Method for manufacturing long-life secondary battery and secondary battery

Abstract

The application provides a preparation method of a long-life secondary battery and the secondary battery, and relates to the technical field of batteries. The preparation method of the long-life secondary battery provided by the application comprises the following steps: mainly taking a positive plate, a negative plate and an electrolyte as raw materials to prepare a secondary battery, so that the secondary battery is in the 88%-98% SOC stage, 3<=negative exchange current density / positive exchange current density<=5; the positive plate comprises a positive current collector and a positive active material layer arranged on the positive current collector; and the negative plate comprises a negative current collector and a negative active material layer arranged on the negative current collector. The preparation method realizes the kinetic performance matching of the positive electrode and the negative electrode in the high SOC stage by limiting the exchange current density relationship of the positive electrode and the negative electrode, reduces the risk of lithium precipitation in the charging process, and thus realizes the rapid evaluation and accurate design of the cycle life of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a method for preparing a long-life secondary battery and the secondary battery itself. Background Technology

[0002] Lithium-ion batteries have become the most widely used power source for electric vehicles due to their small size, high energy density, lack of environmental pollution, low self-discharge rate, absence of memory effect, and eco-friendliness. However, the performance degradation of lithium-ion batteries themselves and issues related to charge and discharge management can lead to the end of their lifespan, often causing overall vehicle system malfunction. Therefore, the reliability of lithium-ion battery life directly affects the effective service life of the entire vehicle.

[0003] The current huge demand in the energy storage field is driving the pursuit of longer lifespans for lithium-ion batteries. How to precisely design a long-life lithium-ion battery, reduce unnecessary design margins, lower costs, and improve battery consistency are paramount in lithium battery research.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] The first objective of this invention is to provide a method for preparing a long-life secondary battery to solve at least one of the above-mentioned problems.

[0006] The second objective of this invention is to provide a secondary battery.

[0007] A third objective of this invention is to provide a method for evaluating the cycle life of a secondary battery.

[0008] In a first aspect, the present invention provides a method for preparing a long-life secondary battery, comprising: mainly using a positive electrode sheet, a negative electrode sheet, and an electrolyte as raw materials to prepare the secondary battery, such that the secondary battery, in the 88%-98% SOC stage, has a negative electrode exchange current density of 3 ≤ J N ) / Positive electrode exchange current density (J p ≤5; The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0009] As a further technical solution, the secondary battery at 90% SOC stage has a negative electrode exchange current density / positive electrode exchange current density ≤ 5.

[0010] As a further technical solution, the secondary battery at 90% SOC stage has a negative electrode exchange current density of 3.5 / positive electrode exchange current density of 4.5.

[0011] As a further technical solution, the negative electrode exchange current density = exchange current / mass of negative electrode active material / area of ​​negative electrode sheet; The positive electrode exchange current density = exchange current / mass of positive electrode active material / area of ​​positive electrode sheet.

[0012] As a further technical solution, the formula for calculating the switching current is as follows: ; Where i0 is the exchange current; R is the ideal gas constant; T is the battery temperature; n is the number of transferred electrons; F is the Faraday constant; and K is the slope of the battery's current-overpotential curve at the open-circuit voltage at the 90% SOC stage.

[0013] As a further technical solution, the battery is subjected to LSV or CV tests at 90% SOC to obtain the current-overpotential curve, and then the slope of the current-overpotential curve at the open circuit voltage is obtained. The range of the LSV or CV test is: open circuit voltage -10mV to open circuit voltage +10mV.

[0014] As a further technical solution, an electrochemical workstation is used to perform LSV or CV tests.

[0015] As a further technical solution, the secondary battery includes a lithium-ion battery.

[0016] In a second aspect, the present invention provides a secondary battery, comprising: a positive electrode and a negative electrode; The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector; During the 88%-98% SOC stage of the secondary battery, the ratio of negative electrode exchange current density to positive electrode exchange current density is 3 ≤ 5.

[0017] Thirdly, the present invention provides a method for evaluating the cycle life of a secondary battery. If the cycle life of the secondary battery is long when it is in the 88%-98% SOC stage and the ratio of negative electrode exchange current density to positive electrode exchange current density is 3≤5, then the cycle life of the battery is long.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The method for preparing a long-life secondary battery provided by this invention achieves kinetic performance matching of the positive and negative electrodes in the high SOC stage by limiting the exchange current density relationship between the positive and negative electrodes, reducing risks such as lithium plating during the charging process, thereby enabling rapid evaluation and precise design of the battery cycle life. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 Cyclic trends for different groups; Figure 2 Results of three-electrode testing during battery charging. Detailed Implementation

[0021] The embodiments and examples of the present invention will be described in detail below. However, those skilled in the art will understand that the following embodiments and examples are for illustrative purposes only and should not be considered as limiting the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise specified, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0022] In a first aspect, the present invention provides a method for preparing a long-life secondary battery, comprising: mainly using positive electrode sheet, negative electrode sheet and electrolyte as raw materials to prepare a secondary battery, such that in the 88%-98% SOC stage, 3≤negative electrode exchange current density / positive electrode exchange current density≤5; The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0023] The higher the SOC, the closer the negative electrode potential is to 0V, increasing the risk of lithium plating. The inventors' research revealed that when (J... N When / Jp) < 3, the negative electrode has insufficient kinetics at high SOC, while the positive electrode has relatively large kinetics. At this time, the Li on the negative electrode surface + Excessive concentration can easily lead to lithium precipitation, reducing cycle life; when (J N When / Jp)>5, it indicates that the negative electrode kinetics are too large or the positive electrode kinetics are too small, leading to an increase in side reactions, an increase in low-capacity risk, and an increase in cost.

[0024] The method for preparing a long-life secondary battery provided by this invention achieves kinetic performance matching of the positive and negative electrodes in the high SOC stage by limiting the exchange current density relationship between the positive and negative electrodes, reducing risks such as lithium plating during the charging process, thereby enabling rapid evaluation and precise design of the battery cycle life.

[0025] In some alternative implementations, the secondary battery at 90% SOC has a negative electrode exchange current density / positive electrode exchange current density ≤ 5.

[0026] In some alternative implementations, the secondary battery at 90% SOC has a negative electrode exchange current density of 3.5 / positive electrode exchange current density of 4.5.

[0027] By further optimizing and adjusting the ratio of negative electrode exchange current density to positive electrode exchange current density in the secondary battery at 90% SOC, the lifespan of the prepared battery is extended.

[0028] In this invention, the negative electrode exchange current density = exchange current / mass of negative electrode active material / area of ​​negative electrode sheet, with units of mA / (g*cm). 2 ); The positive electrode exchange current density = exchange current / mass of positive electrode active material / area of ​​positive electrode sheet, in mA / (g*cm²). 2 ).

[0029] In some alternative implementations, the formula for calculating the exchange current is as follows: ; Where i0 is the exchange current; R is the ideal gas constant; T is the battery temperature in K; n is the number of transferred electrons (for lithium-ion batteries, lithium ions are valence 1, and n is 1; for magnesium-ion batteries, magnesium ions are valence 2, and n is 2); F is the Faraday constant in C / mol; and K is the slope of the current-overpotential curve of the battery at the open circuit voltage at the 90% SOC stage.

[0030] The exchange current can be obtained based on the above calculation formula.

[0031] In some alternative implementations, the slope K is obtained as follows: At 90% SOC, the battery is subjected to LSV or CV tests to obtain the current-overpotential curve, and then the slope of the current-overpotential curve at the open circuit voltage is obtained. The range of the LSV or CV test is: open circuit voltage -10mV to open circuit voltage +10mV.

[0032] Substitute the slope obtained from the test into the above formula to calculate the exchange rate.

[0033] In some alternative implementations, an electrochemical workstation is used for LSV or CV testing.

[0034] In some alternative implementations, the secondary battery includes a lithium-ion battery.

[0035] Secondly, the present invention provides a secondary battery prepared by the above-described preparation method, comprising: a positive electrode and a negative electrode; The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector; During the 88%-98% SOC stage of the secondary battery, the ratio of negative electrode exchange current density to positive electrode exchange current density is 3 ≤ 5.

[0036] Because the kinetic performance of the positive and negative electrodes of this secondary battery is matched during the high SOC stage, the risk of lithium plating during the charging process can be effectively avoided. Therefore, this secondary battery has a longer service life.

[0037] Thirdly, the present invention provides a method for evaluating the cycle life of a secondary battery. If the cycle life of the secondary battery is long when it is in the 88%-98% SOC stage and the ratio of negative electrode exchange current density to positive electrode exchange current density is 3≤5, then the cycle life of the battery is long.

[0038] The evaluation method provided by this invention is simple and convenient, and can quickly evaluate the cycle life of multiple secondary batteries, which is of guiding significance for battery design.

[0039] In the evaluation method, the negative electrode exchange current density / positive electrode exchange current density is described in the first aspect of the present invention.

[0040] The present invention will be further illustrated below with specific embodiments and comparative examples. However, it should be understood that these embodiments are merely for the purpose of more detailed illustration and should not be construed as limiting the present invention in any way.

[0041] Example 1 Lithium-ion batteries A1, A2, A3, A4, and A5 were selected, and the negative electrode exchange current density / positive electrode exchange current density of each battery was measured at 90% SOC. The method is as follows: A1: Positive electrode preparation method: Lithium iron phosphate (a), conductive carbon black (SP), and PVDF are mixed in a mass ratio of 96:2:2, then added to NMP and thoroughly mixed. After uniform mixing, the mixture is coated on both sides onto a (13+1+1)μm carbon-coated aluminum foil. The electrode is then dried, rolled, slit, and cut to obtain the positive electrode. Samples are then assembled into a 12mm diameter half-cell. Negative electrode preparation method: Graphite (a), conductive carbon black (SP), CMC, and SBR are mixed in a mass ratio of 96:1:1.1:1.9, then added to water and thoroughly mixed. After uniform mixing, the mixture is coated on both sides onto a 6μm diameter copper foil. The electrode is then dried, rolled, slit, and cut to obtain the negative electrode. Samples are then assembled into a 12mm diameter half-cell. A2: Positive electrode preparation method: Lithium iron phosphate (a), conductive carbon black (SP), and PVDF are mixed in a mass ratio of 96:2:2, then added to NMP and thoroughly mixed. After uniform mixing, the mixture is coated on both sides onto a (13+1+1)μm carbon-coated aluminum foil. The electrode is then dried, rolled, slit, and cut to obtain the positive electrode. Samples are then assembled into a 12mm diameter half-cell. Negative electrode preparation method: Graphite (b) (D50 radius b > a), conductive carbon black (SP), CMC, and SBR are mixed in a mass ratio of 96:1:1.1:1.9, then added to water and thoroughly mixed. After uniform mixing, the mixture is coated on both sides onto a 6μm diameter copper foil. The electrode is then dried, rolled, slit, and cut to obtain the negative electrode. Samples are then assembled into a 12mm diameter half-cell. A3: Positive electrode preparation method: Lithium iron phosphate (a), conductive carbon black (SP), and PVDF are mixed in a mass ratio of 95:3:2, then added to NMP and mixed thoroughly. After uniform mixing, the mixture is coated on both sides onto a (13+1+1)μm carbon-coated aluminum foil. The electrode is then dried, rolled, slit, and cut to obtain the positive electrode. Samples are then assembled into a 12mm diameter half-cell. Negative electrode preparation method: Graphite (a), conductive carbon black (SP), CMC, and SBR are mixed in a mass ratio of 96:1:1.1:1.9, then added to water and mixed thoroughly. After uniform mixing, the mixture is coated on both sides onto a 6μm diameter copper foil. The electrode is then dried, rolled, slit, and cut to obtain the negative electrode. Samples are then assembled into a 12mm diameter half-cell. A4: Positive electrode preparation method: Lithium iron phosphate b (D50 radius b > a), conductive carbon black (SP), and PVDF are mixed in a mass ratio of 96:2:2. Then, NMP is added and thoroughly mixed. After uniform mixing, the mixture is coated on both sides onto a (13+1+1)μm carbon-coated aluminum foil. The electrode is then dried, rolled, slit, and cut to obtain the positive electrode. Samples are assembled into a 12mm diameter half-cell. Negative electrode preparation method: Graphite a, conductive carbon black (SP), CMC, and SBR are mixed in a mass ratio of 96:1:1.1:1.9. Water is added and thoroughly mixed. After uniform mixing, the mixture is coated on both sides onto a 6μm diameter copper foil. The electrode is then dried, rolled, slit, and cut to obtain the negative electrode. Samples are assembled into a 12mm diameter half-cell. A5: Positive electrode preparation method: Lithium iron phosphate c (D50 radius a > c), conductive carbon black (SP), and PVDF are mixed at a mass ratio of 95:3:2. Then, NMP is added and thoroughly mixed. After uniform mixing, the mixture is coated on both sides onto a (13+1+1)μm carbon-coated aluminum foil. The electrode is then dried, rolled, slit, and cut to obtain the positive electrode. Samples are assembled into a 12mm diameter half-cell. Negative electrode preparation method: Graphite c (D50 radius a > c), conductive carbon black (SP), CMC, and SBR are mixed at a mass ratio of 96:1:1.1:1.9. The mixture is then added to water and thoroughly mixed. After uniform mixing, the mixture is coated on both sides onto a 6μm diameter copper foil. The electrode is then dried, rolled, slit, and cut to obtain the negative electrode. Samples are assembled into a 12mm diameter half-cell. It should be noted that the above "(13+1+1)μm carbon-coated aluminum foil" refers to a 13µm aluminum foil with a 1µm carbon coating on both sides.

[0042] Compared to A2, increasing the D50 radius of the graphite particles in the A2 negative electrode increases the kinetic performance of the negative electrode sheet. Compared to A3, increasing the content of A3 positive electrode conductive agent improves the dynamic performance of the positive electrode sheet; Compared to A4, increasing the D50 particle size of the lithium iron phosphate cathode particles reduces the kinetic performance of the cathode sheet. Compared to A1, A5 reduces the D50 particle size of the negative electrode graphite particles, while also reducing the D50 particle size of the lithium iron phosphate particles and increasing the conductive agent content, thereby improving the negative electrode kinetics and enhancing the positive electrode kinetics.

[0043] 1. At 90% SOC, perform LSV or CV tests on the battery to obtain the current-overpotential curve, and then obtain the slope of the current-overpotential curve at the open circuit voltage based on the curve. 2. Calculate the battery's exchange rate i0 according to the following formula.

[0044]

[0045] Where i0 is the exchange current; R is the ideal gas constant; T is the battery temperature; n is 1; F is the Faraday constant; and K is the slope of the battery's current-overpotential curve at the open-circuit voltage at the 90% SOC stage.

[0046] 3. Obtain the mass of the negative electrode active material, the area of ​​the negative electrode sheet, the mass of the positive electrode active material, and the area of ​​the positive electrode sheet. Calculate the negative electrode exchange current density and the positive electrode exchange current density based on the formula: negative electrode exchange current density = exchange current / mass of negative electrode active material / area of ​​negative electrode sheet, and positive electrode exchange current density = exchange current / mass of positive electrode active material / area of ​​positive electrode sheet. 4. Obtain the ratio of the negative electrode exchange current density to the positive electrode exchange current density for each battery. The results are shown in the table below.

[0047] Table 1 Exchange current density of different groups

[0048] As can be seen from the table above, the ratio of negative electrode exchange current density to positive electrode exchange current density for batteries A2 and A5 at 90% SOC is less than 3. However, the ratio for batteries A1, A3, and A4 at 90% SOC is between 3 and 5.

[0049] 5. The cycle life of the above batteries was tested at 1C / 1C, 2.5V-3.65V, with a cutoff of 70% SOH. The results are as follows. Figure 1 As shown in the figure, batteries A2 and A5 exhibit poor cycle life, and disassembly revealed significant lithium plating at the battery edges. Batteries A1, A3, and A4, on the other hand, demonstrate better cycle life, with only minor lithium plating at the battery edges. Furthermore, the inventors discovered that when the negative electrode exchange current density / positive electrode exchange current ratio exceeds 5, the excess ratio of the negative electrode is very high, which is detrimental to the battery's energy density and increases cost. This indicates that when the negative electrode exchange current density / positive electrode exchange current ratio is between 3 and 5 at 90% SOC, the kinetic performance of the positive and negative electrodes is well-matched, effectively reducing the risk of lithium plating during cycling and resulting in a longer lifespan.

[0050] Example 2 A battery was prepared according to the preparation method of battery A1 in Example 1. During the battery preparation process, a lithium-plated copper wire was placed inside the battery as a reference electrode, resulting in a three-electrode battery with a reference electrode. Charge-discharge studies were conducted on the three-electrode battery, and the results are as follows: Figure 2As shown. The results indicate that the higher the SOC, the closer the negative electrode potential is to 0V, and the higher the risk of lithium plating. Therefore, this invention uses the ratio of negative electrode exchange current density to positive electrode exchange current density at the high SOC stage to evaluate the cycle life of the battery.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for producing a long-life secondary battery, characterized by, include: The secondary battery is mainly prepared using positive electrode sheet, negative electrode sheet and electrolyte as raw materials, so that the secondary battery has 3≤negative electrode exchange current density / positive electrode exchange current density≤5 at 90% SOC stage. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; wherein the positive electrode active material is lithium iron phosphate; The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector; wherein the negative electrode active material is graphite; The negative electrode exchange current density = exchange current / mass of negative electrode active material / area of ​​negative electrode sheet; The positive electrode exchange current density = exchange current / mass of positive electrode active material / area of ​​positive electrode sheet.

2. The production method according to claim 1, characterized by, At 90% SOC, the secondary battery has a negative electrode exchange current density of 3.5 / positive electrode exchange current density of 4.

5.

3. The preparation method according to claim 1, characterized in that, The formula for calculating the switching current is as follows: ； Where i0 is the exchange current; R is the ideal gas constant; T is the battery temperature; n is the number of transferred electrons; F is the Faraday constant; and K is the slope of the battery's current-overpotential curve at the open-circuit voltage at the 90% SOC stage.

4. The production method according to claim 3, characterized by Half-cells of the positive and negative electrodes are obtained respectively. At 90% SOC, the half-cells are subjected to LSV or CV tests to obtain current-overpotential curves, and then the slope of the current-overpotential curves at the open circuit voltage is obtained. The range of the LSV or CV test is: open circuit voltage -10mV to open circuit voltage +10mV.

5. The preparation method according to claim 4, characterized in that, LSV or CV tests were performed using an electrochemical workstation.

6. The method of claim 1, wherein, The secondary battery includes a lithium-ion battery.

7. A secondary battery characterized by comprising: include: Positive and negative electrode plates; The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; wherein the positive electrode active material is lithium iron phosphate; The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector; wherein the negative electrode active material is graphite; At 90% SOC, the secondary battery has a negative electrode exchange current density / positive electrode exchange current density ≤ 5. The negative electrode exchange current density = exchange current / mass of negative electrode active material / area of ​​negative electrode sheet; The positive electrode exchange current density = exchange current / mass of positive electrode active material / area of ​​positive electrode sheet.

8. A method for evaluating cycle life of a secondary battery, characterized by, The secondary battery includes: a positive electrode and a negative electrode; The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector; wherein the positive electrode active material is lithium iron phosphate; The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector; wherein the negative electrode active material is graphite; If the secondary battery is at 90% SOC and the ratio of negative electrode exchange current density to positive electrode exchange current density is 3≤5, then the battery cycle life is long. The negative electrode exchange current density = exchange current / mass of negative electrode active material / area of ​​negative electrode sheet; The positive electrode exchange current density = exchange current / mass of positive electrode active material / area of ​​positive electrode sheet.

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