Recognizing the Mechanism of Sulfurized Polyacrylonitrile Cathode

Nov 6, 2018 - Recognizing the Mechanism of Sulfurized Polyacrylonitrile Cathode Materials for Li–S Batteries and beyond in Al–S Batteries. Wenxi W...
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Recognizing the Mechanism of Sulfurized Polyacrylonitrile Cathode Materials for Li−S Batteries and beyond in Al−S Batteries Wenxi Wang,†,‡ Zhen Cao,†,‡ Giuseppe Antonio Elia,¶,‡ Yingqiang Wu,† Wandi Wahyudi,† Edy Abou-Hamad,# Abdul-Hamid Emwas,# Luigi Cavallo,*,† Lain-Jong Li,*,† and Jun Ming*,†,⊥ ACS Energy Lett. 2018.3:2899-2907. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 01/14/19. For personal use only.

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Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ⊥ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, People’s Republic of China ¶ Research Center of Microperipheric Technologies, Technische Universität Berlin, Gustav-Meyer-Allee 25, D-13355 Berlin, Germany # Core Laboratories, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia S Supporting Information *

ABSTRACT: Sulfurized polyacrylonitrile (SPAN) is the most promising cathode for next-generation lithium−sulfur (Li−S) batteries due to the much improved stability. However, the molecular structure and reaction mechanism have not yet been fully understood. Herein, we present a new take on the structure and mechanism to interpret the electrochemical behaviors. We find that the thiyl radical is generated after the cleavage of the S−S bond in molecules in the first cycle, and then a conjugative structure can be formed due to electron delocalization of the thiyl radical on the pyridine backbone. The conjugative structure can react with lithium ions through a lithium coupled electron transfer process and form an ion-coordination bond reversibly. This could be the real reason for the superior lithium storage capability, in which the lithium polysulfide may not be formed. This study refreshes current knowledge of SPAN in Li−S batteries. In addition, the structural analysis is applicable to analyze the current organic cathodes in rechargeable batteries and also allows further applications in Al−S batteries to achieve high performance.

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polymerized from sulfur and polyacrylonitrile, provides an approach to tackle these issues20,21 However, the molecular structure (i.e., predominant active unit) and reaction mechanism have not yet been fully understood during the (dis-)charge process (i.e., (de-)lithiation). There is a consensus that current knowledge is incapable of interpreting all of the electrochemical behaviors clearly at present. SPAN is a typical sulfur-containing compound, which was supposed to be efficient to eliminate polysulfide dissolution and the shuttling effect due to the existence of a C−S bond.20,22−24 Although the synthetic conditions,25−27 sulfurization process,28,29 structural characterization,30,31 and utilization in lithium batteries32−35 have been widely studied, the predominant active unit of SPAN and the electrochemical

echargeable lithium−sulfur (Li−S) batteries have attracted considerable attention due to the high theoretical capacity of 1675 mAh g−1, low cost of sulfur resources, and environmental friendliness for sustainability.1−3 However, the commercialization of Li−S batteries is plagued seriously by several intrinsic issues, including (i) low utilization of cathode determined by nonconductive sulfur, (ii) the poor cycle stability from the “shuttling effect” of polysulfide intermediates, and (iii) safety concerns from anodic lithium dendrites.4−6 In the past 2 decades, many strategies (e.g., the preparation of a sulfur-based composite,7−10 intercalated layer insertion,11−13 separator modification,14,15 electrolyte design,16,17 and anode protection18,19) have been developed to stabilize the sulfur cathode and/or trap the lithium polysulfides. However, the shuttling effect of polysulfide is hard to overcome completely because the dissolution and migration of polysulfides is an intrinsic process in electrolytes, particularly under the electric field during the (dis-)charge process. Alternatively, sulfurized polyacrylonitrile (SPAN), © 2018 American Chemical Society

Received: October 10, 2018 Accepted: November 6, 2018 Published: November 6, 2018 2899

DOI: 10.1021/acsenergylett.8b01945 ACS Energy Lett. 2018, 3, 2899−2907

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Cite This: ACS Energy Lett. 2018, 3, 2899−2907

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Figure 1. (a) Comparison of stability and capacity retention of the SPAN cathode in different alkali metal−sulfur batteries. Electrochemical performances of the as-prepared SPAN cathode in Li−S batteries. (b) Cycle stability and Coulombic efficiency at a current density of 0.5 A g−1. (Inset) Comparative energy density of a commercial cathode and SPAN. (c) Galvanostatic (dis-)charge curves and (d) rate capabilities.

Figure 2. (a) Experimental 13C DP/MAS solid-state NMR spectra of SPAN and (b) corresponding 13C locations in the unit of SPAN. (c,d) Proposed molecular structure of SPAN based on simulations of 13C NMR spectra.

deduced by combinations of 13C solid-state nuclear magnetic resonance (NMR) and quantum chemical simulation. We find that the S−S bond in the structure is cleaved in the first cycle, and then, the thiyl radical is generated to form a conjugative structure through the characterizations of 7Li solid-state NMR, electron paramagnetic resonance (EPR), and simulations. Thus, the lithium ions (Li+) can react with negative locations around S and N atoms in the conjugative structure through a lithium coupled electron transfer process reversibly. This is confirmed to be the reason for the superior stability, rather than the general belief of forming lithium polysulfide.25,31,38

behaviors in the (dis-)charge process need to be further explored,20,36,37 such as the reason for the low discharge voltage (plateau) in the first discharge process, the structural evolution upon cycling, and the unexpected stability in carbonate-based electrolyte. In addition, the performance of SPAN may still have a lot of room to improve after a full understanding of the molecular structure and electrochemical mechanism. Herein, we propose a new take on the reaction mechanism of the SPAN cathode to store lithium during the (de)lithiation process. The molecular configuration of the active unit is 2900

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interpret aforementioned phenomena. Herein, we analyze the structure in detail through 13C cross-polarization/magic angle spinning (CP/MAS) solid-state NMR. Two characteristic peaks at 122 and 153 ppm corresponding to CC47 and C N48 were observed, respectively (Figure 2a). In addition, the new peak at 165 ppm can be identified as a C−S bond compared with the chemical shift of the PAN precursor. The arrangement and position of carbon, nitrogen, and sulfur atom in the structure are proposed in Figure 2b, where the unit can be connected through the sulfur atom in two forms (Figure 2c,d). Theoretical density functional theory (DFT) computation of NMR further confirms the conjecture and is used to identify which structure (i.e., I or II) is preferable, where the number of peaks and the difference of the chemical shift between peaks (i.e., ΔJ) (Table 1) are two important references. We find that the simulated number of peaks and the tendency of ΔJ for structure I are consistent with those in experiment (Table 1, Figure 2c). By contrast, the simulated number of peaks for structure II only shows two types of peaks (Table 1). Thus, we consider that the active unit of SPAN is structure I. Note that the structure in Figure 2b has been proposed as one portion of the molecular structure in SPAN previously,22,32−34 while herein we speculate that structure I may have a repeated and/or periodic unit (i.e., C3N1S1) in the SPAN structure. This is the main difference from previous results.30,31,45,46 This conjecture is further confirmed by the consistent amount of sulfur in theory and in experiment (i.e., 39 wt % calculated from C3N1S1 vs 39.8 wt % determined by elemental analysis in Table S3), which is also close to the reported value of 41 ± 1 wt %.30 Structural Evolution and Analysis. The structural evolution of SPAN during the (dis-)charge process was studied by 7Li solidstate NMR, where the (dis-)charged products were characterized. We find that the peak position of 7Li shifts from 0.98 to 2.13 ppm gradually in the first discharge from 1D200 to 1Dtotal. This should be ascribed to the deshielding effect on the 7 Li atom49 and can imply the existence of chemical reaction between Li and SPAN (Figure 3).50,51 Later, the chemical shift of SPAN could return to 0.76 ppm when the electrode was fully charged (i.e., the state of 1Ctotal). These results demonstrate that the reaction between Li+ and SPAN is reversible. Note that the chemical shifts of discharged products (i.e., 0.98−2.13 ppm) are smaller than 2.37 ppm of commercial Li2S and larger than 0.00 ppm of Li2S849 (Figure 3). This means that the local environment of Li+ in SPAN is different from that of Li2S and Li2S8. Besides, we find that the half-peak width broadens gradually in the discharge, indicating an increasing number of Li+ located within SPAN. In particular, the broad signals of 7Li NMR show that the discharged component could be in the solid state. This is because a broad signal can always be caused by the anisotropic interactions of the solid compound (e.g., homonuclear dipolar interactions and quadrupole couplings).51 These results support our hypothesis that the Li+ can react with the negative locations around sulfur, and the lithium polysulfide may not be formed in the (dis-)charge process. The absence of lithium polysulfide is further confirmed by the great compatibility of SPAN and carbonate-based electrolyte, where thousands of cycles can be maintained (Figure 1b). Otherwise, severe capacity fading46 could be observed because the lithium polysulfide (i.e., formed from the long-chain sulfur) can induce a strong nucleophilic reaction with carbonate-based solvent.29 This point is further proved by the inferior cycle performance and new discharge

Our point is further demonstrated by the high performances of SPAN for Al−S batteries in this study for the first time. Electrochemical Features. The widespread applications of SPAN in Li−S,22,30,31,37,39,40 Na−S,41,42 and K−S43,44 batteries in Figure 1a confirm its importance as a promising cathode in mobile ion batteries (Table S1). Herein, we not only demonstrate an extraordinary cycling performance of over 2000 cycles with a high capacity retention of 96.8% (Figure 1b) but also explore its electrochemical features in Al−S batteries after a full understanding of the molecular structure and reaction mechanism. In addition, another attractive feature of SPAN is the much higher energy density compared with that of current cathodes (e.g., LiMn2O4, LiFePO4, LiNi1/3Co1/3Mn1/3O2, and LiNiCoAlO2) (inset of Figure 1b, Table S2). However, insight of electrochemical behaviors becomes necessary for further applications. For example, the reason for the superior cycle stability in carbonate-based electrolyte (Figure 1b) and the cause of the voltage (plateau) Table 1. Experimental and Calculated NMR Peaks and the Value of ΔJ for Structures I and II C1 C2 C3

ΔJ(Exp.)/ppm

ΔJ(calc.)/ppmI

ΔJ(calc.)/ppmII

43 (C1−C2) 31 (C3−C2)

36 (C1−C2) 11 (C3−C2)

37 (C1−C2)

Figure 3. Ex situ 7Li solid-state NMR spectra of the SPAN electrode at the state of different (dis-)charged capacities. The abbreviations C and D mean the state of charge and discharge, while the values of x and y represent the cycle number and corresponding capacities, respectively.

difference between the first and second discharge curves (i.e., ΔV = 0.35 V) (Figures 1c and S1) need to be interpreted. Besides, the surprising rate capability with degradation less than 4.5% from 10 to 5000 mA g−1 (i.e., 587 to 444 mAh g−1) is hard to achieve and understand in mobile ion batteries (Figure 1d). Molecular Structural Analysis. SPAN is thermally polymerized from the sulfur and polyacrylonitrile,20,28,30,37 in which a structure of SPAN including the oligo(sulfide)s, 2-pyridythiolates, and vinylogous/phenylogous thioamides,30,31,45,46 has been proposed. However, the molecular structure (i.e., predominant active unit) needs to be further explored to 2901

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Figure 4. (a) Galvanostatic (dis-)discharge curves of SPAN in the first and second cycles. (b) Comparative ex situ EPR spectra of SPAN at different discharge and charge states accordingly. (c) Proposed structural evolution from pristine SPAN to the intermediates of radical and ionic SPAN in the reactions, where the deeper red color represents the higher active sites.

chemical behaviors of SPAN (e.g., the longer cyclability and the value of the discharge voltage (plateau) and number of (dis-)charge curves in Figure 1b,c) compared to those of sulfur-based cathodes in Li−S batteries29 further demonstrate the low possibility of forming lithium polysulfide in the discharge. Thus, the stable cycle performance and different electrochemical behaviors of SPAN both demonstrate the absence of lithium polysulfide. On the basis of these analyses, we show that the long-chain Sx (4 < x < 8) structure should be absent or very limited in the SPAN structure (Figure S3). Besides, we find that the peak position and half-peak width of 15 N solid-state NMR at 238 ppm vary regularly in the discharge (Figure S4), revealing the existence of interaction between Li+ and the local environment surrounding nitrogen.49 Thus, we conclude that lithium polysulfide may not be formed, which is different from the general belief.25,31,38 Alternatively, Li+ can probably react with the negative locations around sulfur and nitrogen atoms in SPAN (i.e., the redox reaction between S/N and Li+ after accepting the electron) to form an ion-coordination bond, as discussed later. Reaction Mechanism. The reaction mechanism and location of Li+ in SPAN is further studied by the ex situ EPR signal and theoretical simulations, in which the electrodes at the state of different (dis-)charge capacities were examined (Figure 4a,b). First, we find that the signal of the radical appears and the intensity increases gradually, which reaches the highest value in the discharge state from pristine SPAN to 1D400 (Figure 4b, i− iii). This is because there is continuous S−S cleavage50,52 and generation of radicals as the reaction between Li+ and pristine SPAN in the discharge process (Figure 4b, ii,iii). Later, the

Figure 5. Potential energy of the SPAN radical to host different units of Li+ in different states. (a,b) 1 Li+, (c−e) 2 Li+, (f,g) 3−n Li+.

plateau (i.e., the reaction of long-chain sulfur to lithium polysulfide) of SPAN when 10 wt % S8 was added into the electrode (Figure S2). In addition, the different electro2902

DOI: 10.1021/acsenergylett.8b01945 ACS Energy Lett. 2018, 3, 2899−2907

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Figure 6. Schematic illustration of the reaction pathway for SPAN to store lithium. (a) Pristine SPAN. (b) Radical SPAN with a conjugative structure and the corresponding minimum repeat unit. (c,d) Ionic SPAN2− + 2Li+. (e) Ionic SPAN3− + 3Li+. (f) Ionic SPANn− + nLi+.

the SPAN structure. The radicals are maintained in the following charge process (Figure 4b, viii), which could be the probable reason for the high reversibility and superior cycle stability, as discussed later. Herein, the type of radicals was confirmed to be thiyl radical by the value of the g tensor of around 2.006 (Figure S5) because the radical species with a value of around 2.01 can be considered the thiyl radical.53,54 However, we preferably speculate the coexistence of radical centers (i.e., S-center/C-center). This is because a conjugative structure can be induced by electron delocalization on the aromatic moieties.54,55 In addition, the tested g value is slightly smaller than that of the typical S center radical (i.e., 2.006 vs 2.01), which also corroborates our conjecture. Note that the thiyl radicals could be stable against storage time because the conjugative structure (i.e., conjugated pyridine backbone) can stabilize radicals.56 Electron delocalization on the SPAN structure can also be interpreted by the steric configuration of radical and ionic SPAN in simulations, with a possible evolution of the SPAN structure presented in Figures 4c and S6a,b. Starting from the pristine SPAN, the linear radical SPAN is formed after cleavage of the S−S bond (Figures 4c and S6a), where a conjugated π structure can easily be formed due to the linear structure, and then the thiyl radicals can be stabilized.56 Meanwhile, radical SPAN can turn into ionic SPAN when SPAN accepts electrons from the external circuit. Then, the linear structure converts to a “zigzag” shape due to electrostatic repulsion (Figures 4c and S6b). Thus, this kind of molecular structure has a large space and is rich with negative locations, enabling the molecules to have high activity to react with lithium. Note that the radicals could be in a negative, positive, or neutral state.57 Thus, radical SPAN can be generated from pristine SPAN through a reductive step. As a result, more details about the Li+ location in ionic SPAN are further investigated based on our model using theoretical simulation. As shown in Figure 5, Li+ probably can bond with the negative site around the sulfur and nitrogen atoms in a different type of model. The different units of Li+ (i.e., one, two, or three) and their locations were discussed and simulated in detail. Several pieces of information can be summarized from the simulations: (i) The negative site around sulfur can host Li+ much easier than that around the nitrogen atom due to the lower potential energy (Figure 5a,b); (ii) the pyridine N atom can host one unit of Li+ when two units of Li+

Figure 7. High-resolution XPS spectra of S2p for SPAN at different discharge and charge states. Calibration of the binding energy referred to the C 1s peak at 284.8 eV, where the S2p3/2 and S2p3/1 doublet has an energy discrepancy of 1.2 eV and intensity ratio of 2:1.

radical intensity weakens when the electrode is discharged further to the state of 1Dtotal (Figure 4b, iv), demonstrating the chemical combination of radicals with Li+ after the coupling electron (i.e., redox reaction between S/N in SPAN and lithium ions). Reversibly, the radical signal increases again when the battery is recharged to 1Ctotal (Figure 4b, vi), implying regeneration of the radical with Li+ extraction from 2903

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Figure 8. Electrochemical performance of SPAN in Al−S battery applications. (a) Cyclic voltammograms. (b) Cycle performance. (c) Galvanostatic (dis-)charge curves. (d) Rate capabilities.

framework can be well maintained. Thus, extremely high stability and superior rate capability were demonstrated. The speculated molecular structure and reaction mechanism can be further confirmed by variation of S−X bonds in SPAN during the reaction process, as analyzed by ex situ XPS. Three groups of bonds (i.e., S−S, S−C, S2−) featured by the S 2p3/2 and S 2p1/2 doublet in SPAN are observed in Figure 7, where the peak of S 2p3/2 is analyzed to confirm the variations of the S-based bond. The first, the presence of bridging S−S45,58 and terminal S−C58,59 species, are reconfirmed by the S2p3/2 peak at 163.5 and 163.2 eV, respectively. Then, the conversion of the bridging S−S bond to the terminal S−C bond in the discharge process is demonstrated by the regular variation of the S2p3/2 peak from 163.5 to 163.2 eV. In addition, the formation of lithium sulfides and sulfate can be judged by the intensity increment of the S2p3/2 peak at 161.745,60 and 169.3 eV,61 respectively. In the following charge process, the S atom may still exist as a thiyl radical because the charge products consist of only terminal S−C (confirmed by 13C solid-state NMR in Figure S8) and Li−S, which is consistent with EPR results. This is rational because the conjugative structured thiyl radical prefers to maintain the intermediate framework, rather than reconnection to form a S−S bond after the first cycle due to the stabilization effect of the conjugated π-accepting group56 (e.g., pyridine functional group) on the thiyl radical. Applications for Al−S Batteries. The reactivity of the SPAN cathode with an Al3+ ion through the proposed radical electrochemical mechanism was further investigated. Although the Al ion and, in particular, the Al−S battery have attracted great attention recently,62−65 an efficient cathode is still needed to make the battery work. This is because the solubility issue and high irreversibility of AlSx always cause limited stability and activity.66,67 Interestingly, herein the activity, reversibility, and stability of SPAN with Al were confirmed by the cathodic/ anodic peaks in cyclic voltammograms (CVs) (Figure 8a). There is an activation process in the initial cycles at about 0.3

are located in the structure because of the lowest potential energy (Figure 5c−e); (iii) ideally, a repeat unit of active structure can host three units of Li+ with a more stable coordination structure according to the potential energy (Figure 5f,g). Two Li+ units can be located in the negative area between two sulfur atoms, while one more Li+ should be located at the negative site around pyridine N atoms in the discharge. The illustrated structure of SPAN with more Li+ (i.e., lithiated SPAN) is presented in Figure 5g. In addition, the theoretical capacity can achieve 654 mAh g−1 when the unit of the deduced structure tends to infinite (Figure S7). This value is consistent with the experimental value of 631 mAh g−1, demonstrating the rationality and accuracy of our conjecture. On the basis of our analysis, the reaction mechanism and probable reaction process are proposed, in which the repeated unit, possible reduced/oxidized structure, and counteranions are presented in Figure 6. The SPAN experiences S−S bond cleavage and reacts with Li+ when SPAN accepts electrons from the external circuit to form lithiated ionic SPAN in the first discharge (Figure 6c−e). The reaction in SPAN with an infinite unit structure is illustrated in Figure 6f. The proposed mechanism may involve two reaction pathways from pristine SPAN to lithiated ionic SPAN in Figure 6a−e (i.e., pathways I and II). Herein, pathway I seems preferable because of the lowest potential energy (Figure 5c) and the existence of radicals (Figure 4b, ii,iii). Note that lithiated ionic SPAN can convert to radical SPAN (conjugative structure with a thiyl radical) in the charge process, rather than returning to initial SPAN. Thus, the discharge voltage (plateau) in the first cycle is different from that in the second cycle and beyond. This is because the discharge process begins from cleavage of the S−S bond in SPAN for the first cycle while all of the following discharge processes start from radical SPAN (Figure 6b). In addition, we could find that the formation of an ioncoordination bond between Li+ and negative locations around sulfur/nitrogen atoms is very fast, where the polymer 2904

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ACS Energy Letters V (vs Al/Al3), and it should be ascribed to breaking of the S−S bond. Thus, the discharged capacity can increase from 320 to 605 mAh g−1 and then stabilizes at 201 mAh g−1 (Figure 8b). Note that the low voltage (plateau) in the first discharge is consistent with the low reduction potential in the first scan in the CV (Figure 8c), which is similar to the voltage drop of SPAN in Li−S batteries. These results further demonstrate breaking of the S−S bond and the active sites of S/N atoms with Al3+. As a result, SPAN demonstrates the superior rate capacities of 343, 258, 160, 93, and 54 mAh g−1 at current densities of 0.025, 0.05, 0.1, 0.2, and 0.5 A g−1, respectively (Figure 8d), which are much higher than those in recent literature.48 Our preliminary results confirm that more intriguing details of SPAN in Al−S batteries deserve further development. In summary, we analyzed the molecular structure (i.e., predominant active unit) of SPAN and the reaction mechanism through thorough analysis of NMR, EPR, and simulations. We confirm that a conjugative structure with a thiyl radical is formed after cleavage of the S−S bond in the first discharge process. The radical structure can react with lithium through the negative locations around sulfur and nitrogen atoms to form an ion-coordination bond. This lithium coupled electron transfer process is fast and reversible; thus, the SPAN can demonstrate superior cycle stability over thousands of cycles and extraordinary rate capability. Our results not only interpret many unexplained electrochemical behaviors including the voltage difference and the reason for high performances but also refresh current knowledge of SPAN in Li−S batteries. The knowledge and understanding are applicable for the analysis of current organic cathodes in rechargeable batteries and also successfully guide us to get high performance in Al−S battery applications

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Author Contributions ‡

W.W., Z.C., and G.A.E. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the King Abdullah University of Science and Technology (KAUST).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b01945. Experimental section and simulation method, comparative performance of SPAN in different metal−sulfur batteries, comparative energy density of SPAN and other conventional cathodes, cycle performance of SPAN mixed with 10 wt % S8 (i.e., elemental sulfur), TGA and elemental analysis of SPAN, 15N and 13C solid-state NMR and molecular structure of SPAN in radical and ionic states, locations of Li+ within SPAN and capacity calculation in theory, and comparison of supercapacitor and battery systems (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.C.). *E-mail: [email protected] (L.-J.L.). *E-mail: [email protected] (J.M.). ORCID

Giuseppe Antonio Elia: 0000-0001-6790-1143 Luigi Cavallo: 0000-0002-1398-338X Jun Ming: 0000-0001-9561-5718 2905

DOI: 10.1021/acsenergylett.8b01945 ACS Energy Lett. 2018, 3, 2899−2907

Letter

ACS Energy Letters

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DOI: 10.1021/acsenergylett.8b01945 ACS Energy Lett. 2018, 3, 2899−2907

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DOI: 10.1021/acsenergylett.8b01945 ACS Energy Lett. 2018, 3, 2899−2907