Electrode forming method

Abstract

The electrode forming method comprises the steps that (1) an electrode is fed into an electromagnetic field, and the electrode comprises a current collector and active material layers located on the two sides of the current collector; adjusting the magnetic field intensity and frequency of the electromagnetic field according to a preset electrode rebound target value R and a formula (I) so as to adjust the temperature t1 of the outer surface of the electrode heated by the electromagnetic field and the temperature t2 of the current collector heated by the electromagnetic field, thereby forming a temperature gradient delta t from inside to outside; and (2) within 5 seconds after the temperature gradient delta t is generated by the electrode, the electrode is rolled so that a preset roll gap h1 can be reached, then rolling is removed, the electrode is cooled to the room temperature, and the formed electrode is obtained. The formula (I) is constructed, so that the magnetic field intensity and frequency of the electromagnetic field can be adjusted according to the R and the formula (I) to control the temperature gradient, reduce rebound and optimize the pore structure, so as to facilitate infiltration of electrolyte and embedding of lithium ions, and improve the fast charging capacity while ensuring high volume energy density.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to an electrode forming method. Background Art

[0002] Lithium-ion batteries have been widely used in mobile phones, computers, new energy vehicles and other fields. People have higher and higher demands on battery performance, especially energy density, fast charging capability, high-power discharge capability and so on. In order to meet these demands, lithium-ion battery designers are constantly exploring new technologies and methods.

[0003] In terms of improving the volume energy density of the battery, the electrode is usually compacted to a preset thickness through a rolling process to improve the volume utilization rate. However, after the electrode is compacted, the porosity of the electrode will decrease, which directly affects the wetting effect of the electrolyte and the embedding rate of lithium ions, thereby reducing the fast charging capability. Therefore, it is difficult to obtain high energy density and fast charging capability at the same time. How to ensure high energy density while not reducing or even improving charging capacity and power has become a research focus in the field of lithium-ion battery manufacturing.

[0004] At present, the roller pressing technology commonly used in the manufacture of lithium-ion batteries has developed a variety of solutions, aiming to balance the contradiction between high energy density and fast charging capability, but all have obvious limitations. Although the non-heating roller pressing method is low-cost, it is easy to cause damage to the electrode material and rebound problems, limiting the improvement of volume energy density. The multiple roller pressing method attempts to avoid material damage by reducing the pressure of a single roller pressing, but this method increases manufacturing costs and time, and the effect of reducing rebound is limited. It still faces the problems of insufficient fast charging capability and low volume energy density.

[0005] The hot rolling technology heats the electrode material by placing heating oil inside the roller to promote the plastic deformation of the material, thereby effectively reducing rebound. Due to uneven temperature distribution and significant differences in the internal and external electrode structures, the electrodes after hot rolling often appear dense on the surface and fluffy in the center, which is not conducive to the infiltration of the electrolyte and the embedding of lithium ions. In addition, the heating oil flow channel needs to be opened inside the roller, and long-term use may cause roller deformation and oil leakage, increasing the difficulty and cost of equipment maintenance.

[0006] In order to overcome the defects of hot rolling technology, some studies have tried to preheat the electrode material before rolling. This method avoids the complexity of drilling channels inside the roller, but there are still key defects in the control of the heating process. Although preheating ensures that the electrode has a certain temperature base before entering the rolling area, how to accurately control the heating process to construct an ideal electrode structure to improve the volume energy density is still the focus and difficulty of current research in the field of lithium-ion battery manufacturing. Summary of the invention

[0007] The purpose of the present invention is to provide an electrode forming method, which aims to overcome the contradiction between high energy density and fast charging capability in the existing lithium-ion battery electrode manufacturing process, especially when the volume energy density is increased by the rolling process, to avoid the reduction of fast charging capability caused by the decrease of electrode porosity. The present invention constructs an electrode with an ideal structure by introducing electromagnetic field heating and accurately controlling the heating process, so as to ensure high energy density while not reducing or even improving the charging capacity and power of the battery.

[0008] To achieve the above object, the present invention adopts the following technical solutions: An electrode forming method comprises the following steps: (a) placing an electrode into an electromagnetic field, the electrode comprising a current collector and active material layers on both sides of the current collector; adjusting the magnetic field strength and frequency of the electromagnetic field according to a preset electrode rebound target value R and formula (I) to adjust the outer surface temperature t1 of the electrode after electromagnetic field heating and the current collector temperature t2 after electromagnetic field heating, thereby forming a temperature gradient Δt from inside to outside; (I) Where r is the electrode rebound rate measured when the electrode is rolled at temperature t0 to reach the preset roll gap h1, k t is the thermal deformation rate of the electrode binder at different temperatures, K is the thermal deformation rate of the electrode binder at temperature t0, Δt = t2-t1, t1 is the outer surface temperature of the electrode after electromagnetic field heating, t2 is the collector temperature after electromagnetic field heating, and t0 is any temperature value in the range of 10~30℃; (2) Within 5 seconds after the electrode generates a temperature gradient Δt, the electrode is rolled to reach a preset roll gap h1, and then the rolling is removed and the electrode is allowed to cool to room temperature to obtain a formed electrode.

[0009] When the electrode is heated by the electromagnetic field, the heat diffuses from the current collector to the active material layers on both sides, forming a temperature gradient from the inside to the outside. The active material layer includes active material particles, conductive agents and electrode binders. The electrode binders are used to bind the active material particles together and maintain their stability in the electrode structure.

[0010] The electrode rebound rate r is determined by rolling the electrode at a temperature of t0 to reach a preset rolling gap h1, then taking out the electrode and placing it at a temperature of t0 until the electrode thickness stabilizes, and then determining the electrode thickness at this time as h2, and the electrode rebound rate r = (h2-h1) / h1.

[0011] GB / T 1634.1-2019 is used to test the thermal deformation rate of electrode binder.

[0012] An electromagnetic induction heating control system is used to provide an electromagnetic field, and a roller press is used to provide roller pressing, wherein the electromagnetic induction heating control system is arranged at the front end of the roller press.

[0013] Furthermore, the electrode is fed into and taken out of the electromagnetic field and the roller press by a reeling and unreeling mechanism.

[0014] The preset roll gap h1 during rolling is controlled by a thickness control system.

[0015] The electromagnetic induction heating control system includes a shielding cover, which plays an electromagnetic shielding function to prevent the electromagnetic field from heating the roller press.

[0016] The pressure during rolling is controlled by the roller pressure control system.

[0017] Compared with the prior art, the present invention has the following advantages: The present invention constructs a preset electrode rebound rate R, electrode rebound rate r, and thermal deformation rate k through a large amount of experimental data. t The formula (I) for the relationship between the thermal deformation rate K at temperature t0 and the temperature gradient Δt allows those skilled in the art to adjust the magnetic field strength and frequency of the electromagnetic field according to the electrode rebound target value R and formula (I) to control the temperature gradient, reduce the rebound, and optimize the pore structure to facilitate the infiltration of the electrolyte and the embedding of lithium ions, thereby improving the fast charging capability while ensuring high volume energy density. This precise control ensures the idealization of the electrode structure and improves the stability and consistency of electrode forming.

[0018] The electrode forming method of the present invention has simple steps, is easy to realize automated production, and improves manufacturing efficiency. In addition, by adjusting the parameters of the electromagnetic field, it can flexibly adapt to the manufacturing needs of electrodes of different types and specifications, thereby enhancing the flexibility and adaptability of the production line. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic diagram of an electromagnetic induction heating roller pressing device.

[0020] Figure 2 This is the thermal deformation rate curve of the positive electrode binder at different temperatures.

[0021] Figure 3 The SEM images of the actual distribution of particles after rolling of Example 1 and Comparative Example 1 (magnification 10000). DETAILED DESCRIPTION

[0022] In order to enable those skilled in the art to better understand the technical solutions in this application, the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only part of the embodiments of this application, rather than all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of this application.

[0023] All descriptions of parts and proportions in this specification are by weight unless otherwise specified.

[0024] like Figure 1 As shown, an electromagnetic induction heating rolling device includes an unwinding device 31, an electromagnetic induction heating control system 1, a rolling machine 2 and a winding device 32 in sequence according to the direction of the coil movement. The unwinding device 31 and the winding device 32 together constitute the unwinding and winding mechanism 3. The rolling machine 2 includes two rollers arranged opposite to each other, and the electrode passes through the middle of the two rollers. The size of the roller gap is set by setting the distance between the two rollers.

[0025] After the coating and baking process of the traditional liquid lithium-ion battery manufacturing, the baked electrode 4 is sent to the electromagnetic induction heating control system 1 through the unwinding device 31. The electromagnetic induction heating control system generates an alternating magnetic field. When the electrode passes through the magnetic field, eddy currents and hysteresis losses will be generated inside the current collector (metal such as copper foil or aluminum foil) in the middle of the electrode, thereby heating the electrode. However, the thermal field first occurs in the current collector located in the middle layer. Due to the characteristics of electromagnetic induction heating, the current collector can reach a predetermined temperature in an instant (seconds), and then the heat diffuses from the current collector to the active material layer on both sides, thereby forming a temperature gradient from the middle layer to the surface layer, thereby achieving the purpose of heating the entire electrode. The electromagnetic induction heating control system 1 includes a shielding cover 11, which plays an electromagnetic shielding function to prevent the electromagnetic field from heating the roller. A non-contact thermometer (not shown in the figure) is provided in the pole ear foil area at the edge of the electrode and the surface of the electrode active material, which is used to accurately measure the temperature of the current collector in the electrode and the temperature of the surface of the electrode active material, and the measured temperature is fed back to the electromagnetic induction heating control system to control the magnetic field strength and frequency of the electromagnetic field, thereby achieving accurate control of the temperature gradient.

[0026] The electromagnetic induction heating control system is arranged at the front end of the roller. After the electrode is heated, it enters the roller pressing immediately (within 5 seconds) to press the electrode to a certain thickness. The thickness is controlled by the preset roller gap. After rolling, the heat is dissipated as the mechanical force load is unloaded. After the electrode is cooled, it is rolled up by the winding device 32.

[0027] Next, the finished batteries are obtained through the processes of slitting, stacking / winding, shelling and packaging, liquid injection, activation, formation, capacity division, degassing, appearance inspection and shipment.

[0028] The electrodes used in the embodiments and comparative examples are as follows: in the lithium-ion battery, the positive electrode active material layer formula is 100 parts, of which the content of lithium iron phosphate (LFP) is 93 parts, the conductive agent (conductive carbon black) is 3 parts, and the electrode binder (polyvinylidene fluoride) is 4 parts. The current collector used in the positive electrode is aluminum foil, and the electrode is prepared by wet method; the negative electrode active material layer formula is 100 parts, of which 95 parts of graphite, 1 part of conductive agent (conductive carbon black), 2 parts of styrene-butadiene rubber latex (SBR), 2 parts of sodium carboxymethyl cellulose (CMC), the current collector used in the negative electrode is copper foil, and the electrode is prepared by wet method. The embodiments and comparative examples use the same cold rolling process to roll the negative electrode, and 30 positive electrodes and 31 negative electrodes are used when stacking the lithium-ion battery.

[0029] In Examples 1-3: t0 is 25°C; r is the electrode rebound rate measured when the positive electrode is rolled at 25°C to reach the preset roller gap h1 (h1=150μm), r=4.7%; K is the thermal deformation rate of the positive electrode binder (polyvinylidene fluoride in the example) at 25°C, K=0.341%; k t is the thermal deformation rate of the positive electrode binder (polyvinylidene fluoride in the embodiment) at different temperatures, k t The values ​​at different temperatures are Figure 2 shown.

[0030] Example 1 After the coating and baking process in traditional liquid lithium-ion battery manufacturing, Figure 1 The device performs positive electrode heating and rolling, specifically: the baked positive electrode is sent into the electromagnetic induction heating control system through the unwinding device, and the magnetic field strength and frequency of the electromagnetic field are adjusted according to the preset electrode rebound target value R=4.2% and formula (I) to adjust the electrode outer surface temperature 25°C (t1) after electromagnetic field heating and the collector temperature 70°C (t2) after electromagnetic field heating, thereby forming a temperature gradient Δt from inside to outside; within 5s after the electrode generates the temperature gradient Δt, the electrode is rolled to reach the preset roller gap h1, and the temperature in the temperature range is obtained. =0.385%, then remove the roller pressing and let the electrode cool to room temperature to obtain the formed positive electrode.

[0031] The formed positive and negative electrodes are subjected to processes such as striping, lamination / winding, shell packaging, liquid injection, activation, formation, capacity division, degassing, appearance inspection, and shipment to obtain finished batteries.

[0032] Example 2 The manufacturing process of lithium-ion battery is basically the same as that of Example 1, except that the set R value is different, and the t1 and t2 obtained after adjustment are also different. In this example, R=3.3%, t1=25℃, t2=90℃, =0.492%.

[0033] Example 3 The manufacturing process of lithium-ion battery is basically the same as that of Example 1, except that the set R value is different, and the t1 and t2 obtained after adjustment are also different. In this example, R=1.8%, t1=25℃, t2=110℃, =0.867%.

[0034] Comparative Example 1 The manufacturing process of the lithium-ion battery is basically the same as that of Example 1, except that the positive electrode of this comparative example does not have an electromagnetic field heating process, and the positive electrode is manufactured by a cold rolling process. Subsequently, the positive electrode and the negative electrode are subjected to the processes of striping, lamination / winding, shell packaging, liquid injection, activation, formation, capacity division, degassing, appearance inspection, and shipment to obtain the finished battery.

[0035] Comparative Example 2 The manufacturing process of the lithium-ion battery is basically the same as that of Comparative Example 1, except that the positive electrode of this comparative example does not have an electromagnetic field heating process, and the positive electrode is manufactured by a hot roller pressing process (heating oil heating, roller surface temperature 70°C), and the positive electrode and the negative electrode are subsequently manufactured by striping, lamination / winding, shell packaging, liquid injection, activation, formation, capacity division, degassing, appearance inspection, and shipment to obtain finished batteries.

[0036] Comparative Example 3 The manufacturing process of the lithium-ion battery is basically the same as that of Comparative Example 2, except that the surface temperature of the roller is 90°C.

[0037] Comparative Example 4 The lithium-ion battery manufacturing process is basically the same as that of Comparative Example 2, except that the roller surface temperature is 110°C.

[0038] After the invention and design, the positive electrode rebound rate, the volume energy density and charging capacity of the battery are significantly optimized.

[0039] Table 1

[0040] In Table 1, the set temperature and roll gap thickness are designed for the positive electrode. The thickness after cooling for 12 hours after rolling refers to the thickness of the positive electrode after the rolling is removed and cooled to room temperature for 12 hours after rolling. The rebound rate refers to the rebound percentage of the thickness of the positive electrode after cooling for 12 hours after rolling relative to the roll gap thickness.

[0041] In Table 1, the test method for charging capacity is as follows: charge at a charge rate of X until the upper limit voltage is reached, then disassemble the battery in a low humidity environment to observe whether there is silvery white lithium on the surface of the negative electrode. If lithium precipitation is not found, discharge to the initial position and charge at a charge rate of (X+0.1) until lithium precipitation is found. If lithium precipitation is found at a charge rate of (Y+0.1), it means that the battery does not have the current charging capacity of the (Y+0.1) rate, and if lithium precipitation is not found at the charge rate of Y, then the charging capacity of the battery is the charge rate of Y.

[0042] It can be seen from Table 1 that the actual rebound rate of the positive electrode prepared by the method of the present invention is equivalent to the rebound target value. The present invention can use formula (I) to control the temperature gradient, reduce the rebound, and optimize the pore structure, thereby improving the fast charging capability while ensuring a high volume energy density.

[0043] At the same time, scanning electron microscope images were taken of the electrodes that were respectively subjected to cold rolling (Comparative Example 1) and electromagnetic induction heating rolling (Example 1) ( Figure 3 ) It can be seen that the positive electrode prepared by cold rolling process has a loose active material layer structure (Figure 3 (a)); the positive electrode prepared by electromagnetic induction heating rolling has a relatively loose surface area of ​​the active material layer (Figure 3 (b)), which is conducive to the infiltration of electrolyte and the embedding of lithium ions, and the density of the active material layer structure is significantly improved compared with cold rolling, thereby increasing the volume energy density of the battery.

Claims

1. An electrode forming method, characterized in that: The following steps are involved: (1) placing an electrode into an electromagnetic field, wherein the electrode includes a current collector and active material layers located on both sides of the current collector; According to the preset electrode rebound target value R and formula (I), the magnetic field strength and frequency of the electromagnetic field are adjusted to adjust the electrode outer surface temperature t1 after electromagnetic field heating and the collector temperature t2 after electromagnetic field heating, thereby forming a temperature gradient Δt from inside to outside; (I) Where r is the electrode rebound rate measured when the electrode is rolled at temperature t0 to reach the preset roll gap h1, k t is the thermal deformation rate of the electrode binder at different temperatures, K is the thermal deformation rate of the electrode binder at temperature t0, Δt = t2-t1, t1 is the outer surface temperature of the electrode after electromagnetic field heating, t2 is the collector temperature after electromagnetic field heating, and t0 is any temperature value in the range of 10~30℃; (2) Within 5 seconds after the electrode generates a temperature gradient Δt, the electrode is rolled to reach a preset roll gap h1, and then the rolling is removed and the electrode is allowed to cool to room temperature to obtain a formed electrode.

2. The electrode forming method according to claim 1, characterized in that: The electrode rebound rate r is determined as follows: the electrode is rolled at a temperature of t0 to reach a preset rolling gap h1, and then the electrode is taken out and placed at a temperature of t0 until the electrode thickness stabilizes, and the electrode thickness at this time is determined as h2, and the electrode rebound rate r = (h2-h1) / h1.

3. The electrode forming method according to claim 1, characterized in that: An electromagnetic induction heating control system is used to provide an electromagnetic field, and a roller press is used to provide roller pressing, wherein the electromagnetic induction heating control system is arranged at the front end of the roller press.

4. The electrode forming method according to claim 3, characterized in that: The electrode is fed into and out of the electromagnetic field and the roller press by the reeling and unreeling mechanism; the preset roller gap h1 during roller pressing is controlled by the thickness control system; and the pressure during roller pressing is controlled by the roller pressure control system.

5. The electrode forming method according to claim 3, characterized in that: The electromagnetic induction heating control system includes a shielding cover.

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