Interface Chemistry Guided Long-Cycle-Life Li–S Battery - Nano

Letter pubs.acs.org/NanoLett

Interface Chemistry Guided Long-Cycle-Life Li−S Battery Lei Wang,†,‡ Dong Wang,‡ Fengxing Zhang,† and Jian Jin*,†,‡ †

Key Laboratory of Synthesis and Natural Functional Molecular Chemistry (Ministry of Education), College of Chemistry & Materials Science, Northwest University, Xi’an, Shaanxi, 710069, China ‡ i-LAB and Nano-bionics Division, Suzhou Institute of Nano-Tech & Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu, 215123, China S Supporting Information *

ABSTRACT: To date, most of the research on electrodes for energy storage has been focused on the active material itself. It is clear that investigating isolated active materials is no longer sufficient to solve all kinds of technological challenges for the development of modern battery infrastructure. From the interface chemistry point of view, a system-level strategy of designing polydopamine coated reduced graphene oxide/sulfur composite cathodes aimed at enhancing cyclic performance was reported in this work. As a soft buffer layer, the polydopamine shell was used to accommodate the volume expansion of S and avoid the leakage of polysulfide during cycling. A cross-link reaction between polydopamine buffer and poly(acrylic acid) binder was further designed to improve the strength of the entire electrode. As a result, the electrode demonstrated excellent cyclic performance with a discharge capacity of 728 mAh/g after 500 cycles at the current density of 0.5 A/g (a very small capacity loss of 0.41 mAh/g per cycle). Most importantly, 530 mAh/g was obtained even at a higher current density of 1 A/g after 800 cycles. Our results indicate the importance of chemically designing interfaces in the whole electrode system on achieving improved performance of electrodes of rechargeable lithium ion batteries. KEYWORDS: Interface chemistry, Li−S battery, long cycle life, polydopamine

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reduced products can also diffuse back to the cathode during cyclic processes. These issues result in the low utilization of active materials and decrease the cycle life of S electrode. To address these issues, various S-based composites have been fabricated such as various carbon−sulfur composites, polymer− sulfur composites, hybrid carbon−polymer−sulfur composites, and nanostructured Li2S electrodes.21−30 Among these composites, the carbon−polymer−sulfur composite is considered as the most promising one. On one hand, the use of carbon materials could improve the conductivity of the entire electrode. On the other hand, the functional groups and unique chain structure of polymer including inter- and/or intrachain bonding could further chemically confine sulfur and polysulfide. In addition, polymers are generally mechanically soft and can even be self-healing, which is beneficial for solving the issues related to volume expansion and material pulverization and allowing for better accommodation of volume expansion than pure carbon coatings. Significant progress has been achieved in the past few years. The rational and creative design of carbon-polymersulfur composites could address to some extent the interrelated issues of volume expansion, poor ionic and electronic transport,

echargeable lithium ion batteries (LIBs) are a favorite energy storage device for upcoming mobile electric devices and hybrid vehicles because of their high performance. Although LIBs are promising for a wide variety of applications, many issues including their energy density, cycle stability, and economic efficiency are still being intensively studied for further improvement. In particular, developing advanced electrode materials with high energy density and cycling stability is still in great demand.1−6 In the past several years, much improvement has been achieved in the development of high-performance anode materials to replace carbon-based anodes including silicon, tin, and transition metal oxide.7−12 In contrast, progress on cathode materials is relatively slow, and the state-of-the-art cathode materials have only a capacity less than a half of carbon anode. Sulfur (S) is a promising cathode material with a high specific energy due to the lithium (Li)/S couple which could yield a theoretical specific energy of about 1675 mAh/g and a theoretical specific energy of 2600 Wh/kg on the assumption of complete reaction of Li with S to form Li2S. In addition, S is inexpensive, abundant, and nontoxic. However, there are still a number of challenges in Li−S batteries.13−20 The main hindrance for utilizing S is that it is electrochemically inactive due to poor conductivity. In addition, the polysulfide ions that are formed during charge and discharge processes are highly soluble. The dissolved intermediate polysulfide can diffuse through electrolytes to the Li metal anode and is reduced therein to solid precipitates in the form of Li2S or Li2S2. These © 2013 American Chemical Society

Received: May 23, 2013 Revised: July 13, 2013 Published: August 5, 2013 4206

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Scheme 1. Schematic of the Formation of PD-Coated RGO/S Compositea

a

Optical image shows as-prepared RGO/S aerogel.

Figure 1. Structural characterization of as-prepared RGO/S aerogel. (a) SEM image; (b) nitrogen adsorption/desorption curves and pore size distribution; (c) TEM image; (d) DF-STEM image and corresponding elemental mapping images of C (red) and S (yellow); (e) S XPS spectrum.

development of modern battery infrastructure. More attention should be focused on the entire electrode system where studying the interface between individual components within the system is of paramount importance.39 In this work, we report a system-level strategy of designing reduced graphene oxide (RGO)/S composite-based cathode electrodes aimed at enhancing the cyclic performance. As shown in Scheme 1, RGO/S aerogel was first prepared by hydrothermal graphene oxide (GO) aqueous dispersion and S nanoparticles. The RGO/S aerogel was then dispersed and coated by polydopamine (PD). Finally, a cross-link reaction was built between PD buffer and poly(acrylic acid) (PAA) binder to integrate individual RGO/S composites into a whole

and polysulfide dissolution in Li−S battery and improve the capacity, cycle life, and power capability of sulfur cathodes.31−37 However, the current performance of Li−S battery is still far from expectations where cycle retention is still a serious problem to be solved. Very recently, Cui and co-workers demonstrated a S-TiO2 yolk-shell nanoarchitecture aiming to accommodate the volume change during cycling. The capacity decay as small as 0.033% per cycle after 1000 cycles has been achieved.38 To date, most of the research on electrodes for energy storage has been focused on the active material itself. It is clear that investigating isolated active materials is no longer sufficient to solve all kinds of technological challenges for the 4207

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system through forming a robust network of covalent bond in the cathode. By such a design, two stable interfaces between active material (RGO/S composite) and buffer layer and between buffer layer and binder are built to improve the stability of electrode. Our results indicate the effectiveness of system engineering to the final performance of the S-based electrode, especially its long-term cycle stability. As a consequence, the designed S-based cathode system exhibits an outstanding cycle performance that an energy capacity of 530 mAh/g at current density of 1 A/g is achieved after 800 cycles. Results and Discussion. In this work, the hydrothermal method was used for the preparation of RGO/S aerogel because this method could prevent the aggregation of RGO during reaction and obtain product with a large Brunauer− Emmett−Teller (BET) surface and open framework. The morphology and microstructure of as-prepared RGO/S aerogel were investigated by scanning electron microscopy (SEM) first. As shown in Figure 1a, a well-defined and interconnected 3D porous network with open pores can be clearly recognized. Pores with diameters in the range from several hundred nanometers to tens of micrometers are embedded with the ultrathin layer of aerogel matrix. In the composite, the content of S is as high as 82 wt % as calculated by thermogravimetry (TG) and derivative thermogravimetry (DTG) analysis (Figure S6 of the Supporting Information). However, there are no obvious S particles or aggregations observed (Figure S3 also). To explore the porous structure and specific surface area of the RGO/S aerogel, N2 sorption investigation is carried out (Figure 1b). The N2 adsorption/desorption plot shows the type IV isotherm suggesting a characteristic open-wedge shaped mesoporous structure, and the BET surface is calculated to be 130 m2/g. The N2 adsorption/desorption plot shows a steep increase of absorption at a high pressure (P/P0 = 0.80−0.99), indicating the pore volume is not only contributed by mesopores at 4 nm, but also by pores in a much larger size around 28 nm. The transmission electron microscopy (TEM) image in Figure 1c indicates that a thin layer of S is homogenously dispersed on the paper-like GO surface with no significant fraction of bulk S on the external surface of the sample. The dark-field scanning transmission electron microscopy (DF-STEM) and corresponding elemental mapping images (Figure 1d) of C and S display very similar intensity distributions, revealing an evenly S coating on GO flakes in the composite. Figure 1e shows the X-ray photoelectron spectroscopy (XPS) of as-prepared RGO/S aerogel. Two peaks were observed at 164.0 eV in the S 2p3/2 region and 228.0 eV in the S 1s region due to the interaction between S and RGO in the RGO/S aerogel according to previous reports (Figure S5 also).40,41 Dopamine has been proved to be a powerful building block for spontaneously coating a wide variety of material surfaces in the form of PD films.42 Recent results also demonstrated that PD could improve a lot of critical properties such as electrolyte wetting, electrolyte uptake, and ionic conductivity.43 The in situ coating of PD layer on RGO/S was carried out via selfpolymerization of dopamine in weak base conditions according to previous reports.44 Compared to bare RGO/S composite, the PD-coated RGO/S composite becomes a bit blurry due to PD covering. DF-STEM and corresponding elemental mapping images demonstrate that the elements C, S, O, and N are evenly distributed in the whole sample (Figure 2). The BET surface of PD-coated RGO/S composite gives a sharply

Figure 2. DF-STEM and corresponding elemental mapping images of PD-coated RGO/S composite.

decrease down to 13 m2/g (Figure S7). These results clearly show that the PD layer uniformly coats on RGO/S composite and there are nearly no free PD and bare RGO/S composite. The final content of S in the composite is about 74 wt % as calculated by TG and DTG analysis, and the corresponding contents of PD and RGO are 10 and 16 wt %, respectively (Figure S6). To achieve cross-link reaction between PD and PAA, the working electrode containing PD-coated RGO/S composite, carbon black, and PAA (weight ratio of 80:10:10) pasted on aluminum foil as current collector was heated under vacuum condition. Considering the low sublimated temperature of S, the temperature for cross-link reaction was conducted at 80 °C, and the reaction time was prolonged to 12 h. Fourier transform infrared spectroscopy (FTIR) was utilized to prove the formation of cross-link reaction as shown in Figure 3. Without

Figure 3. FTIR spectra of electrode materials with and without crosslinking.

cross-linking, the hydrogen bond dominates the range from 3700 to 2800 cm−1, among which the broad absorption band between 3200 to 2800 cm−1 belongs to the hydrogen bond between −COOH groups of PAA and a relatively strong peak around 3400 cm−1 is ascribed to N−H/O−H (or N−H/N−H, O−H/O−H) hydrogen bond of PD. The bands at 1718 and 1402 cm−1 are the characteristic vibrations of CO stretching and O−H bending of carboxyl group in PAA, respectively. The strong absorption at 1572 cm−1 is due to N−H bending vibration of secondary amine in PD. For the spectrum with cross-link, the broad band between 3200 to 2800 cm−1 almost 4208

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Figure 4. Electrochemical performance of cross-linked PD-coated RGO/S composite. (a) Discharge capacity retention of cross-linked PD-coated RGO/S composite electrode cycled at 0.5 A/g, in comparison with noncross-linked PD-coated RGO/S composite electrode and bare RGO/S composite (the first two cycles and the third cycle discharged at the current density of 0.1 A/g and 0.2 A/g for all of three type electrodes, respectively). (b) Charge/discharge capacity and Coulombic efficiency over 800 cycles at 1 A/g. (c) Voltage profiles and rate performance cycled at various current densities from 0.1 A/g to 1 A/g.

capacity is 1343 mAh/g at 0.1 A/g. When the current density is up to 0.2 and 0.5 A/g at the third and fourth cycle, respectively, the cross-linked electrode still holds the discharge capacity of 1007 and 930 mAh/g. Most importantly, after prolonged cycling over 500 cycles, the capacity retention of 78% (728 mAh/g) is achieved, which suggests a very small capacity loss of 0.41 mAh/g per cycle (calculated based on the first cycle at 0.5 A/g). The electrode without cross-linking also displays a good cyclic performance. Over 500 cycles, the capacity retention of 72% (from 861 to 622 mAh/g) at the current density of 0.5 A/ g is obtained. However, the electrode without cross-linking exhibits a more serious decay at the first three cycles. The discharge capacity of 1356 and 1260 mAh/g at 0.1 A/g for first two cycles and 861 mAh/g at 0.2 A/g for the third cycle are obtained (see detail comparison in Table S1). In contrast, the electrode made of bare RGO/S composite suffers seriously from the quick decay of capacity. After 100 cycles, its capacity retention decreases to 62%. Apparently, PD coating contributes a lot to cyclic performance of RGO/S composite electrode because the PD shell could efficiently inhibit the leakage of polysulfide during cycling. This has been further confirmed by DF-STEM and corresponding elemental mapping images of PD-coated RGO/S composite electrode after 500 cycles where a thin PD layer uniformly coating on the surface of RGO/S composite could be still seen clearly (Figure S12). The role of cross-linking reaction in the electrode is to improve the strength of the entire electrode and therefore effectively diminish the capacity loss at the initial cycles. The cyclic performance of the cross-linked electrode was further investigated at current density of 1 A/g as shown in Figure 4b. The discharge capacity of 530 mAh/g after 800 cycles is

disappears, which indicates the hydrogen bond between −COOH groups of PAA is weakened greatly after cross-link reaction. Meanwhile, the bands at 1718, 1572, and 1402 cm−1 are all decreased obviously. Instead, a new peak at 1632 cm−1 appears which is attributed to CO stretching vibration of amide. This result provides a concrete proof for the occurrence of cross-link reaction between binder and buffer through the dehydration of carboxyl group of PAA and amino group of PD to produce a tertiary amide. We therefore conclude that s covalent amide bond is formed between PAA and PD under this condition. To further quantitatively examine the effect of PD and PAA cross-linking on mechanical strength of electrode, stress−strain curves of PD and PAA mixtures with and without cross-link were measured as shown in Figure S9. Tensile strength increases approximately 32% after cross-linking. Apparently, the strength of entire electrode is improved greatly through cross-linking PD and PAA. This will improve the stability of the whole electrode and thus prolong the cycle life according to previous reports.8,44 A series of electrochemical tests were carried out to investigate the cyclic performance of the electrode. 2032 type coin cells were fabricated using lithium foil as the counter electrode. The cells were cycled from 1.5 to 2.8 V versus Li+/Li. The electrolyte was lithium bis(trifluoromethanesulfony)imide in 1,2-dimethoxyethane and 1,3-DOL. Discharge/charge capacities were calculated based on the mass of S. Apparently, the cross-linked PD-coated RGO/S composite electrode exhibits more stable cycling performance and the highest discharge capacity over 500 cycles at 0.5 A/g as shown in Figure 4a after an initial discharge capacity of 1366 mAh/g at 0.1 A/g (Figure S10). In the second cycle, the discharge 4209

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obtained which shows the average Coulombic efficiency of 92.2%. Figure 4c depicts the discharge/charge voltage profiles of the cross-linked electrode at various current densities from 0.1 to 1 A/g between 2.8 and 1.5 V. All of the discharge/charge curves show similar plateaus that are consistent with the peaks in the cyclic voltammetry (CV) (Figure S11) and previously reported data.31 The cross-linked electrode was also subject to cycling at various current densities as shown in Figure 4d. After an initial discharge capacity of 1355 mAh/g, the capacity could stabilize at 1260 mAh/g at 0.1 A/g. Further cyclings at 0.2, 0.3, 0.5, and 1 A/g all exhibit high discharge capacities of 982, 858, 768, and 592 mAh/g, respectively. When the current density is switched abruptly from 1 to 0.3 A/g, the original capacity is almost recovered, indicating robustness and stability of the cross-linked electrode. Two important aspects have been thought to contribute mainly to the stable cycling performance of our PD-coated RGO/S based electrode: by PD coating on RGO/S composite and cross-link reaction built between buffer and binder. The leakage of liquid polysulfide during the charge/discharge process has been considered as a serious issue in Li−S battery which often damages the stability of the whole electrode. As a buffer layer, PD coating on RGO/S composite could effectively avoid the leakage of polysulfide as schematically drawn in Figure S13. In addition, as a relatively soft and elastic polymer, PD could endure large volume change coming from contraction and expansion of S through adjusting its own elastic deformation, thus release partial pressure of S during cycling. Furthermore, just as various motifs inspired by living organisms in nature have been utilized for practical applications, PD have also provided creative motivations for exceptional adhesion properties even on wet surface. The wetness-resistant adhesion ability is also very useful for battery operations because the battery components are also in contact with each other in liquid environments. The cross-link reaction built between PD layer and binder through forming amide bond is also significant to further enhance the cycling stability of the whole anode system. Recent studies have also demonstrated the importance of the rigidity of polymer backbone in retaining the capacity of the electrode during cycling. As compared to hydrogen bond or weak van der Waals interaction which usually appears in reported electrodes, the strong covalent bond formed here could effectively integrate the individual components in the electrode into a whole system and greatly improve the stability of the electrode to withstand structure damaging during cycling. The robustness of covalent bond in our system is confirmed by FTIR spectra of the electrode after many cycles of charge/ discharge process. The result shows that the covalent bond in the electrode is well-maintained (Figure S14). Conclusions. In summary, a functional PD buffer was introduced to the RGO/S system to help trap liquid polysulfide and to accommodate the volume expansion of S during cycles, and a novel chemical approach is employed to build a cross-link reaction between PD buffer and PAA binder to improve the strength of the entire electrode. The designed electrode demonstrated long cycling capability over 800 cycles. This work provides a general strategy for addressing the issues of both anode and cathode electrode materials with high capacity during cycling from the viewpoint of the whole electrode system. The available materials with the low-cost and easily achieved manufacturing process for electrode make our S-based electrode highly promising for practical application in LIBs.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and supplemental figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (grant no. 2013CB933000 and 2010CB934700), the National Natural Science Foundation of China (grant no. 21004076), and the Key Development Project of Chinese Academy of Sciences (grant no. KJZD-EW-M01-3).

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