Lithium ion battery positive electrode sheet and preparation method thereof

Abstract

The application discloses a lithium ion battery positive pole piece and a preparation method thereof, wherein an additive is added into a positive pole active material, the additive comprising one or a combination of several of lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium manganese iron phosphate, Li5FeO4, lithium titanate, lithium vanadium phosphate and the like. In the charging and discharging process of the lithium iron phosphate battery, the voltage change at the end of the voltage curve becomes slow due to the influence of the lithium extraction platform of the additive, so that the purpose of improving the SOC estimation accuracy of the lithium iron phosphate battery is achieved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a positive electrode sheet for a lithium-ion battery and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used in new energy vehicles due to their low cost, high safety, and excellent cycle life. Among them, lithium iron phosphate (LFP) batteries are particularly important because the remaining charge of the battery pack directly affects energy management decisions, while overcharging or over-discharging directly impacts the battery pack's lifespan. Therefore, the state of charge (SOC) of the LFP battery pack is a crucial indicator for energy management. However, due to the inherent properties of lithium iron phosphate materials, the voltage curve changes steeply at the end of the charge / discharge cycle. Currently, the industry generally uses a combination of SOC-OCV and ampere-hour integration to estimate SOC, which presents a challenge for estimating the battery's SOC at the end of the charge / discharge cycle.

[0003] Patent application (publication number CN110895310A) discloses a lithium iron phosphate battery SOC estimation system, which enables real-time and reliable estimation of the state of charge (SOC) of lithium iron phosphate batteries. The system includes a data acquisition module, an SOC calculation module, a temperature compensation module, and an SOC correction module. When the current charge level is determined to be between 30% and 90%, the current SOC value is calculated using the ampere-hour integration method. However, when the current charge level is determined to be ≤30% or ≥90%, the current SOC value is retrieved by querying the open-circuit voltage. This method does not effectively solve the problem of inaccurate SOC estimation caused by the rapid voltage changes at the end of the charging and discharging process of lithium iron phosphate batteries.

[0004] Patent application CN109031147A discloses a method for estimating the state of charge (SOC) of a lithium iron phosphate (LFP) battery pack using a backpropagation (BP) neural network. This method includes the following steps: adjusting the connection weight adjustment stage of the BP neural network by inserting an inertia coefficient, improving the connection weight adjustment through smooth weighting calculation, and adding the original connection weight term with a heavier weight during the calculation. This invention uses current integral correction to modify the parameters of the BP neural network, reducing the impact of LFP battery pack capacity decay on the accuracy of SOC estimation. This method mainly improves the accuracy of SOC estimation due to capacity decay during cycling, but it does not provide a solution for the problem of inaccurate SOC estimation at the end of the charge-discharge cycle of LFP batteries. Summary of the Invention

[0005] Given that the existing technologies mentioned above cannot effectively solve the problem of SOC estimation accuracy at the end of charge and discharge of lithium iron phosphate batteries, this invention proposes a method to improve the SOC accuracy at the end of charge and discharge. By adding a small amount of additives to the battery, the electrochemical performance of the battery is not affected, and the change trend of the voltage curve at the end of charge and discharge can be slowed down, thereby improving the SOC estimation accuracy.

[0006] To achieve the above objectives, the present invention provides a positive electrode sheet for a lithium-ion battery and a method for preparing the same.

[0007] <First Aspect>

[0008] A method for preparing a positive electrode sheet for a lithium-ion battery includes the following steps:

[0009] S1. Add the additive to the positive electrode active material and disperse it to form a positive electrode active material additive mixture;

[0010] S2. Add the positive electrode active material additive mixture and conductive agent from step S1 to the adhesive solution and mix to form a slurry.

[0011] S3. The slurry prepared in step S2 is coated onto aluminum foil and then dried to form an electrode roll.

[0012] S4. Press the electrode roll into a positive electrode sheet;

[0013] The additives include one or a combination of several of lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium manganese iron phosphate, Li5FeO4, lithium titanate, and lithium vanadium phosphate.

[0014] In step S3, the aluminum foil includes carbon-coated aluminum foil.

[0015] The additives include a combination of lithium manganese oxide and lithium iron manganese phosphate, or a combination of lithium nickel cobalt manganese oxide and lithium iron manganese phosphate.

[0016] Furthermore, the additive comprises a combination of lithium manganese oxide and lithium manganese iron phosphate; the mass ratio of lithium manganese iron phosphate to lithium manganese oxide is 0.5:1-5:1.

[0017] Furthermore, the mass ratio of lithium manganese iron phosphate to lithium manganese oxide is 1:1 to 3:1. Specifically, the mass ratio of lithium manganese iron phosphate to lithium manganese oxide can be 1:1, 1.3:1, 1.5:1, 1.8:1, 2:1, 2.3:1, 2.5:1, 2.8:1 or 3:1.

[0018] In step S1, the additive accounts for 0.5wt%-50wt% of the mass fraction of the positive electrode active material. If the additive accounts for less than 0.5wt% of the mass fraction of the positive electrode active material, the effect is not obvious and does not meet the requirements of the present invention; if the additive accounts for more than 50wt% of the mass fraction of the positive electrode active material, the proportion of the positive electrode active material is too small, and the battery's overall performance, such as specific capacity, charge / discharge rate, and safety, is significantly different from that of lithium iron phosphate batteries, which may result in the battery not meeting the relevant standards for lithium iron phosphate batteries.

[0019] In step S2, the mass ratio of the positive electrode active material additive mixture, conductive agent, and binder in the slurry is 80-98:0.5-3:0.5-3, the solid content of the slurry is 30%-80%, and the viscosity is 500-20000cp.

[0020] The conductive agent is selected from one or more of conductive carbon black, superconducting carbon black, conductive graphite, acetylene black, and carbon nanotubes.

[0021] The adhesive solution is prepared by mixing and stirring an adhesive with N-methylpyrrolidone. The solid content of the adhesive solution is 4-12%. The adhesive is selected from one or more of polytetrafluoroethylene, polyvinylidene fluoride, and polyvinyl alcohol.

[0022] In step S1, the positive electrode active material includes lithium iron phosphate.

[0023] Step S4 specifically involves compacting the electrode roll using a roller press to prepare a positive electrode sheet, wherein the compaction density is 1.5-4.0 g / cm³. 3 .

[0024] In step S3, an oven is used for drying at a temperature of 70-150℃.

[0025] <Second aspect>

[0026] This invention provides a lithium-ion battery positive electrode sheet prepared by the method described above.

[0027] <Third aspect>

[0028] The present invention provides a battery comprising a lithium-ion battery positive electrode as described above, and the battery further comprising a negative electrode, a separator, and an electrolyte.

[0029] The positive electrode, separator, and negative electrode are formed into a battery by winding or stacking. The lithium-ion battery can be any of cylindrical, prismatic, or pouch batteries.

[0030] The diaphragm includes any one or more of the following: polyethylene membrane, polypropylene membrane, polyethylene and polypropylene composite membrane, polyimide membrane, and ceramic membrane.

[0031] The electrolyte is an organic solution containing at least one lithium salt, including LiPF6, LiFSI, LiBF6, or LiClO4.

[0032] The active material in the negative electrode is any one or a mixture of several of the following: graphite, hard carbon, mesophase carbon microspheres, and silicon carbon.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The key to this invention lies in adding one, two, or a mixture of additives such as lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium manganese iron phosphate, Li5FeO4, lithium titanate, and lithium vanadium phosphate to lithium iron phosphate batteries. By varying the charge-discharge curves of the additives with those of lithium iron phosphate, the voltage curve of the lithium iron phosphate battery during charge-discharge is altered, thus mitigating the problem of excessively rapid voltage changes at the end of the lithium iron phosphate charge-discharge curve and improving the accuracy of estimating the battery's state of charge (SOC) based on battery voltage. This invention addresses the root cause of the problem by using an additive method, resulting in a simple manufacturing process that does not require changes to existing battery manufacturing processes or SOC estimation methods, and eliminates the need for complex calculations for verification.

[0035] 2. This invention conducts a comparative study on various additives and finds that additives such as lithium nickel cobalt manganese oxide, lithium manganese oxide, lithium iron manganese phosphate, Li5FeO4, lithium titanate, and lithium vanadium phosphate have better effects. This is because the charge and discharge voltage windows of these additives overlap more with those of lithium iron phosphate. During the charge and discharge process, the changes brought about by the additives during charge and discharge are more reflected in the voltage changes of the entire battery system. In addition, these additives are relatively easier to insert and extract lithium, and have better conductivity. In some practical applications of batteries, such as large current charge and discharge, they can play a charge and discharge function and achieve the effect of improving the sharp voltage change at the end of the charge and discharge of lithium iron phosphate.

[0036] 3. Improvement in SOC estimation accuracy can be achieved without changing existing mature SOC estimation methods;

[0037] 4. It solves the problem of inaccurate SOC estimation caused by the rapid voltage change at the end of the charge and discharge of lithium iron phosphate batteries, and can significantly improve the SOC estimation accuracy at the end of the charge and discharge of lithium iron phosphate batteries. Attached Figure Description

[0038] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0039] Figure 1 The charging curves of lithium iron phosphate batteries for Example 1 and Comparative Example 1 are shown.

[0040] Figure 2The discharge curves of lithium iron phosphate batteries for Example 4 and Comparative Example 1 are shown. Detailed Implementation

[0041] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0042] Examples 1-11

[0043] Examples 1-11 provide a method for preparing a positive electrode sheet for a lithium iron phosphate battery, comprising the following steps:

[0044] S1. Weigh the additives and lithium iron phosphate according to Table 1; the additives account for 0.5wt%-50wt% of the mass of lithium iron phosphate.

[0045] S2. Add the additive to lithium iron phosphate, the positive electrode active material of the lithium iron phosphate battery, and disperse it evenly to form a lithium iron phosphate additive mixture.

[0046] S3. Add the lithium iron phosphate additive mixture and conductive agent from step S1 to the adhesive solution and mix to form a slurry; in the slurry, the mass ratio of (lithium iron phosphate + additive): conductive agent: binder is 80-98:0.5-3:0.5-3, the solid content of the slurry is 54%, and the viscosity is 500-20000cp.

[0047] The adhesive solution is prepared by mixing and stirring an adhesive with the solvent N-methylpyrrolidone, and the solid content of the adhesive solution is 6%.

[0048] S4. The slurry prepared in step S3 is coated onto aluminum foil and then dried in an oven to prepare an electrode roll at a temperature of 70-150℃.

[0049] S5. The electrode rolls are compacted using a roller press to prepare positive electrode sheets, wherein the compaction density is 1.5-4.0 g / cm³. 3 .

[0050] The specific parameters, substances, and amounts for each step are shown in Table 1.

[0051] Comparative Examples 1-4

[0052] The preparation methods of Comparative Examples 1-4 are basically the same as those of Example 1, with the differences shown in Table 1.

[0053] Table 1

[0054]

[0055]

[0056]

[0057] Application performance testing example:

[0058] The battery electrodes prepared in Examples 1-11 and Comparative Examples 1-4 are used to prepare batteries, specifically including the following steps:

[0059] 1. According to the battery design, the rolled electrode sheets are slit and die-cut;

[0060] 2. The battery can be assembled into a round shape by winding or stacking, or it can be assembled into a square or pouch battery shape. The round battery is used in this performance test example.

[0061] 3. By performing operations such as welding tabs, encapsulation, baking, electrolyte injection, formation, and capacity testing on the battery cell, an improved battery sample can be obtained, in which the voltage change trend at the charging and discharging ends slows down.

[0062] The voltage changes at the end of the charge and discharge cycles of the battery samples prepared in each embodiment and comparative example were tested.

[0063] The testing method is as follows:

[0064] The prepared battery was left to stand at 25℃-23℃ for 12 hours, then charged with a constant current of 0.1C. The specific charging cutoff voltages for each embodiment and comparative example are shown in Table 2. After standing for 30 minutes, it was discharged with a constant current of 0.1C to 2V to obtain the charge-discharge voltage-capacity curve of the battery sample.

[0065] Table 2

[0066]

[0067] from Figure 1 It can be seen that the sample with additives generated a new voltage plateau at the end of charging, and the voltage change at the end of charging was smoother compared to the sample without additives, which can improve the estimation accuracy of SOC at the end of charging.

[0068] from Figure 2 It can be seen that the voltage change trend of the sample with additives slows down significantly at the end of the discharge, which can improve the estimation accuracy of the SOC at the end of the discharge.

[0069] To evaluate the improvement of SOC estimation accuracy by additives, the charge / discharge voltage-capacity curves of the battery samples obtained using the above test methods are used. Δ represents the steepness of the voltage change trend at the end of the charge / discharge period. 充 = (Charging cutoff voltage - Inflection point voltage) / (Q) 截止 -Q拐点 ), Δ 放 = (Inflection point voltage - 2) / (Q) 截止 -Q 拐点 The specific values ​​of the charging cutoff voltage are shown in Table 2. The inflection point voltage represents the voltage at the point where the slope of the charge / discharge curve changes drastically at the end. Q 截止 Q represents the battery capacity at the end of the charge / discharge cycle. 拐点 The value of Δ represents the battery capacity corresponding to the inflection point voltage. Therefore, it is clear that the larger the value of Δ, the steeper the voltage curve at the end of the battery charge and discharge period, and the more drastic the voltage change. The specific calculation results of each embodiment and comparative example are shown in Table 3.

[0070] Table 3

[0071]

[0072] As can be seen from Table 3, in Examples 1-11, Δ 充 and Δ 放 The voltage difference is significantly smaller than that of comparative examples 1-4, indicating a significantly slower voltage change trend at the end of the battery charge-discharge cycle, which is beneficial for improving the accuracy of SOC estimation. In the preferred combination of lithium manganese iron phosphate and lithium manganese phosphate, the manganese content is increased, extending the voltage difference compared to lithium iron phosphate. Simultaneously, since lithium manganese iron phosphate has a higher specific capacity than lithium manganese phosphate, the capacity of the additives is maximized during charge-discharge, improving the proportion of the slower-than-normal voltage change in the charge-discharge curve.

[0073] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a positive electrode sheet for a lithium-ion battery, characterized in that, Includes the following steps: S1. Adding the additive to the positive electrode active material and dispersing it to form a positive electrode active material additive mixture; the additive is a combination of lithium manganese oxide and lithium iron manganese phosphate; the mass ratio of lithium iron manganese phosphate to lithium manganese oxide is 0.5:1-5:1; the positive electrode active material is lithium iron phosphate; the additive accounts for 0.5wt%-50wt% of the mass fraction of the positive electrode active material. S2. Add the positive electrode active material additive mixture and conductive agent from step S1 to the adhesive solution and mix to form a slurry; the adhesive solution is formed by mixing and stirring a binder and N-methylpyrrolidone. S3. The slurry prepared in step S2 is coated onto aluminum foil and then dried to form an electrode roll. S4. Press the electrode roll into a positive electrode sheet.

2. The method for preparing the positive electrode sheet of a lithium-ion battery according to claim 1, characterized in that, In step S2, the mass ratio of the positive electrode active material additive mixture, conductive agent, and binder in the slurry is 80-98:0.5-3:0.5-3.

3. The method for preparing the positive electrode sheet of a lithium-ion battery according to claim 2, characterized in that, The conductive agent is selected from one or more of conductive carbon black, conductive graphite, acetylene black, and carbon nanotubes.

4. A lithium-ion battery positive electrode sheet prepared by the method described in any one of claims 1-3.

5. A battery, characterized in that, The battery includes the positive electrode sheet of the lithium-ion battery as described in claim 4, and the battery further includes a negative electrode sheet, a separator, and an electrolyte.

6. The battery according to claim 5, characterized in that, The electrolyte is an organic solution containing at least one lithium salt, including LiPF6, LiFSI, LiBF6, or LiClO4.

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