition lead to a blue shift and an intensity reduction of active Raman bands. In the case of sequential irradiations, extended x-ray absorption fine structure measurements reveal a decrease in concentration of coupled TaK-VO defects under subsequent irradiation with 1.1 GeV Au ions.

Formation and Elimination of Anti-site Defects during Crystallization in Perovskite Ba1–xSrxLiF3

The defect density of a material is decisive for its physical, chemical, and mechanical properties. Accordingly, defect tuning is desirable for applications spanning, e.g., batteries, fuel cells, electronics, optics, catalysis, and mechanical strength and resilience. Here, we simulate the mechanochemical synthesis of the perovskite Ba1–xSrxLiF3 by compressing a BaLiF3 nanoparticle with a SrLiF3 nanoparticle under conditions likely to occur during high-energy ball milling. We investigate the crystallization process and the ionic mobility of the system and compare with experiment. Animations of the crystallization, simulated under high pressure, revealed that cations, within the crystallization front, would commonly condense onto “incorrect” lattice sites, in some cases eventually leading to the formation of anti-site defects. However, most of these cations would then re-amorphize and the “correct” cation would take its place—rectifying the defect. Crucially, it is the amorphous/crystalline interface that e...

First principle calculation of helium in La2Zr2O7: Effects on structural, electronic properties and radiation tolerance

First principle calculations based on density functional theory have been employed to study structural effects of trapping helium in La2Zr2O7 pyrochlore. Lattice swelling and the distortion of unit cell have been found in He-La2Zr2O7 systems. By analyzing the electronic structures and chemical bonding of He-La2Zr2O7 systems, weak repulsive and attractive chemical interactions of helium in La2Zr2O7 pyrochlore have been observed. The formation energies have been calculated to assess the relative stability of various helium interstitial configurations and the results show that the octahedral interstitial site is the most stable structure. The cation antisite defect formation energies and the x positional parameter for 48f-site oxygen are calculated to predict the radiation resistance of He-La2Zr2O7 systems. The results indicate that the presence of low concentration of He interstitials may increase the radiation resistance of La2Zr2O7 pyrochlore.

MnNiO3 revisited with modern theoretical and experimental methods.

MnNiO3 is a strongly correlated transition metal oxide that has recently been investigated theoretically for its potential application as an oxygen-evolution photocatalyst. However, there is no experimental report on critical quantities such as the band gap or bulk modulus. Recent theoretical predictions with standard functionals such as LDA+U and HSE show large discrepancies in the band gaps (about 1.23 eV), depending on the nature of the functional used. Hence there is clearly a need for an accurate quantitative prediction of the band gap to gauge its utility as a photocatalyst. In this work, we present a diffusion quantum Monte Carlo study of the bulk properties of MnNiO3 and revisit the synthesis and experimental properties of the compound. We predict quasiparticle band gaps of 2.0(5) eV and 3.8(6) eV for the majority and minority spin channels, respectively, and an equilibrium volume of 92.8 Å3, which compares well to the experimental value of 94.4 Å3. A bulk modulus of 217 GPa is predicted for MnNiO3. We rationalize the difficulty for the formation of ordered ilmenite-type structure with specific sites for Ni and Mn to be potentially due to the formation of antisite defects that form during synthesis, which ultimately affects the physical properties of MnNiO3.

Insights into the radiation behavior of Ln2TiO5 (Ln = La-Y) from defect energetics

First-principles calculations have been performed to study the structural properties and defect energetics of orthorhombic Ln2TiO5 (Ln=La, Pr, Nd, Sm, Gd, Dy and Y). The lattice parameters decrease with increasing lanthanide cation radius. The mean 〈Ln1-O〉 and 〈Ln2-O〉 bond lengths increase systematically by respectively 1.16% and 1.19% from Dy2TiO5 to La2TiO5. Meanwhile, mean 〈Ti-O〉 bond length increases slightly with increasing Ln cation radius from Y2TiO5 to La2TiO5. The calculated results of point defect formation energies suggest that Ln2-Ti antisite defect is the most energetically favorable defect type for Ln2TiO5. The trend of Ln2-Ti antisite defect formation energy of Ln2TiO5 is precisely consistent with the radiation resistance, as demonstrates by the critical amorphization temperature, which indicates that the formation of Ln2-Ti antisite defect may has a significant influence on the radiation behavior of Ln2TiO5.

Understanding the Effects of Defects on Phase Transformation Kinetics in Olivine LiFePO4 Particles

Effects of anti-site defects on reducing Li diffusion anisotropy in LiFePO4 single crystal have been experimentally [1] and theoretically [2, 3] examined. The strongly anisotropic Li diffusion in olivine phosphates were firstly reported by first-principle calculations by Morgan et al. [4], and later confirmed by experiment [5]. However, the theoretical Li diffusivity (D=1x10-8 cm2/s) in defect-free LiFePO4 single crystals are several orders of magnitude larger than experimentally measured values in the range of 10-11 – 10-14 cm2/s. Subsequent calculations by Malik et al. [2] and Dathar et al. [3] confirmed that anti-site defects not only block the [010] diffusion channels but also decrease the energy barrier for inter-channel Li hopping, effectively reducing Li diffusion anisotropy. In particular, Amin et al. [1] reported the comparable Li diffusivities along the three axes in millimeter-sized LiFePO4 single crystals containing 2.5-3% Li-Fe anti-site defects. Although the reduced Li diffusion anisotropy in LiFePO4 single crystal is understood, the effects of anti-site defects on phase transformation kinetics in olivine LiFePO4 particles have not been clarified. Here we show that both the phase transformation kinetics and the surface reaction kinetics of LiFePO4 particles will be significantly affected by the anti-site defects. Recently, we combine operando hard X-ray spectroscopic imaging and phase-field modeling to show two-dimensional (2D) Li diffusion behaviors in micro-sized LiFePO4 rod containing ~3% Li-Fe anti-site defects. We obtain direct evidence that Li ions can be intercalated through the (100)/(001) surfaces, contradicting a common belief that only the (010) surface of LiFePO4 is electrochemically active. This study not only presents the first experimental confirmation of the previously hypothesized surface-reaction-limited (SRL) [6] phase boundary migration in LiFePO4, but also reveals a new hybrid mode of phase transformation, where the growth of new phase is controlled by surface reaction or Li diffusion in different crystallographic directions. Based on these findings, we propose the existence of three distinct kinetic regimes (SRL, hybrid, and bulk-diffusion-limited (BDL)) of phase transformations of LiFePO4 and potentially many other intercalation compounds undergoing first-order phase transformations (as shown in Figure 1a). In particular, our phase-field simulations show that 2D Li diffusion can significantly enhance Li insertion kinetics in SRL regime. In a LiFePO4 nanoparticle of 200 nm x 50 nm x L nm ([100] x [010] x [001]), our simulation results (Figure 1b) show that the traveling constant velocity of (100) straight phase boundary driven by 2D Li diffusion (D=1x10-11 cm2/s along [100] and [010]) is one magnitude larger than the velocity of the phase boundary driven by 1D Li diffusion with infinite diffusivity along [010]. The latter one is calculated by SCB theory [6]. Anti-site defects are commonly believed to deteriorate high rate performances of LiFePO4, due to its blocking to the 1D Li diffusion along [010]. Our counterintuitive finding, however, show that the enhanced Li diffusion along [100] by Li inter-channel hopping around the defects [2, 3] significantly enhance surface reaction kinetics by activating large fraction of electrochemically surface for Li insertion. The maximum phase boundary velocity driven by 2D Li diffusion is linear to particle size along [100] and inversely linear to thickness along [010]. Therefore, we propose that LiFePO4 nanosheets with large (010) surface and small thickness along [010] is beneficial for high rate capability and high energy density. This is consistent to experimental observations. Our studies show the important effects of anti-site defects on phase transformation kinetics and surface reaction kinetics in LiFePO4. This will provide new insights on potential avenues to improve performances of olivine electrodes via engineering anti-site defect formation and distribution.

Investigation of Antisite Defect Formation and Chemical Expansion in LiNiPO4 by in Situ Neutron Diffraction.

In situ neutron diffraction was used to characterize the effect of temperature on the crystal structure of LiNiPO4. LiNiPO4 adopts an ordered olivine structure at room temperature, but, with increasing temperature, this work shows that a significant amount of Li and Ni cation exchange occurs, for example, ∼15% at 900 °C. The antisite disorder is detected by residual nuclear densities on the M1 and M2 octahedral sites in the olivine structure using difference Fourier maps and by changes in cation site occupancies, lattice parameters, and mean ⟨M-O⟩ bond distances. The antisite disorder is also responsible for chemical expansion of the crystal lattice in addition to thermal expansion. Antisite defect formation at high temperature and its reversibility on cooling can be understood as an entropically driven feature of the crystal structure of LiNiPO4. The lithium ion diffusion pathway, that follows a curved trajectory along the b axis in the olivine structure, is, therefore, susceptible to be blocked if synthesis conditions are not carefully controlled and should also be influenced by the chemically expanded lattice of the disordered structure if this is preserved to ambient temperature by rapid cooling.

Speciation of uranium and doping induced defects in Gd1.98U0.02Zr2O7: Photoluminescence, X-ray photoelectron and positron annihilation lifetime spectroscopy

Based on photoluminescence spectroscopy it was inferred that uranium stabilizes as both U(IV) as well as U(VI) in Gd2Zr2O7 which was also corroborated using X-ray photo electron spectroscopy (XPS). Absence of equidistant vibronic structure in emission spectrum of Gd1.98U0.02Zr2O7 confirmed that U(VI) stabilizes in the form of UO66−. Based on luminescence lifetime it was inferred that majority of UO66− stabilizes at both Gd3+/Zr4+ whereas U4+ stabilizes only at Zr4+ sites. The positron lifetime doesn’t change on uranium doping indicating the formation of antisite defect. Infact it is this antisite defect in Gd1.98U0.02Zr2O7 which favours the stabilization of its fluorite phase.

Yttrium-Doped Sb2Te3: A Promising Material for Phase-Change Memory.

Sb2Te3 exhibits outstanding performance among the candidate materials for phase-change memory; nevertheless, its low electrical resistivity and thermal stability hinder its practical application. Hence, numerous studies have been carried out to search suitable dopants to improve the performance; however, the explored dopants always cause phase separation and thus drastically reduce the reliability of phase-change memory. In this work, on the basis of ab initio calculations, we have identified yttrium (Y) as an optimal dopant for Sb2Te3, which overcomes the phase separation problem and significantly increases the resistivity of crystalline state by at least double that of Sb2Te3. The good phase stability of crystalline Y-doped Sb2Te3 (YST) is attributed to the same crystal structure between Y2Te3 and Sb2Te3 as well as their tiny lattice mismatch of only ∼1.1%. The significant increase in resistivity of c-YST is understood by our findings that Y can dramatically increase the carrier's effective mass by regulating the band structure and can also reduce the intrinsic carrier density by suppressing the formation of SbTe antisite defects. Y doping can also improve the thermal stability of amorphous YST based on our ab initio molecular dynamics simulations, which is attributed to the stronger interactions between Y and Te than that of Sb and Te.

Li-ion storage dynamics in metastable nanostructured Li2FeSiO4 cathode: Antisite-induced phase transition and lattice oxygen participation

Abstract Li2FeSiO4 (LFS) has drawn much attention as cathode for high capacity Li-ion batteries. Even though significant volume of study has been devoted to its crystal chemistry and electrochemistry, many questions relating to its Li-ion storage dynamics remain yet to be fully elucidated. In this context, synchrotron-based X-ray diffraction and absorption spectroscopies are employed to characterize the phase stability and charge compensation mechanism in a metastable Li2FeSiO4 nanostructured cathode as a function of state-of-charge (Li2−xFeSiO4, x = 0, 0.25, 0.50, 0.75, 1.0) and cycling at very low current. The results demonstrate (i) no detectable phase transition from monoclinic to orthorhombic phase during the first charge-discharge cycle but rather formation of antisite defects that progressively induce phase transformation after several electrochemical cycles; (ii) characteristics of solid solution Li-ion storage (Li2−xFeSiO4, x = 0–1); and (iii) the charge compensation for the first Li extraction does not come solely from the ferrous to ferric conversion, but interestingly from prominent participation of lattice oxygen as well that appears to destabilize the cycled LFS structure with significant performance implications.

Synthesis and radiation tolerance of Lu2−xCexTi2O7 pyrochlores

As a nonradioactive surrogate for Pu, Ce is selected to study the solubility and radiation tolerance in pyrochlore matrixes. In this paper, we synthesized a series of Lu2−xCexTi2O7 (x = 0–0.7) samples. X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were used to study the structure of Lu2−xCexTi2O7 (x = 0–0.7) with different Ce3+ content x. The results show that the maximum solubility of Ce3+ in Lu2Ti2O7 is 32 mol%. In order to study the radiation tolerance of these compounds, 400 keV Ne2+ was used to perform the irradiation experiments. The grazing incident X-ray diffraction patterns revealed that the radiation tolerance of the Lu2−xCexTi2O7 decreases with increasing Ce-content. This correlation between radiation tolerance and Ce-content was interpreted in terms of the antisite defect formation energies of Lu2−xCexTi2O7 (x = 0, 0.5, 1) based on the density functional theory.

Sodiation Kinetics of Metal Oxide Conversion Electrodes: A Comparative Study with Lithiation

The development of sodium ion batteries (NIBs) can provide an alternative solution to lithium ion batteries (LIBs) for sustainable, low-cost energy storage. Due to the many similarities between sodium and lithium chemistries, the development of sodium-ion batteries (NIBs) – in particular the discovery and development of electrode materials – has been benefited greatly from the knowledge developed in understanding lithium chemistries in LIBs. However, many electrode materials used in LIBs show an inferior performance in sodiation. For example, in transition metal oxides MOx (M = Fe, Co and Ni), the sodiation reactions have shown much lower capacity and slower kinetics than those commonly observed in lithiation reactions. This could not be simply interpreted from the difference in the ionic size of Na+ and Li+ because the reaction happened in this system is not intercalation but conversion. For the conversion compounds, although there were many papers published to investigate their electrochemical performance, the origin of different reaction kinetics is still not clear which hinder the further study on this system. To address this issue, we present herein our comprehensive, systematic study of the sodiation and lithiation of a model MOx conversion compound, viz., nanostructured sheets of NiO. We investigate the sodiation and lithiation processes using a combination of multiple temporal- and spatial-scaled characterization techniques, including electrochemical measurements, in situ transmission electron microscopy (TEM), and ab initio molecular dynamics (MD) simulations. From the in situ TEM, where our previous results showing the lithiation heterogeneous reaction pathways, we found the sodiation only go through a “shrinking-core” pathway and further sodiation was blocked by a crystalline passivation layer of Na2O that formed at an early stage. Our MD simulation shows that the sodiation pathway originates from a layer-by-layer reaction that occurs during the insertion of Na ions; this was the rate-determining step for the entire process. In case of lithiation, however, the formation of Li anti-site defects significantly distorts the local NiO lattice, which thereby facilitates Li insertion and enhance the overall reaction rate. In conclusion, the intrinsic layer-by-layer sodiation reaction and the formation of a crystalline passivation layer of Na2O play an important role for sluggish sodiation reactions.

Structural View of Li Ion and Electron Correlation in Li2FeSiO4 Cathode

Orthosilicates, Li2 M SiO4, where M = Fe, Mn, Co, Ni, are characterized by a theoretical specific capacity that is twice that of LiFePO4, namely 340 vs. 170 mAh/g. In this presentation, we have probed the structural evolution and charge compensation of Li2FeSiO4 cathode using both in-situ/postmortem synchrotron XRD/XANES and first-principle calculations1-3. The results demonstrate that the formation of Li-Fe antisite defects play a key role in destabilizing the Li2FeSiO4 structure and “catalyzing” the monoclinic-to-orthorhombic transition. Then the charge compensation for the first Li extraction does not come solely from the ferrous to ferric conversion due to the strong electron correlation of Fe 3d orbitals, but interestingly from prominent participation of lattice oxygen as well that appears to destabilize the cycled structure. Using scanning transmission X-ray microscopy, it is revealed that the electrochemical lithiation/delithiation of Li2FeSiO4 electrode does not proceed uniformly but it exhibits an unusual solid solution pattern4 with the stored Li content varying locally possibly arising from a combination of inherent LFS material and electrode properties like poor conductivities, variable particle size/distribution and extent of electrolyte penetration. All these findings should have a thought-provoking influence on the study of the material itself and the broader Li-ion battery research community as well.

Antisite defects at oxide interfaces

We use ab initio calculations to estimate the formation energies of cation (transition-metal) antisite defects at oxide interfaces and to understand the basic physical effects that drive or suppress the formation of these defects. Antisite defects are found to be favored in systems with substantial charge transfer across the interface, while Jahn-Teller distortions and itinerant ferromagnetism can prevent antisite defects and help stabilize atomically sharp interfaces. Our results enable identification of classes of systems that may be more and less susceptible to the formation of antisite defects, and they motivate experimental studies and further theoretical calculations to elucidate the local structure and stability of oxide interface systems.

Antisite defects and valence state of vanadium in Na3V2(PO4)3

Electron paramagnetic resonance (EPR) studies have been performed with the aim of determining the valence state and local crystal structure of the nearest environment of vanadium ions in the initial, charged, and discharged samples of the cathode material NaxV2(PO4)3 (1 ≤ x ≤ 3). It has been found that the charged sample (x = 1) is characterized by an intense signal corresponding to V4+ ions located in a highly distorted octahedral crystal field. An EPR signal with the g-factor close to the g-factor of the V4+ ion has also been observed in the initial sample (x = 3), where the intensity of the resonance signal is one order of magnitude lower than that in the charged sample. It has been revealed that the resonance signal under consideration is associated with the formation of antisite defects when a part of vanadium ions are located in sites of sodium ions. It has also been found that the intensity of this signal increases after a complete charge–discharge cycle (x = 3).

Formation mechanism and elimination methods for anti-site defects in LiNbO3/LiTaO3crystals

Kanget al.summarized the mechanism of formation of anti-site defects in LiNbO3/LiTaO3crystals and the measures for growth of anti-site defect free LiNbO3/LiTaO3crystals to give the readers an overview in this field.