JP2007128904A - Organic electrolyte battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic electrolyte battery which has a structure having a lighter battery weight, while maintaining not only a high efficiency discharging property but also excellent battery characteristics. <P>SOLUTION: The organic electrolyte battery is composed of an anode made of an anode mixture layer of a sheet shape mainly made of spherical carbon particles containing a polymer which absorbs and holds organic electrolyte solution on both sides of an anode collector and a cathode, facing the anode, which contains a polymer which absorbs and holds organic electrolyte solution with a polyelectrolyte layer of a porous film in between composed of a polymer which absorbs and holds organic electrolyte on both sides of the anode and forms a cathode mixture layer of a sheet shape mainly made of lithium cobalt complex oxide. All the above is integrated into a lamination of a sheet or a film shape and the organic electrolyte solution is absorbed and held by the above lamination. A thickness of the cathode mixture layer shall be 0.1mm or less, that of the anode mixture layer shall be 0.15 mm or less, and the surfaces of the anode collector and the cathode collector are coated with a mixture of a conductive carbon material and a binder. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a configuration of an organic electrolyte battery formed by laminating a positive electrode and a polymer electrolyte layer made of a polymer that absorbs and holds an organic electrolytic solution and a negative electrode.

  In response to the recent reduction in size, weight, and thickness of portable devices, lithium ion secondary batteries are also strongly required to be small, light, and thin. Lithium ion secondary batteries have been put to practical use and have greatly contributed to the reduction in size and weight of mobile phones and notebook computers, but the demand for smaller, lighter, and thinner devices is increasing. However, the strength of the outer case is maintained in the conventional lithium ion secondary battery in which the electrode and the separator are tightly bound by the strong outer case, and the distance between the electrode plates is minimized and the good contact between the electrode separators is realized. Therefore, there is a limit to reducing the thickness.

  As one of the methods for pursuing thinning, a lithium polymer secondary battery using a porous polymer that absorbs and holds an organic electrolytic solution in an electrolyte layer as a separator has attracted attention. Above all, in the battery system in which the same polymer is joined and integrated to the electrolyte layer and the electrode, the contact between the electrolyte layer and the electrode can be realized without using a strong outer case as described above. Effective for thinning. US Pat. No. 4,830,939 and US Pat. No. 5,478,668 are examples in which the electrolyte layer and the electrode are joined and integrated.

  In the case of U.S. Pat. No. 4,830,939, after applying a mixed solution of a monomer and an electrolytic solution on negative metal lithium or a positive electrode, the monomer is polymerized by ultraviolet rays or electron beams to form a solid polymer electrolyte. In this case, since the electrolyte layer is formed along the minute irregularities on the surface of the electrode plate, good contact is realized between the electrode and the electrolyte layer without applying any binding.

  In the case of US Pat. No. 5,478,668, a copolymer of vinylidene fluoride (VDF) and propylene hexafluoride (HFP) (P (VDF-HFP)) is used as the polymer material, and the positive electrode, negative electrode and separator of the laminated electrode are used. After producing a certain electrolyte layer, the separator and the positive electrode or the negative electrode are integrated by heat fusion.

By joining and integrating the electrode and the electrolyte layer as in the above two examples, it is possible to obtain a thin battery that can be discharged even when a flexible and thin laminate sheet outer package is used.
US Pat. No. 4,830,939 US Pat. No. 5,478,668

However, simply stacking the above-mentioned laminated electrode, which is a laminate of the positive electrode, electrolyte layer, and negative electrode, in order to obtain a practical battery capacity, the proportion of the current collector to the battery weight is larger than that of the lithium ion secondary battery. There is a problem of becoming. In a general lithium ion secondary battery, as shown in FIG. 5, both surfaces of a positive electrode 6 in which a positive electrode mixture layer 6b is formed on both surfaces of an aluminum foil 6a that is a positive electrode current collector and a copper foil 7a that is a negative electrode current collector. A negative electrode 7 on which a negative electrode mixture layer 7b is formed is opposed to a separator 8 with a separator 8 interposed therebetween, and this is wound. For this reason, in the lithium ion secondary battery, one set of battery configuration is formed on both surfaces of the positive electrode current collector 6a and the negative electrode current collector 7a, and the current collector is efficiently used. In contrast, in the lithium polymer secondary battery shown in the conventional example in which the electrolyte layer and the electrode are joined and integrated, it is very difficult to uniformly apply electron beams, ultraviolet rays, or heat over multiple layers. Therefore, this requirement of joining becomes an obstacle, and a repetitive configuration of positive electrode-separator-negative electrode-positive electrode cannot be adopted like a lithium ion secondary battery. Therefore, in the conventional lithium polymer secondary battery, as shown in FIG. 4, the positive electrode 1 having the positive electrode mixture layer 1b formed on one side of the positive electrode current collector 1a and the negative electrode mixture layer on one side of the negative electrode current collector 2a The negative electrode 2 on which 2b is formed faces each other with the electrolyte layer in between, and one battery configuration is constituted by a pair of positive electrode current collector and negative electrode current collector. For this reason, the current collector on the side where the electrode mixture is not applied is not used.

  In addition, although light aluminum or the like can be used as a material for the positive electrode current collector, aluminum cannot be used for the negative electrode current collector, and the current collector becomes heavy due to the use of copper or the like. The negative electrode current collector has a disadvantage that the weight efficiency is poor. Under such circumstances, it is necessary to obtain good battery characteristics including high rate discharge characteristics.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an organic electrolyte battery having a structure in which the weight of the battery is reduced while eliminating the above-mentioned problems of the conventional example and maintaining good battery characteristics such as high rate discharge characteristics.

  In order to achieve the above object, the present invention is characterized by the following electrode structure.

  The present invention absorbs and holds an organic electrolyte solution on both sides of a negative electrode in which a sheet-like negative electrode mixture layer mainly composed of spherical carbon particles is formed, including a polymer that absorbs and holds an organic electrolyte solution on both sides of the negative electrode current collector. A positive electrode on which a lithium-cobalt composite oxide-based sheet-like positive electrode mixture layer containing a polymer that absorbs and retains an organic electrolyte solution through a porous film-like polymer electrolyte layer made of a polymer is opposed to each other. Is laminated and integrated into a sheet or film, and an organic electrolyte is absorbed and held in the sheet or film, wherein the positive electrode mixture layer has a thickness of 0.1 mm or less and the negative electrode An organic electrolyte battery comprising a mixture layer having a thickness of 0.15 mm or less and a mixture of a conductive carbon material and a binder applied to the surfaces of the negative electrode current collector and the positive electrode current collector so That.

  According to the present invention, a structure suitable for obtaining good high-rate discharge characteristics can be achieved, so that the battery can be made thinner and lighter while maintaining good battery characteristics.

  One component of the invention described in claim 1 is that a negative electrode in which a negative electrode mixture layer is formed on both sides of a negative electrode current collector, and a positive electrode on both sides of the negative electrode through a polymer electrolyte layer on both sides of the negative electrode Is an organic electrolyte battery characterized by being laminated. By forming a negative electrode mixture layer on both surfaces of a negative electrode current collector that is incapable of using a lightweight metal such as aluminum and using one negative electrode current collector in two negative electrode mixture layers A lightweight organic electrolyte battery can be obtained. In addition, the positive electrode is dispersed on both sides of the negative electrode so as to face each other, so that the distance of ion movement during charging / discharging can be made delicate, the electrode reaction can proceed smoothly, and a large current can be taken out. Moreover, since the reaction area can be doubled by using two opposing surfaces of the positive electrode and the negative electrode for one negative electrode current collector, the load per unit area can be reduced, and a high rate Charging / discharging can be performed.

Furthermore, one constituent feature of the invention described in claim 1 of the present invention is that the negative electrode current collector includes a polymer that absorbs and holds an organic electrolyte solution on both sides, and a negative electrode layer formed mainly of spherical carbon particles is formed. And a lithium-cobalt composite oxide-based sheet containing a polymer that absorbs and holds the organic electrolyte solution through a porous film-like polymer electrolyte layer made of a polymer that absorbs and holds the organic electrolyte solution on both sides of the negative electrode. This is an organic electrolyte battery in which the positive electrode formed with the positive electrode mixture layer is opposed to each other, and the whole is laminated and integrated into a sheet or film, and the organic electrolyte is absorbed and retained in this. In particular, the electrode and the electrolyte layer are laminated and integrated As a result, sufficient electric conductivity can be obtained without binding the battery, and good battery characteristics can be obtained. The other constituent elements of the invention described in claim 1 of the present invention will be described in detail in the embodiments.

  According to a second aspect of the present invention, a required battery capacity is obtained by laminating at least two or more laminated electrodes.

  The positive electrode according to claim 3 of the present invention is an organic electrolyte battery in which a positive electrode mixture layer is formed on one surface of a porous aluminum current collector. Or it is the organic electrolyte battery which formed the positive mix layer on both surfaces of the porous aluminum electrical power collector, and unevenly distributed the positive mix in thickness. Aluminum is lightweight as a material for the positive electrode current collector, and is preferable as a material for reducing the weight of the battery. In particular, when taking a configuration in which laminated electrodes are laminated in multiple layers, it is preferable to use a porous plate such as a perforated plate.

  Moreover, the surface area of the reaction part of the said positive electrode and the said negative electrode is made the same, and an extra electrode area is not required.

  Moreover, the short circuit in an electrode edge part can be prevented by making the surface dimension of the reaction part of the said negative electrode larger than the surface dimension of the reaction part of the said positive electrode.

  A preferred embodiment of the present invention is a negative electrode in which a spherical carbon particle-based sheet or film-shaped negative electrode mixture layer is formed containing a polymer that absorbs and holds an organic electrolyte solution on both sides of a current collector made of a porous copper foil, Lithium containing a polymer that absorbs and holds organic electrolyte on a current collector made of porous aluminum foil via a porous film-like polymer electrolyte layer made of polymer that absorbs and holds organic electrolyte on both sides of the negative electrode This is a battery in which a cobalt composite oxide-based sheet or a positive electrode in which a film-like positive electrode mixture layer is formed is opposed to each other, and the whole is laminated and integrated into a sheet or film, and the organic electrolyte is absorbed and held in these. The polymer that absorbs and holds the organic electrolyte is selected from the group of polyvinylidene fluoride or a copolymer of vinylidene fluoride and propylene hexafluoride. It is an organic electrolyte battery as a main component or more. As the same polymer material for the positive electrode, the polymer electrolyte layer, and the negative electrode, polyvinylidene fluoride or a copolymer of vinylidene fluoride and propylene hexafluoride as a main component makes the positive electrode by a polymer that absorbs and holds an organic electrolyte solution, The polymer electrolyte layer and the negative electrode can be joined and integrated by heat fusion.

  In addition, one or more selected from the group of polyethylene oxide and polymethacrylic acid ester is added to the polymer that absorbs and retains the organic electrolyte, and by adding these polymers, organics into the polymer are added. Increase the electrolyte absorption retention.

(Embodiment 1)
A part of the configuration of the organic electrolyte battery of the present invention will be described with reference to FIG.

FIG. 1 shows a laminated electrode 4 in which a positive electrode plate 1 and a negative electrode plate 2 are laminated via a polymer electrolyte layer 3. The positive electrode plate 1 is prepared by applying and drying a positive electrode mixture layer 1b made of lithium cobaltate, a conductive material, and a polymer that absorbs and holds an organic electrolyte as a positive electrode active material on one surface of an aluminum core plate 1a that is a positive electrode current collector. To do. The negative electrode plate 2 is prepared by applying and drying a negative electrode mixture layer 2b made of spherical carbon particles, a conductive material, and a polymer that absorbs and holds an organic electrolyte on both surfaces of a copper core plate 2a that is a negative electrode current collector. The polymer electrolyte layer 3 is a porous film made of a copolymer (P (VDF-HFP)) of vinylidene fluoride and propylene hexafluoride, which is a polymer that absorbs and holds an organic electrolyte. Then, the negative electrode mixture layer 2b on both sides of the negative electrode plate 2 is opposed to the positive electrode mixture layer 1b of the positive electrode plate 1 through the polymer electrolyte layer 3, respectively, laminated, and then laminated and integrated by heat fusion to form the laminated electrode 4
Is configured.

  After charging the laminated electrode into the laminate sheet outer package, 1 mol / l of lithium hexafluorophosphate was dissolved in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 3 from the opening of the outer package. Inject electrolyte. After the injection, the inside of the exterior body is decompressed and the laminated electrode 4 is sufficiently impregnated with the electrolytic solution, and then returned to atmospheric pressure, and the opening of the exterior body is sealed with a heat seal. The sealed battery is heated at 45 ° C. for 20 minutes, and the polymer is impregnated with the organic electrolytic solution to obtain the organic electrolyte battery of the present invention. In addition, the laminate sheet exterior body is obtained by arranging an aluminum film for blocking airflow between insulating resin films, and laminating and integrating the whole.

(Embodiment 2)
A part of the configuration of the organic electrolyte battery of another configuration of the present invention will be described with reference to FIG. The positive electrode 1 is formed by laminating the positive electrode 1 and the negative electrode 2 with the polymer electrolyte layer 3 in the same configuration as in the first embodiment except that the positive electrode mixture layer 1b is formed on both surfaces of the positive electrode current collector 1a. Thus, the laminated electrode 5 is configured.

(Embodiment 3)
A part of the configuration of the organic electrolyte battery in which the multilayer electrode 4 of the present invention is laminated in multiple layers will be described with reference to FIG.

  FIG. 3 shows a stacked electrode portion of a multi-layer battery in which five stacked electrodes 4 of Embodiment 1 are stacked.

  In addition, the method of forming each mixture layer on the positive and negative electrode current collectors is a method in which a mixture sheet is prepared in advance, and the current collector and the mixture sheet are joined by heat fusion. There is a method of applying a mixture paste directly to the body. In the case of forming the mixture layer on one side of the current collector, a method of preparing a mixture sheet in advance and bonding the current collector and the mixture sheet by heat fusion is preferable.

As the positive electrode active material, a lithium-containing composite metal oxide capable of reversing lithium ions by charging / discharging such as lithium cobaltate, lithium nickelate, or lithium manganate can be used.
As the negative electrode active material, a carbon material capable of reversibly taking in and out lithium ions by charging / discharging is preferable, and in particular, granular graphite particles obtained by carbonizing and graphitizing carbonaceous mesofused particles are preferable. A material capable of reversing lithium ions by charging and discharging such as nitride can be used.

As the electrolyte, a mixture of a cyclic carbonate such as ethylene carbonate or propylene carbonate and a chain carbonate, a mixture of a cyclic carbonate of ethylene carbonate and propylene carbonate, or the like can be used as a solvent, and LiPF ₆ , LiCF ₃ SO ₃ , LiClO ₄ , LiBF ₄ , LiAsF _6, LiN (CF ₃ SO ₂ ), or the like can be used. Examples of the chain carbonate include ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate.

  Embodiments of the present invention will be described in detail with reference to the drawings.

Example 1
The organic electrolyte battery of the present invention was produced by the method described below.

The polymer electrolyte sheet was produced by the following method. 30 g of a copolymer of vinylidene fluoride and propylene hexafluoride (P (VDF-HFP), propylene hexafluoride ratio 12 wt%) is dissolved in 150 g of acetone, and the pore-forming agent di-n-butyl phthalate (DBP) ) Prepare a mixed solution with 30 g added. After this solution is applied onto a glass plate, acetone is removed by drying to produce a polymer electrolyte sheet having a thickness of 0.02 mm and a size of 35 mm × 65 mm.

  The sheet of the positive electrode mixture layer was prepared by applying a paste prepared by mixing a solution prepared by dissolving 70 g of P (VDF-HFP) in 1000 g of acetone, 1000 g of lithium cobaltate, 50 g of acetylene black, and 100 g of DBP on a glass plate and drying it. By rolling, a sheet having a thickness of 0.1 mm and a size of 30 mm × 60 mm is obtained.

  The sheet of the negative electrode mixture layer is a solution of 40 g of P (VDF-HFP) dissolved in 300 g of acetone and 250 g of spherical graphite particles (made by Osaka Gas) obtained by carbonizing and graphitizing carbonaceous mesophase spheres, and vapor-grown carbon fibers are made of graphite. (VGCF) (Osaka Gas) 20g, DBP 60g mixed paste prepared on a glass plate, dried and then rolled, rolled to a sheet with a thickness of 0.15mm and a size of 30mm x 60mm Get.

  Each of the positive electrode and negative electrode current collectors was coated with a mixture of a conductive carbon material and a binder on the surface. The mixture of the conductive carbon material and the binder to be applied to the current collector is prepared by dispersing and mixing 30 g of acetylene black and an N-methylpyrrolidone solution (12% by weight) of polyvinylidene fluoride. After this mixture is applied to a perforated plate of aluminum and copper each having a thickness of 0.06 mm, a mixture comprising a conductive carbon material and polyvinylidene fluoride is obtained by drying and removing N-methylpyrrolidone at a temperature of 80 ° C. or higher. A current collector bound with is manufactured.

  A sheet of the positive electrode mixture layer laminated on one side of the aluminum current collector is sandwiched between polytetrafluoroethylene sheets (PTFE, thickness 0.05 mm) and heated and heated through two rollers heated to 150 ° C. The positive electrode plate is manufactured by heat-sealing by pressing. PTFE is used to prevent the mixture layer from adhering to the roller, and other materials such as copper foil and aluminum foil may be used.

  In the same manner, a negative electrode plate is prepared by laminating sheets of the negative electrode mixture layer on both surfaces of the copper current collector.

  Finally, a laminated electrode in which the mixture surface of the positive electrode plate is opposed to and laminated on both sides of the negative electrode plate via a polymer electrolyte sheet, and heat fusion is integrated by heating and pressing with two rollers heated to 120 ° C. Is made.

  The laminated electrode integrated as described above is immersed in diethyl ether to extract and remove DBP, thereby providing porosity in the polymer portion, and drying at 50 ° C. in vacuum. After drying, the laminated electrode was placed in a bag-shaped outer package made of a laminate sheet in which an aluminum film was placed between insulating resin films, and ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 3. An electrolyte solution (3.5 g) in which lithium hexafluorophosphate is dissolved at a rate of 1.5 mol / l is poured into the product. After the injection, the electrolyte was absorbed and held in the pores of the polymer under a reduced pressure of 0.5 atm, then returned to atmospheric pressure and sealed with a heat seal, and further heated at 45 ° C. for 20 minutes to enter the polymer. The battery A was in a state capable of being charged and discharged by absorbing the electrolytic solution.

  The weight of the obtained battery A was 3.3 g, and the weight ratio of the current collector was 14.6%. When the obtained battery was charged with a charging current of 22 mA and then discharged with a discharging current of 220 mA, the discharge capacity was 92 mAh.

(Comparative Example 1)
The sheet thickness of the positive electrode mixture layer and the negative electrode mixture layer is set to 0.2 mm and 0.3 mm, which is twice the thickness of Example 1, and the electrode mixture layer sheet is provided only on one surface of the current collector for both the positive electrode and the negative electrode. A polymer electrolyte sheet, a positive electrode plate and a negative electrode plate are produced in the same manner as in Example 1 except that the heat fusion is performed.

  Finally, the mixture layer of the negative electrode plate was opposed to and laminated with the mixture layer of the positive electrode plate through the polymer electrolyte sheet, and heat fusion was integrated by heating and pressing with two rollers heated to 120 ° C. A laminated electrode is produced.

  The laminated electrode was made into a battery B in a chargeable / dischargeable state in the same manner as in Example 1.

  The weight of the obtained battery B was 3.1 g, and the weight ratio of the current collector was 12.6%. When this battery was charged with a charging current of 22 mA and then discharged with a discharging current of 220 mA, the discharge capacity was 35 mA.

(Comparative Example 2)
A polymer electrolyte sheet, a positive electrode mixture layer sheet, a negative electrode mixture layer sheet, and a current collector are prepared in the same manner as in Example 1.

  However, the positive electrode plate is heat-sealed by laminating a positive electrode mixture layer sheet on both surfaces of the aluminum current collector and heating and pressing through two rollers heated to 150 ° C. as in Example 1.

  Further, the negative electrode plate is heat-sealed by laminating a negative electrode mixture layer sheet on one side of a copper current collector and heating and pressurizing it through two rollers heated to 150 ° C. as in Example 1.

  Finally, contrary to the structure of Example 1, the mixture layer of the negative electrode plate is opposed and laminated on both surfaces of the positive electrode plate via the polymer electrolyte sheet, and heated and pressurized with two rollers heated to 120 ° C. Thus, a laminated electrode integrated by heat fusion is produced.

  The above laminated electrode was made into a battery C in a state where charge and discharge were possible in the same manner as in Example 1.

  The weight of the obtained battery C was 3.538 g, and the weight ratio of the current collector was 19.5%. Moreover, when the obtained battery was charged with a charging current of 22 mA and then discharged with a discharging current of 220 mA, the discharge capacity was 90 mAh.

  Example 1 and Comparative Example 1 and Comparative Example 2 are fabricated so that the theoretical values of battery capacity are the same. Comparing the weight ratio of the current collector to the battery weight in each battery, Example 1 is 14.6%, Comparative Example 1 is 12.6%, and Comparative Example 2 is 19.7%. In Example 1, the battery weight is slightly heavier because the number of the positive electrode current collector is one more than that in Comparative Example 1, but the positive electrode thickness is 0.1 mm, which is half that of Comparative Example 1, and the high rate discharge characteristic is Comparative Example 1. It is very good compared to Next, when Example 1 and Comparative Example 2 are compared, since the thickness of the active material layer from the current collector is the same, the high rate discharge characteristics are very good. However, in Comparative Example 2, since the number of negative electrode current collectors is one more than that in Example 1, the weight ratio of the current collectors in the entire battery is high. From the above, it can be seen that according to the configuration of Example 1, the ratio of the current collector weight to the battery weight can be made as small as possible, and the structure is suitable for obtaining good high rate discharge characteristics.

  Furthermore, when the positive electrode mixture layer is prepared on both sides of the current collector, the thickness of the positive electrode mixture layer is halved and the high-rate discharge characteristics are further improved if the same theoretical battery capacity is prepared. .

As described above, according to the present invention, a battery having a single negative electrode current collector can also serve as a current collector of two mixture layers while realizing good battery characteristics such as high rate discharge characteristics. The ratio of the current collector weight in the weight can be kept low, the weight efficiency is good, and an organic electrolyte battery having excellent high rate discharge characteristics can be obtained.

Sectional drawing of the laminated electrode part of the organic electrolyte battery which shows one Example of this invention Sectional drawing of the laminated electrode part of the organic electrolyte battery which shows the other Example of this invention Sectional drawing of the laminated electrode part of the organic electrolyte battery which laminated | stacked five laminated electrodes which show one Example of this invention Sectional view of the layered portion of an organic electrolyte battery having a conventional configuration Sectional view of the constituent electrodes of a conventional cylindrical lithium ion battery

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode plate 1a Positive electrode collector 1b Positive electrode mixture layer 2 Negative electrode plate 2a Negative electrode collector 2b Negative electrode mixture layer 3 Polymer electrolyte layer 4 Laminated electrode 5 Laminated electrode 6 Positive electrode plate 6a Positive electrode collector 6b Positive electrode mixture layer 7 Negative electrode plate 7a Negative electrode current collector 7b Negative electrode mixture layer 8 Separator

Claims (3)

Absorbing the organic electrolyte on at least one side of the positive electrode current collector and the negative electrode in which the negative electrode current collector contains a polymer that absorbs and holds the organic electrolyte solution and includes two negative electrode mixture layers mainly composed of spherical carbon particles A porous film-like polymer electrolyte comprising a polymer in which a mixture layer absorbs and holds an organic electrolyte solution, and a positive electrode in which a lithium-cobalt composite oxide-based sheet-like positive electrode mixture layer is formed. An organic electrolyte battery having laminated electrodes facing each other through a layer, wherein the positive electrode mixture layer has a thickness of 0.1 mm or less and the negative electrode mixture layer has a thickness of 0.15 mm or less An organic electrolyte battery, wherein a mixture of a conductive carbon material and a binder is applied to the surfaces of the negative electrode current collector and the positive electrode current collector. The organic electrolyte battery according to claim 1, wherein at least two of the laminated electrodes are laminated. The organic electrolyte battery according to claim 1, wherein the positive electrode current collector and the negative electrode current collector are made porous.