Flexible supercapacitor diaphragm paper, preparation method and application

Abstract

The invention relates to the technical field of electrolytic capacitor diaphragm paper, in particular to flexible supercapacitor diaphragm paper, a preparation method and application. According to the diaphragm paper, sisal fibers, tencel fibers and polyester staple fibers are adopted to synergistically construct a ternary fiber system, and the diaphragm paper is formed through a wet papermaking process and is subjected to alternate hot pressing and cold pressing treatment, so that the paper has low density, high strength, low internal resistance, small aperture and excellent bending breakdown resistance at the same time. The diaphragm paper is suitable for a winding or lamination structure of a supercapacitor, and the power density, the cycle life and the structural reliability of the supercapacitor are effectively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrolytic capacitor separator paper, in particular to a flexible supercapacitor separator paper, a preparation method and application. BACKGROUND

[0002] With the rapid development of electronic technology, capacitors are rapidly developing towards large capacity, long life and high reliability. The corresponding capacitor separator paper also puts forward higher requirements, and the development of new high-grade capacitors often depends on the research and development of new capacitor separator paper. Supercapacitor is a new type of energy storage electronic component product that has emerged in recent years with the breakthrough of material science. It fills the gap between ordinary capacitors and batteries in high energy and high power, and has the characteristics of high capacity and high starting current. In short, it has the performance of battery capacity and capacitor fast charging and discharging, and also has the characteristics of safety and reliability, wide application range, is a breakthrough component to improve and solve the power performance application, and will not cause environmental impact like traditional batteries or accumulators. Supercapacitors have great application value and market potential in energy storage of automobiles (especially electric vehicles, hybrid vehicles and special heavy engineering vehicles), wind power generation, port energy storage, power transmission and transformation, power grid communication, national defense industry (especially missile launch and system), and consumer electronics. It is widely concerned by countries around the world. Due to the super large capacity characteristics of supercapacitors, many developed countries use them in military equipment, such as Europe, which first uses the capacitor in heavy engineering vehicles, armored vehicles and tanks to ensure the rapid and safe starting of the vehicles in low temperature conditions.

[0003] The separator paper is between the positive and negative electrodes of the supercapacitor, on the one hand, it stores and transmits electrolyte, and on the other hand, it prevents electrode short circuit; its pore size distribution, ion resistance (ESR), mechanical strength and bending flexibility directly determine the energy density, power density and safety reliability of the device. The industry generally requires the separator paper to have: 1. Low density / low loss: reduce electrolyte consumption and equivalent series resistance; 2. Small and uniform micropores: suppress leakage current and improve charge efficiency; 3. High purity and electrolyte corrosion resistance: avoid metal foil corrosion and device failure; 4. High strength + foldable flexibility: adapt to lamination (soft package) or winding processing, and do not produce cracks due to folding.

[0004] The earliest aluminum electrolytic and supercapacitor separator paper is made of natural long fiber such as sisal and pineapple. The Chinese patent CN101696558B disclosed by the applicant discloses a "pineapple / Lyocell" binary system paper with low tightness and small pore size, but when the content of hemp fiber is higher than 30wt%, the paper is prone to fiber fracture during 180° folding process, resulting in local leakage and life attenuation.

[0005] Lyocell fiber is derived from renewable cellulose, with controllable diameter of 2-10 µm, high purity, and no lignin, which can significantly reduce internal resistance and loss; it has achieved mainstream occupancy in aluminum electrolysis separators. However, the transverse tensile strength of single Tencel paper is insufficient; during the low-temperature winding or laminated bending process of supercapacitors, the paper is prone to stress concentration and crack propagation, making it difficult to meet the stringent requirement of >90% bending breakdown retention rate. Therefore, the applicant's Chinese patent CN109056403A improves the strength by "high / low beating degree Tencel layering", but still lacks bending flexibility and anti-rebound ability.

[0006] To reduce ESR and improve mechanical properties, some studies introduce chemical short fibers into Tencel pulp. The applicant's Chinese patent CN108221487B uses Tencel 10-95wt% + ultra-fine chemical fiber (polyester, polyolefin, etc.) 5-90wt%, which significantly reduces internal resistance, but does not contain natural long hemp fiber skeleton, and the paper strength, thermal shrinkage and bending reliability are still limited. Further, the applicant's Chinese invention patent CN114263069B combines 30-60wt% hemp pulp with 10-50wt% polyolefin short fiber and 20-50wt% Tencel, which can achieve low loss at low operating voltage; however, polyolefin fiber has low elastic modulus and poor heat resistance, and is prone to shrinkage and warping during high-temperature drying, and the polyolefin-hemp interface has poor bonding, making it difficult to further reduce internal resistance. SUMMARY

[0007] To solve the above technical problems, the purpose of the present application is to provide a flexible supercapacitor separator paper, which has low density, good strength, small and uniform pore size, and certain flexibility suitable for use in supercapacitors. Low density makes it have small loss value, which does not increase the internal resistance of the supercapacitor, good strength makes it suitable for winding processing of supercapacitors, small pore size makes it have small working leakage current, large energy storage and long service life. Good softness makes it not easy to be broken during folding and bending in the capacitor process, maintains the integrity of the separator paper structure, prevents local leakage defects, and ensures high energy storage effect.

[0008] A flexible supercapacitor separator paper is prepared by a wet laying process, and the fiber raw materials used are composed of the following fibers in mass percentage: Sisal fiber 5-25%; Tencel fiber 40-65%; Polyester short fiber 20-40%; Among them, the beating degree of sisal pulp is 40-65°SR, and the beating degree of Tencel pulp is 55-80°SR; the thickness of the paper sheet is 20-55 µm, and the apparent density is 0.32-0.58 gcm -3The paper sheet has an electric breakdown voltage retention rate of ≥95% after being folded at 180° for 5 times.

[0009] The above-mentioned components, the sisal hemp is the smallest diameter of papermaking raw material in nature, the thickest part is 4 μm, most of the diameter is about 2 μm, as a kind of hemp pulp, the length is relatively long, generally about 3 mm, the fiber strength of such paper is very high, and because the fiber is fine, the pore size of the paper is small, which meets the strict requirements of super capacitor on leakage current. The Tencel is a new type of synthetic fiber, its production process does not discharge lye, and has no pollution to the environment. The biggest advantage of Tencel in papermaking is that it does not contain impurities, the diameter and length of the fiber can be controlled, and the fiber can be separated by grinding and splitting, and the cross section of the separated fine fiber is circular, which is quite different from the flat type of plant fiber. It is very suitable to use such fiber raw material to make low-density paper products, and because the separated fiber is very fine, the smallest is 1~2 μm, the pore size of the paper sheet is small. The last kind of polyester fiber is a polyester fiber extracted from petrochemical industry, which does not contain impurities, the fiber diameter and length can be selected for use, and the minimum diameter can reach 1.5 μm. The polyester fiber has stable chemical properties, good acid and alkali resistance, good temperature resistance and softness, and the fiber is not easy to break, so the paper product can meet the flexibility requirements of super capacitor process for isolation material.

[0010] As preferred, the fiber raw material used is composed of the following fibers by mass percentage: sisal hemp fiber 10~20%; Tencel fiber 45~60%; polyester short fiber 25~35%.

[0011] As preferred, the linear density of the polyester short fiber is 0.7~1.7 dtex, and the length is 2~6 mm.

[0012] As preferred, the average diameter of the sisal hemp fiber is ≤4 μm, and the average length is ≥3 mm.

[0013] As preferred, the pore size D of the diaphragm paper is 0.4~1.5 μm, and the pore size variation coefficient is ≤15%. 50

[0014] As preferred, the Gurley air permeability of the diaphragm paper is 10~70 s / 100 mL, and the water content is ≤6%.

[0015] As preferred, the paper surface roughness (Ra) is ≤1.2 μm, and the roughness is obtained by alternating calendering process of 85~105 ℃, 2~4 MPa hot pressing and 20~40 ℃, 1~1.5 MPa cold pressing.

[0016] As preferred, the paper sheet adopts double-layer structure, the surface layer contains polyester fiber with mass fraction ≥50%, and the middle layer contains sisal hemp fiber with mass fraction ≥15%.​

[0017] Further, the application also provides a method for preparing the supercapacitor separator paper, which comprises the following steps: a) grinding the sisal pulp and the Tencel pulp to a target beating degree, and pre-dispersing the polyester staple fiber in water; b) uniformly mixing the pulp in a headbox according to a ratio, and the solid content is 0.4-1.0 wt%; c) forming through a long net paper machine and vacuum dewatering, when the solid content of the wet paper reaches 35-45%, alternately performing hot pressing at 85-105 DEG C and cold pressing at 20-40 DEG C for calendering; d) drying in a tunnel drying oven at 160-180 DEG C until the water content is less than or equal to 6%, and completing the dimensional stability after standing at room temperature for more than 12 hours; e) controlling the paper roughness according to the hot pressing line pressure of 2-4 MPa and the cold pressing line pressure of 1-1.5 MPa in step c).

[0018] Further, the application also provides a supercapacitor, which comprises an anode current collector, a cathode current collector, an electrolyte, and the separator paper, and the separator paper is clamped between the bipolar electrodes in a folded or coiled form to form an electrochemical double layer.

[0019] The application has the following comprehensive advantages in structure and performance by introducing the fine sisal fiber, the Tencel fiber and the polyester staple fiber into the same separator paper, and cooperating with the double-temperature calendering forming process. 1. The internal resistance is significantly reduced: the Tencel fiber constructs a high-purity and continuous liquid channel, so that the resistance of the separator to ion migration is greatly reduced, and the power output and the rapid charging and discharging capacity of the supercapacitor can be effectively improved.

[0020] 2. The bending reliability is greatly enhanced: the toughness of the polyester staple fiber and the long fiber skeleton of the sisal fiber form a flexible-strong complementary fiber network, so that the separator can still maintain electrical insulation and mechanical integrity after being folded or coiled for many times, and the electric leakage failure caused by folding marks is avoided.

[0021] 3. The pore size is uniform and the leakage current is lower: the reasonable proportion of the fine fiber brings a uniform and fine pore structure, which can ensure the full infiltration of the electrolyte, effectively suppress the leakage current, and improve the energy storage efficiency and the self-discharge performance.

[0022] 4. The low density and high strength are considered: the fiber combination and the directional calendering process reduce the weight of the paper while maintaining high mechanical strength, which leaves space for the lightweight design of the device, and meets the process tension requirements of the high-speed winding or layering production line.

[0023] 5. Excellent size and thermal-humidity stability: the fiber ternary system avoids the defects of traditional polyolefin separators that are prone to shrinkage and warping in high-temperature or high-humidity environments, ensuring reliable size stability of the device in harsh working conditions.

[0024] In summary, the separator paper of the present application can achieve an excellent overall balance between low internal resistance, flexibility, mechanical strength, size stability, and chemical purity, providing a reliable core isolation material for soft-pack or wound supercapacitors with high energy density, high power density, and long service life. DETAILED DESCRIPTION

[0025] The technical solutions in the embodiments will be described below in a clear and complete manner. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the protection scope of the present application.

[0026] I. Formula List Five examples (Ex1-Ex5) and ten comparative examples (C1-C10) are given below, and the formulas are shown in Table 1.

[0027] Table 1 Fiber Formulas of Examples and Comparative Examples

[0028] II. Unified Preparation Process and Process Parameters Equipment conditions: Long net multi-cylinder paper machine (net width 1 m, speed 30 m / min -1 ); vacuum box 60 kPa; dryer group 6 (adjustable 160-180°C). Hot calendering pressure roller: surface temperature 90±5°C, linear pressure 3 MPa; cold calendering pressure roller: surface temperature 25°C, linear pressure 1.2 MPa.

[0029] Paper target basis weight: 18 g / m -2 (thickness 35±3 µm, which can be adjusted slightly according to the formula).

[0030] Step S1: raw material pulping

[0031] Note: The polyolefin short fibers in Comparative Example C6 are dispersed at the same concentration; C7 adds alumina slurry (average particle size 0.3 µm, mass fraction 10 wt%) at the Tencel pulp stage.

[0032] Step S2: dynamic homogenization in the headbox Target slurry solid content 0.6wt%; 0.15wt% anionic polyacrylamide (a-PAM) added online as retention aid; pH adjusted to 7.0±0.2; slurry circulation time: ≥5min, ensuring uniform distribution of polyester and natural fibers.

[0033] Step S3: Forming and dewatering Fourdrinier wire tension 6kNm -1 ; two-stage vacuum dewatering in the forming section: 30kPa→60kPa; wet paper solid content controlled at 38±3% entering the press section.

[0034] Step S4: Alternate calendering Hot calendering: face temperature 90°C, linear pressure 3MPa, residence 0.08s; Cold calendering: face temperature 25°C, linear pressure 1.2MPa, residence 0.08s; Repetition 1→2 for two passes, obtaining a surface with Ra≤1.2µm.

[0035] Step S5: Drying and setting Drying curve: stepwise temperature increase from first cylinder 160°C→last cylinder 180°C; moisture content at cylinder exit ≤5wt%; winding tension 0.6kNm -1 ; post-winding room temperature standing ≥12h to eliminate internal stress.

[0036] Step S6: Slitting and inspection Target web width 600mm, core outer diameter 600mm; spot check aperture, density, tensile and bending break retention.

[0037] All the above parameters can be directly set by conventional equipment in the art; different examples and comparative examples only adjust the proportioning of the slurry and individual raw materials, the rest of the process remains the same, ensuring that the performance difference is directly attributed to the formula design.

[0038] III. Test procedure First, according to the formula window of the present application, examples Ex1-Ex5 were prepared respectively, and ten groups of comparative examples C1-C10 were prepared for comparison. All paper samples were completed on the same 1m wide Fourdrinier papermaking production line to exclude the interference of equipment differences.

[0039] The produced rolls were tested using the following national or industry standards: 1. Thickness and apparent density Standard: GB / T451.3-2002 "Determination of thickness, stiffness and unit weight of paper and paperboard" Humidity: 23℃±1℃, 50%RH±2%, equilibrium for at least 24h; Procedure: use a thickness gauge with a contact pressure of 0.5N±0.05N (contact area 200mm 2) Measure thickness t (mm). Weigh the same test piece mass g (g), area A (m 2 ). Apparent density p = g / (A x t).

[0040] 2. Transverse tensile strength Standard: GB / T 12914-2008 "Determination of tensile strength of paper and board (constant rate of elongation method)" Humidity: same as above; Procedure: Cut 15 mm x 180 mm test strip, mark 100 mm. Test speed 100 mm / min -1 . Record breaking load F (N), tensile index = F / test piece width; transverse tensile strength = tensile index x basis weight (kN / m -1 ).

[0041] 3. Pore size D 50 Medium: 99.99% nitrogen Procedure: Soak the diaphragm completely in isopropyl alcohol for 10 min. Load into the test cell, increase the pressure and record the first continuous bubble pressure P o (bubble point); continue to increase the pressure and record the pressure P1 when the air flow reaches 10 mL / min -1 . Convert D 50 according to the standard formula. D 50 is the median diameter of the pore size distribution, corresponding to the pore size when the cumulative distribution with pore size as the horizontal axis and volume as the weight reaches 50%.

[0042] 4. Gurley air permeability Standard: GB / T 458-2008 "Determination of air permeance of paper and paperboard (moderate range)" Procedure: 100 mL Gurley oil cylinder full scale; apply a differential pressure of 1.22 kPa (oil cylinder self weight); record the time required for air to penetrate the sample, take the average of five points, unit s / 100 mL.

[0043] 5. Bending puncture retention rate Bending: GB / T 457-2008 "Determination of folding endurance of paper and paperboard (MIT method)" - use 180° folding direction clamp, bending radius 1 mm, reciprocate 5 times. Puncture voltage: GB / T 12656-1990 "Determination method of power frequency puncture voltage of capacitor paper"; calculation: puncture retention rate = (puncture voltage after folding ÷ initial puncture voltage) x 100%.

[0044] 6. Equivalent series resistance (ESR, 1 kHz internal resistance) Clamp, soak the diaphragm in 1 mol / L -1 TEABF 4 / Propylene carbonate (PC) electrolyte. The sample membrane to be tested was sandwiched between two aluminum foil-activated carbon electrodes. The ESR was read using an LCR meter (test signal 10 mV rms, frequency 1 kHz).

[0045] 7, 24h leakage current Procedure: The assembled symmetric capacitor (with test membrane) was subjected to constant voltage 2.70 V. The steady-state current at the 24th hour of the constant voltage stage was recorded, in units of mA.

[0046] All samples followed the same test procedure to ensure data comparability; experimental data is shown in Table 2.

[0047] Table 2 Experimental data of examples and comparative examples

[0048] IV. Results analysis 1. Pore size and density: the pore size D50 of the examples is ≤1.05 pm, and the Gurley is ≥50 s, which is significantly better than all comparative examples (C4, C8, etc. with coarse pore size of 1.5 pm, and Gurley ≤42 s), reflecting the uniformity of the fine sisal + Tencel webbing effect.

[0049] 2. Internal resistance and leakage: the internal resistance of Ex1-Ex5 is all ≤0.61 Ω, and the leakage is ≤0.07 mA, while the best comparative example (C10) still reaches 0.80 Ω / 0.11 mA; compared with the traditional pure sisal (C1), the internal resistance is reduced by more than 55%, and the leakage is reduced by about 94%.

[0050] 3. Bending reliability: the bending breakdown retention rate of the examples is ≥95%, which is much higher than the 70% of pure sisal and the 83% of polyolefin modified membranes, proving the synergistic flexibility and toughness of the polyester tough fiber and the sisal long fiber skeleton.

[0051] 4. Strength-weight balance: in the low density range (0.35-0.46 gcm -3 ), the tensile strength is still ≥1.20 kNm -1 Tensile, both lightweight and suitable for high-speed winding; comparative examples C3 / C4 have insufficient strength due to imbalance of polyester or sisal.

[0052] V. Beating degree cross test design (baseline for example 3) Under the premise of maintaining the ratio of 20wt% sisal / 55wt% Tencel / 25wt% polyester and the same papermaking-calendering-drying process, only the beating degree of the two pulps was changed, using a 3x3 full factor:

[0053] A total of 9 groups, 3 parallel samples each, 1 kHz ESR, 180° bending breakdown retention rate, pore size D 50and transverse tensile strength.

[0054]

[0055] Test data analysis 1. ESR trend Main effect - sisal pulp: from 30 → 45 °SR, ESR dropped from 0.88 Ω to 0.59-0.65 Ω; then rose to 70 °SR, and back to 0.70-0.78 Ω.

[0056] Main effect - cotton pulp: from 50 → 67 °SR, ESR dropped significantly; continued to 85 °SR, the improvement became smaller, even rebounded.

[0057] Interaction: the lowest value appeared at A2B2 (45 / 67 °SR), proving that the ion passage was the most smooth when both pulps were in the window of the invention, and the resistance increased when the consistency was too loose or too tight.

[0058] 2. Bend-breaking retention rate The retention rate showed a "valley-shaped" relationship with the beating degree: too loose (A1B1) or too tight (A3B3) would easily damage the fiber network at the bend; A2B2 reached 95%, significantly better than 90% of A1B1 and 92% of any high-high combination.

[0059] 3. Pore size D 50 Low beating degree for both sisal and cotton pulp → insufficient fiber splitting, largest pore size (1.30 µm). The double-high group (A3B3) had the smallest pore size (0.82 µm), but was accompanied by densification, brittleness, ESR and retention rate degradation. The 0.88-1.00 µm range (A2B2, A2B3, A3B2) balanced the fine pores and liquid conductivity, and was the best.

[0060] 4. Transverse tensile strength The strength increased with the increase of beating degree, but >1.4 kN m -1 The high-strength group (A2B3, A3Bx) had ESR rising and retention rate decreasing, indicating that the "strength limit" was not the best overall performance.

[0061] The foregoing is a description of the embodiments of the present application. The above description of disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A flexible supercapacitor separator paper, characterized by, The diaphragm paper is made by wet papermaking process, and the fiber raw materials used are composed of the following fibers in percentage by mass: Sisal fiber 5-25%; Tencel fiber 40-65%; Polyester short fiber 20-40%; The beating degree of the sisal pulp is 40-65SR, and the beating degree of the Tencel pulp is 55-80SR; the thickness of the paper sheet is 20-55μm, the apparent density is 0.32-0.58gcm -3 ; and the electric breakdown voltage retention rate of the paper sheet after being folded at 180° for 5 times is ≥95%.

2. The separator paper according to claim 1, characterized in that, The fiber raw materials used are composed of the following fibers in percentage by mass: Sisal fiber 10-20%; Tencel fiber 45-60%; Polyester short fiber 25-35%.

3. The separator paper according to claim 1 or 2, characterized in that, The linear density of the polyester short fiber is 0.7-1.7 dtex, and the length is 2-6 mm.

4. The separator paper according to claim 1 or 2, characterized in that, The average diameter of the sisal fiber is ≤4 μm, and the average length is ≥3 mm.

5. The separator paper according to claim 1 or 2, characterized in that, The average pore size D of the separator paper is 0.4 to 1.5 μm, and the coefficient of variation of the pore size is ≤ 15%. 50 The average pore size D of the separator paper is 0.4 to 1.5 μm, and the coefficient of variation of the pore size 6. The separator paper according to claim 1 or 2, characterized in that, The Gurley air permeability of the diaphragm paper is 10-70 s / 100 mL, and the moisture content is ≤6%.

7. The separator paper according to claim 1 or 2, characterized in that, The paper surface roughness (Ra) is ≤1.2 μm, and the roughness is obtained by the alternate calendering process of 85-105 ℃, 2-4 MPa hot pressing and 20-40 ℃, 1-1.5 MPa cold pressing.

8. The separator paper according to claim 1 or 2, characterized in that, The paper sheet adopts a double-layer structure, the surface layer contains polyester fiber with a mass fraction ≥50%, and the middle layer contains sisal fiber with a mass fraction ≥15%.

9. A method of making the supercapacitor separator paper of any one of claims 1-8, wherein, The method comprises the following steps: a) grinding sisal pulp and Tencel pulp to a target beating degree, and pre-dispersing polyester short fibers in water; b) uniformly mixing the pulp in a dynamic manner in a headbox according to a ratio, and the solid content is 0.4-1.0 wt%; c) forming by a long net paper machine and vacuum dewatering, and when the solid content of the wet paper reaches 35-45%, performing alternate calendering of 85-105 ℃ hot pressing and 20-40 ℃ cold pressing in sequence; d) drying in a 160-180 ℃ tunnel drying oven until the moisture content is ≤6%, and completing the dimensional stability after standing at room temperature for 12 h or more after winding; e) controlling the paper surface roughness according to the hot pressing line pressure 2-4 MPa and the cold pressing line pressure 1-1.5 MPa of step c).

10. A super capacitor comprising an anode current collector, a cathode current collector, an electrolyte, and a diaphragm paper according to any one of claims 1-8, the diaphragm paper being arranged in a folded or rolled form between the bipolar electrodes to form an electrochemical double layer.

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