Electrochemical cells based on the deposition and unboxing of chalcogen anions

Description

[0001] The present invention relates to the field of electrochemistry and solid-state ion batteries in particular.

[0002] Given global warming and the scarcity of natural resources, the production and storage of renewable energy are crucial and represent one of the greatest challenges for the scientific community.

[0003] Electrical devices and electric transportation are currently based on lithium batteries. However, lithium resources are limited. Therefore, researchers are already considering battery variants containing, for example, sodium.

[0004] In these batteries, the alkali metal cation (Li+, Na+) ensures conduction within the electrolyte: the cation (for example, Li+ ion) is exchanged between two active materials located on the positive and negative electrodes. These materials can intercalate and deintercalate lithium, at least in the positive electrode.

[0005] More recently, Li-ion batteries using elemental sulfur as a cathode have been considered (Manthiram et al. Chem. Rev. 2014, 114, 11751-11787). However, in such lithium-sulfur batteries, the conducting ionic species that moves between the sulfur cathode and the lithium anode remains the lithium cation (Li+).

[0006] Other ionic species have also been studied as conductive species in electrochemical cells, for example, zinc-ion batteries that include zinc at the negative electrode and manganese oxide at the positive electrode. These electrochemical systems are widely studied, but their potential remains undecided: after years of research, the first stable charge / discharge cycle was recently achieved for a battery using aluminum ions as an intercalated species. Magnesium-ion batteries have also been successfully produced. Nevertheless, the use of cation variants remains a challenge.

[0007] Batteries based on the intercalation or deintercalation of an anion have been significantly less studied. Most research focuses on metal ions where O₂ ions migrate from an air cathode to a metal anode during discharge. Organic anions have also been considered (PF₆, BF₄, TFSI). Variants of anion batteries use fluoride, chloride, and metal halide such as BiF₃ and BiCl₃ as cathodes and reactive metals such as lithium as anodes, so that halide anions migrate from the cathode to the anode.

[0008] However, lithium sources are limited and other materials must be considered.

[0009] Therefore, one object of the invention is to propose other electroactive materials which are based on naturally abundant elements, such as chalcogens.

[0010] Sasaki et al Angewandte Chemie International vol.57, No.41, 2018; 13618-13623 describes the intercalation of copper in La 2 O 2 S 2 .

[0011] The present inventors have discovered that it is possible to deintercalate and reintercalate at a moderate temperature (< 300° C) a chalcogen / chalcogenide, such as sulfur, from materials containing chalcogen oligomers, such as sulfur pairs.

[0012] This phenomenon has proven reversible. This discovery opens the door to using such chalcogenized materials to create chalcogen-chalcogen batteries, for example, sulfur-sulfur batteries. Such batteries involve active materials comprising chalcogen oligomers, such as chalcogen pairs, and the exchange of a chalcogen anion between the cathode and the anode.

[0013] Therefore, according to a first object, the present invention relates to an electroactive material for an electrochemical cell electrode in which said material comprises a chalcogen oligomer, characterized in that said material can reversibly disintercalate an anion of said chalcogen by reduction and reintercalate said anion by oxidation, while retaining its overall structure, said structure being lamellar.

[0014] The term "electroactive material" as used herein refers to materials that can be used as electrode material and that can undergo a redox reaction.

[0015] Chalcogens are the chemical elements in group 16 of the periodic table. This group includes, in particular, the following elements: oxygen (O), sulfur (S), selenium (Se), and tellurium (Te). Typically, a chalcogen is sulfur.

[0016] The term "oligomer" refers to a neutral or charged chemical species consisting of chalcogen atoms chemically bonded to each other by strong chemical bonds. Typically, these blocks are made up of a few identical chalcogen atoms, typically between 2 and 6, with a charge that can range from 1 to 2. The chalcogen oligomer thus represents a linear or nonlinear sequence of 2 to 6 chalcogen atoms, typically of the formula Qn, where Q represents a chalcogen atom and n represents an integer between 2 and 6. Typically, the chalcogen oligomer is a sulfur pair (SS).

[0017] Typically, said material comprising a chalcogen oligomer may be chosen in particular from La 2 O 2 S 2 , SrS 2 , SrS 3 , BaS 2 , BaS 3 , Ba 2 S 2 F 2 , FeS 2 , NiS 2 , CoS 2 , MnS 2 , TiS 3 , VS 4 , PbS 2 , BiS 2 but any compound with a sequence of at least 2 chalcogen atoms may be of interest.

[0018] According to the invention, said materials comprise chalcogen oligomers that can be reduced to form chalcogen anions or chalcogen oligomer anions, such as: (Q n ) 2-< + (2n-2)e -< → n Q 2-<

[0019] In the case where the chalcogen is sulfur, the following reductive cleavage can occur: (S 2 ) 2-< + 2e-< → 2 S 2-< or (S 3 ) 2-< + 4e- → 3 S 2-<

[0020] For example, the structure of La₂O₂S₂ is made up of infinite layers of 2 ∞ La 2 O 2 2 + of the fluorite type, separated from each other by isolated sulfur (S₂)₂-< dimers aligned parallel to these 2D blocks. The deinsertion, namely the disintercalation of one sulfur atom per dimer, results in a La₂O₂S compound whose structure is inherited from the layered structure of the La₂O₂S₂ precursor. This entirely new phase crystallizes in the Amm2 space group and is a new polymorph of the well-known hexagonal La₂O₂S (hp-La₂O₂S (P-3m1)). Hereafter, the term oA-La₂O₂S will be used for this specific orthorhombic form, as opposed to the well-known hexagonal form.

[0021] Consequently, La₂O₂S₂ reversibly disintercalates and reintercalates S²⁻ according to the following reaction: La₂O₂S₂ + 2e⁻ ⇆ oA-La₂O₂S + S²⁻, where oA denotes the body-centered orthorhombic crystalline form. oA-La₂O₂S has never been synthesized or identified before. According to another object, the present invention thus also relates to the material of formula: oA-La₂O₂S(I), where oA denotes the body-centered orthorhombic crystalline form.

[0022] Similarly, La₂O₂S₂ also reversibly deintercalates and reintercalates 0.5S-< according to the following reaction: La₂O₂S₂ + e⁻< ⇆ oA-La₂O₂S₁.5 + 0.5S₂-< oA-La₂O₂S₁.5 (space group Amm2) has never been synthesized or identified before. According to another object, the present invention thus also relates to the material of formula (II): OA-La₂O₂S₁.5 (II) where oA denotes the body-centered orthorhombic crystal form.

[0023] According to one embodiment, oA-La₂O₂S(I) can be prepared using a process comprising the steps of mixing La₂O₂S₂ and Rb and heating said mixture. Typically, the reaction can be carried out in a sealed silica tube, preferably in a molar ratio of 1:2. Typically, the heating temperature is between 200 and 350°C, particularly by applying or adapting the procedure discussed in the examples.

[0024] According to one embodiment, the oA -La₄O₄S₃(II) can be prepared similarly. According to an alternative embodiment, it can also be prepared by intercalating sulfur anions into oA-La₂O₂S, namely using a process comprising the steps of mixing oA-The 2 O 2 S with S flakes, and heating of said mixture. Typically, the mixture can be granulated and sealed in a silica tube under vacuum before the heating step. Typically, the heating temperature is between 150 and 200° C.

[0025] Materials of formula (I) and (II) can be characterized by their electron and radiological diffraction spectrum, as illustrated in the attached figures.

[0026] As used herein, disintercalation refers to the removal of an atom or ion from a host lattice where it resided, while re-intercalation refers to the reintroduction (or reinsertion) into the lattice, reversibly and without modification of the overall crystal structure of the host material, namely its lamellar structure.

[0027] The invention relates to an electroactive material for an electrochemical cell electrode in which said material comprises a chalcogen oligomer, characterized in that said material can reversibly disintercalate an anion of said chalcogen by reduction and reintercalate said anion by oxidation, while retaining its lamellar structure.

[0028] According to the invention, a chalcogen (or chalcogenide) anion (typically S 2-< ) disintercalates from the electrode material during discharge (reduction) and re-intercalates within said material during charging (oxidation), at the level of the positive electrode.

[0029] According to one embodiment, said chalcogen anion is sulfur or an oligomer thereof of formula (III): (S n ) x-< (III) where n and x are integers, so that x is equal to 1 or 2, and n is between 1 and 6. Typically, n is 1 and x is 2, so said chalcogen anion is S 2-< .

[0030] In one embodiment, the electrode of an electrochemical cell is a positive electrode. The positive electrode refers to the electrode of an electrochemical cell, called an element herein, into which electrons enter and chalcogen ions exit during discharge.

[0031] "The positive electrode" refers, in discharge, to the electrode functioning as a cathode, and in charge, to the electrode functioning as an anode, the anode being defined as the electrode where an electrochemical oxidation reaction (electron emission) takes place, while the cathode is the site of a reduction.

[0032] According to another object, the present invention also relates to a positive electrode comprising the electroactive material of the invention.

[0033] According to one embodiment, the positive electrode may include a current collector and a coating layer, in which said coating layer includes said electroactive material.

[0034] The positive electrode is generally made of a conductive medium used as a current collector which is coated with a mixture comprising the electroactive material, and, typically, solid electrolyte particles, an electroconductive additive and a binder.

[0035] The term "current collector" means an element such as a pellet, plate, aluminum foil, sheet, mesh, fabric or other, made of a conductive material, connected to the positive or negative electrode, and ensuring the conduction of the flow of electrons between the electrode and the terminals of the battery.

[0036] Typically, the current collector is made of a metal or alloy chosen from the group consisting of aluminum, copper, nickel, carbon, stainless steel and alloys thereof.

[0037] According to one embodiment, the current collector is an aluminum sheet.

[0038] According to another object, the present invention also relates to an electrochemical cell comprising: a positive electrode as defined above; a negative electrode; an electrolytic layer sandwiched between the positive and negative electrodes, characterized in that the electrolyte is a solid electrolyte comprising as a conducting ion the chalcogen anion of the chalcogen oligomer of the electroactive material.

[0039] The term "negative electrode" refers, in discharge, to the electrode functioning as an anode and in charge, to the electrode functioning as a cathode, the anode being defined as the electrode where an electrochemical oxidation reaction (electron emission) takes place, while the cathode is the site of a reduction.

[0040] Typically, the term negative electrode refers to the electrode from which electrons exit, and on which chalcogen anions are collected in discharge.

[0041] By "electrochemical cell" we mean an elementary electrochemical cell composed of the positive electrode / electrolyte / negative electrode assembly, allowing the electrical energy supplied by a chemical reaction to be stored and returned in the form of current.

[0042] According to one embodiment, the electrochemical cell is a cell in the solid state.

[0043] In all solid-type elements, electrolytic compounds can be included in the electrolytic layer in the solid state, but can also be partially included within the electrodes.

[0044] According to another object, the present invention relates to a battery comprising a plurality of electrochemical cells as defined above, in which the cells are electrically connected.

[0045] By "battery" or accumulator, we mean the assembly of several cells according to the invention. Figures

[0046] [ Fig 1 ] There figure 1 illustrates the principle of a sulfur-sulfur battery according to the invention, involving sulfur as a chalcogen and La₂O₂S₂ as the electroactive material. Fig 2 ] There figure 2illustrates (a) the structure of La₂O₂S₂ reported by Ostorero et al. (SG: Cmca) (Acta Cryst. C46, ​​1376–1378 (1990)); (b) the conceptual scheme of SS bond cleavage under the elemental metal M₀< electron donation, which subsequently triggers the deintercalation of the dumbbell sulfur half-atom S₂, eventually allowing the topochemical conversion of La₂O₂S₂ to the new polymorph of La₂O₂S; (c) the two dynamically stable low-energy phases of oA-La₂O₂S predicted by USPEX. Fig 3 ] There figure 3 illustrates (a) the partial intercalation of sulfur in oA-La2O2S and the sulfur deintercalation of La2O2S2 resulting in an intermediate compound of oA-La2O2S1.5; (b) experiments on the intercalation of sulfur in oA -2O2S. Experimental XRD diagrams of oA-Pure La₂O₂S and products of its mixture with sulfur (0.5 or 1 equiv. of S) after heat treatments at 150 or 200°C. The new XRD peaks emerging after heat treatment with 0.5 S are marked with an *; (c) sulfur deintercalation of La₂O₂S₂. Experimental XRD diagrams of La₂O₂S₂ and the product of its mixture with Rb₂O₇, Ag₂O₇, and Ni₂O₇ after heat treatments at 200 or 350°C. The XRD peaks attributed to the derivative products are marked as follows; • = Ag₂S (Für Krist.-Cryst. Mater. 110, 136-144 (1958); ▪ = α -NiS (J. Trahan, RG Goodrich, SF Watkins, Phys. Rev. B 2, 2859-2863 (1970)). [ Fig 4 ] There figure 4 represents the overview of the low-temperature sulfur-rich topochemistry in the La-OS system. Topochemical intercalation and deintercalation of sulfur in the La₂O₂S₂ oxychalcogenide compound results in the formation of two new metastable compounds. Fig 5] There figure 5 represents (a) the EDX spectrum of the oA-La 2 O 2 S powder sample impregnated with epoxy resin and (b) its backscattered electron image (BEI) as well as its elemental composition mapped for La and S.

[0047] On the figure 1 The cell of such a sulfur-sulfur battery is schematically represented during discharge.

[0048] The cell comprises a positive electrode 1 (cathode) and a negative electrode 2 (anode). A sulfur anion conductive electrolyte 3 is sandwiched between electrodes 1 and 2.

[0049] The two electrodes 1 and 2 are electrically connected by means of an electrical circuit including an ammeter 9.

[0050] As described on the figure 1: The positive electrode 1 comprises a current collector 4 and a layer of electroactive material 5. The layer 5 is located at an interface between the conductive electrolyte 3 and the inner face of the current collector 4. Typically, the current collector 4 can be an aluminum foil.

[0051] The negative electrode 2 comprises a current collector 6 and a layer 7 at the interface between the conductive electrolyte 3 and the inner face of the current collector 6.

[0052] In general, the current collector 6 of the negative electrode is made of copper.

[0053] Layer 7 can be made of a sulfur composite or a metal M which can react with a sulfur anion according to the reaction: wM+S 2-< →M w S+2e -< . It can also be made of another material capable of intercalating and deintercalating sulfur anions.

[0054] During a discharge, the positive electrode 1 attracts electrons from the electrical circuit in such a way that a reducing cleavage occurs, such as in the case of La₂O₂S₂: La₂O₂S₂ + 2e⁻ ⇆ oA⁻La₂O₂S + S₂⁻

[0055] The anions of S 2-< migrate through the electrolyte 8 from the positive electrode 1 to the negative electrode 2 and are collected at the negative electrode 2 to undergo oxidation, releasing electrons: S 2-< -> S 0< + 2 e-< or wM+S 2-< →M w S+2e-<

[0056] The resulting electrons then migrate back towards the positive electrode 1 through the electrical circuit 9.

[0057] Although not represented on the figure 1 The opposite reactions occur under charge, where the positive electrode becomes the anode (site of oxidation) and the negative electrode becomes the cathode (site of reduction).

[0058] The following examples are given for illustrative purposes only. Examples

[0059] Du La 2 O 2 S 2 is used as a precursor to test the topochemical reduction of chalcogenides ( Fig. 2a Its structure consists of infinite layers of 2 ∞ La 2 O 2 2 + of the fluorite type separated from each other by isolated sulfur (S 2 ) 2-< dimers aligned parallel to these 2D blocks. The deinsertion of a sulfur atom per dimer should a priori lead to a La 2 O 2 S ( Fig. 2b ) whose structure should be inherited from the layered structure of the La₂O₂S₂ precursor. First, the low-energy structures of the La₂O₂S compound were investigated using an elaborate crystal structure prediction (CSP) methodology. Combining an evolutionary USPEX structure-finding algorithm with initial principle calculations allowed the identification of two polymorphs, namely hP and oA crystal structures, which are respectively stable and metastable (see Fig. 2c ). The two phases are dynamically stable, which justifies their respective locations at global and local minima on the potential energy surface of La₂O₂S. The more stable candidate exhibits a hexagonal stratified structure with a plate of 2 ∞ La 2 O 2 of the fluorite type (111) alternating with sulfur atoms in the octahedral environment of lantane. Interestingly, this is the exact structure of the La₂O₂S compound reported in the literature (Acta Cryst. B29, 2647-2648 (1973)), commonly prepared at high temperature (800-1200 °C). In what follows, this structure will be denoted hP -The 2 O 2 S according to Pearson notation ( h for hexagonal and P for primitive cell). USPEX also predicted the structure of an unknown metastable polymorph with only slightly higher enthalpy. This structure also exhibits a layered characteristic but relies on the stacking of plaques of 2 ∞ La 2 O 2 of the fluorine type (complete reminiscence of the La₂O₂S₂ structure) alternating with sulfur atoms in prismatic environments. Similarly with the hP -The 2 O 2 S, this metastable polymorph with the space group Amm2 orthorhombic is named below oA -The 2 O 2 S. The thermal and kinetic stabilities of these two structures were subsequently confirmed by ab initio molecular dynamics (AIMD) simulation in which hP- And oA- Both 2O2S retained their main structural framework after 10 ps at temperatures up to 600 K. Therefore, theoretical calculations clearly predict the possible existence of oA -The metastable 2 O 2 S in addition to the phase hP -The 2 O 2 S already known.

[0060] The topochemical disinsertion of sulfur in the stratified La₂O₂S₂ precursor was then attempted by reaction with the alkali metal RbO₂ at low temperature in hermetically sealed, vacuum-sealed Pyrex tubes. After the excess Rb (and its salts) was rinsed with dry ethanol (see Synthetic Procedure in SI), powder X-ray diffraction (PXRD) patterns were collected on products synthesized at 200°C and 350°C. Both were found to be very similar and did not reveal any known phases. Furthermore, EDX analyses of the bulk product powder revealed the absence of rubidium and a La / S molar ratio of 2.0(2) / 1.0(1) (See figure 5 These results indicate the formation of a low-sulfur La₂O₂S phase without Rb incorporation into the structure. No peaks were observed. hP -2O2S was not detected in the X-ray diffraction pattern, but the existence of the polymorph oA-The 2 O 2 S predicted by USPEX could be easily established via a Rietveld refinement with a goodness of fit of χ 2< = 1.33 and a Bragg reliability factor of R (obs) = 1.67%: see table below: [Table 1] Table 1. oA Crystallographic parameters determined from the Rietveld refinement of the d'-La 2 O 2 S powder. Crystallographic data Chemical formula The 2 O 2 S Molar mass (g mol⁻¹) 341,87 Symmetry Orthorhombic Color White Space Group Amm2 (N° 38) a (Å) 4,1489(1) b (Å) 3,9750(9) c (Å) 12,728(0) Volume (Å 3< ) 209,9(1) Z 2 Density (g cm⁻³) 5,4088 Anisotropic stress (Å -2< ) 2< S 400 = 11,8(9) ; S 040 = 8,66(0) ; S 004 = 0,0485(9) ; S 2 20 = 2,82(0) ; S 202 = 0,830(7) ; S 022 = 0.636(8) ; March-Dollase parameter (Preferred orientation along <100> ) P md = 0,943(5) Structural refinement Profile reliability factor R p = 6,38% Weighted profile reliability factor R wp = 8,73% Bragg Reliability Factors R (obs) = R (all) = 1,67% Weighted Bragg Reliability Factors R w (obs) = R w (all) = 2,29% Quality of fit χ 2< = 1,33 Atomic position isotropic thermal parameters a< Atom x y z U iso (Å 2< ) La1 0 0 0,6442(4) 0,0054(8) La2 0,5 0,5 0,8379(7) 0,0040(1) O1 0 0,5 0,7350(1) 0,001 b< O2 0,5 0 0,7169(3) 0,001 b< S1 0 0 0,9664(7) 0,0068(4) a< The site occupancy factors of all atoms are fixed at full occupancy. b< These atomic displacement factors are set at 0.001.

[0061] Transmission and scanning electron microscopy (STEM) also supports the conclusion that the newly synthesized phase is from the oA -The 2 O 2 S. The stacking of infinite sheets 2 ∞ La 2 O 2 The fluorite-type (100) plate structure is clearly visible in the wide-angle annular dark-field (HAADF) STEM image. In contrast, the characteristic fluorite-type (111) plate structure of the stable polymorph cannot be found. hP -La₂O₂S on the experimental STEM image. The EDX spectrum of a nanometer-sized single crystal, as well as the EDX analysis of the bulk powder, was consistent with the composition of La₂O₂S. The structural arrangement of the new compound oA -La₂O₂S is directly inherited from that of La₂O₂S₂. This observation undeniably supports the topochemical nature of the deintercalation process. The sulfur deintercalation process does not, under any circumstances, alter the integrity of the plate. 2 ∞ La 2 O 2 but assumes a change in a layer of 2 ∞ La 2 O 2 on two along the direction ½( b + c ) of the structure of clear La 2 O 2 S 2 (SG: CMCA). Raman spectroscopy confirmed the complete loss of sulfur dimers during the topochemical reduction: the band associated with the SS stretching modes, located at 487 and 498 cm⁻¹ in La₂O₂S₂, completely disappeared after the deintercalation of a sulfur atom from La₂O₂S₂, confirming the conclusion made from the XRD diagram that the reaction of hP -The 2 O 2 S with oAThe reaction between La₂O₂S and La₂S was complete. Finally, diffuse reflectance spectra also support the cleavage of (S₂)₂<- dimers. Absorption thresholds shift from 2.50 eV in La₂O₂S₂, a characteristic value of a π*-σ* electronic transition of lone pairs, to 3.88 eV in oA-La₂O₂S, a value slightly lower than that observed in hP-La₂O₂S (4.13 eV). Thus, it was concluded that upon the reaction of La₂O₂S₂ with elemental rubidium, the alkali metal activates a redox reaction with (S₂)₂< dimers that trigger the breaking of SS bonds. However, unlike Cu 0< nanoparticles which intercalate into the La 2 O 2 S 2 host network (Angew. Chem. Int. Ed. 57, 13618-13623 (2018)), Rb 0< leads to the topochemical deinsertion of sulfur to obtain the metastable phase of oA-La 2 O 2 S. The choice of reducing agents is the decisive factor on the consequence of the reaction.No reaction occurred when La₂O₂S₂ was treated at 200–300°C under a reducing atmosphere, namely, a 5% H₂ / A flux. At 350°C, the reduction finally took place, but it terminated with the thermodynamically stable hP-La₂O₂S, where the original fluorite plate (100) was deformed into the fluorite plate (111). This result highlights the contrast between the common reducing agent such as H₂ and the more powerful reducing agent Rb₂O, which even favored the topochemical reduction to oA-La₂O₂S at the same reaction temperature (350°C).

[0062] At this stage, it was hypothesized that the topotactic deintercalation of La₂O₂S₂ may or may not be reversible at low temperatures. To test this possibility, a portion of oA-La₂O₂S was mixed with an equivalent amount of sulfur and heated to 200°C ( figure 3a The product was analyzed using XRD as shown in the diagram. figure 3bThe original La₂O₂S₂ material could be completely recovered without any by-products, confirming the reversibility of temperature-assisted intercalation / deintercalation processes based on the formation / breakdown of sulfur dimers within the layered La₂O₂S / La₂O₂S₂ oxychalcogenides. To gain a better understanding of sulfur intercalation, the reactivity of oA-La₂O₂S towards only half the sulfur equivalent at low temperature was also tested. The XRD diagram of the product obtained from the intercalation of 0.5 S into oA-La₂O₂S at 200°C (see figure 3b) clearly highlights the conversion of oA-La₂O₂S to an unknown intermediate phase in conjugation with a small amount of La₂O₂S₂. The XRD pattern of the intermediate phase was similar to that of oA-La₂O₂S but shifted at lower diffraction angles, suggesting the existence of an intercalated oA-La₂O₂Sx phase (1 < x < 2.0). The same XRD pattern was observed when attempting to deintercalate 0.5S from La₂O₂S₂ using 1.0 equiv. of Rb 0< , 1.0 equiv. of Ag and 0.5 equiv. of Ni 0< ( figure 3cThe diffraction pattern of oA-La₂O₂S₁.5 could be refined with the same space group as oA-La₂O₂S (Amm²) and cell parameters of ~8.4 Å, ~4.0 Å, and ~12.8 Å without any superstructure peaks. This clearly proved the existence of an intermediate phase with a strong reminiscence of the oA-La₂O₂S structure. One reasonable hypothesis is that this new intermediate phase replaced half of the monatomic S²⁻ with (S₂)²⁻ dimer anions, retaining the main structural infrastructure of oA-La₂O₂S. This partial dimerization should lead to the expected composition of oA-La₂O₂S₁,5. Indeed, the intercalation of 0.5 S and the deintercalation of 0.5 S by means of metallic species both gave similar Raman spectra that exhibited the single intense peak at 413–417 cm⁻¹, while Raman peaks from oA-La₂O₂S or La₂O₂S₂ were absent. Since an intense peak on the order of 400–500 cm⁻¹ is characteristic of the SS stretching mode (Angew. Chem. Int. Ed. Engl. 14, 655–720 (1975)), these Raman spectra support the formation of oA-La₂O₂S₁,5 via the partial cleavage of SS bonds.

[0063] To resolve the crystal structure of this new phase, electron precession diffraction tomography (PEDT) analyses were performed. This emerging technique can reduce the effects of dynamic diffraction during data acquisition and allows for the ab initio resolution of complex structures using simple nanocrystals. PEDT data were therefore collected on several nanocrystals of the new phase. All datasets were analyzed using the PETS2.0 (Acta Crystallographica, B75, 512-522 (2019)), Superflip (Journal of Applied Crystallography, 40, 451-456 (2007)), and Jana2006 software programs. (Zeitschrift für Kristallographie, 229, 345-352. (2014). The reconstruction of the reciprocal lattice planes hk0, h0l, and 0kl was observed, which conform to an orthorhombic unit cell a = 8.348 Å, b = 3.961 Å, and c = 12.645 Å (V = 418.1 ų) and a non-centrosymmetric Amm2 space group. The structure was then solved and refined using the Jana2006 program on the electron diffraction database. The structural analysis converged with the Bragg electronic reliability factor. R (obs) = 10.1%, revealing a stratified structure with a composition of oA-La₂O₂S 1.5. This newly obtained structure consists of an infinite plate of 2 ∞ La 2 O 2 of the fluorine (001) type alternating with sulfur layers containing one-third and two-thirds sulfur anions as S2- and (S2)2- species, respectively. Using this structural model oA-The 2 O 2 S 1.5, the two powder XRD diagrams resulting from the intercalation and deintercalation of sulfur, namely from reaction mixtures of oA -La₂O₂S + 0.5S and La₂O₂S₂ + 0.5Ni (see figure 3 ), were both successfully refined. High-strain parameters had to be considered to achieve a satisfactory fit. This can be interpreted as the signature of a stacking problem occurring, as expected, during intercalation or deintercalation processes related to the 2D structure of the host lattice and the possible existence of different stages. The structural analysis was based on data collected from the best-crystallized crystals. However, in most PEDT data, stacking defects result in diffuse scattering features along

[001] . The experimental contrast in the STEM-HAADF image indicates fluorine-type plate stacking 2 ∞ La 2 O 2 (001) infinite. A similar structure was independently predicted by the USPEX evolutionary algorithm for this specific oA-La₂O₂S₁₅ composition. The predicted most stable structure agreed well with the experimental structure obtained by PEDT analysis. The 2nd and 3rd most stable structures displayed 1D and 2D hexagonal plates (of fluorine (111) type) as their units [La₂O₂], and these plates formed intergrowth structures with 2D (quasi-)lattices of dimers / sulfur atoms. However, none of these could be found in the experiments performed.

[0064] This work demonstrates the deintercalation and reintercalation of sulfur in a layered oxychalcogenide compound using a novel topochemical approach. Alkali or transition metals can be used as reducing agents to trigger the reduction of chalcogenide oligomers and the breaking of the chalcogen-chalcogen bond. In the case of La₂O₂S₂, the low-temperature deintercalation of sulfur atoms occurs in two steps to form two new metastable phases, OA-La₂O₂S₁.5 and OA-La₂O₂S₂, which retain the layered characteristic of the precursor. As shown in the figure 4 , this is a completely reversible topotactic process since the sulfur atoms can be re-intercalated at low temperature to reform the precursor La 2 O 2 S 2 . [Table 2] Table 2. x x Summary of crystallographic parameters of La2O2S series (1 ≤ 2.0) The 2 O 2 S 2 hP -La 2 O 2 S 2 oA -La 2 O 2 S oA -La 2 O 2 S 1.5 Source Ostorero et al. (XRD) Morosin et al. (XRD) This study (XRD) This study (PEDT) Space Group CMCA P-3m1 Amm2 Amm2 a (Å) 13,215(2) 4,049(1) 4,148(9) 8,348 b (Å) 5,943(1) - 3,975(1) 3,961 c (Å) 5,938(1) 6,939(2) 12,728(0) 12,645 SS distance (Å) 2,103 4,049 3,975 2,011 Ostero et al Acta Cryst. C46, 1376-1378 (1990) Morosin et al Angew. Chem. Int. Ed. Engl. 14, 655-720 (1975) 1. Synthesis Procedures

[0065] The initial precursor La₂O₂S₂ was synthesized following the procedure described in Angew. Chem. Int. Ed. 2018, 57, 13618-13623 oA-La 2 O 2 S : topochemical deintercalation of S 2- anions by Rb

[0066] Before preparation, all glassware and utensils for the experiment were dried in the oven ( T= 80 °C). Under an argon atmosphere, La₂O₂S₂ and Rb (Aldrich, 98+%) were weighed in a 1:2 molar ratio and introduced into the silica tube, the bottom of which was protected by a carbon coating. All these preparations were carried out under an argon atmosphere. The Pyrex tube was placed under vacuum (~10⁻³ torr) and sealed. The sealed mixture was heated to 200 °C at a rate of 20 °C h⁻¹ and annealed for 2 h. Finally, the sealed mixture was gradually cooled in a furnace to produce the pale gray-blue powder. The excess Rb settled on the opposite side of the silica tube. The silica tube was opened under an argon atmosphere, and the entire contents were rapidly cooled with the excess amount of ethanol ( Caution: Under ambient conditions, Rb ignites upon contact with ethanol.The colorless precipitate was contaminated by small carbon flakes, which were separated by repeated decantation with mechanical stirring. The precipitate was then washed with ethanol, water, and acetone, and then dried under vacuum to obtain the colorless powder. oA -The 2 O 2 S. The product was stable under ambient atmosphere. The same reaction obtained at 350 °C also gave identical results, of the 'o Pure A-La 2 O 2 S without any trace of hP- Neither 2 O 2 S nor other impurities. Intercalation of sulfur anions in oA-La 2 O 2 S

[0067] The colorless powder of oA -The 2 O 2 S ( That. 200 mg) was combined with S flakes (Aldrich, 99.99+%) in oA-La 2 O 2 S: molar ratio S = 1:0.5 and ground on an agate mortar under an argon atmosphere. The mixture was then granulated and sealed in a silica tube (~10⁻³ torr) under vacuum. The sealed mixture was heated to 150–200 °C at a rate of 100 °C h⁻¹ and annealed for 4–48 h (see figure 3b (for the result), followed by gradual cooling in a furnace to obtain the pale yellow granule. When the sulfur was not completely consumed, the residual sulfur was deposited on the opposite side of the silica tube. To complete the intercalation, the resulting granule was ground with an additional 0.5 S equivalent under an argon atmosphere. The mixture was then subjected to heat treatment at 200 °C in the vacuum silica tube. After 160 h of annealing, the mixture was fully converted into the pale yellow granule of pure La₂O₂S₂. oA-La 4 O 4 S 3 : general procedure for topochemical disintercalation of S 2- anions by various metals

[0068] The detailed synthetic conditions (i.e., stoichiometry, annealing time, forms of metallic sources) for the respective metallic species were noted below. To 1.0 equiv. of La₂O₂S₂ (ca. 150–250 mg), 0.5–2.0 equivalents of metallic elements were added and ground together under an argon atmosphere until the powder became grayish and sticky on an agate mortar. The mixture was then granulated and sealed in a silica tube under vacuum (~10⁻³ torr). The sealed mixture was heated to 350 °C at a rate of 300 °C h⁻¹ and annealed for 2–4 h. Finally, the sealed mixture was gradually cooled in a furnace to give the mixture containing oA -The 4 O 4 S 3 (see figure 3c (for its DRX). Reaction with Ag:

[0069] 1.0 equiv. of Ag powder (Aldrich, 2 - 3.5 µm, ≥ 99.9%) was added. Annealing: 4h Neither a small excess (-1.1 equiv.) of Ag nor prolonged / repeated heat treatments led to greater consumption of La2O2S2. Reaction with Ni:

[0070] 0.5 equivalent of Ni nanopowder (Aldrich, < 100 nm, 99%) was added. Annealing: 4 h. Prolonged and repeated heat treatments did not improve the yield of oA-La₄O₄S₃, but ended with partial decomposition into hP -The 2 O 2 S

Claims

1. An electroactive material for electrochemical cell electrode wherein said material comprises a chalcogen oligomer, characterized in that said material can reversibly deintercalate an anion of said chacolgen by reduction and reintercalate said anion by oxidation, while maintaining its lamellar structure.

2. The electroactive material of claim 1 wherein said chalcogen anion is sulfur or an oligomer thereof of formula:         (Sn)x-     (III) where n and x are integers, such that x equals to 1 or 2, and n is comprised between 1 and 6.

3. The electroactive material according to claim 1 or 2 wherein said chalcogen anion is S2-.

4. The electroactive material according to any one of the preceding claims which is chosen from La2O2S2, SrS2, SrS3, BaS2, BaS3, Ba2S2F2, FeS2, NiS2, CoS2, MnS2, TiS3, VS4, PbS2, BiS2.

5. The electroactive material according to any one of the preceding claims wherein said active material is La2O2S2, which reversibly deintercalates and reintercalates S2- according to the following reaction:         La2O2S2 + 2e- ⇆ oA-La2O2S + S2- where oA designates the centered orthorombic crystalline form.

6. A material of formula:         oA-La2O2S     (1) where oA designates the centered orthorombic crystalline form.

7. A material of formula (II):         oA-La2O2S1.5     (II) where oA designates the centered orthorhombic crystalline form.

8. A positive electrode comprising the material according to any one of the preceding claims.

9. The positive electrode according to claim 8 comprising a current collector and a coating layer, wherein said coating layer comprises said electroactive material.

10. The positive electrode according to claim 9, wherein the current collector is an aluminium sheet.

11. An electrochemical cell comprising : - A positive electrode as defined in any one of claims 8 to 10; - A negative électrode; and - An electrolyte layer sandwiched between the positive and the negative electrodes, characterized in that the electrolyte is a solid electrolyte comprising as conducting ion the chalcogen anion of the chalcogen oligomer of the electroactive material.

12. A battery comprising a plurality of electrochemical cells as defined in claim 11, wherein the cells are electrically connected.