Digital multi-layer winding method for lithium iron phosphate cell

Abstract

The invention relates to the technical field of battery cell winding, in particular to a digital multi-layer winding method for a lithium iron phosphate battery cell, which comprises the following steps: conveying a negative plate, a second diaphragm, a positive plate and a first diaphragm to a to-be-wound position of a heating winding needle to form a single-layer lithium iron phosphate battery cell; forming an inner-layer lithium iron phosphate cell; the hot-pressing time phase difference between each roll core and the next adjacent roll core or the friction coefficient of the roll core is determined according to the wavelength and the maximum amplitude; adjusting a heat dissipation mode of heating a winding needle after the inner-layer lithium iron phosphate battery core is formed into a heat dissipation mode of convergent heat dissipation; heat dissipation is conducted according to the heat dissipation mode; forming an initial lithium iron phosphate cell; and carrying out post-treatment to form a finished product lithium iron phosphate battery cell. According to the invention, the winding consistency of the battery cell and the structural stability of the battery cell are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery cell winding, and particularly to a digital multi-layer winding method for lithium iron phosphate battery cells. Background Art

[0002] In the prior art, the winding process refers to winding and extruding the pre-treated positive electrode sheet, separator, and negative electrode sheet in sequence by fixing the winding needle. The specific process is to stack the raw materials in the order of negative electrode, separator, positive electrode, and separator, and directly wind them into a cylindrical or elliptical cylindrical shape by the winding method, and place them in a square shell or cylindrical metal shell; compared with the stacking process, the winding process of lithium batteries has only two electrode sheets, and the production control is relatively simple and the electric welding is easy; the normal operation of the current winding equipment still requires manual intervention, resulting in a decrease in the overall efficiency of power battery production and low consistency, which is an important factor leading to production waste and quality decline. Especially in the winding and forming stage, the control accuracy of tension, deviation correction and alignment, and ear misalignment will directly affect the quality of the battery cell; for example, the "C-angle" problem that may occur during the winding process, which may lead to uneven internal stress of the battery, affecting the performance and life of the battery, and it is necessary to minimize the "C-angle" effect by optimizing the winding technology and electrode sheet design.

[0003] Chinese Patent Publication No.: CN1,175,256,18A discloses a battery cell, a battery cell winding method and a winding device, including: the first step of winding, starting winding and the first part of main winding are carried out at the first station, at least two separators are clamped by the winding needle to start winding, and the first electrode sheet and the second electrode sheet are fed respectively before starting winding and before the first part of main winding. The first part of main winding completes a part of the required winding turns of the separator, the first electrode sheet and the second electrode sheet in the battery cell; transfer, transferring the winding needle and the material after the first step of winding to the second station, and transferring the idle winding needle to the first station; the second step of winding, carrying out the second part of main winding and winding up the material completed in the first step of winding at the second station, cutting off the separator, the first electrode sheet and the second electrode sheet respectively before winding up, and the second part of main winding completes the remaining turns of the required winding turns of the separator, the first electrode sheet and the second electrode sheet; blanking, blanking the material completed in the second step of winding; wherein, after cutting off the separator, the first electrode sheet and the second electrode sheet, the first step of winding and the second step of winding are carried out synchronously. It can be seen that the battery cell, the battery cell winding method and the winding device have problems such as wrinkles in the inner layer negative electrode sheet after the battery cell formation or grading due to insufficient hot pressing, unreasonable tension setting during the winding process, and excessive expansion of the electrode sheet during the hot pressing process, and due to the gap between the edge position of the hot pressing fit during the winding process and the battery cell or the winding extrusion resulting in uneven stress on each part of the battery cell, thereby reducing the consistency and stability of the battery cell winding. Summary of the Invention

[0004] To this end, the present invention provides a digital multi-layer winding method for lithium iron phosphate battery cells, which is used to overcome the problems in the prior art that due to insufficient hot pressing, unreasonable tension setting during the winding process, and excessive expansion of the electrode sheet during the hot pressing process, wrinkles appear on the inner negative electrode sheet after the battery cell formation or grading, and due to the gap between the edge position of the hot pressing fit during the winding process and the battery cell or the winding extrusion, the force on each part of the battery cell is uneven, resulting in a decrease in the winding consistency and stability of the battery cell.

[0005] To achieve the above object, the present invention provides a digital multi-layer winding method for lithium iron phosphate battery cells, including: Step S1, the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator are simultaneously conveyed to the winding position of the heating winding needle in the direction from the outer circumferential surface close to the heating winding needle to the direction away from the outer circumferential surface to form a single winding core, and a hot press is controlled to hot press the single winding core to form a single-layer lithium iron phosphate battery cell; Step S2, on the basis of the single-layer lithium iron phosphate battery cell, the above-mentioned step S1 is repeated a preset first number of times to form an inner-layer lithium iron phosphate battery cell; Wherein, in the current step S1, the winding tensions of the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator are respectively greater than the winding tensions when the previous step S1 is executed; the hot pressing pressure and hot pressing temperature of the hot pressing are respectively greater than the hot pressing pressure and hot pressing temperature when the previous step S1 is executed; the time phase difference of the hot pressing between each winding core and the adjacent lower-layer winding core or the friction coefficient of the winding core is determined according to the wavelength and maximum amplitude of the surface corrugation of the winding core before hot pressing; Step S3, according to the number of inflection points of the surface corrugation of each layer of winding core, the heat dissipation method of the heating winding needle after forming the inner-layer lithium iron phosphate battery cell is adjusted to a convergent heat dissipation method; Step S4, the hot press is withdrawn, and the heating winding needle is controlled to dissipate heat according to the corresponding heat dissipation method; Step S5, a plurality of the winding cores are wound onto the surface of the inner-layer lithium iron phosphate battery cell according to a preset second number of times to form an initial lithium iron phosphate battery cell; Step S6, post-treating the initial lithium iron phosphate battery cell to form a finished lithium iron phosphate battery cell.

[0006] Further, the calculation formula of the winding tension is: ΔA i =A i +a i ×n, Wherein, ΔA i is the winding tension of the i-th winding material, A i is the initial tension of the i-th winding material, a iis the variable tension coefficient of the i-th winding material, and n is the total number of layers of the formed core; The first winding material is the negative electrode sheet, the second winding material is the second separator, the third winding material is the positive electrode sheet, and the fourth winding material is the first separator.

[0007] Further, in step S2, the initial tension of the positive electrode sheet is equal to the initial tension of the negative electrode sheet, and the initial tension of the first separator is equal to the initial tension of the second separator; the variable tension coefficient of the negative electrode sheet is greater than or equal to the variable tension coefficient of the positive electrode sheet, the variable tension coefficient of the positive electrode sheet is greater than the variable tension coefficient of the first separator, and the variable tension coefficient of the first separator is equal to the variable tension coefficient of the second separator.

[0008] Further, the calculation formula for the hot pressing pressure is: F = (q × ( + + + )) × (2 × S × E); where F is the hot pressing pressure, q is the number of layers of the core, is the thickness of the first separator, is the thickness of the second separator, is the thickness of the positive electrode sheet, is the thickness of the negative electrode sheet, S is the anti-pressure effect coefficient of the core, and E is the material elastic modulus of the core.

[0009] Further, the calculation formula for the hot pressing temperature is: T = (W × q × ( + + + )) / (K × A) + ; where T is the anti-pressure temperature, W is the output power of the hot pressing, K is the thermal conductivity of the core, A is the surface area in contact between the hot press and the core, is the ambient temperature.

[0010] Further, the process of determining the time phase difference between the hot pressing of each core and the next adjacent core or the friction coefficient of the core according to the wavelength and maximum amplitude of the surface ripples of the core before hot pressing includes: Obtain the wavelength and maximum amplitude of the surface ripples respectively; If the wavelength is greater than or equal to the preset wavelength and the maximum amplitude is less than or equal to the preset maximum amplitude, then determine that the type of the surface ripples is temperature-dominated ripples; If the wavelength is less than the preset wavelength and the maximum amplitude is greater than the preset maximum amplitude, it is determined that the type of the surface ripple is stress-dominated ripple. Wherein, the wavelength is the total length of the straight line connecting the head and tail of the surface ripple; the maximum amplitude is the maximum value of the sum of the distances perpendicular to the straight line connecting the head and tail of each adjacent pair of ripple inflection points of the surface ripple, or the maximum value of the distance from the ripple inflection point perpendicular to the straight line connecting the head and tail.

[0011] Further, when the surface ripple is the temperature-dominated ripple, increase the time phase difference of the hot pressing; when the surface ripple is the stress-dominated ripple, increase the friction coefficient in the generation area of the surface ripple. Wherein, the time phase difference is positively correlated with the wavelength, and the friction coefficient is positively correlated with the maximum amplitude.

[0012] Further, the step S3 includes: Obtain the number of inflection points of the surface ripple; Compare the number of inflection points with a preset number; If the number of inflection points is greater than or equal to the preset number, adjust the heat dissipation method of the heating coiled needle from the heat dissipation method of forming the outer circumferential surface of the heating coiled needle and then standing still after stopping heating to the heat dissipation method of the outer circumferential surface of the heating coiled needle converging symmetrically on both sides towards the generation area of the surface ripple.

[0013] Further, the time phase difference of the hot pressing is the difference between the starting heating moment of the next layer of the current core for hot pressing and the starting heating moment of the current core for hot pressing.

[0014] Further, the preset second number is an integer multiple of the preset first number.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows. The method of the present invention releases the stress between each layer of the electrode sheet and the separator at the inner layer of the core by performing multi-layer winding and hot pressing on the core, making the laminated materials fit tightly, improving the wrinkling problem after inner-layer winding of the existing lithium battery, and avoiding the problem of poor infiltration that may be caused by excessive hot pressing of the first-layer electrode roll due to repeated hot pressing. The pressure of hot pressing, the temperature of the winding needle and the hot pressing arc-shaped pressing plate, and the winding tension are dynamically adjusted according to the number of winding layers, avoiding insufficient hot pressing and unreasonable tension setting during the winding process, and avoiding excessive expansion of the electrode sheet during the hot pressing and composite process. When the hot pressing of the inner-layer electrode roll is completed, the hot pressing device stops working and withdraws, and the winding of the entire core continues, improving the production efficiency; the dead zone and lithium deposition caused by the core wrinkling are reduced through hot pressing and composite, the voltage polarization is reduced, the safety performance of the battery cell is increased, the distance between the separator and the electrode sheet and between the electrode sheets is reduced, the consumption of the electrolyte is reduced, and the cycle performance of the battery cell is improved; due to the existence of a gap between the edge position of the hot pressing and fitting during the winding process and the battery cell, the heat received by each part of the battery cell is not completely uniform, or due to the winding extrusion, the force received by each part of the battery cell is not uniform, resulting in a decrease in the consistency and stability of the battery cell winding, and the core becomes softer or the wrinkled material becomes softer due to uneven temperature or stress, resulting in interlayer sliding or wrinkling. By adjusting the time phase difference of the hot pressing between each core and its adjacent core, or adjusting the friction coefficient of the core, the expansion difference between the cores caused by the temperature gradient is reduced, and the concentrated local stress is reduced, achieving an improvement in the structural compactness and performance stability of the battery cell.

[0016] Furthermore, in the method of the present invention, by setting the winding tension, due to the fixed tension, the outer layer is too tight, resulting in material tensile failure, or the inner layer is too loose, resulting in an interlayer gap, affecting the contact between the electrode sheet and the separator; by making the tension increase linearly with the number of winding layers, with a higher outer layer tension to offset the material ductility and a lower inner layer tension to avoid breakage, the contact effect between the electrode sheet and the separator is improved.

[0017] Furthermore, in the method of the present invention, by setting the hot pressing pressure and the hot pressing temperature, since the fixed hot pressing pressure cannot adapt to the change in the total thickness after multi-layer material lamination, resulting in weak interlayer bonding caused by insufficient pressure or diaphragm perforation caused by excessive pressure, and since the fixed hot pressing temperature leads to uneven heat distribution, local overheating causing material degradation or insufficient bonding caused by insufficient temperature, by adjusting the hot pressing pressure and the hot pressing temperature of each layer of the core, the structural consistency and performance stability of the battery cell are improved.

[0018] Furthermore, the method of the present invention determines the corresponding adjustment method by determining the type of surface ripples. Since the ambient temperature of the core near the hot press is high, a temperature difference is formed with the side far from the hot press, or during the winding process, due to the mutual pressing of the core and the winding needle during the winding process, the winding action becomes stuck due to the rotational fatigue of the winding needle, resulting in uneven mechanical stress distribution, thus presenting surface ripples formed due to temperature difference or stress unevenness on the core. When it is temperature-dominated ripples, by increasing the time phase difference of hot pressing to offset the temperature gradient, the peak values of the thermal fields of the two layers are spatially staggered, realizing thermal strain hedging, avoiding the overlap of hot pressing areas, and reducing the expansion difference caused by the temperature gradient. When it is stress-dominated ripples, by increasing the friction coefficient within the area where the surface ripples are generated, the mechanical stress is converted into heat energy diffusion, reducing the area of the stress concentration area, and realizing the improvement of the structural compactness and stability of the battery cell.

[0019] Furthermore, the method of the present invention adopts a heat dissipation method of convergent heat dissipation. Due to uneven release of residual stress caused by uniform heat dissipation and layer misalignment caused by a large number of inflection points, by adjusting the heat dissipation method to stop heating symmetrically in the direction of the outer circumference of the heating winding needle towards both sides of the area where the surface ripples are generated, the inner layer lithium iron phosphate battery cell still in a softened state gradually releases stress towards the stress concentration area, reserving deformation space for the area where the deformation is slow due to stress concentration, thus effectively avoiding layer misalignment caused by stress concentration, and further reducing the residual stress caused by temperature difference, realizing the improvement of the structural stability of the battery cell. Description of the Drawings

[0020] Figure 1 It is the overall flowchart of the digital multi-layer winding method for lithium iron phosphate battery cells in the embodiment of the present invention; Figure 2 It is the structural schematic diagram of the unwinding mechanism used in the digital multi-layer winding method for lithium iron phosphate battery cells in the embodiment of the present invention; Figure 3 It is the flowchart for determining the type of surface ripples in the digital multi-layer winding method for lithium iron phosphate battery cells in the embodiment of the present invention; Figure 4 It is the structural schematic diagram of the core wound by the digital multi-layer winding method for lithium iron phosphate battery cells in the embodiment of the present invention; Explanation of the reference numerals in the drawings: 1 - negative electrode sheet unwinding roller, 2 - second separator unwinding roller, 3 - positive electrode sheet unwinding roller, 4 - first separator unwinding roller, 5 - cutter, 6 - industrial camera, 7 - heating winding needle, 8 - arc-shaped hot press, 9 - negative electrode sheet, 10 - second separator, 11 - positive electrode sheet, 12 - first separator. Detailed Description of the Invention

[0021] To make the objectives and advantages of the present invention more clear and understandable, the present invention will be further described below in conjunction with embodiments; it should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0022] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principles of the present invention and do not limit the protection scope of the present invention.

[0023] It should be noted that in the description of the present invention, the terms indicating the direction or positional relationship such as "upper", "lower", "left", "right", "inner", "outer", etc. are based on the direction or positional relationship shown in the drawings. This is only for convenience of description and does not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0024] In addition, it should also be noted that in the description of the present invention, unless otherwise clearly specified and defined, the terms "installation", "connection", and "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0025] Please refer to Figure 1 , Figure 2 , Figure 3 and Figure 4 as shown, which are respectively the overall flowchart of the digital multi-layer winding method for lithium iron phosphate battery cores in the embodiments of the present invention, the structural schematic diagram of the unwinding mechanism used, the flowchart for determining the type of surface ripples, and the structural schematic diagram of the core. An embodiment of the present invention provides a digital multi-layer winding method for lithium iron phosphate battery cores, including: Step S1: Convey the negative electrode sheet 9, the second separator 10, the positive electrode sheet 11, and the first separator 12 to the winding position of the heating winding needle simultaneously in the direction from the outer circumferential surface close to the heating winding needle to the outer circumferential surface far away to form a single core, and control the hot press to hot press the single core to form a single-layer lithium iron phosphate battery core; Step S2: Repeat the step S1 for a preset first number of times on the basis of the single-layer lithium iron phosphate battery core to form an inner-layer lithium iron phosphate battery core; Among them, in the current step S1, the winding tensions of the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator are respectively greater than the winding tensions in the previous execution of step S1; the hot pressing pressure and hot pressing temperature are respectively greater than the hot pressing pressure and hot pressing temperature in the previous execution of step S1; the time phase difference of hot pressing between each core and the adjacent lower-layer core or the friction coefficient of the core is determined according to the wavelength and maximum amplitude of the surface corrugation of the core before hot pressing; Step S3, adjust the heat dissipation method of the heating winding needle after forming the inner-layer lithium iron phosphate battery core to a convergent heat dissipation method according to the number of inflection points of the surface corrugation of each layer of the core; Step S4, withdraw the hot press, and control the heating winding needle to dissipate heat according to the corresponding heat dissipation method; Step S5, wind a plurality of the cores onto the surface of the inner-layer lithium iron phosphate battery core according to a preset second number of times to form an initial lithium iron phosphate battery core; Step S6, perform post-treatment on the initial lithium iron phosphate battery core to form a finished lithium iron phosphate battery core.

[0026] Specifically, the number of single-layer lithium iron phosphate battery cores in the inner-layer lithium iron phosphate battery core = 1 + preset first number of times.

[0027] Specifically, the first separator and the second separator have the same thickness and material.

[0028] Specifically, the single-layer lithium iron phosphate battery core is a composite film of a negative electrode sheet, a second separator, a positive electrode sheet, and a first separator arranged from the inside to the outside.

[0029] Specifically, in step S1, under the conditions that the material of the negative electrode sheet is graphite, the material of the positive electrode sheet is NCM111, the materials of the first separator and the second separator are polyethylene, and the thicknesses do not exceed 150 μm, the value range of the hot pressing time for hot pressing is [3 s, 5 s], and those skilled in the art can adaptively adjust the hot pressing time based on the actual situation during the specific implementation process.

[0030] Specifically, in step S1, the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator are respectively conveyed to the winding needle through a unwinding mechanism; Among them, the unwinding mechanism includes: Negative electrode sheet unwinding roller 1; Second separator unwinding roller 2; Positive electrode sheet unwinding roller 3; First separator unwinding roller 4; Cutter 5, which is perpendicular to the negative electrode sheet and is used to cut the core; Industrial camera 6, which is arranged between the cutter 5 and the heating winding needle 7 and is used to collect the surface image of the core to obtain the surface corrugation.

[0031] Specifically, the winding tension of the positive electrode sheet / negative electrode sheet / first separator / second separator is the surface tension of the positive electrode sheet / negative electrode sheet / first separator / second separator between the unwind roller 3 of the positive electrode sheet / unwind roller 1 of the negative electrode sheet / unwind roller 4 of the first separator / unwind roller 2 of the second separator and the heating winding needle 7. The winding tension of the positive electrode sheet is adjusted by the rotational speed difference between the rotational speed of the unwind roller and the rotational speed of the heating winding needle 7. Among them, the rotational speed difference is the difference between the rotational speed of the heating winding needle 7 and the rotational speed of the unwind roller, and the rotational speed difference and the winding tension are in a positive correlation.

[0032] Specifically, the winding position of the heating winding needle 7 is the converging position of the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator on the heating winding needle 7.

[0033] Specifically, the unit thickness lithium iron phosphate battery cell is the thickness of a single wound core after hot pressing for 10 s to a composite state according to the hot pressing temperature and hot pressing pressure.

[0034] Specifically, the device for hot pressing is the arc-shaped hot press 8.

[0035] Specifically, the winding tension is the corresponding surface tension between the unwind rollers corresponding to the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator to the winding position.

[0036] Specifically, the surface of the heating winding needle 7 is coated with an electric heating film.

[0037] Specifically, the generation area of the surface corrugation of each wound core is the surface corrugation within the surface of the wound core after one turn of winding from the winding position to the end point of the long axis of the winding needle in the direction of the unwind roller 1 of the negative electrode sheet.

[0038] Specifically, step S6 includes: Step S61, putting the initial lithium iron phosphate battery cell into an ultrasonic cleaning tank for cleaning; Step S62, putting the cleaned initial lithium iron phosphate battery cell into a drying oven for hot air drying; Step S63, immersing the initially hot air-dried lithium iron phosphate battery cell in a passivation solution for passivation treatment, and then hot air drying the passivated initial lithium iron phosphate battery cell again to form a finished lithium iron phosphate battery cell; Among them, the cleaning power of the ultrasonic cleaning tank is 30 kHz, and the cleaning time is 8 minutes; the drying temperatures for both hot air dryings are 60 °C, and the drying times are both 30 minutes; the passivation solution is a 10 g / L chromate aqueous solution, and the passivation time for immersing in the passivation solution is 20 minutes.

[0039] In implementation, the method of the present invention fully releases the stress between each layer of the pole piece and the separator at the inner layer of the core by performing multi-layer winding and hot pressing on the core, making the laminated materials fit tightly, improving the wrinkling problem after the inner layer of the existing lithium battery is wound, avoiding the problem of poor infiltration that may be caused by excessive hot pressing of the first layer of pole winding due to repeated hot pressing, dynamically adjusting the pressure of hot pressing, the temperature of the winding needle and the hot pressing arc pressing plate, and the winding tension as the number of winding layers changes, avoiding insufficient hot pressing and unreasonable tension setting during the winding process, and avoiding excessive expansion of the pole piece during the hot pressing and compounding process. When the hot pressing of the inner layer of the pole winding is completed, the hot pressing device stops working and withdraws, and the winding of the entire core is continued, improving the production efficiency; by hot pressing and compounding, the dead zones and lithium deposition caused by core wrinkling are reduced, the voltage polarization is reduced, the safety performance of the battery cell is increased, the spacing between the separator and the pole piece and between the pole pieces is reduced, the consumption of the electrolyte is reduced, and the cycle performance of the battery cell is improved; during the winding process, due to the gap between the edge position of the hot pressing and fitting and the battery cell, the heat absorption of each part of the battery cell is not completely uniform, or due to the winding extrusion, the stress on each part of the battery cell is not uniform, resulting in a decrease in the consistency and stability of the battery cell winding. The core becomes soft or wrinkled due to uneven temperature or stress, and then layer-to-layer sliding or wrinkling occurs. By adjusting the time phase difference of the hot pressing between each core and its adjacent core, or adjusting the friction coefficient of the core, the expansion difference between the cores caused by the temperature gradient is reduced, and the concentrated local stress is reduced, achieving an improvement in the structural compactness and performance stability of the battery cell.

[0040] Specifically, the calculation formula for the winding tension is: ΔA i =A i +a i ×n, where ΔA i is the winding tension of the i-th winding material, A i is the initial tension of the i-th winding material, a i is the variable tension coefficient of the i-th winding material, and n is the total number of layers of the formed core; The first winding material is the negative pole piece, the second winding material is the second separator, the third winding material is the positive pole piece, and the fourth winding material is the first separator.

[0041] Specifically, in step S2, the initial tension of the positive pole piece is equal to the initial tension of the negative pole piece, and the initial tension of the first separator is equal to the initial tension of the second separator; the variable tension coefficient of the negative pole piece is greater than or equal to the variable tension coefficient of the positive pole piece, the variable tension coefficient of the positive pole piece is greater than the variable tension coefficient of the first separator, and the variable tension coefficient of the first separator is equal to the variable tension coefficient of the second separator.

[0042] Specifically, the variable tension coefficient of the i-th winding material is the actual tension increased for the i-th winding material corresponding to an increase in unit thickness. Those skilled in the art can understand that when the number of layers of the core increases, the thickness of the core will increase, resulting in an increase in the length of the winding material between the current winding connection position of the i-th winding material and the formed core and the tangent point of the corresponding winding material and the corresponding unwinding roller, thereby causing an increase in the actual tension.

[0043] Specifically, the variable tension coefficient of the i-th winding material is obtained by analyzing and summarizing the number of layers of the core and the actual tension obtained during the historical test process; the unit of the variable tension coefficient of the i-th winding material is g / layer.

[0044] In practice, during winding, the inner layer material is tightly wrapped by the outer layer material and bears a greater radial pressure. If the tension remains unchanged, the inner layer material will be overstretched or even broken due to the short inner layer path, while the outer layer material will be loose due to insufficient tension, resulting in an interlayer gap and poor contact, and the inner layer material will be pulled tighter and tighter by the outer layer and break. By gradually reducing the winding tension, the stress states of the inner and outer layer materials during the winding process are balanced, preventing the inner layer from breaking or the outer layer from being loose.

[0045] Specifically, the value range of the initial tension of the positive electrode sheet is [1000g, 1500g], the value range of the initial tension of the negative electrode sheet is [1000g, 1500g], the value range of the initial tension of the first separator is [300g, 500g], and the value range of the initial tension of the second separator is [300g, 500g]; The value range of the variable tension coefficient of the positive electrode sheet is [-35, -25], the value range of the variable tension coefficient of the negative electrode sheet is [-35, -25], the value range of the variable tension coefficient of the first separator is [-6, -5], and the value range of the variable tension coefficient of the second separator is [-6, -5].

[0046] In practice, in the method of the present invention, by setting the winding tension, due to the fixed tension, the outer layer is too tight, which leads to material stretching failure, or the inner layer is too loose, which leads to an interlayer gap and affects the contact between the electrode sheet and the separator; by realizing that the tension increases linearly with the number of winding layers, the outer layer has a higher tension to offset the material ductility, and the inner layer has a lower tension to avoid breaking, which improves the contact effect between the electrode sheet and the separator.

[0047] Specifically, the calculation formula for the hot pressing pressure is: F = (q × ( + + + )) × (2 × S × E); Wherein, F is the hot pressing pressure, q is the number of layers of the core, is the thickness of the first separator, is the thickness of the second separator, is the thickness of the positive electrode sheet, is the thickness of the negative electrode sheet, S is the anti-pressure effect coefficient of the core, and E is the material elastic modulus of the core.

[0048] Specifically, under the conditions that the material of the negative electrode sheet is graphite, the material of the positive electrode sheet is NCM111, and the materials of the first separator and the second separator are polyethylene, the anti-pressure effect coefficient of the core is 0.2, and the material elastic modulus of the core is 12 GPa.

[0049] Specifically, the anti-pressure effect coefficient is the compression displacement pressed down by the single-layer core formed after the thermal lamination compression of the four-layer winding material once. Those skilled in the art can understand that when the magnetic layer winding material is wound to the thermal lamination position after one winding, due to the increase in the number of core layers, the actual thermal lamination area in contact with the arc-shaped hot press increases, and the actual thermal pressing pressure also increases accordingly.

[0050] Specifically, the winding needle and the arc-shaped hot press interact to apply bidirectional pressure to the winding material in the middle, and the total compression displacement after bidirectional pressure is 2×S.

[0051] Specifically, the anti-pressure effect coefficient is obtained by analyzing and summarizing the number of core layers and the actual pressure obtained during the historical test process; the unit of the anti-pressure effect coefficient is mm.

[0052] Specifically, the calculation formula for the thermal pressing temperature is: T=(W×q×( + + + )) / (K×A)+ ; wherein, T is the compressive temperature, W is the output power of the thermal pressing, K is the thermal conductivity of the core, A is the surface area in contact between the hot press and the core, is the ambient temperature.

[0053] Specifically, the surface area in contact between the hot press and the core is detected by the pressure sensor on the thermal pressing surface of the hot press.

[0054] Specifically, under the conditions that the material of the negative electrode sheet is graphite, the material of the positive electrode sheet is NCM111, and the materials of the first separator and the second separator are polyethylene, the thermal conductivity of the core is 0.4 W / (m·K).

[0055] In implementation, in the method of the present invention, by setting the hot pressing pressure and hot pressing temperature, since the fixed hot pressing pressure cannot adapt to the change in the total thickness after the superposition of multiple layers of materials, it may lead to weak interlayer bonding caused by insufficient pressure or diaphragm perforation caused by excessive pressure. And due to the fixed hot pressing temperature, the heat distribution is uneven, local overheating causes material degradation or insufficient temperature causes insufficient bonding. By adjusting the hot pressing pressure and hot pressing temperature of each layer of core, the structural consistency and performance stability of the battery cell are improved.

[0056] Specifically, the process of determining the time phase difference of hot pressing between each core and the adjacent lower-layer core or the friction coefficient of the core according to the wavelength and maximum amplitude of the surface corrugation of the core before hot pressing includes: Obtain the wavelength and maximum amplitude of the surface corrugation respectively; If the wavelength is greater than or equal to a preset wavelength and the maximum amplitude is less than or equal to a preset maximum amplitude, then determine that the type of the surface corrugation is a temperature-dominated corrugation; If the wavelength is less than the preset wavelength and the maximum amplitude is greater than the preset maximum amplitude, then determine that the type of the surface corrugation is a stress-dominated corrugation. Wherein, the wavelength is the total length of the straight line connection between the head and tail of the surface corrugation; the maximum amplitude is the maximum value of the sum of the distances from each adjacent two corrugation inflection points of the surface corrugation perpendicular to the straight line connection between the head and tail, or the maximum value of the distance from the corrugation inflection point perpendicular to the straight line connection between the head and tail.

[0057] Specifically, if all the corrugation inflection points of the surface corrugation are located on any one side of the straight line connection between the head and tail, then the maximum amplitude is the maximum value of the distance from the corrugation inflection point perpendicular to the straight line connection between the head and tail; if there is at least one corrugation inflection point and the remaining corrugation inflection points are respectively located on both sides of the straight line connection between the head and tail, then the maximum amplitude is the maximum value of the sum of the distances from each adjacent two corrugation inflection points of the surface corrugation perpendicular to the straight line connection between the head and tail.

[0058] Specifically, if the surface corrugation is a discontinuous corrugation, then the wavelength is the sum of the lengths of the head-tail connections of several segments of corrugations.

[0059] Specifically, under the conditions that the total thickness of the core does not exceed 1.5 mm, the ambient temperature is 25 °C, and the rotation speed of the heating roll needle 7 is 50 mm / s, the general value range of the preset wavelength is [4 mm, 6 mm], the general value range of the preset maximum amplitude is [0.1 mm, 0.4 mm], the preferred embodiment of the preset wavelength is 5 mm, and the preferred embodiment of the preset maximum amplitude is 0.3 mm.

[0060] Those skilled in the art can understand that the optional ranges of the preset wavelength and the preset maximum amplitude and the preferred embodiments provided in this embodiment are the values that best solve the technical problems of the technical solution of the present invention under the conditions that the total thickness of the core does not exceed 1.5 mm, the ambient temperature is 25 °C, and the rotation speed of the heating winding needle 7 is 50 mm / s. In actual applications or experiments, those skilled in the art can adaptively adjust the preset wavelength and the preset maximum amplitude according to the actual application environment and application scenarios.

[0061] Specifically, when the surface corrugation is the temperature-dominated corrugation, increase the time phase difference of the hot pressing; when the surface corrugation is the stress-dominated corrugation, increase the friction coefficient in the generation area of the surface corrugation; Among them, the time phase difference is positively correlated with the wavelength, and the friction coefficient is positively correlated with the maximum amplitude.

[0062] In implementation, the relationship between the time phase difference and the wavelength is: time phase difference = (wavelength - preset wavelength) × 10 / 2 × winding needle rotation speed. For example, when the wavelength is 7 mm, the time phase difference = (7 mm - 5 mm) × 10 / 2 × 50 mm / s = 0.2 s.

[0063] Specifically, the way to increase the friction coefficient is to increase the heat release of the heating winding needle 7 in the generation area of the surface corrugation to increase the friction coefficient of the core, and its increase principle is the friction heat generation formula.

[0064] In implementation, if the difference between the amplitude and the preset amplitude exceeds 0.1 mm each time, the friction coefficient of the core increases by 0.02. For example, when the difference between the amplitude and the preset amplitude is 0.2 mm and the current friction coefficient is 0.2, the friction coefficient increases to 0.2 + 0.02 × 2 = 0.24.

[0065] In implementation, the method of the present invention determines the corresponding adjustment method by determining the type of the surface corrugation. Since the ambient temperature of the core near the hot press is high, a temperature difference is formed with the side far from the hot press, or due to the uneven mechanical stress distribution caused by the jamming of the winding action during the winding process of the core and the winding needle being pressed against each other during the winding process due to the rotational fatigue of the winding needle, thus presenting surface corrugations formed by the temperature difference or stress unevenness on the core; when it is the temperature-dominated corrugation, increase the time phase difference of the hot pressing to offset the temperature gradient, and the thermal field peaks of the two layers are staggered in space, realizing the thermal strain counteraction, avoiding the overlap of the hot pressing areas, and reducing the expansion difference caused by the temperature gradient; when it is the stress-dominated corrugation, increase the friction coefficient in the generation area of the surface corrugation, convert the mechanical stress into heat energy diffusion, reduce the area of the stress concentration area, and improve the structural compactness and stability of the battery cell.

[0066] Specifically, step S4 includes: Obtaining the number of inflection points of the surface corrugation; Comparing the number of inflection points with a preset number; If the number of inflection points is greater than or equal to the preset number, the heat dissipation method of the heating coil needle 7 is adjusted from the heat dissipation method of forming the outer circumferential surface of the heating coil needle 7 and then standing still after stopping heating to the heat dissipation method of the outer circumferential surface of the heating coil needle 7 converging symmetrically on both sides towards the generation area of the surface corrugation.

[0067] Specifically, the heat dissipation method of symmetric two-side converging heat dissipation is that the position symmetrically centered with the center line of the generation area of the surface corrugation on the outer circumferential surface of the heating coil needle 7 is the starting position of heat dissipation. From the starting position of heat dissipation towards the center line of the generation area of the surface corrugation, the heating coil needle 7 gradually stops heating at each position according to the converging speed until the center line of the generation area of the surface corrugation stops heating, and thus the heat dissipation of the heating coil needle 7 is completed.

[0068] Specifically, the end time of heat dissipation is when the surface temperature of the inner layer lithium iron phosphate battery core is equal to the ambient temperature.

[0069] Specifically, under the conditions that the total thickness of the core does not exceed 1.5 mm and the ambient temperature is 25 °C, the general value range of the converging speed of symmetric two-side converging heat dissipation is [10 mm / s, 30 mm / s], and the preferred embodiment of the converging speed of symmetric two-side converging heat dissipation is 20 mm / s.

[0070] Specifically, under the conditions that the total thickness of the core does not exceed 1.5 mm, the ambient temperature is 25 °C, and the rotation speed of the heating coil needle 7 is 50 mm / s, the general value range of the preset number is [2, 5], and the preferred embodiment of the preset number is 4.

[0071] Those skilled in the art can understand that the optional range of the preset number and the preferred embodiment provided in this embodiment are the values that best solve the technical problems of the technical solution of the present invention selected under the conditions that the total thickness of the core does not exceed 1.5 mm, the ambient temperature is 25 °C, and the rotation speed of the heating coil needle 7 is 50 mm / s. In actual applications or experiments, those skilled in the art can adaptively adjust the preset number according to the actual application environment and application scenario.

[0072] Specifically, the time phase difference of the hot pressing is the difference between the starting heating moment of the current core for hot pressing in the next layer and the starting heating moment of the current core for hot pressing.

[0073] Specifically, the preset second number is an integer multiple of the preset first number.

[0074] Specifically, under the conditions that the material of the negative electrode sheet is graphite, the material of the positive electrode sheet is NCM111, and the materials of the first separator and the second separator are polyethylene, the integer multiple of the preferred embodiment is 3 times.

[0075] In implementation, the method of the present invention adopts the heat dissipation method of convergent heat dissipation. Due to the uneven release of residual stress caused by uniform heat dissipation and the interlayer misalignment caused by a large number of inflection points, by adjusting the heat dissipation method to stop heating in a convergent manner on both symmetric sides in the direction of the outer circumferential surface of the heating coiling needle 7 facing the generation area of the surface corrugation, the inner layer lithium iron phosphate battery cell still in a softened state gradually releases stress to the stress concentration area, reserving a deformation space for the area where the deformation is slow due to stress concentration, thereby effectively avoiding the interlayer misalignment caused by stress concentration, and further reducing the residual stress caused by temperature difference, achieving an improvement in the structural stability of the battery cell.

[0076] So far, the technical solution of the present invention has been described in combination with the preferred embodiments shown in the drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the protection scope of the present invention.

Claims

1. A digital multi-layer winding method for lithium iron phosphate battery cells, characterized in that Including: Step S1: Transport the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator to the winding position of the heating winding needle in the direction from the outer circumferential surface close to the heating winding needle to the outer circumferential surface far away, so as to form a single core, and control the hot press to hot press the single core to form a single-layer lithium iron phosphate battery core; Step S2: Repeat the step S1 for a preset first number of times on the basis of the single-layer lithium iron phosphate battery core to form an inner-layer lithium iron phosphate battery core; Wherein, in the current step S1, the winding tensions of the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator are respectively greater than the winding tensions when the step S1 was executed last time; the hot press pressure and hot press temperature of the hot press are respectively greater than the hot press pressure and hot press temperature when the step S1 was executed last time; the time phase difference of the hot press between each core and the adjacent lower-layer core or the friction coefficient of the core is determined according to the wavelength and maximum amplitude of the surface corrugation of the core before hot press; Step S4: Withdraw the hot press, and control the heating winding needle to dissipate heat according to the corresponding heat dissipation method; Step S5: Wind a plurality of the cores on the surface of the inner-layer lithium iron phosphate battery core according to a preset second number of times to form an initial lithium iron phosphate battery core; Step S6: Perform post-treatment on the initial lithium iron phosphate battery core to form a finished lithium iron phosphate battery core. The calculation formula of the winding tension is:

2. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 1, wherein The first winding material is the negative electrode sheet, the second winding material is the second separator, the third winding material is the positive electrode sheet, and the fourth winding material is the first separator. ΔA i = A i + a i × n, Among them, ΔA i is the winding tension of the i-th winding material, A i is the initial tension of the i-th winding material, a i is the variable tension coefficient of the i-th winding material, and n is the total number of layers of the formed core; In step S2, the initial tension of the positive electrode sheet is equal to the initial tension of the negative electrode sheet, and the initial tension of the first separator is equal to the initial tension of the second separator; 3. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 2, characterized in that, The variable tension coefficient of the negative electrode sheet is greater than or equal to the variable tension coefficient of the positive electrode sheet, the variable tension coefficient of the positive electrode sheet is greater than the variable tension coefficient of the first separator, and the variable tension coefficient of the first separator is equal to the variable tension coefficient of the second separator. The calculation formula of the hot press pressure is:

4. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 3, characterized in that The calculation formula of the hot press temperature is: F = (q × ( + + + )) × (2 × S × E); Among them, F is the hot pressing pressure, q is the number of layers of the core, is the thickness of the first separator, is the thickness of the second separator, is the thickness of the positive electrode sheet, is the thickness of the negative electrode sheet, S is the pressure resistance effect coefficient of the core, and E is the material elastic modulus of the core.

5. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 4, characterized in that, The process of determining the time phase difference of the hot press between each core and the adjacent lower-layer core or the friction coefficient of the core according to the wavelength and maximum amplitude of the surface corrugation of the core before hot press includes: T = (W × q × ( + + + )) / (K × A) + ; Wherein, T is the compressive temperature, W is the output power of the hot pressing, K is the thermal conductivity of the core, A is the surface area of contact between the hot press and the core, and is the ambient temperature.

6. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 5, wherein, Respectively obtain the wavelength and maximum amplitude of the surface corrugation; If the wavelength is greater than or equal to a preset wavelength and the maximum amplitude is less than or equal to a preset maximum amplitude, then determine that the type of the surface corrugation is a temperature-dominated corrugation; If the wavelength is less than the preset wavelength and the maximum amplitude is greater than the preset maximum amplitude, then determine that the type of the surface corrugation is a stress-dominated corrugation, Wherein, the wavelength is the total length of the straight line connection between the head and tail of the surface corrugation; the maximum amplitude is the maximum value of the sum of the distances perpendicular to the straight line connection between the head and tail of each adjacent two corrugation inflection points of the surface corrugation, or the maximum value of the distance perpendicular to the straight line connection between the head and tail of the corrugation inflection point. ​ 7. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 6, wherein When the surface corrugation is the temperature-dominated corrugation, increase the time phase difference of the hot pressing; when the surface corrugation is the stress-dominated corrugation, increase the friction coefficient in the generation area of the surface corrugation; Among them, the time phase difference is positively correlated with the wavelength, and the friction coefficient is positively correlated with the maximum amplitude.

8. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 7, characterized in that The step S3 includes: Obtain the number of inflection points of the surface corrugation; Compare the number of inflection points with a preset number; If the number of inflection points is greater than or equal to the preset number, adjust the heat dissipation method of the heating coiled needle from the heat dissipation method of forming the outer circumferential surface of the heating coiled needle and then standing still after stopping heating to the heat dissipation method of the outer circumferential surface of the heating coiled needle converging symmetrically on both sides towards the generation area of the surface corrugation.

9. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 8, characterized in that, The time phase difference of the hot pressing is the difference between the starting heating moment of the next layer of the current core being hot pressed and the starting heating moment of the current core being hot pressed.

10. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 9, characterized in that, The preset second number is an integer multiple of the preset first number.

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