Re-exploration of the transition metals sulfides in the context of anionic redox

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Hydrothermal synthesis of ternary alkaline-earth ruthenium oxides.

As mentioned in the introduction, Hiley et al.116 proposed a hydrothermal process to synthetize ruthenium oxides, the aim of this first part is to deepen the understanding of this reaction procedure and see how the structure of the synthesis products depends on the physico-chemical reaction parameters.
For all the synthesis described in the following sections, we used KRuO4 (Alfa Aesar), BaCl2.2H2O (Alfa Aesar), and Sr(NO3)2 (Sigma Aldrich) as sources of Ru, Ba and Sr, respectively. To prepare the aqueous alkaline solutions, extra pure KOH (> 99.98 metal basis, Alfa Aesar) was used, as well as NaOH (98% metal basis, Sigma-Aldrich) and LiOH (98% metal basis, Alfa Aesar). The synthesis consists in reacting a ruthenium (VII) salt (KRuO4) with alkaline-earth (BaCl2, Sr(NO3)2) salts in aqueous alkaline hydroxides solution (KOH, NaOH or LiOH) at 200°C. Our synthesis procedure throughout this manuscript, if not otherwise mentioned, consists in i) mixing of 5 mg (2.45.10-2 mmol) of KRuO4 with the desired amount of alkaline-earth salt in 1 mL of aqueous solution of alkaline hydroxide with controlled concentration, ii) pouring the mixtures into a hermetically sealed tailor made 2 mL Teflon-lined steel autoclave and iii) placing the autoclave at 200°C in a preheated chamber furnace for 72 hours. After the reaction, powders are cleaned three times: twice with 5 mL of a 10-2 M solution of HCl and then with water before being dried at 100°C overnight. The resulting powder was then characterized for phase purity and composition. Hydrothermal syntheses being known as extremely sensitive to physico-chemical parameters such as pH, reactant ratio, cations in solution or temperature, a survey of these various parameters has been undertaken.
i) Impact of KOH concentration and reactant ratio: speciation diagram
For guidance purpose speciation diagrams were first experimentally drawn (cf. figure II.2 and II.3) as function of the KOH concentration and of the MX2/KRuO4 ratio (with MX2 = BaCl2 or Sr(NO3)2). To build these diagrams, three different molar ratios of BaCl2/KRuO4 reactants (0.5, 1 and 2) were studied. For each of these ratios, nine concentrations of KOH ranging from 0 to 8 mol.L-1 were considered (cf. figure II.2 a)). The same ratios were used for Sr(NO3)2/KRuO4, with seven concentrations of KOH ranging from 0 to 3 mol.L-1(cf. figure II.2 b)). We observed that the higher KOH concentration is, the lower the amount of powder is formed at the end of the reaction (after 72 hours) and this depends on the nature of alkaline earth cation. Consequently, no KOH concentration greater than 8 M and 3 M will be studied from now on for the barium/ruthenium and strontium/ruthenium systems, respectively. The resulting samples from such a survey that correspond to symbols in the diagram (cf. figure II.3) were analysed for phase purity by XRD and single phase domains are defined by different colours. For multiphase samples, the relative ratios are not given, as they slightly fluctuate from one experiment to the other.
For the barium/ruthenium system with a BaCl2/KRuO4 ratio of 1 we observed the formation of five phases (cf. figure II.3 a) upon increasing KOH concentration: BaRu2O6 ([KOH] < 1M), Ba2Ru3O9(OH) (1M < [KOH] < 4M), Ba2Ru3O10 (4M < [KOH] < 6M), 10H-Ba5Ru4O15 (6M < [KOH] < 7M), and finally the layered perovskite Ba4Ru3O10.2(OH)1.8 for [KOH] > 7M. Note that BaRu2O6, Ba2Ru3O10 and 10H-Ba5Ru4O15 are new phases obtained in pure form except for 10H-Ba5Ru4O15 which was contaminated with traces of either Ba2Ru3O10 or Ba4Ru3O10.2(OH)1.8. This contrasts with the strontium/ruthenium system with Sr(NO3)2/KRuO4 of 1 for which we solely found (cf. figure II.3 b) three different phases which are in the order of increasing the KOH concentration : SrRu2O6, Sr2Ru3O9(OH) and Sr2Ru3O10, respectively. Interestingly, none of the isolated phase have the same M/Ru ratio whatever the alkaline earth cation (M = Ba or Sr). Moreover, this ratio is not distributed randomly within the diagram but increases with the KOH concentration taking the values of 0.5, 0.66, 1.1, 1.25, 1.3 for BaRu2O6, Ba2Ru3O9(OH), Ba4Ru3O10.2(OH)1.8, 10H-Ba5Ru4O15, Ba4Ru3O10.2(OH)1.8. The same trend applies as well for the strontium/ruthenium system and this is not fortuitous as discussed latter.
Our results further indicate that the M/Ru ratio in the obtained phases greatly differs from the reactant ratio. Nevertheless, the fact remains that this reactant ratio impacts considerably the domain of OH concentrations, that is the pH, over which these phases are forming as shown in figure II.3 a) and b). For example, when the reactants are alkaline-earth rich (MX2/KRuO4 >1), phases richer in alkaline earth are formed in a broader [KOH] range (red and orange colours), while such phases cannot even form when the reactants are alkaline-earth poor. The inverse observation is made with alkaline-earth poor reactants (i.e MX2/KRuO4 <1) (see green and blue domains figure II.3 a)).
ii) Impact of the counter cation
Having explored the impact of KOH concentration on the nature of the synthesis products within the Ba-Ru and Sr-Ru systems, we next check the importance of the nature of counter cation (K+). New syntheses were performed by replacing KOH by NaOH and LiOH while keeping BaCl2/KRuO4 and Sr(NO3)2/KRuO4 precursor ratios of 1. For the strontium/ruthenium couple, whatever the alkaline hydroxides used, the obtained phases remain the same. This is not any longer true in presence of the barium/ruthenium couple for hydroxides concentrations greater than 1M since with NaOH and LiOH, we obtained the NaBa4Ru3O12 and LiBa4Ru3O12 phases, respectively as opposed to Ba2Ru3O10, 10H-Ba5Ru4O15 and Ba4Ru3O10.2(OH)1.2 for [KOH] > 1 M, as previously described. Let’s recall that the aforementioned Li and Na-based compounds are not new since they have been previously reported using ceramic high temperature process (800°C).120 For completeness we also examined the influence of counter ion concentration, replacing KOH with KCl. Whatever the added KCl concentration we only obtained BaRu2O6. Therefore, the hydroxide ions are essential to guide the reaction. Altogether these results show that the hydroxide counter ions can modify the speciation diagram leading to new compounds in which they could become a component of the structure.
iii) Impact of the temperature
Temperature is another parameter that we have investigated as it is of paramount importance in controlling reaction pathways. Figure II.4 represents XRD patterns of the phases resulting from heating precursor mixtures (BaCl2:KRuO4 = 1:1 at [KOH] = 4 M) at four different temperatures ranging from 120°C to 220°C. Crystalline phase are solely obtained for T > 120°C. The phase formed at 175°C (red diamonds) can easily be identified as Ba2Ru3O10. With increasing temperature to 200°C, there is the appearance of a second phase Ba2Ru3O9(OH) (blue triangles) which grows at the expense of the first one and becomes single phase as the temperature reaches 220°C. This indicates a well-pronounced influence of the temperature on the speciation diagrams and may explain, based on the temperature inhomogeneity of our furnaces, the occasional irreproducibility of the synthesis.

Low temperature cations exchange

Recent studies have reported the low temperature synthesis of the AMO3 compounds (with A = Mg or Zn and M = Mn or Ir). It consists in exchanging the lithium atoms of the lamellar compounds Li2MO3 with divalent cations by the following metathesis reaction123,124:
The thermodynamic driving force of this reaction has to our knowledge not been discussed, although it is clear that the consumption of an equivalent of ACl2 to form two LiCl entropically favors the reaction. Therefore, inspired by these studies, we attempted to synthetize ZnRuO3 and MgRuO3 which could exhibit interesting magnetic properties or possibly be candidates as cathode materials for Zn or Mg-ion based batteries.
Experimentally, these reactions must take place in the complete absence of an oxygen source to avoid the formation of the inert zinc or magnesium oxides. Therefore, the hygroscopic halides (ZnCl2, MgCl2, etc…) have been bought ultra-dried. The Li2RuO3 precursor has been synthetized inspired by already reported procedure:76 an appropriate amount of RuO2 (Alfa Aesar 99.9%) was mixed with a 10%wt excess (to compensate the volatilization at high temperature) of Li2CO3 (Sigma Aldrich 99.0 %) with a mortar and pestle and then ball milled (SPEX). The mixture was then transferred to an alumina crucible and heated to 900° for 12 h using a heating and cooling rate of 2°C.min−1. Then, to perform the cation exchange, Li2RuO3 as well as the magnesium (zinc) precursor were mixed in an argon-filled glove box before being transferred in a quartz tube and subsequently sealed under high vacuum. The resulting sealed tube was then placed in a box furnace at the required temperature for 60 hrs. After the reaction, the quartz tube was opened in air and the resulting powder was rinsed once with water and a second time with 0.1 M HCl solution with centrifugation steps in between, the washed powders were finally dried in an oven at 110°C for 24 hours.
i) MgRuO3
The lithium/magnesium exchange was carried out according to the protocol mentioned above using ultra dried MgCl2 (Alfa Aesar 99.99%) in a molar ratio Li2RuO3:MgCl2 of 1:2 and with a heating temperature of 350°C. XRD analysis (cf. next section) suggests the synthesis of a pure compound with minute amounts of crystalized RuO2.
ii) ZnRuO3
Cation exchange was first attempted using the same conditions as for MgRuO3; however, it lead to the formation of two phases. Of these, one was identified as the expected layered ZnRuO3 and the other one could be indexed with a F-centered cubic unit cell (a = 8.4278(8) Å) and could refer to a spinel (Zn2RuO4, ZnRu2O4 ?). In order to obtain the pure ZnRuO3 phase, the heating temperature was first varied from 200° C to 450°C in 50°C steps. While no sign of intercalation was observed for syntheses performed below 300°C, the best ZnRuO3/”cubic phase” ratio was obtained at 300°C (cf. figure II.7) and any increase in temperature clearly favored the growth of the “cubic phase” at the expense of ZnRuO3 (cf. figure II.7). Following this first test, different reactions parameters were investigated. Briefly, the Zn/Ru ratio in the reagents was changed (form 1:2 to 10:1), ZnCl2 was then replaced by ZnBr2 or ZnI2 and lastly Na2RuO3 has been tried instead of Li2MnO3 without much success. At this point, we have not found a way to obtain pure ZnRuO3 and the best remaining strategy is probably to understand the nature of the “cubic” phase (XRD, EDX) to determine the physicochemical levers available to obtain the desired phase.
Figure II.7: X-ray diffraction patterns of powders obtained after reaction of Li2RuO3 with ZnCl2 in a
1:2 ratio at 300°C (blue) or 400°C (pink). The pattern at 400°C can be indexed with a F-centered cubic unit cell with a = 8.4278(8) Å. The vertical grey dashed lines highlight the positions of reflections from this “cubic phase” with the corresponding indexations; the other reflections observed in the 300°C pattern can be indexed by the expected layered ZnRuO3 structure and are highlighted by an asterix.
Now that the various synthetic processes have been described, the next section focuses on describing the structure of the as-synthetized compounds.

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Structural characterisations

Elemental analysis

The atomic ratio of Ru/M (where M=Sr/Ba) where determined using energy dispersive X-ray spectrometry (EDX) and are reported in the table II.1. ICP-Ms analysis attempts were done to determine the stoichiometry of the compounds, but the phases described above where found to form RuO2 after acidic treatment (aqua regia or HCl 37%). RuO2 is insoluble in acidic media and then the analyses were not convincing.

Structural characterisations

In the following section, unless otherwise stated the synchrotron X-ray diffraction measurements (SXRD) were performed on the 11-BM beamline of the Advanced Photon source at Argonne National Laboratory, with a wavelength at 0.413 Å (the exact wavelength for each materials is given with the refinement). Laboratory powder XRD measurements were performed with a Bruker D8 Advance diffractometer as described in the materials and methods section. Finally, single crystal X-ray diffraction (SCXRD) data were collected at the X-ray diffraction platform of IMPMC, on a Rigaku MM007HF diffractometer equipped with a RAXIS4++ image plate detector, a Mo rotating anode (λ = 0.71073 Å, Varimax multilayer optics) at 293 K (cf. details in the materials and methods).
The crystal structures of 10H-Ba5Ru4O15, Sr2Ru3O10, Ba2Ru3O10 ,Sr2Ru3O9(OH) and BaRu2O6 were determined while the MgRuO3 one is discussed at the end of the section. For sake of clarity, we will handle each compound separately by first reporting the exact synthesis process of the powders used for carrying the structural determination.

Table of contents :

Chapter I: State of the art
I.1 Intercalation chemistry: from the synthesis tool to the Li-ion batteries
I.1.a Intercalation chemistry: definition and principles
I.1.b Intercalation chemistry: modifications of the physical properties
I.1.c Intercalation chemistry to store the energy: Li-ion batteries
I.2 Design principles of a cathode material
I.2. a Tuning the crystal structure
I.2. b Tuning the redox potential
I.2. c Increasing the gravimetric capacity
I.2 d Redox in high-valence systems
Chapter conclusion
Chapter II: Low temperature synthesis of ruthenium ternary oxides 
II.0 Introduction
II.0.a Low temperature synthesis?
II.0.b Ruthenium oxides
II.1 Synthesis
II.1.a Hydrothermal synthesis of ternary alkaline-earth ruthenium oxides.
II.1.b Low temperature cations exchange
II.2 Structural characterisations
II.2.a Elemental analysis
II.2.a Structural characterisations
II.3 Physico-chemical properties
II.3.a Magnetic properties of BaRu2O6
II.3.a Electrochemical insertion of Li+ in BaRu2O6 and SrRu2O6
Chapter conclusion
Chapter III: Re-exploration of the transition metals sulfides in the context of anionic redox
III.0 Introduction
III.0.a General Background
III.0.b How to activate anionic activity in transition metal sulfides?
III.1 Study of the Li3MS4 family (M = V, Nb or Ta)
III.1.a Synthesis
III.1.b Structures
III.1.c Electrochemistry
III.1.d How to explain the difference in electrochemical behavior of the two polymorphs Li3NbS4 polymorphs ?
III.1.e Partial conclusion
III.2 Study of the LiIrS2 and IrS2 compounds
III.2.a Experimental results:
III.2.b Discussion
Chapter conclusion
Chapter IV: Investigation of new chemistries
IV.0 Introduction
IV.1 Oxysulfides
IV.1.a General Considerations
IV.1.b Na3VS4-xOx compounds
IV.2 Vanadium halides
IV.2.a Synthesis
IV.2.b Structure
IV.2.c Lithium intercalation
IV.2.d Magnetic properties of the LixVX3 phases (with x = 0 or 1)
Chapter conclusion

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