Positive electrode sheet and design method thereof, secondary battery, battery module, battery pack and power-consuming device

Abstract

The present application provides a positive electrode sheet and a design method thereof, a secondary battery, a battery module, a battery pack and an electric device. The positive electrode sheet for a secondary battery comprises: a positive electrode current collector; and a positive electrode film layer coated on two surfaces opposite to each other in the thickness direction of the positive electrode current collector and comprising a positive electrode active material, wherein the two surfaces of the positive electrode film layer have different charge and discharge rates.

Description

Technical Field

[0001] The present invention relates to the field of batteries, and more specifically, to a positive electrode plate and a design method thereof, a secondary battery, a battery module, a battery pack and an electrical device. Background Art

[0002] Energy conservation and emission reduction are the key to the sustainable development of the automobile industry. Electric vehicles have become an important part of the sustainable development of the automobile industry due to their advantages in energy conservation and environmental protection. For electric vehicles, battery technology is an important factor in their development.

[0003] During the use of batteries, they will be continuously consumed due to side reactions such as the formation of SEI film, which will affect the battery life and may bring safety hazards. Summary of the invention

[0004] In view of the above problems, the present application provides a positive electrode plate and its design method, a secondary battery, a battery module, a battery pack and an electrical device, which can compensate for the SEI film formation loss during battery use and extend the battery life.

[0005] According to the first aspect of the present application, a positive electrode plate is provided, comprising a positive electrode current collector, and a positive electrode film layer coated on two surfaces opposite to each other in the thickness direction of the positive electrode current collector and comprising a positive electrode active material, wherein the two surfaces of the positive electrode film layer have different charge and discharge rates. By providing a positive electrode plate with surfaces having different charge and discharge rates, electricity can be hidden in the two surfaces of the plate, so that no external lithium source is required, and the hidden electricity can be slowly released in the later stage, which is more effective in extending the battery life, and is easy to manufacture and has high stability.

[0006] According to the positive electrode sheet of the above embodiment, the charge-discharge rate ratio of the two surfaces of the positive electrode film layer is 0.4-2.5; optionally, the charge-discharge rate ratio is 0.8-1.25. The difference in charge-discharge rate provides the electrode sheet with the ability to store electricity, and the appropriate rate can further prevent the surface current difference from being too large.

[0007] According to the positive electrode sheet of any of the above embodiments, the impedance ratio of the two surfaces of the positive electrode film layer is 0.25-4; optionally, the impedance ratio is 0.7-1.4. By adjusting the impedance ratio, the power storage capacity and power storage surface can be controlled under a fixed capacity ratio, while preventing the charge and discharge rate ratio of the two surfaces from being too large.

[0008] According to the positive electrode plate of any of the above embodiments, the single-sided resistance of any one of the two surfaces of the positive electrode film layer is less than 1 ohm; optionally, less than 0.8 ohm. The two sides of the electrode plate are in a parallel circuit, so the smaller resistance can make the overall impedance of the system small enough, so that the internal resistance of the system is reduced.

[0009] According to the positive electrode sheet of any of the above embodiments, the capacity ratio of the two surfaces of the positive electrode film layer is 0.64-1.58; optionally, the capacity ratio is 0.9-1.12. A suitable capacity ratio can ensure that the surface with small capacity does not face too many anodes, so as not to affect the energy density of the battery cell.

[0010] According to the positive electrode sheet of any of the above embodiments, among the two surfaces of the positive electrode film layer, the low impedance surface has a higher charge and discharge rate than the high impedance surface. The high impedance surface thus obtains a low charge and discharge rate, so that electricity is hidden in the high impedance surface, which can meet the fast charging requirements while taking into account the life. At this time, by increasing the amount of anode corresponding to the low impedance surface, the safety of the high rate charging process can be guaranteed. At the same time, the amount of anode corresponding to the high impedance surface can be selectively reduced.

[0011] According to the positive electrode plate of any of the above embodiments, among the two surfaces of the positive electrode film layer, the high impedance surface has a higher charge and discharge rate than the low impedance surface. The low impedance surface thus obtains a low charge and discharge rate, so that electricity is hidden in the low impedance surface. At this time, because the high impedance and high rate surface has a greater polarization and is cut off early, in addition to the low impedance surface storing electricity due to the low rate, the high impedance surface can also further store electricity due to polarization. At this time, the thermal effect at the pole piece level will show a large difference on the two surfaces, and this pole piece level self-heating effect is suitable for low temperature cold start application scenarios.

[0012] According to the second aspect of the present application, a design method for a positive electrode sheet for a secondary battery is provided, the method comprising the following steps: S1: determining the designed power storage capacity of the positive electrode sheet; and S2: designing the positive electrode film layer including the positive electrode active material coated on two surfaces of the positive electrode current collector in opposite thickness directions of the positive electrode sheet to have different charge and discharge rates to achieve the designed power storage capacity. By this method, the positive electrode sheet can have the power storage capacity, thereby improving the battery life.

[0013] According to the method of the above embodiment, step S2 includes:

[0014] The charge and discharge rates of the two surfaces of the positive electrode film are designed according to the following formula:

[0015]

[0016] in,

[0017]

[0018] Among them, the capacity ratio, impedance ratio and charge-discharge rate ratio in the above formula are the ratios of the capacity, impedance and charge-discharge rate of the first surface of the two surfaces of the positive electrode film layer to the capacity, impedance and charge-discharge rate of the second surface. Through the functional relationship of the power storage capacity given in this application, the positive electrode plate can be quantitatively designed by controlling the impedance ratio and capacity ratio so that it has the desired power storage capacity.

[0019] According to the method of any of the above embodiments, step S2 further includes: first determining the capacity ratio, and then realizing the designed power storage capacity by designing the impedance ratio. Relatively speaking, the adjustable space of capacity is small, while the adjustable space of impedance ratio is large, and the adjustment means is easy to control and implement.

[0020] According to the method of any of the above embodiments, step S2 includes: designing the impedance ratio so that the low impedance surface of the two surfaces of the positive electrode film layer has a higher charge and discharge rate than the high impedance surface. As described above, this design can store electricity on the high impedance surface, which can meet the fast charging requirements while taking into account the life.

[0021] According to the method of any of the above embodiments, step S2 includes: designing the impedance ratio so that the high impedance surface of the two surfaces of the positive electrode film layer has a higher charge and discharge rate than the low impedance surface. As described above, this design can store electricity on the low impedance surface, and make the high impedance surface further store electricity due to polarization, and can bring about a self-heating effect at the electrode layer, which is suitable for low temperature cold start application scenarios.

[0022] According to the method of any of the above embodiments, designing the impedance ratio further includes designing the positive electrode sheet resistance of the two surfaces of the positive electrode film layer. The positive electrode sheet resistance plays a significant role in the positive electrode sheet impedance, so designing the positive electrode sheet resistance can easily adjust the single-sided impedance.

[0023] According to the third aspect of the present application, a secondary battery is provided, comprising the positive electrode sheet as described in any embodiment of the first aspect or the positive electrode sheet obtained by the method described in any embodiment of the second aspect. The secondary battery has the ability to store electricity and the corresponding effect of improving battery life due to the use of the positive electrode sheet of the present application.

[0024] According to a fourth aspect of the present application, a battery module is provided, which includes the secondary battery as described in the third aspect of the present application. The electric module has the ability to store electricity and the corresponding effect of improving battery life due to the use of the positive electrode sheet of the present application.

[0025] According to a fifth aspect of the present application, a battery pack is provided, which includes the battery module as described in the fourth aspect of the present application. The battery pack has the ability to store electricity and the corresponding effect of improving battery life due to the use of the positive electrode sheet of the present application.

[0026] According to the sixth aspect of the present application, there is provided an electric device, which includes at least one of the secondary battery as described in the third aspect of the present application, the battery module as described in the fourth aspect of the present application, or the battery pack as described in the fifth aspect of the present application. The electric device has the ability to store electricity and the corresponding effect of improving battery life due to the use of the positive electrode sheet of the present application.

[0027] Compared with the lithium replenishment scheme of the prior art, the present application designs the difference in charge and discharge rates on the two sides of the positive electrode plate, so that electricity is stored in the SOC difference between the two sides of the electrode plate without the need for an additional lithium replenishment source, which significantly improves the slow release in the later stage. At the same time, the present application also provides a functional relationship of the electricity storage capacity, thereby providing a method that allows battery life design.

[0028] These and other features and advantages will become apparent by reading the following detailed description and by reference to the associated drawings.It is to be understood that the foregoing general description and the following detailed description are illustrative only and are not restrictive of the aspects of what is claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to understand in detail the manner in which the above features of the present application are used, the above briefly summarized contents may be described in more detail with reference to various embodiments, some of which are shown in the accompanying drawings. However, it should be noted that the accompanying drawings only show certain typical aspects of the present application and should not be considered to limit its scope, as the description may allow for other equally effective aspects.

[0030] Figure 1 It is a schematic diagram of one embodiment of the secondary battery of the present application.

[0031] Figure 2 yes Figure 1 An exploded view of a secondary battery is shown.

[0032] Figure 3 It is a schematic diagram of an embodiment of a battery module of the present application.

[0033] Figure 4 It is a schematic diagram of an embodiment of the battery pack of the present application.

[0034] Figure 5 yes Figure 4 Exploded view of the battery pack shown.

[0035] Figure 6 This is a schematic diagram of an embodiment of an electric device using the secondary battery of the present application as a power source.

[0036] Figure 7 It is a schematic diagram of the electricity storage principle of the present application.

[0037] Figure 8It is a schematic flow chart of a design method of a positive electrode sheet for a secondary battery according to one embodiment of the present application.

[0038] Fig. 9 It is a schematic diagram showing the design of the power storage capacity by adjusting the impedance ratio after the capacity ratio is fixed.

[0039] Fig.10 It is a schematic diagram showing that the charge and discharge rate ratio is controlled by adjusting the impedance ratio after the capacity ratio is fixed.

[0040] Fig.11 Schematic diagram showing the linear relationship between the membrane resistance and the designed capacity of the lithium iron phosphate material.

[0041] The reference numerals in the specific implementation manner are as follows:

[0042] 1. Battery pack;

[0043] 2. Upper box;

[0044] 3. Lower box;

[0045] 4. Battery module;

[0046] 5. Secondary battery;

[0047] 51. Shell;

[0048] 52. Electrode assembly;

[0049] 53. Cover plate; DETAILED DESCRIPTION

[0050] The present application is described in detail below in conjunction with the accompanying drawings, and the features of the present application will be further revealed in the following specific description.

[0051] "Scope" disclosed herein is defined in the form of lower limit and upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a particular range. The scope defined in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a particular parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0052] In this application, unless otherwise specified, all technical features and preferred features mentioned herein can be combined with each other to form a new technical solution.

[0053] In the present application, unless otherwise specified, all steps mentioned herein may be performed sequentially or randomly, but are preferably performed sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0054] In this application, unless otherwise specified, the terms "include" and "comprising" mentioned herein may be open-ended or closed-ended. For example, the terms "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0055] In the description of this article, it should be noted that, unless otherwise specified, "above" and "below" are inclusive of the number itself, and "several" in "one or several" means two or more.

[0056] In the description herein, unless otherwise specified, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0057] In view of the problem that the service life of the battery is affected by SEI film formation during the use of the battery, some solutions have been proposed in the prior art to extend the service life of the battery by introducing a lithium source for later lithium replenishment. Taking lithium replenishment as an example, the existing lithium replenishment scheme is mainly to add a third-party substance (lithium replenishment additive) to the positive electrode, negative electrode, and isolation membrane through mixing in a stirring process, spraying in a coating process, or adding a pre-lithiation process. However, although the above-mentioned introduction of lithium replenishment methods can make up for the SEI film formation loss during the first cycle of charge and discharge to a certain extent, it also increases the process complexity and cost. For example, the introduction of lithium replenishment additives requires matching formulas, or even requires separate development of formulas. At the same time, external additives only work during the first charge and discharge, and cannot be slowly released later. In addition, the above method may also have many problems such as safety and stability, production process compatibility, and capacity improvement effect.

[0058] The inventors have noticed that in the traditional design and manufacturing process of secondary batteries, for the positive electrode plate, the production consistency of the two sides of the plate (for the convenience of description, any of the two sides will be referred to as side A, and the other side opposite to side A will be referred to as side B) is usually strictly controlled, that is, side A and side B have basically the same impedance and the same capacity. Correspondingly, side A and side B also have basically the same charge and discharge rate and state of charge (SOC). Therefore, during the battery charging and discharging process, side A and side B reach the cutoff SOC at basically the same time, and the lithium electrons on both sides participate in the charging and discharging, that is, there is no SOC difference between side A and side B. The inventors have found that if the two surfaces of the positive electrode plate are designed to have different charge and discharge rates, the positive electrode plate can have the ability to store electricity, that is, some lithium ions do not participate in the charging and discharging process in the early stage, and thus can be released by technical means in the later stage to supplement the source of initial lithium ion loss.

[0059] The positive electrode plate disclosed in the embodiments of the present application can be applied to secondary batteries, battery modules and battery packs containing secondary batteries, and electrical devices that use the secondary batteries, battery modules, battery packs, etc. disclosed in the present application to form a power supply system, including but not limited to vehicles, ships or aircraft, etc., which is beneficial to prolonging the battery life.

[0060] The secondary battery can be a lithium ion secondary battery, a potassium ion secondary battery, a sodium ion secondary battery, etc., and is particularly preferably a lithium ion secondary battery or a sodium ion secondary battery. For ease of explanation, the following description takes a lithium ion secondary battery as an example, but the present application is also applicable to a sodium ion secondary battery or other types of ion batteries. The secondary battery may include a positive electrode, a negative electrode, a separator, an electrolyte, etc. During the battery charge and discharge process, active ions are embedded and removed back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet.

[0061] Figure 1 The secondary battery 5 is a square structure as an example, but those skilled in the art can understand that the shape of the secondary battery of the present application can also be cylindrical or any other shape.

[0062] Figure 2 Shows Figure 1 The exploded view of the secondary battery 5 is shown in FIG. 5 . The outer package may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate. The bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity. The cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly is encapsulated in the receiving cavity, and the electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more.

[0063] In one embodiment of the present application, a plurality of secondary batteries may be assembled together to form a battery module, wherein the battery module includes two or more secondary batteries, and the specific number depends on the application of the battery module and the parameters of a single battery module.

[0064] Figure 3 4 is an example of a battery module. Figure 3 In the battery module 4, the plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0065] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0066] In one embodiment of the present application, two or more of the above-mentioned battery modules can be assembled into a battery pack, and the number of battery modules contained in the battery pack depends on the application of the battery pack and the parameters of the individual battery modules. The battery pack may include a battery box and a plurality of battery modules disposed in the battery box, and the battery box includes an upper box body and a lower box body, and the upper box body can cover the lower box body and match well with it to form a closed space for accommodating the battery modules. Two or more battery modules can be arranged in the battery box in a desired manner.

[0067] Figure 4 and Figure 5 1 is a battery pack 1 as an example. Figure 4 and Figure 5 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3, wherein the upper box body 2 is used to cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

[0068] Electrical devices

[0069] In one embodiment of the present application, the electric device of the present application includes at least one of the secondary battery, battery module, or battery pack of the present application, and the secondary battery, battery module, or battery pack can be used as a power source of the electric device, and can also be used as an energy storage unit of the electric device. The electric device includes but is not limited to mobile digital devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0070] Figure 6 The device is an electric device as an example. The device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the device's requirements for high power and high energy density, a battery pack or a battery module can be used.

[0071] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0072] According to some embodiments of the present application, a positive electrode sheet for a secondary battery is provided. The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer coated on two surfaces opposite to each other in the thickness direction of the positive electrode current collector and including a positive electrode active material. The two surfaces of the positive electrode film layer have different charge and discharge rates.

[0073] The charge and discharge rate (hereinafter referred to as rate) refers to the current value required for the battery to discharge its rated capacity within a specified time. Designing the two surfaces of the positive electrode plate to have different charge and discharge rates can enable the positive electrode plate to have the ability to store electricity. More specifically, unlike the situation where the charge and discharge rates on both sides of the positive electrode plate are consistent, when one side of the positive electrode plate has a high rate and the other side has a low rate, the high rate side reaches the cut-off SOC before the low rate side. At this time, there will be unreleased lithium ions on the low rate side. This part of the lithium ions does not participate in the charge and discharge process in the early stage, and therefore can be released by technical means in the later stage as a source of supplementing the initial lithium ion loss. Therefore, the positive electrode plate of the present application having a difference in charge and discharge rate surfaces on the two surfaces of the positive electrode film layer has the ability to store electricity. In this article, the ability to store electricity can be further understood as the proportion of unreleased lithium ions to the total amount of lithium.

[0074] Figure 7 It is a schematic diagram of the power storage principle of the present application. Assume that of the two sides AB of the positive electrode sheet, side A is the high-rate side and side B is the low-rate side. Figure 7 In the figure, the left side shows the SOC-voltage curve of the charging process of the A side, and the right side shows the SOC-voltage curve of the charging process of the B side. Figure 7 As shown in the middle left figure, the process of charging side A from SOC 0 to SOC 1, the corresponding voltage changes from U start (Starting voltage) to U cut (cut-off voltage). Similarly, Figure 7 The right figure in the figure shows the process of charging side B from SOC 0 to SOC 1. For comparison, the curve of side A is represented by a solid line, and the curve of side B is represented by a dotted line, and they are marked in the curve graph on the other side. It can be seen that since the charge and discharge rate of side B is lower than that of side A, when side A is fully charged (i.e. SOC is 1 and voltage is U cut ), the B side is not fully charged. When the B side is fully charged (i.e., the SOC is 1), the voltage will be higher than U cut , reaching η+U cut Therefore, when the AB surfaces are the front and back sides of the positive electrode, the start and end of SOC will be based on the high-rate surface, so there will always be a part of lithium ions in the low-rate surface that do not participate in charging and discharging, and are therefore hidden in the electrode.

[0075] By setting the two surfaces of the positive electrode film layer of the positive electrode plate to have different charge and discharge rates, some lithium ions can be hidden in the low-rate surface of the two surfaces of the plate, so there is no need for an external lithium supplement source. In this way, the electricity hidden can be slowly released in the later stage, which is more effective in extending the battery life. In addition, this lithium supplement scheme is easy to implement and has high stability.

[0076] According to some embodiments of the present application, optionally, the charge-discharge rate ratio of the two surfaces of the positive electrode film layer may be 0.4-2.5. Further optionally, the charge-discharge rate ratio of the two surfaces of the positive electrode film layer may be 0.8-1.25.

[0077] The charge-discharge rate ratio is the ratio of the charge-discharge rate of one side of the two surfaces of the positive electrode film layer to the charge-discharge rate of the other side. After in-depth research by the inventors, the charge storage capacity of the positive electrode plate is related to the charge-discharge rate ratio of the two surfaces of the positive electrode film layer of the positive electrode plate, and the charge-discharge rate itself is related to the impedance and capacity of the positive electrode film layer. More specifically, under the equal voltage of the parallel circuit, the current will be autonomously divided according to the impedance of the branch, that is:

[0078]

[0079] Therefore, in a parallel circuit, the greater the impedance of the branch, the smaller the shunt. Therefore, when the external circuit is charged with a constant current, the current allocated to the high impedance side will be less than the current allocated to the low impedance side. At the same time,

[0080]

[0081] From this, we can further deduce:

[0082]

[0083] Based on the above derivation, since both sides of the positive electrode are under the same voltage in the parallel circuit, the single-sided rate of the two surfaces is inversely proportional to the product of impedance and single-sided capacity, or proportional to the inverse of the product of impedance and single-sided capacity, that is:

[0084]

[0085] In other words, the product of impedance and capacitance is the key factor affecting the actual rate of single-sided.

[0086] Therefore, a positive electrode with a charge-discharge rate ratio can be obtained by controlling the difference in impedance and capacity on both sides of the positive electrode, and on this basis, various needs such as battery life, capacity, charging speed, and safety can be met through design and balanced between different needs. On the one hand, the difference in charge-discharge rate provides the positive electrode with the ability to store electricity. The greater the difference in the rates between the two sides, the greater the ability to store electricity. On the other hand, too large a rate difference will also lead to too large a current difference on the two sides. Therefore, by setting the charge-discharge rate ratio within a more appropriate range, the current difference on the two sides can be prevented from being too large.

[0087] According to some embodiments of the present application, optionally, the impedance ratio of the two surfaces of the positive electrode film layer may be 0.25 to 4. Further optionally, the impedance ratio of the two surfaces of the positive electrode film layer may be 0.7 to 1.4.

[0088] The impedance ratio of the two surfaces of the positive electrode film layer is the ratio of the impedance of one surface to the impedance of the other surface. By adjusting the impedance ratio, the charge storage capacity and charge storage surface of the positive electrode sheet can be controlled under a fixed capacity ratio, and at the same time, the charge and discharge rate ratio of the two surfaces can be prevented from being too large.

[0089] According to some embodiments of the present application, optionally, the single-sided resistance of any one of the two surfaces of the positive electrode film layer is less than 1 ohm. Further optionally, the single-sided resistance of any one of the two surfaces of the positive electrode film layer is less than 0.8 ohm.

[0090] The single-sided resistance of the positive electrode film layer refers to the cold-pressed resistance of the single side, which can be obtained by using a common fixture two-point test method, that is, through two terminals with a cross-sectional area of ​​S (for example, a diameter of 14mm), clamped at a certain pressure (for example, 20-28MPa), and the sampling time range is, for example, 5s to 17s. For the single-sided resistance of the positive electrode sheet, whether it is after cold pressing or the first cycle of disassembled and dried positive electrode sheet, it is necessary to wipe off the other side with NMP, and then use the two-point method to test the film resistance.

[0091] The resistance of the dry electrode sheet during cold pressing can be composed of the coating main material body, the conductive network inside the coating main material, and the contact resistance between the coating main body and the current collector, and is controllable and adjustable. It can be first assumed that there is a positive correlation between the resistance of the dry electrode sheet during cold pressing of the positive electrode sheet and the resistance of the positive electrode sheet after drying after the first cycle of discharge. Therefore, by adjusting the resistance of the positive electrode dry electrode sheet during production, the resistance after drying after the first cycle of charge and discharge can be adjusted. The resistance of the positive electrode sheet after drying after the first cycle of discharge is greatly affected by the chemical system, such as the SEI film formation effect in the formation stage, the influence of the electrode rebound, the electrolyte infiltration, etc., which needs to be collected in actual production to correct the proportional factor.

[0092] Since the two sides of the positive electrode are in a parallel circuit, the overall impedance of the system can be made small enough by setting a smaller single-sided resistance value, thereby reducing the internal resistance of the system.

[0093] According to some embodiments of the present application, optionally, the capacity ratio of the two surfaces of the positive electrode film layer may be 0.64-1.58. Further optionally, the capacity ratio of the two surfaces of the positive electrode film layer may be 0.9-1.12.

[0094] The capacity ratio of the two surfaces of the positive electrode film layer is the ratio of the impedance of one surface to the capacity of the other surface. The capacity of the positive electrode film layer can be adjusted by the coating weight of the coated positive electrode active material film layer, the active material loading, the gram capacity of the active material, etc. The capacity ratio design can be achieved by the above parameters for both sides of the positive electrode plate or the single-sided stack in parallel.

[0095] By setting a suitable capacity ratio, the surface with small capacity will not face too many anodes, so as not to affect the energy density of the battery cell.

[0096] According to some embodiments of the present application, optionally, among the two surfaces of the positive electrode film layer, the low-impedance surface has a higher charge and discharge rate than the high-impedance surface.

[0097] As described above, the difference in charge and discharge rates on the two sides of the positive electrode plate causes electricity to be hidden in the plate, and the electricity is hidden in the low-rate surface. In this embodiment, when the low-impedance surface has a higher charge and discharge rate than the high-impedance surface, the high-impedance surface has a lower charge and discharge rate, so the electricity is hidden in the high-impedance surface. This can meet the demand for fast charging while taking into account the lifespan. At this time, by increasing the amount of anode corresponding to the low-impedance surface, the safety of the high-rate charging process can be guaranteed. At the same time, the amount of anode corresponding to the high-impedance surface can be selectively reduced.

[0098] According to some embodiments of the present application, optionally, among the two surfaces of the positive electrode film layer, the high-impedance surface has a higher charge and discharge rate than the low-impedance surface.

[0099] As described above, the difference in charge and discharge rates on the two sides of the positive electrode plate allows electricity to be hidden in the plate, and the electricity is hidden on the low-rate side. In this embodiment, when the low-impedance side has a higher charge and discharge rate than the high-impedance side, the low-impedance side thus obtains a lower charge and discharge rate, and thus the electricity is hidden on the low-impedance side. At this time, because the high-impedance and high-rate side has a greater polarization and is cut off early, in addition to the low-impedance side storing electricity due to the low rate, the high-impedance side can further store electricity due to polarization. At this time, the thermal effect at the plate level will show a large difference on the two sides, and this self-heating effect at the plate level is suitable for low-temperature cold start application scenarios.

[0100] According to some embodiments of the present application, a design method for a positive electrode sheet for a secondary battery is provided. Figure 8 As shown in , the method may include the following steps:

[0101] S1: Determine the design charge storage capacity of the positive electrode; and,

[0102] S2: The positive electrode film layer including the positive electrode active material and coated on the two surfaces of the positive electrode current collector in opposite thickness directions of the positive electrode sheet is designed to have different charge and discharge rates to achieve the designed power storage capacity.

[0103] Designing power storage capacity refers to achieving the desired power storage capacity by designing parameters that affect power storage capacity based on the actual demand for power storage capacity of the product. Power storage capacity reflects the proportion of unreleased lithium ions to the total amount of lithium, that is, the ability to replenish lithium ions in the battery at a later stage, which can extend the service life of the battery. Therefore, the design power storage capacity can be determined by reverse calculation from the perspective of the design service life of the battery.

[0104] After the designed power storage capacity is determined, the various parameters of the positive electrode plate can be designed based on the various factors affecting the power storage capacity described in this application. More specifically, the designed power storage capacity can be achieved by designing the difference in charge and discharge rates of the two surfaces of the positive electrode film layer.

[0105] Through the method of the present application, the charge storage capacity of the positive electrode plate can be designed according to the relationship between the charge storage capacity and the difference in charge and discharge rates on both sides of the positive electrode plate, thereby designing the life of the battery.

[0106] According to some embodiments of the present application, optionally, the above step S2 may include:

[0107] The charge and discharge rates of the two surfaces of the positive electrode film layer are designed according to the following formula:

[0108]

[0109] in,

[0110]

[0111] Among them, the capacity ratio, impedance ratio and charge-discharge rate ratio in the above formula are the ratios of the capacity, impedance and charge-discharge rate of the first surface of the two surfaces of the positive electrode film layer to the capacity, impedance and charge-discharge rate of the second surface.

[0112] As mentioned above, when the two surfaces of the positive electrode plate have different charge and discharge rates, the battery will have the ability to store electricity. The following formula will more specifically explain the functional relationship between the impedance and capacity of the positive electrode plate of the present application and the ability to store electricity, as well as the derivation process of the above formula. In all formulas, it is assumed that the two surfaces of the positive electrode plate are the A side and the B side, and all ratios are the A side / B side.

[0113] Formula 1:

[0114]

[0115] Formula 2:

[0116]

[0117] Formula 3a:

[0118]

[0119] Formula 3b:

[0120]

[0121] Formula 4:

[0122]

[0123] Formula 5:

[0124] Estimated heat = external circuit current × min (A surface impedance × A surface capacity, B surface impedance × B surface capacity)

[0125] Formula 6:

[0126]

[0127] Formula 7a:

[0128]

[0129] Formula 7b:

[0130]

[0131] Formula 7c:

[0132]

[0133]

[0134] Through the functional relationship between the power storage capacity and the charge-discharge rate ratio and further the capacity ratio and impedance ratio given in this application, the positive electrode plate can be quantitatively designed by controlling the impedance ratio and capacity ratio so that it has the desired designed power storage capacity.

[0135] According to some embodiments of the present application, optionally, the above step S2 may further include: first determining the capacity ratio, and then realizing the designed power storage capacity by designing the impedance ratio.

[0136] Fig. 9 This is a schematic diagram showing the design of power storage capacity by adjusting the impedance ratio after the capacity ratio is fixed, where the horizontal axis is the impedance ratio of the AB surface and the vertical axis is the power storage capacity. From the perspective of power storage capacity, the impedance ratio and capacity ratio can be adjusted in a wide range. Fig. 9As shown in , five curves are drawn according to different single-sided capacity ratios (the capacity ratios are 1.6, 1.2, 1, 0.8 and 0.6 respectively). As can be seen from the figure, each curve consists of two parts, a dotted line and a solid line. The solid line represents the electricity storage on the A side, and the dotted line represents the electricity storage on the B side. The intersection of the curve and the horizontal axis represents that the value of the impedance ratio × capacity ratio here is 1. Therefore, according to formula 7c, it can be calculated that the electrode does not have the ability to store electricity at this time. Subsequently, with the adjustment of the impedance ratio, on the one hand, the electricity storage capacity can be adjusted, that is, the larger the impedance ratio, the stronger the electricity storage capacity. In other words, after the capacity ratio is determined, the impedance can amplify or reduce the electricity storage capacity, which provides the possibility of adjustment for subsequent life design. On the other hand, the electricity storage side can be switched, that is, by adjusting the impedance ratio, it can be controlled which side has a higher charge and discharge rate, and the electricity will always be hidden on the side with a lower charge and discharge rate.

[0137] Fig.10 is a schematic diagram showing how to control the charge-discharge rate ratio by adjusting the impedance ratio after the capacity ratio is fixed, wherein the abscissa is the impedance ratio of the AB surface and the ordinate is the charge-discharge rate ratio. Fig. 9 similar, Fig.10 Five curves are still drawn according to different single-sided capacity ratios (the capacity ratios are 2, 1.2, 1, 0.8 and 0.6 respectively), and the recommended range of rate ratio is shown. Similarly, each curve consists of two parts, a dotted line and a solid line, where the solid line represents the power storage on side A and the dotted line represents the power storage on side B. It can be seen from the figure that at a given capacity ratio, the closer the impedance ratio is to 1 / capacity ratio, the smaller the difference in charge and discharge rates (and the closer it is to 1), and the critical point of switching the power storage side appears at a rate ratio of 1, at which time both sides will be fully charged, all lithium ions are involved in charging and discharging, and the system has no power storage.

[0138] Among the two main adjustable factors that affect the ability to store electricity, capacity and impedance, because capacity itself is also an important parameter of battery products and has its own design requirements, relatively speaking, the space for adjusting capacity is relatively limited, and the capacity ratio cannot usually be designed to be very large, otherwise it may affect the energy density of the battery cell. Therefore, when designing the impedance ratio and capacity ratio, the single-sided capacity is generally determined according to actual needs, and then the capacity ratio of the two sides is determined. Subsequently, on the basis of the determined capacity ratio, the impedance ratio can be adjusted to achieve the designed power storage capacity. On the other hand, the impedance ratio has a large room for adjustment, so by first determining the capacity ratio and then adjusting the impedance ratio, the subsequent manufacturing process can be easily controlled and implemented.

[0139] According to some embodiments of the present application, optionally, the above step S2 may include: designing an impedance ratio so that, of the two surfaces of the positive electrode film layer, the low impedance surface has a higher charge and discharge rate than the high impedance surface.

[0140] As mentioned above, this design can store electricity in the high impedance surface, thus meeting the fast charging requirements while taking into account the lifespan.

[0141] According to some embodiments of the present application, optionally, the above step S2 may include: designing an impedance ratio so that, among the two surfaces of the positive electrode film layer, the high impedance surface has a higher charge and discharge rate than the low impedance surface.

[0142] As mentioned above, this design can store electricity in the low-impedance surface, and allow the high-impedance surface to further store electricity due to polarization, and can bring about a self-heating effect at the electrode level, which is suitable for low-temperature cold start application scenarios.

[0143] According to some embodiments of the present application, optionally, designing the impedance ratio further includes designing the positive electrode sheet resistance on two surfaces of the positive electrode film layer.

[0144] The impedance of the secondary battery mainly includes ohmic impedance, electrochemical polarization impedance and concentration polarization impedance. Electrochemical polarization impedance and concentration polarization impedance mainly depend on the system environment and are basically not adjustable. Therefore, in this application, the impedance is mainly adjusted by adjusting the ohmic impedance. Ohmic impedance further includes the resistance of the positive electrode sheet (including SEI film), the ion diffusion resistance and the negative electrode membrane resistance, among which the positive electrode sheet resistance has a significant effect on the ohmic impedance. Therefore, the single-sided impedance of the positive electrode sheet can be adjusted by adjusting the resistance of the positive electrode sheet. In addition, compared with the resistance of the negative electrode membrane, the resistance of the positive electrode sheet is larger, so it is easier to regulate.

[0145] According to some embodiments of the present application, the present application further provides a secondary battery, which may include the positive electrode sheet described in any of the above schemes or the positive electrode sheet obtained by the method described in any of the above schemes.

[0146] According to some embodiments of the present application, the present application also provides a battery module, which may include a secondary battery as described in any of the above schemes.

[0147] According to some embodiments of the present application, the present application also provides a battery pack, which may include a battery module as described in any of the above schemes.

[0148] According to some embodiments of the present application, the present application also provides an electrical device, which may include at least one of the secondary battery described in any of the above schemes, the battery module described in any of the above schemes, or the battery pack described in any of the above schemes.

[0149] [Secondary battery]

[0150] In the following, the battery life improvement effect brought about by the impedance difference design on both sides of the battery positive electrode plate proposed in the present application is characterized mainly based on secondary batteries, especially lithium (sodium) ion secondary batteries. However, it should be emphasized here that the impedance difference design of the positive electrode plate of the present application can be used for any electrical device including a carbon-based electrode, and the electrical device can benefit from it.

[0151] In one embodiment of the present application, the secondary battery includes a positive electrode, a negative electrode, a separator, an electrolyte, etc. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode and the negative electrode. The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode.

[0152] [Positive electrode]

[0153] In the secondary battery of the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on two surfaces in the thickness direction of the positive electrode current collector and including a positive electrode active material. In the secondary battery of the present application, the positive electrode current collector may be a metal foil or a composite current collector, for example, the metal foil may be an aluminum foil, and the composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and its copolymers, etc.).

[0154] In the secondary battery of the present application, the positive electrode active material may adopt the positive electrode active material for secondary batteries known in the art. For example, the positive electrode active material may include one or more of the following: lithium-containing phosphates of olivine structure, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for secondary batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.5 Co0.25 Mn 0.25 O2(NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811)), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and one or more of its modified compounds, etc. Examples of lithium phosphates containing olivine structures may include, but are not limited to, lithium iron phosphate (such as LiFePO4 (LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. In some embodiments, the positive electrode film layer may also optionally include a binder. Non-limiting examples of binders that can be used for the positive electrode film layer may include one or more of the following: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin. In some embodiments, the positive electrode film layer may also optionally include a conductive agent. Examples of conductive agents for positive electrode film layers may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In one embodiment of the present application, the positive electrode can be prepared in the following manner: the above-mentioned components for preparing the positive electrode, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0155] [Negative electrode]

[0156] The secondary battery of the present application includes a negative electrode plate, which includes a negative electrode collector and a negative electrode film layer arranged on at least one surface of the negative electrode collector, and the negative electrode film layer contains the negative electrode active material mentioned above in the present application.

[0157] In one embodiment of the present application, the negative electrode film layer may include a certain amount of other commonly used negative electrode active materials in addition to the negative electrode active materials described above, for example, one or more of natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from one or more of elemental silicon, silicon oxide, and silicon-carbon composites. The tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys.

[0158] In the secondary battery of the present application, the negative electrode membrane comprises a negative electrode active material and an optional binder, an optional conductive agent and other optional auxiliary agents, and is usually formed by coating and drying a negative electrode slurry. The negative electrode slurry coating is usually formed by dispersing the negative electrode active material and the optional conductive agent and binder in a solvent and stirring them evenly. The solvent can be N-methylpyrrolidone (NMP) or deionized water.

[0159] As an example, the conductive agent may include one or more of superconducting carbon, carbon black (eg, acetylene black, Ketjen black), carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0160] As an example, the binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA) and carboxymethyl chitosan (CMCS). As an example, the binder may include one or more of styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS). Other optional additives are, for example, thickeners (such as sodium carboxymethyl cellulose CMC-Na), PTC thermistor materials, etc.

[0161] In addition, in the secondary battery of the present application, the negative electrode plate does not exclude other additional functional layers besides the negative electrode film layer. For example, in some embodiments, the negative electrode plate of the present application may also include a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the first negative electrode film layer and disposed on the surface of the negative electrode current collector. In some other embodiments, the negative electrode plate of the present application may also include a covering protective layer covering the surface of the second negative electrode film layer.

[0162] In the secondary battery of the present application, the negative electrode current collector may be a metal foil or a composite current collector, for example, the metal foil may be a copper foil, a silver foil, an iron foil, or a foil composed of an alloy of the above metals. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer, and may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material base layer (such as a base layer made of materials such as polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and copolymers thereof).

[0163] [Electrolytes]

[0164] The electrolyte plays the role of conducting ions between the positive electrode plate and the negative electrode plate. The electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte). In some embodiments, the electrolyte uses an electrolyte. The electrolyte includes an electrolyte salt and a solvent. In some embodiments, the electrolyte salt can be selected from LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonyl imide), LiTFSI (lithium bistrifluoromethanesulfonyl imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro oxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluoro dioxalate phosphate) and LiTFOP (lithium tetrafluoro oxalate phosphate) One or more. In one embodiment of the present application, the solvent may be selected from one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE). In one embodiment of the present application, based on the total weight of the electrolyte, the content of the solvent is 60-99% by weight, such as 65-95% by weight, or 70-90% by weight, or 75-89% by weight, or 80-85% by weight. In one embodiment of the present application, based on the total weight of the electrolyte, the content of the electrolyte is 1-40% by weight, such as 5-35% by weight, or 10-30% by weight, or 11-25% by weight, or 15-20% by weight.

[0165] In one embodiment of the present application, the electrolyte may optionally contain additives. For example, the additives may include one or more of the following: negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high temperature performance, additives that improve battery low temperature performance, etc.

[0166] [Isolation film]

[0167] In one embodiment of the present application, the secondary battery also includes a separator, which separates the anode side of the secondary battery from the cathode side, and provides selective permeation or blocking for substances of different types, sizes and charges in the system. For example, the separator can insulate electrons, physically isolate the positive and negative active substances of the secondary battery, prevent internal short circuits and form an electric field in a certain direction, and allow ions in the battery to pass through the separator and move between the positive and negative electrodes. In one embodiment of the present application, the material used to prepare the separator may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0168] Example

[0169] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0170] Benchmark Group

[0171] Preparation of positive electrode

[0172] Lithium iron phosphate (LiFePO4 or LFP), conductive carbon black (Super-P), polyvinylidene fluoride (PVDF) are mixed with N, N-dimethylpyrrolidone (NMP) in a weight ratio of 96:2.5:1.5, and stirred evenly to obtain a slurry for coating the positive electrode sheet. The viscosity can be adjusted by NMP during the stirring process. Then, the slurry is evenly coated on the A and B surfaces of the 13μm thick positive electrode collector (aluminum foil) with a certain width. In this process, the consistency of the A and B surfaces is strictly controlled, that is, the capacity and impedance of the AB surfaces are consistent, that is, the capacity ratio is 1 and the impedance ratio is 1.

[0173] The positive electrode sheet is manufactured by cold pressing and slicing (ie, the electrode sheet is trimmed and cut into the required size).

[0174] Preparation of negative electrode

[0175] Artificial graphite, conductive carbon black (Super-P), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR) are mixed with water at a weight ratio of 97:0.5:1:1.5, and stirred evenly to obtain a slurry for coating the negative electrode sheet. The viscosity can be adjusted by water during the stirring process. Then, the slurry is coated on both sides of a 6μm thick negative electrode current collector (copper foil) with a certain width to ensure that the capacity per unit area on both sides is the same, and the negative electrode sheet is obtained after cold pressing and slicing.

[0176] Preparation of lithium-ion secondary batteries

[0177] 7μm polyethylene (PE) is used as a separator. The positive electrode sheet, separator and negative electrode sheet are wound into a battery cell, and a lithium-ion secondary battery is obtained through ear welding, aluminum shell packaging, liquid injection, packaging formation and vacuum molding. The battery cell has a width of 148mm, a thickness of 28mm, a height of 98mm and a capacity of 40Ah. The electrolyte is a solution of 1M LiPF6 ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:2).

[0178] In this benchmark group, the capacity ratio and impedance ratio of the AB surface are consistent, so the charge and discharge rate is obviously consistent, so the positive electrode plate will not store electricity. By substituting the impedance ratio and capacity ratio into the above formula, the calculated power storage capacity is 0.00%, that is, the electrode will not store electricity. In addition, it should be noted that when the battery cell is charged and discharged at a high rate, the available capacity of the battery cell will also decrease, and this part of the capacity is not calculated in the power storage capacity.

[0179] In the following Examples 1-5 and the comparative group, a positive electrode sheet preparation method similar to that of the reference group is adopted, wherein the positive electrode sheet in Examples 1-5 is coated on two surfaces opposite to each other in the thickness direction of the positive electrode current collector itself and the positive electrode film layer including the positive electrode active material is realized to have different charge and discharge rates on both surfaces, while the two surfaces of the positive electrode film layer of the reference group and the comparative group are realized to have the same charge and discharge rate. The difference between Examples 1-5, the comparative group and the reference group is that the different charge and discharge rate ratios shown in the following Table 1 are achieved by adjusting the capacity ratio and the impedance ratio:

[0180]

[0181] Table 1

[0182] Example 1

[0183] Compared with the reference group, Example 1 controls the capacity of the AB surface to be consistent, but by slightly changing the proportion of the conductive carbon black based on the weight ratio of 96:2.5:1.5 of lithium iron phosphate (LiFePO4 or LFP), conductive carbon black (Super-P), and polyvinylidene fluoride (PVDF) used in the reference group, the impedance ratio of the slurry film layer coated on the A surface and the B surface of the positive electrode sheet is adjusted, and the impedance ratio of the AB surface is 0.8, 0.4, 1.2 and 1.6 respectively. Substituting the impedance ratio and capacity ratio into the above formula, the charge and discharge rate ratios of the AB surface can be calculated to be 1.25, 2.50, 0.83 and 0.63 respectively.

[0184] Example 2

[0185] Compared with the reference group, in the second group of embodiments 2a-2d of the present application, the AB surface is controlled to have different capacity ratios by changing the coating weight, which are 0.8, 0.4, 1.2 and 1.6 respectively. When the capacity of the AB surface is different, the impedance difference itself will also be introduced. In this group of embodiments, the impedance of the AB surface is adjusted by further adopting the same impedance adjustment method as in Example 1, so that the impedance of the AB surface is consistent, that is, the impedance ratio is 1. At this time, substituting the impedance ratio and capacity ratio into the above formula, it can be calculated that the charge and discharge rate ratios of the AB surface are 1.25, 2.50, 0.83 and 0.63 respectively.

[0186] Example 3-4

[0187] In the third group of embodiments 3a-3d and the fourth group of embodiments 4a-4d of the present application, the capacity of the AB surface is adjusted by adopting the same capacity adjustment method as in embodiment 2, so that the AB surface has different capacity ratios, which are 0.64, 0.94, 1.23 and 1.54 respectively. The difference between the two groups of embodiments is that in the third group of embodiments 3a-3d, the impedance of the AB surface is further adjusted by adopting the same impedance adjustment method as in embodiment 1, so that the impedance ratio × capacity ratio of the AB surface has the same value greater than 1, 1.25, that is, the A surface has a high impedance ratio × capacity ratio. At this time, substituting the impedance ratio and capacity ratio into the above formula, it can be calculated that the charge and discharge rate ratio of the AB surface is 0.80. Correspondingly, in the fourth group of embodiments 4a-4d, by further adjusting the impedance of the AB surface, the impedance ratio × capacity ratio of the AB surface has the same value less than 1, 0.8, that is, the B surface has a high impedance ratio × capacity ratio. At this time, substituting the impedance ratio and capacity ratio into the above formula, it can be calculated that the charge and discharge rate ratio of the AB surface is 1.25.

[0188] As mentioned above, when the capacity of the AB surface is different, the impedance difference will be introduced. Under the same formula, on the one hand, at the same compaction density, the thicker the coating film, the heavier the weight, and the more active materials actually introduced, the larger the capacity of the electrode. On the other hand, the increase in the thickness of the electrode itself causes the increase in the resistance of the membrane, and thus the impedance also increases accordingly. Fig.11 Schematic diagram showing the linear relationship between the membrane resistance and the designed capacity of lithium iron phosphate material. Fig.11 As shown in , the diaphragm impedance has a linear relationship with the capacity per unit area.

[0189] Example 5

[0190] In the fifth group of examples 5a and 5b of the present application, the case where only the coating weight of the AB surface is changed is shown respectively. At the same compaction density, the membrane coated on the A surface of Example 5a is thinner than the B surface, and the actual capacity ratio is 0.8, which is less than 1, and its impedance ratio is also correspondingly 0.8, the impedance ratio × capacity ratio is 0.64, and the charge and discharge rate ratio is 1.56. In Example 5b, on the contrary, the membrane coated on the A surface is thicker than the B surface, and the actual capacity ratio is 1.6, which is greater than 1, and its impedance ratio is also correspondingly 1.6, the impedance ratio × capacity ratio is 2.56, and the charge and discharge rate ratio is 0.39.

[0191] Comparison group

[0192] In the control groups 1a-1d, the AB surfaces have different capacity ratios, which are similar to the third and fourth groups of embodiments, 2, 1.25, 0.91 and 0.8 respectively. In this group of control examples, by further adjusting the impedance of the AB surface, the impedance ratio × capacity ratio of the AB surface is made consistent, that is, the impedance ratio × capacity ratio is 1. At this time, substituting the impedance ratio and capacity ratio into the above formula, it can be calculated that the charge and discharge rate ratio of the AB surface is also 1.

[0193] Power storage capacity test

[0194] The power storage ability described in this application comes from the difference in charge and discharge rates between the AB sides of the electrode, which is different from the polarization caused by the high-rate charge and discharge process in actual applications. In other words, the user usage rate acting on the entire electrode is not evenly distributed to the AB sides in this application, but this non-uniformity can be designed by the method described in this application. Therefore, the capacity loss measured in the finished battery cell includes two parts: polarization and power storage. Polarization comes from the rate itself, which can be reduced by reducing the rate, but it cannot be completely eliminated; power storage comes from the rate difference between the AB sides, which can be considered to be basically unchanged.

[0195] According to one embodiment of the present application, the power storage capacity can be non-destructively tested using the following steps:

[0196] (a) The actual charging capacity Cap1 is obtained by charging to the cut-off voltage at a low charging rate (recommended temperature 25°C, rate 0.04C);

[0197] (b) Calculate the actual mass of the positive electrode active material in the battery cell;

[0198] (c) Calculate the theoretical charge capacity Cap2 based on the gram capacity data of the powder; and

[0199] (d) Actual power storage capacity = 1-actual charging capacity / theoretical charging capacity = 1-Cap1 / Cap2.

[0200] It should be noted here that the electricity is hidden in the low-rate surface of the positive electrode, so it is recommended to use the charging process for calculation; of course, the discharge process can also be calculated using the same rate discharge according to steps (a)-(d). At this time, it is necessary to pay attention to the film loss of lithium (sodium) ions on the AB surface.

[0201] According to another embodiment of the present application, the power storage capacity can also be tested destructively using the following steps:

[0202] (a) The actual charging capacity Cap1 is obtained by charging to the cut-off voltage at a low charging rate (recommended temperature 25°C, rate 0.04C);

[0203] (b) disassembling the battery cell, obtaining two positive electrode sheets, and retaining the electrolyte and the separator;

[0204] (c) using NMP to wipe off one side to obtain a single-sided positive electrode sheet with only side A and only side B;

[0205] (d) using the positive electrode, separator, electrolyte and metallic lithium of steps b and c to make a buckle battery;

[0206] (e) Continue charging at a rate of 0.04C to the cut-off potential to obtain capacities Cap2a and Cap2b (to be converted according to the mass of the active material); and,

[0207] (f) Actual power storage capacity = stored power / total power = (Cap2a+Cap2b) / (Cap1+Cap2a+Cap2b).

[0208] Comparison of electricity storage effects

[0209] Table 2 below shows the differences in electricity storage effects achieved by Examples 1 to 5, the reference group, and the comparison group.

[0210]

[0211]

[0212] Table 2

[0213] It can be seen from Table 2 above that different charge storage surfaces and charge storage capacities can be achieved by designing the impedance ratio and capacity ratio of the two surfaces of the positive electrode film layer.

[0214] In the first group of embodiments 1a-1d, the achieved power storage capacity is 10.00%, 30.00%, 8.33% and 18.75%, wherein in embodiments 1a and 1b, surface A is a low impedance surface and a high rate surface, and surface B is a high impedance surface and a low rate surface, while in embodiments 1c and 1d, surface A is a high impedance surface and a low rate surface, and surface B is a low impedance surface and a high rate surface. According to the power storage principle described above, electricity will be hidden in the low rate surface of the two surfaces of the positive electrode. Therefore, when the capacity of the AB surface is the same, the electricity can be hidden in the high impedance surface by adjusting the impedance size of the AB surface.

[0215] In the second group of embodiments 2a-2d, the achieved power storage capacity is 8.89%, 17.14%, 7.58% and 14.42%, wherein in embodiments 2a and 2b, the A side is a low-capacity side and a high-rate side, and the B side is a high-capacity side and a low-rate side, while in embodiments 2c and 2d, the A side is a high-capacity side and a low-rate side, and the B side is a low-capacity side and a high-rate side. According to the power storage principle described above, electricity will be stored in the low-rate side of the two surfaces of the positive electrode. Therefore, when the capacity of the AB side is different, the electricity can be stored in the high-capacity side by adjusting the impedance of the AB side.

[0216] In the third group of embodiments 3a-3d of the present application, the achieved power storage capabilities are 12.12%, 10.32%, 9.10% and 7.81% respectively, and the A surface has a high impedance ratio × capacity ratio. In the fourth group of embodiments 4a-4d, the achieved power storage capabilities are 7.80%, 9.74%, 11.06% and 12.08% respectively, and. According to the power storage principle described above, electricity will be hidden in the low-rate surface of the two surfaces of the positive electrode. Therefore, in the case of different capacities of the AB surfaces, the electricity can be hidden in the surface with a high impedance ratio × capacity ratio by adjusting the impedance of the AB surface.

[0217] In the fifth group of embodiments 5a and 5b of the present application, the achieved electricity storage capacity is 16.00% and 23.44%. According to the electricity storage principle described above, electricity will be stored in the low-rate surface of the two surfaces of the positive electrode. Therefore, this group of embodiments shows that, if the design conditions permit, electricity can also be stored by coating membranes of different weights (i.e., thicknesses) on both sides AB, and electricity will be stored on the side with higher coating weight.

[0218] In the control groups 1a-1d, the achieved power storage capacity is 0.00%. This group of control examples indicates that, when the capacities of the AB surfaces are different, if necessary, the functional relationship between the power storage capacity and the capacity ratio and the impedance ratio provided in the present application can be used to adjust the impedance of the AB surfaces to achieve no power storage, thereby using all the designed power.

[0219] What has been described above includes examples of various aspects of the claimed subject matter. Of course, it is not possible to describe every conceivable combination of components or methods for the purpose of describing the claimed subject matter, but one of ordinary skill in the art will recognize that many further combinations and permutations of the claimed subject matter are possible. Thus, the disclosed subject matter is intended to encompass all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

Claims

1. A positive electrode sheet, characterized in that: The invention comprises a positive electrode current collector and a positive electrode film layer coated on two surfaces of the positive electrode current collector in opposite directions in thickness direction and comprising positive electrode active material, wherein the two surfaces of the positive electrode film layer have different charge and discharge rates.

2. The positive electrode sheet according to claim 1, characterized in that: The charge-discharge rate ratio of the two surfaces of the positive electrode film layer is 0.4-2.

5.

3. The positive electrode sheet according to claim 2, characterized in that: The charge-discharge rate ratio is 0.8-1.

25.

4. The positive electrode sheet according to claim 1, characterized in that: The impedance ratio of the two surfaces of the positive electrode film layer is 0.25-4.

5. The positive electrode sheet according to claim 4, characterized in that: The impedance ratio is 0.7-1.

4.

6. The positive electrode sheet according to claim 1, characterized in that: The single-sided resistance of any one of the two surfaces of the positive electrode film layer is less than 1 ohm.

7. The positive electrode sheet according to claim 1, characterized in that: The single-sided resistance of any one of the two surfaces of the positive electrode film layer is less than 0.8 ohms.

8. The positive electrode sheet according to claim 1, characterized in that: The capacity ratio of the two surfaces of the positive electrode film layer is 0.64-1.

58.

9. The positive electrode sheet according to claim 8, characterized in that: The capacity ratio is 0.9-1.

12.

10. The positive electrode sheet according to any one of claims 1 to 9, characterized in that: Of the two surfaces of the positive electrode film layer, the low-impedance surface has a higher charge and discharge rate than the high-impedance surface.

11. The positive electrode sheet according to any one of claims 1 to 9, characterized in that: Of the two surfaces of the positive electrode film layer, the high-impedance surface has a higher charge and discharge rate than the low-impedance surface.

12. A method for designing a positive electrode sheet for a secondary battery, characterized in that: The method comprises the following steps: S1: Determine the designed power storage capacity of the positive electrode plate; and S2: The positive electrode film layer including the positive electrode active material and coated on two surfaces of the positive electrode current collector in opposite thickness directions of the positive electrode sheet is designed to have different charge and discharge rates to achieve the designed power storage capacity.

13. The method according to claim 12, characterized in that Step S2 includes: The charge and discharge rates of the two surfaces of the positive electrode film layer are designed according to the following formula: in, Among them, the capacity ratio, impedance ratio and charge-discharge rate ratio in the above formula are the ratios of the capacity, impedance and charge-discharge rate of the first surface of the two surfaces of the positive electrode film layer to the capacity, impedance and charge-discharge rate of the second surface.

14. The method according to claim 13, characterized in that Step S2 further comprises: The capacity ratio is first determined, and then the impedance ratio is designed to achieve the designed power storage capacity.

15. The method according to any one of claims 13 to 14, characterized in that: Step S2 includes: The impedance ratio is designed so that, among the two surfaces of the positive electrode film layer, the low impedance surface has a higher charge and discharge rate than the high impedance surface.

16. The method according to any one of claims 13 to 14, characterized in that: Step S2 includes: The impedance ratio is designed so that, among the two surfaces of the positive electrode film layer, the high impedance surface has a higher charge and discharge rate than the low impedance surface.

17. The method according to any one of claims 13 to 14, characterized in that: Designing the impedance ratio further includes designing the positive electrode sheet resistance on two surfaces of the positive electrode film layer.

18. A secondary battery, characterized in that: It includes the positive electrode sheet as described in any one of claims 1 to 11 or the positive electrode sheet obtained by the method according to any one of claims 12 to 17.

19. A battery module, characterized in that: Comprising the secondary battery as claimed in claim 18.

20. A battery pack, characterized in that: Comprising the battery module as claimed in claim 19.

21. An electrical device, characterized in that: Includes at least one of the secondary battery according to claim 18, the battery module according to claim 19, or the battery pack according to claim 20.

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