Sodium-ion Electrolyte Research Articles

On the Thermal Stability of Selected Electrode Materials and Electrolytes for Na-Ion Batteries

Sodium-ion batteries are a technology rapidly approaching widespread adoption, so studying the thermal stability and safety of their components is a pressing issue. In this work, we employed differential scanning calorimetry (DSC) and ex situ powder X-ray diffraction to study the thermal stability of several types of sodium-ion electrolytes (NaClO4 and NaPF6 solutions in PC, EC, DEC, and their mixtures) and various cathode and anode materials (Na3V2(PO4)3, Na3(VO)2(PO4)2F, β-NaVP2O7, and hard carbon) in combination with electrolytes. The obtained results indicate, first, the satisfactory thermal stability of liquid Na-ion electrolytes, which start to decompose only at 270~300 °C. Second, we observed that charged vanadium-based polyanionic cathodes, which appear to be very stable in the “dry” state, demonstrate an increase in decomposition enthalpy and a shift of the DSC peaks to lower temperatures when in contact with 1 M NaPF6 in the EC:DEC solution. However, the greatest thermal effect from the “electrode–electrolyte” interaction is demonstrated by the anode material: the heat of decomposition of the soaked electrode in the charged state is almost 40% higher than the sum of the decomposition enthalpies of the electrolyte and dry electrode separately.

Doping effects of conductivity improvement in anti-perovskite Na3OBr solid electrolytes

Improving the ionic conductivity of solid-state sodium (Na) ion electrolytes is an urgent issue, given their widespread application in all solid-state commercial batteries, and the problems facing this industry, including source shortage, high cost, and safety issues. Substituting halogen and oxygen ions (O2−) with larger atoms is expected to enlarge this bottleneck, as the introduction of distortions in the material can result in an improvement in its ionic conductivity. In this paper, two approaches to introduce distortions into Na3OBr solid electrolytes are provided. Adding either I− or S2− to replace the smaller ions, Br− or O2−, can achieve this result. The lattice distortion increases with increasing concentration of I− or S2− in Na3OBr electrolytes, improving their ionic conductivity. We also discuss the crystallinity of electrolytes, which is an important factor for the diffusion of mobile ions.

Sodium-ion electrolytes based on poly-ethylene oxide oligomers in dual-carbon cells: Anion size drives the charging behavior

Sodium-ion dual-carbon electrochemical cells are an environmentally friendly technology for energy storage purposes. In such systems, the migration of mutual ions in both electrodes is crucial for performance and determines the cell’s behavior. To investigate this effect, we have performed force-field molecular dynamics simulations of dual-carbon sodium-ion systems containing poly-ethylene oxide (PEO6) as the solvent and two different anions, bis(trifluoromethanesulfonyl)imide (TFSI−) and bis(fluorosulfonyl)imide (FSI−). We used a constant potential model that allows simulating the charging behavior of these cells at the potential differences, ΔΨ=1, 2, 3, and 5 volt (V). Our results showed that considering the neat solution, NaTFSIPEO6 exhibited the best transport properties. However, NaFSIPEO6 is a better choice of electrolyte to be employed in dual-carbon cells since it provides higher and faster charge accumulation. Structural analysis also confirmed that the system containing NaFSIPEO6 stored more ions inside their respective electrodes with Na+ cations having fewer molecules composing their first solvation shell. Overall, our findings suggest that the smaller molecular volume of the FSI− anion is the key factor to properly fill the positive electrode, and the NaFSIPEO6 combination is the best choice for electrolytes of dual-carbon cells.

Low-Temperature Properties of the Sodium-Ion Electrolytes Based on EC-DEC, EC-DMC, and EC-DME Binary Solvents

Sodium-ion batteries are a promising class of secondary power sources that can replace some of the lithium-ion, lead–acid, and other types of batteries in large-scale applications. One of the critical parameters for their potential use is high efficiency in a wide temperature range, particularly below 0 °C. This article analyzes the phase equilibria and electrochemical properties of sodium-ion battery electrolytes that are based on NaPF6 solutions in solvent mixtures of ethylene carbonate and diethyl carbonate (EC:DEC), dimethyl carbonate (EC:DMC), and 1,2-dimethoxyethane (EC:DME). All studied electrolytes demonstrate a decrease in conductivity at lower temperatures and transition to a quasi-solid state resembling “wet snow” at certain temperatures: EC:DEC at −8 °C, EC:DMC at −13 °C, and EC:DME at −21 °C for 1 M NaPF6 solutions. This phase transition affects their conductivity to a different degree. The impact is minimal in the case of EC:DEC, although it partially freezes at a higher temperature than other electrolytes. The EC:DMC-based electrolyte demonstrates the best efficiency at temperatures down to −20 °C. However, upon further cooling, 1 M NaPF6 in EC:DEC retains a higher conductivity and lower resistivity in symmetrical Na3V2(PO4)3-based cells. The temperature range from −20 to −40 °C is characterized by the strongest deterioration in the electrochemical properties of electrolytes: for 1 M NaPF6 in EC:DMC, the charge transfer resistance increased 36 times, and for 1 M NaPF6 in EC:DME, 450 times. For 1 M NaPF6 in EC:DEC, the growth of this parameter is much more modest and amounts to only 1.7 times. This allows us to consider the EC:DEC-based electrolyte as a promising basis for the further development of low-temperature sodium-ion batteries.

Shorter-chained trialkylsulfonium cations are preferable as admixtures to lithium-ion and sodium-ion electrolytes in acetonitrile

The development and improvement of electrolytes is a persisting research agenda in the context of electrochemical energy sources. The sodium-ion is a possible and partial substitute for Li-ion in alkali ion batteries of the future. Many large-scale applications urge electrolytes featuring a competitive interplay of ionic conductivity, shear viscosity, and cohesion energy. The general applicability of sulfonium ionic liquids (ILs) reinforced with Li-ions to energy storage was experimentally exemplified in recent years. Herein, we employed potential energy surface exploration bundled with electronic-structure simulations to shed light on the ion-ionic and ion-molecular interactions in perspective electrolyte systems involving trialkylsulfonium cation, Li/Na salts, and acetonitrile (ACN). By analyzing a wealth of supramolecular conformations and characterizing them via thermodynamics, structure, charge density distributions, electrostatic potential surfaces, atom–atom couplings, and reactivity descriptors, we outline the most promising sulfonium-based electrolytes. We provide a critical discussion of the future of sodium-ion batteries and compare them to lithium-ion batteries directly. The oxygen atoms of bis(trifluoromethanesulfonyl)imide anion [TFSI] coordinate Li+ and Na+ to remarkably various extents. Surprisingly, the size of the chosen trialkylsulfonium-based cation impacts the ion-ionic interactions weakly, contrary to expectations. Whereas the difference between the behavior of the Li-ion and Na-ion is paramount irrespective of all other components and their stationary points. [TFSI] was confirmed to be a highly flexible chemical structure although its conformations (one cis and two trans) were unraveled to differ only within an amount of thermal energy at room conditions. The reported discussion drives ongoing trials of the alkali-ion electrolytes based on sulfonium ILs and ACN.

Sodium-Ion Batteries: Exploration of Electrolyte Materials

In recent years, as fossil energy sources such as oil and coal continue to be consumed, the issue of resources and the environment has become one of the main challenges to the sustainable development of human society. People's electricity consumption has increased dramatically, and the demand for energy storage batteries has also increased. Sodium-ion batteries (SIBs) are a very worthwhile development because of high Na reserves in the world, which can bring many advantages. The electrolyte can control the battery's inherent electrochemical window and performance, influence the nature of the electrode/electrolyte interface, and is one of the most important material choices for SIBs. The electrolyte simultaneously influences the electrochemical performance and safety of SIBs. This paper focuses on electrolyte materials in SIBs, explaining the fundamental needs and categorization of sodium ion electrolytes and highlighting the most recent advances in liquid and solid electrolytes. It is found that SIBs still have problems such as lower energy density, narrower electrochemical stability windows, poorer solid electrolyte interphase (SEI) stability, etc. Solving the related technical problems is of great significance for commercializing SIBs.

Sodium-ion conducting pseudosolid electrolyte for energy-dense, sodium-metal batteries

A liquid electrolyte based on the addition of a sodium salt in glyme solvent is encapsulated within a polymer-modified mesoporous silica matrix enabling the fabrication of a pseudosolid, sodium-ion electrolyte. This ionogel electrolyte is stable in contact with sodium metal leading to low overpotentials for sodium metal plating/stripping along with high Na+ conductivity and > 4 V electrochemical stability window. Sodium-metal batteries with sodium vanadium fluorophosphate positive electrode achieve over 400 Wh kg−1 at 0.5C.

(Digital Presentation) Electrochemical Measurements of Solvent Co-Intercalation Phenomenon Enabling Sodium-Ion Electrolyte Optimization for Graphite Anodes

Sodium-ion batteries (SIBs) is one of the more promising battery technologies that are starting to see commercialisation, were the low cost, abundance of necessary raw materials, higher safety and power and similar energy densities compared to lithium-ion batteries (LIBs) are often mentioned as their key advantages. SIBs, due to their great similarities with LIBs have seen a rapid development. One of the key differences, however, is the carbonaceous negative electrode, where SIBs, due to Na+ not forming stable intercalation compounds with graphite, the anode of choice for LIBs, are forces to use hard carbons. But, it was recently discovered by Jache et al. 1 that Na+ can form stable ternary graphite intercalation compounds through a solvent co-intercalation mechanism, thus enabling the use of graphite in SIBs. This sparked several follow-up studies exploring solvent co-intercalation as a way to use graphite in SIBs2, but the question of how many solvent molecules are intercalated along with the cation, and thus the stoichiometry of the redox reaction, was never settled. Here, we present a novel electrochemical method to directly measure the number of solvents that are brought along by the cation into the graphite host structure.The SIBs using the co-intercalation mechanism to enable the use of graphite has so far shown great kinetics, power densities and extreme cyclability, with great capacity retention over thousands of cycles2. They are, however, plagued by a low specific capacity, a great volume expansion, and high average voltage in half-cells vs. Na+/Na, leading to poor energy density. But, the knowledge of how many glymes entering the graphite host gained from our measurements enabled us to rationally design new electrolytes, composed of low amounts of glymes mixed with another co-solvent, that both increases the energy density of the system, by lowering the average voltage while retaining the capacity, and reduces the volume expansion, while also reducing the cost of the electrolyte (Figure 1). The impact of the new electrolytes on the structure of graphite is studies using operando dilatometry and XRD, while the local structure in the electrolyte is investigated with ab initio methods – together giving a comprehensive view of the system. Figure 1. Voltage profile of new electrolytes, showing a lowering of the average voltage a second plateau, and snap shot from ab initio molecular dynamics.

Boosting the cycling stability of hard carbon with NaODFB-based electrolyte at high temperature

The optimization of electrolyte formulation and the resulting change in the properties of the solid electrolyte interfacial film (SEI) are the key to affecting the cycle stability of sodium ion batteries at high temperatures. Traditional sodium ion electrolytes are prone to decomposition at high temperatures, which leads to a rapid decline in battery performance. Herein, we use an effective strategy to construct a SEI film on hard carbon anodes by introducing self-developed synthetic sodium-difluoro(oxalate)borate (NaODFB)-based ethers electrolyte. This study aims to analyze the compatibility between NaODFB-based electrolyte and hard carbon by theoretical calculations and experimental analysis including Na/Cu cells，In-suit EIS and cyclic voltammetry curves at different scan rates. The results indicate that the Na/HC cells with NaODFB-based electrolyte has excellent cycling stability at 55 °C. The battery delivers a high reversible capacity of 249.9 mAh/g at 100 mA/g due to the stable SEI riched in inorganic substances. This work provides guidance and ideas for the design of sodium-ion battery electrolyte at high temperatures in the future.

Effect of Concentration and Temperature on the Structure and Ion Transport in Diglyme-Based Sodium-Ion Electrolyte.

Glyme-based sodium electrolytes show excellent electrochemical properties and good chemical and thermal stability compared with existing carbonate-based battery electrolytes. In this investigation, we perform classical molecular dynamics (MD) simulations to examine the effect of concentration and temperature on ion-ion interactions and ion-solvent interactions via radial distribution functions (RDFs), mean residence time, ion cluster analysis, diffusion coefficients, and ionic conductivity in sodium hexafluorophosphate (NaPF6) salt in diglyme mixtures. The results from MD simulations show the following trends with concentration and temperature: The Na+---O(diglyme) interactions increase with concentration and decrease with temperature, while the Na+---F(PF6-) interactions increase with concentration and temperature. The mean residence time suggests that Na+---O(diglyme) are significantly longer lived compared with that of Na+---F(PF6-) and H (diglyme)---F(PF6-), which shows the affinity of diglyme to the Na+ ions. The ion cluster analysis suggests that the Na+ ions largely exist as solvated ions (coordinated to diglyme molecules), whereas some fractions exist as contact-ion pairs, and negligible fractions as aggregated ion pairs, with the latter two increasing slightly with temperature and more with ion concentration. The magnitude of the diffusion coefficients of Na+ and PF6- ions decreases with concentration and increases with temperature, where the Na+ ion has slightly lower mobility compared with the PF6- anion. The simulated total ionic conductivities show qualitative trends comparable to experimental data and highlight the need for the inclusion of ion-ion correlations in the Nernst-Einstein equation, especially at higher concentrations and lower temperatures.

A high capacity dual-carbon battery universal design for ultrafast lithium/sodium storage

Dual-ion batteries that store energy through anion/cation intercalation are characterized by their wide working window, high safety and low costs. However, the unsatisfied capacity of dual-ion batteries seriously inhibits their practical applications. Herein, a novel dual‑carbon battery based on lithium-ion electrolyte, utilizing reduced oxide graphene (rGO) as the cathode material and mesocarbon microbead (MCMB) as the anode material is designed for efficient energy storage. The resulting dual‑carbon battery delivers a high reversible capacity of 280 mA h g−1 at 1 A g−1 over a voltage window of 3.0–5.0 V after 400 cycles. Moreover, the universal dual‑carbon battery structure is also suitable for sodium-ion electrolyte and shows a discharge specific capacity of 190 mA h g−1 at 1 A g−1 over a voltage window of 0.7–5.0 V. This universal design about dual‑carbon battery opens up a new way for cheap, safe and practical energy storage system.

Progress and Opportunities in the Application of Cold Sintering to Solid-State, Electrochemical, and Ceramic Devices

The high temperature conventional sintering process has been practiced by humans for thousands of years, but only in recent decades has there been significant progress in modifying the sintering process to achieve lower temperatures or shorter dwell times.In this talk, we focus on the newest addition to the family of alternative sintering techniques; cold sintering. Cold sintering is characterized by the addition of a small amount of a transient sintering aid, such as an alkali hydroxide, to the parent ceramic powder. The mixture is then subjected to low temperatures (Tsinter < 400°C) and moderate uniaxial pressure (100 - 300 MPa) which results in a remarkable degree of densification for a wide range of ceramics. Broadly speaking, cold sintering is able to densify even the most refractory technical ceramics at around 30% of the melting temperature, whereas conventional wisdom asserts that the solid-state sintering process requires temperatures near 70% of the melting temperature.To illustrate the benefits of this dramatic reduction in sintering temperature, we will present recent examples of cold sintering applied to conductive ceramics for high temperature electrochemical devices. These examples include sodium ion electrolytes for solid-state batteries (e.g. Na3Zr2Si2PO12, β’’-Al2O3) and oxygen-ion electrolytes for solid oxide fuel cells (e.g. CeO2). For these electrolytes, we illustrate how the density and conductivity of the cold sintered electrolytes is comparable to the conventionally fired ceramics. In doing so, we also discuss the opportunities and open questions surrounding these ceramic materials which have been densified at low temperatures.These examples show how the elimination of elemental volatilization and grain size control can be easily achieved with lower temperature processing. We will conclude with a forward-looking assessment of some of the opportunities that low temperature sintering may enable, such as co-firing device layers and the inclusion of thermally fragile additives into the sintered ceramics. The low temperatures afforded by cold sintering presents new opportunities for the processing of many ceramic-dominated solid-state electrochemical devices.Figure Caption: The relative density is plotted as a function of sintering temperature for the prototypical refractory solid electrolyte, sodium-ion conducting β’’-Al2O3. Conventional sintering methods include hot pressing, spark plasma sintering, liquid phase sintering, and solid-state sintering. Cold sintering achieves high densities at exceedingly low temperatures for this refractory material. Figure 1

Ionic Liquid (IL) Laden Metal-Organic Framework (IL-MOF) Electrolyte for Quasi-Solid-State Sodium Batteries.

An ionic liquid (IL) laden metal-organic framework (MOF) sodium-ion electrolyte has been developed for ambient-temperature quasi-solid-state sodium batteries. The MOF skeleton is designed according to a UIO-66 (Universitetet i Oslo) structure. A sodium sulfonic (-SO3Na) group grafted to the UIO-based MOF ligand improves the Na+-ion conductivity. Upon lading with a sodium-based ionic liquid (Na-IL), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) in 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Bmpyr-TFSI), the Na-IL laden sulfonated UIO-66 (UIOSNa) quasi-solid electrolyte exhibits a Na+-ion conductivity of 3.6 × 10-4 S cm-1 at ambient temperature. Quasi-solid-state sodium batteries with the Na-IL/UIOSNa electrolyte are demonstrated with a layered Na3Ni1.5TeO6 cathode and sodium-metal anode. The quasi-solid-state Na∥Na-IL/UIOSNa∥Na3Ni1.5TeO6 cells show remarkable cycling performance.

Transport studies of NaPF6 carbonate solvents-based sodium ion electrolytes

Characterizing fundamental ion transport is necessary for the modification and development of electrolytes. In this study, we investigated the local and bulk transport properties of 1 M NaPF6 in the binary EC:PC (1:1), and ternary 5EC:3PC:2DEC (5:3:2) solvents, as well as their 2 wt.% FEC modulated versions. We determined 1H, 19F and 23Na Nuclear Magnetic Resonance (NMR) spin-lattice relaxation time (T1), self-diffusion coefficient (D), linewidth and chemical shift. These were complimented with density, viscosity and ionic conductivity measurements, all as a function of temperature. Both the viscosity and ionic conductivity displayed VFT behavior and their Walden products showed a dependence on temperature. This coupled with their having the lower Walden products indicated less salt dissociation in the 5EC:3PC:2DEC and 5EC:3PC:2DEC + 2 wt.% electrolytes. NMR D19F and 23Na values for these electrolytes were also similar at 298K indicating cation-anion association, but diverged with increasing temperatures, with that for the anion being greater than the cation. The addition of the FEC provides greater local dynamics for the individual solvents in the less polar electrolyte but appeared to impede that of the ions.

Analysis of the conductive properties and structural surroundings of sodium ions in GeGaSbS sulfur glass system

Sodium-ion-doped glass ceramics may potentially be used as solid electrolytes. In this study, a novel sulfide-based sodium-ion electrolyte, namely Ga4Ge12Sb64S128–xNaI (x = 5–25), with an optimized network structure and good physical properties, has been prepared by a melting and milling route. The properties of this novel electrolyte have been investigated by Raman spectroscopy, differential scanning calorimetry, X-ray diffraction analysis, and electrochemical methods. The ionic conductivity of the conductors at room temperature proved to be superior to that of the GaSbS-NaI glass system and increased to the order of 10−5 S/cm. The differentiation in conductivity originated from microstructural changes in ion channels and crystal growth. The optimal glass composition for harnessing the characteristics of sulfide sodium-ion conductor materials was designed by extrapolation of the analytical results.

Effect of Electrolyte Anion on Electrochemical Behavior of Nickel Hexacyanoferrate Electrode in Aqueous Sodium-Ion Batteries

Nickel hexacyanoferrate (NiHCF) has a three-dimensional open framework structure, excellent long-cycling stability and rate performance as a cathode for aqueous sodium-ion batteries. However, the specific capacity of NiHCF is lower than that of present cathodes for aqueous batteries. A sodium-ion electrolyte was explored to achieve optimum capacity with NiHCF. Powder-type NiHCF was fabricated by coprecipitation with the atomic composition K0.065Ni1.44Fe(CN)6·4.4H2O. The presence of Fe vacancies in the atomic composition is attributable to the inclusion of coordinating and zeolitic water during coprecipitation. Two sodium-ion electrolytes, 1 M Na2SO4 and 1 M NaNO3, were employed to analyze the electrochemical behavior of the NiHCF electrode. Identical redox potentials to 0.58 V (vs. NHE) were measured in both electrolytes. However, a lower overpotential was observed in NaNO3 compared to Na2SO4 as a result of the smaller interfacial charge transfer resistance. The lower charge transfer resistance in the NaNO3 solution produced a higher specific capacity of 57 mAh g&lt;sup&gt;-1&lt;/sup&gt; (1 C-rate) and the superior capacity retention of 46.6% at 20 C-rate. The anion in the aqueous electrolyte changed the charge transfer resistance at the electrode/electrolyte interface, confirming the electrolyte anion has a crucial effect on the charge capacity and rate performance of NiHCF.

Rectification effects of Nafion-backed micropore-voltammograms by difference in migrational modes

Rectified current-voltage curves were observed by micro-hole voltammetry in various concentrations of sodium ion electrolyte for transport through a micro-hole and a cationic exchange membrane (Nafion). Data suggest a linear current-concentration relation, implying electric migration-control. The rectification is observed as difference in current-potential slopes at positive and at negative applied voltages. The voltammograms are analyzed quantitatively in the context of Ohm's law as a function of both the geometry of the micro-hole and of the entire cell. The conductivity in the direction of the ion transport from the micro-hole to Nafion, corresponding to the lower conductance (closed diode), is proportional both to the cross-section area of the micro-hole and to the inverse micro-hole length. This is consistent with control by electro-migration within the micro-hole. In contrast, the conductivity in the opposite cation transport direction (open diode) is almost independent of the micro-hole geometry but varies with the length of the glass tube of the cell. The current-potential slope is consistent with the value calculated for the NaCl concentration and the geometry of the measurement cell. Under these conditions, the micro-hole is filled with a high concentration of NaCl as the flux of Na+ from the Nafion to the hole, and the high concentration causes the migration current larger that the current in the opposite direction. Consequently, the rectification ratio can be enhanced by careful design of both (i) the cell geometry including ion-exchange membranes and (ii) the micro-hole geometry rather than nano-scaled chemistry in holes.

Sodium-ion start-up raises $35 million

Natron Energy, a developer of batteries featuring sodium-ion electrolyte and Prussian blue dye to enhance the performance of its electrodes, has raised $35 million in series D funding. Investors in...

Densification of a Solid-State NASICON Sodium-Ion Electrolyte Below 400 °C by Cold Sintering With a Fused Hydroxide Solvent

A modified cold sintering process is described, which permits the densification of the prototypical sodium-ion electrolyte, Na3Zr2Si2PO12, to greater than 90% relative density at a process temperat...

Novel layered K0.7Mn0.7Ni0.3O2 cathode material with enlarged diffusion channels for high energy density sodium-ion batteries

As promising, low-cost alternatives of lithiumion batteries for large-scale electric energy storage, sodiumion batteries (SIBs) have been studied by many researchers. However, the relatively large size of Na+ leads to sluggish diffusion kinetics and poor cycling stability in most cathode materials, restricting their further applications. In this work, we demonstrated a novel K+-intercalated Mn/Ni-based layered oxide material (K0.7Mn0.7Ni0.3O2, denoted as KMNO) with stabilized and enlarged diffusion channels for high energy density SIBs. A spontaneous ion exchange behavior in forming K0.1Na0.7Mn0.7Ni0.3O2 between the KMNO electrode and the sodium ion electrolyte was clearly revealed by in situ X-ray diffraction and ex situ inductively coupled plasma analysis. The interlayer space varied from 6.90 to 5.76 Å, larger than that of Na0.7Mn0.7Ni0.3O2 (5.63 Å). The enlarged ionic diffusion channels can effectively increase the ionic diffusion coefficient and simultaneously provide more K+ storage sites in the product framework. As a proof-of-concept application, the SIBs with the as-prepared KMNO as a cathode display a high reversible discharge capacity (161.8 mA h g-1 at 0.1 A g-1), high energy density (459 W h kg-1) and superior rate capability of 71.1 mA h g-1 at 5 A g-1. Our work demonstrates that the K+ pre-intercalation strategy endows the layered metal oxides with excellent sodium storage performance, which provides new directions for the design of cathode materials for various batteries.