Carbon fiber composite material battery shell as well as preparation method and application thereof

Abstract

The invention provides a carbon fiber composite material battery shell as well as a preparation method and application thereof. The carbon fiber composite material battery shell comprises an upper cover and a box body; the box body is provided with side walls and an internal reinforcing beam; a skin of the box body is formed by compression molding of a plurality of layers of biaxial warp-knitted carbon fiber cloth; the internal reinforcing beam is a prefabricated carbon plate; the prefabricated carbon plate is embedded and fused in the skin of the box body in the compression molding process of the box body to form an integrated structure; when the maximum height-width ratio H / L of the side walls is larger than 0.2, the included angle between the main fiber guiding direction and the length direction of the box body ranges from + / -55 degrees to + / -65 degrees. When the maximum height-width ratio H / L of the side walls is smaller than or equal to 0.2, the included angle between the main fiber guiding direction and the length direction of the box body ranges from + / -30 degrees to + / -50 degrees. The weight is reduced, the consumption of the carbon fibers is reduced, meanwhile, the mechanical property is kept unchanged, the material can be applied to new energy electric vehicles, and the cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber composite materials technology, and more specifically, to a carbon fiber composite battery casing, its preparation method, and its application. Background Technology

[0002] The metal battery casings used in new energy electric vehicles are quite heavy, making lightweighting of the battery system crucial for improving the driving range of electric vehicles. Carbon fiber composites are ideal lightweight materials, but their high cost—the unit price of carbon fiber is much higher than that of metal—means that even with a 50% weight reduction, the raw material cost would increase by approximately five times, severely limiting large-scale application. Currently, carbon fiber battery casings are generally formed by hot pressing unidirectional prepreg tape layups, often using unidirectional prepreg tape laid up at specific angles (such as 0°, ±45°, 90°) before hot pressing. This traditional method has two problems: first, 90° layups contribute little to the mechanical properties of the main battery casing structure, resulting in material redundancy; second, using unidirectional tape for multi-angle layups is cumbersome, and the interlayer performance needs improvement.

[0003] Therefore, there is an urgent need for a carbon fiber composite battery casing that can significantly reduce the amount of carbon fiber used and lower costs while ensuring mechanical properties. Summary of the Invention

[0004] To address the technical problems existing in the prior art, this invention provides a carbon fiber composite battery casing, its preparation method, and its application.

[0005] The purpose of this invention is to provide a lightweight, low-cost, and high-strength carbon fiber battery casing. This invention selects the dominant fiber direction of the biaxial warp-knitted carbon fiber fabric by using the height H of the battery casing sidewall and the length L of the bottom edge. When the maximum H / L > 0.2, the sidewall of the casing is relatively high, and torsional resistance is crucial. In this case, a biaxial warp-knitted fabric with the dominant fiber direction at ±55° to ±65° to the length direction of the casing should be selected. This angle maximizes the in-plane shear modulus of the material, effectively resisting torsional deformation. In ±55° and ±65°, +55° and +65° refer to the direction of the dominant fiber rotating counterclockwise by 55° and 65° respectively from the length of the box. -55° and -65° refer to the direction of the dominant fiber rotating clockwise by 55° and 65° respectively from the length of the box. The biaxial warp-knitted carbon fiber fabric contains two sets of fiber bundles, symmetrically arranged with respect to the length of the box. Specific angles could be [+55 / -55°], [+60° / -60°], or [+65° / -65°]. The length of the box refers to the longest dimension of the box in the horizontal plane. When H / L ≤ 0.2, the sidewalls of the box are relatively short, closer to a flat plate. In this case, bending is the primary factor. A biaxial warp-knitted fabric with the dominant fiber direction at ±30° to ±50° of the length of the box is selected, achieving a good balance between bending, shearing, and processability.

[0006] Meanwhile, to optimize stress concentration at the junction of the precast carbon plate and the skin, a transition slope can be machined on the edge of the precast carbon plate, with the slope width W satisfying W≥2T with the precast carbon plate thickness T. After molding, the skin covering the slope and the carbon plate form a smooth transition zone in stiffness. Asymmetric layup can be used according to different functional requirements of the inner and outer sides of the battery casing (e.g., impact resistance on the outer side, fatigue resistance and good adhesion on the inner side). For example, the outermost layer of the casing skin uses [+60° / -60°] carbon cloth for impact resistance, while the innermost layer can use glass fiber nonwoven fabric to improve adhesion and damping performance.

[0007] One objective of this invention is to provide a carbon fiber composite battery casing, comprising a top cover and a housing; the housing has side walls and internal reinforcing beams; the skin of the housing is molded from multiple layers of biaxial warp-knitted carbon fiber fabric; the internal reinforcing beams are prefabricated carbon plates; the prefabricated carbon plates are embedded and fused into the skin of the housing during the molding process, forming an integrated structure; the dominant fiber direction of the biaxial warp-knitted carbon fiber fabric is selected based on the height-to-width ratio of the side wall of the housing, wherein the height-to-width ratio is the ratio of the side wall height H to its bottom edge length L, wherein when the maximum height-to-width ratio H / L > 0.2, the dominant fiber direction is ±55° to ±65° with the length direction of the housing; when the maximum height-to-width ratio H / L ≤ 0.2, the dominant fiber direction is ±30° to ±50° with the length direction of the housing.

[0008] Fusion refers to the process in which the precast carbon plate and the upper and lower carbon fiber fabrics of the skin are combined into a dense and continuous integral structure during the molding process of the box body. Under hot pressing conditions, the matrix resin in the precast carbon plate and the matrix resin in the carbon fiber fabric of the skin melt simultaneously, diffuse into each other, and solidify together to form a unified continuous resin phase. At the same time, under pressure, the resin and fibers of the skin layer impregnate and wrap the edges and surfaces of the precast carbon plate, realizing the mechanical interlocking and physical bonding of the fiber-resin layer.

[0009] For high-walled box structures, torsional instability is the primary risk; for low-walled and wide box structures, bending deformation is the primary risk. Therefore, this invention selects the sidewall with the largest height-to-width ratio as the criterion. When this value is greater than 0.2, it is determined to be a high-walled box structure, and priority is given to enhancing torsional resistance. Therefore, a fiber direction of ±55° to ±65° is selected to maximize the in-plane shear modulus. When this value is less than or equal to 0.2, it is determined to be a low-walled box structure, and priority is given to ensuring bending stiffness. Therefore, a fiber direction of ±30° to ±50° is selected to achieve the optimal balance between bending and shear performance.

[0010] In a preferred embodiment of the present invention, The carbon fiber plate is embedded within a skin molded from multiple layers of biaxially oriented warp-knitted carbon fiber fabric; and / or, The top cover is formed by molding multiple layers of biaxial warp-knitted carbon fiber fabric.

[0011] In a preferred embodiment of the present invention, After cutting biaxial warp-knitted carbon fiber fabric at a selected angle, the layers are laid out and then molded to obtain the top cover; and / or, The number of layers in the top cover is 2 to 6.

[0012] In a preferred embodiment of the present invention, The number of layers in the box body is 3 to 8, preferably 4 to 6; More preferably, the beam structure is first formed into a carbon plate, 1 to 2 layers of biaxial warp-knitted carbon fiber cloth are laid in the mold, the carbon plate is added in the preset groove, and 1 to 7 layers, more preferably 2 to 5 layers of biaxial warp-knitted carbon fiber cloth are laid and molded to obtain the box body.

[0013] The inner surface of the mold is provided with a groove that matches the shape of the precast carbon plate. This groove is used to accurately position and temporarily support the precast carbon plate during the layup process, ensuring that it maintains the preset position before and during molding, so that the precast carbon plate can be accurately embedded between the upper and lower skins.

[0014] In a preferred embodiment of the present invention, The biaxial warp-knitted carbon fiber fabric is woven from unidirectional prepreg tape of carbon fiber.

[0015] In a preferred embodiment of the present invention, The edge of the prefabricated carbon plate is machined with a transition slope, and the width W of the transition slope and the thickness T of the carbon plate satisfy the relationship: W ≥ 2T.

[0016] In a preferred embodiment of the present invention, When the number of layers in the housing is odd, the middle layer serves as a plane of symmetry, and the biaxial warp-knitted carbon fiber fabric is laid in an asymmetrical manner; preferably. Asymmetric layups employ angular asymmetry; and / or, Asymmetric layup employs asymmetric material arrangement; and / or, When the number of layers in the housing is even, the biaxial warp-knitted carbon fiber fabric is laid in an asymmetric manner; preferably. Asymmetric layups employ angular asymmetry; and / or, Asymmetric layup employs asymmetric material properties.

[0017] Depending on the different functional requirements of the inner and outer sides of the enclosure, different layup angles or different materials can be set. For different total number of layers, this principle can be used to make adaptive adjustments. For example, for odd-numbered layers, the middle layer can be used as the plane of symmetry, and the number of layers on both sides can be set to be asymmetrical in terms of angle or material. For even-numbered layers, asymmetrical angle or material can be adopted.

[0018] Taking a maximum H / L ≤ 0.2 and a total of 4 layers for the box skin as an example, three feasible asymmetric layup schemes are shown. The layup sequence is described from the outside to the inside.

[0019] Asymmetrical angle scheme one: emphasizing external rigidity and internal toughness; Layer 1 (outermost layer): [+50° / -50°] Biaxial warp-knitted carbon fiber fabric; Reason: The ±50° layup, close to the upper limit of the range, provides good impact and shear resistance to the outer surface of the enclosure.

[0020] Layer 2: [+50° / -50°] Biaxial warp-knitted carbon fiber fabric; Reason: To enhance the impact resistance of the outer layer.

[0021] Layer 3: [+40° / -40°] Biaxial warp-knitted carbon fiber fabric; Reason: Introducing ±40° ply creates an angle difference with the outer layer, enhancing the structure's ability to withstand complex loads in a balanced manner.

[0022] 4th layer (innermost layer): [+40° / -40°] Biaxial warp-knitted carbon fiber fabric; Reason: The inner side adopts ±40° ply, which has good conformability and can form a more flexible connection with the internal reinforcing beam, which is beneficial to improving fatigue resistance.

[0023] Asymmetric Scheme 2: Emphasizing high bending stiffness; Layer 1 (outermost layer): [+45° / -45°] Biaxial warp-knitted carbon fiber fabric; Layer 2: [+45° / -45°] Biaxial warp-knitted carbon fiber fabric; Layer 3: [+45° / -45°] Biaxial warp-knitted carbon fiber fabric; 4th layer (innermost layer): [+30° / -30°] Biaxial warp-knitted carbon fiber fabric; Reason: The innermost layer uses a ±30° layup close to the lower limit of the range, which can significantly improve the bending stiffness of the casing in the length direction, providing a more solid base for fixing the internal battery modules and forming a performance gradient of "external shear resistance and internal high bending resistance".

[0024] Material asymmetry solution three: functional integration and cost reduction; Layer 1 (outermost layer): [+50° / -50°] Biaxial warp-knitted carbon fiber fabric; Reason: To ensure that the outer surface has excellent impact resistance.

[0025] Layer 2: [+50° / -50°] Biaxial warp-knitted carbon fiber fabric; Layer 3: [+50° / -50°] Biaxial warp-knitted carbon fiber fabric; 4th layer (innermost layer): Glass fiber nonwoven fabric (or chopped strand mat) impregnated with resin; Reason: The innermost layer uses low-cost glass fiber material.

[0026] Function 1 (Improved interfacial adhesion): After curing, this layer forms a resin-rich layer, which has better adhesion reliability with materials such as thermal pads and insulating sheets inside the battery pack.

[0027] Function 2 (Improved Damping and Fatigue Resistance): Fiberglass materials are more resilient and help absorb vibrational energy.

[0028] Function 3 (Cost Reduction): Partially replace carbon fiber with low-cost materials.

[0029] The second objective of this invention is to provide a method for preparing a carbon fiber composite battery casing, comprising the following steps: (1) The top cover is obtained by molding the biaxial warp-knitted carbon fiber fabric layer; (2) A prefabricated carbon fiber plate is used as an internal reinforcing beam. A layer of biaxial warp-knitted carbon fiber cloth is laid in the box mold as a lower skin. The prefabricated carbon fiber plate is placed in a preset position on the lower skin. After laying another layer of biaxial warp-knitted carbon fiber cloth, it is molded and cured so that the prefabricated carbon fiber plate is embedded and fused into the upper and lower skins to form an integrated box. The dominant fiber direction of the biaxial warp-knitted carbon fiber cloth is selected based on the height-to-width ratio of the side wall of the box. When the height-to-width ratio H / L > 0.2, the dominant fiber direction is ±55° to ±65° with the length direction of the box. When the height-to-width ratio H / L ≤ 0.2, the dominant fiber direction is ±30° to ±50° with the length direction of the box. (3) The top cover obtained in step (1) and the box obtained in step (2) are assembled together by adhesive bonding and / or mechanical connection to obtain the carbon fiber composite battery case.

[0030] The prefabricated carbon plate is placed at a predetermined position on the lower skin, which corresponds to the location of the groove on the inner surface of the mold. This position is determined based on the overall structural mechanics design of the battery box, specifically corresponding to the main stress areas on the bottom and side walls of the box, the key support parts of the battery module, and the layout of the internal beam structure. By placing the prefabricated carbon plate at this predetermined position, it is ensured that it forms a precisely aligned internal reinforcing beam after molding and curing, thereby effectively reinforcing the key parts of the box.

[0031] In a preferred embodiment of the present invention, The number of layers in the top cover is 2 to 6; and / or, The number of layers in the box body is 3 to 8, preferably 4 to 6; more preferably, the beam structure is first formed into a carbon plate, 1 to 2 layers of biaxial warp-knitted carbon fiber cloth are laid in the mold, the carbon plate is added in the preset groove, and then 1 to 7 layers, more preferably 2 to 5 layers of biaxial warp-knitted carbon fiber cloth are laid and molded to obtain the box body.

[0032] The third objective of this invention is to provide an application of the above-mentioned carbon fiber composite battery casing or the carbon fiber composite battery casing obtained by the above-mentioned preparation method in new energy electric vehicles.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Significant weight and cost reduction: Compared with the traditional 8-layer layup scheme, the areal density of the 4-layer skin scheme in this invention is reduced from 7.0 kg / m² to 4.5 kg / m², which is about 36% lighter, directly proving the reduction in the amount of carbon fiber used.

[0034] 2. Comprehensive performance and outstanding safety: While maintaining comparable basic bending performance to traditional solutions, the compressive strength (CAI) after impact is increased from 240 MPa to over 285 MPa, an improvement of over 18%; the interlaminar shear strength is increased from 55 MPa to 65 MPa. This demonstrates its higher damage tolerance and structural integrity.

[0035] 3. The flexural strength (720 MPa) and post-impact compressive strength (285 MPa) of Example 1 (±45°) are significantly higher than those of Comparative Example 2 (680 MPa and 260 MPa, respectively) using ±60° on the same low-walled box, proving the necessity of the angle selection rule; the roller peel strength of Example 2 reaches 45 N·mm / mm, which is 28% higher than that of Example 1 (35 N·mm / mm) without gradient; the interlaminar shear strength (65 MPa) and post-impact compressive strength (285 MPa) of Example 1 are far superior to the secondary adhesive bonding scheme of Comparative Example 3 (52 MPa and 235 MPa, respectively), proving the comprehensive superiority of the technical solution of the present invention in terms of material utilization, interface bonding and overall structural performance. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the carbon fiber composite battery box of the present invention; In the diagram, 1 represents the top cover, and 2 represents the box body; Figure 2 This is a schematic diagram of the structure of the carbon fiber composite battery box of the present invention; In the figure, B is a magnified view of a part, and 3 is an internal reinforcing beam, which is embedded and integrated into the skin of the box body by prefabricated carbon plates during the box body molding process to form an integrated structure; Figure 3 This is a schematic diagram of the [+45° / -45°] biaxial warp-knitted carbon fiber fabric used to prepare the box skin in Example 1. Detailed Implementation

[0037] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0038] The raw materials used in the examples and comparative examples were all commercially available or prepared according to existing technical methods.

[0039] The carbon fiber used is T700 grade, and the resin is an epoxy resin system.

[0040] The precast carbon plate is made by laying several layers of unidirectional prepreg tape of carbon fiber in the design direction and then curing it by hot pressing. Performance testing shall follow the following national standards or detailed test methods: Areal density: determined according to GB / T 38519-2020 "Method for determination of areal density of fiber reinforced plastic composites".

[0041] Bending performance (bending strength): Tested according to GB / T 1449-2005 "Test method for bending performance of fiber reinforced plastics", using the three-point bending method.

[0042] Interlaminar shear strength: Tested according to GB / T 1450.1-2005 "Test Method for Interlaminar Shear Strength of Fiber Reinforced Plastics".

[0043] Post-impact compressive strength: The test was conducted according to GB / T 21239-2007 "Test Method for Compression Properties of Fiber Reinforced Plastic Laminates After Drop Impact". A drop-weight impact testing machine was used to impact the specimen with a specified energy (e.g., 6.7 J / mm), and the remaining compressive strength was immediately measured in a compression fixture.

[0044] Test method for torsional stiffness of the housing: The housing assembly is fixed at both ends to simulate the actual vehicle condition. A torque M is applied to one end, and the deformation δ along the diagonal of the housing is measured. The torsional stiffness K is calculated using the formula K = M / θ, where the torsional angle θ is obtained by converting δ and the geometric dimensions of the housing. The test is conducted on a self-made test bench, and the method is stable and reliable.

[0045] Roller peel strength test method (used to characterize the bonding performance between the skin and the reinforcing beam): Cut a test strip containing the edge of the reinforcing beam from the molded box body, fix the carbon plate, and roll the free end of the skin into the roller fixture. Peel the skin at a specific rate (e.g., 25 mm / min) on a universal testing machine, and record the force-displacement curve during the peeling process. The peel strength is taken as the average force value of the stable segment of the curve, expressed in N·mm / mm.

[0046] Example 1 Target product: Long strip battery pack box (side wall height H=150mm, side wall width L=1200mm, maximum H / L=0.125).

[0047] Angle selection: Since the maximum H / L = 0.125 ≤ 0.2, we select [+45° / -45°] biaxial warp-knitted carbon fiber cloth.

[0048] Layering scheme: Box skin: 4 layers of [+45° / -45°] fabric, symmetrically laid.

[0049] Internal reinforcing beams: The prefabricated carbon fiber plate is made of 6 layers of T700 carbon fiber unidirectional prepreg tape (0° layup) hot-pressed together, with a thickness of T=4mm. The edges of the carbon fiber plate are not beveled.

[0050] Preparation process: Precast carbon fiber panels are manufactured according to the requirements for internal reinforcing beams mentioned above.

[0051] Two layers of [+45° / -45°] fabric are laid in the box mold as the lower skin.

[0052] Place the pre-made carbon plate inside.

[0053] Then lay two more layers of [+45° / -45°] fabric as the upper skin.

[0054] The entire unit is molded and cured to form a single, integrated box.

[0055] Top cover: Molded from 4 layers of [+45° / -45°] biaxial warp-knitted carbon fiber fabric.

[0056] Final assembly: The top cover is bonded to the box body with sealant and then connected with bolts.

[0057] Example 2 The difference from Example 1 is that the vertical edge of the precast carbon plate is CNC milled to produce a bevel with a width W=15mm (W≈2.2T>2T). Apart from the differences mentioned above, the materials, layup schemes, and processes in Example 2 are exactly the same as those in Example 1.

[0058] Example 3 The difference from Example 1 is that the layering sequence of the box skin is different; it adopts an asymmetrical layering from the outside to the inside. The specific scheme is as follows: Layer 1 (outermost layer): [+50° / -50°] fabric; Layer 2: [+50° / -50°] fabric; Layer 3: [+40° / -40°] fabric; 4th layer (innermost layer): [+40° / -40°] fabric; Apart from the differences mentioned above, the materials, layup schemes, and processes in Example 3 are exactly the same as those in Example 1.

[0059] Example 4 Target product: Long strip battery pack box (side wall height H=220mm, width L=1200mm, maximum H / L≈0.18≤0.2).

[0060] Angle selection: Since the maximum H / L ≤ 0.2, biaxial warp-knitted carbon fiber fabric with the dominant fiber direction of ±30° is selected.

[0061] Layering scheme: Box skin: 6 layers of [+30° / -30°] fabric, symmetrically laid.

[0062] Internal reinforcing beams: The prefabricated carbon plates are made of 8 layers of T700 carbon fiber unidirectional prepreg tape (0° direction lay-up) hot-pressed together, with a thickness of T=5mm.

[0063] Preparation process: Same as in Example 1.

[0064] Top cover: Molded from 3 layers of [+30° / -30°] biaxial warp-knitted carbon fiber fabric.

[0065] Example 5 Target product: Long strip battery pack box (side wall height H=300mm, width L=1000mm, maximum H / L=0.3>0.2).

[0066] Angle selection: Since the maximum H / L > 0.2, biaxial warp-knitted carbon fiber fabric with the dominant fiber direction of ±60° is selected.

[0067] Layering scheme: Box skin: 6 layers of [+60° / -60°] fabric, symmetrically laid.

[0068] Internal reinforcing beams: The prefabricated carbon plates are made of 8 layers of T700 carbon fiber unidirectional prepreg tape (0° direction lay-up) hot-pressed together, with a thickness of T=5mm.

[0069] Preparation process: Same as in Example 1.

[0070] Top cover: Molded from 6 layers of [+60° / -60°] biaxial warp-knitted carbon fiber fabric.

[0071] Example 6 Target product: Long strip battery pack box (side wall height H=300mm, width L=1200mm, H / L=0.25>0.2).

[0072] Angle selection: Since H / L > 0.2, select ±65° biaxial warp-knitted carbon fiber cloth.

[0073] Layering scheme: Box skin: 4 layers of [+65° / -65°] fabric, symmetrically laid.

[0074] Internal reinforcing beams: The prefabricated carbon plates are made of 6 layers of T700 carbon fiber unidirectional prepreg tape (0° direction lay-up) hot-pressed together, with a thickness of T=4mm.

[0075] Preparation process: Same as in Example 1.

[0076] Top cover: Molded from 4 layers of [+65° / -65°] biaxial warp-knitted carbon fiber fabric.

[0077] Comparative Example 1 Target product: The same long, narrow box as in Example 1 (maximum H / L = 0.125).

[0078] Material: Traditional [0° / 90°] plain weave carbon fiber cloth (T700 grade).

[0079] Layering scheme: The box skin adopts a [0° / +45° / -45° / 90°] sequential layering. Calculations show that 8 layers are required to achieve a reference stiffness similar to that of Example 1.

[0080] Preparation process: conventional lay-up molding, with reinforcing beams and skin co-cured.

[0081] Top cover: Molded from 8 layers of [0° / 90°] plain weave fabric.

[0082] Comparative Example 2 Target product: The same long, narrow box as in Example 1 (maximum H / L = 0.125).

[0083] Material: [+60° / -60°] Biaxial warp-knitted carbon fiber cloth (T700 grade).

[0084] Layering scheme: The box lining consists of 4 layers of [+60° / -60°] fabric. The reinforcing beams and their fabrication process are the same as in Example 1.

[0085] Comparative Example 3 Target product: The same long, narrow box as in Example 1.

[0086] Materials: Exactly the same as in Example 1 ([+45° / -45°] fabric).

[0087] Layering scheme: The box skin consists of 4 layers of [+45° / -45°] fabric.

[0088] Preparation process: First, four layers of fabric are molded into a beamless box shell. Then, the same prefabricated carbon plate as in Example 1 is bonded to the corresponding position inside the box a second time using high-strength epoxy structural adhesive.

[0089] Performance testing methods and results analysis: The standards and methods used to test the above samples are as follows: Areal density: determined according to GB / T 38519-2020 "Method for determination of areal density of fiber reinforced plastic composites".

[0090] Bending performance (bending strength): Tested according to GB / T 1449-2005 "Test method for bending performance of fiber reinforced plastics", using the three-point bending method.

[0091] Interlaminar shear strength: Tested according to GB / T 1450.1-2005 "Test Method for Interlaminar Shear Strength of Fiber Reinforced Plastics".

[0092] Post-impact compressive strength: The test was conducted according to GB / T 21239-2007 "Test Method for Compression Properties of Fiber Reinforced Plastic Laminates After Drop Impact". A drop-weight impact testing machine was used to impact the specimen with a specified energy (e.g., 6.7 J / mm), and the remaining compressive strength was immediately measured in a compression fixture.

[0093] Test method for torsional stiffness of the housing: The housing assembly is fixed at both ends to simulate the actual vehicle condition. A torque M is applied to one end, and the deformation δ along the diagonal of the housing is measured. The torsional stiffness K is calculated using the formula K = M / θ, where the torsional angle θ is obtained by converting δ and the geometric dimensions of the housing. The test is conducted on a self-made test bench, and the method is stable and reliable.

[0094] Roller peel strength test method (used to characterize the bonding performance between the skin and the reinforcing beam): Cut a test strip containing the edge of the reinforcing beam from the molded box body, fix the carbon plate, and roll the free end of the skin into the roller fixture. Peel the skin at a specific rate (e.g., 25 mm / min) on a universal testing machine, and record the force-displacement curve during the peeling process. The peel strength is taken as the average force value of the stable segment of the curve, expressed in N·mm / mm.

[0095] The test results are shown in Table 1: Table 1

[0096] As can be seen from Table 1: The areal density of Examples 1-3 is approximately 4.5 kg / m², which is significantly lower than that of Comparative Example 1 (7.0 kg / m²), achieving a weight reduction of approximately 36%. This directly demonstrates that while the amount of carbon fiber used was reduced, the mechanical properties not only did not decrease but also slightly increased, achieving the goal of reducing weight while maintaining mechanical properties.

[0097] The flexural strength (720 MPa) and post-impact compressive strength (285 MPa) of Example 1 (±45°) are significantly higher than those of Comparative Example 2 (680 MPa and 260 MPa, respectively), which has the same number of plies but an angle of ±60°. This demonstrates the importance of selecting plies within the angle range specified in this invention for low and wide box bodies (H / L≤0.2).

[0098] The interlaminar shear strength of Example 2 was 68 MPa and the post-impact compressive strength was 295 MPa, both of which were higher than those of Example 1. Its roller peel strength reached 45 N·mm / mm, which was about 28% higher than the 35 N·mm / mm of Example 1. This directly proves that the gradient transition design of the carbon plate edge can significantly improve the interfacial bonding force and damage tolerance.

[0099] The interlaminar shear strength of Example 1 is 65 MPa and the post-impact compressive strength is 285 MPa, which are much higher than those of Comparative Example 3, which has the same material and layup but uses secondary bonding (52 MPa and 235 MPa, respectively). This fully demonstrates the great advantages of the integrated structure formed by integral molding and embedding in terms of integrity and toughness.

[0100] Example 3, while maintaining a lightweight design, exhibits different performance tendencies through asymmetric layup, demonstrating the potential of asymmetric layup to meet specific needs.

[0101] The carbon fiber composite battery casings prepared in Examples 1-6 achieved weight reduction with a small amount of carbon fiber, without compromising mechanical properties. This demonstrates that the technical solution of the present invention, through the customization of fiber angles based on the sidewall aspect ratio, the integrated structure of prefabricated carbon plates, and the gradient transition design of carbon plate edges, significantly reduces the amount of carbon fiber used (weight reduction of 36%) while comprehensively ensuring and improving the mechanical and safety performance of the product. It achieves a balance between lightweight and high reliability, with significant technical effects, outstanding substantive features, and remarkable progress.

Claims

1. A carbon fiber composite battery case comprising a top cover and a box; the box having sidewalls and internal stiffening beams; the box skin being molded from multiple layers of biaxial carbon fiber woven fabric; the internal stiffening beams being preformed carbon plates; the preformed carbon plates being embedded and fused within the box skin during the box molding process to form an integrated structure; the biaxial carbon fiber woven fabric having its primary fiber direction selected based on the aspect ratio of the box sidewalls, the aspect ratio being the ratio of the sidewall panel height H to its base edge length L, wherein, When the maximum height-width ratio H / L of the side wall is greater than 0.2, the main fiber direction is ±55° to ±65° to the length direction of the box; when the maximum height-width ratio H / L of the side wall is less than or equal to 0.2, the main fiber direction is ±30° to ±50° to the length direction of the box. 2.The carbon fiber composite battery case of claim 1, wherein: The upper cover is obtained by molding a plurality of layers of biaxial warp-knitted carbon fiber cloth. 3.The carbon fiber composite battery case of claim 2, wherein: The upper cover is obtained by cutting the biaxial warp-knitted carbon fiber cloth at a selected angle, stacking and laying the cloth, and then molding; and / or, The number of layers of the upper cover is 2-6. 4.The carbon fiber composite battery case of claim 1, wherein: The number of layers of the box is 3-8, preferably 4-6; Further preferably, the beam structure is first formed into a carbon plate, 1-2 layers of biaxial warp-knitted carbon fiber cloth are laid in a mold, the carbon plate is added at a pre-set groove, and 1-7, more preferably 2-5, layers of biaxial warp-knitted carbon fiber cloth are continuously laid and molded to obtain the box. 5.The carbon fiber composite battery case of claim 1, wherein: The biaxial warp-knitted carbon fiber cloth is obtained by weaving carbon fiber unidirectional prepreg tape. 6.The carbon fiber composite battery case of claim 1, wherein: The edge of the pre-prepared carbon plate is processed with a transition bevel, and the width W of the transition bevel and the thickness T of the carbon plate satisfy the relationship: W≥2T. 7.The carbon fiber composite battery case of claim 4, wherein: When the number of layers of the box is odd, the middle layer serves as a symmetry plane, and the biaxial warp-knitted carbon fiber cloth is laid in an asymmetric manner; preferably, The asymmetric layer is in an angle-asymmetric manner; and / or, The asymmetric layer is in a material-asymmetric manner; and / or, When the number of layers of the box is even, the biaxial warp-knitted carbon fiber cloth is laid in an asymmetric manner; preferably, The asymmetric layer is in an angle-asymmetric manner; and / or, The asymmetric layer is in a material-asymmetric manner. 8.A method for preparing the carbon fiber composite battery case of any one of claims 1-7, comprising the following steps: (1) molding the upper cover by laying and molding biaxial warp-knitted carbon fiber cloth; (2) A prefabricated carbon fiber plate is used as an internal reinforcing beam. A layer of biaxial warp-knitted carbon fiber cloth is laid in the box mold as a lower skin. The prefabricated carbon fiber plate is placed in a preset position on the lower skin. After laying another layer of biaxial warp-knitted carbon fiber cloth on it, it is molded and cured so that the prefabricated carbon fiber plate is embedded and fused into the upper and lower skins to form an integrated box. The dominant fiber direction of the biaxial warp-knitted carbon fiber cloth is selected based on the height-to-width ratio of the side wall of the box. When the height-to-width ratio H / L > 0.2, the dominant fiber direction is ±55° to ±65° with the length direction of the box. When the height-to-width ratio H / L ≤ 0.2, the dominant fiber direction is ±30° to ±50° with the length direction of the box. (3) The top cover obtained in step (1) and the box obtained in step (2) are assembled together by adhesive bonding and / or mechanical connection to obtain the carbon fiber composite battery case.

9. The method for preparing the carbon fiber composite battery casing as described in claim 8, characterized in that: The number of layers in the top cover is 2 to 6; and / or, The number of layers in the box body is 3 to 8, preferably 4 to 6; more preferably, the beam structure is first formed into a carbon plate, 1 to 2 layers of biaxial warp-knitted carbon fiber cloth are laid in the mold, the carbon plate is added in the preset groove, and then 1 to 7 layers, more preferably 2 to 5 layers of biaxial warp-knitted carbon fiber cloth are laid and molded to obtain the box body.

10. The application of a carbon fiber composite battery casing in a new energy electric vehicle, wherein the carbon fiber composite battery casing is the carbon fiber composite battery casing according to any one of claims 1 to 7, or the carbon fiber composite battery casing obtained by the preparation method according to any one of claims 8 to 9.

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