Sodium Secondary Batteries Research Articles

Inorganic–Organic Hybrid Ionic Liquid Electrolytes for Na Secondary Batteries

Sodium secondary batteries are attractive energy devices owing to the natural abundance of Na resources. The use of ionic liquid electrolytes can solve the safety problems associated with Na secondary batteries, especially for large-scale applications. The present study investigates the physical and the electrochemical properties of three inorganic–organic hybrid ionic liquid electrolytes for Na secondary batteries (Na[FSA]-[TMHA][FSA], Na[FSA]-[DBDM][FSA], and Na[FSA]-[AS(4.5)][FSA] (TMHA+ = trimethylhexylammonium cation, DBDM+ = dibutyldimethylammonium cation, AS(4.5)+ = 5-azoniaspiro(4.5)nonane cation and FSA− = bis(fluorosulfonyl)amide anion) have been investigated. In all the three systems, the liquid phase is observed at room temperature in a certain range of the Na[FSA] fraction. The ionic conductivities and the viscosities of the three systems obey the Vogel–Tammann–Fulcher equation, and the highest ionic conductivity among the three is observed for the Na[FSA]-[AS(4.5)][FSA] system. Their cathode limit on a Cu plate is Na metal deposition around 0 V vs. Na+/Na and anode limit on a carbon electrode is observed at ∼5 V. The Coulombic efficiency of Na metal electrodeposition/dissolution is improved by elevation of temperature.

Charge-discharge Performance of an Ionic Liquid-based Sodium Secondary Battery in a Wide Temperature Range

Charge-discharge Performance of an Ionic Liquid-based Sodium Secondary Battery in a Wide Temperature Range

A high-capacity TiO2/C negative electrode for sodium secondary batteries with an ionic liquid electrolyte

Carbon-coated TiO2 (TiO2/C) nanopowders were synthesized as a negative electrode material for sodium secondary batteries. The TiO2/C negative electrode delivers a high reversible discharge capacity of 275 mA h g−1 at 10 mA g−1 at 363 K.

Improved Electrochemical Performance of NaVOPO4 Positive Electrodes at Elevated Temperature in an Ionic Liquid Electrolyte

Sodium secondary batteries operating in a wide temperature range are attractive as large-scale energy storage devices, and ionic liquid electrolytes are suitable for this purpose. In this study, NaVOPO4 has been investigated as positive electrode material for Na secondary batteries, and its electrochemical performance has been examined in the Na[FSA]–[C3C1pyrr][FSA] ionic liquid (C3C1pyrr = N-methyl-N-propylpyrrolidinium and FSA = bis(fluorosulfonyl)amide) at 298 and 363 K. The NaVOPO4 electrode exhibits a reversible capacity of 60 and 101 mAh g−1 at 298 and 363 K, respectively. Acceptably good rate capability is achieved at 363 K, as 76% of the maximum capacity is maintained at 5 C rate. Cyclability tests prove good reversibility of the material, in which 74% of the initial specific capacity maintains over 300 cycles at 363 K. XRD measurements reveal that the charge–discharge process of NaVOPO4 involves a single-phase reaction. Galvanostatic intermittent titration technique (GITT) analysis highlights a 3–5-fold increase of the apparent Na chemical diffusion coefficient in NaVOPO4 upon increasing the temperature from 298 to 363 K, which is reflected in the superior electrochemical performance at 363 K than at 298 K.

Inorganic-Organic Hybrid Ionic Liquid Electrolytes for Na Secondary Batteries

Sodium secondary batteries are attracting more and more attraction as energy storage devices for stationary use and electric vehicles due to high abundance and low cost of sodium resources as well as the low redox potential of Na+/Na. Ionic liquid electrolytes are attractive to improve the safety issues of sodium secondary batteries, owing to their unique properties such as low flammability and low vapor pressure [1-3]. Our recent works suggest FSA- (FSA- = bis(fluorosulfonyl)amide anion) based ionic liquids, such as Na[FSA]-K[FSA] and Na[FSA]-[C3C1pyrr][FSA] (C3C1pyrr+ = N-propyl-N-methylpyrrolidinium cation) systems, exhibit good physical and electrochemical properties as electrolytes for Na secondary batteries [4-6]. This study will present a new series of inorganic-organic hybrid ionic liquids as electrolytes for Na secondary batteries. Four FSA salts of organic cations, N6111 + (hexyltrimethylammonium), N4422 + (dibutyldimethylammonium), AS[4.5]+ (azoniaspiro[4.5]nonane), and C2C1im+ (1-ethyl-3-methylimidazolium), are mixed with Na[FSA]. In all the cases, a wide liquid temperature range around room temperature was observed in a certain x(Na[FSA]) range. Ionic conductivity and viscosity increase and decrease with decreasing x(Na[FSA]), respectively. The highest ionic conductivity was observed for the Na[FSA]-[C2C1im][FSA] system. Electrochemical windows are around 5 V and Na metal deposition and dissolution are observed at the cathode limit. The efficiency of Na metal deposition and dissolution is improved by increasing temperature due to suppression of dendritic Na metal deposition.[1] L.G. Chagas, D. Buchholz, L.M. Wu, B. Vortmann, S. Passerini, J. Power Sources, 247 (2014) 377-383.[2] S.A.M. Noor, P.C. Howlett, D.R. MacFarlane, M. Forsyth, Electrochim. Acta, 114 (2013) 766-771.[3] D. Monti, E. Jonsson, M.R. Palacin, P. Johansson, J. Power Sources, 245 (2014) 630-636.[4] C. Ding, T. Nohira, K. Kuroda, R. Hagiwara, A. Fukunaga, S. Sakai, K. Nitta, S. Inazawa, J. Power Sources, 238 (2013) 296-300.[5] T. Yamamoto, T. Nohira, R. Hagiwara, A. Fukunaga, S. Sakai, K. Nitta, S. Inazawa, J. Power Sources, 217 (2012) 479-484.[6] C.Y. Chen, K. Matsumoto, T. Nohira, R. Hagiwara, A. Fukunaga, S. Sakai, K. Nitta, S. Inazawa, J. Power Sources, 237 (2013) 52-57.

Tin-Based Negative Electrodes for Sodium Secondary Battery Using Na[FSA]-K[FSA] Ionic Liquid Electrolyte

Sodium secondary battery is promising as a large-scaled energy storage device owing to an abundant reserve of sodium and its large energy density. Our group has developed a new class of electrolyte for a sodium secondary battery: Na[FSA]–K[FSA] (FSA: bis(fluorosulfonyl)amide) ionic liquid with a wide electrochemical window (ca. 5 V) and practically low melting point (334 K) [1,2]. We have already reported that a Na/Na[FSA]–K[FSA]/NaCrO2 cell exhibits excellent cyclability and rate capability at 353–363 K [2,3]. However, a sodium metal negative electrode has its potential risk of dendritic sodium formation. Thus, the authors focused on tin as a new negative electrode material that has high theoretical capacity (847 mAh (g-Sn)–1) and safety [4,5]. In our previous study, we investigated the sodiation/desodiation process of tin thin films (ca. 10 µm thickness) electrodeposited on an Al foil at 363 K. Fig. 1 shows the initial charge–discharge curve of the film [4]. Three plateaus (a–c) were observed both during charging and discharging process in the 1st cycle. Each plateau corresponds to coexisting state of two Na–Sn alloy phases; (a)β-Sn/α-NaSn, (b)α-NaSn/Na9Sn4, (c)Na9Sn4/Na15Sn4. Although the initial charge (sodiation) and discharge (desodiation) capacities were 796 and 729 mAh (g-Sn)–1, respectively, the reversible capacity rapidly dropped below 100 mAh (g-Sn)–1 within 20 cycles, due to the large volume changes during sodiation/desodiation process (ΔV = 430% for Na15Sn4). In the present study, we used thinner tin films (ca. 1 µm thickness) electrodeposited on Cu foils for the improvement of its cyclability. Sn–Cu alloys inactive to sodium are expected to be formed at the Sn/Cu interfaces by the utilization of Cu foils as a current collector. These alloys simultaneously work as a buffer for the significant volume change and suppress the pulverization of the β-Sn layer reacted with sodium. Therefore, we fabricate various Sn–Cu films by annealing tin-coated copper foils at 463 K for 0–45 h. Fig. 2 shows the cyclability of the negative electrode made of annealed Sn–Cu thin films. Sn–Cu films without annealing (0h-annealed film) showed an initial reversible capacity of ca. 400 mAh (g-Sn)–1, equivalent to a half of the theoretical capacity of tin. This is originated from the low reactivity of Sn–Cu alloys to sodium. The films annealed for 10, 25 and 45h exhibited no β-Sn phase by XRD and much lower capacities than the other films. The best capacity retention was observed for the films annealed for 4 h. The film retained its reversible capacity of approximately 100 mAh (g-Sn)–1 for 100 cycles. This good cyclability would be attributed to the best mixed structure of active β-Sn and less active Sn–Cu alloys. Acknowledgments This study was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan Science and Technology Agency (JST), and MEXT program "Elements Strategy Initiative to Form Core Research Center" (since 2012), MEXT; Ministry of Education Culture, Sports, Science and Technology, Japan.

Na[FSA]-[C3C1pyrr][FSA] ionic liquids as electrolytes for sodium secondary batteries: Effects of Na ion concentration and operation temperature

As electrolytes for sodium secondary batteries operating over a wide temperature range, Na[FSA]-[C3C1pyrr][FSA] (FSA = bis(fluorosulfonyl)amide, C3C1pyrr = N-methyl-N-propylpyrrolidinium) ionic liquids have been investigated. The effects of Na ion concentration (0–60 mol% Na[FSA]) and operation temperature (253–363 K) on the viscosity and ionic conductivity and charge–discharge performance of Na/Na[FSA]-[C3C1pyrr][FSA]/NaCrO2 cells are studied. Results show that Na ion concentration strongly affects the rate capability of the cells, and that the best rate capability at 363 K is obtained at 40 mol% Na[FSA]. The operation temperature also significantly influences the charge–discharge performance, especially at low temperatures. At operation temperatures below 273 K, 25 mol% Na[FSA] is found to be the optimum Na ion concentration. There exist different optimum ranges of Na ion concentration depending on the operation temperatures.

Room Temperature Sodium-Sulfur Batteries with Glyme-Na Salt Solvate Ionic Liquid Electrolytes

Room temperature ionic liquids (RTILs) are desirable electrolyte materials for advanced batteries because of their unique properties such as non-volatility, non-flammability and high thermal and electrochemical stability. We have reported that glyme-Li salt complexes, comprising equimolar mixtures of a glyme (CH3O(CH2CH2O) n CH3) and a Li salt, have similar properties to RTILs and classified as solvate ionic liquids. Certain glyme-Li salt complexes such as lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) and tetraglyme (G4) or triglyme (G3) are liquid at room temperature and show high thermal stability, high lithium ion transference number, and high lithium ion concentration. We demonstrated reversible charge-discharge of lithium secondary batteries using glyme-Li salt complex as an electrolyte. Recently, the similar results were obtained with equimolar mixtures of pentaglyme (G5) and Na[TFSA] abbreviated as [Na(G5)1][TFSA] was able to use as an electrolyte of sodium secondary battery. Elemental sulfur has been studied as the post-lithium ion battery cathode material for its high theoretical capacity of 1672 mAh g-1. However, the major drawback of this material is the poor cycle stability because of dissolution of lithium polysulfides into typical organic electrolyte. One facile approach to overcome this problem is trapping the sulfur inside a polymer network such as poly(acrylonitrile) (PAN) to prevent the dissolution. By mixing PAN and excess amount of sulfur and heating at around 300 °C, cyclization and dehydrogenation of PAN is promoted by sulfur and results in sulfur contained p-conjugated polymer (PAN-S). The PAN-S has been studied as cathode active material with high specific capacity, good efficiency and cycling stability. However, most reports of PAN-S dealt with lithium batteries and not so many reports have been done in PAN-S sodium system. In this study, we tested the charge-discharge property of PAN-S composite cathode with [Na(G5)1][TFSA] electrolyte at 30 °C. The PAN-S was synthesized by mixing PAN and elemental surfur in 1:4 weight ratio and heated at 350 °C for 6 hours under Ar atmosphere. The composite cathode was fabricated by mixing PAN-S, AB, PVA in the weight ratio of 70:20:10 with NMP used as homogenizer to obtain slurry and pasted onto Al sheet. The electrolyte was prepared by mixing pentaglyme (G5) and sodium bis(trifluoromethylsulfonyl)amide (Na[TFSA]) in 1:1 molar ratio ([Na(G5)1][TFSA]) and mixed with a hydrofluoroether (HFE) in 1:4 molar ratio in order to improve ionic conductivity. Anode material Na15Sn4was prepared by mixing sodium metal and Sn powder in 15:4 molar ratio by ball mill for 5 hours, 510 rpm. The result of charge discharge test is shown in the figure. The initial discharge capacity was about 500 mA h g(PANS) -1 and charge-discharge capacity after that was around 400 mA h g(PANS) -1with excellent coulombic efficiency for 30 cycles. Although there are capacity decrease as the cycle number increase, the cycle stability is much improved compared to that of elemental sulfur composite cathode and sodium metal anode system.ReferencesT.Tamura et al., Chem. Lett., 2010, 39, 753.K.Yoshida et al.,J. Am. Chem. Soc., 2011, 133, 13121.X. Yu et al., J. Electroanal. Chem., 2004, 573,121-128

Na2MnSiO4 as a positive electrode material for sodium secondary batteries using an ionic liquid electrolyte

Sodium manganese orthosilicate, Na2MnSiO4, was synthesized by a sol–gel method and investigated for use as a positive electrode material for Na secondary batteries using Na[FSA]–[C3C1pyrr][FSA] (FSA=bis(fluorosulfonyl)amide anion and C3C1pyrr=N-methyl-N-propylpyrrolidinium cation) ionic liquid electrolyte in the temperature range 298–363K. Carbon coating and elevation of operational temperatures significantly improved the electrode performance. A reversible capacity of 125mAhg−1 (90% of the theoretical value based on a one-electron transfer process) was found at a rate of C/10 (13.9mAg−1) within 2.0–4.0V at 363K, with an acceptably high rate capability. Na2MnSiO4 became amorphous upon the electrochemical removal of sodium, exhibiting a similar behavior as its lithium equivalent. Both Na2MnSiO4 and its desodiated form (Na0.8MnSiO4) possess remarkable thermal stability, suggesting their safety characteristic.

Improved cyclability of Sn–Cu film electrode for sodium secondary battery using inorganic ionic liquid electrolyte

Thin films of Sn–Cu alloys with various compositions were prepared by annealing tin-coated copper foils at 463K for 0–45h. Their charge–discharge behaviors as negative electrodes for sodium secondary batteries were investigated using an inorganic ionic liquid electrolyte, i.e., NaFSA–KFSA (FSA=bis(fluorosulfonyl)amide), at 363K. A Sn–Cu film annealed for 4h exhibited a stable reversible capacity of approximately 100 mAh (g-Sn)−1. The improvement in the cyclability is discussed based on X-ray diffraction, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy results.

The Na[FSA]–[C2C1im][FSA] (C2C1im+:1-ethyl-3-methylimidazolium and FSA−:bis(fluorosulfonyl)amide) ionic liquid electrolytes for sodium secondary batteries

Physical and electrochemical properties of the Na[FSA]–[C2C1im][FSA] (C2C1im+:1-ethyl-3-methylimidazolium and FSA−:bis(fluorosulfonyl)amide) ionic liquids have been investigated in view of their application as electrolytes for sodium secondary batteries operating in a wide temperature range. The Na[FSA]–[C2C1im][FSA] ionic liquids in the range of 0.0 ≤ x(Na[FSA]) ≤ 0.5 are in the liquid state at room temperature, where x(Na[FSA]) is the mole fraction of Na[FSA]. In the case of x(Na[FSA]) = 0.3, the ionic conductivity, viscosity, and electrochemical window at 298 K are 5.4 mS cm−1, 78 mPa s, and 5.1 V, respectively. Sodium metal deposition/dissolution test in the ionic liquid at x(Na[FSA]) = 0.3 resulted in average cycle efficiencies of 69% and 96% at 298 K and 363 K, respectively, at a current density of 1.0 mA cm−2.

Characterization of Electrochemical Cycling-Induced New Products of NaCuO2 Cathode Material for Sodium Secondary Batteries

Using a Na/NaCuO2 cell, we investigate the conversion of NaCuO2 during charge and discharge reactions and the new products formed by the conversion. In the voltage range of 0.75 to 3.0 V, the results of ex-situ XRD analysis indicate that Na2CuO2, an unstable amorphous discharge product, converted into CuO and Cu2O. Moreover, an XPS analysis reveals Na2O is formed on the surface of a NaCuO2 electrode. From 1.7 to 4.2 V, on the other hand, the first charge product, Na1-xCuO2, should partially form CuO. This behavior is similar to the reaction in which the charge of a Li/LiCuO2 cell forms CuO. Then, after the discharge, CuO and Cu2O are observed as the main components in XRD patterns of the electrode. NaCuO2 phase appeared again after the subsequent charge. NaCuO2 is gradually converted into CuO as the main component as the cycles proceed. The cycling-induced new products of NaCuO2 change, depends on the voltage ranges.

Electrochemical Properties and Structural Changes of NaCuO2 for Sodium Secondary Batteries

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Properties of sodium-based ionic liquid electrolytes for sodium secondary battery applications

Abstract The enormous demands on available global lithium resources have raised concerns about the sustainability of the supply of lithium. Sodium secondary batteries have emerged as promising alternatives to lithium batteries. We describe here sodium bis(trifluoromethylsulfonyl) amide (NaNTf 2 ) electrolyte systems based on 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (C 4 mpyrNTf 2 ) ionic liquids. The electrochemical stability of the system was examined; a pair of facile cathodic and anodic processes around 0 V vs. Na/Na + were observed in cyclic voltammetry measurements and interpreted as deposition and dissolution of sodium metal. Density, viscosity and conductivity of the electrolytes were studied. It was found that the ionic conductivity of electrolytes reached as high as 8 mS/cm, decreasing slowly as the salt content increased due to increasing of viscosity and density of the electrolyte. Therefore, sodium electrolytes based on C 4 mpyrNTf 2 appear to be promising for secondary sodium battery applications.

Discharge/charge reaction mechanism of a pyrite-type FeS2 cathode for sodium secondary batteries

The structure changes of iron disulfide (FeS2) during discharge/charge process were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and synchrotron-based X-ray adsorption near the edge structure (XANES). Although the cyclability of the FeS2/Na battery is poor, FeS2/Na battery has a large first discharge capacity of 758 mA h g−1. It corresponds to 85% of the theoretical capacity based on the 4.0 Na reaction per FeS2 (893 mA h g−1). According to the Fe K-edge XANES spectra of discharged FeS2 pellets, there were no clear traces of iron valence change. However, the clear S K-edge jump shifted to lower energy with increasing Na content in FeS2. This is because the binding energy of S 1s electrons decrease as the oxidation number of S decreases, suggesting that the charge neutrality of the cathode after 2.0 Na discharge is kept by an S valence change from (S–S)2− to S2−. Moreover, Na2S diffraction peaks were detected at the 3.0 Na discharged state as a trace of Na conversion reaction in the FeS2 cathode.

Pyrophosphate Na2FeP2O7 as a low-cost and high-performance positive electrode material for sodium secondary batteries utilizing an inorganic ionic liquid

The electrochemical performance of a Na2FeP2O7 positive electrode has been evaluated in an inorganic ionic liquid NaFSA–KFSA (FSA = bis(fluorosulfonyl)amide) at 363 K. The electrode delivers a reversible capacity of 91 mAh g−1 with excellent rate capability (59 mAh g−1 at 2000 mA g−1) and a capacity retention of 91% over 1000 cycles, which facilitates the development of low-cost and high-safety sodium secondary batteries for large-scale energy storage applications. The average oxidation state of iron increases upon sodium extraction, as evidenced by the edge shift of an X-ray absorption spectroscopy analysis. According to an extended X-ray absorption fine structure analysis, the sodium extraction is accompanied by a shortening of Fe–O bonds.

A safe and high-rate negative electrode for sodium-ion batteries: Hard carbon in NaFSA-C1C3pyrFSA ionic liquid at 363 K

Charge–discharge properties of hard carbon (HC) for sodium secondary batteries using NaFSA-C1C3pyrFSA (FSA: bis(fluorosulfonyl)amide, C1C3pyr: N-methyl-N-propylpyrrolidinium) ionic liquids were investigated. A Na/NaFSA-C1C3FSA/HC cell showed reversible capacity of 260 mAh (g-HC)−1 at a constant rate of 50 mA (g-HC)−1 at 363 K, maintaining cell capacities of approximately 95.5% of the initial capacity after 50 cycles. Coulombic efficiency was higher than 99.5% except for the initial few cycles, reaching 99.9% after 40 cycles. The Na/NaFSA-C1C3pyrFSA/HC cell also exhibited a high rate capability of 211 mAh (g-HC)−1 at a rate of 1000 mA (g-HC)−1, which corresponds to a charge rate of approximately 4C.

NaFSA–C1C3pyrFSA ionic liquids for sodium secondary battery operating over a wide temperature range

The physicochemical properties of NaFSA–C1C3pyrFSA ionic liquids were investigated to explore their potential as new electrolytes for sodium secondary batteries operating over a wide temperature range. The viscosity and ionic conductivity of NaFSA–C1C3pyrFSA with a molar ratio of 2:8 were 16.7 cP and 15.6 mS cm−1, respectively, at 353 K. The electrochemical window of this ionic liquid was 5.2 V at 353 K with sodium metal deposition determining the cathode limit. A Na/NaFSA–C1C3pyrFSA/NaCrO2 cell exhibited stable charge–discharge behaviour at 298 and 353 K. The discharge capacities of 92 and 106 mAh (g-NaCrO2)−1 were observed at 298 and 353 K, respectively, at 20 mA g−1. The Coulombic efficiency was higher than 99% during the charge–discharge tests except for the initial few cycles. The cell also showed high discharge rates, reaching 500 mA g−1 at 353 K.

Electrochemical and structural investigation of NaCrO2 as a positive electrode for sodium secondary battery using inorganic ionic liquid NaFSA–KFSA

The electrochemical performance of NaCrO2 as a positive electrode material for an intermediate-temperature sodium secondary battery was evaluated in an inorganic ionic liquid, NaFSA–KFSA (FSA = bis(fluorosulfonyl)amide), at 363 K. The positive electrode using NaCrO2, acetylene black, and polytetrafluoroethylene exhibited a stable discharge capacity of 113 mA h g−1 at a current density of 125 mA g−1. The Coulombic efficiency and the capacity retention at the 100th cycle were 99.6% and 98.5%, respectively. Even at the very high current density of 2000 mA g−1, the discharge capacity was maintained at 63 mA h g−1. X-ray diffraction analyses revealed that the deintercalation of Na+ ion from NaCrO2 was associated with several phase transitions in the following sequence: rhombohedral O3, monoclinic O′3, and monoclinic P′3.

Charge–discharge behavior of tin negative electrode for a sodium secondary battery using intermediate temperature ionic liquid sodium bis(fluorosulfonyl)amide–potassium bis(fluorosulfonyl)amide

The charge–discharge behavior of a Sn negative electrode for use in a sodium secondary battery was investigated in an intermediate temperature ionic liquid, NaFSA–KFSA (xNaFSA = 0.56, xKFSA = 0.44, FSA = bis(fluorosulfonyl)amides), at 363 K. A Na/Sn half-cell was prepared using a Sn-film (10–12 μmt) electrode. The charge–discharge curves at 0.619 mA cm−2 with cut-off voltages of 0.005 and 1.200 V exhibited three potential plateaus in both the charging (alloying) and discharging (de-alloying) processes, indicating the formation of various alloy phases. Charge and discharge capacities as high as 790 and 729 mA h (g-Sn)−1, respectively, were obtained for the 1st cycle; however, the capacities decreased rapidly due to volume change during the alloying and de-alloying processes. The best cycleability was achieved when the lower and higher cut-off voltages were set to 0.005 and 0.200 V, respectively. In this case, a reversible capacity of about 300 mA h (g-Sn)−1 was obtained for the initial 15 cycles.