Charge Compensation Process Research Articles

Accessing Cation and Anion Redox in a 1-D Iron-Chalcogenide, Na3Fe2S4-XSex (x = 0, 2, & 4), in Na-Ion Batteries

The utilization of anionic redox activity is gaining significant interest in chalcogen based alkali-ion batteries. In this study, we are reporting reversible Na-ion (de)intercalation in one-dimensional structure of Na3Fe2S4-xSex (x = 0, 2, and 4). The crystal structure of Na3Fe0.5 2+Fe0.5 3+Q4 (Q = S and Se) can be visualized as a mixed valent iron, Fe2+/Fe3+ , forming infinite edge-shared chain of FeQ 4 tetrahedra with Na-ion residing in the interchain voids. While cycling the cathodes within a potential range of 1.5 – 2.5 V vs. Na/Na+, we could reversibly (de)intercalate one mole of Na from the structure. Mössbauer spectroscopy reveals a complete Fe2+ to Fe3+ oxidation in all the charged samples. Substitution of less electronegative Se2– in the structure systematically lowers the Fe2+/3+ redox couple during cycling. After 100 cycles at 1C rate, the cathode retains 83 % of its initial reversible capacity. While cycling the same cathode at a higher voltage range of 1.5 – 3.0 V, we observed a flat plateau profile at ~2.6 V with capacity reaching 255 mAh/g for Na3Fe2S4 cathode. This high capacity can be directly attributed to the anion oxidation for the charge compensation process during ~3.0 Na-ion deintercalation as supported by XPS analysis. The flat plateau profile further decreases from 2.6 to 2.5 V with the substitution of Se2–. The GITT results show two distinct kinetic behavior for the cation (fast) and anion (sluggish) redox. The decoupling of cation and anion redox in this system can enable us to understand and optimize the utilization of both redox chemistries to achieve a high energy density cathode.

Towards the Direct Observation of the Local and Electronic Structure Evolution of Positive Electrode Materials by X-Ray Raman Scattering and X-Ray Emission Spectroscopy

The emergence of high brilliance synchrotron sources and the availability of more sophisticated infrastructure have opened doors to advanced material characterization, enabling a profound understanding of new processes and mechanisms in battery research. [1] More specifically, non-resonant Valence-to-Core X-ray emission spectroscopy (VtC-XES) and X-ray Raman scattering (XRS) stand out as complementary photon-in/photon-out X-ray spectroscopy techniques. The unique capability of these techniques lies in their access to soft X-ray edges using a hard X-ray beam, providing comprehensive information on both the electronic and local structure, due to their high sensitivity to local structure and coordination. [2,3] In this work, two different families of materials with different compositions and crystal structures are studied using XES and XRS.The first part focuses on the detailed study of oxygen substituted KVPO4F1-xOx (x = 0, 0.25, 0.5, 0.75, 1) using VtC-XES combined to ab initio theoretical calculations. The average working potential increases with increasing oxygen content by activating the V3+/4+ and V4+/5+ redox couples and replacing the VO4F2 “ionic” entity by {V=O}O5 unit with a highly “covalent” vanadyl-type bond. [4] VtC-XES allowed us to distinguish the contributions of V-F, V-O, and V=O bonds in the local environment, with spectral intensities highly correlated to the F- and O2- anionic composition. In addition, specific spectral evolutions are also associated to the presence of highly covalent V=O bonds, strongly influencing the electrode potential of the material.The second part examines a series of electrochemically obtained LiyNiO2 layered oxide materials (y = 0.02, 0.25, 0.33, 0.5, 0.67, 1.0). VtC-XES performed at the Ni Kβ emission line allows to study the local structure evolution around Ni, whereas XRS spectroscopy at both the O K-edge and Ni L2,3 L-edges gives an access to the interplay between Ni and O through the hybridized states of Ni 3d and O 2p. [5] This work provides a survey on both the cationic and anionic redox processes, and unveils the underlying phenomena related to the charge compensation processes occurring in this material.Overall, we present a comprehensive study of different polyanionic and layered oxide positive electrode materials using complementary X-ray spectroscopic techniques to probe both the local and electronic structures. We demonstrate that they allow to follow and understand evolution and changes in the local structure in materials of intricate crystal structures.AcknowledgementThis work was supported by the DESTINY Marie Skłodowska-Curie Actions COFUND PhD Programme (Grant Agreement #945357) co-funded by the European Union's Horizon2020 research and innovation program and the Synchrotron SOLEIL. ANR is also acknowledged for funding the RS2E network through the STORE-EX Labex Project ANR-10-LABX-76-01. Alistore-ERI network is also acknowledged.

Regulating electronic structure of anionic oxygen by Ti4+ doping to stabilize layered Li-rich oxide cathodes for Li-ion batteries

Abstract Layered Li-rich oxide cathodes enable to activate lattice oxygen anions redox in the charge compensation process and provide superior high specific capacity over 250 mAh g−1 due to their unique configuration, and thus attracting great attentions as promising cathode candidates for Li-ion batteries. However, how to better stabilize the bulk lattice oxygen framework and surface structure, and slow down the release of oxygen, is still major bottleneck to develop high performance Li-rich materials. Transition metal ions with outer d0 electronic configuration have distortable configuration, which can accommodate the local structure and chemical environment of the material, and then improve structural stability. Herein this work, the d0 transition metal Ti4+ is used as doping element to improve the chemical and structural stability, capacity retention and lithium ion diffusion kinetics of Li-rich material. The role of Ti in the material modification is revealed through synchrotron-based soft x-ray absorption spectroscopy, XRD, XPS and electrochemical tests. The improvement in structural stability can be attributed to that Ti doping can adjust the hybridization of O2p and TM3d to regulate the local electronic structure of both bulk lattice oxygen and surface oxygen vacancies. It is hoped that this work should shed light on the development of high-performance cathode materials for Li-ion Batteries.

Dual-Site Doping in Transition Metal Oxide Cathode Enables High-Voltage Stability of Na-Ion Batteries.

Designing cathode materials that effectively enhancing structural stability under high voltage is paramount for rationally enhancing energy density and safety of Na-ion batteries. This study introduces a novel P2-Na0.73K0.03Ni0.23Li0.1Mn0.67O2 (KLi-NaNMO) cathode through dual-site synergistic doping of K and Li in Na and transition metal (TM) layers. Combining theoretical and experimental studies, this study discovers that Li doping significantly strengthens the orbital overlap of Ni (3d) and O (2p) near the Fermi level, thereby regulates the phase transition and charge compensation processes with synchronized Ni and O redox. The introduction of K further adjusts the ratio of Nae and Naf sites at Na layer with enhanced structural stability and extended lattice space distance, enabling the suppression of TM dissolution, achieving a single-phase transition reaction even at a high voltage of 4.4V, and improving reaction kinetics. Consequently, KLi-NaNMO exhibits a high capacity (105 and 120 mAh g-1 in the voltage of 2-4.2V and 2-4.4V at 0.1 C, respectively) and outstanding cycling performance over 300 cycles under 4.2 and 4.4V. This work provides a dual-site doping strategy to employ synchronized TM and O redox with improved capacity and high structural stability via electronic and crystal structure modulation.

Integrating K+ into Eu and Tb doped GdCa4O(BO3)3: A dual study on photoluminescence and structure

In this study, we investigate the structural and photoluminescence (PL) properties of rare-earth-doped GdCa4O(BO3)3 (GdCOB) phosphors, specifically focusing on the spectral behaviour induced by doping with Eu³⁺ and Tb³⁺ ions. The powder X-ray diffraction (XRD) spectra confirm the formation of a monoclinic phase. The XRD data were also refined by a Rietveld refinement method. The existence of B, O, Ca, Gd, Tb, Eu and K elements was verified by EDS spectra. We introduce a detailed examination of the charge compensation process using Kröger-Vink notation to clarify the mechanisms essential for tailoring the optical properties of the phosphors. The PL excitation spectrum of GdCOB:Eu3+, monitored at 611 nm, reveals sharp excitation peaks at 319, 361, 380, and 392 nm, corresponding to 7F0→5H3, 7F0→5D4, 7F0→7F0, and 7F0→5L6 transitions, respectively. The PL spectrum under excitation of 392 nm exhibits that phosphors doped with Eu3+ a significant red emission at 611 nm, which is attributed to the 5D₀→7F₂ transition. This emission intensity is particularly enhanced at non-centrosymmetric sites of the Eu³⁺ ions. Similarly, the PL excitation spectrum of GdCOB:Tb3+, monitored at 552 nm, displays distinct excitation peaks at 316, 341, 353, and 379 nm, which correspond to the transitions 7F₆→5D₀, 7 F₆→5L₇, 7F₆→5D₂, and 7F₆→5D₃, respectively. Tb³⁺-doped phosphors display a bright green emission, with a dominant peak at 552 nm, resulting from the 5D₄→7F₅ transition. Additionally, the introduction of K⁺ ions as co-dopants results in modifications to the local environments of Eu³⁺ and Tb³⁺ ions. These changes allow for fine-tuning of the emission peaks, significantly enhancing the luminescent output of the phosphors. Optimal doping concentrations of 5 mol% for Eu³⁺ and 1 mol% for Tb³⁺ enhance luminescent intensity and prevent concentration quenching. This phenomenon, often resulting in reduced PL intensity at higher dopant levels, is primarily due to dipole-dipole interactions, consistent with Dexter's theory of energy transfer. Strategic modulation of doping concentrations, coupled with a deep understanding of energy transfer mechanisms are critical for the development of advanced luminescent materials Our analysis of the Commission de l′Eclairage (CIE) chromaticity coordinates reveals enhanced energy transfer dynamics in rare-earth-doped borates, facilitating the tuning of luminescent properties. These results not only deepen our understanding of the fundamental physics governing such phosphors but also open pathways for the development of optoelectronic applications requiring consistent color output, such as LED technologies and solid-state lighting.

Unraveling the chemical and structural evolution of novel Li-rich layered/rocksalt intergrown cathode for Li-ion batteries

The prototype material, Li1.23Ru0.41Ni0.36O2, is proposed to gain the deep and comprehensive understanding of chemical and structural changes of the novel layered/rocksalt intergrown cathodes. Synchrotron-based X-ray absorption spectra and resonant inelastic X-ray scattering reveal that both cationic and anionic redox evolves in the charge compensation process of the intergrown material, while synchrotron-based extended X-ray fine structure spectra and in situ X-ray diffraction measurements demonstrates that the intergrown material undergoes minimal local- and long-range structural variations at deep de/lithiation. This work highlights the great potential of the intergrown structure to inspire the design of advanced cathode materials for lithium-ion batteries.

Time-pH and time-humidity scaling of ionic conductivity spectra of polyelectrolyte multilayers.

In this systematic study, ionic conductivity spectra of poly(diallyl-dimethylammonium)/poly(acrylic acid) (PDADMA/PAA)n polyelectrolyte multilayers (PEMs) are investigated regarding superposition principles. In this context, charge transport as well as charge compensation processes in polyelectrolyte assemblies are discussed. The validity of different scaling concepts is tested to differentiate between changes in the mobility and charge carrier density, caused by the variation of a parameter X, where X is either relative humidity during measurement, or salt concentration or pH during preparation. For the first time, time-X scaling for conductivity spectra of PEMs is reported for all three parameters X, resulting in individual mastercurves. Furthermore, a super-mastercurve can be obtained including variations of all three parameters. Changes in plasticization caused by either varied humidity, pH or ionic strength imply non-constant charge carrier mobilities in accordance with a Summerfield-type of scaling, while the charge carrier density remains constant. Interestingly, for preparation conditions which favor extrinsic charge compensation, significant deviations from such Summerfield-type scaling are observed, indicating a variation of the number density of mobile charge carriers with humidity.

Improving the blue emission from Eu2+ by codoping Re3+ ions (Re[dbnd]Y, La, Gd, Lu) in NaMgPO4

Codoping Ce3+ is an effective strategy to improve the blue emission from Eu2+ in NaMgPO4:Eu2+ phosphor, while the origin of the luminescence enhancement is still ambiguous. Here, Re3+ (ReY, La, Gd, Lu) ions instead of Ce3+ are codoped into NaMgPO4:Eu2+ phosphor and the effect of Re3+ substituting for Na+ on the photo-luminescence and photo-phosphorescence properties is investigated. Codoping Re3+ enhances the emission intensity of Eu2+ greatly, as observed by measuring the optical properties. We suggest that codoped Re3+ ions could stabilize Na + vacancies via a charge compensation process, prompt the reduction of Eu3+ to Eu2+, increase the number of Eu2+ ions, and finally improve the luminescence properties. Meanwhile, a blue- or red-shift behavior of Eu2+ emission is witnessed in NaMgPO4:Eu2+,Re3+ system, depending on the size of Re3+ ions. This could be attributed to the alterable crystal field splitting for Eu2+ caused by the incorporation of Re3+ ions. All the results suggest that codoping Re3+ has an obvious influence on the luminescent behavior of Eu2+ in this system. This work also promotes research on the effect of codoping Ce3+ on the luminescence properties of NaMgPO4:Eu2+ phosphor, and offers new ideas for developing Eu2+-activated phosphors.

Origin of over-cycling tolerance achieved by metal phosphate coating for transition metal oxide lithium-ion batteries

Surface coating methods with metal phosphates (e.g., AlPO4, NiPO4, CoPO4, Co3(PO4)2, FePO4) have been widely employed to improve the electrochemical performance of the layered LiNi1-x-yCoxMnyO2 (NMC, where x < 1 and y < 1) battery system. However, the key mechanism that underpins the performance/stability improvement is not fully understood. Herein, an amorphous FePO4 coating method is chosen to investigate the effects of the coating on the electrochemical property and the surface environment of the cathode. A particular attention has been devoted to the origin of the over-charge tolerance achieved by the metal phosphate coating. To gain further insight into the improvement process by the FePO4, a density functional theory (DFT) simulation has been conducted. The enhanced stability during the high voltage cycling can be ascribed to the increased Fe(3d)-O(2p) covalency. Consequently, the TM(3d)-O(2p) character become less covalent, which in turn minimizes the electron extractions from the O(2p) band during the charge compensation process. This study offers a crucial aspect of the optimized materials design for metal phosphate coatings to enhance the battery stability and performance.

Implication of magnetic property on the degradation process of layered Ni-rich transition metal oxide cathode for high-capacity lithium-ion batteries

Layered LiNi0.8Mn0.1Co0.1O2 (NMC811) has received considerable attention as a high-energy density cathode for next generation lithium-ion batteries (LiBs). The charge compensation process underlying the achievement of high capacity is generally described by a two-stage redox reaction between Ni2+/Ni3+ and Ni3+/Ni4+. However, the increase of the Ni for the high battery capacity often leads to a significant degradation of the NMC811 material. In this study, the magnetic property of the NMC811 battery system has been compared at low (4.3 V) and high (4.7 V) cut-off voltages. There has been a notable drop in the magnetic moment from 2.33 μB (pristine) to 2.11 μB and 2.07 μB for the cathodes cycled at 4.3 V and 4.7 V, respectively. The amount of low-spin Ni4+ increased in the cathode material after the high voltage cycling. A density-functional theory (DFT) simulation has been adopted to further understand the discrepancy in the magnetic moment upon the different voltages. The local magnetic moment () of the transition metals are evaluated with different atomic environments including the Li/Ni exchange. Also, the corresponding density of states (DOS) are provided. It is shown that the electrochemical performance will significantly deteriorate when a high antiferromagnetic interaction occurs with the Li/Ni exchange.

(Digital Presentation) The Influence of Alkali Metal Cations on the Redox Processes of Copper Hexacyanoferrate in Rechargeable Aqueous Zinc-Ion Batteries

Rechargeable aqueous batteries are of high interest for future stationary grid-scale energy storage applications where high safety is needed (1). Copper hexacyanoferrate (CuHCF), a Prussian Blue Analogue (PBA) material, has recently gained interest as positive electrode for aqueous Zn-ion batteries (ZIBs) (2,3). Reversible Zn2+ ion insertion is facilitated by its open-framework with large channels that can host a variety of monovalent and divalent cations. Recent studies have shown that the charge compensation process takes place via Zn2+ ions swapping position between tunnel sites and vacancy sites (4).Among aqueous ZIBs, the Zn/CuHCF cell has attracted noticeable attention, as it can combine abundant and inexpensive materials with a good trade-off between capacity and performance. Zinc can supply a high capacity (820 mAh g-1), while CuHCF typically provides a moderate capacity of ca. 60-80 mAh g-1.However, it exhibits one of the highest operating voltages among PBA-type cathodes (1.7 V vs. Zn2+/Zn) and exhibits a cubic-type structure with wide channels. In comparison to many other materials, CuHCF undergoes minimal volume and structural changes during ion insertion/de-insertion, exhibits a high Coulombic efficiency of ca. 99%, and can be cycled at high rates without compromising the capacity, which makes it an interesting material for high-power applications.Nevertheless, CuHCF suffers from capacity fade owing to instabilities of Cu, which we have demonstrated in details in a recent investigation (5). In that study, we also highlighted that the characteristic aging effect, observed as a growth of a two-phase plateau in the charge/discharge profiles and associated with capacity loss (6), can be explained by Cu dissolution and thereby a displacement of the two Cu2+/Cu+ and Fe3+/Fe2+ redox-couples (5). Alkali metal cations (Li+, Na+, K+, Rb+, Cs+) have recently been shown to impact the capacity of the Zn/CuHCF cell along with a modulation of the characteristic redox-features in the voltammetric profiles (7). Cycling of large cations (Rb+, Cs+) was linked to a reduced capacity, while moderately sized cations (K+) resulted in optimal capacity and higher charge retention. This has motivated us to investigate the effect of the alkali metal cations on the charge compensation and redox processes in CuHCF in more detail in this study.By employing X-ray photoelectron spectroscopy (XPS), we show that small cations (Li+) have negligible impact on the Cu and Fe redox processes, while moderately sized cations (K+) suppress Cu redox and enhance Fe redox, which optimizes the capacity and improves the cycling stability, accordingly. Large cations (Cs+), on the other hand, prevent reversible redox and lock both metal centers in their most reduced states (Cu+, Fe2+), which impedes the charge compensation process and reduces the capacity. Our study unveils how alkali metal cations influence the performance of ZIBs by affecting the synergy of the Cu2+/Cu+ and Fe3+/Fe2+ redox couples in CuHCF and demonstrates how tailoring the electrolyte formulation can conveniently impact the capacity retention of this compound in PBA-type ZIBs. Figure 1. The aqueous Zn/CuHCF cell. (a) Schematic figures illustrating the structure of the CuHCF cathode and its wide channels that can host ions. (b) Cyclic voltammograms (CV) of the Zn/CuHCF cell in pure 1M ZnSO4 and in presence of 0.2 M alkali metal cation additives (Li+, K+, Cs+). (c) X-ray photoelectron spectroscopy (XPS) showing the Cu 2p3/2 and Fe 2p3/2 spectra of the same electrolytes as shown in (b). References J. Shin, and J. W. Choi, Advanced Energy Materials, 10, 2001386–2001386 (2020).R. Trócoli, and F. La Mantia, ChemSusChem, 8, 481–485 (2015).Z. Jia, B. Wang, and Y. Wang, Materials Chemistry and Physics, 149–150, 601–606 (2015).V. Renman, D. O. Ojwang, M. Valvo, C. P. Gómez, T. Gustafsson, G. Svensson, Journal of Power Sources, 369, 146–153 (2017).M. Görlin, D. O., Ojwang, M.-T. Lee, V. Renman, C.-W. Tai, and M. Valvo, ACS Applied Materials & Interfaces, 13, 59962–59974 (2021)R. Trócoli, G. Kasiri, and F. La Mantia, Journal of Power Sources, 400, 167–171 (2018).D. Phadke, R. Mysyk, and M. Anouti, Journal of Energy Chemistry 40, 31–38 (2020). Figure 1

Serum albumin/hyaluronic acid nanoconjugate: Evaluation of concentration-dependent structural changes to form an efficient drug carrier particle

In actual work we clearly highlight in the first time the experimental factors (dominantly the concentration of the macromolecule components) influencing the formation of human serum albumin/hyaluronic acid (HSA/HyA) complex carrier particles which is not available in the literature despite the several published articles. We have shown that the charge compensation process of the oppositely charged macromolecules results in the formation of drug carrier particles (d ~ 200–350 nm), where the size, size distribution and structure can be largely controlled by changing the concentration of the macromolecule components and the experimental conditions (pH, reaction time, presence of inert salt etc.). Moreover, we firstly demonstrate the encapsulation capability of this optimized HSA/HyA carrier particles - which possess dominantly hydrophilic character - using vitamin D3 (D3) as a model compound having hydrophobic property, where the kinetic properties of the dissolution were studied as well.

First‐principles insights on anion redox activity in NaxFe1/8Ni1/8Mn3/4O2: Toward efficient high‐energy cathodes for Na‐ion batteries

AbstractIn the search for high‐energy cathode materials for Na‐ion batteries (NIBs), Fe‐doped layered transition metal oxides have been recently proposed as promising systems that can ensure improved reversible capacity at high working voltage. Exploiting the anionic redox chemistry in this class of materials represents a great advance for the energy storage community, but uncontrolled oxidation process can lead to the formation of unbound molecular oxygen, with detrimental effects on overall capacity and stability upon cycling. The higher TM–O covalency provided by Fe doping seems to prevent oxygen loss and ensure full capacity recovery. Understanding anionic processes and the underlying mechanism with atomistic details can reinforce the experimental efforts and help to outline rational design strategies for novel high‐performing NIB cathodes. To this end, we present a state‐of‐the‐art first‐principles study on the P2‐type NaxTMO2 (TM = Fe, Ni, and Mn—NFNMO) oxide. We compare structural and electronic features of stoichiometric (NaxFe0.125Ni0.125Mn0.75O2) and Mn‐deficient (NaxFe0.125Ni0.125Mn0.68O2) NFNMO to identify and discuss the contribution of each element sublattice on charge compensation processes. Although Mn deficiency is predicted to increase the cathode working voltage, we find the charge compensation being mostly exerted by the Ni and Fe sublattices. Oxygen redox is unfold via the formation of superoxide species at low Na loads with a preferential breaking of more labile Ni–O bonds and binding to Fe atoms. Our calculations predict no release of molecular O2 upon desodiation, thus highlighting the key role of Fe dopant that provides a good TM–O bond strength, preventing oxygen loss while still enabling anionic redox reactions at high voltages with extra reversible capacity.

Charge Compensation Mechanisms and Oxygen Vacancy Formations in LiNi1/3Co1/3Mn1/3O2: First-Principles Calculations.

Charge compensation mechanisms in the delithiation processes of LiNi1/3Co1/3Mn1/3O2 (NCM111) are compared in detail by the first-principles calculations with GGA and GGA+U methods under different U values reported in the literature. The calculations suggested that different sets of U values lead to different charge compensation mechanisms in the delithiation process. Co3+/Co4+ couples were shown to dominate the redox reaction for 1 ≥ x ≥ 2/3 by using the GGA+U1 method (U1 = 6.0 3.4 3.9 for Ni, Co, and Mn, respectively). However, by using the GGA+U2 (U2 = 6.0 5.5 4.2) method, the results indicated that the redox reaction of Ni2+/Ni3+ took place in the range of 1 ≥ x ≥ 2/3. Therefore, according to our study, experimental charge compensation processes during delithiation are of great importance to evaluate the theoretical calculations. The results also indicated that all the GGA+Ui (i = 1, 2, 3) schemes predicted better voltage platforms than the GGA method. The oxygen anionic redox reactions during delithiation are also compared with GGA+U calculations under different U values. The electronic density of states and magnetic moments of transition metals have been employed to illustrate the redox reactions during the lithium extractions in NCM111. We have also investigated the formation energies of an oxygen vacancy in NCM111 under different values of U, which is important in understanding the possible occurrence of oxygen release. The formation energy of an O vacancy is essentially dependent on the experimental conditions. As expected, the decreased temperature and increased oxygen partial pressure can suppress the formation of the oxygen vacancy. The calculations can help improve the stability of the lattice oxygen.

Modulating anionic activities in layered Li-rich cathode materials with inverse spinel MnFe2O4 coating

Layered Li-rich cathode oxides can provide high specific capacity due to oxygen anion redox involving in charge compensation process during cycling, but there is a severe structural transition from layered to spinel accompanying with irreversible oxygen loss during cycling, which lead to electrochemical degradation. The current researches show that the irreversible oxygen evolution reaction of layered lithiumrich materials in the first cycle mainly comes from the surface lattice oxygen, so the surface modification by the materials with more stable structure is one of the effective ways to improve the electrochemical performance of layered lithium-rich materials. In this paper, we report a modified layered lithium-rich cathode material by surface coating of inverse spinel MnFe2O4. The inverse spinel has strong polarization effect on anion migration due to its different atoms occupying octahedral sites from layered structure and it can also modulate the Fermi level and stretching the O-O bond, thereby increasing the energy barrier for surface oxygen oxidization. Furthermore, the three-dimensional connected tunnel structure of the inverse spinel also makes the surface layer of the material have a faster lithium ion transferring rate, and a large number of lithium storable vacancies inside of it improved the Li+ intercalation efficiency, initial coulombic efficiency and rate performance. Compared with the unmodified sample, the modified material coated with 2% MnFe2O4 has superior cycle stability and outstanding rate performance. It is hoped our work would provide the knowledge for the future development of high-performance cathode materials for Li-ion batteries.

Aging and Charge Compensation Effects of the Rechargeable Aqueous Zinc/Copper Hexacyanoferrate Battery Elucidated Using In Situ X-ray Techniques

The zinc/copper hexacyanoferrate (Zn/CuHCF) cell has gained attention as an aqueous rechargeable zinc-ion battery (ZIB) owing to its open framework, excellent rate capability, and high safety. However, both the Zn anode and the CuHCF cathode show unavoidable signs of aging during cycling, though the underlying mechanisms have remained somewhat ambiguous. Here, we present an in-depth study of the CuHCF cathode by employing various X-ray spectroscopic techniques. This allows us to distinguish between structure-related aging effects and charge compensation processes associated with electroactive metal centers upon Zn2+ ion insertion/deinsertion. By combining high-angle annular dark-field-scanning electron transmission microscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy, and elemental analysis, we reconstruct the picture of both the bulk and the surface. First, we identify a set of previously debated X-ray diffraction peaks appearing at early stages of cycling (below 200 cycles) in CuHCF. Our data suggest that these peaks are unrelated to hypothetical ZnxCu1–xHCF phases or to oxidic phases, but are caused by partial intercalation of ZnSO4 into graphitic carbon. We further conclude that Cu is the unstable species during aging, whose dissolution is significant at the surface of the CuHCF particles. This triggers Zn2+ ions to enter newly formed Cu vacancies, in addition to native Fe vacancies already present in the bulk, which causes a reduction of nearby metal sites. This is distinct from the charge compensation process where both the Cu2+/Cu+ and Fe3+/Fe2+ redox couples participate throughout the bulk. By tracking the K-edge fluorescence using operando XAS coupled with cyclic voltammetry, we successfully link the aging effect to the activation of the Fe3+/Fe2+ redox couple as a consequence of Cu dissolution. This explains the progressive increase in the voltage of the charge/discharge plateaus upon repeated cycling. We also find that SO42– anions reversibly insert into CuHCF during charge. Our work clarifies several intriguing structural and redox-mediated aging mechanisms in the CuHCF cathode and pinpoints parameters that correlate with the performance, which will hold importance for the development of future Prussian blue analogue-type cathodes for aqueous rechargeable ZIBs.

Investigation of surface defects in BaTiO3 nanopowders studied by XPS and positron annihilation lifetime spectroscopy

Defect structures of BaTiO3 nanoparticles annealed in the air and their correlation with magnetic properties have been studied. As main research techniques, complementary methods such as XPS and positron annihilation lifetime spectroscopy were used. It was found that low-temperature annealing creates additional oxygen vacancies in nanoparticles both inside and on the nanoparticle surface, and high-temperature annealing favors the reduction of Ti. These changes influence nanoparticle magnetism and occur due to the charge compensation process and grain growth mechanism. Performed studies show that complementary XPS and positron annihilation spectroscopy are the effective tools to determine nanoparticle surface defect structure, which can be used to fabricate new rare room-temperature multiferroics materials.

Origin of magnetic, magnetoelectric effect and the influence of reentrant ferroelectric phase on the structural and multiferroic properties of Dy3+-Fe3+ co-substituted BaTiO3 ceramics

Retain of phase pure ferroelectric tetragonal phase (P4mm) achieved from the coexistence of paraelectric hexagonal phase (P63/mmc) + ferroelectric phase without the loss of polarisation and magnetization under the Dy3+-Fe3+ co-substitution on BaTiO3, Ba1-yDyyTi1-xFexO3 (x = 0.05; y = 0; 0.005; 0.010 and 0.015) ceramics. A small amount of Dy substitution enhances the inhibition of vacancy formation and suppresses the defect concentration through the decrease of the hexagonal phase. The unexpected ferromagnetic nature in the non-magnetic tetragonal phase in y = 0.015 ceramics unambiguously depends on the formation of the paramagnetic defect, Ti3+ for the charge compensation process that induces magnetic moment. The observed ferromagnetic nature in y = 0.01 ceramics originated from the Fe3+-O2−-Fe3+ super-exchange and Dy3+-O2−-Dy3+ interactions which are stronger than the magneto-crystalline anisotropy (y = 0.015). For the first time, multiferroic properties (magnetodielectric and magnetoelectric) were reported in this work. The magnetoelectric coupling effect originated from the co-contribution of magnetocrystalline anisotropy due to spin–orbit interaction through symmetry breaking and microstrain.

Broadband NaK2Li[Li3SiO4]4:Ce Alkali Lithosilicate Blue Phosphors

Phosphors with a rigid and symmetrical structure are urgently needed. The alkali lithosilicate family (A[Li3SiO4]) has been extensively studied with a narrow emission band due to its unique cuboid-coordinated environment and rigid structure. However, here we demonstrate for the first time Ce-doped NaK2Li[Li3SiO4]4 phosphors with a broad emission band, a high internal quantum efficiency (85.6%), and excellent thermal stability. Photoluminescence indicates the Ce's preference to occupy the Na+ site, leading to a strong blue color emission with peak maxima at 417 and 450 nm. Temperature- and pressure-dependent photoluminescence reveals thermal stability and a phase transition. Moreover, the X-ray absorption near-edge structure reveals the mixing of Ce3+ and Ce4+ in the materials; this result differs from that of Eu2+-doped A[Li3SiO4] phosphors. The charge compensation process is then proposed to explain this difference. This study not only provides insights into Ce-doped UCr4C4-type phosphors but also explains the charge compensation mechanism of the aliovalent doping process.

Effect of Iron Doping on Structural, Magnetic, and Electrical Characteristics of Manganites in La0.7Sr0.3Mn0.9Zn0.1 –xFexO3 (0 ≤ x ≤ 0.1) System

Experimental data on the structure and properties of manganites of La0.7Sr0.3Mn0.9Zn0.1 –xFexO3 (0 ≤ x ≤ 0.1) system synthesized by ceramic processing and annealed under conditions ensuring the achievement of stoichiometric oxygen concentrations are presented. All obtained manganites have a rhombohedral structure. The substitution of iron for zinc leads to an increase in the unit cell volume and average cation–anion distance in the octahedral sublattice, which is determined by the increase in the concentration of Mn3+ ions due to the decrease in the concentration of Mn4+ as a result of charge compensation processes. The angles between the Mn–O–Mn bonds change slightly. Curie point, magnetization, and temperature of the metal–semiconductor transition as a function of the concentration of iron are characterized by the presence of a maximum. The maximum value of the resistance of manganite with x = 0.1 is a factor of 800–1000 higher than the maximum values of the resistance of other samples in the studied range of temperatures. Temperature dependences of magnetic permeability are indicative of the existence of magnetic inhomogeneities in manganites, especially in the samples with x = 0 and 0.1, which may be due to the nonuniform distribution of Zn2+ ions and the presence of ferromagnetic and paramagnetic clusters. These data are in agreement with the results of the investigation of the electron paramagnetic resonance (EPR) spectra containing additional and/or broadened lines. Characteristic features of the dependences of the properties of studied manganites on the concentration of iron are explained by the influence of a series of competing factors, namely, the decrease in the concentration of Mn4+ ions and probability of antiferromagnetic interaction between them, change in the number of disturbed exchange bonds between Mn4+ and Mn3+ ions, and variations of the characteristics of magnetic inhomogeneities.