EP4128388A1

EP4128388A1 - Nanoporous electrode

Info

Publication number: EP4128388A1
Application number: EP21714905.3A
Authority: EP
Inventors: Vincent PELE; Christian Jordy
Original assignee: SAFT Societe des Accumulateurs Fixes et de Traction SA
Current assignee: SAFT Societe des Accumulateurs Fixes et de Traction SA
Priority date: 2020-03-31
Filing date: 2021-03-30
Publication date: 2023-02-08
Also published as: WO2021198271A1; FR3108793A1; US20230125633A1; FR3108793B1

Abstract

The present application relates to an electrode comprising stacks of conductors covered with at least two layers, making it possible to improve the deposition of lithium, as well as to electrochemical elements and batteries comprising same.

Description

TITLE: Nanoporous electrode

The present invention relates to the field of energy storage, and more specifically to accumulators, in particular of the lithium type.

Rechargeable lithium-ion batteries indeed offer excellent energy and volume densities and today occupy a prominent place in the market for portable electronics, electric and hybrid vehicles and stationary energy storage systems.

Solid electrolytes also offer a significant improvement in terms of safety as they present a much lower risk of flammability than liquid electrolytes.

The operation of lithium batteries is based on the reversible exchange of lithium ion between a positive electrode and a negative electrode, separated by an electrolyte, the lithium being deposited at the negative electrode during operation under charge.

It is therefore desirable to promote the deposition of lithium and to obtain a deposition that is as homogeneous as possible.

Capacitor electrodes comprising aluminum current collectors on which carbon nanotubes (CNTs) are deposited have been described by Arcila-Velez et al Nano Energy 2014, 8, 9-16.

KR101746927 describes an electrode comprising a protective layer containing a lithium salt, in order to prevent corrosion of lithium by the liquid electrolyte. The protective layer can also include CNTs. However, due to its very high nucleation energy, lithium dendrites will form on the carbon surface. This structure therefore does not allow homogeneous deposition of lithium.

US 2019/088981 describes a cell for a battery, such that the negative electrode comprises conductive elements: here again, the deposition of lithium or of lithiated alloy will greatly increase the thickness of the electrode, which leads to weakening the structure during charge and discharge cycles.

US2017 / 133662 describes a lithium battery comprising a composite type anode in which the lithium metal is inserted within the porous matrix. However, lithium is present here initially and this type of solution does not allow lithium to be kept in the porosity of the carbon during cycling, which causes a significant variation in the thickness of the negative electrode. It was actually observed that after several cycles, lithium no longer fits into the porous carbon structure but between the porous carbon layer and the current collector (YG Lee et al, Nat Energy (2020). https://doi.org /10.1038/s41560-020-0575-z). This phenomenon is explained by the migration of particles of the material forming alloys with lithium towards the surface of the current collector under the layer of carbon particles. This solution does not allow lithium to be stored in the porosity of the carbon and therefore will generate significant variations in thickness of the negative electrode causing a degradation of the service life and moreover, it requires '' apply high mechanical pressures to the accumulator during operation.

Terranova et al Journal of Power Sources 246 (2014) 167-177 and WO 2017/034650 describe electrodes based on carbon nanotubes but do not in any way describe the inhibition of the variation in thickness of said electrodes during charging and charging cycles. dump.

It is therefore desirable to provide a negative electrode whose structure and composition make it possible to increase the quantity and quality of the lithium deposit while avoiding large variations in thickness.

The invention therefore aims in particular to provide a nanoporous negative electrode comprising conductor pillars arranged on the current collector, said electrode being characterized in that the surface of said pillars is at least partially covered with a layer of a material consisting of '' at least one element forming alloys with lithium

According to one embodiment, the pillars are such that they consist of electronically conductive particles which are in direct contact with the current collector.

Typically, that is to say that for the vast majority of the particles of the invention (typically more than 90%), during a SEM observation, no space is perceived between the current collector and the one of the surfaces of the particles (corresponding to the base of the pillar); taking into account the resolution of the SEM, this space must be less than 1 nm.

Due to the fact that there is no space between the collector and the base of the particles (here, the pillars), the compound forming alloys with lithium cannot migrate between the collector and the material constituting the pillar and therefore Consequently, the formation of lithium metal in the porosity of the electrode will be maintained throughout the cycling. The electrode structure according to the invention thus allows the deposit of lithium in a homogeneous manner within the nanoporous structure while greatly limiting the volume variations of the electrode.

The term "nanoporous" according to the invention means a pore size of less than 300 nm.

The pore size corresponds to the structure of the material having an organized network of channels of very small variable pore size (typically less than 300 nanometers), which gives them a particularly large active area per unit electrode area.

The term “negative electrode” designates when the accumulator is discharging, the electrode operating as an anode and when the accumulator is charging, the electrode operating as a cathode, the anode being defined as the electrode where a reaction takes place. electrochemical reaction of oxidation (emission of electrons), while the cathode is the seat of the reduction.

As used herein, the term "conductor pillars" particularly refers to pillars as described by Wei et al (Microelectronic Engineering, Vol. 158, 2016, 22-25). In particular, this term illustrates the arrangement of several elements made of a conductive material, such that said elements are generally parallel to each other, and such that they are arranged on a surface at an angle varying between approximately 70 and 90 ° with the surface, for example at a right angle. The pillars form the porous structure and serve as a support for the alloy forming compound.

Typically, said pillars are arranged in a comb shape, such that the spaces located between said pillars form channels of length which may vary from 1 μm to 1 mm, typically from a few micrometers to several hundred micrometers. Said pillars can have sizes and spacings of a few nanometers to several hundred nanometers, preferably from 10 to 10Onm.

According to one embodiment, the conductor pillars are chosen from copper pillars, carbon nanotubes or microporous carbons.

According to one embodiment, the carbon nanotubes are vertically aligned carbon nanotubes (VACNT). The expression “material forming alloys with lithium” or “lithiophilic” material defines a material exhibiting an affinity for lithium.

According to one embodiment, the electrode according to the invention does not contain lithium metal before it is put into operation.

According to one embodiment, the lithiophilic element is chosen from silver, zinc and magnesium. Typically, the alloys formed by these elements with lithium include Li _x Zn _y , Li _x Mg _y and Li _x Ag _y type alloys, with variable x / y atomic ratios.

According to one embodiment, a second nanometric conductive layer of lithium is deposited on at least part of the surface of the first layer.

The "nanoscale layer" mentioned here refers to the thickness of the second layer, which can vary from a few nanometers to less than 10Onm, typically about less than 50nm.

Typically, the second layer comprises a polymer, ceramic or gel.

The second layer conducts lithium, in that it allows the transit of Li ⁺ ions from the electrolyte layer to the first layer. It can also allow homogenization of the lithium deposit by allowing the formation of local batteries: in fact during charging, a potential difference is created in the thickness of the electrode; this difference in potentials can then allow an electrochemical rebalancing on the thickness of the electrode by oxidation of lithium in the areas of the most positive potentials and a reduction of Li + in the areas of the most negative potentials.

According to one embodiment, the porosity of the electrode is between 45 and 98% to make it possible, on the one hand, to accommodate the lithium metal in the porosity and at the same time to maintain mechanical strength of the electrode.

According to one embodiment, the negative electrode according to the invention further comprises a third layer comprising an electrolyte.

According to one embodiment, the surface density of CNT is between 10 ⁹ and 2.10 ¹¹ CNT / cm ² .

According to one embodiment, the porosity of the electrode is such that:

[Math 1] E xs> 4.85 x C where C is the surface capacity of the positive electrode (in mAh / cm ² ); E is the thickness of the negative electrode in the discharged state expressed in µm; and s is the porosity of the negative electrode in the discharged state (the porosity being defined as the ratio of the difference between the total volume of the electrode (excluding the current collector) and the volume of material divided by the volume of electrode).

According to one embodiment, said electrode has a thickness in the charged state (Ec) and a thickness in the discharged state (Ed), and such that:

[Math 2]

Ec - Ed <4.85 x C xh, where C is the surface capacity of the positive electrode (in mAh / cm ² ); h is a dimensionless number between 0 and 0.3.

As used herein, the term "areal capacitance (or C)" refers to the amount of electricity that the electrode can deliver per unit area.

According to another object, the present invention also relates to a method for preparing a negative electrode according to the invention, said method comprising the step of successively depositing the first layer then the second layer, each of the deposition steps being carried out. by physical or chemical vapor route (PVD or CVD, respectively), or liquid.

By way of physical or chemical vapor deposition (CVD or PVD), mention may in particular be made of atomic layer deposition (ALD). Mention may also be made of deposits by liquid, in particular electrolytic.

According to another object, the present invention also relates to an electrochemical element comprising a negative electrode according to the invention, characterized in that it is a battery of the all-solid or hybrid type (containing at least one inorganic electrolyte and one electrolyte organic polymer), for example a Li free type battery.

According to another object, the present invention also relates to an electrochemical element comprising a negative electrode according to the invention, characterized in that it is a battery of the "lithium free" type.

It is understood that the term “lithium free” defines the fact that the battery does not contain lithium metal during assembly of the accumulator, but that lithium is deposited in metallic form and then consumed in a controlled and reversible manner in situ during battery operation. Typically, lithium is deposited within the negative electrode during charging and consumed during discharge.

By "electrochemical element" is meant an elementary electrochemical cell made up 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.

According to another object, the present invention also relates to an electrochemical module comprising the stack of at least two elements according to the invention, each element being electrically connected with one or more other element (s). The term "module" therefore designates here the assembly of several electrochemical elements, said assemblies being able to be in series and / or parallel.

Another object of the invention is still a battery comprising one or more modules according to the invention.

The term "battery" or accumulator is understood to mean the assembly of several modules according to the invention.

According to one embodiment, the batteries according to the invention are accumulators whose capacity is greater than 100 mAh, typically 1 to 100Ah.

Figures

[Fig 1] Figure 1 shows a schematic representation of the structure of an electrode according to the invention. The current collector (1) such as a metal strip has a flat surface, on which stand pillars of conductive material (2), such as copper pillars or carbon nanotubes.

These pillars (2) are covered with at least one layer:

- first, in direct contact with the pillar, a first layer (3) made of a material capable of forming alloys with lithium; and

- on this first layer and in contact with the electrolyte: optionally a second layer (4) which conducts Li ⁺ ions.

In operation, in charge, the Li ⁺ ions arrive from the solid electrolyte layer separating the 2 positive and negative electrodes, and react at the ends of the pillar. When the pillar is made of carbon, the latter forms a lithiated compound of varying formula (for example, in the case of graphite, its composition is CL10 _.17 ), When the potential of the negative electrode reaches potentials below 0V, a deposit of lithium should form; nevertheless the formation of lithium metal requires a nucleation energy which can be relatively high on the carbon corresponding to an overvoltage. A supersaturation of carbon with lithium can thus take place and the lithium will thus diffuse in the pillar; adding a material forming alloys with lithium to the surface of the CNT will reduce the lithium metal formation overvoltage. The lithium in the structure of the supersaturated lithiated carbon will thus be able to transform into lithium metal on the layer deposited on the surface of the CNT. It should be noted in passing that the material forming alloys will have already lithiated before the precipitation of lithium metal because its formation potential is greater than that of lithium metal. In parallel with this process, the lithium ions can also pass through said possible second layer to be deposited, in the form of Li metal under this layer.

[Fig 2] Figure 2 is a schematic representation of the structure of an electrode according to Figure 1 above, in the charged state, where Li (5) is present around the pillars.

The following examples are given by way of illustration and without limitation of one embodiment of the present invention.

Examples

Realization of electrochemical cells:

Regarding the preparation of the negative electrode, the deposition of VACNT can be carried out as described in the article by Arcila-Velez et al., Nano Energy, Volume 8, 2014, 9-16: the VACNT tubes are made in a quartz tube with a diameter of 5cm; the 2 ends of the tube are partially closed with stainless steel. The tube is placed in an oven with 2 hot zones, the first zone serving as preheating and in the second the reaction takes place. A pump makes it possible to inject the precursor (solution of ferrocene in xylene, containing for example 0.5 at% iron) in the center of the preheating zone. Acetone-cleaned copper sheets, a few cm wide and long, are placed in the center of the reaction zone in the furnace. The system is brought to 600 ° C. under a flow of argon and hydrogen (17% by volume of H2). The precursor is injected at 600 ° C with a low flow rate, for example from 0.1 to 1 5ml / h, in a flow of C2H2 with a flow rate of 30cm ³ / min. The duration of treatment varies from 5 to 50 min depending on the length of the CNTs desired.

Deposits of the first and second layer can be carried out according to the following 5 methodologies:

[Table 1]

The preparation of the electrolyte layer and of the positive electrode is carried out under an argon atmosphere (<1 ppm H2O). 0 The electrolyte membrane is obtained in several stages. A first step of mixing sulfide electrolyte of U6PS5CI argyrodite type with 2% by mass of a copolymer binder based on polyvinylidene fluoride is carried out in a planetary mill. This mixing is carried out at a speed of 1000 rpm for 10 min with several solvents: xylene and isobutyl isobutyrate previously dried using molecular sieves (pore size of 3 Å). In a second step, the ink thus obtained is coated on a PET film allowing the membrane to be peeled off after drying. The thickness of this membrane is 50 µm.

Similarly, the positive electrode is made from an NMC type material of composition LiNiO.6OMnO.2OCoO.2OO ₂ covered with a 10 nm layer of LiNbOs, mixed with solid electrolyte U ₆ PS ₅ CI, carbon fibers (VGCF) and copolymer binder based on polyvinylidene fluoride in mass proportions NMC: Li ₆ PS ₅ CI: VGCF: binder 70: 30: 3: 3. These materials are dispersed in a mixture of xylene and isobutyl isobutyrate solvents. A homogeneous ink is obtained after passing through the planetary mixer. This ink is then coated on an aluminum current collector previously covered with a thin layer of carbon. The weight of the electrode is varied between 15 and 95 mg / cm ² .

After drying, a 12mm diameter disc is cut from the electrolytic membrane along with a 10mm diameter positive electrode disc. These two discs are pressed against each other in a mold under a pressure of 5.61.

For a given example, a 10 mm diameter disc is cut from the negative electrode corresponding to the example and placed on the other side of the electrolytic membrane. This stack is then compressed at a pressure of 1 t / cm ² and can be subjected to a heat treatment of between 80 and 130 ° C. for 12 h.

The stack is then placed in a Swagelok-type cell compressed at a pressure of between 1 and 5 MPa. For the electrical tests, the charge and the discharge are carried out at a rate of C / 20.

Counter examples:

For Comparative Example No. 1, CNT powder 40 nm in diameter and between 20 and 50 μm in length is dispersed in an organic solvent (for example NMP) in the presence of 2% PVDF. The mixture is deposited on a copper collector then dried at 120 ° C. and compressed; the layer thickness is 25pm. A silver deposit is then made by PECVD on the layer thus obtained followed by a deposit of LiPON by ALD. The negative electrode is then obtained by cutting a 10mm diameter disc of coated collector.

For Comparative Examples 2 to 5, the method of preparing the negative electrodes is similar to that used for Examples 1 to 6.

The electrochemical cells are then prepared in an identical manner to Examples 1 to 6. The examples described in Tables 2 and 3 show that the variations in thickness are significantly smaller than Comparative Examples 1 and 4 described in Tables 4 and 5; in fact the negative electrodes of comparative examples 4 and 5 correspond to an increase in thickness corresponding to more than 60% of the initial thickness. Comparative Examples 2, 3 and 5 show too high inter CNT distances which give rise to problems with the mechanical strength of the CNTs under pressure associated with a heterogeneous lithium deposit resulting in a shorter lifetime.

11

[Table 2]

The following features are observed for the embodiments of Table 2 above: Example 1: interCNT distance of the order of 300nm; - Example 2: low thickness of the surface layers;

Example 3: high surface capacity;

Example 4: Ec-Ed> 0 but <0.2 ^* 4.85 ^* 0;

Example 5: small pore size = small interCNT distance;

Example 6: very high surface capacity.

[Table 3]

12

( ^* ) thickness of the negative electrode without the thickness of the collector for an electrode face Comparative examples:

The following examples were obtained from cells comprising negative electrodes, the characteristics of which are given in Tables 4 and 5.

[Table 4]

13

The following features are observed for the embodiments of Table 4 above:

Example 1: CNT powder deposited on a Cu collector => all the Li is deposited between the CNTs and the copper;

Example 2: Poor penetration of lithium into the porosity => poor lifetime and swelling of the electrode;

Example 3: InterCNT distance> 300nm => low mechanical resistance;

Example 4: E ^* poro> 4.85.C and Ec-Ed> 0.2.4.85.C: strong swelling of the electrode corresponding to more than 60% of the thickness of the electrode => poor service life;

Example 5 Poor penetration of lithium into the porosity and low developed surface area of carbon leading to the formation of dendrite.

[Table 5]

It thus appears in particular that when E ^* poro> 4.85.C (example 4), a strong swelling of the electrode is observed.

Claims

1. Nanoporous negative electrode comprising conductor pillars arranged on the current collector, such that the porosity of said negative electrode is such that:

E xs> 4.85 x C where C is the surface capacity of said positive electrode (in mAh / cm ² );

E is the thickness of said negative electrode in the discharged state expressed in µm; and s is the porosity of said negative electrode in the discharged state (the porosity being defined as the ratio of the difference between the total volume of the electrode (excluding the current collector) and the volume of said material divided by the volume of l 'electrode), said electrode being characterized in that the surface of said pillars is at least partially covered with a layer of a material consisting of at least one element forming alloys with lithium.

2. Negative electrode according to claim 1 comprising a second nanometric conductive layer of lithium deposited on at least part of the surface of said layer.

3. Negative electrode according to any one of the preceding claims, such that the conductor pillars are chosen from copper pillars, carbon nanotubes or microporous carbons.

4. Negative electrode according to claim 3 such that the carbon nanotubes are vertically aligned carbon nanotubes (VACNT).

5. Negative electrode according to claim 2, such that the second layer comprises a polymer, a ceramic or a gel.

6. Negative electrode according to claim 2 such that the thickness of the second layer has a thickness between 0 and 100 nm.

7. Negative electrode according to any one of the preceding claims, such that the lithiophilic element is selected from silver, zinc and magnesium.

8. Negative electrode according to any one of claims 2 to 7 such that it further comprises a third layer comprising an electrolyte.

9. An electrode according to any one of the preceding claims having a thickness in the loaded state (Ec) and a thickness in the discharged state (Ed), and such that:

10. A method of preparing an electrode according to any one of the preceding claims comprising the step of successively depositing the first layer and then optionally of the second layer, each of the deposition steps being carried out physically or chemically in the vapor phase. (PVD or CVD, respectively), or liquid.

11. An electrochemical element comprising a negative electrode according to any one of claims 1 to 9, characterized in that it is an all-solid or hybrid type battery.

12. An electrochemical element according to claim 11 such that it is a "Li free" type battery, in that it does not contain lithium metal during assembly.

13. An electrochemical module comprising the stack of at least two elements according to claim 11 to 12, each element being electrically connected with one or more other element (s).

14. A battery comprising one or more modules according to claim 13.