Electrolytes For Sodium-ion Batteries Research Articles

Achieving the Inhibition of Aluminum Corrosion by Dual-Salt Electrolytes for Sodium-Ion Batteries.

Sodium bis(fluorosulfonyl)imide (NaFSI) electrolytes are renowned for their superior physicochemical and electrochemical properties, making them ideal for high-performance sodium-ion batteries (SIBs). However, severe oxidative dissolution of aluminum current collectors (commonly known as Al corrosion) in NaFSI-based electrolytes occurs at high potentials. To address this challenge, aiming to understand the Al corrosion mechanism and develop strategies to inhibit corrosion, we propose dual-salt electrolytes using 0.8 mol L-1 (M) NaFSI and 0.2 M of a second fluorine-containing sodium salt dissolved in EC/PC solutions (1:1, v/v) to construct an insoluble deposits layer on the Al. Dual-salt electrolytes adopting a second sodium salt capable of passivating the Al collector have been extensively investigated through various techniques, such as cyclic voltammetry, scanning electron microscopy, chronoamperometry, X-ray photoelectron spectroscopy, and charge-discharge tests. Our findings demonstrate that introducing sodium difluoro(oxalato)borate (NaDFOB) into the NaFSI electrolytes inhibits Al corrosion, which is attributed to the formation of insoluble deposits of Al-F (AlF3) and B-F containing polymers. Moreover, the capacity retention of Na||Na3V2(PO4)3 (NVP) cells using the NaFSI-NaDFOB dual-salt electrolyte reaches 99.2% along with a Coulombic efficiency over 99.3% at a 1 C rate after 200 cycles. This research provides a practical solution for passivating Al collectors in SIBs with NaFSI electrolytes and promotes the development of sodium batteries with long calendar lifetimes.

Investigating the Influence of Sintering Temperature and Ascertaining Hopping Path in Sodium Superionic Conducting Electrolytes for Sodium-Ion Batteries

Recently, sodium super-ionic conducting (NASICON) type solid electrolytes have gained significant interest as potential candidates for next-generation batteries due to their exceptional properties such as high ionic conductivity, affordability, and remarkable chemical and electrochemical stability. Therefore, NASICON type solid electrolyte (Na3Zr2Si2PO12) is synthesized by solution assisted-solid state reaction (SA-SSR) method and sintered at three different temperatures, 1200 ℃, 1250 ℃ and 1300 ℃. Various experimental techniques are used to characterize and optimize the sintered electrolytes. The Rietveld refinement of X-ray diffraction pattern of prepared electrolytes revealed the monoclinic crystal structure having 3-dimension hopping channel for sodium ion migration through triangular bottleneck area. Complex impedance spectroscopy revealed, the highest bulk and grain boundary conductivity are found, 4.05 mS cm-1 and 0.31 mS cm-1, respectively, for the sample sintered at 1250 ℃. Using optimized electrolyte, sodium half-cell is demonstrated with Na3V2P3O12 (NVP) as cathode. The assembled cell delivers maximum discharge capacity 101.6 and 84.45 mAh g-1 at the current density 10 and 40 mA g-1, respectively with good capacity retention (~95.02%) upto 50 cycles. Figure 1

Developing High-Purity Sodium Oxychloride Solid-State Electrolyte for Sodium-Ion Batteries

Sodium-ion batteries have attracted significant attention as an alternative energy source to lithium-ion batteries. Solid-state sodium oxyhalides (Na3OX, X = Cl, Br, I) have the advantages of easy synthesis from cheap raw materials. In this study, sodium oxychloride (Na3OCl) antiperovskite was synthesized using a microwave furnace, the first instance in currently available literature. Using the microwave furnace also significantly decreased the reaction time when compared to other published procedures. The ionic conductivity of Na3OCl pellets was determined using electrochemical impedance spectroscopy (EIS). Na3OCl was hot pressed between sodiated hard carbon in a PEEK lined split cell with stainless steel current collectors. The symmetric cell was tested at room temperature with varying external pressures (1 MPa to 12 MPa). Symmetric cells were also tested at constant external pressure (6 MPa) from room temperature to 80˚C.Arrhenius plots demonstrated linear behavior in the whole temperature range with a calculated activation energy of 0.98 eV. The melting and crystallization temperatures were determined using differential scanning calorimetry (DSC). The purity of as-synthesized Na3OCl was evaluated using x-ray diffraction (XRD). The results on chemical composition and crystal-to-glass phase transformations were produced from in-situ X-ray diffraction (XRD) spectroscopy in the temperature range from 25˚C up to the melting point of Na3OCl. Cross-sectional SEM images of the Na3OCl pellets showed regions that were dense, but also regions with many pores (≤ 5 μm). SEM also showed microcracks that ranged from 1-5 μm. Future work will investigate methods of minimizing pores and microcracks during the cell manufacturing process as these surface features can impede efficient ionic movement.

Overview of Inorganic Electrolytes for All-Solid-State Sodium Batteries.

All-solid-state sodium batteries (AS3B) emerged as a strong contender in the global electrochemical energy storage market as a replacement for current lithium-ion batteries (LIB) owing to their high abundance, low cost, high safety, high energy density, and long calendar life. Inorganic electrolytes (IEs) are highly preferred over the conventional liquid and solid polymer electrolytes for sodium-ion batteries (SIBs) due to their high ionic conductivity (∼10-2-10-4 S cm-1), wide potential window (∼5 V), and overall better battery performances. This review discusses the bird's eye view of the recent progress in inorganic electrolytes such as Na-β"-alumina, NASICON, sulfides, antipervoskites, borohydride-type electrolytes, etc. for AS3Bs. Current state-of-the-art inorganic electrolytes in correlation with their ionic conduction mechanism present challenges and interfacial characteristics that have been critically reviewed in this review. The current challenges associated with the present battery configuration are overlooked, and also the chemical and electrochemical stabilities are emphasized. The substantial solution based on ongoing electrolyte development and promising modification strategies are also suggested.

Electrolytes for Sodium Ion Batteries: The Current Transition from Liquid to Solid and Hybrid systems.

Sodium-ion batteries (NIBs) have recently garnered significant interest in being employed alongside conventional lithium-ion batteries, particularly in applications where cost and sustainability are particularly relevant. The rapid progress in NIBs will undoubtedly expedite the commercialization process. In this regard, tailoring and designing electrolyte formulation is a top priority, as they profoundly influence the overall electrochemical performance and thermal, mechanical, and dimensional stability. Moreover, electrolytes play a critical role in determining the system's safety level and overall lifespan. This review delves into recent electrolyte advancements from liquid (organic and ionic liquid) to solid and quasi-solid electrolyte (dry, hybrid, and single ion conducting electrolyte) for NIBs, encompassing comprehensive strategies for electrolyte design across various materials, systems, and their functional applications. The objective is to offer strategic direction for the systematic production of safe electrolytes and to investigate the potential applications of these designs in real-world scenarios while thoroughly assessing the current obstacles and forthcoming prospects within this rapidly evolving field.

Tailoring the Properties of Gel Polymer Electrolytes for Sodium-Ion Batteries Using Ionic Liquids: A Review.

Ionic liquids are an extraordinary group of compounds, fully ionic in structure like inorganic salts but with low melting points, that resemble organic molecular solvents. Their chemical, electrochemical, and thermal stability is what draws the attention and enables their use in many applications, including electrochemical power sources. Even though they are no longer considered eco-friendly because of nonnegligible toxicity and long bioaccumulation, they can still be efficiently recovered, purified, and reused. These attributes can be harvested to enhance the properties of gel polymer electrolytes for the emerging sodium-ion batteries. The variety of anions and cations for ILs and their influence on the final properties of the compound opens the road to tuning the properties of gel polymer electrolytes. Ionic liquids as plasticizers constitute a major part of gel polymer electrolytes (average of 70 wt%) and hence, they affect the fundamental properties of gel electrolytes like ionic conductivity and electrochemical window. They also improve the safety features of sodium-ion batteries, which is relevant for their anticipated applications in stationary energy storage and electric vehicles. The presented review paper aims to explain the relationship between the cation and anion in ionic liquid and the properties of gel electrolytes for sodium-ion batteries.

Sodium superionic conductors NaTi2(PO4)3 as a solid electrolyte: A combined experimental and theoretical study

The natrium superionic conductors (NASICON) family of materials is being widely examined as a solid-state electrolyte for sodium-ion batteries (SIBs). We present a combined experimental and theoretical study of the crystal structure and ionic conductivity of a NASICON NaTi2(PO4)3. Rietveld refinement of the x-ray diffraction pattern reveals that the prepared single phase NaTi2(PO4)3 exhibits rhombohedral symmetry in the R3¯c space group. The elemental analysis by energy dispersive spectroscopy (EDS) confirms the desired composition of NaTi2(PO4)3. The ionic conductivity evaluated from the electrochemical impedance spectroscopy measurements as a function of temperature shows Arrhenius-type relaxation behaviour with activation energy ∼0.47(2) eV. A sizeable ionic conductivity (∼1.16×10−3S/cm at 600 K) has been observed for NaTi2(PO4)3. Further, the migration of Na+ via a sigmoidal path in the skeleton of NaTi2(PO4)3 has been investigated by the density functional theory (DFT). The estimated migration energy for the hopping of Na+ between two crystallographic sites is found to be 0.79 eV. The calculated conductivity (3.03×10−7S/cm) resulting from the interstitial diffusivity of the Na-ion is consistent with the measured ionic conductivity at room temperature (∼1.34×10−7S/cm). While the wide calculated electronic band gap (2.52 eV) suggests poor electronic conductivity of NaTi2(PO4)3, it could be beneficial for solid electrolyte applications.

Ionic Competition between Na+ and H+ in Aqueous Sodium-Ion Battery Electrolytes.

Aqueous electrolytes have become a research hotspot because of their high safety and low cost, while the inevitable ionization phenomenon of water in aqueous solution leads to the existence of competitive ions (H+) except the active ions. In this article, we take aqueous Na base electrolyte as an example to clear the ion competition behavior by modeling, simulating together with experimental verification. First, the reaction tendency of the two ions (Na+ and H+) is obtained by calculating the Gibbs energy change of the reaction. Furthermore, the properties of electrolytes with different concentrations including transportation are obtained by modeling. After that, relevant experiments are also proceeded to verify the simulation results. Then, the ion competition behavior is analyzed by in situ observation by controlling the constant concentration of Na+: the high concentration of Na+ can reduce the proportion of H+ and reduce the competitiveness of H+; a high concentration of Na+ causes the increased viscosity and reduces the ion diffusion. Based on this, the correlation between ion competitiveness and ion ratio is also confirmed by keeping the concentration of Na+ unchanged and adjusting the concentration of H+ (adjusting pH). The influence of the ion competition phenomenon (Na+ and H+) is the reaction characteristics of the substance itself and the ratio of ion concentration. Finally, the electrochemical performance is further verified in 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDI) symmetric cells and in full-cells with vanadium phosphate sodium (NVP) as the cathode and PTCDI as the anode.

Interfacial Chemistry of Perfluorinated-Anion Additives Deciphering Ether-Based Electrolytes for Sodium-Ion Batteries

Interfacial Chemistry of Perfluorinated-Anion Additives Deciphering Ether-Based Electrolytes for Sodium-Ion Batteries

The problems with solid-state sodium ion battery electrolytes and their solutions

In the context of the continuous growth of global demand for energy storage, electrochemical energy storage systems stand out among many energy storage technologies due to their excellent performance. At present, batteries as one of the most mainstream forms of electrochemical energy storage, such as fuel cells, lithium-ion batteries, and solid-state batteries, have been widely used. In particular, as the safety hazards existing in lithium-ion batteries due to the inclusion of organic electrolytes have gradually been recognized, solid-state batteries have received extensive attention and in-depth research for their higher safety performance. As an emerging electrochemical energy storage device, the most prominent feature of solid sodium-ion batteries is that they use solid electrolytes instead of traditional liquid electrolytes, which greatly improves the safety of batteries theoretically. Unfortunately, while solid electrolytes exhibit excellent chemical stability and good electrochemical performance, they still face a number of challenges in practical applications, including low ionic conductivity, poor interface contact, and short life span. In this paper, the characteristics, challenges, and solutions of electrolytes in solid-state sodium-ion batteries are discussed.

Communication—High-Capacity Hard Carbons Enabled by a Sodium Carborane Solid Electrolyte for Sodium-Ion Batteries

High-energy-density sodium-ion batteries, which are promising alternatives to lithium-ion batteries, require high-capacity hard carbons as negative electrodes. Conventional sodium-ion batteries use carbonate liquid electrolytes that decompose at hard-carbon surfaces, which limits the use of high-surface-area hard carbons. Herein, we report that high-surface-area hard carbons synthesized using magnesium oxide templates exhibit high capacities when combined with a sodium carborane solid electrolyte. Unlike carbonate liquid electrolytes, the solid electrolyte is highly stable and enables reversible electrochemical reactions in hard carbons to deliver high capacities of approximately 500 mA h g−1.

Comparative analysis of thermal stability and electrochemical performance of NaNi1/3Fe1/3Mn1/3O2 cathode in different electrolytes for sodium ion batteries

The electrochemical compatibility and thermal stability of cathode and electrolyte guarantee the intrinsic safety of batteries. The NaNi1/3Fe1/3Mn1/3O2 (NaNFM) cathode for sodium ion batteries (SIBs) has not received, sufficient attention concerning its thermal safety compared to its electrochemical properties. Herein, we investigated the thermal stability of NaNFM, exothermic reaction and electrochemical performance of NaNFM with various electrolytes. The comprehensive electrochemical performance of NaNFM in different electrolytes followed this sequence: sodium hexafluorophosphate (NaPF6) > sodium bis(trifluoromethyl-sulfonyl)imide (NaTFSI) > sodium perchlorate (NaClO4)- based electrolytes. Additionally, ethylene carbonate (EC): propylene carbonate (PC)- type electrolytes displayed superior cycling stability compared to ethylene carbonate (EC): diethyl carbonate (DEC)- type electrolytes. Moreover, the heat flow profiles demonstrated that the thermal stability between NaNFM and various electrolytes could be ordered according to the total heat release as: NaClO4 < NaPF6 < NaTFSI-based electrolyte. X-ray diffractometer (XRD), scanning electron microscope (SEM) and x-ray photoelectron spectrometer (XPS) analyses were carried out to investigate the changes in crystal structure, morphology and valence state of thermal reaction products after microcalorimetric analysis experiments. These findings provide valuable insights for the safe application of the NaNFM cathode in SIBs.

Improving the Performance of Flame-Retarding Sodium Bis(Oxalato)Borate in Triethyl Phosphate Electrolyte for Sodium Ion Batteries

An ever-growing demand for energy storage has highlighted potential future resource constraints for Li-ion batteries (LIBs), in particular with respect to Li.1-3 With this in mind Na-ion batteries (NIBs) hold promise as a complementary technology to LIBs, particularly for stationary energy storage.4 To be competitive with LIBs, NIBs need to become fully competitive on cost and on capacity density.5-7 NIBs can however offer more than simply a cost competitive option to LIBs, with potential to offer improvements on both safety and sustainability. A number of cathode materials exist that can avoid many of the unanswered question around the sustainability of state-of-the-art LIB cathodes.3 However, to address safety concerns of LIBs it is impossible to ignore the electrolyte. State of the art electrolyte in LIBs, regularly LiPF6 in a mix of carbonates, is both toxic (by virtue of LiPF6 and to a limited extent ethylene carbonate) and flammable (by virtue of the electrolyte solvents). Importing the Na analogue of state-of-the-art LIB electrolytes (i.e. NaPF6 in carbonates) creates the same problems of flammability and toxicity currently seen in LIBs (albeit the toxicity of NaPF6 is lower than that of LiPF6).Recently Mogensen et al.8 proposed sodium bis(oxalato)borate (NaBOB) in trimethyl phosphate (TMP) electrolyte that overcame the flammability concerns of NaPF6 in carbonate electrolytes. In addition to non-flammability, the use of NaBOB instead of NaPF6 allowed for higher temperatures to be used and produced an electrolyte that was halogen free. Switching from TMP to triethyl phosphate (TEP) allowed for a relatively less-toxic electrolyte (NaBOB and TEP are listed by the European Chemical Agency as ‘harmful if swallowed’ in comparison to ‘harmful if swallowed, in contact with skin or inhaled’ for NaPF6 in carbonates, although limited research has been conducted on the toxicity of NaBOB and so this should be received with limited certainty) without sacrificing cell performance.9 However, hurdles remain for NaBOB in organophosphate electrolytes. Principal amongst these has been the poor solubility of NaBOB in most solvents and the difficulties transferring promising low mass-loading full cell results to higher mass-loading full cells. The low solubility of NaBOB limits the concentration of NaBOB in organophosphate electrolytes to < 0.4 M, producing electrolytes with relatively low conductivities (for NaBOB in TMP, 4.5 mS/cm, and in TEP, 5 mS/cm). N-methyl pyrrolidone (NMP) has previously been shown to increase the solubility of NaBOB when used as a cosolvent with TMP, with conductivity improved to 6.5 mS/cm without sacrificing non-flammability and promising cycling shown in low mass-loading full-cells.10 A range of alternative cosolvents were here investigated to increase the solubility of NaBOB in TEP. Of these, NMP, N,N-dimethyl formamide (DMF) and N,N-dimethyl acetamide (DMAc) were evaluated as cosolvents with TEP for near-commercial mass loading full-cells (Prussian white||hard carbon at ~ 2 mAh/cm2). To maintain non-flammable characteristics, the cosolvents had to be kept below 40% (v/v) for DMAc and DMF and below 60% (v/v) for NMP, resulting in electrolyte conductivities of 5.7, 6.9 and 6.0 mS/cm respectively. These nearly doubled the conductivity we observed for NaBOB in TEP of 3.5 mS/cm. However, all electrolytes examined showed large polarisation, suspected to be due a thick, resistive SEI. A range of strategies have been employed in order to reduce this polarisation, with particular focus on improved formation protocols (studying the effects of temperature, current density and electrochemical method of formation) and their use in concert with film forming additives. Future work will aim to understand the interplay between formation conditions, the resultant SEI formation and the cell polarization in an effort to translate promising low mass-loading full cell results to near-commercial mass-loading full cells.

Non-Flammable Ether and Phosphate-Based Liquid Electrolytes for Sodium-Ion Batteries

The demand on the lithium reserves has been increasing rapidly due to the continuous growth in energy storage devices. Since the lithium reserves are limited and only accessible at certain geographical areas, the costs of lithium precursors are increasing significantly.1 Therefore, alternative battery chemistries need to be investigated. One of the next generation battery solutions is sodium-ion batteries (SIBs).2 However, current commercial electrolytes in SIBs are based on highly flammable carbonate-based solvents, such as ethylene carbonate, propylene carbonate, diethyl carbonate and dimethyl carbonate.3 To enhance the safety of SIBs, it is of utmost importance to develop non-flammable electrolytes. In this work, NaPF6 and NaBF4 salts are used in combination with a solvent mixture of ethers (diglyme, tetraglyme) and phosphates (triethylphosphate, trimethylphosphate) to obtain non-flammable liquid electrolytes. The electrolytes are evaluated in terms of physicochemical properties and electrochemical performance full-cells using high-mass loading Prussian White (12 mg/cm2) and Hard Carbon (7 mg/cm2). Flammability tests have shown that at least 70% of phosphate co-solvents are required to make the glyme based electrolyte non-flammable. In summary, the combination of glymes and phosphates provide promising electrolyte formulations in terms of non-flammability characteristics and ionic conductivities, but still lack long-term cycling stability. This work opens avenues to optimizing non-flammable electrolytes for high-performing and safe SIBs. References 1 R. Schmuch, R. Wagner, G. Hörpel, T. Placke and M. Winter, Nat. Energy, 2018, 3, 267–278.2 H. S. Hirsh, Y. Li, D. H. S. Tan, M. Zhang, E. Zhao and Y. S. Meng, Adv. Energy Mater., 2020, 10, 1–8.3 R. Gond, W. Van Ekeren, R. Mogensen, A. J. Naylor and R. Younesi, Mater. Horizons, 2021, 8, 2913–2928. Acknowledgement The authors would like to acknowledge the financial support by the Swedish Energy Agency via project no. 50177-1, VINNOVA via projects no. 2022-01465 and 2019-00064 (Batteries Sweden), and SuSaNa project under M-ERA-Net call 2021 (G.A. no 958174).

Doped Nasicon-Type Solid Electrolytes for Sodium-Ion Batteries from Scalable Spray-Flame Synthesis

Due to the limited availability of raw materials, sodium-ion batteries (SiB) are a viable alternative to lithium-ion batteries. However, liquid-electrolyte-based SiBs have a comparably low energy density and suffer from safety issues similar to their lithium-ion-based counterparts. To address these issues, solid-state SiBs are a promising option and sodium super ionic conductors (NaSICON) are suitable solid electrolytes for such systems. They exhibit good mechanical properties and chemical stability, high ionic conductivity and compatibility with sodium metal based anodes enabling high energy density [1]. One of the best studied NaSICON materials is Na3Zr2Si2PO12 (NZSP) with a high ionic conductivity in the order of 10-4 –10-3 S·cm-1 at room temperature [2]. Conventional synthesis methods of this class of materials such as solid-state reaction and liquid-phase synthesis have several drawbacks due to time-consuming process steps such as milling, high temperature sintering, precipitation, washing and drying steps to obtain the final product.We present spray flame synthesis (SFS) as a new approach for the synthesis of nanosized NaSICON materials. Recent studies have shown that sintering of nanoparticular NZSP precursors offers several advantages: A high specific surface area, which increases the sintering activity, and short atomic diffusion paths, allowing high homogeneity and phase purity to be achieved at comparatively low sintering temperatures [3]. In SFS, metal salts dissolved in organic solvents are combusted resulting in fine metal oxide particles. They are characterized by transmission electron microscopy (TEM), X-Ray diffraction (XRD) and Raman-Spectroscopy for structural and morphological investigation. Elemental information is obtained via energy-dispersive X-Ray spectroscopy (EDX). Ionic conductivities of sintered NZSP pellets are measured by impedance spectroscopy.In our approach, nanoparticles with a median diameter of around 5–9 nm are obtained. The pristine particles consist of crystalline ZrO2, homogeneously covered with an amorphous layer consisting of the elements Na, Si, P and O. After a short annealing step for 1h at 1000°C, this mixture can be converted almost quantitatively into the desired rhombohedral NZSP phase. Moreover, aliovalent dopants were successfully added for the synthesis of Na3+2xAxZr2−xSi2PO12 with A = Mg or Ca. Pressed pellets sintered at 1100°C for 3h to a relative density of ~92% showed - for a material sintered for such a short period of time - a surprisingly high ionic conductivity of up to 7.9×10-4 S·cm-1 (Mg-doped).In conclusion, a novel approach for the preparation of NZSP allowing the phase formation at relatively low temperatures is demonstrated. Spray flame synthesis is an elegant and promising possibility for the scalable production of solid electrolytes, which also holds great potential, especially with regard to further improvement of ionic conductivity through targeted doping.[1] Zhang et al., ACS Appl. Energy Mater., 3 (2020) 7427; doi.org/10.1021/acsaem.0c00820[2] Narayanan et al., Solid State Ion., 331 (2019), 22; doi.org/10.1016/j.ssi.2018.12.003[3] Jalalian-Khakshour et al. J.Mater.Sci., 55, (2019) 2291; doi.org/10.1007/s10853-019-04162-8

CEI Optimization: Enable the High Capacity and Reversible Sodium‐Ion Batteries for Future Massive Energy Storage

Sodium‐ion batteries (SIBs) have attracted attention due to their potential applications for future energy storage devices. Despite significant attempts to improve the core electrode materials, only some work has been conducted on the chemistry of the interface between the electrolytes and essential electrode materials. Therefore, the different cathode–electrolyte interfaces (CEIs) that form between various cathode materials and liquid electrolytes for SIBs are briefly reviewed, and the requirements of CEI performance in low‐temperature SIBs. Modifying the cathode materials and electrolyte formulas offers an efficient strategy for CEI enhancement. The summary and analysis, given in this article, may serve as a reference for the development of better sodium‐ion batteries.

Ester-based anti-freezing electrolyte achieving ultra-low temperature cycling for sodium-ion batteries

With the continuous advancement of industrialization, sodium-ion batteries (SIBs) need to operate in various challenging circumstances, particularly in extremely cold conditions. However, at ultra-low temperatures, the reduced ionic conductivity and sluggish Na+ migration of commonly carbonate-based electrolytes will inevitably lead to a sharp decrease in the capacity of SIBs. Herein, we design a carboxylate ester-based electrolyte with excellent ultra-low temperature performance by straightforward cosolvent strategy. Due to the low viscosity, melting point, and sufficient ionic conductivity of the designed electrolyte, the resulting Na||Na3V2(PO4)2O2F can achieve the capacity retention of 96% (100 cycles at 0.1 C) at –40 °C and can also operate stably even at –50 °C. Besides, galvanostatic intermittent titration technique (GITT), ex-situ X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (TEM) tests are employed to analyze and confirm that the carboxylate ester-based electrolyte promotes robust and uniform cathode/electrolyte interface layer formation and accelerates ion diffusion kinetics, which collectively facilitates the better low-temperature performance. In addition, the assembled hard carbon||NVPOF full cells further prove the practicability of the carboxylate ester-based electrolyte at low-temperature, which delivers high discharge capacity of 108.4 and 73.0 mAh g–1 at –25 and –40 °C. This work affords a new avenue for designing advanced low-temperature electrolytes for SIBs.

Deciphering Electrolyte Dominated Na+ Storage Mechanisms in Hard Carbon Anodes for Sodium-Ion Batteries.

Although hard carbon (HC) demonstrates superior initial Coulombic efficiency, cycling durability, and rate capability in ether-based electrolytes compared to ester-based electrolytes for sodium-ion batteries (SIBs), the underlying mechanisms responsible for these disparities remain largely unexplored. Herein, ex situ electron paramagnetic resonance (EPR) spectra and in situ Raman spectroscopy are combined to investigate the Na storage mechanism of HC under different electrolytes. Through deconvolving the EPR signals of Na in HC, quasi-metallic-Na is successfully differentiated from adsorbed-Na. By monitoring the evolution of different Na species during the charging/discharging process, it is found that the initial adsorbed-Na in HC with ether-based electrolytes can be effectively transformed into intercalated-Na in the plateau region. However, this transformation is obstructed in ester-based electrolytes, leading to the predominant storage of Na in HC as adsorbed-Na and pore-filled-Na. Furthermore, the intercalated-Na in HC within the ether-based electrolytes contributes to the formation of a uniform, dense, and stable solid-electrolyte interphase (SEI) film and eventually enhances the electrochemical performance of HC. This work successfully deciphers the electrolyte-dominated Na+ storage mechanisms in HC and provides fundamental insights into the industrialization of HC in SIBs.

Scalable Synthesis of Selenide Solid-State Electrolytes for Sodium-Ion Batteries.

Solid-state sodium-ion batteries employing superionic solid-state electrolytes (SSEs) offer low manufacturing costs and improved safety and are considered to be a promising alternative to current Li-ion batteries. Solid-state electrolytes must have high chemical/electrochemical stability and superior ionic conductivity. In this work, we employed precursor and solvent engineering to design scalable and cost-efficient solution routes to produce air-stable sodium selenoantimonate (Na3SbSe4). First, a simple metathesis route is demonstrated for the production of the Sb2Se3 precursor that is subsequently used to form ternary Na3SbSe4 through two different routes: alcohol-mediated redox and alkahest amine-thiol approaches. In the former, the electrolyte was successfully synthesized in EtOH by using a similar redox solution coupled with Sb2Se3, Se, and NaOH as a basic reagent. In the alkahest approach, an amine-thiol solvent mixture is utilized for the dissolution of elemental Se and Na and further reaction with the binary precursor to obtain Na3SbSe4. Both routes produced electrolytes with room temperature ionic conductivity (∼0.2 mS cm-1) on par with reported performance from other conventional thermo-mechanical routes. These novel solution-phase approaches showcase the diversity and application of wet chemistry in producing selenide-based electrolytes for all-solid-state sodium batteries.

Anion-derived cathode interface engineering enables ether-based electrolytes for sodium-ion batteries

High-performance sodium-ion batteries (SIBs) require not only non-aqueous electrolytes with high ion conductivity but also high interfacial compatibility with both electrodes. Ether-based electrolytes are considered one of the most promising electrolyte systems for SIBs due to the advantages of high reduction stability and weakly desolvating power. However, conventional ether-based electrolytes are easy to decompose and deteriorate on the cathode/electrolyte interface under high voltage, which greatly restricts their practical application in SIBs. Herein, a novel and effective interfacial stabilization strategy is proposed and systemically investigated. A homogeneous and compact cathode electrolyte interphase (CEI) film rich in fluorides and borides can be successfully constructed on the cathode surface by pre-cycling of NVP@C cathode in the NaODFB-DEGDME electrolyte. The ODFB anion-derived CEI film can effectively inhibit the continuous oxidation decomposition of NaPF6-DEGDME electrolytes. As a result, the Na||NVP@C half-cell can stably cycle for 1500 cycles with high average Coulombic efficiency (CE, >99.7 %) at 5C. This work illustrates the significance of the composition and structure of CEI film for improving the interfacial stability of high-voltage SIBs with ether-based electrolytes.