Positive electrode sheet and lithium secondary battery

Abstract

In order to overcome the problem of low pass rate of lithium-ion battery needle puncture test in the prior art, the present application provides a positive electrode sheet and a lithium secondary battery, the positive electrode sheet including a current collector, a first active material layer and a second active material layer, the first active material layer is arranged on the surface of the current collector, the second active material layer is arranged on the surface of the first active material layer away from the current collector, and the membrane conductivity of the first active material layer satisfies the following relationship: 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm, wherein 0.01≤β≤0.1, 80%≤W1≤90%, 0.005S / cm≤δ1≤1S / cm, 1×10 ‑5 S / cm≤δ2≤5×10 ‑5 S / cm, 5%≤W2≤20%; the positive electrode sheet provided in the present application can increase the membrane resistance of the positive electrode sheet, reduce the short-circuit current during acupuncture, thereby improving the battery acupuncture pass rate and safety performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a positive electrode sheet and a lithium secondary battery. Background Art

[0002] Efficient and reliable energy storage systems are crucial to our modern society. While lithium-ion batteries offer excellent performance and are widely used in portable electronics and electric vehicles, frequent fires and explosions limit their further and broader application. Lithium-ion battery safety has become a key issue in the industry.

[0003] There have been numerous recent reports of fire accidents and failures involving lithium-ion batteries. Due to their high operating voltage, high energy density, and use of flammable organic electrolytes, lithium-ion batteries (LIBs) are energy devices with the potential to catch fire. Therefore, lithium-ion batteries designed for commercial use must meet safety standards and undergo safety testing, such as electrical, environmental, and mechanical testing. These safety standards evaluate battery safety performance and ensure safe use. The needle penetration test is a widely used lithium-ion safety test used to assess internal short circuits, a major cause of battery fires. Existing lithium-ion batteries have a low pass rate for needle penetration testing. This is because during penetration, a short-circuit current is generated within the cell, causing a rapid temperature rise, which subsequently leads to thermal decomposition and failure of the internal materials. Analysis of the short-circuit mode reveals that the highest temperature rise occurs when the aluminum foil shorts against the negative electrode, representing the primary point of thermal runaway failure. Summary of the Invention

[0004] In order to solve the problem of low pass rate of lithium-ion battery needle penetration test in the prior art, the present application provides a positive electrode sheet and a lithium secondary battery.

[0005] To solve the above technical problems, the present application provides a positive electrode sheet, including a current collector, a first active material layer, and a second active material layer, wherein the first active material layer is disposed on the surface of the current collector, and the second active material layer is disposed on a surface of the first active material layer facing away from the current collector, the first active material layer includes a first active material and a first resistance increasing agent, and the membrane conductivity of the first active material layer satisfies the following relationship:

[0006] 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm

[0007] Among them, 0.01≤β≤0.1, 80%≤W1≤90%, 0.005S / cm≤δ1≤1S / cm, 1×10 -5 S / cm≤δ2≤5×10 -5 S / cm, 5%≤W2≤20%;

[0008] β is the impact factor;

[0009] δ1 is the powder conductivity of the first active material, in S / cm;

[0010] δ2 is the powder conductivity of the first resistance increasing agent, in S / cm;

[0011] W1 is the mass percentage of the first active material in the first active material layer, in %;

[0012] W2 is the mass ratio of the first resistance increasing agent in the first active material layer, in %.

[0013] Preferably, the membrane conductivity of the first active material layer satisfies the following relationship:

[0014] 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.005S / cm.

[0015] Preferably, the impact factor β is 0.01S / cm to 0.05S / cm.

[0016] Preferably, the first active material includes lithium iron phosphate and its derivatives;

[0017] The powder conductivity δ1 of the first active material is 0.004 S / cm to 1 S / cm;

[0018] The mass proportion W1 of the first active material in the first active material layer is 85% to 90%.

[0019] Preferably, the first resistance increasing agent comprises a ceramic material;

[0020] The mass ratio W2 of the first resistance increasing agent in the first active material layer is 5% to 15%.

[0021] Preferably, the first active material layer further includes a first binder and a first conductive agent, the mass proportion of the first binder in the first active material layer is 2.0% to 10.0%; the mass proportion of the first conductive agent in the first active material layer is 0% to 1%.

[0022] Preferably, the second active material layer includes a second active material, a second binder and a second conductive agent;

[0023] Based on the mass of the second active material layer being 100%, the mass ratios of the second active material, the second binder, and the second conductive agent are (94-97):(1-2):(2-5).

[0024] Preferably, the positive electrode sheet further includes an insulating layer, which is disposed on the current collector and arranged side by side with the first active material layer, and the insulating layer includes a ceramic material.

[0025] Preferably, the thickness of the insulating layer is 10um to 30um.

[0026] On the other hand, the present application provides a lithium secondary battery, comprising a negative electrode sheet, a separator, and the positive electrode sheet as described above.

[0027] Beneficial effects:

[0028] Compared with the prior art, the positive electrode sheet of the present application is coated with a first active material layer and a second active material layer on the surface of the current collector, and the membrane conductivity of the first active material layer is required to satisfy the relationship 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm, which can increase the membrane resistance of the positive electrode sheet and reduce the short-circuit current during acupuncture, thereby improving the battery acupuncture pass rate and safety performance; at the same time, the first active material layer also cooperates with the second active material layer to ensure that the electrical performance of the lithium-ion battery is not lost. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural diagram of the positive electrode;

[0030] 1. Current collector; 2. First active material layer; 3. Second active material layer; 4. Insulating layer. DETAILED DESCRIPTION

[0031] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0032] The present application provides a positive electrode sheet, including a current collector 1, a first active material layer 2, and a second active material layer 3. The first active material layer 2 is disposed on the surface of the current collector 1, and the second active material layer 3 is disposed on the surface of the first active material layer 2 facing away from the current collector 1. The first active material layer 2 includes a first active material and a first resistance increasing agent. The membrane conductivity of the first active material layer 2 satisfies the following relationship:

[0033] 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm

[0034] Among them, 0.01≤β≤0.1, 80%≤W1≤90%, 0.005S / cm≤δ1≤1S / cm, 1×10 -5 S / cm≤δ2≤5×10-5 S / cm, 5%≤W2≤20%;

[0035] β is the impact factor;

[0036] δ1 is the powder conductivity of the first active material, in S / cm;

[0037] δ2 is the powder conductivity of the first resistance increasing agent, in S / cm;

[0038] W1 is the mass percentage of the first active material in the first active material layer 2, in %;

[0039] W2 is the mass ratio of the first resistance increasing agent in the first active material layer 2 , in %.

[0040] The positive electrode sheet of the present application is coated with a first active material layer 2 and a second active material layer 3 on the surface of the current collector 1, wherein a first resistance increasing agent is added to the first active material layer 2, and the membrane conductivity of the first active material layer 2 is required to satisfy the relationship 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm, thereby increasing the membrane resistance of the positive electrode sheet of the lithium-ion battery and reducing the short-circuit current during needle puncture, thereby improving the battery needle puncture pass rate and safety performance. The first active material layer 2 provided on the positive electrode sheet not only has the effect of increasing the positive electrode membrane resistance and reducing the short-circuit current during needle puncture, but also cooperates with the second active material layer 3 to ensure that the electrical performance of the lithium-ion battery is not compromised.

[0041] In some preferred embodiments, the membrane conductivity of the first active material layer 2 satisfies the following relationship:

[0042] 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.005S / cm.

[0043] Specifically, when the membrane conductivity of the first active material layer 2 satisfies the relationship 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.005S / cm, the membrane resistance of the positive electrode sheet is higher, and its pass rate is higher when the battery is subjected to a needle penetration test, which is more conducive to reducing the short-circuit current of the battery during a needle penetration test and improving the safety performance of the battery.

[0044] The membrane conductivity of the first active material layer 2 is calculated according to the relationship β×(δ1×W1+δ2×W2), and its value can be 0.001S / cm, 0.003S / cm, 0.005S / cm, 0.007S / cm, 0.009S / cm, 0.01S / cm, 0.002S / cm, 0.004S / cm, 0.006S / cm, 0.008S / cm, etc., as long as the membrane conductivity of the first active material layer 2 satisfies the relationship 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm.

[0045] In some embodiments, β is an influence factor, and the range of β is 0.01≤β≤0.1.

[0046] Specifically, β can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, etc.

[0047] The greater the mass fraction of the first conductive agent in the first active material layer 2, the higher the conductivity of the resulting first active material layer 2 membrane, and the higher the short-circuit current of the resulting battery when punctured; the smaller the mass fraction of the first conductive agent, the lower the conductivity of the first active material layer 2 membrane, and the lower the short-circuit current when punctured. After extensive research, the inventors have discovered that controlling the conductivity and mass fraction of the first active material, adding a first resistance increasing agent, and controlling the mass fraction of the first conductive agent can increase the membrane resistance of the first active material layer 2. The inventors have discovered that the magnitude of the β value is related to the mass fraction of the first binder in the first active material layer 2: the greater the mass fraction of the first binder, the smaller the β value; the greater the mass fraction of the first conductive agent, the larger the β value. After extensive research, the inventors found that when β is in the range of 0.01 to 0.1, the conductivity and mass ratio of the first active material and the first resistance increasing agent are limited, and the membrane conductivity of the prepared first active material layer 2 satisfies the relationship 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm, which can increase the membrane resistance of the lithium-ion battery, reduce the short-circuit current during battery puncture, and improve the safety performance of the battery.

[0048] In some preferred embodiments, the impact factor β is 0.01 to 0.05.

[0049] In some embodiments, the powder conductivity δ1 of the first active material is 0.005 S / cm≤δ1≤1 S / cm; and the mass proportion W1 of the first active material in the first active material layer 2 is 80%≤W1≤90%.

[0050] Specifically, the powder conductivity of the first active material is within the range of 0.005 S / cm to 1 S / cm, which helps improve the sheet resistance of the first active material layer 2. For example, the powder conductivity of the first active material can be 0.005 S / cm, 0.01 S / cm, 0.04 S / cm, 0.06 S / cm, 0.08 S / cm, 0.1 S / cm, 0.3 S / cm, 0.5 S / cm, 0.7 S / cm, 0.8 S / cm, 0.9 S / cm, 1.0 S / cm, etc. The mass proportion W1 of the first active material in the first active material layer 2 is 80% ≤ W1 ≤ 90% to ensure that the battery's electrical performance, such as capacity, is not compromised.

[0051] If the powder conductivity δ1 of the first active material is greater than 1S / cm, the sheet resistance of the prepared first active material layer 2 will be lower, and the battery needle puncture pass rate will be reduced. If the mass proportion W1 of the first active material layer 2 is greater than 90%, although the battery capacity can be correspondingly increased, the sheet resistance of the first active material layer 2 does not satisfy the relationship 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm. The sheet resistance of the first active material layer 2 is reduced, the conductivity of the positive electrode sheet is high, and the short-circuit current of the prepared battery during needle puncture is high.

[0052] In some preferred embodiments, the first active material includes lithium iron phosphate and its derivatives; the mass proportion W1 of the first active material in the first active material layer 2 is 85% to 90%; and the powder conductivity δ1 of the first active material is 0.004S / cm to 1S / cm.

[0053] In some embodiments, the mass proportion W2 of the first resistance increasing agent in the first active material layer 2 is 5% to 20%; the powder conductivity δ2 of the first resistance increasing agent is 1×10 -5 S / cm~5×10 -5 S / cm.

[0054] Specifically, the added mass percentage of the first active material layer 2 is 5% to 20%, and the conductivity is 1×10 -5 S / cm~5×10 -5 The first resistance increasing agent, with a range of 100 S / cm, reduces the conductivity of the first active material layer 2, thereby reducing the short-circuit current during battery puncture and improving the battery's puncture pass rate. The weight percentage of the first resistance increasing agent can be 5%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20%, etc.

[0055] If the mass ratio of the first resistance increasing agent is less than 5%, the sheet resistance of the first active material layer 2 is reduced; if the mass ratio of the first resistance increasing agent is greater than 20%, the content of the first active material is reduced, the capacity of the battery is reduced, and the electrical performance of the battery is lost. If the conductivity of the first resistance increasing agent is higher than 5×10 -5 S / cm, the membrane conductivity of the first active material layer 2 is relatively high, and the effect of reducing the membrane conductivity of the first active material layer 2 is not achieved.

[0056] In some preferred embodiments, the first resistance increasing agent includes a ceramic material; and the mass proportion W2 of the first resistance increasing agent in the first active material layer 2 is 5% to 15%.

[0057] Specifically, the ceramic material may be one or more of aluminum oxide, silicon oxide, silicon nitride, zirconium oxide, silicon carbide, boron nitride, titanium oxide, magnesium oxide, beryllium oxide, aluminum carbide alloy, silicon carbide alloy, zirconium carbide, titanium carbide, and titanium nitride.

[0058] In some embodiments, the first active material layer 2 further includes a first binder and a first conductive agent, wherein the mass proportion of the first binder in the first active material layer 2 is 2.0% to 10.0%; the mass proportion of the first conductive agent in the first active material layer 2 is 0% to 1%.

[0059] Specifically, the first binder includes one or more of styrene-butadiene rubber, water-based acrylic resin, carboxymethyl cellulose, polyvinylidene fluoride, ethylene-vinyl acetate copolymer, and polyvinyl alcohol. The first conductive agent includes one or more of graphite, carbon black, graphene, carbon nanotubes, carbon nanofibers, SuperP, and acetylene black.

[0060] The mass proportion of the first binder in the first active material layer 2 is between 2% and 10%. The content of the first binder is relatively high, which increases the bonding force between the first active material layer 2 and the current collector 1, prevents the first active material layer 2 from separating from the current collector 1 when the battery is punctured, and reduces the risk of contact between the aluminum foil and the negative electrode sheet.

[0061] In some embodiments, the second active material layer 3 includes a second active material, a second binder and a second conductive agent; based on the mass of the second active material layer 3 being 100%, the mass ratio of the second active material, the second binder and the second conductive agent is (94-97): (1-2): (2-5).

[0062] The second active material includes one or more of lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and lithium vanadium phosphate. The second binder includes one or more of styrene-butadiene rubber, aqueous acrylic resin, carboxymethyl cellulose, polyvinylidene fluoride, ethylene-vinyl acetate copolymer, and polyvinyl alcohol. The second conductive agent includes one or more of graphite, carbon black, graphene, carbon nanotubes, carbon nanofibers, SuperP, and acetylene black.

[0063] In the second active material layer, the second active material accounts for between 94% and 97%. This high second active material content and high lithium ion content ensure the battery's capacity. The second conductive agent, which accounts for between 2% and 5% by weight, increases the conductivity of the second active material layer 3, facilitating the battery's electrochemical reactions and improving its electrical performance.

[0064] In some embodiments, the positive electrode sheet further includes an insulating layer 4 , which is disposed on the current collector 1 and arranged side by side with the first active material layer 2 , and the insulating layer 4 includes a ceramic material.

[0065] Specifically, the insulating layer 4 is coated on the empty foil portion to avoid the risk of the aluminum foil contacting the negative electrode sheet during the battery puncture test, thereby improving the safety performance of the lithium-ion battery. It should be noted that if the empty foil portion is N, the number of the insulating layer 4 coated on the empty foil portion is less than or equal to N. Figure 1 As shown, both hollow foils are coated with an insulating layer 4 .

[0066] In some embodiments, the thickness of the insulating layer 4 is 10 um to 30 um.

[0067] Specifically, the insulating layer 4 prevents contact between the negative electrode sheet and the aluminum foil during battery puncture. If the thickness of the insulating layer 4 is less than 10 mm, the layer 4 can easily be punctured during battery puncture, allowing the aluminum foil to contact the negative electrode sheet, causing a short circuit risk. If the thickness of the insulating layer 4 exceeds 30 mm, the thickness of the battery cell increases, reducing the energy density of the battery.

[0068] The positive electrode current collector 1 is selected from a metal material that can conduct electrons. Preferably, the positive electrode current collector 1 includes one or more of Al, Ni, tin, copper, and stainless steel. In a more preferred embodiment, the positive electrode current collector 1 is selected from aluminum foil.

[0069] On the other hand, the present application provides a lithium secondary battery, comprising a negative electrode sheet, a separator and the above-mentioned positive electrode sheet.

[0070] The negative electrode sheet is obtained by using an existing preparation method, which will not be described in detail here.

[0071] The lithium secondary battery provided in the present application uses the positive electrode sheet described above, and the membrane resistance of the positive electrode sheet is increased, which reduces the short-circuit current of the battery during acupuncture, and can effectively improve the battery's acupuncture pass rate and safety performance without reducing the battery's electrical performance.

[0072] The specific embodiments of the present invention will be further explained below through examples, but it does not mean that the protection scope of the present invention is limited to the scope described in the examples.

[0073] Example

[0074] This embodiment is used to illustrate the lithium secondary battery disclosed in this application.

[0075] Preparation of a lithium secondary battery, comprising the following steps:

[0076] Preparation of positive electrode:

[0077] Preparation of the first positive electrode active material layer slurry:

[0078] The first active material (lithium iron phosphate with W1 of 80% to 90% and δ1 of 0.025 S / cm to 1 S / cm), the first conductive agent (0 wt% to 1 wt% conductive carbon black), the first resistance increasing agent (W2 of 5% to 20% and δ2 of 1×10 -5 S / cm~5×10 - 5 S / cm alumina ceramic particles) and a first binder (2-10 wt% polyvinylidene fluoride) are mixed, and then N-methyl pyrrolidone is added and stirred to disperse into a first positive electrode active material layer slurry.

[0079] Preparation of the second positive electrode active material layer slurry:

[0080] The second positive electrode active material (94-97 wt% lithium cobaltate), the second conductive agent (2-5 wt% conductive carbon black) and the second binder (1-2 wt% polyvinylidene fluoride) are mixed, and then N-methylpyrrolidone is added and stirred to disperse into a first positive electrode active material layer slurry.

[0081] Preparation of insulation layer 4 slurry:

[0082] (70-90) wt% alumina ceramic material, (10-30) wt% polyvinylidene fluoride or polybutadiene acrylonitrile, and N-methylpyrrolidone are stirred and dispersed to form an insulating layer 4 slurry.

[0083] The first positive electrode active material layer slurry is then applied to the aluminum foil surface using a coating device, and the second positive electrode active material layer slurry is applied to the surface of the first active material layer 2 facing away from the aluminum foil. Simultaneously, the insulating layer 4 slurry is applied to the bare area of ​​the aluminum foil to form the insulating layer 4 slurry. After drying, rolling, and slitting, the positive electrode sheet is prepared. The thickness of the first active material layer 2 is 5 to 20 μm, the thickness of the second active material layer 3 is 60 to 90 μm, and the thickness of the insulating layer 4 is 10 to 30 μm.

[0084] Preparation of negative electrode sheet:

[0085] According to the mass ratio of 94:1:2.5:2.5, the negative electrode active material hard carbon, conductive carbon black, binder styrene-butadiene rubber and carboxymethyl cellulose are mixed, and then dispersed in an appropriate amount of deionized water to obtain a negative electrode slurry; the slurry is coated on both sides of the copper foil, and after drying, rolling, slitting, a negative electrode sheet is prepared. The thickness of the negative electrode sheet is between 70-120μm.

[0086] Preparation of lithium secondary batteries:

[0087] The positive electrode sheet, negative electrode sheet, separator and aluminum-plastic film prepared above are made into a battery, and then the processes of liquid injection and formation are carried out. Finally, the battery is subjected to electrical performance test and needle penetration test.

[0088] Examples 1-10 and Comparative Examples 1-10

[0089] Batteries of Examples 1-10 and Comparative Examples 1-10 were prepared according to the above preparation method, wherein the mass content W1 and conductivity δ1 of the first active material, the mass content W2 and conductivity δ2 of the first resistance increasing agent, and the influencing factor β value are shown in Table 1.

[0090] Example 11

[0091] The difference between Example 11 and Example 1 is that, in Example 11, the insulating layer 4 is not coated on the hollow foil area of ​​the current collector 1 .

[0092] Comparative Example 11

[0093] The difference between Comparative Example 11 and Example 1 is that in Comparative Example 1, two layers of second active material layer 3 slurry are coated on the surface of the current collector 1, and the rest is the same as Example 1.

[0094] Table 1 Data of the first active material layer 2 of Examples 1-11 and Comparative Examples 1-11

[0095]

[0096]

[0097] Battery performance test:

[0098] (1) Battery Internal Resistance and Energy Density: At room temperature, the batteries prepared in Examples 1-11 and Comparative Examples 1-11 were charged at a constant current of 1C to 4.2V, then charged at a constant voltage of 4.3V with a cutoff current of 0.05C to measure the internal resistance of the batteries. The batteries were then discharged at a constant current of 1C to 3.0V, and the discharge capacity C1 of the batteries was measured. The energy density of the batteries was calculated. Specific data are shown in Table 2.

[0099] (2) 25℃ normal temperature cycle test:

[0100] The batteries prepared in Examples 1-11 and Comparative Examples 1-11 were placed at room temperature (25°C) and charged at a constant current of 1C to 4.2V, then charged at a constant voltage of 4.2V with a cutoff current of 0.05C, and then discharged at a constant current of 1C to 3.0V, and this cycle was repeated for 500 cycles.

[0101] Calculate the 500-cycle capacity retention rate = discharge capacity at the 500th cycle / average discharge capacity of the 1st to 3rd cycle × 100%.

[0102] (3) Acupuncture test:

[0103] The batteries prepared in Examples 1-11 and Comparative Examples 1-11 were subjected to a needle penetration test. The test procedure was as follows: a high-temperature resistant steel needle with a diameter of 4±0.5mm was used to penetrate the battery cell plates perpendicularly at a speed of (30mm / s±5mm / s), with the penetration position preferably close to the geometric center of the surface being penetrated (the needle remained in the battery cell). After 1 hour of penetration, the battery cell was observed for penetration and the test was terminated.

[0104] The electrical performance test data are shown in Table 2.

[0105] Table 2 Battery performance test data of Examples 1-11 and Comparative Examples 1-11

[0106]

[0107]

[0108] It can be seen from Table 1-2 that there is no first active material layer 2 in Comparative Example 11. Although the battery room temperature cycle capacity retention rate is higher than that of Example 1, the battery's needle penetration rate is 0; in Comparative Example 2, the first active material layer 2 is coated on the surface of the current collector 1, and the membrane conductivity relationship of the first active material layer 2 is 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm, which is higher than 0.01. The battery's needle penetration rate is increased to 2. Compared with Comparative Example 11, the resistance in Comparative Example 2 increases. It is speculated that increasing the conductivity of the positive electrode membrane can increase the internal resistance of the battery and increase the battery's needle penetration rate; in Examples 1-9, the first active material layer 2 is coated on the surface of the current collector 1, and the membrane conductivity relationship of the first active material layer 2 is 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm, which is higher than 0.01. The diaphragm conductivity relationship of the material layer 2 is 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm. The value is between 0.001-0.01, and the battery's puncture pass rate reaches 100%. In comparative example 1, the value of the diaphragm conductivity relationship of the first active material layer 2 is lower than 0.001, and the battery's cycle performance is reduced; it shows that the diaphragm resistivity of the first active material layer 2 coated on the surface of the current collector 1 is between 0.001 and 0.01. It is speculated that during the battery puncture process, when the current passes through the first active material layer 2, the electrode has a larger resistance, which prevents the battery from rapidly generating heat and burning, thereby improving the battery's puncture pass rate.

[0109] Comparing Comparative Examples 3 and 4 with Example 9, the first active material W1 in Comparative Example 3 is relatively low, resulting in a lower battery energy density. Comparative Example 4 has a relatively high W1, resulting in a lower battery needle puncture rate. It is speculated that a high W1 indicates a low binder content in the battery, which reduces the adhesion between the first active material layer 2 and the current collector 1, thereby reducing the battery needle puncture rate. Comparing Example 10 with Comparative Example 5, the first active material conductivity δ1 is higher than 1 S / cm, resulting in a lower needle puncture rate. It is speculated that a δ1 higher than 1 S / cm reduces the resistance to current flow during needle puncture. Comparative Example 6 has a first active material conductivity δ1 lower than 0.005 S / cm, resulting in a lower battery cycle performance. It is speculated that a δ1 lower than 0.005 S / cm increases the battery's internal resistance, reducing battery cycle performance. Comparing Comparative Examples 7 and 8 with Examples 1-10, the battery capacity retention rates at room temperature cycles are lower than those of the examples, indicating that the first resistance increasing agent content W2 affects the battery's cycle performance. Comparing Comparative Examples 9 and 10 with Examples 1-10, the first resistance-increasing agent δ1 below 0.00001 S / cm affects battery cycling performance; the first resistance-increasing agent δ1 above 0.00005 S / cm reduces the battery needle puncture pass rate. Comparing Examples 1-10 with Example 11, the bare foil area of ​​the current collector 1 in Example 11 is not coated with the insulating layer 4, resulting in a lower battery pass rate.

[0110] By comparing Examples 1-10 and Comparative Examples 1-11, it is found that the conductivity of the membrane of the first active material layer 2 coated on the surface of the current collector 1 satisfies the relationship 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.01S / cm, and 0.01≤β≤0.1, 80%≤W1≤90%, 0.005S / cm≤δ1≤1S / cm, 1×10 -5 S / cm≤δ2≤5×10 -5 S / cm, 5%≤W2≤20%; it can increase the sheet resistance of the positive electrode sheet and reduce the short-circuit current during acupuncture, thereby improving the battery acupuncture pass rate and safety performance; at the same time, the first active material layer 2 also cooperates with the second active material layer 3 to ensure that the electrical performance of the lithium-ion battery is not lost.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet includes a current collector, a first active material layer, and a second active material layer, wherein the first active material layer is disposed on the surface of the current collector, and the second active material layer is disposed on the surface of the first active material layer away from the current collector, wherein the first active material layer includes a first active material and a first resistance increasing agent, wherein the first active material is lithium iron phosphate and its derivatives; and the first resistance increasing agent is a ceramic material. The positive electrode sheet also includes an insulating layer, which is disposed on the current collector and arranged side by side with the first active material layer, and the insulating layer includes a ceramic material. The membrane conductivity of the first active material layer satisfies the following relationship: 0.001 S / cm≤β×(δ1×W1+δ2×W2)≤0.0087 S / cm Among them, 0.01≤β≤0.1, 80%≤W1≤90%, 0.005S / cm≤δ1≤0.9S / cm, 1×10 -5 S / cm≤δ2≤5×10 -5 S / cm, 5%≤W2≤20%; β is the impact factor; δ1 is the powder conductivity of the first active material, in S / cm; δ2 is the powder conductivity of the first resistance increasing agent, in S / cm; W1 is the mass ratio of the first active material in the first active material layer, unit: %; W2 is the mass ratio of the first resistance increasing agent in the first active material layer, in %.

2. The positive electrode sheet according to claim 1, characterized in that The membrane conductivity of the first active material layer satisfies the following relationship: 0.001S / cm≤β×(δ1×W1+δ2×W2)≤0.005 S / cm.

3. The positive electrode sheet according to claim 1, characterized in that The impact factor β is 0.01~0.

05.

4. The positive electrode sheet according to claim 1, characterized in that The mass proportion W1 of the first active material in the first active material layer is 85% to 90%.

5. The positive electrode sheet according to claim 1, characterized in that: The mass ratio W2 of the first resistance increasing agent in the first active material layer is 5% to 15%.

6. The positive electrode sheet according to claim 1, characterized in that The first active material layer further includes a first binder and a first conductive agent. The mass proportion of the first binder in the first active material layer is 2.0% to 10.0%; the mass proportion of the first conductive agent in the first active material layer is 0% to 1%.

7. The positive electrode sheet according to claim 1, characterized in that: The second active material layer includes a second active material, a second binder and a second conductive agent; Taking the mass of the second active material layer as 100%, the mass ratios of the second active material, the second binder, and the second conductive agent are (94-97): (1-2): (2-5).

8. The positive electrode sheet according to claim 7, characterized in that: The thickness of the insulating layer is 10um~30um.

9. A lithium secondary battery, characterized in that: The invention comprises a negative electrode sheet, a separator and a positive electrode sheet as claimed in any one of claims 1 to 8.

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