JP6016512B2 - Lithium secondary battery separator and lithium secondary battery - Google Patents

Description

  The present invention relates to a separator for a lithium secondary battery and a lithium secondary battery.

Conventionally, as a separator for a lithium secondary battery, a resin-made microporous film such as polypropylene or polyethylene has been used. Recently, non-woven fabrics and papers mainly composed of cellulose fibers have been used as separators in lithium ion batteries comprising specific electrodes. Batteries using paper containing regenerated cellulose fibers beaten as separators (for example, see Patent Document 1), separators made of cellulose fibers with a maximum fiber thickness of 1000 nm or less (for example, see Patent Document 2), specific Separator made of a nonwoven fabric having a thickness of 20 μm or less, including cellulose fibers having an average fiber diameter in a range and polyolefin having a specific average fiber diameter (see, for example, Patent Document 3), synthetic fibers and modified filtered water in a specific range A separator containing solvent-spun cellulose fibers having a degree (for example, see Patent Document 4) is disclosed. The separator made of nonwoven fabric particles are filled (e.g., see Patent Document 5), a separator comprising the particles composed mainly of a compound represented by AlOOH or _{Al 2 O 3 · H 2 O} ( for example, patent Document 6) is disclosed.

  Since the separator of Patent Document 1 does not consider the relationship between the beating degree of the regenerated cellulose fiber, the average fiber length, and the transferability to the felt, when the regenerated cellulose fiber is 100% regenerated, the beating progresses too much. When the average fiber length is short, the transferability to the felt is poor and papermaking may not be stable. In addition, when regenerated cellulose fibers that have been beaten and other cellulose pulps are mixed, problems such as poor and uneven formation, low charge / discharge efficiency, and large variations in battery characteristics There was a problem that pinholes were easily generated. The separators of Patent Documents 2 and 3 are composed only of fine cellulose fibers. However, in a normal drying method, the cellulose fibers are bound to each other to form a film, and the gaps between the fibers are blocked. For this reason, there has been a restriction that the moisture of the wet paper must be replaced with an organic solvent and dried at a low temperature. Therefore, the process of replacing the moisture in the wet paper with the organic solvent takes time, and the concentration of the organic solvent in the vat decreases with time due to solvent replacement, and the replacement efficiency decreases with time. It was difficult to process the paper continuously, and there was a problem that it was necessary to work in a batch type, making it difficult to manufacture by winding.

  In the separator of Patent Document 4, when the average fiber length of the cellulose fibers is too short due to beating, the fibers are not sufficiently entangled, so that the fibers are taken into the papermaking net, and the wet paper felt is fed to the felt. In some cases, transfer defects occurred, and show-through and pinholes occurred. Conversely, when beating is sweet and the average fiber length is long, a large number of thick fibers are included, resulting in a tight formation and coarse and dense spots, resulting in low charge / discharge efficiency problems and variations in battery characteristics. There was a problem of getting bigger.

  The separators of Patent Documents 5 and 6 are produced by a method in which a nonwoven fabric is substantially impregnated or coated with a solution containing particles. A binder for fixing the particles to the nonwoven fabric or for fixing the particles to each other is added to the solution. When the amount of the binder added is large, the binder covers the particles and closes the gaps between the particles. There was a problem. On the other hand, when the amount of the binder added is small, there is a problem that the particles easily fall off. In the method of painting particles on a nonwoven fabric, it has been difficult to produce a thin separator without causing pinholes. Once the particles are applied to the nonwoven fabric, the method of peeling the coating layer from the nonwoven fabric has a problem that the peeled coating layer is brittle, lacks flexibility, and is easily broken.

  The lithium secondary battery may be overcharged by some accident. When the battery is overcharged, acicular metal lithium called lithium dendrite is deposited on the negative electrode surface, which may cause a short circuit between the electrodes or battery characteristics. Therefore, there is a demand for a lithium secondary battery in which battery characteristics are unlikely to deteriorate even if overcharged.

JP-A-8-306352 JP 2006-49797 A JP 2012-36518 A International Publication No. WO2012 / 8559 Pamphlet JP 2010-538173 A JP 2008-4439 A

  An object of the present invention is a lithium secondary battery separator having excellent papermaking stability, high uniformity with a thin film, high charge / discharge efficiency, and high discharge capacity retention even when overcharged, and using the same To provide a lithium secondary battery.

  As a result of diligent research to solve the above problems, by using solvent-spun cellulose fibers having a specific range of modified freeness and an average fiber length of a specific range, excellent papermaking stability, thin and uniform It has been found that a separator for a lithium secondary battery having a high charge and discharge efficiency can be realized. Further, it has been found that by including an inorganic filler, a lithium secondary battery separator having a high discharge capacity retention rate can be realized even when overcharged.

  The separator for a lithium ion secondary battery of the present invention is a paper made of solvent-spun cellulose fibers having a modified freeness of 75 to 250 ml and a length weighted average fiber length of 0.80 to 1.80 mm. Therefore, fibers are not easily taken on the papermaking net, the wet paper is easily transferred to the felt, excellent in papermaking stability, thin and highly uniform, and high charge / discharge efficiency. Moreover, when it contains an inorganic filler, even if it is a case where a lithium secondary battery is overcharged, a high discharge capacity maintenance factor is obtained.

  In the present invention, the expression “separator” means a lithium secondary battery separator. In the present invention, when “predetermined solvent-spun cellulose fiber” is described, a solvent-spun cellulose having a modified freeness of 75 to 250 ml and a length-weighted average fiber length of 0.80 to 1.80 mm. Means fiber.

The lithium secondary battery in the present invention refers to a lithium ion battery, a lithium ion polymer battery, or the like. Examples of the negative electrode active material of the lithium secondary battery include carbon materials such as graphite and coke, metallic lithium, aluminum, silica, tin, nickel, and an alloy of lithium and lithium, SiO, SnO, Fe Metal oxides such as ₂ O ₃ , WO ₂ , Nb ₂ O ₅ , Li _4/3 Ti _5/3 O ₄ , and nitrides such as Li _0.4 CoN are used. As the positive electrode active material, lithium cobaltate, lithium manganate, lithium nickelate, lithium titanate, lithium nickel manganese oxide, or lithium iron phosphate is used. Further, the lithium iron phosphate may be a composite with one or more metals selected from manganese, chromium, cobalt, copper, nickel, vanadium, molybdenum, titanium, zinc, aluminum, gallium, magnesium, boron, and niobium.

  The electrolyte of the lithium secondary battery includes propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, dimethoxymethane, acetonitrile, γ-butyrolactone, dimethylformamide, tetrahydrofuran, sulfolane, and mixed solvents thereof. A solution in which a lithium salt is dissolved is used. Examples of the lithium salt include lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium perchlorate. As solid electrolyte, what melt | dissolved lithium salt in gel-like polymers, such as polyethyleneglycol, its derivative (s), polymethacrylic acid derivative, polysiloxane, its derivative (s), polyvinylidene fluoride, is used.

  The solvent-spun cellulose fiber used in the present invention means a cellulose fiber obtained by spinning by dissolving directly in an organic solvent without passing through a cellulose derivative. In JIS L0204-2, the term “Lyocell” is used as the name of this fiber. In the present invention, solvent-spun cellulose fibers having various modified freeness and length-weighted average fiber length were prepared, and as a result of investigating the relationship between transferability and denseness, the modified freeness was 75 to 250 ml, and The present inventors have found that solvent-spun cellulose fibers having a length-weighted average fiber length of 0.80 to 1.80 mm are excellent in transferability and denseness, are less likely to be picked up by a papermaking net, and can be easily transferred to felt. If the modified freeness is less than 75 ml or the length-weighted average fiber length is less than 0.80 mm, fibers are taken into the papermaking net and cannot be transferred to the felt, and there is a problem of see-through and pinholes. Occur. If the modified freeness exceeds 250 ml or the length-weighted average fiber length exceeds 1.80 mm, it is difficult to reduce the thickness of the paper, causing problems of formation of uneven spots and pinholes.

  The modified freeness of the solvent-spun cellulose fiber is more preferably 90 to 220 ml, further preferably 90 to 175 ml. The length weighted average fiber length is more preferably 0.90 to 1.60 mm, and further preferably 0.90 to 1.30 mm. The fiber length distribution of a given solvent-spun cellulose fiber may have one peak or two or more peaks.

The modified freeness in the present invention is that a wire mesh (made by PULP AND PAPER RESEARCH INSTITUTE OF CANADA) having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as a sieve plate, and the sample concentration is 0.1%. The freeness measured in accordance with JIS P8121. Here, “%” of the sample concentration is “mass fraction (%)”. The reason why the modified freeness is adopted in the present invention is that the Canadian standard freeness specified in JIS P8121 is around several tens to 0 ml and the difference in the freeness cannot be discriminated any more. This is because the fiber length is shortened by beating, and the pulp sample slips through the holes in the sieve plate used in the Canadian standard freeness measurement, and the precise freeness is measured. This is because it is possible to clarify the beating degree of the sample that cannot be performed.

  The length-weighted average fiber length can be measured by using a commercially available fiber length measuring instrument that is obtained by using polarization characteristics obtained by applying laser light to the fiber. In the present invention, JAPAN TAPPI paper pulp test method no. It was measured using Kajaani Fiber Lab V3.5 (manufactured by Metso Automation) according to 52 “Fiber length test method for paper and pulp (automatic optical measurement method)”. “Length-weighted average fiber length” of solvent-spun cellulose fiber means “length-weighted average fiber length” measured and calculated according to the above.

  The predetermined solvent-spun cellulose fiber in the present invention is fixed to a refiner, a beater, a beat refiner, a mill, an attritor, a rotary blade homogenizer that applies a shearing force by a high-speed rotary blade, and a cylindrical inner blade that rotates at high speed. Double-cylindrical high-speed homogenizer that generates shearing force with the outer blade, ultrasonic crusher that is refined by impact by ultrasonic waves, high-pressure homogenizer, etc. It is produced by adjusting conditions such as processing speed.

  In the separator of the present invention, the paper preferably contains an inorganic filler in addition to the predetermined solvent-spun cellulose fiber. Examples of inorganic fillers include calcium carbonate, sodium carbonate, alumina, gibbsite, boehmite, magnesium oxide, magnesium hydroxide, silica, titanium oxide, barium titanate, zirconium oxide, and other inorganic oxides, inorganic hydroxides, aluminum nitride, and nitride. Examples thereof include inorganic nitrides such as silicon, aluminum compounds, zeolites, and mica. The shape of the inorganic filler may be any of a spherical shape, a substantially spherical shape, a plate shape, a rectangular parallelepiped shape, a star shape, a scale shape, and an indefinite shape. The average primary particle diameter of the inorganic filler is preferably 0.01 to 3.00 μm, more preferably 0.1 to 1.00 μm. If it is less than 0.01 μm, it may be difficult to carry. If it exceeds 3.00 μm, it may be difficult to reduce the thickness of the separator or a pinhole may occur.

  In the separator of the present invention, when the paper contains an inorganic filler, it is possible to prevent the cellulose fibers from being in close contact with each other and block the gap in the depth direction and the planar direction of the separator, thereby improving the permeability of the electrolytic solution. Since the ion movement becomes smooth, the charge / discharge efficiency and the discharge capacity retention rate become higher. In addition, the mechanism of whether there is an effect of suppressing the generation of lithium dendrite or an effect of suppressing the decomposition of the electrolytic solution is not well understood, but the discharge capacity maintenance rate is excellent even when overcharged.

  0-30 mass% is preferable and, as for the content rate of the inorganic filler in the separator of this invention, 5-20 mass% is more preferable. If it exceeds 30 mass%, pinholes may be generated in the separator or the strength may be insufficient.

The paper relating to the separator of the present invention is produced by a papermaking method. Specifically, the fiber is dispersed in water to form a uniform slurry, and this slurry is rolled up by a paper machine. You may add chemical | medical agents, such as a dispersing aid, an antifoamer, a thickener, a flocculant, a paper strength enhancer, and a release agent, to a slurry as needed. Examples of the paper machine include a paper machine that independently uses a paper net such as a circular net, a long net, an inclined type, and an inclined short net, and a composite paper machine that combines a plurality of these paper nets. The basis weight of the paper is preferably ^{12.0~23.0g / m 2, 13.0~20.0g / m} 2 is more preferable. If the basis weight is less than 12.0 g / m ² , pinholes may occur. If it exceeds 23.0 g / m ² , it may be difficult to reduce the thickness of the separator.

  The thickness of the paper of the present invention may be adjusted by calendering as necessary after paper making. The thickness is preferably 18 to 35 μm, and more preferably 20 to 30 μm. If the thickness is less than 18 μm, pinholes may be generated, or the density may be too high, and the electrical resistance of the separator may be increased. If the thickness exceeds 35 μm, the lithium secondary battery may not be sufficiently reduced in size and thickness. The calendar process may be performed at room temperature or by heating.

  The separator of the present invention preferably has a porosity of 50 to 75%, more preferably 56 to 70%. When the porosity is less than 50%, the electrolyte solution retention becomes insufficient and the internal resistance may increase. If it exceeds 75%, the strength of the separator may be insufficient.

  The separator of the present invention preferably has an air permeability of 1.0 to 20.0 s / 100 ml, and more preferably 2.0 to 15.0 s / 100 ml. If it is less than 1.0 s / 100 ml, pinholes may occur. If it exceeds 20.0 s / 100 ml, the electrolyte permeability may be insufficient.

  The separator of the present invention preferably has a tensile strength of 4.0 N / 15 mm or more, and preferably 5.0 N / 15 mm or more. If it is less than 4.0 N / 15 mm, the separator may be cut during winding.

  The separator of the present invention preferably has a maximum pore size defined by ASTM-F316-86 of 0.50 to 5.00 μm, and more preferably 0.50 to 3.00 μm. If it is less than 0.50 μm, the electrolyte permeability may deteriorate. If it is larger than 5.00 μm, pinholes may be formed in the separator. The average pore diameter of the separator of the present invention is preferably 0.20 to 0.90 μm, and more preferably 0.30 to 0.75 μm. When the average pore diameter is less than 0.20 μm, electrolyte permeability may be deteriorated. If it exceeds 0.90 μm, pinholes may be formed in the separator.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a present Example. Examples 1 to 8 are reference examples.

  Table 1 shows the modified freeness, length weighted average fiber length, and Canadian standard freeness of solvent-spun cellulose fibers used in Examples and Comparative Examples. The Canadian standard freeness was measured in accordance with JIS P8121. The weight-weighted average fiber length and Canadian standard freeness of Manila hemp pulp and esparto pulp used in the comparative examples are shown. F1 to F9 were produced using solvent-spun cellulose fibers (fineness: 1.7 dtex, fiber length: 4 mm, manufactured by Coatles Co., Ltd.), beating using a double disc refiner, and changing the beating time. Manila hemp pulp and esparto pulp were also beaten using a double disc refiner.

  Table 2 shows the papermaking slurries used in Examples and Comparative Examples. The “raw material” symbol in Table 2 corresponds to the “symbol” in Table 1. B1 of “raw material” in Table 2 means boehmite (average primary particle size 0.5 μm). In Table 2, “raw material” M1 means magnesium oxide (average primary particle size 0.3 μm). The average primary particle diameters of boehmite and magnesium oxide used in Examples and Comparative Examples mean the particle diameter when the mass ratio is 50%, that is, D50, as measured with a laser diffraction particle size distribution analyzer.

Examples 1-5, 7, 8
Slurries 1 to 5, 6, and 7 were prepared using a pulper, fed to a long paper machine, paper-made, and dried with a 150 ° C. Yankee dryer. After paper making, the thickness was adjusted by calendar treatment, and separators of Examples 1 to 5, 7, and 8 were produced.

Example 6
2% by mass of a paper strength enhancer (trade name: Polystron (registered trademark) 1250, manufactured by Arakawa Chemical Co., Ltd.) is added to the slurry 5 with respect to the solvent-spun cellulose fiber, and the mixture is stirred for a predetermined time, and is fed to a long paper machine. The paper was made and dried with a Yankee dryer at 150 ° C. After papermaking, calendar treatment was performed to adjust the thickness, and the separator of Example 6 was produced.

Examples 9-12
While a predetermined solvent-spun cellulose fiber is disaggregated with a pulper, a predetermined amount of boehmite dispersed in advance with a mixer is added, and a flocculant (trade name: Sanfloc (registered trademark) C-009P, manufactured by Sanyo Kasei) is added. Add 0.3 mass% of the solvent-spun cellulose fiber and boehmite, stir for a predetermined time, and then add a paper strength enhancer (trade name: Polystron (registered trademark) 1250, manufactured by Arakawa Chemical) to solvent-spun cellulose. 2 mass% was added with respect to the fiber, and it stirred for the predetermined time, and prepared the slurry 8-11. Slurries 8 to 11 were fed to a long paper machine to make paper, and dried with a Yankee dryer at 150 ° C. After paper making, the thickness was adjusted by calendering to produce separators of Examples 9-12. When the separators of Examples 9 to 12 were rubbed with fingers, the inorganic filler did not fall off.

Example 13
While a predetermined solvent-spun cellulose fiber is disaggregated with a pulper, a predetermined amount of magnesium oxide previously dispersed with a mixer is added, and a flocculant (manufactured by Sanyo Chemical Co., Ltd., trade name: Sanfloc (registered trademark) C-009P) Is added in an amount of 0.3% by mass based on the total mass of the solvent-spun cellulose fiber and magnesium oxide, and the mixture is stirred for a predetermined time. Further, a paper strength enhancer (trade name: Polystron (registered trademark) 1250, manufactured by Arakawa Chemical Co., Ltd.) is used as the solvent. A slurry 12 was prepared by adding 2% by mass to the spun cellulose fiber and stirring for a predetermined time. Slurry 12 was fed to a long paper machine to make paper, and dried with a Yankee dryer at 150 ° C. After papermaking, calendar treatment was performed to adjust the thickness, and the separator of Example 13 was produced. When the separator was rubbed with a finger, the inorganic filler did not fall off.

Comparative Examples 1-4
Slurries 13 to 16 were prepared using a pulper, sent to a long paper machine to make paper, and dried with a 150 ° C. Yankee dryer. After paper making, the thickness was adjusted by calendering to produce separators of Comparative Examples 1 to 4.

Comparative Example 5
While disaggregating the Manila hemp pulp with a pulper, a predetermined amount of boehmite dispersed in advance with a mixer is added, and a flocculant (manufactured by Sanyo Chemical Co., Ltd., trade name: Sanfloc (registered trademark) C-009P) is added to the Manila hemp pulp and boehmite. The slurry 17 was prepared by adding 0.3% by mass with respect to the total mass and stirring for a predetermined time. The slurry 17 was fed to a long net paper machine to make paper, and dried with a Yankee dryer at 150 ° C. After paper making, a calendar process was performed to adjust the thickness, and a separator of Comparative Example 5 was produced. When the separator was rubbed with a finger, the inorganic filler did not fall off.

[Evaluation]
The separators of Examples and Comparative Examples were evaluated as follows, and the results are shown in Table 3. “-” In Table 3 means not applicable.

<Transferability>
When the separator is made, there is almost no fiber taken on the paper web, and when the wet paper can be stably transferred to the felt `` ◎ '', when there is little fiber taken on the paper web and the paper can be transferred stably, or In some cases, the fibers are slightly picked up on the paper net, but “○” indicates that the fibers are stably transferred, and the fibers are continuously picked up on the entire surface or most of the paper net. The case where the paper could not be transferred to the felt was indicated as “x”.

<Thickness>
The thickness was measured according to JIS P8118, and the average value was calculated.

<Density>
The basis weight of the separator was measured in accordance with JIS P8124, and the value obtained by dividing the basis weight by the thickness and multiplying by 100 was defined as the density.

<Pinhole>
Light was applied from the back side of the separator, and it was visually determined whether or not the separator had a pinhole. When there is a pinhole, it is “Yes”, and when there is no pinhole, it is “No”.

<Porosity>
The porosity of the separator was calculated by dividing the value obtained by subtracting the density of the separator from the specific gravity of the separator by the specific gravity of the separator and multiplying by 100. About the specific gravity of the separator of Examples 1-8 and Comparative Examples 1-4, it was set as 1.5 specific gravity of a cellulose fiber. About the specific gravity of the separator of Examples 9-12 and the comparative example 5, the specific gravity of boehmite was set to 3.04, and it computed according to the compounding ratio of a cellulose fiber and boehmite. The specific gravity of the separator of Example 13 was calculated according to the blending ratio of cellulose fiber and magnesium oxide, with the specific gravity of magnesium oxide being 3.65.

<Air permeability>
Using a Gurley air permeability meter having a circular hole with an outer diameter of 28.6 mm, 10 Gurley air permeability values were measured in the width direction for each separator, and the average value was shown. The Gurley air permeability was measured according to JIS P8117.

<Tensile strength>
The separator was trimmed to a width of 15 mm and a length of 200 mm. The 200 mm length was the paper flow direction. The upper and lower sides of the test piece were fixed to a chuck of a tabletop material testing machine (Orientec) at 100 mm intervals, and the maximum load when the test piece was pulled up at a constant speed of 100 mm / min until it was cut. Five or more test pieces were measured for each separator, and the average value was shown.

<Average pore diameter>
The pore size distribution of the separator was determined using a pore size measuring device (manufactured by PMI, device name: palm porometer (CFP-1500-A)), and the obtained average pore size was shown.

<Uniformity>
Observe light from the back side of the separator and visually observe that the uniformity of the formation is very good compared to `` ◎ '' and ◎, but there is no pinhole, `` ○ '', pinhole is generated “X” indicates that the texture and roughness of the texture are moderate.

[Production of lithium-ion batteries]
A positive electrode sheet, a separator, and a negative electrode sheet were laminated in this order, accommodated in an aluminum laminate bag, the electrolyte was injected, the bag was sealed, and a lithium ion battery was produced. For the positive electrode, use is made of a slurry in which an active material lithium cobaltate, a conductive auxiliary agent acetylene black, and a binder polyvinylidene fluoride mixed in a mass ratio of 90: 5: 5 are applied to both sides of an aluminum current collector. It was. As the negative electrode, a slurry in which natural graphite and polyvinylidene fluoride were mixed at a mass ratio of 95: 5 was applied to both surfaces of a copper foil current collector. As the electrolytic solution, a mixed solution in which 1 mol / liter of LiPF ₆ was dissolved was used. The mixed solution is a mixture of ethylene carbonate and diethyl carbonate in a mass ratio of 3: 7.

<Charge / discharge efficiency>
The lithium ion battery was charged at a constant current of up to 4.2 V at 25 ° C. and 0.5 mA / cm ² , further charged at a constant voltage of 4.2 V, and charged until the current value was attenuated to 0.2 mA or less. The capacity obtained at this time was defined as the initial charge capacity. After completion of charging, the battery was paused for 20 minutes, and discharged to 3.0 V at 0.5 mA / cm ² . The capacity obtained at this time was used as the initial discharge capacity, and the ratio with respect to the initial charge capacity was calculated as the charge / discharge efficiency. Higher charge / discharge efficiency is preferable.

<Discharge capacity maintenance rate>
The lithium ion battery was charged at a constant current until it reached 4.3 V at 25 ° C. and 0.5 mA / cm ² , and further overcharged by applying 4.3 V for 24 hours. Subsequently, it rested for 30 minutes and discharged to 3.0 V at a constant current of 0.5 mA / cm ² . The ratio of the discharge capacity obtained at this time to the initial discharge capacity was defined as the discharge capacity maintenance ratio. The higher the discharge capacity retention rate, the better.

  The separators of Examples 1 to 8 are made of solvent-spun cellulose fibers, have a modified freeness of 75 to 250 ml, and a length-weighted average fiber length of 0.80 to 1.80 mm. Transferability was good and papermaking stability was excellent. Further, a thin and highly uniform separator without pinholes was obtained.

  Since the separators of Examples 9 to 13 contain solvent-spun cellulose fibers having a modified freeness of 75 to 250 ml and a length-weighted average fiber length of 0.80 to 1.80 mm and an inorganic filler, cellulose Since the adhesion between the fibers was suppressed and the gaps between the cellulose fibers were retained, the ion migration of the electrolyte was smooth, and the charge / discharge efficiency was higher and superior to the separators of Examples 1-8. Furthermore, even when overcharged, the discharge capacity maintenance rate was high and excellent.

  The separator of Comparative Example 1 was composed of solvent-spun cellulose fibers and had good transferability, but was thick because the modified freeness was more than 250 ml and the length-weighted average fiber length was more than 1.80 mm. A lot of fibers are included, the entanglement between cellulose fibers is insufficient, it becomes a stiff and uneven texture, the thickness cannot be reduced, there are pinholes, strength, charge and discharge efficiency, The discharge capacity retention ratio was inferior to those of Examples 1 to 8 (paper made of a predetermined solvent-spun cellulose fiber).

  The separator of Comparative Example 2 is composed of solvent-spun cellulose fibers, but has a modified freeness of less than 75 ml and a length-weighted average fiber length of less than 0.80 mm, so that the fibers are removed from the papermaking network. In many cases, the transfer of the wet paper to the felt became unstable, and the paper could not be stably produced.

  Since the separator of Comparative Example 3 is composed of a predetermined solvent-spun cellulose fiber and hemp pulp, the transfer property of the wet paper is good and the paper making stability is excellent, and the strength is stronger than those of Examples 1 to 13, but hemp pulp A part of the film formed a film and the like was uneven and had pinholes, and the charge / discharge efficiency and the discharge capacity retention rate were inferior to those of Examples 1-13.

  Since the separator of Comparative Example 4 is composed of a predetermined solvent-spun cellulose fiber and esparto pulp, the wet paper has good transferability and excellent papermaking stability, but the esparto pulp is stiff and there is little entanglement between fibers. Therefore, the formation is non-uniform, there are pinholes, the charge / discharge efficiency and the discharge capacity retention rate are inferior to those of Example 5 consisting only of solvent-spun cellulose fibers, and the strength when converted to the same basis weight is also inferior. It was.

  Since the separator of Comparative Example 5 does not contain solvent-spun cellulose fibers and is composed of hemp pulp and inorganic filler, the transfer property of the wet paper is good and the papermaking stability is excellent, and the strength is strong, but the formation is The charge and discharge efficiency and the discharge capacity maintenance rate were inferior to Examples 9 to 13 which were non-uniform and had pinholes and contained an inorganic filler.

  The separator for lithium secondary batteries of the present invention can be used for lithium secondary batteries such as lithium ion batteries and lithium ion polymer batteries.

Claims (2)

A paper comprising a solvent-spun cellulose fiber having a modified freeness of 75 to 250 ml defined below and a length-weighted average fiber length of 0.80 to 1.80 mm and an inorganic filler. The separator for lithium secondary batteries is characterized in that the content of the inorganic filler in the separator is 5 to 30% by mass and contains only the solvent-spun cellulose fiber as a fiber component .
Modified freeness: Freeness measured in accordance with JIS P8121, except that an 80-mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm was used as the sieve plate, and the sample concentration was 0.1%. A lithium secondary battery comprising the lithium secondary battery separator according to claim 1.