high-strength binder interactions enabling durable high-loading lithium–sulfur batteries

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Highly integrated sulfur cathodes with strong sulfur/high-strength binder interactions enabling durable high-loading lithium-sulfur batteries Arif Rashid , Xingyu Zhu , Gulian Wang , Chengzhi Ke , Sha Li , Pengfei Sun , Zhongli Hu , Qiaobao Zhang , Li Zhang PII: DOI: Reference:

S2095-4956(20)30046-2 https://doi.org/10.1016/j.jechem.2020.01.031 JECHEM 1084

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

30 December 2019 21 January 2020 22 January 2020

Please cite this article as: Arif Rashid , Xingyu Zhu , Gulian Wang , Chengzhi Ke , Sha Li , Pengfei Sun , Zhongli Hu , Qiaobao Zhang , Li Zhang , Highly integrated sulfur cathodes with strong sulfur/high-strength binder interactions enabling durable high-loading lithium-sulfur batteries, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.01.031

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Highlights:  Highly integrated sulfur cathodes with excellent mechanical properties are devised  Super-adhesion polydopamine nanointerlayer connects sulfur and binder tightly together  The integrated sulfur cathodes show strong ability to capture soluble polysulfides  Integrated sulfur cathodes deliver excellent cycling stability and rate capability  A 9.1 mg cm-2 sulfur-loaded cathode shows high reversible capacity at 0.2 C after 50 cycles

Highly integrated sulfur cathodes with strong sulfur/high-strength binder interactions enabling durable high-loading lithium-sulfur batteries Arif Rashida,1, Xingyu Zhua,1, Gulian Wanga,1, Chengzhi Keb, Sha Lic, Pengfei Suna, Zhongli Hua, Qiaobao Zhangb,*, Li Zhanga,* a

College of Energy, Soochow Institute for Energy and Materials Innovations,

Soochow University, Suzhou 215006, Jiangsu, China. b

Department of Materials Science and Engineering, College of Materials, Xiamen

University, Xiamen, 361005, Fujian, China. c

State Key Laboratory of Physical Chemistry of Solid Surface and Department of

Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China 

Corresponding authors. [email protected] (Q. Zhang),

[email protected] (L. Zhang). 1

These authors contributed equally to this work.

Abstract: The development of high-sulfur-loading Li-S batteries is a key prerequisite for their commercial applications. This requires to surmount the huge polarization, severe polysulfide shuttling and drastic volume change caused by electrode thickening. High-strength polar binders are ideal for constructing robust and long-life high-loading sulfur cathodes but show very weak interfacial interaction with non-polar sulfur materials. To address this issue, this work devises a highly integrated sulfur@polydopamine/high-strength binder composite cathodes, targeting long-lasting and high-sulfur-loading Li-S batteries. The super-adhesion polydopamine (PD) can form a uniform nano-coating over the graphene/sulfur (G-S) surface and provide strong affinity to the cross-linked polyacrylamide (c-PAM) binder, thus tightly integrating sulfur with the binder network and greatly boosting the overall mechanical strength/conductivity of the electrode. Moreover, the PD coating and c-PAM binder rich in polar groups can form two effective blockades against the effusion of soluble polysulfides. As such, the 4.5 mg cm-2 sulfur-loaded G-S@PD-c-PAM cathode achieves a capacity of 480 mAh g-1 after 300 cycles at 1 C, while maintaining a capacity of 396 mAh g-1 after 50 cycles at 0.2 C when the sulfur loading rises to 9.1 mg cm-2. This work provides a system-wide concept for constructing high-loading sulfur cathodes through integrated structural design. Keywords: Cross-linked high-strength polar binder; Highly integrated electrode structure; High-sulfur-loading Li-S battery; Polydopamine nano-bonding layer; Strong sulfur/binder interaction

1. Introduction Lithium-sulfur (Li-S) batteries are favorable energy storing contrivances owing to their exceptional specific capacity and high energy density. Moreover, sulfur takes several worthful qualities, for example extremely inexpensive, equipollent weightiness, risk-free, and ecological compatibility [1−6]. However, Li-S batteries are probably the most complex systems in existing secondary batteries, with a range of inherent shortcomings such as insulating nature of sulfur and lithium sulfides, long-chain polysulfide dissolution and shuttling, and dramatic volume changes during cycling. More seriously, these issues will become more pronounced with the thickening of the electrodes, rendering the high-sulfur-loading sulfur cathodes to suffer from huge polarization, limited utilization, high mechanical instability, and poor cyclability [7−12]. Apparently, the commercialization process of Li-S batteries should be achieved in two phases according to different sulfur loadings. In the past decade, the vast majority of Li-S battery research was conducted under low sulfur mass loadings (normally, <2 mg cm-2) [4,13−17]. By developing high-conductive carbon supports [1−2,6,18−20], inorganic/organic polar host materials [13,15,21−26], high-efficient interlayer materials [27−29] and advanced electrolyte additives [30−32], the polysulfide shuttling and electrode polarization issues were greatly suppressed, endowing the low-loading sulfur cathodes with long-term cycling durability and outstanding rate capability. On this basis, to meet the basic requirements of commercial applications (specific energy density >350 Wh kg-1), the next phase of the task will naturally fall

on the construction and performance optimization of high-sulfur-loading sulfur cathodes (no less than 5 mg cm-2 areal sulfur loading and 75 wt.% sulfur content) [1,4,33−34]. In response, tremendous efforts have been devoted to mitigating the key challenges of thick sulfur cathodes and prime examples include interface engineering, particle design, and electrode architecture optimization [4,35-36]. One of the most important principles for ensuring the efficient working of high-sulfur-loading sulfur cathodes is the close integration of conductive skeletons, sulfur materials, host materials, and current collectors. This is highly beneficial for reducing polarization, restricting the polysulfide shuttling and maintaining the overall mechanical stability [1,4,33−34,36]. Therefore, the role of binder, a traditional component in the electrode, becomes more critical and is being given a richer function [37−39]. In general, binders go through a trajectory from single-component [37,40−44] to multi-component (i.e., composite binder) [37,45−46], and finally to a three-dimensional (3D) cross-linking type [14,38,47−48], accompanied by a significant increase in mechanical strength. Nazar et al. proposed that a cross-linked sodium methylene cellulose/citric acid (CMC/CA) binder guarantees 10 cycles of 14.9 mg cm-2 sulfur-loaded Li-S battery at 1.0 mA cm-2 [14]. On the other hand, the introduction of polar groups has also become an important strategy for the performance improvement of binders. For instance, polyethyleneimine (PEI) [44], polycation

β-Cyclodextrin

polymer

poly(acrylamide-co-diallyldimethylammonium

(β-CDp-N+) chloride)

[49], [50],

poly[(N,N-diallyl-N,N-dimethylammonium bis(trifluoromethanesulfonyl) imide [51]

and

polyamidoamine

dendrimer

binders

[52]

containing

-quaternary

ammonium/-amide/-amine groups can effectively suppress the soluble polysulfide shuttling and well support the operation of sulfur cathodes with sulfur loadings of more than 3 mg cm-2 and up to 8.6 mg cm-2 under low current densities (e.g., 0.05 C). More impressively, the carrageenan binder riching in -OSO3- groups can support a 17.0 mg cm-2 sulfur-loaded cathode for 8 cycles at 0.01 C [53]. Further, a combination of 3D high-strength binders and polar groups, e.g., cross-linked polyacrylamide hydrogel binder (c-PAM) [13], polyethyleneimine/hexamethylene diisocyanate (PEI/HDI) [54] and guar gum/xanthan gum (GG/XG) polymeric networks [55], can even assist a 19.8 mg cm-2 sulfur-loaded cathode for 5 cycles at 0.8 mA cm-2 (ca. 0.0005 C). Given the above, high-strength polar binders are ideal for constructing high-sulfur-loading Li-S batteries. Nevertheless, most of the current high-loading Li-S batteries can be only cycled for limited times under very low current densities. This could be attributed to a fact that the binders show very weak interfacial interaction with the non-polar sulfur surface, therefore sulfur materials are difficult to effectively integrate into the conductive network formed by the binder and conductive additives despite the high mechanical strength of cross-linked binders. In this regard, enhancing the interaction between sulfur and high-strength polar binders is the key to guaranteeing

the

achievement

[1−2,4,17,38,56−57].

of

a

highly

integrated

electrode

structure

Herein, we devise an interface assembly strategy to construct a highly integrated sulfur@polydopamine/cross-linked polyacrylamide (c-PAM) binder composite cathodes, targeting durable high-sulfur-loading Li-S batteries (Scheme 1). The catechol and amine functional groups on polydopamine (PD) can be covalently and non-covalently correlated with any organic/inorganic surfaces, endowing the PD layer with powerful adhesion and film-forming capability. Hence, the high-adhesion PD can be firmly anchored onto the graphene/sulfur (G-S) surface and form a uniform and ultra-thin coating layer. Meanwhile, the PD coating also presents a strong affinity to the c-PAM binder, thus tightly integrating sulfur with the binder and greatly boosting the overall mechanical strength of the laminate. Moreover, the PD coating and c-PAM network rich in polar amine and amide groups can act as two effective blockades against the effusion of soluble polysulfides through powerful capture. The PD-c-PAM double-layer structure can also effectively release stress from volume changes by adjusting its own elastic deformation during cycling. Consequently, the G-S@PD-c-PAM cathodes present an excellent anti-deformation upon folding and pressing. The 2.2 mg cm-2 sulfur-loaded G-S@PD-c-PAM cathode exhibits a capacity of 500 mAh g-1 after 800 cycles at 4 C while retaining the capacity of 480 mAh g-1 after 300 cycles at 1 C upon the sulfur loading rises to 4.5 mg cm-2 (1 C=1600 mA g-1). Even at a high sulfur loading of 9.1 mg cm-2 (66.4 wt. % sulfur content), the integrated cathode can still achieve a capacity of 396 mAh g-1 after 50 cycles at 0.2 C. Impressively, the high-sulfur-loading Li-S coin cells can power 120 light-emitting diodes and an electric fan, verifying their high energy density for potential practical

applications. This work provides a system-wide concept for constructing high-loading sulfur cathodes through integrated structural design. 2. Experimental 2.1 Preparation of graphene/sulfur (G-S) composites G-S nanosheet composites were prepared via the melt-diffusion method. Nano-sulfur powders (99.95%, Aladdin) were mixed with graphene nanosheets (XF NANO, Nanjing) with a mass ratio of 9:1. After 30 min of ball grinding, the mixture was transferred to an autoclave and thermostatically heated for 6 h at 155 °C to ensure that the sulfur is dispersed evenly on the graphene surface, resulting in G-S nanosheet composites. 2.2 Synthesis of G-S@PD composite The PD coating on the G-S nanosheet surface was prepared by the in situ polymerization method. 1 g G-S composite was uniformly dispersed in 2 L tris(hydroxymethyl)aminomethane-HCl buffer solution with a pH of 8.5. Then, 1 g dopamine hydrochloride was added

under

strong magnetic stirring. The

polymerization reaction reacted overnight under stirring conditions at 25 °C. The reaction precipitates were thoroughly washed to remove the residual dopamine and vacuum dried at 60 °C. 2.3 Synthesis of the c-PAM binder The preparation procedure of the cross-linked high-strength c-PAM hydrogel binder was proposed in our previous paper [14]. Briefly, PAM was first achieved by polymerizing the acrylamide monomers with the aid of ammonium persulfate (APS).

The

PAM

chains

were

further

covalently

cross-linked

by

N,

N-methylenebisacrylamide (MBAA) using N, N, N′, N′-tetramethylethylenediamine as an accelerator. The molar ration of MBAA to PAM was ca. 0.5‰ in this work (i.e., the cross-linking degree). 2.4 Characterization Morphologies and elemental distribution of bare G-S, G-S@PD, GS@PD-c-PAM, G-S@PD-LA133, and GS-c-PAM were characterized by SEM (Hitachi SU8010) and TEM (FEI Tecnai G2 F20). Thermogravimetric analysis (TGA) was carried out using a TG/DTA7300 Thermogravimetric/Differential Thermal Analyzer. The elemental and structural analysis was performed by XRD (PANalytical X’Pert PRO) and FTIR (TENSOR 27, Bruker Optics). 2.5 Visualized adsorption tests 7 mmol L-1 Li2S6-DME/DOL solution was prepared by dissolving stoichiometric amounts of S8 and Li2S in a DME/DOL binary solvent under vigorous stirring. 20 mg G-S@PD-c-PAM powders that were scraped from the electrode laminate were added into 4 mL Li2S6-DME/DOL solution and placed in the glove box. 2.6 Electrochemical performance evaluation G-S nanosheets and G-S@PD composites were evenly mixed with binders (c-PAM or LA133) and conductive reagents (carbon nanotubes: super P carbon blacks=1:1) with a mass ratio of 8:1:1 using a Thinky mixer (AR-100, Japan). The as-obtained slurry was cast onto the Al current collector and vacuum dried at 60 °C for 12 h. The sulfur mass loadings were determined by the gap of the scrapers and controlled in a

range of 2.2 to 9.1 mg cm-2 in this work. The sulfur content in the whole G-S@PD-c-PAM cathode is ca. 66.4 wt. %. CR-2032 coin cells were assembled to estimate the electrochemical performance of GS@PD-c-PAM, G-S@PD-LA133 and GS-c-PAM cathodes. CR-2032 coin cells were assembled to estimate the electrochemical performance of GS@PD-c-PAM, G-S@PD-LA133 and GS-c-PAM cathodes. Celgard 2400 membrane was used as the separator and Li metal was employed as the anode material. The electrolyte was 1.0 M LiTFSI in dimethoxyethane (DME)/1,3-dioxolane (DOL) (1:1 in v:v) with a 5 wt.% LiNO3 addition. The electrolyte/sulfur ratio was controlled around 10:1 (μL: mg). Galvanostatic charge/discharge (GCD) tests were conducted using a LAND CT2001A battery testing system (Lanhe, Wuhan) at various current rates (1 C=1600 mA g-1). Cyclic voltammetry (CV) measurements were performed in a potential window of 1.7−2.8 V at a scan rate of 50 μV m-1. Electrochemical impedance spectroscopy (EIS) was recorded on an Autolab potentiostat (PGSTAT302N, Netherland), and frequency ranged from 105−10-2 Hz with a modulation amplitude of 5 mV. 3. Result and discussion The flow chart of the whole synthetic process of G-S@PD-c-PAM integrated cathodes is displayed in Scheme 1. Graphene/sulfur (G-S) composites are first synthesized via a simple melt-diffusion approach at 155 °C with a mass ratio of 10:90 [6]. Scanning electron microscopy (SEM) images verify that the nano-sulfur is completely and evenly dispersed over the graphene sheets, forming a translucent composite structure (Fig. 1a and b). Transmission electron microscopy (TEM) image

further demonstrates that no apparent sulfur flecks or aggregation can be observed on the G-S composite surface, confirming the perfect combination of sulfur and graphene (Fig. 1e). Clearly, this flat composite surface is highly favourable for the next stage of PD film cladding. After the in situ polymerization of dopamine monomers, a spontaneous PD coating is formed over the G-S composite surface owing to its super-adhesion ability (Scheme 1c and Fig. S1, supporting information) [56−57]. The morphology of the as-derived G-S@PD nano-sheets is provided in Fig. 1(c, d, and f). Apparently, the G-S@PD composite is slightly blurred compared to that of uncoated G-S composite due to the presence of the PD layer (Fig. 1d). The TEM image further demonstrates that an ultra-thin PD coating is firmly anchored onto the G-S surface. The scanning transmission electron microscope (STEM) and corresponding element maps show that the elements C, S, N, and O are uniformly dispersed throughout the sample, manifesting the full coverage of PD coating on the G-S surface (Fig. 1g and h).

Scheme 1. Schematic of the flow chart of the whole synthetic process of G-S@PD-c-PAM integrated cathode. (a, b) Schematic of the preparation of G-S nanosheets via ball milling and melt diffusion at 155 °C. (c) Schematic of the G-S@PD composites after in situ polymerization of dopamine over the G-S surface. (d) Schematic of the G-S@PD-c-PAM cathode using a high-strength cross-linked c-PAM polar binder. X-ray diffraction (XRD) patterns of G-S and G-S@PD composites are presented in Fig. (2a). All diffraction peaks of G-S sheets can be well indexed to the orthorhombic phase of S8 (08-0247). Interestingly, the G-S@PD composite exhibits exactly the same spectrum as G-S sheets, reflecting the amorphous nature or tiny proportion of the ultra-thin PD coating. Thermogravimetric analysis (TGA) measurements are performed to evaluate the mass contents of the elemental S8 and PD nano-coating. As Fig. (2b and c) demonstrates, the G-S and G-S@PD samples lose 85 and 83 wt.% of

their weight between 150 and 300 °C, corresponding to the sublimation of sulfur (i.e., the sulfur mass content). Moreover, the mass content of the PD thin layer can be deduced by the difference between the two remaining mass, approximately 2 wt.-%, which is well consistent with TEM observation in Fig. (1f). Obviously, the introduction of PD nano-coating can only cause a slight reduction in the total sulfur content and has a negligible effect on the overall energy density of sulfur cathodes. The PD modified G-S nanosheets are further mixed with a c-PAM binder and conductive additives (super P carbon blacks and carbon nanotubes) with a mass ratio of 8:1:1 and cast onto an aluminium substrate (Scheme 1d). The synthetic method of the high-strength c-PAM hydrogel binder was proposed in our previous work [14]. Briefly, PAM is first achieved by polymerizing the acrylamide monomers with the aid of ammonium persulfate (APS). The PAM chains are further covalently cross-linked by

N,

N-Methylenebisacrylamide

(MBAA)

using

N,

N,

N′,

N′-tetramethyl-ethylenediamine as an accelerator (Fig. S2, supporting information). The cross-linking degree is ca. 0.5‰, i.e., the molar ration of MBAA to PAM. The as-derived c-PAM hydrogel with a 3D hyperbranched polymer network demonstrates exceptional mechanical properties. As displayed in Fig. 2(d and e), the c-PAM binder can be stretched up to 7 times their original length, indicating a super anti-deformation capability.

Fig. 1. Morphologies and elemental distribution of bare G-S, G-S@PD, GS@PD-c-PAM and G-S@PD-LA133. (a, b) SEM images of G-S nanosheets at different magnifications. (c, d) SEM images of G-S@PD composites at different magnifications. (e, f) TEM images of G-S nanosheet and G-S@PD composite. (g, h) STEM images and corresponding elemental maps of G-S@PD composites. (i-l) SEM images

of

G-S@PD-c-PAM

and

G-S@PD-LA133

electrodes

at

different

magnifications. The morphology of the as-prepared G-S@PD-c-PAM electrode is given in Fig. 1(i and j). Regardless of the natural voids in the porous electrode, the entire electrode presents a highly compact structure. As shown in Fig. 1(j), the G-S@PD nanosheets and conductive reagents are well “confined” within a robust c-PAM binder network, forming interconnected integrated granules. Fourier transform infrared (FTIR) spectroscopy measurements are performed to verify the strong interaction between PD coating and c-PAM binder (Fig. 2f). The spectrum of G-S@PD nanosheets shows four characteristic absorption peaks at 3300, 1612, 1512 and 1292 cm-1, corresponding to the stretching vibration of NH/OH, resonant vibration of C=C,

stretching vibration of N-H and vibration of OH in the catechol group, respectively [58]. In sharp contrast, only C=C vibration shows an enhanced signal in the G-S@PD-c-PAM spectrum, and the other three peaks disappeared completely. More interestingly, the G-S@PD-c-PAM composites display exactly the same spectrum as the bare c-PAM binder. Clearly, the FTIR information of the PD coating is completely covered by the c-PAM network, and the enhanced C=C signal is originated from the ample C=C groups in the c-PAM binder. FTIR analysis provides direct evidence of the strong interaction between PD coating and the c-PAM network, resulting in full coverage of high-strength binder on G-S@PD nanosheets. Given the above, the synergistic effect of the PD coating and cross-linked c-PAM binder is the key to guaranteeing the construction of high-strength integrated electrodes. The PD ultra-thin layer provides a strong affinity to both the non-polar G-S sheets and the c-PAM binder, and the 3D c-PAM network tightly bonds the active

materials

and

conductive

reagents

together.

For comparison,

the

G-S@PD-LA133 cathode using a traditional linear binder (LA133 means [R1-R2-CH2-CH (CN)]n-) [41] and G-S-c-PAM electrode without PD coating layer are prepared and characterized (Figs. 1(k, l) and Fig. S3, supporting information). Note that the electrochemical performance of sulfur cathodes using c-PAM and conventional poly(vinylidene fluoride) (PVDF) binders have been compared in detail in our previous work, and the sulfur cathode with c-PAM binder shows significantly improved cycling stability [13]. Therefore, PVDF has not been chosen as the reference binder in this work. As displayed in Fig. (1l), the G-S@PD-LA133 cathode

shows a loose electrode architecture owing to the absence of a 3D cross-linked binder network. Conversely, the high-strength c-PAM network can endow the G-S-c-PAM electrode with a compact structure, however, no interconnected granules can be observed due to the lack of strong interaction between G-S nanosheets and c-PAM binder (Fig. S3).

Fig. 2. Physicochemical characterizations. (a) XRD patterns of G-S nanosheets and G-S@PD composites. (b, c) TGA curves of G-S nanosheets and G-S@PD composites. (d, e) Photographs showing the mechanical stretching ability of the c-PAM hydrogel binder. (f) FTIR spectra of G-S@PD composites, bare c-PAM binder and G-S@PD-c-PAM composites. (g-m) Digital images of as-obtained G-S-c-PAM, G-S@PD-c-PAM and G-S@PD-LA133 electrodes and their corresponding states after being folded twice, pressed and then unfolded. (n) Visualized Li2S6 adsorption test of G-S@PD-c-PAM powders that was scraped from the electrode laminate.

To gain insight into the effect of a highly integrated electrode structure on the mechanical behavior of cathode laminates, the anti-deformation of the G-S-c-PAM, G-S@PD-LA133 and G-S@PD-c-PAM laminates is performed through simple folding experiments. All three types of electrodes are folded twice, pressed and then unfolded (Fig. 2g-m). Apparently, no any mechanical damages (e.g., fissures and materials spalling) can be observed in the G-S@PD-c-PAM electrode (Fig. 2m). By contrast, both the G-S-c-PAM, G-S@PD-LA133 electrodes show discernible cracks and expose the bright Al substrate after twice folding (Fig. 2h and j). The folding experiments clearly verify the importance of the combination of the PD interlayer and 3D cross-linked binder in constructing a highly integrated and high-strength electrode. The close integration of G-S, PD coating, and c-PAM will offer two additional benefits: On one hand, as a relatively soft and elastic polymer, PD can withstand large volume change and expansion of its own elastic deformation [56−58]. Moreover, the c-PAM binder is mechanically elastic and durable. Therefore, the PD-c-PAM double-layer structure can greatly release stress caused by the volume change of internal sulfur materials during cycling. On the other hand, both PD coating and c-PAM network are rich in polar amine and amide groups [14,56−58], which can act as two effective barriers against the free diffusion of soluble polysulfides through powerful capture capability. Visual adsorption experiments are conducted to intuitively confirm the anchoring effect of the G-S@PD-c-PAM on soluble polysulfides. As demonstrated in Fig. 2(n), the addition of 20 mg G-S@PD-c-PAM powders that are scraped from the electrode laminate can quickly decolorize the

Li2S6-DME/DOL solution (7 mmol L-1) after static adsorption for only 10 min, explicitly indicating the strong affinity of the PD-c-PAM double-layer to Li2S6 molecules [8,59]. As discussed above, the highly integrated G-S@PD-c-PAM cathode is featured for its outstanding mechanical properties, double-layer protection, and powerful polysulfide immobilization capability. To further substantiate the critical role of the integrated electrode structural design in promoting the electrochemical performance of Li-S batteries, G-S-c-PAM, G-S@PD-LA133 and G-S@PD-c-PAM cathodes with distinct electrode architectures are analyzed by assembling CR2032 coin cells comprising a metallic Li anode. The typical areal mass loading of sulfur is controlled at around 2.2 mg cm-2 (Fig. S4a). The reaction reversibility and reaction activity of the three electrodes are first investigated by cyclic voltammogram (CV) measurements in a potential range of 1.7-2.8 V at a sweep rate of 50 μV m-1 (Fig. 3a). As for the G-S@PD-c-PAM electrode, two sharp cathodic peaks appear at 2.328 and 2.05 V in the first negative scan, corresponding to the reaction process from S8 to soluble lithium polysulfides (Li2Sx, 4≤x≤8), followed by Li2Sx to solid-state lithium sulfides (Li2S2/Li2S). During the reverse scan, two pronounced oxidation peaks situated at 2.34 and 2.39 V, associated with the successive transformation from Li2S2/Li2S to elemental sulfur [13−14,22,29,33,51,53,60]. By contrast, the G-S-c-PAM and G-S@PD-LA133 electrodes exhibit markedly lower cathodic/anodic current densities, indicating a lower reaction activity. Moreover, the first reduction

and oxidation peaks of the two electrodes show significant negative and positive shifts, corresponding to a higher polarization [14,24−25].

Fig. 3. Effect of integrated electrode structural design on the Li-S battery performance. (a) CV curves of G-S-c-PAM, G-S@PD-c-PAM and G-S@PD-LA133 electrodes at 0.05 mV s-1. (b) Rate performance of G-S-c-PAM, G-S@PD-c-PAM, and G-S@PD-LA133 electrodes at varous current densities. (c) Discharge/charge profiles of the G-S@PD-c-PAM electrode at various C rates (1 C=1600 mAh g-1). (d) Nyquist plots of G-S-c-PAM, G-S@PD-c-PAM and G-S@PD-LA133 electrodes after the first-three cycles. (e) Long-term cycling stability of G-S-c-PAM, G-S@PD-c-PAM, and G-S@PD-LA133 electrodes at 2 C and corresponding Coulombic efficiency vs. cycle number profiles. (f) Long-term cycling stability of the G-S@PD-c-PAM electrode at 4 C and corresponding Coulombic efficiency vs. cycle number profiles. Note that the mass loading of sulfur was controlled at around 2.2 mg cm-2 in all of the above tests.

Apparently, the larger peak intensity and low polarization are a good illustration of the structural advantages of the integrated G-S@PD-c-PAM electrode. The PD coated G-S nanosheets have strong interaction with the conductive network formed by the c-PAM binder and conductive agents, thus greatly boosting the overall electrical conductivity of the electrode laminate and its reaction kinetics. Rate performance of the G-S-c-PAM, G-S@PD-LA133 and G-S@PD-c-PAM electrodes is further studied. As shown in Fig. 3(b), the G-S@PD-c-PAM electrode provides reversible capacities of 1576, 1401, 1287, 1135, 879 and 744 mAh g-1 respectively when cycled at 0.1, 0.2, 0.5, 1, 2 and 3 C, correspondingly. When the current density returns back to 0.1 C, a capacity of 1233 mAh g-1 can be recovered. Fig. 3(c) shows the corresponding discharge/charge curves of the G-S@PD-c-PAM electrode at various current rates from 0.1 to 3 C. Two featured discharge voltage plateaus and charge voltage platforms are well consistent with the observations in the CV analysis and can be perfectly preserved even if the current rate rises to 3C, suggesting the low polarization of the integrated G-S@PD-c-PAM electrode under high power conditions. Conversely, the G-S-c-PAM electrode presents the lowest reversible capacities under all current rates (Fig. 3b), which accords well with the CV measurements and can be ascribed to the poor interactions between the non-polar G-S nanosheets and c-PAM binder. Moreover, the G-S@PD-LA133 electrode shows the intermediate level of rate capability, which can be attributed to the linear structure, low strength and poor polysulfide capturing ability of the LA133 binder [28,38,41,60]. The enhanced reaction kinetics and low polarization of the G-S@PD-c-PAM electrode can be

further verified by comparing the Nyquist plots of the G-S-c-PAM, G-S@PD-LA133 and G-S@PD-c-PAM electrodes after the first-three cycles. As shown in Fig. 3(d), the G-S@PD-c-PAM electrode demonstrates the lowest charge-transfer resistance (Rct) among the three electrodes. This provides direct evidence of the unique advantages of the integrated electrode structural design [45]. The long-lasting cycling stability of the G-S-c-PAM, G-S@PD-LA133 and G-S@PD-c-PAM electrodes is compared at 1 C and 2 C rates (Fig. S5, supporting information and Fig. 3e). As displayed in Fig. 3(e), the G-S@PD-c-PAM electrode shows the highest initial charge capacity of 1145 mAh g-1 and still remains 646 mAh g-1 after 500 cycles at a current density of 2 C. In stark contrast, the G-S@PD-LA133 and G-S-c-PAM electrodes exhibit initial charge capacities of 831 and 661 mAh g-1 and maintain 358 and 276 mAh g-1 after 500 cycles under the same conditions, respectively. Upon cycling at a low current density of 1 C, the G-S@PD-c-PAM electrode also shows significant advantages in terms of initial reversible capacity and cycling durability, thus retaining a high reversible capacity of 757 mAh g-1 after 300 cycles (Fig. S5). The capacity versus cycle number profiles and the corresponding Coulombic efficiency of the G-S@PD-c-PAM electrodes under high current rates of 3, 4, 5 C are further investigated and summarized in Fig. 3(f), Figs. S6 and S7. As displayed in Fig. 3(f), the G-S@PD-c-PAM electrode exhibits a high reversible capacity of 850.9 mAh g-1 and retains the capacity of 495 mAh g-1 after 800 cycles at 4 C, indicating a low capacity decay rate of only 0.052% per cycle. More importantly, the nearly 100%

efficiency manifests the outstanding reaction reversibility and significantly suppressed polysulfide shuttling and side reactions. Similarly, the G-S@PD-c-PAM electrode presents an average capacity decline rate of 0.053% per cycle at 3 C over 800 cycles (Fig. S6). Upon the current rate rises to 5 C, the reversible capacity of the G-S@PD-c-PAM electrode gradually dwindles from 616 to 302 mAh g-1 during 1000 cycles (Fig. S7), with a high Coulombic efficiency above 99.8% (apart from the first-four cycles). Given the above, the integrated electrode structural design is confirmed to significantly boost the reaction kinetics and cycling durability of 2.2 mg cm-2 sulfur-loaded Li-S batteries at a variety of current densities, especially under high power conditions such as 5 C. In particular, no any inorganic/organic polar host materials are introduced here, in other words, the Li-S battery performance can be greatly promoted by simply achieving a close integration of sulfur materials, high-strength polar binder and conductive skeletons without reducing the actual sulfur content in the overall electrode laminate. Achieving high-sulfur-loading Li-S batteries (no less than 5 mg cm-2) with high material utilization and long life span is a key prerequisite for its commercial application [4,13−17]. In this regard, the G-S@PD-c-PAM cathodes with high sulfur loadings from 4.5 to 9.1 mg cm-2 are further investigated (Figs. S4b and 4a). Fig. 4(a) and Fig. S8 show typical cross-sectional SEM images of a 9.1 mg cm-2 sulfur-loaded G-S@PD-c-PAM cathode that is prepared via a common doctor blade coating method on the Al current collector. Apparently, the 208.5 μm-thick electrode laminate shows

a very tight and compact architecture owing to the strong interaction between the PD coated G-S sheets and c-PAM binder network, and no visible voids or cracks can be observed in such a thick electrode.

Fig. 4. Integrated electrode structure enabling durable high-sulfur-loading Li-S batteries. (a) Typical cross-sectional SEM images of a 9.1 mg cm-2 sulfur-loaded G-S@PD-c-PAM cathode. (b, c) Digital images of the “SIEMIS” pattern comprising 108 red LEDs and DC-mini fan powered by Li-S batteries with a 9.1 mg cm-2

sulfur-loaded

G-S@PD-c-PAM

cathode.

(d-f)

Cycling

performance

of

G-S@PD-c-PAM cathodes with sulfur mass loadings of 4.5, 8.2 and 9.1 mg cm-2 at 1C, 0.1 C and 0.2 C, respectively, and corresponding Coulombic efficiency vs. cycle number profiles. The correlation between the electrochemical performance of Li-S batteries and the rising sulfur loading in cathodes are systematically investigated and summarized in Fig. 4(d-f). As Fig. 4(d) demonstrates, a 4.5 mg cm-2 sulfur-loaded G-S@PD-c-PAM electrode exhibits favourable cycling stability up to 300 cycles at 1 C with a high initial reversible capacity of ca. 1001 mAh g-1, reflecting a considerable material utilization. The electrode still maintains a charge capacity of 480 mAhg-1 after 300 cycles, corresponding to a capacity decay rate of 0.17% per cycle. More importantly, the nearly 100% efficiency manifests the high reaction reversibility and greatly reduced side reactions including soluble polysulfide shutting. The discharge/charge curves at various cycles provide more information about the capacity decay (Fig. S9, supporting information). Apparently, both the two featured discharge voltage plateaus and two charge voltage platforms are getting shorter with the increase of cycle number, however, no distinct polarization can be found. This convincingly indicates that the integrated electrode structural design can effectively preserve the structural/electrical integrity of the laminate under high sulfur load conditions. Hence, the capacity degradation can be primarily ascribed to a decrease in the number of active materials available, including irreversible loss of partial Li2S2/Li2S and soluble polysulfide consumption reacting with Li metal anode.

Operating stability under higher sulfur mass loadings has been further attempted in view of commercialization requirements. The G-S@PD-c-PAM cathode with a sulfur loading of 7.0 mg cm-2 shows superior cycling stability and delivers reversible capacities of 1136 and 920 mAh g-1 at the 1st and 35th cycle at 0.1 C with an average capacity decay rate of 0.54% per cycle (Fig. S10, supporting information). Moreover, the 8.2 mg cm-2 sulfur-loaded G-S @PD-c-PAM electrode delivers an initial capacity of 924 mAh g-1 and a slightly higher capacity decay rate of 0.65% upon cycling at 0.1 C for 50 cycles (Fig. 4e). To further confirm the concepts presented in this work, we increase the sulfur mass loading up to 9.1 mg cm-2, the battery exhibits an initial capacity of 681 mAh g-1 with a capacity retention of 58.2% at 0.2 C after 50 cycles (Fig. 4f). The discharge/charge profiles at various cycles demonstrate the decreasing discharge/charge plateaus and slow-rising polarization (Fig. S11, supporting information). In view of the above, some important conclusion can be obtained: Ⅰ) The initial reversible capacity decreases with the increase of sulfur loadings, but the capacity decay rate is the opposite. Ⅱ) The G-S@PD-c-PAM electrode can maintain a low polarization even under high sulfur load conditions, revealing the robust combination of the PD coated G-S nanosheets and the high-strength conductive network formed by the c-PAM binder and conductive additives. Ⅲ) The highly integrated electrode structural design indeed improves the overall electrochemical properties of high-sulfur-loading sulfur cathodes. However, the huge volume change, irreversible loss of active materials and soluble polysulfide consumption still results in gradually increased polarization and rapid capacity decay under high sulfur mass

loadings such as 9.1 mg cm-2. This can be confirmed by the morphology evolution of the high-sulfur-loading sulfur cathodes after 50 cycles (Fig. S12, supporting information). As Figs. S(12a and b) demonstrate, the G-S@PD composites show significant volume shrinkage owing to the loss of active sulfur. The cross-sectional SEM images further indicate that the sulfur cathodes experienced severe volume change and active material loss (Figs. S12c and d). The total thickness of the cathode electrode is significantly lower than that before cycling (208.5 μm). Digital images of the Celgard separator after cycling verify that part of the loss of active materials can be ascribed to the formation of the agglomerated sulfur (Figs. S12e and f). On other hand, Fig. S13 presnets the morphology evolution of Li metal anodes before and after cycling in the high-sulfur-loading Li-S batteries. Apparently, the Li metal electrode becomes very rough, but there is no ovbious presence of Li dendirtes. The interfacial film on the Li metal surface is very likely to come from the reaction between soluble polysulfides and metallic Li. In this sense, constructing a more reasonable conductive network in the highly integrated electrode and introducing the right amount of high-efficient inorganic polar hosts will hopefully further improve the electrochemical performance of high-sulfur-loading Li-S batteries [61−62]. As a proof-of-concept demonstration, Li-S coin cells with a 9.1 mg cm-2 sulfur-loaded G-S@PD-c-PAM cathode can support a “SIEMIS” pattern comprising 108 red light emitting diode (LED, Fig. 4b) and power a DC-mini fan for more than 25 s (Fig. 4c and Supplementary video 1), implying their great potential in a commercially viable prototype.

In the past decade, the importance of functional binders in Li-S batteries has been widely recognized. Supplementary Table 1 summarizes the state-of-the-art binders in recent years and their performance impact on the high-sulfur-loading Li-S batteries. Obviously, Li-S batteries with sulfur loadings of more than 4 mg cm-2 can be supported only if the binder has polar groups or cross-linking structures (blue label). Moreover, almost all high-loading sulfur cathodes operate at very small current densities and have very few cycles. It should be noted that the very weak interaction between non-polar sulfur and functional binder is a problem that has been overlooked, however, this directly results in the poor binding of sulfur with the conductive network, even if a large number of conductive reagents are added to the electrode. In stark contrast, our highly integrated electrode structural design significantly enhances the relationship between sulfur and the conductive network. As such, the as-obtained G-S@PD-c-PAM electrodes can allow stable operation at high current densities under high sulfur loadings, e.g., 4.5 mg cm-2/1 C (1.6 A g-1) and 9.1 mg cm-2/0.2 C (320 mA g-1). The comparison with the existing cutting-edge binders again convincingly confirms the importance of integrated electrode structural design in improving the overall performance of high-loading sulfur cathodes. 4. Conclusions In summary, to address the key challenge of weak interactions between non-polar sulfur and high-strength polymeric binder, we put forward a solution to the design of highly integrated electrode structure. The super-adhesion PD nano-coating over the G-S nanosheets shows a high affinity to the conductive network formed by the

c-PAM binder and conductive additives. Moreover, the PD coating and c-PAM network rich in polar amine and amide groups serve as two effective blockades against the effusion of soluble polysulfides through powerful capture. Consequently, the as-derived G-S@PD-c-PAM electrode features with its robust structural/electrical integrity and strong polysulfide immobilization capability and can withstand drastic volume change such as repeated folding and pressing. The integrated electrode structural design is confirmed to significantly boost the reaction kinetics and cycling durability of 2.2 mg cm-2 sulfur-loaded Li-S batteries at a variety of current densities, especially under high power conditions such as 5 C. More importantly, the G-S@PD-c-PAM cathodes can allow stable operation at high current densities under high sulfur loadings, e.g., 4.5 mg cm-2/1 C (1.6 A g-1) and 9.1 mg cm-2/0.2 C (320 mA g-1), outperforming those of the state-of-the-art binders. The integrated electrode structure demonstrated in our work solves the long-neglected problem in high-sulfur-loading sulfur cathodes and is expected to promote the commercialization of high-loading Li-S batteries. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work.

Acknowledgments This work was supported by the National Natural Science Foundation of China (21875155, 51675275, 21703185 and 21473119). Q. B. Z. acknowledges the Leading Project Foundation of Science Department of Fujian Province (2018H0034) and Shenzhen Science and Technology Planning Project (JCYJ20170818153427106).

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TOC

A highly integrated graphene/sulfur@polydopamine/cross-linked polyacrylamide (G-S@ PD-c-PAM) composite cathode is designed for long-lasting and high-sulfur-loading Li-S batteries, which allows the stable operation of a 9.1 mg cm-2 sulfur-loaded Li-S battery at 0.2 C.