Electronic Structure of Sodium Superoxide Bulk, (100) Surface, and Clusters using Hybrid Density Functional: Relevance for Na–O₂ Batteries

Clarifying the electronic structure of sodium superoxide (NaO₂) is a key step in understanding the electrochemical behavior of Na–O₂ batteries. Here we report a density functional theory study to explore the effect of atomic structure and morphology on the electronic properties of different model systems: NaO₂ bulk, (100) surface, and small (NaO₂)_<i>n</i> clusters (<i>n</i> = 3–8). We found that a correct description of the open-shell 2p electrons of O₂^– requires the use of a hybrid functional, which reveals a clear insulating nature of all of the investigated systems. This sheds light onto the capacity limitations of pure NaO₂ as a discharge product and highlights the need for developing new strategies to enhance its electron transport in the optimization of Na–O₂ cells

Silicon-Reduced Graphene Oxide Self-Standing Composites Suitable as Binder-Free Anodes for Lithium-Ion Batteries

Silicon-reduced graphene oxide (Si-rGO) composites processed as self-standing aerogels (0.2 g cm^–3) and films (1.5 g cm^–3) have been prepared by the thermal reduction of composites formed between silicon nanoparticles and a suspension of graphene oxide (GO) in ethanol. The characterization of the samples by different techniques (X-ray diffraction, Raman, thermogravimetric analysis, and scanning electron microscopy) show that in both cases the composites are formed by rGO sheets homogeneously decorated with 50 nm silicon nanoparticles with silicon contents of ∼40% wt. The performances of these self-standing materials were tested as binder-free anodes in lithium-ion batteries (LIBs) in a half cell configuration under two different galvanostatic charge–discharge cutoff voltages (75 and 50 mV). The results show that the formation of a solid electrolyte interphase (SEI) is favored in composites processed as aerogels due to its large exposed surface, which prevents the activation of silicon when they are cycled within the 2 to 0.075 V voltage windows. It is also found that the composites processed in the form of self-standing films exhibit good stability over the first 100 cycles, high reversible specific capacity per mass of electrode (∼750 mAh g^–1), areal capacities that reach 0.7 mAh cm^–2, and high Coulombic efficiencies (80% for the first charge–discharge cycle and over 99% in the subsequent cycles)

Toward Safe and Sustainable Batteries: Na₄Fe₃(PO₄)₂P₂O₇ as a Low-Cost Cathode for Rechargeable Aqueous Na-Ion Batteries

The electrochemical properties of Na₄Fe₃(PO₄)₂P₂O₇ in aqueous and organic electrolyte are compared under similar conditions. Na₄Fe₃(PO₄)₂P₂O₇ is able to deliver almost the same capacity in both types of electrolytes despite the smaller electrochemical window entailed by the aqueous one. As shown by electrochemical impedance spectroscopy (EIS), this is possible thanks to the lower overpotential that this material exhibits in aqueous electrolyte. It is shown here that the main contribution to overpotential in organic electrolyte mainly originates from a SPI (Solid Permeable Interphase) layer formed below 3.5 V vs Na⁺/Na that acts as a blocking layer and hinders Na⁺ diffusion and that is absent in aqueous electrolyte. Overall, the obtained results highlight the positive attributes of using low-cost and environmentally friendly aqueous electrolytes and the challenges to be overcome in terms of air and moisture stability of the studied material

Composition and Evolution of the Solid-Electrolyte Interphase in Na₂Ti₃O₇ Electrodes for Na-Ion Batteries: XPS and Auger Parameter Analysis

Na₂Ti₃O₇ is considered a promising negative electrode for Na-ion batteries; however, poor capacity retention has been reported and the stability of the solid-electrolyte interphase (SEI) could be one of the main actors of this underperformance. The composition and evolution of the SEI in Na₂Ti₃O₇ electrodes is hereby studied by means of X-ray photoelectron spectroscopy (XPS). To overcome typical XPS limitations in the photoelectron energy assignments, the analysis of the Auger parameter is here proposed for the first time in battery materials characterization. We have found that the electrode/electrolyte interface formed upon discharge, mostly composed by carbonates and semicarbonates (Na₂CO₃, NaCO₃R), fluorides (NaF), chlorides (NaCl) and poly­(ethylene oxide)­s, is unstable upon electrochemical cycling. Additionally, solid state nuclear magnetic resonance (NMR) studies prove the reaction of the polyvinylidene difluoride (PVdF) binder with sodium. The powerful approach used in this work, namely Auger parameter study, enables us to correctly determine the composition of the electrode surface layer without any interference from surface charging or absolute binding energy calibration effects. As a result, the suitability for Na-ion batteries of binders and electrolytes widely used for Li-ion batteries is questioned here

Flexible and Dynamic Thermal Behavior of Self-Catenated [{Ni₃(H₂O)₃(Bpa)₄}(V₆O₁₈)]·8H₂O Constructed from 10-c Heterometallic Inorganic–Organic Clusters

The hydrothermal treatment of Ni­(NO₃)₂·6H₂O, NaVO₃, and Bpa (1,2-Di­(pyridyl)­ethane) (C₁₂H₁₂N₂) at 120 °C during 3 days leads to green single crystals of the title compound. The single crystal X-ray diffraction reveals that [{Ni₃(H₂O)₃(Bpa)₄}­(V₆O₁₈)]·8H₂O crystallizes in the monoclinic system, <i>P</i>2₁/<i>c</i> space group, with <i>a</i> = 13.5536 (2), <i>b</i> = 19.0463 (2), <i>c</i> = 27.7435 (3) Å, β = 112.3880 (10)°, <i>V</i> = 6622(3) ų, with R1­(obs) = 0.0558, wR2­(obs) = 0.1359, for 10278 observed reflections. The complexity of the crystal structure is based on different points, as the existence of: both “gauche” and “trans” conformations of the organic ligand, the [V₁₂O₃₆]⁻¹² cycles, formed by 12 corner-sharing VO₄ tetrahedra, and, finally, the combination of both three-dimensional metal–organic and inorganic substructures, giving rise to a self-catenated highly connected net. The crystallization water molecules are semi-encapsulated in the channels along the [100] direction, and their loss gives rise to a dynamical and reversible structural contraction. Moreover, after the removal of the crystallization water molecules, the compound exhibits a negative thermal behavior in the 85–155 °C temperature range, and irreversible structural transformation due to the loss of coordinated water molecules up to 200 °C. The IR and UV–vis spectra were determined for the as-synthesized sample, after the removal of crystallization water molecules and after the irreversible transformation due to the loss of coordinated water molecules. The thermal evolution of χ_m was adjusted to a magnetic model considering an isotropic dimer plus two Ni­(II) d⁸ isolated octahedra

Flexible and Dynamic Thermal Behavior of Self-Catenated [{Ni₃(H₂O)₃(Bpa)₄}(V₆O₁₈)]·8H₂O Constructed from 10-c Heterometallic Inorganic–Organic Clusters

The hydrothermal treatment of Ni­(NO₃)₂·6H₂O, NaVO₃, and Bpa (1,2-Di­(pyridyl)­ethane) (C₁₂H₁₂N₂) at 120 °C during 3 days leads to green single crystals of the title compound. The single crystal X-ray diffraction reveals that [{Ni₃(H₂O)₃(Bpa)₄}­(V₆O₁₈)]·8H₂O crystallizes in the monoclinic system, <i>P</i>2₁/<i>c</i> space group, with <i>a</i> = 13.5536 (2), <i>b</i> = 19.0463 (2), <i>c</i> = 27.7435 (3) Å, β = 112.3880 (10)°, <i>V</i> = 6622(3) ų, with R1­(obs) = 0.0558, wR2­(obs) = 0.1359, for 10278 observed reflections. The complexity of the crystal structure is based on different points, as the existence of: both “gauche” and “trans” conformations of the organic ligand, the [V₁₂O₃₆]⁻¹² cycles, formed by 12 corner-sharing VO₄ tetrahedra, and, finally, the combination of both three-dimensional metal–organic and inorganic substructures, giving rise to a self-catenated highly connected net. The crystallization water molecules are semi-encapsulated in the channels along the [100] direction, and their loss gives rise to a dynamical and reversible structural contraction. Moreover, after the removal of the crystallization water molecules, the compound exhibits a negative thermal behavior in the 85–155 °C temperature range, and irreversible structural transformation due to the loss of coordinated water molecules up to 200 °C. The IR and UV–vis spectra were determined for the as-synthesized sample, after the removal of crystallization water molecules and after the irreversible transformation due to the loss of coordinated water molecules. The thermal evolution of χ_m was adjusted to a magnetic model considering an isotropic dimer plus two Ni­(II) d⁸ isolated octahedra

Thermal Response, Catalytic Activity, and Color Change of the First Hybrid Vanadate Containing Bpe Guest Molecules

Four isomorphic compounds with formula [{Co₂(H₂O)₂(Bpe)₂}­(V₄O₁₂)]­·4H₂O·Bpe, CoBpe <b>1</b>; [{CoNi­(H₂O)₂(Bpe)₂}­(V₄O₁₂)]­·4H₂O·Bpe, CoNiBpe <b>2</b>; [{Co_0.6Ni_1.4(H₂O)₂(Bpe)₂}­(V₄O₁₂)]­·4H₂O·Bpe, NiCoBpe <b>3</b>; and [{Ni₂(H₂O)₂(Bpe)₂}­(V₄O₁₂)]­·4H₂O·Bpe, NiBpe <b>4</b>, have been obtained by hydrothermal synthesis. The crystal structures of CoBpe <b>1</b> and NiBpe <b>4</b> were determined by single-crystal X-ray diffraction (XRD). The Rietveld refinement of CoNiBpe <b>2</b> and NiCoBpe <b>3</b> XRD patterns confirms that those are isomorphic. The compounds crystallize in the <i>P</i>1̅ space group, exhibiting a crystal structure constructed from inorganic layers pillared by Bpe ligands. The crystal structure contains intralayer and interlayer channels, in which the crystallization water molecules and Bpe guest molecules, respectively, are located. The solvent molecules establish a hydrogen bonding network with the coordinated water molecules. Thermodiffractometric and thermogravimetric studies showed that the loss of crystallization and coordinated water molecules takes place at different temperatures, giving rise to crystal structure transformations that involve important reduction of the interlayer distance, and strong reduction of crystallinity. The IR, Raman, and UV–vis spectra of the as-synthesized and heated compounds confirm that the structural building blocks and octahedral coordination environment of the metal centers are maintained after the structural transformations. The color change and reversibility of the water molecules uptake/removal were tested showing that the initial color is not completely recovered when the compounds are heated at temperatures higher than 200 °C. The thermal evolution of the magnetic susceptibility indicates one-dimensional antiferromagnetic coupling of the metal centers at high temperatures. For NiCoBpe <b>3</b> and NiBpe <b>4</b> compounds magnetic ordering is established at low temperatures, as can be judged by the maxima observed in the magnetic susceptibilities. CoNiBpe <b>2</b> was proved as catalyst being active for cyanosilylation reactions of aldehydes

Electrochemical Na Extraction/Insertion of Na₃V₂O_2<i>x</i>(PO₄)₂F_{3–2<i>x</i>}

A mixed-valence V³⁺/V⁴⁺ composite material belonging to the Na₃V₂O_2<i>x</i>(PO₄)­2F_{3–2<i>x</i>}/C family is synthesized and the electrochemical Na extraction/insertion mechanism is determined using a combination of high-resolution synchrotron X-ray diffraction (XRD) data, X-ray absorption spectroscopy (XAS), ²³Na and ¹⁹F solid state nuclear magnetic resonance (NMR), double titration (for the elucidation of the vanadium oxidation state), and electrochemical measurements. The vanadium oxidation state is found to be +3.8 for the as-prepared sample. Detailed analysis of the cathode structural evolution illustrated that the V⁴⁺/V⁵⁺ couple is active in this compound during electrochemical cycling between 2.8 V and 4.3 V. This study demonstrates how the sodium-ion extraction and insertion pathways in cathode materials can be followed (and verified) using several experimental techniques, especially when multiple potential oxidation states are present in the parent compound

Sodium Distribution and Reaction Mechanisms of a Na₃V₂O₂(PO₄)₂F Electrode during Use in a Sodium-Ion Battery

Ambient temperature sodium-ion batteries are emerging as an exciting alternative to commercially dominant lithium-ion batteries for larger scale stationary applications. In order to realize such a sodium-ion battery, electrodes need to be developed, understood, and improved. Here, Na₃V₂O₂(PO₄)₂F is investigated from the perspective of sodium. Reaction mechanisms for this cathode during battery function include the following: a region comprising at least three phases with subtly varying sodium compositions that transform via two two-phase reaction mechanisms, which appears at the lower potential plateau-like region during both charge and discharge; an extended solid solution region for majority of the cycling process, including most of the higher potential plateau; and a second two-phase region near the highest charge state during charge and between the first and second plateau-like regions during discharge. Notably, the distinct asymmetry in the reaction mechanism, lattice, and volume evolution on charge relative to discharge manifests an interesting question: Is such an asymmetry beneficial for this cathode? These reaction mechanisms are inherently related to sodium evolution, which shows complex behavior between the two sodium crystallographic sites in this compound that in turn mediate the lattice and reaction evolution. Thus, this work relates atomic-level sodium perturbations directly with electrochemical cycling

High-Performance P2-Phase Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ Cathode Material for Ambient-Temperature Sodium-Ion Batteries

High-performance Mn-rich P2-phase Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ is synthesized by a ceramic method, and its stable electrochemical performance is demonstrated. ²³Na solid-state NMR confirms the substitution of Ti⁴⁺ ions in the transition metal oxide layer and very fast Na⁺ mobility in the interlayer space. The pristine electrode delivers a second charge/discharge capacity of 146.57/144.16 mA·h·g^–1 and retains 95.09% of discharge capacity at the 50th cycle within the voltage range 4.0–2.0 V at C/10. At 1C, the reversible specific capacity still reaches 99.40 mA·h·g^–1, and capacity retention of 87.70% is achieved from second to 300th cycle. In addition, the moisture-exposed electrode reaches reversible capacities of more than 130 and 80 mA·h·g^–1 for C/10 and 1C, respectively, with excellent capacity retention. The correlation between overall electrochemical performance of both electrodes and crystal structural characteristics are investigated by neutron powder diffraction. The stability of pristine electrode’s crystallographic structure during the charge/discharge process has been investigated by in situ X-ray diffraction, where only a solid solution reaction occurs within the given voltage range except for a small biphasic mechanism occurring at or below 2.2 V during the discharge process. The relatively small substitution (20%) at the transition metal site leads to stable electrochemical performance, which is in part derived from the structural stability during electrochemical cycling. Therefore, the small cosubstitution (e.g., with Ti and Fe) route suggests a possible new scope for the design of sodium-ion battery electrodes that are suitable for long-term cycling