Manganese-rich Oxides Research Articles

Lithium substitution modulation of P2-type manganese-rich oxide toward high-stable and high-voltage cathode for sodium-ion batteries

P2-type manganese-rich layered oxide have attracted much attention as cathodes of sodium-ion batteries owing to the low cost, high capacity and fast sodium ion diffusion kinetics. However, the Jahn-Teller effect of Mn3+, P2-O2 phase transition and anionic redox on charging to the high-voltage, resulted in a severe capacity delay. Herein, an effective Li substitution strategy is proposed to suppress the Jahn-Teller effect of Mn3+, the harmful phase transition of P2-O2 and the anionic redox. As a result, the as-prepared sample Na0.67(Ni0.25Mn0.75)0.9Li0.1O2 (67L10) displays a discharge specific capacity of 168.6 mA h g−1 at 0.1 C, enhanced average working voltage, and improved cycling stability at 1 C after 200 cycles within 1.5–4.5 V voltage window (capacity retention of 83.3 %, specific capacity of 106.8 mA h g−1) compared with that of pristine Na0.67Ni0.25Mn0.75O2(capacity retention of 34.2 %, specific capacity of 51.3 mA h g−1). This work demonstrates the favorable effect of Li doping in P2-type layered materials to suppress the phase transition and inhibit the redox activity of Mn and O ions, and it provides an effective approach for the development of cathodes for sodium-ion batteries with high specific energy and long life.

Insight into the Hot Corrosion Behavior of FeMnCrSi/TiC Coatings at 900 °C

This study explores the deposition of an Fe-MnCrSi/TiC coating on 45 steel surfaces using high-velocity arc spraying technology, examining the microstructure and hot corrosion behavior of the resultant layer. The microstructure of the FeMnCrSi/TiC coating primarily consists of an α-Fe (BCC) solid solution, composed of Fe, Mn, Cr, Si, C, and other elements, with a minor presence of β-Fe (FCC) solid-solution phase and unmelted TiC particles. Following 100 h of cyclic 900 °C hot corrosion, Mn on the coating surface preferentially oxidizes, forming a manganese-rich oxide layer. This process reduces the oxygen partial pressure (O2) within the coating, prompting the formation of a dense Cr2O3 layer on the inner side of the oxide layer. Concurrently, the rapid diffusion of Mn and Cr elements triggers the generation of Mn- and Cr-deficient regions at the metal/oxide layer interface, inducing the transformation of the coated metal primary matrix from an FCC + BCC dual phase to an α-Fe (BCC) single phase. After the reaction, the hot corrosion weight gain of the coating reached 12.43 mg/cm2, approximately one-fourteenth of the weight gain of the 45 steel substrates. This weight gain adheres to the parabolic law, suggesting that the FeMnCrSi/TiC coating exhibits excellent corrosion resistance under the given conditions.

Stabilizing the Deep Sodiation Process in Layered Sodium Manganese Cathodes by Anchoring Boron Ions.

Advanced high-energy-density sodium-ion batteries (SIBs) are inseparable from cathode materials with high specific capacities. Layered manganese-rich oxides (NaxMnO2, 0.6≤x≤1) are promising cathode materials owing to their ease of intercalation and extraction of a considerable amount of sodium ions. However, lattice interactions, especially electrostatic repulsive forces and anisotropic stresses, are usually caused by deep desodiatin/sodiation process, resulting in intragranular cracks and capacity degradation in SIBs. Here, boron ions are introduced into the layered structure to build up B─O─Mn bonds. The regulated electronic structure in Na0.637B0.038MnO2 (B-NMO) materials inhibits the deformation of MnO6 octahedra, which finally achieves a gentle structural transition during the deep sodiation process. B-NMO electrode exhibits a high capacity (141mAhg-1) at 1C with a capacity retention of 81% after 100 cycles. Therefore, anchoring boron to manganese-rich materials inhibits the detrimental structural evolution of deep sodiation and can be used to obtain excellent cathode materials for SIBs.

Double-layer phosphates coated Mn-based oxide cathodes for highly stable potassium-ion batteries

Manganese rich oxides with layered structure are now being developed as cathodes for potassium ion batteries (PIBs). Nevertheless, poor cycling stability and sluggish electrochemical kinetics have hindered their practical applications. Here, we construct a thin K3PO4/MnPO4 layer, which has high conductivity of both electron and potassium-ion, on the surface of K0.5Mn0.8Co0.2O2 (KMCO) and achieve improved cycling stability and capacity rate when the composite is used as cathodes in PIBs. Comparing to the pure KMCO, the K3PO4/MnPO4-coated K0.5Mn0.8Co0.2O2 (P-KMCO) has higher electron and K-ion conductivity, less surficial oxygen loss and layered-to-spinel-to-rock salt tri-phase transition, and less internal lattice expansion and contraction in cycling, as studied by aberration-corrected scanning transmission electron microscopy and in-situ X-ray diffraction. The synthesized P-KMCO exhibits a highly reversible specific capacity (101.3 mAh g−1 at 0.1 A g−1), excellent rate performance and stellar capacity retention of 80% after 500 cycles at 1 A g−1 compared to pure KMCO (78.6 mAh g−1 at 0.1 A g−1 with 67.3% capacity retention). The P-KMCO electrode shows also a supreme electrochemical performance when assembled in pouch cells. First-principal calculations imply that the surface modification can effectively prevent TM ions dissolution and oxygen release while cycling. Like those developed in lithium-ion batteries, this work demonstrates that the appropriate surface engineering is also an effective approach to development stable cathodes of PIBs.

Studies on sodium-ion batteries: Searching for the proper combination of the cathode material, the electrolyte and the working voltage. The role of magnesium substitution in layered manganese-rich oxides, and pyrrolidinium ionic liquid

The influence of the nature of the cathode, the electrolyte and the operating voltage range on the electrochemical performance of sodium cells has been studied. Manganese-rich P2-Na0.67Mn0.67Ni0.33O2 (MN) layered oxide and the magnesium substituted P2-Na0.67Mn0.67Ni0.23Mg0.1O2 (Mg-MN) have been prepared by solid-state reaction and adequately characterized by X-ray diffraction, SEM and EDX techniques. Cycling performances at the low current of 0.2C (40 mAhg−1) of MN and Mg-MN cathodes have been compared in two voltage ranges. We have found a positive effect on cyclability at high voltages (above 4 V) by partial substitution of Ni2+ by Mg2+, which is in line with previous results. In order to study the influence of the electrolyte, and the working voltage, the electrochemical performance of Mg-MN half-cells with an alkyl carbonate-based electrolyte (1 M NaPF6 in PC) and an ionic liquid (IL)-based electrolyte (1 M NaFSI in Pyr14FSI), has been thoroughly studied in three voltage ranges (2.0V-4.0 V, 2.0V-4.5 V and 1.5V-4.0 V). Furthermore, flammability tests indicate a large improvement in safety for the IL-electrolyte when compared with the alkyl carbonate one. Results from galvanostatic cycling at increasing currents have shown that parameters such as the rate capability, and cyclability depend strongly on both the operation voltage range and the nature of the electrolyte. The discharge capacity, the average working voltage and the cyclability have been superior for the Mg-MN half-cells assembled with the non-flammable IL-electrolyte. Overall, the proper combination of the cathode (Mg-doped layered manganese-rich oxide), a pyrrolidinium-based IL electrolyte, and an adequate working voltage (2.0–4.5 V) shows promising results for the development of safe Na-ion batteries.

Critical Barriers to Successful Implementation of Mn-Rich Cathodes: A Study of Low SOC Impedance

The development of sustainable materials for lithium-ion batteries is of paramount significance with the ever-growing demand of energy storage for the transportation sector. Lithium- and manganese-rich oxides are of great interests because of inherently high capacities/energies and the potential economic benefits associated with earth-abundant manganese-based compositions. With several challenges hindering the commercialization of these cathodes, there have been many studies on understanding the mechanisms of voltage fade, hysteresis, and oxygen activity. However, relatively little attention has been given to understanding the impedance of these materials. In particular, the drastic rise in area specific impedance at lower states of charge, and the overall impedance rise on extended cycling due to surface issues, are critical barriers that need to be understood and addressed to accelerate deeper market penetration.This poster will present a detailed study of the impedance behavior in cobalt-free, lithium- and manganese-rich cathodes. A combined surface-treatment/electrolyte-additive has been used to probe the behavior over long-term cycling in the absence of (i.) surface issues, (ii.) the impedance rise due to electrolyte interactions, and (iii.) capacity loss. The results presented in the poster support the hypothesis that the anomalous rise in impedance at lower states of charge is directly associated with the bulk processes of voltage fade and voltage hysteresis. Furthermore, a preliminary conceptual model is presented for understanding the impedance behavior of these activated materials, which can be described as layered-type, ordered-domain network with facile Li diffusion, where interspersed, disordered, low-voltage sites within this network hinder the Li insertion.

Development of Earth-Abundant Cathodes for Vehicle Applications and Beyond

The predicted growth in electrification of the grid-storage and transportation sectors has seemingly left the research community a significant responsibility in developing a more robust portfolio of materials solutions. Specifically, it has become clear that beyond simply meeting/exceeding the current technical requirements, new technologies would do well to align with the goals of sustainable development in supporting market growth. Current state-of-the-art, Li-ion cathodes, based heavily in Ni, will continue to play an important role. However, alternatives to these chemistries will be critical in diversifying supply chains and expanding the available portfolio of electrode materials towards a more sustainable future. Among the potential candidates for next-generation cathodes are Lithium- and Manganese-rich oxides. These oxides remain attractive because of their intrinsically high, specific capacities/energies and the economic benefits they offer through earth-abundant, Mn-rich (>50%) compositions.This presentation will discuss research and development efforts at Argonne National Laboratory aimed at advancing cathode materials that are rich in manganese and have the potential to be cost competitive for electric vehicle applications and beyond. Research efforts into mitigation of issues related to several identified critical barriers, such as impedance, will be discussed.

Iron isotope constraints on the metal source and depositional environment of the Neoproterozoic banded iron- and manganese deposits in Urucum, Brazil

The Urucum area of Brazil hosts a series of Cryogenian ironstones intercalated by oxide-dominated manganese layers. The Urucum iron and manganese formations (IF-MnF) are among the largest sedimentary iron and associated manganese deposits of the Neoproterozoic, however, the depositional model and the source of metals for the IF-MnF in this area are highly controversial. In this study, we performed systematic Fe isotope analysis on fresh and geochemically characterized drill core samples of the Urucum iron and manganese formation deposited in the center of the ancient Urucum graben system. The samples have a large variation in Fe isotope composition, with a δ56Fe range of −2.04‰ to +0.75‰, and exhibit a general trend of decreasing δ56Fe values with increasing manganese contents. The low δ56Fe values of the IF and MnF samples reflect Rayleigh fractionation processes of contineous partial oxidation of aqueous Fe(II) prior to deposition at the sampling site. Using a mixing model and previously published Nd isotope data on the same samples, we estimated that benthic (i.e., porewaters released from submarine sediments in the Urucum basin) Fe fluxes provided 7–50% of total Fe in the Urucum IF-MnF, and the rest of Fe source was from low-temperature hydrothermal vents. Based on combined Fe and Nd isotope data of the Urucum IF-MnF, we propose that low-temperature hydrothermal fluids and benthic fluxes of pore waters were mixed and transported by an upwelling current. The fluid subsequently experienced partial oxidation during the transportation process and became enriched in light Fe isotopes. In the Urucum graben basin, the iron- and manganese-rich oxides deposition occurred progressively under increasingly oxidizing conditions, and such process could have operated repeatedly to produce the alternation of iron and manganese formations. The chemical sediments of the Urucum IF-MnF deposits thus reflect the existence of a sharp redox gradient in the marine environment during the late Cryogenian period.

Equimolar AlCuFeMn: A novel oxidation resistant alloy

High-temperature oxidation behavior of an equimolar AlCuFeMn multicomponent alloy was investigated at 500 °C and 1000 °C for 50 h of exposure in ambiance. Relative weight gain after oxidation was 0.5 mg cm−2 and 0.3 mg cm−2 at 500 °C and 1000 °C, respectively. At 500 °C, the oxidized surface comprised of a manganese-rich oxide layer with an aluminum oxide sublayer, while only an aluminum-rich oxide layer was observed at 1000 °C. The alloy microstructure was temperature stable and consisted of an FCC, BCC, and a minor Al11Mn4 phase. The alloy's oxidation resistance compared well with 9% Cr steel and nickel-based superalloys suitable for high-temperature applications.

High-resolution topochemical analysis and thermochemical simulations of oxides and nitrides at grain boundaries and within the grains of a low alloy Mn-Cr hot-rolled steel sheet

The selective oxidation underneath the scale layer of an industrially hot rolled Fe-1.8Mn-0.8Cr steel at temperatures between 600 and 700 °C has been investigated. The spatial distribution and composition of formed precipitates has been studied by high-resolution topochemical analysis via TEM-EELS and NanoSIMS and revealed heterogeneities in chemical composition, especially along grain boundaries. It could be shown that grain boundary oxides are predominantly composed of aluminium, chromium or silicon oxides/nitrides, surrounded by manganese-rich oxides. Experimental results of phase stability have been compared to numerical simulations, considering the distribution of more than 40 potentially stable oxide-, nitride- and carbide phases, and differences are critically discussed.

A green water-induced spinel heterostructure interface enabling high performance lithium and manganese rich oxides

Lithium and manganese rich oxides (LMROs) are promising candidates for next-generation high-energy lithium-ion batteries.

High Power Energy Storage via Electrochemically Expanded and Hydrated Manganese-Rich Oxides.

Understanding the materials design features that lead to high power electrochemical energy storage is important for applications from electric vehicles to smart grids. Electrochemical capacitors offer a highly attractive solution for these applications, with energy and power densities between those of batteries and dielectric capacitors. To date, the most common approach to increase the capacitance of electrochemical capacitor materials is to increase their surface area by nanostructuring. However, nanostructured materials have several drawbacks including lower volumetric capacitance. In this work, we present a scalable “top-down” strategy for the synthesis of EC electrode materials by electrochemically expanding micron-scale high temperature-derived layered sodium manganese-rich oxides. We hypothesize that the electrochemical expansion induces two changes to the oxide that result in a promising electrochemical capacitor material: (1) interlayer hydration, which improves the interlayer diffusion kinetics and buffers intercalation-induced structural changes, and (2) particle expansion, which significantly improves electrode integrity and volumetric capacitance. When compared with a commercially available activated carbon for electrochemical capacitors, the expanded materials have higher volumetric capacitance at charge/discharge timescales of up to 40 s. This shows that expanded and hydrated manganese-rich oxide powders are viable candidates for electrochemical capacitor electrodes.

Exploring the activating voltages on the electrochemical performances of Li1.17Ni0.139Co0.139Mn0.552O2 cathode materials

Lithium and manganese-rich oxide (LMR) is regarded as one of the next generation of the high energy density cathode materials. However, the activating voltages have a great impact on the electrochemical performances. Herein, we explore the effects of the activating voltages on the electrochemical performances of Li1.17Ni0.139Co0.139Mn0.552O2 cathode materials. The activating voltages can mainly affect the capacity release, rather than the cycling characteristics. The comparison study reveals that the layered structure can transform into spinel and/or nickel enriched structure due to the cation migration and the hardly-being reduced transition metals, especially for the Ni4+ during activation. Meanwhile, the surface side-reactions would also deteriorate the lithium kinetic characteristics, but the formed by-products coated on the cathode surface can resist the further attack from electrolyte, which favors to the following cycles. The Li1.17Ni0.139Co0.139Mn0.552O2 cathode materials activated to 4.6 V with 25 mA·g−1 can deliver capacities of 262.0 and 209.2 mAh·g−1 over 2.0–4.8/4.4 V at 1C (250 mA·g−1) and the highest energy densities. The appropriate activating voltages play critical roles in the optimization of the structure of LMR cathode materials and improving the electrochemical performances.

Renovating the electrode-electrolyte interphase for layered lithium- & manganese-rich oxides

Layered lithium- & manganese-rich oxides (LMR), with their high capacity and cost-effective advantage, are considered as a potent alternative of the next-generation cathode material for lithium-ion batteries. The behaviors of the electrode-electrolyte interphase (EEI) are crucial to the electrochemical properties of LMR as a cathode material operating at wide voltage regions (from 2 to 4.8 ​V). Nonetheless, the understanding of EEI for LMR materials and the related renovation techniques are somewhat lacking. Herein, we gain insight into the EEI change mechanism for LMR materials during long electrochemical cycles and demonstrate a renovating method to mitigate its deterioration. As for the pristine electrode based on LMR materials, the increasing amount of POxFyz− and metal fluorides lead to unpleasant degradation for both the EEI and the active material particle, causing evident performance decay. Whereas, the lithium phosphate, if employed in the electrode, effectively enhances the lithium ions transfer, impedes the decomposition of electrolyte salt, and leads to a more stable EEI, thus promoting the electrochemical performances of LMR materials. All results indicate that the EEI should be one of the critical components for comprehensively understanding the LMR material, and the success renovation by the lithium phosphate offers a new orientation for those intrinsic drawbacks of LMR material.

Reconstructing the Surface Structure of Li-Rich Cathodes for High-Energy Lithium-Ion Batteries.

Reconstructing a favorable surface layer could contribute to superior charge transfer and stabilize bulk structure and thus achieve the excellent electrochemical performance of lithium- and manganese-rich oxides, but it is still challenging. In this work, the surface structures of Li-rich oxides have been successfully reconstructed via a facile strategy utilizing hydrothermal glucose carbonization and the subsequent reduction procedure. Surface microstructure and chemical state analyses reveal that the reconstruction process involves roughening of the surface connects with the extraction of lithium ions and the reduction of Mn ions as well as the formation of a spinel phase due to the distortion of oxygen anions or the presence of oxygen deficiencies. The reconstructed Co-free Li-rich oxide using 0.025 g of glucose exhibits superior electrochemical performance. Its maximum discharge capacities are 237 and 193 mAh/gat 100 and 600 mA/g, respectively, and their corresponding capacity retention ratios are higher than 93% at the 100th cycle. Furthermore, reconstructing the surface structure also enhances the discharge capacity and cycling performance of Co-containing Li-rich cathodes. The findings in this work would offer hints for surface structure reconstruction of many oxides used in energy and other fields.

Effect of Cu doping on the structural and electrochemical properties of lithium-rich Li1.25Mn0.50Ni0.125Co0.125O2 nanopowders as a cathode material

Lithium- manganese-rich oxides are believed to be suitable candidate for cathode materials in the next generation of lithium-ion batteries. However, they have some disadvantages such as low initial coulombic efficiency, low rate capacity, and deficient cyclability. Different approaches such as elemental doping and surface coating have been adopted to overcome these shortcomings. In this study, Cu-doped Li1.25Mn0.50Ni0.125Co0.125O2 was synthesized by sol-gel method. The prepared samples were characterized using thermal analysis, X-ray diffraction, Fourier transform infrared spectroscopy, surface area analysis, and field emission scanning electron microscopy. Galvanostatic charge-discharge measurement and electrochemical impedance spectroscopy were also conducted to investigate the electrochemical performance of the prepared samples. The XRD patterns revealed that all the samples had two phase structures. By doping Cu, the lattice parameters and the volume of the samples changed. The first discharge capacity of the doped samples was found to be lower than that of the undopped sample. In comparison with the pristine material, Cu-doped ones displayed better cycling performance and rate capability. The Li1.25Mn0.50Ni0.125Co0.125-xCuxO2 sample (with x = 0.05), which delivers an initial discharge capacity of 225.2 mAhg−1 at 0.1 C, was found to have the highest capacity retention with the best cycling performance (207.4 mAhg−1), after 50 cycles. The optimum performance of the doped samples could be related to its lower charge transfer resistance and better structural stability.

Trace Amounts of Phosphorus Removal Based on the in-suit Oxidation Products of Iron or Manganese in a Biofilter

In order to remove trace amounts of phosphorus from water bodies, a lab-scale biofilter was constructed to investigate the capacity of in situ oxidation products of iron or manganese for phosphorus adsorption. SEM, EDS, BET, and zeta technologies were employed to reveal the adsorption mechanisms. The results indicated that phosphorus could be removed by the oxide products generated from the iron or manganese removal process, at 106.28 μg·mg-1 and 77.98 μg·mg-1, respectively, as shown by the linear relationships between phosphorus removal and the two oxides. SEM, EDS, and BET analysis demonstrated that the BET specific surface areas for the iron- and manganese-rich oxides were 96 m2·g-1 and 67 m2·g-1, respectively, with the former accumulated between the pore spaces of the filtering sand and easily washed out of the layer by backwashing, whereas the latter coated the surface of the filtering sand. Thus, backwashing was favorable for phosphorus adsorption in the iron oxidation process to avoid overaccumulation. Moreover, the zero point of charge of the two oxides indicated electrostatic attraction may have occurred between iron-rich oxide and phosphorus; however, inner-sphere complex reactions obviously occurred for the two oxides because the zero point of charge after phosphorus adsorption decreased to a lower level. In addition, other anions were negatively complexed with the phosphorus on the surface of the oxides, it demonstrated that phosphorus adsorption on the surface of the two oxides seemed to be a specific adsorption.

Influence of the Composition of Lithium- and Manganese-Rich Layered Oxides on the Oxygen Release and the Electrochemical Performance

Lithium- and manganese-rich layered oxides, also referred to as high energy NCM (HE-NCM), are a promising class of materials for applications in future Li-ion batteries. Due to their high specific discharge capacity (~250 mAh/g) and the high operating voltage, the energy density of next-generation batteries could be increased using these materials [1]. However, these materials exhibit several problems that hinder their commercialization such as low first cycle coulombic efficiency, poor rate capabilities, as well as a significant voltage-hysteresis between charge and discharge accompanied by a rapid voltage decay [2]. So far it is well known that the first cycle activation is a crucial step and strongly influences the further cycling performance of the material [3]. The activation plateau was initially referred to the activation of Li2MnO3 to electrochemically active MnO2 accompanied by a release of oxygen and excess lithium [1]. Strehle et al. [4] recently showed by on-line electrochemical mass spectrometry (OEMS) that the amount of released oxygen, for the material with intermediate Li2MnO3 content, is an order of magnitude smaller than the oxygen release expected for the bulk activation of Li2MnO3. Therefore, the oxygen release of the material seems to be related to oxygen depletion at the surface rather than to a conversion of Li2MnO3 in the bulk material. On the other hand, there is evidence that also changes in the bulk materials occur during activation, which might play a crucial role for the understanding of the material and the development of strategies to overcome these issues. In this study, we compare different HE-NCM compositions, i.e., different Li2MnO3 contents for xLi2MnO3•(1-x)LiMO2 (M = Ni, Co and Mn). In Figure 1, the potential curve for the first charge is shown for the three different materials, showing clearly the activation plateau at around 4.5 V vs. Li+/Li, extending over different capacity ranges for the three compositions. We further investigate the changes of lithium- and manganese-rich oxides using OEMS as well as other electrochemical methods, such as electrochemical impedance spectroscopy (EIS). From these comparisons we aim to get a profound understanding about the influence of the additional lithium within the materials. Furthermore, we expect to get evidence, which phenomena can be assigned to occur at the surface and which processes occur in the bulk material. Figure 1: First cycle potential curves for different Li2MnO3 contents in HE-NCM//Graphite full cells. The Activation was carried out at 25°C between 2.0 and 4.7 V at C/15. The data were collected in Coin Cells with Celgard separator. Literature: [1] M. M. Thackeray, S.-H. Kang, C. S. Johnson, J. T. Vaughey, R. Benedek and S. A. Hackney, J. Mater. Chem. 17, 3112-3125 (2007). [2] J. R. Croy, M. Balasubramanian, K. G. Gallagher and A. K. Burrell, Acc. Chem. Res. 48, 2813-2821 (2015). [3] J. R. Croy, K. G. Gallagher, M. Balasubramanian, Z. Chen, Y. Ren, D. Kim, S.-H. Kang, D. W. Dees and M. M. Thackeray, J. Phys. Chem. C 117, 6525-6536 (2013). [4] B. Strehle, K. Kleiner, R. Jung, F. Chesneau, M. Mendez, H. A. Gasteiger and M. Piana, submitted. Acknowledgement: The authors want to thank BASF SE for their financial support of this work. Furthermore, we want to thank Dr. Karin Kleiner for the contribution to this work. Figure 1

Correlating Local Structure with Electrochemical Activity in Li2MnO3

Li2MnO3 is believed to be a critical component of the high capacity Li-rich–manganese-rich oxide materials; however, the mechanism of its electrochemical activity remains controversial. Here, Raman spectroscopy and mapping are used to follow the chemical and structural changes that occur in Li2MnO3 during electrochemical cycling. Conventional composite electrodes cast from a slurry and thin films are studied as a function of the state of charge (voltage) and cycle number. Thin films have similar electrochemical properties as electrodes prepared from slurries but enable spectroscopy of uniform samples without carbon additives and binder. First-principles density functional theory is used to calculate the phonon spectra and identify the Raman-active modes. On the basis of the calculations of phonon spectra for pristine Li2MnO3 and structures with Li vacancies, we discuss the origin of Raman-active peaks observed during the electrochemical cycling. The spectral changes correlate well with the electrochemical...

Selective oxidation of Al rich Fe–Mn–Al–C low density steels

The surface quality of steels containing solutes that have a strong affinity for oxygen can be markedly compromised during processing at elevated temperatures. Here, we examine a class of steels that are being developed for low density applications and hence, have relatively large concentrations of aluminium and manganese. These two elements compete for what little oxygen is available in the predominantly hydrogen–nitrogen mixture used to protect the steel during heat treatment. It is found that although the general sequence of oxidation does not seem to vary as the aluminium concentration is increased, the stoichiometry of the oxides changes. Manganese rich oxide is always the first to form for all the dew points examined, but is rapidly overtaken by aluminium oxide when the dew point is kept below −30°C. In the latter case, the eventual formation of an alumina film also halts the internal oxidation of aluminium. The results obtained using a variety of high resolution microscopy and analytical techniques have been analysed by calculating phase diagrams and by creating a finite difference model to reveal aspects of the internal oxidation problem.