A flexible supercapacitor separator paper, method of making and use

Abstract

The present application relates to the technical field of electrolytic capacitor separator paper, and particularly relates to a flexible supercapacitor separator paper, a preparation method and application. The separator paper adopts synergistically constructed ternary fiber system of sisal fiber, Tencel fiber and polyester staple fiber, is formed through a wet papermaking process, and is subjected to alternating hot pressing and cold pressing treatment, so that the paper has low density, high strength, low internal resistance, small pore size and excellent bending breakdown resistance. The separator paper is suitable for the winding or laminated structure of supercapacitors, and effectively improves the power density, cycle life and structural reliability of the supercapacitors.

Description

Technical Field

[0001] This invention relates to the field of electrolytic capacitor separator paper technology, and in particular to a flexible supercapacitor separator paper, its preparation method, and its application. Background Technology

[0002] With the rapid development of electronic technology, capacitors are rapidly evolving towards larger capacity, longer lifespan, and higher reliability. Correspondingly, higher requirements are being placed on capacitor separator paper, and the development of new high-end capacitors often relies on the research and development of novel capacitor separator paper. Supercapacitors are a new type of energy storage electronic component that has emerged in recent years with breakthroughs in materials science. They fill the gap between ordinary capacitors and batteries in terms of high energy and high power, possessing high capacity and high starting current characteristics. In short, they simultaneously possess the capacity of batteries and the rapid charging and discharging performance of capacitors, while also being safe, reliable, and widely applicable. They are a breakthrough component for improving and solving the application of electrical power performance, and unlike traditional batteries or accumulators, they do not cause environmental impact. Supercapacitors have enormous application value and market potential in areas such as automotive energy storage (especially electric vehicles, hybrid vehicles, and special heavy-duty engineering vehicles), wind power generation, port energy storage, power transmission and transformation, power grid communication, defense industry (especially missile launch and system integration), and consumer electronics, attracting widespread attention from countries around the world. Because of their ultra-large capacity, supercapacitors are used in military equipment by many developed countries. For example, European countries were the first to use them in heavy engineering vehicles, armored vehicles and tanks to ensure that the vehicles can start quickly and safely in low-temperature conditions.

[0003] Separator paper, positioned between the positive and negative electrodes of a supercapacitor, serves two purposes: storing and transporting the electrolyte while preventing short circuits. Its pore size distribution, ionic resistance (ESR), mechanical strength, and bending flexibility directly determine the device's energy density, power density, and reliability. Industry standards generally require separator paper to possess the following characteristics: 1. Low density / low loss: reducing electrolyte consumption and equivalent series resistance; 2. Small and uniform micropores: suppressing leakage current and improving charging efficiency; 3. High purity and resistance to electrolyte corrosion: preventing metal foil corrosion and device failure; 4. High strength + foldable flexibility: adaptable to lamination (soft packaging) or winding processes, without cracking due to bending.

[0004] The earliest aluminum electrolytic capacitor and supercapacitor separator paper was mostly made from natural long hemp fibers such as sisal and pineapple hemp. The "pineapple hemp / Tencel (Lyocell)" binary system paper disclosed in the applicant's Chinese patent CN101696558B has low density and small pore size. However, when the hemp fiber content is higher than 30wt%, the paper is prone to fiber breakage during the 180° folding process, resulting in local leakage and reduced lifespan.

[0005] Lyocell fiber, derived from renewable cellulose, has a controllable diameter of 2–10 µm, high purity, and is lignin-free, significantly reducing internal resistance and losses; it has already achieved mainstream market share in aluminum electrolytic separators. However, the transverse tensile strength of single-layer Tencel paper is insufficient; during the low-temperature winding or lamination bending process of supercapacitors, the paper is prone to stress concentration and crack propagation, making it difficult to meet the stringent requirement of >90% folding breakdown retention. To address this, the applicant's Chinese patent CN109056403A improves strength through "high / low beating degree Tencel layering," but still lacks folding flexibility and resilience.

[0006] To reduce ESR and improve mechanical properties, some studies have introduced short chemical fibers into Tencel pulp. The applicant's Chinese patent CN108221487B uses 10–95 wt% Tencel plus 5–90 wt% ultrafine chemical fibers (polyester, polyolefin, etc.), significantly reducing internal resistance. However, it does not include a natural long hemp fiber skeleton, limiting paper strength, heat shrinkage, and bending reliability. Furthermore, the applicant's Chinese invention patent CN114263069B combines 30–60 wt% hemp pulp with 10–50 wt% polyolefin short fibers and 20–50 wt% Tencel, achieving low loss at low operating voltages. However, polyolefin fibers have low elastic modulus and poor heat resistance, easily shrinking and warping during high-temperature drying. Additionally, the poor bonding between polyolefin and hemp makes further reduction of internal resistance difficult. Summary of the Invention

[0007] To address the aforementioned technical problems, the present invention aims to provide a flexible supercapacitor separator paper. This separator paper possesses low density, high strength, small and uniform pore size, and a certain degree of flexibility suitable for use in supercapacitors. Its low density results in low loss values, preventing an increase in the internal resistance of the supercapacitor; its high strength makes it suitable for winding processes in supercapacitors; its small pore size results in low operating leakage current, high energy storage capacity, and long lifespan; and its good flexibility prevents breakage during capacitor manufacturing by folding and bending, maintaining the integrity of the separator paper structure, preventing localized leakage defects, and ensuring high energy storage performance.

[0008] A flexible supercapacitor separator paper, which is manufactured by a wet papermaking process, and the fiber raw material used is composed of the following fibers by mass percentage:

[0009] Sisal fiber content: 5-25%;

[0010] Tencel fiber 40-65%;

[0011] Polyester staple fiber 20-40%;

[0012] The sisal pulp has a freeness of 40–65°SR, and the Tencel pulp has a freeness of 55–80°SR; the paper thickness is 20–55 μm, and the apparent density is 0.32–0.58 g / cm³. -3 The paper retains ≥95% of its electrical breakdown voltage after being folded 5 times at 180°.

[0013] Of the components mentioned above, sisal is the smallest papermaking raw material in nature, with a maximum diameter of 4μm and most of it around 2μm. As a type of pulp, its length is relatively long, generally around 3mm. Such fibers result in paper with very high strength, and due to their fineness, the paper has small pores, meeting the stringent leakage current requirements of supercapacitors. Tencel, on the other hand, is a new type of synthetic fiber. Its production process does not release alkali and does not pollute the environment. Tencel's biggest advantage in papermaking is that it is free of impurities, its fiber diameter and length are controllable, and it can be separated into fine fibers through pulping. Moreover, the cross-sections of the separated fine fibers are all circular, which is very different from the flat type of plant fibers. Using such fiber raw materials to make low-density paper products is very suitable. Furthermore, because the separated fibers are very fine, with the smallest being 1~2μm, the resulting paper sheets have small pores. The last type of polyester fiber is refined from petrochemicals. It is free of impurities, and the fiber diameter and length can be selected, with a minimum diameter of 1.5μm. Polyester fiber has stable chemical properties, is resistant to acids and alkalis, has good temperature resistance, good flexibility, and is not easily broken. Paper products can meet the requirements for the flexibility of the insulating material in the manufacturing process of supercapacitors.

[0014] Preferably, the fiber raw material used consists of the following fibers by weight percentage:

[0015] Sisal fiber content: 10-20%;

[0016] Tencel fiber 45-60%;

[0017] Polyester staple fiber 25-35%.

[0018] Preferably, the polyester staple fiber has a linear density of 0.7 to 1.7 dtex and a length of 2 to 6 mm.

[0019] Preferably, the sisal fibers have an average diameter ≤4μm and an average length ≥3mm.

[0020] Preferably, the pore size D of the diaphragm paper 50 The pore size ranges from 0.4 to 1.5 μm, with a coefficient of variation of ≤15%.

[0021] Preferably, the diaphragm paper has a Grellley air permeability of 10–70 s / 100 mL and a moisture content of ≤6%.

[0022] Preferably, the paper surface roughness (Ra) is ≤1.2μm, which is obtained by alternating hot pressing at 85~105℃ and 2~4MPa and cold pressing at 20~40℃ and 1~1.5MPa.

[0023] Preferably, the paper has a double-layer structure, with the surface layer containing ≥50% polyester fiber by mass and the middle layer containing ≥15% sisal fiber by mass.

[0024] Furthermore, the present invention also provides a method for preparing the supercapacitor separator paper, the method comprising the steps of:

[0025] a) Grind the sisal pulp and Tencel pulp to the target beating degree, and pre-disperse the polyester staple fiber in water;

[0026] b) The mixture is dynamically homogenized in the pre-mesh box according to the specified ratio, with a solid content of 0.4–1.0 wt%.

[0027] c) The paper is formed by a wire paper machine and vacuum dewatered. When the solid content of the wet paper reaches 35-45%, it is alternately calendered by hot pressing at 85-105°C and cold pressing at 20-40°C.

[0028] d) Dry in a tunnel drying oven at 160-180℃ until the moisture content is ≤6%, and after winding, let stand at room temperature for more than 12 hours to achieve dimensional stability;

[0029] e) Control the paper surface roughness according to step c) with a hot pressing line pressure of 2-4 MPa and a cold pressing line pressure of 1-1.5 MPa.

[0030] Furthermore, the present invention also provides a supercapacitor, including an anode current collector, a cathode current collector, an electrolyte, and the aforementioned separator paper, wherein the separator paper is sandwiched between bipolar electrodes in a folded or rolled form to form an electrochemical double layer.

[0031] By employing the above-mentioned technical solution, this invention achieves the following comprehensive advantages in terms of structure and performance by synergistically introducing fine-diameter sisal fibers, Tencel fibers, and polyester staple fibers into the same separator paper and combining it with a dual-temperature calendering process:

[0032] 1. Significantly reduced internal resistance: Tencel fibers construct high-purity, continuous liquid-conducting channels, which greatly reduces the obstruction of ion migration by the membrane, effectively improving the power output and fast charge / discharge capability of the supercapacitor.

[0033] 2. Significantly enhanced bending reliability: The toughness of polyester staple fibers and the long fiber skeleton of sisal fibers form a flexible-strong complementary fiber network, which enables the diaphragm to maintain electrical insulation and mechanical integrity after multiple folds or rolls, avoiding leakage failure due to creases.

[0034] 3. Uniform pore size and lower leakage current: The reasonable ratio of fine-diameter fibers results in a uniform and micro-pore structure, which effectively suppresses leakage current and improves energy storage efficiency and self-discharge performance while ensuring full electrolyte wetting.

[0035] 4. Balancing low density and high strength: Fiber combination and directional calendering process reduce paper weight while maintaining high mechanical strength, leaving room for lightweight device design and adapting to the process tension requirements of high-speed winding or lamination production lines.

[0036] 5. Excellent dimensional and thermal stability: The fiber ternary system avoids the defects of traditional polyolefin membranes that are prone to shrinkage and warping under high temperature or high humidity conditions, ensuring that the device still has reliable dimensional stability under harsh operating conditions.

[0037] In summary, the separator paper of this invention achieves an excellent overall balance between low internal resistance, flexibility, mechanical strength, dimensional 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 life. Detailed Implementation

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present invention.

[0039] I. Formula Summary

[0040] Five examples (Ex1-Ex5) and ten comparative examples (C1-C10) are given below, with formulations shown in Table 1.

[0041] Table 1. Fiber formulations of the examples and comparative examples.

[0042]

[0043] II. Standardize the preparation process and parameters

[0044] Equipment requirements:

[0045] Long-wire multi-cylinder paper machine (wire width 1m, machine speed 30min) -1 Vacuum chamber 60kPa; drying cylinder assembly 6 sections (adjustable 160-180°C). Hot pressing light pressing roller: surface temperature 90±5°C, linear pressure 3MPa; cold pressing light pressing roller: surface temperature 25°C, linear pressure 1.2MPa.

[0046] Target basis weight for paper: 18gm -2 (Thickness 35±3µm, can be finely adjusted according to the formula).

[0047] Step S1: Raw material pulping

[0048]

[0049] Note: In Comparative Example C6, the polyolefin short fibers were dispersed at the same concentration; in C7, alumina slurry (average particle size 0.3µm, mass fraction 10wt%) was added during the Tencel slurry stage.

[0050] Step S2: Dynamic homogenization in the pre-mesh box

[0051] The target slurry has a solid content of 0.6 wt%; 0.15 wt% anionic polyacrylamide (a-PAM) is added online as a retention aid; the pH is adjusted to 7.0 ± 0.2; the slurry circulation time is ≥ 5 min to ensure uniform distribution of polyester and natural fibers.

[0052] Step S3: Shaping and Dehydration

[0053] Long-term tension 6kNm -1 The forming section undergoes two-stage vacuum dewatering: 30kPa→60kPa; the wet paper solid content is controlled at 38±3% before entering the pressing zone.

[0054] Step S4: Alternate pressing

[0055] Hot pressing: surface temperature 90°C, linear pressure 3MPa, residence time 0.08s;

[0056] Cold pressing: surface temperature 25°C, linear pressure 1.2MPa, residence time 0.08s;

[0057] Repeat steps 1 and 2 twice to obtain a surface with Ra ≤ 1.2µm.

[0058] Step S5: Drying and Setting

[0059] Drying profile: temperature gradually increased from 160°C in the first cylinder to 180°C in the last cylinder; moisture content upon exiting the cylinder ≤ 5wt%; winding tension 0.6kNm -1 After winding, allow to stand at room temperature for ≥12 hours to eliminate internal stress.

[0060] Step S6: Slicing Inspection

[0061] The target width is 600mm and the outer diameter of the core is 600mm; the sampling inspection includes aperture, density, tensile strength, and bending breakdown retention rate.

[0062] The parameters mentioned above can be directly set using conventional equipment in this field; different embodiments and comparative examples only adjust the slurry ratio and individual raw materials, while keeping the rest of the process consistent, ensuring that performance differences are directly attributable to the formulation design.

[0063] III. Experimental Procedure

[0064] First, Examples Ex1-Ex5 and ten comparative examples C1-C10 were prepared according to the formulation window of the present invention. All paper samples were produced on the same 1m wide wire paper production line to eliminate interference from equipment differences.

[0065] The produced rolls of paper are tested according to the following national or industry standards:

[0066] 1. Thickness and apparent density

[0067] Standard: GB / T451.3-2002 "Determination of thickness, density and unit weight of paper and paperboard"

[0068] Humidity conditioning: 23℃±1℃, 50%RH±2%, equilibrate for at least 24 hours;

[0069] Procedure: Use a thickness gauge with a contact pressure of 0.5N ± 0.05N (contact area 200mm²) 2 The thickness t (mm) was measured. The mass g (g) and area A (m²) of the same sample were weighed. 2 Apparent density ρ = g / (A × t).

[0070] 2. Transverse tensile strength

[0071] Standard: GB / T12914-2008 "Determination of Tensile Strength of Paper and Paperboard (Constant Rate Tensile Method)"

[0072] Humidity adjustment: Same as above;

[0073] Procedure: Cut a 15mm × 180mm test strip with a gauge length of 100mm. Test speed: 100mm / min. -1 Record the fracture load F (N), tensile index = F / specimen width; transverse tensile strength = tensile index × quantitative (kN / m³). -1 ).

[0074] 3. Aperture D 50

[0075] Medium: 99.99% nitrogen

[0076] Procedure: Completely immerse the diaphragm in isopropanol for 10 minutes. Place it in the test cell, pressurize, and record the first continuous bubble pressure P. o (Bubble point); continue pressurizing and record the gas flow rate up to 10 mL / min. -1 The pressure P1. D is obtained by converting according to the standard formula. 50 D 50 It is the median diameter of the aperture distribution, the aperture corresponding to 50% of the cumulative distribution with aperture as the horizontal axis and volume as the weight.

[0077] 4. Gurley air permeability

[0078] Standard: GB / T458-2008 Paper and paperboard - Determination of air permeability (medium range)

[0079] Procedure: 100 mL Grley cylinder full scale; apply a differential pressure of 1.22 kPa (cylinder weight); record the time required for air to penetrate the sample, and take the average of five points, in seconds per 100 mL.

[0080] 5. Bending breakdown retention rate

[0081] Bending: GB / T457-2008 "Paper and Paperboard - Determination of Folding Degree (MIT Method)"—Use a 180° folding clamp, bending radius 1mm, 5 reciprocations. Breakdown Voltage: GB / T12656-1990 "Determination of Power Frequency Breakdown Voltage of Capacitor Paper"; Calculation: Breakdown retention rate = (Breakdown voltage after folding ÷ Initial breakdown voltage) × 100%.

[0082] 6. Equivalent series resistance (ESR, internal resistance at 1kHz)

[0083] Clamping, immersing the diaphragm in 1 mol / L solution for 1 hour. -1 TEABF 4 / Propylene carbonate (PC) electrolyte. The membrane to be tested is sandwiched between two aluminum foil-activated carbon electrodes. The ESR is read using an LCR meter (test signal 10 mVrms, frequency 1 kHz).

[0084] 7. 24-hour leakage current

[0085] Procedure: Apply a constant voltage of 2.70V to the assembled symmetrical capacitor (including the test diaphragm). Record the steady-state current (mA) at the 24th hour of the constant voltage period.

[0086] All samples followed the same testing procedure to ensure data comparability; the experimental data are shown in Table 2.

[0087] Table 2. Experimental data for the examples and comparative examples.

[0088]

[0089] IV. Results Analysis

[0090] 1. Micropores and compactness: The pore size of the example is D50≤1.05µm and Gurley ≥50s, which is significantly better than all comparative examples (C4, C8, etc. with pore size coarsened to 1.5µm and Gurley ≤42s), demonstrating the uniform web formation effect of fine sisal + Tencel.

[0091] 2. Internal resistance and leakage current: The internal resistance of Ex1-Ex5 is ≤0.61Ω and the leakage current is ≤0.07mA, while the best comparative example (C10) still reaches 0.80Ω / 0.11mA; compared with traditional pure sisal (C1), the internal resistance is reduced by more than 55% and the leakage current is reduced by about 94%.

[0092] 3. Bending reliability: The bending breakdown retention rate of the example is ≥95%, which is much higher than the 70% of pure sisal and the 83% of polyolefin modified diaphragm, proving the synergistic effect of polyester toughness fiber and sisal long fiber skeleton in enhancing flexibility and toughness.

[0093] 4. Strength-weight balance: In the low-density region (0.35–0.46 g / cm³) -3 It remains at ≥1.20 kNm -1 It is tensile-resistant, lightweight, and adaptable to high-speed winding; the comparative example C3 / C4 has insufficient strength due to the imbalance of polyester or sisal.

[0094] V. Cross-test design for freeness (based on Example 3)

[0095] While maintaining the same formulation of 20wt% sisal / 55wt% Tencel / 25wt% polyester and the same papermaking-calendering-drying process, only the freeness of the two pulps is changed, using a 3×3 total factor:

[0096]

[0097] A total of 9 groups were prepared, with 3 parallel tests for each group. The 1kHz ESR, 180° bending breakdown retention rate, and pore size D were measured. 50 And transverse tensile strength.

[0098]

[0099] Experimental Data Analysis

[0100] 1. Internal Resistance (ESR) Trend

[0101] Main effect—Sisal paste: From 30 to 45 °SR, ESR decreased from 0.88 Ω to 0.59-0.65 Ω; after increasing to 70 °SR, it rebounded to 0.70-0.78 Ω.

[0102] Main effect—Tencel pulp: From 50 to 67 °SR, ESR decreased significantly; continuing to 85 °SR, the improvement became smaller or even rebounded.

[0103] Interaction: The lowest value was found at A2B2 (45 / 67 °SR), which proves that the ion pathway is most unobstructed when both slurries are simultaneously within the window of this invention. Excessive or insufficient compression will increase resistance.

[0104] 2. Bending breakdown retention rate

[0105] The retention rate and freeness have a "valley-shaped" relationship: too loose (A1B1) or too tight (A3B3) fiber networks are prone to damage at bends; A2B2 reaches 95%, which is significantly better than A1B1's 90% and any high-high combination's 92%.

[0106] 3. Aperture D 50

[0107] Both sisal and Tencel have low beating degrees, resulting in insufficient fiber splitting and the largest pore size (1.30 µm). While the high-porosity group (A3B3) has the smallest pore size (0.82 µm), it is accompanied by densification and embrittlement, and deterioration in ESR and retention. The 0.88-1.00 µm range (A2B2, A2B3, A3B2) balances fine pores and liquid conductivity, making it the optimal choice.

[0108] 4. Transverse tensile strength

[0109] Strength increases with increasing freeness, but >1.4 kN / m -1 The high-strength groups (A2B3, A3Bx) have shown an increase in internal resistance and a decrease in retention rate, indicating that the "strength limit" is not the optimal comprehensive performance.

[0110] The foregoing description of embodiments of the present invention, through which those skilled in the art are able to implement or use the present invention, will be readily apparent to those skilled in the art. Various modifications to these embodiments will be readily apparent to those skilled in the art. The general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novelty disclosed herein.

Claims

1. A flexible supercapacitor separator paper, characterized in that, The separator paper is produced by a wet papermaking process, and the fiber raw materials used are composed of the following fibers by weight percentage: Sisal fiber content: 5-25%; Tencel fiber 40-65%; Polyester staple fiber 20-40%; The sisal pulp has a freeness of 40–65°SR, the Tencel pulp has a freeness of 55–80°SR, the polyester staple fiber has a linear density of 0.7–1.7 dtex and a length of 2–6 mm; the separator paper has an average pore size D. 50 The pore size ranges from 0.4 to 1.5 μm, the coefficient of variation is ≤15%, the paper thickness is 20 to 55 μm, and the apparent density is 0.32 to 0.58 g / cm³. -3 The paper retains ≥95% of its electrical breakdown voltage after being folded 5 times at 180°.

2. The diaphragm paper according to claim 1, characterized in that, The fiber raw materials used consist of the following fibers by weight percentage: Sisal fiber content: 10-20%; Tencel fiber 45-60%; Polyester staple fiber 25-35%.

3. The diaphragm paper according to claim 1 or 2, characterized in that, The sisal fibers have an average diameter of ≤4μm and an average length of ≥3mm.

4. The diaphragm paper according to claim 1 or 2, characterized in that, The diaphragm paper has a gas permeability of 10–70 s / 100 mL and a moisture content of ≤6%.

5. The diaphragm paper according to claim 1 or 2, characterized in that, The paper surface roughness (Ra) is ≤1.2μm, which is obtained by alternating hot pressing at 85~105℃ and 2~4MPa and cold pressing at 20~40℃ and 1~1.5MPa.

6. The diaphragm paper according to claim 1 or 2, characterized in that, The paper has a double-layer structure, with the surface layer containing ≥50% polyester fiber by mass and the middle layer containing ≥15% sisal fiber by mass.

7. A method for preparing the supercapacitor separator paper according to any one of claims 1-6, characterized in that, The method includes the following steps: a) Grind the sisal pulp and Tencel pulp to the target beating degree, and pre-disperse the polyester staple fiber in water; b) The mixture is dynamically homogenized in the pre-mesh box according to the specified ratio, with a solid content of 0.4–1.0 wt%. c) The paper is formed by a wire paper machine and vacuum dewatered. When the solid content of the wet paper reaches 35-45%, it is alternately calendered by hot pressing at 85-105°C and cold pressing at 20-40°C. d) Dry in a tunnel drying oven at 160-180℃ until the moisture content is ≤6%, and after winding, let stand at room temperature for more than 12 hours to achieve dimensional stability; e) Control the paper surface roughness according to step c) with a hot pressing line pressure of 2-4 MPa and a cold pressing line pressure of 1-1.5 MPa.

8. A supercapacitor comprising an anode current collector, a cathode current collector, an electrolyte, and a separator paper according to any one of claims 1-6, wherein the separator paper is sandwiched between bipolar electrodes in a folded or rolled form to form an electrochemical double layer.

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