Paula Serras

Na-ion batteries, recent advances and present challenges to become low cost energy storage systems

Energy production and storage have become key issues concerning our welfare in daily life. Present challenges for batteries are twofold. In the first place, the increasing demand for powering systems of portable electronic devices and zero-emission vehicles stimulates research towards high energy and high voltage systems. In the second place, low cost batteries are required in order to advance towards smart electric grids that integrate discontinuous energy flow from renewable sources, optimizing the performance of clean energy sources. Na-ion batteries can be the key for the second point, because of the huge availability of sodium, its low price and the similarity of both Li and Na insertion chemistries. In spite of the lower energy density and voltage of Na-ion based technologies, they can be focused on applications where the weight and footprint requirement is less drastic, such as electrical grid storage. Much work has to be done in the field of Na-ion in order to catch up with Li-ion technology. Cathodic and anodic materials must be optimized, and new electrolytes will be the key point for Na-ion success. This review will gather the up-to-date knowledge about Na-ion battery materials, with the aim of providing a wide view of the systems that have already been explored and a starting point for the new research on this battery technology.

Crystal chemistry of Na insertion/deinsertion in FePO4–NaFePO4

In this paper we examine the mechanism of Na insertion and extraction in the FePO4–NaFePO4 system. Chemical preparation of the intermediate Na1−xFePO4 phase has revealed the existence of a range of stable compositions with different Na+/vacancy arrangements. The mechano-chemical aspects of the charge and discharge reactions are also discussed.

High voltage cathode materials for Na-ion batteries of general formula Na3V2O2x(PO4)2F3−2x

Different samples of the sodium–vanadium fluorophosphate cathodic materials have been synthesized via the hydrothermal method, varying the type and content of carbon used in the synthesis. Structural characterization of the composites was performed by powder X-ray diffraction. Magnetic susceptibility measurements and EPR (Electron Paramagnetic Resonance) polycrystalline spectra indicate that some of the samples exhibit V3+/V4+ mixed valence, with the general formula Na3V2O2x(PO4)2F3−2x where 0 ≤ x < 1. The morphology of the materials was analyzed by Transmission Electron Microscopy (TEM). A correlation between the type and content of carbon with the electrochemical behavior of the different samples was established. Electrochemical measurements conducted using Swagelok-type cells showed two voltage plateaux at 3.6 and 4.1 V vs. Na/Na+. The best performing sample, which comprised a moderate percentage of electrochemical grade carbon as additive, exhibited specific capacity values of about 100 mA h g−1 at 1C (≈80% of theoretical specific capacity). Cyclability tests at 1C proved good reversibility of the material that maintained 98% of initial specific capacity for 30 cycles.

Structural evolution of high energy density V3+/V4+ mixed valent Na3V2O2x(PO4)2F3−2x (x = 0.8) sodium vanadium fluorophosphate using in situ synchrotron X-ray powder diffraction

Sodium-ion batteries have become good candidates for energy storage technology. For this purpose it is crucial to search for and optimize new electrode and electrolyte materials. Sodium vanadium fluorophosphates are considered promising cathodes but further studies are required to elucidate their electrochemical and structural behavior. Therefore, this work focuses on the time-resolved in situ synchrotron X-ray powder diffraction study of Na3V2O2x(PO4)2F3−2x (x = 0.8) while electrochemically cycling. Reaction mechanism evolution, lattice parameters and sodium evolution, and the maximum possible sodium extraction under the applied electrochemical constraints, are some of the features that have been determined for both a fresh and an offline pre-cycled cell. The reaction mechanism evolution undergoes a solid solution reaction with a two-phase region for the first lower-potential plateau while a predominantly solid solution behavior is observed for the second higher-potential plateau. Lattice and volume evolution is clearly dependent on the Na insertion/extraction mechanism, the sodium occupancy and distribution amongst the two crystallographic sites, and the electrochemical cycling history. The comparison between the fresh and the pre-cycled cell shows that there is a Na site preference depending on the cell and history and that Na swaps from one site to the other during cycling. This suggests sodium site occupancy and mobility in the tunnels is interchangeable and fluid, a favorable characteristic for a cathode in a sodium-ion battery.

Structural evolution of mixed valent (V3+/V4+) and V4+ sodium vanadium fluorophosphates as cathodes in sodium-ion batteries: comparisons, overcharging and mid-term cycling

Sodium vanadium fluorophosphates belonging to the Na3V2O2x(PO4)2F3−2x family of compounds have recently shown very good electrochemical performance versus Na/Na+ providing high working voltages (3.6 and 4.1 V) and good specific capacity values. In this work the electrochemical behaviour and structural evolution of two compositions, Na3V2O1.6(PO4)2F1.4 (V3.8+) and Na3V2O2(PO4)2F (V4+), are detailed using time-resolved in situ synchrotron X-ray powder diffraction. For the first time in sodium-ion batteries the effects of overcharging and mid-term cycling are analyzed using this technique. Differences in the composition of both materials lead to different combinations of biphasic and single-phase reaction mechanisms while charging up to 4.3 V and overcharging up to 4.8 V. Moreover, the analysis of particle size broadening of both samples reveals the higher stress suffered by the V4+ compared to the more disordered V3.8+ sample. The more “flexible” structure of the V3.8+ sample allows for maximum sodium extraction when overcharging up to 4.8 V while in the case of the V4+ sample no evidence is shown of more sodium extraction between 4.3 V and 4.8 V. Furthermore, the analysis of both materials after 10 cycles shows the appearance of secondary phases due to the degradation of the material or the battery itself (e.g. electrolyte degradation). This study shows examples of the possible degradation mechanisms (and phases) while overcharging and mid-term cycling which is in turn crucial to making better electrodes, either based on these materials or generally in cathodes for sodium-ion batteries.

Vanadyl-type defects in Tavorite-like NaVPO4F: from the average long range structure to local environments

Tavorite-type compositions offer rich crystal chemistry for positive electrodes in rechargeable batteries, among which LiVIIIPO4F has the highest theoretical energy density (i.e. 655 Wh kg−1). In this article, we report for the first time the synthesis of the related Na-based phase crystallizing in the Tavorite-like structure. Its in-depth structural and electronic characterization was conducted by a combination of several techniques, spanning electron and X-ray powder diffraction as well as infrared and X-ray absorption spectroscopy. The magnetic susceptibility measurement reveals an average oxidation state for vanadium slightly higher than V3+. This slight oxidation is supported by infrared and X-ray absorption spectroscopies which highlight the presence of V4+[double bond, length as m-dash]O vanadyl-type defects leading to an approximated NaVIII0.85(VIVO)0.15(PO4)F0.85 composition. In this material, the profile of the diffraction lines is governed by a strong strain anisotropic broadening arising from the competitive formation between the ionic V3+–F and the covalent V4+[double bond, length as m-dash]O bonds. This material shows a limited extraction of sodium, close to 15% of the theoretical capacity. Indeed, its electrochemical properties are strongly inhibited by the intrinsic low sodium mobility in the Tavorite framework.

High-Voltage Cathodes for Na-Ion Batteries: Sodium– Vanadium Fluorophosphates

This chapter analyses the main advances made in the field of sodium–vanadium fluorophosphates as cathodes for Na-ion batteries and tries to clarify some discrepan‐ cies and common errors published about these compounds. The sodium–vanadium fluorophosphate family can be divided in two main members: Na3V2(PO4)2F3 (V+3 extreme phase) and Na3V2O2(PO4)2F (V4+ extreme phase). Na3V2O2x(PO4)2F3-2x, where 0 < x < 1 would correspond to intermediate V3+/4+ mixed valence phases. Among them, the V3+ extreme has demonstrated to be difficult to isolate, whereas the V4+ and mixed valence phases can be more easily prepared by different synthesis methods and from different vanadium sources. In terms of electrochemical performance, mixed valent compound provides good performance, with high specific capacity at moderate/high cycling rates, and long cycle life. The future perspectives for this family of compounds are discussed in terms of raw materials availability, price, and performance relative to other cathode systems for Naion batteries.