Unusual Passivation Ability of Superconcentrated Electrolytes toward

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Unusual passivation ability of superconcentrated electrolytes toward hard carbon negative electrodes in sodium-ion batteries Koji Takada, Yuki Yamada, Eriko Watanabe, Jianhui Wang, Keitaro Sodeyama, Yoshitaka Tateyama, Kazuhisa Hirata, Takeo Kawase, and Atsuo Yamada ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08414 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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ACS Applied Materials & Interfaces

Unusual passivation ability of superconcentrated electrolytes toward hard carbon negative electrodes in sodium-ion batteries Koji Takada,a Yuki Yamada,a,b Eriko Watanabe,a Jianhui Wang,a Keitaro Sodeyama,b,c,d Yoshitaka Tateyama,b,c Kazuhisa Hirata,e Takeo Kawase,e and Atsuo Yamada*,a,b. a

Department of Chemical System Engineering, The University of Tokyo, 7-3-1, Hongo,

Bunkyo-ku, Tokyo 113-8656, Japan. b

Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, 1-30,

Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan. c

Center for Green Research on Energy and Environmental Materials (GREEN) and Research

and Services Division of Materials Data and Integrated System (MaDIS), National Institute for Materials Scinence (NIMS), 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan. d

PRESTO, Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi, Saitama

333-0012, Japan e

Innovation & Business Development Division, Nippon Shokubai Co., Ltd., 5-8, Nishi Otabi-

cho, Suita, Osaka 564-8512, Japan.

ACS Paragon Plus Environment

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KEYWORDS. Sodium-ion batteries, electrolytes, hard carbon, passivation, high concentration

ABSTRACT. The passivation of negative electrodes is key to achieving prolonged chargedischarge cycling with Na-ion batteries. Here, we report the unusual passivation ability of superconcentrated

Na-salt

electrolytes.

For

example,

a

50

mol%

sodium

bis(fluorosulfonyl)amide (NaFSA)/succinonitrile (SN) electrolyte enables highly reversible Na+ insertion into a hard carbon negative electrode without any electrolyte additive, functional binder, or electrode pretreatment. Importantly, an anion-derived passivation film is formed via preferential reduction of the anion upon charging, which can effectively suppress further electrolyte reduction. As a structural characteristic of the electrolyte, most anions are coordinated to multiple Na+ cations at high concentration, which shifts the lowest unoccupied molecular orbitals (LUMOs) of the anions downwards, resulting in preferential anion reduction. The present work provides new understanding of the passivation mechanism with respect to the coordination state of the anion.


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Introduction Lithium (Li)-ion batteries are widely used in portable electronic devices, such as notebook computers and cell phones, and are finding large-scale application in electric vehicles. Despite the growing demand for Li-ion batteries, Li resources are limited and unevenly distributed worldwide, which may cause serious supply shortages in the future. In contrast, sodium (Na) resources are abundantly available from the Earth’s crust and the sea and thus have minimal supply risk. From this perspective, Na-ion chemistries are attracting much attention as a post Liion battery suitable for large-scale application.1,2 Based on the similar intercalation mechanism to Li-ion batteries, Na-ion batteries have, at least in principle, lower gravimetric/volumetric capacity (heavier and larger Na ion) and lower operation voltage (higher Na+/Na reaction potential). However, a variety of positive electrode materials (including those not obtainable when using Li counterparts as stable phases) have been reported based on the diversity of coordination environments available for the larger Na+ ions,3,4 and some of these materials exhibit higher capacity and/or voltage than state-of-the-art electrodes for Li-ion batteries.1,2,4–11 In addition, in a weaker Lewis acid, Na+ ions only weakly coordinate with Lewis-basic solvent molecules in the electrolyte,12 which enables higher rate capability by virtue of lower interfacial resistance (owing to the smaller activation energy of de-solvation)13 and higher ionic conductivity (owing to the smaller solvated ionic radius).14,15 As a result, Na-ion batteries are increasingly recognized as an inexpensive and high-performance battery that can possibly replace current Li-ion batteries in the future. A liquid electrolyte design is a crucial to improve the cyclability and rate capability of batteries.16–18 For Na-ion battery electrolytes, the ability to passivate negative electrodes (i.e., by forming a rigid solid electrolyte interphase (SEI) via sacrificial electrolyte decomposition) is


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particularly important because i) a charged negative electrode (e.g., sodiated hard carbon) is a strong reducing substance, as with Li-ion batteries, and ii) SEI components based on Na+ cations are more soluble in electrolyte than those based on Li cations.19,20 Currently, the Na-ion counterpart of commercial Li-ion electrolytes (e.g., NaPF6 in organic carbonates, such as ethylene carbonate (EC)) is widely used as a standard electrolyte,21 but its passivation ability toward hard carbon negative electrodes is considerably poorer than that of Li-ion electrolytes.19– 21

Functional additives (e.g., fluoroethylene carbonate, FEC)22–24 and binders (e.g., sodium

carboxymethylcellulose, Na-CMC)25,26 have been widely studied to compensate for the poor passivation ability. However, there are few works on Na-ion electrolyte design utilizing its inherent passivation ability. Recently, superconcentrated (highly concentrated) Li-salt solutions have attracted much attention as a new class of liquid electrolytes for battery applications.27–41 Conventionally, the salt concentration is optimized at approximately 1 mol dm−3 with an eye to ionic conductivity, because increasing the concentration over 1 mol dm−3 results in a monotonic decrease in the ionic conductivity. However, various new functionalities were discovered at the highly concentrated region (e.g., over 3 mol dm−3). Of particular interest is their unusual passivation ability toward negative electrodes (e.g., graphite and Li metal), enabling highly reversible reactions.35–37 In contrast to an EC-derived SEI in conventional EC-based electrolytes, an SEI film rich in anion-derived products is formed in highly concentrated electrolytes, which is considered to kinetically stabilize the electrode/electrolyte interface.36,38 A similar concept was recently applied to Na-based batteries.42–45 Nevertheless, the mechanism behind the unusual passivation is not understood for either Li- or Na-salt electrolytes.


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In this work, we report the unusual passivation stability of a Na salt-based superconcentrated electrolyte in correlation with its solution structure. We used the combination of sodium bis(fluorosulfonyl)amide (NaN(SO2F)2, NaFSA) salt and succinonitrile (SN) solvent (Fig. 1). The NaFSA salt can be mixed with solvent in a wide concentration range owing to its low cation-anion binding energy.46 The SN solvent can dissolve various salts at high concentrations owing to its high dielectric constant and two nitrile groups, which act as cation coordination sites.47 In addition, SN forms a plastic crystal phase at room temperature but can function as a fast ionic conductor due to its molecular reorientational dynamics.48–50 We demonstrate that highly concentrated NaFSA/SN electrolyte can be stabilized in liquid form at room temperature and effectively passivate a hard carbon negative electrode to enable highly stable chargedischarge cycling. Based on both experimental and theoretical approaches, the origin of the unusual passivation is discussed in detail from the viewpoint of the electronic structure of the electrolyte.

Figure 1. Chemical structures of NaN(SO2F)2 (NaFSA) and succinonitrile (SN).

Experimental Methods Materials. The NaFSA salt was provided by Nippon Shokubai Co., Ltd. The SN solvent and a conventional 1.0 mol dm−3 NaPF6/EC:diethyl carbonate (DEC) (1:1 by vol.) electrolyte were


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purchased from Kishida Chemical Co., Ltd. The materials were all of battery grade and used without further purification or pretreatment. The electrolyte solutions were prepared by adding the NaFSA salt to the SN solvent under heating because the melting point of the SN solvent is approximately 60 °C. Physicochemical properties. The density and viscosity of the solutions were measured with DMA 35 (Anton Paar) and Lovis 2000 M (Anton Paar), respectively. The molar concentration (mol dm−3) was calculated from the molar ratio and density. The ionic conductivity was measured in a two-electrode cell with a pair of platinum plates using the ac impedance method with a Solartron 147055BEC electrochemical measurement system (Solartron Analytical). The thermal properties of the solutions were studied by differential scanning calorimetry (DSC) using a DSC 8230 calorimeter (Rigaku). To prevent any contamination from air, the solutions were sealed in an Al pan in an Ar-filled glove box. The weight of the solutions was approximately 20 mg. To standardize the temperature history, all samples were heated to 90 °C and then cooled to −120 °C. After that, the DSC curve was recorded by heating the sample from −120 °C to 90 °C at 5 °C min−1. The obtained heat flow was divided by the actual sample weight. Electrochemical properties. Hard carbon (Carbotron P(J)) was purchased from Kureha Corporation and was used without any further purification or pretreatment. To prepare the hard carbon electrode, the hard carbon powder was well mixed with polyvinylidene difluoride (PVdF, Kureha Corporation) in a weight ratio of 9:1 in N-methylpyrrolidone (NMP, Wako Pure Chemical Industries, Ltd.). The slurry was uniformly spread onto a Cu foil (Fuchikawa Material) with a 50-µm doctor blade. The obtained sheet was punched to form a 12-mmφ disk electrode. The active material loading was approximately 1.0 mg cm−2. The relatively low loading was used


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to minimize the effect of Na counter electrode (e.g., reaction irreversibility). The electrode was dried at 120 °C under vacuum overnight and then transferred into an Ar-filled glove box. A 2032-type coin cell was assembled in the glove box. A glass fiber filter (GC-50, ADVANTEC) was used as a separator. Na metal (Wako Pure Chemical Industries, Ltd.) was used as a counter electrode. The volume of electrolyte was 80 µL. The charge-discharge test of the hard carbon/Na metal half cell was conducted using a charge-discharge unit (TOSCAT, Toyo System Co., Ltd.) in a 25 °C thermostatic oven. The coin cells were charged and discharged between 0.01-2.5 V at a current rate of 10 mA g−1 or 20 mA g−1. Electrochemical impedance spectroscopy. The impedance test of the hard carbon electrodes was performed using a sealed three-electrode cell. Na metal was used as counter and reference electrodes. A glass fiber filter was used as a separator. The charge-discharge cycling and impedance test was conducted with a Solartron 147055BEC electrochemical measurement system (Solartron Analytical) in a 25 °C thermostatic oven. All impedance experiments of the hard carbon electrodes were performed at 0.1 V vs Na/Na+ after the cells were charged and discharged between 0.01 V− 2.5 V at a current rate of 10 mA g−1 for 1 cycle. Morphologies of the harvested electrodes. The surface morphologies of hard carbon electrodes were analyzed by scanning electron microscopy (SEM) using S4800 (Hitachi) at 3 kV. The hard carbon/Na metal cells, charged and discharged for 3 cycles at a current rate of 20 mA g−1, were disassembled in an Ar-filled glove box, and the cycled hard carbon electrodes were washed twice with dimethyl carbonate (DMC) to minimize the amount of a residual NaFSA salt. The washed electrodes were dried and attached to the SEM sample holder with carbon tape. The samples were transferred into SEM machine without exposure to air.


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Surface chemistry of the electrodes. The electrode surface was analyzed by X-ray photoelectron spectroscopy (XPS) using a PHI 5000 VersaProbe Scanning XPS System (ULVAC-PJI, Inc.) equipped with a monochromatic Al Kα X-ray source. The hard carbon/Na metal cells, charged and discharged for 3 cycles at a current rate of 10 mA g−1, were disassembled in an Ar-filled glove box, and the cycled hard carbon electrodes were washed twice with dimethyl carbonate (DMC) to minimize the amount of a residual NaFSA salt. The washed electrodes were dried and attached to the XPS sample holder with carbon tape. The samples were transferred into the XPS chamber without exposure to air. Curve fitting of the spectra was performed with a Gaussian-Lorentz function after a Shirley-type background subtraction. The doublet-separated peak in the S2p spectra was analyzed with an energy separation of 1.18 eV and intensity ratio of 0.50. Computational details. The liquid structures and electronic structures of the dilute (4 NaFSA and 36 SN) and superconcentrated (20 NaFSA and 20 SN) systems were analyzed by density functional theory-based molecular dynamics (DFT-MD) simulations. Car-Parrinello-type DFTMD simulations were carried out using the CPMD code.51,52 Cubic supercells with 17.310 Å and 17.845 Å linear dimensions were used for the dilute and superconcentrated systems, respectively. These values were based on the experimentally obtained densities. The calculation was carried out under constant volume and temperature. The temperature was controlled using a Nosé thermostat53,54 with a target temperature of 350 K. The electronic wave function was quenched to the Born-Oppenheimer surface approximately every 1 ps or 0.5 ps for dilute and superconcentrated systems, respectively. The energy cutoff of the plane wave basis was set to 90 Ry. Goedecker-Teter-Hutter-type norm-conserving pseudopotentials55–57 for C, H, O, N, S, F and


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Na were used. A time step of 4 au (0.10 fs) was chosen. The projected density of states (PDOS) was calculated for the selected geometry in the equilibrium trajectory. Coordination states. The coordination states of the solutions were studied via Raman spectroscopy using a NRS-5100 spectrometer (JASCO) with a laser excitation of 532 nm. The solution was sealed in a quartz cell in an Ar-filled glove box, and the laser was directed through the quartz crystal window. The obtained spectra were deconvoluted with a Gaussian-Lorentzian function and a linear background correction. Oxidative Al corrosion. The stability of an Al current collector was studied via linear sweep voltammetry (LSV). LSV was conducted by using a VMP3 (BioLogic) with a three−electrode cell equipped with an Al foil as a working electrode and Na metal as counter and reference electrodes. The surface area was defined to be 1.1 cm2 with an O-ring. Scan rate was 0.1 mV s−1.

Results and Discussion Physicochemical properties. The basic physicochemical properties of the NaFSA/SN solutions are presented in Table 1. The solubility of NaFSA in SN solvent was over 50 mol% (NaFSA molar fraction) at 25 °C. The ionic conductivity decreased with increasing concentration because of a rapid increase in the viscosity (see also Fig. S1). The ionic conductivity of the most concentrated 50 mol% NaFSA/SN was 0.31 mS cm−1 at 25 °C, which is not so low, as expected from the approximately 103 times higher viscosity compared with the conventional 1.0 mol dm−3 NaPF6/EC:DEC (1:1 by vol.) electrolyte (9.5 mS cm−1 and 0.00486 Pa s at 20 °C). This is because SN solvent molecules have structural flexibility, which facilitates ion transport due to


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their molecular reorientational dynamics.48,50 Despite its high viscosity, the 50 mol% NaFSA/SN could fully permeated a glass fiber separator within 1 hour (Fig. S2). Table 1. Physicochemical properties of NaFSA/SN solutions at 25 °C Mole fraction of Na salt

Density

Concentration

Ionic conductivity

Viscosity

[g cm−3]

[mol dm−3]

[mS cm−1]

[Pa s]

11

1.185

1.40

5.1

0.0294

20

1.287

2.46

3.4

0.0690

33

1.456

4.01

1.3

0.358

50

1.656

5.86

0.31

2.52

[mol%]

The phase behavior of the NaFSA/SN mixtures was studied via DSC (Fig. 2). The pure SN solvent displayed two endothermic peaks at −31 °C and 60 °C, which correspond to the rigid-toplastic crystal transition (TPC) and the melting point (Tm), respectively.58 The 4.8 mol% NaFSA/SN mixture displayed almost the same TPC (−33 °C) and a lower Tm (37 °C). On the other hand, the 11 mol% NaFSA/SN mixture exhibited a glass-transition point (Tg) at −93 °C, which crystallized at −44 °C with an exothermic peak and then melted at −4 °C. The more concentrated 20 mol%, 33 mol%, and 50 mol% NaFSA/SN mixtures exhibited Tgs at −77 °C, −61 °C, and −61 °C, respectively, and retained stable amorphous liquid phases up to at least 90 °C. Hence, the liquidus temperature range can be remarkably widened at high concentrations, which has also been reported for other concentrated solutions (e.g., LiFSA/acetonitrile).59


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Figure 2. a) DSC curves of the NaFSA/SN mixtures at various concentrations obtained by heating at 5 °C min−1. b) The corresponding phase diagram for (1−x) SN-(x) NaFSA mixtures. The circles, cross points, squares, and triangles indicate the melting point, plastic-crystal transition point, crystallization point, and glass-transition point, respectively.

Enhanced reductive stability. To study the reductive stability of the NaFSA/SN electrolyte, the reversibility of the Na+ insertion reaction into a hard carbon electrode was investigated. Fig. 3 shows the charge-discharge voltage curves and cycling performance of the hard carbon/Na metal half cells with various NaFSA/SN electrolytes. All the electrolytes showed sloping voltage plateaus below 1.0 V during charge and discharge, which were attributed to a Na+ insertion/extraction reaction at the hard carbon electrode. However, its reversibility strongly depended on the salt concentration. In the dilute 11 mol% NaFSA/SN mixture, the reversible capacity of the hard carbon electrode was only

Unusual Passivation Ability of Superconcentrated Electrolytes toward

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