Combination of solid state NMR and DFT calculation to elucidate the state of sodium in hard carbon electrodes

We examined the state of sodium electrochemically inserted in HC prepared at 700–2000 °C using solid state Na magic angle spinning (MAS) NMR and multiple quantum (MQ) MAS NMR. The 23Na MAS NMR spectra of Na-inserted HC samples showed signals only in the range between +30 and −60 ppm. Each observed spectrum was ascribed to combinations of Na+ ions from the electrolyte, reversible ionic Na components, irreversible Na components assigned to solid electrolyte interphase (SEI) or non-extractable sodium ions in HC, and decomposed Na compounds such as Na2CO3. No quasi-metallic sodium component was observed to be dissimilar to the case of Li inserted in HC. MQMAS NMR implies that heat treatment of HC higher than 1600 °C decreases defect sites in the carbon structure. To elucidate the difference in cluster formation between Na and Li in HC, the condensation mechanism and stability of Na and Li atoms on a carbon layer were also studied using DFT calculation. Na3 triangle clusters standing perpendicular to the carbon surface were obtained as a stable structure of Na, whereas Li2 linear and Li4 square clusters, all with Li atoms being attached directly to the surface, were estimated by optimization. Models of Na and Li storage in HC, based on the calculated cluster structures were proposed, which elucidate why the adequate heat treatment temperature of HC for high-capacity sodium storage is higher than the temperature for lithium storage

A Reversible Phase Transition for Sodium Insertion in Anatase TiO₂

International audienceAnatase TiO2 is a potential negative electrode for sodium-ion batteries. The sodium storage mechanism is, however, still under debate, yet its comprehension is required to optimize the electrochemical properties. To clarify the sodium storage mechanism occurring in anatase, we have used both electrochemical and chemical routes from which we obtained similar trends. During the first discharge, an irreversible plateau region is observed which corresponds to the insertion of Na + within the interstitial sites of anatase and is accompanied by a drastic loss of the long-range order as revealed by x-ray diffraction, high resolution of high angle annular dark field scanning transmission electron microscope (HAADF-STEM) and pair distribution function (PDF) analysis. Further structural analysis of the total scattering data indicates that the sodiated phase displays a layered-like rhombohedral R-3m structure built from the stacking of Ti and Na slabs. Because of the initial 3D network of anatase, the reduced phase shows strong disorder due to cationic inter-mixing between the Ti and Na slabs and the refined chemical formula is (Na0.43Ti0.57)3a(0.22Na0.39Ti0.39)3bO2 where refers to vacancy. The presence of high valence Ti ions in the Na layers induces a contraction of the c-parameter as compared to the ordered phase. Upon de-sodiation, the structure further amorphized and the local structure probed by PDF is shown to be similar to the anatase TiO2 suggesting that the 3D network is recovered. The reversible sodium insertion/de-insertion is thus attributed to the rhombohedral active phase formed during the first discharge, and an oxidized phase featuring the local structure of anatase. Due to the amorphous nature of the two phases, the potential-composition curves are characterized by a sloping curve. Finally, a comparison between the intercalation of lithium and sodium into anatase TiO2 performed by DFT calculations confirmed that for the sodiated phase, the rhombohedral structure is more stable than the tetragonal phase observed during the lithiation of nanoparticles. In many areas of modern life, lithium-ion batteries are ubiquitous as energy-storage solutions. The growing demand for higher energy density and lower cost of electro-chemical energy storage devices, however, has motivated a search for auxiliary technologies based on alternative chemistries. 1,2 One possible candidate is the sodium-ion battery, which is attractive because of the high earth– abundance of sodium, and lower cost versus lithium-ion batteries, due to compatibility with aluminum as the an-odic current collector. 3-5 Development of sodium-ion batteries has been largely stimulated by knowledge of lithium-ion analogues. The intercalation of Na + or Li + ions into a host lattice can, however, give qualitatively different voltage profiles, corresponding to different intercalation mechanisms. For example, lithium insertion in Li4Ti5O12 is accompanied by a spinel to rock-salt phase transition. 6,7 The equivalent sodium insertion, however, proceeds via a complex three-phase–separation mechanism (spinel to two rock-salt phases of Li7Ti5O12 and Na6LiTi5O12). 8 Such differences in intercalation behaviour can often be attributed to different properties of Li versus Na, such as ionic radius and polarizability. 9, 10 In general, however, the performance of electrodes in sodium-ion batteries cannot be understood by simply extrapolating from their behaviour versus lithium, when it is necessary to carefully reexamine the sodium-intercalation behaviour

Insights into Li+ , Na+ , and K+ Intercalation in Lepidocrocite-Type Layered TiO 2 Structures

International audienceA lamellar lepidocrocite-type titanate structure with ∼25% Ti4+ vacancies was recently synthesized, and it showed potential for use as an electrode in rechargeable lithium-ion batteries. In addition to lithium, we explore this material’s ability to accommodate other monovalent ions with greater natural abundance (e.g., sodium and potassium) in order to develop lower-cost alternatives to lithium-ion batteries constructed from more widely available elements. Galvanostatic discharge/charge curves for the lepidocrocite material indicate that increasing the ionic radius of the monovalent ion results in a deteriorating performance of the electrode. Using first-principles electronic structure calculations, we identify the relaxed geometries of the structure while varying the placement of the ion in the structure. We then use these geometries to compute the energy of formations. Additionally, we determine that all ions are favorable in the structure, but interlayer positions are preferred compared to vacancy positions. We also conclude that the exchange between the interlayer and vacancy positions is a process that involves the interaction between interlayer water and surface hydroxyl groups next to the titanate layer. We observe a cooperative effect between structural water and OH groups to assist alkali ions to move from the interlayer to the vacancy site. Thus, the as-synthesized lepidocrocite serves as a prototypical structure to investigate the migration mechanism of ions within a confined space along with the interaction between water molecules and the titanate framework

LiBr-coated Air Electrodes for Li-air Batteries

Li–air batteries (LAB) have a theoretical energy density as high as 3500 Wh kg−1; however, many problems remain to be addressed before their practical application. Introduction of a redox mediator (RM) is commonly applied to reduce the high overpotential of the air electrode (AE) during the charge process. We try to fix an RM on the AE by coating it with a slurry of carbon black and binder on a carbon paper substrate to enable us not only to suppress the shuttle effect but also to concentrate the RM on the surface of the AE where it works. We use LiBr as the RM in this study and compare two types of LAB cells: one with a LiBr-coated AE and the other with LiBr dissolved in the electrolyte solution. The cell with the LiBr-coated AE exhibits a better cell performance than that with the dissolved LiBr

Insights into Li⁺, Na⁺, and K⁺ Intercalation in Lepidocrocite-Type Layered TiO₂ Structures

A lamellar lepidocrocite-type titanate structure with ∼25% Ti⁴⁺ vacancies was recently synthesized, and it showed potential for use as an electrode in rechargeable lithium-ion batteries. In addition to lithium, we explore this material’s ability to accommodate other monovalent ions with greater natural abundance (e.g., sodium and potassium) in order to develop lower-cost alternatives to lithium-ion batteries constructed from more widely available elements. Galvanostatic discharge/charge curves for the lepidocrocite material indicate that increasing the ionic radius of the monovalent ion results in a deteriorating performance of the electrode. Using first-principles electronic structure calculations, we identify the relaxed geometries of the structure while varying the placement of the ion in the structure. We then use these geometries to compute the energy of formations. Additionally, we determine that all ions are favorable in the structure, but interlayer positions are preferred compared to vacancy positions. We also conclude that the exchange between the interlayer and vacancy positions is a process that involves the interaction between interlayer water and surface hydroxyl groups next to the titanate layer. We observe a cooperative effect between structural water and OH groups to assist alkali ions to move from the interlayer to the vacancy site. Thus, the as-synthesized lepidocrocite serves as a prototypical structure to investigate the migration mechanism of ions within a confined space along with the interaction between water molecules and the titanate framework

Impact of the Cut-Off Voltage on Cyclability and Passive Interphase of Sn-Polyacrylate Composite Electrodes for Sodium-Ion Batteries

Reversibility of electrochemical sodiation for Sn-based electrodes consisting of Sn powder, graphite, and sodium polyacrylate was examined at different upper cutoff voltages of 0.65 and 0.70 V in nonaqueous Na cells. The upper cutoff voltage is one of the key factors to improve the electrochemical reversibility. In case of a cutoff voltage of 0.70 V, the sodiation/desodiation cycle performance was not stable and accompanied by capacity decay, indicating that the anodic decomposition of passivation layer is led to the dissolution and reformation at 0.68 and 0.40 V, respectively, on Sn particles that were catalyzed by pure Sn metal. The repeated dissolution and reformation brought a thicker and resistive surface layer, resulting from the accumulation of electrolyte decomposition products, which was clarified by X-ray photoelectron spectroscopy. In contrast, the capacity retention and stability were improved by simply changing the upper cutoff voltage to 0.65 V due to exclusion of the SEI decomposition at 0.68 V. The results of time-of-flight secondary ion mass spectroscopy measurements suggests that the surface passivation layer containing polymer/oligomer on the Sn electrode was successfully formed and enhanced the SEI functionality for 0.65 V cutoff. The Sn-based electrode delivered ∼700 mAh g^–1 reversible capacity over 100 cycles