This is the submitted version of the journal article: Li, Mengyao; Yang, Dawei; Jacas Biendicho, Jordi; [et al.]. «Enhanced polysulfide conversion with highly conductive and electrocatalytic iodine-doped Bismuth selenide nanosheets in lithium-sulfur batteries». Advanced functional materials, Vol. 32, issue 26 (June 2022), art. 2200529. DOI 10.1002/adfm.202200529 This version is available at https://ddd.uab.cat/record/270827 under the terms of the license Page 1 of 34 ACS Energy Letters 1 2 3 4 5 6 Enhanced Polysulfide Conversion with Highly Conductive and 7 8 9 Electrocatalytic Iodine-Doped Bismuth Selenide Nanosheets 10 11 12 13 in Lithium-Sulfur Batteries 14 15 16 17 18 Mengyao Li,†,‡,+ Dawei Yang,*,‡,‡,+, Jordi Jacas Biendicho,† Xu Han,§ Chaoqi Zhang,†,‡ Kun 19 20 Liu,○ Jiefeng Diao, Junshan Li,// Marc Heggen,# Rafal E. Dunin-Borkowski,# Jiaao Wang,*, 21 22 23 Graeme Henkelman, Joan Ramon Morante,†,‡ Jordi Arbiol,§, Shu-Lei Chou,*,& Andreu 24 25 26 Cabot*,†, 27 28 † Catalonia Institute for Energy Research-IREC, Sant Adrià de Besòs, Barcelona, 08930, Spain. 29 30 31 ‡ Department of Electronic and Biomedical Engineering, Universitat de Barcelona, 08028, 32 33 34 Barcelona, Spain. 35 36 § Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus, UAB, 37 38 39 Bellaterra, 08193, Barcelona, Spain. 40 41 ○ Institute of Materials and Technology, Dalian Maritime University, Dalian 116026, China. 42 43 44  Department of Chemistry and the Oden Institute for Computational Engineering and Sciences, 45 46 47 The University of Texas at Austin, Austin, TX, 78712 USA. 48 49 // 50 Institute of Advanced Study, Chengdu University, 610106 No. 2025, Chengluo Avenue, 51 52 Chengdu, Sichuan Province, China. 53 54 # Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg 55 56 57 Institute, Forschungszentrum Jülich GmbH, Jülich, 52425 Germany. 58 59 60 1 ACS Paragon Plus Environment ACS Energy Letters Page 2 of 34 1 2 3 4 & Institute for Carbon Neutralization, College of Chemistry and Materials Engineering, 5 6 7 Wenzhou University, Wenzhou, Zhejiang 325035, China. 8 9  ICREA, Pg. Lluís Companys 23, 08010, Barcelona, Spain. 10 11 + These authors contributed equally to this work 12 13 14 15 16 Email: daweiirec@163.com 17 18 Email: wangjiaao0720@utexas.edu 19 20 21 Email: chou@wzu.edu.cn 22 23 24 Email: acabot@irec.cat 25 26 27 ABSTRACT 28 29 30 Lithium-sulfur batteries (LSBs) are a particularly promising candidate for next-generation 31 32 33 energy storage systems owing to their high energy density and large potential for low cost and 34 35 36 minimum environmental impact. However, their commercialization is still hindered by 37 38 limitations of the sulfur cathode, mainly the lithium polysulfide (LiPS) dissolution and shuttling, 39 40 41 and the sluggish sulfur redox kinetics. Herein, we propose an innovative sulfur host, based on 42 43 an iodine-doped bismuth selenide (I-Bi2Se3), able to solve these limitations by immobilizing 44 45 46 the LiPS and catalytically activating the redox conversion at the cathode. We detail here the 47 48 49 synthesis of I-Bi2Se3 nanosheets and thoroughly characterize their morphology, crystal 50 51 structure and composition. We use density-functional theory and experimental tools to 52 53 54 demonstrate that I-Bi2Se3 nanosheets are characterized by a proper composition and micro- and 55 56 nano-structure to facilitate Li+ diffusion and fast electron transportation, and to provide 57 58 59 numerous surface sites with strong LiPS adsorbability and extraordinary catalytic activity. 60 2 ACS Paragon Plus Environment Page 3 of 34 ACS Energy Letters 1 2 3 4 Overall, I-Bi2Se3/S electrodes exhibit outstanding initial capacities up to 1500 mAh g-1 at 0.1 C 5 6 7 and cycling stability over 1000 cycles, with an average capacity decay rate of only 0.012% per 8 9 cycle at 1 C. Besides, at a sulfur loading of 5.2 mg cm-2, a high areal capacity of 5.70 mAh cm-2 10 11 12 at 0.1 C is obtained with an electrolyte/sulfur ratio of 12 µL mg-1. 13 14 Keywords: Bismuth selenide, nanosheet, iodine-doped, lithium polysulfide, lithium-sulfur 15 16 17 battery 18 19 20 There is an urgent need for a new generation of rechargeable energy storage devices that offer 21 22 23 a step higher capacity and durability, well beyond the limitations of current lithium-ion batteries 24 25 (LIBs). Among the possible candidates, lithium-sulfur batteries (LSBs) attract significant 26 27 28 attention due to their very high theoretical specific energy, 2600 Wh Kg-1, their potential for 29 30 31 low cost associated with the abundance of sulfur, and their lack of toxicity.1–5 However, their 32 33 practical use is still hindered by the electrically insulating nature of sulfur and its discharge 34 35 36 products (Li2S/Li2S2),6–8 a low utilization of the active material, and a moderate cycling stability 37 38 associated with the large volume expansion of the cathode upon lithiation9–12 and the shuttle of 39 40 41 the soluble intermediate lithium polysulfides (LiPS).13–15 These drawbacks call for an improved 42 43 44 design and engineering of the sulfur cathode.16–20 45 46 Conductive porous carbons have been widely used to improve the electrical conductivity of the 47 48 49 cathode, encapsulate sulfur and restrain the dissolution and diffusion of LiPS.21,22 However, the 50 51 encapsulation of sulfur within porous carbon can be detrimental to the cathode redox reaction 52 53 54 kinetics by reducing sulfur reactivity and slowing the desolvation process.23,24 To overcome 55 56 57 current cathode limitations, metal-based compounds and metal-organic frameworks have been 58 59 added to strengthen the chemical interaction with LiPS and reduce the shuttle effect.25–29 60 3 ACS Paragon Plus Environment ACS Energy Letters Page 4 of 34 1 2 3 4 Among the tested metal compounds, sulfides such as MoS2, VS2, CoS2, Ni3S2, and Sb2S3, have 5 6 7 demonstrated particularly strong sulfiphilic ability toward LiPS trapping and low lithiation 8 9 voltages.30–35 However, cathodes based on metal sulfides are generally characterized by 10 11 12 moderate electrical conductivities, limited sulfur utilization, insufficient cycling stability and 13 14 low rate capabilities.36–40 15 16 17 Recently, we and others have demonstrated metal selenides, such as ZnSe, NbSe2 and NiCo2Se4, 18 19 20 as promising sulfur hosts in LSB cathodes, owing to their notable polarity, excellent catalytic 21 22 activity, and high electrical conductivity.3,5,33,41 However, among the extended family of 23 24 25 possible chalcogenides, a particularly interesting candidate has been so far overlooked. Bismuth 26 27 selenide (Bi2Se3) is generally an n-type degenerated semiconductor, with a low bandgap of 0.3 28 29 30 eV, that is widely used in the field of thermoelectricity due to its high electrical conductivity.[41, 31 32 42] 33 Its n-type electronic behavior is related to the presence of Se vacancies that act as electron 34 35 donors. Bi2Se3 has a layered crystal structure consisting of stacks of covalently bonded 36 37 38 quintuple atomic layers, Se-Bi-Se-Bi-Se, that are held together by weak van der Waals 39 40 interactions. Such high electrical conductivity and layered structure is highly suitable for its use 41 42 43 as sulfur hosts in LSB cathodes. Besides, the variable chemical valences of Bi and Se anticipates 44 45 46 a high potential catalytic activity. 47 48 In this work, we evaluate the potential of iodine-doped Bi2Se3 nanosheets as sulfur host in LSBs. 49 50 51 We first detail the synthesis of Bi2Se3 nanosheets using a high yield and scalable solution-based 52 53 method. These Bi2Se3 nanosheets are doped with iodine to further improve their adsorption and 54 55 56 catalytic activity. The obtained material is used as sulfur host in LSB cathodes and its 57 58 59 performance is experimentally analyzed and computationally rationalized. Results demonstrate 60 the I-Bi2Se3 nanosheets to provide high electrical conductivity, enhanced confinement of LiPS, 4 ACS Paragon Plus Environment Page 5 of 34 ACS Energy Letters 1 2 3 4 the mitigation of the sulfur volume expansion effects, and an excellent electrocatalytic activity 5 6 7 towards the redox reaction of sulfur and lithium sulfide. 8 9 10 RESULTS AND DISCUSSION 11 12 13 Bi2Se3 nanosheets, assembled into flower-like structures (Figure S1a), were produced at 180 ºC 14 15 16 from a basic solution containing bismuth nitrate, sodium selenite and polyvinylpyrrolidone (see 17 18 19 details in the experimental section in the supporting information). X-ray diffraction (XRD) 20 21 analysis showed the nanosheets to have high crystallinity and their crystal structure to match 22 23 24 the rhombohedral Bi2Se3 phase (JCPDS No. 00-033-0214, Figure S1b). High resolution 25 26 transmission electron microscopy (HRTEM) characterization confirmed the Bi2Se3 27 28 29 rhombohedral phase (space group =Pbnm) with a= 11.6230 Å, b=11.8310 Å and c=4.0900 Å 30 31 32 (Figure S1c). Besides, electron energy loss spectroscopy (EELS) chemical composition maps 33 34 demonstrated a uniform distribution of both elements, Bi and Se, through the nanosheet (Figure 35 36 37 S1d). 38 39 Iodine ions were incorporated to pre-synthesized Bi2Se3 by dispersing the Bi2Se3 nanosheets in 40 41 42 an aqueous solution of potassium iodide under vigorous stirring at room temperature for 48 h. 43 44 45 Iodine-doped Bi2Se3 (I-Bi2Se3) nanosheets preserved the flower-like morphology and the 46 47 rhombohedral crystal structure of undoped Bi2Se3 (Figure 1a-c). EDX analysis quantified the 48 49 50 iodine concentration at a 5 at%. Besides, EELS chemical composition maps displayed a 51 52 homogeneous distribution of Bi, Se and I within the I-Bi2Se3 nanosheets (Figure 1d). 53 54 55 X-ray photoelectron spectroscopy (XPS) was used to determine the surface composition of I- 56 57 58 Bi2Se3 and the chemical environment of its elements (Figure 1e,f,g). The high-resolution Bi 4f 59 60 XPS spectrum was fitted with two doublets (Figure 1e). The lowest energy doublet was 5 ACS Paragon Plus Environment ACS Energy Letters Page 6 of 34 1 2 3 4 associated with Bi3+ within a Bi2Se3 chemical environment (Bi 4f7/2 binding energy at 157.5 5 6 7 eV). The highest energy doublet was associated with Bi3+ within a more electronegative 8 9 environment, as it could be Bi2O3, Bi2(SeO3)3, Bi2SeO2 or another oxidized form of Bi2Se3 (Bi 10 11 12 4f7/2 binding energy at 158.5 eV).[44, 45] The presence of an oxide component was related to the 13 14 transportation and handling of the particles in the air. The high-resolution Se 3d XPS spectrum 15 16 17 was also fitted with two doublets (Figure 1f), which were associated with Se2- within Bi2Se3 18 19 20 (Se 3d5/2 at 52.8 eV) and SeO2, Bi2(SeO3)3, Bi2SeO2 or another form of Se within a more 21 22 electronegative environment (Se 3d5/2 at 53.7 eV), related to the partial oxidation of the material 23 24 25 surface.42,44 The high resolution I 3d XPS spectrum was fitted with two peaks, at 618.5 eV (I 26 27 3d5/2) and 630 eV (3d3/2), which were associated with an I-1 chemical state (Figure 1i).46 28 29 30 The electrical conductivity of both Bi2Se3 and I-Bi2Se3 samples decreased with temperature, 31 32 33 implying a degenerated semiconductor behavior (Figure 1f). Besides, the Seebeck coefficient 34 35 of both samples was negative in the whole temperature range, implying an n-type conductivity. 36 37 38 With the introduction of iodine, a twofold increase of the Bi2Se3 electrical conductivity was 39 40 measured. The rise of the electrical conductivity was correlated with a decrease of the absolute 41 42 43 value of the Seebeck coefficient, which is consistent with an increase of the charge carrier 44 45 46 concentration with the iodine doping. 47 48 Sulfur was introduced via a melt-diffusion process (see details in the experimental section).5,33 49 50 51 The morphology of the I-Bi2Se3/S composite resembled that of the original I-Bi2Se3 nanosheets 52 53 assembled into flowers (Figure 1g). EDX elemental maps displayed a homogeneous 54 55 56 distribution of the four elements, I, Bi, Se and S, within the composite (Figure 1g), with no 57 58 59 independent sulfur particle. Upon sulfur incorporation, the nitrogen adsorption-desorption 60 isotherms showed the Brunauer–Emmett–Teller (BET) specific surface area to be reduced from 6 ACS Paragon Plus Environment Page 7 of 34 ACS Energy Letters 1 2 3 4 128.6 m2 g−1 for I-Bi2Se3 to 13.2 m2 g−1 for I-Bi2Se3/S (Figure S3). Besides, the pore volume of 5 6 7 I-Bi2Se3, decreased from 0.5 cm3 g−1, to 0.026 cm3 g−1 with the incorporation of sulfur, which 8 9 further demonstrated the successful loading of sulfur within the pore structure of the assembled 10 11 12 I-Bi2Se3 nanosheets. The loaded mass of sulfur was quantified at ca. 70.2 wt% using 13 14 thermogravimetry analysis (TGA, Figure S2). 15 16 17 a b (051) 18 19 (05-1) 20 21 (002) 22 23 2 μm [201] Pnma [100] 24 c 25 c Bi Se I Bi-Se-I 26 27 28 29 30 31 32 d e Bi 4f Se 3d I 3d 33 I-Bi2Se3 Bi2Se3 Bi2Se3 g 34 Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) 35 Bi2SeO2 36 Bi2O3 37 Bi2Se3 JCPDS No. 00-033-0214 38 39 10 20 30 40 50 60 70 80 166 164 162 160 158 156 57 56 55 54 53 52 51 633 630 627 624 621 618 615 40 2 (Degree) Binding Energy (eV) Binding Energy (eV) Binding Energy (eV) 41 f 2.0 0 g 42 -30 I-Bi2Se3 43 1.5  (10 S m ) -1 I-Bi2Se3 44 S (V K ) -60 -1 1.0 45 5 -90 46 0.5 -120 47 Bi2Se3 Bi2Se3 48 0.0 -150 49 300 400 500 600 300 400 500 600 3 µm T (K) T (K) 50 51 S 52 53 54 55 56 57 58 59 Figure 1. Characterization of I-Bi2Se3 nanosheets: (a) SEM image. (b) HRTEM image and its 60 corresponding power spectrum. (c) EELS chemical composition maps obtained from the red 7 ACS Paragon Plus Environment ACS Energy Letters Page 8 of 34 1 2 3 4 squared area of the STEM micrograph. Individual Bi N6,7-edges at 157 eV (red), Se M1-edge at 5 6 232 eV (green), I M4,5-edge 619 eV (blue) and composite of Bi-Se-I. (d) XRD pattern. (e) Bi 7 8 4f, Se 3d and I 3d high-resolution XPS spectra. (f) Temperature dependence of the electrical 9 10 conductivity () and Seebeck coefficient (S) of Bi2Se3 and I-Bi2Se3. (g) SEM image EDX 11 elemental maps of I-Bi2Se3/S composite. 12 13 14 The interaction between I-Bi2Se3 and LiPS was analyzed using Li2S4 adsorption tests (see 15 16 17 details in experimental section). Figure 2a,b displays an optical image and the UV-vis spectra 18 19 20 of several flasks that contain 3 mL of a 0.01 M Li2S4 solution and the exact same amount (20 21 22 mg) of each of the tested adsorbers, Super P, Bi2Se3 or I-Bi2Se3, after overnight ageing. We 23 24 25 observe that the color of the Li2S4 solution, and the absorbance intensity in the 400–500 nm 26 27 region associated with Li2S4, was almost unchanged in the presence of Super P. On the contrary, 28 29 30 the color of the Li2S4 solutions and the visible absorbance became much lighter in the presence 31 32 33 of Bi2Se3 and especially I-Bi2Se3, suggesting a strong chemical interaction of LIPS with Bi2Se3 34 35 and particularly with I-Bi2Se3. I-Bi2Se3 exhibited the clearest solution and the lowest visible 36 37 38 absorption, indicating the presence of the lowest amount of Li2S4 in the solution and thus the 39 40 greatest LiPS adsorbability. 41 42 43 Figure 2c displays the high-resolution Bi 4f XPS spectra of I-Bi2Se3 before and after the Li2S4 44 45 46 adsorption test. Compared with the original spectra of I-Bi2Se3, the Bi 4f binding energies in I- 47 48 Bi2Se3-Li2S4 shifted to lower values, revealing the interaction of S with surface Bi.5 49 50 51 Density functional theory (DFT) calculations were carried out to further evaluate the interaction 52 53 between LiPS and I-Bi2Se3. The optimized LiPS adsorption configuration at six different 54 55 56 lithiation stages (Li2S, Li2S2, Li2S4, Li2S6, Li2S8 and S8) on the Bi2Se3 and I-Bi2Se3 surfaces are 57 58 59 displayed in Figures S4 and S5. DFT calculations showed the binding energies (Eb) of LiPS 60 species at the six tested lithiation stages to be higher on I-Bi2Se3 than on Bi2Se3 (Figure 2e). 8 ACS Paragon Plus Environment Page 9 of 34 ACS Energy Letters 1 2 3 4 Figure 2d displays the relaxed adsorption structure of Li2S4 on the (110) surface of Bi2Se3 and 5 6 7 I-Bi2Se3, respectively. The calculated Eb for the relaxed configuration of Li2S4 absorbed on 8 9 Bi2Se3 (110) and I-Bi2Se3 (110) surfaces are -1.27 eV and -2.09 eV, respectively. These DFT 10 11 12 results demonstrate a strong interaction between S within LiPS and Bi within Bi2Se3, which 13 14 corroborates the sulfiphilic character of Bi2Se3. These results also demonstrate that the presence 15 16 17 of I on the Bi2Se3 surface further increases the absolute value of the binding energy, thus 18 19 20 favoring LiPS adsorption and hence potentially reducing the shuttle effect. 21 22 Figure 2f displays the band structure and density of states (DOS) of Bi2Se3 and I-Bi2Se3. 23 24 25 Calculations show I-Bi2Se3 to be characterized by a significantly smaller bandgap than Bi2Se3. 26 27 Besides, consistently with results from electrical transport measurements, DFT calculations 28 29 30 show the presence of iodine to upward shift the Fermi level, which appears lying within the I- 31 32 33 Bi2Se3 conduction band thus promoting the intrinsic conductivity and degenerated/metallic 34 35 character of I-Bi2Se3.47 36 37 38 Figure 2g and Figure S6 exhibit the initial state, transition state and final state of Li2S 39 40 decomposition on Bi2Se3 and I-Bi2Se3. The calculated energy barrier for Li2S decomposition 41 42 43 on Bi2Se3 and I-Bi2Se3 surfaces was 0.56 and 0.98 eV, respectively (Figure 2g). These results 44 45 46 demonstrate that I-Bi2Se3 can greatly reduce the Li2S decomposition energy barrier and enhance 47 48 the redox reversibility between Li2S and LiPS. 49 50 51 The S reduction pathways on both Bi2Se3/S and I-Bi2Se3/S cathodes were calculated 52 53 considering the overall reversible reaction between Li2S and S8 + Li. The first step of the 54 55 56 discharge process involves the reduction of S8 with two Li+ to form Li2S8. Subsequently, Li2S8 57 58 59 undergoes further reduction to three intermediate LiPS, Li2S6, Li2S4, Li2S2 and finally Li2S as 60 the end product. Figure 2h displays the optimized configuration of the intermediates and their 9 ACS Paragon Plus Environment ACS Energy Letters Page 10 of 34 1 2 3 4 Gibbs free energy profiles. For both materials, Bi2Se3/S and I-Bi2Se3/S, the largest increase of 5 6 7 Gibbs free energy was obtained for the conversion from Li2S2 to Li2S, suggesting this step as 8 9 the rate-limiting for the discharge process.48 The free energy increase of this step was lower for 10 11 12 I-Bi2Se3 (0.71 eV) than for Bi2Se3 (0.89 eV), suggesting that the reduction of S was more 13 14 thermodynamically favorable on I-Bi2Se3 than on Bi2Se3. 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 Figure 2. (a) Optical photograph of flasks containing 3 mL of a 0.01 M Li2S4 solution and 20 59 60 mg of Super P, Bi2Se3 or I-Bi2Se3 after overnight ageing. (b) UV–vis spectra of the supernatants. 10 ACS Paragon Plus Environment Page 11 of 34 ACS Energy Letters 1 2 3 4 (c) Bi 4f XPS spectra of I-Bi2Se3 before and after Li2S4 adsorption. (d) Optimized geometrical 5 6 configurations of Li2S4 on Bi2Se3 and I-Bi2Se3 surfaces. (e) Calculated binding energies of LiPS 7 8 (Li2S, Li2S2, Li2S4, Li2S6, Li2S8 and S8) on Bi2Se3 and I-Bi2Se3 surfaces. (f) HSE06 band 9 10 structure and density of states of Bi2Se3 and I-Bi2Se3. (g) Optimized adsorption configuration 11 for the Li2S decomposition on I-Bi2Se3 and energy barrier profiles of Li2S cluster 12 13 decomposition on Bi2Se3 and I-Bi2Se3 along with different reaction coordinates. (h) Gibbs free 14 15 energy profiles and optimized adsorption conformation of LiPS species on Bi2Se3 and I-Bi2Se3, 16 17 showing a much lower reaction free energy from Li2S2 to Li2S on I-Bi2Se3 than on Bi2Se3. 18 19 20 The electrochemical performance of Li-S coin cells containing an I-Bi2Se3/S cathode was 21 22 23 evaluated and compared with that of cells based on Bi2Se3/S and Super P/S cathodes. Figure 3a 24 25 displays the CV curves obtained at 0.1 mV s-1. All cathodes displayed two well-defined cathodic 26 27 28 peaks associated with the reduction of solid-state S into soluble long-chain LiPS (Li2Sx, 4 < x 29 30 31 < 8, peak C1) and the subsequent conversion of these LiPS to insoluble Li2S2/Li2S (peak C2).3 32 33 Besides, an anodic peak is observed (peak A), corresponding to the reverse oxidation 34 35 36 conversion from Li2S to LiPS and ultimately to S.33 The peak current density of the reduction 37 38 and oxidation peaks of I-Bi2Se3/S was clearly higher than that of the two reference cathodes, 39 40 41 all containing the same amount of sulfur, thus suggesting a higher sulfur utilization in the I- 42 43 44 Bi2Se3/S cathode. The C2 peak of the I-Bi2Se3/S cathode at about 2.042 V shows a much larger 45 46 current density and is slightly shifted to a higher potential compared with the other cathodes, 47 48 49 suggesting a promoted conversion of LiPS to insoluble Li2S2/Li2S within the I-Bi2Se3/S cathode. 50 51 Besides, the A oxidation peak of I-Bi2Se3/S is shifted to a lower potential, 2.33 V, and displays 52 53 54 a larger current density, indicating an enhanced Li2S decomposition (Figure 3a,b). 55 56 57 The onset potentials at a current density of 10 μA cm-2 beyond the baseline current were used 58 59 to quantitatively estimate the electrocatalytic activity (Figure 3b and S8).23 Cells based on I- 60 11 ACS Paragon Plus Environment ACS Energy Letters Page 12 of 34 1 2 3 4 Bi2Se3/S cathodes showed the highest onset potentials of cathodic peaks and the lowest onset 5 6 7 potentials for the anodic peak, which further demonstrates the accelerated redox kinetics 8 9 obtained with the I-doped Bi2Se3 nanosheets. Figure S7 displays the first five CV cycles 10 11 12 measured from I-Bi2Se3/S cells. All CV curves almost overlapped, which suggests excellent 13 14 reversibility of the sulfur redox reactions. 15 16 17 CV tests at different scan rates in the range 0.1-0.5 mV s-1 were conducted to investigate the 18 19 20 reaction kinetics and the lithium-ion diffusion properties of I-Bi2Se3/S (Figure 3c). The peak 21 22 current density of the I-Bi2Se3/S cathode increases with the scan rate, and its shape is maintained, 23 24 25 which denotes good electrochemical stability. In contrast, Bi2Se3/S and especially Super P/S 26 27 cathodes display more remarkable changes in the CV curve when increasing the scan rate, 28 29 30 denoting limited reaction kinetics (Figure S9). 31 32 33 I-Bi2Se3/S cathodes exhibited a linear relationship between the cathodic and anodic peak 34 35 currents and the square root of the scanning rate, as it corresponds to a diffusion-limited process 36 37 38 (Figure 3d-f). From the fitting of this linear dependence, the diffusion constant of the rate- 39 40 limiting species, i.e. lithium ions (DLi+), was calculated according to the Randles-Sevcik 41 42 43 equation:5 44 45 46 𝐼𝑃 = (2.69 ∗ 105)𝑛1.5𝐴𝐷0.5 𝐿𝑖 + 𝐶𝐿𝑖 + 𝑣 0.5 (1) 47 48 49 where Ip is the peak current density, n is the number of charges transferred, A is the geometric 50 51 area of the electrode, CLi+ is the concentration of lithium ions in the cathode, and v is the scan 52 53 54 rate. I-Bi2Se3/S electrodes displayed the sharpest IP vs 𝑣0.5 slopes, indicating the fastest 55 56 57 lithium-ion diffusion in both the reduction and oxidation processes. According to the Randles- 58 59 Sevcik equation, the values of DLi+ calculated for I-Bi2Se3/S at peaks C1, C2 and A were 2.3 × 60 12 ACS Paragon Plus Environment Page 13 of 34 ACS Energy Letters 1 2 3 4 10−7, 3.7 × 10−7, and 5.9 × 10−7 cm2 s-1, respectively. All these values were well above those 5 6 7 obtained for Bi2Se3/S and Super P/S electrodes (Figure 3g). Taking into account the similar 8 9 morphology of I-Bi2Se3 and Bi2Se3, the enhanced lithium-ion diffusivities must be related to 10 11 12 the improved catalytic activity of the former, which accelerates the LiPS conversion, and to the 13 14 stronger LiPS adsorption that suppresses the shuttle effect and prevents the deposition of an 15 16 17 insulating layer at the anode side.49 18 19 20 To further analyse the electrocatalytic activity of the different materials toward the polysulfide 21 22 conversion, CV profiles were measured on symmetric cells within the voltage window -1.0 to 23 24 25 1.0 V and using a 0.5 M Li2S6 electrolyte (Figure 3h).5,33 To eliminate the capacitive 26 27 contribution, the CV curve of I-Bi2Se3 in a Li2S6-free electrolyte was also measured as a 28 29 30 reference (Figure S10). The CV curve of the symmetric cells with a Li2S6-free electrolyte 31 32 33 delivered a low current, well below that of CV curves of all symmetric cells having Li2S6 in 34 35 their electrolyte, which indicates that the lithiation/delithiation reactions dominate the current 36 37 38 response.50 Besides, the I-Bi2Se3 symmetric cells including Li2S6 additive exhibited a much 39 40 higher current density than those of Bi2Se3 and Super P cells, which we associate with an 41 42 43 enhanced polysulfide absorption and a higher catalytic activity. 44 45 46 The Li2S nucleation and dissolution processes were analysed to investigate the liquid-solid 47 48 reaction kinetics. Compared with the Bi2Se3 cathode, the I-Bi2Se3 cathode exhibited 49 50 51 significantly shorter nucleation and growth times and a higher discharging peak current during 52 53 the potentiostatic measurement at 2.05 V (Figure 3i). Based on Faraday´s law, the capacity of 54 55 56 the I-Bi2Se3 electrode was calculated at 287.5 mAh g-1, well above that of the Bi2Se3 electrode, 57 58 59 206.8 mAh g-1.51 Taking into account the similar morphology and crystal structure of the two 60 13 ACS Paragon Plus Environment ACS Energy Letters Page 14 of 34 1 2 3 4 materials, the significant differences obtained must be ascribed to the effect of iodine doping, 5 6 7 which significantly improves the Li2S precipitation kinetics. 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 Figure 3. (a) CV profiles of Li–S cells with I-Bi2Se3/S, Bi2Se3/S and Super/S cathodes at a 0.1 38 39 mV s-1 scan rate. (b) Peak potential and onset potential of asymmetrical Li–S cells obtained 40 41 from CV curves. (c) CV profiles of the I-Bi2Se3/S electrode at scan rates from 0.1 to 0.5 mV 42 43 s-1. (d) Peak current vs the square root of the scan rate () for the cathodic reduction processes 44 (C1, C2) and anodic oxidation process (A) in Li–S cells with different electrodes. (e) Li+ 45 46 diffusion coefficient calculated from the CV redox peaks according to the Randles−Sevcik 47 48 equation. (f) CV curves of symmetric cells at a scan rate of 20 mV s-1. (g) Potentiostatic 49 50 discharge profile at 2.05 V on different electrodes with Li2S8 catholyte for evaluating the 51 52 nucleation kinetics of Li2S. 53 54 55 The galvanostatic charge/discharge curves of I-Bi2Se3/S, Bi2Se3/S and Super P/S electrodes at 56 57 58 a current density of 0.1 C (1672 mA g-1) are shown in Figure 4a. Charge/discharge curves 59 60 display two discharge plateaus and one charge plateau, consistently with CV curves. The first 14 ACS Paragon Plus Environment Page 15 of 34 ACS Energy Letters 1 2 3 4 discharge plateau at around 2.3 V is related to the reduction of sulfur to soluble LiPS (S8 → S62- 5 6 7 → S42-).52 The second discharge plateau corresponds to the conversion of soluble LiPS to 8 9 lithium sulfide (S4 → Li2S2 → Li2S) at about 2.1 V.53 The voltage difference between the 10 11 12 oxidation and the second reduction plateaus is considered as the polarization potential, ΔE.54 13 14 Among the tested materials, I-Bi2Se3/S showed the lowest ΔE, at 131 mV, well below that of 15 16 17 Bi2Se3/S, 162 mV, and Super P/S, 205 mV (Figure 4b). 18 19 20 Q1 and Q2 are defined as the capacity of the first and the second discharge plateaus, respectively. 21 22 Q1 measures the amount of soluble polysulfides created and potentially diffusing to the Li metal 23 24 25 anode. Q2 measures how efficiently the created polysulfides are reduced to Li2S. Thus the ratio 26 27 Q2/Q1 can be used as a measure of the catalytic activity for LiPS conversion reaction.55 The 28 29 30 Q2/Q1 ratio for I-Bi2Se3/S was 2.71, close to the theoretical limit of 3, and well above the values 31 32 33 obtained from Bi2Se3/S and Super P/S, 2.23 and 1.85, respectively (Figure 4b). The high Q2/Q1 34 35 ratio of I-Bi2Se3/S implies a high catalytic activity toward polysulfide’s redox reaction. 36 37 38 The charge and discharge profiles of the different electrodes displayed in Figure S11show the 39 40 I-Bi2Se3/S electrode to be characterized by significantly lower overpotentials for phase 41 42 43 conversion between soluble LiPS and insoluble Li2S2/Li2S, which further demonstrates the 44 45 46 enhanced electrochemical kinetics of I-Bi2Se3/S.56 47 48 The galvanostatic charge/discharge voltage profiles of I-Bi2Se3/S electrodes at various current 49 50 51 densities, from 1.0 C to 4 C, are shown in Figure 4c. Two discharge plateaus and one charge 52 53 plateau were maintained even at the highest current rates tested, which is in contrast with the 54 55 56 results obtained from Super P/S electrodes (Figure S12). An outstanding initial specific capacity 57 58 59 of 1496 mAh g-1 was measured from the I-Bi2Se3/S electrode. Additionally, this electrode 60 demonstrated a superior rate performance, with average discharge capacities of 1361, 1168, 15 ACS Paragon Plus Environment ACS Energy Letters Page 16 of 34 1 2 3 4 1002, 897, 795 and 651 mAh g-1 at current rates from 0.1 C to 4 C, respectively (Figure 4d). 5 6 7 Even when the current density was returned to 0.2 C, the specific capacity of the I-Bi2Se3/S 8 9 electrode was recovered to 1138 mAh g-1, suggesting high stability even after the high rate 10 11 12 charge/discharge process. All these values were clearly above those obtained from Bi2Se3/S and 13 14 Super P/S electrodes. 15 16 17 Among the tested materials, the cells based on I-Bi2Se3/S were also characterized by the highest 18 19 20 energy efficiencies (Figure 4e), as calculated from E = UIdt.57 I-Bi2Se3/S based cells displayed 21 22 an energy efficiency of 89.7% at 4 C, higher than Bi2Se3/S (85.7%) and Super P/S (66.1%) 23 24 25 cells. The high energy efficiency measured for I-Bi2Se3/S was consistent with the lower 26 27 polarization potential and higher catalytic properties of I-Bi2Se3. 28 29 30 Figure 4f shows the cycling performance of the different cathodes. In this test, the first three 31 32 33 cycles were measured at a current density of 0.1C, and then 100 cycles were tested at a current 34 35 density of 0.2C. The three electrodes showed a notably stable cycling performance during 100 36 37 38 cycles, with a coulombic efficiency (CE) above 99%. At 0.2C, I-Bi2Se3/S electrodes displayed 39 40 a high initial specific capacity (1274 mAh g-1), well above that of Bi2Se3/S (1045 mAh g-1), and 41 42 43 Super P/S (727 mAh g-1). After 100 cycles, 97.7% of the initial capacity was still retained for 44 45 46 the I-Bi2Se3/S electrodes (1245 mAh g-1). In contrast, the Bi2Se3/S cathode maintained just an 47 48 84.4% (882 mAh g-1) of the initial discharge capacity, and the Super P/S cathode preserved just 49 50 51 a 69.6% (506 mAh g-1) of the initial capacity. The capacity loss in these cathodes was mainly 52 53 associated with the dissolution of polysulfides into the electrolyte. Thus the high cycling 54 55 56 stability of I-Bi2Se3/S was related to the effective inhibition of this dissolution. 57 58 59 Electrochemical impedance spectroscopy (EIS) was used to gain understanding of the 60 parameters behind the enhanced redox kinetics of I-Bi2Se3/S electrodes. Figure 4g and S13 16 ACS Paragon Plus Environment Page 17 of 34 ACS Energy Letters 1 2 3 4 display the Nyquist plots of the EIS data obtained from I-Bi2Se3/S, Bi2Se3/S and Super P/S coin 5 6 7 cells before and after cycling. The fresh electrodes displayed a semicircle in the high-frequency 8 9 region associated with the charge-transfer resistance (Rct), followed by a linear dependence in 10 11 12 the low-frequency region that is related to the diffusion of lithium ions.58 After charge/discharge 13 14 loops, an additional semicircle appeared in the high-frequency range, which is associated with 15 16 17 the deposition of the insulating discharge products of Li2S on the electrode surface (Rin) during 18 19 20 cycling. Based on the equivalent circuit displayed in Figure S13,59 the I-Bi2Se3/S electrode has 21 22 a considerably lower Rin and Rct compared with Bi2Se3/S and Super P/S electrodes. These lower 23 24 25 resistances indicate that the I-Bi2Se3/S electrode is characterized by a faster polysulfide 26 27 conversion reaction, as well as a faster charge transfer kinetics during lithiation/delithiation 28 29 30 reaction. 31 32 33 Even at a high current rate of 1C, I-Bi2Se3/S electrodes displayed a high specific capacity and 34 35 stable cycling performance, with an average 0.012% capacity decay per cycle after 1000 cycles 36 37 38 and CEs above 99.6%. The performance of I-Bi2Se3/S electrodes in this direction was also 39 40 clearly above that of Bi2Se3/S and Super P/S (Figure 4h). 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 17 ACS Paragon Plus Environment ACS Energy Letters Page 18 of 34 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 Figure 4. (a) Galvanostatic charge/discharge profiles of I-Bi2Se3/S, Bi2Se3/S and Super/S 31 32 electrodes at a 0.1 C current rate. (b) ∆E and Q2/Q1 obtained from the charge/discharge curves. 33 34 (c) Galvanostatic charge/discharge profiles at various rates, from 0.1 C to 4 C. (d) Rate 35 36 capability of different electrodes at various C rates, from 0.1 C to 4 C. (e) Energy efficiency of 37 different cathodes at different current rates. (f) Cycling performances of I-Bi2Se3/S, Bi2Se3/S, 38 39 and Super/S electrodes at 0.2 C. (g) EIS spectrum of the I-Bi2Se3/S electrode after and before 40 41 cycling. (h) Cycling performance and coulombic efficiency of the three cathodes at a current 42 43 rate of 1 C. 44 45 46 I-Bi2Se3 electrodes were further tested with a high sulfur loading (5.2 mg cm-2) and a lean 47 48 49 electrolyte (8 µL mg-1) to demonstrate their potential application in practical LSBs. As shown 50 51 in Figure 5a, high sulfur loading cells exhibited an excellent rate performance, with areal 52 53 54 capacities of 5.70 mAh cm-2 at 0.1C, and 4.12 mAh cm-2 at 0.5C, i.e. exceeding the industrially 55 56 57 requested areal capacity of LIBs (~4 mAh cm-2).60 The corresponding charge/discharge profiles 58 59 at different current rates are shown in Figure 5b. With the current rate increasing to 0.5C, the 60 18 ACS Paragon Plus Environment Page 19 of 34 ACS Energy Letters 1 2 3 4 discharge profile still presented the intact second plateau, demonstrating excellent 5 6 7 electrochemical kinetics within the cell. The performance of the cell under lean electrolyte 8 9 conditions, with an electrolyte/sulfur ratio of 8 µL mg-1, is displayed in Figure 5c. At a current 10 11 12 rate of 0.5C, the initial discharge capacity was 527 mAh g-1 and it rapidly increased to 686 mAh 13 14 g-1 after 15 cycles. After 300 cycles, a capacity of 576 mAh g-1 was still measured, which 15 16 17 corresponds to an ultralow capacity decay of 0.053% per cycle with respect to the maximum 18 19 20 achieved capacity. Besides, I-Bi2Se3-based cells maintained the voltage profile during the 300 21 22 cycles, with just a small potential hysteresis, indicating a minor LiPS shuttling and stable sulfur 23 24 25 electrochemistry (Figure S14). 26 27 Figure 5d and Table S1 display a comparison of several parameters of state-of-the-art Bi-based 28 29 30 and Se-based materials as cathode hosts for LSBs. Notice that the I-Bi2Se3 host presented here 31 32 33 is characterized by the highest capacities and stabilities. This strategy offers a new route to 34 35 design multifunctional sulfur immobilizer with highly adoptive and catalytic capabilities for 36 37 38 rapid LiPS adsorption-conversion process, holding a great promise in promoting the practical 39 40 application of LSBs. 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 19 ACS Paragon Plus Environment ACS Energy Letters Page 20 of 34 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 Figure 5. (a) Rate performance of the Li–S coin cells at different current rates with a sulfur 29 30 loading of 5.2 mg cm-2. (b) Initial charge/discharge profiles at the various current rates. (c) 31 32 Cycling performances of I-Bi2Se3/S cathodes at a current rate of 0.5C under lean electrolyte 33 34 conditions. (d) Decay rate per cycle compared with other reported works. 35 36 37 38 CONCLUSIONS 39 40 41 42 In summary, we rationally designed and engineered new LSBs cathodes based on I-Bi2Se3 43 44 sulfur hosts through a facile and efficient strategy. This unique architecture not only establishes 45 46 47 a conductive framework for electron/ion transfer and buffers the volume variation of the sulfur 48 49 50 cathode during cycling, but also constructs a multifunctional LiPS barrier that effectively 51 52 suppresses polysulfide shuttling. I-Bi2Se3 was characterized by a high electrical conductivity 53 54 55 due to the doping of iodine and significantly enhanced the redox kinetics of sulfur species, 56 57 especially for the conversion between soluble LiPS species and Li2S2/Li2S during 58 59 60 charge/discharge processes. As a result of these excellent qualities, I-Bi2Se3/S cathodes deliver 20 ACS Paragon Plus Environment Page 21 of 34 ACS Energy Letters 1 2 3 4 excellent long-term cycling stability with a capacity decay of 0.012% per cycle over 1000 5 6 7 cycles at 1C. Moreover, at a sulfur loading of 5.2 mg cm−2, I-Bi2Se3/S cathodes delivered a high 8 9 areal capacity of 5.70 mAh cm−2. Overall, this work shows an effective way to enhance the 10 11 12 performance of LSB cathodes by the heteroatom doping of a metal selenide catalyst. 13 14 15 16 17 18 Supporting Information 19 20 The supporting information is available free of charge via the Internet at http://pubs.acs.org. 21 22 23 The supporting information includes experimental details, Figure S1-S14 and Table S1 as 24 25 described in the text, specifically, additional SEM, TEM, XRD, TGA, BET, CV, 26 27 28 electrochemical performance, DFT simulation results, and crystal structure. 29 30 31 Corresponding Author 32 33 34 Email: daweiirec@163.com 35 36 37 Email: wangjiaao0720@utexas.edu 38 39 Email: chou@wzu.edu.cn 40 41 42 Email: acabot@irec.cat 43 44 45 ACKNOWLEDGMENTS 46 47 48 The authors thank support from the projects ENE2016-77798-C4-3-R and NANOGEN 49 50 (PID2020-116093RB-C43), funded by MCIN/ AEI/10.13039/501100011033/ and by “ERDF 51 52 53 A way of making Europe”, by the “European Union”. M. Li, D. Yang, X. Han and C. Zhang 54 55 thank the China Scholarship Council for the scholarship support. ICN2 acknowledges the 56 57 58 support from the Severo Ochoa Programme (MINECO, grant no. SEV-2017-0706). IREC and 59 60 ICN2 are both funded by the CERCA Program/Generalitat de Catalunya. This project has 21 ACS Paragon Plus Environment ACS Energy Letters Page 22 of 34 1 2 3 4 received funding from the European Union’s Horizon 2020 research and innovation program 5 6 7 under grant agreement No 823717–ESTEEM3. Calculations at UT Austin were supported by 8 9 the Welch Foundation (F-1841) and the Texas Advanced Computing Center. Part of the present 10 11 12 work has been performed in the framework of Universitat Autònoma de Barcelona Materials 13 14 Science PhD program. 15 16 17 18 19 20 REFERENCES 21 22 23 (1) Zheng, J.; Tian, J.; Wu, D.; Gu, M.; Xu, W.; Wang, C.; Gao, F.; Engelhard, M. H.; Zhang, 24 25 26 J. G.; Liu, J.; Xiao, J. 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