Letter pubs.acs.org/JPCL
Porous Carbon Mat as an Electrochemical Testing Platform for Investigating the Polysulfide Retention of Various Cathode Configurations in Li−S Cells Sheng-Heng Chung,†,# Richa Singhal,‡,# Vibha Kalra,*,‡ and Arumugam Manthiram*,† †
Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States ‡ Department of Chemical and Biological Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: Two optimized cathode configurations (a porous current collector and an interlayer) are utilized to determine the better architecture for improving the cycle stability and reversibility of lithium−sulfur (Li−S) cells. The electrochemical analysis on the upperplateau discharge capacity (QH) and the lower-plateau discharge capacity (QL) is introduced for assessing, respectively, the polysulfide retention and the electrochemical reactivity of the cell. The analysis results in line with the expected materials chemistry principles suggest that the interlayer configuration offers stable cell performance for sulfur cathodes. The significance of the interlayer is to block the free migration of the dissolved polysulfides, which is a key factor for immobilizing and continuously utilizing the active material in sulfur cathodes. Accordingly, the carbon mat interlayers provide sulfur cathodes with a high discharge capacity of 864 mA h g−1 at 1 C rate with a high capacity retention rate of 61% after 400 cycles.
T
volume change of 80%. Such a volume change leads to a disruption in the contact among insulating sulfur, conductive carbon, binder, and the current collector, resulting in rapid capacity fade during cycling.3,6,9 The progress on the cell configuration design has almost conquered the intrinsic deficiencies of the sulfur cathodes and accomplished high active material utilization, electrochemical reversibility, and cycle stability.2,3 The enhanced cycling performance results from (i) the improved redox accessibility of the active material with the support of conductive additives and (ii) the immobilization of polysulfides within the cathode region of the cell with the use of porous substrates or polysulfide diffusion barriers.9−11 At the microscopic level, composite cathodes have demonstrated the ability to encapsulate the active material within the nanospace of either the carbon hosts12−19 or the functional polymer frameworks. 12,20−23 At the macroscopic level, new cathode configurations, porous current collectors,24−28 and interlayers,29−33 have employed conductive and porous substrates to increase the cathode conductivity and immobilize the active material within the cathode. Recently, our group has reported several Li−S cells with optimized cathode architectures.27−30 The porous current collectors hold the active material within the cathode with their porous framework and are able to
he insertion compound cathodes used in current Li ion batteries are limited by their charge-storage capacities, restricting the overall energy density. Faced with this reality, there is an immense demand to develop high-capacity cathodes. Sulfur, a solid conversion reaction cathode, is an appealing candidate as it provides an order of magnitude higher chargestorage capacity (1675 mA h g−1) than insertion reaction cathodes.1−4 Thus, sulfur cathodes offer a promising solution to boost the energy and power of next-generation rechargeable batteries. Moreover, sulfur is inexpensive, abundant, and environmentally friendly.2,3 However, several technical challenges have delayed the commercialization of Li−S batteries.3−6 First, the insulating nature of sulfur and its end-discharge product (Li2S) limits the effective active material utilization, lowering the practical discharge capacity.2−6 Second, the conversion reaction between S and Li2S involves the formation of Li polysulfides (Li2Sx, x = 4−8), which are highly soluble in the liquid electrolyte used in Li−S cells. When dissolved, the polysulfides easily diffuse out from the cathode, resulting in a significant loss of active material and electrochemical instability.7,8 The dissolved polysulfides penetrate easily through the polymer separator and react with the Li−metal anode. This creates a harmful redox shuttle reaction within the cell and degradation of the Li−metal anode. The shuttling polysulfides subsequently accumulate on the cathode surface and convert to inactive and insoluble Li2S/Li2S2, lowering the redox accessibility and electrochemical reversibility.2−7 Moreover, the phase transition of sulfur−polysulfide−sulfide causes a © XXXX American Chemical Society
Received: May 5, 2015 Accepted: May 24, 2015
2163
DOI: 10.1021/acs.jpclett.5b00927 J. Phys. Chem. Lett. 2015, 6, 2163−2169
Letter
The Journal of Physical Chemistry Letters
Figure 1. Microstructural analysis of the porous carbon mat: (a) SEM observation (the inset is the high-magnification SEM); (b) EDX analysis and elemental mapping; (c) isotherm; and (d) pore size distributions (PSDs).
abundant nanopores throughout the carbon nanofibers and microcracks on the carbon nanofibers, which increase the amount of micro/mesopores and the porosity of the fibrous substrate.34,35 The co-continuous porous morphology of carbon nanofibers fabricated similarly have been studied in detail previously in the Kalra research group.34,35 A representative TEM image of the porous carbon mat is shown in Figure S1 (Supporting Information). The surface area and total pore volume of the porous carbon mat are, respectively, 521 m2 g−1 and 0.64 cm3 g−1. In Figure 1c, the type-I isotherm and type-H4 hysteresis loop are indicative of, respectively, the high micropore filling behavior and slit-like narrow mesopores.36 Figure 1d summarizes the pore size distribution (PSD) based on different models. The broad PSD that was analyzed by the Barrett−Joyner−Halenda (BJH) method demonstrates the high micro/mesoporosity of the porous carbon mat. The adsorption characterization of micro/mesopores was analyzed with the density functional theory (DFT) model. The DFT model PSD reveals a micropore size of 1.34 nm and mesopore sizes between 2.64 and 5.69 nm.36,37 A tail raised at 0.72 nm denotes high microporosity at the small micropore region. Thus, the use of the Horvath−Kawazoe (HK) model is imperative to investigate the micropore size. The HK model PSD shows major micropore sizes of 0.55 and 1.26 nm, which agrees with the DFT model PSD in the micropore region. The micropore surface area and corresponding micropore volume are, respectively, 110 m2 g−1 and 0.23 cm3 g−1.36,38 As a result, the microstructure and porosity analyses demonstrate the hierarchically micro/meso/macroporous structure of the porous carbon mat. To examine the relationship between the cathode configuration and the cell performance, porous carbon mats were used as either the porous current collector (porous current collector cell, Figure S2a, Supporting Information) or as the interlayer (interlayer cell, Figure S2b, Supporting Information).
enhance the redox accessibility by functioning as an inner conductive matrix.10,11,24−28 The interlayers inserted between the sulfur cathode and the separator aim at filtering out the polysulfides by their nanospace and then reutilizing the trapped active material by serving as an upper-current collector.11,29−33 Thus, it is instructive to conduct a comparative analysis between these two optimized cathode architectures to understand the origin of improved cell cyclability.3,11 Herein, we report the use of a porous carbon mat as either the porous current collector or the interlayer in investigating the electrochemical behavior of Li−S cells. We also introduce the analysis of the upper-plateau and lower-plateau discharge capacities (QH/QL) as a simple electrochemical tool to provide quantified information for better evaluating the polysulfide retention and reaction accessibility. The free-standing porous carbon mats exhibit a hierarchically porous structure consisting of internal micro/mesopores within the nanofibers and external intrafiber macropores. Thus, the use of a micro/meso/ macroporous structure aims at achieving a fair comparison of the cell performance and the QH/QL analysis of the cells by employing different cathode configurations. The free-standing porous carbon mat with a nonwoven fibrous network was fabricated using a facile electrospinning method.31,34 The electrospinning solution contained 20 wt % polyacrylonitrile (as the carbon precursor) and 80 wt % Nafion (as the sacrificial polymer) in N,N-dimethylformamide (DMF). The carbonization treatment at 1000 °C was used for the selective decomposition of the sacrificial polymer. This created a high-porosity framework with abundant nanopores and microcracks within the carbon mat.34,35 The scanning electron microscopy (SEM) images (Figure 1a) and the corresponding elemental mapping image performed with energy-dispersive Xray spectroscopy (EDX, Figure 1b) show that the porous carbon mats possess a coalescing carbon nanofiber network. The inserted high-magnification SEM image demonstrates 2164
DOI: 10.1021/acs.jpclett.5b00927 J. Phys. Chem. Lett. 2015, 6, 2163−2169
Letter
The Journal of Physical Chemistry Letters
Figure 2. Electrochemical analyses of Li−S cells with porous carbon mats: (a) discharge/charge curves of the current collector cell; (b) discharge/ charge curves of the interlayer cell; (c) reversible QH; (d) reversible QL; and (e) cycle stability at a C/5 rate.
has a relatively lower polarization (ΔE = 0.18 V) than the current collector cell (ΔE = 0.25 V).33,40 The enhanced cyclability in the interlayer cell is further confirmed by (i) the overlap of the scanning curves in the cyclic voltammograms (CVs) during repeated scans (Figure S4, Supporting Information) and (ii) the overlapping curves in the discharge/charge voltage profiles at various cycling rates (Figure S5, Supporting Information).14,40 In addition to identifying the cycle stability by the discharge/ charge voltage and the CV profiles, the QH/QL analysis provides quantified data for better evaluating the electrochemical stability and reversibility. First, the upper-discharge plateau that involves the fast reaction kinetics (solid-to-liquid phase transition) relates to the production and diffusion of polysulfides, which impacts mainly the loss of capacity and the electrochemical stability of cells.41−43 Therefore, the QH analysis is used to determine the polysulfide retention level of the cells employing different cell configurations. In Figure 2c, both cell configurations attain a high initial QH utilization of 90%. The theoretical value of QH is 419 mA h g−1. After 200 cycles, the interlayer cell has a highly reversible QH, retaining 74% of its original value as compared to that of the current collector cell (retention rate of QH (RQH): 29%, Figure S6a, Supporting Information). Therefore, the Q H and the corresponding RQH values suggest that the interlayer cell has
The comparison of the electrochemical performance of the cells employing different configurations starts from the electrochemical impedance spectroscopy (EIS) measurements (Figure S3, Supporting Information). The EIS data show a chargetransfer resistance of
