Layered TiS2 Positive Electrode for MgBatteriesXiaoqi 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 energystorage systems, but the limited discovery of functional positive electrodematerials beyond the seminal Chevrel phase (Mo6S8) has slowed theirdevelopment. Herein, we report on layered TiS2 as a promising positiveelectrode intercalation material, providing 115 mAh g−1 stabilized capacity ina Mg full cell. Reversible Mg2+ intercalation into the structure is proven byelemental analysis combined with X-ray diffraction studies that elucidate thephase behavior upon cycling. The voltage profiles reveal distinct Mg2+ cationordering, unlike the solid solution behavior exhibited by Li+. Our findings notonly point to the important role of “soft” lattices to facilitate divalent solid-state cation mobility but also now provide an alternative sulfide to serve as aplatform for the fundamental understanding of Mg2+ intercalation in lattices.

Rechargeable Mg batteries are one of the candidatesystems to possibly surpass Li-ion in volumetric energydensity.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 volumetriccapacity (3833 mAh cm−3) and dendrite free deposition duringcharging. Metals that alloy with Mg have also been explored asnegative electrodes, such as Sn4 and recently Pb, which has thelowest voltage and highest volumetric capacity of any Mg alloyyet reported.5 However, available positive electrode materialshave been limited to very few compounds, the first one to cyclewell being the Chevrel phase (CP) Mo6S8 reported by Aurbachet al. in 2000.6 Recently our group also identified the titaniumthiospinel as a Mg insertion host.7 In contrast to the favorableMg mobility in sulfide structures, sluggish Mg diffusion8−13 orconversion 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 insertionstructures that will contribute to the fundamental under-standing of divalent ion diffusion in lattices, as well as Mgdesolvation at the electrolyte/electrode interface.16−21 Forexample, it has been shown that the low Mg desolvation barrieron 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 electrodematerials, here we re-examine the first Li insertion positiveelectrode material: layered TiS2.

24 Chemical Mg insertion intothis structure was demonstrated long ago, but the Mg sites werenot identified.25,26 TiS2 nanotubes, cycled in Mg(ClO4)2/acetonitrile with a Mg negative electrode, were reported as aMg positive electrode in 2004.27 Mg deposition in such asystem will be limited,17 however, owing to the passivation of

the Mg electrode, preventing further transport of Mg2+

ions.28,29 Another seminal report on TiS2 showed low initialcapacity in a Mg cell, followed by a significant capacity dropduring the second cycle.30 Although previous computationalstudies have suggested a high migration barrier to Mg diffusionin layered TiS2 (>1 eV),

31 this can be overcome by cycling at anelevated temperature as observed in previous studies on the CPand 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 thestandard P3̅m1 space group (Figure S1). The plateletmorphology (Figure S2) results from the layered crystalstructure shown in Figure 1a. Possible octahedral andtetrahedral sites for Mg occupation are indicated on thediagram. Electrochemical cycling was performed using 2325coin cells with APC in THF (APC/THF) electrolyte and a Mgnegative electrode at 60 °C. The resulting discharge and chargevoltage 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 dischargecapacity of 270 mAh g−1 (Mg0.56TiS2) was obtained (Figure 1binset), twice that of the CP phase, at a similar voltage.6 Theaverage Mg content, as measured by energy dispersive X-rayspectroscopy 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 thefirst charge, however, suggesting they are trapped in thestructure. The Mg content after charge to 1.8 V vs Mg (the

Received: May 14, 2016Accepted: June 8, 2016Published: June 23, 2016

Letterhttp://pubs.acs.org/journal/aelccp

© 2016 American Chemical Society 297 DOI: 10.1021/acsenergylett.6b00145ACS Energy Lett. 2016, 1, 297−301

limit of the APC electrolyte) is Mg0.26TiS2, in agreement withthe EDX value (x = 0.3 ± 0.02; Table 1). From the secondcycle onward, the reversibility improved dramatically, and thecapacity stabilized at around 160 mAh g−1 (0.33Mg/TiS2,Figure 1b). Thus, cation entrapment did not continue uponcycling; instead, reversible Mg2+ de/intercalation was achievedwith the partially magnesiated TiS2 formed on the first cycle.Three distinct processes were observed in the voltage profileand its associated differential capacity curve (Figure 1c),indicating a multistep Mg insertion mechanism that is discussedin 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 duringthe initial discharge, and stabilized at 140 mAh g−1 and 90mAh g−1, respectively, during subsequent cycles. Long-termcycling of the material was examined in APC/tetraglyme(APC/G4) electrolyte, where similar capacities were obtainedwith slightly larger overpotentials (Figure S3). A relativelystable capacity of 115 mAh g−1 was obtained after a few cyclesof stabilization (Figure 1d).The detailed Mg insertion mechanism was examined by in

situ XRD, with scans taken at the end of each pronouncedelectrochemical step (Figure 2a). The first three intense peaksof the P3 ̅m1 phase(001), (011), and (012)were used toextract the a and c lattice parameters by Le Bail34 full-profile

fitting (Figure S4). Owing to preferred orientation, limitedpositive electrode thickness in the in situ cell and multiplephases, Rietveld analysis could not be readily carried out. Figure2b shows the evolution of those three peaks; the correspondinga and c values are summarized in Figure 2c,d and Table S1. Inthe pristine material (point A, phase 1), cell parameters fornearly stoichiometric TiS2,

33 a = 3.4058(2) Å and c =5.6987(4) Å, were obtained. We note that preventingnonstochiometry in TiS2 (where typical lattice parameters areas high as a = 3.419 Å and c = 5.713 Å), due to excess Ti atomsoccupying interlayer sites and pinning the layers, is crucial forgood Li intercalation properties.35,36 Similar logic likely appliesto Mg intercalation. During the discharge process, the peakintensities 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 weakshoulders at lower angle to the phase 1 reflections. Phase 2exhibits a notably increased c parameter (5.903(7) Å), but onlya small increase in a (3.416(3) Å). Upon further discharge topoint C, 0.17Mg/TiS2, a third phase (phase 3) appears withsharper reflections. Again, the c parameter increases greatly(6.112(3) Å), while a remains almost constant (3.431(2) Å). Atpoint C, the asymmetric peak shapes indicate a small amount ofphase 2 persists. At the end of discharge (point D, 0.56Mg/TiS2), the (011) and (012) peaks of phase 3 shift further butthe (001) reflection remains at the same position. This resultsfrom a major increase in a to 3.4934(5) Å and a minor increasein c (6.123(1) Å) of the new phase (phase 4) by comparisonwith phase 3. The cell parameters of phase 4 are in agreementwith those obtained by chemical Mg insertion into layered TiS2

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

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

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reported by Bruce et al.25,26 Structural analysis of that materialwas not provided. At this state of discharge, we also note thatsome phase 1 still remains. Because the TiS2 electrode particlesare micrometer-sized, diffusion lengths between the surface andthe interior of individual TiS2 particles are large enough thatpresumably phase 1 remains within the particle core. Thefraction of phase 1 to phase 4, estimated by comparison of theintensity of the well-resolved (012) reflections of phase 1 andphase 4, is about 15%.The overall discharge process involves an initial expansion of

the c parameter from point A to point C, followed by anincrease in the a parameter during the final step (point D).Based on previous computational results, the octahedral siteenergy for Mg occupation is much lower than the tetrahedralsite during the initial stages of intercalation.31 This suggests

preferred Mg insertion onto the octahedral sites during earlydischarge (A to C). In fact, at point C the Mg stoichiometry islikely larger than x = 0.17. When the intensities of the (012)reflections at this intermediate point are compared, phase 3comprises about 50% of the material. As such, the Mg contentof the outer shell of the active material is more likely x = 2 ×0.17 = 0.34, corresponding to the ordered compositionMg1/3TiS2 where Mg occupies octahedral sites and whoseexistence was suggested by first-principles calculations.31 Thesecalculations also showed that as the unit cell increases in sizewith increasing Mg2+/e− content, the Mg2+ preference for theoctahedral site over the tetrahedral site is lessened,31 suggestingan increased likelihood of tetrahedral occupation at high xvalues. Meanwhile, we note a possible parallel with lithiuminsertion in TiS2 where a switchover in siting as a function ofdepth of intercalation occurs. On insertion of between 0−1 Li/TiS2,

37 Li+ occupies octahedral sites (causing an increase inonly the c parameter).38−41 Subsequent insertion of 1−2 Li/TiS2 populates the tetrahedral sites, accompanied by anincrease in the a parameter.41 These changes in latticeparameters match those in our study, namely, an increase ofthe c parameter in phase 1 to 3, followed by an increase in the aparameter from phase 3 to 4. Thus, the increase in a may alsoresult from additional Mg occupation on the tetrahedral sites(i.e., mixed occupation driven by subtle thermodynamic andkinetic factors). Understanding of the detailed Mg sitingbehavior in TiS2 requires full and detailed structural analyses ofthe single phases at the intermediate multisteps, coupled withquasi-equilibrium electrochemical measurements. Such a studyis 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 3start to appear. This trend continues on the next plateau (E toF), while the c parameters of both phases decrease. Upon finalcharge (G), phase 4 disappears completely and phase 3 is themajor phase along with a minor contribution of phase 2. Theintensity of the residual phase 1 reflections remain constant oncharging; thus, the pristine structure is not regenerated. Thelack of conversion of phase 2/3 back to phase 1 indicates someMg is trapped (presumably on octahedral sites), accounting forsome of the capacity loss during the first cycle. The overallprocess is summarized in Figure 3. Such cation entrapment is

potentially due to an increase of the Mg diffusion barrier as cellparameters decrease during Mg extraction, effects of Mg-vacancy ordering, and/or kinetic constraints imposed by theneed for the migration of octahedral Mg to pass throughadjacent tetrahedral sites31 (partially populated on discharge).The behavior contrasts sharply with the highly reversible solidsolution Li de/insertion process in TiS2

24 where the mobility isless affected by the Li concentration and ordering does notprevail.42,43 The higher charge on the multivalent Mg2+ cationsleads both to a stronger Coulombic attraction to the anions in

Figure 2. XRD studies of TiS2 structure evolution during cycling.(a) Electrochemical discharge−charge profile at C/20 on the firstcycle showing labeling of the points at which diffraction patternswere collected. (b) XRD profiles. Note that a trace of the pristinematerial remains throughout cycling. (c) Extracted cell parametersfor 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 XRDdata. 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 iselectrochemically reversible, but the first is not.

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the lattice and greater cation repulsion. These interactionsretard diffusion and lead to the complex ordering observed inthe voltage profile as predicted by first-principles calculations.31

In summary, we report a new insertion positive electrodematerial, layered TiS2, for nonaqueous Mg batteries. At a C/10rate 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 partialMg entrapment within the structure owing to complex orderingand kinetic effects that will be the subject of upcoming studies.XRD analysis and a comparison with previous computation andLi insertion results suggest that the multistep Mg insertion weobserve during cycling can be assigned to progressive changesin Mg siting. The multiple phases shown by XRD indicate aninhomogeneous Mg distribution throughout the particles butalso suggest the capability of the material to accept more Mgthan the average value we currently obtain. Future studies willreport on decreasing the particle size and thus the ion diffusionlength. Nonetheless, the high electrochemical reversibility ofour bulk MgxTiS2 after the first cycle indicates that other sulfidematerials are potentially excellent hosts for Mg2+ if cationentrapment is minimized. Furthermore, our work identifiesTiS2 as new platform for the study of Mg diffusion behavior inthe bulk and at interfaces, which will undoubtedly provideguidance in the search for new Mg positive electrode materials.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsenergy-lett.6b00145.

Experimental methods, EDX, full profile fitting, XRD,SEM image, crystal structure, and electrochemicalperformance of TiS2 (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: lfnazar@uwaterloo.ca.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

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