Verónica Palomares

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.

Update on Na-based battery materials. A growing research path

This work presents an up-to-date information on Na-based battery materials. On the one hand, it explores the feasibility of two novel energy storage systems: Na-aqueous batteries and Na–O2 technology. On the other hand, it summarises new advances on non-aqueous Na-ion systems. Although all of them can be placed under the umbrella of Na-based systems, aqueous and oxygen-based batteries are arising technologies with increasing significance in energy storage research, while non-aqueous sodium-ion technology has become one of the most important research lines in this field. These systems meet different requirements of energy storage: Na-aqueous batteries will have a determining role as a low cost and safer technology; Na–O2 systems can be the key technology to overcome the need for high energy density storage devices; and non-aqueous Na-ion batteries have application in the field of stationary energy storage.

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.

Influence of Carbon Content on LiFePO4 / C Samples Synthesized by Freeze-Drying Process

The influence of the carbon content in LiFeP0 4 /C composites synthesized by the freeze-drying method was studied by varying the citric acid (chelating agent): Fe ratio. Diminishing this ratio from 1:1 to 0.33:1 led to a gradual reduction of the carbon content from 16.1 to 7.2% wt and different morphologies. Transmission electron microscopy micrographs of the composite with the greatest carbon percentage (16%) show mainly 30 nm LiFeP0 4 particles homogeneously embedded in a carbon network. Samples containing less carbon exhibit only one type of morphology, 200-700 nm aggregates made up of an intimate mixture of LiFeP0 4 particles and carbon. Galvanostatic cycling from 2 to 4 V vs Li/Li + evidences the typical LiFePO 4 redox behavior at 3.4 V, and a second contribution at 2.65 V probably related to the carbon content. At a high rate, a good specific capacity value is observed for the nanoparticulate sample (16% wt C), whereas poorer performance is observed for low carbon content samples (11 and 7.2 wt % C). Heterogeneous and insufficient carbon covering together with phosphate particle aggregation in these latter samples can account for this behavior. Two carbon distribution models are proposed to explain different electrochemical responses. In all cases, a good capacity retention is observed after prolonged cycling.

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.

Challenges and perspectives on high and intermediate-temperature sodium batteries

Energy storage systems are selected depending on factors such as storage capacity, available power, discharge time, self-discharge, efficiency, or durability. Additional parameters to be considered are safety, cost, feasibility, and environmental aspects. Sodium-based batteries (Na–S, NaNiCl2) typically require operation temperatures of 300–350 °C. The high operating temperatures substantially increase the operating costs and raise safety issues. This updated review describes the state-of-the-art materials for high-temperature sodium batteries and the trends towards the development and optimization of intermediate and low-temperature devices. Recent advances in inorganic solid electrolytes, glass-ceramic electrolytes, and polymer solid electrolytes are of immense importance in all-solid-state sodium batteries. Systems such as Na+ super ionic conductor (NASICON, Na1+xZr2P3–xSixO12 (0 ≤ x ≤ 3)), glass-ceramic 94Na3PS4·6Na4SiS4, and polyethylene oxide (PEO)–sodium triflate (NaCF3SO3) are also discussed. Room temperature ionic liquids (RTILs) are also included as novel electrolyte solvents. This update discusses the progress of on-going strategies to enhance the conductivity, optimize the electrolyte/electrode interface, and improve the cell design of emerging technologies. This work aims to cover the recent advances in electrode and electrolyte materials for sodium–sulfur and sodium–metal-halide (zeolite battery research Africa project (ZEBRA)) batteries for use at high and intermediate temperatures.

Structural, magnetic and electrochemical study of a new active phase obtained by oxidation of a LiFePO4/C composite

In this work, a new phase produced by controlled oxidation of a LiFePO4/C composite has been isolated and characterized. This new compound preserves mainly an olivine structure, but the complete oxidation of Fe is implied. A significant iron mis-site disorder and vacancy formation is proposed. The new phase demonstrated possession of different spectroscopic, magnetic and electrochemical properties from triphilite–heterosite. AC and DC magnetic susceptibility and specific heat measurements showed that the new phase presented spin-glass behavior. Electrochemical cycling showed that the new phase reacted at 2.5 V, which is a different potential to the heterositeversus a Li anode. Moreover, it provoked reversion to the triphilite–heterosite system, although a fraction of the material remained as the oxidized phase.

Synthesis Processes for Li-Ion Battery Electrodes – From Solid State Reaction to Solvothermal Self-Assembly Methods

Since 1990, Li-ion batteries became essential for our daily life, and the scope of their applications is currently expanding from mobile electronic devices to electric vehicles, power tools and stationary power grid storage. The ever-enlarging market of portable electronic products and the new demands of the transportation market and stationary storage require cells with enhanced energy density, power density, cyclability and safety. In short, to get better performance. These new needs have boosted research and optimization of new materials for Li-ion batteries.