This is the submitted version of the journal article: Yang, Dawei; Li, Mengyao; Zheng, Xuejiao; [et al.]. «Phase engineering of defective copper selenide toward robust lithium-sulfur batteries». ACS nano, Vol. 16, issue 7 (July 2022), p. 11102-11114. DOI 10.1021/acsnano.2c03788 This version is available at https://ddd.uab.cat/record/270834 under the terms of the license Page 1 of 37 ACS Nano 1 2 3 4 5 6 7 Synergistically Enhancement of Electrochemical 8 9 10 11 Kinetics by Phase Engineering of Defective 12 13 14 Copper Selenide toward Robust Lithium-Sulfur 15 16 17 18 Batteries 19 20 21 22 Dawei Yang,†,‡ Mengyao Li,*,†,‡ Xuejiao Zheng,& Xu Han,§ Chaoqi Zhang,†,‡ Jordi Jacas 23 24 25 Biendicho,† Jordi Llorca,// Jiaao Wang,*,$ Hongchang Hao,± Junshan Li,○ Graeme Henkelman,$ 26 27 28 Jordi Arbiol,§, Joan Ramon Morante,†,‡ David Mitlin,*,± Shu-Lei Chou,*, Andreu Cabot*,†, 29 30 31 † Catalonia Institute for Energy Research – IREC, Sant Adrià de Besòs, Barcelona, 08930, Spain. 32 33 ‡ Department of Electronic and Biomedical Engineering, Universitat de Barcelona, 08028, 34 35 36 Barcelona, Spain. 37 38 39 & Nanjing Hydraulic Research Institute, Nanjing 210029, China. 40 41 § Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus, UAB, 42 43 44 Bellaterra, 08193, Barcelona, Spain. 45 46 // Institute of Energy Technologies, Department of Chemical Engineering and Barcelona 47 48 49 Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, 50 51 52 EEBE, 08019, Barcelona, Spain. 53 54 $ 55 Department of Chemistry and the Oden Institute for Computational Engineering and Sciences, 56 57 The University of Texas at Austin, Austin, TX, 78712 USA. 58 59 60 1 ACS Paragon Plus Environment ACS Nano Page 2 of 37 1 2 3 4 ± Materials Science and Engineering Program & Texas Materials Institute, The University of 5 6 7 Texas at Austin, Austin, Texas 78712, United States. 8 9 ○ Institute of Advanced Study, Chengdu University, 610106, Chengdu, China. 10 11  Institute for Carbon Neutralization, College of Chemistry and Materials Engineering, 12 13 14 Wenzhou University, Wenzhou, Zhejiang 325035, China. 15 16 17  ICREA, Pg. Lluís Companys 23, 08010, Barcelona, Spain. 18 19 20 Email: limengyao@irec.cat 21 22 Email: wangjiaao0720@utexas.edu 23 24 25 Email: david.mitlin2@utexas.edu 26 27 Email: chou@wzu.edu.cn 28 29 30 Email: acabot@irec.cat 31 32 33 34 35 36 37 Abstract 38 39 40 The shuttling of soluble lithium polysulfides (LiPS) and the sluggish Li-S conversion kinetics 41 42 are two main barriers toward the practical application of lithium-sulfur batteries (LSBs). Herein, 43 44 45 we propose the addition of copper selenide nanoparticles at the cathode to trap LiPS and 46 47 48 accelerate the Li-S reaction kinetics. Using both computational and experimental results, we 49 50 demonstrate the crystal phase and concentration of copper vacancies to control the electronic 51 52 53 structure of the copper selenide, its affinity towards LiPS chemisorption, and its electrical 54 55 conductivity. The adjustment of the defect density also allows tuning the electrochemically 56 57 58 active sites for the catalytic conversion of polysulfide. The optimized S/Cu1.8Se cathode 59 60 efficiently promotes and stabilizes the sulfur electrochemistry, thus improving significantly the 2 ACS Paragon Plus Environment Page 3 of 37 ACS Nano 1 2 3 4 LSB performance, including an outstanding cyclability over 1000 cycles at 3 C with a capacity 5 6 7 fading rate of just 0.029% per cycle, a superb rate capability up to 5 C, and a high areal capacity 8 9 of 6.07 mAh cm-2 under high sulfur loading. Overall, the present work proposes a crystal phase 10 11 12 and defect engineering strategy toward fast and durable sulfur electrochemistry, demonstrating 13 14 great potential in developing practical LSBs. 15 16 17 Keywords: Copper selenide, phase engineering, copper vacancies, lithium-sulfur battery, 18 19 20 lithium polysulfide 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 ACS Paragon Plus Environment ACS Nano Page 4 of 37 1 2 3 4 Lithium-sulfur battery (LSB) has emerged as one of the most promising energy storage 5 6 7 technologies owing to its large energy density (2600 Wh kg-1) and high potential for low cost 8 9 and environmental friendliness.1–3 Despite these attractive advantages, the deployment of 10 11 12 commercial LSBs is still hampered by several limitations, including the intrinsically low 13 14 conductivity of sulfur and lithium sulfides, the dissolution and shuttling of lithium polysulfide 15 16 17 (LiPS) intermediates, the large volume change of sulfur during the charge-discharge process, 18 19 20 and the slow Li-S reaction kinetics, which result in insufficient cycling stabilities and rate 21 22 capabilities.4–6 23 24 25 To address these issues, extensive efforts have been devoted to synthesizing porous/hollow 26 27 carbon-based cathode hosts to physically confine sulfur and polysulfides.7–10 However, the 28 29 30 shuttle effect of soluble polysulfides cannot be effectively suppressed by means of carbon 31 32 33 encapsulation due to the weak interaction of LiPS with nonpolar carbon materials. Beyond 34 35 encapsulation, a more effective strategy to suppress the shuttle effect is the use of sulfur host 36 37 38 materials able to chemisorb and catalyze the LiPS reaction. In this direction, several polar 39 40 inorganic compounds including metal oxides,11,12 nitrides,13 phosphides,14 and sulfides15 have 41 42 43 been incorporated into the host to chemically immobilize polysulfide species, accelerate their 44 45 46 conversion, and thus reduce their dissolution and shuttling. Among the proposed compounds, 47 48 transition metal chalcogenides (TMCs) have been demonstrated particular effective.16,17 49 50 51 Copper chalcogenides have recently drawn extensive attention as functional materials in several 52 53 energy conversion and storage applications, including photovoltaics, thermoelectrics and 54 55 56 several electrochemical technologies.18–20 The suitability of copper chalcogenides arises from 57 58 59 the abundance of their constituent elements, the strong influence of Cu 3d states on the 60 4 ACS Paragon Plus Environment Page 5 of 37 ACS Nano 1 2 3 4 electronic band structure near the Fermi level, the contribution of the redox reactions between 5 6 7 Cu0, Cu1+ and Cu2+ to catalytically activating several processes, and the low defect formation 8 9 energies that result in large densities of defects that control the optical, charge transport and 10 11 12 catalytic properties of the material.21,22 13 14 Among the different copper chalcogenides, sulfides offer advantages in terms of cost but are 15 16 17 characterized by moderate electrical conductivities. At the other extreme, copper telluride 18 19 20 shows a metallic character, but the low abundance of tellurium prevents its use in applications 21 22 involving large volumes of material. Selenides thus represent the best compromise, as 23 24 25 recognized in the fields of photovoltaics and thermoelectrics among others. Copper selenides 26 27 have been also recently reported to show excellent electrocatalytic activity towards the oxygen 28 29 30 reduction reaction among other electrocatalytic reactions.23,24 31 32 33 In the field of LSBs, copper sulfides have been explored as sulfur hosts able to expedite the 34 35 polysulfides transformation, with notable success.25-28 On the other hand, surprisingly, despite 36 37 38 its high potential as LiPS adsorbent and Li-S electrocatalysts, no previous work on the use of 39 40 copper selenides as sulfur host in LSB cathodes has been reported. 41 42 43 Herein, we produced copper selenide electrocatalyst and optimized them towards the LiPS 44 45 46 adsorption and catalytic conversion through adjusting their crystal phase and Cu vacancy 47 48 concentration. Computational and experimental analyses are used to demonstrate that tailoring 49 50 51 the crystal phase and defect concentration allows modulating the electrical conductivity of the 52 53 material, its chemical affinity towards LiPS adsorption, and its catalytic activity towards LiPS 54 55 56 conversion reactions. The optimized materials are loaded with sulfur and tested as cathode 57 58 59 material in LSBs, which capacity, stability and rate capability are thoroughly investigated. 60 5 ACS Paragon Plus Environment ACS Nano Page 6 of 37 1 2 3 4 RESULTS AND DISCUSSION 5 6 7 A rapid synthetic protocol based on the injection of TOPSe complex into a copper precursor 8 9 solution at 220 °C was developed to produce the copper selenide NPs, as schematized in 10 11 12 Scheme1 (see details in the experimental section). 13 14 Figures 1a and S1 display TEM images of the 20±10 nm quasi-spherical NPs obtained. EELS 15 16 17 chemical composition maps displayed a uniform distribution of Cu and Se within each particle 18 19 20 (Figure 1c). HRTEM characterization showed the crystal structure of the NPs to match the 21 22 Cu1.8Se cubic phase (Figure 1b).29 XRD patterns confirmed the cubic Cu1.8Se phase (JCPDS 23 24 25 No. 01-071-0044, Figure 1g).29 26 27 Cu1.8Se NPs were annealed in a reducing atmosphere at 600ºC for 3h. Figures 1d,f and S2 28 29 30 display TEM and HRTEM images of the annealed material, which was characterized by larger 31 32 33 crystal domains of a different phase identified as orthorhombic Cu2Se.30 XRD analysis 34 35 confirmed the crystal structure of the annealed material to match that of orthorhombic Cu2Se 36 37 38 (JCPDS No. 00-019-0401, Figure 1g).30 EELS chemical composition maps displayed a uniform 39 40 distribution of Cu and Se within the Cu2Se sample (Figure 1e). 41 42 43 Figure 1h displays the high-resolution Cu 2p XPS spectra of Cu1.8Se and Cu2Se. A unique Cu 44 45 46 2p double was identified from both samples and it was associated with Cu+ within a copper 47 48 selenide environment.31 The Cu 2p doublet was located at 954.5 (2p1/2) and 934.5 eV (2p3/2) in 49 50 51 Cu2Se, and it was slightly shifted toward lower binding energies in Cu1.8Se. The Se 3d XPS 52 53 spectra of Cu1.8Se and Cu2Se also displayed a single chemical state ascribed Se2-, with a unique 54 55 56 doublet located at 54.3 eV (3d3/2) and 53.2 eV (3d5/2) in the case of Cu2Se and shifted to slightly 57 58 59 higher energies in Cu1.8Se.32,33 The correlated redshift of the Cu 2p spectrum and blue shift of 60 6 ACS Paragon Plus Environment Page 7 of 37 ACS Nano 1 2 3 4 the Se 3d spectrum when decreasing the Cu:Se ratio from Cu2Se to Cu1.8Se is associated with 5 6 7 a transfer of charge from Se to Cu. 8 9 Sulfur was introduced within the host materials by a melt-diffusion process (see the 10 11 12 experimental section for details). XRD patterns confirmed the presence of crystalline sulfur 13 14 (JCPDS No. 01-85-0799) within the S/Cu1.8Se composites (Figure S3).34 EELS elemental maps 15 16 17 displayed a homogeneous distribution of the three elements, Cu, Se and S, within the composite 18 19 20 (Figure 1j), with no independent sulfur particle. Nitrogen adsorption-desorption isotherms 21 22 showed the Brunauer–Emmett–Teller (BET) specific surface area to be reduced from 64.3 m2 23 24 25 g-1 for Cu1.8Se to 9.5 m2 g-1 for S/Cu1.8Se, which further demonstrated the successful loading of 26 27 sulfur (Figure S4). Thermogravimetric analysis (TGA) allowed quantifying the sulfur content 28 29 30 in the S/Cu1.8Se composite at about 70.0 wt%, consistent with the nominal amount introduced 31 32 33 (Figure S5). 34 35 To evaluate LiPS adsorption affinity, the same amount of the different hosts was immersed into 36 37 38 a 0.01 M Li2S4 solution. After 12 h, clear differences in the colour of the solutions containing 39 40 the different materials were appreciated both by direct observation and using UV-vis 41 42 43 spectroscopy, indicating a different degree of LiPS adsorption (Figure 2a,b). As observed from 44 45 46 the optical images in Figure 2a, the 0.01 M Li2S4 solution had an intense orange colour. In the 47 48 presence of the carbon black Super P, the colour of the solution remained unchanged after 12 49 50 51 h, as it was expected from the low affinity of the nonpolar carbon surface to LiPS. On the other 52 53 hand, the two solutions containing copper selenides displayed a much more pallid colour, 54 55 56 indicating that most of the Li2S4 in the solution had been captured by the adsorber. More precise 57 58 59 conclusions can be obtained from the UV-vis spectra (Figure 2b). Li2S4 presents a strong 60 7 ACS Paragon Plus Environment ACS Nano Page 8 of 37 1 2 3 4 absorption band in the range of 400-500 nm.35,36 The absorbance in this region strongly 5 6 7 decreased in the presence of Cu2Se, but especially with Cu1.8Se, indicating a much stronger 8 9 polysulfide adsorption ability of the latter. 10 11 12 The surface chemical states of Cu1.8Se after the Li2S4 adsorption test (labelled as Cu1.8Se-Li2S4) 13 14 were analyzed and compared with those of pristine Cu1.8Se. Figure 2c displays the high- 15 16 17 resolution Cu 2p XPS spectra of Cu1.8Se-Li2S4. Compared with Cu1.8Se, the Cu 2p spectrum of 18 19 20 Cu1.8Se-Li2S4 shifted toward lower binding energies, which is attributed to the less 21 22 electronegative chemical environment created by the chemical adsorption of negatively charged 23 24 25 polysulfide species on the Cu1.8Se surface.37 Besides, a second Cu 2p doublet was identified in 26 27 the Cu 2p spectra of Cu1.8Se-Li2S4, at 951.2 eV and 931.3 eV, corresponding to the 2p1/2 and 28 29 30 2p3/2 levels of Cu0. Thus a partial reduction of the surface copper ions in Cu1.8Se took place 31 32 33 during the LiPS chemisorption process.31 Figure 2d shows the high-resolution Se 3d spectrum 34 35 of Cu1.8Se before and after Li2S4 adsorption (Cu1.8Se-Li2S4). Compared with pristine Cu1.8Se, 36 37 38 the Se 3d XPS spectrum of Cu1.8Se-Li2S4 shifted to higher binding energies, which indicated 39 40 an increase of the chemical environment electronegativity during the LiPS adsorption and 41 42 43 simultaneous Cu partial reduction.38,39 The Li 1s XPS spectra of Cu1.8Se-Li2S4 is compared with 44 45 46 that of Li2S4 in Figure 2e. While the Li 1s band at 55.2 eV is attributed to the Li-S bonding 47 48 within Li2S4., the new peak appearing in the Li 1s XPS spectra of Cu1.8Se-Li2S4 is associated 49 50 51 with the formation of Li-Se chemical bonds during the adsorption process.40 Overall, the 52 53 significant variations in the XPS spectra of Cu1.8Se before and after the Li2S4 adsorption test 54 55 56 clearly demonstrate the chemical interaction and thus affinity between the Cu1.8Se surface and 57 58 59 Li2S4. 60 8 ACS Paragon Plus Environment Page 9 of 37 ACS Nano 1 2 3 4 DFT calculations were conducted to assess the affinity between LiPS species and Cu1.8Se and 5 6 7 Cu2Se. The calculated geometrically stable configurations of Cu2Se-Li2S4 and Cu1.8Se-Li2S4 8 9 are illustrated in Figure 2f. The results revealed that the S and Li atoms in Li2S4 preferentially 10 11 12 bind with the Cu and Se atoms on the (002) surface of Cu2Se and (111) surface of Cu1.8Se, 13 14 respectively. While in both cases negative binding energies were obtained, a significantly 15 16 17 higher absolute value of the binding energy (-3.21 eV) is achieved for Cu1.8Se-Li2S4 than for 18 19 20 Cu2Se-Li2S4 (-1.62 eV), which is consistent with the Li2S4 test results. 21 22 To gain additional insight into the interactions between LiPS species and copper selenide 23 24 25 surfaces, the charge density difference of Li2S4 species adsorbed on Cu2Se (220) and Cu1.8Se 26 27 (111) were calculated and are displayed in Figure 2g. The cyan isosurfaces around the Li–S 28 29 30 bonds in Li2S4 indicated a decreased charge density and weakened bonds between Li and S 31 32 33 atoms, which benefits its further conversion toward lower-order LiPS. In addition, the yellow 34 35 isosurface suggests that the charge density between Cu1.8Se and the Li atom of Li2S4 was 36 37 38 significantly increased, indicating a strong interaction between Cu1.8Se and Li2S4, which is 39 40 consistent with the stronger affinity of the Cu1.8Se surfaces to LiPS.41 41 42 43 The results of computational simulations of the interactions between CuxSe and different LiPS 44 45 46 covering the full range of sulfur chemical states upon battery operation (Li2S, Li2S2, Li2S4, Li2S6, 47 48 Li2S8, and S8) are displayed in Figure S6 and S7. The S-Cu and Li-Se attractions can be 49 50 51 confirmed and are maintained in all the LiPS-adsorbent couples, while Cu1.8Se exhibits stronger 52 53 adsorbability for the LiPS by showing systematically higher binding energies (Figure 2h) and 54 55 56 shorter bond lengths for both the S-Cu and Li-Se bonds than Cu2Se (Figure S8). 57 58 59 60 9 ACS Paragon Plus Environment ACS Nano Page 10 of 37 1 2 3 4 DFT results showed the Cu1.8Se Fermi level to lay within a band of states, which was consistent 5 6 7 with its degenerated/metallic character and is related to the high density of Cu vacancies that 8 9 provide high hole densities.33,42 In contrast, the Fermi level of Cu2Se lies within its bandgap 10 11 12 showing a p-type semiconductor behaviour (Figure 3a). 13 14 DFT calculations were also used to evaluate the lithium-ion and Li2S4 diffusion on the (002) 15 16 17 facet of Cu2Se and the (111) facet of Cu1.8Se. The geometrical configurations of the Li-ion 18 19 20 diffusion paths are shown in Figure 3b and the corresponding energy profiles are displayed in 21 22 Figure 3c. The calculated Li-ion diffusion energy barriers (Ebarrier) were relatively low, just 0.34 23 24 25 eV for Cu2Se and even a threefold smaller, 0.11 eV, for Cu1.8Se. Figure 3d,e displays the 26 27 geometrical configurations of the path for Li2S4 migration on the (002) facet of Cu2Se and the 28 29 30 (111) facet of Cu1.8Se and the Ebarrier for Li2S4 migration along different adsorption sites. 31 32 33 Calculations showed the Ebarrier for Li2S4 migration on Cu2Se to be 0.89 eV and just 0.30 eV on 34 35 Cu1.8Se. These low diffusion barriers allow fast Li-ion and LiPS diffusion rates on the copper 36 37 38 selenide surfaces, which promotes the Li - S redox reactions. 39 40 Figure 3f exhibits the initial state, transition state, and final state of Li2S decomposition based 41 42 43 on different sulfur hosts. The calculated energy barrier for Li2S decomposition on Cu2Se and 44 45 46 Cu1.8Se surfaceS was 1.27 and 0.63 eV, respectively (Figure 3g). Thus, the defective cubic 47 48 Cu1.8Se can greatly reduce the Li2S decomposition energy barrier and enhance the redox 49 50 51 reversibility between Li2S and LiPS. 52 53 The calculated Gibbs free energy changes during the S reduction on Cu2Se and Cu1.8Se are 54 55 56 displayed in Figure 3h. The overall reversible reaction from S8 and Li to Li2S was considered. 57 58 59 During the discharge process, the first step involves the double reduction of S8 with two Li+ to 60 10 ACS Paragon Plus Environment Page 11 of 37 ACS Nano 1 2 3 4 form Li2S8. Then Li2S8 undergoes further reduction to three intermediate LiPS, Li2S6, Li2S4, 5 6 7 and Li2S2. Finally, Li2S is obtained as the end product. The largest increase of Gibbs free energy 8 9 was obtained for the conversion from Li2S2 to Li2S, suggesting this step as the rate-limiting one 10 11 12 during the discharge process.43,44 Both copper selenides exhibited low Gibbs free energy 13 14 changes, demonstrating their potential as Li-S catalysts. Among them, Cu1.8Se was 15 16 17 characterized with the lowest Gibbs free energy change, 0.62 eV, well below that of Cu2Se, 18 19 20 0.96 eV, which suggests that the reduction of S is more thermodynamically favourable on 21 22 Cu1.8Se than on Cu2Se. Overall, DFT calculations pointed at the defective cubic Cu1.8Se 23 24 25 structure as the most effective to trap LiPS and catalytically accelerate the Li-S reversible 26 27 reaction. 28 29 30 To experimentally evaluate the ability of host materials to catalytically accelerate the kinetics 31 32 33 of polysulfide conversion, CV tests of Li2S6 symmetrical cells were conducted in the voltage 34 35 window -0.8 to 0.8 V. As shown in Figure 4a, Cu1.8Se, Cu2Se, and Super P electrodes showed 36 37 38 significantly different CV curves. The symmetric curves of Cu1.8Se electrodes exhibited higher 39 40 peak current densities than Cu2Se and Super P electrodes, illustrating the faster reaction kinetics 41 42 43 of Cu1.8Se during liquid-to-solid (Li2S ↔ S62− ↔ S8) conversion.16,45 When similar experiments 44 45 46 were carried out on Cu1.8Se electrodes without Li2S6 addition in the electrolyte, approximately 47 48 linear-shaped CV curves were obtained, pointing at a pure capacitive behaviour rather than a 49 50 51 chemical reaction. This result implies that Li2S6 was the sole electrochemically active species 52 53 in the system. The well-maintained CV profile of symmetric cells based on Cu1.8Se after 10 54 55 56 cycles points at proper reaction stability (Figure S9). 57 58 59 60 11 ACS Paragon Plus Environment ACS Nano Page 12 of 37 1 2 3 4 The electrochemical performance of the electrodes was evaluated using coin-type cells. Figure 5 6 7 4b displays the CV profiles of Li-S cells based on S/Cu1.8Se, S/Cu2Se and S/Super P cathodes 8 9 at a scan rate of 0.1 mV s-1. All curves displayed two reduction peaks (peak C1 and C2) that 10 11 12 were attributed to a two-step S reduction process upon cathodic scanning: peak C1 was 13 14 associated with the transformation from sulfur to soluble long-chain LiPS (Li2Sx, 4 ≤ x ≤ 8), 15 16 17 and peak C2 to a further reduction of sulfur to insoluble Li2S2/Li2S.46,47 The anodic peak (peak 18 19 20 A) corresponded to the reverse multistep oxidation process of short-chain sulfides to LiPS and 21 22 eventually to sulfur.48 S/Cu1.8Se electrodes exhibited the most intense peak currents and the 23 24 25 most positive/negative potentials of the cathodic/anodic peaks (Figure 4c). This result indicates 26 27 that among the tested materials, Cu1.8Se was the most effective catalyst in promoting the 28 29 30 polysulfides redox reaction kinetics. 31 32 33 The excellent catalytic activity of Cu1.8Se was further confirmed by measuring the onset 34 35 potentials at a current density of 10 µA cm-2 beyond the baseline current (Figure S10).48,49 As 36 37 38 illustrated in Figure 4c, among the three electrodes tested, S/Cu1.8Se exhibited the highest onset 39 40 potentials of reduction peaks and the lowest onset potentials of oxidation peaks, evidencing the 41 42 43 ability of Cu1.8Se to reduce the overpotentials of the Li-S reversible reaction. Besides, the CV 44 45 46 curves of S/Cu1.8Se almost overlapped, showing no obvious peak shift or current changes, 47 48 which points to excellent stability and reversibility (Figure S11). 49 50 51 The lithium-ion (Li+) diffusion rate, which is another important factor affecting the conversion 52 53 kinetics of LiPSs, was investigated by measuring CV curves at different scan rates, from 0.1 to 54 55 56 0.5 mV s-1. Figure S12 displays the results obtained from S/Cu1.8Se, S/Cu2Se, and S/Super P 57 58 59 electrodes containing similar amounts of sulfur. All reduction and oxidation peaks current were 60 12 ACS Paragon Plus Environment Page 13 of 37 ACS Nano 1 2 3 4 observed to linearly change with the square root of the scan rates, demonstrating the reaction 5 6 7 to be diffusion-limited. Thus the Li+ diffusivity can be estimated according to the classical 8 9 Randles-Sevcik equation:35,50 10 11 0.5 12 I p  (2.69 *105 )n1.5 ADL i  C Li  v 0.5 13 14 where Ip is the peak current density, n is the number of reaction electrons, A is the geometric 15 16 17 area of the electrode, DLi+ is the Li+ diffusion coefficient, CLi+ is the concentration of Li+ in the 18 19 20 electrolyte, and ν is the scan rate. 21 22 As shown in Figure 4d-f, S/Cu1.8Se electrodes exhibited the sharpest slopes for the three peaks 23 24 25 (peak C1, peak C2, and peak A), thus the highest Li+ diffusivity during the redox reactions. 26 27 Based on the Randles-Sevcik equation, S/Cu1.8Se electrodes were characterized by a DLi+ at 28 29 30 peaks C1, C2, and A of 3.3 × 10-7, 5.0 × 10-7, and 8.6 × 10-7 cm2 s−1, respectively, well above 31 32 33 the values obtained for S/Cu2Se and S/Super P (Figure 4g). The Li+ diffusivity strongly depends 34 35 on the accumulation of insulating Li2S/Li2S2 on the electrode and the viscosity of the electrolyte 36 37 38 that varies with the concentration of soluble LiPS.48 The highest Li+ diffusivities obtained for 39 40 the Cu1.8Se host reflect a more effective catalytic activity towards Li-S reaction and a more 41 42 43 effective trapping of soluble LiPS that prevented them from dissolving into the electrolyte. 44 45 46 A very significant part of the discharge capacity, up to 75%, stems from the transformation of 47 48 the Li2S4 intermediate into Li2S. Thus the rate of Li2S precipitation on the electrode matrix is a 49 50 51 very significant conversion kinetic indicator for LSBs. From the potentiostatic discharge 52 53 profiles in Figure 4h,i, both copper selenide electrodes displayed very sharp nucleation peaks 54 55 56 and fast responsivity toward Li2S nucleation, being the Cu1.8Se performance significantly better 57 58 59 60 13 ACS Paragon Plus Environment ACS Nano Page 14 of 37 1 2 3 4 than that of the Cu2Se electrodes. According to Faraday's law, the Li2S deposition capacity of 5 6 7 Cu1.8Se (304 mAh g-1) was larger than that of Cu2Se (136 mAh g-1). 8 9 Galvanostatic charge/discharge curves of S/Cu1.8Se, S/Cu2Se, and S/Super P at 0.1 C (1 C = 10 11 12 1672 mA g-1) exhibited one charge plateau and two discharge plateaus (Figure 5a), consistently 13 14 with CV. The voltage gap between the charge plateaus and the second discharge is generally 15 16 17 referred to as the polarization potential (ΔE).51 As displayed in Figure 5b, S/Cu1.8Se electrodes 18 19 20 showed a lower polarization potential (ΔE = 150 mV) than S/Cu2Se (ΔE = 175 mV) and 21 22 S@Super P electrodes (ΔE = 238 mV). 23 24 25 The catalytic activity of the host materials toward the LiPS conversion reaction can be further 26 27 quantified by the ratio QH/QL, where QL and QH denote the capacity of the two discharge 28 29 30 plateaus (Figure 5a). QL is related to the reduction of sulfur to soluble LiPS (S8 + 4Li+ + 4e- → 31 32 33 2Li2S4) and QH corresponds to the subsequent transformation to insoluble sulfide (2Li2S4 + 34 35 12Li+ + 12e- → 8Li2S).43 Thus the ratio QH/QL can be interpreted in terms of the catalytic ability 36 37 38 for LiPS conversion reaction: sluggish kinetics during the solid → liquid → solid process and 39 40 shuttle effect caused by the diffusion of soluble LiPS give rise to capacity fading during QH 41 42 43 stage. Thus, the higher QH/QL, the better the catalytic ability. As shown in Figure 5b, QH/QL 44 45 46 ratios obtained for copper selenides were much larger than those of the carbon black 47 48 (QH/QL=1.71), being the QH/QL ratio of the S/Cu1.8Se electrode (QH/QL=2.71) significantly 49 50 51 larger than that of S/Cu2Se (QH/QL=2.43). 52 53 The electrochemical kinetics were further analyzed during the phase transformation between 54 55 56 soluble LiPS and insoluble Li2S2/Li2S in the charge/discharge processes. Figure S13a shows 57 58 59 the voltage jump at the initial charging period, reflecting the overpotentials of Li2S activation.9 60 14 ACS Paragon Plus Environment Page 15 of 37 ACS Nano 1 2 3 4 Besides, the galvanostatic discharge curves displayed a voltage dip at the beginning of the Li2S 5 6 7 precipitation voltage plateaus, which is associated with the overpotential of Li2S nucleation 8 9 (Figure S13b).33 In both charge and discharge processes, the S/Cu1.8Se electrode exhibited 10 11 12 lower overpotentials than S/Cu2Se and S/Super P electrodes, indicating a promoted Li-S 13 14 reaction. 15 16 17 The galvanostatic charge/discharge profiles of S/Cu1.8Se, S/Cu2Se, and S/Super P electrodes at 18 19 20 various current densities, from 0.1 to 5 C are displayed in Figure 5c and Figure S14. Figure 5d 21 22 compares the rate performances of the three types of electrodes tested. Copper selenide 23 24 25 electrodes displayed significantly higher capacities than carbon black. The S/Cu1.8Se electrode 26 27 showed the largest capacity among the different electrodes tested, with two clear discharge 28 29 30 plateaus at all current rates. The S/Cu1.8Se electrode displayed the best rate capability with a 31 32 33 high capacity of 1218 mAh g-1 at 0.2 C and 588 mAh g-1 at 5 C. When switched back to 0.2 C, 34 35 the capacity recovered to 1148 mAh g-1, corresponding to 94.3% of its initial value, indicating 36 37 38 remarkable electrochemical reversibility. In comparison, the cells based on S/Super P undergo 39 40 rapid capacity fading when increasing the current rate to 5 C. The excellent rate capability of 41 42 43 the copper selenides, especially the S/Cu1.8Se electrode, is consistent with their high ability to 44 45 46 accelerate the charge transfer and promote the conversion of polysulfides. 47 48 The energy conversion efficiency of the cells in the charge/discharge process was calculated by 49 50 51 the ratio of energy output/input (E = UIdt).35,52 At 0.1 C current rate, the three types of electrode 52 53 tested exhibited a high energy efficiency of around 90% (Figure 5e). When increasing the 54 55 56 current rate, S/Cu1.8Se displayed the highest and most stable energy efficiency, with 88.5% 57 58 59 efficiency at 1 C and 82.8% at 5 C. In contrast, the S/Super P electrode was characterized with 60 15 ACS Paragon Plus Environment ACS Nano Page 16 of 37 1 2 3 4 just a 71.3% efficiency at1 C. The high energy efficiency of the cathodes based on copper 5 6 7 selenides, and particularly the S/Cu1.8Se electrode, is associated with their low polarization 8 9 potential and excellent catalytic activity. 10 11 12 Figure 5f displays the galvanostatic cycling performances of coin cells with different cathodes. 13 14 Consistent with previous results, the S/Cu1.8Se electrode enabled a remarkable initial capacity 15 16 17 of 882 mAh g−1 at 1C and 837 mAh g−1 after 200 cycles, which corresponds to an outstanding 18 19 20 capacity retention of 94.9 %. The S/Cu2Se electrode also showed a notable initial capacity (770 21 22 mAh g−1) and decent capacity retention of ca. 80% after 200 cycles. In contrast, the S/Super P 23 24 25 electrode showed a moderate initial capacity (375 mAh g−1) and low capacity retention (52.8%) 26 27 after 200 cycles. 28 29 30 Figure 5g and Figure S15 display the Nyquist plot of the electrochemical impedance 31 32 33 spectroscopy (EIS) data obtained from S/Cu1.8Se, S/Cu2Se, and S/Super P coin cells before and 34 35 after cycling at 1 C. For the fresh electrodes, the semicircle in the high-frequency region is 36 37 38 related to the charge-transfer resistance (Rct), and the linear feature in the low-frequency region 39 40 is associated with the diffusion of lithium ions.48,53 Based on the equivalent circuit shown in 41 42 43 Figure S15c, a relatively low Rct was obtained for the S/Cu1.8Se electrodes (39.5 Ω), well below 44 45 46 that of S/Cu2Se (71.3 Ω) and S/Super P (87.8 Ω). After 200 charge/discharge cycles at 1C, an 47 48 additional semicircle in the high-frequency range was evidenced. This new feature is associated 49 50 51 with the resistance of an insoluble Li2S2/Li2S passivation layer (Rp in the equivalent circuit 52 53 shown in Figure S15d) grown during cycling.53 After cycling, the S/Cu1.8Se electrodes were 54 55 56 characterized by smaller Rct (9.8 Ω) and Rp (16.1 Ω), than S/Cu2Se (Rct=32.5 Ω, Rp=22.9 Ω) 57 58 59 and S/Super P (Rct=53.5 Ω, Rp=109 Ω) (Figure S16). This result confirms the important role 60 16 ACS Paragon Plus Environment Page 17 of 37 ACS Nano 1 2 3 4 played by copper selenides and particularly Cu1.8Se in accelerating the LiPS conversion 5 6 7 reaction and facilitating the charge transfer kinetics during lithiation/delithiation reactions. 8 9 Figure 5h displays the long term cycling performance of the electrodes at 3C. Copper selenide 10 11 12 electrodes displayed an excellent cycling performance at this relatively high current rate, well 13 14 above that of the cell based on a carbon cathode. Among the two copper selenides tested, the 15 16 17 highly defective cubic Cu1.8Se showed the best cyclability, retaining a high reversible capacity 18 19 20 of 527 mAh g-1 after 1000 cycles at 3 C, which corresponds to a tiny capacity fading rate of 21 22 0.029% per cycle. The outstanding capacity, rate capability and cyclability of the Cu1.8Se 23 24 25 cathodes are attributed to the highly conductive nature, rapid ion transfer, excellent catalytic 26 27 activity, and good LiPS affinity of this material, which immobilizes LiPS species and 28 29 30 effectively accelerate their electrochemical conversion. 31 32 33 A key step toward the real-world application of LSBs is increasing the sulfur loading to rise the 34 35 energy density of the overall device. Thus, the performance of S/Cu1.8Se cathodes with higher 36 37 38 sulfur loadings of 3.2, 5.1, and 7.2 mg cm−2 was also evaluated. As shown in Figure 6a, at a 39 40 0.1C current rate, two-plateau voltage profile is discerned under high the highest sulfur loadings 41 42 43 tested, up to 7.2 mg cm−2, with a high areal capacity of 6.07 mAh cm-2, that largely exceeds the 44 45 46 industrially requested areal capacity of LIBs (~4 mAh cm-2). S/Cu1.8Se-based cells containing 47 48 high sulphur loads also demonstrated a notable rate performance (Figure 6b), achieving areal 49 50 51 capacities of 3.01, 4.54, and 5.40 mAh cm-2 at 0.2 C when containing 3.2, 5.1, and 7.2 mg cm−2 52 53 of sulfur, respectively. As shown in Figure 6c, the high sulfur loaded Li-S cells based on 54 55 56 S/Cu1.8Se cathodes also showed a notably stable Li-S electrochemistry, with just a slight 57 58 59 potential hysteresis after 50 cycles that indicates a minor LiPS shuttling. Figure 6d displays the 60 17 ACS Paragon Plus Environment ACS Nano Page 18 of 37 1 2 3 4 cycling performances of the high sulphur loaded cells. S/Cu1.8Se cathodes showed high capacity 5 6 7 retention after 100 cycles, with areal capacities of 3.32, 4.27, and 5.00 mAh cm-2 at sulfur 8 9 loadings of 3.2, 5.1, and 7.2 mg cm−2, respectively. 10 11 12 As a practical demonstration of the potential of LSB cells based on Cu1.8Se cathodes, an electric 13 14 fan with a light pattern was powered with a single coin cell (Figure 6e). Besides, the excellent 15 16 17 performance of Cu1.8Se-based cathodes is placed in context in Figure 6f and Table S1, where 18 19 20 results obtained in the present work are compared with those reported in previous publications. 21 22 23 24 25 CONCLUSIONS 26 27 28 In summary, we demonstrated copper selenides produced by a facile solution-based method to 29 30 31 be an efficient multifunctional sulphur host material in LSB cathodes. Both experimental results 32 33 and computational data demonstrated the excellent electrochemical performance of copper 34 35 36 selenides, and particularly Cu1.8Se, to be attributed to: i) The presence of a combination of 37 38 lithiophilic/sulfiphilic sites which contributes to an effective trapping of LiPS and the related 39 40 41 promotion of the Li+ diffusion; ii) high electrical conductivity of Cu1.8Se associated to its high 42 43 44 density of copper vacancies; and iii) A large improvement of the Li-S reaction kinetics during 45 46 the battery charge/discharge processes, including LiPS phase change and Li2S 47 48 49 deposition/dissolution. Among the copper selenides tested, experimental data and DFT 50 51 calculations pointed at the defective cubic Cu1.8Se structure as the most effective to trap LiPS 52 53 54 and to catalytically accelerate the Li-S reversible reaction. Attributed to these beneficial 55 56 57 features, S/Cu1.8Se cathodes delivered excellent long-term cycling stability with a low capacity 58 59 fading of 0.029% per cycle over 1000 cycles, a remarkable rate capability up to 5 C, and a high 60 18 ACS Paragon Plus Environment Page 19 of 37 ACS Nano 1 2 3 4 areal capacity of 6.07 mAh cm-2 under a high sulfur load (7.2 mg cm−2). This work demonstrates 5 6 7 that multifunctional sulphur hosts, with high adsorption and catalytic capabilities for rapid LPS 8 9 adsorption-conversion process, hold a great promise in promoting the practical application of 10 11 12 LSBs. 13 14 15 16 17 EXPERIMENTAL SECTION 18 19 20 Chemicals: Copper(II) chloride dihydrate (CuCl2.2H2O, 99+%), selenium pieces (99.999%), 21 22 and 1-octadecene (ODE, technical grade 90%) were purchased from Aldrich. 23 24 25 Triocthiylphosphine (TOP, 97%) was purchased from Strem Chemicals. Oleylamine (OAm, 26 27 C18 content approximately 80-90%) was purchased from Fisher. Analytical grade ethanol and 28 29 30 hexane were obtained from various sources. All chemicals were used as received, without 31 32 33 further purification. A stock solution of TOPSe (1 M) was prepared by dissolving 0.789 g of 34 35 selenium pieces in 10 mL of TOP. All syntheses were carried out using standard vacuum/argon 36 37 38 Schlenk lines. 39 40 Synthesis of Cu1.8Se and Cu2Se: In a typical synthesis, 9 mmol of CuCl2.2H2O was mixed 41 42 43 with 30 mL of ODE and 70 mL of OAm in a 250 mL three-neck flask. The mixture was 44 45 46 degassed under vacuum for 20 min at room temperature and then heated to 120 °C for 1h under 47 48 vacuum. Afterwards, the solution was flushed with Ar and heated up to 220 °C. At this 49 50 51 temperature, 5 mL of the TOP-Se stock solution was quickly injected. After injection, the 52 53 reaction temperature was maintained at 220 °C for 20 min. Then, the solution was cooled using 54 55 56 a water bath. Cu1.8Se nanoparticles (NPs) were precipitated and redispersed several times using 57 58 59 ethanol and hexane. Then, Cu1.8Se NPs were dried under vacuum and kept in an Ar-filled 60 19 ACS Paragon Plus Environment ACS Nano Page 20 of 37 1 2 3 4 glovebox until their posterior use. Cu2Se was obtained by annealing the Cu1.8Se NPs under a 5 6 7 reducing atmosphere (5% H2 in Ar) at 600ºC for 3h. 8 9 Synthesis of S/Cu1.8Se, S/Cu2Se and S/Super P: Typically, Cu1.8Se and sulfur powder were 10 11 12 mixed with a weight ratio of 1:3, and then the mixture was heated at 155 °C for 8 h in a sealed 13 14 glass bottle under Ar protection. Then the powder was immersed in a 10 mL CS2 and ethanol 15 16 17 solution (1:4, volume ratio) several times to remove the redundant sulfur not incorporated into 18 19 20 Cu1.8Se. S/Cu2Se and S@Super P were prepared by the same method. 21 22 Materials Characterization: X-ray diffraction (XRD) patterns were obtained using a Bruker 23 24 25 AXS D8 Advance X-ray diffractometer with Cu K radiation (λ = 1.5106 Å) operating at 40 kV 26 27 and 40 mA. The morphology and microstructure of samples were characterized in a ZEISS 28 29 30 Auriga Field emission scanning electron microscopy (FESEM) equipped with an energy- 31 32 33 dispersive X-ray spectroscopy (EDS) detector operated at 20 kV. Transmission electron 34 35 microscopy (TEM) characterization was carried out on a Zeiss Libra 120 (Carl Zeiss, Jena, 36 37 38 Germany) operating at 120 kV. High-resolution TEM (HRTEM) and scanning TEM (STEM) 39 40 were measured using a field emission gun FEI Tecnai F20 microscope at 200 kV. High angle 41 42 43 annular dark-field (HAADF) STEM was combined with electron energy loss spectroscopy 44 45 46 (EELS) in the Tecnai microscope by using a GATAN QUANTUM filter. X-ray photoelectron 47 48 spectroscopy (XPS) measurements were performed using an Al anode XR50 source operating 49 50 51 at 150 mW and a Phoibos 150 MCD-9 detector. Thermogravimetric (TGA) (PerkinElmer 52 53 Diamond TG/DTA instrument.) experiment was carried out to record the ratio of S within the 54 55 56 prepared composites. UV-vis absorption spectra were identified by the PerkinElmer LAMBDA 57 58 59 950 UV-vis spectrophotometer. Nitrogen adsorption-desorption isotherms were recorded to 60 20 ACS Paragon Plus Environment Page 21 of 37 ACS Nano 1 2 3 4 calculate the specific surface area and analysis of the pore size distribution by Brunauer- 5 6 7 Emmett-Teller method (Tristar II 3020 Micromeritics system). 8 9 Electrochemical Measurements: LSB performance was tested at room temperature in 10 11 12 CR2032 coin-type cells. Active materials (S/Cu1.8Se, S/Cu2Se and S/Super P), PVDF binders 13 14 and Super P were mixed (weight ratio = 8:1:1) in NMP to form a black slurry and then coated 15 16 17 on Al foils and dried at 60 °C overnight. The prepared homogeneous slurry was coated on an 18 19 20 aluminium foil and vacuum dried at 60 °C overnight. Subsequently, the coated aluminium foil 21 22 was punched into small disks with a diameter of 12.0 mm as the cathode. Sulfur loading was 23 24 25 about 1.0 mg cm-2 (E/S = 20 μL mg-1). Lithium foils were used as anode and Celgard 2400 26 27 membranes as separators. The electrolyte was prepared by dissolving 1.0 M lithium 28 29 30 bis(trifluoromethanesulfonyl)imide in a mixture of DOL and DME (v/v = 1/1) with 0.2 M 31 32 33 LiNO3 additive. Galvanostatic charge-discharge (GCD) tests were performed on a Neware 34 35 BTS4008 battery tester with different C rates. Cyclic voltammetry (CV) measurements were 36 37 38 performed on a battery tester BCS-810 from Bio Logic at a scan rate of 0.1-0.4 mV s-1with an 39 40 electrochemical window of 1.7-2.8 V, and electrochemical impedance spectroscopy (EIS) tests 41 42 43 with a voltage amplitude of 10 mV in the frequency range 100 kHz to 0.01 Hz were performed. 44 45 46 Preparation of Li2S4 solutions for adsorption test: Sulfur and Li2S with a molar ratio of 3:1 47 48 were dissolved into a certain amount of DME and DOL (volume ratio of 1:1) for continuous 49 50 51 stirring overnight in a glove box, which eventually yielded a homogeneous dark brown solution. 52 53 To evaluate the polysulfide absorption ability, 15 mg of Cu1.8Se, Cu2Se and Super P were 54 55 56 poured into 3.0 mL 10 mM Li2S4 solution, respectively. The obtained mixtures were vigorously 57 58 59 stirred for 2 h and aged overnight under Ar atmosphere. 60 21 ACS Paragon Plus Environment ACS Nano Page 22 of 37 1 2 3 4 Symmetric Cell Assembly and Measurements: Symmetric cells were assembled and tested 5 6 7 using the same process as for LSBs. Two pieces of the same electrode (average S loading about 8 9 0.5 mg cm-2) were used as identical working and counter electrodes. Each coin cell contained 10 11 12 40 μL of electrolyte (0.5 M Li2S6 and 1 M LiTFSI in 1:1 DOL/DME (v/v)). CV measurements 13 14 were carried out at a scan rate of 20 mV s−1. 15 16 17 Measurement of Nucleation of Li2S: Standard 2032 coin cells were assembled to analyze the 18 19 20 nucleation of Li2S. A certain amount of host materials (Cu1.8Se and Cu2Se) were dissolved in 21 22 ethanol. Then the catalysts were coated on the carbon papers applied as the working electrode. 23 24 25 Li foil worked as the counter electrode. The catholyte consisted of 20 μL of 0.25 M Li2S8 26 27 dissolved in tetraglyme with 1.0 M LiTFSI. The anolyte contained 20 μL of 1.0 M LiTFSI in 28 29 30 tetraglyme solution without Li2S8. The cells were first discharged at a current of 0.112 mA to 31 32 33 2.06 V and then hold the voltage at 2.05 V until the current decreased to 0.01 mA for Li2S 34 35 nucleation and growth. 36 37 38 Supporting Information 39 40 The supporting information is available free of charge via the Internet at http://pubs.acs.org. 41 42 43 The supporting information includes Figure S1-S16 and Table S1 as described in the text, 44 45 46 specifically, additional SEM, TEM, XRD, TGA, CV, electrochemical performance, DFT 47 48 simulation results, and crystal structure. 49 50 51 Corresponding Author 52 53 Email: limengyao@irec.cat 54 55 56 Email: wangjiaao0720@utexas.edu 57 58 59 Email: david.mitlin2@utexas.edu 60 22 ACS Paragon Plus Environment Page 23 of 37 ACS Nano 1 2 3 4 Email: chou@wzu.edu.cn 5 6 7 Email: acabot@irec.cat 8 9 10 ACKNOWLEDGMENTS 11 12 13 The authors thank support from the projects ENE2016-77798-C4-3-R and NANOGEN 14 15 (PID2020-116093RB-C43), funded by MCIN/ AEI/10.13039/501100011033/ and by “ERDF 16 17 18 A way of making Europe”, by the “European Union”. D. Yang, M. Li, X. Han and C. Zhang 19 20 21 thank the China Scholarship Council for the scholarship support. ICN2 acknowledges the 22 23 support from the Severo Ochoa Programme (MINECO, grant no. SEV-2017-0706). IREC and 24 25 26 ICN2 are both funded by the CERCA Program/Generalitat de Catalunya. This project has 27 28 received funding from the European Union’s Horizon 2020 research and innovation program 29 30 31 under grant agreement No 823717 – ESTEEM3. Calculations at UT Austin were supported by 32 33 34 the Welch Foundation (F-1841) and the Texas Advanced Computing Center. Part of the present 35 36 work has been performed in the framework of Universitat Autònoma de Barcelona Materials 37 38 39 Science PhD program. J. Llorca is a Serra Húnter Fellow and is grateful to MICINN/FEDER 40 41 RTI2018-093996-B-C31, GC 2017 SGR 128 and to ICREA Academia program. 42 43 44 45 46 47 REFERENCES 48 49 50 (1) Bai, S.; Liu, X.; Zhu, K.; Wu, S.; Zhou, H. 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Bimetallic Metal- 5 6 Organic Framework with High-Adsorption Capacity toward Lithium Polysulfides for 7 8 Lithium–Sulfur Batteries. Energy Environ. Mater. 2021, 0, 1–9. 9 10 (56) Zhubing, X.; Zhi, Y.; Linjie, Z.; Hui, P.; Ruihu, W. Sandwich-Type NbS2@S@I-Doped 11 12 Graphene for High-Sulfur-Loaded, Ultrahigh-Rate and Long-Life Lithium-Sulfur 13 14 Batteries. ACS Nano 2017, 11, 8488–8498. 15 16 (57) Hao, Q.; Cui, G.; Zhang, Y.; Li, J.; Zhang, Z. Novel MoSe2/MoO2 Heterostructure as an 17 18 Effective Sulfur Host for High-Performance Lithium/Sulfur Batteries. Chem. Eng. J. 19 20 2020, 381, 122672. 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Scheme 1. Schematic illustration of the synthetic procedure used to produce copper selenide. 30 ACS Paragon Plus Environment Page 31 of 37 ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 Figure 1. (a) TEM image of Cu1.8Se. (b) HRTEM micrograph of Cu1.8Se, detail of the orange 42 43 squared region and its corresponding power spectrum. (c) EELS chemical composition maps 44 45 obtained from the red squared area of the STEM micrograph of Cu1.8Se. Individual Cu L2,3- 46 47 edges at 931 eV (red) and Se M1,2-edges at 168 eV (green) and composites of Cu-Se. (d) TEM 48 49 image of Cu2Se. (e) EELS chemical composition maps obtained from the red squared area of 50 51 the STEM micrograph of Cu2Se. Individual Cu L2,3-edges at 931 eV (red) and Se M1,2-edges at 52 53 168 eV (green) and composites of Cu-Se. (f) HRTEM image of Cu2Se and corresponding FFT 54 55 spectrum of the enlarged image of the orange squared region. (g) XRD patterns of Cu2Se and 56 57 Cu1.8Se. (h) Cu 2p high-resolution XPS spectra, and (i) Se 3d high-resolution XPS spectra from 58 59 Cu2Se and Cu1.8Se. (j) EELS chemical composition maps obtained from the STEM micrograph 60 31 ACS Paragon Plus Environment ACS Nano Page 32 of 37 1 2 3 4 of a S/Cu1.8Se composite. Individual Cu L2,3-edges at 931 eV (red) and Se M1,2-edges at 168 5 6 eV (green), S L2,3-edge 165 eV (blue) and composites of Cu-Se-S. 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 Figure 2. LiPS adsorption ability. (a) Optical image of Cu1.8Se, Cu2Se, and Super P materials 50 51 soaked in Li2S4 solution after 12 h adsorption tests. (b) UV−vis spectrum of the absorbed Li2S4 52 53 solution containing the different adsorbents. (c-e) High-resolution XPS spectrum at (c) Cu 2p, 54 55 (d) Se 3d, and (e) Li 1s core levels of the pristine Cu1.8Se and Cu1.8Se-Li2S4, respectively. (f) 56 57 Optimized adsorption configurations of LiPS species on the (220) facet of Cu2Se and (111) 58 59 facet of Cu1.8Se, respectively. (g) Charge density difference of Li2S4 adsorbed on Cu1.8Se (111) 60 and Cu2Se (220). The yellow and cyan regions represent the accumulation and depletion of the 32 ACS Paragon Plus Environment Page 33 of 37 ACS Nano 1 2 3 4 electron, respectively. (h) Binding energies of LiPS species (Li2S, Li2S2, Li2S4, Li2S6, Li2S8 and 5 6 S8) absorbed on Cu1.8Se and Cu2Se surfaces, respectively, calculated with DFT. 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 Figure 3. Lithium-ion diffusion and catalytic conversion kinetics studies of polysulfides in Li-S 39 40 cells. (a) HSE06 band structures of Cu2Se and Cu1.8Se, respectively. (b) The total Li-ion 41 42 diffusion paths on the optimized (220) facet of Cu2Se and (111) facet of Cu1.8Se, respectively. 43 44 (c) Energy barrier of Li-ion diffusion on (220) facet of Cu2Se and (111) facet of Cu1.8Se, 45 46 respectively. (d) Illustration of stages of Li2S4 diffusing on (002) facet of Cu2Se and (111) facet 47 48 of Cu1.8Se, respectively. (e) Energy profiles of Li2S4 migration on (002) facet of Cu2Se and (111) 49 50 facet of Cu1.8Se along different adsorption sites. (f) Initial state, transition state and final state 51 52 of Li2S decomposition on (220) facet of Cu2Se and (111) facet of Cu1.8Se, respectively. (g) 53 54 Energy barrier profiles of Li2S cluster decomposition on Cu2Se and Cu1.8Se along with different 55 56 reaction coordinates. (h) Gibbs free energy profiles of LiPS species on Cu2Se and Cu1.8Se, 57 58 showing a lower reaction free energy from Li2S2 to Li2S on Cu1.8Se than that on Cu2Se. 59 60 33 ACS Paragon Plus Environment ACS Nano Page 34 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Figure 4. (a) CV profiles of symmetrical cells at a scan rate of 20 mV s-1 with three different 32 33 host materials using an electrolyte containing 0.5 mol L-1 Li2S6 and 1 mol L-1 LiTFSI dissolved 34 35 in DOL/DME (v/v = 1/1). (b) CV profiles of Li-S coin cells with different electrodes. (c) Peak 36 37 voltages and onset potentials obtained from CV curves of Li-S coin cells. (d-f) CV peak current 38 39 values of peaks C1, C2, and A for S/Cu1.8Se, S/Cu2Se, and S/Super P electrodes versus the 40 41 square root of scan rates. (g) Li+ diffusion coefficient value at peaks C1, C2, and A. (h,i) 42 43 Potentiostatic discharge profile at 2.05 V with Li2S8 catholyte for evaluating the nucleation 44 45 kinetics of Li2S on Cu1.8Se and Cu2Se electrodes. 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 34 ACS Paragon Plus Environment Page 35 of 37 ACS Nano 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 5. (a) Galvanostatic charge/discharge profiles of S/Cu1.8Se, S/Cu2Se, and S/Super P 31 32 electrodes at a current rate of 0.1 C. (b) ΔE and QH/QL values obtained from charge/discharge 33 34 curves. (c) Charge/discharge curves of the S/Cu1.8Se electrodes at various C rates from 0.1 C to 35 36 5 C. (d) Rate capability of S/Cu1.8Se, S/Cu2Se, and S/Super P electrodes at various C rates from 37 0.1 C to 5 C. (e) Energy efficiencies of three different electrodes at different current densities. 38 39 (f) Cycling performances of different electrodes at 1 C over 200 cycles. (g) EIS plot of S/Cu1.8Se 40 41 electrode before and after 200 cycles at 1 C. (h) Long-term cycling behaviour of Li–S cells 42 43 based on S/Cu1.8Se, S/Cu2Se, and S/Super P electrodes at 3 C. 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 35 ACS Paragon Plus Environment ACS Nano Page 36 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Figure 6. (a) Charge/discharge profiles at 0.1 C of S/Cu1.8Se electrodes under raised sulfur 33 34 loadings of 3.2, 5.1, and 7.2 mg cm−2. (b) Rate capabilities with different sulfur loadings at 35 36 various current densities. (c) Galvanostatic charge/discharge profiles at 0.1 C with a sulfur 37 38 loading of 5.1 mg cm−2. (d) High-loading cycling performances with sulfur loadings of 3.2, 5.1, 39 40 and 7.2 mg cm−2 at 0.1 C of S/Cu1.8Se electrodes. (e) Photograph of an electric fan with an “I 41 42 LOVE YOU” pattern being charged by Li-S cells based on S/Cu1.8Se electrodes. (f) Decay rate 43 44 per cycle compared with other reported works. 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 36 ACS Paragon Plus Environment Page 37 of 37 ACS Nano 1 2 3 4 5 6 7 8 9 Abstract Graphics 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 37 ACS Paragon Plus Environment