This application is a 35 U.S.C. § 371 filing of International Application No. PCT/JP2017/010662, filed Mar. 16, 2017 which application claims priority to Japanese Patent Application No. 2016-072964 filed Mar. 31, 2016, which is incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a porous sheet, and in particular, relates to a porous sheet suitable for a separator for an electrochemical device.

In the specification of the present application, an electrochemical device means an electrochemical device equipped with a positive electrode, a negative electrode, and a separator. As examples thereof, mention may be made of, for example, various types of secondary batteries such as a lithium ion secondary battery and a polymer lithium battery; various types of capacitors such as an aluminum electrolytic capacitor, an electrical double-layered capacitor and a lithium ion capacitor; and the like.

BACKGROUND ART

Recently, use of electricity as a source of energy has increased for addressing environmental problems related to CO2reduction, depletion of fossil fuel resources and the like. Therefore, for example, electric vehicles utilizing secondary batteries are being actively developed in the automobile industry. In addition, secondary batteries are featured also in view of efficient use of natural energy such as solar or wind energy.

In general, lithium ion secondary batteries are used as the secondary batteries for driving electric vehicles, at present, in view of the relationship between power and energy density. On the other hand, various companies have focused on the development of next-generation batteries in view of increased energy density, output, safety and the like. The next-generation batteries are in the fields with high future growth in the market.

In secondary batteries including lithium ion secondary batteries, primary batteries, capacitors (condensers) and the like, separators formed from paper, non-woven fabrics, porous films or the like are used. The performances required for the separators are, in general, short circuit protection between positive and negative electrodes, chemical stability with respect to electrolytic solutions, low inner resistivity and the like. The aforementioned requisite performances are universal ones required in separators regardless of types thereof, although they differ in degree in accordance with devices.

Separators of almost all lithium ion secondary batteries use porous membranes formed from a polymer organic compound such as polypropylene, polyethylene, or the like. The aforementioned porous membranes possess some characteristics suitable for lithium ion secondary batteries. For example, the following characteristics can be mentioned.

(1) Chemical stability with respect to electrolytic solutions is exhibited, and no fatal failures occur by separators.

(2) Thickness of a separator can be freely designed, and for this reason, separators responding to various demands can be provided.

(3) The diameter of pores can be designed to be reduced, and for this reason, superior lithium shielding properties are exhibited, and short circuit caused by lithium dendrite hardly occurs.

(4) When thermal runaway of lithium ion secondary batteries occurs, the initial thermal runaway can be controlled by melting polypropylene or polyethylene and thereby narrowing pores.

However, conventional research for lithium ion secondary batteries cannot identify an underlying cause of an occurrence of thermal runaway. Under present circumstances, various companies have studied and proposed a means for avoiding risks of thermal runaway with regard to various materials used in secondary batteries by empirical tools. Developments of materials suitable for vehicles having increased safety would be advanced in the future by clarifying the principle of thermal runaway and establishing a common evaluation method thereof, and if so, problems with respect to safety are expected to be overcome.

On the other hand, the second problem in secondary batteries for use in vehicles is cost. A separator is a material accounting for 20% of the battery cost, and further cost reduction is required under the present circumstances.

Next, in the field of rechargeable transportation units such as electric vehicles, and in the field of portable electronic terminals such as mobile phones, an electrical energy-storage device having an increased amount of storage electrical energy per unit volume is required in order to be operational for a long period of time even with a reduced volume. As an example of the aforementioned electrical energy-storage device, mention may be made of an electrical double-layered capacitor in which an electrolyte dissolved in an electrolytic solution is adsorbed by an electrode, and electrical energy is stored on the interface (electrical double layer) formed between the electrolyte and the electrode.

Main roles of separators in the electrical double-layered capacitors are short circuit protection of electrodes (separability), non-blocking movement of ions in the electrolytic solution (low inner resistivity), and the like. However, the aforementioned porous membranes have high density, and for this reason, the inner resistivity tends to increase. On the other hand, it is known that non-woven fabrics are used as a separator of a capacitor, but there are problems in that when a fiber diameter is reduced or a fiber density is increased in order to maintain separability, an inner resistivity increases. For this reason, development of a separator with a reduced inner resistivity is desirable. In addition, in order to increase the capacity of the electrical double-layered capacitor, it is necessary to increase the thickness of the electrode layer, and for this reason, it is required to reduce the thickness of the separator therefor.

There are two major processes for preparing polymer porous membranes of polypropylene, polyethylene or the like, namely a wet process and a dry process. The aforementioned preparation processes have respective characteristics. In the wet process, a plasticizer is added to a polymer such as polyethylene to form a film, subsequently, the film is biaxially drawn, the plasticizer is removed by cleaning with a solvent, and thereby, pores are provided. In this process, there are advantages in that pore size or film thickness can be superiorly adjusted, and response to various demands for all individual types of batteries can be carried out. On the other hand, there is a problem in that the preparation process is complicated, and for this reason, cost increases. In contrast, in the dry process, a polymer such as polyolefin is dissolved, the polymer is extruded on a film, the film with the polymer is subjected to annealing, the annealed film is drawn at a low temperature to form pores at the initial stage, and subsequently, drawing at a high temperature is carried out to form a porous product. In this process, there are advantages in that polymers having different melting points can be laminated, and the process is easy, and for this reason, the product can be produced at a reasonable cost. On the other hand, there is a problem in that sensitive adjustment of pores or thickness cannot be carried out.

A separator using non-woven fabrics formed from synthetic fibers, inorganic fibers or the like, other than the polymer porous films, has also been proposed. Conventional non-woven fabrics include dry types of non-woven fabrics and wet types of non-woven fabrics, and both of these have also been utilized as separators. It is believed that dry types of non-woven fabrics by which uniformity of fiber distribution cannot be obtained has a low effect of isolating electrodes, and for this reason, they cannot be used for lithium ion secondary batteries. On the other hand, wet types of non-woven fabrics have characteristics in that fiber distribution is uniform, as compared with dry types of non-woven fabrics. In addition, a higher porosity can be adjusted, as compared with porous films due to the characteristics of the preparation process, and for this reason, a sheet with reduced impedance can be produced. However, it is substantially difficult to use the dry types of non-woven fabrics in batteries using graphite negative electrodes which are widely applied to lithium ion secondary batteries at present. This is due to the characteristics of lithium ion secondary batteries which produce lithium dendrite at the negative electrode side. The aforementioned lithium dendrite has a property in which lithium dendrite is easily produced on the surface of a negative electrode which many lithium ions in a separator pass. For this reason, in non-woven fabrics in which roughness in a range with several dozen μm order is formed in the sheet itself, the parts at which lithium dendrite is easily formed are course. Therefore, shielding properties of controlling short circuit at the time of producing lithium dendrite may be reduced, as compared with a film type one. In addition, it is required to reduce the thickness of the separator as described above, but in particular, as the separator using the nonwoven fabric becomes thinner, the strength decreases. As a result, problems in safety such as a short circuit may occur, or there may be difficulties in the processability of the separator at the time of manufacturing the electric double-layered capacitor.

In order to overcome the aforementioned problems, specifying a pore size to a specified range is carried out, as described in Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. H11-040130). However, the pore size depends on a fiber diameter. For this reason, the fiber diameter needs to be reduced in order to control the pore size to a small size. However, in the case of using fine fibers, strength is generally deteriorated.

PRIOR ART DOCUMENTS

Patent Documents

[Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H11-040130

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

The present invention was made considering the aforementioned circumstances, and has an objective to provide a porous sheet exhibiting a superior strength (in particular, both tear strength and tensile strength), and exhibiting a superior performance as a separator for an electrochemical device.

Means for Solving the Problems

As a result of diligent studies in order to achieve the aforementioned objective, the present inventors discovered that a porous sheet containing a specific combination of a specific cellulose fiber, a specific cut fiber, and a specific binder has a superior strength (in particular, both tear strength and tensile strength), and exhibits a superior performance as a separator for an electrochemical device, thus completing the present invention.

The present invention relates to a porous sheet containing:

cellulose fibers having an average fiber diameter ranging from 20 to 500 nm,

cut fibers having an average fiber diameter ranging from 1.5 to 20 μm, and

a hydrophilic polymer binder,

wherein

an amount of the aforementioned cut fibers contained is 1% by weight or more and 80% by weight or less based on the total weight of the aforementioned cellulose fibers and the aforementioned cut fibers, and

an amount of the aforementioned hydrophilic polymer binder is 5 parts by weight or more and 30 parts by weight or less with respect to 100 parts by weight of the total weight of the aforementioned cellulose fibers and the aforementioned cut fibers. The term “average fiber diameter” used herein means an average fiber diameter obtained by magnification-observing a plurality of points on the surface of the porous sheet by means of an electron microscope, randomly selecting a predetermined number of fibers from each of the electron microscopic images, measuring the diameters of the selected fibers, and averaging the fiber diameters. The number of fibers to be screened is 100 or more, preferably 150 or more, more preferably 200 or more, and still more preferably 300 or more.

It is preferable that the aforementioned porous sheet has a specific tear strength of 15.0 mN·m2/g or more.

It is preferable that the aforementioned porous sheet has a tensile strength exceeding 3.0 N/15 mm.

It is preferable that the aforementioned porous sheet has the Gurley permeability resistance per 10 μm of thickness of the porous sheet, which is 10 seconds or more and 200 seconds or less.

It is preferable that the aforementioned cellulose fibers contain fibers having a diameter of 400 nm or less in an amount of 50% by weight or more based on the total weight of the cellulose fibers.

It is preferable that the aforementioned cellulose fibers contain fibers having a diameter of 1 μm or more in an amount ranging from 5 to 40% by weight based on the total weight of the cellulose fibers.

It is preferable that the aforementioned cut fibers have an average fiber length of 10 mm or less.

It is preferable that the aforementioned cut fibers are selected from the group consisting of polyester fibers, aramid fibers, PPS resin fibers, regenerated cellulose fibers, vinylon fibers, and combinations thereof.

It is preferable that the aforementioned porous sheet has a porosity ranging from 30 to 70%.

It is preferable that the aforementioned porous sheet has a thickness ranging from 3 to 50 μm.

The porous sheet of the present invention can be further provided with a layer containing an inorganic filler on the at least one surface of the porous sheet.

The present invention also relates to a laminate containing the aforementioned porous sheet and a porous substrate (which is different from the porous sheet mentioned above).

It is preferable that the porous substrate include a woven fabric or a nonwoven fabric. In addition, it is preferable that the porous substrate include cellulose fibers.

It is preferable that the aforementioned porous sheet or the aforementioned laminate has a volume resistivity of 1,500 Ω·cm or less, which is determined by alternate current with a frequency of 20 kHz in the state in which the porous sheet or the laminate is impregnated with a 1 mol/LiPF6/propylene carbonate solution.

The present invention also relates to a separator for an electrochemical device including the aforementioned porous sheet or the aforementioned laminate. Furthermore, the present invention also relates to an electrochemical device such as a battery, a capacitor, or the like, including the aforementioned separator for an electrochemical device.

Effects of the Invention

The porous sheet of the present invention is excellent in strength, and in particular, both tear strength and tensile strength, and in addition, the porous sheet is excellent in performance as a separator for an electrochemical device. In particular, the porous sheet of the present invention can exhibit both high tear strength and high tensile strength.

The porous sheet of the present invention contains fine cellulose fibers, and for this reason, it has superior blocking properties for lithium dendrite. In addition, the porous sheet of the present invention contains specific cut fibers, and for this reason, superior strength, and in particular, superior tear strength can be exhibited, even if the thickness is small. In addition, the porous sheet of the present invention contains a hydrophilic polymer binder, and for this reason, the uniformity of the fiber distribution can be further improved, and thereby, the blocking property of the lithium dendrite is further enhanced and the tensile strength can also be improved.

In addition, the laminate of the porous sheet and the porous substrate according to the present invention has an increased property of preventing short circuit as a whole, and exhibits superior performance as a separator for an electrochemical device, since for example, even in the case where the porous substrate is mainly composed of fibers having a relatively large diameter and a poor blocking property of lithium dendrite is exhibited by the porous substrate, per se, the porous sheet containing fine cellulose fibers and exhibiting superior blocking properties of lithium dendrite is laminated on the porous substrate.

Therefore, by using the porous sheet of the present invention or the laminate of the porous sheets and the porous substrate of the present invention, a separator for an electrochemical device having high lithium barrier properties which was difficult with conventional nonwoven fabrics, paper and the like, can be produced easily or at a low cost.

In addition, when the porous sheet mentioned above partially contains fibers with a relatively large diameter, the porous sheet and the laminate including the porous sheet and the porous substrate of the present invention can have further superior strength properties as a separator for an electrochemical device. In the laminate of the present invention, for example, even in the case where the physical properties of each of the porous substrate and the porous sheet tend to be deteriorated under a high temperature environment, the porous sheet and the porous substrate are laminated, and for this reason, thermal deterioration of the laminate as a whole can be reduced or avoided.

