Positive electrode sheet and secondary battery

Abstract

A positive electrode sheet (10) and a secondary battery. The positive electrode sheet (10) comprises: a current collector (11); a multifunctional coating (12), provided on at least one side of the current collector (11); and a positive electrode active substance layer (13), provided on the surface of the side of the multifunctional coating (12) away from the current collector (11). The components of the multifunctional coating (12) comprise a solid-state electrolyte, a first conductive agent, and a first binder. The resistance of the multifunctional coating (12), the resistance of the positive electrode sheet (10), and the thickness of the multifunctional coating satisfy the following relational expression: 2≤R1*R2*d≤45, where R1 is the resistance of the multifunctional coating (12), R2 is the resistance of the positive electrode sheet (10), and d is the total thickness of the multifunctional coating (12). The mass ratio c of the solid-state electrolyte to the multifunctional coating (12) and the ionic conductivity б of the solid-state electrolyte at room temperature satisfy the following relational expression: 0.1≤(10c3-21c2+14c+0.08) / (lgб)2≤0.45. The low-temperature discharge performance and cycle performance of a battery are effectively improved while ensuring the safety performance of a battery cell.

Description

Positive electrode sheet, secondary battery TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy density, no memory effect, long cycle life, environmental friendliness, and can adapt to various environments, and are widely used in 3C, electric vehicles and electric tools and other products.

[0003] With the increasing competition in the lithium battery industry, customers have increasingly high requirements for the safety of lithium batteries, and coating a safety coating on the positive current collector is one of the common measures to improve battery safety. Generally, the safety coating contains a certain proportion of inorganic fillers, conductive agents and binders. The inorganic fillers can be inert substances such as alumina and boehmite, or active substances such as lithium iron phosphate and manganese lithium iron phosphate that can provide a certain capacity and have good thermal stability.

[0004] The use of safety coatings can greatly improve the safety of the battery, but the introduction of safety coatings will significantly increase the internal resistance of the battery cell, resulting in deterioration of the low-temperature discharge performance and cycle performance of the battery cell.

[0005] SUMMARY

[0006] The main purpose of the present application is to provide a positive electrode sheet and a secondary battery, which aims to solve the problem that the safety coating of the existing lithium battery causes the low-temperature discharge performance and cycle performance of the battery cell to deteriorate.

[0007] To achieve the above-mentioned purpose, the present application provides a positive electrode sheet, which comprises:

[0008] a current collector;

[0009] a multifunctional coating arranged on at least one side surface of the current collector;

[0010] a positive active material layer arranged on the surface of the multifunctional coating;

[0011] wherein the composition of the multifunctional coating comprises a solid-state electrolyte, a first conductive agent and a first binder, and the resistance, total thickness of the multifunctional coating and the resistance of the positive electrode sheet satisfy the relationship: 2≤R1*R2*d≤45, wherein R1 is the resistance of the multifunctional coating, R2 is the resistance of the positive electrode sheet, and d is the total thickness of the multifunctional coating;

[0012] The mass ratio c of the solid-state electrolyte in the multifunctional coating and the ionic conductivity б of the solid-state electrolyte at room temperature satisfy the relationship: 0.1≤(10c3-21c 2+14c+0.08) / (lgб) 2 ≤0.45.

[0013] In some embodiments, the resistance of the multifunctional coating, the total thickness of the multifunctional coating and the resistance of the positive electrode sheet satisfy the relationship: 15≤R1*R2*d≤35.

[0014] In some embodiments, the mass ratio c of the solid-state electrolyte in the multifunctional coating and the ionic conductivity б of the solid-state electrolyte at room temperature satisfy the relationship: 0.12≤(10c 3 -21c 2 +14c+0.08) / (lgб) 2 ≤0.22.

[0015] In some embodiments, the total thickness d of the multifunctional coating is 1 μm to 20 μm; more preferably, the total thickness d of the multifunctional coating is 7.5 to 10.3 μm.

[0016] And / or, the resistance R1 of the multifunctional coating is 0.5 Ω to 5 Ω, more preferably 1.2 Ω to 1.9 Ω, tested under a test pressure of 0.4 tons.

[0017] In some embodiments, the mass ratio of the solid-state electrolyte, the first conductive agent and the first binder in the multifunctional coating is (1 to 95):(0.5 to 5):(0.5 to 10).

[0018] And / or, the total thickness d of the multifunctional coating is 7.5 to 10.3 μm.

[0019] In some embodiments, the first conductive agent is selected from one or both of conductive carbon black and carbon nanotubes; and / or, the first binder is selected from at least one of polyvinylidene fluoride, butadiene rubber, sodium carboxymethyl cellulose and polyacrylate.

[0020] In some embodiments, the multifunctional coating further comprises at least one of a heat-stable active substance and an inert inorganic filler;

[0021] And / or, the mass ratio of the solid-state electrolyte, the first conductive agent, the first binder, the heat-stable active substance and the inert inorganic filler in the multifunctional coating is (1 to 95):(0.5 to 5):(0.5 to 10):(1 to 95):(1 to 95).

[0022] In some embodiments, the heat-stable active substance is selected from one or both of lithium iron phosphate and lithium manganese iron phosphate, and the inert inorganic filler is selected from one or both of alumina and boehmite;

[0023] And / or, the double-sided coating area density of the multifunctional coating is 2 mg / 1540.25 mm2 ~35mg / 1540.25mm 2 .

[0024] In some embodiments, the solid electrolyte is selected from Li 1+x Al x M 2-x (PO4)3, Li7La3N2O 12 Li 3y La 2 / 3-y TiO3, Li 7-z La3Zr 2-z Ta z O 12 and Li 2+a-b ZrCl 6-a-b O a At least one of the following, wherein 0 < x < 0.5, M is selected from Ti and / or Ge, N is selected from at least one of Zr, Sn, and Ta, 0 < y < 0.16, 0 < z < 2, 0 ≤ a ≤ 2, and 0 ≤ b ≤ 0.75;

[0025] And / or, the particle size D of the solid electrolyte v50 The range is 0.2μm to 1.5μm.

[0026] This application further provides a secondary battery, including a positive electrode, a negative electrode, and a separator spaced between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode as described above.

[0027] The beneficial effects of this application's technical solution are as follows: This application sets a multifunctional coating between the current collector and the active material layer of the positive electrode, the multifunctional coating containing a solid electrolyte, and adjusts 2≤R1*R2*d≤45 and 0.1≤(10c 3 -21c 2 +14c+0.08) / (lgб) 2 With a concentration of ≤0.45, the battery's low-temperature discharge performance and cycle performance are effectively improved, while also ensuring the safety performance of the battery cell. Attached Figure Description

[0028] Figure 1 is a schematic diagram of the positive electrode structure according to an embodiment of this application. Detailed Implementation

[0029] The solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments in this application, and not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0030] It should be noted that all the direction indications (such as up, down, left, right, front, back, top, bottom, side, etc.) in the embodiments of the present application are only used to explain the relative position relationship, movement condition, etc. between components in a certain specific posture (as shown in the drawings), and if the specific posture changes, the direction indications will also change accordingly.

[0031] It should also be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it can be directly on the other element or there can be a middle element. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or there can be a middle element.

[0032] In addition, the descriptions involving "first", "second", etc. in the embodiments of the present application are only for descriptive purposes and cannot be understood as indicating or implying the relative importance of the indicated technical features or implying the number of the indicated technical features.

[0033] The internal short circuit of a lithium ion battery can be generally classified into several types: 1) short circuit between positive and negative current collectors; 2) short circuit between positive and negative active materials; 3) short circuit between negative active material and positive current collector; and 4) short circuit between positive active material and negative current collector. Among them, the impedance of the short circuit point between the negative active material and the positive current collector is small, and the trigger temperature of the negative exothermic reaction is low, so the short circuit between the negative active material and the positive current collector is the most dangerous. By introducing a safety coating to the surface of the positive current collector, the probability of contact between the positive current collector and the negative active material can be reduced, which can effectively improve the safety performance of the battery cell. However, the introduction of the safety coating in the prior art will significantly increase the internal resistance of the battery cell, resulting in deterioration of the low-temperature discharge performance and cycle performance of the battery cell.

[0034] Based on the defects in the prior art, an embodiment of the present application provides a positive electrode sheet, referring to FIG. 1, the positive electrode sheet 10 comprises:

[0035] a current collector 11;

[0036] a multifunctional coating 12 disposed on at least one side surface of the current collector 11;

[0037] a positive active material layer 13 disposed on a side surface of the multifunctional coating 12 away from the current collector 11;

[0038] wherein the composition of the multifunctional coating 12 comprises a solid-state electrolyte, a first conductive agent and a first binder, the resistance, total thickness of the multifunctional coating and the resistance of the positive electrode sheet satisfy the relationship: 2≤R1*R2*d≤45, wherein R1 is the resistance of the multifunctional coating 12, R2 is the resistance of the positive electrode sheet 10, and d is the total thickness of the multifunctional coating 12.

[0039] The mass ratio c of the solid-state electrolyte in the multifunctional coating and the ionic conductivity b of the solid-state electrolyte at room temperature satisfy the relationship: 0.1≤(10c 3 -21c 2 +14c+0.08) / (lgб) 2 ≤0.45.

[0040] In the present embodiment, by arranging the multifunctional coating between the current collector and the positive active material layer of the positive electrode sheet, and by using the characteristic that the solid-state electrolyte has good ionic conductivity at low temperature, the low-temperature discharge performance and the cycle performance of the battery can be improved. Generally speaking, the effect of the solid-state electrolyte is related to its content and ionic conductivity, and if the content or the ionic conductivity is too low, it cannot produce good effects, and when (10c 3 -21c 2 +14c+0.08) / (lgб) 2 ≥0.1, the solid-state electrolyte can produce better effects. In addition, the three parameters R1, d and R2 are positively correlated with the safety of the battery, and are negatively correlated with the cycle performance of the battery, and when R1*R2*d<2, the safety performance of the battery is poor, and when R1*R2*d>45, the cycle performance of the battery is poor. The present application limits the parameters of the multifunctional coating as described above, which can take into account the safety performance and the cycle performance of the battery without changing the original process, so that a good balance between the safety performance and the cycle performance of the battery is achieved.

[0041] Further, the room temperature in the present embodiment is 25°C. Generally speaking, the larger the c and b are, the more excellent the low-temperature discharge performance of the battery is, and the larger the b is, the smaller the c is to achieve the same effect. In the present embodiment, considering the marginal diminishing effect of the amount of solid-state electrolyte and its expensive price, when the above relationship is satisfied, the use of less solid-state electrolyte can greatly improve the low-temperature discharge performance of the battery. When the parameters of the multifunctional coating satisfy the above two relationships at the same time, the battery can achieve a needle penetration pass rate of more than 90%, a 3.4V discharge capacity at -10°C of not less than 79%, and a capacity retention rate after 800 cycles at 25°C of not less than 89%.

[0042] Preferably, the resistance, total thickness of the multifunctional coating and the resistance of the positive electrode sheet satisfy the relationship: 15≤R1*R2*d≤35, and in this range, the performance of the sample is more excellent, the needle test pass rate can reach 100%, and the capacity retention rate after 800 cycles at room temperature is >90%.

[0043] Preferably, the mass ratio c of the solid-state electrolyte in the multifunctional coating and the ionic conductivity b of the solid-state electrolyte at room temperature satisfy the relationship: 0.12≤(10c3 -21c 2 +14c+0.08) / (lgб) 2 ≤0.22, in this range, the amount of solid-state electrolyte is less, and the low-temperature discharge performance is obviously improved, and the comprehensive benefit is better.

[0044] The selection of raw materials will affect the resistance R1 and total thickness d of the multifunctional coating, and the resistance R2 of the positive plate, and then affect whether the parameters of the multifunctional coating can meet the above relationship, therefore, the raw materials and the amount of the multifunctional coating also need to be further limited.

[0045] Therefore, in some embodiments, the total thickness d of the multifunctional coating is 1 μm-20 μm; and / or the resistance R1 of the multifunctional coating is 0.5 Ω-5 Ω under a test pressure of 0.4 tons.

[0046] In this embodiment, the sheet resistance of the multifunctional coating is the primary factor affecting the safety performance of the battery, and when the value is too small, the safety performance of the battery will deteriorate, and when the value is too large, the cycle performance of the battery will deteriorate, and when the value is between 0.5 Ω and 5 Ω (the test pressure is 0.4 tons), the safety performance and the cycle performance can be considered, and more preferably, the value is between 1.2 Ω and 1.9 Ω.

[0047] In some embodiments, the mass ratio of the solid-state electrolyte, the first conductive agent and the first binder in the multifunctional coating is (1-95):(0.5-5):(0.5-10); and / or the total thickness d of the multifunctional coating is 7.5-10.3 μm.

[0048] In this embodiment, the amount of the first conductive agent is 0.5%-5% of the weight of the multifunctional coating, and preferably 2%-3.5%, and when the content of the conductive agent is too low, the sheet resistance of the multifunctional coating will be too large, and the cycle performance of the battery will be poor, and when the content of the conductive agent is too high, the sheet resistance of the multifunctional coating will be too low, and the safety of the battery will be poor, and when the content of the conductive agent is between 0.5% and 5%, the safety performance and the cycle performance can be considered. The amount of the first binder is 0.5%-10% of the weight of the multifunctional coating, and preferably 4%-6%, and when the amount of the first binder is too low, the adhesion will be too low, and there will be a risk of falling off of the coating during use, and when the amount of the first binder is too high, the adhesion will be improved, and the safety performance of the battery will be improved, but the cycle performance of the battery will be poor.

[0049] In some embodiments, the first conductive agent is selected from one or both of conductive carbon black and carbon nanotubes; and the first binder is selected from at least one of polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC) and polyacrylate (PAA).

[0050] In some embodiments, the multifunctional coating further includes at least one of a thermally stable active substance and an inert inorganic filler; and / or, the mass ratio of the solid electrolyte, the first conductive agent, the first binder, the thermally stable active substance, and the inert inorganic filler in the multifunctional coating is (1-95):(0.5-5):(0.5-10):(1-95):(1-95).

[0051] In this embodiment, the heat-stabilized active material and the inert inorganic filler are not essential components, but considering cost and processability, a certain amount of heat-stabilized active material and inert inorganic filler can be added to the multifunctional coating.

[0052] In some embodiments, the thermally stable active material is selected from one or both of lithium iron phosphate and lithium manganese iron phosphate; the inert inorganic filler is selected from one or both of alumina and boehmite.

[0053] In some embodiments, the double-sided coating surface density of the multifunctional coating is 2 mg / 1540.25 mm. 2 ~35mg / 1540.25mm 2 .

[0054] In this embodiment, there is a positive correlation between the surface density and thickness of the double-sided coating of the multifunctional coating. If the thickness is too low, the safety performance of the battery cell will be poor, and if the thickness is too high, the energy density of the battery cell will be reduced.

[0055] In some embodiments, the solid electrolyte is selected from Li 1+x Al x M 2-x (PO4)3, Li7La3N2O 12 Li 3y La 2 / 3-y TiO3, Li 7-z La3Zr 2-z Ta z O 12 and Li 2+a-b ZrCl 6-a-b O a At least one of the following, wherein 0 < x < 0.5, M is selected from Ti and / or Ge, N is selected from at least one of Zr, Sn, and Ta, 0 < y < 0.16, 0 < z < 2, 0 ≤ a ≤ 2, 0 ≤ b ≤ 0.75; and / or, the particle size D of the solid electrolyte is... v50 The range is 0.2μm to 1.5μm.

[0056] In this embodiment, the selected solid electrolyte has an ionic conductivity greater than 10 at room temperature. -5 S / cm, preferably, the solid electrolyte can be Li 1+x Alx Ti 2-x (PO4)3(LATP), Li7La3Zr2O 12 (LLZO) and Li 3x La 2 / 3-x TiO3(LLTO) and Li 2+a-b ZrCl 6-a-b O a (LZCO, 0≤a≤2, 0≤b≤0.75). The particle size D v50 of the solid-state electrolyte is between 0.2-1.5 pm. Too small particles are not conducive to subsequent processing and dispersion, and too large particles can lead to a longer ion conduction path inside the electrolyte, thereby reducing the conduction efficiency of ions. In addition, larger particles can not form good contact with the electrode material, leading to increased interfacial impedance and affecting the charge and discharge performance of the battery. In addition, larger particles can cause greater stress during the cycling process, thereby accelerating the aging of the material and reducing the cycle life of the battery.

[0057] In some embodiments, the method for preparing the multifunctional coating comprises:

[0058] mixing the raw materials of the multifunctional coating with a solvent to obtain a multifunctional coating slurry;

[0059] transferring the multifunctional coating slurry to at least one side surface of the current collector by gravure printing to obtain the multifunctional coating.

[0060] In the present embodiment, the solvent can be N-methyl pyrrolidone (NMP). Since the present application has strict requirements for the resistance and thickness of the multifunctional coating, the gravure printing method can achieve precise control of the parameters of the multifunctional coating. The temperature of the oven during gravure printing is 90-110°C, and the printing speed is 10-50 m / min. When the temperature of the oven is lower than 90°C, the production efficiency is reduced, and when the temperature of the oven is higher than 110°C, problems such as cracking and migration of the conductive agent to the surface can occur. Too low printing speed will also reduce the production efficiency, and too high printing speed will cause the multifunctional coating to be missed, thereby negatively affecting the safety performance of the battery.

[0061] In some embodiments, the solid content of the multifunctional coating slurry is not less than 10%, and the viscosity is not less than 50 mPa·s. Too low solid content not only reduces the production efficiency, but also makes it difficult to obtain a multifunctional coating with a specified thickness and area density, and too low viscosity is not conducive to the progress of the intaglio printing process. The multifunctional coating can firmly adhere to the current collector surface, not only can reduce the surface contact resistance, but also can reduce the generation of positive current collector burrs during the safety test of the battery cell, reduce the contact short circuit between the positive current collector burrs and the unstable negative active material in the charged state, thereby improving the safety of the battery cell. At the same time, the solid-state electrolyte contained in the multifunctional coating can also conduct ions, reduce the interface impedance, and make the battery cell maintain good low-temperature discharge performance and cycle performance.

[0062] In the present embodiment, the adhesion between the multifunctional coating 12 and the current collector 11 is not less than 100 N / m. When the adhesion is less than 100 N / m, the multifunctional coating has a risk of falling off during subsequent use. The current collector 11 can be one or both of an aluminum foil and a composite aluminum foil; the active material layer 13 includes a positive active material, a second conductive agent, and a second binder.

[0063] In some embodiments, the positive active material can be one or more of common lithium cobaltate, lithium iron phosphate, and ternary materials, and the amount is 90% to 98% of the total weight of the positive active material layer 13. In the case of ensuring that the positive active material layer has a certain conductivity and adhesion, the higher the content of the positive active material, the better, so that the battery cell has a higher energy density; the second conductive agent is a commonly used conductive agent, including one or both of conductive carbon black and carbon nanotubes, and the amount is 0.5% to 5% of the total weight of the positive active material layer 13, preferably 0.5% to 2%. Too low content of the second conductive agent will make the positive plate have a large resistance R2 and poor cycle performance, and too high content will lead to poor safety performance of the battery cell; the second binder can be one or more of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), and polyacrylate (PAA), and the amount is 0.5% to 5% of the total weight of the positive active material layer 13. The influence of the amount of the second binder is similar to that of the first binder, but considering that it does not directly contact the current collector, the upper limit of the amount can be appropriately reduced to ensure that the positive active material does not fall off during use of the battery cell. At the same time, a lower amount can also improve the cycle performance of the battery cell and increase the amount of the positive active material to improve the energy density of the battery cell.

[0064] The application further provides a secondary battery cell, which includes a positive plate, a negative plate, and a separator interposed between the positive plate and the negative plate, and the positive plate is the positive plate as described above.

[0065] In the present embodiment, the positive electrode sheet comprises the above-mentioned positive electrode current collector with multifunctional coating and a positive electrode active material layer coated on at least one surface of the positive electrode current collector. The positive electrode active material layer comprises a positive electrode active material, which is a positive electrode active material commonly used in lithium ion batteries at present, including but not limited to compounds with chemical formula such as Li x Ni h Co y M z O 2-d N d a combination of one or more of compounds represented by the formula (I) (wherein 0.95≤x≤1.2, h>0, y≥0, z≥0, and h+y+z=1, 0≤d≤1, M is selected from a combination of one or more of Mn, Al, and N is selected from a combination of one or more of F, P, and S), and the positive electrode active material can also be a combination of one or more of LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material can also be subjected to a modification treatment, and the method for modifying the positive electrode active material is known to those skilled in the art, for example, the positive electrode active material can be modified by coating or doping, and the material used for the modification treatment can be a combination of one or more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc.

[0066] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector, and the negative electrode current collector is a copper foil. The negative electrode active material layer comprises a negative electrode active material, which comprises a silicon-based material and can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon alloys, or a mixture of the silicon negative electrode material and other commonly used negative electrode active materials, including but not limited to one or more of graphite, soft carbon, hard carbon, carbon fibers, mesocarbon microbeads, tin-based materials, lithium titanate, or other metals capable of forming alloys with lithium, etc. Among them, the graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; and the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys.

[0067] The separator can be any of a variety of materials suitable for use as a separator in a lithium-ion battery, for example, can be a combination of one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyesters, and natural fibers, among others.

[0068] The secondary battery also includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt, and an additive. The electrolyte lithium salt can be LiPF6 and / or LiBOB used in a high-temperature electrolyte; can be at least one of LiBF4, LiBOB, LiPF6 used in a low-temperature electrolyte; can be at least one of LiBF4, LiBOB, LiPF6, LiTFSI used in an overcharge-preventing electrolyte; can be at least one of LiClO4, LiAsF6, LiCF3SO3, LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC, EC; can be a chain carbonate, including DEC, DMC, or EMC; can be a carboxylic acid ester, including PP, MA, EA, EP, and the like. The additive includes, but is not limited to, at least one of a film-forming additive, a conductive additive, a flame-retardant additive, an overcharge-preventing additive, an additive for controlling the content of H2O and HF in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

[0069] In order to make the technical solutions and advantages of the present application clearer, the following will describe the present application and its beneficial effects in further detail with reference to specific embodiments, but the embodiments of the present application are not limited thereto.

[0070] Example 1

[0071] A positive electrode sheet includes a positive electrode active material layer, a multifunctional coating layer, and an aluminum foil. The multifunctional coating layer can firmly adhere to the surface of the aluminum foil, not only reducing the surface contact resistance, but also reducing the generation of positive electrode current collector burrs during safety testing of the battery cell, reducing the contact short circuit between the positive electrode current collector burrs and the unstable negative electrode active material in the charged state, thereby improving the safety of the battery cell. At the same time, the solid-state electrolyte contained in the multifunctional coating layer can also conduct ions, reduce the interface impedance, and maintain good low-temperature discharge performance and cycle performance of the battery cell.

[0072] The multifunctional coating is arranged on both surfaces of the aluminum foil. The double-sided tab resistance R2 (Ω) of the positive electrode sheet, the double-sided tab resistance R1 (Ω) of the multifunctional coating of the positive electrode sheet, and the total thickness d (μm) of the multifunctional coating should satisfy 2≤R1*R2*d≤45. In addition, the mass content ratio c of the solid-state electrolyte in the multifunctional coating and the ionic conductivity б (S / cm) of the solid-state electrolyte at room temperature should satisfy 0.1≤(10c3-21c2+14c+0.08) / (lgб)2≤0.45. The multifunctional coating comprises, in addition to the solid-state electrolyte, a certain proportion of the first conductive agent conductive carbon black SuperPLi, the first binder PVDF, and boehmite. Specifically, the preparation method is as follows:

[0073] In NMP (N-methyl pyrrolidone) as a solvent, 76 parts of boehmite, 15 parts of solid-state electrolyte Li 1.4 Al 0.4 Ti 1.6 (PO4)3 (lithium aluminum titanium phosphate, LATP, ionic conductivity is 1.83×10 -4 S / cm, D v 50 is 0.5 μm), 6 parts of the first binder PVDF, and 3 parts of the first conductive agent SuperPLi are uniformly mixed to obtain a multifunctional coating slurry with a solid content of 20% and a viscosity of 100 mPa·s. The multifunctional coating slurry is coated on both sides of the aluminum foil by gravure printing. The temperature of the oven is set to 95°C, and the printing speed is 30 m / min. Finally, an aluminum foil comprising a multifunctional coating is obtained, which has a double-sided coating area density of 15 mg / 1540.25 mm 2 , a double-sided total thickness (referring to the thickness of the multifunctional coating arranged on both surfaces of the aluminum foil) of 8.1 μm, a double-sided tab resistance of 1.5 Ω (test pressure is 0.4 tons), and a bonding force between the multifunctional coating and the aluminum foil of 215 N / m.

[0074] The obtained aluminum foil comprising a multifunctional coating is applied to a lithium ion battery. The preparation method of the lithium ion battery is as follows:

[0075] (1) Preparation of positive electrode sheet:

[0076] The lithium cobaltate, conductive agent (mixture of conductive carbon black and carbon nanotube, mass ratio 6:5), PVDF binder and NMP were mixed uniformly in a mass ratio of 97.6:1.1:1.3:35 to prepare a positive electrode slurry with a solid content of 75% and a viscosity of 7000 mPa·s. The positive electrode slurry was coated on one side surface of the above-mentioned aluminum foil containing a multifunctional coating, dried at 85°C and wound, and then the other side surface of the aluminum foil containing a multifunctional coating was coated with the positive electrode slurry and dried in the above-mentioned manner, and then the positive electrode sheet coated with the positive electrode active material layer on both sides was subjected to cold pressing treatment; then edge cutting and striping were performed to prepare a lithium ion battery positive electrode sheet (double-sided sheet resistance 1.7Ω, test pressure 0.4t).

[0077] (2) Preparation of negative electrode sheet:

[0078] A lithium ion battery negative electrode slurry with a solid content of 50% and a viscosity of 5000 mPa·s was prepared by uniformly mixing graphite, thickening agent and SBR binder in a mass ratio of 97.7:1.1:1.2 with water as the solvent, coated on one side surface of a copper foil, dried at 80°C and wound, and then the other side of the copper foil was coated with the negative electrode slurry and dried in the above-mentioned manner to obtain a negative electrode sheet coated with an active material on both sides.

[0079] (3) Preparation of electrolyte:

[0080] Lithium hexafluorophosphate (LiPF6) was dissolved in a mixed solvent of dimethyl carbonate (DMC), ethylene carbonate (EC) and methyl ethyl carbonate (EMC) (mass ratio of DMC, EC and EMC 3:5:2) to obtain an electrolyte.

[0081] (4) Preparation of battery:

[0082] The above-prepared positive electrode sheet, negative electrode sheet and separator were wound into an electric core with a capacity of about 5 Ah. The separator was located between adjacent positive electrode sheets and negative electrode sheets, the positive electrode was led out by spot welding with an aluminum tab, and the negative electrode was led out by spot welding with a nickel tab; then the electric core was placed in an aluminum plastic packaging bag, and after baking, the above-mentioned electrolyte was injected, and after packaging, formation, capacity distribution and other processes, finally a lithium ion battery was prepared.

[0083] Example 2

[0084] The difference between Example 1 and Example 2 is that the amount of components of the multifunctional coating is different, specifically: the amount of LATP is 50 parts, and the amount of boehmite is 41 parts.

[0085] The rest is the same as Example 1, which will not be repeated here.

[0086] Example 3

[0087] The difference from Example 1 is that the components of the multifunctional coating are different, specifically: 15 parts of LATP is replaced by 15 parts of Li7La3Zr2O 12 (lithium lanthanum zirconium oxide (LLZO), ion conductivity is 1.87 x 10 -4 S / cm, D v50 is 0.5 μm)

[0088] The rest is the same as Example 1, which will not be repeated here.

[0089] Example 4

[0090] The difference from Example 1 is that the components and content of the multifunctional coating are different, specifically: 15 parts of LATP is replaced by 30 parts of Li 0.33 La 0.57 TiO3 (lithium lanthanum titanium oxide (LLTO), ion conductivity is 2.29 x 10 -5 S / cm, D v50 is 0.5 μm), and the amount of boehmite is 61 parts.

[0091] The rest is the same as Example 1, which will not be repeated here.

[0092] Example 5

[0093] The difference from Example 1 is that the components and content of the multifunctional coating are different, specifically: 15 parts of LATP is replaced by 60 parts of Li 0.33 La 0.57 TiO3 (lithium lanthanum titanium oxide (LLTO), ion conductivity is 2.29 x 10 -5 S / cm, D v50 is 0.5 μm), and the amount of boehmite is 31 parts.

[0094] The rest is the same as Example 1, which will not be repeated here.

[0095] Example 6

[0096] The difference from Example 1 is that the components and content of the multifunctional coating are different, specifically: 15 parts of LATP is replaced by 80 parts of Li 0.33 La 0.57 TiO3 (lithium lanthanum titanium oxide (LLTO), ion conductivity is 2.29 x 10 -5 S / cm, D v50 is 0.5 μm), and the amount of boehmite is 11 parts.

[0097] The rest is the same as Example 1, which will not be repeated here.

[0098] Example 7

[0099] The difference from Example 1 is that the LATP particle size of the multifunctional coating is different, specifically: the particle size D of the LATP is 1.3 μm. v50 The difference from Example 1 is that the LATP particle size of the multifunctional coating is different, specifically: the particle size D of the LATP is 1.3 μm.

[0100] The rest is the same as Example 1, which will not be repeated here.

[0101] Example 8

[0102] The difference from Example 1 is that the LATP particle size of the multifunctional coating is different, specifically: the particle size D of the LATP is 1.3 μm. v50 The difference from Example 1 is that the LATP particle size of the multifunctional coating is different, specifically: the particle size D of the LATP is 1.3 μm.

[0103] The rest is the same as Example 1, which will not be repeated here.

[0104] Example 9

[0105] The difference from Example 1 is that the amount of components of the multifunctional coating is different, specifically: the amount of the first conductive agent SuperPLi is reduced from 3 parts to 2 parts, and the amount of boehmite is increased from 76 parts to 77 parts.

[0106] The rest is the same as Example 1, which will not be repeated here.

[0107] Example 10

[0108] The difference from Example 1 is that: the solid content of the multifunctional coating slurry prepared is 23%, the viscosity is 157 mPa·s, the temperature of the oven is set to 95°C during gravure printing, the printing speed is 25 m / min, and the final multifunctional coating thickness is 10.3 μm.

[0109] The rest is the same as Example 1, which will not be repeated here.

[0110] Example 11

[0111] The difference from Example 1 is that the amount of components of the multifunctional coating is different, specifically: the amount of the first conductive agent SuperPLi is increased from 3 parts to 4 parts, and the amount of boehmite is reduced from 76 parts to 75 parts.

[0112] The rest is the same as Example 1, which will not be repeated here.

[0113] Example 12

[0114] The difference from Example 1 is that the amount of components of the multifunctional coating is different, specifically: the amount of the first conductive agent SuperPLi is increased from 3 parts to 4 parts, and the amount of boehmite is reduced from 76 parts to 75 parts, the solid content of the multifunctional coating slurry prepared is 12%, the viscosity is 81 mPa·s, and the final multifunctional coating thickness is 1 μm.

[0115] The rest is the same as example 1, which will not be repeated here.

[0116] Example 13

[0117] The difference from example 1 is that the amount of components of the multifunctional coating is different, specifically: the amount of the first conductive agent SuperPLi is increased from 3 parts to 5 parts, and the amount of boehmite is reduced from 76 parts to 74 parts. The prepared multifunctional coating slurry has a solid content of 38% and a viscosity of 165 mPa·s. When performing intaglio printing, the temperature of the oven is set to 95℃, and the printing speed is 25 m / min. The final multifunctional coating thickness is 20 μm.

[0118] The rest is the same as example 1, which will not be repeated here.

[0119] Example 14

[0120] The difference from example 1 is that the amount of components of the multifunctional coating is different, specifically: the amount of LATP is 11 parts, and the amount of boehmite is 80 parts.

[0121] The rest is the same as example 1, which will not be repeated here.

[0122] Example 15

[0123] The difference from example 1 is that the components and amount of the multifunctional coating are different, specifically: 15 parts of LATP are replaced by 91 parts of Li 1.75 ZrCl 4.75 O 0.5 (zirconium lithium oxychloride, LZCO, ionic conductivity is 2.42×10 -3 S / cm, D v50 is 0.5 μm), without boehmite.

[0124] The rest is the same as example 1, which will not be repeated here.

[0125] Comparative example 1

[0126] The current collector used in the preparation of the positive electrode sheet is an aluminum foil, without a multifunctional coating.

[0127] The rest is the same as example 1, which will not be repeated here.

[0128] Comparative example 2

[0129] The difference from example 1 is that: the amount of LATP is 10 parts, and the amount of boehmite is 81 parts.

[0130] The rest is the same as example 1, which will not be repeated here.

[0131] Comparative example 3

[0132] The difference from Example 1 is that the amount of the first conductive agent SuperP Li is reduced from 3 parts to 0.3 parts, and the amount of boehmite is increased from 76 parts to 78.7 parts.

[0133] The rest is the same as Example 1, which will not be repeated here.

[0134] Comparative Example 4

[0135] The difference from Example 1 is that the amount of the first conductive agent SuperP Li is increased from 3 parts to 6 parts, and the amount of boehmite is reduced from 76 parts to 73 parts.

[0136] The rest is the same as Example 1, which will not be repeated here.

[0137] Comparative Example 5

[0138] The difference from Example 1 is that the amount of the first binder PVDF is increased from 6 parts to 15 parts, and the amount of boehmite is reduced from 76 parts to 67 parts.

[0139] The rest is the same as Example 1, which will not be repeated here.

[0140] Comparative Example 6

[0141] The difference from Example 1 is that the solid content of the prepared multifunctional coating slurry is 45%, the viscosity is 191 mPa·s, the temperature of the oven is set to 95°C when performing intaglio printing, the printing speed is 25 m / min, and the final thickness of the multifunctional coating is 25 μm.

[0142] The rest is the same as Example 1, which will not be repeated here.

[0143] The parameters in Examples 1-15 and Comparative Examples 1-6 are shown in Table 1.

[0144] Film sheet resistance test method: The instrument used for film sheet resistance test is Hangzhou Chuanyuan Technology Co., Ltd. ACCFILM film sheet resistance test system (model TT-ACCF-G2A), the pressure during testing is 0.4 t, and the pressure holding time is 10 s.

[0145] Table 1 Parameters of Examples 1-15 and Comparative Examples 1-6

[0146] To verify the influence of the introduction of the multifunctional coating described in the present application on the performance of the battery cell, the safety performance (needle test), low temperature discharge performance and cycle performance of the battery cell were tested.

[0147] Puncture test method: at room temperature, charge to 4.45V at 1.0C constant current and constant voltage, with a cutoff rate of 0.05C, then perform a puncture test on the fully charged cell, with the cell's deep pit facing upwards, use a steel nail with a diameter of 4.0mm to completely puncture the cell at one time at a speed of 40mm / s, the puncture position is at the left, middle and right positions of the cell's largest face (5 cells are tested at each position), maintain for 1h, if the cell does not catch fire or explode, it is considered to pass the test.

[0148] Low-temperature discharge performance test method: discharge to 3V at 1.0C constant current, rest for 5min; set the oven to 25℃, rest for 60min; charge to 4.45V at 1.0C constant current and constant voltage, with a cutoff rate of 0.02C; rest for 5min, discharge to 3V at 0.2C constant current, and record the capacity at 0.2C discharge to 3V as the initial capacity C0; set the oven to 25℃, rest for 60min; charge to 4.45V at 1.0C constant current and constant voltage, with a cutoff rate of 0.02C; rest for 5min, set the oven to -10℃, rest for 120min, then discharge the cell to 3.0V at 0.2C, record the capacities C1 and C2 at 3.4V and 3.0V, the ratio of C1 to C0 is the capacity retention rate at -10℃ discharge to 3.4V.

[0149] Cycle performance test method: at an ambient temperature of 25±2℃, discharge the cell to 3.0V at 0.2C constant current, then charge to 4.45V at 3C constant current and constant voltage, with a cutoff rate of 0.05C, record the voltage, internal resistance, capacity and thickness (use 600g PPG to measure thickness) of the fully charged cell. The cycle process is as follows: discharge to 3V at 0.2C constant current; charge to 4.25V at 3.0C constant current; charge to 4.25V at 2.5C constant current; charge to 4.45V at 2.0C constant current; charge to 4.50V at 1.4C constant current and constant voltage, with a cutoff rate of 0.3C; charge to 4.45V at 2A constant current and constant voltage, with a cutoff rate of 0.05C; discharge to 3V at 1.0C constant current. Complete the above steps for 1 cycle, after 49 cycles, perform a small current recovery as follows: charge to 4.25V at 3.0C constant current; charge to 4.25V at 2.5C constant current; charge to 4.45V at 2.0C constant current; charge to 4.50V at 1.4C constant current and constant voltage, with a cutoff rate of 0.3C; charge to 4.45V at 2A constant current and constant voltage, with a cutoff rate of 0.05C; discharge to 3V at 0.2C constant current; charge to 4.45V at 3.0C constant current and constant voltage, with a cutoff rate of 0.05C. Record the voltage, internal resistance and thickness (use 600g PPG to measure thickness) of the fully charged cell every 100 weeks.

[0150] The secondary batteries obtained from Examples 1-15 and Comparative Examples 1-6 were tested, and the test results are shown in Table 2 below.

[0151] Table 2 Test results of Examples 1-15 and Comparative Examples 1-6

[0152] From the comparison of the test results in the above table, compared with Comparative Example 1, the low-temperature discharge performance of the battery cell is significantly improved by about 8% after the multifunctional coating is introduced in Examples 1-15, and the capacity retention rate after 800 cycles at room temperature is also increased by about 2% (which is a significant improvement in the lithium battery field). Compared with the samples with R1*R2*d<2, the safety performance of the examples is also significantly improved, and the needle test pass rate is increased from 20% to >90%; compared with the samples with R1*R2*d>45, the capacity retention rate of the examples after 800 cycles at room temperature is >89%. In particular, when R1*R2*d is in the preferred range, i.e., 15≤R1*R2*d≤35, the performance of the sample is more excellent, the needle test pass rate is 100%, and the capacity retention rate after 800 cycles at room temperature is >90%.

[0153] In addition, it is also found from Comparative Example 2 that when (10c 3 -21c 2 +14c+0.08) / (lgб) 2 <0.1, the improvement of the low-temperature discharge performance of the battery cell becomes not obvious. Example 4 shows that when the ionic conductivity of the solid-state electrolyte is poor, in order to achieve the same effect, the amount thereof needs to be significantly increased (from 15% in Example 1 to 30%). It can be known from Examples 5-6 and Examples 14-15 that when the amount of the solid-state electrolyte reaches a certain degree, further increasing the amount thereof will no longer obviously improve the low-temperature performance of the battery cell, and when 0.12≤(10c 3 -21c 2 +14c+0.08) / (lgб) 2 ≤0.22, the amount of the solid-state electrolyte is less, and the improvement of the low-temperature discharge performance is obvious, and the comprehensive benefit is better.

[0154] In summary, by introducing the solid-state electrolyte into the multifunctional coating, and limiting the relationship between the double-sided film resistance R1 and the thickness d of the multifunctional coating, the double-sided film resistance R2 of the positive plate and the relationship between the content c of the solid-state electrolyte in the multifunctional coating and the ionic conductivity б at room temperature, the internal resistance of the battery is reduced, not only the safety performance of the coating is maintained, but also the solid-state electrolyte has excellent ionic conductivity at low temperature, and the low-temperature discharge performance and the cycle performance of the battery are considered. The battery cell provided in the present application satisfies 2≤R1*R2*d≤45, and 0.1≤(10c 3 -21c 2 +14c+0.08) / (lgб) 2When the content of Li2CO3 is less than or equal to 0.45%, the needle penetration pass rate is greater than 90%, the discharge capacity at-10 DEG C is not less than 79% at 3.4V, and the capacity retention rate after 800 cycles at 25 DEG C is not less than 89%.

[0155] The above only describes some or preferred embodiments of the present application, neither the text nor the drawings can limit the scope of protection of the present application, any equivalent structural transformation using the content of the present application specification and drawings, or direct / indirect application in other related technical fields is included in the scope of protection of the present application.

Claims

1. A positive electrode sheet characterized by comprising: The positive electrode sheet comprises: a current collector; a multifunctional coating layer arranged on at least one side surface of the current collector; a positive active material layer arranged on a surface of the multifunctional coating layer; wherein the multifunctional coating layer comprises a solid-state electrolyte, a first conductive agent and a first binder, and the resistance of the multifunctional coating layer, the total thickness of the multifunctional coating layer and the resistance of the positive electrode sheet satisfy the relationship: 2≤R1*R2*d≤45, wherein R1 is the resistance of the multifunctional coating layer, R2 is the resistance of the positive electrode sheet, and d is the total thickness of the multifunctional coating layer. The mass ratio c of the solid-state electrolyte to the multifunctional coating and the ionic conductivity of the solid-state electrolyte at room temperature satisfies the relationship:

2. The positive electrode sheet according to claim 1, characterized by The resistance of the multifunctional coating layer, the total thickness of the multifunctional coating layer and the resistance of the positive electrode sheet satisfy the relationship: 15≤R1*R2*d≤35.

3. The positive electrode sheet according to claim 1, characterized by The mass ratio c of the solid-state electrolyte to the multifunctional coating and the ionic conductivity of the solid-state electrolyte at room temperature satisfies the relationship:

4. The positive electrode sheet according to any one of claims 1 to 3, characterized by, The total thickness d of the multifunctional coating layer is 1 μm to 20 μm. And / or, the resistance R1 of the multifunctional coating layer is 0.5 Ω to 5 Ω when tested under a test pressure of 0.4 tons.

5. The positive electrode sheet according to claim 4, characterized by The mass ratio of the solid-state electrolyte, the first conductive agent and the first binder in the multifunctional coating layer is (1-95):(0.5-5):(0.5-10). And / or, the total thickness d of the multifunctional coating layer is 7.5-10.3 μm.

6. The positive electrode sheet according to claim 5, characterized by The first conductive agent is selected from one or both of conductive carbon black and carbon nanotubes; and / or, the first binder is selected from at least one of polyvinylidene fluoride, butadiene-styrene rubber, sodium carboxymethyl cellulose and polyacrylate.

7. The positive electrode sheet according to claim 1, characterized by The multifunctional coating layer further comprises at least one of a heat-stable active material and an inert inorganic filler; And / or, the mass ratio of the solid-state electrolyte, the first conductive agent, the first binder, the heat-stable active material and the inert inorganic filler in the multifunctional coating layer is (1-95):(0.5-5):(0.5-10):(1-95):(1-95).

8. The positive electrode sheet according to claim 7, characterized by The double-sided coated area density of the multifunctional coating is 2 mg / 1540.25 mm 2 ~ 35 mg / 1540.25 mm 2 ; And / or, the heat-stable active material is selected from one or both of lithium iron phosphate and lithium manganese iron phosphate; and the inert inorganic filler is selected from one or both of alumina and boehmite.

9. The positive electrode sheet according to claim 1, characterized by The solid-state electrolyte is selected from Li 1+x Al x M 2-x (PO4)3, Li7La3N2O 12 , Li 3y La 2 / 3-y TiO3, Li 7-z La3Zr 2-z Ta z O 12 and Li 2+a -b ZrCl 6-a-b O a , wherein 0 The particle size of the solid-state electrolyte is 0.2 μm to 1.5 μm.

10. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator interposed between the positive electrode sheet and the negative electrode sheet, characterized by The positive electrode sheet is the positive electrode sheet according to any one of claims 1 to 9.

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