Bulk Oxygen Research Articles

Pseudo‐Bonding and Electric‐Field Harmony for Li‐Rich Mn‐Based Oxide Cathode

AbstractThe practical application of Li‐rich Mn‐based oxide cathode is predominately retarded by the capacity decline and voltage fading, associated with the structure distortion and anionic redox reactions. Here, a linkage‐functionalized modification approach to tackle these challenges via a synchronous lithium oxidation strategy is reported. The doping of Ce in the bulk phase activates the pseudo‐bonding effect, effectively stabilizing the lattice oxygen evolution and suppressing the structure distortion. Interestingly, it also induces the formation of spinel phase Li4Mn5O12 in the subsurface, which in turn constructs the phase boundaries, thereby arousing the interior self‐built‐in electric field to prevent the outward migration of bulk oxygen anions and boost the charge transfer. Moreover, the formed coating layer Li2CeO3 with oxygen vacancies accelerates Li+ diffusion and mitigates electrolyte cauterization. The corresponding cathode exhibits superior long‐cycle stability after 300 cycles with only a 0.013% capacity drop and 1.76 mV voltage decay per cycle. This work sheds new light on the development of Li‐rich Mn‐based oxide cathodes toward high energy density applications.

Tracing bulk elemental ratios in exoplanetary atmospheres with TiO chemistry

Deciphering the bulk elemental abundances of exoplanetary atmospheres is not an easy task, yet it is crucial to understanding the formation history of planets. The purpose of this work is to show that the observability of TiO features at optical wavelengths in the transmission spectra of hot Jupiter atmospheres is sensitive to the bulk chemical properties of the atmosphere. To this end, we ran a grid of chemical models, which include TiO formation and destruction, for the ultra-hot Jupiter WASP-19b and an ultra-hot version of HD 209458b. We take into account non-equilibrium chemistry and changes in the temperature and pressure structure of these atmospheres caused by different C/O ratios. We calculated synthetic transmission spectra for these models, and studied the relative strengths of TiO and H2O features quantitatively. To compare with observations, we used a model-independent metric for molecular abundances, ΔZTiO−H2O/Heq, which has previously been used in observational studies of exoplanetary atmospheres. We find that with this metric we can differentiate between different chemical models and place constraints on the bulk carbon and oxygen abundances of the atmosphere. From chemical considerations, we expected the TiO abundance to depend on the bulk nitrogen. However, we find that changes in N/H do not result in changes in the resulting TiO. We applied our method to a set of known exoplanets that have been observed in the relevant optical wavelengths and find good agreement between low-resolution observations and our model for WASP-121b, marginally good agreement with WASP-79b, WASP-76b, and WASP-19b, and poorer agreement with HD 209458b. Our method will be particularly helpful for indirect studies of the bulk abundances of carbon and oxygen.

SBA-16-Mediated Nanoparticles Enabling Accelerated Kinetics in Cyclic Methane Conversion to Syngas at Low Temperatures

To meet the globally growing energy demands, it is essential to develop schemes with higher fuel conversion efficiency at temperatures <1000 °C while suppressing emissions of CO2. Iron oxide nanoparticles supported by mesoporous silica SBA-16 (Fe2O3@SBA-16) is conceived and developed in this study for chemical looping partial oxidation, yielding syngas selectivity above 95% with operating temperatures as low as around 400 °C is achieved, a temperature that is 600 °C lower than the conventional operating temperature. The methane conversion rate for Fe2O3@SBA-16 is 52 and 660% higher than those for established nanoparticle oxygen carriers and bulk oxygen carriers, respectively. Dynamic Monte Carlo simulations are conducted that demonstrate the distinct effects of nanoparticle loading and particle size distribution on 3-D interconnected Fe2O3@SBA-16, affirming its accelerated reaction kinetics. This finding has significant implications in mesoporous materials and broadens research domains in other cyclic redox energy systems.

Valence Engineering via In Situ Carbon Reduction on Octahedron Sites Mn3O4 for Ultra‐Long Cycle Life Aqueous Zn‐Ion Battery

AbstractIn recent years, rechargeable aqueous zinc‐ion batteries (ZIBs) have received much attention. However, the disproportionation effect of Mn2+ seriously affects the capacity retention of ZIBs during cycling. Here, the capacity retention of the Mn3O4 cathode is improved by effective valence engineering. The valence engineering of Mn3O4 is caused by bulk oxygen defects, which are in situ derived from the Mn‐metal organic framework during carbonization. Bulk oxygen defects can change the (MnO6) octahedral structure, which improves structural stability and inhibits the dissolution of Mn2+. The ZIB assembled from bulk oxygen defects Mn3O4@C nanorod arrays (Od‐Mn3O4@C NA/CC) exhibits an ultra‐long cycle life, reaching 84.1 mAh g−1 after 12 000 cycles at 5 A g−1 (up to 95.7% of the initial capacity). Furthermore, the battery has a high specific capacity of 396.2 mAh g−1 at 0.2 A g−1. Ex situ characterization results show that initial Mn3O4 is converted to ramsdellite MnO2 for insertion and extraction of H+ and Zn2+. First‐principles calculations show that the charge density of Mn3+ increases greatly, which improves the conductivity. In addition, the flexible quasi‐solid‐state ZIB is successfully assembled using Od‐Mn3O4 @ C NA/CC. Valence engineering induced by bulk oxygen defects can help develop advanced cathodes for aqueous ZIB.

Oxygen vacancy engineering with flame heating approach towards enhanced photoelectrochemical water oxidation on WO3 photoanode

Abstract Oxygen vacancies are a double-edged sword for photoelectrochemical (PEC) devices and a comprehensive understanding on their role in PEC processes is important. Although there is existing work that illustrates their effect on PEC processes, clarifying the influence of surface and bulk oxygen vacancies, respectively, on the PEC performance is challenging. Herein, we present the fabrication of WO3 photoanodes with tunable bulk and surface oxygen vacancy with a novel two-step flame heating approach that is proving to be a powerful platform for this purpose. We found that both the conductivity and trapping state density of the WO3 photoanode increase with bulk oxygen vacancy density, leading to a volcano-like relationship between the kinetics of charge separation/transport and bulk oxygen vacancy density. Moreover, both the interfacial charge transfer rate constant and the charge recombination rate constant on the surface of the WO3 photoanode increase with surface oxygen vacancy density, which also leads to a volcano-like relationship between the charge injection efficiency and surface oxygen vacancy density. By tuning the surface and bulk oxygen vacancy density simultaneously, the PEC performance of the WO3 photoanode was increased by ca. 10 times, which illustrates the strength of delicate oxygen vacancy engineering in optimizing PEC devices.

Isotopic and textural analysis of giant unmelted micrometeorites – identification of new material from intensely altered 16O-poor water-rich asteroids

Bulk oxygen isotope data has the potential to match extraterrestrial samples to parent body sources based on distinctive δ18O and Δ17O ratios. We analysed 10 giant (>500 μm) micrometeorites using combined micro-Computer Tomography (μCT) and O-isotope analysis to pair internal textures to inferred parent body groups. We identify three ordinary chondrite particles (L and LL groups), four from CR chondrites and the first micrometeorite from the enstatite chondrite (EH4) group. In addition, two micrometeorites are from hydrated carbonaceous chondrite parent bodies with 16O-poor isotopic compositions and plot above the terrestrial fractionation line. They experienced intense aqueous alteration, contain pseudomorphic chondrules and are petrographically similar to the CM1/CR1 chondrites. These micrometeorites may be members of the newly established CY chondrites and/or derived from the enigmatic “Group 4” micrometeorite population, previously identified by Yada et al., 2005 [GCA, 69:5789-5804], Suavet et al., 2010 [EPSL, 293:313-320] (and others). One of our 16O-poor micrometeorite plots on the same isotopic trendline as the CO, CM and CY chondrites – “the CM mixing line” (with a slope of ∼0.7 and a δ17O intercept of -4.23‰), this implies a close relationship and potentially a genetic link to these hydrated chondrites. If position along the CM mixing line reflects the amount of 16O-poor (heavy) water-ice accreted onto the parent body at formation, then the CY chondrites and these 16O-poor micrometeorites must have accreted at least as much water-ice as CM chondrites but potentially more. In addition, thermal metamorphism could have played a role in further raising the bulk O-isotope compositions through the preferential loss of isotopically light water during phyllosilicate dehydration. The study of micrometeorites provides insights into asteroid belt diversity through the discovery of material not currently sampled by larger meteorites, perhaps as a result of atmospheric entry biases preventing the survival of large blocks of friable hydrated material.

Highly reversible oxygen redox in layered compounds enabled by surface polyanions

Oxygen-anion redox in lithium-rich layered oxides can boost the capacity of lithium-ion battery cathodes. However, the over-oxidation of oxygen at highly charged states aggravates irreversible structure changes and deteriorates cycle performance. Here, we investigate the mechanism of surface degradation caused by oxygen oxidation and the kinetics of surface reconstruction. Considering Li2MnO3, we show through density functional theory calculations that a high energy orbital (lO2p’) at under-coordinated surface oxygen prefers over-oxidation over bulk oxygen, and that surface oxygen release is then kinetically favored during charging. We use a simple strategy of turning under-coordinated surface oxygen into polyanionic (SO4)2−, and show that these groups stabilize the surface of Li2MnO3 by depressing gas release and side reactions with the electrolyte. Experimental validation on Li1.2Ni0.2Mn0.6O2 shows that sulfur deposition enhances stability of the cathode with 99.0% capacity remaining (194 mA h g−1) after 100 cycles at 1 C. Our work reveals a promising surface treatment to address the instability of highly charged layered cathode materials.

Ionospheric Outflow During the Substorm Growth Phase: THEMIS Observations of Oxygen Ions at the Plasma Sheet Boundary

AbstractIonospheric outflow is an important plasma source that feeds the near‐Earth magnetotail with heavy oxygen ions. Because these ions can significantly alter the structure and stability of the magnetotail current sheet, the characteristics of this outflow are important for accurate magnetosphere modeling, including modeling of substorms—a key element of magnetosphere dynamics. Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft measurements in the magnetotail (around the plasma sheet boundary), we investigate characteristics oxygen outflows observed during substorm growth phases. The observed oxygen ion temperature and flow energy indicate that the outflow is marginally stable to ion acoustic wave generation: The oxygen temperature is slightly lower than the electron temperature and slightly higher than the oxygen flow energy. Moreover, the observed outflows are accompanied by low frequency electrostatic waves that may contribute to outflow thermalization. The oxygen bulk velocity has a significant component directed toward the equatorial plane, originating from the cross‐field drift in the convection electric field. The estimated radial distances at which oxygen ions reach the plasma sheet are ∼20–40RE downtail, that is, outflows during substorm growth phases can alter current sheet characteristics around the potential magnetic reconnection region.

Surface chemical analysis of copper powder used in additive manufacturing

Additive manufacturing (AM) has during years gained significant interest owing to its endless component design possibilities. One of the most popular AM techniques is laser powder bed fusion (LPBF), which selectively melts metal powder layer‐by‐layer in a chamber with protective argon atmosphere. This technique is attractive for realizing Cu‐based products in which the high electrical conductivity of Cu is combined with component design possibilities. The successful use of Cu powder not only poses challenges owing to the high reflectivity and thermal conductivity of Cu but also involves the important concern of controlling the powder surface chemistry since the powder surface constitutes the main source of oxygen. It is of crucial importance to control the oxygen level in order to maintain good electrical conductivity and brazing ability of the AM‐fabricated Cu‐part. In LPBF, fine spherical powder with size of 10–60 μm is used, providing significant specific surface area, and this powder is also usually recycled several times, and hence, the role of powder surface chemistry is evident. Two kinds of copper powder with purities 99.70 and 99.95 wt% were analysed in both virgin and in used conditions after numerous printing cycles using LPBF. The powder was analysed by X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). A clear difference between the two powder grades in terms of surface chemistry was observed. The oxide thickness and bulk oxygen content increased for both powder grades after recycling. The surface oxides under different conditions are identified and the effect of powder purity on the oxide formed is discussed.

Creation of oxygen vacancies to activate lanthanum-doped bismuth titanate nanosheets for efficient synchronous photocatalytic removal of Cr(VI) and methyl orange

Bi3.25La0.75Ti3O12 (BLTO) nanosheets were synthesized via a simple hydrothermal process, followed by an economically facile vacuum-deoxidized treatment to fabricate Bi3.25La0.75Ti3O12-x (BLTO12-x) nanosheets with abundant surface oxygen vacancies. Relative ratio of surface and bulk oxygen vacancy, as evidenced with positron annihilation lifetime and X-ray photoelectron spectra, was controllable via adjusting vacuum-deoxidized temperature and time. Photocatalytic performance of the pristine and deoxidized nanosheets for synchronous photocatalytic removal of Cr(VI) and methyl orange (MO) were thoroughly investigated. Control experimental results show that the photocatalytic activities of the BLTO12-x nanosheets significantly relied on vacuum-deoxidized temperature and time. The optimal deoxidized nanosheets as activated at 330 °C for 3 h, which has the largest relative concentration of surface oxygen vacancy, shows highest activity under visible light irradiation. Moreover, the synergetic effect for synchronous enhanced photocatalytic degradation of Cr(VI) and MO were observed over all BLTO12-x nanosheets. The detailed synergistic reduction−oxidation removal mechanism is proposed. The surface oxygen vacancy led to rise of valence band maximum, which is responsible for both widening of valence band and decreasing of band gap. The former enhances separation efficiency of photoinduced e−/h + pairs, and the latter extends photoresponse range into visible light region. As a result, compared with pristine BLTO, the visible-light catalytic activity of deoxidization-activated BLTO12-x was significantly enhanced. The vacuum-deoxidized strategy proposed in this work can introduce oxygen vacancy into the matrix of catalyst, which provides an effective way to design and prepare high-efficiency oxides based photocatalyst.

Temperature-dependent Rutile Solubility in Garnet and Clinopyroxene from Mantle Eclogite: Implications for Continental Crust Formation and V-based Oxybarometry

Abstract Despite its accessory mineral status in metabasaltic rocks, rutile controls the whole-rock Ti, Nb and Ta budget. These are key elements used to trace fluid- and melt-mediated mass transfer across the mantle–crust boundary. Rutile also contains significant amounts of the redox-sensitive element V, which is increasingly used to estimate oxygen fugacity. Kimberlite-borne mantle eclogite xenoliths, which are frequently rutile-bearing, have been interpreted as residues from the extraction of silicic partial melt similar in composition to the average continental crust. Published mineral compositions for eclogite xenoliths from various cratons combined with geothermobarometrical calculations show that TiO2 contents in garnet and clinopyroxene increase with increasing temperature of last residence in the lithospheric mantle, whereas apparent clinopyroxene–garnet distribution coefficients decrease. This implies that (1) increasing TiO2 contents in eclogitic garnet or clinopyroxene are not a signature of increasing metasomatism with depth, (2) whole-rock eclogites reconstructed without rutile will increasingly underestimate TiO2, Nb and Ta contents with decreasing temperature, and (3) low-temperature eclogites are more likely to contain free rutile. Only about a third of the ∼250 samples considered here would have whole-rock TiO2 contents (reconstructed with calculated rutile modes) required for rutile saturation during subduction and partial melting. If there is a role for subducting oceanic crust now sampled as mantle eclogite, the characteristic Ti–Nb–Ta depletion in continental crust may require fluid-dominated processes, where these elements are not efficiently mobilised. In garnet, Ti uptake on the octahedral site is accommodated primarily by coupled substitution with Na and subordinately with a divalent metal cation, and there is no evidence for substitution on the tetrahedral site. For samples equilibrated to the conductive geotherm, Ti in addition to Na enrichment may be indicative of equilibration in the diamond stability field. The jadeite component in clinopyroxene as a function of temperature is a good indicator of the geotherm to which the various samples equilibrated, and can be used to reveal samples within each suite that have been affected by isobaric heating. The distribution of V in eclogitic garnet, clinopyroxene and rutile is affected by bulk composition, temperature and oxygen fugacity. In carefully vetted, low-temperature samples with TiO2 contents &amp;gt;0·8 wt%, V-based oxybarometry may monitor redox conditions prevailing during metamorphism of oceanic crust or, at lower TiO2, during (secular) cooling-related exsolution of rutile from garnet or clinopyroxene, whereas in higher-temperature ilmenite-bearing samples metasomatic conditions may be recorded.

Defects evolution of Ca doped La2NiO4+δ and its impact on cathode performance in proton-conducting solid oxide fuel cells

As a typical Ruddlesden-Popper oxide, La2NiO4+δ draws special attention for its high oxygen ion conducting behavior and special interstitial oxygen defects which enables it a promising electrocatalyst toward oxygen reduction reaction. In this work, Ca-doped La2NiO4+δ samples are prepared and their structure and defect evolution are investigated as Ca content. Electrical conductivity and electron conduction relaxation (ECR) investigations suggest that La1.9Ca0.1NiO4+δ has the great electronic conductivity and the highest oxygen surface exchange coefficient and oxygen bulk diffusion coefficient at intermediate temperatures. These results may imply that more gas oxygen has inserted into La1.9Ca0.1NiO4+δ sample, suggesting the native interstitial oxygen defects in it. X-ray photon spectroscopy (XPS) results confirm that La1.9Ca0.1NiO4+δ have more active oxygen species when compared with La2NiO4+δ and La1.8Ca0.2NiO4+δ. The great oxygen exchange and bulk diffusion properties along with its great stability in steam involved atmosphere enables it a promising for cathode for H–SOFCs. Compared with that using La2NiO4+δ cathode, peak powder density of H–SOFC using La1.9Ca0.1NiO4+δ single phase cathode improves about 30.5% at 700 °C, suggesting that accelerating oxygen reduction reaction can effectively improve cathode performance of H–SOFCs.

FeO6 Octahedral Distortion Activates Lattice Oxygen in Perovskite Ferrite for Methane Partial Oxidation Coupled with CO2 Splitting.

Modulating lattice oxygen in metal oxides that conducts partial oxidation of methane in balancing C-H activation and syngas selectivity remains challenging. This paper describes the discovery of distorting FeO6 octahedra in La1-xCexFeO3 (x = 0, 0.25 0.5, 0.75, 1) orthorhombic perovskites for the promotion of lattice oxygen activation. By combined electrical conductivity relaxation measurements and density functional theory calculations studies, this paper describes the enhancement of FeO6 octahedral distortion in La1-xCexFeO3 promoting their bulk oxygen mobility and surface oxygen exchange capability. Consequently, La0.5Ce0.5FeO3 with the highest FeO6 distortion achieves exceptional syngas productivity of ∼3 and 8 times higher than LaFeO3 and CeFeO3, respectively, in CH4 partial oxidation step with simultaneous high CO2 conversion (92%) in the CO2-splitting step at 850 °C. The results exemplify the feasibility to tailor the active lattice oxygen of perovskite by modulating the distortion of BO6 in ABO3, which ultimately influences their reaction performance in chemical looping processes.

Electrochemical characterization of La2NiO4-infiltrated La0.6Sr0.4Co0.2Fe0.8O3-δ by analysis of distribution of relaxation times

Mixed ionic-electronic conductor LSCF is a popular cathode material for intermediate to low temperature solid oxide fuel cells; nevertheless, there are concerns about its surface-limited oxygen reduction reaction. In this work, Ruddlesden-Popper-structured La2NiO4+δ (LNO) nanoparticles are introduced onto the surface of (La0.6Sr0.4)0.995Co0.2Fe0.8O3-δ (LSCF) cathode and the electrode performance is examined by means of electrochemical impedance spectroscopy (EIS) in the temperature range of 550–750 °C. Impedance spectra analysis shows that an 81% reduction in polarization resistance can be achieved by infiltration of LNO nanoparticles. The distribution of relaxation times (DRT) technique is used to gain insights on the physicochemical properties of the system under study. Obtained results reveal that by correlating oxygen surface exchange, bulk diffusivity, ionic conduction, and particle size of nano-structured LNO with the electronic and ionic conductivity of LSCF, the electrochemical behavior of the cathodes is improved.

Revealing the role of oxygen vacancies in bimetallic PbBiO2Br atomic layers for boosting photocatalytic CO2 conversion

The effect of surface defects on photogenerated carriers separation and CO2 activation is not always clear. Herein, the well-defined bimetallic oxyhalide PbBiO2Br atomic layers with tunable oxygen vacancies (OVs) concentrations have been synthesized and investigated to reveal the role of OVs for photocatalytic CO2 conversion. Ultrafast transient absorption spectroscopy and theoretical calculations revealed that the rich oxygen vacancies (ROV) in PbBiO2Br atomic layers facilitated the transfer of photogenerated electrons from the internal structure to surface and prolonged their lifetime on the surface OVs. PbBiO2Br atomic layers with ROV demonstrated superior CO2 reduction performance with enhanced CO evolution rate of 4.58 μmol g−1 h−1 in water under visible light irradiation, which was 2.05 and 10.90 times higher than that of deficient oxygen vacancies (DOV) PbBiO2Br atomic layers and bulk PbBiO2Br, respectively. Intermediate products of CO2 conversion were investigated by in-situ Fourier-transform infrared spectroscopy and the corresponding reaction mechanism was proposed.

Oxygen diffusion in Mg-doped Sm and Gd zirconates with pyrochlore structure

Oxides with pyrochlore structure are promising materials for SOFCs, oxygen and hydrogen separation membranes, and other devices. Ln zirconates attract interest due to their oxygen and protonic mobility. In this work, oxygen transport mechanism in Mg-doped Sm and Gd zirconates was studied. Sm1.9Mg0.1Zr2O6.95 and Gd1.9Mg0.1Zr2O6.95 oxides have been prepared via the mechanical activation and characterized by XRD, TGA, and SEM. Oxygen transport features were studied by the oxygen isotope exchange with C18O2. Isotope substitution of bulk oxygen was demonstrated to be described by the 2D diffusion model. Very fast diffusion within grain boundaries provides fast isotope label distribution over the oxide domains, then tracer distributes due to a slower diffusion within domains’ bulk. Three forms of oxygen were distinguished within grains’ bulk. Very fast diffusion along grain boundaries (Dgb ~ 10−7 cm2/s at 700 °C) can provide high rates of oxygen transport in oxygen separation membranes.

Europium-Doped Ceria Nanowires as Anode for Solid Oxide Fuel Cells

CeO2-based materials have been studied intensively as anodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this work, pristine and europium (Eu)-doped CeO2 nanowires were comprehensively investigated as anode materials for IT-SOFCs, by a combination of theoretical predictions and experimental characterizations. The results demonstrate: (1) Oxygen vacancies can be energetically favorably introduced into the CeO2 lattice by Eu doping; (2) The lattice parameter of the ceria increases linearly with the Eu content when it varies from 0 to 35 mol.%, simultaneously resulting in a significant increase in oxygen vacancies. The concentration of oxygen vacancies reaches its maximum at a ca. 10 mol.% Eu doping level and decreases thereafter; (3) The highest oxygen ion conductivity is achieved at a Eu content of 15 mol.%; while the 10 mol.% Eu-doped CeO2 sample displays the highest catalytic activity for H2-TPR and CO oxidization reactions. The conducting and catalytic properties benefit from the expanded lattice, the large amount of oxygen vacancies, the enhanced reactivity of surface oxygen and the promoted mobility of bulk oxygen ions. These results provide an avenue toward designing and optimizing CeO2 as a promising anode for SOFCs.

Correlation between oxygen surface exchange rate and surface structure in the La1.5Sr0.5Ni1-yCoyO4+δ ceramic membranes

The La1.5Sr0.5Ni1-yCoyO4+δ (y = 0, 0.2, 0.4) complex oxides were synthesized by the citrate-nitrate method. The disk-shaped dense ceramic membranes with thickness of 0.8 and 1.2 mm for each composition were prepared by uniaxial pressing of the powders and sintering at 1350 °C in air. The phase purity of the samples was confirmed by the X-ray diffraction (XRD). The oxygen excess in the samples gradually increased with cobalt substitution and was almost independent of temperature. Oxygen permeation studies through the membranes with different thickness revealed that the cobalt doping eliminated the surface exchange limitations leading to a significant increase in the oxygen surface exchange coefficients (kex). On the contrary, the bulk oxygen diffusion was deteriorated by the cobalt doping, although the oxygen-ion conductivity remained almost unchanged. The electron backscatter diffraction (EBSD) analysis of the surface for the studied ceramic membranes indicated that the observed increase in the kex values could be explained by the effect of cobalt on the crystallographic orientation of crystallites. It is suggested that the cobalt doping increases the surface area of the (1‾10), (1‾11) and (011) faces leading to a higher number of 3d-metals in the surface layer of the membranes.

Dielectric Polarization in Inverse Spinel-Structured Mg2 TiO4 Coating to Suppress Oxygen Evolution of Li-Rich Cathode Materials.

High-energy Li-rich layered cathode materials (≈900 Wh kg-1 ) suffer from severe capacity and voltage decay during cycling, which is associated with layered-to-spinel phase transition and oxygen redox reaction. Current efforts mainly focus on surface modification to suppress this unwanted structural transformation. However, the true challenge probably originates from the continuous oxygen release upon charging. Here, the usage of dielectric polarization in surface coating to suppress the oxygen evolution of Li-rich material is reported, using Mg2 TiO4 as a proof-of-concept material. The creation of a reverse electric field in surface layers effectively restrains the outward migration of bulk oxygen anions. Meanwhile, high oxygen-affinity elements of Mg and Ti well stabilize the surface oxygen of Li-rich material via enhancing the energy barrier for oxygen release reaction, verified by density functional theory simulation. Benefited from these, the modified Li-rich electrode exhibits an impressive cyclability with a high capacity retention of ≈81% even after 700 cycles at 2 C (≈0.5 A g-1 ), far superior to ≈44% of the unmodified counterpart. In addition, Mg2 TiO4 coating greatly mitigates the voltage decay of Li-rich material with the degradation rate reduced by ≈65%. This work proposes new insights into manipulating surface chemistry of electrode materials to control oxygen activity for high-energy-density rechargeable batteries.

The Role of Surface Oxygen Vacancies in BiVO4

Bismuth vanadate (BiVO4) is a widely studied oxide in solar water splitting, known for its ease of synthesis, high charge extraction yields, and advantageous band alignment with water. We present a combined first-principles and experimental study of the electronic structure of the (010) surface of BiVO4 aimed at disentangling the impact of the surface and bulk oxygen vacancies on the electronic structure and transport properties. We found that oxygen vacancies are deep donors at the surface as they are in the bulk; our calculations on defect and polaron formation energies suggest that, while polarons formed from oxygen vacancies in the bulk can contribute to conductivity, those at the surface likely do not. Our results also show that out-of-plane structural relaxations at the surface contribute to the relatively immobile nature of electron polarons derived from surface oxygen vacancies. The structural model derived from first-principles calculation was validated by comparing computed results with experimental measurements of single-crystal and epitaxially grown single-crystalline BiVO4 samples. We also found a reasonably good agreement between our calculated and measured work functions for BiVO4 samples with and without oxygen vacancies.