Multidimensional Polycation Î²-Cyclodextrin Polymer as an Effective

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Multidimensional Polycation #-Cyclodextrin Polymer as An Effective Aqueous Binder for High Sulfur Loading Cathode in Lithium-Sulfur Batteries Fanglei Zeng, Weikun Wang, Anbang Wang, Keguo Yuan, Zhaoqing Jin, and Yu-Sheng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08537 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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ACS Applied Materials & Interfaces

Multidimensional

Polycation

β-Cyclodextrin

Polymer as an Effective Aqueous Binder for High Sulfur Loading Cathode in Lithium-Sulfur Batteries Fanglei Zeng,1,2 Weikun Wang,*2 Anbang Wang,*2 Keguo Yuan,2 Zhaoqing Jin,2 Yu-sheng Yang2 1

School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081,

China. 2

Military Power Sources Research and Development Center, Research Institute of Chemical

Defense, Beijing 100191, China. KEYWORDS: lithium sulfur battery, binder, β-cyclodextrin polymer, polycation, high sulfur loading.

ABSTRACT: Although the lithium-sulfur battery has attracted significant attention due to its high theoretical energy density and low cost of elemental sulfur, its real application is still hindered by multiple challenges, especially the polysulfides shuttle between the cathode and anode electrodes. By originating from β-cyclodextrin and introducing quaternary ammonium cation into β-cyclodextrin polymer, a new multifunctional aqueous polycation binder (β-CDp-N+) for the sulfur cathode is obtained. The unique hyperbranched network structure of the new binder β-CDp-N+ as well as its multidimensional noncovalent interactions and the introduced cations

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endowed β-CDp-N+ with some new abilities: a sulfur electrode-stabilized ability, a polysulfidesimmobilized ability and a volume-accommodated ability, which help to ease the primary problem of lithium sulfur battery, the shuttle of polysulfides and the volume change of the sulfur during charge and discharge. It is demonstrated that cycling performance and rate capability of the cathodes can be the improved by using β-CDp-N+ as the binder compared to other wellknown binders. Even with high sulfur loading of 5.5 mg cm-2, the cathode with β-CDp-N+ still can deliver an areal capacity of 4.4 mAh cm-2 at 50 mA g-1 after 45 cycles, which is much higher than the cathode with the conventional binder (0.9 mAh cm-2).

Introduction Driven by the rapid development and minimization of portable energy storage devices, it is urgent to require batteries with high energy density, long-term durability, high safety, low cost, and nontoxicity.1-4 Although lithium-ion batteries (LIBs) have been the dominant power sources in the commercial energy storage devices, they cannot sufficiently satisfy long-term storage requirements due to the inherent limitation of gravimetric energy density which is lower than 300 Wh kg-1.5-8 In this case, the lithium-sulfur (Li-S) batteries are seriously considered as one of the most promising candidates for the next generation system owing to its extraordinarily high theoretical capacity of 1675 mAh g-1 and energy density of 2600 Wh kg-1 (based on pure electrochemical active materials).4,9,10 Additionally, sulfur as an electrode material has other advantages such as low cost, environmental friendliness, naturally abundance.9-11 However, the practical application of Li-S batteries still has some critical barriers to overcome.9,10,12-14 One is the low electrical conductivity of elemental sulfur (5×10-30 S cm-1, 25 ℃) and the discharge products, which leads to poor rate capability and low sulfur utilization. Another issue is the large volume change of sulfur particles during charge and discharge which can lead to the structural


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collapse of the cathode. The major problem is the high solubility of intermediate polysulfide anions Li2Sx (2 < X ≤ 8) generated on reduction of S8 or oxidation of Li2S. These various polysulfide anions can freely migrate between the cathode and anode, leading to a so-called polysulfide shuttle effect, which could cause the loss of sulfur materials and lower the coulombic efficiency as well. To address these above mentioned problems, tremendous efforts have been made toward improving the electrochemical performance of Li-S batteries by embedding sulfur in conductive polymers15,16 or in carbon of various morphologies including micro / mesoporous carbon,4,17,18 porous carbon,19 carbon fiber cloth,20 carbon nanotubes (CNTS),21,22 and functionalized graphene.23-25 Additionally, researchers also make lots of efforts in electrolyte,26-29 separators30-33 used in Li-S batteries and cell configurations34,35 of the Li-S batteries. However, very little attention has been paid to electrochemically inactive components such as binders. The binder functions to enhance the electrical contact between active materials and conductive agents and bond the active materials on the current collector. The ideal binder used in battery technology should be of low cost, low resistance, strong bonding strength, and high physical and electrochemical stability in electrolyte.36,37 Moreover the binder would be more appropriate for the sulfur cathode if it can limit the dissolution of polysulfides as well as buffer the volume change of the cathode during charge / discharge processes and maintain the stability of the electrode structure.38 In the literatures, polyvinylidene fluoride (PVDF) is the most commonly used binder,9,10,12 but it still poses some problems. For example, the PVDF binder is often dissolved in N-methyl-2-pyrrolidone (NMP), which is toxic, flammability and difficult to vaporize due to its high boiling point.39 Moreover, the price of NMP is high, undoubtedly increasing the manufacture cost of the batteries. To address these issues, water-soluble binders


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for sulfur cathodes have recently been investigated due to their nontoxicity, nonflammability, and low-cost. The main kinds of water-soluble binders for sulfur cathodes are gelatin,40,41 styrene butadiene rubber (SBR),42 polyacrylonitrile binder LA13243 and cyclodextrin binder, ect. These kind of binders have made progress in improving the adhesion and dispersion property and improving cycling performance. Especially, Wu44 et al. found that the sulfur cathode with βcyclodextrin (β-CD) as binder showed better performance compared with other conventional binders. Wang et al.38 utilized carbonyl-β-cyclodextrin (C-β-CD) as a water-soluble binder by modifying β-cyclodextrin in H2O2 solution. When the C-β-CD binder was applied, the sulfur cathode with a sulfur content of 45 wt% exhibited a high initial discharge capacity of 1543 mAh g-1 and a reversible capacity of 1456 mAh g-1 over 50 cycles which is related to the fact that C-βCD can tightly wrap the sulfur composite and suppress its aggregation during cycling. Several research groups45-47 proposed that the binders owning three-dimensional network and massive functional groups such as hydroxyl would provide superior mechanical strength against the disintegration of Si induced by volume expansion thereby stabilize the Si electrode. The volume expansion of the Si particles could be up to ~300%1-3 which is larger than the volume expansion of sulfur particles (about 80%)48 between fully charged and discharged states. These works inspired us to design a multidimensional aqueous binder with massive functional groups such as hydroxyl to ease the volume expansion of the sulfur particles. In addition, Song et al.49 employed cetyltrimethyl ammonium bromide (CTAB) and graphene oxide to anchor sulfur and mitigate the loss of sulfur from the electrode. This work also showed that the CTAB play an important role in stabilizing polysulfides and enhancing the cycling performance of the Li-S batteries. Taking these aspects into consideration, we designed a multidimensional aqueous polymer with quaternary ammonium cations as the effective binder for the sulfur cathodes. By


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originating from the hydroxyl-enriched β-cyclodextrin (β-CD) and introducing quaternary ammonium cation into β-cyclodextrin polymer (β-CDp), a new multidimensional aqueous binder (β-CDp-N+) for the sulfur cathode is obtained. In this paper, we found that the unique multidimensional network structure of the new binder β-CDp-N+ as well as the massive functional groups and the introduced cations endowed the new binder β-CDp-N+ with some new abilities: a sulfur electrode-stabilized ability, a polysulfides-immobilized ability and a volumeaccommodated ability. Herein, we found that the new binder β-CDp-N+ based cathode with a high sulfur content of 90 wt% showed smaller polarization, improved cycling performance and rate capability compared to those cathodes with other well-known binders.

Results and discussion

Figure 1. Schematic illustration of the synthesis of the cationic β-CDp-N+.


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β-CD is a seven-membered sugar macrocycle incorporating glucose as monomeric units. In the practical viewpoint, the use of β-CD is meaningful because it is the only large-scale commercialized cyclodextrin and the cheapest cyclodextrin derivative. Additionally, the multidimensional polycation β-CDp-N+ can be easily synthesized by reacting of β-CD with epichlorohydrin (EPI) and then with 2,3-Epoxypropyl-trimethyl ammonium chloride (EPTMAC). The synthesis schematic illustration of the cationic β-CDp-N+ can be found in Figure 1. Due to the undiscriminating reactions of hydroxyl groups in β-CD with EPI, the prepared β-CDp-N+ incorporate a series of hydroxyl and ether groups, which are physically entangled and grasp each other as shown in Figure 1, leading to generate multidimensional contacts with active materials and conductive agents via noncovalent interactions, such as hydrogen bonding and the van der Waals force. These interactions can help to keep intimate contact between the active material particles and the multidimensional binder during discharge and charge ( Figure 2 ). In particular, once the active material particles that lost the original contacts with the binder at certain point of discharge could recover the interaction within the multidimensional binder at other points, thus the stability of the sulfur electrode be enhanced.45-47 Moreover, the introduced quaternary ammonium ion in β-CDp-N+ binder could anchor the polysulfides and suppress the shuttle effect utilizing the electrostatic interaction, resulting in good cycling performance. In a word, the unique hyperbranched network structure, multidimensional noncovalent interactions and the introduced cations endowed the new binder β-CDp-N+ with new abilities: a sulfur electrodestabilized ability, a volume-accommodated ability and a polysulfides-immobilized ability, superior to the conventional binders. However, the conventional binders could not generate multidimensional interactions to against the volume change of sulfur during discharge. What’s


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more, the conventional binder such as PVDF does not possess roust groups to suppress the diffusion and shuttle of polysulfides during charge / discharge processes ( Figure 2 ).

Figure 2. Schematic representations of cathode configurations with the new binder β-CDp-N+ and the conventional binder. The color of the prepared β-CDp-N+ is same as the β-CD, which does not change much on the appearance showed in Figure S1a, but the solubility of β-CDp-N+ and β-CD in water has changed. As Figure S1b shown, it was found that unlike β-CD, which is poorly dissolved in water, the β-CDp-N+ has a good solubility in water. For battery technology, the good solubility of the binder will help to disperse active materials and conductive agents and bond them on the current collector. Meanwhile, the β-CDp-N+ binder was insoluble in the electrolyte which was consist of 1,2-dimethoxy-ethane ( DME ) and 1,3-dioxolane ( DOL ) ( volume ratio 1:1 ) as shown in Figure S1c, which is also important for maintaining the stability of the structure of the electrodes during charge and discharge.


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(a)

(b)

β-CD

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H2-6,   

 CDp H1

2920

1380 5.5

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Figure 3.(a) FTIR spectra of β-CD and β-CDp and (b) 1H NMR spectra of β-CDp and β-CDp-N+. Figure 3a is the infrared spectra for β-CD and β-CDp. From the infrared spectra, the chemical bond changes of β-CD with EPI are clearly investigated. The peak at 2920cm-1 is characteristic of the -CH2- stretch, and which is strengthened in the β-CDp than the same peak in the β-CD. The peak of C-O-C stretching vibration between 1000~1300cm-1 and the peak of CH2- stretching vibration at 1380cm-1 in the β-CDp are broader than the same peaks in the β-CD. Expect for these peaks, no other obvious changes exist between the FTIR spectra of β-CD before and after treatment. The broader and strengthener peaks indicate that β-CD has reacted with EPI, meanwhile maintaining the functional groups of β-CD. 1H NMR spectra of the precursor β-CDp and β-CDp-N+ can be found in Figure 3b. For β-CDp, a broad signal at δ = 4.6-5.4 ppm ascribes to the cyclodextrin anomeric protons H-1, another broad signal at δ = 3.6-4.4 ppm ascribes to the cyclodextrin protons H-2 to H-6 and the EPI poly-chain protons. In Figure 3b, a new signal at 3.59 ppm ascribes to methyl (CH3) protons which ensure the trimethyl ammonium ion has been introduced into β-CDp and the polycation β-CDp-N+ polymer is obtained.


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(a)

(b)

3.59 ppm -CDp-N+ -CDp-N++25 μL Li2S4

3.62 ppm

-CDp-N++125 μL Li2S4

3.65 ppm

4.4

4.2 4.0 3.8  (1H) (ppm)

3.6

Figure 4. (a) The visual discrimination of the interactions of (A, A1) Li2Sx-containing electrolyte with (B, B1) PVDF and (C, C1) β-CDp-N+ immediately upon contact(A, B, C) and after 2 h (A1, B1, C1). (b) 1H NMR spectra of β-CDp-N+ with different concentration Li2Sx in DMF-d7. The interactions of the new binder β-CDp-N+ and the conventional binder, taking PVDF as the representative, with lithium polysulfides were probed using a visual discrimination. The deep yellow-colored Li2S4-containing electrolyte was separated equally into three bottles A, B and C. When the equivalent amount of β-CD-N+ and PVDF were added to bottle B and C, respectively, the capability of the β-CD-N+ to adsorb Li2S4 was clearly obvious as shown in Figure 4a. The addition of the β-CD-N+ rendered the Li2S4 solution light yellow immediately and almost completely colorless after 2 h, indicating strong adsorption ability for lithium polysulfides. Whereas the PVDF solution remained deep yellow, indicative of no interaction between PVDF and lithium polysulfides. To further prove the interactions between the quaternary ammonium cations of β-CDp-N+ binder and polysulfides, the 1H NMR spectra of β-CDp-N+ with different concentration Li2Sx was introduced, which taking Li2S4 as the representative polysulfide showed in Figure 4b. When the β-CDp-N+ DMF-d7 solution was physically blended with 25 μL Li2S4 containing DMF-d7 solution, the signal of methyl (CH3) protons of trimethyl ammonium ion in β-CDp-N+ at 3.59


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ppm was seem to shift by nearly 0.03 ppm ( from 3.59 ppm to 3.62 ppm ), and shift to 3.65 ppm as mixing with 125 μL Li2S4 solution. This observation is an expected consequence of the strong electrostatic interactions between the quaternary ammonium cations in β-CDp-N+ and lithium polysulfide species. As a binder, the binding strength and mechanical property are two basic parameters. 36,37 Shearing strength was adopted to quantify the binding strength of the binders and nanoindentation test was conducted to evaluate the mechanical properties of the binders. The shearing strength of β-CDp-N+ is close to the β-CDp and LA132 binders, which are 1.85 MPa, 1.82 MPa and 1.80 MPa, respectively. The shearing strength of PVDF is much lower than the above binders, which is 0.94 MPa. β-CD cannot bond two Al strips together at all and the shearing strength of β-CD approaches zero. This demonstrates the higher adhesion force of β-CDp-N+ binder than that of other binders such as PVDF and β-CD. To confirm that the electrolyte solvent does not have a significant influence on the mechanical properties of the binder, we have studied the stiffness of β-CDp-N+ in a dry state as well as in contract with electrolyte solvent using nano-indentation measurement. And the studies were performed with β-CDp, PVDF and LA132 for comparison. β-CD film is difficult to prepare due to its poor water solubility and the β-CD binder leaves a crystalline solid after the water is evaporated. In a dry state, films made of β-CDp-N+ exhibited higher stiffness than the dry films of PVDF (Table S1). Young’s moduli of polymers were on the level of 5.5 GPa and 2.0 GPa for β-CDp-N+ and PVDF, respectively. The β-CDp and LA132 film showed stiffness close to the βCDp-N+ film. Interestingly, when immersed into the electrolyte solvent, the stiffness of β-CDpN+, β-CDp and LA132 did not change much, whereas the stiffness of PVDF was significantly decreased. This result suggests that the electrolyte solvent has a significant influence on


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mechanical property of PVDF. After immersing into electrolyte, PVDF binder exhibits weak resistance to both elastic and plastic deformations. The PVDF’s weak resistance to deformations may explain its reported poor performance in the electrodes which undergo large volume changes such as Si anode. Moreover, the Young’s modulis of β-CDp-N+ and β-CDp are higher than PVDF and do not change much in the wet state with electrolyte solvent. The result demonstrates that the binder β-CDp-N+ can help to endure the mechanical stress which was brought with the large volume change of sulfur particles during charge and discharge. In order to prove the effectiveness of the β-CDp-N+ binder on the performance of Li-S batteries, the sulfur composite materials with high sulfur content 90wt% was prepared as in our previous papers.15,16,50 The rise of sulfur content in the composite cathode renders an increase of the energy density of lithium-sulfur batteries. However, the cathodes with high sulfur content generally lead to more severe polarization, poor cycling stability and lower capacity.51 The sulfur content of the sulfur composite is determined by TGA as shown in Figure S2. The sulfur cathode contains an areal sulfur loading of 2.5 mg cm-2. Coin cells were assembled in an argonfilled glove box with lithium metal anode and 1.0 M LiTFSI / DOL / DME electrolytic solution containing 0.1 M LiNO3 as the additive. The electrolyte amount in each coin cell is about 7 μL mg-1 S.


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(a)0.006

(b)

CDp-N

+

-0.002

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

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Figure 5. Electrochemical performance of sulfur electrodes with different binders. (a) CV curves of the cathode with β-CDp-N+ during the first four cycles at a scan rate of 0.05 mV s -1. (b) Charge-discharge curves of cathodes with β-CDp-N+ from 1st to 100th cycle at 100mA g-1(c) Cycle performances of cathodes with β-CD-N+, β-CDp, β-CD binder at 100mA g-1. (d) Cycle performances of cathodes with β-CD-N+, LA132, PVDF binders at 100mA g-1. (e) The rate


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capability of sulfur cathodes with different binders at various current density. (f) Cycle performance of sulfur cathode with β-CD-N+ binder at 600 mA g-1 and 1000 mA g-1. Figure 5a shows the cyclic voltammetry (CV) curves of the cathode with β-CDp-N+ during the first four cycles at a scan rate of 0.05 mV s-1. The CV curves exhibit typical characteristic oxidation and reduction peaks of elemental sulfur. During the first cycle scan, the two reduction peaks at 2.30 V and 2.03V (vs. Li+ / Li) are related with the reduction steps of sulfur: the first step at 2.30 V represents the solid elemental sulfur accepts electrons generating long chain lithium polysulfides (Li2Sn, 4 < n < 8 ), and the second step at 2.03 V corresponds to the further reduction of these soluble polysulfides to insoluble lower-order Li2S2 and Li2S. In the subsequent anodic scan, a broad oxidation peak at around 2.5 V attributes to the conversion of lithium sulfide into elemental sulfur and lithium. After the second cycle, both the peak currents and CV peak positions undergo very small changes, which suggest a stable structure of the sulfur cathode with β-CDp-N+ binder. The CV redox peaks are only attributed to the oxidation and reduction of elemental sulfur, and no additional peaks are found in the CV curves, indicating that β-CDp-N+ binder will not participate in the reduction / oxidation process of sulfur. The charge-discharge profiles at the 1st, 2nd, 10th, 20th, 50th and 100th cycle are shown in Figure 5b. The dischargecharge voltage plateaus, which are the short upper plateau at 2.30 V, the long lower plateau at 2.10 V during the discharge process, and the long plateau during the charge process, are correspond well with the peaks in the CV curves in Figure 5a. The cycling performance of the sulfur-based cathode with β-CDp-N+ is shown in Figure 5c. Also, in order to clearly illuminate the importance of the introduced cations and network structure, β-CDp and β-CD itself were tested as the control binders at the same conditions. All of the electrodes were measured at 100mA g-1 for both charge and discharge. The initial capacity of


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β-CDp and β-CD binder based cathodes were 1271 and 1118 mAh g-1 respectively (normalized to S, the same hereafter) and decreased to 842 and 591 mAh g-1 after 100 cycles respectively, thus reconfirming the importance of the hyperbranched network and multidimensional noncovalent interactions for stable cycling. The cathode with β-CDp-N+ binder delivered an initial discharge capacity of as high as 1380 mAh g-1, corresponding to 82.4% of the theoretical capacity 1675 mAh g-

1

of sulfur, higher than the β-CDp and the β-CD binder based cathodes.

The high initial discharge capacity of β-CDp-N+ based cathode might rise from that the new binder β-CDp-N+ can help to disperse sulfur composites and conductive agents homogeneously, which could ensure an intimate contact between the sulfur composites and conductive agents and improve the utilization of sulfur. The β-CDp-N+ based cathode demonstrated stable cycling performance, and the discharge capacity remained at 928 mAh g-1 at the 100th cycle, which still higher than the β-CDp and the β-CD binder based cathodes. It indicated that the quaternary ammonium ion could immobilize the lithium polysulfides and play an important role in stabilizing the sulfur cathode. Accordingly, the β-CDp-N+-based cathode presented a much better cycling stability than the β-CDp and the β-CD binder based cathodes. As depicted in Figure 5d, the cycling performance of the cathode with β-CDp-N+ binder was also compared with that of the same cathodes but with conventional binders including LA132 and PVDF. After 100 cycles, the capacity of LA132 and PVDF binder based cathodes were only 750 and 684 mAh g-1, which were lower about 200 mAh g-1 compared with the βCDp-N+ binder based cathode. The cycling performance of PVDF binder based cathode was worse than other binders based cathodes, which can be explained by the following two aspects. Firstly, the weak van der waals interactions between the PVDF binder and sulfur surface cannot accommodate well significant volume change of sulfur during the charge / discharge. The PVDF


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binder cannot suppress the diffusion of the dissolved polysulfides out of the cathode, thus also cannot retard the shuttle of polysulfides. The LA132 binder based cathode showed better cycling performance compared to the PVDF binder based cathode, but which was inferior to the β-CDpN+ binder based cathode. It was worth noting that a capacity decrease during the initial 20 cycles was observed at all binders based cathodes. The reason is mainly caused by the dissolution of the polysulfide anions into the electrolyte. Mostly, the dissolution of polysulfide anions into electrolyte could not be fully suppressed, and partly dissolved polysulfide anions could not return to the sulfur cathode. Especially, when the sulfur areal mass loading of the sulfur cathode is up to 2.5 mg cm-2, more polysulfide anions dissolved into the electrolyte. However, the β-CDp-N+ binder based cathodes exhibit better performance than other binder based cathodes. The results show that the β-CDp-N+ binder play a crucial role in stabilizing polysulfides and reducing active mass loss and finally improving the sulfur utilization. The β-CDp-N+ binder based cathode also exhibited superior rate capability. The rate performances of sulfur cathodes with different binders, β-CDp-N+, β-CDp, LA132, PVDF and βCD, were compared in Figure 5e. As Figure 5e shown, the capacity of the β-CDp-N+ binder based cathode decreased slowly from the reversible capacity 1323 mAh g-1 at a current density of 100 mA g-1 to 912, 846, 852, 855 and 854 mAh g-1 at a current density of 200, 400, 600, 1000 and 1600mA g-1, respectively, higher than other binders based cathodes. It was worth noting that the capacity of the β-CDp-N+ based cathode was almost not decreased at the current density of 400, 600 and 1000 mA g-1, even some increased. However, the LA132, PVDF and β-CD based cathodes were decaying all the time, which only obtained lower reversible capacity of 566, 501, 405 mAh g-1 respectively after 10 cycles at 1600 mA g-1. They were far below the capacity of βCDp-N+ based cathode which is 805 mAh g-1. When the current was switched back to 100 mA


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g-1, a capacity of 936 mAh g-1 of β-CDp-N+ based cathode was obtained which closed to the capacity of the 11th cycle at 100 mA g-1, 941 mAh g-1, superior to other kind cathodes. The β-CDp-N+ cathode with the sulfur loading of 2.5 mg cm-2 and sulfur content of 90 wt% in sulfur composite was also evaluated for the cycling at higher current density as shown in Figure 5f, including 4 cycles at 50mA g-1 and subsequently 100 cycles at 600 mA g-1 and 1000 mA g-1. The initial discharge capacity of the cathode reached up to 1395 mAh g-1. With an elevated current density of 600 mA g-1, the discharge capacity decreased to 973 mAh g-1 while maintaining well as 873 mAh g-1 after 100 cycles. Even at high current density of 1000 mA g-1, the β-CDp-N+ binder based cathode also demonstrated good cycling performance with 824 mAh g-1 after 100 cycles. From the above results, the β-CDp-N+ based cathode exhibits the best electrochemistry performance in Li-S batteries. These results indicate that the new binder βCDp-N+ is responsible for the effective inhibition of polysulfide shuttling effect and stabilization of the electrode. c

b

a

a1

c1

b1

10μm

10μm

10μm

10μm

10μm


10μm

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Figure 6. Surface morphologies of the sulfur cathodes with (a, a1) LA132, (b, b1) β-CDp and (c, c1) β-CDp-N+: (a, b, c) as prepared and (a1, b1, c1) after 100 cycles. The surface morphology of each binder based cathode was characterized using scanning electron microscopy (SEM) to see the change of the physical structure of the cathode before and after cycling. Figure 6 shows the morphology images of cathodes using LA132, β-CDp and βCDp-N+ binders as prepared and after 100 cycles. The morphology images of PVDF and β-CD based cathodes after cycling are shown in Figure S3. It was clear that the sulfur composites and acetylene black were uniformly distributed in both β-CDp and β-CDp-N+ binder based cathodes in their initial states before cycling. However, the sulfur composites and conductive agents appeared agglomeration to some extent in the LA132 binder based cathode (Figure 6a). The diameter of the agglomerate is approximately 12 μm. After 100 cycles, the sulfur cathode using LA132 binder appeared dense precipitates on its surface filling the void space within sulfur composite. These precipitates demonstrated that the binder LA132 cannot successfully suppress the diffusion of polysulfides within the composite and even in the composite cathode region. In addition, some micrometer-scale cracks were observed on the surface of the LA132 binder based cathode after 100 cycles (Figure 6a1) and also can be found on the surface of PVDF and β-CD binder based cathodes (Figure S3) whereas not be found on the β-CDp binder based cathode(Figure 6b1). The precipitates and the cracks may give rise to the structural destruction of the cathode, gradual loss of the active sulfur, and finally leading to quick capacity degradation. The cathode using β-CDp binder showed a better surface morphology. The size of the sulfur composite particles did not change much, but the cathode surface still showed some dense precipitates. For the β-CDp-N+ binder based cathode, it still preserved uniform particle distribution to a large extent after 100 cycles, as shown in Figure 6c1. The size of the sulfur


17


composite particles was much smaller and there were not cracks and precipitates on the cathode surface. It is easy to infer that the β-CDp-N+ binder can help to distribute sulfur composites and their reductive products homogeneously and then mitigate their aggregation during cycling. The homogeneous distribution of active composites could be a reason for the good electrical conductivity (Figure S4) and stable cycling performance of the β-CDp-N+ based cathode with high sulfur content of 90 wt%. Moreover, the hyperbranched network structure of the β-CDp-N+ binder could accommodate the large volume change of sulfur composite particles during charge and discharge and provide multidimensional noncovalent interaction with active material particle surfaces through the crowd bulky side groups, which enable the binder to maintain the interactions with sulfur composite particles during continuous volume change of sulfur. Hence, the structure of the β-CDp-N+ binder based cathode with high sulfur content preserved well with no cracks. More important, the exist of the quaternary ammonium ion can anchor the polysulfides and inhibit the diffusion of the polysulfides out of the cathode, thus retarding the

1400 -CDpN PVDF

+

1200 1000

7 6 5

800

4

-2

5.5 mg cm

600

3

400

2

200

1

0

0

10

20

30

40

0

-2

Specific Capacity/mAhg -1

polysulfide shuttle effect.

Areal Capcity/mAh cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cycle Number

Figure 7. Cycle performance of the β-CDP-N+ cathode and the PVDF cathode with sulfur loading of 5.5 mg cm-2 and electrolyte amount of 7 μL mg-1 S.


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It was worth noting that when the sulfur mass loading of the β-CDp-N+ cathode was increased from 2.5 mg cm-2 to 5.5 mg cm-2, the discharge capacity still remained at 800 mAh g-1 at 50 mA g-1 after 45 cycles (Figure 7), corresponding to areal capacity of 4.4 mAh cm-2, which is higher than the values of the commercial LIBs for practical application. The PVDF cathode with sulfur mass loading of 5.5 mg cm-2 can deliver an initial discharge capacity of 1130 mAh g-1. However, in the subsequent cycles, the capacity did not exceed 160 mAh g-1. The corresponding areal capacity of PVDF cathode is only 0.9 mAh cm-2 far below the β-CDp-N+ cathode. As Figure S5 shown, the cathode with PVDF showed larger polarization than the cathode with βCDp-N+ at the first cycle. And in the subsequent cycles of the PVDF cathode, the long lower plateau at 2.1 V during the discharge process and the long plateau during the charge process disappeared. It may can be explained as followed. With the increasing of the sulfur mass loading, the viscosity of the electrolyte increased dramatically, leading to more severe polarization of the battery. And the severe polarization causes the second long discharge voltage plateau much lower than the cut-off voltage 1.7 V, so the second long discharge plateau disappeared. Moreover, when increasing the electrolyte amount from 7 μL mg-1 S to 20 μL mg-1 S, the polarization of the cathode with PVDF is also severe after 15 cycles showed in Figure S6. However, for the β-CDpN+ cathode, the β-CDp-N+ could act as a continuous multidimensional network, which could provide efficient accessibility of active material to the electrolyte. Even with high concentration of polysulfides in electrolyte, the conductive agents could fully contact with active sulfur and reduce the polarization of the battery, so the cathode with β-CDp-N+ exhibited a better electrochemistry performance even at high sulfur mass loading. These results demonstrate that the multidimensional binder β-CDp-N+ is an effective binder for sulfur cathode. Conclusion


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In conclusion, we have introduced a novel multidimensional aqueous polymer with quaternary ammonium cation as an effective binder for sulfur cathodes in Li-S batteries. This new binder β-CDp-N+ originates from the only large-scale commercialized cyclodextrin β-CD and is obtained by chemical modification of β-CD polymer. The unique hyperbranched network structure of the β-CDp-N+ binder helps to endure the mechanical stress which was brought with the large volume change of sulfur particles during charge and discharge. The massive functional hydroxyl and ether groups of the β-CDp-N+ binder contribute to enhance the interactions between active materials and conductive agents and bond the active materials on the current collector. The introduced quaternary ammonium cations play an important role in immobilizing polysulfides and suppressing the shuttle effect. Utilizing these advantageous features, the β-CDpN+ based cathode demonstrates improved cycling performance and rate capability compared to those cathodes with other well-known binders. Even with a high sulfur mass loading of 5.5 mg cm-2, the cathode with β-CDp-N+ still can deliver a higher areal capacity of 4.4 mAh cm-2. Moreover, the new β-CDp-N+ binder is easily prepared, low cost, and environmental friendly. Therefore, it is possible for β-CDp-N+ binder to be adopted in actual battery manufacturing processes. ASSOCIATED CONTENT Supporting Information Experimental details. Solubility test of binders in water and electrolyte solvents. TGA curves of sulfur composite. SEM images of sulfur cathodes with PVDF and β-CD after 100 cycling. Nyquist plots of sulfur cathodes with different binder before and after cycling. 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] (Anbang Wang); [email protected] (Weikun Wang). Author Contributions These authors contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Fund from Beijing Science and Technology Project.

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Multidimensional Polycation Î²-Cyclodextrin Polymer as an Effective

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