Effect of the Calcination Duration on the Electrochemical Properties of Na2Ti3O7 as Anode Material for Na-Ion Batteries

peer reviewedThe growing interest in Na-ion batteries as a “beyond lithium” technologies for energy storage drives the research for high-performance and environment-friendly materials. Na2Ti3O7 (NTO) as an eco-friendly, low-cost anode material shows a very low working potential of 0.3 V vs. Na+/Na but suffers from poor cycling stability, which properties can be significantly influenced by materials synthesis and treatment. Thus, in this work, the influence of the calcination time on the electrochemical performance and the reaction mechanism during cycling were investigated. NTO heat-treated for 48 h at 800 °C (NTO-48h) demonstrated enhanced cycling performance in comparison to NTO heat-treated for only 8 h (NTO-8h). The pristine material was thoroughly characterized by X-ray diffraction, laser granulometry, X-ray photoelectron spectroscopy, and specific surface area measurements. The reaction mechanisms induced by sodiation/desodiation and cycling were investigated by operando XRD. Electrochemical impedance spectroscopy was used to evidence the evolution of the solid electrolyte interface layer (SEI) and modification of charge transfer resistances as well as the influence of cycling on capacity decay. The evolution of the crystallographic structure of NTO-48h revealed a more ordered structure and lower surface contamination compared to NTO-8h. Moreover, the residual Na4Ti3O7 phase detected after the sodium extraction step in NTO-8h seems correlated to the lower electrochemical performance of NTO-8h compared to NTO-48h

Lithium Intercalation in Anatase Titanium Vacancies and the Role of Local Anionic Environment

The structure of bulk and nondefective compounds is generally described with crystal models built from well mastered techniques such the analysis of an X-ray diffractogram. The presence of defects, such as cationic vacancies, locally disrupt the long-range order, with the appearance of local structures with order extending only a few nanometers. To probe and describe the electrochemical properties of cation-deficient anatase, we investigated a series of materials having different concentrations of vacancies, i.e., Ti_{1–<i>x</i>–<i>y</i>}□_{<i>x</i>+<i>y</i>}O_{2–4(<i>x</i>+<i>y</i>)}F_4<i>x</i>(OH)_4<i>y</i>, and compared their properties with respect to defect-free stoichiometric anatase TiO₂. At first, we characterized the series of materials Ti_{1–<i>x</i>–<i>y</i>}□_{<i>x</i>+<i>y</i>}O_{2–4(<i>x</i>+<i>y</i>)}F_4<i>x</i>(OH)_4<i>y</i> by means of pair distribution function (PDF), ¹⁹F nuclear magnetic resonance (NMR), Raman and X-ray photoelectron spectroscopies, to probe the compositional and structural features. Second, we characterized the insertion electrochemical properties vs metallic lithium where we emphasized the beneficial role of the vacancies on the cyclability of the electrode under high C-rate, with performances scaling with the concentration of vacancies. The improved properties were explained by the change of the lithium insertion mechanism due to the presence of the vacancies, which act as host sites and suppress the phase transition typically observed in pure TiO₂, and further favor diffusive transport of lithium within the structure. NMR spectroscopy performed on lithiated samples provides evidence for the insertion of lithium in vacancies. By combining electrochemistry and DFT-calculations, we characterized the electrochemical signatures of the lithium insertion in the vacancies. Importantly, we found that the insertion voltage largely depends on the local anionic environment of the vacancy with a fluoride and hydroxide-rich environments, yielding high and low insertion voltages, respectively. This work further supports the beneficial use of defects engineering in electrodes for batteries and provides new fundamental knowledge in the insertion chemistry of cationic vacancies as host sites

Role of Segregated Iron at Grain Boundaries on Mg Corrosion

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Effect of Cr Content on Corrosion Resistance of Low-Cr Alloy Steels Studied by Surface and Electrochemical Techniques

The electrochemical behavior of low alloyed Fe-Cr steels with 3 and 5% wt. of Cr in neutral Na2SO4 electrolyte combined with a detailed chemical and morphological characterization of these alloys performed by Auger electron spectroscopy, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry and scanning electron microscopy are presented here. The corrosion of low alloyed Fe-Cr steels proceeds in the prepassive range, with the formation of corrosion surface films having a duplex structure with outer iron oxide/hydroxide layer and inner Cr oxide-rich layer. The thickness, composition, and the morphology of the surface films vary as a function of chromium content in the alloy as well as conditions of electrochemical tests (temperature). Even a low chromium content shows a beneficial effect on the corrosion performances of the Fe-Cr steels. The chromium as a more active component than iron of ferrite increases the anodic activity of this phase, which results in a rapid saturation of the surface with the anodic reaction products forming a fine crystalline-like and compact layer of corrosion products. In this way, the chromium acts as a modifier of formation/crystallization of the iron-rich surface film (mainly magnetite) in the prepassive range

Effect of Cr Content on Corrosion Resistance of Low-Cr Alloy Steels Studied by Surface and Electrochemical Techniques

The electrochemical behavior of low alloyed Fe-Cr steels with 3 and 5% wt. of Cr in neutral Na2SO4 electrolyte combined with a detailed chemical and morphological characterization of these alloys performed by Auger electron spectroscopy, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry and scanning electron microscopy are presented here. The corrosion of low alloyed Fe-Cr steels proceeds in the prepassive range, with the formation of corrosion surface films having a duplex structure with outer iron oxide/hydroxide layer and inner Cr oxide-rich layer. The thickness, composition, and the morphology of the surface films vary as a function of chromium content in the alloy as well as conditions of electrochemical tests (temperature). Even a low chromium content shows a beneficial effect on the corrosion performances of the Fe-Cr steels. The chromium as a more active component than iron of ferrite increases the anodic activity of this phase, which results in a rapid saturation of the surface with the anodic reaction products forming a fine crystalline-like and compact layer of corrosion products. In this way, the chromium acts as a modifier of formation/crystallization of the iron-rich surface film (mainly magnetite) in the prepassive range

The effect of Na2S additive in alkaline electrolyte on improved performances of Fe-based air batteries

International audienceFe-based air alkaline batteries are promising candidates for large scale energy storage due to their low cost, eco-friendliness and high energy density. In this work, pure iron was studied as negative electrode of Fe-air batteries with and without Na 2 S added to the KOH alkaline electrolyte in order to bring new insight on the mechanisms of inhibition of the hydrogen evolution reaction (HER) and improvement of the electrochemical performances of the Fe electrode. Electrochemical (CV, galvanostatic discharge), spectroscopic (XPS, ToF-SIMS) and microscopic (SEM) analysis were applied to characterize the effect of the sulfide additive (Na 2 S) on the electrochemical behavior of Fe electrodes and their chemical and morphological modifications. Surface analysis performed by XPS and ToF-SIMS revealed the formation of a markedly thicker oxide/hydroxide layer. The mechanism of formation of this thick oxide/hydroxide layer is initiated by the adsorption of sulfur (S ads) at the oxide/metal interface that inhibits the HER during the first stages of electrochemical process and promotes subsequent anodic oxidation. With the Na 2 S electrolyte additive, Fe(0)/Fe(II) conversion/deconversion is suppressed and charge/discharge proceeds mostly by Fe(II)/Fe(III) conversion/deconversion with improved capacity owing to the porous morphology of the oxide/hydroxide layer

Promises and challenges of alloy-type and conversion-type anode materials for sodium–ion batteries

International audienceSodium-ion batteries (SIBs) are considered as one of the most promising candidates for competing with lithium-ion batteries owing to significant natural abundance of sodium and similar reaction mechanism. The large radius of Na+ (1.02 Å) severely impedes the electrochemical performance, especially high-rate capability and long-cycle stability of SIBs. Together with the cathode and electrolyte, the anode determines full battery's operating voltage and plays a vital role in avoiding sodium dendrite in terms of redox potential. On this account, the anode holds a prime importance toward the development of advanced SIBs with high rate capability and superior durability. Herein, we present a review on the exciting advances in alloy- and conversion-type anode materials as well as solid-electrolyte interphase layer for SIBs aiming at the improvement of rate capability and cycling life. Finally, the challenges and some of the critical issues in alloy-type and conversion-type anode materials for sodium-ion batteries are addressed

Self-standing V2O5/Polydopamine/CNT film as high-performance cathode material for advanced zinc-ion batteries

peer reviewedRechargeable aqueous zinc-ion battery (ZIB) is regarded as a strategic technology for energy storage and conversion in high-power, high-safety applications. Nonetheless, the primary challenge lies in the development of high-performance cathode materials. In this work, we report for the first time a self-standing V2O5/polydopamine/CNT (V2O5/PDA/CNT) electrode that shows outstanding electrochemical performance as cathode material for zinc-ion batteries. A polydopamine coating was applied to the surface of V2O5 particles through a facile self-polymerization process of dopamine. The PDA coating was confirmed and studied by TEM, TGA, FTIR-ATR, and XPS (O 1s & V 2p, C 1s and N 1s). Polydopamine thin layer leads to the partial reduction of V5+ to V4+. Combining V2O5/PDA with CNT allowed us to obtain a self-standing, binder-free cathode material with a high capacity of ∼ 530 mAh g−1 at 0.1 A g−1. The developed electrode demonstrates excellent rate capability. Indeed, even with a 25-fold increase in current density (from 0.2 A g−1 to 5 A g−1), V2O5/PDA/CNT-b retains 94 % of its capacity. In addition, the new electrode exhibits long-term cycle stability enduring up to 1000 cycles at high current density with excellentcapacity retention. The combination of PDA coating with the incorporation of carbon nanotube appears to be an excellent strategy for enhancing the electrochemical performance and stability of V2O5 for ZIBs