LithiumâSulfur Capacitors

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Lithium–Sulfur Capacitors Mok-Hwa Kim, Hyun-Kyung Kim, Kai Xi, R. Vasant Kumar, Dae Soo Jung, Kwang-Bum Kim, and Kwang Chul Roh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09833 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Applied Materials & Interfaces

Lithium–Sulfur Capacitors Mok-Hwa Kima,c, Hyun-Kyung Kimb, Kai Xib, R. Vasant Kumarb, Dae Soo Junga, KwangBum Kimc, Kwang Chul Roha* a

Energy and Environmental Division, Korea Institute of Ceramic Engineering and

Technology, Jinju 660-031, Republic of Korea b

Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3

0FS, UK c

Department of Materials Science and Engineering, Yonsei University, Seoul 120-749,

Republic of Korea *Corresponding Author Tel: +82-55-792-2625, Fax: +82-55-792-2643, E-mail: [email protected]

ABSTRACT: Although many existing hybrid energy storage systems demonstrate promising electrochemical performances, imbalances between the energies and kinetics of the two electrodes must be resolved to allow their widespread commercialization. As such, the development of a new class of energy storage systems is a particular challenge, since future systems will require a single device to provide both a high gravimetric energy and a high power density. In this context, we herein report the design of novel lithium–sulfur capacitors. The resulting asymmetric systems exhibited energy densities of 23.9–236.4 Wh kg−1 and power densities of 72.2–4097.3 W kg−1, which are the highest reported values for an 1

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asymmetric system to date. This approach involved the use of a pre-lithiated anode and a hybrid cathode material exhibiting anion adsorption–desorption in addition to the electrochemical reduction and oxidation of sulfur at almost identical rates. This novel strategy yielded both high energy and power densities, and therefore establishes a new benchmark for hybrid systems. KEYWORDS: electrical energy storage, hybrid capacitor, ultracapacitor, carbon/sulfur composite, hybridized capacitive mechanism

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1. Introduction The development of novel sources of renewable and sustainable energy systems constitute a globally important technological challenge for addressing issues such as resource exhaustion, acute environmental pollution, and the risks of climate change associated with fossil fuel usage. For example, in recent decades, the use of solar, tidal, and wind power has increased, and the development of electric vehicles (e.g., hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and all-electric vehicles (EVs)) with low CO2 emissions has been ongoing. Thus, for the effective use of renewable energy sources, the development of highperformance, safe, inexpensive, and environmentally benign energy conversion and storage systems is required. Such systems include lithium-ion batteries (LIBs) and ultracapacitors (UCs), which are at the forefront of development.1–5 In this context, Figure 1 shows a plot of power versus energy density (i.e., a Ragone plot) for LIBs and UCs, alongside the performance targets for HEVs, PHEVs, and EVs specified by the U.S. Advanced Battery Consortium.5–7 Indeed, it is apparent that both the power and energy densities for each type of device must be improved to meet such performance targets, with lithium–sulfur capacitors (LSCs) potentially providing a unique platform to achieve these goals. LIBs are common electrochemical devices that are employed to store electrical energy. Despite their commercial success, however, they fail to meet the high power demand required for applications such as power tools, EVs, and efficient storage devices of renewable energy. In contrast, UCs exhibit promise for application in systems that require large quantities of power, as they instantaneously provide higher power densities than batteries, in addition to providing higher energy densities than conventional dielectric capacitors. However, their energy densities remain insufficient for novel applications where high energy and power densities are required.8–11 3



To overcome these shortcomings, studies into LIBs have focused on improving electrode materials by using, for example, silicon anodes and Li-rich cathodes. However, these materials suffer from several restrictions, including low initial coulombic efficiencies, unsatisfactory rate performances, poor cycle lives, poor thermal characteristics, and pronounced voltage decays.12–14 Indeed, alternative battery systems, such as lithium–air, lithium–sulfur, and sodium/magnesium-ion batteries, have been demonstrated to surpass LIBs in terms of energy/power densities, safety, and affordability. However, these systems also present their own individual disadvantages.15–18 Thus, to address such issues, a novel class of asymmetric energy storage systems has been proposed that combine the fast charging rates of UCs and the high energy densities of LIBs. These systems contain two different types of electrode materials, namely a faradaic redox LIB material and a porous UC carbon material. To date, two key approaches have been reported to achieve such hybrid systems, including the use of pseudocapacitive metal oxides with a capacitive carbon electrode, and lithium-ion intercalation/de-intercalation in the electrode bulk combined with anion adsorption/desorption on the surface of an activated carbon (AC) cathode. This second type of hybrid system is known as a lithium-ion capacitor (LIC). Although LICs based on lithiumdoped carbon anodes and AC-based cathodes exhibit higher working voltages and energy densities than UCs, the imbalance between the kinetics of the two electrodes must be resolved before their commercial application can be realized.19–21 In response to these major challenges, we herein report the preparation of LSCs as novel devices that are expected to exhibit enhanced energy and power densities. In our system, the LSCs will utilize redox reactions between a graphite anode and an AC/sulfur composite cathode with selective sulfur infiltration. Indeed, AC has been widely used as an electrode material for UCs due to its excellent electrochemical properties, high specific surface area, 4


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and relatively low cost. Unfortunately, the amorphous and porous structure of AC significantly lowers the specific capacitances of the resulting UCs.11 In contrast, elemental sulfur has a high theoretical specific capacity (1675 mAh g−1) when employed as a cathode in batteries.16 Thus, we herein propose a novel strategy based on the selective infiltration of AC with small quantities of S to form a superior cathode material. We expect that the LSCs prepared using this material will be able to overcome the challenges posed by conventional AC to ultimately achieve stable and high capacities with good rate properties and cycling efficiencies. Thus, as advanced asymmetric energy storage devices, the prepared LSCs should undergo both non-faradaic and faradaic reactions, thereby providing greater energy densities than UCs and higher power densities than LIBs. 2. Experimental 2.1. Material preparation The AC/S composite (AC@S) was prepared using a melt-diffusion method. More specifically, AC (MSP20, Kansai Chemical, Japan) was ground with elemental sulfur (Sigma-Aldrich) to produce homogeneous mixtures containing AC:S mass ratios of 95:5 and 90:10, and the resulting mixtures were heated at 160 °C for 3 h under an Ar atmosphere. 2.2. Material preparation Thermogravimetric analysis (TGA) was then carried out using a TGA Q50 instrument under a N2 atmosphere between room temperature and 600 °C at a scan rate of 10 °C min−1. The surface chemical compositions of the AC@S samples were determined by X-ray photoelectron spectroscopy (XPS) at 1.1 × 10−7 Pa using a PHI 5000 VersaProbe with Al Kα radiation (ULVAC-PHI, Inc., Japan). The morphologies and structures of the prepared samples were characterized using field-emission scanning electron microscopy (FESEM, 5



JSM-7000F, JEOL, Japan), while elemental mapping was performed using energy-dispersive X-ray spectroscopy combined with transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). The crystal structures of the samples were identified by X-ray diffraction (XRD, D/Max 2500/PC, Rigaku, Japan), and the specific surface areas of the samples were calculated according to the Brunauer–Emmett–Teller (BET) method. In addition, The pore size distributions (PSDs) were calculated via a non-local density functional theory model (NLDFT). 2.3. Material preparation Rubber electrodes were fabricated from AC@S samples using polytetrafluoroethylene (D60, Daikin Industries, Japan) as the binder and Super-P black (MMM Carbon Co., Belgium) as the conducting agent. The prepared electrodes were then punched using a 12 mm-diameter punch, and dried in a vacuum oven. For the half-cell tests, the sulfur and graphite electrodes were prepared under similar conditions. The anode was prepared by mixing 90 wt% graphite (DRAMX), 5 wt% Super-P, and 5 wt% polytetrafluoroethylene prior to connection to a Limetal foil. Polypropylene separators (Celgard 2400) were also employed, along with an electrolyte composed of a 1 M solution of bis(trifluoromethane) sulfonimide lithium salt dissolved in a mixture of 1,2-dioxolane and dimethoxymethane (1:1 vol%), which also contained 0.2 M lithium nitrate. The prepared coin cells were then tested in galvanostatic mode at various currents in the voltage range of 1.5–3.8 V at room temperature. The current density for the tests was set according to the area of the electrode. Prior to assembly of the LSCs, the properties and electrochemical performances of all electrode materials were examined. More specifically, the half-cells of AC, S, the AC@S composite, and graphite were tested in galvanostatic mode at 0.1 mA cm−2 at room temperature. The LICs were also tested at various currents in the voltage range of 1.5–3.8 V at room temperature. 6


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3. Results and discussion Sulfur-infiltrated AC is known to have a complex charge mechanism that indicates electrical double-layer capacitive behavior and pseudocapacitive properties. A schematic representation of the LSC system prepared herein is shown in Figure 2a. Although we employed commercial AC for the purpose of this study, other materials could also be used for the cathode. As described above, AC@S was prepared via a simple melt-diffusion process, and the obtained composites containing AC:S mass ratios of 95:5 and 90:10 were denoted as “AC@S (5% S)” and “AC@S (10% S),” respectively. As AC typically has a wide range of pore sizes, including micro-, meso-, and macropores, during the melt-diffusion process, liquid S readily diffuses into the micro- and mesopores within the AC walls. Consequently, small quantities of S infiltrate these pores and make contact with the carbon walls. Subsequent prelithiation of the anode was carried out by placing graphite adjacent to the Li foil. Upon immersion in the electrolyte, Li ions were immediately intercalated into the graphite anode. As such, the graphite was always lithiated at equilibrium and maintained a pseudo-equal potential with the Li metal.22 To demonstrate the combination of capacitive and faradaic electrodes in these devices, a schematic representation of a typical charge–discharge potential profile is shown in Figure 2b. More specifically, UCs display linear charge–discharge profiles typical of capacitor electrodes. In contrast, LICs are Li ion-based hybrid systems with two representative asymmetric configurations. In the first configuration, the hybrid system employs a faradaic Li-ion-intercalated electrode and a non-faradaic anion adsorption–desorption AC electrode. In this case, Li ions intercalate into the graphite electrode during charging of the LIC at 0.01 V vs. Li/Li+, which results in a higher working voltage and energy density than those displayed by UCs. In addition, in the case of LICs, the energy density can be effectively 7



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enhanced by increasing the voltage due to the quadratic dependence of these systems on voltage.19–21 However, LICs tend to encounter a kinetic imbalance due to the significantly different charge storage mechanisms of the cathodes and the anodes. In the second type of asymmetric configuration of a Li-ion-based hybrid system, which we propose for the first time herein, LSCs employ selectively sulfur-infiltrated AC as the cathode material to enhance their energy density and kinetics for charge storage. As shown in Figure 2b, the charge– discharge profile of the LSC cathode shows a combination of typical capacitor-like electrode behavior and typical battery-type electrode behavior. In this well-balanced asymmetric system based on a pre-lithiated anode and a hybrid cathode material, the processes of anion adsorption–desorption and the electrochemical reduction–oxidation of S occur at comparable rates. This novel strategy simultaneously yields high energy and power densities, thereby establishing a new benchmark for hybrid systems. A schematic illustration of the procedure used to fabricate the LSCs is shown in Figure S1. Although several types of electrodes have been proposed or are currently employed in commercial devices, we herein prepared kneaded rubber-type electrodes to verify the potential practical applications of this system, and the electrochemical properties of these electrodes were evaluated. Thus, for their preparation, electrode sheets with thicknesses of 100–150 µm

were

bound

to

the

active material, a

conductive

agent,

and a

polytetrafluoroethylene binder. Following the successful preparation of AC@S (5% S) and AC@S (10% S), TGA and XPS were carried out to determine the chemical states and S contents of the ACs. Interestingly, the S contents as calculated from the TGA curves were ~3.9 and 7.7 wt% for AC@S (5% S) and AC@S (10% S), respectively (Figure 3a). According to the TGA weight-loss curves, sulfur loss in the AC@S composites began at temperatures slightly higher than those observed for 8


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the evaporation of pure S (160–270 °C),23 which can be attributed mainly to the strong interactions between the AC and S particles.23 The surface chemical compositions of pure AC and AC@S were then identified using XPS. As shown in Figure 3b, C 1s and O 1s peaks were detected in the survey spectra for pure AC, with minimal changes being observed in the AC@S materials. However, new peaks located at 164.5 and 228.2 eV for AC@S were attributed to S 2p and S 2s, respectively, thereby suggesting the efficient infiltration of S into the AC.24 In addition, the C 1s spectrum of AC@S (5% S) (Figure 3c) consists of six peaks, which correspond to C=C bonding (284.5 eV), C-C/C-S bonding (285.5 eV), C-O bonding (286.6 eV), C=O bonding (287.8 eV), O-C=O bonding (289.0 eV), and π-π* transitions in the aromatic rings (shake-up satellite peaks, 290.7 eV). The S 2p XPS spectrum was then deconvoluted into four peaks (Figure 3d) characterized by a S 2p3/2 and 2p1/2 doublet with an energy separation of 1.2 eV, which confirmed the presence of C-S bonds. In addition, the weak signal at 165.5 eV corresponds to S-O species, while the peak at 168.6 eV was assigned to the sulfate species formed by the oxidation of S in air.25–26 Furthermore, Figures 3e–3g show scanning electron microscopy (SEM) images of AC@S (5% S) and the corresponding energy dispersive X-ray spectroscopy (EDS) maps for C and S, which further confirmed the distribution of S on the AC surface. Prior to assembly of the LSCs, the properties and electrochemical performances of all electrode materials were examined. As shown in Figure S2a, the specific capacity of AC was only 165.8 mAh g−1 at 0.1 mA cm−2, whereas that of AC@S (5% S) was 224.1 mAh g−1, thereby representing a 35% increase. These results demonstrate the important role of AC@S in improving the electrochemical performance of the device. Subsequently, cyclic voltammetry (CV) and galvanostatic charge–discharge measurements were conducted on the prepared LSCs using AC@S (5% S) and pre-lithiated graphite as the cathode and anode 9



materials, respectively. Thus, Figure 4a shows the CV curves of these asymmetric energy storage systems measured in the potential range of 1.5–3.8 V at a scan rate of 0.2 mV s−1. As indicated, the CV curves show quasi-rectangular shapes with faradaic redox reactions of S, suggesting that AC@S (5% S) exhibits both electrical double-layer capacitor-type behavior and pseudocapacitive properties, thereby indicating that these devices function via a hybrid charge storage mechanism that could be attributed to the different properties of AC and S. In addition, the observation of a pair of sharp redox peaks indicates that the electrochemical reduction–oxidation of S occurred in two stages during charge–discharge. According to a previously reported mechanism, the two separated reduction peaks at 2.33 and 2.00 V correspond to the conversion of higher-order lithium polysulfides (e.g., Li2S8) to lower-order species (Li2Sx, 4≤x≤8) and the conversion of lithium polysulfides to solid-state Li2S2/Li2S, respectively. Furthermore, the oxidation peak at 2.24 V was associated with the formation of Li2Sx (x>2).23–24 Notably, no changes in the CV peak positions or peak intensities (see the inset of Figure 4a) were observed, thereby demonstrating the electrochemical stability and effectiveness of the AC@S (5% S) composite in preventing the loss of S to the electrolyte. Thus, a high utilization rate of active S was maintained in the redox reactions. This process continued until lithium polysulfide was completely consumed and elemental S was produced at 2.35 V. As shown in Figure 4b, the CV peak current and potential both exhibit polarization behavior upon increasing the scan rate. However, with the exception of peak IV, which overlaps with peak III at high scan rates, complete and well-separated peaks were observed even at a high scan rate of 5 mV s−1. These results thereby indicate that the LSCs exhibited both superior rate abilities and excellent reversibilities. Figure 4c presents the charge/discharge voltage profiles of the LSC at various fixed current densities (per unit electrode area) within a potential window of 1.5–3.8 V. As indicated, the LSC delivered high 10


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capacities of 128.9 mAh g–1 at 0.1 mA cm−2 (30 mA g−1) and 103.0 mAh g−1 at 1.0 mA cm−2 (303 mA g−1). In addition, the reversible discharge capacity reached 89.1 mAh g−1 at a high current density of 10 mA cm−2 (3025 mA g−1), and a similar pattern of charge and discharge plateaus was observed even at particularly high current rates. To demonstrate the rate capabilities of this system, the current density was varied between 0.1 and 50 mA cm−2, as shown in Figure 4d. At the maximum discharge rate of 50 mA cm−2 (15123 mA g−1), the capacity remained high (i.e., 67.7 mAh g−1), with only a 30.8% decrease in capacity being observed compared to that at 1 mA cm−2 (303 mA g−1). The degree of graphite pre-lithiation in the LSC system was then examined using discharge curves of the graphite anode in a three-electrode system and in a graphite half-cell. Figure S2c shows the charge–discharge profile for the graphite half-cell, where the specific capacity of graphite was 278 mAg g−1, while Figure S2d shows the charge–discharge curves of the three-electrode LSC. In accordance with the half-cell result, the charge–discharge curve indicates that the intercalation/de-intercalation processes occurred at low potentials (

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