Intercalated Chevrel Phase Mo6S8 as a Janus Material for Energy

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Intercalated Chevrel Phase MoS as a Janus Material for Energy Generation and Storage Michael L. Agiorgousis, Yi-Yang Sun, Damien J. West, and ShengBai Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00092 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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ACS Applied Energy Materials

Intercalated Chevrel Phase Mo6S8 as a Janus Material for Energy Generation and Storage Michael L. Agiorgousis,† Yi-Yang Sun,‡,* Damien West, † and Shengbai Zhang†,* †

Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute,

Troy, New York 12180, USA ‡

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Y.-Y. Sun), [email protected] (S. B. Zhang)

ACS Paragon Plus Environment

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ABSTRACT. An approach to designing solar absorber materials based on intercalation-induced metal-to-semiconductor transition has been proposed. Using hybrid density functional calculation, Al-intercalated Chevrel phase Mo6S8 with end product Al4/3Mo6S8 is predicted to be a semiconductor with a 1.18 eV indirect band gap. Compared with GaAs and Si, Al4/3Mo6S8 shows significantly higher optical absorption over the solar spectrum as a result of the parallel bands across the band gap with direct transitions around 1.35 eV, nearly ideal for an absorber material in a solar cell. Chevrel phase Mo6S8 has recently been explored as a cathode material in rechargeable Al-ion batteries. Thus, Al4/3Mo6S8 is a potential dual-function (or Janus) material for both energy generation and storage. Possible device structure utilizing this unique property has been proposed.

KEYWORDS. Solar energy, Secondary batteries, Chevrel phases, Intercalation, Janus materials, Chalcogenides 1. Introduction Conversion of solar energy to electrical energy through the photovoltaic (PV) effect is one of the most promising technologies for renewable energy generation. The intermittent nature of solar energy, however, poses a great challenge to traditional power grid, which usually lacks the storage capacity to balance the peaks and valleys of energy demand and supply. One possible design is to combine a solar cell with an electrochemical battery in a monolithic device that works as a solar cell during the daytime, charging the battery in excess of demand, and as a battery during the night, utilizing the stored solar energy. Combining the two devices offers the possibility of a simplified grid design wherein the energy management can be mostly realized locally. Also, a monolithic device could potentially be more power efficient and cost effective.


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While significant progress has been made in both PV and battery technologies, which are mainly based on Si solar cells and Li-ion batteries (LIBs), respectively, the developments on the two technologies so far have been independent with little crossover. There have been few materials that possess the unique properties to be used in the monolithic device for both electricity generation and storage. Mature PV absorber materials usually lack the electrochemical properties to be used as an electrode material in a secondary battery. The most successful absorbers to date are Si, GaAs, CdTe, and Cu(In,Ga)Se2.1 All of these materials have closepacked tetrahedral coordination between their constituent atoms, which lack diffusion channels for intercalants as required by a cyclable electrode material. Si has been explored as an anode material in LIBs, but large volume expansion upon Li intercalation (> 300%) has prevented commercialization.2 On the other hand, secondary battery materials usually lack the optical and electronic properties to be used as a solar absorber. The most common cathode materials used in LIBs are metal oxides such as CoO2, LiFePO4, and Mn2O4. The oxide materials, however, usually possess band gaps that are too large for effective PV operation.3,4 A sulfur based cathode material would yield a smaller band gap than commonly used oxides. CuS is a sulfur based cathode that has been used in Li-ion batteries. LiCuS, an intermediate discharge product of a Li-CuS secondary battery, is a semiconductor with a 1.95 eV direct band gap which is suitable for an absorber material in a solar cell.5 Another interesting class of sulfide materials is Chevrel phase Mo6S8,6–8 which has been studied as an alternative cathode material for Li+,9–11 Na+,12–14 Mg2+,15–21 Zn2+,13,22–24 Ca2+,25,26 and recently Al3+ ions.

27,28

Al-ion batteries

offer a promising alternative energy storage technology due to the high specific capacity, second only to Li, and superior volumetric capacity of Al anode,29 as well as the chemical stability and earth abundance of Al metal. The PV related properties of the intercalated Chevrel phases,


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especially their optical properties, have so far been much less known as they are usually noncritical for battery operations. In this paper, using first-principles calculations the structural, thermodynamic, kinetic, electronic, and optical properties of Mo6S8 during Al intercalation are studied. The chemical formula of the material at full intercalation is determined to be Al4/3Mo6S8. At full intercalation the cathode undergoes a metal-to-semiconductor transition. Even though Al4/3Mo6S8 is found to have an indirect gap, which is about 1.18 eV, its optical absorption is superior to GaAs and Si within the solar spectrum. In such a case, the indirect gap has the advantage that radiative recombination can be significantly quenched. The kinetic barrier for Al diffusion in Mo6S8 is predicted to be 0.33 eV. Given the capability of working as a cathode material for an Al-ion secondary battery,27,28 Al4/3Mo6S8 represents a promising dual-function (or Janus) material that can be used for both energy generation and energy storage. This finding also points to an effective approach to designing high-performance Janus materials in future works. 2. Computational details Our first-principles calculations were based on the density functional theory (DFT) implemented in the VASP program30 using a planewave basis set. The ionic cores were represented by the projector augmented wave (PAW) potentials.31 Structural optimizations during the intercalation process were carried out with the PBEsol functional32 and a kinetic energy cutoff of 30 Ry for the planewave basis set. A 3×3×3 Γ-centered k-point grid was used to sample the Brillouin zone of the hexagonal conventional cell. The lattice parameters and atomic positions were fully relaxed until the force on each atom was smaller than 0.5 mRy/Bohr. The diffusion barriers were calculated by using the climbing-image nudged elastic band (NEB) method and the code developed by Henkelman’s group.33 In NEB calculations, a smaller cutoff


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energy of 22.5 Ry and a larger force criterion of 1 mRy/Bohr were used. We used eight intermediate images between two end configurations. If metastable configurations were found in between, the path was separated into two sub-paths, until no more metastable configuration was found. To accurately predict the electronic and optical properties we employed the HeydScuseria-Ernzerof (HSE) hybrid functional.34,35 In HSE calculations, the kinetic energy cutoff was reduced to 22.5 Ry.

Figure 1. (a) A 2D view of atomic structure of Chevrel phase Mo6S8 highlighting the framework formed by 3D-connected Mo6S8 clusters. The location of intercalation Sites 1 and 2 are represented by red and blue balls, respectively. (b) Enlarged view of the structure of a Mo6S8 cluster. (c) A 3D view of the framework formed by Mo6S8 clusters. A simple model of the framework structure can be viewed as a nearly simple cubic cell with a Mo6S8 cluster sitting at each corner. Intercalation Site 1 possesses six symmetrically equivalent positions at the center of the cube. Intercalation Site 2 also has six symmetrically equivalent positions, though they are located on each face of the cube. Each face has a pair of Site 2 positions corresponding to the


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current Mo6S8 cube and adjacent cube. The occupied positions (for illustration purposes) are indicated by darker coloring. 3. Results and discussion Figure 1a shows the crystal structure of the Chevrel phase Mo6S8, which is a framework structure composed of Mo6S8 clusters linked three-dimensionally by Mo-S bonds. Figure 1b highlights the structure of each cluster, which can be viewed as a face-centered cube with Mo atoms occupying the faces and S atoms occupying the corners. Two distinct intercalation sites are shown in Figure 1a and they can be differentiated by observing the structure as an approximately simple cubic cell with a Mo6S8 cluster occupying each corner, as shown in Figure 1c. Site 1 consists of six symmetrically equivalent positions forming a hexagon around the center of the cell. Site 2 also consists of six equivalent positions, but is closer to the faces of the cell than Site 1. For Site 1, only one of the equivalent positions can be occupied by an intercalant atom, while for Site 2, up to three of the equivalent positions within a cell can be occupied, as highlighted in Figure 1c. It is noted that not all intercalants can occupy Site 1 and Site 2 locations in the framework structure. Large intercalants such as Pb with an ionic radius of 1.19 Å36 can only intercalate into Site 1 positions. In contrast, Al is a small ion with an ionic radius of 0.48 Å and as a result can occupy both Site 1 and Site 2 positions. The hexagonal conventional cell containing three formula units of Mo6S8 was used in our calculations. Twelve possible intercalation sites for Al are present in the cell, three of Site 1 and 9 of Site 2.


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Figure 2. Calculated diffusion path and barriers for an Al atom in Mo6S8. Three different steps are involved in the diffusion: from Site 1 to Site 1 (labeled as 1-1), from Site 1 to Site 2 (1-2), and from Site 2 to Site 2 (2-2). The diffusion kinetics of Al intercalated Mo6S8 were investigated using NEB methods. Given the possible intercalation sites shown in Figure 1, the transport of an Al atom involves three steps: 1) from a Site 1 to another Site 1 within the hexagon close to the cavity center, 2) from a Site 1 to a neighboring Site 2 and 3) from this Site 2 to its neighboring Site 2. From this second Site 2 the Al atom can hop to a neighboring Site 1 to form a closed path. The calculated diffusion barriers for the three steps are shown in Figure 2. Hopping within the hexagon of Site 1 has a small barrier of 0.08 eV, while hopping from a Site-1 to a neighboring Site 2 needs to overcome a barrier of 0.25 eV. As shown in Figure 2, from a Site 2 to another Site 2 in an adjacent cavity, the barrier is 0.24 eV. The overall barrier for an Al atom to diffuse from a Site 1 to another Site 1 in a neighboring cell, i.e, from energy zero to the highest transition state in Step 2-2, is about 0.33 eV, which is lower than the theoretically calculated barrier of above 0.5 eV for Mg intercalated Mo6S8.37,38 We estimated the diffusivity according to


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= exp (− / ) where is the vibrational frequency of atoms at Site 1 or Site 2, a0 is the lattice constant of the rhombohedral primitive cell (6.43 Å), EA is the diffusion barrier (0.33 eV), and kB is the Boltzmann constant. We calculated the frequencies of Al at Site-1 and Site-2 by fixing all atoms in the supercell except for the Al atom and the neighboring S atoms. The highest frequency was found to be 12 and 13 THz at Site 1 and Site 2, respectively. At = 300 , the obtained diffusivity is D≈1.6×10–7 cm2/s which is comparable to Li diffusion in graphite along the basal plane.39 We studied the thermodynamic stability of Al intercalation at different Al concentrations in the Mo6S8 host lattice. This was completed by adding Al atoms to intercalation sites throughout the lattice, one at a time. The sampling accuracy for the composition parameter x as in AlxMo6S8 is limited by the chosen supercell size. Here, each additional Al atom in our calculation corresponds to an increase of x by 1/3. The intercalation energy for the addition of each Al atom was calculated by Eint =

E (Al x2 Mo 6S8 ) − E (Al x1 Mo 6S8 ) − ( x2 − x1 ) E (Al) x2 − x1

where E(AlxMo6S8) and E(Al) are the total energy per formula unit of AlxMo6S8 and per Al atom in bulk phase, respectively. A negative value for Eint means that the intercalation is exothermic. Figure 3a shows the calculated intercalation energy as a function of composition parameter x. For x between 0 and 1, we considered the intercalation at both Site 1 and Site 2. It can be seen that, adding Al to Site 2 yields higher intercalation energy than Site 1 suggesting that Site 1 is more stable than Site 2. There is a turning point in the curve from 1 to 4/3, which is because Site 1 is fully occupied and Al atoms start to occupy Site 2 for x greater than 1. It is worth noting that in the case of x=4/3, even though the occupation of Site 2 shows an up-turn of the intercalation


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energy, it is still an exothermic process with an intercalation energy of about –0.46 eV per Al atom. An interesting finding at x=4/3 is that, upon insertion of the fourth Al in the hexagonal convention cell, the Site-1 Al atom that was closest to the added Site-2 Al atom relaxed to another Site-2 location resulting in equal occupation between Site 1 and Site 2 in the hexagonal conventional cell (i.e., two at Site 1 and two at Site 2). Starting from x = 5/3, the intercalation energy becomes positive suggesting thermodynamic instability even though geometrically there are still unoccupied Site 2 in the structure. Experimentally, two voltage stages have been observed in Chevrel phase intercalation,11,13,40 which were attributed to the occupation of Site 1 and Site 2.

Figure 3. Calculated intercalation energy (a) and lattice constants (b) as a function of the composition parameter x in AlxMo6S8. The intercalation energies of Al occupying Site 2 for x=1/3 to x=1 were shown in the plot for a comparison purpose. At lower x, Site-1 positions are more favorable to be filled compared with Site 2. (c) Density of states (DOS) of AlxMo6S8 with varying x. The vertical dashed line indicates the Fermi energy.


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Figure 3b shows the calculated lattice constants a and c for the hexagonal conventional cell as a function of x. Experimental results are available for pristine Mo6S8 (a= 9.193 Å, c= 10.856 Å), AlMo6S8 (a= 9.582 Å, c= 10.027 Å), and Al4/3Mo6S8 (a= 9.635 Å, c= 10.005 Å) which show excellent agreement with theoretical predictions.27,28 Significant increase in a and decrease in c are predicted at concentrations higher than x = 4/3 and such deviations were not observed experimentally. Thus, combined with the intercalation energy results in Figure 3a, we conclude that the thermodynamically stable end composition after Al intercalation is Al4/3Mo6S8, on which there have been different reports in previous experimental works.27,28 The end product composition can be explained by the electronic structure of Mo6S8. Figure 3c shows the density of states (DOS) of AlxMo6S8 with varying x. It can be seen that pristine Mo6S8 is electron deficient such that the Fermi level is below the band gap by about 1 eV. The number of empty states between the Fermi level and the gap is able to accommodate 12 electrons as judged by an integration. Intercalation from x = 0 to 4/3 gradually fills these empty states. While the band gap value varies with x, a well-defined band gap is maintained as can be seen from the DOS plots. At x = 4/3, each conventional cell contains four Al atoms. By contributing three valence electrons per Al atom, a semiconductor is obtained. Further intercalation would involve the occupation of the high-energy states above the band gap, which will increase the total energy and make further intercalation unstable. It is well known that standard DFT underestimates semiconductor band gaps by roughly 50%. Here we employ the HSE hybrid functional, a screened-exchange hybrid functional,34,35 to correct the DFT band gap error. The HSE functional was found to yield an indirect gap of 1.18 eV for Al4/3Mo6S8 and a direct gap of 1.35 eV, which is the ideal band gap for single-junction solar cells according to the Shockley-Quiesser limit.41


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Figure 4. (a) Imaginary part of the dielectric constant (ε2) of Al4/3Mo6S8, compared with Zn2Mo6S8, GaAs, and Si. Arrows are used to indicate calculated band gaps, where unfilled arrows are used for indirect gaps and solid arrows are used for direct gaps. (b) Band structure (left) and DOS (right) of Al4/3Mo6S8. Top valence band and bottom conduction band are highlighted to show the parallel band dispersion. Results shown in both (a) and (b) are obtained using HSE functional. Another important property for solar-cell materials is optical absorption. Figure 4a shows the calculated imaginary part of the dielectric constant (ε2), which determines optical absorption properties of semiconductor materials. Results for high efficiency solar-cell materials GaAs and Si are included in Figure 4a for comparison. Al4/3Mo6S8 shows the largest ε2 over most of the solar spectrum. Only at energy values above 3.8 eV do we observe Si overtaking Al4/3Mo6S8. The high absorption of Al4/3Mo6S8 can be explained by the band structure, shown in Figure 4b, where we see that the top valence band and bottom conduction band show the same trend of dispersion, i.e., either both increasing or both decreasing. The conduction and valence band states are dominated by Mo d states, as shown in Figure 4b. The small dispersion of the Mo d states results in relatively large carrier effective masses. However, the parallel bands with direct


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transition energy around 1.35 eV are responsible for the large optical absorption of this material. It is worth noting that the high absorption caused by parallel dispersion of edge bands can only occur in an indirect-gap material. In such a case, photogenerated electrons and holes will quickly relax to conduction band minimum and valence band maximum, respectively, which are located at different k-points. Thus, radiative recombination can be significantly quenched to enhance the quantum efficiency. As stated earlier, Mo6S8 has been studied as a cathode in secondary battery for various intercalant elements, partly due to its framework structure. We also considered the intercalation of other elements to search for other compounds that are suitable for PV application. Figure 5 shows the band gaps of intercalated Mo6S8 at end compositions with various intercalants using PBEsol functional. For singly charged intercalants, such as Li and Na, the end composition M4Mo6S8 corresponds to full occupation of all the 12 intercalant sites in a hexagonal conventional cell.

Figure 5. Calculated band gaps of Chevrel phase end compositions with different intercalant elements. PBEsol functional is used here.


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For doubly charged intercalants, such as Mg, Ca, and Zn, six of the intercalation sites are occupied, three of Site 1 and three of Site 2, resulting in the end compound M2Mo6S8. Ca intercalation into Mo6S8 has only been experimentally realized up to CaMo6S8.25,26 While it may not be possible to intercalate Ca up to Ca2Mo6S8 due to the large ionic radius of Ca (1.00 Å) the material has been theoretically proposed.42,43 Our results are included here for comparison with other divalent intercalated compounds where a metal-to-insulator transition is accomplished. From Figure 5, it can be seen that the band gap decreases as the oxidation state of intercalants increases. This trend could be correlated to the altering of the cubic cluster structure depending on how the intercalants occupy interstitial positions around each cluster.44,45 Another consideration is the ionic radius of the intercalated atom, as they may affect the distortion of the Mo6S8 clusters.46 The Shannon ionic radii for the studied intercalants are Al (0.48 Å) < Mg (0.72 Å) < Zn (0.74 Å) < Li (0.76 Å) < Ca (1.00 Å) < Na (1.02 Å).36 The correlation pattern between the calculated band gap and the ionic radius appears to be subtle. The Li and Na intercalated compounds have similar gaps despite the large size difference between Li and Na, while Zn and Mg have different gaps despite their similar size, valence, and Mo-Mo bond distances of 2.66 Å and 2.62 Å, respectively.16,23 Considering the PBEsol band gap as shown in Figure 5, Zn2Mo6S8 is also a promising material. We also performed HSE calculation on its optical absorption. The calculated ε2 is shown in Figure 4a. Similar to Al4/3Mo6S8, it has large absorption over the solar spectrum though its absorption onset occurs at a higher energy than Al4/3Mo6S8 because of its larger band gaps (1.51 eV indirect gap and 1.67 eV direct gap).


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Figure 6. Schematic of monolithic device based on Janus material Mo6S8 that integrates a solar cell and a secondary battery. Since Al4/3Mo6S8 is a discharge product of an Al-ion battery with favorable optical properties, it can be a Janus energy material that can be used during the day and night. Figure 6 shows a schematic for a monolithic device that integrates a solar cell and a secondary battery. Previous works have integrated Si photovoltaic cells with Li-ion battery in a single device connected by a metal contact.47,48 A possible advantage of the proposed device in Figure 6 is the minimization of ohmic losses due to the absence of an electrical contact as well as the prospect of a smaller device since the generation and storage segments can be sealed in a single apparatus.49,50 The device consists of two circuits, one for solar cell operation and another for battery operation. The battery circuit is the first phase of device operation. During this process the metallic Mo6S8 cathode is electrochemically converted to Al4/3Mo6S8 by intercalating Al ions that diffuse from the Al anode. Once discharge is complete the metal cathode is fully converted to the solar absorber. At this point the battery circuit is opened and the solar cell circuit is closed.


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The absorber Al4/3Mo6S8 is placed below the transparent conducting oxide (TCO) and p-n junction. The junction will be a heterojunction. The choice of the materials forming the p-n junction with Al4/3Mo6S8 requires further study so that the band alignments are suited for carrier separation. Typical junction-forming materials, such as CdS or CdSe in CdTe solar cells51, could be used, although other options are also possible. As light is absorbed, Al3+ ions are deintercalated from the cathode and diffuse back to the Al metal anode. Once the amount of photogenerated energy is below a certain value the solar cell circuit is disconnected and the battery can be discharged. 4. Conclusions In summary, we present a new method of designing PV materials where an electron deficient metallic framework is identified (or designed) first and a semiconducting state is then achieved through intercalation, which induces a metal-to-semiconductor transition. Based on this approach and first-principles calculations, we propose a solar absorber material, Al4/3Mo6S8, which is predicted to be a semiconductor with a 1.18 eV indirect gap and 1.35 eV direct gap. Comparison of optical absorption with PV materials GaAs and Si shows superior absorption over the solar spectrum, which is a result of the parallel bands across the band gap with direct transitions around 1.35 eV. Al4/3Mo6S8 can be obtained by reversibly Al intercalation into the Chevrel phase Mo6S8, which represents the essential process for a rechargeable battery. The unique properties of Mo6S8 could thus be utilized in a monolithic device that integrates an Al-ion battery with a solar cell where Al4/3Mo6S8 serves as a dual-function Janus material for both energy generation and storage. The designing approach is expected to be applied in future works to design higher performance Janus materials. Acknowledgement


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The authors acknowledge support by National Science Foundation (NSF) under Award CBET1510948. Y.Y.S. is also supported by National Natural Science Foundation of China under Grant No. 11774365. The supercomputer time was provided by the National Energy Research Scientific Computing Center (NERSC) under DOE Contract DE-AC02-05CH11231. References (1)

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Intercalated Chevrel Phase Mo6S8 as a Janus Material for Energy

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