Layered TiS2 Positive Electrode for Mg Batteries

Layered TiS2 Positive Electrode for Mg Batteries Xiaoqi Sun, Patrick Bonnick, and Linda F. Nazar* Department of Chemistry and the Waterloo Institute of Nanotechnology, University of Waterloo, Waterloo, ON N2L 3G1, Canada S Supporting Information *

ABSTRACT: Magnesium batteries are a good candidate for high energy storage systems, but the limited discovery of functional positive electrode materials beyond the seminal Chevrel phase (Mo6S8) has slowed their development. Herein, we report on layered TiS2 as a promising positive electrode intercalation material, providing 115 mAh g−1 stabilized capacity in a Mg full cell. Reversible Mg2+ intercalation into the structure is proven by elemental analysis combined with X-ray diffraction studies that elucidate the phase behavior upon cycling. The voltage profiles reveal distinct Mg2+ cation ordering, unlike the solid solution behavior exhibited by Li+. Our findings not only point to the important role of “soft” lattices to facilitate divalent solidstate cation mobility but also now provide an alternative sulfide to serve as a platform for the fundamental understanding of Mg2+ intercalation in lattices.

R

the Mg electrode, preventing further transport of Mg2+ ions.28,29 Another seminal report on TiS2 showed low initial capacity in a Mg cell, followed by a significant capacity drop during the second cycle.30 Although previous computational studies have suggested a high migration barrier to Mg diffusion in layered TiS2 (>1 eV),31 this can be overcome by cycling at an elevated temperature as observed in previous studies on the CP and the thiospinel.7,32 This strategy is used here. Micrometer-sized, nearly stoichiometric TiS2 crystals were prepared by literature methods33 with some modifications. Their X-ray diffraction (XRD) pattern was indexed in the standard P3̅m1 space group (Figure S1). The platelet morphology (Figure S2) results from the layered crystal structure shown in Figure 1a. Possible octahedral and tetrahedral sites for Mg occupation are indicated on the diagram. Electrochemical cycling was performed using 2325 coin cells with APC in THF (APC/THF) electrolyte and a Mg negative electrode at 60 °C. The resulting discharge and charge voltage profiles at various rates are shown in Figure 1b. At a C/ 20 rate (1C = 1 e−/TiS2 in 1 h, black curve), an initial discharge capacity of 270 mAh g−1 (Mg0.56TiS2) was obtained (Figure 1b inset), twice that of the CP phase, at a similar voltage.6 The average Mg content, as measured by energy dispersive X-ray spectroscopy analysis (EDX), at the end of discharge was x = 0.54 ± 0.02 (Table 1), which matches the discharge capacity (x = 0.56). Not all of these Mg cations can be extracted on the first charge, however, suggesting they are trapped in the structure. The Mg content after charge to 1.8 V vs Mg (the

echargeable Mg batteries are one of the candidate systems to possibly surpass Li-ion in volumetric energy density.1−3 Their most significant advantage comes from the potential use of Mg metal as the negative electrode, which, in addition to its low-cost, also offers high volumetric capacity (3833 mAh cm−3) and dendrite free deposition during charging. Metals that alloy with Mg have also been explored as negative electrodes, such as Sn4 and recently Pb, which has the lowest voltage and highest volumetric capacity of any Mg alloy yet reported.5 However, available positive electrode materials have been limited to very few compounds, the first one to cycle well being the Chevrel phase (CP) Mo6S8 reported by Aurbach et al. in 2000.6 Recently our group also identified the titanium thiospinel as a Mg insertion host.7 In contrast to the favorable Mg mobility in sulfide structures, sluggish Mg diffusion8−13 or conversion reactions14,15 are generally observed in oxide hosts. Hence, shifting to “softer” lattices (i.e., S, Se, etc. instead of O) is a promising means of discovering new Mg insertion structures that will contribute to the fundamental understanding of divalent ion diffusion in lattices, as well as Mg desolvation at the electrolyte/electrode interface.16−21 For example, it has been shown that the low Mg desolvation barrier on the CP surface from the all-phenyl complex (APC)22 electrolyte is a key factor for its good performance.23 In our search for sulfide-based Mg positive electrode materials, here we re-examine the first Li insertion positive electrode material: layered TiS2.24 Chemical Mg insertion into this structure was demonstrated long ago, but the Mg sites were not identified.25,26 TiS2 nanotubes, cycled in Mg(ClO4)2/ acetonitrile with a Mg negative electrode, were reported as a Mg positive electrode in 2004.27 Mg deposition in such a system will be limited,17 however, owing to the passivation of © XXXX American Chemical Society

Received: May 14, 2016 Accepted: June 8, 2016

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Figure 1. (a) Crystal structure of TiS2. Titanium atoms (blue) are at the center of octahedra constructed of sulfur atoms (yellow). Empty interlayer octahedral (orange) and tetrahedral (green) sites that can be occupied by Mg2+ are illustrated. (b) Discharge and charge profiles of layered TiS2 examined at 60 °C with an APC/THF electrolyte and a Mg negative electrode at various current densities; (c) corresponding differential capacity curve at C/20; (d) discharge (red) and charge (blue) capacity and Coulombic efficiency (green) evolution at a C/10 rate in an APC/G4 electrolyte.

fitting (Figure S4). Owing to preferred orientation, limited positive electrode thickness in the in situ cell and multiple phases, Rietveld analysis could not be readily carried out. Figure 2b shows the evolution of those three peaks; the corresponding a and c values are summarized in Figure 2c,d and Table S1. In the pristine material (point A, phase 1), cell parameters for nearly stoichiometric TiS2,33 a = 3.4058(2) Å and c = 5.6987(4) Å, were obtained. We note that preventing nonstochiometry in TiS2 (where typical lattice parameters are as high as a = 3.419 Å and c = 5.713 Å), due to excess Ti atoms occupying interlayer sites and pinning the layers, is crucial for good Li intercalation properties.35,36 Similar logic likely applies to Mg intercalation. During the discharge process, the peak intensities of phase 1 decrease, while a new set of peaks evolve. At the end of the first discharge plateau (point B, 0.05Mg/ TiS2), a second phase (phase 2) appears, denoted by weak shoulders at lower angle to the phase 1 reflections. Phase 2 exhibits a notably increased c parameter (5.903(7) Å), but only a small increase in a (3.416(3) Å). Upon further discharge to point C, 0.17Mg/TiS2, a third phase (phase 3) appears with sharper reflections. Again, the c parameter increases greatly (6.112(3) Å), while a remains almost constant (3.431(2) Å). At point C, the asymmetric peak shapes indicate a small amount of phase 2 persists. At the end of discharge (point D, 0.56Mg/ TiS2), the (011) and (012) peaks of phase 3 shift further but the (001) reflection remains at the same position. This results from a major increase in a to 3.4934(5) Å and a minor increase in c (6.123(1) Å) of the new phase (phase 4) by comparison with phase 3. The cell parameters of phase 4 are in agreement with those obtained by chemical Mg insertion into layered TiS2

Table 1. EDX Results for Layered TiS2 sample

pristine

discharged

charged

EDX

Ti1.09(3)S2

Mg0.54(2)Ti1.10(4)S2

Mg0.30(2)Ti1.02(5)S2

limit of the APC electrolyte) is Mg0.26TiS2, in agreement with the EDX value (x = 0.3 ± 0.02; Table 1). From the second cycle onward, the reversibility improved dramatically, and the capacity stabilized at around 160 mAh g−1 (0.33Mg/TiS2, Figure 1b). Thus, cation entrapment did not continue upon cycling; instead, reversible Mg2+ de/intercalation was achieved with the partially magnesiated TiS2 formed on the first cycle. Three distinct processes were observed in the voltage profile and its associated differential capacity curve (Figure 1c), indicating a multistep Mg insertion mechanism that is discussed in more detail later. As the cycling rates increased to C/10 and C/5, the capacities dropped to 250 mAh g−1 and 140 mAh g−1 during the initial discharge, and stabilized at 140 mAh g−1 and 90 mAh g−1, respectively, during subsequent cycles. Long-term cycling of the material was examined in APC/tetraglyme (APC/G4) electrolyte, where similar capacities were obtained with slightly larger overpotentials (Figure S3). A relatively stable capacity of 115 mAh g−1 was obtained after a few cycles of stabilization (Figure 1d). The detailed Mg insertion mechanism was examined by in situ XRD, with scans taken at the end of each pronounced electrochemical step (Figure 2a). The first three intense peaks of the P3̅m1 phase(001), (011), and (012)were used to extract the a and c lattice parameters by Le Bail34 full-profile 298

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preferred Mg insertion onto the octahedral sites during early discharge (A to C). In fact, at point C the Mg stoichiometry is likely larger than x = 0.17. When the intensities of the (012) reflections at this intermediate point are compared, phase 3 comprises about 50% of the material. As such, the Mg content of the outer shell of the active material is more likely x = 2 × 0.17 = 0.34, corresponding to the ordered composition Mg1/3TiS2 where Mg occupies octahedral sites and whose existence was suggested by first-principles calculations.31 These calculations also showed that as the unit cell increases in size with increasing Mg2+/e− content, the Mg2+ preference for the octahedral site over the tetrahedral site is lessened,31 suggesting an increased likelihood of tetrahedral occupation at high x values. Meanwhile, we note a possible parallel with lithium insertion in TiS2 where a switchover in siting as a function of depth of intercalation occurs. On insertion of between 0−1 Li/ TiS2,37 Li+ occupies octahedral sites (causing an increase in only the c parameter).38−41 Subsequent insertion of 1−2 Li/ TiS2 populates the tetrahedral sites, accompanied by an increase in the a parameter.41 These changes in lattice parameters match those in our study, namely, an increase of the c parameter in phase 1 to 3, followed by an increase in the a parameter from phase 3 to 4. Thus, the increase in a may also result from additional Mg occupation on the tetrahedral sites (i.e., mixed occupation driven by subtle thermodynamic and kinetic factors). Understanding of the detailed Mg siting behavior in TiS2 requires full and detailed structural analyses of the single phases at the intermediate multisteps, coupled with quasi-equilibrium electrochemical measurements. Such a study is under investigation but is beyond the scope of this Letter. Upon charging, at the end of the first plateau (point E), the intensities of phase 4 reflections decrease and those of phase 3 start to appear. This trend continues on the next plateau (E to F), while the c parameters of both phases decrease. Upon final charge (G), phase 4 disappears completely and phase 3 is the major phase along with a minor contribution of phase 2. The intensity of the residual phase 1 reflections remain constant on charging; thus, the pristine structure is not regenerated. The lack of conversion of phase 2/3 back to phase 1 indicates some Mg is trapped (presumably on octahedral sites), accounting for some of the capacity loss during the first cycle. The overall process is summarized in Figure 3. Such cation entrapment is

Figure 2. XRD studies of TiS2 structure evolution during cycling. (a) Electrochemical discharge−charge profile at C/20 on the first cycle showing labeling of the points at which diffraction patterns were collected. (b) XRD profiles. Note that a trace of the pristine material remains throughout cycling. (c) Extracted cell parameters for a at each state. (d) Extracted cell parameters for c at each state. See the Supporting Information for details on the fitting.

Figure 3. Proposed Mg2+ (de)insertion mechanism based on XRD data. This involves a multistep Mg insertion mechanism into TiS2, with initial occupation of primarily the octahedral sites (phases 2/ 3) followed by occupation of the octahedral and tetrahedral sites (conversion of phases 2/3 to phase 4). The latter process is electrochemically reversible, but the first is not.

reported by Bruce et al.25,26 Structural analysis of that material was not provided. At this state of discharge, we also note that some phase 1 still remains. Because the TiS2 electrode particles are micrometer-sized, diffusion lengths between the surface and the interior of individual TiS2 particles are large enough that presumably phase 1 remains within the particle core. The fraction of phase 1 to phase 4, estimated by comparison of the intensity of the well-resolved (012) reflections of phase 1 and phase 4, is about 15%. The overall discharge process involves an initial expansion of the c parameter from point A to point C, followed by an increase in the a parameter during the final step (point D). Based on previous computational results, the octahedral site energy for Mg occupation is much lower than the tetrahedral site during the initial stages of intercalation.31 This suggests

potentially due to an increase of the Mg diffusion barrier as cell parameters decrease during Mg extraction, effects of Mgvacancy ordering, and/or kinetic constraints imposed by the need for the migration of octahedral Mg to pass through adjacent tetrahedral sites31 (partially populated on discharge). The behavior contrasts sharply with the highly reversible solid solution Li de/insertion process in TiS224 where the mobility is less affected by the Li concentration and ordering does not prevail.42,43 The higher charge on the multivalent Mg2+ cations leads both to a stronger Coulombic attraction to the anions in 299

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(5) Periyapperuma, K.; Tran, T. T.; Purcell, M. I.; Obrovac, M. N. The Reversible Magnesiation of Pb. Electrochim. Acta 2015, 165, 162− 165. (6) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Prototype Systems for Rechargeable Magnesium Batteries. Nature 2000, 407, 724−727. (7) Sun, X.; Bonnick, P.; Duffort, V.; Liu, M.; Rong, Z.; Persson, K. A.; Ceder, G.; Nazar, L. F. A High Capacity Thiospinel Cathode for Mg Batteries. Energy Environ. Sci. 2016, DOI: 10.1039/C6EE00724D. (8) Amatucci, G. G.; Badway, F.; Singhal, A.; Beaudoin, B.; Skandan, G.; Bowmer, T.; Plitz, I.; Pereira, N.; Chapman, T.; Jaworski, R. Investigation of Yttrium and Polyvalent Ion Intercalation into Nanocrystalline Vanadium Oxide. J. Electrochem. Soc. 2001, 148, A940−A950. (9) Levi, E.; Levi, M. D.; Chasid, O.; Aurbach, D. A Review on the Problems of the Solid State Ion Diffusion in Cathodes for Rechargeable Mg Batteries. J. Electroceram. 2009, 22, 13−19. (10) Rong, Z.; Malik, R.; Canepa, P.; Gautam, G. S.; Liu, M.; Jain, A.; Persson, K.; Ceder, G. Materials Design Rules for Multivalent Ion Mobility in Intercalation Structures. Chem. Mater. 2015, 27, 6016− 6021. (11) Gautam, G. S.; Canepa, P.; Abdellahi, A.; Urban, A.; Malik, R.; Ceder, G. The Intercalation Phase Diagram of Mg in V2O5 from FirstPrinciples. Chem. Mater. 2015, 27, 3733−3742. (12) Bo, S.-H.; Grey, C. P.; Khalifah, P. G. Defect-Tolerant Diffusion Channels for Mg2+ Ions in Ribbon-Type Borates: Structural Insights into Potential Battery Cathodes MgVBO4 and MgxFe2−xB2O5. Chem. Mater. 2015, 27, 4630−4639. (13) Levi, E.; Gofer, Y.; Aurbach, D. On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials. Chem. Mater. 2010, 22, 860−868. (14) Arthur, T. S.; Zhang, R.; Ling, C.; Glans, P.-A.; Fan, X.; Guo, J.; Mizuno, F. Understanding the Electrochemical Mechanism of KαMnO2 for Magnesium Battery Cathodes. ACS Appl. Mater. Interfaces 2014, 6, 7004−7008. (15) Sun, X.; Duffort, V.; Mehdi, B. L.; Browning, N. D.; Nazar, L. F. Investigation of the Mechanism of Mg Insertion in Birnessite in Nonaqueous and Aqueous Rechargeable Mg-Ion Batteries. Chem. Mater. 2016, 28, 534−542. (16) Kim, H. S.; Arthur, T. S.; Allred, G. D.; Zajicek, J.; Newman, J. G.; Rodnyansky, A. E.; Oliver, A. G.; Boggess, W. C.; Muldoon, J. Structure and Compatibility of a Magnesium Electrolyte with a Sulphur Cathode. Nat. Commun. 2011, 2, 427. (17) Tran, T. T.; Lamanna, W. M.; Obrovac, M. N. Evaluation of Mg[N(SO2CF3)2]2/Acetonitrile Electrolyte for Use in Mg-Ion Cells. J. Electrochem. Soc. 2012, 159, A2005−A2009. (18) Doe, R. E.; Han, R.; Hwang, J.; Gmitter, A. J.; Shterenberg, I.; Yoo, H. D.; Pour, N.; Aurbach, D. Novel, Electrolyte Solutions Comprising Fully Inorganic Salts with High Anodic Stability for Rechargeable Magnesium Batteries. Chem. Commun. 2014, 50, 243− 245. (19) Tutusaus, O.; Mohtadi, R.; Arthur, T. S.; Mizuno, F.; Nelson, E. G.; Sevryugina, Y. V. An Efficient Halogen-Free Electrolyte for Use in Rechargeable Magnesium Batteries. Angew. Chem., Int. Ed. 2015, 54, 7900−7904. (20) Liao, C.; Sa, N.; Key, B.; Burrell, A. K.; Cheng, L.; Curtiss, L. A.; Vaughey, J. T.; Woo, J.-J.; Hu, L.; Pan, B.; Zhang, Z. The Unexpected Discovery of the Mg(HMDS)2/MgCl2 Complex as a Magnesium Electrolyte for Rechargeable Magnesium Batteries. J. Mater. Chem. A 2015, 3, 6082−6087. (21) Pan, B.; Zhang, J.; Huang, J.; Vaughey, J. T.; Zhang, L.; Han, S.D.; Burrell, A. K.; Zhang, Z.; Liao, C. A Lewis Acid-Free and Phenolate-Based Magnesium Electrolyte for Rechargeable Magnesium Batteries. Chem. Commun. 2015, 51, 6214−6217. (22) Mizrahi, O.; Amir, N.; Pollak, E.; Chusid, O.; Marks, V.; Gottlieb, H.; Larush, L.; Zinigrad, E.; Aurbach, D. Electrolyte Solutions with a Wide Electrochemical Window for Rechargeable Magnesium Batteries. J. Electrochem. Soc. 2008, 155, A103−A109.

the lattice and greater cation repulsion. These interactions retard diffusion and lead to the complex ordering observed in the voltage profile as predicted by first-principles calculations.31 In summary, we report a new insertion positive electrode material, layered TiS2, for nonaqueous Mg batteries. At a C/10 rate and 60 °C, it provides a stabilized capacity of 115 mAh g−1. A capacity drop after the first cycle appears to be due to partial Mg entrapment within the structure owing to complex ordering and kinetic effects that will be the subject of upcoming studies. XRD analysis and a comparison with previous computation and Li insertion results suggest that the multistep Mg insertion we observe during cycling can be assigned to progressive changes in Mg siting. The multiple phases shown by XRD indicate an inhomogeneous Mg distribution throughout the particles but also suggest the capability of the material to accept more Mg than the average value we currently obtain. Future studies will report on decreasing the particle size and thus the ion diffusion length. Nonetheless, the high electrochemical reversibility of our bulk MgxTiS2 after the first cycle indicates that other sulfide materials are potentially excellent hosts for Mg2+ if cation entrapment is minimized. Furthermore, our work identifies TiS2 as new platform for the study of Mg diffusion behavior in the bulk and at interfaces, which will undoubtedly provide guidance in the search for new Mg positive electrode materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00145. Experimental methods, EDX, full profile fitting, XRD, SEM image, crystal structure, and electrochemical performance of TiS2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences. NSERC is acknowledged by L.F.N. for a Canada Research Chair.

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REFERENCES

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