EP4173067A1

EP4173067A1 - Electrochemical cell with solid electrolyte comprising a reference electrode

Info

Publication number: EP4173067A1
Application number: EP21737447.9A
Authority: EP
Inventors: Romain DUGAS; Jean-Marie Tarascon
Original assignee: Centre National de la Recherche Scientifique CNRS; Sorbonne Universite; College de France
Current assignee: Centre National de la Recherche Scientifique CNRS; Sorbonne Universite; College de France
Priority date: 2020-06-30
Filing date: 2021-06-29
Publication date: 2023-05-03
Also published as: WO2022002972A1; US20230261347A1; FR3112027B1; FR3112027A1

Abstract

The invention relates to an electrochemical cell (1), comprising a positive electrode (11) and a negative electrode (12) disposed face to face in the cell body (10), and a stack of electrolytic layers (13, 14, 15) interposed between the positive electrode (11) and the negative electrode (12). The stack of electrolytic layers (13, 14, 15) comprises a first solid electrolyte layer (13) disposed at one end of the positive electrode (11), a second solid electrolyte layer (14) disposed at one end of the negative electrode (12), a layer of composite powder (15) interposed between the first (13) and the second layer (14), the layer of composite powder (15) comprising an electrolyte powder and a powder of electroactive material functioning as a reference electrode of the electrochemical cell.

Description

DESCRIPTION

Title of the invention: Electrochemical cell with solid electrolyte comprising a reference electrode.

The invention relates to an electrochemical cell, to a composite powder intended to be interposed between two layers of solid electrolyte in an electrochemical cell, to a process for preparing a composite powder, and to a process for assembly of an electrochemical cell.

In the present application, we will call "electrochemical cell" a device consisting of two electrodes separated by an electrolyte, which are the site of redox reactions, and we call "battery" a device composed of one or more cells electrochemical.

For the study and development of new electrochemical cells, a third electrode, commonly called the reference electrode, is necessary to independently study the electrochemical reactions occurring at the anode (negative electrode) and at the cathode (positive electrode ). This third electrode, remains at a constant potential during cell operation and is a reference against which the potential of the anode and the cathode is measured. We can thus study the contribution of each electrode to the overall performance of the cell. With the development of electric vehicles and hybrid vehicles, attempts are made to design batteries having an increasingly high energy storage capacity, in an increasingly limited volume. However, with higher energy density, more safety concerns arise, especially with lithium ion batteries using liquid electrolytes which are toxic, flammable and volatile. In this context, a significant research effort is currently devoted to the development of all-solid batteries (also called semiconductor batteries), which rely on a solid lithium ion conductor as an electrolyte instead of a liquid. Solid electrolytes also provide a stronger physical barrier against short circuits. The fact of having a reference electrode in an electrochemical cell with a solid electrolyte poses several problems. Indeed, it should be remembered that the electrochemical cells with solid electrolyte generally have the form of pellets assembled between two pistons sliding inside a cylinder, and subjected to high pressure during assembly (from the order of one ton per cm ² ). The introduction of a third electrode into such a cell is thus made complicated by the pressure necessary to assemble the cell, liable to break a thin wire such as that usually used as a reference electrode in cells with liquid electrolyte.

In addition, assuming that the third electrode withstands the pressure exerted, it is also necessary, during the assembly of the cell, to extract from the cell the wire acting as a reference electrode, in order to take a electrical contact, either through a tiny hole in the edge of the cylinder, or between the cylinder and one of the pistons, which is also tricky given the pressure exerted.

[0008] Finally, if one wishes to study more particularly the electrode / electrolyte interfaces, the use of the technique known as electrochemical impedance spectroscopy, described in particular in [Costard2017] is then recommended. This technique brings additional constraints, in particular with regard to the symmetry of the electrochemical cell, including the symmetry of the reference electrode, to avoid the appearance of artefacts on the impedance spectra. In the aforementioned article, a grid-shaped reference electrode makes it possible to obtain electronic conduction on a section of the cell located between the positive electrode and the negative electrode, while the openings of the grid allow ionic conduction. between the two electrodes. This type of arrangement cannot, however, be transposed to a solid electrolyte cell, for the high pressure reasons explained above.

[0009] There are a few articles in the scientific literature proposing to add a reference electrode in a solid electrolyte cell.

In [Nam2018], two configurations are reported, which allow the monitoring of the potential of each of the electrodes during galvanostatic cycles (ie at constant current). The first configuration, illustrated by diagram 2a of the article, consists of a point reference electrode arranged on the side of the electrolyte without being completely integrated into the cell. In the second configuration, illustrated by diagram 2b of the article, the reference electrode is placed above the cathode, the current collector of which must be carefully drilled. The reference electrodes are made of lithium or a _0.5 ln Li alloy. In both configurations, the reference electrode is added after assembly of the anode and the cathode with the electrolyte. The resulting configurations do not allow distortion-free impedance measurements, the first configuration having low symmetry (showing artifacts on the impedance spectra) and the second configuration positioning the reference electrode above the cathode rather than between anode and cathode. [0011] In [Kasemchainan2019], the three-electrode cell which is employed has an electrolyte of larger diameter than the working electrodes and a smaller reference electrode is placed next to a working electrode on the pellet of electrolyte. The three electrodes are made of lithium. Again, the geometry is not ideal for impedance measurements due to its asymmetry. In [Schlenker2020], the reference electrode is a gold-plated tungsten wire inserted into the electrolyte before densification. This configuration makes it possible to obtain correct impedance spectra, thanks to the fact that the reference electrode passes through the cell (symmetrical configuration). The use of a gold-plated tungsten electrode on the other hand poses the problem that the potential is not well defined and risks being unstable in the long term, since the reference material does not contain lithium. To improve it, it would be possible to alloy gold with lithium but this solution is known not to be systematically reliable, as it is reported in [Pritzl2018]. In fact, with this configuration, only a very small quantity of lithium can be present in the reference electrode, and parasitic reactions taking place in the cell can therefore be sufficient to detach the reference electrode.

The invention therefore aims to provide an electrochemical cell with a solid electrolyte, allowing both monitoring of the potential of the two electrodes over the long term and measurements of impedance without distortions. An object of the invention is therefore an electrochemical cell, comprising a positive electrode and a negative electrode arranged face to face in the cell body, a stack of electrolytic layers interposed between the electrode positive and negative electrode, the stack of electrolyte layers comprising a first solid electrolyte layer disposed at one end of the positive electrode, a second solid electrolyte layer disposed at one end of the negative electrode, a layer of composite powder interposed between the first and the second layer, the layer of composite powder comprising an electrolyte powder and a powder of electroactive material having a function of reference electrode of the electrochemical cell.

Advantageously, the electrochemical cell further comprises a current collector disposed on at least part of the periphery of the layer of composite powder and in electrical contact therewith.

Advantageously, the current collector is an annular collector, arranged around the entire periphery of the layer of composite powder.

Advantageously, the composite powder layer is arranged symmetrically with respect to an axis coinciding with the stacking direction of the electrolytic layers and passing through the center of the cell.

Advantageously, the first electrolyte layer, the second electrolyte layer and the electrolyte powder comprise a sulfide.

Advantageously, the sulfide is p-Li ₃ PS4 (b-LiPS).

Advantageously, the powder of electroactive material comprises lithium, or an alloy based on lithium and indium.

Advantageously, the lithium-indium-based alloy is Li _x ln, where 0 <x <1, and preferably Li _0.5 ln.

Advantageously, the powder of electroactive material comprises Li ₄ Ti ₅ 0i ₂ or Li ₇ Ti ₅ 0i ₂ (LTO). Advantageously, the negative electrode and the powder of electroactive material have an identical composition.

The invention also relates to a composite powder intended to be interposed between two layers of solid electrolyte in an aforementioned electrochemical cell, the composite powder comprising an electrolyte powder and a powder of electroactive material having a function of reference electrode of the electrochemical cell. The invention also relates to a process for preparing the aforementioned composite powder, in which the electrolyte powder comprises P-Li ₃ PS ₄ (b-LPS), the powder of electroactive material comprises Li _x ln, where 0 <x <1, the process comprising the following steps: a1) plating, by cold co-rolling, under a protective atmosphere, of a sheet of lithium between two sheets of indium in proportions by mass corresponding to the composition Li _x ln, then folding of the product thus obtained; b1) repeating the previous step a plurality of times, until a stable state of the product is obtained; c1) grinding of the product with the electrolyte powder in proportions by mass of

50-70% electroactive material powder and 30-50% electrolyte powder, preferably 60% electroactive material powder and 40% electrolyte powder, until a homogeneous powder is obtained.

The invention also relates to a process for preparing the aforementioned composite powder, the electrolyte powder comprises p-Li ₃ PS ₄ (b-LPS), the powder of electroactive material comprises Li _{4 + x} Ti ₅ 0i ₂ (LTO), where 0 <x <1, the process comprising the steps of: a2) mixing a powder of Li ₄ Ti ₅ 0i ₂ with carbon black, in a ratio of 3: 1 in mass, to form an LTO: C powder b2) reduction of part of the LTO: C powder in a liquid electrolyte cell with a lithium counter-electrode, until a Li ₇ Ti ₅ 0i powder is obtained ₂ :

VS ; c2) rinsing and centrifuging the Li ₇ Ti powder ₅ 0i _2: C with dimethyl carbonate; d2) mixing the powder obtained in step c2) with an unreduced part of the LTO: C powder, in proportions corresponding to the composition Li _{4 + x} Ti ₅ 0i ₂ , and 60% electrolyte powder en masse.

The invention also relates to a method of assembling an electrochemical cell comprising an insulating cell body equipped with a current collector, a first piston and a second piston, the method also comprising the following steps: a) insertion of the second piston into the cell body so that it is flush with the current collector in abutment, introduction of the aforementioned composite powder, then of an electrolyte pellet, and application of pressure between the two pistons, so that the composite powder is in electrical contact with the current collector, while preventing any sliding of the composite powder in the cell body in the event of withdrawal of at least one of the two pistons; b) inversion of the cell body, withdrawal of the second piston, addition of an electrolyte powder, insertion of the second piston into the cell body 10, and application of pressure between the two pistons so as to compact the powder d 'electrolyte; c) withdrawal of the second piston, deposition, in the cell body, of a powder of electroactive positive electrode material, insertion of the second piston into the cell body, inversion of the cell body, withdrawal of the first piston, deposition, in the cell body, a powder of electroactive material of negative electrode, insertion of the first piston in the cell body, and application of pressure between the two pistons, so as to compact the two powders of electroactive material.

[0028] Other features, details and advantages of the invention will become apparent on reading the description given with reference to the accompanying drawings given by way of example and which represent, respectively:

[0029] Figure 1 shows a sectional view of an electrochemical cell according to the invention;

[0030] Figure 2 shows a sectional view of an electrochemical cell according to the invention, housed in an insulating cell body;

FIG. 3 represents the potential of each electrode, as well as the impedances at the end of charging and discharging, for different galvanostatic cycles, with the following triplet of electrodes: NMC / Lio _. sln / Lio _. sln;

FIG. 4 represents the potential of each electrode, as well as the impedances at the end of charging and discharging, for different galvanostatic cycles, with the following triplet of electrodes: LTFS / Lio _. sln / Lio _. sln;

FIG. 5 represents the potential of each electrode, as well as the impedances at the end of the charge, for a galvanostatic cycle, with the following triplet of electrodes: LTFS / LTO / Lio _. sln; Figures 6, 7 and 8 illustrate the different steps of the assembly process of an electrochemical cell according to the invention.

[0035] Figure 1 shows a sectional view of the electrochemical cell according to the invention. Cell 1 is made up of a positive electrode 11 and a negative electrode 12.

In the present application, it will be considered that the positive electrode 11 is the working electrode (WE), namely the electrode on which the reaction of interest occurs, and the negative electrode 12 is against -electrode (CE). Depending on whether the reaction on the electrode is reduction or oxidation, the working electrode is therefore called the cathode or anode, respectively.

In general, the names "working electrode" and "counter-electrode" are used more for a test cell used to study the properties of the working electrode, where the counter-electrode is used only to supply the current. .

For a complete electrochemical cell, such as the one that can be found in an on-board application (for example in a vehicle), this distinction no longer applies and we will instead use the names “positive electrode” and “negative electrode”. The convention for electrochemical cells is to use the term "cathode" for the positive electrode and "anode" for the negative electrode, although these terms are only correct when the cell is discharging.

Advantageously, the positive electrode 11 comprises carbon fibers of VGCF type (for "Vapor Phase Growth Carbon Fibers", or vapor phase growth carbon fibers).

[0039] According to a variant, the positive electrode 11 comprises carbon black, in particular carbon black of the "Super-P" type (registered trademark).

[0040] Carbon fibers, and carbon black, serve to increase the electronic conductivity of the positive electrode composite, in order to improve capacitance and power.

The electrodes are spaced apart by a stack of electrolytic layers (13, 14, 15), intended for the transfer of ions between the electrodes. The stack of electrolytic layers (13, 14, 15) consists of three layers, namely two layers (13, 14) of solid electrolyte, and a layer of composite powder 15, interposed between the two layers ( 13, 14) of solid electrolyte.

The composite powder layer 15 itself comprises an electrolyte powder and a powder of electroactive material having a function of a reference electrode of the electrochemical cell. By "electroactive material having a function of reference electrode of the electrochemical cell" is meant a material whose potential is constant during use of the cell.

The innovative aspect thus comes from the use of a composite powder as a reference electrode located in the middle of the cell in the form of a

" slice ". The proportions of the electrolyte powder and of the powder of electroactive material are determined so that there is both electronic conduction towards a current collector arranged at the periphery of the layer of composite powder 15 (constant voltage ), and ionic conduction between the working electrode and the counter-electrode.

By using a composite powder in an all-solid battery, it takes advantage of the fact that the electrolyte is solid to overcome the presence of the current collector in the middle of the battery and the resulting constraints, all by having a reference electrode placed in the heart of the battery. The geometry of the cell can be fully axisymmetric to provide the best conditions for impedance measurements. The electrical contact on the reference electrode can be taken at any location on its periphery or all around it using a ring-shaped current collector.

Instead of having the form of a wire, the reference electroactive material is used in the form of powder and mixed with solid electrolyte. After cold pressing, the reference electrode takes the form of a layer in the middle of the cell, located between two of the two electrolyte layers (13, 14).

The mass distribution of the first electrolyte layer 13, of the second electrolyte layer 14 and of the composite powder layer 15 can be identical (one third for each layer), without this condition being necessary . It is thus possible to lighten one of the electrolyte layers. Figure 2 shows the electrochemical cell 1, arranged in a cell body 10. The cell consists of a hollow insulating body 10, the shape of which corresponds to that of the pistons (17, 18) arranged on both sides other of the electrodes. For example, the cell body can be cylindrical, as can the pistons (17, 18). The pistons can be made of stainless steel. A current collector 16 is disposed on at least part of the periphery of the composite powder layer 15. The composite powder layer 15 is advantageously disposed symmetrically with respect to the stacking axis of the layers, and as the composite powder has good electronic conductivity, it is possible to make contact only on part of the circumference without risk of the appearance of distortions. It is also possible to opt for a current collector arranged over the entire periphery of the composite powder layer 15. The current collector 16 can thus be annular, if the other parts are cylindrical.

The height of the composite powder layer 15 must be less than the sum of the heights of the two electrolyte layers (13, 14) and of the composite powder layer 15, with a safety margin, to ensure that the layer of composite powder 15 arrives directly in line with the current collector, and avoids any risk of short-circuit.

The electrolyte used in the composite powder must be of the same type (of the oxide type or of the sulphide type) as that used in the other electrolyte layers (13, 14). It is preferable that the materials are exactly the same. The electrolyte used can advantageously be p-Li ₃ PS4 (b-LPS), which has a high ionic conductivity (of the order of 1.6 × 10 ⁴ S. cm ¹ at room temperature). This material also has good stability when it is coupled to lithium-based anodes.

As a variant, the electrolyte can be Li ₆ PS5CI. This component exhibits an ionic conductivity approximately ten times higher than b-LPS, which directly induces greater capacities in terms of energy density and power. Li ₆ PS ₅ CI can be found, for example, in argyrodite.

The material chosen for the powder of electroactive material should preferably have a sufficiently large plate with which the variation in potential of the material of the positive or negative electrodes would be compared. Indeed, the fact of having, at the level of the reference electrode, a material exhibiting a complex electrochemical signature would complicate the characterization of the electrodes.

The Li _x ln alloy, for 0 <x <1, has a plateau at 0.622 V vs. Li + / Li, making it a suitable reference material for electroactive material powder.

[0054] It is advantageous to take two-phase systems for office electroactive material making reference electrode, in particular of In / LiLn, or the Ü4Ti50i2 / Li7Ti ₅ 0i2. It is also possible to use lithium directly.

Thus, the composite powder can comprise a mixture of b-LPS and Lio.sin.

The electrochemical cell according to the invention does not impose any particular material for the positive electrode, one of the objectives of the cell with three electrodes being precisely to study in the laboratory the behavior and performance of this electrode, in particular at for experimental purposes.

Likewise, the choice of the negative electrode is more dictated by the general operating constraints of all-solid-state batteries than by the use of a three-electrode assembly. As a general rule, care should be taken to ensure that the negative electrode can accommodate lithium if the positive electrode is a source of lithium and vice versa.

The In-Li alloy can be used as a negative electrode in an all-solid cell. In particular, it is advantageous, for practical reasons of preparation for the experiment, to use the same powder for the negative electrode and for the electrolyte material.

[0059] Alternatively, the negative electrode 12 can comprise exclusively lithium, which makes it possible to increase the cell voltage and to decrease the mass for more energy.

Figures 3, 4 and 5 show different galvanostatic cycles, for different electrode triplets. Three configurations were tested, each triplet designating, in the following order, the positive electrode, the electroactive material having the function of reference electrode, and the negative electrode. NMC / Lio _. sln / Lio _. sln in Figure 3, LTFS / Lio _. sln / Lio _. sln in Figure 4 and LTFS / LTO / Lio _. sln in FIG. 5. In all the configurations tested, the composite powder also comprises b- LPS as an electrolyte material, although it could be considered to use another material.

The NMC corresponds to the following compound: Nio _.6 Mno _.2 Coo _.2 O ₂ The LTFS corresponds to the following compound: Li _1.13 Tio _.57 Feo _.3 S ₂ LTO corresponds to the following compound : LUTisO ^

The galvanostatic cycles are carried out by applying a constant current between the positive electrode (represented by the mention WE in Figures 3 to 5 and the negative electrode (represented by the mention CE in Figures 3 to 5). the potential of the positive electrode reaches a limit value depending on the material, the direction of the current is reversed. The graphs show the potentials of the positive and negative electrodes with respect to the reference electrode (represented by the mention RE in figures 3 to 5) as a function of the capacity (in the “battery” sense of the term, that is to say the integral of i.dt) per unit mass of electroactive material in the positive electrode. or quasi-consecutive (cycles 1, 2 and 4 in Figures 3 and 4) are superimposed without the potential dropping or shifting on the x-axis.

The current used was 13.8 mA / g for the example of Figure 3, and 9.25 mA / g for the example of Figure 4. The composite powders consist of 70% of electroactive material and 30% solid electrolyte by mass and prepared by mechanical grinding for 20 min (Spex type grinder).

Impedance measurements were carried out at regular intervals during the galvanostatic charges and discharges, every two hours. These measurements consist in applying a sinusoidal voltage signal between the positive electrode and the reference electrode around their equilibrium voltage, and in measuring the current between the positive electrode and the negative electrode, which is also sinusoidal and of the same frequency but out of phase with respect to the voltage. Impedance, which is the ratio between voltage and current, is therefore a complex quantity. As these measurements are made around an equilibrium point, it is necessary to let the cell relax before performing the measurement, which results in the vertical lines on the galvanostatic cycles. The impedance was measured for frequencies ranging from 200 kHz to 10 mHz and an amplitude of 50 mV. Only the impedances at the end of charge and end of discharge are reported in the right part of each Figures 3 to 5 (identified by solid discs on the galvanostatic cycles) although multiple intermediate measurements have been made. The impedance spectra are plotted in the form of Nyquist diagrams, on which the imaginary part of the impedance is plotted against its real part. The frequency that does not appear explicitly decreases as the real part increases.

[0067] The results obtained first of all show a good decoupling between the curves of the positive and negative electrodes, which makes it possible to properly characterize each of the active materials at the positive and negative electrodes.

The second and third configurations (positive LTFS sulphide electrode) have a capacity four times higher than the first configuration, with excellent reversibility. The impedance, for the second and third configurations, remains of the same order of magnitude throughout the cycling. Variations are however observable, in particular at the end of the discharge, where an intermediate frequency contribution tends to increase. The impedance is also lower for the second and third configurations, without really increasing at the end of the load.

In all three configurations, the impedance of the negative electrode is much more stable although it also tends to increase. The negative electrode is not perfectly stable either, which appears both on impedance measurements and on galvanostatic cycles, where polarization tends to increase during cycles. This is particularly visible on the right side of Figure 4, where the negative electrode curve tends to open up.

Without the presence of the composite powder layer, only the point-to-point sum of the two impedance curves of the same graph could be measured, making the attribution of the variations to one or the other of the electrodes questionable. Likewise, without the presence of the composite powder layer, only the increase in polarization could be observed without it being possible to attribute it to the positive or negative electrode.

Finally, the use of LTO as powder of electroactive material (FIG. 5) gives similar results with a shift of 940 mV on the scale of the potentials, which corresponds to the difference between the equilibrium potential of the Li / Liln and LUTisC Li _/ TisO- ^ systems. Consequently the negative electrode of Lio. ₅ ln, which works around 0 V relative to a reference of the same nature, is found in negative potentials relative to an LTO reference. It exhibits a greater polarization than for the preceding cells which is attributable to a change of lot. Since impedance measurements only involve variations around an equilibrium point, they are not affected by the absolute value of the potential. The spectra obtained with one or the other type of reference are thus similar.

The invention also relates to a composite powder intended to be interposed between two layers of solid electrolyte in an electrochemical cell. The composite powder comprises an electrolyte powder and an electroactive material powder having a function of the reference electrode of the electrochemical cell. The use of such a powder in an electrochemical cell thus allows easy interaction with an external current collector.

The invention also relates to a process for preparing a composite powder. Two embodiments can be envisaged, depending on whether the composite powder comprises Li _x ln or LU ₊ xTisO ^ (LTO) (where 0 <x <1), corresponding respectively to the measurements of FIGS. 3 and 4 on the one hand. , and Figure 5 on the other hand. In both embodiments, the composite powder also comprises p-Li ₃ PS ₄ (b-LPS), as the electrolyte material.

According to a first embodiment, the composite powder comprises Li _x ln (where 0 <x <1), and the method comprises the following steps:

The powder of electroactive material having a function of reference electrode is first prepared. In a first step a1), a plating is carried out, by cold co-rolling, under a protective atmosphere, of a sheet of lithium between two sheets of indium in proportions by mass corresponding to the composition Li _x ln, then the product thus obtained is folded. The co-rolling can be carried out, for example, using a glass tube on a polyethylene sheet.

The co-rolling and folding are repeated a plurality of times, until a stable state of the product is obtained (step b1). Indeed, after a few repetitions the material becomes brittle, a sign that the alloy has been formed. The material is initially soft and spreads out quite similar to a paste, whereas once the alloy is formed it starts to crack during lamination. It is also less bright. The successive co-laminations make it possible to increase the exchange surface between the lithium and indium sheets, and to accelerate the process of obtaining stability of the Li _x ln alloy.

In a third step c1), the product obtained in the previous step is ground with the electrolyte powder with the following proportions by mass: from 50 to 70% of powder of electroactive material and from 30 to 50% of powder of electrolyte, and preferably 60% of powder of electroactive material and 40% of powder of electrolyte, until a homogeneous powder is obtained, which is used as it is for the assembly of the battery.

All of these operations can be carried out in a glove box, under a controlled atmosphere, for example argon.

According to a second embodiment, the powder of electroactive material comprises LU ₊ xTisO · ^ (LTO), where 0 <x <1, and the composite powder is prepared as follows.

In a first step a2), the LUTisO ^ powder is mixed with an electronic conductor, preferably carbon black, in a ratio of 3: 1 by weight to form an LTO: C powder.

In a second step b2), part of the powder obtained in the previous step is reduced in a liquid electrolyte cell facing a lithium counter electrode in order to form the Li _/ TisO ^ phase. The liquid electrolyte can be LP30.

In a third step c2), when the reduction mentioned in step b2) is completed, the powder of ίί ₇ Tί ₅ 0i ₂ : 0 is recovered, rinsed and centrifuged with dimethyl carbonate. Centrifugation can be repeated several times (eg three times). This powder is mixed (step d2) with the portion of LTO powder: C which has not been subject to reduction; the mixture is carried out in proportions corresponding to the composition Li _{4 + x} Ti ₅ 0i ₂ , and to 60% electrolyte by mass.

The preparation process according to the first embodiment is particularly advantageous, since it does not require the assembly of a liquid electrolyte cell to prepare one of the two phases of the reference material. He nor does it require the addition of an electronic conductor (carbon black) since lithium and indium are metallic conductors.

The invention also relates to a method of assembling an electrochemical cell, illustrated by Figures 6, 7 and 8.

An insulating cell body 10 is provided, in which is disposed a current collector 16. The cell body 10 may be cylindrical, and the current collector 16 annular. A first piston 17 and a second piston 18 are also provided, intended to serve as a support, respectively, for the negative electrode and the positive electrode.

In a first step a), the second piston 18 is inserted into the cell body so that it is flush with the current collector 16 in abutment. The composite powder 15 which has been defined above is poured into the cell body 10, then an electrolyte pellet 13 is placed in the cell body 10. The mass of composite powder is 45 mg for the powder Li _0, 5ln: p-LPS or 35 mg for the powder LTO: C : p-LPS, less dense, for a cell body 10 of 8 mm in diameter.

Prior to the assembly of the cell, we will therefore have prepared an electrolyte pellet 13, of the same diameter as the diameter of the pistons. To obtain an 8 mm diameter pellet, a pressure of 1 t cm-2 is exerted on 45 mg of electrolyte material, for example b-LPS.

Pressure is then applied between the two pistons (17, 18), so that the composite powder 15 is in electrical contact with the current collector 16, while preventing any sliding of the composite powder 15 in the cell body 10 in the event of withdrawal of at least one of the two pistons (17, 18), and in particular in the event of withdrawal of the first piston 17 when the cell body 10 will be returned, to the next step. The pressure between the two pistons is increased to 1 t / cm ² for three minutes. At the end of this step, the composite powder is densified and is located at its current collector.

In a second step b), illustrated in Figure 7, the cell body 10 is turned over. The first piston 17 is now found in the lower position. The second piston 18 is withdrawn from the cell body 10. An electrolyte powder 14 is poured into the cell body 10. For example, 40 mg of b-LPS can be poured. This mass can be reduced, in order to limit the ohmic drop between the reference electrode and the positive electrode.

The second piston 18 is then replaced in the cell body 10, and a pressure is applied between the two pistons (17, 18) so as to compact the electrolyte powder 14. It is possible by applying a pressure of 1 T / cm ² - for a few seconds.

In the last step c), illustrated in Figure 8, the second piston 18 is removed, the electroactive material of the positive electrode 11 (for example NMC or LTFS) is poured, the second piston 18 is replaced in cell body 10. The cell is inverted (not visible in Fig. 8) and, similarly, negative electrode material 12 (e.g. _0.5 ln Li) is poured into cell body 10. The assembly is then maintained under a pressure such that the two powders of electroactive material (14, 15) become compact. For example, a pressure of 4 T / cm ² can be applied for 15 minutes.

If a pure lithium electrode is to be used, the electrode, which could not withstand a pressure of 4 T / cm ² , is placed in the cell only after the application of the pressure mentioned in l 'step c). This problem does not arise for LTO or L1 _0.5 ln electrodes.

The cell 10 can then be placed in a frame maintaining the desired pressure for the cycling (here 1 T / cm ² ) and ready to be used by taking the electrical contacts on the pistons and the annular manifold.

[0094] All of the assembly operations are carried out in a glove box.

The assembly process thus makes it possible to obtain good mechanical strength of the various components of the electrochemical cell, with interfaces between the layers allowing good ionic conduction from one electrode to another, while ensuring conductivity. to the current collector.

As a variant, the method of assembling an electrochemical cell 1 comprises a first step a) comprising two sub-steps a1) and a2).

The first sub-step a1) consists in inserting the second piston 18 into the cell body so that it is flush with the current collector 16 in abutment, in introducing the composite powder 15, and in applying pressure between the two pistons (17, 18), so that the composite powder 15 is in electrical contact with the current collector 16, while preventing any sliding of the composite powder 15 in the cell body 10 in the event of removal of at least one minus one of the two pistons (17, 18).

In a second sub-step a2), the first piston 17 is removed, an electrolyte powder is added 13. Then, the second piston 17 is inserted into the cell body 10, and pressure is applied between the two pistons 17, 18 so as to compact the electrolyte powder 13.

The rest of the process is identical to the aforementioned assembly process. This variant therefore does not require the prior preparation of an electrolyte tablet.

[0100] The electrochemical cell that is the subject of the invention makes it possible first of all to characterize electrodes at the laboratory scale. Beyond the laboratory scale, the composite powder according to the invention, which has the advantage of not needing to be mechanically supported by a current collector, can also be integrated as an on-line diagnostic element in batteries.

With a view to an on-board application, with strong constraints in terms of volume, it can be envisaged to approach the percolation threshold by varying the proportion of electroactive material in the composite powder, in order to affect as little as possible the ion transport between anode and cathode

[0101] With a view to an on-board application, it is also possible to seek to significantly reduce the quantity of composite powder, in order to affect the transport of ions between the anode and the cathode as little as possible. It is also conceivable to interpose the composite powder layer only on the inner periphery of the cell, for example with a ring or rectangle shape depending on the geometry of the battery.

[0102] For on-board use, the pistons, which are used to assemble the cell, can be replaced by lighter and less bulky current collectors.

[0103] List of references cited:

[0104] [Costard2017]: “Three-Electrode Setups for Lithium-Ion Batteries II. Experimental Study of Different Reference Electrode Designs and Their Implications for Half-Cell Impedance Spectra ”(J. Costard et al.), Journal of The Electrochemical Society, 164 (2) A80-A87 (2017)

[0105] [Nam2018]: “Diagnosis of failure modes for all-solid-state Li-ion batteries enabled by three-electrode cells” (Young Jin Nam et al.), Mater. Chem. A, 2018, 6, 14867

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Claims

1. Electrochemical cell (1), comprising a positive electrode (11) and a negative electrode (12) arranged face to face in the cell body (10), a stack of electrolytic layers (13, 14, 15) interposed between the cell body (10). 'positive electrode (11) and negative electrode (12), characterized in that the stack of electrolyte layers (13, 14, 15) comprises a first solid electrolyte layer (13) disposed at one end of the positive electrode (11), a second layer of solid electrolyte (14) disposed at one end of the negative electrode (12), a layer of composite powder (15) interposed between the first (13) and of the second layer (14), the composite powder layer (15) comprising an electrolyte powder and a powder of electroactive material having a function of reference electrode of the electrochemical cell.

2. An electrochemical cell (1) according to claim 1, further comprising a current collector (16) disposed on at least part of the periphery of the layer of composite powder (15) and in electrical contact therewith.

3. An electrochemical cell (1) according to claim 2, wherein the current collector (16) is an annular collector, arranged around the entire periphery of the composite powder layer (15).

4. Electrochemical cell (1) according to one of the preceding claims, wherein the composite powder layer (15) is arranged symmetrically with respect to an axis coinciding with the stacking direction of the electrolytic layers (13, 14, 15) and passing through the center of the cell (1).

5. An electrochemical cell (1) according to one of the preceding claims, wherein the first electrolyte layer (13), the second electrolyte layer (14) and the electrolyte powder comprise a sulfide.

6. An electrochemical cell (1) according to claim 5, in which the sulphide is P-Li ₃ PS ₄ (b-LiPS).

7. Electrochemical cell (1) according to one of the preceding claims, wherein the powder of electroactive material comprises lithium, or an alloy based on lithium and indium.

8. Electrochemical cell (1) according to the preceding claim, wherein the lithium-indium-based alloy is Li _x ln, where 0 <x <1, and preferably Lio.5ln.

9. An electrochemical cell (1) according to one of claims 1 to 7, wherein the powder of electroactive material comprises Li ₄ Ti ₅ 0i ₂ or Li ₇ Ti ₅ 0i ₂ (LTO).

10. Electrochemical cell (1) according to one of the preceding claims, wherein the negative electrode (12) and the powder of electroactive material have an identical composition.

11. Composite powder intended to be interposed between two layers of solid electrolyte in an electrochemical cell according to one of the preceding claims, the composite powder comprising an electrolyte powder and a powder of electroactive material having a function of reference electrode of the electrochemical cell.

12. A method of preparing a composite powder according to claim 11, wherein the electrolyte powder comprises p-Li ₃ PS4 (b-LPS), the electroactive material powder comprises Li _x ln, where 0 <x <1, the method comprising the following steps: a1) plating, by cold co-rolling, under a protective atmosphere, of a lithium sheet between two indium sheets in proportions by mass corresponding to the composition Li _x ln, then folding of the product thus obtained; b1) repeating the previous step a plurality of times, until a stable state of the product is obtained; c1) grinding of the product with the electrolyte powder in proportions by mass of 50 to 70% of powder of electroactive material and of 30 to 50% of powder of electrolyte, preferably 60% of powder of electroactive material and 40% of electrolyte powder, until a homogeneous powder is obtained.

13. A method of preparing a composite powder according to claim 11, wherein the electrolyte powder comprises -Li ₃ PS4 (b-LPS), the powder of electroactive material comprises Li _{4 + x} Ti ₅ 0i ₂ (LTO), where 0 <x <1, the process comprising the following steps: a2) mixing a powder of Li ₄ Ti ₅ 0i ₂ with black of carbon, in a ratio of 3: 1 by mass, to form an LTO: C powder b2) reduction of part of the LTO: C powder in a liquid electrolyte cell with a lithium counter electrode, until obtained a powder of Li ₇ Ti ₅ 0i _2: C; c2) rinsing and centrifuging the Li ₇ Ti powder ₅ 0i _2: C with dimethyl carbonate; d2) mixing the powder obtained in step c2) with an unreduced part of the LTO: C powder, in proportions corresponding to the composition Li _{4 + x} Ti ₅ 0i ₂ , and 60% electrolyte powder en masse.

14. A method of assembling an electrochemical cell (1) comprising an insulating cell body (10) equipped with a current collector (16), a first piston (18) and a second piston (17), the method also comprising the following steps: a) insertion of the second piston (18) into the cell body so that it is flush with the current collector (16) in abutment, introduction of the composite powder (15) according to claim 11 , then an electrolyte tablet (13), and application of pressure between the two pistons (17, 18), so that the composite powder (15) is in electrical contact with the current collector ( 16), while preventing any sliding of the composite powder (15) in the cell body (10) in the event of withdrawal of at least one of the two pistons (17, 18); b) inverting the cell body (10), removing the second piston (18), adding an electrolyte powder (14), inserting the second piston (18) into the cell body (10), and applying d 'a pressure between the two pistons (17, 18) so as to compact the electrolyte powder (14); c) withdrawal of the second piston (18), deposit, in the cell body (10), of a powder of electroactive material of positive electrode (11), insertion of the second piston (18) in the cell body 10) , inversion of the cell body (10), withdrawal of the first piston (17), deposit, in the cell body (10), of a powder of electroactive material of negative electrode (12), insertion of the first piston (17) ) in the cell body (10), and application of pressure between the two pistons, so as to compact the two powders of electroactive material (11, 12).

15. A method of assembling an electrochemical cell (1) comprising an insulating cell body (10) equipped with a current collector (16), a first piston (18) and a second piston (17), the method also comprising the following steps: a1) insertion of the second piston (18) into the cell body so that it is flush with the current collector (16) in abutment, introduction of the composite powder (15) according to claim 11 and applying pressure between the two pistons (17, 18), so that the composite powder (15) is in electrical contact with the current collector (16), while preventing any sliding of the composite powder (15) in the cell body (10) in the event of withdrawal of at least one of the two pistons (17, 18); a2) removing the first piston (17), adding an electrolyte powder (13), inserting the second piston (17) into the cell body (10), and applying pressure between the two pistons (17) , 18) so as to compact the electrolyte powder (13); b) inverting the cell body (10), removing the second piston (18), adding an electrolyte powder (14), inserting the second piston (18) into the cell body (10), and applying d 'a pressure between the two pistons (17, 18) so as to compact the electrolyte powder (14); c) withdrawal of the second piston (18), deposit, in the cell body (10), of a powder of electroactive material of positive electrode (11), insertion of the second piston (18) in the cell body 10) , inversion of the cell body (10), withdrawal of the first piston (17), deposit, in the cell body (10), of a powder of electroactive material of negative electrode (12), insertion of the first piston (17) ) in the cell body (10), and application of pressure between the two pistons, so as to compact the two powders of electroactive material (11, 12).