High-toughness low-impedance structural energy storage member and method of forming same

Abstract

The application provides a high-toughness low-impedance structural energy storage component and a forming method thereof, which comprises a positive electrode carbon fiber, delignified bamboo or wood, a negative electrode carbon fiber, an electrolyte and a structural resin; the delignified bamboo or wood is obtained by delignifying bamboo or wood after chlorination; each layer is laid in the order of the positive electrode carbon fiber, the delignified bamboo or wood and the negative electrode carbon fiber; in a water-free and oxygen-free environment, the electrolyte liquid and the structural resin liquid are mixed in a certain proportion to form a structural electrolyte, and then the solidification is completed by pouring. The application can improve the mechanical and electrical properties of the structural energy storage composite material, improve the toughness and reduce the impedance. Meanwhile, the natural capillary pressure of the delignified bamboo or wood can absorb the structural electrolyte into the interlayer area, so that the structural energy storage composite material without defects can be obtained without high-pressure environment, and the quality of the liquid-formed structural energy storage component is effectively improved.

Description

Technical Field

[0001] This invention relates to the technical field of manufacturing multifunctional composite materials, specifically to a high-toughness, low-resistivity structural energy storage component and its molding method, which is a low-cost, high-quality molding method. Background Technology

[0002] Lightweight and high-performance structural materials are core to the design and manufacturing of cutting-edge equipment, crucial for withstanding harsher loads and conserving energy. As a typical example of lightweight, high-performance structures, energy storage structures can store energy and power equipment while simultaneously bearing the mechanical loads during service. This concept has been widely adopted; for instance, aircraft wings serve as both fuel tanks and wings; motorcycle fuel tanks also support the rider; and current electric vehicles use their battery packs as load-bearing components, with the battery covers directly forming the vehicle chassis. However, these structures are essentially still assembly structures with limited performance.

[0003] Therefore, integrated energy storage structural composite materials have been proposed. The earliest integrated energy storage structural composites involved co-curing existing battery or capacitor elements into carbon fiber prepreg, utilizing the adhesive properties of the resin for encapsulation. This is an embedded energy storage structure. However, traditional batteries and capacitors are heavy and weak, and their interface performance with the composite is poor, resulting in unsatisfactory performance in terms of lightweighting and structural strength. Recently, a highly integrated carbon fiber energy storage structure concept has been proposed. By fully utilizing the lightweight, high modulus, high strength, high conductivity, and high electrochemical capacity of carbon fiber, high-strength carbon fiber serves as the positive and negative electrodes of the energy storage element, while insulating high-strength glass fiber acts as the separator. The electrolyte and resin are thoroughly mixed to form a bicontinuous phase. The electrolyte acts as an ion channel for ion transport, and the resin acts as the structural matrix to encapsulate and fix the reinforcing fibers. The cured structural energy storage composite material can serve as both an energy storage element for charging and discharging and a high-performance load-bearing structure, making it a typical advanced material for integrated structural energy storage.

[0004] However, due to the poor electrolyte carrying capacity, the overall mechanical properties of the composite material are poor after mixing with the structural resin. On the other hand, the presence of the resin also affects the electrolyte's ion transport capacity, significantly increasing impedance and reducing the electrical performance of the energy storage structure, creating a contradiction in material properties. Furthermore, due to the complex multiphase composition of structural energy storage composite materials, current methods only allow for liquid molding. However, liquid molding under vacuum pressure results in numerous porosity defects, while high-pressure injection places high demands on molds, equipment, and atmosphere, creating a conflict between cost and quality. Therefore, balancing the contradictions between material properties and process cost and quality, and fully improving its electrical and mechanical properties, is the key to the practical application of energy storage structural components.

[0005] Patent document CN112652737A discloses a carbon fiber-based composite material structure battery and mobile phone casing. The battery structure includes a modified carbon fiber negative electrode and a modified carbon fiber positive electrode. The modified carbon fiber negative electrode is formed by forming a first modified coating layer on the carbon fiber filaments of a first carbon fiber cloth. The modified carbon fiber positive electrode is formed by forming a second modified coating layer on the carbon fiber filaments of a second carbon fiber cloth. The modified carbon fiber negative electrode and the modified carbon fiber positive electrode are stacked and completely isolated by a solid electrolyte, forming an integral structure. This patent proposes a fully solid electrolyte encapsulation method, eliminating the need for an additional separator. The fully solid electrolyte serves as both the overall matrix and the separator, preventing short circuits between the positive and negative electrodes. However, the extremely poor mechanical properties, low ionic conductivity, and moisture absorption problems of the fully solid electrolyte remain unresolved. More advanced and effective design and manufacturing methods for high-toughness, low-resistance structural energy storage composite materials are still needed in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a high-toughness, low-impedance structural energy storage component and its molding method.

[0007] The objective of this invention can be achieved through the following methods:

[0008] This invention provides a high-toughness, low-impedance structural energy storage component, comprising: positive carbon fiber, delignified bamboo or delignified wood, negative carbon fiber, and structural electrolyte; positive carbon fiber and negative carbon fiber are respectively laid on both sides of the delignified bamboo or delignified wood; the positive carbon fiber, delignified bamboo or delignified wood, and negative carbon fiber are integrated as a whole, impregnated in the structural electrolyte, and cured; the structural electrolyte is a mixed solution of structural resin and electrolyte liquid (ionic electrolyte or solid electrolyte solution).

[0009] Preferably, the delignified bamboo or wood is obtained by removing lignin from bamboo or wood after chlorination. The thickness of the delignified bamboo or wood is 0.05-0.2 mm.

[0010] Preferably, carbon nanotubes are grown on the surfaces of the positive and negative carbon fibers through surface modification. Growing carbon nanotubes through surface modification yields a higher specific surface area, allowing for more thorough contact with the ionic solution. Furthermore, the high conductivity and high strength of carbon nanomaterials can further improve the electrical and interlayer mechanical properties of the energy storage structure.

[0011] Preferably, the delignified bamboo and delignified wood are grafted with copper ions. The delignified bamboo and delignified wood can be further grafted with copper ions to improve ion transport capacity.

[0012] Preferably, if the surface of the positive carbon fiber is coated with the positive electrode material of the battery, then the high-toughness, low-impedance energy storage component is a structural battery; if the surface of the positive carbon fiber is grafted with pseudocapacitive material or has no energy storage material, then the high-toughness, low-impedance energy storage component is a structural capacitor.

[0013] Preferably, the structural resin material includes one or more of epoxy resin, bismaleimide resin, polyimide resin, polyetheretherketone resin, and phenolic resin.

[0014] The present invention also provides a method for molding the high-toughness, low-resistivity structural energy storage component, the molding method comprising the following steps:

[0015] Step S1: Perform delignification treatment on bamboo or wood to obtain delignified bamboo or delignified wood;

[0016] Step S2: Weave and lay positive and negative carbon fibers; coat the surface of the positive carbon fiber with positive material, graft pseudocapacitor material, or leave it uncoated with energy storage material;

[0017] Step S3: Lay out each layer in the order of positive carbon fiber, delignified bamboo or wood, and negative carbon fiber to obtain the preform;

[0018] Step S4: Mix the structural resin with the electrolyte liquid (ionic electrolyte or solid electrolyte solution) to form a structural electrolyte;

[0019] Step S5: Inject the structural electrolyte obtained in step S4 into the preform in step S3 and cure it. After curing, seal the pores again with structural resin to obtain the structural energy storage component.

[0020] Preferably, in step S1, the delignification treatment involves chlorination in a high-temperature sodium chlorite solution to delignify the bamboo or wood. Specifically, the bamboo or wood is immersed in a 2-8% sodium chlorite or sodium hypochlorite solution at 80-120°C for 2-4 hours. After delignification, the delignified bamboo or wood is soaked in deionized water for 24-48 hours.

[0021] Preferably, in step S2, the weaving structure of the positive and negative carbon fibers includes one of the following: unidirectional fabric, plain weave fabric, twill weave fabric, satin weave fabric, and three-dimensional woven fabric. The positive and negative carbon fibers are desized carbon fibers.

[0022] Preferably, in step S2, carbon nanotubes are grown on the surface of the woven positive and negative carbon fibers through surface modification. The modification method includes one of flame growth, chemical vapor deposition, or direct coating. Growing carbon nanotubes through surface modification can achieve a higher specific surface area, allowing for more thorough contact with the ionic solution. Furthermore, the high conductivity and high strength of carbon nanomaterials can further improve the electrical performance and interlayer mechanical properties of the energy storage structure.

[0023] Preferably, in step S2, the cathode material is one or more selected from ternary lithium metal oxide, lithium iron phosphate, lithium manganese oxide, and lithium cobalt oxide. Ternary lithium metal oxide includes LiNi... 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.3 Co 0.3 Mn 0.3 One or more of O2 (NCM111); the grafted pseudocapacitor material includes MnO2, Fe2O3, and Ti3C2. When the positive electrode carbon fiber is coated with the battery positive electrode material, the structural energy storage component is a structural battery; when the positive electrode carbon fiber is grafted with pseudocapacitor material or has no energy storage material coated, the structural energy storage component is a structural capacitor.

[0024] Preferably, in step S3, when laying delignified bamboo and delignified wood, the fiber growth direction of the bamboo and wood is aligned with the lamination direction. The longitudinally oriented porous structure also avoids delamination defects and inefficient ion transfer caused by the lateral expansion of electrolytes.

[0025] Preferably, in step S3, the preform is also laid up with positive and negative current collectors on both sides.

[0026] Preferably, in step S4, the mass ratio of the structural resin to the electrolyte liquid is in the range of 3:7 to 7:3. The structural resin includes one or more of epoxy resin, bismaleimide resin, polyimide resin, polyetheretherketone resin, and phenolic resin. The electrolyte liquid is an ion-exchange electrolyte or a solid electrolyte solution; the ion-exchange electrolyte is an ester or ether electrolyte containing LiPF6 or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI); the solid electrolyte solution is an N-methylpyrrolidone (NMP) solution containing LiTFSI and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) or polyethylene oxide (PEO). When using an ion-exchange electrolyte, curing at room temperature or low temperature (20-60℃) is required to prevent the ion-exchange electrolyte from failing. When using a solid electrolyte solution, curing at medium to high temperature (80-150℃) is required to allow the organic solvents in the solid electrolyte solution to evaporate.

[0027] Preferably, steps S4 and S5 are carried out in an anhydrous and oxygen-free environment.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) This invention utilizes the highly oriented porous structure of insulating bamboo or wood and its natural liquid transport capability to achieve full wetting of liquid resin and electrolyte solution. At the same time, the capillary effect of the pores can promote rapid ion transport and reduce the impedance of the energy storage structure composite material.

[0030] (2) By adopting a highly oriented porous structure of insulating bamboo or wood, the present invention utilizes the capillary effect unique to micro-nano porous structures to increase interlayer pressure and draw structural electrolytes into the interlayer region, so that the component can obtain a dense and defect-free energy storage structure composite material without a high-pressure environment.

[0031] (3) The present invention utilizes the highly oriented high-strength cellulose structure of insulating delignified bamboo or wood to reinforce the energy storage structure composite material in the thickness direction, which greatly improves the interlayer fracture toughness. The longitudinally oriented porous structure also avoids the delamination defects and inefficient ion transfer caused by the lateral expansion of electrolyte.

[0032] (4) The delignified bamboo and wood used in this invention will have their thickness compressed when the structural battery is pressurized and solidified, which can form a thinner structural battery. At the same time, the two ends of the cellulose fiber will be partially embedded in the gap between the positive and negative carbon fibers, which reduces the ion transfer distance on the one hand and further improves the interlayer toughness on the other.

[0033] (5) The delignified bamboo and wood used in this invention can be easily grafted with metal ions to further improve ion transfer ability.

[0034] (6) The natural bamboo and wood used in this invention can be treated with a simple chlorine bleaching method to remove lignin, which is low in cost and suitable for actual production.

[0035] (7) The high-toughness and low-impedance structural energy storage component realized by the present invention has both higher mechanical properties and energy storage capacity, and uses structural resin to seal the pores in one piece, so the whole can be used in the air. Attached Figure Description

[0036] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0037] Figure 1 A schematic diagram of a high-toughness, low-impedance structural energy storage component;

[0038] Figure 2This is a schematic diagram of the bamboo structure used in Example 1;

[0039] Among them, 1-positive carbon fiber, 2-delignified bamboo or wood, 3-negative carbon fiber, 4-structural resin, and 5-ionic electrolyte or solid electrolyte polymer. Detailed Implementation

[0040] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0041] This invention provides a high-toughness, low-impedance structural energy storage component, such as... Figure 1 As shown, the materials used include: positive carbon fiber 1, delignified bamboo or wood 2, negative carbon fiber 3, structural resin 4, and ion electrolyte or solid electrolyte polymer 5; positive carbon fiber 1 and negative carbon fiber 3 are laid on both sides of delignified bamboo or wood 2 respectively, and positive carbon fiber 1, delignified bamboo or wood 2 and negative carbon fiber 3 are impregnated with a mixed solution of structural resin 4 and ion electrolyte or solid electrolyte polymer 5 and then cured.

[0042] A method for fabricating a high-toughness, low-impedance energy storage structure includes the following steps:

[0043] Step S1: In a high-temperature sodium chlorite solution, the bamboo or wood is treated with delignification. After the lignin is removed, the delignified bamboo or wood 2 is soaked in deionized water until the residual sodium chlorite is completely dissolved.

[0044] Step S2: Weave and lay the positive electrode carbon fiber 1 and the negative electrode carbon fiber 3;

[0045] Step S3: Lay out each layer in the order of the positive carbon fiber 1, the delignified bamboo or wood 2, and the negative carbon fiber 3;

[0046] Step S4: In an anhydrous and oxygen-free environment, mix the structural resin liquid 4 and the electrolyte liquid 5 in a certain proportion to form a structural electrolyte;

[0047] Step S5: In an anhydrous and oxygen-free environment, inject the structural electrolyte described in step S4 into the preform in step S3 and cure it. After curing, seal the pores again with structural resin.

[0048] Step S6: Conduct mechanical and electrical performance tests and verifications to complete the performance experiment verification.

[0049] In some embodiments, by growing carbon nanotubes in situ on the surfaces of positive carbon fiber 1 and negative carbon fiber 3 in a directional or non-directional manner, the modified positive carbon fiber 1 and negative carbon fiber 3 can obtain a higher specific surface area, which allows for more thorough contact with the ionic solution. Furthermore, by utilizing the high conductivity and high strength properties of carbon nanomaterials, the electrical performance and interlayer mechanical properties of the energy storage structure can be further improved.

[0050] In other embodiments, delignified bamboo and delignified wood 2 can be further grafted with copper ions to improve ion transport capacity.

[0051] Example 1

[0052] This invention provides a high-toughness, low-impedance energy storage battery component, comprising positive and negative carbon fibers coated with positive electrode material, delignified bamboo, and a room-temperature curing structural resin mixed with LiPF6 ion electrolyte. The bamboo structure used is as follows: Figure 2 As shown.

[0053] The specific steps are as follows:

[0054] T1. Place a 0.1 mm thick bamboo sheet in a 4% sodium chlorite solution at 90℃ for 3 hours to remove lactone, then soak it in deionized water for 48 hours, and dry and store it.

[0055] T2. The conductive carbon black particles (Super P): polyvinylidene fluoride (PVDF): LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was mixed at a mass ratio of 1:1:8, dissolved in NMP, and stirred for 20 hours to form a positive electrode slurry.

[0056] T3. T700 carbon fiber unidirectional tape that has been desized by calcining at 400°C for 1.5 hours in air is used as negative carbon fiber. A portion of the desized carbon fiber is coated with the positive slurry described in T2, dried in a vacuum at 120°C for 12 hours, and then dried with NMP to obtain positive carbon fiber coated with positive active material.

[0057] T4. Lay 10-micron thick stainless steel foil on the concave mold in sequence as the positive electrode current collector, one part of positive electrode carbon fiber unidirectional tape containing positive electrode active material in T3 as the positive electrode (positive electrode carbon fiber), one part of delignified bamboo sheet in T1 with fiber growth direction consistent with the axial direction (lamination direction) and laid parallel to the axial direction, another part of carbon fiber unidirectional tape in T3 as the negative electrode (negative electrode carbon fiber), and lay stainless steel foil again as the negative electrode current collector.

[0058] T5. In the glove box, under the atmosphere of argon, mix commercial room temperature curing structural epoxy resin (E-51) and LiPF6 ion electrolyte in equal mass, pour into the preform stacked in T4, seal and evacuate, and cure at 40°C. After curing, use room temperature curing structural epoxy resin again to seal the holes.

[0059] T6. Perform electrical and mechanical performance tests on the structural battery obtained in T5 to complete the performance verification.

[0060] The resulting battery structure has a tensile modulus of 30.8 GPa, a tensile strength of 378.0 MPa, a flexural modulus of 35.1 GPa, a flexural strength of 280.5 MPa, an energy density of 153.2 Wh / kg, and an impedance of 126 Ω, exhibiting both high mechanical and electrical properties.

[0061] Example 2

[0062] This embodiment provides a high-toughness, low-impedance energy storage capacitor component. Its preparation method is basically the same as that of Embodiment 1, except that in steps T2, T3, and T4, the carbon fiber unidirectional tape is not coated with positive electrode slurry and is directly laid as positive electrode carbon fiber to form a structural capacitor.

[0063] The resulting structural capacitor has a tensile modulus of 32.9 GPa, a tensile strength of 418.4 MPa, a flexural modulus of 42.5 GPa, a flexural strength of 326.1 MPa, and a capacitance of 7.05 mF / g, and also possesses high mechanical and electrical properties.

[0064] Comparative Example 1

[0065] This comparative example provides a high-toughness, low-impedance energy storage battery component, the preparation method of which is as follows:

[0066] T1. The conductive carbon black particles (Super P): polyvinylidene fluoride (PVDF): LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was mixed at a mass ratio of 1:1:8 and dissolved in N-methylpyrrolidone (NMP). The mixture was stirred for 20 hours to form a positive electrode slurry.

[0067] T2. T700 carbon fiber unidirectional tape after desizing at 400℃ for 1.5h in air is used to obtain negative electrode carbon fiber. A portion of the negative electrode carbon fiber is coated with the positive electrode slurry described in T1, dried under vacuum at 120℃ for 12h, and NMP is dried to obtain positive electrode carbon fiber coated with energy storage material.

[0068] T3. Lay 10-micron thick stainless steel foil on the concave mold as the positive electrode current collector, one part of positive electrode carbon fiber unidirectional tape containing positive electrode active material in T2 as the positive electrode, one part of plain glass fiber with a thickness of 0.1mm, and another part of carbon fiber unidirectional tape in T2 as the negative electrode, and lay stainless steel foil again as the negative electrode current collector.

[0069] T4. In a glove box under an argon atmosphere, mix equal amounts of structural epoxy resin (E-51) and LiPF6 ion electrolyte, pour the mixture into the preform stacked in T3, seal and evacuate, and cure at 40°C. After curing, seal the holes again with structural resin.

[0070] T5. Perform electrical and mechanical performance tests on the structural battery obtained in T4 to complete the performance verification.

[0071] The resulting battery structure has a tensile modulus of 24.5 GPa, a tensile strength of 260.8 MPa, a flexural modulus of 16.1 GPa, a flexural strength of 235.2 MPa, an energy density of 107.7 Wh / kg, and an impedance of 523 Ω, which is worse than that of Example 1.

[0072] Comparative Example 2

[0073] This comparative example provides a high-toughness, low-impedance energy storage battery component, the preparation method of which is basically the same as that of Example 1, the only difference being:

[0074] In step T4, the fiber growth direction of the delignified bamboo sheet is perpendicular to the axial direction (lamination direction) and laid parallel to the axial direction.

[0075] The resulting battery structure has a tensile modulus of 34.7 GPa, a tensile strength of 371.2 MPa, a flexural modulus of 28.6 GPa, a flexural strength of 183.1 MPa, an energy density of 82.3 Wh / kg, and an impedance of 609 Ω, which is worse than that of Example 1.

[0076] In the description of this application, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0077] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A high-toughness, low-impedance structural energy storage component, characterized in that, include: Positive carbon fiber, delignified bamboo or delignified wood, negative carbon fiber, and structural electrolyte; positive carbon fiber and negative carbon fiber are laid on both sides of the delignified bamboo or delignified wood respectively; positive carbon fiber, delignified bamboo or delignified wood and negative carbon fiber are impregnated in structural electrolyte and cured as a whole; structural electrolyte is a mixed solution of structural resin and electrolyte liquid. When laying delignified bamboo or delignified wood, ensure that the fiber growth direction of the bamboo or wood is consistent with the stacking direction.

2. The high-toughness, low-impedance structural energy storage component according to claim 1, characterized in that, The delignified bamboo or wood is obtained by removing lignin from bamboo or wood after chlorination.

3. The high-toughness, low-impedance structural energy storage component according to claim 1, characterized in that, If the surface of the positive carbon fiber is coated with the positive electrode material of the battery, then the high-toughness, low-impedance energy storage component is a structural battery; if the surface of the positive carbon fiber is grafted with pseudocapacitive material or has no energy storage material, then the high-toughness, low-impedance energy storage component is a structural capacitor.

4. The high-toughness, low-impedance structural energy storage component according to claim 1, characterized in that, The structural resin material includes one or more of epoxy resin, bismaleimide resin, polyimide resin, polyetheretherketone resin, and phenolic resin.

5. A method for molding a high-toughness, low-resistivity structural energy storage component as described in claim 1, characterized in that, The molding method includes the following steps: Step S1: Perform delignification treatment on bamboo or wood to obtain delignified bamboo or delignified wood; Step S2: Weave and lay positive and negative carbon fibers; coat the surface of the positive carbon fiber with positive material, graft pseudocapacitor material, or leave it uncoated with energy storage material; Step S3: Lay out each layer in the order of positive carbon fiber, delignified bamboo or delignified wood, and negative carbon fiber to obtain the preform; Step S4: Mix the structural resin with the electrolyte liquid to form a structural electrolyte; Step S5: Inject the structural electrolyte obtained in step S4 into the preform in step S3 and cure it. After curing, seal the pores again with structural resin to obtain the structural energy storage component.

6. The molding method according to claim 5, characterized in that, In step S1, the delignification treatment is carried out by chlorination in a high-temperature sodium chlorite solution to delignify bamboo or wood. The specific steps are as follows: put the bamboo or wood into a sodium chlorite or sodium hypochlorite solution with a concentration of 2-8% at 80-120℃ and delignify for 2-4 hours.

7. The molding method according to claim 5, characterized in that, In step S2, the weaving structure of the positive and negative carbon fibers includes one of the following: unidirectional fabric, plain weave fabric, twill weave fabric, satin weave fabric, and three-dimensional woven fabric.

8. The molding method according to claim 5, characterized in that, In step S2, the positive electrode material is one or more of ternary lithium metal oxide, lithium iron phosphate, lithium manganese oxide, and lithium cobalt oxide; the grafted pseudocapacitor material includes one or more of MnO2, Fe2O3, and Ti3C2.

9. The molding method according to claim 5, characterized in that, In step S4, the mass ratio of structural resin to electrolyte liquid is 3:7-7:3.

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