Sodium Metal Electrodes Research Articles

Reversible Sodium Metal Electrodes: Is Fluorine an Essential Interphasial Component?

Alkaline metals are an ideal negative electrode for rechargeable batteries. Forming a fluorine‐rich interphase by a fluorinated electrolyte is recognized as key to utilizing lithium metal electrodes, and the same strategy is being applied to sodium metal electrodes. However, their reversible plating/stripping reactions have yet to be achieved. Herein, we report a contrary concept of fluorine‐free electrolytes for sodium metal batteries. A sodium tetraphenylborate/monoglyme electrolyte enables reversible sodium plating/stripping at an average Coulombic efficiency of 99.85 % over 300 cycles. Importantly, the interphase is composed mainly of carbon, oxygen, and sodium elements with a negligible presence of fluorine, but it has both high stability and extremely low resistance. This work suggests a new direction for stabilizing sodium metal electrodes via fluorine‐free interphases.

Anion amphiprotic ionic liquids as protic electrolyte matrices allowing sodium metal plating.

The sodium-ion battery (SIB) is proposed as a complementary technology to today's commercially dominant lithium-ion battery (LIB). While much know-how can be transferred from LIBs to SIBs, adjustments are still necessary, not the least for the electrolytes employed. Here the use of anion amphiprotic ionic liquid (AAIL) based electrolytes is proposed for SIB application. Two different AAILs, based on organic trifluoromethylsulfonylamide (TFSAm) and inorganic HSO4- anions, respectively, doped with NaTFSI salt have been studied, focusing on electrochemical stability and transport properties, complemented by studies of the ion-ion interactions, and final sodium-ion battery performance via stripping/plating vs. sodium metal electrodes.

On the Reliability of Sodium Metal Anodes: The Influence of Neglected Parameters

Sodium metal is an auspicious, sustainable alternative to be used in next generation rechargeable batteries. During the past years, a considerable effort was put into finding suitable chemistries, electrode designs, cell setups, and test configurations to enable the safe and efficient use of sodium metal electrodes. With the hunt for ever-increasing coulombic efficiencies and dendrite suppression, the understanding of basic influencing factors is thereby often neglected. Here, we show that not only the basic and so far most studied cell elements, namely electrolyte, separator and substrate, have an impact on the performance of sodium metal anodes. Smallest variations in the amount of electrolyte, current density, cell architecture, and electrode preparation, which are often overlooked, are shown to make the difference between instant cell failure and stable long-term cycling. By optimizing all discussed parameters, we apply the findings and show that stable cycling for more than 1.000 cycles can be achieved with a commercially available electrolyte, if seemingly unimportant factors are considered.

Improving the Cycling Stability of Sodium Metal Electrode By Making an Sodium-Carbon Composite Electrode

The use of metallic anodes such as lithium and sodium has attracted large attention in the battery field because of their extremely high specific capacity and low operation potential. Indeed, the formation of metal dendrite upon repeated electrochemical plating/stripping cycling was early identified as obstacle of the development of safe metal anode secondary batteries. Also, due to dendrite formation, the low cycle efficiency and unstable cycle life happen, which hinder their practical application as anode materials. Therefore, to suppress the growth of metal dendrite is one of the most critical issues from a practical view point. Recently, lithium metal has been extensively researched as anode candidate for secondary batteries due to its lowest redox potential and highest specific capacity among metallic anodes. Despite these promising properties, lithium sources are relatively limited and unevenly distributed across the globe, which lead to high costs of lithium metal batteries. Therefore, inevitably, lithium-based batteries will become unaffordable and large-scale production will falter. Therefore, sodium (Na) is one of the promising alternatives to lithium for energy storage technologies because of its relatively high theoretical specific capacity of 1166 mAh g-1 and natural abundance. Also, it has comparable redox potential, that is -2.70 V vs. SHE. However, like lithium metal anode, inhomogeneous electrochemical deposition of Na is a crucial problem that have to be solved for the usage of sodium metal as anode safely. Furthermore, Na metal has extremely high reactivity with liquid organic electrolyte, so high interfacial resistance could be induced due to continuous electrolyte decomposition during battery cycling, which also leads to earlier cell failure. In this research, we present a stabilized sodium electrode by introducing electrically connected carbon network (ECN) to a bulk Na electrode with a simple folding & rolling method. As a results of introducing ECN, we observed that a cell overpotential was decreased as the amount of ECN inside bulk Na metal was increased. Also, we found that the cycling stability of Na/Na symmetric cell with ECN was almost 4.5 times increased compared to bare Na/Na cell, which is consistent with the results of EIS during cycling. By using SEM and CA analysis, we confirmed that an introduced ECN could act as not only an electrical network but also artificial uniform seed for Na nucleation.

Thin film manganese oxide polymorphs as anode for sodium-ion batteries: An electrochemical and DFT based study

In the present work, we have studied phase formation behavior and electrochemical characteristics of various polymorphs of Manganese oxides in thin films forms deposited on stainless substrate by pulsed laser deposition. Investigation of these thin films using x-ray diffraction, X-ray photo electron spectroscopy and Raman spectroscopy reveals that the growth of various polymorphs of MnO2 and their mixture depend on the substrate temperatures. The films deposited at 300, 400, and 450 °C contains a mixture of γ and β, β and R, β and R MnO2 phases respectively, Whereas, film deposited at 550 and 650 °C shows a mixture of R-MnO2 and α-Mn2O3. The film containing β- and R-MnO2 phases deposited at 450 °C shows optimum electrochemical property against sodium metal electrode. The density functional theory based analysis shows that MnO2 polymorphs can accommodate sodium ions by intercalation mechanism in the ratio of 2:1 of Na and Mn, followed by a conversion mechanism into Mn metal and Na2O. Collectively, electrochemical and first principle based calculations suggests mixture of β- and R-MnO2 is suitable as anode material.

Enhancing the Cycling Stability of Sodium Metal Electrodes by Building an Inorganic-Organic Composite Protective Layer.

Owing to the natural abundance of sodium resources and their low price, next-generation batteries employing an Na metal anode, such as Na-O2 and Na-S systems, have attracted a great deal of interest. However, the poor reversibility of an Na metal electrode during repeated electrochemical plating and stripping is a major obstacle to realizing rechargeable sodium metal batteries. It mainly originates from Na dendrite formation and exhaustive electrolyte decomposition due to the high reactivity of Na metal. Herein, we report a free-standing composite protective layer (FCPL) for enhancing the reversibility of an Na metal electrode by mechanically suppressing Na dendritic growth and mitigating the electrolyte decomposition. A systematic variation of the liquid electrolyte uptake of FCPL verifies the existence of a critical shear modulus for suppressing Na dendrite growth, being in good agreement with a linear elastic theory, and emphasizes the importance of the ionic conductivity of FCPL for attaining uniform Na plating and stripping. The Na-Na symmetric cell with an optimized FCPL exhibits a cycle life two times longer than that of a bare Na electrode.

Identification and Quantification of the Main Electrolyte Decomposition By-product in Na-Ion Batteries through FEC: Towards an Improvement of Safety and Lifetime

Electrolyte is a crucial point for the safety and the lifetime of sodium-ion batteries (NIBs). The electrolyte decomposition must be avoided in order to guarantee NIB development in the future. In this work, the responsibility of the well-known by-product ethylene glycol bis-(methyl carbonate) (EGMC) in creating overcharging is discussed. A liquid GC/MS technique was used to detect EGMC in 1 M NaPF6 in EC:DMC (1:1) as the electrolyte in a half cell configuration, i.e with sodium metal as a counter electrode. The lack of reproducible electrochemical results due to random overcharges is linked to the presence of EGMC. It is demonstrated that FEC (fluoroethylene carbonate) additive could increase SEI stability on the sodium metal electrode and prevent EGMC formation. The strong impact of this by-product has been observed in the case of two types of positive electrode materials: a layered oxide and a fluorophosphate. Moreover, an optimization of the FEC quantity was studied, depending on the electrode material and the cell design.

Interfacial Reactivity Benchmarking of the Sodium Ion Conductors Na3PS4 and Sodium β-Alumina for Protected Sodium Metal Anodes and Sodium All-Solid-State Batteries.

The interfacial stability of solid electrolytes at the electrodes is crucial for an application of all-solid-state batteries and protected electrodes. For instance, undesired reactions between sodium metal electrodes and the solid electrolyte form charge transfer hindering interphases. Due to the resulting large interfacial resistance, the charge transfer kinetics are altered and the overvoltage increases, making the interfacial stability of electrolytes the limiting factor in these systems. Driven by the promising ionic conductivities of Na3PS4, here we explore the stability and viability of Na3PS4 as a solid electrolyte against metallic Na and compare it to that of Na-β″-Al2O3 (sodium β-alumina). As expected, Na-β″-Al2O3 is stable against sodium, whereas Na3PS4 decomposes with an increasing overall resistance, making Na-β″-Al2O3 the electrolyte of choice for protected sodium anodes and all-solid-state batteries.

The Performance of Hard Carbon in a Sodium Ion Battery and Influence of the Sodium Metal in Observed Properties

This investigation includes the electrochemical testing protocols for a hard carbon sodium ion anode material, and discusses the use of a sodium metal counter electrode with respect to the material and cell stability during the electrochemical testing. A 3-electrode cell in compression fitting (‘Swagelok’) configuration was used with hard carbon as the working electrode and sodium metal as the counter and reference electrode. The cells were charged and discharged at high rates to investigate the polarisation effects of the sodium metal. The results indicate a significant contribution from the sodium metal electrode to the observed cell characteristics, depending on state-of-charge and rate. These contributions are significant and may lead to misinterpretation of the material and electrode performance when tested in 2-electrode arrangements with a sodium metal counter electrode.

Sb2O3 Microstructures As Efficient Anode Material for Sodium-Ion Batteries

With the rapid growth in various advanced technologies such as microelectronics, development of high energy and high power energy storage devices is very essential to power portable electronic devices and the the demand is even more pressing to meet the higher end energy needs for vehicular and large-scale grid storage applications. Lithium ion batteries continue to serve as an excellent choice amongst electrochemical energy storage devices for several decades. However, because of the low abundance and increasing demand for lithium metal, a competent alternative for lithium-ion batteries is being sought after, for quite some time now. Among these alternatives, sodium ion battery (SIB) turns out to be a wise choice, owing to the high abundance of sodium on Earth’s crust, suitable redox properties and similar electrochemical behaviour as that of lithium.[1] Though research on SIBs have started in 1970’s, increasing interest on these systems gained attention over the last few years. Several novel cathode materials have been studied for SIBs in recent years,[2] however, development of efficient anode materials remain a big challenge for the successful implementation of Na-ion battery technology. In the continued search for anodes for SIBs, metal and metal-oxide materials are of great interest because of their potential to deliver high, and stable specific capacities. In particular, antimony-based materials (Sb, Sb2O3) have attracted recent attention due to their high theoretical capacities (660 mAh/g for Sb)[3]. Antimony trioxide (Sb2O3), compared to other metal oxides is unique in such a way that the typical volume expansion issue occurring due to sodiation, gets addressed by the formation of a Na2O ‘buffer matrix’ during the coupled conversion-alloying reaction[4]. Herein, we have synthesized Sb2O3 microstructures that exhibits a unique star-shaped morphology, through hydrothermal method. As-synthesized Sb2O3 has been characterized by using powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Detailed studies on its electrochemical performance as anode material for sodium ion batteries have been carried out by using cyclic voltammetry (CV), galvanostatic charge/discharge cycling and electrochemical impedance spectroscopy (EIS) vs. sodium metal electrode in a standard electrolyte of 1 M NaClO4 in propylene carbonate (PC). The as-synthesized Sb2O3 electrode delivered a stable specific capacity of 510 mAh/g for 30 cycles, at a current rate of 50 mA/g. In order to enhance both the specific capacity and the cycling stability, a graphene network that completely encapsulates Sb2O3 microstructures, forming a 3D interconnected network around it, has also been synthesized and tested as anode for sodium-ion batteries. The composite electrode of graphene-Sb2O3 hydrogel, prepared through simple hydrothermal route, showed excellent rate performance owing to the synergistic effects of highly conducting and interconnected graphene networks along with the pristine high capacity Sb2O3microstructures. The results are discussed and the key challenges will be addressed. The whole approach appears to be quite promising towards fabricating high performance anodes for Na-ion batteries.

Superconcentrated Electrolytes for Sodium-Ion Batteries

Sodium-ion batteries are receiving intense attention as an inexpensive yet high-performance rechargeable battery to possibly replace current lithium-ion chemistries in future. A myriad of positive and negative electrode materials have so far been reported for sodium-ion batteries and, interestingly, some of them exhibit superior performance (e.g., voltage, capacity, kinetics, or cycleability) than state-of-the-art electrodes for lithium-ion batteries. On the other hand, there are only a few studies on liquid electrolyte design for sodium-ion batteries. In particular, high reductive/oxidative stability is strongly required for electrolyte materials to achieve prolonged charge-discharge cycling of hard carbon anodes and high-voltage cathodes. Recently, superconcentrated electrolytes are being recognized as a new class of highly stable electrolytes for lithium-based batteries.1-3 In the present work, we report basic physicochemical and electrochemical properties of sodium salt-based superconcentrated electrolytes. First, we study the effects of cations, anions, and solvents on the ionic conductivity and viscosity of electrolytes at various concentrations. Based on the results, important factors to dominate the physicochemical properties are discussed. Next, we study charge-discharge behavior of hard carbon and sodium metal electrodes in sodium salt-based superconcentrated electrolytes. We have found much better cycleability of the electrodes in superconcentrated electrolytes than in conventional dilute electrolytes, suggesting that the sodium salt-based superconcentrated electrolytes have enhanced reductive stability. 1. K. Yoshida et al., J. Am. Chem. Soc., 133, 13121 (2011). 2. Y. Yamada et al., J. Am. Chem. Soc., 136, 5039 (2014). 3. Y. Yamada et al., J. Electrochem. Soc., 162, A2406 (2015).

The Performance of Hard Carbon in a Sodium Ion Battery and Influence of the Sodium Metal in Observed Properties

Since the 1990’s and the commercialisation of the first lithium ion cell by Sony there has been a large focus on new materials for lithium ion batteries, and the work in the 1970’s and 80’s in room temperature sodium ion materials was lapsed. However, due to the high current cost of lithium ion batteries, alternative sodium ion technologies may offer a cost and safety advantage over the typical lithium ion cells.1,2 Sodium is a much more abundant metal when compared to lithium which should result in lower cost materials. Although the mass of sodium is higher than lithium, the observed specific capacity of a sodium ion battery compared to lithium is not hugely compromised; this is because the proportion of the sodium and lithium content is small enough that the difference is minimal. Sharp Laboratories of Europe Ltd have been developing a new sodium ion cell chemistry based upon a layered oxide cathode3 and a hard carbon anode. The hard carbon anode is extremely sensitive to processing conditions and testing methods. In two electrode cells we have observed high hysteresis in the charge and discharge profiles and have experienced difficulties in reaching the low voltages required for complete sodiation of the hard carbon. In addition we have also observed high Coulombic inefficiencies during the testing in a sodium metal anode cell, and the instability of the sodium in certain solvents. By using 3-electrode cells we have been able to investigate the specific properties of the hard carbon in considerably more detail, and note that much of the hysteresis observed in two electrode measurements originates from the sodium metal anode. 3-electrode galvanic and potenstiostatic intermittent titration techniques have been used to investigate the properties of the hard carbon and sodium metal electrodes in more detail, and much of the hysteresis with regards to the performance of the carbon material in a 2-electrode cell can be shown to be related to the sodium metal deposition and stripping. Figure 1 shows the galvanic intermittent titration technique (G.I.T.T.) for hard carbon vs sodium metal in a 3-electrode arrangement visualising the difference between cell potential (red) and potential of the working electrode vs sodium reference (hard carbon, blue). V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez and T. Rojo, Energy Environ. Sci. 2012, 5, 588,J. Barker, M.Y. Saidi and J. Swoyer, Electrochem. Solid-State Chem. 2003, 6, A1,E. Kendrick, R. Gruar et al, Tin containing compounds, WO2015177568A1, 2015-11-26 Figure 1

Discharge properties of all-solid sodium–sulfur battery using poly (ethylene oxide) electrolyte

An all-solid sodium/sulfur battery using poly (ethylene oxide) (PEO) polymer electrolyte are prepared and tested at 90 °C. Each battery is composed of a solid sulfur electrode, a sodium metal electrode, and a solid PEO polymer electrolyte. During the first discharge, the battery shows plateau potentials at 2.27 and at 1.76 V. The first discharge capacity is 505 mAh g −1 sulfur at 90 °C. The capacity drastically decreases by repeated on charge–discharge cycling but remains at 166 mAh g −1 sulfur after 10 cycles. The latter value is higher than that reported for a Na/poly (vinylidene difluoride)/S battery at room temperature.

Room-Temperature Solid-State Sodium∕Sulfur Battery

Solid-state sodium/sulfur batteries using polyvinylidene-fluoride-hexafluoropropene (PVDF) polymer electrolyte were prepared and tested at room temperature. Solid sodium/sulfur batteries may be composed of solid-composite-type sulfur electrodes, sodium metal electrodes, and PVDF gel polymer electrolyte. The PVDF gel polymer electrolyte with tetraglyme plasticizer and NaCF 3 SO 3 salt had a high sodium ion conductivity of 5.1 X 10 -4 S cm -1 at 25°C. During the first discharge, the sodium/sulfur battery showed two plateau potentials of 2.27 and 1.73 V, respectively. The first discharge capacity was 489 mAh/g sulfur at room temperature, which was similar to the high temperature battery. The discharge capacity drastically decreased by repeated charge-discharge cycling, and remained at 40 mAh/g after 20 cycles.

Observation of shell structure in sodium nanowires

The fact that the rare gas atoms at the end of each row in the periodic table of the elements are exceptionally stable is explained by the closed-shell configuration of their electronic structure. In general, the quantum states of a system of particles in a finite spacial domain form a set of discrete energy eigenvalues, which are usually grouped into bunches of degenerate or close-lying levels, called shells [1]. For fermions this gives rise to a local minimum in the total energy of the system when all states of a given shell are occupied. Shell effects have previously been observed for protons and neutrons in nuclei and for clusters of metal atoms [2], [3], [4]. Here, we report the first observation of a shell effect in an open system, a metal nanowire. When recording the statistical distribution of conductance values observed while pulling off the contact between two bulk sodium metal electrodes, the histogram shows oscillations up to contacts larger than 100 atoms in cross section. The period follows the law expected for the electronic shell-closing effect similar to that determining the “magic numbers” in metal clusters [3], [4]