Electrolytes For Sodium-ion Batteries Research Articles

Electrochemical Properties of Micron-Sized SnO Anode Using a Glyme-Based Electrolyte for Sodium-Ion Battery.

Tin monoxide (SnO) anodes are promising candidates for use in sodium-ion batteries because of their high theoretical capacities and stable cycle performance. In previous reports, electrodes with excellent performance have been prepared by using nano-sized SnO particles. However, the synthesis of nano-sized SnO particles is complex, time-consuming, and expensive. In this paper, an anode of micron-sized SnO is prepared by using commercial micron-sized SnO particles. The electrode exhibits a reversible capacity of 450 mAh g-1 in the 1st cycle at a current rate of 100 mA g-1. We used a tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte, which is well known for its superior electrochemical performance in sodium-ion batteries. The mechanism of operation of the anode containing micron-sized SnO particles has been confirmed by a detailed study using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). During initial discharge, the SnO changed to Sn and sodium oxide, and the surface of the electrode became covered with a film. The electrode composed of micron-sized SnO is a potential candidate for use in sodium-ion batteries.

Functionalized Carboxyl Carbon/NaBOB Composite as Highly Conductive Electrolyte for Sodium Ion Batteries

AbstractSodium‐ion batteries are the most promising candidates of reusable batteries because of the huge abundant sodium resources and low cost. So the new types of sodium electrolyte materials are attracted to be identified. In this paper, a series of sodium composites are synthesized by the carboxyl carbon spheres (HTC‐COOH) and sodium bis(oxalate)‐borate (NaBOB) using the solid‐state reaction. FTIR, TG, XRD, elemental analysis, SEM and AFM are used to prove the successful synthesis of functionalized carboxyl carbon/NaBOB (HTC/NaBOB) composite. The thermal stability and ionic conductivity of the composites are also studied and discussed. The test results show the HTC/NaBOB composites have high thermal stability, which possess the thermal decomposition temperature higher than 300°C in N2. When the weight ratio of oxalic acid, boric acid, sodium hydroxide and HTC‐COOH reaches appropriate proportion, the conductivity of HTC/NaBOB composite electrolyte solution is higher than NaBOB in N, N‐Dimethylformamide (DMF) with the same the content of element boron and the ionic conductivity of 0.10 mol dm−3 electrolyte solution is 4.87 mScm−1 in DMF. The study shows that these composites are non‐absorbent water and easy to be recycle and store as electrolytes. They may have great potential as a new class of electrolyte for high‐performance sodium ion batteries.

A porous poly (vinylidene fluoride-co-hexafluoropropylene) based separator-cum-gel polymer electrolyte for sodium-ion battery

We report the preparation of amorphous poly (vinylidene fluoride-co-hexafluoropropylene) [P(VdF-co-HFP)] based electrospun fibrous membrane by a simple electrospinning technique for sodium-ion battery applications. The physicochemical characteristics of as-prepared fibrous polymer membranes were characterized by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The separator-cum gel polymer electrolyte (SGPE) was activated by soaking the freshly prepared electrospun fibrous membranes in a liquid electrolyte solution containing 1 M NaClO4, EC/DEC (1:1 by volume) in an argon-filled glove box. The highly porous electrospun P(VdF-co-HFP) based polymer exhibited a high ionic conductivity of 1.13 × 10−3 S cm−1 at ambient temperature in contrast to the commercial Celgard membrane. The electrochemical stability window of the SGPE was above 4.8 V versus Na+/Na. Further, the electrochemical investigations of the sodium half-cell (NaNi0.5Mn0.5O2 as a cathode and unadulterated sodium as an anode) assembled from SGPE are illustrated. It showed 97% retention columbic efficiency from initial to even after 50 cycles at 0.1 C rate, however, the capacity loss is inherent for the cathode material.

Fast Ion Conducting Solid-State Electrolytes for Sodium Ion Batteries — from Elementary Steps to Macroscopic Transport

All-solid-state batteries and especially solid electrolytes stand in the spotlight of current energy research.[1,2] An extremely high ion mobility, which is known for liquid electrolytes, is needed to realize batteries with ceramic electrolytes. Apart from Li-bearing materials also a range of sodium compounds exhibit extremely high ion conductivities. With regard to availability and costs Na-based energy storage systems have attracted great interest over the last years.[3] Here we re-investigated the most prominent representatives of this kind of functional materials viz. Na-β''-alumina[4] and Na3Zr2(SiO4)2PO4 (NASICON), which are known to show conductivity values in the order of mS/cm at RT.[5,6] We used high precision impedance measurements and nuclear magnetic resonance (NMR) spectroscopy to study the diffusion mechanisms and long-range ion transport. In the case of Na-β''-alumina we focused on a commercially available sintered pellets to investigate both bulk dynamics and grain boundary effects on ionic conductivity. Impedance spectroscopy was helpful to interpret ion dynamics revealed by NMR on both short as well as long-range length scales. For well-sintered and highly dense samples ion transport across grain boundaries has only little impact on the overall fast ion transport seen.

(Keynote) Development of Novel Lithium and Sodium Salts Based on Huckel Type Anions

Lithium salts are essential components of all electrolytes used in lithium-ion batteries. In our group in co-operation with CIC (Michel Armand) and Chalmers University (Patrik Johansson) various salts based on heterocyclic compounds with five-membered rings, such as 4, 5-dicyanoimidazole or 4, 5-dicyanotriazoles derivatives were obtained and investigated. The novel, promising concept of the application of new anions is based on the application of so called “Hückel anions”. The name came from the transposition of the Hückel rule predicting the stability of the aromatic systems. One of the most common examples of this type of anions is 4,5-dicyano-triazole (DCTA). This particular structure is completely covalently bonded and shows very stable 6p (or 10p electron if CN bonds are involved in calculations) configuration. It can be produced from commercially available precursor and even more importantly does not comprise fluorine atoms. Salts of this type of anion were found to exhibit high (~300oC) thermal stability. LiDCTA was successfully tested in PEO matrices systems as a promising, improved electrolyte for lithium ion batteries. Unfortunately DCTA failed as a component of the EC/DMC (1:1) battery electrolyte. Our idea was to design structure that would not have disadvantages of big bulky anions causing high viscosity when dissolved in organic solvents, therefore a decrease in conductivity. Also, ions of new salts should not form agglomerates after dissolution, due to ion pairs’ and triplet’s which negatively affect ionic conductivity of an electrolyte, and high transference number of a lithium cation. Molecular modelling studies showed that benzimidazolide and imidazolide anions show a typical behaviour of heterocyclic anion alternatives to PF6 -: if tailored correctly they offer more dissociative lithium ion pairs compared to LiPF6 -, but are not as electrochemically stable. The data obtained for the benzimidazolides, indicate that with very small alterations, such as the positioning of –CN substituent’s on the heterocycle, ion pairing can be changed drastically. With equal predicted properties for analogous imidazolides and benzimidazolides, the difference in anion size is an important variable for choosing an appropriate lithium salt. Overall the potential of using cyano chemistry to create new lithium salts with excellent electrochemical stabilities and ion pairing properties is evident. The previously recognized improvement of both the electrochemical stability and the lithium ion pair dissociation ability, by increasing the number of cyano groups, is apparent also for the benzimidazole salts and further strengthened by the predictions made for P (CN) 6 -. However, an increase in the number of –CN groups leads to a larger anion size and require increased synthesis efforts. Electrochemical properties of the systems doped with 4, 5-dicyano-2-trifluoromethyl imidazole lithium salt (LITDI) were most intensively studied. Crystallographic studies allowed for a better understanding of electrochemical properties of TDI salts. Despite of these studies more detailed examination of the interactions between [Li(glyme)]+ complex cations and weakly Lewis basic anions are still necessary to gain further insight into the ion coordination structure and transport mechanism in liquid and polymeric electrolytes containing LITDI as a dopant. In this presentation some recent results which enable us to design new electrolyte systems will be highlighted. As an example the new class of ionic liquid systems in which lithium salt is introduced into the solution as a lithium cation−glyme solvate. This modification leads to the reorganization of solution structure, what entails release of free mobile lithium cation solvate and hence leads to the enhancement of ionic conductivity and lithium cation transference numbers. This method enables also even three-fold increase of salt concentration in ionic liquids or dissolution of salts previously insoluble in some ionic liquids. As a further extension of the previously mentioned studies we have introduced a new family of fluorine-free solid polymer electrolytes for use in sodium battery application. Systems are based on three novel sodium salts: sodium pentacyanopropenide (NaPCPI), sodium 2,3,4,5-tetracyanopirolate (NaTCP) and sodium 2,4,5-tricyano-1,3-imidazolate (NaTIM) which were dissolved in poly (ethylene oxide) to simulate viable solid-polymer electrolytes for sodium-ion batteries. Salts were selected with respect to their fluorine-free composition and diffuse negative charges and in consequence offered special „tailored“ structure leading to better performance in electrolyte sytems. Due to this “liquid-like” high conductivities (> 1 mS cm-1) were obtained above 70 ˚C for solid-polymer electrolytes. All presented salts showed high thermal stability and suitable electrochemical stability windows up to 5V. Finally some examples of the lithium-ion and sodium –ion batteries using newly design anions will be presented and discussed.

Unveil the mechanism of solid electrolyte interphase on Na3V2(PO4)3 formed by a novel NaPF6/BMITFSI ionic liquid electrolyte

Sodium-ion batteries (SIBs) are gaining much attentions due to their potentials to achieve large scale energy storage coupled with relatively low cost. However, typically used organic electrolytes with high flammability and poor thermal stability have impeded further developments of SIBs in large-scale energy storage. Ionic liquids (ILs), featuring excellent thermal stability, non-flammability and wide electrochemical window have been promising alternative electrolytes for SIBs. Herein, rechargeable Na/Na3V2(PO4)3 cells with NaPF6-incorporated 1-butyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide BMITFSI IL electrolyte are demonstrated to reduce the flammability of the electrolyte and improve the safety of cells. Besides, the electrochemical performances of SIBs are enhanced by forming a stable solid-electrolyte interphase (SEI) layer on the cathode. The optimized Na/Na3V2(PO4)3 cells with the IL electrolyte exhibit high initial discharge specific capacity of 107.2 mA h/g and good cycling performance, which is superior to that using carbonate organic electrolyte. Remarkably, the formation mechanism of the SEI layer on the Na3V2(PO4)3 cathode in NaPF6/BMITFSI IL electrolyte for SIBs is put forward in this work according to density functional theory (DFT) calculations and confirmed by Raman, energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and fourier-transform infrared spectra (FTIR) tests, where NaOH, Na2SO4, Na2S2O7 and NaF are assigned as the major inorganic components of SEI. These results show that NaPF6/BMITFSI is a promising electrolyte to improve the safety and electrochemical performances of SIBs.

Glyme–Sodium Bis(fluorosulfonyl)amide Complex Electrolytes for Sodium Ion Batteries

The physicochemical and electrochemical properties of an equimolar complex of pentaglyme (G5) and sodium bis(fluorosulfonyl)amide (NaFSA), [Na(G5)][FSA], mixed with a hydrofluoroether (HFE) were investigated. Ab initio calculations and Raman spectroscopy showed that the coordination structure of [Na(G5)][FSA] was similar to that of [Na(G5)][TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide). The ligand G5 remained coordinated to Na+ and was not liberated from the cationic [Na(G5)]+ complex even in the presence of HFE. The charge transport property was greater in [Na(G5)][FSA]/HFE than in [Na(G5)][TFSA]/HFE. A prominent difference was found in the Na metal deposition/dissolution behavior. Highly reversible Na deposition/dissolution with a Coulombic efficiency (95%) could be achieved in [Na(G5)][FSA]/HFE; however, the reversibility in [Na(G5)][TFSA]/HFE was very low. X-ray photoelectron spectroscopy (XPS) of the deposited Na metal in each electrolyte revealed that a thin and compact layer of electrolyte decom...

Aqueous/Nonaqueous Hybrid Electrolyte for Sodium-Ion Batteries

Here, we report an aqueous/nonaqueous hybrid electrolyte based on sodium trifluoromethanesulfonate with an expanded electrochemical window up to 2.8 V and high conductivity (∼25 mS cm–1 at 20 °C). The hybrid electrolyte inherits the safety characteristic of aqueous electrolytes and the electrochemical stability of nonaqueous systems, enabling stable and reversible operation of the Na3V2(PO4)3/NaTi2(PO4)3 sodium-ion battery.

Diglyme Based Electrolytes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are currently being considered for large-scale energy storage. Optimization of SIB electrolytes is, however, still largely lacking. Here we exhaustively evaluate NaPF6 in diglyme as an electrolyte of choice, via both physicochemical properties and extensive electrochemical tests including half as well as full cells. Fundamentally, the ionic conductivity is found to be quite comparable to carbonate based electrolytes and to obey the fractional Walden rule with viscosity. We find Na metal to work well as a reference electrode and the electrochemical stability, evaluated potentiostatically for various electrodes and corroborated by DFT calculations, to be satisfactory in the entire voltage range 0–4.4 V. Galvanostatic cycling at C/10 of half and full cells using Na3V2(PO4)3 (NVP) or Na3V2(PO4)2F3 (NVPF) as cathodes and hard carbon (HC) as anodes indicates rapid capacity fading in cells with HC anodes, possibly originating in a lack of a stable SEI or by trapping of sodium. Aiming ...

A first principle study of the phase stability, ion transport and substitution strategy for highly ionic conductive sodium antipervoskite as solid electrolyte for sodium ion batteries

Materials in the Na-rich antiperovskite family are promising candidates as solid electrolytes (SEs) for all-solid-state Na-ion batteries (NIBs). In this work, we carry out ab-initio calculations to study various properties of Na3OCl, namely the formation energies of various neutral defect pairs, the defect hopping barriers, the solution energies of high valence alkali earth metal substitutes to the Na atoms, and the effect of such substitution to the Na migration. While the introduction of alkali earth metal ions increases the Na vacancy concentration, the activation energy of Na transport also increases. Furthermore, we identify Ca as the most promising alkali earth metal to be doped into Na3OCl due to its low binding energy and relatively small impact on the migration barrier. Our work provides a theoretical framework for further improving the Na conductivity of materials in the antiperovskite family for all-solid-state NIBs.

Three-dimensional carbon framework anode improves sodiation–desodiation properties in ionic liquid electrolyte

Ionic liquid (IL), characterized by wide potential window, high thermal stability, negligible volatility, and low flammability, is a promising electrolyte for sodium-ion batteries (NIBs). However, the typical NIB anodes, such as hard carbon, show unsatisfactory capacities and poor charge–discharge rate capability in IL electrolyte at room temperature. A three-dimensional carbon framework (3-D CF) constructed with graphene nanosheets and carbon nanospheres is developed to address this issue. With this unique nano-architecture, the interfacial charge transfer, IL electrolyte infiltration, and Na+ transport can be improved at room temperature. As a consequence, this electrode shows superior sodiation–desodiation properties, with a charge–discharge efficiency being stable over 250 cycles in N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide IL electrolyte. We also confirm that the 3-D CF electrode exhibits higher Coulombic efficiency and better cycling stability in the IL electrolyte than those found in conventional organic carbonate electrolyte, especially at an elevated temperature, due to the different chemistry of the solid–electrolyte interphase. The total heat releases of the sodiated 3-D CF samples contacted with IL and carbonate electrolytes, measured with differential scanning calorimetry up to 300 °C, are 250 and 805 J g–1, respectively. The combination of the 3-D CF electrode and IL electrolyte shows great potential for NIB applications.

Multiscale Modeling Approach to Identify Glassy Electrolytes for Sodium Ion Batteries

Sodium is an earth-abundant element, which makes sodium-ion batteries a promising technology for grid-scale energy storage applications. Conventional liquid electrolytes pose safety concerns due to poor stability, which has led to an extensive research to explore solid-state electrolytes. Developing novel solid electrolytes with ionic conductivity comparable to that of liquid electrolytes has been a major challenge over the past decade. Sulfide glasses have been reported to possess excellent ionic conductivity making them potential electrolytes for high performance sodium ion batteries. Optimizing electrolyte compositions through series of fabrication and experimental characterization cycles is not an economical approach. Atomistic simulations provide an economical route to analyze wide range of compositions and get crucial insights into ion transport in these glasses. With this motivation, we have analyzed the local structure of sulfide glasses and its subsequent impact on the ionic conductivity. We have employed ab initio molecular dynamics (MD) simulations that perform quantum calculations on the fly while solving equations of motions at each MD step to accurately capture the complex chemistry in sodium thiophosphate glasses. We modeled a range of sodium thiophosphate glasses with composition x Na2S – (1-x) P2S5 (x = 0, 0.33, 0.5, 0.6, 0.67, 0.75). The glasses were formed using melt-quench technique and the calculated density of the glasses from simulations matched well (within 5%) with the experimentally measured value. We characterized the local structure of these glasses by calculating the radial pair distribution functions (RDF’s) from ab initio MD and compared it to those obtained through X-ray scattering experiments to validate the model. The FTIR and NMR characterization of these glasses help in identifying the different structural units dominating the local structure at a composition. We calculated the relative fractions of each of these structural units present in the different glasses and correlated the results to change in composition. The dominant structural units influence the ion transport in these glassy electrolytes. These atomistic simulations thereby allow us to predict the trend in ionic conductivity with composition. Our calculations indicate that an optimal room temperature ionic conductivity of ~ 10-6 S/cm is obtained at 75 Na2S – 25 P2S5 composition. A relatively high ionic conductivity (~ 10-5 S/cm) can be achieved at slightly elevated temperatures of around 60 °C, making these sodium thiophosphate glasses promising electrolytes for solid-state sodium ion batteries. Our calculations provide crucial insights into ion transport that can help develop solid electrolytes with high ionic conductivity. We developed a kinetic Monte Carlo (kMC) model to scale up our calculations. Crucial inputs such as Na+ ions distribution and migration energies obtained from ab initio MD are the key inputs to this kMC model. This kMC model is able to predict the ionic conductivity of these glasses in agreement with experimental values. A separate Monte Carlo based model was developed to determine the charge-discharge plots for the entire battery employing these solid-state electrolytes. This novel multiscale approach of coupling atomistic simulations with Monte Carlo model provides an economical route for developing novel electrolytes for sodium ion batteries.

(Invited) Modeling Glassy Electrolytes for All-Solid-State Sodium Ion Batteries

Solid-state sodium ion batteries, a relatively safe and potentially cost-effective technology for electrical grid-scale energy storage, has attracted significant scientific attention in recent times. However, identifying solid electrolytes with high ionic conductivity at room temperature is a requirement in order to commercialize solid state sodium ion batteries. Amorphous sulfide glasses are considered as promising candidates with relatively high ionic conductivity. However, the properties of these electrolytes are highly sensitive to the chemical composition. We present a multiscale approach, which employs atomistic simulations coupled with mesoscale modeling, to analyze and evaluate transport properties of solid glassy electrolytes. As a model system, we studied sodium sulfide – silicon sulfide [xNa2S – (1-x)SiS2] based electrolytes with varying compositions. We determined the ionic conductivities and activation energies for sodium ion hops using ab initio and classical molecular dynamics (MD) approach. These simulations provide fundamental insights into ion conduction mechanisms as well as help to correlate ionic conductivity with glass structure and composition. We further developed a Monte Carlo model to scale up our calculations to a much larger length-scales that are inaccessible by molecular dynamics simulations. Crucial inputs such as Na+ ions distribution and migration energies obtained from classical and ab initio MD served as key inputs to the Monte Carlo model. This model accurately predicts the ionic conductivity of the glasses in agreement with experimental values. Overall, the multiscale model shows excellent promise in identifying optimal glassy electrolytes for sodium ion batteries.

Fundamental Investigations into Na+ Behavior in Aqueous and Non-Aqueous Electrolytes

Sodium ion batteries present a meaningful opportunity to apply knowledge gained from lithium ion batteries. In order to produce functional sodium ion batteries that present real advantages compared to today’s commercial lithium ion batteries, the sodium ion and its interactions in solution must be exploited. From previous work on the subject, our work has shown fundamental differences in the solvation of Na+ as compared to Li+. Specifically, we were able to show that Na+ experiences weaker attractive forces and longer bond lengths between itself and its solvent molecules in organic and aqueous solution, which affects contact ion pairing and aggregation behavior. In this work, we will explore the relationship between ion-solvent interactions as they relate to the transference number and electrochemical stability of sodium organic and aqueous electrolytes. Ion interaction has a significant effect on the reduction potential of both solvent molecules and anions, and so there may be opportunities to take advantage of Na+ to affect both transference number and stability with the goal of producing effective electrolytes for sodium ion batteries.

Investigations of Thermal Stability and SEI on Different Anodes for Sodium-Ion Battery Using Non-Flammable Ether-Based Electrolyte

The accelerating consumption of the limited lithium resources may affect future prices of lithium-ion batteries. In contrast, sodium-ion battery is considered to be a cheaper alternative to lithium-ion battery on account of the globally abundant sodium resources1. In order to become commercially viable, the sodium-ion battery needs to deliver long cycling life with good capacity and energy density while still ensuring safety. In this study, the interactions between selected sodium-ion anodes and electrolytes were carefully investigated. A newly designed ether-based non-flammable electrolyte, 1M NaBF4 in tetraglyme2 was tested with three types of anodes (Na2Ti3O7/C, Graphite and Hard Carbon), and the results were compared with those obtained with the popularly used carbonate-based electrolyte, 1M NaClO4 in EC:PC (v:v=1:1). With 1M NaBF4 in tetraglyme electrolyte, stable half-cell cycling performance was achieved for all three anodes. Being glyme-based, this electrolyte also enabled excellent cycling of graphite anode through solvent co-intercalation reaction mechanism (with conventional carbonate-based electrolytes, graphite cannot store sodium)2,3. Compared with 1M NaClO4 in EC:PC, 1M NaBF4 in tetraglyme showed much higher first cycle coloumbic efficiencies for Na2Ti3O7/C and Hard Carbon half-cells, indicating less solid -electrolyte interphase (SEI) formation on the surface of the electrodes. Furthermore, the cycling stability and capacity of Na2Ti3O7/C were similar with both of the two electrolytes while better performance was achieved with 1M NaBF4 in tetraglyme for Hard Carbon. Thermal stability studies were conducted for all the electrodes at the states of pristine, fully sodiated and fully desodiated using Differential Scanning Calorimetry (DSC). Furthermore, investigations on the SEI formation on the various anodes were also performed using the techniques of Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR), Field Emission Scanning Electron Microscope (FESEM), Energy-Dispersive X-ray Spectroscopy (EDX), Electrochemical Impedance Spectroscopy (EIS) and DSC. The DSC analysis showed that the anodes cycled with 1M NaBF4 in tetraglyme were more thermally stable than their counterparts cycled with 1M NaClO4 in EC:PC. The SEI investigations corroborated the cycling and DSC results, indicating that the SEI formed on the various anodes using 1M NaBF4 in tetraglyme was not only thinner, but also thermally more stable than the SEI formed using 1M NaClO4 in EC:PC. Such details from the various studies, which will be revealed in the presentation, unambiguously proved that 1M NaBF4 in tetraglyme was a much safer choice as an electrolyte for sodium-ion batteries compared with 1M NaClO4 in EC:PC especially for low voltage anodes. [1] Kubota, K., & Komaba, S. (2015). Review—Practical Issues and Future Perspective for Na-ion Batteries. Journal of The Electrochemical Society, 162(14), A2538–A2550. [2] Rudola, A., Du, K., & Balaya, P. (2017). Monoclinic Sodium Iron Hexacyanoferrate Cathode and Non-Flammable Glyme-Based Electrolyte for Inexpensive Sodium-ion Batteries, Journal of The Electrochemical Society, 164 (6), A1098-A1109. [3] Jache, B.; Adelhelm, P. (2014) Use of Graphite as a Highly Reversible Electrode with Superior Cycle Life for Sodium-Ion Batteries by Making Use of Co-Intercalation Phenomena. Angew. Chemie - Int. Ed., 53 (38), 10169–10173.

A High-Voltage Aqueous Electrolyte for Sodium-Ion Batteries

Due to their inherent safety, environmental friendliness, and potential low cost, rechargeable batteries based on aqueous electrolytes are being developed as a large-scale energy storage option for grid applications to enable wide spread integration of renewables. In this field cost and safety are more important than energy density, which is why water based systems are a promising candidate. So far, operational voltages of aqueous batteries have been too low to enable market penetration due to the narrow electrochemical stability window of water (~1.23 V). Using highly-concentrated aqueous sodium bis(fluorosulfonyl)imide (NaFSI) solutions (35m), we recently reported a stability window of 2.6 V on stainless steel current collectors1. Meanwhile addressing the raw materials supply chain as important challenge for scaling, we developed an aqueous sodium-ion battery employing only non-critical raw materials, namely NaTi2(PO4)3 2 and Na3(VOPO4)2F3 on the anode and cathode side, respectively. The average discharge voltage is 1.5 V, whereas the 35-molal NaFSI aqueous electrolyte allowed us to use the full capacity of the high-voltage cathode material. Due to the enhanced oxidative stability of the electrolyte, this battery displays an energy density that is almost twice as high as that of previously reported aqueous sodium-ion batteries4. Kühnel, R. S.; Reber, D.; Battaglia, C., A High-Voltage Aqueous Electrolyte for Sodium-Ion Batteries. ACS Energy Lett. 2017, 2 , 2005.Li, Z.; Young, D.; Xiang, K.; Carter, W. C.; Chiang, Y.-M., Towards High Power High Energy Aqueous Sodium-Ion Batteries: The NaTi2(PO4)3/Na0.44MnO2 System. Adv. Energy Mater. 2013, 3, 290.Qi, Y.; Mu, L.; Zhao, J.; Hu, Y. S.; Liu, H.; Dai, S., Superior Na-Storage Performance of Low-Temperature-Synthesized Na3(VO(1-x)PO4)2F(1+2x) (0≤x≤1) Nanoparticles for Na-Ion Batteries. Angew. Chem. Int. Ed. 2015, 54, 9911.Kumar, P. R.; Jung, Y. H.; Wang, J. E.; Kim, D. K., Na3V2O2(PO4)2F-MWCNT nanocomposites as a stable and high rate cathode for aqueous and non-aqueous sodium-ion batteries. J. Power Sources 2016, 324, 421.

Sodium ionic conductivity and stability of amorphous Na2O·2SiO2 added with MxOy (M = Zr, Y, and Sm)

Amorphous Na2O·2SiO2 (A-Na2O·2SiO2) is one of the promising sodium ionic conductors for solid-state sodium-ion battery electrolytes owing to its easy preparation and low production cost. However, Na2O·2SiO2 glass suffers from relatively low conductivity and poor thermal stability as a result of crystallization and other defects. Herein, we improved the ionic conductivity and thermal stability of A-Na2O·2SiO2 by adding this material with metal oxides (MxOy, M=Y, Sm, and Zr) at varying loadings. The physical properties and stability of the A-Na2O·2SiO2 added with different oxides were systematically investigated. The results indicated that oxide adding on A-Na2O·2SiO2 resulted in larger density values that increased with the adding content. Additionally, oxide adding changed the strength of the SiO bond in A-Na2O·2SiO2, thereby affecting the overall network structure and uniformity. Among the different oxides prepared, added with 3mol% Sm2O3, 3mol% Y2O3, and 2mol% ZrO2 showed maximum hardness and elasticity modulus values. Additionally, oxide adding on A-Na2O·2SiO2 resulted in significantly improved ionic conductivity values. 3mol% Y2O3-added A-Na2O·2SiO2 material showed the highest ionic conductivity value (5.2×10−3Scm−1) and optimum thermal stability, indicating that Y2O3-added Na2O·2SiO2 has high potential to be high-performance and stable sodium ionic conductors.

Novel safer phosphonate-based gel polymer electrolytes for sodium-ion batteries with excellent cycling performance

Phosphonate containing GPEs for safer sodium ion batteries with high capacity and stable long term cycling performance.

Microporous ceramic coated separators with superior wettability for enhancing the electrochemical performance of sodium-ion batteries

Finding an alternative to glass fiber (GF) separators is a crucial factor for the fast commercialization of sodium-ion batteries (SIBs), because GF separators are too thick for use in SIBs, thereby decreasing the volumetric and gravimetric energy density. Here we propose a microporous composite separator prepared by introducing a polymeric coating layer of polyvinylidene fluoride–hexafluoropropylene (PVdF-HFP co-polymer) with ZrO2 nanoparticles to a polyethylene (PE) separator. The coated separator efficiently enhances the cell performance of SIBs. The ZrO2 nanoparticles, finely dispersed on the polymeric coating layer, induce the formation of many micropores on the polymeric coating layer, suggesting that micropore formation on the coating layer renders the composite separator more open in structure. An ethylene carbonate/propylene carbonate liquid electrolyte for SIBs is not absorbed by PE separators even after 1 h of electrolyte droplet testing, while the proposed separator with many micropores is completely wetted by the electrolyte. Sodium ion migration across the composite separator is therefore effectively enhanced by the formation of ion transfer pathways, which improve ionic conductivity. As a result, the microporous composite separator affords stable cycle performances and excellent specific capacity retention (95.8%) after 50 cycles, comparable to those offered by a SIB with a GF separator.

Coordination Abilities of 4,5-Dicyano-2-(trifluoromethyl)imidazolate Anion toward Sodium Cation: Structural and Spectroscopic Studies of Solid and Liquid Glyme-Solvated Electrolyte Systems

Comprehensive structural analysis of sodium 4,5-dicyano-2-(trifluoromethyl)imidazolate (NaTDI) solvates with glymes (1–4), tetrahydrofuran, and crown ethers has been performed. Several structural motifs obtained from single-crystal X-ray analysis of complementary series of crystalline adducts with varying O:Na ratios were correlated with spectroscopic and thermal data to provide new information about the coordination ability of heterocyclic anions toward sodium cations. Presented results provide a basis for developing models of poly(ethylene oxide) electrolytes and liquid systems for sodium ion battery electrolytes. We have found a wide variety of anion–cation coordination types which allow us to compare them with analogous lithium solvates in terms of Brown’s valence-matching principle and Lewis acid strength (Sa) parameters. Noticed aggregation modes of sodium salts confirm the occurrence of a solvate disproportionation conductivity mechanism at high salt concentrations which can be used for developing ...