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.

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In this paper we examine the mechanism of Na insertion and extraction in the FePO 4 -NaFePO 4 system. Chemical preparation of the intermediate Na 1Àx FePO 4 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.

Rechargeable sodium-sulfur (Na-S) batteries are regarded as a promising energy storage technology due to their high energy density and low cost. High-temperature sodium-sulfur (HT Na-S) batteries with molten sodium and...

are proving to be an emergent technology with potentially very attractive properties. They are potentially low cost and environmentally friendly with reduced supply risk. However, the development of NIBs faces various challenges such as low gravimetric and volumetric energy densities and difficulty in achieving broader voltage windows. Although early studies in NIBs date back to the 1970s, just like Li-ion battery (LIB) research, the commercialization of the former systems in 1991 by the team formed by Sony and Asahi Kasei marked a milestone not only in the field of energy storage technology but also in the evolution of the modern society. This technological breakthrough had been possible thanks to several preceding contributions, particularly the works by M. S Whittingham, [1] J. Goodenough, [2,3] and A. Yoshino [4] on the discovery of Li-ion intercalation materials (Nobel Laureates in Chemistry 2019). This important historical event polarized material science research toward Li-ion technology and slowed down considerably the advances in the field of sodium. However, at the end of the 2000s, mainly driven by the concerns about future lithium supply and the uneven worldwide distribution of its reserves and resources, the research on Na-ion reemerged and so did the number of articles published (Figure 1). The intercalation chemistry of both metal ions is very alike, and thus, the materials tested for NIBs could be similar to those used in Li-ion systems. Both systems share the same working principle, and therefore, in terms of manufacturing, the industry producing LIBs can be easily tuned towards NIB fabrication, which is an important asset to invest in and support this technology. NIBs started to reach the market in the early 2010s, about two decades after their Li counterparts. Nevertheless, the progress has been relatively fast due to the straightforward LIB equipment and facility transfer just mentioned. In the search of high performance, low cost, abundance, low environmental impact, long-term cyclability and safety, layered metal oxides, polyanionic compounds and Prussian blue analogues (PBAs) are among the most studied families of Na-ion cathode materials. On the anode side, metallic sodium exhibits the same operation and safety problems as lithium, and therefore, it cannot be considered an option in conventional NIBs. Thus, in this scenario, most of the research has been dominated by the use of disordered carbons, mainly hard carbons (HCs). Other prospective anodes

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 V 3+ /V 4+ mixed valence, with the general formula Na 3 V 2 O 2x (PO 4 ) 2 F 3À2x 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 À1 at 1C (z80% of theoretical specific capacity). Cyclability tests at 1C proved good reversibility of the material that maintained 98% of initial specific capacity for 30 cycles.

A mixed-valence V3+/V4+ composite material belonging to the Na3V2O2x (PO4)2F3–2x /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.

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, Na 3 V 2 O 2 (PO 4 ) 2 F 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 atomiclevel sodium perturbations directly with electrochemical cycling.