An electrode forming method

Abstract

The present invention discloses an electrode forming method, comprising: (1) feeding 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; 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 temperature t1 of the outer surface of the electrode after electromagnetic field heating and the temperature t2 of the current collector after electromagnetic field heating, so as to form a temperature gradient Δt from the inside to the outside; (2) within 5 s after the temperature gradient Δt is generated in the electrode, rolling the electrode to reach a preset roll gap h1, then removing the rolling and cooling the electrode to room temperature to obtain the formed electrode. The present invention constructs formula (I), so that the magnetic field strength and frequency of the electromagnetic field can be adjusted according to R and formula (I) to control the temperature gradient, reduce rebound, optimize the pore structure, facilitate the infiltration of the electrolyte and the insertion of lithium ions, and improve the fast charging ability while ensuring a high volumetric energy density.

Description

Technical Field

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

[0002] Lithium-ion batteries have been widely used in many fields such as mobile phones, computers, and new energy vehicles. People's demand for battery performance is getting higher and higher, especially in terms of energy density, fast charging ability, high-power discharging ability, etc. In order to meet these demands, lithium-ion battery designers are constantly exploring new technologies and methods.

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

[0004] Currently, a variety of solutions have been developed for the commonly used rolling technology in lithium-ion battery manufacturing, aiming to balance the contradiction between high energy density and fast charging ability, but all have obvious limitations. The non-heated rolling method is inexpensive, but it is prone to electrode material damage and rebound problems, limiting the improvement of volumetric energy density. The multiple rolling method attempts to avoid material damage by reducing the pressure of a single rolling, but this method increases the manufacturing cost and time, and the effect of reducing rebound is limited, still facing the problems of insufficient fast charging ability and low volumetric energy density.

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

[0006] In order to overcome the defects of the hot rolling technology, some studies have tried the method of preheating the electrode material before rolling. This method avoids the complexity of digging 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 basis before entering the rolling area, how to precisely control the heating process to construct an ideal electrode structure to improve the volumetric energy density remains the focus and difficulty of current research in the field of lithium-ion battery manufacturing. Summary of the Invention

[0007] The object of the present invention is to provide an electrode forming method, aiming to overcome the contradiction between high energy density and fast charging ability in the manufacturing process of existing lithium-ion battery electrodes. Especially when improving the volumetric energy density through the rolling process, it is necessary to avoid the reduction of fast charging ability caused by the decrease of electrode porosity. The present invention constructs an electrode with an ideal structure by introducing electromagnetic field heating and precisely controlling the heating process, so as to ensure high energy density while not reducing or even improving the charging ability and power of the battery.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] An electrode forming method, comprising the following steps:

[0010] (a) Feed the electrode into the electromagnetic field. The electrode includes a current collector and active material layers on both sides of the current collector; adjust the magnetic field strength and frequency of the electromagnetic field according to the preset electrode rebound target value R and formula (I) to adjust the temperature t1 of the outer surface of the electrode after electromagnetic field heating and the temperature t2 of the current collector after electromagnetic field heating, so as to form a temperature gradient Δt from the inside to the outside.

[0011] (I)

[0012] 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 temperature of the outer surface of the electrode after electromagnetic field heating, t2 is the temperature of the current collector after electromagnetic field heating, and t0 is any temperature value within the range of 10 - 30 °C;

[0013] (2) Within 5 s after the electrode generates the temperature gradient Δt, roll the electrode to reach the preset roll gap h1, then remove the rolling and let the electrode cool to room temperature to obtain the formed electrode.

[0014] When the electrode is heated by the electromagnetic field, 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, a conductive agent, and an electrode binder. The electrode binder is used to bond the active material particles together and maintain their stability in the electrode structure.

[0015] Among them, the measurement method of the electrode rebound rate r is: roll the electrode at temperature t0 to reach the preset roll gap h1, then take out the electrode and place it at temperature t0 until the electrode thickness is stable, and then measure the electrode thickness at this time as h2. The electrode rebound rate r = (h2 - h1) / h1.

[0016] The thermal deformation rate of the electrode binder is tested according to GB / T 1634.1 - 2019.

[0017] An electromagnetic induction heating control system is used to provide an electromagnetic field, and a roll press is used to provide rolling pressure. Among them, the electromagnetic induction heating control system is arranged at the front end of the roll press.

[0018] Furthermore, the electrode is fed into and out of the electromagnetic field and the roll press through a winding mechanism.

[0019] The preset roll gap h1 during rolling is controlled by a thickness control system.

[0020] The electromagnetic induction heating control system includes a shielding cover, and the shielding cover functions as electromagnetic shielding to prevent the electromagnetic field from heating the roll press.

[0021] The pressure during rolling is controlled by a roll pressure control system.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] Through a large amount of experimental data, the present invention constructs a formula (I) for the relationship between the preset electrode bounce rate R, the electrode bounce rate r, the thermal deformation rate k t , the thermal deformation rate K at temperature t0, and the temperature gradient Δt. Thus, those skilled in the art can adjust the magnetic field intensity and frequency of the electromagnetic field according to the electrode bounce target value R and formula (I) to control the temperature gradient, reduce bounce, and at the same time optimize the pore structure to facilitate the infiltration of the electrolyte and the insertion of lithium ions, thereby improving the fast charging ability while ensuring a high volumetric energy density. This precise control ensures the idealization of the electrode structure and improves the stability and consistency of electrode forming.

[0024] 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 requirements of different types and specifications of electrodes, enhancing the flexibility and adaptability of the production line. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of an electromagnetic induction heating and rolling device.

[0026] Figure 2 It is a thermal deformation rate curve of the positive electrode binder at different temperatures.

[0027] Figure 3 It is a SEM diagram of the actual particle distribution after rolling in Example 1 and Comparative Example 1 (magnification 10000). DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] To enable those skilled in the art to better understand the technical solutions in this application, the following will further illustrate the present invention in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all of them. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of this application.

[0029] In the relevant descriptions of parts and proportions in this specification, unless otherwise specified, they are all by weight.

[0030] As Figure 1 shown, an electromagnetic induction heating and rolling equipment, in the direction of the movement of the coil material, successively includes an unwinding device 31, an electromagnetic induction heating control system 1, a rolling press 2, and a winding device 32. Among them, the unwinding device 31 and the winding device 32 together form a winding and unwinding mechanism 3. The rolling press 2 includes two relatively arranged rollers, and the electrode passes through the middle of the two rollers. By setting the distance between the two rollers, the size of the roll gap is set.

[0031] After the coating and baking processes in the manufacture of traditional liquid lithium-ion batteries, the baked electrode 4 is sent into 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 this magnetic field, eddy currents and hysteresis losses will be generated inside the current collector (such as copper foil or aluminum foil and other metals) in the middle of the electrode, thereby realizing the heating of the electrode. However, this heat 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 the predetermined temperature instantly (in seconds), and then the heat diffuses from the current collector to the active material layers on both sides, thereby forming a temperature gradient from the middle layer to the surface layer, so as to achieve the purpose of heating the entire electrode. The electromagnetic induction heating control system 1 includes a shielding cover 11, and the shielding cover 11 functions as electromagnetic shielding to avoid heating the rollers by the electromagnetic field. Non-contact temperature measuring instruments (not shown in the figure) are provided on the tab foil area at the edge of the electrode and the surface of the electrode active material, which are 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 feedback the measured temperature to the electromagnetic induction heating control system to control the magnetic field strength and frequency of the electromagnetic field, so as to achieve precise control of the temperature gradient.

[0032] The electromagnetic induction heating control system is arranged at the front end of the roller. After the electrode is heated, it immediately (within 5 s) enters the rolling process, and the thickness of the electrode is pressed to a certain thickness. This thickness is controlled by the preset roll gap. After rolling, as the mechanical force load is unloaded, heat is dissipated. After the electrode is cooled, it is wound by the winding device 32.

[0033] Next, finished batteries are obtained through processes such as strip cutting, stacking / winding, casing encapsulation, liquid injection, activation, formation, grading, degas pumping, appearance inspection, and shipment.

[0034] The electrodes used in the examples and comparative examples are as follows: In a lithium-ion battery, based on 100 parts of the positive electrode active material layer formulation, 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 for the positive electrode is aluminum foil, and the electrode is prepared by a wet method; based on 100 parts of the negative electrode active material layer formulation, the content of graphite is 95 parts, the conductive agent (conductive carbon black) is 1 part, styrene-butadiene rubber latex (SBR) is 2 parts, and sodium carboxymethyl cellulose (CMC) is 2 parts. The current collector used for the negative electrode is copper foil, and the electrode is prepared by a wet method. The same cold roll pressing process is used for the negative electrode in the examples and comparative examples. When stacking the lithium-ion battery, 30 positive electrodes and 31 negative electrodes are used in both cases.

[0035] In Examples 1-3: t0 is 25 °C; r is the electrode rebound rate measured when the positive electrode is roll pressed at 25 °C to reach the preset roll gap h1 (h1 = 150 μm), r = 4.7%; K is the thermal deformation rate of the positive electrode binder (polyvinylidene fluoride in the examples) at 25 °C, K = 0.341%; k t is the thermal deformation rate of the positive electrode binder (polyvinylidene fluoride in the examples) at different temperatures, k t The values at different temperatures are as Figure 2 shown.

[0036] Example 1

[0037] After the coating and baking processes in the manufacture of traditional liquid lithium-ion batteries, the positive electrode is heated and roll pressed using Figure 1 the described device. Specifically, through the unwinding device, the baked positive electrode is fed into the electromagnetic induction heating control system. According to the preset electrode rebound target value R = 4.2% and formula (I), the magnetic field strength and frequency of the electromagnetic field are adjusted to adjust the outer surface temperature of the electrode 25 °C (t1) and the current collector temperature 70 °C (t2) after electromagnetic field heating, thereby forming a temperature gradient Δt from the inside to the outside; within 5 s after the temperature gradient Δt is generated in the electrode, the electrode is roll pressed to reach the preset roll gap h1, obtaining the = 0.385%, and then the roll pressing is removed and the electrode is cooled to room temperature to obtain the formed positive electrode.

[0038] The formed positive electrode and negative electrode are processed through processes such as slitting, stacking / winding, casing encapsulation, liquid injection, activation, formation, grading, degassing, appearance inspection, and shipping to obtain the finished battery.

[0039] Example 2

[0040] The manufacturing process of the lithium-ion battery is basically the same as that of Example 1, except that the set R value is different, and the obtained t1 and t2 after adjustment are also different. In this example, R = 3.3%, t1 = 25 °C, t2 = 90 °C, = 0.492%.

[0041] Example 3

[0042] The manufacturing process of the lithium-ion battery is basically the same as that of Example 1, except that the set R value is different, and the obtained t1 and t2 after adjustment are also different. In this example, R = 1.8%, t1 = 25 °C, t2 = 110 °C, = 0.867%.

[0043] Comparative Example 1

[0044] The manufacturing process of the lithium-ion battery is basically the same as that of Example 1, except that in this comparative example, the positive electrode has no electromagnetic field heating process, and the positive electrode is manufactured by a cold roll pressing process. Subsequently, the positive electrode and the negative electrode are processed through processes such as slitting, stacking / winding, casing encapsulation, liquid injection, activation, formation, grading, degassing, appearance inspection, and shipping to obtain the finished battery.

[0045] Comparative Example 2

[0046] The manufacturing process of the lithium-ion battery is basically the same as that of Comparative Example 1, except that in this comparative example, the positive electrode has no electromagnetic field heating process, and the positive electrode is manufactured by a hot roll pressing process (heated by heating oil, the surface temperature of the roller is 70 °C). Subsequently, the positive electrode and the negative electrode are processed through processes such as slitting, stacking / winding, casing encapsulation, liquid injection, activation, formation, grading, degassing, appearance inspection, and shipping to obtain the finished battery.

[0047] Comparative Example 3

[0048] 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.

[0049] Comparative Example 4

[0050] 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 110 °C.

[0051] After the design of the present invention, the rebound rate of the positive electrode, the volume energy density and the charging ability of the battery are significantly optimized.

[0052] Table 1

[0053]

[0054] In Table 1, the set temperature and roll gap thickness are both designed for the positive electrode. The thickness after cooling for 12 h after rolling refers to the thickness of the positive electrode after rolling, removing the rolling effect, and cooling to room temperature for 12 h. The rebound rate refers to the percentage of the thickness of the positive electrode after cooling for 12 h after rolling relative to the roll gap thickness.

[0055] In Table 1, the test method for the charging ability is as follows: Charge at the X charging rate until the upper limit voltage is reached, then disassemble the battery in a low-humidity environment and observe whether there is silver-white lithium on the surface of the negative electrode. If no lithium deposition is found, after discharging to the initial position, charge at the (X + 0.1) charging rate until lithium deposition is found. If lithium deposition is found at the (Y + 0.1) charging rate, it means that the battery does not have the charging ability at the current (Y + 0.1) rate, and if no lithium deposition is found at the Y charging rate, the charging ability of the battery is the Y charging rate.

[0056] As can be seen from Table 1, for the positive electrode prepared by the method of the present invention, its actual rebound rate is comparable to the rebound target value. The present invention can use formula (I) to control the temperature gradient, reduce rebound, and at the same time optimize the pore structure, so as to improve the fast charging ability while ensuring a high volume energy density.

[0057] At the same time, electrodes that have undergone cold roll pressing (Comparative Example 1) and electromagnetic induction heating roll pressing (Example 1) respectively were used to take scanning electron microscope images ( Figure 3 ) It can be seen that for the positive electrode prepared by the cold roll pressing process, the structure of the active material layer is loose ((a) in Figure 3); for the positive electrode prepared by electromagnetic induction heating roll pressing, the surface area of the active material layer is relatively loose ((b) in Figure 3), which is beneficial to the infiltration of the electrolyte and the insertion of lithium ions, and the denseness of the structure of the active material layer is significantly improved compared with cold roll pressing, 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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