Buffer layer for electrochemical elements

Abstract

Buffer layer for electrochemical elements The present invention relates to an electrochemical element comprising at least one compressible porous layer acting as a buffer layer making it possible to homogenize its operation. Figure for abstract: None

Description

Title of the invention: Buffer layer for electrochemical elements

[0001] The present invention relates to the field of energy storage, and more specifically to accumulators, in particular of the lithium-ion type with solid electrolyte.

[0002] Rechargeable lithium-ion accumulators offer excellent energy and volume densities and today occupy a predominant place in the market for portable electronics, electric and hybrid vehicles and stationary energy storage systems.

[0003] Solid electrolytes also offer a significant improvement in terms of safety in that they present a much lower risk of flammability than liquid electrolytes.

[0004] The operation of lithium accumulators is based on the reversible exchange of the lithium ion between a positive electrode and a negative electrode, separated by an electrolyte, the lithium being deposited or inserted at the negative electrode during charging operation.

[0005] In the case of lithium metal accumulators, it is therefore desirable to promote the deposition of lithium and to obtain a deposit that is as homogeneous as possible.

[0006] During the operation of all-solid electrochemical elements using active materials or electrodes with high volume variations (Li-free; alloy reaction; conversion reaction), the overall volume of the stack varies. These cumulative variations at the beam scale lead to significant pressure variations during operation which, in combination with the volume variations, facilitate contact losses in the different layers (electrodes, separator, current collectors) or at their interfaces.

[0007] In particular, by using a ceramic solid electrolyte, local differences in weights within the electrodes and separators are present. Isostatic pressure can avoid local pressure inhomogeneities during the manufacture of the electrochemical elements. However, during operation, these weight inhomogeneities will induce heterogeneities in terms of thickness variations which will be reflected from layer to layer. They will be all the more significant when materials with high volume variation are used.

[0008] These pressure variations remain macroscopic considerations which translate locally into pressure gradients and mechanical constraints leading to inhomogeneous current densities and therefore inhomogeneous operation of the electrochemical elements. In addition, in an all-solid ceramic configuration, the mechanical stresses formed can generate cracks in the separating membrane and in the electrodes.

[0009] It has been demonstrated that these deformations cause contact losses between active material and electrolyte within the negative electrode which also have repercussions on the opposite positive electrode despite the use of thick separators (> 500 pm in pressurized electrochemical element). Similarly, in multi-layer "pouch" type electrochemical elements, the thinner separators (20-50 pm) used to achieve the energy density objectives increase the risk of these undesirable effects. In addition, the stacking of multiple unitary electrochemical elements (multi-layer pouches) in contact with each other leads to local deformations which propagate directly from layer (electrode, separator) to layer.

[0010] Unlike liquid electrolyte elements where the separator can be partially compressible by its porosity and its mechanical properties and thus absorb part of the pressure and volume variations, ceramic type electrolytes do not have these deformation properties to absorb volume variations and contact losses therefore risk being irreversible.

[0011] Pressure absorption has been investigated for solid electrolyte elements:

[0012] WO 2023 / 153582 concerns the damage caused to solid sulfide electrolytes during the pressurization step during the manufacture of electrochemical elements which requires very high pressures (>100MPa). To protect the electrolytes, WO 2023 / 153582 proposes to cover the external face of the current collectors with an elastic sheet.

[0013] However, WO 2023 / 153582 essentially concerns the problem of pressure absorption during pressurization and therefore proposes thick sheets capable of withstanding very high pressures and requiring thicknesses greater than 100 pm, which affects the electrochemical performance by reducing the energy density. To minimize the loss of energy density with thicknesses greater than 100 pm, it is necessary to use electrodes with a high surface capacity (> 6-7 mAh / cm2). However, it is known that the lifetime of lithium metal electrochemical elements decreases when the surface capacity is high. Consequently, we seek to achieve both optimum energy densities and a lifetime compatible with automotive applications, for example.

[0014] In addition, said elastic sheet must be positioned on the external face of the cathode.

[0015] It is therefore desirable to be able to compensate for variations in volume and pressure during operation.

[0016] An aim of the invention is then to propose a buffer layer making it possible to absorb these inhomogeneities, in particular for electrode surface capacities of less than 6 mAh / cm2.

[0017] For this purpose, the invention relates to an electrochemical element comprising the stack of at least one unit, each unit comprising:

[0018] two positive half-electrodes, each positive half-electrode comprising a current collector and a layer of positive active material;

[0019] two separators;

[0020] two negative half-electrodes, each negative half-electrode comprising a current collector and a layer of negative active material;

[0021] said element being characterized in that each unit comprises at least one compressible porous layer, located between two current collectors of the positive half-electrode and / or the negative half-electrode, each compressible porous layer having a thickness under zero pressure (EOMPa) strictly less than 100 μm.

[0022] The compressible porous layer (also called here "buffer layer") according to the invention allows a mechanical "spring" function making it possible to compensate for the strong volume variations which can occur during the operation of elements, in particular completely solid ones, and to minimize the operating inhomogeneities, the volume variations of the element and the risks of local contact losses caused by delaminations.

[0023] In the discharged state where the volume of the electrodes is the smallest, these buffer layers are at rest and compress during the swelling of the electrodes occurring during charging. The elasticity of these buffer layers makes it possible to compensate for local variations in volume and therefore in pressure which can cause variations in operation, while maintaining the contacts.

[0024] The invention thus improves the homogeneity of the mechanical stresses in the element to avoid the accumulation of defects causing the creation of cracks and therefore a degradation of the power and capacity of the element, while limiting the loss of volumetric energy linked to the addition of this additional device.

[0025] In particular, the element according to the invention advantageously achieves these results with a loss of volumetric energy of less than 20% compared to a reference element (ie) without a compressible porous layer.

[0026] According to other advantageous aspects of the invention, said element comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations.

[0027] The electrochemical element

[0028] The term "electrochemical element" means an elementary electrochemical cell comprising an assembly of positive and negative electrodes, electrolyte, separators, container and terminals making it possible to store the electrical energy supplied by a chemical reaction and to restore it in the form of current. The electrolyte may be solid or liquid, preferably solid.

[0029] In all-solid-state electrochemical elements, the separator may consist mainly of solid electrolyte. Also, the electrolytic compounds may be included in the electrolytic layer, but may also be included partly within the electrodes.

[0030] The term positive electrode designates the electrode where the electrons enter, and where the cations (Li+) arrive in discharge.

[0031] The positive electrode generally consists of a conductive support used as a current collector which is coated with a layer of positive electrode material containing the positive electrode active material, to which an electronically conductive carbon additive and solid electrolyte particles may be added in the case of an all-solid electrochemical element.

[0032] Typically the positive electrode may also comprise a binder.

[0033] The term "negative electrode" refers to the electrode functioning as an anode when the accumulator is discharging, the anode being defined as the electrode where an electrochemical oxidation reaction takes place (emission of electrons). The term negative electrode also refers to the electrode from which the electrons leave, and from which the cations (Li+) are released during discharge.

[0034] In the context of the present invention, the negative electrode may be of any known type, but preferably with anode active materials exhibiting a high volume variation during the charge / discharge process such as lithium metal and its alloys and silicon but may also be chosen from graphite and / or carbon-based electrodes, titanium oxide, and mixtures thereof. The negative electrode typically consists of a conductive support used as a current collector on which is deposited the layer of negative electrode material comprising a negative electrode active material to which solid electrolyte particles and an electronically conductive material may be added. A binder may also be incorporated into the mixture. It is understood that in anode-free systems called "anode free", a negative electrode is also present (generally initially limited to the current collector alone).

[0035] According to the invention, a unit is thus defined as an assembly of:

[0036] two positive half-electrodes, each positive half-electrode comprising a current collector and a layer of positive active material;

[0037] two separators;

[0038] two negative half-electrodes, each negative half-electrode comprising a current collector and a layer of negative active material.

[0039] It is conventionally understood that each half-electrode has a so-called internal face located opposite the layer of active material, and an external face located opposite the current collector.

[0040] It is understood that each separator is conventionally comprised between a positive half-electrode and a negative half-electrode, at the interface between the positive active material layer and the negative active material layer.

[0041] According to the invention, each unit comprises one or more compressible porous layers for which different positions within the unit can be envisaged.

[0042] Thus, a compressible porous layer can be positioned:

[0043] either between two current collectors of two positive half-electrodes;

[0044] either between two current collectors of two negative half-electrodes; or

[0045] either between the current collector of a negative half-electrode and the current collector of a positive half-electrode in the case of a bipolar electrochemical element.

[0046] Typically, said compressible porous layer may be positioned on the external face of said half-electrodes.

[0047] According to one embodiment, said compressible porous layer is positioned between two current collectors of two negative half-electrodes.

[0048] According to the invention, each compressible porous layer has a thickness under zero pressure (EoMPa) strictly less than 100 μm, in particular less than or equal to 95 μm, in particular less than 90 μm, more particularly less than 80 μm, preferably less than 70 μm, more preferably less than 50 μm.

[0049] The term “compressible porous layer” means a layer whose compression, elasticity and / or porosity properties allow:

[0050] - to compensate for variations in the volumes of the electrodes;

[0051] - to absorb the inhomogeneities of mechanical stress and pressure;

[0052] - to maintain contacts between the different constituents of the unit.

[0053] Furthermore, according to one embodiment, said compressible porous layer limits the loss of volumetric energy linked to the addition of this additional layer.

[0054] Thus, a reduced thickness of said compressible porous layer as mentioned previously advantageously makes it possible to reduce the volumetric energy by at most 20%, typically by at most 15%, preferably by at most 12% compared to a reference element (ie) without compressible porous layer.

[0055] According to one embodiment, each unit comprises one or two compressible porous layers.

[0056] According to an alternative, when each unit comprises a single compressible porous layer, when the surface capacity of the positive electrode (Cs) is less than 5 mAh / cm2, the thickness under zero pressure of said compressible porous layer is generally such that 10 pm < E0Mpa <100 pm.

[0057] According to another alternative, when each unit comprises two porous layers compressible, when the surface capacity of the positive electrode (Cs) is between 5 and 6 mAh / cm2, the thickness under zero pressure of said compressible porous layer is such that 30 pm < E0Mpa <100 pm.

[0058] According to one embodiment, the thickness under pressure of 1 MPa of said compressible porous layer EiMPa (pm) is typically between 5 and 25 pm.

[0059] According to one embodiment, at least one compressible porous layer is located between a positive half-electrode and a negative half-electrode of each unit, typically between each external face of said electrode, so that said compressible porous layer is positioned between a current collector of the negative half-electrode and a current collector of the positive half-electrode.

[0060] The electrochemical element may have different stacking configurations of the units that constitute it, such as unipolar or bipolar stacks.

[0061] The term “bipolar configuration” means a configuration in which the external face of the positive half-electrodes faces the external face of the negative half-electrodes (i.e.) the current collector of the positive half-electrodes is positioned opposite the current collector of the negative half-electrodes.

[0062] The term “unipolar configuration” means a configuration in which the external face of the positive half-electrodes face each other and the external face of the negative half-electrodes also face each other (i.e.) the current collector of the positive half-electrodes, and respectively the current collector of the negative half-electrodes, face each other.

[0063] In a bipolar configuration, in order to optimize the energy and power of the electrochemical element, the compressible porous layer is electronically conductive with an electronic conductivity > IS / m, preferably >100S / m, which on the one hand makes it possible to avoid having to resort to an external connection of the 2 positive and negative half-electrodes but also makes it possible to reduce the electrical resistance of the assembly. The compressible porous layer is generally positioned between the facing current collectors.

[0064] Thus, according to one embodiment, the electrochemical element is of bipolar configuration, where each unit comprises at least one electronically conductive compressible porous layer, each compressible porous layer being located between the current collector of said negative electrode and the current collector of said positive electrode.

[0065] In a unipolar configuration, the compressible porous layer may be electronically insulating. It is generally positioned between two facing current collectors. Typically, in a unipolar configuration, each unit comprises one or two compressible porous layers.

[0066] Thus, according to one embodiment, the electrochemical element is of configuration unipolar where each unit comprises one or two compressible porous layers, each compressible porous layer being located between each external face of the half-electrodes of the same polarity.

[0067] Advantageously, in a unipolar configuration, the use of an electronically conductive compressible porous layer makes it possible to connect only one of the 2 half-electrodes, and not both, leading to simpler implementation and lighter connections making it possible to increase the energy density.

[0068] The compressible porous layer

[0069] According to one embodiment, said compressible porous layer is made of foam having a porosity of between 60% and 97%, typically between 70 and 85%.

[0070] Porosity corresponds to the structure of a material having an organized network of channels, of very small variable pore size. Porosity can be conventionally calculated.

[0071] Porosity can be calculated from dimensions measured by scanning electron microscopy (SEM).

[0072] Porosity can also be calculated from the thickness, mass and density of the components. Porosity can be conventionally calculated by the formula (em / d) / e * 100

[0073] in which:

[0074] e represents the thickness of said compressible porous layer in cm,

[0075] m represents the mass of 1 cm2 of said compressible porous layer expressed in grams and

[0076] d represents the average density of the compressible porous layer in g / cm3, this being determined on the basis of the mass ratio of the different constituents of said compressible porous layer.

[0077] According to one embodiment, said compressible porous layer is made of foam having a compressibility of between 50 and 95%, typically between 60 and 80%.

[0078] Compressibility can typically be calculated as the ratio (E0Mpa - E iMPa) / EoMPa *100, where EiMPa and EoMPa are as defined above.

[0079] According to one embodiment, said compressible porous layer is made of foam having an elastic modulus under compression of between 0.1 and 5 MPa, preferably between 0.15 and 1.2 MPa.

[0080] According to one embodiment, said compressible porous layer is made of foam comprising a polymer, in particular a polymer chosen from:

[0081] - polyurethanes,

[0082] - thermoplastic elastomers,

[0083] - polysulfide rubbers such as thiokols,

[0084] - polyolefin elastomers, in particular those composed of at least 95% in weight of partially crosslinked macromolecules consisting of ethylene and at least one other olefin,

[0085] - polybutadienes,

[0086] - neoprenes,

[0087] - natural rubbers,

[0088] - silicone elastomers.

[0089] An example of the production of a compressible and thin porous layer is described in the following publication:

[0090] “Cellular Thermoplastic Polyurethane Thin Film: Preparation, Elasticity, and Thermal Insulation Performance”, C. Ge, W. Zhai*, Ind. Eng. Chem. Res. 2018, 57, 13,4688-4696

[0091] According to one embodiment, said foam does not comprise polyacrylate.

[0092] According to one embodiment, said compressible porous layer has a electronic conductivity > 1 S / m, preferably >1000 S / m.

[0093] According to one embodiment, said foam may also be made up of or comprise electronically conductive compressible particles such as carbon nanotubes (CNTs) and their agglomerates, preferably vertically aligned CNTs (VACNTs), the VACNTs being able to be deposited directly on the collector of a half electrode or on an independent support.

[0094] According to one embodiment, said foam may also comprise one or more additives such as flame retardant additives in particular.

[0095] The elements according to the invention are particularly advantageous when the components used are subject to large variations in thickness, in particular during the first cycle. This is particularly the case for batteries with a positive electrode containing a sacrificial cathode material. In the case of a sacrificial positive electrode, part of the capacity of the positive electrode will not cycle reversibly during subsequent cycles. Thus, the sacrificial positive electrode serves to compensate for lithium losses during the first cycle.

[0096] Thus, according to one embodiment, said positive half-electrodes contain a sacrificial cathode material, such as for example Li2O, Li2S, Li5FeO4, Li3N, Li2 NiO2.

[0097] According to another embodiment, the collector of the half-electrodes is integral with said compressible porous layer and the thickness of the metal layer acting as a current collector is less than 5 μm, preferably less than 3 μm.

[0098] According to another embodiment, the 2 collectors of the half-electrodes are integral with said compressible porous layer and the thickness of each of these layers metallic is less than 5 qm.

[0099] For these 2 embodiments, the metallic deposition of the compressible porous layers can, for example, be carried out by magnetron cathode sputtering or by wet metallization process, which consists of immersing the substrate in a solution containing aluminum ions and reducing agents.

[0100] The separator

[0101] The nature of the separators suitable for the invention is not limited. The separators may in particular be chosen from:

[0102] porous polymeric separators optionally impregnated with a liquid or gelled electrolyte, and

[0103] layers of inorganic solid electrolytes, including in particular

[0104] sulfide, oxide, oxysulfide or halide electrolytes or

[0105] polymer electrolytes or

[0106] mixtures of inorganic and polymeric electrolytes.

[0107] The buffer properties of the compressible porous layer being particularly suitable for variations in thickness and pressure, solid electrolytes are particularly advantageous in the context of the electrochemical elements of the invention.

[0108] The current collector

[0109] It is understood that in a unipolar configuration, two collectors of the same polarity can be combined into a single collector.

[0110] The nature of the current collectors suitable for the invention is not limited. Current collectors conventionally used for positive and negative electrodes for solid lithium or Li-ion electrochemical cells can thus be envisaged. For example, the current collectors can be made of copper or a copper-based alloy, aluminum or an aluminum-based alloy, stainless steel.

[0111] Typically, the collectors may have a thickness of between 1 and 5 qm.

[0112] According to another object, the invention also relates to the method of preparing an element according to the invention.

[0113] Conventionally, the collectors are deposited directly on each face of the compressible porous layer.

[0114] The invention will be described more precisely, in an illustrative manner, with reference to the Figures and examples given below. Figures

[0115] [Fig.l] [Fig.l] represents an electrochemical element according to the invention, in a unipolar configuration comprising a single compressible porous layer (6).

[0116] [Fig.2] [Fig.2] represents an electrochemical element according to the invention, in a unipolar configuration comprising two compressible porous layers (6) and (6').

[0117] [Fig.3] [Fig.3] represents an electrochemical element according to the invention, in a bipolar configuration comprising two compressible porous layers (6) and -6').

[0118] [Fig.4] [Fig.4] an element according to the invention before operation (Al) and during operation (A2), compared to a reference element without compressible porous layer before operation (Bl) and during operation (B 2).

[0119] In Figures 1, 2 and 3 a unit as defined above is shown in the dotted box.

[0120] The elements are therefore made up of the stacking of several units, according to unipolar (Figures 1 and 2) or bipolar ([Fig.3]) configurations.

[0121] As shown in Figures 1 to 3, each unit comprises:

[0122] two positive half-electrodes, each positive half-electrode comprising a current collector (2) and (2') respectively and a positive active material layer (1) and (1') respectively;

[0123] two separators (3) and (3');

[0124] two negative half-electrodes, each negative half-electrode comprising a current collector (5) and (5') respectively and a negative active material layer (4) and (4') respectively.

[0125] In the unipolar configuration shown in [Fig.l], the two collectors (2) and (2') can be combined into a single collector (here represented by (2+2')).

[0126] In the unipolar configuration shown in [Fig.l], the unit comprises a single compressible porous layer (6) located between each external face of the two negative half-electrodes, i.e. between the two current collectors (5) and (5').

[0127] In the unipolar configuration shown in [Fig.2], the unit comprises two layers (6) and (6') respectively positioned - Between each external face of the two negative half-electrodes, that is to say between the two current collectors (5) and (5'), and - Between each external face of the two positive half-electrodes, that is to say between the two current collectors (2) and (2').

[0128] In the bipolar configuration shown in [Fig.3], the unit comprises two compressible porous layers (6) and (6'), each being positioned between the external face of a negative half-electrode and the external face of a positive half-electrode, i.e. between the two current collectors (2) and (5) on the one hand, and the two current collectors (2') and (5') on the other hand.

[0129] [Fig.4] further illustrates the operation of an electrochemical element according to the invention in a unipolar configuration with a compressible porous layer, compared to a reference electrochemical element. In [Fig.4], the half-electrode (7) comprises the current collector and the positive active material layer. The negative half-electrode is represented by its negative active material layer (4) and its current collector (5). A separator (3) is positioned between the internal face of this half-electrode (7) and the internal face of the negative half-electrode, i.e. opposite the negative active material layer (4).

[0130] In the representations (A1) and (A2) of an electrochemical element according to the invention, a compressible porous layer (6) is present between each external face of the negative half-electrodes, i.e. between the negative current collectors (5) and (5').

[0131] In operation (A2) and (B2) respectively, lithium is inserted or deposited in the active material layer of the negative half-electrode, leading to a layer (8) with a volume expansion of said layer.

[0132] In an electrochemical element according to the invention (A2), this increase in volume is then absorbed by the compressible porous layer (6) which compresses and therefore compensates at the overall level for the external apparent volume of a unit.

[0133] In the absence of a buffer layer (B2), the expansion of the lithiated negative layer (8) causes an increase in the volume of each unit which propagates within the electrochemical element. The apparent external volume of the unit and of the electrochemical element varies during cycling. In addition, the electrode may preferentially lithiate in certain areas (not shown), leading to a risk of loss of contact within the adjacent unit (for example at the separator / electrode interface) and other areas with excess thicknesses creating mechanical stresses.

[0134] Thus, local deformations propagate in cascade and can accelerate the end of life of the electrochemical element, for example due to contact losses; short circuit through a separator; delamination; etc. Examples:

[0135] Preparation of the positive half electrodes:

[0136] The electrode consists of powders of active material LiNi0.80Ni0.IOCoO. 1002 coated with LiNbO3, solid electrolyte (Li6PS5Cl), a binder (PVDF), and a conductive carbon (VGCF) whose respective quantities in mass percentages of these components are 83%, 15%, 1% and 1%. These components are mixed in a solvent (isobutylisobutyrate) using a Thinky type mixer. A film of this mixture is then deposited on an aluminum current collector. The electrode is then dried at room temperature, calendered and then cut. The surface area of ​​the positive electrodes is 25cm2.

[0137] The desired surface capacity of the half-electrode is obtained by adapting the weight of the positive material calculated from the relation:

[0138] Cs = weight of active material of the electrode in mg / cm2 multiplied by the capacity of the active material (0.2 mAh / mg)

[0139] Preparation of electrolyte layers (separators):

[0140] The solid electrolyte powder of composition Li6PS5Cl is mixed with 2%(*) of binder (PVDF) in a solvent (isobutylisobutyrate) using a Thinky type mixer. A film of this mixture is then deposited on a PET support and then dried at room temperature and then calendered.

[0141] Preparation of the negative half-electrodes:

[0142] The negative half-electrode consists of a stainless steel collector on which a carbon layer is deposited. This layer is prepared by mixing the carbon powder with 6%(*) of PVDF in NMP, which is then deposited on the collector and then dried at 120°C, calendered and then cut. The surface area of ​​the negative electrodes is 25cm2.

[0143] (*): as a percentage of the total quantity of dry matter.

[0144] Preparation of the compressible porous layer:

[0145] A polyurethane-based layer is prepared based on polyurethane thermoplastic

[0146] The details of the preparation are described in the following publication (“Cellular Thermoplastic Polyurethane Thin Film: Preparation, Elasticity, and Thermal Insulation Performance”, C. Ge, W. Zhai*, Ind. Eng. Chem. Res. 2018, 57, 13, 4688-4696).

[0147] Pre-dried pellets of Austin TPU 380A polyurethane thermoplastic are placed between 2 layers of polyimide (quantity: 2 to 10g of TPU per 400cm2) and then compressed at 190°C under a pressure of between 15 and 20MPa for 3 min. After this operation, the 3 layers are placed in an enclosure under a CO2 atmosphere of 25 to 45 bar for 12 hours, then heated to between 90 and 120°C.

[0148] Characterization of the compressible porous layer:

[0149] For the measurement of the thickness under zero pressure E(0 MPa), this can be measured using a scanning electron microscope.

[0150] For the measurement of the thickness under pressure of 1 MPa E(1 MPa), this is carried out using a thickness gauge; the surface of the contact keys with the layer to be measured is chosen knowing the force applied by the gauge and in such a way that the contact pressure is equal to IMPa.

[0151] Assembly of the electrochemical element:

[0152] First, subassemblies (half positive electrode / separator layers / half negative electrode) are compressed to 400MPa. The thickness of each subassembly is measured using a thickness gauge.

[0153] The electrochemical elements are produced in a unipolar configuration, that is to say that the subassemblies (half positive electrode / separator / half negative electrode) are stacked by applying the faces of the half electrodes opposite the half electrodes

[0154]

[0155]

[0156]

[0157]

[0158]

[0159]

[0160]

[0161]

[0162] of the same polarity. For examples 8 to 12, a compressible porous layer is inserted between each pair of positive half-electrodes and each pair of negative half-electrodes and for examples 1 to 7, the compressible porous layer is inserted only between each pair of negative half-electrodes. The assembly of the electrochemical elements is then carried out by combining 8 sub-assemblies. After soldering the connections, the electrochemical element is sealed in a "pouch" made of polymer-coated aluminum sheets, allowing a waterproof container to be created after heat sealing. The electrochemical elements are then placed between steel plates, which are clamped under an initial pressure of 0.1 MPa. The assembly is equipped with a pressure sensor to monitor the pressure changes during electrochemical operation. The electrochemical elements are then charged at a rate of C / 20 at room temperature up to a voltage of 4.2V. At the end of the charge, the clamping system is dismantled. The electrochemical element is then compressed under a pressure of IMPa and then the thickness of the electrochemical element under pressure is measured. Calculation of volumetric energy loss relative to the theoretical value of an electrochemical element without a compressible porous layer: Adding compressible porous layers to the electrochemical cell will induce a loss of volumetric energy density. Since the volume of the electrochemical cell varies during charging and discharging, the volumetric energy to be considered is the energy of the electrochemical cell in the state for which the volume is the highest, i.e. in the charged state. In this state, as a first approximation, the volume of the electrochemical cell is equal to the volume in the discharged state to which the volume of lithium formed during charging should be added. Furthermore, with the increase in volume of the electrochemical element, depending on the mechanical holding system, the pressure will increase. In practice, in order to minimize the mass and volume of this system which penalizes the overall energy density, it is preferable not to exceed a pressure of 1 MPa. Therefore, the energy density considered in this invention is estimated in the charged state under a pressure of IMPa The loss of volumetric energy density related to the insertion of compressible porous layers is estimated for all examples in Table 1. It is calculated from the relationship DE (in %) (Ewithout compressible porous layer Ewith compressible porous layers) / Ewithout compressible porous layer *100

[0163] With Ewithout compressible porous layer: theoretical volumetric energy density without insertion of compressible porous layer calculated in the loaded state

[0164] and Ewithcompressibleporouslayers 7 theoretical volumetric energy density with the insertion of compressible porous layers at a pressure of IMPa; this energy density is estimated from the measurement of the thickness of the electrochemical element in the compressed charged state under a pressure of IMPa.

[0165] The characteristics of the electrochemical elements according to the invention and of reference are summarized below in Table 1.

[0166] [Tables 1] Surface capacity (Cs, in mAh / cm2 ) N (*) E(0MPa ) in microns E(lMPa) in microns Pressure of the electrochemical element at the end of charge (MPa) (**) Loss of volumetric energy density DE (in %) Example 1 2 1 10 2.5 »1 1.7 Example 2 2 1 30 7.5 <1 6.0 Example 3 (Reference) 5 1 110 49.5 <1 23.5 Example 4 5 1 90 22.5 <1 10.7 Example 5 (Reference) 2 1 100 29 <1 23.1 Example 6 6 1 80 35 »1 15.1 Example 7 (Reference) 6 1 100 55 »1 23.0 Example 8 6 2 50 17.5 <1 14.6 Example 9 6 2 10 1.5 »1 1.3 Example 10 6 2 40 10 <1 8.4 Example 11 (Reference) 6 2 100 29 <1 24.3 Example 12 6 2 85 12.75 <1 10.7

[0167] (*) N: number of layers per unit (2 half positive - 2 half negative - 2 se- parators)

[0168] Due to swelling of the negative electrode, an increase in pressure is expected at the end of charging.

[0169] (**) the expression »1 means much greater than 1 MPa (for example 2MPa)

[0170] In order to achieve both an electrochemical element pressure not exceeding IMPa at the end of charging and a minimal volumetric energy loss, it appears from Table 1:

[0171] For a unit comprising a single compressible porous layer with a surface capacity of the positive electrode (Cs) of less than 5 mAh / cm2, the thickness under zero pressure of said compressible porous layer is preferably greater than 10 pm and the thickness under pressure of IMPa is greater than 5 pm

[0172] For a unit comprising two compressible porous layers with a surface capacity of the positive electrode Cs of between 5 and 6 mAh / cm2, the thickness under zero pressure of said compressible porous layer is generally such that 30 pm < EoMPa < 100 pm.

[0173] The thickness under pressure of 1 MPa of said compressible porous layer EiMPa (pm) is typically between 5 and 25 pm.

[0174] It should be noted that in each case, a compressible porous layer with a thickness of 100 μm leads to too great a loss of volumetric energy (greater than 20%).

Claims

Claims

1. Electrochemical element comprising the stack of at least one unit, each unit comprising: two positive half-electrodes, each positive half-electrode comprising a current collector and a layer of positive active material; two separators; two negative half-electrodes, each negative half-electrode comprising a current collector and a layer of negative active material; said element being characterized in that each unit comprises at least one compressible porous layer, located between two current collectors of the positive half-electrode and / or the negative half-electrode, each compressible porous layer having a thickness under zero pressure (E0Mpa) strictly less than 100 μm.

2. An electrochemical element according to claim 1 such that: i. each unit comprises a compressible porous layer, the surface capacity of the positive electrode (Cs) is less than 5 mAh / cm2 and the thickness under zero pressure of said layer is such that 10 pm < E0Mpa <100 pm; or ii. each unit comprises two compressible porous layers, the surface capacity of the positive electrode Cs is between 5 and 6 mAh / cm2 and the thickness under zero pressure of said layer is such that 30 pm < E0Mpa <100 pm.

3. An element according to any preceding claim wherein the compressible porous layer is electronically conductive.

4. An electrochemical element according to claim 1, 2 or 3 such that the electrochemical element is of bipolar configuration, wherein each unit comprises at least one electronically conductive compressible porous layer, each layer being located between a current collector of said negative electrode and a current collector of said positive electrode.

5. An electrochemical element according to claim 1, 2 or 3 such that the electrochemical element is of unipolar configuration where each unit comprises one or two compressible porous layers, each layer being located between each external face of the two half-electrodes of the same polarity.

6. Element according to any one of the preceding claims such that the thickness under pressure of 1 MPa of said compressible porous layer (EiMpa) is between 5 and 25 pm.

7. An element according to any one of the preceding claims such that said compressible porous layer is made of foam having a porosity of between 60% and 97%.

8. Element according to any one of the preceding claims such that said porous layer is made of foam having a compressibility of between 50 and 95%.

9. Element according to any one of the preceding claims such that said compressible porous layer is made of foam having an elastic modulus under compression of between 0.1 and 5 MPa.

10. Element according to any one of the preceding claims such that said compressible porous layer has an electronic conductivity > 1 S / m, preferably >1000 S / m.

11. An element according to any one of the preceding claims such that said compressible porous layer comprises a polymer such as a polyurethane, and optionally compressible particles such as carbon nanotubes (CNTs) and their agglomerates, and / or one or more additives such as flame retardant additives.

12. Element according to any one of the preceding claims such that said porous layer is made of or comprises carbon nanotubes (CNT) preferably aligned vertically (VACNT).

13. Element according to any one of the preceding claims such that the collector of the half-electrodes is integral with said layer and such that the thickness of the collector is less than 5 μm, preferably less than 3 μm.

14. An element according to any preceding claim such that said positive half-electrodes contain sacrificial cathode materials.

15. Element according to any one of the preceding claims such that the separators are chosen from porous polymeric separators optionally impregnated with a liquid or gelled electrolyte, and layers of inorganic solid electrolytes comprising: sulfide, oxide, oxysulfide or halide electrolytes or polymer electrolytes or mixtures of inorganic and polymer electrolytes.

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