CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201402329
Free-Standing Nitrogen-doped Graphene Paper as Electrodes for High-Performance Lithium/Dissolved Polysulfide Batteries Kai Han,[a, b] Jingmei Shen,[b] Shiqiang Hao,[c] Hongqi Ye,[a] Christopher Wolverton,[c] Mayfair C. Kung,*[b] and Harold H. Kung*[b] Free-standing N-doped graphene papers (NGP), generated by pyrolysis of polydiallyldimethylammonium chloride, were successfully used as binder-free electrodes for the state-of-the-art Li/polysulfide-catholyte batteries. They exhibited high specific capacities of approximately1000 mA h g1 (based on S) after 100 cycles and coulombic efficiencies great than 98 %, significantly better than undoped graphene paper (GP). These NGP were characterized with XRD, X-ray photoelectron spectroscopy, thermogravimetric analysis, AFM, electron microscopy, and Raman and impedance spectroscopy before and after cycling.
Spectroscopic evidence suggested stronger binding of sulfide to NGP relative to GP, and modelling results from DFT calculation, substantiated with experimental data, indicated that pyrrolic and pyridinic N atoms interacted more strongly with Li polysulfides than quaternary N atoms. Thus, more favorable partition of polysulfides between the electrode and the electrolyte and the corresponding effect on the morphology of the passivation layer were the causes of the beneficial effect of N doping.
Introduction The demands of emerging energy storage applications in electric vehicles and large-scale stationary grid necessitate the development of new materials and electrochemistry beyond state-of-the-art lithium-ion batteries.[1] Lithium–sulfur (Li–S) batteries are promising candidates for the next generation rechargeable systems owing to their low cost and high theoretical specific energy of 2567 Wh kg1,[2] which is a consequence of their conversion electrochemistry in which sulfur undergoes the redox reaction: S8 + 16 e + 16 Li + !8 Li2S, to form lithium sulfide (Li2S) via a series of intermediate polysulfides.[3] However, there exist issues with Li–S batteries that have to be resolved before this technology can become practical.[4] Among the most critical issues is the dissolution and shuttling of longchain polysulfide intermediates in the organic electrolyte, which results in the formation of passivation films on the electrodes, loss of active materials, and, consequently, rapid capacity degradation and low coulombic efficiencies.[5] Other issues [a] K. Han, Prof. H. Ye College of Chemistry and Chemical Engineering Central South University Changsha, 410083 (P.R. China) [b] K. Han, Dr. J. Shen, Prof. M. C. Kung, Prof. H. H. Kung Department of Chemical and Biological Engineering Northwestern University Evanston, IL, 60208 (United States) E-mail: [email protected] [c] Dr. S. Hao, Prof. C. Wolverton Department of Material Science and Engineering Northwestern University Evanston, IL, 60208 (United States) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402329.
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include the insulating nature of sulfur and its reduced products, both ionic and electronic, and deposition of dense Li2S2/ Li2S passivation films on both cathode and lithium anode surfaces, leading to low capacity utilization and poor rate capability of the active material.[6] In addition, the large volume changes of sulfur during charge/discharge result in pulverization of active material and poor cell reversibility.[3a, 7] In order to address these issues, strategies ranging from new electrolyte formulation,[8] new electrode configuration,[9] and physical confinement[10] of sulfur within the cathode have been explored.[11] A relatively successful approach is to form carbon–sulfur composite cathodes, in which sulfur is infiltrated into different carbon matrices, such as mesoporous carbon,[12] hollow carbon nanofiber,[13] carbon nanotube,[14] and graphene stack.[15] A conducting carbon matrix with tailored porous morphology could minimize diffusion of polysulfide from the electrode to the electrolyte and accommodate volume changes during cycling. However, although relatively high discharge capacities and improved cycling stability have been reported with carbon–sulfur composite cathode,[15a, 16] it appears that physical confinement alone is insufficient to solve the polysulfide shuttling problem. Thus, significant research effort has been expended towards chemical confinement of the polysulfide species within the cathode, such as by judicious functionalization of the carbon surface to enhance its interaction with polysulfide.[17] Such a chemical trapping approach permits tunability at the molecular level, and potentially offers a high degree of control that might be necessary for the fabrication of practical Li–S batteries. Within this strategy, the approach to modify carbon with nitrogen-containing functionalities appears to be pursued intensely.[17a–c, e, 18] Thus, one objective of this ChemSusChem 2014, 7, 2545 – 2553
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Figure 1. Preparation Scheme of free-standing N-doped graphene paper (NGP).
work is to generate a deeper understanding of how nitrogen doping of carbon material affects various aspects of electrochemical performance of a Li–S battery, such as charge transfer, efficiency in the use of active material, and reversibility. Here, the effects were evaluated in the form of an electrode in a catholyte cell, as recent reports have shown that when sulfur source is in the form of dissolved polysulfide in the electrolyte (catholyte), the capacity retention and sulfur utilization were markedly better than the conventional carbon–sulfur composite cathode in many cases.[9d, 19] For example, recent studies by the Manthiram group have reported outstanding performance for catholyte cells constructed with self-standing, interwoven carbon nanotube and nanofiber paper electrodes.[19b, d] Since cathodes constructed with nanofibers or nanotubes may have a wide distribution of interspaces and pre-existing nitrogen functionalities, we opted to use graphene paper that had a narrow range of interlayer spacings and introduced the nitrogen dopants by treatment with a polydiallyldimethylammonium salt. Graphene, a two-dimensional carbon material with high conductivity, large surface area, and excellent mechanical flexibility, has been widely studied in Li-ion batteries.[20] Free-standing graphene papers (GP), made from stacks of graphene sheets, have been successfully assembled as flexible anode or 3D scaffolds to embed electrochemically active material, such as silicon and sulfur.[18a, 21] Thus, they can be easily integrated into the catholyte configuration. In this study, we prepared and compared the lithium polysulfide adsorption capacity of GP and nitrogen-doped graphene paper (NGP), as well as their intrinsic conductivities and impedance as an electrode, and first principles calculation was used to identify which nitrogen species was responsible for the higher charge capacity and improved cycling stability of NGP than GP.
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Results and Discussion Free-standing N-doped graphene was prepared by pyrolysis of poly(diallyldimethylammonium chloride) (PDDA) using the procedure shown in Figure 1. It has been shown that PDDA can act both as a reducing agent for GO and a stabilizer for the resultant rGO suspension,[22] and generate different types of N in the graphene framework via control of the pyrolysis temperature.[23] When a solution of PDDA was added to an aqueous suspension of GO prepared by the modified Hummers method, the positively charged PDDA was adsorbed onto the negatively charged GO via electrostatic interaction. At the same time, GO was slowly reduced, as the color of the suspension became increasingly darker. The resultant material is termed PDDA-rGO, where rGO stands for partly reduced GO. Free PDDA, not adsorbed on the rGO surface, could be removed by repeated washing with DDI water followed by centrifugation. The zeta-potential of PDDA-rGO was found to be + 50 mV, versus 36.5 mV for GO, implying the presence of positively charged surface species (Figure S1 in Supporting Information). The presence of adsorbed PDDA in PDDA-rGO paper was confirmed by comparing the height profiles of GO and PDDA-rGO using AFM (Figure 2). GO nanosheets showed a flat surface with a typical height of approximately 1 nm, indicating relatively effective exfoliation. The significantly rougher surface observed for PDDA-rGO that reached a height of > 9 nm was consistent with the presence of adsorbed PDDA molecules on the surface. Vacuum filtration of the suspension generated the PDDA-rGO paper. Heating this paper in Ar at 700 8C further reduced the sample, and the adsorbed PDDA was either pyrolyzed with incorporation of nitrogen into the graphene framework to form a free-standing NGP or decomposed and desorbed, as confirmed by thermogravimetric analysis (TGA) measurements (Figure S2). ChemSusChem 2014, 7, 2545 – 2553
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Figure 2. AFM image (a) and height profile (b) of graphene oxide sheets, and image (c) and height profile (d) of PDDA-rGO sheets.
1570 cm1 in graphene to 1574 cm1 in NGP[26] was consistent with the nature of the nitrogen species present as shown by the X-ray photoelectron spectroscopy (XPS) data. As expected, the ratio of XPS O 1s to C 1s signal intensities decreased from GO to PDDA-rGO to graphene and NGP (Figure S5) as the sample was increasingly more reduced, similar to the trend in the CO versus CC peaks within the C 1s spectra (Figure S6). For the NGP sample, an additional composite N 1s peak was detected (Figure 3 c), indicative of the presence of multiple nitrogen species. The different types of nitrogen species reported for N-doped graphene are pyridinic, pyrrolic, and Figure 3. (a) XRD patterns of the NGP electrode together with graphite, GO and PDDA-rGO (inset shows the plot quaternary N atoms, with their for NGP with expanded Y axis); (b) Raman spectra of GP and NGP; (c) N 1s XPS spectrum of the NGP. respective XPS binding energies at 398.6 0.3, 400.3 0.3,[27] and [28] The XRD pattern of a GO paper (Figure 3 a) showed a diffracPyridinic N atoms present at the edges of gra401.5 eV. tion peak at 10.88, indicating a layered structure with an interphene sheets are sp2 hybridized N atoms with two of its layer spacing of 0.82 nm.[24] The diffraction peak shifted to 6.38 three hybridized orbitals engaged in bonding with carbon in PDDA-rGO, corresponding to an increased average interlayer atoms and the third as a lone-pair in the plane of the aromatic spacing of 1.4 nm that was consistent with the presence of adring. The remaining p electron becomes part of the p elecsorbed PDDA. After high temperature heating to form NGP, tron cloud. A pyrrolic N atom is located in a five-carbon ring the XRD showed a weak and broad diffraction peak centered and its three sp2 hybridized orbitals are bonded to two C at 25.68 that corresponded to an interlayer spacing of 0.35 nm, atoms and a H atom. The two remaining p electrons particisuggesting some degree of reconstitution of graphitic dopate in the conjugated p cloud of the delocalized p orbitals.[23a] [23b] mains. The quaternary N atom substitutes for a C atom in the graphene plane and is bonded to three C atoms. In the NGP Raman spectroscopy was used to probe the density of desample, only pyrrolic and pyridinic species were detected and fects in graphene. In a Raman spectrum, the D band at approxtheir population distribution was estimated by curve fitting to imately 1350 cm1 corresponds to structural defects and disorbe 75 % and 25 %, respectively. It should be noted that the ders in the carbon matrix, and the G band at approximately dominant 402.3 eV peak present in the N 1s spectrum of 1570 cm1 is related to the E2g vibration mode of sp2-bonded PDDA-rGO (Figure S4) was absent in NGP. This peak is a signacarbon atoms.[25] The intensity ratios of ID/IG were 0.99 and 1.15 ture of N-containing ammonium group. Its absence indicated for GP and NGP, respectively (Figure 3 b), consistent with somecomplete decomposition of the PDDA precursor as concluded what higher densities of structural defects and disorder conseearlier. quent of doping. The slight shift of the G band from 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 4. (a) Digital photographs of a free-standing NGP and a bent strip of the NGP (inset); (b) FESEM images of the top view of NGP surface; (c) crosssectional view of NGP (d) FESEM image of N-doped graphene nanosheets; (e) TEM images of N-doped graphene nanosheets at low magnification, and (f) at high magnification.
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Figure 5. UV/Vis absorption spectra of lithium polysulfide (Li2S6) solution before and after addition of NGP and GP. There were no absorption features beyond 400 nm.
A strip of free-standing NGP can be flexed repeatedly withtribution. It is known that reduced graphene oxide has a high out breaking (Figure 4 a). Its surface was wrinkled (Figure 4 b– adsorption capacity for polysulfide due to the high surface f). Wrinkles on graphene sheets have been reported to enarea and the residual oxygen-containing groups.[15b] Here, we hance chemisorption energy due to the appearance of mid[23a] gap states. Thus, its presence could also facilitate the addemonstrated additional adsorption capacity due to the nitrosorption of polysulfide onto the electrode. The NGP consisted gen heteroatoms in the graphene layer. Further evidence of of multiple stacks of N-doped graphene nanosheets and was enhanced adsorption was provided by the impedance measapproximately 8 mm thick (Figure 4 c). urements. Adsorption of polysulfide (Li2S6) was used as a measure of its The electrochemical performance of the NGP as a free-standinteraction with GP and NGP. This was determined by monitoring electrode without the use of binder was evaluated using ing with UV/Vis spectroscopy the changes in the concentration a Li/dissolved polysulfide battery in a coin cell, using a solution of Li2S6 in a DME/DOL (1:1 vol. ratio) battery electrolyte soluof Li2S6 in DME/DOL as catholyte and a metallic lithium foil as tion after adding GP or NGP. In this experiment, all handling of the counter electrode (Figure S7). Typically, the mass ratio of S the samples was performed in an Ar glove box to avoid expoto graphene in the cell was 1:1, and we used the total mass of sure of the components to air. As shown in Figure 5, a fresh S in the cell as the basis to calculate specific capacities. polysulfide solution exhibited two characteristic peaks of S62 Figure 6 shows the electrochemical properties of a NGP electrode. When tested at a rate of 0.2C (1C = 1675 mA g1), an inispecies at approximately 260 and 280 nm.[29] Two hours after adding GP or NGP, the intensities of these peaks decreased, tial discharge (lithiation) capacity of approximately and a larger decrease was observed with NGP ( 12 %) than 1100 mA h g1 was obtained, which increased gradually to apGP ( 7 %). The changes continued such that after 24 h, the decrease was 20 % for NGP and 13 % for GP. The data suggested that the presence of N in NGP resulted in a higher affinity by providing additional adsorption sites for polysulfide. Since attempts to determine the specific surface area of NGP were unsuccessful due to limited sample availability, it is possible that NGP had a higher specific surface area that also contributed to higher adsorption of sulfur. However, surface area alone should not affect the manner in which the storage capacity decreases with cycling and could not account for the Figure 6. (a) Specific capacity and coulombic efficiencies of NPG and GP electrodes in prolonged testing at 0.2C observed difference for NGP and rate between 1.6 and 3.0 V; (b) charge/discharge voltage profiles for cycling tests in a; (c) specific capacity and GP (Figure 6). It appears that coulombic efficiencies of NPG electrode tested at different C rates; and (d) charge/discharge voltage profiles at sulfur affinity was the major con- the 20th cycle at different C rates in c. (C rate is based on the sulfur theoretical capacity of 1675 mA h g1). 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMSUSCHEM FULL PAPERS proximately 1300 mA h g1 after 5 cycles. The latter value corresponded to 78 % usage of sulfur in the catholyte. The initial increase in the capacity suggested that there existed a stabilization and activation process in the initial several cycles, which may possibly be attributed to the redistribution of active materials on the NGP electrode surface, similar to that in the report of CNF/dissolved polysulfide cells.[19d] The capacity remained at the high value for the next 25 cycles before degrading slowly to 1000 mA h g1 after 100 cycles (Figure 6 a). For comparison, the initial capacity of a graphene paper (GP) electrode was 750 mA h g1, which degraded rapidly to 500 mA h g1 after 30 cycles, and to 410 mA h g1 after 100 cycles. The significantly higher reversibility and coulombic efficiency of NGP compared with GP electrodes reaffirm the premise that enhanced interaction between polysulfides and graphene is important for high capacity utilization of active material and long cycle life. The lithiation voltage curves (Figure 6 b) for NGP showed a smaller plateau at 2.35 V and a larger one at 2.0 V, which have been assigned to the reduction of elemental sulfur to high molecular weight polysulfides Li2Sn (4 n 8) and the formation of Li2S2/Li2S from soluble polysulfide species, respectively.[15b] The two step lithiation process was consistent with the CV curve (Figure S8 a), and the lower capacity of the graphene electrode was observed for both steps in lithiation and delithiation (Figure S8 b). Except for the first 5 cycles, the coulombic efficiency was consistently > 98 % for the 100 cycles, and slightly better than the GP electrode. After the 0.2C test, the effect of C rate was examined using the same cell, and the results are shown in Figure 6 c and 6 d. The accessible capacity fell with increasing C rate, and the capacity in cycles 5 to 30 at 2C was about one third that when the C rate was 0.2C. Interestingly, the retention of capacity with cycling and coulombic efficiencies were better at higher C rates, and there was no obvious degradation after 100 cycles at 2C. These effects were probably due to the fact that at high rates, diffusion of polysulfide in the electrolyte that led to the shuttle effect became less important compared with the electrochemical process, and that the total test time and the time for undesirable chemical reactions between the Li electrode and polysulfide were shorter. Nonetheless, the capacities of NGP at high rates were still about twice that of GP (Figure S9 b). The galvanostatic charge/discharge voltage profiles for NGP at the 20th cycle at different C rates are shown in Figure 6 d. With increasing C rate, the magnitude of the hysteresis, and thus the degree of polarization, increased. Electrochemical impedance spectroscopy (EIS) measurements were conducted to gain additional insight into the effect of N doping. Figure 7 shows the Nyquist plots of the cells with NGP or GP electrodes at their OCV before cycling and after 100 cycles. All of the Nyquist plots were in the form of two or more flattened semicircles and a diagonal line at the low frequency, and they were modelled with the equivalent circuit shown in Figure S10 d. The high frequency semicircle is assigned to be associated with charge-transfer resistance (Rct) while the one in the mid frequency region corresponded to the resistance of a passivation film on the electrodes.[28] The intersection of the beginning point of the high frequency semi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 7. Nyquist plots of a Li/dissolved polysulfide cell with NGP or graphene electrode before cycling, and after 100 cycles ending at discharge.
circle and the real component axis reflects the electrolyte solution resistance or viscosity (Rs).[19d] Before cycling, the chargetransfer resistances of NGP and GP electrodes were similar because graphene was already highly conducting with or without N doping. However, the electrolyte resistance of the NGP cell was noticeably smaller than the GP cell (inset of Figure 7), suggesting a decreased amount of Li2S6 in the electrolyte (thus lower viscosity), which was consistent with a higher amount of adsorbed Li2S6 on the NGP. The presence of the second small semicircle for both NGP and GP before cycling suggested that a passivation film was formed upon standing. After 100 cycles at 0.2C, there was no obvious increase in the charger-transfer resistance for the cell with NGP, implying its good electrochemical stability. The change in the semicircle in the mid-frequency range was also small. In contrast, a pronounced increase in the semicircle in the mid-frequency region and an apparent shift of the high-frequency semicircle were observed for the GP electrode. The much more prominent mid-frequency semicircle was possibly due to the development of a dense Li2S2/Li2S passivation film on the graphene surface. To verify this, the morphology of NGP and GP electrodes after 100 cycles were examined with SEM and their images are shown in Figure 8. Figure 8 a shows the presence of a dense corrugated film on the GP surface. The consumption of polysulfide in forming the film depleted its concentration in the electrolyte and lowered the viscosity. Thus, the electrolyte impedance also decreased as manifested by the shift of the highfrequency semicircle. In contrast, the SEM image of NGP electrode showed a relatively smooth surface with occasional presence of nanoclusters of Li2S; the latter confirmed by XRD (Figure 8 e). EDX elemental mapping of carbon and sulfur (Figure 8 c and d) showed a uniform distribution of sulfur on the carbon, consistent with the presence of a uniform layer of Li2Sx coating the graphene. Since deposition of Li2Sx occurs where current passes through the electrode, a more uniform deposition implied a higher density of electrochemically active sites on the NGP electrode surface, and consequently lower current flux per site and formation of a thinner layer of insulating Li2Sx. This improved the cycling stability of the electrode and lowered the resistance due to the passivation layer. ChemSusChem 2014, 7, 2545 – 2553
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Figure 9. Results of first principle calculations showing the most stable configurations and calculated binding energies of Li2S, Li–S, or S atom with graphene sheet containing: (a) pyrrolic N, and (b) pyridinic N. Figure 8. (a) SEM images of GP electrode after 100 cycles; (b) SEM images, and corresponding EDX elemental mapping for carbon (c) and sulfur (d), and (e) XRD pattern of the NGP electrode after 100 cycles at 0.2C rate.
atoms, and the binding energy was 0.91 and 2.24 eV, respectively, whereas the strongest binding of S atom was with pyrrolic N atoms, at 1.28 eV. This stronger binding was facilitated by the presence of a higher density of delocalized electrons contributed by the N atoms, and explained the greater polysulfide adsorption capacity of NGP than GP. This led to a more uniform distribution of current density across the graphene sheet and improved electrochemical performance, as explained earlier. The computational results also suggested that the effect of N-doping needs not be confined to the location of the N atoms, since their presence affects the charge density distribution of the graphene framework, especially for the nearby carbon atoms (Figure S13). The conclusion from the DFT calculations that pyrrolic and pyridinic N sites are much more effective in enhancing the electrochemical performance was tested by comparing electrodes containing different distributions of N species. It has been reported that pyrrolic N species could be converted to quaternary N sites above 800 8C.[28] Thus, it should be possible to alter the N distribution in our samples by raising the pyrolysis temperature of the PDDA-rGO from 700 to 850 8C to form the sample NGP-850. Comparing the XPS N1 s spectrum of NGP850 in Figure 10 a with the corresponding spectrum for NGP (Figure 3 c), the main peak at 400.3 eV was shifted to 401.5 eV and broadened, indicating appearance of quaternary N species. By curve fitting, NGP-850 was found to possess 19 % pyridinic
The changes in impedance during cycling were also monitored as a function of depth of charge and discharge in the second cycle (Figure S10). The changes in the electrolyte resistance during the cycling were as expected for standard sulfur chemistry.[19d] During charging the impedances of the electrolyte solution increased continuously until the formation of Li2S, and during discharging the highest resistance coincided with the highest polysulfide concentration in the electrolyte, that is, at the end of the first discharge plateau. The total energies and relaxed geometries of sulfur-containing species adsorbed on different N sites in a N-doped graphene sheet were calculated using first principle DFT methods within the generalized gradient approximation, using periodic boundary conditions and a plane wave basis set as implemented in the Vienna ab initio simulation package (see the Supporting Information for details). The NGP was modeled with a single-layer graphene (typically 6 carbon rings) doped with pyrrolic, pyridinic (Figure 9), or quaternary (Figure S12) N atoms. The adsorption of Li2S, Li–S (to represent polysulfide species[30]), or S atom was investigated. The results were compared with those for graphene. Figure 9 a and b show the most stable adsorption configurations for sulfur-containing species on a pyrrolic N- and a pyridinic N-doped graphene framework, respectively. In all cases, binding of the sulfur-containing species at the N sites was more stable than binding on a carbon site. Comparing the different N sites, interaction of Scontaining species was stronger with pyridinic N sites or pyrrolic N sites than with quarternary N sites on graphene. The strongest binding for Li2S and Li–S species Figure 10. (a) N 1s XPS spectrum of the NGP-850; (b) cycling performance and coulombic efficiency of the cell was observed with pyridinic N with NGP-850 electrode at 0.2C. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMSUSCHEM FULL PAPERS N species (398.8 eV), 30 % pyrrolic N species (400.3 eV), and 51 % quaternary N species (401.5 eV), and the N/C ratio was 3.95 % compared with 4.54 % for NGP. That is, roughly half of the pyrrolic/pyridinic N species were converted to quaternary N species. When NGP-850 was tested as an electrode in a Li/ dissolved polysulfide battery at a rate of 0.2C under the same condition as that for NGP, an initial discharge capacity of 814 mA h g1 and a capacity of 650 mAh g1 after 100 cycles were obtained (Figure 10 b). Similar to NGP, NGP-850 also exhibited an increase in capacity in the first ten cycles. Although these values were respectable, they were roughly 65 % those of NGP, the same order of magnitude as the decrease in the pyrrolic and pyridinic N species in the electrode. These results also showed that the effect of N-doping was not due to increased electrical conductivity, since the conductivity of NGP850 was 2000 S m1, higher than 1700 S m1 for NGP in spite of its lower capacity. Since high energy density is essential in practical devices, we explored the effect of increasing the sulfur content in the cell by increasing the Li2S6 concentration in the electrolyte from 50 % to 66 %. Within experimental uncertainties, except for the first few cycles, the specific capacity (per unit of S) was identical for both concentrations (Figure 11). That is, the cell
Figure 11. Cycling performances of the cells with NGP electrodes of different sulfur loadings.
with a higher S loading also had a proportionally higher energy density without degradation in cycling stability. The cells also exhibited very similar charge/discharge voltage profiles at different cycles and rates (Figure S14). The only discernible difference was the larger number of cycles ( 20) needed for the cell with a higher sulfur loading to reach a steady state, implying a longer stabilization and activation process.
Conclusions A flexible, free-standing nitrogen-doped graphene paper prepared by pyrolysis of PDDA onto a reduced graphene oxide sheet has been demonstrated to perform effectively as a binder-free electrode for a Li/dissolved polysulfide battery. Ndoping significantly improved the charge capacity and cycling stability, and combined experimental and computational results suggested that the improvement was due to enhanced binding of S-containing species to the pyrrolic and pyridinic N sites of the electrode and the corresponding decrease in poly 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org sulfide concentration in the electrolyte. The enhanced absorptivity increased partition of the active material into the electrode relative to the electrolyte, and the nitrogen doping significantly improved the uniformity and thickness of the Li2S film coating on the cathode. The new insight into the beneficial effect of nitrogen doping could open up further avenues to improve the cathode performance.
Experimental Section Preparation of graphene paper: Graphene oxide (GO) was synthesized from flake graphite by a modified Hummers method following a previously published procedure.[21b, 31] (See details in the Supporting Information). The obtained GO was diluted to a 5 mg mL1 aqueous dispersion for storage. To make the free-standing graphene paper, 4 mL of the as-obtained GO suspension (20 mg GO) was ultrasonicated for half an hour and vacuum filtered through an anodic membrane filter (AAO, 47 mm diameter, 0.2 mm pore size, Whatman), followed by air drying at room temperature. The solid was peeled off from the AAO membrane to obtain a freestanding GO paper. Small circular disks (normally 1.4 cm diameter) were punched out from this paper and placed in a quartz tube for thermal reduction in a flow of argon (100 mL min1) at 700 8C for 1 h to obtain the free-standing graphene paper. Preparation of N-doped graphene paper: Typically, 4 mL of GO suspension (20 mg GO) was diluted with DDI water to a total volume of 20 mL (1 mg mL1). After the suspension was ultrasonicated for 0.5 hour and transferred to a stirring plate, 126 mg of NaCl (0.5 wt %) was added to increase the ionic strength. Then, 1 mL of an aqueous solution of poly(diallyldimethylammonium chloride) (PDDA, 20 wt %, MW 400 000 ~ 500 000, Sigma Aldrich) was added to the GO suspension slowly under vigorous stirring. The weight ratio of PDDA to GO was controlled at about 10:1. The GO/PDDA suspension was then heated to 90 8C for 3 h during which time the color changed from yellow–brown to black. After cooling, a stable black PDDA-rGO suspension was obtained. It was purified by several cycles of centrifugation and washing by DDI water and stored as an aqueous suspension. Filtration of the PDDA-rGO suspension with an AAO membrane produced the PDDA-rGO paper, which was further heated in a flow of argon, as described above for graphene, to obtain the NGP paper. Preparation of Li2S6 catholyte: Appropriate amounts of sublimed sulfur powder (Alfa Aesar) and Li2S (Alfa Aesar) were added to a 1:1 solvent mixture of 1, 3-dioxolane (DOL) and dimethoxy ethane (DME) to form a solution of 1.2 or 2.4 m sulfur in the form of Li2S6 in the solution. The mixture was stirred at 50 8C for 20 h under Ar to produce a blood-red Li2S6 catholyte solution, which was stored under Ar until use. Material characterization: The morphology and thickness of the free-standing NGP electrode before and after cycling were examined using a Hitachi S-8030 field emission scanning electron microscope (FE-SEM) with energy dispersive spectroscopy (EDS) detectors and a JEOL 2100F field emission transmission electron microscopy (FE-TEM). AFM images of GO and PDDA-rGO were collected with a Bruker ICON System using the standard tapping model in air. XRD patterns were collected with a Scintag XDS2000 diffractometer (CuKa, l = 1.5418 ) at 40 kV, a step size of 0.028, and a step time of 1 s. For samples after cycling, the cycled cells were disassembled inside an Ar-filled glove box, and the electrodes ChemSusChem 2014, 7, 2545 – 2553
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CHEMSUSCHEM FULL PAPERS were washed with DME/DOL (1:1 v/v) mixture solvent three times to remove the lithium salt and dried inside the glovebox at room temperature before analysis. Raman spectroscopy was conducted using an Acton TriVista CRS Confocal Raman System with a 100 objective lens and a 514.5 nm laser beam at room temperature. XPS was conducted using Thermo Scientific ESCALAB 250Xi equipped with an electron flood gun and a scanning ion gun, using the AlKa radiation (1486.6 eV) as the excitation source. The binding energy scale was calibrated with respect to adventitious carbon (C 1s). TGA was performed with TGA/SDTA851e analyzer (Mettler Toledo) at a heating rate of 10 8C min1 from 30 to 500 8C in Ar. The zeta-potential of the GO and PDDA-rGO aqueous suspensions was measured using a Malvern Zeta Sizer at the concentration of 0.1 mg mL1 and room temperature. To evaluate the adsorption polysulfide onto NGP and GP, Li2S6 solution (10 mL , 0.12 mmol L1, prepared by dissolving Li2S and sulfur in 1:1 v/v DME and DOL at 50 8C under Ar was divided evenly into 3 sealed vials (air-free). 1 mg of NGP or GP was separately put into two of the vials, and the third was left as the control. UV/Vis absorption spectra of these solutions were collected using a Perkin Elmer LAMBDA 1050 spectrophotometer at different times. Electrochemical tests: Electrochemical measurements were carried out using two electrode 2032-type coin cells with Li metal as the counter electrode. The cells were assembled in an Ar-filled glove box. The free-standing GP and NGP were directly used as positive electrode without any further treatment. They were typically 0.4– 0.5 mg cm2 and 8 mm thickness. To assemble the cell, 20 mL of the Li2S6 polysulfide catholyte was dropped carefully onto the freestanding GP or NGP paper electrode, corresponding to 0.8 mg (50 wt %, 0.53 mg cm2) or 1.6 mg (66 wt %, 1.06 mg cm2) of sulfur respectively, for the 1.2 and 2.4 m [S] catholytes. The electrolyte consisted of 1.0 m lithium bis(trifluoromethanesulfonyl) imide (LITFSI) and 0.1 m LiNO3 in 1:1 v/v DOL and DME. A Celgard 2500 membrane was used as separator and another 20 mL of electrolyte was added into the Celgard, so that the total liquid volume in the cell was fixed at 40 mL (20 mL Li2S6 solution + 20 mL electrolyte). An Al foil was used as backing for the paper electrodes. Cyclic voltammetry data were recorded using Solartron 1260/1287 electrochemical interface in the voltage range of 1.5–3.0 V at a scan rate of 0.05 mV s1. The galvanostatic cycling measurements were conducted with a Arbin BT2000 potentiostat/galvanostat system (Arbin Instruments) at various current density rates, typically in the voltage range of 1.6–3.0 V vs Li/Li + . The capacities shown in this paper were calculated based on the total mass of sulfur within the catholyte in the cell. Electrochemical impedance data were collected also using the two electrode coin cell on a Solartron 1260 impedance analyzer coupled with a Solartron 1286 electrochemical interface by applying an AC voltage of 10 mV amplitude and DC open circuit voltage in the frequency range of 100 kHz to 0.1 Hz at room temperature.
Acknowledgements This research was supported by the U.S. Department of Energy, Basic Energy Sciences, grant DE-AC02-06CH11357 through the Center for Electrical Energy Storage, an Energy Frontier Research Center. K.H. acknowledges funding support from China Scholarship Council and Hunan Provincial Innovation Foundation for Postgraduate. H.Y. acknowledges funding support from National Natural Science Foundation of China (21276284). 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org Keywords: binder-free · catholyte · free-standing · Li/ polysulfide battery · N-doped graphene [1] a) J.-M. Tarascon, M. Armand, Nature 2001, 414, 359 – 367; b) N. S. Choi, Z. Chen, S. A. Freunberger, X. Ji, Y. K. Sun, K. Amine, G. Yushin, L. F. Nazar, J. Cho, P. G. Bruce, Angew. Chem. Int. Ed. 2012, 51, 9994 – 10024; Angew. Chem. 2012, 124, 10134 – 10166; c) J. B. Goodenough, Y. Kim, Chem. Mater. 2010, 22, 587 – 603; d) M. M. Thackeray, M. K. Y. Chan, L. Trahey, S. Kirklin, C. Wolverton, J. Phys. Chem. Lett. 2013, 4, 3607 – 3611; e) M. M. Thackeray, C. Wolverton, E. D. Isaac, Energy Environ. Sci. 2012, 5, 7854 – 7863. [2] P. G. Bruce, S. A. Freunberger, L. J. Hardwick, J.-M. Tarascon, Nat. Mater. 2012, 11, 19 – 29. [3] a) Y. X. Yin, S. Xin, Y. G. Guo, L. J. Wan, Angew. Chem. Int. Ed. 2013, 52, 13186 – 13200; Angew. Chem. 2013, 125, 13426 – 13441; b) X. Ji, L. F. Nazar, J. Mater. Chem. 2010, 20, 9821 – 9826. [4] A. Manthiram, Y. Fu, Y. Su, Acc. Chem. Res. 2013, 46, 1125 – 1134. [5] a) Y. Diao, K. Xie, S. Xiong, X. Hong, J. Electrochem. Soc. 2012, 159, A1816 – A1821; b) Y. Diao, K. Xie, S. Xiong, X. Hong, J. Power Sources 2013, 235, 181 – 186. [6] J. Shim, K. A. Striebel, E. J. Cairns, J. Electrochem. Soc. 2002, 149, A1321 – A1325. [7] X. He, J. Ren, L. Wang, W. Pu, C. Jiang, C. Wan, J. Power Sources 2009, 190, 154 – 156. [8] a) J. Gao, M. A. Lowe, Y. Kiya, H. D. AbruÇa, J. Phys. Chem. C 2011, 115, 25132 – 25137; b) Z. Lin, Z. Liu, W. Fu, N. J. Dudney, C. Liang, Adv. Funct. Mater. 2013, 23, 1064 – 1069; c) R. Xu, I. Belharouak, J. Li, X. Zhang, I. Bloom, J. BareÇo, Adv. Energy Mater. 2013, 3, 833 – 838. [9] a) J. Hassoun, B. Scrosati, Adv. Mater. 2010, 22, 5198 – 5201; b) Y. Yang, M. T. McDowell, A. Jackson, J. J. Cha, S. S. Hong, Y. Cui, Nano Lett. 2010, 10, 1486 – 1491; c) S. Zheng, Y. Chen, Y. Xu, F. Yi, Y. Zhu, Y. Liu, J. Yang, C. Wang, ACS Nano 2013, 7, 10995 – 11003; d) R. Demir-Cakan, M. Morcrette, Gangulibabu, A. Guguen, R. Dedryvre, J.-M. Tarascon, Energy Environ. Sci. 2013, 6, 176 – 182; e) Z. Yang, J. Guo, S. K. Das, Y. Yu, Z. Zhou, H. D. AbruÇa, L. A. Archer, J. Mater. Chem. A 2013, 1, 1433. [10] a) Y. Yang, G. Zheng, Y. Cui, Chem. Soc. Rev. 2013, 42, 3018 – 3032; b) Y. Yang, G. Yu, J. J. Cha, H. Wu, M. Vosgueritchian, Y. Yao, Z. Bao, Y. Cui, ACS Nano 2011, 5, 9187 – 9193; c) S. Xin, L. Gu, N.-H. Zhao, Y.-X. Yin, L.-J. Zhou, Y.-G. Guo, L.-J. Wan, J. Am. Chem. Soc. 2012, 134, 18510 – 18513; d) X. Ji, K. T. Lee, L. F. Nazar, Nat. Mater. 2009, 8, 500 – 506; e) W. Zhou, Y. Yu, H. Chen, F. J. DiSalvo, H. D. Abruna, J. Am. Chem. Soc. 2013, 135, 16736 – 16743; f) Z. Wei Seh, W. Li, J. J. Cha, G. Zheng, Y. Yang, M. T. McDowell, P. C. Hsu, Y. Cui, Nat. Commun. 2013, 4, 1331 – 1336. [11] S. Evers, L. F. Nazar, Acc. Chem. Res. 2013, 46, 1135 – 1143. [12] J. Schuster, G. He, B. Mandlmeier, T. Yim, K. T. Lee, T. Bein, L. F. Nazar, Angew. Chem. Int. Ed. 2012, 51, 3591 – 3595; Angew. Chem. 2012, 124, 3651 – 3655. [13] a) G. Zheng, Y. Yang, J. J. Cha, S. S. Hong, Y. Cui, Nano Lett. 2011, 11, 4462 – 4467; b) L. Ji, M. Rao, S. Aloni, L. Wang, E. J. Cairns, Y. Zhang, Energy Environ. Sci. 2011, 4, 5053 – 5059. [14] J. Guo, Y. Xu, C. Wang, Nano Lett. 2011, 11, 4288 – 4294. [15] a) H. Wang, Y. Yang, Y. Liang, J. T. Robinson, Y. Li, A. Jackson, Y. Cui, H. Dai, Nano Lett. 2011, 11, 2644 – 2647; b) L. Ji, M. Rao, H. Zheng, L. Zhang, Y. Li, W. Duan, J. Guo, E. J. Cairns, Y. Zhang, J. Am. Chem. Soc. 2011, 133, 18522 – 18525; c) S. Evers, L. F. Nazar, Chem. Commun. 2012, 48, 1233 – 1235; d) R. Chen, T. Zhao, J. Lu, F. Wu, L. Li, J. Chen, G. Tan, Y. Ye, K. Amine, Nano Lett. 2013, 13, 4642 – 4649. [16] a) S. Moon, Y. H. Jung, W. K. Jung, D. S. Jung, J. W. Choi, D. K. Kim, Adv. Mater. 2013, 25, 6547 – 6553; b) G. He, X. Ji, L. Nazar, Energy Environ. Sci. 2011, 4, 2878 – 2883. [17] a) J. Song, T. Xu, M. L. Gordin, P. Zhu, D. Lv, Y. B. Jiang, Y. Chen, Y. Duan, D. Wang, Adv. Funct. Mater. 2014, 24, 1243 – 1250; b) F. Sun, J. Wang, H. Chen, W. Li, W. Qiao, D. Long, L. Ling, ACS Appl. Mater. Interfaces 2013, 5, 5630 – 5638; c) F. Sun, J. Wang, H. Chen, W. Qiao, L. Ling, D. Long, Sci. Rep. 2013, 3, 2823 – 2830; d) J. Yang, J. Xie, X. Zhou, Y. Zou, J. Tang, S. Wang, F. Chen, L. Wang, J. Phys. Chem. C 2014, 118, 1800 – 1807; e) Z. Wang, X. Niu, J. Xiao, C. Wang, J. Liu, F. Gao, RSC. Adv. 2013, 3, 16775 – 16780; f) Z. W. Seh, H. Wang, P.-C. Hsu, Q. Zhang, W. Li, G. Zheng, H. Yao, Y. Cui, Energy Environ. Sci. 2014, 7, 672 – 676.
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Received: April 19, 2014 Revised: May 25, 2014 Published online on July 22, 2014
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Free-Standing Nitrogen-doped Graphene Paper as Electrodes for High-Performance Lithium/Dissolved Polysulfide Batteries Kai Han,[a, b] Jingmei Shen,[b] Shiqiang Hao,[c] Hongqi Ye,[a] Christopher Wolverton,[c] Mayfair C. Kung,*[b] and Harold H. Kung*[b] Free-standing N-doped graphene papers (NGP), generated by pyrolysis of polydiallyldimethylammonium chloride, were successfully used as binder-free electrodes for the state-of-the-art Li/polysulfide-catholyte batteries. They exhibited high specific capacities of approximately1000 mA h g1 (based on S) after 100 cycles and coulombic efficiencies great than 98 %, significantly better than undoped graphene paper (GP). These NGP were characterized with XRD, X-ray photoelectron spectroscopy, thermogravimetric analysis, AFM, electron microscopy, and Raman and impedance spectroscopy before and after cycling.
Spectroscopic evidence suggested stronger binding of sulfide to NGP relative to GP, and modelling results from DFT calculation, substantiated with experimental data, indicated that pyrrolic and pyridinic N atoms interacted more strongly with Li polysulfides than quaternary N atoms. Thus, more favorable partition of polysulfides between the electrode and the electrolyte and the corresponding effect on the morphology of the passivation layer were the causes of the beneficial effect of N doping.
Introduction The demands of emerging energy storage applications in electric vehicles and large-scale stationary grid necessitate the development of new materials and electrochemistry beyond state-of-the-art lithium-ion batteries.[1] Lithium–sulfur (Li–S) batteries are promising candidates for the next generation rechargeable systems owing to their low cost and high theoretical specific energy of 2567 Wh kg1,[2] which is a consequence of their conversion electrochemistry in which sulfur undergoes the redox reaction: S8 + 16 e + 16 Li + !8 Li2S, to form lithium sulfide (Li2S) via a series of intermediate polysulfides.[3] However, there exist issues with Li–S batteries that have to be resolved before this technology can become practical.[4] Among the most critical issues is the dissolution and shuttling of longchain polysulfide intermediates in the organic electrolyte, which results in the formation of passivation films on the electrodes, loss of active materials, and, consequently, rapid capacity degradation and low coulombic efficiencies.[5] Other issues [a] K. Han, Prof. H. Ye College of Chemistry and Chemical Engineering Central South University Changsha, 410083 (P.R. China) [b] K. Han, Dr. J. Shen, Prof. M. C. Kung, Prof. H. H. Kung Department of Chemical and Biological Engineering Northwestern University Evanston, IL, 60208 (United States) E-mail: [email protected] [c] Dr. S. Hao, Prof. C. Wolverton Department of Material Science and Engineering Northwestern University Evanston, IL, 60208 (United States) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201402329.
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include the insulating nature of sulfur and its reduced products, both ionic and electronic, and deposition of dense Li2S2/ Li2S passivation films on both cathode and lithium anode surfaces, leading to low capacity utilization and poor rate capability of the active material.[6] In addition, the large volume changes of sulfur during charge/discharge result in pulverization of active material and poor cell reversibility.[3a, 7] In order to address these issues, strategies ranging from new electrolyte formulation,[8] new electrode configuration,[9] and physical confinement[10] of sulfur within the cathode have been explored.[11] A relatively successful approach is to form carbon–sulfur composite cathodes, in which sulfur is infiltrated into different carbon matrices, such as mesoporous carbon,[12] hollow carbon nanofiber,[13] carbon nanotube,[14] and graphene stack.[15] A conducting carbon matrix with tailored porous morphology could minimize diffusion of polysulfide from the electrode to the electrolyte and accommodate volume changes during cycling. However, although relatively high discharge capacities and improved cycling stability have been reported with carbon–sulfur composite cathode,[15a, 16] it appears that physical confinement alone is insufficient to solve the polysulfide shuttling problem. Thus, significant research effort has been expended towards chemical confinement of the polysulfide species within the cathode, such as by judicious functionalization of the carbon surface to enhance its interaction with polysulfide.[17] Such a chemical trapping approach permits tunability at the molecular level, and potentially offers a high degree of control that might be necessary for the fabrication of practical Li–S batteries. Within this strategy, the approach to modify carbon with nitrogen-containing functionalities appears to be pursued intensely.[17a–c, e, 18] Thus, one objective of this ChemSusChem 2014, 7, 2545 – 2553
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Figure 1. Preparation Scheme of free-standing N-doped graphene paper (NGP).
work is to generate a deeper understanding of how nitrogen doping of carbon material affects various aspects of electrochemical performance of a Li–S battery, such as charge transfer, efficiency in the use of active material, and reversibility. Here, the effects were evaluated in the form of an electrode in a catholyte cell, as recent reports have shown that when sulfur source is in the form of dissolved polysulfide in the electrolyte (catholyte), the capacity retention and sulfur utilization were markedly better than the conventional carbon–sulfur composite cathode in many cases.[9d, 19] For example, recent studies by the Manthiram group have reported outstanding performance for catholyte cells constructed with self-standing, interwoven carbon nanotube and nanofiber paper electrodes.[19b, d] Since cathodes constructed with nanofibers or nanotubes may have a wide distribution of interspaces and pre-existing nitrogen functionalities, we opted to use graphene paper that had a narrow range of interlayer spacings and introduced the nitrogen dopants by treatment with a polydiallyldimethylammonium salt. Graphene, a two-dimensional carbon material with high conductivity, large surface area, and excellent mechanical flexibility, has been widely studied in Li-ion batteries.[20] Free-standing graphene papers (GP), made from stacks of graphene sheets, have been successfully assembled as flexible anode or 3D scaffolds to embed electrochemically active material, such as silicon and sulfur.[18a, 21] Thus, they can be easily integrated into the catholyte configuration. In this study, we prepared and compared the lithium polysulfide adsorption capacity of GP and nitrogen-doped graphene paper (NGP), as well as their intrinsic conductivities and impedance as an electrode, and first principles calculation was used to identify which nitrogen species was responsible for the higher charge capacity and improved cycling stability of NGP than GP.
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Results and Discussion Free-standing N-doped graphene was prepared by pyrolysis of poly(diallyldimethylammonium chloride) (PDDA) using the procedure shown in Figure 1. It has been shown that PDDA can act both as a reducing agent for GO and a stabilizer for the resultant rGO suspension,[22] and generate different types of N in the graphene framework via control of the pyrolysis temperature.[23] When a solution of PDDA was added to an aqueous suspension of GO prepared by the modified Hummers method, the positively charged PDDA was adsorbed onto the negatively charged GO via electrostatic interaction. At the same time, GO was slowly reduced, as the color of the suspension became increasingly darker. The resultant material is termed PDDA-rGO, where rGO stands for partly reduced GO. Free PDDA, not adsorbed on the rGO surface, could be removed by repeated washing with DDI water followed by centrifugation. The zeta-potential of PDDA-rGO was found to be + 50 mV, versus 36.5 mV for GO, implying the presence of positively charged surface species (Figure S1 in Supporting Information). The presence of adsorbed PDDA in PDDA-rGO paper was confirmed by comparing the height profiles of GO and PDDA-rGO using AFM (Figure 2). GO nanosheets showed a flat surface with a typical height of approximately 1 nm, indicating relatively effective exfoliation. The significantly rougher surface observed for PDDA-rGO that reached a height of > 9 nm was consistent with the presence of adsorbed PDDA molecules on the surface. Vacuum filtration of the suspension generated the PDDA-rGO paper. Heating this paper in Ar at 700 8C further reduced the sample, and the adsorbed PDDA was either pyrolyzed with incorporation of nitrogen into the graphene framework to form a free-standing NGP or decomposed and desorbed, as confirmed by thermogravimetric analysis (TGA) measurements (Figure S2). ChemSusChem 2014, 7, 2545 – 2553
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Figure 2. AFM image (a) and height profile (b) of graphene oxide sheets, and image (c) and height profile (d) of PDDA-rGO sheets.
1570 cm1 in graphene to 1574 cm1 in NGP[26] was consistent with the nature of the nitrogen species present as shown by the X-ray photoelectron spectroscopy (XPS) data. As expected, the ratio of XPS O 1s to C 1s signal intensities decreased from GO to PDDA-rGO to graphene and NGP (Figure S5) as the sample was increasingly more reduced, similar to the trend in the CO versus CC peaks within the C 1s spectra (Figure S6). For the NGP sample, an additional composite N 1s peak was detected (Figure 3 c), indicative of the presence of multiple nitrogen species. The different types of nitrogen species reported for N-doped graphene are pyridinic, pyrrolic, and Figure 3. (a) XRD patterns of the NGP electrode together with graphite, GO and PDDA-rGO (inset shows the plot quaternary N atoms, with their for NGP with expanded Y axis); (b) Raman spectra of GP and NGP; (c) N 1s XPS spectrum of the NGP. respective XPS binding energies at 398.6 0.3, 400.3 0.3,[27] and [28] The XRD pattern of a GO paper (Figure 3 a) showed a diffracPyridinic N atoms present at the edges of gra401.5 eV. tion peak at 10.88, indicating a layered structure with an interphene sheets are sp2 hybridized N atoms with two of its layer spacing of 0.82 nm.[24] The diffraction peak shifted to 6.38 three hybridized orbitals engaged in bonding with carbon in PDDA-rGO, corresponding to an increased average interlayer atoms and the third as a lone-pair in the plane of the aromatic spacing of 1.4 nm that was consistent with the presence of adring. The remaining p electron becomes part of the p elecsorbed PDDA. After high temperature heating to form NGP, tron cloud. A pyrrolic N atom is located in a five-carbon ring the XRD showed a weak and broad diffraction peak centered and its three sp2 hybridized orbitals are bonded to two C at 25.68 that corresponded to an interlayer spacing of 0.35 nm, atoms and a H atom. The two remaining p electrons particisuggesting some degree of reconstitution of graphitic dopate in the conjugated p cloud of the delocalized p orbitals.[23a] [23b] mains. The quaternary N atom substitutes for a C atom in the graphene plane and is bonded to three C atoms. In the NGP Raman spectroscopy was used to probe the density of desample, only pyrrolic and pyridinic species were detected and fects in graphene. In a Raman spectrum, the D band at approxtheir population distribution was estimated by curve fitting to imately 1350 cm1 corresponds to structural defects and disorbe 75 % and 25 %, respectively. It should be noted that the ders in the carbon matrix, and the G band at approximately dominant 402.3 eV peak present in the N 1s spectrum of 1570 cm1 is related to the E2g vibration mode of sp2-bonded PDDA-rGO (Figure S4) was absent in NGP. This peak is a signacarbon atoms.[25] The intensity ratios of ID/IG were 0.99 and 1.15 ture of N-containing ammonium group. Its absence indicated for GP and NGP, respectively (Figure 3 b), consistent with somecomplete decomposition of the PDDA precursor as concluded what higher densities of structural defects and disorder conseearlier. quent of doping. The slight shift of the G band from 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 4. (a) Digital photographs of a free-standing NGP and a bent strip of the NGP (inset); (b) FESEM images of the top view of NGP surface; (c) crosssectional view of NGP (d) FESEM image of N-doped graphene nanosheets; (e) TEM images of N-doped graphene nanosheets at low magnification, and (f) at high magnification.
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Figure 5. UV/Vis absorption spectra of lithium polysulfide (Li2S6) solution before and after addition of NGP and GP. There were no absorption features beyond 400 nm.
A strip of free-standing NGP can be flexed repeatedly withtribution. It is known that reduced graphene oxide has a high out breaking (Figure 4 a). Its surface was wrinkled (Figure 4 b– adsorption capacity for polysulfide due to the high surface f). Wrinkles on graphene sheets have been reported to enarea and the residual oxygen-containing groups.[15b] Here, we hance chemisorption energy due to the appearance of mid[23a] gap states. Thus, its presence could also facilitate the addemonstrated additional adsorption capacity due to the nitrosorption of polysulfide onto the electrode. The NGP consisted gen heteroatoms in the graphene layer. Further evidence of of multiple stacks of N-doped graphene nanosheets and was enhanced adsorption was provided by the impedance measapproximately 8 mm thick (Figure 4 c). urements. Adsorption of polysulfide (Li2S6) was used as a measure of its The electrochemical performance of the NGP as a free-standinteraction with GP and NGP. This was determined by monitoring electrode without the use of binder was evaluated using ing with UV/Vis spectroscopy the changes in the concentration a Li/dissolved polysulfide battery in a coin cell, using a solution of Li2S6 in a DME/DOL (1:1 vol. ratio) battery electrolyte soluof Li2S6 in DME/DOL as catholyte and a metallic lithium foil as tion after adding GP or NGP. In this experiment, all handling of the counter electrode (Figure S7). Typically, the mass ratio of S the samples was performed in an Ar glove box to avoid expoto graphene in the cell was 1:1, and we used the total mass of sure of the components to air. As shown in Figure 5, a fresh S in the cell as the basis to calculate specific capacities. polysulfide solution exhibited two characteristic peaks of S62 Figure 6 shows the electrochemical properties of a NGP electrode. When tested at a rate of 0.2C (1C = 1675 mA g1), an inispecies at approximately 260 and 280 nm.[29] Two hours after adding GP or NGP, the intensities of these peaks decreased, tial discharge (lithiation) capacity of approximately and a larger decrease was observed with NGP ( 12 %) than 1100 mA h g1 was obtained, which increased gradually to apGP ( 7 %). The changes continued such that after 24 h, the decrease was 20 % for NGP and 13 % for GP. The data suggested that the presence of N in NGP resulted in a higher affinity by providing additional adsorption sites for polysulfide. Since attempts to determine the specific surface area of NGP were unsuccessful due to limited sample availability, it is possible that NGP had a higher specific surface area that also contributed to higher adsorption of sulfur. However, surface area alone should not affect the manner in which the storage capacity decreases with cycling and could not account for the Figure 6. (a) Specific capacity and coulombic efficiencies of NPG and GP electrodes in prolonged testing at 0.2C observed difference for NGP and rate between 1.6 and 3.0 V; (b) charge/discharge voltage profiles for cycling tests in a; (c) specific capacity and GP (Figure 6). It appears that coulombic efficiencies of NPG electrode tested at different C rates; and (d) charge/discharge voltage profiles at sulfur affinity was the major con- the 20th cycle at different C rates in c. (C rate is based on the sulfur theoretical capacity of 1675 mA h g1). 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMSUSCHEM FULL PAPERS proximately 1300 mA h g1 after 5 cycles. The latter value corresponded to 78 % usage of sulfur in the catholyte. The initial increase in the capacity suggested that there existed a stabilization and activation process in the initial several cycles, which may possibly be attributed to the redistribution of active materials on the NGP electrode surface, similar to that in the report of CNF/dissolved polysulfide cells.[19d] The capacity remained at the high value for the next 25 cycles before degrading slowly to 1000 mA h g1 after 100 cycles (Figure 6 a). For comparison, the initial capacity of a graphene paper (GP) electrode was 750 mA h g1, which degraded rapidly to 500 mA h g1 after 30 cycles, and to 410 mA h g1 after 100 cycles. The significantly higher reversibility and coulombic efficiency of NGP compared with GP electrodes reaffirm the premise that enhanced interaction between polysulfides and graphene is important for high capacity utilization of active material and long cycle life. The lithiation voltage curves (Figure 6 b) for NGP showed a smaller plateau at 2.35 V and a larger one at 2.0 V, which have been assigned to the reduction of elemental sulfur to high molecular weight polysulfides Li2Sn (4 n 8) and the formation of Li2S2/Li2S from soluble polysulfide species, respectively.[15b] The two step lithiation process was consistent with the CV curve (Figure S8 a), and the lower capacity of the graphene electrode was observed for both steps in lithiation and delithiation (Figure S8 b). Except for the first 5 cycles, the coulombic efficiency was consistently > 98 % for the 100 cycles, and slightly better than the GP electrode. After the 0.2C test, the effect of C rate was examined using the same cell, and the results are shown in Figure 6 c and 6 d. The accessible capacity fell with increasing C rate, and the capacity in cycles 5 to 30 at 2C was about one third that when the C rate was 0.2C. Interestingly, the retention of capacity with cycling and coulombic efficiencies were better at higher C rates, and there was no obvious degradation after 100 cycles at 2C. These effects were probably due to the fact that at high rates, diffusion of polysulfide in the electrolyte that led to the shuttle effect became less important compared with the electrochemical process, and that the total test time and the time for undesirable chemical reactions between the Li electrode and polysulfide were shorter. Nonetheless, the capacities of NGP at high rates were still about twice that of GP (Figure S9 b). The galvanostatic charge/discharge voltage profiles for NGP at the 20th cycle at different C rates are shown in Figure 6 d. With increasing C rate, the magnitude of the hysteresis, and thus the degree of polarization, increased. Electrochemical impedance spectroscopy (EIS) measurements were conducted to gain additional insight into the effect of N doping. Figure 7 shows the Nyquist plots of the cells with NGP or GP electrodes at their OCV before cycling and after 100 cycles. All of the Nyquist plots were in the form of two or more flattened semicircles and a diagonal line at the low frequency, and they were modelled with the equivalent circuit shown in Figure S10 d. The high frequency semicircle is assigned to be associated with charge-transfer resistance (Rct) while the one in the mid frequency region corresponded to the resistance of a passivation film on the electrodes.[28] The intersection of the beginning point of the high frequency semi 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 7. Nyquist plots of a Li/dissolved polysulfide cell with NGP or graphene electrode before cycling, and after 100 cycles ending at discharge.
circle and the real component axis reflects the electrolyte solution resistance or viscosity (Rs).[19d] Before cycling, the chargetransfer resistances of NGP and GP electrodes were similar because graphene was already highly conducting with or without N doping. However, the electrolyte resistance of the NGP cell was noticeably smaller than the GP cell (inset of Figure 7), suggesting a decreased amount of Li2S6 in the electrolyte (thus lower viscosity), which was consistent with a higher amount of adsorbed Li2S6 on the NGP. The presence of the second small semicircle for both NGP and GP before cycling suggested that a passivation film was formed upon standing. After 100 cycles at 0.2C, there was no obvious increase in the charger-transfer resistance for the cell with NGP, implying its good electrochemical stability. The change in the semicircle in the mid-frequency range was also small. In contrast, a pronounced increase in the semicircle in the mid-frequency region and an apparent shift of the high-frequency semicircle were observed for the GP electrode. The much more prominent mid-frequency semicircle was possibly due to the development of a dense Li2S2/Li2S passivation film on the graphene surface. To verify this, the morphology of NGP and GP electrodes after 100 cycles were examined with SEM and their images are shown in Figure 8. Figure 8 a shows the presence of a dense corrugated film on the GP surface. The consumption of polysulfide in forming the film depleted its concentration in the electrolyte and lowered the viscosity. Thus, the electrolyte impedance also decreased as manifested by the shift of the highfrequency semicircle. In contrast, the SEM image of NGP electrode showed a relatively smooth surface with occasional presence of nanoclusters of Li2S; the latter confirmed by XRD (Figure 8 e). EDX elemental mapping of carbon and sulfur (Figure 8 c and d) showed a uniform distribution of sulfur on the carbon, consistent with the presence of a uniform layer of Li2Sx coating the graphene. Since deposition of Li2Sx occurs where current passes through the electrode, a more uniform deposition implied a higher density of electrochemically active sites on the NGP electrode surface, and consequently lower current flux per site and formation of a thinner layer of insulating Li2Sx. This improved the cycling stability of the electrode and lowered the resistance due to the passivation layer. ChemSusChem 2014, 7, 2545 – 2553
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Figure 9. Results of first principle calculations showing the most stable configurations and calculated binding energies of Li2S, Li–S, or S atom with graphene sheet containing: (a) pyrrolic N, and (b) pyridinic N. Figure 8. (a) SEM images of GP electrode after 100 cycles; (b) SEM images, and corresponding EDX elemental mapping for carbon (c) and sulfur (d), and (e) XRD pattern of the NGP electrode after 100 cycles at 0.2C rate.
atoms, and the binding energy was 0.91 and 2.24 eV, respectively, whereas the strongest binding of S atom was with pyrrolic N atoms, at 1.28 eV. This stronger binding was facilitated by the presence of a higher density of delocalized electrons contributed by the N atoms, and explained the greater polysulfide adsorption capacity of NGP than GP. This led to a more uniform distribution of current density across the graphene sheet and improved electrochemical performance, as explained earlier. The computational results also suggested that the effect of N-doping needs not be confined to the location of the N atoms, since their presence affects the charge density distribution of the graphene framework, especially for the nearby carbon atoms (Figure S13). The conclusion from the DFT calculations that pyrrolic and pyridinic N sites are much more effective in enhancing the electrochemical performance was tested by comparing electrodes containing different distributions of N species. It has been reported that pyrrolic N species could be converted to quaternary N sites above 800 8C.[28] Thus, it should be possible to alter the N distribution in our samples by raising the pyrolysis temperature of the PDDA-rGO from 700 to 850 8C to form the sample NGP-850. Comparing the XPS N1 s spectrum of NGP850 in Figure 10 a with the corresponding spectrum for NGP (Figure 3 c), the main peak at 400.3 eV was shifted to 401.5 eV and broadened, indicating appearance of quaternary N species. By curve fitting, NGP-850 was found to possess 19 % pyridinic
The changes in impedance during cycling were also monitored as a function of depth of charge and discharge in the second cycle (Figure S10). The changes in the electrolyte resistance during the cycling were as expected for standard sulfur chemistry.[19d] During charging the impedances of the electrolyte solution increased continuously until the formation of Li2S, and during discharging the highest resistance coincided with the highest polysulfide concentration in the electrolyte, that is, at the end of the first discharge plateau. The total energies and relaxed geometries of sulfur-containing species adsorbed on different N sites in a N-doped graphene sheet were calculated using first principle DFT methods within the generalized gradient approximation, using periodic boundary conditions and a plane wave basis set as implemented in the Vienna ab initio simulation package (see the Supporting Information for details). The NGP was modeled with a single-layer graphene (typically 6 carbon rings) doped with pyrrolic, pyridinic (Figure 9), or quaternary (Figure S12) N atoms. The adsorption of Li2S, Li–S (to represent polysulfide species[30]), or S atom was investigated. The results were compared with those for graphene. Figure 9 a and b show the most stable adsorption configurations for sulfur-containing species on a pyrrolic N- and a pyridinic N-doped graphene framework, respectively. In all cases, binding of the sulfur-containing species at the N sites was more stable than binding on a carbon site. Comparing the different N sites, interaction of Scontaining species was stronger with pyridinic N sites or pyrrolic N sites than with quarternary N sites on graphene. The strongest binding for Li2S and Li–S species Figure 10. (a) N 1s XPS spectrum of the NGP-850; (b) cycling performance and coulombic efficiency of the cell was observed with pyridinic N with NGP-850 electrode at 0.2C. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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CHEMSUSCHEM FULL PAPERS N species (398.8 eV), 30 % pyrrolic N species (400.3 eV), and 51 % quaternary N species (401.5 eV), and the N/C ratio was 3.95 % compared with 4.54 % for NGP. That is, roughly half of the pyrrolic/pyridinic N species were converted to quaternary N species. When NGP-850 was tested as an electrode in a Li/ dissolved polysulfide battery at a rate of 0.2C under the same condition as that for NGP, an initial discharge capacity of 814 mA h g1 and a capacity of 650 mAh g1 after 100 cycles were obtained (Figure 10 b). Similar to NGP, NGP-850 also exhibited an increase in capacity in the first ten cycles. Although these values were respectable, they were roughly 65 % those of NGP, the same order of magnitude as the decrease in the pyrrolic and pyridinic N species in the electrode. These results also showed that the effect of N-doping was not due to increased electrical conductivity, since the conductivity of NGP850 was 2000 S m1, higher than 1700 S m1 for NGP in spite of its lower capacity. Since high energy density is essential in practical devices, we explored the effect of increasing the sulfur content in the cell by increasing the Li2S6 concentration in the electrolyte from 50 % to 66 %. Within experimental uncertainties, except for the first few cycles, the specific capacity (per unit of S) was identical for both concentrations (Figure 11). That is, the cell
Figure 11. Cycling performances of the cells with NGP electrodes of different sulfur loadings.
with a higher S loading also had a proportionally higher energy density without degradation in cycling stability. The cells also exhibited very similar charge/discharge voltage profiles at different cycles and rates (Figure S14). The only discernible difference was the larger number of cycles ( 20) needed for the cell with a higher sulfur loading to reach a steady state, implying a longer stabilization and activation process.
Conclusions A flexible, free-standing nitrogen-doped graphene paper prepared by pyrolysis of PDDA onto a reduced graphene oxide sheet has been demonstrated to perform effectively as a binder-free electrode for a Li/dissolved polysulfide battery. Ndoping significantly improved the charge capacity and cycling stability, and combined experimental and computational results suggested that the improvement was due to enhanced binding of S-containing species to the pyrrolic and pyridinic N sites of the electrode and the corresponding decrease in poly 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org sulfide concentration in the electrolyte. The enhanced absorptivity increased partition of the active material into the electrode relative to the electrolyte, and the nitrogen doping significantly improved the uniformity and thickness of the Li2S film coating on the cathode. The new insight into the beneficial effect of nitrogen doping could open up further avenues to improve the cathode performance.
Experimental Section Preparation of graphene paper: Graphene oxide (GO) was synthesized from flake graphite by a modified Hummers method following a previously published procedure.[21b, 31] (See details in the Supporting Information). The obtained GO was diluted to a 5 mg mL1 aqueous dispersion for storage. To make the free-standing graphene paper, 4 mL of the as-obtained GO suspension (20 mg GO) was ultrasonicated for half an hour and vacuum filtered through an anodic membrane filter (AAO, 47 mm diameter, 0.2 mm pore size, Whatman), followed by air drying at room temperature. The solid was peeled off from the AAO membrane to obtain a freestanding GO paper. Small circular disks (normally 1.4 cm diameter) were punched out from this paper and placed in a quartz tube for thermal reduction in a flow of argon (100 mL min1) at 700 8C for 1 h to obtain the free-standing graphene paper. Preparation of N-doped graphene paper: Typically, 4 mL of GO suspension (20 mg GO) was diluted with DDI water to a total volume of 20 mL (1 mg mL1). After the suspension was ultrasonicated for 0.5 hour and transferred to a stirring plate, 126 mg of NaCl (0.5 wt %) was added to increase the ionic strength. Then, 1 mL of an aqueous solution of poly(diallyldimethylammonium chloride) (PDDA, 20 wt %, MW 400 000 ~ 500 000, Sigma Aldrich) was added to the GO suspension slowly under vigorous stirring. The weight ratio of PDDA to GO was controlled at about 10:1. The GO/PDDA suspension was then heated to 90 8C for 3 h during which time the color changed from yellow–brown to black. After cooling, a stable black PDDA-rGO suspension was obtained. It was purified by several cycles of centrifugation and washing by DDI water and stored as an aqueous suspension. Filtration of the PDDA-rGO suspension with an AAO membrane produced the PDDA-rGO paper, which was further heated in a flow of argon, as described above for graphene, to obtain the NGP paper. Preparation of Li2S6 catholyte: Appropriate amounts of sublimed sulfur powder (Alfa Aesar) and Li2S (Alfa Aesar) were added to a 1:1 solvent mixture of 1, 3-dioxolane (DOL) and dimethoxy ethane (DME) to form a solution of 1.2 or 2.4 m sulfur in the form of Li2S6 in the solution. The mixture was stirred at 50 8C for 20 h under Ar to produce a blood-red Li2S6 catholyte solution, which was stored under Ar until use. Material characterization: The morphology and thickness of the free-standing NGP electrode before and after cycling were examined using a Hitachi S-8030 field emission scanning electron microscope (FE-SEM) with energy dispersive spectroscopy (EDS) detectors and a JEOL 2100F field emission transmission electron microscopy (FE-TEM). AFM images of GO and PDDA-rGO were collected with a Bruker ICON System using the standard tapping model in air. XRD patterns were collected with a Scintag XDS2000 diffractometer (CuKa, l = 1.5418 ) at 40 kV, a step size of 0.028, and a step time of 1 s. For samples after cycling, the cycled cells were disassembled inside an Ar-filled glove box, and the electrodes ChemSusChem 2014, 7, 2545 – 2553
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CHEMSUSCHEM FULL PAPERS were washed with DME/DOL (1:1 v/v) mixture solvent three times to remove the lithium salt and dried inside the glovebox at room temperature before analysis. Raman spectroscopy was conducted using an Acton TriVista CRS Confocal Raman System with a 100 objective lens and a 514.5 nm laser beam at room temperature. XPS was conducted using Thermo Scientific ESCALAB 250Xi equipped with an electron flood gun and a scanning ion gun, using the AlKa radiation (1486.6 eV) as the excitation source. The binding energy scale was calibrated with respect to adventitious carbon (C 1s). TGA was performed with TGA/SDTA851e analyzer (Mettler Toledo) at a heating rate of 10 8C min1 from 30 to 500 8C in Ar. The zeta-potential of the GO and PDDA-rGO aqueous suspensions was measured using a Malvern Zeta Sizer at the concentration of 0.1 mg mL1 and room temperature. To evaluate the adsorption polysulfide onto NGP and GP, Li2S6 solution (10 mL , 0.12 mmol L1, prepared by dissolving Li2S and sulfur in 1:1 v/v DME and DOL at 50 8C under Ar was divided evenly into 3 sealed vials (air-free). 1 mg of NGP or GP was separately put into two of the vials, and the third was left as the control. UV/Vis absorption spectra of these solutions were collected using a Perkin Elmer LAMBDA 1050 spectrophotometer at different times. Electrochemical tests: Electrochemical measurements were carried out using two electrode 2032-type coin cells with Li metal as the counter electrode. The cells were assembled in an Ar-filled glove box. The free-standing GP and NGP were directly used as positive electrode without any further treatment. They were typically 0.4– 0.5 mg cm2 and 8 mm thickness. To assemble the cell, 20 mL of the Li2S6 polysulfide catholyte was dropped carefully onto the freestanding GP or NGP paper electrode, corresponding to 0.8 mg (50 wt %, 0.53 mg cm2) or 1.6 mg (66 wt %, 1.06 mg cm2) of sulfur respectively, for the 1.2 and 2.4 m [S] catholytes. The electrolyte consisted of 1.0 m lithium bis(trifluoromethanesulfonyl) imide (LITFSI) and 0.1 m LiNO3 in 1:1 v/v DOL and DME. A Celgard 2500 membrane was used as separator and another 20 mL of electrolyte was added into the Celgard, so that the total liquid volume in the cell was fixed at 40 mL (20 mL Li2S6 solution + 20 mL electrolyte). An Al foil was used as backing for the paper electrodes. Cyclic voltammetry data were recorded using Solartron 1260/1287 electrochemical interface in the voltage range of 1.5–3.0 V at a scan rate of 0.05 mV s1. The galvanostatic cycling measurements were conducted with a Arbin BT2000 potentiostat/galvanostat system (Arbin Instruments) at various current density rates, typically in the voltage range of 1.6–3.0 V vs Li/Li + . The capacities shown in this paper were calculated based on the total mass of sulfur within the catholyte in the cell. Electrochemical impedance data were collected also using the two electrode coin cell on a Solartron 1260 impedance analyzer coupled with a Solartron 1286 electrochemical interface by applying an AC voltage of 10 mV amplitude and DC open circuit voltage in the frequency range of 100 kHz to 0.1 Hz at room temperature.
Acknowledgements This research was supported by the U.S. Department of Energy, Basic Energy Sciences, grant DE-AC02-06CH11357 through the Center for Electrical Energy Storage, an Energy Frontier Research Center. K.H. acknowledges funding support from China Scholarship Council and Hunan Provincial Innovation Foundation for Postgraduate. H.Y. acknowledges funding support from National Natural Science Foundation of China (21276284). 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: April 19, 2014 Revised: May 25, 2014 Published online on July 22, 2014
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