Manipulating the Redox Kinetics of Li-S Chemistry by Tellurium

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Letter

Manipulating the Redox Kinetics of Li-S Chemistry by Tellurium Doping for Improved Li-S Batteries Kangli Xu, Xiaojing Liu, Jianwen Liang, Jinyan Cai, Kailong Zhang, Yue Lu, Xun Wu, Maogen Zhu, Yun Liu, Yongchun Zhu, Gongming Wang, and Yitai Qian ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01249 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Energy Letters

Manipulating the Redox Kinetics of Li-S Chemistry by Tellurium Doping for Improved Li-S Batteries Kangli Xu,‡ Xiaojing Liu,‡ Jianwen Liang, Jinyan Cai, Kailong Zhang, Yue Lu, Xun Wu, Maogen Zhu, Yun Liu, Yongchun Zhu,* Gongming Wang* and Yitai Qian* Department of Chemistry, National Laboratory for Physical Science at Micro-scale, University of Science and Technology of China, Hefei, Anhui 230026 (P.R. China)

AUTHOR INFORMATION Corresponding Author [email protected] [email protected] [email protected] Author Contributions ‡ These authors contributed equally.

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ABSTRACT Fundamentally altering the essential properties of material itself is always of great interest, but challenging as well. Herein, we demonstrate a simple tellurium doping method to intrinsically reshape the electronic properties of the sulfur and manipulate the kinetics of Li-S chemistry for improving the performance of Li-S batteries. DFT calculation indicates Te doping can effectively facilitate the lithiation/delithiation reactions and lower the lithium ion diffusion energy barrier in Li2S. Additionally, electrochemical studies prove that the reaction kinetics of Li-S chemistry and cycling performance of Li-S batteries have been significantly improved with Te dopants. An exceptional specific capacity of ~656 mA h g-1 and a high coulombic efficiency of ~99% have been achieved at 5 A g-1 even after 1000 cycles. More importantly, the capability to manipulate the intrinsic properties of materials and explore the synergistic effects between conventional strategies and element doping provides new avenues for LiS batteries and beyond. TOC GRAPHIC


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Lithium-sulfur (Li-S) battery, as one of the most promising candidates for the next-generation energy storage devices, has attracted intense interest over the past decades.1-4 Yet, the complicated Li-S chemistry and the intrinsic features of sulfur as well as the intermediates during the cell operating severely limit its performance for practical application.5-7 For example, the “shuttle effect” induced by the soluble polysulfide (PS) intermediates, inevitably results in undesired parasitic reactions, low columbic efficiency and poor cycling performance.6,8,9 Additionally, the insulating nature of sulfur and polysulfides causes high polarization and low energy efficiency. Moreover, the heterogeneous redox reactions occurring on sulfur cathodes during charge/discharge processes are typically accompanied by sluggish reaction kinetics, also endowing the Li-S batteries with unsatisfactory rate capability.10-13 To this end, numerous strategies have been developed to improve the performance of Li-S batteries.13-16 The basic design principle is to increase the conductivity of sulfur electrodes and alleviate the PS shuttling by adding conductive scaffolds and/or polysulfide absorbents.17-25 For instance, carbon materials such as mesoporous carbon, carbon nanotubes and graphene have been used as the loading scaffolds for sulfur, owing to its excellent conductivity and interconnected structures as a physical barrier to trap PSs.22,26,27 Additionally, various metal oxides and metal sulfides were also used as PSs absorbents to relieve the “shuttle effect” and achieved an improved cycling performance.28-31 Recently, catalysts such as noble metals were also added in the sulfur cathodes to improve the redox kinetics.32,33 For example, Salem et al. used Pt nanoparticles as electrocatalysts to facilitate the PSs transformation with reduced redox overpotentials.32 Although significant progress has been achieved in the past decades, the ever-increasing amounts of the additives in the sulfur cathodes inevitably decrease the content of the active sulfur in the whole electrode, making it deviate from the way toward practical application with high loading density of active electrode materials.10,34 Besides, the structural engineering by employing additives cannot change the essential properties of sulfur itself. Moreover, 3 ACS Paragon Plus Environment


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intrinsically altering the properties of material itself for specific applications is always of great interest, but challenging as well to the researchers in chemistry and materials.35,36 Unfortunately, it is still rarely reported to fundamentally refine the Li-S chemistry of sulfur cathodes such as the surface lithiation/delithiation kinetics and lithium ion diffusion in the inner of electrode materials for battery application by manipulating the intrinsic properties of the sulfur itself. Herein, we demonstrate a facile approach to reshape the electronic properties of S and Li2S by a simple tellurium (Te) doping, and further refine the Li-S chemistry to improve the electrochemical performance of Li-S batteries. Given the higher p orbitals of Te, Te doping can introduce more energy states in the electronic structures of S and Li2S, which is beneficial to improve the intrinsic electrical conductivities37,38. More importantly, the electron density redistribution of the S sites induced by the incorporation of Te with weak electron localization could facilitate the lithiation/delithiation process of the sulfur cathode and thus achieve the modulation of the Li-S chemistry occurring in the sulfur cathode.


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Figure 1. (a) Electron density differences for S, Li2S, and their Te-doped counterparts. (b) The formation energies of lithium insertion and lithium extraction required for the initial lithiation (left panel) and delithiation reactions (right panel). The insets are the corresponding structures. (c) Schematic illustration of the possible Li migration pathways in Te-Li2S. (d) Energy profiles for Li migration pathway 1 in Li2S and Te-Li2S . First-principle calculation was first carried out to theoretically probe the effects of Te on the electronic properties of S and Li2S at the atomic scale (see Computational details, Supplementary Information). Figure 1a maps the electron density distribution of S and Li2S, and their doped counterparts. Clearly, the electron densities of S and Li2S have been substantially redistributed after Te doping, in which the electron cloud tends to migrate from Te to S, and the S atoms thus own more electron densities in Te-S than in S. Additionally, the electrons tend to be delocalized between Te and Li in Te-Li2S, which helps relax the interaction between Te and Li. Meanwhile, the density of states (DOS) plots of S and Li2S as well as their doped counterparts reveal that Te with higher p orbitals could narrow band gaps between the valence and conduction bands, indicating the improved conductivity after Te doping (Figure S1). Given that the surface electronic properties of S and Li2S are closely related to the Li-S chemistry occurring in the lithiation and delithation processes, the formation energies of lithium insertion and extraction in S and Li2S were also calculated, as shown in Figure 1b. Obviously, the Te doped S (Te-S) exhibits a smaller formation energy (-3.31 eV) than S (-3.24 eV), suggesting the lithiation process in Te-S is more preferable. The negative energy means the lithiation is a thermodynamically favorable process, corresponding to the discharge process in Li-S battery. On the other hand, the formation energies (5.33 eV) of the initial delithiation reaction of Li2S is much larger than the 4.48 eV of Te-S, also indicating Te dopant could initiate the charge reaction with a lower energy barrier. Furthermore, the impact of Te dopant on the Li ion diffusion in the solid-phase Li2S was also studied. Figure 1c shows five (pathway 1-5) Li migration pathways near to the Te in Te-Li2S, while 5 ACS Paragon Plus Environment


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three conceivable Li migration pathways (pathway 1-3) between the adjacent sites in Li2S are illustrated in Figure S2a. The calculated energy profiles and the corresponding trajectories for all the possible Li migration pathways are shown in Figure S2b and Figure S3. Among them, the pathway 1 goes through the most favorable potential energy surface with the lowest migration energy barrier for both Li2S (0.37 eV) and Te-Li2S (0.08 eV), which is shown in Figure 1d. Apparently, the Li diffusion rate of Li2S can be much improved by Te doping, which benefits a fast reaction kinetics of the delithiation process in TeLi2S. Moreover, the fast reaction kinetics may also contribute to a better cycling performance by shortening the lifespan of the polysulfide intermediates, and thus alleviate the “shuttle effect”. Based on the theoretical calculations, we further experimentally studied the effects of Te dopant on the redox reaction kinetics of Li-S chemistry and explored the potential of Te doped sulfur cathode for Li-S batteries.

Figure 2. (a) XRD patterns of S, Te-1-S, Te-3-S and Te-5-S, respectively. The insets are the corresponding digital images. (b) SEM images of the Te-3-S /KB materials and the spatial elemental 6 ACS Paragon Plus Environment

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distribution images acquired by HAADF-STEM analysis. (c) The TGA curve of the Te-3-S /KB composite. (d) The XPS 2p spectrum of S (upper panel) and Te-3-S (lower panel). Belonging to the same group as sulfur, Te doped sulfur (Te-S) can be easily achieved by heating Te and S powder with controlled ratio in a sealed tube (see Experimental Section, Supplementary Information). X-ray powder diffraction (XRD) patterns of S and Te-S are shown in Figure 2a. The Te-S samples with different weight contents of Te consistently display the same diffraction profiles as S, and no other distinguishable diffraction peaks from Te are observed, suggesting Te doping did not change the crystal structure of S. The insets are the corresponding digital pictures of S, and Te-S with Te weight percentage of 1% (Te-1-S), 3% (Te-3-S) and 5% (Te-5-S). With the increase of the Te content, the color of the samples becomes darker and darker, and finally black when the weight content reaches 5%. The prepared Te-S was further loaded into Ketjen Black (Te-S/KB) by a melting-diffusion method. Figure 2b displays scanning electron microscopy (SEM) image and the elemental spatial distribution in Te-3-S /KB acquired by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). The homogenous elemental distribution indicates that Te is well distributed in S. Interestingly, when S or Te-S was infused into the porous Ketjen Black to form S/KB or Te-S/KB, all the characteristic diffraction peaks originated from S were disappeared. It reveals that the S or Te-S was well penetrated into the mesoporous channels of Ketjen Black (Figure S4), which can also be verified by the analysis of Brunauer-Emmett-Teller (BET). Nitrogen absorption-desorption isotherm plot displayed in Figure S5 indicates that the specific surface area and pore volume of the Ketjen Black significantly decrease after being infused with Te-S, illustrating that the Te-S has been well filled into the porous structures of Ketjen Black. The weight content of Te-S in Te-S/KB was determined to be around 64.45 wt% by the thermogravimetric analysis (TGA), as shown in Figure 2c. The TGA curves of the Te-S shift to the higher temperature region with the increase of Te amount, implying that the Te-S may lose their weight together (Figure S6). X-ray photoelectron spectroscopy (XPS) was further used to 7 ACS Paragon Plus Environment


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characterize the chemical states of S and Te in the Te-3-S. The XPS 2p spectrum of S can be well fitted into a pair of peaks located at 164 eV and 165.2 eV with a spin-orbit splitting of ~1.2 eV, which can be assigned to the 2p 3/2 and 2p 1/2 of the element S (Figure 2d, upper panel).39,40 Yet, the XPS S 2p spectrum of Te-3-S exhibits two resolved doublets (Figure 2d, lower panel), suggesting the presence of two kinds of chemical environments for S atoms. The predominant doublet located at 164 eV and 165.2 eV is the characteristic of element S, while the other one located at 163.6 eV and 164.8 eV is attributed to the Te-S interaction. Consistently, the XPS Te 3d profile of Te-3-S (Figure S7) displays a positive shift in binding energy, also indicating the strong interaction between S and Te. Given the strong electronegativity of S, the Te-S interaction would result in higher electron density on S and lower electron density on Te, which thus leads to the positive shift in binding energy for Te and negative shift for S. Moreover, the absence of the characteristic of Te-Te bond in XPS Te 3d spectrum of Te-3-S further proves Te has been successfully and uniformly doped into S.


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Figure 3. (a) Cyclic voltammograms of S/KB and Te-3-S/KB electrodes with a scan rate of 0.1 mV s−1. (b) Tafel plots of the initial lithiation reaction of S/KB and Te-3-S/KB. (c) Tafel plots of the initial delithiation of S/KB and Te-3-S/KB. (d) The comparison of the onset potentials and the derived exchange current densities for lithiation and delithiation reaction. The Li-S chemistry of the Te-S was further evaluated in Li-S batteries, with the studied materials as the working electrode and metallic lithium foil as the counter electrode (see Experimental Section, Supplementary Information). Figure 3a displays the cyclic voltammograms (CV) of S/KB and Te-3S/KB with the same loading amount of active materials at the scan rate of 0.1 mV s-1, which both exhibit the characteristic lithiation/delithiation features of sulfur cathodes.41 Interestingly, Te-3-S/KB shows a distinguishable positive shift in potential during the lithiation (discharge) sweep and a negative shift during lithiation (charge) sweep, indicating that the Li-S chemical reactions have been facilitated with the assistance of Te dopants. Basically, the potential shifts suggest a decrease of cell polarization and an improved round-trip efficiency. Besides, the peak shifts are also in good agreement with the galvanostatic charge/discharge profiles, where the initial charge plateau is decreased and the initial discharge voltage is also improved (Figure S8). Moreover, the S/KB shows obvious potential barriers at the beginning of the second plateau in discharge process and the first plateau in charge process, suggesting a sluggish reaction kinetics, while the Te-3-S/KB operates without the voltage bump (highlighted in Figure S8).

42

In addition, the second discharge plateau of the Te-3-S/KB cathode

becomes higher than that of S/KB from the second cycle, which may suggest it requires an activation process. The polarization change of the Te-3-S/KB in the discharging process is slighter than that of the charging process after Te doping (Figure S8). It is consistent to the DFT calculation results that the lithiation formation energy difference of Te-S is only 0.07 eV with Te, while the delithiation formation energy difference is 0.85 eV, which suggests the dopant impacts are mainly played on the charge process (Figure 1b). To further analyze the reactivity of Te-S, the onset potentials (Eonset) were derived using the 9 ACS Paragon Plus Environment


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tangent method.43 The determined onset potentials for Te-3-S and S are 2.395 V and 2.369 V in lithiation process, while 2.201 V and 2.234 V in delithiation process. The larger onset potential values in lithiation process and smaller in delithiation process, indicate a more active reaction occurring in the Te3-S electrodes. To evaluate the redox kinetics during the charge and discharge process, Tafel plots and exchange current densities have been determined.33 Figure 3b and 3c compare the Tafel plots of S/KB and Te-3-S/KB electrodes for the initial cathodic and anodic reactions, respectively. Clearly, the lithiation and delithiation overpotentials of S have been significantly decreased with the assistance of Te dopant. The Tafel slopes do not show obvious changes, suggesting that the lithiation and delithiation reaction mechanisms still keep the same.44-46 Furthermore, exchange current densities can be derived from Tafel plots, which reflects the intrinsic electron transfer rate between electrode and electrolyte. The derived exchange current densities of Te-3-S/KB and S/KB are 70.7 and 38.2 µA cm−2 for the discharge process, and 63.1 and 35.5 µA cm−2 for the charge process, respectively. The increase of exchange current densities for Te-3-S in both charge and discharge process indicates the faster charge transfer kinetics induced by Te dopants. Figure 3d summarizes the onset potentials and exchange current densities of Te-3-S/KB and S/KB in the lithiation and delithiation process. It clearly illustrates that the Te doping can effectively decrease the polarization of charge/discharge reaction and increase the redox reaction kinetics of the Li-S chemistry.


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Figure 4. (a) Nyquist plots of Te-3-S/KB and S/KB. (b) Dopant content dependent charge transfer resistances. The inset is the Nyquist plots of Te-S with different Te contents and the corresponding equivalent circuit. (c) GITT voltage profiles of the Te-3-S/KB (lower panel) and S/KB (upper panel). (d) Reaction resistance (R. R.) comparison of the Te-3-S/KB and S/KB during charge (lower panel) and discharge processes (upper panel). To reveal the effects of Te on the charge transfer properties of Te-S/KB electrode, electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) were further conducted. Figure 4a shows the EIS Nyquist plots of S/KB and Te-3-S/KB. The smaller semicircle for Te-3-S/KB, suggests a lower interfacial charge transfer resistance. The charge transfer resistance can be obtained by fitting the EIS Nyquist plots in accordance to the equivalent circuit (Figure 4b, inset). With the increase of the dopant contents, the charge transfer resistance gradually decreases, from 301 Ω for pristine S to 48 Ω for Te-5-S (Figure 4b), which is consistent with the theoretical calculation results that Te doping could manipulate the surface electronic properties and Li-S reaction kinetics. In addition, it is found that the Te-3-S/KB displays a relatively stable impedance after 11 ACS Paragon Plus Environment


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the first five cycles, indicating that a superior and stable reaction interphase can be maintained on the Te-3-S/KB cathode (Figure S9). To in-situ monitor the interfacial reaction resistances at different charge/discharge stages, the GITT was used.47,48 Figure 4c shows the potential response profiles of the S/KB and Te-3-S/KB in the first cycle. The reaction resistances (R. R.) at the different lithiation/delithiation stages were obtained through dividing the overpotential by the pulse current, as presented in Figure 4d. Apparently, the reaction resistances of Te-3-S/KB are consistently smaller than S/KB, especially during the charge process, which is also in good agreement with the improvement of the reaction polarization and exchange current densities (Figure 3d). The lower reaction resistance means the faster charge and mass transfer rate, which both contribute to the improved lithiation/delithiation kinetics.48-50 CVs under different scan rates were further employed to investigate the reaction kinetics with respect to the lithium ion diffusion (Figure S10). The linear relationship with the square root of the scan rates indicates a diffusion-limited process and the higher slope of Te-3-S represents the faster lithium ion diffusion rate, which can be related to the beneficial lithium ion diffusion property of Te doped S by theoretical calculation (Figure 1c, d).18,42 Overall, all these results clearly indicate Te doping can efficiently manipulate the Li-S redox reaction kinetics.


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Figure 5. (a) The rate capability of Te-3-S/KB and S/KB at various current densities. (b) The dopant content dependent discharge capacities at the 1st and 50th cycle and the corresponding capacity retention over the first 50 cycles. (c) The cycling performance of S/KB and Te-3-S/KB. (d) The longterm cycling performance of Te-3-S/rGO. The rate capability and cycling performance of Te-S were also evaluated to further elucidate the Te effects on the redox kinetics and cycling stability of Li-S batteries. Figure 5a, Figure S11 and Figure S12 display the rate performance of the Te-3-S/KB and S/KB under various current densities. With the increase of current densities from 0.5 A g-1 to 20 A g-1, the specific capacity of 579 mAh g-1 can be maintained for Te-3-S/KB at 20 A g-1, while S/KB only retains 287 mAh g-1. Moreover, the specific capacity can be restored after resetting the current back to 0.5 A g-1, indicating that the Te doping can significantly improve the rate capability of Te-3-S/KB, which can be attributed to the improved the 13 ACS Paragon Plus Environment


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lithiation/delithiation reaction activity and ionic diffusion properties. Additionally, the dopant content dependent specific capacities were also studied. Figure 5b and Figure S13 show the specific capacities of Te-S/KB with various dopant contents at the 1st and 50th cycle. As the dopant content increases, the specific capacity retention increases over the initial 50 cycles, suggesting that Te doping can also improve the cycling performance. Further, the long-term discharge/charge cycle performance was tested at 5 A g-1, as presented in Figure 5c. It should be noted that 0.5 A g-1 was applied in the first several cycles to suffciently activate the electrode before cycling test and the capacity contributions from the carbon host and the dopant are very slight (Figure S14). The discharge capacities of ~673 mAh g-1 for Te-3-S/KB can be maintained after 400 cycles, corresponding to a slight capacity fading rate of 0.026% per cycle from the initial cycle at 5 A g-1. Moreover, the cell also exhibited a high coulombic efficiency of above 98% even after 400 cycles. As comparison, S/KB exhibited a faster capacity fading rate with only 322 mAh g-1 maintained and a lower coulombic efficiency of 91.3% at 400th cycle. It suggests that the improved kinetics of Li-S chemistry induced by Te doping can help relieve the “shuttle effect” by shortening the lifespan of PS intermediates, which can also be proved by the lighter color change of electrolyte after cycling (Figure S15). More importantly, the Te doping can also combine with conventional carbon encapsulation method to achieve a synergistic benefit, and further push the performance of Li-S batteries to an even higher level. We finally used graphene encapsulation to prepare Te-3-S/rGO, and the weight content of Te-3-S was determined to be 66.57 wt% (Figure S16). As shown in Figure 5d and Figure S17, an exceptional specific capacity of ~656 mA h g-1 and a high coulombic efficiency of ~99% can be reached at the current density of 5 A g-1 even after 1000 cycles, suggesting a synergistic effect can be achieved in Te-3-S/rGO. In conclusion, we have developed an effective method to intrinsically manipulate the Li-S chemistry of sulfur cathodes by Te doping and demonstrated its promising potential for the development of high performance Li-S batteries. Theoretical calculations reveal that Te dopants can efficiently lower 14 ACS Paragon Plus Environment

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the energy barriers of lithiation and delithiation process in Li-S batteries. Additionally, the lithium ion diffusion property in Li2S is also improved after incorporation with Te, potentially endowing the Li-S batteries with a high rate capability. Furthermore, electrochemical studies verify that Te doping can decrease the redox reaction polarization and increase the exchange current densities, suggesting the enhanced redox reaction kinetics of Li-S chemistry. More importantly, the capability to manipulate the intrinsic properties of electrode materials by element doping could provide new avenues to improve the performance of Li-S batteries and beyond. ACKNOWLEDGMENT This work was supported by the National Natural Science Fund of China (No. 11704365, 21671183, 21521001, 21771169, GG2060190212), the Fundamental Research Funds for the Central Universities (WK2060190053, WK2060190074, WK2060190081), USTC start-up funding and Recruitment Program of Global Expert. The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of University of Science and Technology of China. We also appreciate the Photoemission Endstations (BL10B) in National Synchrotron Radiation Laboratory for the analysis of X-ray photoelectron spectroscopy. Supporting Information Detailed computational methods, experimental procedures, photograph of samples, XRD, N2 adsorption–desorption isotherms, XPS, CV, EIS, TGA and galvanostatic discharge-charge tests. REFERENCES 1. Chu, S.; Cui, Y.; Liu, N. The Path Towards Sustainable Energy. Nat. Mater. 2016, 16, 16. 2. Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S. Rechargeable Lithium–Sulfur Batteries. Chem. Rev. 2014, 114, 11751-11787.



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