High-power lithium polysulfide-carbon battery

Accepted Manuscript High-power Lithium Polysulfide-Carbon Battery Hwang-dong Shin, Marco Agostini, Ilias Belharouak, Jusef Hassoun, Yang-Kook Sun PII:

S0008-6223(15)30253-0

DOI:

10.1016/j.carbon.2015.09.034

Reference:

CARBON 10304

To appear in:

Carbon

Received Date: 19 June 2015 Revised Date:

29 July 2015

Accepted Date: 7 September 2015

Please cite this article as: H.-d. Shin, M. Agostini, I. Belharouak, J. Hassoun, Y.-K. Sun, High-power Lithium Polysulfide-Carbon Battery, Carbon (2015), doi: 10.1016/j.carbon.2015.09.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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High-power Lithium Polysulfide-Carbon Battery

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Hwang-dong Shin†, Marco Agostini‡, Ilias Belharouak*,§, Jusef Hassoun*,‡ and Yang-Kook Sun*,†

† Department of Energy Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 133-791, South Korea

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‡ Department of Chemistry, University of Rome Sapienza, Piazzale Aldo Moro, 5, 00185, Rome, Italy

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§ Qatar Environment and Energy Research Institute, Qatar Foundation, P.O. Box 5825, Doha, Qatar

*Corresponding authors:

[email protected], J Hassoun, Tel: +39-06-4991-3664, Fax: +39-06-491769

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[email protected], Y-K Sun, Tel: +82-2-2220-0524, Fax: +82-2-2282-7329

Abstract

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[email protected], I Belharouak, Tel: +974 44541455, Fax: +974 4454 1528

We report a lithium battery using activated carbon on gas diffusion layer (GDL) electrode as host for

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lithium polysulfide conversion reaction. The cell operates within 2.8 and 2.1 V and delivers a capacity ranging from 400 mAh g-1 at 1C to 150 mAh g-1 at 40C over 100 cycles. These characteristics allow the achievement of high energy and power density, i.e. practically estimated to reach the maximum values of the order of 300 Wh kg-1 and 12 kW kg-1, respectively. These values, exceeding those delivered by the conventional lithium ion batteries, make our battery of sure interest for practical applications.

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1. Introduction Increasing demand of the sustainable, clean energy technologies so far triggered large interest for efficient energy storage systems and, in particular, for rechargeable lithium-ion batteries (LiBs) [1].

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However, the high costs, the safety issues and the limited energy density hindered the large-scale diffusion of LiBs in emerging markets, such as electric vehicles (EVs) and stationary energy storage (ESS) [2-4]. Li-sulfur battery is one of the most attracting alternatives to the present technology due to

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the high theoretical specific capacity and energy density, i.e. of 1675 mAh g-1 and 2600 Wh kg-1 respectively [5]. Furthermore, sulfur is an abundant element in nature, eco-friendly and characterized

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by low costs. Despite of these favorable properties, Li-S cell has intrinsic limitations regarding the insulating nature of sulfur, the formation of polysulfide species during the electrochemical process, the consequent “shuttle reaction” at the lithium surface and the slow kinetics of charge-discharge processes at the lower voltage levels [6-10]. Among the several strategies suggested to overcome these issues, the

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synthesis of sulfur-carbon electrodes comprising microporous-mesoporous carbons [11-15] hierarchical structures [16,17] and hollow-carbon architectures [18-20], actually buffered the sulfur dissolution and mitigated the electrode deterioration by volume expansion. However, the slow kinetics of the

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electrochemical process still hinder the full electrode utilization and shorten the cell cycle life [21,22]. An efficient lithium-sulfur process may be achieved limiting the Li/S reaction to the formation of

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dissolved species, thus avoiding the presence of insulating products such as Li2S2 and Li2S and enhancing the electrochemical properties of the system [22-24]. Indeed, the reduction of Li2S4 at the lower voltage region, i.e. a process characterized by slow kinetics due to the phase transformation from liquid polysulfide to solid Li2S, leads to the growth of insulating species at the electrode surface and consequent cycle life limitation [10,6]. Herein, we reported a cell operating within a region characterized by the faster lithium-sulfur reaction kinetics, i.e. limiting the low voltage cutoff to values 2

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higher than 2.1V vs Li+/Li corresponding to the reaction S8 ↔ Li2S4 with a theoretical capacity of 419 mAh g-1 [21,25], in order to hinder the formation of insoluble species (Li2S2, Li2S). We have exploited a porous cathode material, formed by coating activated carbon (AC) on gas diffusion layer (GDL),

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acting as support for the electrochemical reaction of lithium polysulfide species dissolved in the electrolyte. The data demonstrated that this unique porous structure might enhance the electrochemical characteristic of the cell in terms of cycle life and rate capability, by improving the Li-ion transport

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within the electrode-electrolyte interface.

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2. Experimental Electrode preparation

Activated carbon (AC, GS Energy, CEP 21KS) and a polyvinylidene fluoride binder (PVdF 6020, Solvay) were intimately mixed with a weight ratio of 8:2 in N-methyl-2-pyrrolidone (NMP) solvent. The resulting slurry was coated by doctor blade deposition on a gas diffusion layer (GDL, SGL

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company, density 0.32 g cm-3). Following, the electrodes, indicated by the acronyms PCG, were dried overnight at 100 °C under vacuum in order to remove the residual solvent. The electrodes were of 14 mm diameter and the resulting sulfur areal concentration was of about 1 mg cm-2 (1.62 mg of Li2S8 in

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the solution). The final carbon loading at the electrode surface was of 2.0 ± 0.1 mg cm-2.

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Li2S8-containing electrolyte preparation The Li2S8 was prepared by dissolving elemental sulfur and small pieces of metal lithium (molar ratio 2:8) in 1,3-dioxolane (DOL, Sigma Aldrich) and 1,2-dimethoxyethane (DME, Sigma Aldrich), volume ratio of 1:1, with a final concentration of 0.075 mole L-1. The mixture was heated for 24 h to produce a red colored solution with no residual of sulfur or lithium. Following 0.4 mole of LiNO3 and 1 mole of bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, Sigma Aldrich) were added to 1 L of DOLDME-Li2S8 solution. 3

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Characterization The structure and morphology of the composite powders were determined by field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi). Li-metal surface has been analyzed by X-ray

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photoelectron Spectroscopy (XPS, theta probe base system, Thermo-Fisher Scientific Co.) using an ESCALAB 250 spectrometer equipped by monochromatized Al Kα X-ray source operating at 12 kV and 20 mA. The surface area and porosity of activated carbon were measured by using a Quantachrome

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Autosorb-iQ-MP automated gas adsorption system with liquid nitrogen (at 77 K). The specific surface area was calculated using the Brunauer-Emmett-Teller (BET, Autosorb-iQ-Mp, Quantachrome)

Barrett-Joyner-Halenda (BJH) method.

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method, and the pore diameters were obtained from the adsorption branch of the isotherm using the

The electrochemical tests were performed using CR2032 coin-type cells assembled in an argon-filled glove box (MBRAUN, H2O <0.1 ppm, O2 < 0.1 ppm). Li metal was used as a counter and reference

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electrode while the separator was a Glass fiber (Advantec) imbibed by 80 µL (corresponding to 1 mg cm-2 of S at the electrode surface).

Galvanostatic cycling tests of the lithium cells were conducted using a current ranging from 0.5C to

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40C (1 C = 419 mA g-1 vs sulfur weight) in a 2.8 – 2.1 V voltage range using TOSCAT-3000 cycler. The 1C current (419 mA g-1), calculated to simplify the discussion by taking into account the reaction

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4Li+S8 = 2Li2S4, corresponds to 1/4 of the current 1C referred to the overall capacity delivered by complete lithium sulfur reaction 16Li+S8 = 8Li2S (1675mAh g-1), i.e. a condition not exploited by our cell configuration. Cyclic voltammetry (CV) was performed at a scan rate of 0.1 mV/s within 2.8V 1.6V and 2.1V – 2.8V potential ranges, respectively, by using a VMP3 Biologic instrument. Electrochemical impedance spectroscopy (EIS) of the lithium-sulfur cells were performed within 100

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mHz - 100 kHz frequency range using a signal with voltage amplitude of 10 mV and a VMP3 Biologic instrument. 3. Results and discussion

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Elemental sulfur (S8) electrochemically reacts in lithium cell through the formation of a series of polysulfide ions (S2-8, S2-7, S2-6, S2-5, S2-4), and finally Li2S2 and Li2S. The lithium-sulfur discharge electrochemical process shows two regions, differing by reaction kinetics, nature of the formed

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products and potential value. The first region, characterized by higher potential, i.e. at 2.35V vs. Li, involves the fast-kinetic formation of soluble polysulfide species, such as Li2S6, Li2S6 and Li2S4 [21-25].

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The second region, evolving with potential value of about 2.05V, is characterized by the formation of insoluble, insulator species such as Li2S2 and Li2S. The nature of the products formed within the lower voltage region is responsible of the slow kinetic of the reaction that, in addition, is characterized by high polarization, low rate capability and poor cycle life. Herein, we developed a lithium cell design

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employing carbon substrates in a polysulfide containing. The cell has been characterized using a voltage range in which the discharge products are into a dissolved configuration, i.e. by limiting the minimum cell voltage to 2.1V vs. Li. Figure 1 shows the morphological characteristics of the carbon

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powder, used for PCG electrode assembly, in terms Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption-desorption isotherms, SEM (a) and corresponding pore size distribution (b). The

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activated carbon (AC) shows particles size ranging from 1 to 5 µm and a porous nano-sized structure with a surface area exceeding 2000 m2/g. Therefore, it is expected that the AC may host large fraction of dissolved lithium polysulfide, due to the enhanced surface area and fast-kinetic reaction promoted by nano-pores sites.

5

700

0.006

Adsorption Desorption

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BET=2031.962 m /g Pore volume= 0.92 cc/g at P/P0= 0.992

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Volume Adsorbed (cc g )

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10

100

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Pore Diameter (nm)

(b)

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(a)

Figure 1 Morphological and structural characterization of the PCG electrode. (a) Brunauer-EmmettTeller (BET) adsorption-desorption isotherms and, in inset, SEM image. (b) Pore-size distribution Figure 2 reports the characteristics of the lithium polysulfide cell using the PCG electrode in terms of cyclic voltammetry (CV) within two different potential ranges, i.e. 2.1V-2.8V (a) and 1.6V-2.8V (b),

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the corresponding impedance spectra upon 1st discharge ande (c) and the XPS analysis evidencing the products formed at the PCG electrode surface during the discharge (d). The first discharge, using the

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higher voltage cutoff (2.1V in Fig. 2a), is expected to limit the reaction products to dissolved polysulfide species, such as Li2S8, Li2S6 and Li2S4, while the following charge up to 2.8V may lead to

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the formation of S8. The corresponding CV curve in Fig. 2a shows very sharp reduction and reverse oxidation peaks occuring at 2.28 V and 2.40V, respectively. During the following voltammetric cycles the reduction peak improves in intensity and slightly shifts in potential due to possible formation of S8 at the carbon electrode surface by the initial charge process, and following cell stabilization at the steady state condition. Figure 2b, reporting the CV curves in the full potential range with discharge extended down to 1.6V, shows broad oxidation and reduction peaks with increasing polarization and 6

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decreasing intensity upon cycling, as expected by the low-kinetic formation of insoluble species (Li2S2, Li2S) at the lower potential levels effecting the Li-S cell behavior [21]. Figure 2c reports the impedance spectra of lithium sulfur cells using the PCG electrode discharged both at 1.6 V (red plain circles) and

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2.1 V (black empty circles). It is well known that a depth of discharge (DOD) lowered to 1.6 V leads the formation of a solid electrolyte interphase (SEI) layer, due to the decomposition of the LiNO3, DOL [26] and to the contemporary deposition of Li2S2 and Li2S formed at the cathode on the lithium metal

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anode side. The massive products deposition at the lithium surface leads to a great increase of the cell resistance, as indeed demonstrated in Fig. 2c, in particular in respect to cell cycled within voltage limits

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corresponding to an electrochemical process limited to soluble species (i.e. 2.1-2.8V) [26,27]. In order to investigate the SEI layer composition at the anode side, we detected the surface of PCG electrodes collected from cycled cells by X-ray photoelectron spectroscopy (XPS). Figure 2d illustrates the S2p spectrum of PCG after cycling at different depth of discharge (DOD) in lithium-sulfur cells using the

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PCG electrode. The XPS spectra of the cell discharged down to 2.1 V reveals the presence of -SO bonds ascribed to products of the native film formed principally during the first contact of the electrode with the polysulfide-containing solution at the OCV state of the cell [22,25], as well as S–S bonds

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associated to Li2S8 and Li2S6. Lowering the DOD from 2.1 V to 1.6 V results in a relevant precipitation of insoluble discharge products (i.e. Li2S2, Li2S) at the electrode surface, as indeed evidenced by the

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signals at binding energy of about 161.8 and 160 eV in figure 2d. The Li2S2 and Li2S precipitation at the lithium side generally leads to irreversible behavior, active mass loss and consequent capacity fading [27-29]. Hence, we may reasonably assume that the restricted operating voltage range may greatly enhance the electrochemical performances of the Li-S cell, as indeed demonstrated in the following paragraph.

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2.1 -2.8 V

1.6 - 2.8 V

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discharge to 2.1V vs. Li discharge to 1.6V vs. Li

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Figure 2 Cyclic voltammetry profiles of a lithium-sulfur cell using two different potential regions, i.e.

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(a) 2.1V to 2.8V and (b) 1.6V to 2.8V at a constant scan rate of 0.1 mV/s, and corresponding impedance spectra (c) and XPS spectra of the PCG electrode (d) upon 1st discharge at 1.6V, 2.1V.

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Electrolyte: 1M LiTFSI, 0.4 M LiNO3, 0.075 M Li2S8 in DME/DOL (1:1 = v/v). Room temperature. In order to verify the ability of the PCG electrode in trapping the sulfur produced by polysulfide oxidation following the charge process, we collected scanning electron microscopy (SEM) images and elemental mappings of carbon and sulfur both at the pristine state of the cell and following the discharge/charge process. Figure 3 reports the SEM image (a), the carbon mapping (b) and the sulfur mapping (c) of the PCG electrode before cycling and the corresponding images after discharge/charge 8

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process (d, e and f, respectively). The figure evidences the formation of sulfur at the electrode surface upon charge (compare Fig. 3c and 3f), thus suggesting the effective role of the PCG electrode in

(b)

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(e)

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5 µm

(d)

(c)

S

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(a)

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efficiently confining the lithium-sulfur reaction products.

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(f)

S

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5 µm 5 µm

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Figure 3 Morphologies and elemental mappings of the PCG electrodes. (a) SEM and elemental mappings of carbon (b) and sulfur (c) of the PCG electrode before cycling. (d) SEM and elemental

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mappings of carbon (e) and sulfur (f) of the PCG electrode after discharge/charge cycle. Voltage limits 2.1V and 2.8V, current rates 1C (419 mA g-1). Electrolyte: 1M LiTFSI, 0.4 M LiNO3, 0.075 M Li2S8 in DME/DOL (1:1 = v/v). Room temperature This important hosting action is allowed by the relevant surface area of the activated carbon (i.e., 2000 m2/g) and to the porous nano-sized structure (see figure 1). Furthermore, the configuration adopted here, including also a gas diffusion layer as support for the activated carbon slurry, is expected not only to 9

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host the sulfur but also to allow its reversible electrochemical process to form back the dissolved polysulfide by an excellent efficiency (see following paragraphs). Indeed, the gas diffusion layer facilitates the diffusion of the dissolved electro-active polysulfide species to the active carbon surface

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into PCG electrode. Figure 4 reports the performances of the lithium-sulfur cell using the PCG electrode in terms of voltage profile (a) and cycling behaviour (b) at 1C current (419 mA g-1) and at increasing C-rates increasing from 0.5C to 40C (c,d). Fig. 4a shows a limited discharge capacity during

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the 1st cycle, due to the activation process of the Li2S8 (e.g. Li2S8 → Li2S4; 1st discharge; S8 ↔ Li2S4; following cycles), and a stable, steady state capacity of about 400 mAh g-1, as referred to the overall

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sulfur mass dissolved in the electrolyte, i.e. about 95% of the theoretical value associated to the above reported electrochemical process (419 mAh g-1). Fig. 4b shows excellent cycle life and capacity retention (about 91%), extended up to 100 cycles, and a coulombic efficiency approaching the 100% [30-31]. The lithium-sulfur cell is further characterized by evaluating the effect of the C-rate increase

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(i.e. from 0.5C to 40C). Figures 4 c-d report the voltage profiles and cycling response, respectively, at various C-rates. The Figure shows remarkable capacity, even at c-rate as high as 40C with a delivered capacity of about 150 mAh g-1, i.e. the 36% of the theoretical value. Furthermore, the cell recovers

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about the 97 % of the initial capacity, i.e. about 400 mAh g-1, by moving back the lower current rate, thus confirming an excellent cycle life and capacity retention. The enhanced cycling performance and

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rate capability of the electrode using the GDL (PCG) in respect to the electrode using the common Al support (PCA) in lithium sulfur cell are evidenced by Figures S1 and S2 in Supplementary Information section, respectively. Figure S1 shows a remarkable improvement of the cell using the GDL support in terms of delivered capacity and polarization, while figure S2 reveals a great increase of the cell rate capability.

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Figure 4 Galvanostatic cycling performance of the lithium sulfur cell using the PCG electrode within 2.1V and 2.8V: (a) voltage profiles at 1C current; (b) corresponding cycling behavior and coulombic

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efficiency; (c) voltage profiles at various c-rate, increasing from 0.5C to 40C (1C = 419 mA g-1) and

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(d) corresponding cycling behavior. Electrolyte: 1M LiTFSI, 0.4 M LiNO3, 0.075 M Li2S8 in DME/DOL (1:1 = v/v). Room temperature Further proof of the excellent cycle life and rate capability of the cell here proposed is demonstrated by Figure 5a, reporting the prolonged galvanostatic cycling performances of the lithium sulfur cell using the PCG electrode within 2.1V and 2.8V at current rates of 1C, 7C, 25C, 30C and 40C. The figure shows high capacity, i.e., ranging from 150 to 400 mAh g-1, by increasing the current rate as well as an 11

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excellent cycle life. The practical energy and power densities of the Li-S cell have been calculated, at the various currents, by taking into account the delivered capacity, the corresponding voltage and c-rate values as well as considering a reduction factor of 1/3 comprising the three main components of the

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cell, i.e.: i) cathode; ii) anode; iii) electrolyte and cell case. The first component (cathode) is considered as active part for theoretical energy and power calculation, while the others two are taken as side components. Hence, the practical energy and power density is calculated basing on the delivered

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capacity at the cathode, the corresponding voltage and C-rate values. Figure 5b shows an energy

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Discharge capacity / mAh gS

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density as high as 300 Wh kg-1 and a power density of about 0.3 kW kg-1 at a 1C rate.

Figure 5 (a) Prolonged Galvanostatic cycling performances of the lithium sulfur cell using the PCG

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electrode within 2.1V and 2.8V at current rates of 1C, 7C, 25C, 30C and 40C (1C = 419 mA g-1). (b) Energy and power density curve of the lithium sulfur cell using the PCG electrode cycled within 2.1V and 2.8V voltage limits at various current rates. Electrolyte: 1M LiTFSI, 0.4 M LiNO3, 0.075 M Li2S8 in DME/DOL (1:1 = v/v) electrolyte. Room temperature. The figure evidences also the range corresponding to the conventional lithium-ion battery.

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The energy density decreases to a still satisfactory value of 180 Wh kg-1 at 10C rate, while the power density remarkably increases up to 3 kW kg-1. At the very high current rate, i.e. 40C, the figure shows a residual energy density of about 106 Wh kg-1 and an extremely high power of about 12 kW kg-1 at 40C.

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Considering a capacity in line with the best performances known for the new generation lithium-ion batteries and a higher energy density, the cell here reported is considered a suitable candidate as a new, high-power Li-battery exploiting a polysulfide solution.

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4. Conclusion

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In this work, we studied a lithium sulfur cell using porous carbon GDL electrode (PCG) and lithium polysulfide solution. The results demonstrated that the cell has a very stable cycling performance up to 100th cycles at a current of 1C, and high rate capability extending up to 40C. XPS and EIS measurements have shown that cycling the cell within voltage range restricted to 2.1–2.8 V efficiently

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mitigates the formation of the insoluble species such as Li2S2, Li2S to and avoid capacity decay. The cell evidenced high energy density and outstanding power density, suitable for application in advanced energy storage systems. Previous papers demonstrated the possibility of strongly increasing the sulfur

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loading using glass-type cell and a Li2S8-concentrated solution [23]. We demonstrated in our paper the suitability of the cell configuration here reported, however further efforts aimed to increase the sulfur

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content in the solution are required. Acknowledgements

This work has been supported in part by a grant from the Human Resources Development Program (No. 20124010203310) of the Korea Institute of Energy Technology Evaluation and Planning, funded by the Korean government; by the Ministry of Trade, Industry and Energy; by the National Research

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Foundation

of

Korea

(NRF)

grant

funded

by

the

Korea

government

(MEST)

(No.

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2014R1A2A1A13050479).

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Supplementary Information for

High-power Lithium Polysulfide-Carbon Battery

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Hwang-dong Shin†, Marco Agostini‡, Ilias Belharouak*,§, Jusef Hassoun*,‡ and Yang-Kook Sun*,†

† Department of Energy Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu,

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Seoul 133-791, South Korea

‡ Department of Chemistry, University of Rome Sapienza, Piazzale Aldo Moro, 5, 00185, Rome,

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Italy

§ Qatar Environment and Energy Research Institute, Qatar Foundation, P.O. Box 5825, Doha, Qatar

*Corresponding authors:

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[email protected], J Hassoun, Tel: +39-06-4991-3664, Fax: +39-06-491769 [email protected], Y-K Sun, Tel: +82-2-2220-0524, Fax: +82-2-2282-7329

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[email protected], I Belharouak, Tel: +974 44541455, Fax: +974 4454 1528

ACCEPTED MANUSCRIPT The gas diffusion layer, used as the support for the activated carbon slurry, allows the transport of the polysulfide to the carbon electrode. Indeed, the reversible electrochemical process occurs at the active carbon electrode surface depositing sulfur and forming back the dissolved polysulfide with excellent efficiency. This is demonstrated by the SEM and EDX analysis of the carbon slurry

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surface covering the gas diffusion layer before and after cycling reported in Figure 3(c-f) of the manuscript. The gas diffusion layer role is mainly focused in enhancing the diffusion of the dissolved electro-active polysulfide species to the active carbon surface into PCG electrode that is,

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instead, limited by using the common Al support. This difference is reflected by enhanced cycling performances of the PCG electrode in respect to an electrode using the common Al, as

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demonstrated by Figures S1 and S2 reporting the performances comparison of the Li/S cells using the PCG (carbon electrode supported by GDL) and the PCA (carbon electrode supported by conventional Al). The figures show a remarkable improvement of the cell using the GDL support in terms of lower polarization (Fig. S1 a), higher capacity (Fig. S1 b) and increased rate capability

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(Fig. S2).

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Figure S1: (a) Voltage profiles and (b) cycling behavior comparison of the Li-S cell using the PCG (red line) and the PCA (blue line) electrodes. Voltage range 2.1 V and 2.8V; current rate 1C

ACCEPTED MANUSCRIPT (corresponding to 419 mA g-1). Electrolyte: 1M LiTFSI, 0.4 M LiNO3, 0.075 M Li2S8 in DME/DOL

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PCG PCA

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Figure S2: Cycling performances of the lithium-sulfur cell using the PCG (red line) and the PCA (blue line) electrodes at various C-rate. Current rate from 0.5C to 40C (1C= 419 mA g-1); voltage profiles between 2.1V and 2.8V. Electrolyte: 1M LiTFSI, 0.4 M LiNO3, 0.075 M Li2S8 in

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DME/DOL (1:1 = v/v). Room temperature, 25 °C.