A digital multi-layer winding method for lithium iron phosphate batteries

Abstract

The present invention relates to the field of battery cell winding technology, and in particular to a digital multi-layer winding method for lithium iron phosphate battery cells, comprising: conveying a negative electrode sheet, a second separator, a positive electrode sheet, and a first separator to a waiting position of a heated winding needle to form a single-layer lithium iron phosphate battery cell; forming an inner layer of lithium iron phosphate battery cells; determining the time phase difference between the hot pressing of each winding core and the adjacent next layer of winding cores or the friction coefficient of the winding cores based on the wavelength and maximum amplitude; adjusting the heat removal method of the heated winding needle after forming the inner layer of lithium iron phosphate battery cells to a convergent heat removal method; performing heat removal according to the heat removal method to form an initial lithium iron phosphate battery cell; and performing post-processing to form a finished lithium iron phosphate battery cell. The present invention achieves improved consistency in battery cell winding and improved stability of the battery cell structure.

Description

Technical Field

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

[0002] In the existing technology, the winding process refers to the process of winding and extruding the positive electrode sheets, diaphragms, and negative electrode sheets that have been processed in the early stage in sequence by a fixed winding needle. The specific process is to stack the raw materials in the order of negative electrode, diaphragm, positive electrode, and diaphragm, and directly roll them into a cylindrical or elliptical cylinder by winding, and place them in a square or cylindrical metal shell. Compared with the stacking process, the lithium battery of the winding process has only two electrodes, the production control is relatively simple, and the 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. It is an important factor leading to production waste and quality degradation, especially in the winding forming stage. The control accuracy of tension, correction alignment, and tab misalignment will directly affect the quality of the battery cell. For example, the "C angle" problem that may occur during the winding process may cause uneven stress inside the battery, affecting the performance and life of the battery. It is necessary to minimize the "C angle" effect by optimizing the winding technology and electrode design.

[0003] Chinese Patent Publication No.: CN117525618A discloses a battery cell, a battery cell winding method and a winding device, including: a first winding step, in which winding and the first part of the main winding are performed at a first station, at least two diaphragms are clamped by a winding needle for winding, and the first electrode piece and the second electrode piece are respectively fed from before winding to before the first part of the main winding, and the first part of the main winding completes a part of the required number of winding turns of the diaphragm, the first electrode piece and the second electrode piece in the battery cell; transfer, transfer the winding needle and the material after the first step of winding is completed to the second station, and transfer the idle winding needle to the first station; a second winding step, in which the second part of the main winding and winding of the material completed by the first step are performed at the second station, and the diaphragm, the first electrode piece and the second electrode piece are respectively cut before winding, and the second part of the main winding completes the remaining number of winding turns of the diaphragm, the first electrode piece and the second electrode piece; unloading, unloading the material completed by the second step of winding; wherein, after the diaphragm, the first electrode piece and the second electrode piece are cut, the first step of winding and the second step of winding are performed simultaneously. It can be seen that the battery cell, battery cell winding method and winding equipment have problems such as insufficient hot pressing, unreasonable tension setting during winding and excessive expansion of the electrode during hot pressing, which lead to wrinkles in the inner negative electrode sheet after the battery cell is formed or divided. In addition, due to the gap between the edge position of the hot pressing bonding and the battery cell during winding or winding extrusion, the force on various parts of the battery cell is uneven, which leads to a decrease in 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 such as wrinkles on the inner negative electrode sheets after the battery cells are formed or divided due to insufficient hot pressing, unreasonable tension setting during the winding process, and excessive expansion of the electrodes during the hot pressing process, as well as uneven force on various parts of the battery cells due to gaps between the edge positions of the hot pressing bonding and the battery cells during the winding process or winding extrusion, thereby overcoming the problems of decreased consistency and stability of the battery cell winding.

[0005] To achieve the above objectives, the present invention provides a digital multi-layer winding method for lithium iron phosphate batteries, comprising:

[0006] Step S1: The negative electrode sheet, the second separator, the positive electrode sheet, and the first separator are distributed in a direction from the outer circumference of the heated winding needle to the direction away from the outer circumference of the heated winding needle to form a single winding core, and the hot press is controlled to hot-press the single winding core to form a single-layer lithium iron phosphate battery cell;

[0007] Step S2, repeating the first preset number of steps S1 on the basis of the single-layer lithium iron phosphate battery cell to form an inner-layer lithium iron phosphate battery cell;

[0008] 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 during the previous step S1; the hot pressing pressure and hot pressing temperature of the hot pressing are respectively greater than the hot pressing pressure and hot pressing temperature during the previous step S1; and the time phase difference of the hot pressing between each core and the next adjacent core or the friction coefficient of the core is determined based on the wavelength and maximum amplitude of the surface ripples of the core before hot pressing;

[0009] Step S3, adjusting the heat dissipation mode of the heating winding needle after forming the inner layer lithium iron phosphate battery cell to a convergent heat dissipation mode according to the number of inflection points of the surface corrugations of each layer of the winding core;

[0010] Step S4, withdrawing the hot press and controlling the heating coil needles to remove heat according to a corresponding heat removal method;

[0011] Step S5, winding a plurality of the winding cores onto the surface of the inner layer lithium iron phosphate battery cell according to a preset second number to form an initial lithium iron phosphate battery cell;

[0012] Step S6: post-processing the initial lithium iron phosphate battery cell to form a finished lithium iron phosphate battery cell.

[0013] Furthermore, the calculation formula of the winding tension is:

[0014] ΔA i =A i +ai ×n,

[0015] 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 winding core;

[0016] The first wound material is the negative electrode sheet, the second wound material is the second separator, the third wound material is the positive electrode sheet, and the fourth wound material is the first separator.

[0017] 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 diaphragm is equal to the initial tension of the second diaphragm; 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 diaphragm, and the variable tension coefficient of the first diaphragm is equal to the variable tension coefficient of the second diaphragm.

[0018] Furthermore, the calculation formula of the hot pressing pressure is:

[0019] F=(q×( + + + ))×(2×S×E);

[0020] Wherein, F is the hot pressing pressure, q is the number of layers of the core, is the thickness of the first diaphragm, is the thickness of the second diaphragm, is the thickness of the positive electrode sheet, is the thickness of the negative electrode sheet, S is the pressure resistance coefficient of the winding core, and E is the elastic modulus of the material of the winding core.

[0021] Furthermore, the calculation formula of the hot pressing temperature is:

[0022] T=(W×q×( + + + )) / (K×A)+ ;

[0023] Wherein, T is the pressing temperature, W is the output power of the hot press, K is the thermal conductivity of the core, and A is the surface area of ​​the hot press in contact with the core. is the ambient temperature.

[0024] Furthermore, the process of determining the time phase difference of hot pressing between each core and the next adjacent 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:

[0025] respectively obtaining the wavelength and maximum amplitude of the surface ripples;

[0026] 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 the type of the surface ripple is determined to be a temperature-dominated ripple;

[0027] If the wavelength is smaller than the preset wavelength and the maximum amplitude is larger than the preset maximum amplitude, it is determined that the type of the surface corrugation is a stress-dominated corrugation.

[0028] Among them, the wavelength is the total length of the straight line connecting the head and tail of the surface corrugation; the maximum amplitude is the maximum value of the sum of the distances between each two adjacent corrugation inflection points of the surface corrugation perpendicular to the straight line connecting the head and tail, or the maximum value of the distance between the corrugation inflection point perpendicular to the straight line connecting the head and tail.

[0029] Furthermore, when the surface ripples are the temperature-dominated ripples, the time phase difference of the hot pressing is increased; when the surface ripples are the stress-dominated ripples, the friction coefficient in the generation area of ​​the surface ripples is increased;

[0030] The time phase difference is positively correlated with the wavelength, and the friction coefficient is positively correlated with the maximum amplitude.

[0031] Furthermore, the step S3 includes:

[0032] Obtaining the number of inflection points of the surface ripples;

[0033] comparing the number of inflection points with a preset number;

[0034] If the number of inflection points is greater than or equal to a preset number, the heat dissipation method of the heating coil needle is adjusted from a heat dissipation method in which the outer circumferential surface of the heating coil needle is formed and the heating is stopped and then left to stand, to a heat dissipation method in which the outer circumferential surface of the heating coil needle is symmetrically converged on both sides in the direction of the surface ripple generation area.

[0035] Furthermore, the time phase difference of the hot pressing is the difference between the starting heating time of the hot pressing of the current core and the starting heating time of the hot pressing of the current core.

[0036] Furthermore, the preset second number is an integer multiple of the preset first number.

[0037] Compared with the prior art, the beneficial effect of the present invention is that the method of the present invention fully releases the stress between each layer of the pole piece and the diaphragm at the inner layer of the core by performing multi-layer winding and hot pressing on the core, so that the superimposed materials fit tightly together, improves the wrinkle problem after the inner layer of the existing lithium battery is wound, avoids the problem of poor wetting caused by excessive hot pressing of the first layer of pole coil due to repeated hot pressing, and dynamically adjusts the hot pressing pressure, the temperature of the winding needle and the hot pressing arc pressure plate, and the winding tension according to the number of winding layers, avoids insufficient hot pressing, unreasonable tension setting during winding, and avoids excessive expansion of the pole piece during hot pressing and compounding. When the hot compounding of the inner layer pole coil is completed, the hot pressing device stops working and withdraws, and the winding of the entire core is continued, thereby improving production efficiency; the dead zone and lithium precipitation caused by the wrinkles of the core are reduced by hot pressing and compounding. Reduce voltage polarization, increase battery cell safety performance, and reduce the distance between the diaphragm and the electrode, and between the electrode, reduce the consumption of electrolyte, and improve the cycle performance of the battery cell; due to the gap between the edge position of the hot pressing bonding and the battery cell during the winding process, the heating of the various parts of the battery cell is not completely uniform, or the force of the various parts of the battery cell is uneven due to the winding extrusion, which leads to the decrease in the consistency and stability of the battery cell winding; the core becomes softer or wrinkled due to uneven temperature or uneven stress, which leads to inter-layer 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, thereby achieving the improvement of the structural compactness and performance stability of the battery cell.

[0038] Furthermore, the method of the present invention sets the winding tension. Due to the fixed tension, the outer layer is too tight, which leads to tensile failure of the material, or the inner layer is too loose, which leads to interlayer gaps, affecting the contact between the electrode and the diaphragm. By achieving a linear increase in tension with the number of winding layers, the outer layer tension is higher to offset the ductility of the material, and the inner layer tension is lower to avoid breakage, thereby improving the contact effect between the electrode and the diaphragm.

[0039] Furthermore, the method of the present invention sets the hot pressing pressure and hot pressing temperature. Since the fixed hot pressing pressure cannot adapt to the change in total thickness after the multi-layer materials are stacked, insufficient pressure may cause weak interlayer bonding or excessive pressure may cause diaphragm perforation. The fixed hot pressing temperature may cause uneven heat distribution, local overheating may cause material degradation, or insufficient temperature may cause insufficient bonding. By adjusting the hot pressing pressure and hot pressing temperature of each layer of the core, the structural consistency and performance stability of the battery cell are improved.

[0040] Furthermore, the method of the present invention determines the type of surface ripples and then determines the corresponding adjustment method. Since the ambient temperature of the core close to the hot press is high, a temperature difference is formed with the side away from the hot press, or the core and the winding needle are pressed against each other during the winding process, the winding action is jammed due to fatigue of the winding needle rotation during the winding process, resulting in uneven mechanical stress distribution, thereby showing surface ripples on the core due to temperature difference or uneven stress; by increasing the time phase difference of hot pressing to offset the temperature gradient in temperature-dominated ripples, the thermal field peaks of the two layers are staggered in space, thermal strain offset is achieved, hot pressing area overlap is avoided, and expansion differences caused by temperature gradients are reduced; by increasing the friction coefficient in the generation area of ​​surface ripples in stress-dominated ripples, mechanical stress is converted into thermal energy diffusion, and the area of ​​stress concentration area is reduced, thereby improving the structural compactness and stability of the battery cell.

[0041] Furthermore, the method of the present invention adopts a convergent heat dissipation method. Due to the uneven release of residual stress caused by uniform heat dissipation and the large number of inflection points, interlayer dislocation is caused. By adjusting the heat dissipation method to stop heating in a symmetrical convergent manner on both sides of the outer circumference of the heating coil toward the area where surface ripples are generated, the inner layer of the lithium iron phosphate battery cell, which is still in a softened state, gradually releases stress toward the stress concentration area, reserving deformation space for the area that deforms slowly due to stress concentration, thereby effectively avoiding interlayer dislocation caused by stress concentration, and then reducing the residual stress caused by temperature difference, thereby improving the stability of the battery cell structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is an overall flow chart of a digital multi-layer winding method for lithium iron phosphate batteries according to an embodiment of the present invention;

[0043] Figure 2 This is a schematic structural diagram of an unwinding mechanism used in a digital multi-layer winding method for lithium iron phosphate batteries according to an embodiment of the present invention;

[0044] Figure 3 This is a flow chart for determining the type of surface ripples in a digital multi-layer winding method for lithium iron phosphate batteries according to an embodiment of the present invention;

[0045] Figure 4 A schematic diagram of a core structure wound using a digital multi-layer winding method for lithium iron phosphate batteries according to an embodiment of the present invention;

[0046] Explanation of the accompanying numbers: 1-negative electrode sheet unwinding roller, 2-second diaphragm unwinding roller, 3-positive electrode sheet unwinding roller, 4-first diaphragm unwinding roller, 5-cutter, 6-industrial camera, 7-heating winding needle, 8-arc hot press, 9-negative electrode sheet, 10-second diaphragm, 11-positive electrode sheet, 12-first diaphragm. DETAILED DESCRIPTION

[0047] In order to make the objects and advantages of the present invention more clearly understood, the present invention is further described below in conjunction with embodiments; it should be understood that the specific embodiments described herein are merely used to explain the present invention and are not intended to limit the present invention.

[0048] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0049] It should be noted that, in the description of the present invention, terms such as "up", "down", "left", "right", "inside", and "outside" indicating directions or positional relationships are based on the directions or positional relationships shown in the accompanying drawings. This is only for the 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. Therefore, it cannot be understood as a limitation on the present invention.

[0050] Furthermore, it should be noted that, in the description of the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0051] See also Figure 1 、 Figure 2 、 Figure 3 as well as Figure 4 The figures are as follows, which are respectively an overall flow chart of a digital multi-layer winding method for lithium iron phosphate battery cells according to an embodiment of the present invention, a structural schematic diagram of the unwinding mechanism used, a flow chart for determining the type of surface ripples, and a schematic diagram of the winding core structure. A digital multi-layer winding method for lithium iron phosphate battery cells according to an embodiment of the present invention comprises:

[0052] Step S1: The negative electrode sheet 9, the second separator 10, the positive electrode sheet 11, and the first separator 12 are distributed in a direction from the outer circumference of the heating winding needle to the direction away from the outer circumference of the heating winding needle to form a single winding core, and the hot press is controlled to hot press the single winding core to form a single-layer lithium iron phosphate battery cell;

[0053] Step S2, repeating the first preset number of steps S1 on the basis of the single-layer lithium iron phosphate battery cell to form an inner-layer lithium iron phosphate battery cell;

[0054] 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 during the previous step S1; the hot pressing pressure and hot pressing temperature of the hot pressing are respectively greater than the hot pressing pressure and hot pressing temperature during the previous step S1; and the time phase difference of the hot pressing between each core and the next adjacent core or the friction coefficient of the core is determined based on the wavelength and maximum amplitude of the surface ripples of the core before hot pressing;

[0055] Step S3, adjusting the heat dissipation mode of the heating winding needle after forming the inner layer lithium iron phosphate battery cell to a convergent heat dissipation mode according to the number of inflection points of the surface corrugations of each layer of the winding core;

[0056] Step S4, withdrawing the hot press and controlling the heating coil needles to remove heat according to a corresponding heat removal method;

[0057] Step S5, winding a plurality of the winding cores onto the surface of the inner layer lithium iron phosphate battery cell according to a preset second number to form an initial lithium iron phosphate battery cell;

[0058] Step S6: post-processing the initial lithium iron phosphate battery cell to form a finished lithium iron phosphate battery cell.

[0059] Specifically, the number of layers of single-layer lithium iron phosphate battery cells in the inner-layer lithium iron phosphate battery cell = 1 + the preset first number.

[0060] Specifically, the first diaphragm and the second diaphragm have the same thickness and material.

[0061] Specifically, the single-layer lithium iron phosphate battery cell is a composite film of a negative electrode sheet, a second separator, a positive electrode sheet, and a first separator distributed from the inside to the outside.

[0062] Specifically, in step S1, under the conditions that the negative electrode sheet material is graphite, the positive electrode sheet material is NCM111, the first diaphragm and the second diaphragm material are polyethylene, and the thickness does not exceed 150 μm, the hot pressing time is in the range of [3s, 5s]. Those skilled in the art can adaptively adjust the hot pressing time based on actual conditions during the specific implementation process.

[0063] 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 by the unwinding mechanism;

[0064] Among them, the unwinding mechanism includes:

[0065] Negative electrode sheet unwinding roller 1;

[0066] Second diaphragm unwinding roller 2;

[0067] Positive electrode sheet unwinding roller 3;

[0068] First diaphragm unwinding roller 4;

[0069] A cutter 5, which is arranged perpendicular to the negative electrode sheet and is used to cut the winding core;

[0070] The industrial camera 6 is arranged between the cutter 5 and the heated winding needle 7 to capture the surface image of the winding core to obtain the surface corrugation.

[0071] Specifically, the winding tension of the positive electrode sheet / negative electrode sheet / first diaphragm / second diaphragm is the surface tension of the positive electrode sheet / negative electrode sheet / first diaphragm / second diaphragm between the positive electrode sheet unwinding roller 3 / negative electrode sheet unwinding roller 1 / first diaphragm unwinding roller 4 / second diaphragm unwinding roller 2 and the heating winding needle 7. The winding tension of the positive electrode sheet is adjusted by the speed difference between the speed of the unwinding roller and the speed of the heating winding needle 7, wherein the speed difference is the difference between the speed of the heating winding needle 7 and the speed of the unwinding roller, and the speed difference is positively correlated with the winding tension.

[0072] Specifically, the waiting position of the heating winding needle 7 is the confluence position of the negative electrode sheet, the second separator, the positive electrode sheet and the first separator on the heating winding needle 7 .

[0073] Specifically, the unit thickness of the lithium iron phosphate battery cell is the thickness of a single core that is hot-pressed for 10 seconds at a hot-pressing temperature and hot-pressing pressure to a composite state.

[0074] Specifically, the device for hot pressing is an arc-shaped hot press 8 .

[0075] Specifically, the winding tension is the corresponding surface tension between the unwinding rollers corresponding to the negative electrode sheet, the second separator, the positive electrode sheet, and the first separator and the waiting position.

[0076] Specifically, the surface of the heating coil needle 7 is covered with an electric heating film.

[0077] Specifically, the surface corrugation of each winding core is generated in the surface corrugation within the area from the waiting position to the end point of the long axis of the winding needle in the direction of the negative electrode sheet unwinding roller 1 after winding once.

[0078] Specifically, step S6 includes:

[0079] Step S61, placing the initial lithium iron phosphate battery cell into an ultrasonic cleaning tank for cleaning;

[0080] Step S62, placing the cleaned initial lithium iron phosphate battery cell into a drying oven for hot air drying;

[0081] Step S63, immersing the initial lithium iron phosphate battery cell after hot air drying in a passivation solution to perform a passivation treatment, and hot air drying the initial lithium iron phosphate battery cell after the passivation treatment again to form a finished lithium iron phosphate battery cell;

[0082] Among them, the cleaning power of the ultrasonic cleaning tank is 30kHz, and the cleaning time is 8 minutes; the drying temperature of the two hot air dryings is 60°C, and the drying time is 30 minutes; the passivation solution is a chromate aqueous solution with a concentration of 10g / L, and the passivation time immersing in the passivation solution is 20 minutes.

[0083] During implementation, the method of the present invention fully releases the stress between each layer of the pole piece and the diaphragm at the inner layer of the core by performing multi-layer winding and hot pressing on the core, so that the superimposed materials fit tightly together, improves the wrinkle problem after the inner layer of the existing lithium battery is wound, avoids the problem of poor wetting caused by excessive hot pressing of the first layer of pole coil due to repeated hot pressing, and dynamically adjusts the hot pressing pressure, the temperature of the winding needle and the hot pressing arc pressure plate, and the winding tension according to the number of winding layers, so as to avoid insufficient hot pressing, unreasonable tension setting during winding, and excessive expansion of the pole piece during hot pressing and compounding. When the hot compounding of the inner layer pole coil is completed, the hot pressing device stops working and withdraws, and the winding of the entire core is continued, thereby improving production efficiency. The dead zone and lithium plating caused by the wrinkles of the core are reduced by hot pressing and compounding, and the voltage polarization is reduced. Increase the safety performance of the battery cell, reduce the distance between the diaphragm and the electrode, and between the electrode, reduce the consumption of the electrolyte, and improve the cycle performance of the battery cell; due to the gap between the edge position of the hot pressing and the battery cell during the winding process, the heating of the various parts of the battery cell is not completely uniform, or the force of the various parts of the battery cell is uneven due to the winding extrusion, which leads to the decrease in the consistency and stability of the battery cell winding; the core becomes softer or wrinkled due to uneven temperature or uneven stress, which leads to inter-layer 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, thereby achieving the improvement of the structural compactness and performance stability of the battery cell.

[0084] Specifically, the calculation formula of the winding tension is:

[0085] ΔA i =A i +a i ×n,

[0086] 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 winding core;

[0087] The first wound material is the negative electrode sheet, the second wound material is the second separator, the third wound material is the positive electrode sheet, and the fourth wound material is the first separator.

[0088] Specifically, 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 diaphragm is equal to the initial tension of the second diaphragm; 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 diaphragm, and the variable tension coefficient of the first diaphragm is equal to the variable tension coefficient of the second diaphragm.

[0089] Specifically, the variable tension coefficient of the i-th winding material is the actual tension of the i-th winding material corresponding to the increase in unit thickness.

[0090] It will be understood by those skilled in the art that when the number of layers of the core increases, the thickness of the core will increase, thereby causing the length of the winding material between the current winding connection position of the i-th winding material and the already formed core and the tangent point between the corresponding winding material and the corresponding unwinding roller to become longer, thereby causing the actual tension to increase.

[0091] Specifically, the variable tension coefficient of the i-th winding material is obtained by analyzing and summarizing the number of layers of the winding core and the actual tension obtained during historical tests; the unit of the variable tension coefficient of the i-th winding material is g / layer.

[0092] During implementation, since the inner layer material is tightly wrapped by the outer layer material during winding, it is subjected to greater radial pressure. If the tension remains unchanged, the inner layer material will be over-stretched or even broken due to the short inner layer path, while the outer layer material will relax due to insufficient tension, resulting in interlayer gaps 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 state of the inner and outer layer materials during the winding process is balanced to prevent the inner layer from breaking or the outer layer from relaxing.

[0093] Specifically, the initial tension of the positive electrode sheet is in the range of [1000g, 1500g], the initial tension of the negative electrode sheet is in the range of [1000g, 1500g], the initial tension of the first diaphragm is in the range of [300g, 500g], and the initial tension of the second diaphragm is in the range of [300g, 500g].

[0094] The variable tension coefficient of the positive electrode sheet ranges from [-35, -25], the variable tension coefficient of the negative electrode sheet ranges from [-35, -25], the variable tension coefficient of the first diaphragm ranges from [-6, -5], and the variable tension coefficient of the second diaphragm ranges from [-6, -5].

[0095] In practice, the method of the present invention sets the winding tension. Due to the fixed tension, the outer layer is too tight, which leads to tensile failure of the material, or the inner layer is too loose, which leads to interlayer gaps, affecting the contact between the electrode and the diaphragm. By achieving a linear increase in tension with the number of winding layers, the outer layer tension is higher to offset the ductility of the material, and the inner layer tension is lower to avoid breakage, thereby improving the contact effect between the electrode and the diaphragm.

[0096] Specifically, the calculation formula of the hot pressing pressure is:

[0097] F=(q×( + + + ))×(2×S×E);

[0098] Wherein, F is the hot pressing pressure, q is the number of layers of the core, is the thickness of the first diaphragm, is the thickness of the second diaphragm, is the thickness of the positive electrode sheet, is the thickness of the negative electrode sheet, S is the pressure resistance coefficient of the winding core, and E is the elastic modulus of the material of the winding core.

[0099] Specifically, under the conditions that the negative electrode sheet material is graphite, the positive electrode sheet material is NCM111, and the first and second separator materials are polyethylene, the pressure effect coefficient of the core is 0.2, and the elastic modulus of the core material is 12GPa.

[0100] Specifically, the anti-pressure effect coefficient is the compression displacement of the single-layer core formed by adding a four-layer winding material after heat sealing and compression.

[0101] Those skilled in the art will understand that when the magnetic layer winding material is wound to the heat-sealing position after one winding, the actual heat-sealing area in contact with the arc-shaped hot press increases due to the increase in the number of core layers, and the actual hot pressing pressure also increases accordingly.

[0102] Specifically, the winding needle and the arc-shaped hot press interact with each other to apply bidirectional pressure to the winding material in the middle, and the total compression displacement after bidirectional pressure is 2×S.

[0103] Specifically, the pressure resistance effect coefficient is obtained by analyzing and summarizing the number of layers of the core and the actual pressure obtained during historical tests; the unit of the pressure resistance effect coefficient is mm.

[0104] Specifically, the calculation formula for the hot pressing temperature is:

[0105] T=(W×q×( + + + )) / (K×A)+ ;

[0106] Wherein, T is the pressing temperature, W is the output power of the hot press, K is the thermal conductivity of the core, and A is the surface area of ​​contact between the hot press and the core. is the ambient temperature.

[0107] Specifically, the surface area of ​​the hot press in contact with the core is detected by a pressure sensor on the hot press surface of the hot press.

[0108] Specifically, under the conditions that the negative electrode sheet material is graphite, the positive electrode sheet material is NCM111, and the first and second separator materials are polyethylene, the thermal conductivity of the winding core is 0.4 W / (m·K).

[0109] During implementation, the method of the present invention sets the hot pressing pressure and hot pressing temperature. Since the fixed hot pressing pressure cannot adapt to the change in total thickness after the multi-layer materials are stacked, insufficient pressure may cause weak interlayer bonding or excessive pressure may cause diaphragm perforation. The fixed hot pressing temperature may cause uneven heat distribution, local overheating may cause material degradation, or insufficient temperature may cause insufficient bonding. By adjusting the hot pressing pressure and hot pressing temperature of each layer of the core, the structural consistency and performance stability of the battery cell are improved.

[0110] Specifically, 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 based on the wavelength and maximum amplitude of the surface corrugation of the core before hot pressing includes:

[0111] respectively obtaining the wavelength and maximum amplitude of the surface ripples;

[0112] 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 the type of the surface ripple is determined to be a temperature-dominated ripple;

[0113] If the wavelength is smaller than the preset wavelength and the maximum amplitude is larger than the preset maximum amplitude, it is determined that the type of the surface corrugation is a stress-dominated corrugation.

[0114] Among them, the wavelength is the total length of the straight line connecting the head and tail of the surface corrugation; the maximum amplitude is the maximum value of the sum of the distances between each two adjacent corrugation inflection points of the surface corrugation perpendicular to the straight line connecting the head and tail, or the maximum value of the distance between the corrugation inflection point perpendicular to the straight line connecting the head and tail.

[0115] Specifically, if all the corrugation inflection points of the surface corrugation are located on any side of the line connecting the head and tail straight lines, the maximum amplitude is the maximum value of the distance between the corrugation inflection point and the line connecting the head and tail straight lines; if there is at least one corrugation inflection point and the rest of the corrugation inflection points are located on both sides of the line connecting the head and tail straight lines respectively, then the maximum amplitude is the maximum value of the sum of the distances between each two adjacent corrugation inflection points of the surface corrugation and the line connecting the head and tail straight lines respectively.

[0116] Specifically, if the surface corrugation is a discontinuous corrugation, the wavelength is the sum of the lengths of the lines connecting the end to end of several segments of the corrugation.

[0117] Specifically, under the conditions that the total thickness of the winding core does not exceed 1.5mm, the ambient temperature is 25°C, and the rotation speed of the heated winding needle 7 is 50mm / s, the general value range of the preset wavelength is [4mm, 6mm], the general value range of the preset maximum amplitude is [0.1mm, 0.4mm], the preferred embodiment of the preset wavelength is 5mm, and the preferred embodiment of the preset maximum amplitude is 0.3mm.

[0118] Those skilled in the art will understand that the optional range of the preset wavelength and the preset maximum amplitude provided in this embodiment and the preferred embodiment are the values ​​selected under the conditions that the total thickness of the winding 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, which are the best values ​​for the technical problem solved by the technical solution of the present invention. 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.

[0119] Specifically, when the surface corrugation is the temperature-dominated corrugation, the time phase difference of the hot pressing is increased; when the surface corrugation is the stress-dominated corrugation, the friction coefficient in the generation area of ​​the surface corrugation is increased;

[0120] The time phase difference is positively correlated with the wavelength, and the friction coefficient is positively correlated with the maximum amplitude.

[0121] In practice, the relationship between the time phase difference and the wavelength is: time phase difference = (wavelength - preset wavelength) × 10 / 2 × winding needle speed. For example, if the wavelength is 7mm, then the time phase difference = (7mm-5mm) × 10 / 2 × 50mm / s = 0.2s.

[0122] Specifically, the friction coefficient is increased by increasing the heat released by the heating winding needle 7 in the area where the surface corrugations are generated, thereby increasing the friction coefficient of the winding core. The principle of the increase is based on the friction heat generation formula.

[0123] In practice, if the difference between the amplitude and the preset amplitude exceeds 0.1 mm, the friction coefficient of the core increases by 0.02. For example, if 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.

[0124] In implementation, the method of the present invention determines the type of surface ripples and then determines the corresponding adjustment method. Since the ambient temperature of the core close to the hot press is high, a temperature difference is formed with the side away from the hot press, or the core and the winding needle are pressed against each other during the winding process, the winding action is jammed due to fatigue of the winding needle rotation during the winding process, resulting in uneven mechanical stress distribution, thereby showing surface ripples on the core due to temperature difference or uneven stress; by increasing the time phase difference of hot pressing to offset the temperature gradient in temperature-dominated ripples, the thermal field peaks of the two layers are staggered in space, thermal strain offset is achieved, hot pressing area overlap is avoided, and expansion differences caused by temperature gradients are reduced; by increasing the friction coefficient in the generation area of ​​surface ripples in stress-dominated ripples, mechanical stress is converted into thermal energy diffusion, and the area of ​​stress concentration area is reduced, thereby improving the structural compactness and stability of the battery cell.

[0125] Specifically, step S4 includes:

[0126] Obtaining the number of inflection points of the surface ripples;

[0127] comparing the number of inflection points with a preset number;

[0128] If the number of inflection points is greater than or equal to a preset number, the heat dissipation method of the heating coil needle 7 is adjusted from a heat dissipation method in which the outer circumferential surface of the heating coil needle 7 is formed and the heating is stopped and then left to stand, to a heat dissipation method in which the outer circumferential surface of the heating coil needle 7 is symmetrically converged on both sides in the direction of the surface ripple generation area.

[0129] Specifically, the symmetrical double-sided convergence type deheating method is that the position on the outer circumference of the heating coil 7 that is centrally symmetrical with the center line of the surface ripple generation area is the starting position of deheating, and the heating coil 7 is heated from the starting position of deheating toward the center line of the surface ripple generation area according to the convergence speed. The heating is gradually stopped at each position until the center line of the surface ripple generation area stops heating, and the deheating of the heating coil 7 is completed.

[0130] Specifically, the end time of heat dissipation is when the surface temperature of the inner lithium iron phosphate battery cell is equal to the ambient temperature.

[0131] 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 convergence speed of the symmetrical double-sided convergence type heat removal is [10 mm / s, 30 mm / s], and the preferred embodiment of the convergence speed of the symmetrical double-sided convergence type heat removal is 20 mm / s.

[0132] Specifically, under the conditions that the total thickness of the winding 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, the general value range of the preset number is [2, 5], and the preferred embodiment of the preset number is 4.

[0133] 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 ​​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 winding needle 7 is 50 mm / s, which are the best values ​​for the technical problem solved by the technical solution of the present invention. In actual applications or experiments, those skilled in the art can adaptively adjust the preset number according to the actual application environment and application scenarios.

[0134] Specifically, the time phase difference of the hot pressing is the difference between the start heating time of the hot pressing of the current core and the start heating time of the hot pressing of the current core.

[0135] Specifically, the preset second number is an integer multiple of the preset first number.

[0136] Specifically, under the conditions that the negative electrode sheet material is graphite, the positive electrode sheet material is NCM111, and the first separator and the second separator material are polyethylene, a preferred embodiment of the integer multiple is 3 times.

[0137] In practice, the method of the present invention uses a convergent heat dissipation method. Due to uniform heat dissipation, the residual stress is released unevenly, and when there are many inflection points, interlayer dislocation is caused. By adjusting the heat dissipation method to stop heating in a symmetrical convergent manner on both sides of the outer circumference of the heating coil 7 toward the direction of the surface ripple generation area, the inner layer of the lithium iron phosphate battery cell, which is still in a softened state, gradually releases stress toward the stress concentration area, and reserves deformation space for the area that deforms slowly due to stress concentration, thereby effectively avoiding interlayer dislocation caused by stress concentration, and then reducing the residual stress caused by temperature difference, thereby improving the stability of the battery cell structure.

[0138] Thus far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art may make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will fall within the scope of protection of the present invention.

Claims

1. A digital multi-layer winding method for lithium iron phosphate batteries, characterized in that: include: Step S1: The negative electrode sheet, the second separator, the positive electrode sheet, and the first separator are distributed in a direction from the outer circumference of the heated winding needle to the direction away from the outer circumference of the heated winding needle to form a single winding core, and the hot press is controlled to hot-press the single winding core to form a single-layer lithium iron phosphate battery cell; Step S2, repeating the first preset number of steps S1 on the basis of the single-layer lithium iron phosphate battery cell 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 during the previous step S1; the hot pressing pressure and hot pressing temperature of the hot pressing are respectively greater than the hot pressing pressure and hot pressing temperature during the previous step S1; and the time phase difference of the hot pressing between each core and the next adjacent core or the friction coefficient of the core is determined based on the wavelength and maximum amplitude of the surface ripples of the core before hot pressing; Step S3, adjusting the heat dissipation mode of the heating winding needle after forming the inner layer lithium iron phosphate battery cell to a convergent heat dissipation mode according to the number of inflection points of the surface corrugations of each layer of the winding core; Step S4, withdrawing the hot press and controlling the heating coil needles to remove heat according to a corresponding heat removal method; Step S5, winding a plurality of the winding cores onto the surface of the inner layer lithium iron phosphate battery cell according to a preset second number to form an initial lithium iron phosphate battery cell; Step S6, post-processing the initial lithium iron phosphate battery cell to form a finished lithium iron phosphate battery cell; The process of determining the time phase difference of hot pressing between each core and its adjacent next core or the friction coefficient of the core based on the wavelength and maximum amplitude of the surface corrugation of the core before hot pressing includes: respectively obtaining the wavelength and maximum amplitude of the surface ripples; 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 the type of the surface ripple is determined to be a temperature-dominated ripple; If the wavelength is smaller than the preset wavelength and the maximum amplitude is larger than the preset maximum amplitude, it is determined that the type of the surface corrugation is a stress-dominated corrugation. The wavelength is the total length of the line connecting the head and tail of the surface corrugation; the maximum amplitude is the maximum value of the sum of the distances between each two adjacent corrugation inflection points of the surface corrugation and the line connecting the head and tail, or the maximum value of the distances between the corrugation inflection points and the line connecting the head and tail; When the surface corrugations are the temperature-dominated corrugations, the time phase difference of the hot pressing is increased; when the surface corrugations are the stress-dominated corrugations, the friction coefficient in the generating area of ​​the surface corrugations is increased; Wherein, the time phase difference is positively correlated with the wavelength, and the friction coefficient is positively correlated with the maximum amplitude; The time phase difference of the hot pressing is the difference between the starting heating time of the hot pressing of the next layer of core and the starting heating time of the hot pressing of the current core.

2. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 1, characterized in that: The calculation formula of the winding tension is: ΔA i = Yes 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 winding core; The first wound material is the negative electrode sheet, the second wound material is the second separator, the third wound material is the positive electrode sheet, and the fourth wound material is the first separator.

3. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 2, characterized in that: 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 diaphragm, and the variable tension coefficient of the first diaphragm is equal to the variable tension coefficient of the second diaphragm.

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 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 diaphragm, is the thickness of the second diaphragm, is the thickness of the positive electrode sheet, is the thickness of the negative electrode sheet, S is the pressure resistance coefficient of the winding core, and E is the elastic modulus of the material of the winding core.

5. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 4, characterized in that: The calculation formula of the hot pressing temperature is: T=(W×q×( + + + )) / (K×A)+ ; Wherein, T is the pressing temperature, W is the output power of the hot press, K is the thermal conductivity of the core, and A is the surface area of ​​contact between the hot press and the core. is the ambient temperature.

6. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 5, characterized in that: The step S3 includes: Obtaining the number of inflection points of the surface ripples; comparing the number of inflection points with a preset number; If the number of inflection points is greater than or equal to a preset number, the heat dissipation method of the heating coil needle is adjusted from a heat dissipation method in which the outer circumferential surface of the heating coil needle is formed and the heating is stopped and then left to stand, to a heat dissipation method in which the outer circumferential surface of the heating coil needle is symmetrically converged on both sides in the direction of the surface ripple generation area.

7. The digital multi-layer winding method for lithium iron phosphate battery cells according to claim 1, characterized in that: The preset second number is an integer multiple of the preset first number.

Images