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Heterogeneous Catalysis for Lithium-Sulfur Batteries: Enhanced Rate Performance by Promoting Polysulfide Fragmentations Tae-Gyung Jeong, Dong Shin Choi, Hannah Song, Jihwan Choi, ShinAe Park, Si Hyoung Oh, Heejin Kim, Yousung Jung, and Yong-Tae Kim ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00603 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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ACS Energy Letters

Heterogeneous Catalysis for Lithium-Sulfur Batteries: Enhanced Rate Performance by Promoting Polysulfide Fragmentations Tae-Gyung Jeong,a† Dong Shin Choi,b† Hannah Song,a† Jihwan Choi,a Shin-Ae Park,a Si Hyoung Oh,d Heejin Kim,*c Yousung Jung,*b Yong-Tae Kim*a a.

Department of Energy Systems, Pusan National University, Busan 46241, Republic of Korea.

b.

Graduate School of EEWS, Korea Advanced institute of Science and Technology, Daejeon

34141, Republic of Korea. c.

Center for Electron Microscopy Research, Korea Basic Science Institute, Suncheon 57922,

Republic of Korea. d.

Centre for Energy Convergence, Korea Institute of Science and Technology, Seoul 02792,

Republic of Korea. Corresponding Author *E-mail: [email protected] (Y. K) *Email: [email protected] (Y. J) *Email: [email protected] (H. K)

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Abstract

A spatial confiment of polysulfides using the metal compound additives having polar surfaces has been considered to be a promising approach to address the insufficient rate capability and cyclability of lithium-sulfur batteries. Herein, we report more effective approach outperforming this conventional one; a heterogeneous catalysis to promote polysulfide fragmentations. It was revealed using the combined computational and experimental approaches that an ultra-strong adsorption of elemental sulfur on TiN surfaces resulted in a spontaenous fragmentation into shorter chain of polysulfides. This heterogeneous catalysis reaction improved the sluggish kinetics of polysulfide reduction due to the chemical disproportionation at the second plateau. A markedly enhanced rate capability was finally obtained, exhibiting a discharge capacity of 700 mAh g-1 at a scan rate of 5C.

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Lithium-sulfur batteries (LSBs) have attracted a significant recent attention due to a high theoretical capacity of 1675 mAh g-1 and specific energy of 2600 Wh kg-1 compared to the conventional intercalation-based lithium-ion batteries (LIBs). Sulfur is a naturally abundant element and an inexpensive material, making it even more attractive for large scale energy storage applications.1-5 The main drawbacks in commercializing the LSBs, however, are known to be a low cycle life and poor rate capability, originating partly from the insulating nature of active materials and from the dissolution of polysulfides into the electrolytes during the cell operations.6-8 In an effort to overcome these issues, exciting progresses have been made in the cathodes side by developing new structures and composites to prevent the loss of active materials due to dissolution as well as to improve the conductivity of the cathode at the same time. To physically trap the polysulfides, carbon hosts with high porosity,9-12 carbon interlayers,13-15 hollow carbon nanostructures16-18 , graphene-based structures19-21 and polysulfide absorbent addition22 have been employed due to their high electrical conductivity. While these efforts have increased the capacities and cyclabilities of the existing sulfur cathodes to some extent, nearly 20% of the active material is still lost in the electrolytes.9 Especially at a reasonably high current density, not all the sulfur molecules participate in reduction reactions during discharge but rather remains as an elemental form, leading to a low capacity.23 Recently, chemical modifications of the electrode surfaces with oxides, sulphides, nitrides or polymers have been applied to retain the polysulfides near the cathode.24-30 Also, the use of metal/metal oxides modified carbon to adsorb and re-activate polysulfide species has exhibited excellent capacity and cycle performances.31-34 The addition of these materials provides the polar surface that can strongly interact with the polysulfides and spatially confine them near the


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electrode surfaces, addressing the shuttle effects more aggressively. Most of previous studies have explained the effects of such additives mainly by relying on their increased binding affinities to the polysulfides although they may have more complex and richer functionalities and thus can affect the battery performance via different aspects. Herein, we show that a heterogeneous catalysis on metal compound surfaces having an ultra-strong bond formation with elemental sulfur like nitrogen-terminated TiN results in a spontaneous fragmentation of sulfur into shorter chain of polysulfides. In particular, we demonstrate that this unreported phenomenon can markedly enhance the rate capability with accelerated feeding of short polysulfides by the facilitated fragmentation of longer polysulfides particularly at about 2.0 V region corresponding to the second plateau in the charge-discharge curves, where the further reduction is restricted by the insufficient supply of polysulfides to the electrode due to the sluggish chemical disproportionation.35 We begin with the demonstration of battery performance changes with various additives in sulfur-carbon composite cathodes before discussing the heterogeneous catalysis in LSB in detail. We prepared four composite cathodes for LSB with and without Ti-based additives: 1) bare sulfur–carbon mixture (bare), 2) sulfur–carbon mixture with TiO2 additive (C/TiO2), 3) sulfur– carbon mixture with Ti4O7 additive (C/Ti4O7), and 4) sulfur–carbon mixture with TiN additive (C/TiN). All components in each cathode were homogenously mixed as evidenced by the EPMA images (Figure S1 in the ESI). Also, their morphology was nearly identical as shown in Figure S2 in the ESI, suggesting that the influences from the particle morphology between samples could be neglected. The electrochemical properties of the C/TiN cathode were compared with the other composite cathodes via the conventional galvanostatic cycling. Figure 1 shows the voltage profile of LSBs


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Figure 1. Charge-discharge profiles at (a) 0.2C, (b) 0.5C, (c) 1C, (d) 2C, (e) 5C, and (f) discharge capacity ratio of C/TiN to bare electrodes. (g) Cyclability of the bare, C/TiN, C/Ti4O7 and C/TiO2 electrodes.


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on different C-rates varying from 0.2 C to 5 C. In every C-rate, all the LSBs present typical voltage profiles as reported in literatures, which include a rather short plateau at 2.4 V and a long plateau at 2.0 V. Among the Ti-based additives considered here, TiN markedly improves the battery properties in all aspects, such as discharge capacity, cycle performance, rate capability and polarization. The discharge capacities of the bare, C/TiO2, C/Ti4O7, and C/TiN cathodes were 724, 954, 946, and 1069 mAh g-1 at a scan rate of 0.2 C (Figure 1a), and 231, 294, 288, and 579 mAh g-1 at a scan rate of 2 C (Figure 1d), respectively. As proposed in previous works, the increased capacity for the latter samples can be attributed to the strong interaction between the polysulfide and polar surface of the Ti-based additives.27,36 The C/TiN cathode also outperforms the other LSBs in the cycle retention as shown in Figure 1f. After 50 cycles, the C/TiN cathode still retains 70% of the initial discharge capacity, while the bare, C/TiO2 and C/Ti4O7 cathodes respectively exhibit only 55%, 28% and 47% of each initial capacity after the same cycle. Furthermore, the C/TiN cathode exhibits a superior rate capability, achieving the discharge capacity of 780 mAh g-1 at a rate of 1 C; and a capacity of 411 mAh g-1 is preserved even at a higher scan rate of 5 C. On the other hand, C/TiO2 and C/Ti4O7 cathodes lose their functionalities at fast discharge conditions. See Figure S3 in the ESI for more detailed comparison between Tibased additives. In this set of measurements, it is noteworthy that the capacity improvement by TiN additive is more significant at a faster cell operation (Figure 1g). The discharge capacity ratio of the C/TiN to bare cathode, namely the effect of TiN, increases from 1.5 at a scan rate of 1 C to 5.5 at a scan rate of 5 C. It is also remarkable that the C/TiN cathode shows smaller polarizations than the other samples at high C-rates, especially on the second plateau. These results all point to demonstrate that the TiN additive improves the cell kinetics more effectively than the other additives.


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Figure 2. Battery performance enhancement with TiN additives. (a) Capacity enhancement in the first and second plateaus on different c-rate, compared to the bare cathode. (b) Polarization for the second plateau in bare and C/TiN cathodes, in reference to the values at 0.2 C. (c) CV curves with and without TiN. For further details, we compared the variances in discharge capacity and polarization on the first (2.4 V) and second (2.0 V) plateaus separately. As shown in Figure 2, both discharge capacity and polarization are improved more significantly on the second plateau with adding TiN, suggesting that the TiN additive contributes mainly to this 2 V region. This result is consistent with the previous studies that the later parts of the discharge processes, corresponding to the


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second plateau, is relatively slower and is more influenced by the kinetic factors compared to the first plateau.37,38 The CV measurements of Figure 2c also clearly show that the enhancement by TiN is notable for the second plateau (2.03 V at negative sweep) compared to the first plateau (2.4 V at negative sweep). Those for the other cells (S-TiO2, S-Ti4O7) are shown in Figure S4. Several aspects have been suggested as the origin of slow reaction kinetics at the second plateau region, such as insoluble and insulating nature of the Li2S products,39,40 slow rates of disproportionation reactions,41 and the shuttling effects.42,43 Thus, considering the remarkable improvements shown for the second plateau behavior simply by adding TiN, it is conceivable that the TiN additive may improve these existing issues of slow kinetics of the sulfur battery by, for example, helping to form Li2S, facilitating the disproportionation of long polysulfides, and anchoring polysulfides to prevent the shuttle effect. To gain further insights into the role of TiN additives along these mechanisms, we investigated the interaction between calculations. As shown in Figure 3a, the binding energy of a single S8 molecule on the TiN surface is 6.60 eV, which is substantially greater than that on the graphitic (0.76 eV), TiO2 (1.78 eV), and Ti4O7 (1.28 eV) surfaces (binding geometries are presented in Figure S6 in the ESI). This binding strength of 6.60 eV is also considerably larger than the dispersion interactions appearing in other sulfide or oxide surfaces of the previous work (~0.8 eV).44 This unprecedentedly strong binding of S to TiN lies in the fact that all Ti atoms on the reconstructed TiN(001) surface are coordinatively unsaturated, leading to the complete collapsing of S8 molecules into the atomic sulfur by forming Ti–S bonds to stabilize the unfavorable coordination. The ab initio molecular dynamics simulation (See ESI) consistently predicts the passivation of TiN surface with S atoms as much as ~1 monolayer as shown in Figure 3b. Note that both C and Li atoms, which are the most abundant elements in this


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Figure 3. (a) Binding energy of S8 molecule on different Ti-based surfaces. (b) Sulfur passivated TiN (S–TiN) surface. (c) XPS S 2p spectra of elemental sulfur and (d) S8 on TiN. (e) Formation energy of LiPS with and without S-TiN, and their difference. (f) Sulfur reduction reaction on TiN (g) Binding geometry of Li2S4 on the S–TiN surface.


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this composite cathode besides S, are unstable on the TiN surface, allowing the Ti-S bond primarily. We confirmed the existence of surficial Ti–S bonds using the XPS measurements. As shown in Figure 3c and 3d, the binding energy of the S–2p electron shifts from 164.09 eV in the elemental S8 to 163.10 eV in the toluene-washed TiN/S composite without any trace peak of S8. This red shift of the peak can be ascribed to the increased electron density around the S atoms, as in the various metal sulfides.45 The coincident shifts of calculated charge density (0, +0.16e, and +0.66e for S8, S-TiN, and TiS2, respectively) and experimental XPS data (164.09, 163.10, and 161.9 eV for S8, S-TiN, and TiS2, respectively) suggests that the surface-bound S8 rings are completely disrupted and all S atoms make the chemical bond with Ti. We note that S prefers to form a bond with Ti than N on the TiN surface by 0.51 eV. This sulfur passivated TiN (which we denote as S–TiN) surface shows a favorable interaction with both lithium polysulfides (LiPS) and a sulfur molecule. It has been proposed that the anchoring of LiPS on electrode materials is most effective when they exhibit a moderate binding affinity to the surface, as for sulfides.44 The S–TiN surface shows a comparable (albeit slightly smaller) binding affinity to the LiPS when compared to the TiS2 surface, one of the promising anchoring materials with optimal binding strengths.44 (see Figure S7) The S–TiN surface also shows the same characteristic trend in binding affinity as the other moderate anchoring materials; that is, the binding affinity for shorter LiPS increases overall while the affinity for Li2S6 slightly decreases. The structural similarity of the LiPS conformations on the previous TiS2 and the present S–TiN surfaces is another clear indication for stable anchoring of active materials on the S–TiN (Figure S8 in the ESI) surface.


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Figure 3e shows the formation energy of LiPS when they are in a molecular form (Efmolecule) and when they are bound on the S–TiN surface (EfS-TiN), in reference to the lithium metal and alpha sulfur phases, defined as below. Efmolecule (Li2Sn) = E(Li2Sn) – 2 E(Limetal) – n/8 E(S8)

(1)

EfS-TiN (Li2Sn) = E(Li2Sn@S-TiN) – E(S-TiN) – 2E(Limetal) – n/8 E(S8)

(2)

In the molecular form without the S–TiN surface, the long LiPS (n ≥ 4 in Li2Sn) are more stable than the short LiPS (n ≤ 2 in Li2Sn), impeding the formation of short LiPS. Recently, Wang et al. demonstrated that the clustering of LiPS stabilizes the short LiPS more drastically than the long LiPS,46 leading to the desired precipitation of Li2S2 and Li2S. In our calculations, the Li2S cluster, denoted as [Li2S]m, becomes more stable than the long chain LiPS molecules when m > 4, which can be considered as a critical size for the crystallization (details are in Figure S9 in the ESI). In the presence of the S–TiN surface, however, this stability trend of LiPS is reversed, that is, the short LiPS are rather more stable than the long LiPS even without clustering. It is due to the fact that all atoms in the short LiPS form strong chemical bonds with the surface, while the long LiPS interacts with the surface via relatively weak dispersion forces. From equation (1) and (2), the formation energy difference with and without S-TiN surface can be arranged as a follow: EfS-TiN (Li2Sn) – Efmolecule (Li2Sn) = E(Li2Sn@S-TiN) – E(S-TiN) – E(Li2Sn) where the term on the right-hand side corresponds to the binding energy of LiPS on the S-TiN surface. The difference plot, i.e., binding energy of LiPS, show a clear effect of S-TiN surface as drawn with a dotted line in Figure 3e, where the short LiPS are stabilized more than the long


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LiPS. This peculiar stabilization of the short LiPS on the S–TiN surface promotes the later part of the discharge process that is related to the fragmentation into short LiPS. Figure 3f is the CV curves for the pure polycrystalline TiN thin film formed on bulk Ti pellet without any conductor or binder. The TiN itself does not show any electrochemical activity toward Li, consistent with the aforementioned result that the Li does not bind on the TiN surface. Contrarily, when 4mM of S8 is added to the electrolyte, the TiN cathode exhibits a small but sharp peak at 1.5–1.8 V vs. Li/Li+ in a negative (discharge) sweep. This voltage value agrees with the calculated lithiation voltage of the S–TiN surface (1.70 V vs. Li/Li+), implying that the reduction reaction can readily be proceeded on the S–TiN surface due to a high electronic conductivity of TiN (>103 S cm-1 for bulk).42 In other words, the role of TiN is not only promoting the fragmentation of LiPS, but also participating in the electrochemical reactions, in contrast to the other insulating anchoring materials. Nevertheless, the electronic conductivity of Ti4O7 (~103 S cm-1 for bulk)48,49 is also sufficient for sustaining the electron transfer, yet still showed a much smaller enhancement in LSB performance compared to the C/TiN electrode (Figure 1), suggesting that the high electronic conductivity of additives can be helpful but not the primary factor that enhances the LBS performance presently observed. The improved rate capability of TiN compared to the Ti4O7 is clearly shown in Figure S3 in the ESI. Since the second plateau is related to the short LiPS (n ≤ 4 in Li2Sn),37,38 we further investigated the binding of Li2S4 and its fragmentation on the S–TiN surface. As shown in Figure 3g, the Li2S4 adheres to the S-TiN surface via a side-on mode as in the case of other sulfides. When forming these bonds, approximately 0.65 e- (from S of Li2S4) transfer to the S–TiN substrate (by 0.31 e-) and to the Li of Li2S4 (by 0.34 e-), weakening the intramolecular bond strengths of Li2S4 when adsorbed on the S-TiN surface. The Li–S and S–S bond lengths in the S-


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TiN-adsorbed Li2S4 are also elongated by 0.15 Å and 0.03 Å, respectively, compared to those in the unbound Li2S4. This destabilization of Li2S4 on the S-TiN surface then promotes the further fragmentation of this polysulfide to shorter LiPS with an ultra-strong adsorption which can be considered to be a kind of heterogeneous catalysis. It is obvious that this process is very helpful for the subsequent reduction reaction and/or disproportionation reactions. For example, the reduction reaction, Li2S4 + 2Li + 2e- 2Li2S2, is endothermic (nonspontaneous) by 0.41 eV without the substrate, but it becomes exothermic (spontaneous) by 2.33 eV on the S–TiN surface. Also, although the disproportionation reaction such as Li2S4 Li2S2 + 2/8S8, for example, is well known to be a source of slow kinetics for the second plateau and indeed is calculated here to be energetically uphill (endothermic) by 0.87 eV without the help of the additives, the same reaction on the S–TiN surface becomes significantly less difficult, now requiring only 0.24 eV on the S–TiN surface. On the basis of these thermodynamic driving forces, the activation barrier for fragmentations that correspond to the intramolecular bond breaking of Li2S4 are also expected to be lower on the S-TiN surface, improving the reaction kinetics, as shown experimentally here. The interesting role of TiN as a promoter of chemical disproportionation was clearly confirmed with in-situ UV-Vis spectroscopy. In order to compare the rate of chemical disproportionation on the surface of carbon and TiN, glassy carbon and TiN bulk surfaces were immersed into the electrolyte containing chemically synthesized 30 mM Li2S4. As shown in Figure S5, a strong absorption peak corresponding to the Li2S4 was initially demonstrated at around 410 nm and the peak was decreased with time by progress of chemical disproportionation. Interestingly, the rate of peak decrease is much faster for TiN than for glassy carbon, indicating that TiN can promote the chemical disproportionation.


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Lastly, to verify compatibility of the TiN additive to other polysulfide capturing techniques, we additionally inserted acetylene black mesh (ABM) between the cathode and separator as demonstrated in the previous work.15 Figure 4 shows a comparison of LSB cathodes for the bare, C/TiN, ABM-inserted, and ABM-inserted C/TiN (C/TiN–ABM) cases. The order of capacity

Figure 4. Rate capability of bare cathode, C/TiN cathode, cathode with a carbon interlayer (ABM), and C/TiN cathode with ABM (C/TiN–ABM). *ABM from reference 15. retention with varying the scan rate is C/TiN–ABM > C/TiN > ABM > bare cathodes, where the C/TiN–ABM LSBs exhibits an improved discharge capacity by a factor of 10 compared to the bare cathode at a scan rate of 5C (700 mAh g-1 vs. 70 mAh g-1). This clearly shows that the combination of physical trapping and chemical functionalities of additives markedly enhances the performance of LSBs. To the best of our knowledge, this discharge capacity at a high C-rate, e.g. 700 mAh g-1 at 5C, is among the highest in reported data available.50-52


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In summary, we discovered that the heterogeneous catalysis to promote the fragmentation of LiPS based on the ultra-strong adsorption on TiN can markedly improve the discharge capacity and rate capability of lithium sulfur batteries. The TiN- added cathode exhibits 5.5 times higher discharge capacity compared to the conventional cathode even without pretreatment for encapsulating the active materials; and furthermore, when it combines with the carbon interlayer, a discharge capacity reaches to 700 mAh g-1 at a scan rate of 5C, hich is one of the best rate capability results in the present literature. The distinguished catalytic properties of TiN from other Ti-based compounds were revealed to be the ultra-strong chemical bonding between S and the TiN surfaces using the combined computational and experimental analyses. Hence, the TiN surfaces stabilized the short-chain lithium polysulfides preferentially and assists the fragmentation reactions of long-chain polysulfides into shorter chains required and known to be responsible for the slow kinetics of the second discharge plateau. We also confirmed that the heterogenous catalysis approach for LSB can be readily combined with various conventional LiPS capturing techniques like the insertion of carbon mesh or the encapsulation of elemental sulfur to achieve the acceptable cyclability and rate capability for the future commercialization of lithium sulfur batteries ASSOCIATED CONTENT Supporting Information. Experimental section, SEM and EPMA images, additional electrochemical data, UV-Visible spectroscopy, XPS data, computational details and models AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y. K)


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*E-mail: [email protected] (Y. J) Author Contributions †

T.-G. J, D. S. C and H. S contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by Samsung future technology foundation (SRFC-TA 1403-04)

REFERENCES

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