Sodium Secondary Batteries Research Articles

Designing sodium alloys for dendrite‐free sodium‐metal batteries

AbstractSodium metal, with a high theoretical specific capacity (∼1165 mA h g−1) and a low redox potential (−2.71 V vs. SHE) as well as low cost, becomes an attractive option for high‐energy‐density sodium secondary batteries. However, the practical application of sodium metal anodes is hindered by dendrite growth, which results in low energy efficiency, poor lifetime and serious safety issues. To address this challenge, researchers propose various strategies, including the formation of sodium alloys (Na‐M alloys, M = Sn, Sb, Bi, In, etc.) through alloying reaction. The alloying effect has a positive impact in terms of reducing the local current density, mitigating the volume expansion, and inhibiting the dendrite growth. It is thus an effective solution for constructing high‐performance sodium secondary batteries. This review systematically describes the mechanism of dendrite growth and the alloying process of Na‐M alloys, summarizes recent research progress and strategies for applying Na‐M alloys to create dendrite‐free sodium secondary batteries, as well as presents prospects for the development of Na‐M alloys and offers clear suggestions for future research. This review aims to inspire further efforts to build dendrite‐free, high‐performance sodium secondary batteries and broaden a new aspect for the next‐generation battery systems.

Cover Feature: Use of Ferrocenyl Ni(II) and Zn(II) Porphyrins as Active Organic Electrode Materials for Sodium Secondary Batteries (Batteries & Supercaps 6/2024)

Cover Feature: Use of Ferrocenyl Ni(II) and Zn(II) Porphyrins as Active Organic Electrode Materials for Sodium Secondary Batteries (Batteries & Supercaps 6/2024)

Synthesis of High‐Sodium‐Content Oxythiosilicate Glass Electrolytes Via Sodium Polysulfide

AbstractSulfide glass electrolytes with high ionic conductivities and formabilities are key materials for practical use in all‐solid‐state sodium secondary batteries. Both sealing and quenching are considered to be necessary for producing high‐alkali‐content sulfide glass electrolytes. In this study, the glass‐forming ability of sodium thiosilicate glasses is enhanced by adding a small quantity of SiO2. Consequently, high‐sodium‐content oxythiosilicate glass is synthesized without sealing and quenching processes under Ar atmosphere. The prepared Na4SiS4·0.200SiO2 (molar ratio) glass exhibits ionic conductivity of 4.0 × 10−5 S cm−1 at 25 °C, surpassing that of Na4SiS4 glass. The Na4SiS4·0.200SiO2 glass shows high reduction stability to sodium metal. The all‐solid‐state Na metal/Na4SiS4·0.200SiO2 glass/Na2.88Sb0.88W0.12S4/TiS2 cell shows a reversible capacity of 140 mAh g−1 for 5 cycles. The amount of H2S gas generated in Na4SiS4·0.0830SiO2 and Na4SiS4·0.200SiO2 glasses is one‐tenth that of the Na4SiS4 glass, suggesting increased chemical stability to humidity by incorporating oxygen in the structure. High‐sodium‐content oxythiosilicate glass with high ionic conductivity, high chemical stability against humidity, and high reduction stability is successfully prepared under ambient pressure without sealing and quenching processes.

Excellent sodium metal deposition enabled by three-dimensional porous structures with natrophilic Ni-Sn alloy

Na metal has long been an ideal potential anode material for sodium secondary batteries due to its own superiority; however, the Na dendrite problem during cycling makes it face a great obstacle in application. Here, we grow sodiophilic thin film materials with tin and nickel-tin components uniformly on three-dimensional nickel foam (3D-Ni) to obtain a 3D sodiophilic composite framework (namely 3D-NiSn) using a reproducible and mass-produced electrodeposition strategy. The combination of sodiophilic film (NiSn) and 3D porous framework could lower nucleation overpotential of Na and accelerate ions diffusion, hence inducing the uniform deposition and reversible stripping of sodium and inhibiting the growth of Na dendrites, which makes the 3D-NiSn/Na composite anode exhibit good performance compared to 3D-Ni/Na and Na-foil. The symmetrical cells with 3D-NiSn/Na can maintain up to 2500 h at 2 mA cm−2 and 1 mAh cm−2 with a low overpotential of around 30 mV during the whole cycling process. Additionally, the 3D-NiSn/NaǁNa3V2(PO4)3 full cells deliver good cycle stability with a high specific capacity of around 98.1 mAh g−1 at 3C for over 600 cycles. This work provides an idea for constructing low-cost and commercially available Na metal electrodes toward high-performance SIBs.

Use of Ferrocenyl Ni(II) and Zn(II) Porphyrins as Active Organic Electrode Materials for Sodium Secondary Batteries

AbstractThis study investigates the potential use of ferrocenyl‐substituted porphyrins as organic electrode materials for sodium secondary batteries. Expressly, 5,15‐di(ferrocenyl)‐10,20‐di(pentafluorophenyl)porphyrins (free base, Fc−H2Por), along with its zinc (Zn[II] complex, Fc−ZnPor) and nickel (Ni[II] complex, Fc−NiPor) derivatives were synthesized and employed as the electrode materials. The charge‐discharge processes disclosed that the ferrocenyl‐substituted porphyrin compound performs electrochemical processes poorly without chelating metals. In contrast, when the ferrocenyl‐substituted porphyrins were metalated with Ni(II) and Zn(II), they exhibited dual ionic charge‐discharge capabilities, indicating their stable electrochemical properties. In particular, long‐term cycle tests emphasized the superior stability and high Coulombic efficiency of Fc−ZnPor electrodes. Computational calculations provided conclusive evidence of variances in aromatic stabilization energy based on the metal center, significantly influencing the chemical stability and electrochemical performance. These findings can deliver valuable insight for further development and optimizations in energy storage applications.

(Invited) Solid State Hybrid Inorganic Organic Polymer Electrolytes for Advanced Sodium Secondary Batteries

The large-scale rollout of electric vehicles, smart grids and portable electronics is expected to require an amount of energy storage devices that the Li-ion technology alone will not be able to satisfy. In this concern, a quest for novel electrochemical energy storage technologies has started [1]. Sodium secondary batteries appear to be a good choice due to: (i) the high availability of raw materials; (ii) the low cost of sodium; (iii) the low sodium standard reduction potential; and (iv) the similarity between the chemistries of sodium and lithium, which facilitates the transition between the two technologies. Up to date, the best-performing electrolytes for the reversible deposition of sodium are based on organic solvents, which suffer from safety concerns and a poor stability towards sodium metal. Thus, research activities in this field are devoted to the development of safe and stable solid state electrolytes able to efficiently transport Na+ ions.Inspired by the pioneering work done by Di Noto and co-workers [2-4], herein we present a family of hybrid inorganic-organic polymer electrolytes (HIOPEs) for advanced solid state sodium secondary batteries. The initial HIOPE is obtained by means of a reaction between zirconium ethoxide and polyethylene glycol (PEG400). The resulting material is doped with sodium perchlorate as source of Na+ ions. In this system a 3D network is obtained, where inorganic zirconium metal nodes are interconnected by means of organic PEO chains. The latter ensure flexibility to the overall structure. Na+ ions are coordinated by and exchanged between the oxygen atoms of the ethereal functionalities of PEO chains. In addition, to guarantee a good structural stability, positively charged Zr nodes partially coordinate perchlorate anions, thus raising the sodium transference number. After doping with the poly(ethylene glycol) dimethyl ether (PEGDME250) plasticizer, a room temperature conductivity higher than 10-4 S cm-1 is demonstrated. An advanced study of the thermal and structural properties of the proposed materials is presented, with a particular focus on the interactions established between the different chemical species and complexes composing the HIOPEs. The conduction mechanism is elucidated starting from the results obtained in a wide range of temperatures from broadband electrical spectroscopy studies. Taking all together, this study offers insights on the application of non-traditional solid state electrochemical functional components as replacements for conventional solvents in the emerging field of sodium secondary batteries. Acknowledgments The project “Interplay between structure, properties, relaxations and conductivity mechanism in new electrolytes for secondary Magnesium batteries” (Grant Agreement W911NF-21-1-0347-(78622-CH-INT)) of the U.S. Army Research Office. The project “ACHILLES” (prot. BIRD219831) of the University of Padua. The project “VIDICAT” (Grant Agreement 829145) of the FET-Open call of Horizion 2020. The project TRUST (protocol 2017MCEEY4) of the Italian MIUR funded in the framework of “PRIN 2017” call.

A Hexafluorophosphate‐Based Ionic Liquid as Multifunctional Interfacial Layer between High Voltage Positive Electrode and Solid‐State Electrolyte for Sodium Secondary Batteries

AbstractSodium secondary batteries have gained accolades as future energy storage devices due to their low costs and environmental benignity, but are heavily impeded by the poor anodic stabilities of most electrolytes, including solid‐state electrolytes (SSE), that render them incompatible with high‐voltage positive materials. This study reports, for the first time, a new synthesis technique using a fluorohydrogenate ionic liquid (IL)precursor to prepare a [DEME][PF6] ([DEME]+: N,N‐diethyl‐N‐methyl‐N‐(2‐methoxyethyl) ammonium) with high yield and high purity. Herein, a Na[PF6]‐[DEME][PF6] IL is formulated and subjected to a series of electrochemical tests to validate its performance in battery applications. The present IL harbors a strong oxidative stability (up to 5.2 V on Pt and >4.5 V on conductive carbon electrodes) that aids in the suppression of oxidative decompositions of one typical SSE, for example, beta alumina solid electrolyte (BASE), thereby extending their electrochemical window in hybrid SSE systems. A hybrid solid‐state Na secondary battery employing a high voltage positive electrode, Na3V2(PO4)2F3, is assembled using the BASE/IL configuration, and features energy density and superior cycling performance. This work demonstrates that sandwiching an SSE between the oxidatively stable [PF6]− IL can be an effective design for high voltage operation Na secondary batteries.

Crystalline/amorphous LiCoO2–Li2SO4 nanoheterostructured materials for high-voltage all-solid-state lithium batteries

Amorphous active materials have been developed for high-performance lithium and sodium secondary batteries to achieve a higher capacity, higher rate performance, and longer cycle life than those of crystalline active materials. In this study, a new function of amorphous active materials was explored through the heat treatment of pseudobinary amorphous materials (for example, LiCoO2–Li2SO4). Additionally, active materials with crystalline/amorphous nanoheterostructures, in which the nano-LiCoO2 domains were dispersed in a Li2SO4-rich matrix, were demonstrated. In all-solid-state batteries with sulfide solid electrolytes (SEs), the crystalline/amorphous active materials reduced the interface resistance of the active material and SE when compared with that of Li. High electrochemical performance was achieved using the Li2SO4-rich matrix, which served as an interfacial stabilizer. In summary, this paper reports a new strategy based on annealing amorphous materials for developing active materials with nanoheterostructures for high-performance energy storage devices.

Electrochemical Investigation of Spherical Hard Carbon Negative Electrodes for Sodium Secondary Batteries

Hard carbon (HC) is receiving widespread attention as a negative electrode material for sodium secondary batteries. In this study, spherical HC with three different diameters (1 µm, 1.5 µm, and 5 µm) are investigated using ionic liquid (IL) and organic electrolytes. The HC with a diameter of 5 µm demonstrated the best performance with IL at 90 °C (capacities of 327 mAh g−1 at 20 mA g−1 and 77 mAh g−1 at 2000 mA g−1 and initial Coulombic efficiency of 78.9 %). The HCs showed stable cycling in ILs as compared to the organic electrolytes in the first few cycles.

Solid Gel Electrolytes with Highly Concentrated Liquid Electrolyte in Polymer Networks and Their Physical and Electrochemical Properties and Application to Sodium Secondary Batteries

Gel polymer electrolytes consisting of sulfolane (SL)-NaN(SO2CF3)2 liquid electrolyte and a polyether-based host polymer were prepared, and their physicochemical and electrochemical properties were investigated. The prepared gel electrolytes generally exhibited high thermal stability regardless of the NaN(SO2CF3)2 concentrations. The glass transition temperature decreased with the NaN(SO2CF3)2 concentration owing to the strong interaction between SL and Na+. The ionic conductivities of all gel polymer electrolytes were higher than 10−4 S cm−1 at 303.15 K as a result of the plasticizer effect of SL. Although a relatively large interfacial resistance of the electrolyte/Na metal electrode was observed owing to the high reactivity of the SL-NaN(SO2CF3)2 electrolyte, the fabricated [Na metal negative electrode∣gel polymer electrolyte∣sulfur-modified polyacrylonitrile positive electrode] cell, i.e., the Na-S battery, achieved reversible charge-discharge operation at 333 K and demonstrated its potential to serve as an electric power storage system capable of low-temperature operation.

Molybdenum polysulfide electrode with high capacity for all-solid-state sodium battery

All-solid-state sodium secondary batteries have attracted significant attention as an alternative for post‑lithium-ion batteries. However, to obtain all-solid-state sodium secondary batteries with high-energy density, it is necessary to develop a high-capacity positive electrode material. In this study, two types of amorphous (a-)MoSx (x ≃ 6) active electrode materials were prepared using a mechanochemical (MC) process and through the thermal decomposition (TD) of (NH4)2Mo2S12·2H2O. Herein, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) analyses were conducted, and the results revealed significantly different local structures of a-MoSx (MC) and a-MoSx (TD). The all-solid-state sodium cells with a-MoSx (MC) and a-MoSx (TD) possessed high reversible capacities of 510 and 690 mAh g−1, as well as high-capacity retention rates of 80% and 92% for the 2nd to 30th cycles, respectively. XPS analysis of the discharge–charge products further suggested that dissociation and formation of disulfide bonds occurred reversibly during the discharge–charge reaction. This study shows that a-MoSx (MC) and a-MoSx (TD) are promising active electrode materials for all-solid-state sodium batteries.

Electrode performance of amorphous MoS3 in all-solid-state sodium secondary batteries

All-solid-state Na–S secondary batteries that use sodium and sulfur, both available in abundance, are the most attractive next-generation batteries. In this study, two types of amorphous MoS3 (a-MoS3) were prepared as electrode active materials for use in all-solid-state sodium secondary batteries using the thermal decomposition (TD) of (NH4)2MoS4 and mechanochemical (MC) processes, denoted a-MoS3 (TD) and a-MoS3 (MC), respectively. X-ray diffraction, thermogravimetric-differential thermal analysis, and X-ray photoelectron spectroscopy (XPS) analyses revealed that a-MoS3 (TD) and a-MoS3 (MC) had different local structures. The a-MoS3 (TD) and a-MoS3 (MC) electrodes showed high reversible capacities of 310 mAh g−1 and 260 mAh g−1, respectively, for five cycles in all-solid-state sodium secondary batteries. XPS analysis of the discharge–charge products suggested that the dissociation and formation of disulfide bonds occurred during the discharge–charge reaction. The results show that a-MoS3 is a promising active electrode material for all-solid-state sodium batteries.

High Performance Sodium Secondary Batteries Using Ionic Liquid Electrolytes

Sodium secondary batteries are attracting more and more attention as large-scale energy storage devices owing to high abundance and low cost of sodium resources.[1-3] Ionic liquid electrolytes have unique properties such as low volatility, low flammability, and thermal stability and enable construction of safe electrochemical devices, including sodium secondary batteries.[4,5] In particular, intermediate temperature operation of sodium secondary batteries using ionic liquid electrolytes is attractive by considering the improvement of battery performance. Such environments are ubiquitous in our daily life or industries and can be achieved with minimal extra energy. Self-heating of batteries can also contribute to operation at elevated temperatures.Highly improved performance is observed for some electrode materials by intermediate-temperature operation of sodium secondary batteries with bis(fluorosulfonyl)amide-based ionic liquid electrolytes. As for the sodium metal negative electrode, morphology of sodium metal electrodeposit is significantly dependent on operation temperature; smooth sodium metal deposition with a high deposition/dissolution efficiency is observed at 90 deg C. High rate performance is observed for a number of positive electrode materials. The NASICON-type Na3V2(PO4)3 especially provides a long cycle life, exhibiting more than 5000 cycles at 90 deg C in a half cell configuration at a rate of 2C, which is also due to the high cycleability of sodium metal electrode.[6] Metal phosphide negative electrodes also show improved rate and cycle performance by intermediate-temperature operation. The copper phophide-carbon composite, CuP2/C, electrode prepared by high-energy ball-milling shows a stable cycling based on a conversion mechanism at 90 deg C over 300 cycles.[7]Na metal is used as a counter electrode in a half-cell configuration to test electrode performance for Na secondary batteries, but its unstability and large interfacial resistance causes unreliable results, especially at high rates. The Na3V2(PO4)3 and NaV2(PO4)3 electrode prepared by mixing them in a target ratio exhibits a flat plateau at 3.4 V vs. Na+/Na and lower polarization than the Na metal. This new electrode enables trustable electrochemical evaluation of a target electrode even at a temperature above the melting point of Na metal.[8]

Sodium Niobium Sulfides As Active Materials for High Capacity Sodium Secondary Batteries

All-solid-state sodium batteries with high safety and high energy density are expected as next generation rechargeable batteries. Active materials play the key role to develop the capacity and the reversibility of the batteries. We have focused on the transition metal sulfides as the active materials. We have reported that the all-solid-state cells using TiS2 operated reversibly[1]. In term of the practical use, the further increase of the capacity is important. One of the solutionsll is the application of sodium rich transition metal sulfides as active materials. In previous reports, the several new crystal polymorphs of Na2TiS3 were produced by melt quenching and mechanochemical reaction [2, 3]. P2-type layered Na2TiS3 obtained by melt quenching showed twice higher capacity than that of the stable phase of O3-type layered Na2TiS3. Cubic rocksalt phase Na2TiS3 prepared by mechanochemical process was also a metastable phase with high capacity. The metastable phases with high ionic conductivity are advantageous for insertion/extraction. Sodium niobium sulfides have sodium-rich compositions such as Na3NbS4. In lithium batteries, cubic rocksalt Li3NbS4 synthesized by mechanochemical process showed the reversible capacity of 400 mAh g-1 [4]. In this study, we have synthesized Na3NbS4 and optimized their crystal polymorphs by melt quenching and mechanochemical process for high capacity electrode.Electrode materials with the composition of Na3NbS4 were prepared by solid phase reaction or mechanochemical reaction of Na2S, NbS2 and S. DTA curves of the mixture of Na2S, NbS2 and S with the Na3NbS4 composition suggested that phase transition occurred at 630oC. XRD patterns of Na3NbS4 prepared by melt quenching from 700oC showed that O3-type layered phase were obtained. The all-solid-state cells using the layered Na3NbS4 as active materials showed 80 mAh g-1, suggesting that the O3-type sturucture was not suitable for sodium extraction/insertion. On the other hand, the mechanochemical process produced the new metastable phase of Na3NbS4. The all-solid-state cells using the new phase showed the increment on the capacity of 240 mAh g-1. The new crystal polymorph of Na3NbS4 has also been profitable for sodium extraction/insertion. The newly developed Na3NbS4 is a promising active material with high capacity for all-solid-state sodium secondary batteries. Reference s : [1] A. Hayashi, K. Noi, A. Sakuda and M. Tatsumisago, Nat. Commun., 3 (2012) 856.[2] A. Nasu, M. Otoyama, A. Sakuda, A. Hayashi, and M. Tatsumisago, Chem. Lett., 48 (2019) 288.[3] A. Nasu, M. Otoyama, A. Sakuda, A. Hayashi, and M. Tatsumisago, J. Ceram. Soc. Jpn., 127 (2019) 514.[4] A. Sakuda, T. Takeuchi, K. Okamura, H. Kobayashi, H. Sakaebe, K. Tatsumi and Z. Ogumi, Sci. Rep., 4 (2014) 2.

Deep Eutectic Solvents As Sodium-Ion Battery Electrolytes with Increased Durability at Higher Temperatures

Sodium-ion batteries (SIBs) are a potential cost-effective alternative for lithium-ion batteries (LIBs) in applications which require large-scale energy storage. In these applications, low cost and sustainability are of prime importance. The extensive use of LIBs for large-scale storage would drive up lithium prices because of the latter’s limited production. As the lightest and smallest alkali metal after lithium, sodium is abundant and widely available, and is therefore more cost-efficient and sustainable. SIBs can use light and inexpensive aluminum current collectors for both the anode and cathode, as opposed to LIBs, which usually require more expensive copper current collectors for the anodes. This further increases the SIB's viability in applications where cost and sustainability are the most crucial factors.Viable battery electrolytes should have sufficient ionic conductivity, be electronically insulating, have a large electrochemical and thermal stability window, and not show reactivity with the other components of the battery. They should be safe, non-toxic, and inexpensive. Ionic liquids, defined as materials which consist entirely out of ions and which remain liquid below 100 °C, were previously studied as electrolytes for SIBs as they have a high intrinsic ionic conductivity, can dissolve high amounts of salt, are non-flammable and have a high thermal and (electro)chemical stability. Ionic liquids are typically expensive due the difficulty of their synthesis. Deep eutectic solvents (DESs) are closely related to ionic liquids and consist of a mixture of Lewis/Brönsted acids and bases, where the melting temperature of the mixture is below that of the individual components. Whereas DESs and ionic liquids can have similar physical properties, DESs can have additional advantages such as ease of preparation and wide availability of reagents, although this depends on the exact composition. Binary and ternary mixtures of different XFSI (FSI = bis(fluorosulfonyl)imide) and XTFSI (TFSI = bis(trifluoromethanesulfonyl)imide) inorganic salts were previously applied as DES electrolyte in sodium-ion half cells, which were constrained to operation at 80 °C or higher because of their relatively high melting points.1,2 These cells showed high reversibility and high Coulombic efficiency.Other DESs may also offer an increase of the stability of battery operation, and may be used at less stringent temperature conditions (with acceptable ionic conductivity even at room temperature). This stability problem is particularly pronounced at 55 °C or higher for conventional electrolytes, where battery performance rapidly degrades when linear carbonates are used as electrolyte solvent.3 Our series of DESs as SIB electrolytes were based on the dissolution of NaTFSI in an amide which is solid at room temperature. For each electrolyte, the effect of the solution structure on its electrochemical properties was studied. The amide was spontaneously reduced when contacted with sodium metal. By increasing the salt concentration, we observe a lower reactivity with sodium metal, a broader electrochemical stability window and a decreased ionic conductivity due to the strong Coulombic interactions among the amide molecules, Na+, and TFSI- ions (Figure). The sample containing 10 mol% of NaTFSI shows an anodic stability limit of ~3.6 V vs Na+/Na and a conductivity of 10.3 mS cm-1 at 55 °C. A DES with 30 mol% NaTFSI is stable up to ~4.6 V vs Na+/Na and has a conductivity of 3.8 mS cm-1 at 55 °C. The variation of conductivity with temperature of both DESs could be fitted with the Vogel-Tamman-Fulcher equation. At 55 °C, (Na3V2(PO4)3/C)/(Na2+x Ti4O9/C) full cells containing DES as electrolyte demonstrate a considerably higher durability and Coulombic efficiency than cells containing a conventional organic solvent-based electrolyte. As such, these DESs form a new class of electrolytes for application in sodium-ion batteries, offering a more durable performance at 55 °C than conventional systems.References(1) Nohira, T.; Ishibashi, T.; Hagiwara, R. Properties of an Intermediate Temperature Ionic Liquid NaTFSA – CsTFSA and Charge – Discharge Properties of NaCrO2 Positive Electrode at 423 K for a Sodium Secondary Battery. J. Power Sources 2012, 205, 506–509.(2) Fukunaga, A.; Nohira, T.; Kozawa, Y.; Hagiwara, R. Intermediate-Temperature Ionic Liquid NaFSA-KFSA and Its Application to Sodium Secondary Batteries. J. Power Sources 2012, 209, 52–56.(3) Yan, G.; Dugas, R.; Tarascon, J. The Na3V2(PO4)2F3/Carbon Na-Ion Battery: Its Performance Understanding as Deduced from Differential Voltage Analysis. J. Electrochem. Soc. 2018, 165 (2), 220–227. Figure 1

Highly Sodium-Ion Conductive Sulfide Electrolyte Film Prepared By Pulsed Laser Deposition

All-solid-state sodium secondary batteries exhibit significant potential for use in next-generation batteries that offer advantages associated with safety and cost. The key material in this battery technology is a solid electrolyte that conducts sodium ions. The most typical solid electrolyte for all-solid-state sodium batteries is Na3PS4 [1]. All-solid-state batteries using Na3PS4 and several electrode active materials operate at room temperature. At their present stage of development, the operation of all-solid-state sodium batteries under high current densities is challenging. It was recently reported that Na3SbS4 solid electrolytes showed a high ionic conductivity that exceeded 10-3 S cm-1 [2-4], which was higher than that of Na3PS4 glass-ceramic [1]. The evaluation of bulk ionic conductivity is important for determining the potential of the electrolytes. However, most previous studies have reported on the ionic conductivities of compressed powder pellets.Numerous studies have employed pulsed layer deposition (PLD) for fabricating lithium-ion-conducting sulfide electrolyte films. However, PLD has been seldom used for developing sodium-ion-conducting solid electrolyte films thus far. Herein, a thin film of highly dense Na3SbS4 solid electrolyte was prepared by PLD to evaluate its potential sodium-ion conductivity. The as-deposited film was observed to be amorphous, whereas a cubic crystalline phase of Na3SbS4 was formed in the thin film after heat treatment at 190°C. Crystallization of the thin film after heat treatment increased ionic conductivity and decreased activation energy. The heat-treated film showed a conductivity of 1.7×10-2 S cm-1 at 25°C and an activation energy of 15 kJ mol-1 for conduction. Further, it was discovered that the Na3SbS4 solid electrolyte has a high conductivity of more than 10-2 S cm-1 at room temperature. The evaluation of the chemical composition of the film suggested that there are some vacancies in sodium site. Acknowledgements: This work was supported by the MEXT program "Elements Strategy Initiative to Form Core Research Center (ESICB)" and JSPS KAKENHI Grant Nos. 18H05255 and 19H05816.

Vanadium diphosphide as a negative electrode material for sodium secondary batteries

The abundance of sodium resources has sparked interest in the development of sodium-ion batteries for large-scale energy storage systems, amplifying the need for high-performance negative electrodes. Although transition metal phosphide electrodes have shown remarkable performance and great versatility for both lithium and sodium batteries, their electrochemical mechanisms in sodium batteries, particularly vanadium phosphides, remain largely elusive. Herein, we delineate the performance of VP2 as a negative electrode alongside ionic liquids in sodium-ion batteries. The polycrystalline VP2 is synthesized via one-step high energy ball-milling and characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Electrochemical tests ascertained improved performance at intermediate temperatures, where the initial cycle was conducted at 100 mA g−1 yielded a significantly higher discharge capacity of 243 mAh g−1 at 90 °C compared to the limited capacity of 49 mAh g−1 at 25 °C. Enhanced rate and cycle performance are also achieved at 90 °C. Electrochemical impedance spectroscopy and scanning electron microscopy further reveal a reduced charge transfer resistance at 90 °C and the formation of a uniform and stable solid electrolyte interface (SEI) layer after cycling. X-ray diffraction and nuclear magnetic resonance spectroscopy are used to confirm the conversion-based mechanism forming Na3P after charging.

Electrolytes toward High‐Voltage Na3V2(PO4)2F3 Positive Electrode Durable against Temperature Variation

AbstractHigh power and energy density, long cyclability, and tolerance for wide temperature (seasonal and daily operational temperature differences) must be considered to construct large‐scale sodium secondary batteries. In this regard, Na3V2(PO4)2F3 (NVPF) has become a subject of interest as a high‐performance positive electrode material owing to its high energy density. However, the high operating voltage of NVPF causes continuous decomposition of electrolytes during cycles, resulting in significant capacity fading and low Coulombic efficiency. In this study, the electrochemical performance of the NVPF electrode in organic solvent electrolytes with and without additives and an ionic liquid is investigated at high voltage regimes over a wide temperature range (−20 °C to 90 °C). The results reveal that the performance of organic electrolytes is still insufficient even with additives, and the ionic liquid electrolyte demonstrates high electrochemical stability and cyclability with NVPF electrodes over a temperature range from −20 °C to 90 °C, achieving stable cycling over 500 cycles. The detailed electrochemical analysis combined with X‐ray photoelectron and energy dispersive X‐ray spectroscopy indicates that a sturdy cathode electrolyte interphase layer around the electrode protects it from capacity fading at high voltage and elevated temperature, resulting in high Coulombic efficiency.

Optimization of the Carbon Content in Copper Phosphide–Carbon Composites for High Performance Sodium Secondary Batteries Using Ionic Liquids

AbstractNegative electrode materials with a large capacity and long cycle life are required for practical large‐scale sodium secondary batteries. Copper phosphide‐carbon composites containing various carbon contents were investigated for this purpose using an ionic liquid (IL) electrolyte. The composites were prepared by a two‐step high energy ball‐milling process: preparation of copper phosphide (CuP2) and composite formation between CuP2 and acetylene black (AB) at different ratios. A CuP2 to Cu3P phase transition was observed at AB contents of 30 and 40 wt%. The sample containing 30 wt% AB gave an optimal electrochemical performance (98.9 % capacity retention, 350 cycles, reversible capacity=358 mAh g−1 at 8000 mA g−1 and 90 °C), and upon cycling at 25 °C, a high capacity retention of 111 % was achieved (350 cycles). Ex situ XRD revealed the reversible conversion of Cu3P and the formation of Na3P during charging. The high performance was attributed to the P−O−C bond and the robust interfacial layer formed in the IL.

Oxalate-Based High-Capacity Conversion Anode for Potassium Storage

Conversion anode materials have been applied in lithium and sodium secondary batteries owing to their high capacities; however, there are limited reports on their use in potassium-ion batteries. He...