Insulating separator for electric battery

Abstract

The present invention relates to a separator (1) capable of separating two cells (2) of a battery (3), for example of an electric or hybrid electric vehicle, said separator (1) comprising at least one insulation layer comprising a composite material (4), said composite material (4) comprising a binder mixed with aerogel particles (5), the volume content of said aerogel particles (5) in said composite material (4) being greater than 20%, the sum of the weight contents of said aerogel particles (5) and said binder in said composite material (4) being greater than 80%. The present invention also relates to a battery comprising a separator according to the invention, and a method for manufacturing the separator according to the invention. Abstract figure: Figure No. 2

Description

Title of the invention: Insulating separator for electric battery

[0001] The present invention relates to the field of electric batteries. It relates more particularly to a cell separator for an electric battery.

[0002] With the advantage of low CO2 emissions, electric and hybrid electric vehicles are experiencing significant growth. These vehicles contain batteries, usually lithium-ion batteries. These batteries may experience thermal runaway, during which the temperature spontaneously increases inside the battery. The battery can then heat up to the point of catching fire or exploding. Managing this risk is therefore an important safety issue. To achieve this, one of the challenges is to limit and / or slow down the spread of heat and / or fire between adjacent cells in a battery.

[0003] Document WO2006137935 discloses a thermal insulating separator that can be placed between the cells of a battery. It comprises a fiber-reinforced aerogel, encapsulated in a polymer. This solution is very expensive, and has poor thermal insulation performance.

[0004] An object of the present invention is to provide an insulating separator with improved thermal insulation performance, fire resistance, and durability.

[0005] An object of the present invention is to provide a thermally, electrically, and highly compressible insulating separator.

[0006] Another object of the present invention is to propose a simple, multi-performance insulating separator constituting a single part which is easy to integrate into an existing battery.

[0007] The object of the present invention is to meet at least in part the aforementioned objects by proposing a separator composed mainly of a binder, for mechanical strength, and of aerogel, for thermal and fire resistance. To this end, it proposes a separator capable of separating two cells of a battery, for example of an electric or hybrid electric vehicle, said separator comprising at least one insulation layer comprising a composite material, said composite material comprising a binder mixed with aerogel particles, the volume content of said aerogel particles (5) in said composite material (4) being greater than 20%, the sum of the weight contents of said aerogel particles (5) and said binder in said composite material (4) being greater than 80%.

[0008] Thanks to these provisions, the speed of propagation of heat and / or fire between two battery cells can be reduced, the risks of fire spreading outside of the battery, or of battery explosion, are also reduced. This solution also has good durability over time, the stresses applied to the battery do not damage the separator; the solution is simple to manufacture.

[0009] According to other characteristics: - said separator may comprise at least one volume reduction management layer, said volume reduction management layer being, at any square zone with an area greater than or equal to 4 cm2 located on one of its external faces, capable of deforming in order to absorb at least part of a reduction in the volume of said separator, which makes it possible to avoid wear of battery elements due to expansion of the cells, - at least one of said at least one volume reduction management layer may comprise at least one layer of ceramic fiber paper, which is a simple and robust means of implementing the invention, - at least one of said at least one volume reduction management layer may have on at least one of its faces a three-dimensional shape, for example in the shape of a honeycomb, which is a simple solution to implement, and makes it possible to produce a separator with improved durability, - said insulation layer may comprise at least one mechanical reinforcing element, for example in the form of a honeycomb, which ensures that this layer holds well, for example when the binder tends to crumble, - said binder may comprise an organic material, for example a silicone elastomer, the volume content of said aerogel particles in said composite material being between 20% and 80%, the weight content of said organic material in said binder being greater than 50%, which is a solution particularly suitable for absorbing vibrations without deteriorating. - said binder may comprise a mineral material, for example a calcium hydroxide, the volume content of said aerogel particles in said composite material being greater than 90%, the weight content of said mineral material in said binder being greater than 50%, which is a solution making it possible to reduce the weight of the separator, - the aerogel can be a hydrophilic aerogel, which improves the separator's ability to form a fire barrier.

[0010] The present invention also relates to an electric or hybrid electric vehicle battery comprising at least two cells, and at least one insulating separator according to the invention arranged between said cells.

[0011] Thanks to these provisions, the speed of propagation of heat and / or fire between two battery cells can be reduced, the risks of fire spreading outside the battery or of explosion of the battery are also reduced. This solution also has good resistance over time, the stresses applied to the battery do not damage the separator; the solution is simple to manufacture.

[0012] The present invention finally relates to a method of manufacturing an insulating separator according to the invention, comprising the following steps: - production of aerogel particles, - mixing the aerogel particles with said binder in the liquid state, - hardening of the mixture obtained in a mold in order to obtain said material composite.

[0013] Thanks to these arrangements, the separator according to the invention can be produced in a simple manner.

[0014] According to other characteristics: - the manufacture of said aerogel may include the following sub-steps: - mixing a precursor with a synthesis solvent and a hydrolysis agent such as water, and where appropriate a catalyst, to obtain a gel, - granulation of the product obtained by cutting a jet of said gel, to obtain particles,

[0015] which makes it possible to obtain less angular particles, and therefore less brittle and less likely to split, - the manufacture of said aerogel may include the following sub-step: - drying of the particles, carried out entirely at a pressure higher than the critical point of CO2.

[0016] which makes it possible to avoid breaking the aerogel particles, and makes it possible to obtain a hydrophilic aerogel, - the manufacture of said aerogel may include the following sub-steps: - mixing a precursor with a synthesis solvent and a hydrolysis agent such as water, and where appropriate a catalyst, to obtain a gel, - granulation of the product obtained, to obtain particles, - keeping the particles in contact with the synthesis solvent and the hydrolysis agent, - washing the particles by adding a washing solvent to extract in particular the hydrolysis agent and, where appropriate, the catalyst, - drying of the particles to extract the synthesis and / or washing solvents by sending excess supercritical CO2,

[0017] the sub-steps of granulation, holding, washing and drying are operated at a pressure higher than the critical point of CO2, and these conditions are maintained between these steps, which allows the aerogel to be manufactured continuously; the manufacturing time as well as the costs are significantly reduced, and the quality of the product is improved, due to a reduction in the risky stages of change of state and depressurization.

[0018] The present invention will be better understood on reading the detailed description which follows, with reference to the appended figures in which:

[0019] [Fig-1] [Fig.l] is a schematic sectional view of a battery comprising a separator according to the invention.

[0020] [Fig.2] [Fig.2] is a schematic sectional view of an insulator according to the invention.

[0021] The separator 1 according to the invention, shown in a preferred embodiment in [Fig.2], is intended to be integrated between the cells 2 of a battery 3, as shown in [Fig.l],

[0022] In a battery 3, the separator 1 may be arranged between all adjacent cells 2, which provides maximum efficiency in preventing the spread of heat or fire in the battery. Alternatively, for example to obtain a smaller battery, the separator 1 may be arranged between groups of cells 2, and for example placed every two or three cells 2.

[0023] The shape and dimensions of the separator may vary depending on the applications.

[0024] The dimensions of the face of a cell 2 adjacent to the separator are for example 200 x 100 mm, or 300 x 100 mm. The separator is then preferably in the form of a rectangular parallelepiped of these dimensions.

[0025] The thickness of the separator can for example be between 2 and 4 mm, which is suitable for a large number of applications.

[0026] The present invention can be applied to any battery, particularly batteries requiring heat management and batteries subject to expansion of their cells.

[0027] The present invention applies in particular to batteries for electric or hybrid electric vehicles. An electric vehicle is a vehicle whose propulsion is provided exclusively by one or more electric motors, and a hybrid electric vehicle is a vehicle which comprises one or more electric motors capable of providing propulsion of the vehicle, and one or more other types of motors, generally thermal, capable of providing propulsion of the vehicle.

[0028] The present invention can also be applied to hydrogen vehicle batteries.

[0029] The present invention can finally be applied to domestic batteries, used for example to store electricity produced by solar panels.

[0030] The battery 3 is preferably a lithium-ion battery. This type of battery is currently commonly used for its performance in terms of autonomy and its low cost. However, it is also particularly prone to the risks of thermal runaway.

[0031] The vehicle can be of any category, including a car, a truck, a van, or even a motorcycle.

[0032] The separator according to the invention comprises at least one insulation layer. The insulation layer comprises a composite material 4, comprising a binder mixed with aerogel particles 5.

[0033] The binder makes it possible to maintain the aerogel particles 5 in an optimal distribution within the separator.

[0034] The separator 1 preferably complies with the UL 94 V0 standard, which relates to flame resistance.

[0035] The separator 1 preferably has a thermal resistance which allows it to withstand two minutes with one side at 800°C, and the other side kept below 150°C.

[0036] The separator 1 preferably has a thermal and flame resistance which allows it to withstand ten minutes with one side at 1400°C with the presence of flames, and the other side maintained below 300°C.

[0037] The aerogel particles 5 have a size between 0.015 and 3 mm.

[0038] The aerogel 5 may be a hydrophobic or hydrophilic aerogel.

[0039] The aerogel 5 is preferably a hydrophilic aerogel, in particular when the binder is an organic type binder. The hydrophilic aerogel has the advantage of not being combustible, and therefore gives the separator 1 the ability to slow down or even prevent the spread of fire between the cells 2 separated by the separator 1.

[0040] A hydrophobic aerogel 5 may be preferred when the binder is a mineral type binder.

[0041] For the purposes of the present invention, a "hydrophilic" aerogel is defined as an aerogel that is readily dispersible in water under ambient conditions such as 20°C and atmospheric pressure. Accordingly, the hydrophilic aerogel forms a stable dispersion with water, unlike hydrophobic aerogels which, when dispersed in water, separate into a layer of material on the surface of the water. The hydrophilic character is due to the presence of hydrophilic groups having -OH radicals on the surface of the aerogel, including the inner surface of its pores.

[0042] For example, the hydrophilic aerogel according to the invention forms a stable dispersion with water which is characterized in that after mixing 1, 2, 5 or 8 g of material, with 60 g of distilled water in a plastic pot for 1 min at 20°C using a SpeedMixer™ DAC 150.1FV mixer operating at 2750 rpm and allowing the resulting mixture to stand for 60 min, no visible phase separation or formation of a separate layer of material on the water surface occurs.

[0043] The aerogel 5 may be based on any material relevant for this application. It may be, for example, a silica aerogel, a silica-polymer hybrid aerogel, a carbon aerogel, or a mixture of some of these aerogels.

[0044] The aerogel 5 is preferably a silica aerogel, which is a very good thermal insulator, with a thermal conductivity for example of the order of 0.012 W / mK. The silica aerogel can for example be produced with raw materials of which more than 75% by weight are recycled materials from the demolition industry, which makes it possible to reduce the manufacturing cost and to reduce the energy consumption necessary for the manufacturing of the raw materials.

[0045] The binder may be of the organic type, that is to say it comprises an organic material whose content by weight represents at least 50% of the binder. It is for example an elastomer, preferably a silicone elastomer, which has the advantage of having good mechanical resistance, in particular in the context of a battery 3 placed on a vehicle, which is subject to vibrations during its use. The silicone elastomer therefore makes it possible to extend the service life of the separator 1.

[0046] Alternatively, the binder may be of the mineral type, that is to say it comprises a mineral material whose content by weight represents at least 50% of the binder. This is for example a calcium hydroxide, which has the advantage of having a reduced weight.

[0047] In certain embodiments, the insulation layer may comprise a reinforcing element, for example in the form of a honeycomb such as the product “Nomex Honeycomb” (registered trademark) marketed by the company DuPont, made of paper covered with phenolic resin. Different types of matrices may be used, in different materials that the person skilled in the art will be able to choose, for example a ceramic paper. Such a reinforcing element is particularly advantageous when the binder, for example of the mineral type, has a tendency to crumble.

[0048] The volume content of the aerogel particles 5 in the composite material 4 is greater than 20%, which makes it possible to provide the composite material 4 with good thermal insulation.

[0049] If an organic type binder is used, the volume content of the aerogel particles 5 in the composite material 4 is preferably between 20% and 80%, which corresponds to a weight content for example between 1 and 15%.

[0050] If a mineral type binder is used, the volume content of the aerogel particles 5 in the composite material 4 may be greater than 90%, which corresponds to a weight content for example of between 10 and 65%.

[0051] The binder and the aerogel particles 5 represent the largest part of the composite material 4, in particular the sum of the weight content of the aerogel particles 5 and the elastomer in the composite material 4 is greater than 80%, preferably greater than 90%. The composite material 4 may, in addition to the binder and the aerogel particles 5, comprise different additives, for example a surfactant making it possible to optimize the distribution of the aerogel particles 5 in the composite material 4.

[0052] The separator 1 may also comprise at least one volume reduction management layer 6. Indeed, the volume of the cells 2 of a battery 3, in particular their thickness, may vary during the charge / discharge cycle. Since the battery pack, which comprises the cells 2 and the separators 3, has a constant volume, the separators 3 must be able to cope with a reduction in the volume allocated to them. When the volume of the separator 1 is reduced, the volume reduction management layer 6 makes it possible to limit the pressure build-up inside the composite material 4, particularly at the level of the aerogel particles 5 which risk breaking under the effect of high pressure.

[0053] In certain particular embodiments, the separator 1 comprises two volume reduction management layers, each located on either side of the insulation layer.

[0054] In other embodiments, the separator 1 comprises a volume reduction management layer disposed between two insulation layers.

[0055] The volume reduction management layer 6 is capable of deforming in order to absorb a reduction in the volume of the separator 1. In order to obtain efficiency over the entire surface of the separator 1, the volume reduction management layer 6 is capable of deforming at any square zone with an area greater than or equal to 4 cm2 located on one of its external faces.

[0056] In a preferred embodiment of the invention, the volume reduction management layer 6 comprises at least one layer of ceramic fiber paper. This is for example EST C30 or EST C310, marketed by the company Morgan Advanced Materials. The ceramic fiber paper is for example arranged on one side of the insulation layer, for example by gluing or by lamination, or on both sides, that is to say between the insulation layer and the walls of the cells 2 arranged on either side of the separator 1. It is then this layer of ceramic fiber paper which absorbs the reduction in volume induced by the reduction in thickness of the separator 1, and the insulation layer undergoes no or almost no reduction in volume.

[0057] The volume reduction management layer 6 may also comprise at least one layer of an elastomer, which may be identical to the binder where appropriate, for example a silicone elastomer.

[0058] The volume reduction management layer 6 may also comprise a silicone foam, such as that marketed by the company Saint-Gobain under the name Norseal (registered trademark).

[0059] In another embodiment, the volume reduction management layer 6 has a three-dimensional shape. Such a layer may be made of an elastomer, if appropriate identical to that used for the binder. This shape then comprises compression zones; for example, the shape may be a honeycomb shape, and the edges formed by the hexagons constitute the compression zones. These compression zones make it possible to absorb the volume variations of the separator 1 by deforming a portion of the separator 1, the remainder of the separator 1 being able to retain a volume close to its initial volume. For example, the edges of the hexagons widen in a plane perpendicular to the force so that the thickness of the separator 1 can decrease without a significant increase in pressure. The volume reduction management layer 6 may be directly molded or cast in a shape having the compression zones.

[0060] The various embodiments of the volume reduction management layer 6, i.e., the ceramic fiber paper, the elastomer, the silicone foam, or the three-dimensional shape, can be used individually, or in combination.

[0061] The separator 1 according to the invention may comprise a protective envelope, for example made of PET, making it possible to protect in particular its insulation and volume reduction management layers. This protection is particularly useful when the binder is of the mineral type which tends to crumble.

[0062] The present invention also relates to a battery 3 for an electric or hybrid electric vehicle comprising at least two cells 2, and at least one insulating separator 1 arranged between said cells 2.

[0063] The present invention finally relates to a method of manufacturing a separator 1 comprising the following steps: - production of aerogel particles, - mixing the aerogel particles with said binder, for example a silicone elastomer or a calcium hydroxide. Depending on the type of binder, it may be in the liquid state, in particular when it is an organic binder, or dissolved in a solvent, in particular when it is a mineral binder, - curing the mixture obtained in a mold in order to obtain said composite material. A mechanical reinforcement element, for example a honeycomb-shaped structural matrix, can be added to the mold. The Hardening is achieved, for example, by increasing the temperature, by adding a catalyst, or by evaporating a solvent.

[0064] When manufacturing the aerogel particles, the above process may comprise the following sub-steps: - mixing a precursor with a synthesis solvent and a hydrolysis agent such as water, and where appropriate a catalyst, to obtain a gel, - granulation of the product obtained by cutting a jet of said gel, to obtain particles.

[0065] The gel passes, for example, through an opening whose size corresponds to the desired particle size. The jet forming at the outlet of the opening is then cut at a frequency also depending on the particle size.

[0066] Granulation by cutting a jet makes it possible to obtain aerogel particles having a relatively regular shape and comprising few angles. This means that once integrated into the separator 1, the risks of the aerogel particles 5 breaking or cracking, in particular under the effect of a force suffered resulting from the expansion of the cells 2 of the battery 3, are reduced.

[0067] When manufacturing the aerogel particles, the method for manufacturing a separator 1 may include the following sub-step: - drying of particles, carried out entirely at a pressure higher than the critical point of CO2.

[0068] Such drying makes it possible, on the one hand, to avoid deterioration of the particles during drying, and on the other hand to ensure that the aerogel particles remain hydrophilic. Indeed, other types of drying comprising, for example, a step of evaporation in ambient air, can result in a loss of the hydrophilic character of the aerogel particles.

[0069] Finally, during the manufacture of the aerogel particles, the method for manufacturing a separator 1 may be a continuous method, known to those skilled in the art and described in document FR1670366. This method comprises the following steps: - mixing a precursor with a synthesis solvent and a hydrolysis agent such as water, and where appropriate a catalyst, to obtain a gel, - granulation of the product obtained, to obtain particles, - keeping the particles in contact with the synthesis solvent and the hydrolysis agent, - washing the particles by adding a washing solvent to extract in particular the hydrolysis agent and, where appropriate, the catalyst, - drying of the particles to extract the synthesis and / or washing solvents by sending excess supercritical CO2,

[0070] the granulation, holding, washing and drying sub-steps are operated at a pressure above the critical point of CO2, and these conditions are maintained between these steps.

[0071] Thanks to these provisions, the aerogel manufacturing process can be carried out continuously, the pressure being able to be applied at a stage where the products are still fluids. Indeed, as soon as the products are solid (immediately after granulation), a pressure increase can no longer be carried out continuously. Thanks to the invention, the products do not require any pressure increase once they are solid, nor depressurization, apart from the final depressurization. The manufacturing time as well as the costs are significantly reduced, and the quality of the product is improved, due to a reduction in the risky stages of change of state and depressurization. In addition, this process makes it possible to implement a drying stage entirely operated at a pressure higher than the critical point of CO2, without increasing costs.

[0072] In this continuous process, the following characteristics can be implemented: - the mixing step can also be carried out at a pressure higher than the critical point of CO2, which allows a slight acceleration of this step, - during the drying step, the solvent-laden particles can be subjected to a jet of supercritical CO2 so as to put them in fluidized bed conditions, under temperature and pressure conditions such that the CO2 is supercritical, and the solvent-laden particles are heavier than the CO2-laden particles, this allowing the drying step to be carried out continuously, and to accelerate the drying step, - the synthesis and / or washing solvent may be an organic solvent and the drying step may be carried out under a pressure of between 100 and 200 bars and a temperature of between 35 and 50°C, ethanol being an inexpensive product suitable for the process, and the conditions between 100 and 200 bars and 35 and 50°C allowing that at certain CO2 injection speeds in the fluidized bed, the particles comprising ethanol do not fly away, while those containing only supercritical CO2 fly away from the top of the tower, and can be recovered for the rest of the process, - the aerogel manufacturing process may comprise, after the drying step, a step of replacing the supercritical CO2 with an inert gas, preferably nitrogen, then a decompression step, preferably in stages, this additional step making it possible to carry out rapid decompression, without damaging the aerogel particles, - during the step of replacing the supercritical CO2 with an inert gas, the particles charged with supercritical CO2 can be subjected to a jet of said inert gas, so as to place them in fluidized bed conditions, under temperature and pressure conditions such that the CO2 is supercritical, and that the particles charged with supercritical CO2 are heavier than the particles charged with the inert gas, this making it possible to carry out the step of replacing the supercritical CO2 with an inert gas continuously and to accelerate this step.

[0073] The continuous manufacturing process for aerogel particles can be implemented in a plant for manufacturing an aerogel in particles from a precursor, comprising: - a mixing reactor, - a granulation device, capable of forming particles from a jet of gelled liquid coming from the mixing reactor, where appropriate located inside the aging reactor, - an aging reactor, - a washing reactor, - a drying device, - a decompression device.

[0074] This installation is particular in that the aging reactor, the washing reactor and the drying reactor, as well as the means for transferring products between these reactors, are configured to operate and allow the maintenance of said products from one reactor to another at a pressure above the critical point of CO2.

[0075] Thanks to these provisions, the installation makes it possible to manufacture aerogel particles continuously, the pressure input being able to take place at a stage where the products are still fluids.

[0076] In this installation, the following characteristics can be implemented: - the mixing reactor can also be configured to operate and allow the maintenance of said products from one reactor to another at a pressure higher than the critical point of CO2; this in particular makes it possible to further reduce the reaction time, - the installation may further comprise a first fluidized bed tower configured to allow the replacement of the solvent contained in the particles with supercritical CO2, this allowing the particles to be dried continuously, and to accelerate the drying step, - the installation may also include a second fluidized bed tower configured to allow the replacement of the supercritical CO2 contained into the particles by a pressurized inert gas, preferably nitrogen, allowing rapid decompression without damaging the aerogel particles.

[0077] Although the above description is based on particular embodiments, it is in no way limiting of the scope of the invention, and modifications may be made, in particular by substitution of technical equivalents or by different combination of all or part of the characteristics developed above.

Claims

Claims

1. Separator (1) capable of separating two cells (2) of a battery (3), for example of an electric or hybrid electric vehicle, said separator (1) comprising at least one insulation layer comprising a composite material (4), said composite material (4) comprising a binder mixed with aerogel particles (5), the volume content of said aerogel particles (5) in said composite material (4) being greater than 20%, the binder and the aerogel particles (5) representing the majority of the composite material (4), characterized in that the binder is of mineral type, the weight content of said mineral matter in said binder is greater than 50%.

2. Separator (1) according to the preceding claim, wherein the sum of the weight contents of said aerogel particles (5) and said binder in said composite material (4) is greater than 80%.

3. Separator according to one of the preceding claims, comprising at least one volume reduction management layer (6), said volume reduction management layer (6) being, at any square zone with an area greater than or equal to 4 cm2 located on one of its external faces, capable of deforming in order to absorb at least part of a reduction in the volume of said separator.

4. A separator according to any preceding claim, wherein at least one of said at least one volume reduction management layer (6) comprises a ceramic fiber paper.

5. Separator according to one of the preceding claims, in which at least one of said at least one volume reduction management layer (6) has on at least one of its faces a three-dimensional shape, for example in the form of a honeycomb.

6. Separator according to one of the preceding claims, in which said insulation layer comprises at least one mechanical reinforcing element, for example in the form of a honeycomb.

7. A separator according to any preceding claim, wherein said mineral material is calcium hydroxide.

8. Separator according to one of the preceding claims, wherein the volume content of said aerogel particles (5) in said composite material (4) is greater than 90%.

9. Battery (3) for an electric or hybrid electric vehicle comprising at least two cells (2), and at least one separator (1) according to one of the preceding claims arranged between said cells (2).

10. Method for manufacturing a separator (1) according to one of claims 1 to 5, comprising the following steps: - manufacturing the aerogel particles (5), - mixing the aerogel particles (5) with said binder in the liquid state, - hardening the mixture obtained in a mold in order to obtain said composite material (4).

11. Manufacturing method according to the preceding claim, in which the manufacture of the aerogel particles (5) comprises the following sub-steps: - mixing a precursor with a synthesis solvent and a hydrolysis agent such as water, and where appropriate a catalyst, to obtain a gel, - granulation of the product obtained by cutting a jet of said gel, to obtain particles.

12. Manufacturing method according to one of claims 10 to 11, in which the manufacturing of the aerogel particles (5) comprises the following sub-step: - drying of the particles, carried out entirely at a pressure higher than the critical point of CO2.

13. Manufacturing method according to the preceding claim, wherein the manufacturing of the aerogel particles (5) comprises the following sub-steps: - mixing a precursor with a synthesis solvent and a hydrolysis agent such as water, and where appropriate a catalyst, to obtain a gel, - granulation of the product obtained, to obtain particles, - keeping the particles in contact with the synthesis solvent and the hydrolysis agent, - washing the particles by adding a washing solvent to extract in particular the hydrolysis agent and, where appropriate, the catalyst, - drying of the particles to extract the synthesis and / or washing solvents by sending excess supercritical CO2, the granulation, holding, washing and drying sub-steps are operated at a pressure above the critical point of CO2, and these conditions are maintained between these steps.

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