Understanding Interactions between Lead Iodide Perovskite Surfaces

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C: Energy Conversion and Storage; Energy and Charge Transport

Understanding Interactions between Lead Iodide Perovskite Surfaces and Lithium Polysulfide toward New-Generation Integrated Solar-Powered Lithium Battery: an ab-Initio Investigation Lei Zhang, Bo Wu, and Jingfa Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09114 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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The Journal of Physical Chemistry

Understanding Interactions between Lead Iodide Perovskite Surfaces and Lithium Polysulfide toward New-Generation Integrated Solar-Powered Lithium Battery: an ab-Initio Investigation

Lei Zhangab*, Bo Wuab and Jingfa Liab

a

Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information Science & Technology, Nanjing 210044, China. Email: [email protected] b

School of Physics and Optoelectronic engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China.

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ABSTRACT

Energy conversion devices such as perovskite solar cells and energy storage devices such as lithium sulfur battery flourish in these decades owing to their capabilities to deliver large power conversion efficiency and store superior specific energy, with potentials to solve the global energy crisis and environmental issues. Compared with conventional energy conversion devices and energy storage devices that have limited performances, integrating the energy conversion devices and energy storage devices into a single unit is advantageous to present enhanced performance in multiple applications and satisfy the commercial needs. However, further development of the integration relies on a deeper understanding of the interactions between the functional materials in the energy conversion devices and energy storage devices. In this study, we try to bridge the gap by investigating the interactions between the light absorbing halide perovskite material CH3NH3PbI3 and the lithium polysulfide intermediates (S8, Li2S8, Li2S6, Li2S4, Li2S2 and Li2S) formed during the charging/discharging processes in lithium sulfur batteries via ab-initio calculations. We find that the CH3NH3PbI3 and lithium polysulfide species have decent interactions, with the lithium polysulfide species residing stably on the halide perovskite surfaces and such interactions are strengthened by the charge transfer characters between the adsorbates and the adsorbents. We propose that the light absorbing halide perovskite materials represented by the CH3NH3PbI3 absorber exhibit potentials to be integrated into the lithium sulfur battery cathode to serve as an anchoring material to harness the solar power and mitigate the battery degradation problem, since the dissolution of intermediate lithium polysulfide (Li2Sn) is a severe problem in lithium sulfur batteries. The resulting integrated device is superior in capturing the solar energy due to the presence of the halide perovskite moiety and exhibits a large specific energy, low cost and low toxicity due to the sulfur materials. The comprehensive understanding of the light absorbing halide perovskite material and the lithium polysulfide species in this theoretical work forms a foundation for the further development and


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commercialization of integrated device that captures solar energy and can be charged/discharged efficiently.

Introduction The overuse of fossil fuels, minerals and natural resources by human beings might ultimately lead to a global energy crisis. At this point, it is imperative to exploit renewable energy, especially the energy conversion systems that captures the solar energy and energy storage systems that can store and transfer green energy efficiently. Solar cells work as effective energy conversion systems that harvest the solar energy and convert them into electricity,1,2 while lithium batteries can store and transfer energy efficiently for vehicles and portable devices such as cell phones and personal computers.3–5 At this stage, the energy conversion systems and energy storage systems are separately devised and have seldom been integrated into one unit. Integrating the energy conversion system and the energy storage system into a single unit is an attractive idea, since the integration can bring out better performance in multiple applications and satisfy the commercial needs.6 Prototypes of integrated device have been developed, for example, the design of an integrated power pack system based on double-sided TiO2 arrays.7 However, such integrated power pack is essentially two separate devices with an upper part as the solar cell and the lower part as the lithium ion battery for storing the generated energy by the solar cell. It would be interesting to design a battery electrode that is light sensitive to generate electricity from the incident photons and supply energy for external electrical equipment, such that the developed device could realize the direct storage of solar energy in a lithium battery without using PV cells.6,8 Selecting a superior light conversion material and an energy storing component is critical to realize this goal. Perovskite solar cell is a disruptive technology in the solar cell industry owing to its outstanding power conversion efficiency reaching 22% and lower cost.9–16 The active layer in perovskite solar cells consist of lead halide perovskite materials mainly based on CH3NH3PbI3, which exhibits excellent light absorption properties, long carrier diffusion length and defect tolerance. Many types of lithium batteries ACS Paragon Plus Environment

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are available at the moment,17–20 while the lithium sulfur battery stands out as a promising candidate for the next-generation energy storage devices.21–23 In the lithium sulfur batteries, the sulfur cathode provides high specific capacity that is more than five times the conventional lithium-ion batteries. Furthermore, the low cost and low toxicity of sulfur makes it attractive for commercial applications. The lithium sulfur batteries rely on the efficient conversion between lithium polysulfide and sulfur during charging and discharging, while the polysulfide species should be preserved with minimal dissolution to maintain the proper device functionality. 7,24–30 Considering the benefits from the perovskite solar cell and the lithium sulfur batteries, it would be tempting to integrate the light-sensitive halide perovskite material into the lithium sulfur battery electrode, such that the perovskite material and the sulfur electrode are in direct contact to provide the photocharging capabilities to realize the fully integrated device. Nevertheless, such integration relies on a deeper understanding of the perovskite materials and the battery materials, which is unavailable at the moment. In this manuscript, we theoretically investigate the interactions between the prototypical halide perovskite material CH3NH3PbI3 and the lithium polysulfide species which are present in the lithium sulfur batteries. Firstly, a series of the lithium polysulfide/perovskite composite structures combining lithium polysulfide species and CH3NH3PbI3 substrates are determined via first principles calculations. The binding energies between the adsorbates and the adsorbents are compared. The impacts of the lithium polysulfide species (S8 and Li2Sn) on the perovskite surface structures are explained, together with the structural changes in the Li2Sn intermediates. After that, the electronic and optical properties of the hybrid perovskite/Li2Sn systems are discussed via the plots of density of states, orbital distributions, charge density difference and UV-vis light absorption spectra. The decomposition of Li2Sn as an exemplar Li2Sn intermediate on the perovskite surfaces is also considered. The suitability of integrating the halide perovskite CH3NH3PbI3 into the lithium sulfur battery is discussed.


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Computational details The structure of CH3NH3PbI3 is obtained from the literature that has a tetragonal crystal structure.31 The perovskite (001) surface direction is cleaved to prepare the perovskite surface, since this direction has been proved to be a very stable surface from many perovskite solar cell studies.32–34 Two perovskite surfaces with different terminations are prepared, including a PbI-terminated perovskite surface exposing the PbI-rich layer, and a Cation-terminated perovskite surface exposing the methylammonium cation (Figure 1). Other terminations also exist in halide perovskite surfaces such as the one exposing the vacant/defected layer or the iodine dimer; however, the PbI-terminated perovskite surface and Cation-terminated perovskite surface are very stable that has been confirmed by various investigations.35–38 The PBE functional and DND basis set including all electron relativistic effects are employed in DMol339 for the geometrical optimization of the various Li2Sn/perovskite composites, with a 2 x 2 x 1 k-point set for the electronic minimization. A vacuum layer of 25 Å is constructed to avoid unnecessary top-down interactions between neighbouring layers. The bottommost layer in the unit cell is fixed d to mimic the real scenario of the constrains. The geometrical optimization step is completed until the 0.002 Ha/Å maximum force and the 0.005 Å maximum displacement thresholds are reached. The k-point sampling is 4 x 4 x 1 for the properties calculation. The potential energy surface (PES) on the perovskite surface is obtained by moving the adsorbates along the vertical axis in the unit cell. In the process of simulating the PES, the surface atoms and the internal degrees of freedom of the adsorbates are not allowed to relax. Since no relaxation effects are allowed, these energies are only estimates to the real scenario. The density of states spectra, charge density difference and the orbital distributions are calculated based on the optimized structures. Kramers-Kronig transformation is employed to obtain the UV-vis light absorption spectra from dielectric constants.40 The Li2Sn species investigated in this study are simplified models representing the species generated at various lithiation stages (Figure 2). Unless otherwise specified, the Li2Sn species include S8, Li2S8, Li2S6, Li2S4, Li2S2 and Li2S in this manuscript.


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In total, 20 systems are optimized, including twelve lithium polysulfide/perovskite hybrid system based on the PbI-terminated perovskite surface and the Cation-terminated perovskite surface (S8/PbI, Li2S8/PbI, Li2S6/PbI, Li2S4/PbI, Li2S2/PbI, Li2S/PbI, S8/Cation, Li2S8/Cation, Li2S6/Cation, Li2S4/Cation, Li2S2/Cation and Li2S/Cation), two bare perovskite surfaces with different terminations (bare PbI surface and bare Cation surface) and six lithium polysulfide species in free form (S8, Li2S8, Li2S6, Li2S4, Li2S2, Li2S).

Figure 1 The side view of the bare PbI surface exposing the PbI-rich layer (left) and the bare Cation surface exposing the methylammonium cation (right).

Figure 2 Simplified lithium sulfide models at various lithiation stages. From left to right: S8, Li2S8, Li2S6, Li2S4, Li2S2, Li2S. Yellow: sulfur. Purple: lithium.

Results and Discussion The Li2Sn species can reside stably on both PbI- and Cation-terminated perovskite surfaces and the optimized interfacial structures are depicted in Figure 3. The sulfur atoms in the lithium polysulfide adsorbates tend to approach the under-coordinated lead atom at the perovskite surface (Figure S1), while the lithium atoms in the lithium polysulfide adsorbates lean towards iodine atoms at the perovskite surface. The adsorption geometries of the lithium polysulfide species depend on the perovskite surface ACS Paragon Plus Environment

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terminations, with the PbI-terminated perovskite surface and the Cation-terminated perovskite surface presenting dissimilar adsorption structures (Figure 3).

Figure 3 Optimized interfacial structures of the lithium polysulfide/perovskite systems. Top: the systems based on the PbI-terminated perovskite surface. Bottom: the system based on the Cation-terminated perovskite surface. From left to right: S8, Li2S8, Li2S6, Li2S4, Li2S2, Li2S. Table 1 lists the lengths of the S-S and S-Li bonds in the lithium polysulfide as the free form, and their bond lengths when these species are adsorbed onto the PbI-terminated perovskite surface and the Cation-terminated perovskite surface. The S-Li bond lengths in the lithium polysulfide species increase when the lithium polysulfide molecules are adsorbed on the perovskite surface, especially in the case of the PbI-terminated perovskite surface. For instance, the S-Li bond length increases by 0.3 Å (compare 2.72 Å with 2.42 Å) in the Li2S8/PbI case compared with the free molecule, while the S-Li bond length increases by 0.1 Å (compare 2.18 Å with 2.08 Å) in the Li2S/cation case compared with the free molecule. Nevertheless, the increase in the S-Li bond length is not significant and the structural integrity of the adsorbate is maintained upon adsorption. Also, only slight changes in the S-S bond length in the lithium polysulfide species can be observed upon adsorption in both terminations. For example, the S-S bond lengths in Li2S4 decreases by ca. 0.03 Å in the Li2S4/PbI system compared with the free ACS Paragon Plus Environment

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form, and the S-S bond lengths in Li2S2 decreases by ca. 0.08 Å in the Li2S2/PbI system. In addition, the S-S bond lengths in all the lithium polysulfide species present negligible variations when they are adsorbed onto the Cation-terminated perovskite system. Ideally, the bond length of the lithium polysulfide should not increase/decrease significantly: if the S-Li bond lengths in the lithium polysulfide increase significantly, there should be a higher possibility of lithium polysulfide decomposition, which is undesirable for the lithium sulfur battery. In our case, the increases in the bond lengths of the S-S and S-Li in the lithium polysulfide species upon adsorption are comparable with the reported optimal value in the lithium sulfur battery literature.41 Table 1 The S-S and S-Li bond lengths in the adsorbates for the free lithium polysulfide molecules, the lithium polysulfide/PbI systems and the lithium polysulfide/Cation systems.

S-S

S-Li

S8 Li2S8 Li2S6 Li2S4 Li2S2 Li2S8 Li2S6 Li2S4 Li2S2 Li2S

Free Molecule (Å) 2.16 2.11 2.11 2.13 2.23 2.42 2.41 2.39 2.25 2.08

PbI Surface (Å) 2.14 2.11 2.11 2.10 2.15 2.72 2.94 3.29 2.51 2.42

Cation Surface (Å) 2.14 2.11 2.11 2.12 2.22 2.42 2.40 2.42 2.27 2.18

The surface structures of the perovskite substrate are modified upon the lithium polysulfide adsorption. The variations in the bond lengths and angles in the substrate are dependent on the perovskite surface termination, and uppermost layer in the perovskite surfaces are more prone to the adsorbate effects. For the PbI-terminated perovskite surface, the average Pb-I-Pb angles and I-Pb-I angles of the uppermost layer decreases upon lithium polysulfide adsorption while the average of the Pb-I bond lengths increase upon lithium polysulfide adsorption (Table 2). For example, the Pb-I-Pb angles of the uppermost layer in the PbI-terminated perovskite surface decrease by 10°-18° upon the adsorption of Li2S8, Li2S6, Li2S4, Li2S2 and Li2S. The only exception is the adsorption of S8, where the Pb-I-Pb angle, I-Pb-I angle


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and Pb-I bond length in the S8/PbI system exhibit negligible structural changes, which is ascribed to the relatively weak van der Waals effects. For the Cation-terminated perovskite surface, the modifications of the average of Pb-I-Pb angles, I-Pb-I angles and Pb-I bond lengths are marginal. Regarding the bond lengths and angles in the vertical directions (c-axis), the angles of the uppermost Pb-I-Pb along the vertical c-axis in the PbI-terminated perovskite surface decrease upon molecular adsorption, while the I-Pb-I angles and Pb-I bond length along the vertical c-axis in the uppermost layer of the Cation-terminated perovskite surface exhibit negligible variations upon molecular adsorption (Figure S2). In addition, the PbI-terminated perovskite surfaces exhibit larger sum of the squared deviations from the average values of bond length and angles upon the lithium polysulfide adsorption compared with the Cation-terminated perovskite surface (Table S2). Table 2 The average values of the Pb-I-Pb, I-Pb-I angles and PbI bond lengths of the uppermost layer in the perovskite surfaces for all the hybrid systems.

bare PbI surface S8/PbI Li2S8/PbI Li2S6/PbI Li2S4/PbI Li2S2/PbI Li2S/PbI bare Cation surface S8/Cation Li2S8/Cation Li2S6/Cation Li2S4/Cation Li2S2/Cation Li2S/Cation

Pb-I-Pb (°) 148 148 137 136 138 136 130 150 148 147 148 145 147 154

I-Pb-I (°) 171 170 169 169 166 165 155 176 178 175 174 171 174 175

Pb-I (Å) 3.24 3.24 3.37 3.38 3.36 3.37 3.51 3.22 3.23 3.25 3.24 3.27 3.25 3.19

The structural changes in the perovskite substrate can be quantified by the rotation angles θ and φ in the surface methylammonium cations upon lithium polysulfide adsorption (Table 3). For both perovskite surfaces investigated in this study, the lithium polysulfide adsorption induces methylammonium cation rotation. Since the position and direction of the methylammonium cation is associated with the surface


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polarization properties,35,42,43 the interaction between the lithium polysulfide molecules and the perovskite surfaces might lead to the variation in the perovskite surface polarization properties. Table 3 The rotation of the cation in the PbI- and Cation-terminated perovskite surfaces upon lithium polysulfide adsorption in terms of θ and φ. Two different methylammonium cation molecules (Cation1 and Cation2) are listed.

S8/PbI Li2S8/PbI Li2S6/PbI Li2S4/PbI Li2S2/PbI Li2S/PbI S8/Cation Li2S8/Cation Li2S6/Cation Li2S4/Cation Li2S2/Cation Li2S/Cation

Cation1 Δθ -11.4 -19.0 -13.5 -20.7 -29.0 -6.1 -1.4 -8.9 -2.3 31.8 -8.6 98.8

Δϕ 0.8 2.5 -2.5 2.2 15.9 2.1 -1.6 5.7 14.7 42.6 7.4 10.1

Cation2 Δθ 0.1 -16.1 -2.2 -8.2 -1.7 -5.1 -1.1 5.7 -2.5 2.0 -6.3 -0.5

Δϕ -2.3 4.1 -4.7 4.7 5.3 -1.7 0.7 -1.5 -5.7 -13.2 -0.6 -16.1

Most of the lithium polysulfide molecules investigated in this study bind onto the PbI-terminated perovskite surface strongly, with the binding energies in the range from 2.99 eV to 3.99 eV (Table 4). The Li2S specie binds to the PbI-terminated perovskite surface the most strongly (3.99 eV). In contrast, the S8 molecule has a relatively weak binding energy at 0.76 eV with the PbI-terminated perovskite surface compared with other species. The binding energies between the lithium polysulfide molecules and the PbI-terminated perovskite surface are superior to those based on the Cation-terminated perovskite surface: the energy values for the lithium polysulfide/Cation systems are from 0.61 eV to 2.22 eV. Nevertheless, these energies values are non-negligible and are comparable with those with the benchmark anchoring materials in the cathode of the lithium sulfur batteries, suggesting the potential application of the lead halide perovskite material as a suitable anchoring material in lithium sulfur batteries to immobilize the lithium polysulfide molecules and prevent lithium polysulfide dissolution.41 Table 4 Binding energies of various lithium polysulfide/perovskite systems. ACS Paragon Plus Environment

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S8/PbI Li2S8/PbI Li2S6/PbI Li2S4/PbI Li2S2/PbI Li2S/PbI S8/Cation Li2S8/Cation Li2S6/Cation Li2S4/Cation Li2S2/Cation Li2S/Cation

Binding Energy (eV) 0.76 3.11 2.79 3.00 3.25 3.99 0.61 1.66 1.33 1.32 1.18 2.22

In order to fully understand the potential energy surface of the lithium polysulfide/perovskite systems, a series of energies of the lithium polysulfide/perovskite systems are calculated by moving the adsorbate along the vertical c-axis of the unit cell (Figure 4). The energy of the most stable position is set to 0 eV; the negative value of the reaction coordinate refers to moving the adsorbate toward the perovskite surface, while the positive value of the reaction coordinate refers to moving the adsorbate away from the perovskite surface. When the adsorbate is approaching the perovskite surface, the relative energy rises rapidly, with the energy of the system based on the PbI-terminated perovskite surface growing faster than that of the system based on the Cation-terminated perovskite surface. For the example, the relative energies are 10.5 eV and 24 eV for the S8/PbI and Li2S4/PbI systems at the -0.5 Å reaction coordinate, while the relative energies are 5 eV and 6 eV for the S8/Cation and Li2S4/Cation systems at the same reaction coordinate (Figure S2). The PbI-terminated perovskite surface presents larger repellence with the approaching lithium polysulfide since the methylammonium cations in the Cationterminated perovskite surface is less constrained and free to rotate, which alleviate the steric effects induced by the adsorbates. When the adsorbates are moving further away from the stable position, the energies of the PbI systems based on the Li2S8, Li2S6, Li2S4, Li2S2, Li2S increases by ca. 3 eV at the 3.5 Å reaction coordinate (Figure S2); in contrast, the energies of the Cation systems based on the Li2S8, Li2S6, Li2S4 and Li2S2 increase by ca. 1 eV. Slightly larger energies can be observed for the Li2S/PbI ACS Paragon Plus Environment

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and Li2S/Cation systems when the molecules are moving away, leading to a 3 eV ~ 5 eV increase at the 3.5 Å reaction coordinate, probably due to the larger columbic interactions between the lithium atoms in the Li2S and the perovskite surfaces. The S8 adsorbate mainly interacts with the perovskite surfaces via the weak van der Waals interactions; therefore the increase in the energies of the S8/PbI and S8/Cation systems is marginal when the adsorbate is moving toward/away from the surface.

Figure 4 Relative energies of S8/PbI, Li2S8/PbI, Li2S/PbI, S8/Cation, Li2S8/Cation and Li2S/Cation systems with the adsorbate moving along the vertical c-axis. 0 Å refer to the most stable position. The relative energies for other polysulfide/perovskite systems are described in Figure S2. The PDOS plots of the hybrid systems based on the PbI-terminated perovskite surface (Figure 5) demonstrate that the top of the valence band is mainly contributed by the iodine atoms, while the bottom of the conduction band is predominantly contributed by the lead atoms. In addition, the contribution from the sulfur atoms is present at the top of the valence band from -1 eV to 0 eV, and the conduction band from 1.5 eV to 2.5 eV. The band gaps of these systems are ca. 1.5 eV. Regarding the hybrid systems based on the Cation-terminated perovskite surface, the band gaps of the adsorbate/adsorbent hybrid systems demonstrate slightly smaller band gaps ranging from 1 eV to 1.4 eV (Figure S3). In the case of the Li2S2/Cation system, there are defect states contributed by the sulfur atoms in the middle of the band ACS Paragon Plus Environment

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gap (Figure S3), indicating the electronic properties of the lithium polysulfide/perovskite systems depend on perovskite surface terminations. To sum up, charge transfer characteristics in the hybrid lithium polysulfide/perovskite systems can be observed from the PDOS spectra, and the electronic characteristics of the hybrid systems depend on the perovskite surface terminations.

Figure 5 PDOS of the hybrid systems based on the PbI-terminated perovskite surface, including: S8/PbI, Li2S8/PbI, Li2S6/PbI, Li2S4/PbI, Li2S2/PbI and Li2S/PbI. Total: total density of states. S: PDOS from sulfur. Li: PDOS from lithium. Pb: PDOS from lead. I: PDOS from iodine. MA: PDOS from the methylammonium cation. The spatial orbital distributions of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMO) of all the hybrid systems (Figure 6) indicate that the HOMOs are essentially I-2p orbitals that are present near the iodine atoms; the LUMOs are essentially Pb-2p orbitals that are present near the lead atoms. In addition, the lithium polysulfide adsorbates also contribute to the LUMOs in the lithium polysulfide/perovskite hybrid systems, which agrees with the density of states plot (Figure S3). In general, the charge transfer characters of all the lithium


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polysulfide/perovskite hybrid systems revealed by the HOMO-LUMO orbital distributions are consistent with the PDOS spectra (Figure 5 and Figure S3).

Figure 6 The HOMO-LUMO orbital distributions for all the lithium polysulfide/perovskite systems. The HOMO (left) and LUMO (right) spatial distributions are shown for each species. The isovalue is 0.02. The charge density difference plots (Figure 7) confirm that the charge transfer characters exist for all the hybrid systems. The charge density differences are mainly concentrated in the uppermost surface layer. For the lithium polysulfide/PbI systems, the increase in the charge density localizes in the region near the surface lead atoms, and partially near the molecular adsorbate. Regarding the lithium polysulfide/Cation systems, the increase in the charge density can be observed in the region near the surface iodine atoms and the adsorbates. The charge transfer character is weak in the S8/PbI and S8/Cation systems, due to the weak van der Waals adsorbate/adsorbent interactions in the S8-based interfacial systems


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Figure 7 The charge density difference plots of various lithium polysulfide/perovskite systems. Blue and yellow regions refer to an increase and a decrease of electronic density, respectively. The isovalue is 0.015. The Hirshfeld charges in the lithium polysulfide adsorbates of the hybrid systems generally become more negative upon adsorption on to the halide perovskite surfaces (Table 5), indicating the electron accumulation upon lithium polysulfide adsorption. The only exception is the hybrid systems with the S8, which exhibit no Hirshfeld charge changes or even electron depletion upon adsorption on the perovskite surface due to the weak van der Waals effects. Nevertheless, in general, the Hirshfeld charges together with the HOMO-LUMO orbital distributions and the PDOS plots, suggest the existence of the interfacial electron transfer states when the lithium polysulfide is immobilized onto the halide perovskite surfaces. These charge transfer characteristics stabilize the adsorbate/adsorbent structures based on the lithium polysulfide species and the halide perovskite material, such that the lithium polysulfide species tend to reside stably onto the halide perovskite surfaces,44–46 which might mitigate the lithium polysulfide dissolution problem which is the major bottleneck in the lithium sulfur batteries.


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Table 5 The changes in Hirshfeld charge in the lithium polysulfide species upon adsorption onto the halide perovskite surface. The negative sign suggests that the electrons accumulate in the adsorbates upon adsorption, while the positive sign indicate electron depletion upon adsorption.

PbI Cation

S8 0.01 0.11

Li2S8 -0.09 -0.07

Li2S6 -0.10 -0.10

Li2S4 -0.23 -0.06

Li2S2 -0.05 -0.12

Li2S -0.16 -0.18

The UV-vis absorption properties are essential factors of solar energy conversion devices and the UVvis absorption spectra for all the lithium polysulfide/perovskite systems and the bare halide perovskite surfaces are presented in Figure 8. The UV-vis light absorption spectra of the lithium polysulfide/perovskite systems exhibit small variations compared with the bare halide perovskite surfaces. Nevertheless, the adsorption of the polysulfide molecules leads to inferior light absorption intensities between 500 nm and 600 nm compared with those of the bare perovskite surfaces; the light absorption intensities between 600 nm and 700 nm of the lithium polysulfide/perovskite systems are equivalent or superior to those of the bare perovskite surfaces. In other word, the UV-vis light absorption performances of the hybrid systems based on the lithium polysulfide species are comparable with the light sensitive untreated halide perovskites, and the adsorption of the lithium polysulfide species bring out minimal deterioration in the light absorbing capabilities of the halide perovskite materials.


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Figure 8 UV-vis light absorption spectra of various hybrid systems based on the PbI-terminated perovskite surface (left) and the Cation-terminated perovskite surface (right). Bare PbI: the bare perovskite PbI-terminated surface. Bare Cation: the bare perovskite Cation-terminated surface The lithium polysulfide molecules should be kept with minimal mobilization and the destruction of these species should be prevented to maintain the stability of the lithium sulfur battery. In order to understand the destruction behavior of these lithium polysulfide species in the presence of the halide perovskite surface, the Li2S4 species is selected as a case study and the energies of the destructed states Li+LiS4 and Li+Li+S4 relative to the energy of the intact state Li2S4 are listed (Table 6). For the hybrid systems based on the PbI-terminated perovskite surface, the destructed states Li+LiS4 and Li+Li+S4 are associated with higher energy values compared with Li2S4, indicating that the lithium polysulfide decomposition is difficult in the presence of the PbI-terminated perovskite surface. The Cationterminated perovskite surface exhibit a slight different behavior (the energies of the destructed states of the hybrid systems based on the Cation-terminated perovskite surface are smaller than those of the intact counterpart); nevertheless, the energy difference is small and the stabilities of the hybrid systems based on the Cation-terminated perovskite surface are similar. Considering the fact that these polysulfide species predominantly reside on the PbI-terminated perovskite surface evidenced by larger binding energies between the adsorbates and the PbI-terminated perovskite surface, it is concluded that the methylammonium iodide perovskite present minimal decomposition in the polysulfide species. Therefore, the methylammonium iodide perovskite could act as a potential anchoring material that immobilizes Li2Sn species, achieving long-term cycling stability and high-rate performance in the lithium sulfur battery. The superior light absorbing and solar energy conversion properties of the halide perovskite materials can potentially be integrated into the lithium sulfur batteries to achieve an outstanding integrated device that combines excellent energy conversion/ storage behaviors in a single department.


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Table 6 The energy values of the destructed states Li+LiS4 and Li+Li+S4 relative to those of the intact state Li2S4 on the PbI-terminated perovskite surface and the Cation-terminated perovskite surface. The values are the energy differences between the destructed states and the intact states. The positive sign represents that the energy of the destructed state is higher than the intact state and vice versa Li+LiS4 PbI (eV) 0.19 Cation (eV) -0.26

Li+Li+S4 1.23 -0.12

Conclusions The ab-initio calculations on the polysulfide/perovskite hybrid systems demonstrate that large interactions exist between the methylammonium lead iodide surfaces and the polysulfide species including S8, Li2S8, Li2S6, Li2S4, Li2S2 and Li2S. The halide perovskite material can immobilize the polysulfide species, serving as a suitable anchoring material to enhance the stability of the lithium sulfur battery, once the methylammonium lead iodide is introduced into the cathode of the battery. The lithium polysulfide moieties exhibits structural integrity when they are adsorbed onto the perovskite surfaces. The stabilization of the polysulfide/perovskite system is helped by the charge transfer characters between the adsorbate and the adsorbent. The binding energies between the PbI-terminated perovskite surface and the lithium polysulfide moieties are larger than those between the Cation-terminated perovskite surface and the lithium polysulfide moieties; therefore, it is suggested that the lithium polysulfide moieties can predominantly reside on the PbI-terminated perovskite surface. In addition, the superior light absorption properties of the halide perovskite material are maintained when the lithium polysulfide species are adsorbed onto the perovskite surfaces. The halide perovskite is demonstrated to be a potential suitable anchoring material in the sulfur cathode of a lithium sulfur battery to mitigate the sulfur dissolution problem and enhance the device stability. We believe that this theoretical work will pave a new avenue for the integrated device combining the perovskite solar cell and the lithium sulfur battery into a single cell. Such single-unit device is superior in capturing solar energy owing to the presence of ACS Paragon Plus Environment

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the halide perovskite moiety, and exhibiting a large specific energy, low cost and low toxicity owing to the sulfur materials.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 51702165), the Jiangsu province “Double Plan” project (R2016SCB02), and the Jiangsu Provincial Natural Science Foundation (Grant No. BK20160942 and BK20160941). The authors acknowledge computational support from NSCCSZ Shenzhen, China.

ASSOCIATED CONTENT Details of top view of S8/PbI, the average values of Pb-I-Pb, I-Pb-I angles and PbI bond lengths in the hybrid systems along the vertical c-axis of the outermost layer in the perovskite surfaces, the mean values and the sum of the squared deviations from the sample mean of the Pb-I-Pb, I-Pb-I angles and the PbI bond lengths of the outermost layer in the PbI- and Cation-terminated perovskite surfaces with and without lithium polysulfide adsorption, relative potential energies of S8/PbI, Li2S8/PbI, Li2S/PbI, S8/Cation, Li2S8/Cation and Li2S/Cation systems with the adsorbate moving along the vertical c-axis, PDOS of the hybrid systems based on the Cation-terminated perovskite surface, including: S8/Cation, Li2S8/Cation, Li2S6/Cation, Li2S4/Cation, Li2S2/Cation and Li2S/Cation are deposited in the Supporting Information.

AUTHOR INFORMATION Corresponding Author


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*Email: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Understanding Interactions between Lead Iodide Perovskite Surfaces

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