Lithium intercalation into TiS2 cathode material: phase equilibria in a Li–TiS2 system Evgeny A. Suslov, Olga V. Bushkova, Elena A. Sherstobitova, Olga G. Reznitskikh & Alexander N. Titov Ionics International Journal of Ionics The Science and Technology of Ionic Motion ISSN 0947-7047 Ionics DOI 10.1007/s11581-015-1566-0
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Author's personal copy Ionics DOI 10.1007/s11581-015-1566-0
ORIGINAL PAPER
Lithium intercalation into TiS2 cathode material: phase equilibria in a Li–TiS2 system Evgeny A. Suslov 1 & Olga V. Bushkova 1 & Elena A. Sherstobitova 1,2 & Olga G. Reznitskikh 1 & Alexander N. Titov 2,3
Received: 2 September 2015 / Accepted: 20 September 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Lithium-intercalated titanium disulfide LixTiS2 had been extensively studied as prototypical cathode material for high-energy-density reversible lithium batteries with moderate voltage. Today, this phase is one of the leading candidates for all-solid-state lithium batteries with durable high energy and high rate performance. However, fundamental knowledge of Li+ insertion into TiS2 is still incomplete; available information on the phase diagram of the Li–TiS2 system is limited by x=1 due to difficulties in preparing lithium-rich equilibrated samples. The goals of this work were to re-examine LixTiS2 single phase boundaries and to clarify the existence of lithiumrich intercalates with x>1. А new preparation technique was developed to obtain 1T-LixTiS2 samples as good-quality equilibrated products with desirable lithium content. Phase equilibria in a quasibinary Li–TiS2 system were studied by X-ray diffraction, emf measurements (25 °C) and thermal analysis (25–350 °C) over a wide range of Li:Ti ratios (0 to 4). Keywords Intercalation compounds . Material preparations . Electrochemical characterisations . XRD . Thermal analyses Electronic supplementary material The online version of this article (doi:10.1007/s11581-015-1566-0) contains supplementary material, which is available to authorized users. * Olga V. Bushkova
[email protected] 1
Institute of High-Temperature Electrochemistry, Ural Branch of the Russian Academy of Sciences, Akademicheskaya st. 20, Ekaterinburg 620990, Russia
2
M.N. Mikheev Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, Sofii Kovalevskoi st. 18, Ekaterinburg 620990, Russia
3
Ural Federal University, Mira st. 19, Ekaterinburg 620002, Russia
Introduction Layered dichalcogenides with the general formula MX2 (M= transition metal, X=S, Se, Te) form a group of materials that can be intercalated with a wide range of both organic and inorganic materials without undergoing major structural modifications [1–3]. The intercalation reaction is driven by charge transfer from the intercalant to the host layered compound conduction band; thus, electron-donating species can take part in such a reaction. Lithium-intercalated dichalcogenides LixMX2 have been extensively studied as cathode materials for high-energy-density reversible lithium batteries, and TiS2 is the prototypical cathode material, demonstrating all of the desired characteristics except for overall cell voltage, 1.7– 2.5 V [1, 4–7]. Exxon combined TiS2 with a LiAl anode and an organic-based electrolyte to create safe, rechargeable cells in 1977–1979 [4, 5]. Today, transition-metal-based cathode materials such as LiCoO2, LiNixCoyMnzO2, LiNi0.53Co0.3Al0.17O2, LiMn2O4 and LiFePO4 are the most commonly used in commercial lithium-ion cells [4, 8–10]. Nevertheless, TiS2-based systems having favourable intercalation kinetics and high energy density (which have still not been exceeded by systems such as LiCoO2 and LiMn2O4 [5]) are currently considered attractive candidates for high-power lithium batteries. Recently, it was demonstrated that titanium disulfide is a particularly interesting cathode material in conjunction with a metallic lithium anode in all-solid-state cells [11–14] or with lithiated silicon anode in Li-ion cells comprising modified liquid electrolyte [15]. Moreover, TiS2 was incorporated as a second active component in sulfur–carbon cathodes of Li–S cells; serving as active reaction sites during cycling, TiS2 suppresses agglomeration of sulfur and facilitates better ionic/electronic transport within the cathode structure [16, 17]. In addition to their practical importance, the Li– TiS2 system has attracted considerable attention from theorists
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because the TiS2 host provides an important model for elucidating lithium diffusion mechanisms and the structural, elastic and electronic effects of lithium intercalation [18–25]. The goal of the present work was to investigate phase equilibria in the Li–TiS2 system up to a Li:Ti ratio of 4 in order to re-examine LixTiS2 single phase boundaries and to clarify the existence of lithium-rich phases with x > 1. Lithiumintercalated compounds LixTiS2 were obtained by a new preparation technique based on direct reaction between Li metal and TiS2 that does not set limits on the composition range. Because the structure of the titanium disulfide host remains essentially unchanged during lithium intercalation, X-ray data were combined with thermodynamic emf studies of phase equilibria. To evaluate thermal behaviour of the LixTiS2 intercalates, DSC measurements were carried out over the range of 25–350 °C.
Lithium-intercalated titanium disulfide Structure of titanium dichalcogenides TiX2 Titanium disulfide, diselenide and ditelluride are isomorphous members of a group of compounds that crystallise in a characteristic 1T-CdI2 layer structure (P3 m1 space group) [1]. Their structures (Fig. 1(a)) consist of stacked composite layers, each layer comprising two sheets of hexagonally close-packed chalcogenium atoms sandwiching a sheet of metal atoms. Because the primary valency is saturated within the sandwich X–Ti–X structure, adjacent layers are held together only by relatively weak van der Waals forces. The sandwich layers therefore form extended sheets running through the lattice, forming a strongly anisotropic structure. Thus, the crystals are characterised by a platy habit, with extended growth and pronounced cleavage perpendicular to the trigonal c-axis [1, 23, 26–28]. When intercalation occurs, the intercalant Li enters the van der Waals gaps between the X– Ti–X layers. The system can then be considered to be more three-dimensional [29]. Because the transfer of charge (electrons) from the intercalant Li atoms to the host TiX2 (b)
(a)
(c)
c a
lattice occurs, the relatively weak van der Waals forces are replaced by stronger Coulomb interactions, and the lattice does not expand very much along the c direction normal to the layers. In Fig. 1(b, c), the three interstitial sites available for Li intercalation in the van der Waals gap are shown: one octahedral, which is surrounded by six chalcogen atoms, and two tetrahedrals, which lie in the projections below and above the chalcogen atoms [1, 28]. Thus, from structural considerations, the overall limiting value of intercalated lithium in the TiX2 structure can reach x=3. Lithium intercalation into TiS2 It is generally accepted that the reaction between lithium and titanium disulfide is the perfect intercalation reaction, with the reactant and product having the same structure over the entire range of the reaction, 0≤x≤1, in LixTiS2 [1, 4, 5, 27, 30–36]. This single phase is characterised by only a 10 % expansion of the crystalline lattice along the c-axis (perpendicular to the basal plane) and approximately 1 % along the a-axis [27, 30, 34] which is typical for solid solutions. More recently, a single phase was not excluded for a wider range, at least, up to x= 1.24 [37]. In addition to the likely extension of this solidsolution region, the results regarding the charge–discharge profiles of Li/TiS2 cells [38–40] make it possible to propose formation of a lithium-rich compound with x>1. The range of 1≤x≤2, with a low-voltage plateau (near 0.5 V vs. Li/Li+) in the discharge profile, had been suggested to be a region where LiTiS2 (accepted now as the end member of LixTiS2 solid solution) and hypothetical stoichiometric Li2TiS2 coexist [38–41]. The intercalation process for the range 0≤x≤2 is reversible [38, 39, 41] while electrochemical intercalation up to x=3 results in hysteresis [39]. It should be noted, however, that all attempts to prepare the lithium-rich Li2TiS2 phase to study its physical properties have been unsuccessful [38, 40]; the proposed XRD pattern of Li2TiS2 was obtained ex situ for electrochemically intercalated samples [38]. In contrast, LixTiS2 solid solutions with 0≤x≤1 have been repeatedly synthesised and studied [1, 27, 34, 42, 43]. Thus, it is valid to say that despite numerous studies on lithium intercalation into TiS2, the phase diagram of the Li– TiS2 quasibinary system clearly requires further investigation. This is particularly true for Li:Ti ratios greater than 1, for which very poor previous information (mainly, charge–discharge profiles) is available. It seems that a key problem on this way is synthesis of lithium-rich intercalate samples. LixTiS2 preparation methods
Li
TiS6 octahedron
Fig. 1 Structure of h-TiX2 (X=S, Se, Te) (а) and sequential lithium filling of octahedral (b) and tetrahedral (с) sites in van der Waals gap
Commonly used electrointercalation in Li/TiS2 cells with a non-aqueous electrolyte [1, 22, 30, 44–46] inevitably yields a composite product, Li x TiS 2 , with a polymer binder,
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acetylene black, electrolyte, etc. Therefore, a number of chemical intercalation techniques using non-aqueous solutions of lithiating reagents have been proposed to prepare pure LixTiS2 intercalate compounds with a preset Li:Ti ratio. One such technique is the treatment of parent TiS2 with a solution of lithium in liquid ammonia at low temperatures followed by heat treatment at 250 °C in vacuo to remove NH3 molecules inserted between the S–Ti–S layers [1, 42, 47, 48]. In addition to ammonia co-intercalation, this technique is characterised by obvious experimental difficulties. Therefore, the most frequently used chemical intercalation technique to date remains the immersion of TiS2 powder in the appropriate amount of nbutyllithium solution in hexane (or other simple hydrocarbons) for a few days [1, 18, 31, 37, 47–54]. The procedure is carried out at ambient temperatures. After the filtration and washing of the solid with hexane, vacuum drying is used to remove residual solvent. It is a Bclean^ method: only lithium penetrates between the layers, the butyl radicals combining to yield n-octane, which is easily eliminated together with the solvent. It is worth noting, however, that this method allows the maximum possible reaction product of one lithium per titanium [34], i.e. only LixTiХ2 with 03 (scheme 1)
Preset Li:Ti ratio
1.0 2.0 3.0
Starting materials
TiS2 +Li
Stage 1 Temperature, °C
Time, days
Temperature, °C
Time, days
235
14
235
14
0.97–1.01
14 28
2.13 2.99–3.08
3.1; 3.2; 3.4 4.0 a
Li/Ti ratio by ICP/MS dataa
Stage 2
28 35
The interval of values is given for a series of syntheses
thermal treatment for each reaction mixture is presented in Table 1 for stage 1. After the first stage of annealing (when Li metal was fully transformed), the ampoules were cooled, transferred into a glove box with an inert argon atmosphere and opened. The charges were ground into powder for homogenisation, pressed into pellets and placed into Pyrex ampoules, evacuated to 10−5 Torr, and annealed again at the same temperatures (Table 1, stage 2). The stage 2 operations were repeated until the diffractogram of the product stopped changing; the overall durations of the thermal treatment are presented in Table 1. Since we have proposed here a novel preparation technique for the LixTiS2 intercalates, it is of interest to consider a sequence of chemical reactions, occurring in the reaction mixture during thermal treatment. An example of changes in XRD patterns during thermal treatment is shown in Fig. 2(a, b) for reaction mixture of Li metal and TiS2 with a Li:Ti ratio of 3. The resulting chemical processes may be schematically summarised as follows: x x x xLi þ TiS2 ¼ Li2 S þ Ti; 4 2 4
ð1Þ
x x x Li2 S þ 1− TiS2 þ Ti ¼ Lix TiS2 : 2 4 4
ð2Þ
Equation (1) describes the reaction which is the main content of stage 1 of the synthesis process while Eq. (2) relates to the interaction which proceeds mainly during stage 2. Intermediates in such a way are Li2S and h-Ti metal (incorporation of excess titanium into the van der Waals gap of TiS2 also cannot be excluded). It is easy to see that stage 2 is chemically identical to the process used in some solid-state preparation techniques [38, 59–62] described above in Section BLixTiS2 preparation methods^ but occurs at a much lower temperature due to fine h-Ti intermediate (for this reason, h-Ti peaks are hardly distinguishable in XRD patterns; see Fig. 2(a)) or excess Ti in Ti1+yS2. Because the sample preparation technique employed can influence the experimental results (as it was shown in Section BLixTiS2 preparation methods^), it is necessary to specify some of the details of the thermal treatments. A muffle furnace with a uniform distribution of temperature along vertically placed ampoules was used to prevent a violation of
stoichiometry due to the deposition of chalcogen vapour on the cooler end of the ampoules. After each thermal treatment was completed, the ampoules were slowly cooled together with the muffle furnace to room temperature to prevent the quenching of the high-temperature phases. In all cases, any
Fig. 2 XRD patterns for the reaction mixture of Li metal and TiS2 with different Li:Ti ratios after annealing at 235 °C: a thermal treatment for 28 days (intermediates for Li:Ti=3); b thermal treatment for 42 days (Li3 −δTiS2 as a final product); c thermal treatment for 42 days (equilibrium products for Li:Ti = 3.4); d thermal treatment for 49 days (nearly equilibrium products for Li:Ti=4). The dashed grey bar at the abscissa indicates the reflections from the sample holder sealed with Al foil
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deposits on the ampoule walls or their turbidity due to chemical interactions with lithium vapour were absent. Therefore, it was reasonable to propose that the resulting product composition was similar to the preset value. To be confident in the composition for the intercalates obtained, the lithium content for samples with nominal compositions of LiTiS2, Li2TiS2 and Li 3 TiS 2 was analysed by ICP/MS, as described in Section BExperimental^. The results are presented in Table 1. One can see that the uncertainty in composition associated with ampoule synthesis was approximately 1–7 %. LixTiS2 intercalates with fractional х values less than 3 were synthesised as follows. Powdered TiS2 and synthesised products with nominal compositions of LiTiS2, Li2TiS2 and Li3TiS2 were used as starting materials (Table 2) to minimise the error in the preset x values, which is extremely important if taking into account a small increment in the lithium composition Δх=0.05–0.20. Stoichiometric amounts of the reagents were homogenised, pressed into pellets, placed into Pyrex ampoules, evacuated to 10−5 Torr and then annealed at 235 °C (Table 2). These operations were repeated until the diffractograms of the products stopped changing. After completing the synthesis, all lithium-intercalated products were stored in an argon glove box for 0.5–1 year at room temperature for equilibration. Phase composition of lithium-intercalated products XRD patterns of the samples with Li:Ti ratio 0 to 3 are shown in Fig. 3. The formation of 1T phase with a nominal composition of LixTiS2 without any impurities of the starting materials or foreign phases (including 3R-LixTiS2) was observed for all samples with x3), titanium metal and Li2S were found together with LixTiS2 phase, and the intensity of the Ti and Li2S peaks increased with the Li:Ti ratio (Fig. 2(c, d)). It is reasonable to conclude that the limiting intercalated lithium content in the TiS2 structure can be really close to the expected value of x=3 (see Section BStructure of titanium dichalcogenides TiX2^). Intercalated lithium content in this sample may be expressed as x=3−δ, with δ
