Graphene–Li2S Aerogel ... - ACS Publications

Three-Dimensional CNT/Graphene−Li2S Aerogel as Freestanding Cathode for HighPerformance Li−S Batteries Jiarui He,† Yuanfu Chen,*,† Weiqiang Lv,‡ Kechun Wen,‡ Chen Xu,† Wanli Zhang,† Wu Qin,§ and Weidong He*,†,‡,∥ †

State Key Laboratory of Electronic Thin Films and Integrated Devices, ‡School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China § National Engineering Laboratory for Biomass Power Generation Equipment, School of Renewable Energy Engineering, North China Electric Power University, Beijing 102206, P. R. China ∥ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: Three-dimensional (3D) CNT/graphene-Li 2 S (3DCG−Li2S) cathodes with 81.4 wt % record Li2S loading have been realized through solvothermal reaction and a subsequent liquid-infiltration-evaporation coating method. The highly flexible, conductive 3D mesoporous interconnected network based on twodimensional (2D) graphene nanosheets and one-dimensional (1D) carbon nanotubes (CNTs) provides highly efficient channels for electron transfer and ionic diffusion, and leads to a low solubility of polysulfides in electrolytes in charges/discharges. Without polymeric binders or conductive additives, the freestanding 3DCG−Li2S cathode exhibits record electrochemical performances including reversible discharge capacities of 1123.6 mAh g−1 and 914.6 mAh g−1, 0.02% long-term capacity decay per cycle and a high-rate capacity of 514 mAh g−1 at 4 C. The reported 3DCG−Li2S aerogel with ultrahigh Li2S content presents promising application potentials in high-performance Li−S batteries.

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graphene have been widely used as conductive matrix to enhance the conductivity of Li2S composites.1,13,14,17,19,20,23−28 Most of those studies have demonstrated that the specific capacity of Li2S is high, however, the content of Li2S in the whole cathode electrode is relatively low (80 w%). In our previous report,3 by introducing carbon nanotubes (CNTs) into 3DG to form highly conductive threedimensional CNTs−graphene (3DCG) matrix, the content of

framework to alleviate the shuttle effect of Li2S during cycling. Although the highest ever reported content of Li2S in the whole electrode reaches 60 wt %,32 it is still far lower than the ideal ratio 821

DOI: 10.1021/acsenergylett.6b00272 ACS Energy Lett. 2016, 1, 820−826

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ACS Energy Letters

Figure 3. (a) CVs of the 3DCG−Li2S cathode at 0.1 mV s−1 in a potential window from 1.5 to 3.6 V vs Li+/Li for the initial cycle and 1.5 to 3 V vs Li+/Li for the 2nd and 3th cycles. (b) Discharge/charge voltage profiles of 3DCG−Li2S composite for the first, 50th, 100th, 200th and 300th cycles at 0.2 C. (c) Cyclic performances of 3DG−Li2S and 3DCG−Li2S cathodes at 0.2 C for 300 cycles. (d) Rate performances of 3DG−Li2S and 3DCG−Li2S cathodes at various C-rates.

Figure 4. SEM images of the lithium plate paired with (a) 3DG−Li2S and (b) 3DCG−Li2S after 300 cycles. Corresponding EDS spectra of the lithium plate paired with (c) 3DG−Li2S and (d) 3DCG−Li2S after 300 cycles.

composite cathodes, because the conductive additive, polymer

sulfur was effectively increased, since CNTs not only enhanced the overall conductivity, but also formed bimodal mesopore structure, leading to increased active surface of sulfur and polysulfides. 3DCG is also an excellent template for preparing Li2S-based composites with higher Li2S content. In addition, to further increase the content of Li2S in the whole electrode, it is crucial to fabricate free-standing and binder-free Li2S-based

binder and metallic current collector are unnecessary for such cathodes. However, so far, it has remained to be a challenge for synthesizing freestanding Li2S composite cathodes with ultrahigh Li2S content while maintaining excellent electrochemical performances. 822

DOI: 10.1021/acsenergylett.6b00272 ACS Energy Lett. 2016, 1, 820−826

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ACS Energy Letters

Figure 5. SEM images of (a) 3DG−Li2S and (b) 3DCG−Li2S composites after cycling.

nanosheets. In the infiltration-evaporation coating process, the deficient pores cannot host the abundant Li2S nanoparticles, and the surface of 3DG was consequently covered by Li2S agglomeration, as shown in Figure 1d. Because of the abundant mesopores in 3DCG, no Li2S agglomeration is observed in the 3DCG−Li2S (Figure 1e and Figure S2b). As shown in the TEM images of the 3DCG−Li2S in Figure 1f, the 3DCG forms a three-dimensional porous conductive architecture with CNTs and graphene sheets cross-linked. The Li2S nanoparticles are uniformly anchored on 3DCG, as shown in the high-magnification TEM images in Figure 1g. The Li2S nanoparticles are ∼8 nm in diameter, the minimum value ever reported for the material,19,33 and the small size could be attributed to the sufficient cites in 3DCG for Li2S loading as well as periodical infiltration. In addition, such a small size can effectively alleviate the initial charge barrier of 3DCG−Li2S. The inset of Figure 1g shows the high-resolution TEM image of the Li2S nanoparticles, and the 0.33 nm lattice spacing corresponds to the (111) plane of Li2S. Several sharp preaks at 27°, 31°, 45°, 53°, 65°, 72°, and 74° in commercial Li2S correspond to the characteristic peaks of Li2S (according to JCPSD No. 26-1188), suggesting the highly crystallinity of commercial Li2S before being encapsulated in 3DCG and 3DG, as presented in Figure 2a.20 The characteristic peaks of Li2S are also observed in 3DG−Li2S and 3DCG−Li2S, which indicates that the Li2S nanoparticles were introduced in 3DG and 3DCG. Peak broadening in 3DCG−Li2S illustrates that the Li2S nanoparticles were effectively encapsulated in the abundant pores of 3DCG. Figure 2b shows the energy dispersive X-ray spectroscopy (EDS) of 3DCG−Li2S with 81.4 wt % Li2S content. The correlated elemental mapping of sulfur and carbon are shown in Figure S3. It can be clearly seen that the Li2S nanoparticles are uniformly distributed in the 3DCG aerogel. Full nitrogen sorption isotherms of the 3DG and 3DCG are shown in Figure 2. In the relative pressure range of 0.45−1.0 P/ P0, a type-IV isotherm with a type-H3 hysteresis loop shown in Figure 2c, illustrates the mesoporous structure of 3DG and 3DCG. The specific surface area of 3DCG is 370.8 m2 g−1, which is well above that of 3DG (251.6 m2 g−1), as measured with the Brunauer−Emmett−Teller (BET) method. The unique 3D architecture ensures sufficient surface area for Li2S loading. The pore size distribution as derived from the BJH method is illustrated in Figure 2d. 3DCG possesses abundant bimodal pores of 29.8 and 3.7 nm, whereas 3DG only owns monodal mesopores. The difference of the pore size distribution between 3DCG and 3DG shows that the one-dimensional CNTs can effectively tune the structure and density of the mesopores in the 3D architecture. Smaller mesopores in the 3DCG confine the dissolved polysulfides for suppressing the severe capacity fade and the larger pores in the 3DCG allow for efficient charge

In this work, a well-designed 3D CNT/graphene-Li2S (3DCG−Li2S) hybrid aerogel with ultrahigh Li2S content (81.4 wt %) has been fabricated by facile solvothermal reaction and subsequent liquid infiltration−evaporation coating. The unique mesoporous 3DCG structure with high specific area provides ultrahigh Li2S content and abundant electrochemical nanoreactors, and facilitates efficient charge transport as well as electrolyte penetration. Owing to such advantages, as freestanding and binder-free cathode the 3DCG−Li2S aerogel owns an ultrahigh capacity, a high rate capability as well as a low initial charge barrier. This work provides insights into realizing highcapacity cathodes for next-generation flexible Li−S batteries. The reported route to preparing freestanding, binder-free 3DCG−Li2S composite was illustated schematically in Figure 1a. The 3DCG aerogel was obtained by ultrasonication of the GO/ CNTs suspension, solvothermal reduction, and assembly, as well as subsequent freeze-drying. Corresponding photographs are shown in Figure S1 in the Supporting Information. 3DCG−Li2S composite was fabricated by a liquid-infiltration-evaporation coating method, as shown in Figure 1a. 3DG owns an interconnected 3D conductive porous network, as shown in Figure 1b. The images in Figure 1c and Figure S2a demonstrate

Figure 6. EIS spectra of the 3DG−Li2S and 3DCG−Li2S composite. The inset is the fitting relevant equivalent circuit models.

that the highly conductive CNTs are homogeneously distributed in the 3DCG. Compared to the 3DG, the 3DCG possess substantially more mesopores that provide abundant accessible active sites for hosting Li2S. Such abundant mesopores existing in 3DCG were resulted from the self-assembly process between CNTs and graphene sheets, where the one-dimensional CNTs acted as pillars supporting the two-dimensional graphene 823

DOI: 10.1021/acsenergylett.6b00272 ACS Energy Lett. 2016, 1, 820−826

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ACS Energy Letters

Figure 7. Predicted optimized configurations (left column) and electron density maps (right column) between (a) graphene sheet and Li2S, (b) a CNT and Li2S, and (c) a Li2S molecule intercalated into graphene/CNT composite structure.

capacity of 672 mAh g−1 (2.69 mAh cm−2), as presented in Figure 3c. Then it rapidly decreases to 365.5 mAh g−1 (1.46 mAh cm−2) after 100 cycles and decreases to 287.9 mAh g−1 (1.15 mAh cm−2) after 300 cycles. For the 3DCG−Li2S, a high initial specific capacity of 1052.1 mAh g−1 (4.21 mAh cm−2) is obtained. Even after 300 cycles, a capacity of 958.3 mAh g−1 (3.83 mAh cm−2) is obtained, corresponding to a high capacity retention of 94.5% and a low capacity fading rate of 0.02% per cycle with nearly 100% Columbic efficiency. Such pronounced performances are attributed to the inhibition of the shuttle effect that is regarded as the biggest shortcoming of the Li−S battery. In Figure 4, the post-mortem SEM of the lithium plate paired with 3DCG−Li2S after 300 cycles shows slight corrosion and a relatively smooth surface, which confirms the good confinement of polysulfides. The EDS spectrum in Figure 4 also illustrates the sufficient protection from shuttle effect. The better cyclic performance, as compared to that of 3DG−Li2S, correlates with the abundant bimodal mesopores in 3DCG which provide sufficient confinements for polysulfides in charges/discharges. The high-rate capability of the 3DCG−Li2S cathode at various rates is shown in Figure 3d. The 3DCG−Li2S exhibits excellent rate performances with a high capacity of 1123.6 mAh g−1 (4.49 mAh cm−2, 96.4% of its theoretical capacity), 847.4 mAh g−1 (3.39 mAh cm−2), and 514 mAh g−1 (2.06 mAh cm−2) at 0.1C, 1C, and 4C, respectively. The correlated voltage profiles are provided in Figure S5. When the rate is reduced back to 0.1C, the

transport and electrolyte penetration, resulting in high utilization of Li2S. Figure 3a shows the cyclic voltammetry (CV) curves of 3DCG−Li2S cathodes in the first three cycles. Previous reports reveal that the initial charge barrier exists in Li2S, therefore, the 3DCG−Li2S was scanned to 3.6 V to confirm the presence of the common peaks in the initial cycle. As shown in Figure 3a, two anodic peaks for 3DCG−Li2S are observed in the initial scanning. The peaks at 2.53 and 2.62 V are attributed to the conversions of Li2S to Li2S8 and S.22 The value above 3 V for 3DCG−Li2S is smaller than that for 3DG−Li2S (in Figure S4), indicating a significantly reduced initial charge barrier due to less agglomeration of Li2S in 3DCG−Li2S, as compared with 3DG−Li2S. In the cathodic scanning, the peaks at 2.36 and 2.01 V are attributed to the conversions of S to soluble Li2Sn (4 ≤ n ≤ 8) and Li2S2/Li2S.32 After the first sweep, the potential window was then switched from 1.5 to 3.0 V in the subsequent cycles. The CV curves overlapped after the first cycle, implying the relatively stable cyclic performance of 3DCG−Li2S. Figure 3b demonstrates the charge/discharge profiles of 3DCG−Li2S for the different cycles at 0.2 C. The voltage plateaus are consistent with the CV results. The potential plateaus change only slightly even after 300 cycles, indicating the high capacity reversibility of 3DCG−Li2S. Of crucial importance to Li−S battery applications is the cyclic performance. The 3DG−Li2S delivers a low initial discharge 824

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discharge capacity can be well recovered (1108.3 mAh g−1), showing a high rate capability of the 3DCG−Li2S cathode. The excellent rate capability indicates that the degradation of of 3DCG−Li2S has been mitigated efficiently, as confirmed by the postmortem SEM images in Figure 5. In strong contrast, the 3DG−Li2S displays disadvantageous rate performances, as shown in Figure 3d. The specific capacity of 842.7 mAh g−1 (3.37 mAh cm−2) for 3DG−Li2S at 0.1C quickly drops to 155.3 mAh g−1 (0.62 mAh cm−2) as the rate increases to 1C. It is noted that the 3DCG−Li2S with a high Li2S content still displays a high rate capability even at 4C, which is significantly higher compared with previous Li2S hybrids, as shown in Table S1.13,17,19,20,23−27 The excellent rate capability of 3DCG−Li2S is correlated with the unique architecture of 3DCG, the interconnected mesopores of which provide efficient paths for fast ion diffusion and electron transfer. The Nyquist plots of 3DCG−Li2S and 3DG−Li2S after 300 cycles are shown in Figure 6. Both cells exhibit a depressed semicircle in the high frequency region, corresponding to the charge transfer resistance (Rct) of the Li2S cathode and an inclined line at low frequency, correlated with the Warburg diffusion process (Zw). The intersections of the semicircle at the real axis at the high frequency region (denoted as Rs) are attributed to the internal resistance. The cell with 3DCG−Li2S exhibits a Rct of 57.81 Ω and a Rs of 0.95 Ω, which are well below those of the cell with 3DG−Li2S (72.47 Ω and 3.13 Ω, respectively). Such EIS data are again suggestive of the efficient electron transfer and ion conduction due to the interconnected pores in 3DCG−Li2S. The structural advantages of the 3DCG−Li2S are confirmed by the theoretical calculation, as shown in Figure 7. To realize efficient calculation, the structure of one piece of graphene sheet and one carbon nanotube with one Li2S molecule placed inbetween is established to represent the microstructure of 3DCG−Li2S. Compared with normal Li2S−graphene interaction in Figure 7a, molecular Li2S exhibits substantially larger interaction with graphene and CNT in the hierarchical 3DCG−Li2S, which is demonstrated by the shorter separation between Li2S and CNT or graphene/CNT, as shown in Figure 7b−c. The larger overlap of electron density between Li2S and CNT/graphene compared with Li2S/graphene indicates that the charge transport of Li2S can be dramatically enhanced. As seen in the optimized structure in Figure 7c, the interacting graphene sheet and CNT construct a closely packed microstructure, as consistent with the TEM images in Figure 1. The structure not only ensures a high energy capacity of the 3DCG−Li2S cathode, but also mitigates the dissolution of Li2S in the electrolyte, a major cause of capacity decay for a Li−S battery. In summary, a facile method has been presented to synthesize three-dimensional carbon nanotube/graphene−Li2S (3DCG− Li2S) aerogel with ultrahigh Li2S content. The well-designed flexible, highly conductive matrix and the unique 3D bimodal− mesoporous architecture of 3DCG−Li2S give rise to the pronounced electrochemical performances of 3DCG−Li2S. As the 3DCG−Li2S is employed as free-standing and binder-free cathode without any polymer binders or conductive additives, the reversible discharge capacity is as large as 1123.6 mAh g−1, and the capacity decay is as low as 0.02% per cycle. Moreover, the high-rate capacity up to 4C is as large as 514 mAh g−1. The 3DCG−Li2S aerogel with high Li2S content is promising as freestanding cathodes for high-performance lithium−sulfur batteries.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00272. Experimental details for the preparation of 3DCG−Li2S; electrochemical and materials characterization; electrochemical properties of Li2S−C composites in Table S1; characterization of materials by digital camera, SEM and EDS; further electrochemical characteristics of electrode materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (Grant Nos. 51372033, 21403031, 51202022, and 61378028), the Fundamental Research Funds for the Central Universities (Grant Nos. ZYGX2013Z001, ZYGX2014J088 and ZYGX2015Z003), National High Technology Research and Development Program of China (Grant No. 2015AA034202), the 111 Project (Grant No. B13042), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20120185120011), the Science & Technology Support Funds of Sichuan Province (Grant No. 2016GZ0151). Sichuan Youth Science and Technology Innovation Research Team Funding (Grant No. 2011JTD0006), and the Sino-German Cooperation PPP Program of China (Grant No. 201400260068).

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