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TiS Magnesium Battery Material: Atomic-Scale Study of Maximum Capacity and Structural Behaviour Maxim Arsentev, Alexander Missyul, Andrey Vitalievich Petrov, and Mahmoud Hammouri J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01575 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 8, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Figure 1. Band structure (a) and corresponding total density of states (DOS) (b) for TiS3. The Fermi energy is set to zero. 40x20mm (600 x 600 DPI)

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Figure 2. (a) Structure of TiS3 with 1-9 sites capable for Mg intercalation. The blue and yellow balls represent the Ti and S atoms, respectively. The corresponding intercalated structures of Mg0.5TiS3, obtained by full geometry optimization, for the (1, 4), (2, 3, 9), (5), (7) and (8) sites, respectively (b-f). The structure with Mg at P6 position is not shown because of not converged calculation. 82x64mm (300 x 300 DPI)


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Figure 3. Mg-ion migration pathway obtained by TOPOS software (a,b) and (c) corresponding representation of sites capable for migration. The blue and yellow balls represent the Ti and S atoms, respectively. 1, 2 and 7 sites and the corresponding trigonal prisms are coloured green and pink respectively. 82x56mm (300 x 300 DPI)



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Figure 4. (a) Structure of TiS3 with 1-2 and 7-8 sites capable for Mg intercalation: the blue and yellow balls represent the Ti and S atoms, respectively. Structure of TiS3 extending over about eight unit cells (b). The corresponding intercalated structures of Mg0.5TiS3 and MgTiS3, respectively (c-d). The possible mechanism of phase transformation for TiS3 → Mg0.5TiS3 (e). 82x63mm (300 x 300 DPI)


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Figure 5. (a) The Mg–S equilibrium bond length (dMg–S) with the nearest neighbor S atoms (Å) for the TiS3 at different Mg content. ∆c means the expansion of lattice parameters c after Mg absorbed in TiS3. (b) The Mg binding energy, Eb(Mg), between the Mg atom and the TiS3 at different Mg content. 330x99mm (300 x 300 DPI)



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Figure 6. The energy above hull, Ehull values for MgxTiS3 (x = 0.0-2.0). 57x40mm (600 x 600 DPI)


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Figure 7. Possible mechanism of the structural decomposition of the Mg-intercalated TiS3 into MgS and TiS2. 82x53mm (300 x 300 DPI)



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Figure 8. Mg hop in Mg0.375TiS3. (a) Migration pathway from a trigonal prismatic site with “down” orientation (α) to the neighbour trigonal prismatic site with “up” orientation (γ, green) through the agjastent trigonal prismatic sites (β, δ). (b) Diffusion energy barrier and (c) hop path as projected along the c-axis into the plane of the intercalation layer. 82x82mm (300 x 300 DPI)


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Figure 9. Mg hop in Mg0.5TiS3 in the [010] direction calculated using the SIESTA code. (a) Migration pathway from a trigonal prismatic site (α) to the neighbour trigonal prismatic site (γ) (c) through the saddle-poin site (β) (b). Diffusion energy barrier (d). 82x86mm (300 x 300 DPI)



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Figure 10. Calculated voltage profile for TiS3. 57x40mm (600 x 600 DPI)


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For Table of Contents Only 82x43mm (300 x 300 DPI)



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TiS3 Magnesium Battery Material: Atomic-Scale Study of Maximum Capacity and Structural Behaviour Maxim Arsentev†*, Alexander Missyul‡, Andrey V. Petrov§ and Mahmoud Hammouriǁ †

Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg 199034, Russia

‡

ALBA Synchrotron Light Source, Carrer de la Llum 2-26, 08290, Cerdanyola del Vallès,

§

Institute of Chemistry, Saint Petersburg State University, St. Petersburg 198504, Russia

ǁ

Chemistry and Biochemistry Department, California State Polytechnic University at Pomona,

Pomona, California 91768, United States

ABSTRACT: Good cyclability is essential for the potential application of cathode materials. We investigated electrochemical properties of Mg in layered intercalation compound from firstprinciples using TiS3 as a model system. The calculations showed exothermic phase transformation upon intercalation of Mg from the electrolyte: the geometry optimization of the structure containing 0.5 Mg showed she shift of layers accompanied by change of Mg coordination from square pyramidal to trigonal prismatic. Further increase of the Mg content


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leads to break of the S-S bonds in the disulphide ion and conversion of the TiS3 layers into a ribbons. The obtained phase is metastable and can easily and irreversibly decompose to MgS and TiS2.This means that in order to achieve full theoretical capacity of TiS3 this decomposition has to be suppressed. A very low migration barrier of 0.292-0.698 eV (depending on the Mg content) was found in the [010] direction, which is much lower than the value of analogs, such as layered and spinel TiS2. This finding reveal the potential of TiS3 to become Mg cathode with superior performance compared to similar analogs.

INTRODUCTION Nowadays, much attention is paid to secondary or rechargeable batteries due to their usage as an integral part in several modern applications ranging from miniature and portable devices; such as in cell phones, laptops, medium scale; such as in hybrid (HV), plug-in hybrid (PHEV) and electric vehicles (EV) to large scale stationary and grid applications.1-2 Among these, Mg batteries are attracting much attention today owing to their potential as a safe and low-cost energy storage systems.3-4 In particular, earth abundance, low cost (ca. 24 times cheaper than Li), and potentially high volumetric capacity due to the divalent nature of Mg2+ are its most attractive characteristics as a cathode. Mg, as an anode material, inherently possesses some advantages over lithium: it is safe and does not exhibit dendrite formation during charging. One of the major drawbacks limiting the progress in this field is the absence of practical electrolytes. Fortunately a new Mg(CB11H12)2/tetraglyme electrolyte geared towards overcoming the existing hurdles.5 However, very sluggish kinetics induced by the slow diffusion of magnesium ions still makes magnesium batteries far from being practical. Most of the


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experimental works are devoted to the synthesis of nanometer-sized electrode particles but not the improvement of intrinsic Mg2+ diffusion in bulk materials. The improvement of Mg2+ diffusion can be a significant problem because of the strong electrostatic interactions with the anions and the cations of the hosts, which induces lattice strain.6 There are two possible solutions to tackle these problems: the one relies on shielding of the Mg2+ ion charge with e. g. H2O molecules and the other on the moderation of the external electrons through electron delocalization over multiple atoms in the metal clusters (such as the Chevrel phase (Mo6X8, X = S, Se)).7,4,8 In the latter route, considered to be the most promising, the spread electronic structure over multiple atoms is proposed as a key factor for the reversible cathode performance of Mg batteries. One such example is TiSe2 in which delocalized electronic states formed by d–p orbital hybridization between Ti and Se.9 But selenides are not good in terms of practical use because of environmental concerns, cost and natural abundance. In a recent study TiS3 was synthesized from Ti and S by a solid-state reaction, and a spread electronic structure over Ti and S atoms by d–p orbital hybridization was proposed to act as an effective factor for reversible Mg insertion reactions.10 Despite the relative success, the specific capacity was only about 80 mAh/g for the first 50 cycles at room temperature. It is evident that the better understanding of the properties of this material is necessary. There is only one publication on the performance and mechanism of Li/Na adsorption in bulk and monolayer TiS3.11 However, no experimental and theoretical study has been focused on the mechanism of Mg adsorption in TiS3. This study intends to fill the gap. In this work, for the first time first principles calculations were used to comparatively examine the Mg adsorption and diffusion in bulk and monolayer TiS3.


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COMPUTATIONAL DETAILS In this study, we investigated the stability of MgxTiS3 (x = 0.0-2.0) phases and determined the electronic structure of those phases using ABINIT12-13, a pseudopotential self-consistent DFT code. The exchange–correlation term has been described within the generalized gradient approximation (GGA) parameterized by Perdew–Burke–Ernzerhof (PBE) functional.14 The 15 Hartree mesh cut-off energy was used. All the atoms in the unit cell were fully relaxed until the force on each atom was less than 0.01 eV Å-1. Electronic minimization was performed with a tolerance of 10-8 eV. The Brillouin-zone (BZ)15 sampling was carried out with a 9 × 6 × 3 Monkhorst-Pack grid for the stability calculations. RESULTS AND DISCUSSION The crystal structure of TiS3 consists of the infinite two-dimensional double layers of the TiS8 polyhedra. Each polyhedron includes two S22- ions forming a rectangular face and four noncoplanar S2- ions. The polyhedra within the layer share (S22-)S2- faces along b axis and S2corners along a axis. The double layers are formed by sharing the S2--S2- edges between the two single layers, leaving the space between the (S22-)2 ions rectangles free for the intercalation of the Mg2+ cations. Structural optimization resulted in a relatively large modification of the c parameter from the reported one.16 The optimized values are given in Table 1 together with experimental data. The reason behind the poor agreement of c of the layered compounds seems to be linked to the neglecting a weak van der Waals (vdW) interaction between the layers, and was corrected by us using the DFT-D2 approach.11,17


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The energy band dispersion and DOS are shown in Figs. 1(a) and 1(b), respectively. The calculated electronic band structure indicates that TiS3 is a semiconductor with indirect band gap (0.93 eV). The valence band maximum is at the Brillouin zone (BZ) center, Γ-point, and the conduction band minimum – at the BZ boundary, Y-point (Fig. 1(a)). For the stability study of the intercalated structure, we consider 9 possible configurations by placing the magnesium cation at 1-9 sites, as shown in Figure 2(a). The coordinates of these sites were obtained using TOPOS software18, are listed in Table 2. These coordinates were entered into SIESTA code, followed by the geometry optimization. Optimization of these structures resulted in 5 stable configurations shown in Figure 2(b-f). Mg ions in positions 1, 2 and 9 appear to be unstable and shift to the nearest positions 3 and 4. We calculated the relative energies for all these configurations (Table 3). It shows that in fact there are four equally stable positions of Mg in the Mg0.5TiS3 (Figure 2(b,c,e,f)). The differences in total energy for them are about 0.04 eV and very small, therefore these positions are equally preferable. Table 1. Optimized and experimental lattice parameters (Å). Optimized

Experiment16

a

5.118

4.948

b

3.444

3.379

c

8.918

8.748


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Figure 1. Band structure (a) and corresponding total density of states (DOS) (b) for TiS3. The Fermi energy is set to zero.

Figure 2. (a) Structure of TiS3 with 1-9 sites capable for Mg intercalation. The blue and yellow balls represent the Ti and S atoms, respectively. The corresponding intercalated structures of Mg0.5TiS3, obtained by full geometry optimization, for the (1, 4), (2, 3, 9), (5), (7) and (8) sites, respectively (b-f). The structure with Mg at P6 position is not shown because of not converged calculation.


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The decreased stability can be attributed to the increased electrostatic repulsion between Mg and Ti atoms due to the shorter Mg-Ti distance (2.94 Å vs. 3.74 Å for the most stable ones).

Table 2. Fractional coordinates of sites capable for Mg intercalation. Site number

x

y

z

1

0.828

0.250

0.330

2

0.172

0.750

0.670

3

0.165

0.750

0.624

4

0.836

0.250

0.376

5

0.572

0.750

0.098

6

0.983

0.750

0.111

7

0.144

0.250

0.494

8

0.643

0.750

0.579

9

0.601

0.250

0.456

Interestingly, the tetrahedral site within the TiS3 layer appears to be energetically favorable, even though it is most likely inaccessible in the real material (Figure 2(d)). The presence of Mg atom in this position results in the distortion of the TiS3 host structure (Table 3) (parameter a increased by 16%). The role of this site is not clear at the moment. It may be important during the destruction of the TiS3 at high Mg content. The Mg-ion migration pathway was obtained by TOPOS software (Figure 3a,b).18 In summary, Mg atoms could diffuse from 1 site to a neighboring 2 site by passing through the 7 site (Figure 3c). All these sites have a trigonal


7

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prismatic coordination by the sulfur atoms, which becomes available after the filling of TiS3 with 0.5 Mg (Figure 2(b-c) and (e-f)). So finally we found that among the 9 determined positions there are only 4 unequivalent positions: 1, 2, 7 and 8. We used them as positions for Mg intercalation and calculated relative energies of 15 intercalated configurations based on them (Table 3). It shows that there are the most stable positions in the intercalation/deintercalation configurations, in some cases almost 0.8 eV more stable than the least stable one. The optimized structures of the most stable configurations are shown in Figure 4(c-d). The overall filling sequence of the different Mg positions is summarized in Table 4. As the most stable sites of adsorbed Mg atoms in TiS3 are 1 and 2 sites, they are occupied first. Meanwhile, the equilibrium bond length between Mg and nearest neighbor S atoms for TiS3, as well as the expansion of lattice parameter c after Mg absorbed in TiS3 were also calculated (Figure 5(a)). Upon intercalation of Mg the coordination of Ti changes from octahedral to 6-fold (Figure 4c,d). The change of the coordination is caused by the reduction of the disulfide ions to sulfide: S22- + 2e- = 2S2This leads to break of initial 2D Ti-S layer to 1D ribbons. Further intercalation leads to complete destruction of the structure. The possible mechanism of phase transformation for TiS3 → Mg0.5TiS3 is shown in Figure 4e: upon intercalation of Mg layers of TiS3 slide, so that coordination of Mg changes from square pyramidal to trigonal prismatic. Simultaneously, the overall symmetry of the crystal structure changes from monoclinic to orthorhombic.


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Figure 3. Mg-ion migration pathway obtained by TOPOS software (a,b)18 and (c) corresponding representation of sites capable for migration. The blue and yellow balls represent the Ti and S atoms, respectively. 1, 2 and 7 sites and the corresponding trigonal prisms are coloured green and pink respectively.

Figure 4. (a) Structure of TiS3 with 1-2 and 7-8 sites capable for Mg intercalation: the blue and yellow balls represent the Ti and S atoms, respectively. Structure of TiS3 extending over about


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eight unit cells (b). The corresponding intercalated structures of Mg0.5TiS3 and MgTiS3, respectively (c-d). The possible mechanism of phase transformation for TiS3 → Mg0.5TiS3 (e).

Compared with bulk TiS3, the expansion of lattice parameters for Mg adsorption is not high at moderate Mg content (x < 0.6 in MgxTiS3). The higher values of x will result in the structure deformation of TiS3 and then the capacitance fading. From these considerations up to 0.6 Mg per formula unit is reasonable to use. As a result, 171 mA h g-1 and 844 A h l-1 for Mg storage were obtained for TiS3. This is consistent with experimental result that TiS3 is suitable as electrodes for Mg ion batteries for up to 80 mA h g-1 (this corresponds 0.3 Mg per formula unit).10 The disagreement with the experiment10 can be due to sluggish diffusion kinetics of Mg2+. Unfortunately Taniguchi et al. did not provide the X-ray diffraction patterns of the intercalated TiS3 cathode: they can indicate phase transformations of the part of the material to inactive phases. Also the degree of intercalation for fully charged structure is not known, so the authors write only about x=0.22 value for MgxTiS3, which corresponds to the charging capacity of 83.7 mA h g-1.

Table 3. Relative energies (∆E) of the intercalated structures compared to the lowest energy configurations for the Mg0.5TiS3 structure. The Mg–Ti equilibrium distance (dMg–Ti) with the nearest neighbor Ti atoms (Å) and optimized lattice parameters. Site number

a

b

c

α

β

γ

dMg-Ti

∆E (eV/f.u.)

1-4,9

3.387

4.828

9.312

83.841

90.000

90.000

3.24

0.24


10


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5,6

6.363

3.299

8.377

90.030

96.452

90.009

3.38

0.38

7,8

5.304

3.348

9.385

89.380

73.340

89.707

3.19

0.00

Table 4. Relative energies (∆E) of the intercalated structures compared to the lowest energy configurations for the TiS3 structure at different Mg content (Occ., occupied; Not occ., not occupied site). Site

Mg0.5TiS3

MgTiS3

Mg1.5TiS3

Mg2TiS3

∆E (eV/f.u.)

1

2

7

8

Occ.

Not occ.

Not occ.

Not occ.

0.24

Not occ.

Occ.

Not occ.

Not occ.

0.24

Not occ.

Not occ.

Occ.

Not occ.

0.00

Not occ.

Not occ.

Not occ.

Occ.

0.00

Not occ.

Not occ.

Occ.

Occ.

0.25

Not occ.

Occ.

Not occ.

Occ.

0.00

Not occ.

Occ.

Occ.

Not occ.

0.67

Occ.

Not occ.

Not occ.

Occ.

0.21

Occ.

Not occ.

Occ.

Not occ.

0.67

Occ.

Occ.

Not occ.

Not occ.

0.08

Not occ.

Occ.

Occ.

Occ.

0.00

Occ.

Not occ.

Occ.

Occ.

0.56

Occ.

Occ.

Not occ.

Occ.

0.56

Occ.

Occ.

Occ.

Not occ.

0.00

Occ.

Occ.

Occ.

Occ.

0.00


11

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In order to analyze the possible application of TiS3 as a cathode for Mg ion batteries, we calculated the properties of Mg adsorption in TiS3 as follows: Mg binding energy, Eb(Mg), between the Mg atom and the TiS3 is calculated according to the following definition: Eb(Mg) = Etot(TiS3) + Etot(Mg) – Etot(TiS3–Mg), where Etot(TiS3–Mg) and Etot(TiS3) are the total energies of Mg-intercalated TiS3 and pure TiS3, respectively. Etot(Mg) is the total energy of bulk Mg. If Eb is positive, the intercalation reaction is exothermic (favorable), which indicates the Mg atoms tend to bind to the TiS3. Results from Figure 5b indicate that TiS3 in Mg electrolyte tend to intercalate up to 2.0 Mg atoms per formula unit, leading to destruction of the host structure.

Figure 5. (a) The Mg–S equilibrium bond length (dMg–S) with the nearest neighbor S atoms (Å) for the TiS3 at different Mg content. ∆c means the expansion of lattice parameters c after Mg absorbed in TiS3. (b) The Mg binding energy, Eb(Mg), between the Mg atom and the TiS3 at different Mg content.

The difference between Figure 4b,c and Figure 4d is changing of coordination of Ti from 8-fold to 6-fold, corrupting 2D Ti-S layerers to 1D ones. But the destruction of the structure induced by


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continual filling of the structure with Mg still exists, and can be the possible reason of the experimental observation of huge capacity loss for the first few cycles.10 Because the subsequent cycles show retention of some value of capacity, we can address this destruction to the phase transition to more stable cycled phase similar to Li2FeSiO4.19 Unfortunately, no experimental triple Mg-Ti-S phase diagram exists, only the one calculated by Materials Project team.20 In this phase diagram there are 8 metastable triple compounds. So it can be that in the real experiment the initial TiS3 structure transforms to one of these structures. It can also even transform to the amorphous titanium sulfide, which is known to be a good cathode material for Li batteries.21 Another possible route for the creation of new cathode materials could be searching for the metastable material with a sufficiently firm framework to withstand cycling conditions. This is the case in the search of new lithium cathode materials, such as LiFeSO4F cathode with KTiOPO4 (KTP) structure22 and even Mg cathodes like C60.23 Analyzing the process of phase transformation further, we show that the most intense phase transformations begin during the transformation from Mg0.5TiS3 to MgTiS3 (Figure 4d): the change of coordination of Ti from 8-fold to trigonal prismatic starts, as well as trigonal prismatic to octahedral for Mg. This leads to destruction of the 2D Ti-S layers to 1D ones. The covalent SS bonds of the disulfide ions break resulting formation of the separate S2- ions. In order to evaluate the amorphization risks and risks of undesirable phase transformations upon magnesiation/demagnesiation, we calculated the energy above hull (Ehull). Ehull is a computed descriptor of the stability of a compound, and equals the thermodynamic decomposition energy of the compound into the most stable phases.24-27 Ehull equal to zero


13

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indicates stable compound, while its positive values indicate thermodynamical instability. Ehull is calculated at 0K and entropic contributions can stabilize a structure at higher temperatures. The energy above the convex ground state hull (Ehull) of TiS3 intercalated with Mg was evaluated with respect to stable compounds in the Ti-Mg-S ternary phase diagram. The values are listed in Table 1 and were calculated for compounds, suggested as stable in the Materials Project resource.20 The trends in Table 5 and Figure 6 indicate that TiS3 becomes highly unstable upon intercalation with Mg.

Figure 6. The energy above hull, Ehull values for MgxTiS3 (x = 0.0-2.0). Table 5. The Ehull values (in eV/f.u.) and the corresponding decomposition products are listed as a function of Mg content in the TiS3 structure, as obtained from the Materials Project database. The comments column indicates experimental observations available in the literature. Composition Ehull

Decomposition

Comments

products


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TiS3

0.000

-

Stable16

Mg0.5TiS3

0.945

TiS2 + TiS3 + MgS

-

MgTiS3

2.001

TiS2 + MgS

-

Mg1.5TiS3

2.635

Ti7S12 + MgS + TiS

-

Mg2TiS3

3.249

MgS + TiS

-

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The decomposition process can be shown as: 2Mg0.5TiS3 → MgS + TiS2 + TiS3 The enthalpy of this process was calculated by us to be -0.59 eV/f.u., thus exothermic. MgS crystallizes in rocksalt Fm-3m structure and TiS2 in layered P-3m1.28,29 As we can see for Mg2TiS3 optimized metastable structure there are structural fragments resembling the formation of the local coordination corresponding to these two phases (Figure 7). TiS2 fragments, in principle, can be assembled into the TiS2 layered P-3m1 structure, known as one of a few successful Mg cathodes.30


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Figure 7. Possible mechanism of the structural decomposition of the Mg-intercalated TiS3 into MgS and TiS2.

To model the migration of Mg in TiS3 it was necessary to create the vacancy in Mg0.5TiS3 to allow empty space for migrating cation for the continuous movement (Figure 7c). From Figure 8 it is seen that migration through the trigonal prismatic site with direction opposite than the major orientation causes the rather big migration barrier about 1.2 eV. It is important to note that migration in any direction comes through such sites. Comparing with the other works about TiS2 analogue, it is seen that in its dilute form Mg1/3TiS2 the migration barrier about the same value as ours (1.2 eV), but for more comparable with our object Mg1/3TiS2 it has some lower 1.1 eV value.31 It can be concluded that the use of TiS3 has no gain in performance in comparison with TiS2 in this direction. However we can see that the migration barrier in the [010] direction is quite low 0.292 eV, which for Mg becomes surprising. To check that we recalculated this migration barrier with another software (SIESTA)32 in the Mg0.5TiS3 composition (Figure 9). The results gave us 0.698


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eV value. The opposite lowering trend with increasing Mg content was observed for layered TiS2 by the authors Emly et al. from 1.2 eV for Mg1/32TiS2 to 1.0 eV for Mg1/2TiS2. But even for spinel TiS2 they got 0.85 eV. For Mg0.375TiS3 we got 0.292 eV. These results allow us to conclude that for TiS3 the migration of Mg is much easier than for other analogs. This can lead to superior performance of the TiS3 based Mg cathode.

Figure 8. Mg hop in Mg0.375TiS3. (a) Migration pathway from a trigonal prismatic site with “down” orientation (α) to the neighbour trigonal prismatic site with “up” orientation (γ, green) through the agjastent trigonal prismatic sites (β, δ). (b) Diffusion energy barrier and (c) hop path as projected along the c-axis into the plane of the intercalation layer.


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Figure 9. Mg hop in Mg0.5TiS3 in the [010] direction calculated using the SIESTA code. (a) Migration pathway from a trigonal prismatic site (α) to the neighbour trigonal prismatic site (γ) (c) through the saddle-poin site (β) (b). Diffusion energy barrier (d).

We calculated Mg chemical potentials for TiS3 at 0K temperature using density-functional theory. Figure 10 shows the calculated voltage profile for TiS3. It is known that at finite temperature the calculated voltage profiles become to smear.33 To approach this limit we need to consider as much as possible configurations in MgxTiS3 with the lowest ∆x, but this requires much computer resources and not reasonable. More reasonable is to take some finite step size. Thus for ∆x = 0.5 taken here we will have only averaged values. The initial value thus in average correspond that of experimental of Taniguchi et al.10 However, the steps in curve remain and here they correspond to phase decomposition processes.


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Figure 10. Calculated voltage profile for TiS3.

CONCLUSIONS In this paper, first-principle calculations have been used to study the structural properties and phase stability of TiS3 and its characteristics as electrode materials in rechargeable Mg ion batteries. The geometry optimization of the structure containing 0.5Mg showed the shift of layers accompanying change of Mg coordination from square pyramidal to trigonal prismatic. Further increase in Mg content goes as exothermic process and leads to break of TiS3 layers into a ribbons and then the phase decomposition into MgS and TiS2. Suppressing of this decomposition is the most important challenge for creation of the reversible TiS3-based cathode. A very low migration barrier of 0.292-0.698 eV was found in the [010] direction. This value is much lower than that of common analogs like layered and spinel TiS2 (0.8-1.2 eV). This can lead to superior performance of the TiS3 based Mg cathode. AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Petr Tikhonov at ISC RAS for useful discussions. REFERENCES (1) Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. (2) Andre, D.; Kim, S. J.; Lamp, P.; Lux, S. F.; Maglia, F.; Paschos, O.; Stiaszny, B. Future Generations of Cathode Materials: An Automotive Industry Perspective. J. Mat. Chem. A 2015, 3, 6709-6732. (3) Shterenberg, I.; Salama, M.; Gofer, Y.; Levi, E. Aurbach, D. The challenge of developing rechargeable magnesium batteries. MRS Bull., 2014, 39, 453-460. (4) 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. (5) 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. Engl., 2015, 54, 7900.


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