A positive electrode sheet and use thereof

Abstract

The application provides a positive electrode sheet and application thereof, and the positive electrode sheet comprises a positive electrode current collector and n layers of positive electrode active layers arranged on at least part of the surface of the positive electrode current collector, wherein n is an integer greater than or equal to 3; the n layers of positive electrode active layers are arranged in sequence in the direction perpendicular to the surface of the positive electrode current collector, each layer of the positive electrode active layers comprises positive electrode active material, and the positive electrode active material comprises lithium iron phosphate and a ternary positive electrode material; in the thickness direction of the positive electrode active layer, from the side close to the surface of the positive electrode current collector to the side far from the surface of the positive electrode current collector, the mass content of the ternary positive electrode material in each layer of the positive electrode active layers gradually decreases, and the mass content of the lithium iron phosphate gradually increases. The special structure can make the lithium ion battery have high cycle performance and safety performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and relates to a positive electrode sheet, in particular to a positive electrode sheet and application thereof. BACKGROUND

[0002] Lithium ion batteries are widely used in portable electronic devices (such as notebook computers, tablet computers, smart phones, etc.), electric vehicles, marine affairs, aerospace and other technical fields due to their high voltage platform, long service life, low self-discharge rate, no memory effect, green environmental protection and other advantages. However, with the rapid development of commercial society, the energy density and safety performance of the battery cannot meet the demand.

[0003] As one of the key components of lithium ion batteries, the positive electrode active material plays a crucial role in improving the performance of the battery. The current mainstream positive electrode active material is mainly lithium iron phosphate and ternary positive electrode material. Lithium iron phosphate has the advantages of high safety, long service life and low cost, but its battery consistency is poor, the energy density is low, and the low-temperature performance needs to be improved. The ternary positive electrode material has higher energy density than lithium iron phosphate, and also has better rate performance, but the cost is higher and the safety performance is poor, and there is a risk of fire and spontaneous combustion during the cycle use.

[0004] At present, the lithium iron phosphate and the ternary positive electrode material are directly mixed to make the battery have high energy density and safety performance at the same time, but this method is difficult to fully exert the advantages of the two; and direct mixing is easy to cause lithium precipitation on the surface of the negative electrode, affecting the cycle performance of the battery.

[0005] Therefore, how to improve the lithium precipitation problem and effectively improve the cycle performance of the battery on the basis of ensuring high energy density and safety performance is a technical problem to be solved at present. SUMMARY

[0006] In view of the above defects, the application provides a positive electrode sheet. The positive electrode active layer in the positive electrode sheet is not less than 3 layers, and in the thickness direction thereof, the mass content of lithium iron phosphate gradually increases from the side close to the surface of the positive electrode current collector to the side away from the surface of the positive electrode current collector, and the mass content of the ternary positive electrode material gradually decreases. This special structure can make the lithium ion battery have high cycle performance and safety performance.

[0007] The application also provides a lithium ion battery comprising the above positive electrode sheet, so that the lithium ion battery has high cycle performance and safety performance.

[0008] The application provides a positive electrode sheet, which comprises a positive electrode current collector and n layers of positive electrode active layers arranged on at least part of the surface of the positive electrode current collector, wherein n is an integer greater than or equal to 3.

[0009] The n layers of positive electrode active layers are sequentially stacked on the surface of the positive electrode current collector, each of the positive electrode active layers comprising a positive electrode active material, the positive electrode active material comprising lithium iron phosphate and a ternary positive electrode material;

[0010] In the thickness direction of the positive electrode active layer, from the side close to the positive electrode current collector surface to the side away from the positive electrode current collector surface, the mass content of the ternary positive electrode material in each of the positive electrode active layers gradually decreases, and the mass content of the lithium iron phosphate in each of the positive electrode active layers gradually increases.

[0011] Further, in the positive electrode active material of the first layer of positive electrode active layer, the mass content of the ternary positive electrode material is not less than 99%;

[0012] And / or, in the positive electrode active material of the n layer of positive electrode active layer, the mass content of the lithium iron phosphate is not less than 99%.

[0013] Further, the thickness of the first layer of positive electrode active layer is equal to the thickness of the n layer of positive electrode active layer;

[0014] Preferably, the single-sided total thickness H of the n layers of positive electrode active layers is ≤400 μm.

[0015] Further, the outer surface of the positive electrode active layer is provided with a recess;

[0016] Preferably, the depth d of the recess is 1-20 μm. n Satisfies: 0.01h n ≤d≤h n ;

[0017] More preferably, the depth d of the recess is 1-20 μm.

[0018] Further, the maximum size s of the recess is 5-200 μm, and / or the pitch t of the recess is 100-2000 μm.

[0019] Further, the median particle size Dv50 of the ternary positive electrode material is 5-15 μm, and / or the median particle size Dv50 of the lithium iron phosphate is 1-2.5 μm.

[0020] The application also provides a lithium ion battery comprising the positive electrode sheet of any one of the above.

[0021] Further, the lithium ion battery further comprises an electrolyte, the electrolyte comprising a nitrile additive and an ether nitrile additive;

[0022] The nitrile additive includes at least one of succinonitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluorohexanedinitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, and 1,2,3-tris(cyanoethyloxy)propane.

[0023] The ether nitrile additive includes at least one of 1,2,3-tris(cyanoethyloxy)propane, ethylene glycol bis(2-cyanoethyl) ether, and diethylene glycol bis(2-cyanoethyl) ether.

[0024] Further, the sum of the mass percentage content i of the nitrile additive and the mass percentage content j of the ether nitrile additive in the electrolyte is 2-18%;

[0025] Preferably, 2i≤j;

[0026] More preferably, i is 0.1-10%, and / or j is 0.1-8%.

[0027] Further, the lithium ion battery satisfies: 0.5h n / H≤i≤5h n / H+8, 0.5h n / H≤j≤5h n / H+8.

[0028] The positive electrode sheet in the application includes not less than 3 layers of positive electrode active layers, and each layer of the positive electrode active layer includes positive electrode active materials lithium iron phosphate and ternary positive electrode materials. By making the lithium iron phosphate and the ternary positive electrode materials present a gradient distribution, i.e. from the side close to the positive electrode current collector to the side away from the positive electrode current collector, the content of the lithium iron phosphate gradually increases, and the content of the ternary positive electrode material gradually decreases in the thickness direction of the positive electrode active layer, the uniformity of the lithium iron phosphate and the ternary positive electrode material per unit area is effectively improved, and the problem of black spots or lithium precipitation on the negative electrode surface is improved. Therefore, the positive electrode sheet with the special structure in the application can effectively improve the safety performance and cycle performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

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

[0030] Figure 2 It is a schematic diagram of the positive electrode sheet including the positive electrode tab of the application;

[0031] Figure 3 It is an SEM diagram of the recessed part of the surface of the positive electrode active layer in Example 1 of the application.

[0032] BRIEF DESCRIPTION OF DRAWINGS:

[0033] 1-positive electrode current collector;

[0034] 2 - the first layer of positive active layer;

[0035] 3 - the second layer of positive active layer;

[0036] 4 - the n layer of positive active layer;

[0037] 5 - the positive tab;

[0038] 6 - the positive tab adhesive tape;

[0039] 7 - the negative tab adhesive tape 7. DETAILED DESCRIPTION

[0040] In order to make the objects, technical solutions and advantages of the present application clearer, the following will combine the embodiments of the present application to make a clear and complete description of the technical solutions in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0041] The first aspect of the present application provides a positive electrode sheet, which comprises a positive electrode current collector and n layers of positive active layers arranged on at least part of the surface of the positive electrode current collector, wherein n is an integer greater than or equal to 3;

[0042] The n layers of positive active layers are arranged in sequence on the surface perpendicular to the positive electrode current collector, each layer of positive active layer comprises positive active material, and the positive active material comprises lithium iron phosphate and ternary positive material;

[0043] In the thickness direction of the positive active layer, from the side close to the surface of the positive electrode current collector to the side away from the surface of the positive electrode current collector, the mass content of the ternary positive material in each layer of positive active layer gradually decreases, and the mass content of the lithium iron phosphate in each layer of positive active layer gradually increases.

[0044] Figure 1 Fig. 1 is a structural schematic diagram of the positive electrode sheet, Figure 1 In the thickness direction of the positive active layer, from the side close to the surface of the positive electrode current collector to the side away from the surface of the positive electrode current collector, the mass content of the ternary positive material in each layer of positive active layer gradually decreases, and the mass content of the lithium iron phosphate in each layer of positive active layer gradually increases.

[0045] The inventor has found through research that, in order to achieve high energy density and high safety performance, ternary positive electrode materials and lithium iron phosphate are often directly blended and coated at present, but the capacity of the ternary positive electrode material is higher than that of the lithium iron phosphate, so the attenuation of the ternary positive electrode material is faster than that of the lithium iron phosphate material at high voltage, and due to the difference in particle size between the ternary positive electrode material and the lithium iron phosphate, it is difficult to achieve complete uniformity of the ternary positive electrode material and the lithium iron phosphate material, in this case, in the area rich in ternary positive electrode material, more lithium ions need to be absorbed in the initial stage of the cycle because the capacity of the ternary positive electrode material is higher, but the negative electrode as a whole is uniform, and the concentration difference generated by the transfer of lithium ions to the ternary positive electrode material is getting larger and larger, thereby exacerbating the uneven lithium intercalation of the negative electrode sheet; in the later stage of the cycle, the attenuation of the ternary positive electrode material is accelerated, which causes the area rich in ternary positive electrode material to have poor ability to absorb lithium ions, and the corresponding negative electrode area can receive less lithium ions, so the area rich in ternary positive electrode material is prone to black spots or lithium precipitation.

[0046] In the present application, at least part of the surface of the positive electrode current collector is provided with not less than 3 layers of positive electrode active layers, and each layer of positive electrode active layer includes lithium iron phosphate and ternary positive electrode material, which can effectively improve the uniformity of the distribution of lithium iron phosphate and ternary positive electrode material in the thickness direction, reduce the difference in slurry viscosity and surface tension between different layers, reduce the transfer or diffusion resistance of lithium ions between layers, and effectively improve the cycle performance and fast charging performance of the battery.

[0047] At the same time, in the thickness direction of the positive electrode active layer, the content of lithium iron phosphate gradually increases from the side close to the positive electrode current collector to the side away from the positive electrode current collector, and the content of ternary positive electrode material gradually decreases, which helps to improve the uniformity of lithium iron phosphate and ternary positive electrode material per unit area, thereby effectively avoiding black spots or lithium precipitation caused by uneven distribution. Moreover, since the ternary positive electrode material is alkaline, it is easy to absorb water and carbon dioxide in the air, and more side reactions are likely to occur in the later stage of the cycle, affecting the safety performance of the battery; this special structure can reduce the negative effects of alkalinity, to some extent, reduce the side reactions between the positive electrode side and the electrolyte, and improve the cycle stability. In addition, by controlling the highest content of ternary positive electrode material on the surface close to the positive electrode current collector, a high energy density can be ensured, and by controlling the highest content of lithium iron phosphate on the surface away from the positive electrode current collector, when the temperature is transferred from the outside to the inside of the electrode, the ternary positive electrode material on the inside reaches the failure temperature, and the lithium iron phosphate material on the outermost side has not failed, thereby delaying the increase in the internal temperature of the battery caused by the failure of the ternary positive electrode material, and improving the safety performance of the battery.

[0048] In summary, the positive electrode sheet in the present application can effectively improve the cycle performance and safety performance of the battery.

[0049] The positive current collector in this invention can be a material commonly used by those skilled in the art, such as aluminum foil.

[0050] This invention does not specifically limit the source of lithium iron phosphate and ternary cathode materials; products prepared by commercially available products or conventional preparation methods known to those skilled in the art are acceptable.

[0051] In one specific embodiment, the ternary cathode material content in the cathode active material of the first cathode active layer is not less than 99%. This allows for further improvement of the battery's energy density while ensuring safety performance.

[0052] In one specific embodiment, the lithium iron phosphate content in the positive electrode active material of the nth positive electrode active layer is not less than 99%. This not only allows the battery to have a higher energy density but also further improves its safety performance.

[0053] In one specific implementation, the n-layer positive electrode active layer satisfies: y = p * ln(m) + q, where p and q are constants, and y is the mass percentage of the ternary positive electrode material in the positive electrode active material in the m-th positive electrode active layer.

[0054] When the n-layer positive electrode active layer satisfies the aforementioned relationship, the uniformity of the distribution of lithium iron phosphate and ternary positive electrode materials in the thickness direction can be further improved, which helps to reduce the difference in slurry viscosity and surface tension between different layers, reduce the transport or diffusion resistance of lithium ions between layers, and further improve the cycle performance and fast charging performance of the battery.

[0055] It should be noted that the constants p and q mentioned above can be calculated by substituting the mass content of the ternary cathode material in the first positive electrode active layer and the mass content of the ternary cathode material in the nth positive electrode active layer into the relation y = p * ln(m) + q. Furthermore, the mass content of the ternary cathode material in the mth positive electrode active layer can be obtained simply by substituting m into the aforementioned relation.

[0056] Exemplarily, when n is 10, the mass content of the ternary positive electrode material in the first layer of the positive electrode active layer is 100%, the mass content of the ternary positive electrode material in the tenth layer of the positive electrode active layer is 0, y=p*ln(m)+q is obtained by substituting y=p*ln(m)+q, p=-0.434, q=1, the relationship is y=-0.434*ln(m)+1, the mass content of the ternary positive electrode material in the positive electrode active material in the second layer is -0.434*ln(2)+1=0.6992, which is 69.92%; the mass content of the ternary positive electrode material in the positive electrode active material in the third layer is -0.434*ln(3)+1=0.5232, which is 52.32%; and so on; the mass content of the ternary positive electrode material in the positive electrode active material in the ninth layer is -0.434*ln(9)+1=0.0464, which is 4.64%.

[0057] In a specific embodiment, the thickness of the first layer of the positive electrode active layer is equal to the thickness of the nth layer of the positive electrode active layer.

[0058] The n layers of the positive electrode active layer satisfy: h m =am 2 +bm+c, wherein h (n+1) / 2 =0, a, b, c are constants, and h m is the thickness of the mth layer of the positive electrode active layer.

[0059] Preferably, the total thickness H of the n layers of the positive electrode active layer on one side is ≤400 μm.

[0060] When the positive electrode active layer satisfies the above relationship, the thickness of the first layer and the nth layer of the positive electrode active layer can be moderate, which can not only ensure high thermal stability, but also further improve the energy density of the battery, and greatly improve the overall energy density and safety performance of the battery cell.

[0061] Further, when the total thickness H of the n layers of the positive electrode active layer on one side is ≤400 μm, the lithium ion transport channel is short, and the wettability of the electrolyte is good, which is beneficial to the transmission of lithium ions, reduces the polarization and lithium precipitation risk.

[0062] Exemplarily, H is 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm or 400 μm.

[0063] It can be understood that the total thickness of the n layers of the positive electrode active layer on one side in the present application refers to the total thickness of the n layers of the positive electrode active layer on one side of the positive electrode current collector after rolling.

[0064] It should be noted that the constants a, b, and c described above can be obtained by substituting the thickness of the first layer of the positive electrode active layer, the thickness of the nth layer of the positive electrode active layer, and h (n+1) / 2 =0 into the relationship h m =am 2The thickness of the m-th positive electrode active layer can be obtained by substituting m into the aforementioned formula.

[0065] For example, when n is 10, the thickness of the first positive electrode active layer is 30 μm, the thickness of the 10th positive electrode active layer is 30 μm, and h (10+1) / 2 When = 0, substitute h m =am 2 In the equation +bm+c, we obtain a = 1.4815, b = -16.296, and c = 44.815. Therefore, the relation is h. m =1.4815m 2 Given -16.296m + 44.815, the thickness of the second positive electrode active layer is 1.4815 * 2. 2 -16.296*2+44.815=18.15, which is 18.15μm. The thickness of the third positive electrode active layer is 1.4815*3. 2 -16.296*3+44.815=9.26, which is 9.26μm, ..., the thickness of the 9th positive electrode active layer is 1.4815*9 2 -16.296*9+44.815=18.15, which is 18.15μm.

[0066] In one specific embodiment, a recess is provided on the outer surface of the positive electrode active layer;

[0067] Preferably, the depth d of the recess is equal to the single-sided thickness h of the nth positive electrode active layer. n Satisfying: 0.01h n ≤d≤h n ;

[0068] More preferably, the depth d of the recess is 1 to 20 μm.

[0069] By setting a recess on the outer surface of the positive electrode active layer, on the one hand, the wettability and electrolyte retention of the electrolyte on the electrode can be effectively improved, avoiding the impact of poor electrolyte wetting and insufficient electrolyte during long cycles on lithium-ion transport, thus further improving the cycle performance of the battery; on the other hand, it can reserve a certain space for the volume expansion of the negative electrode active material, improving the problem of cycle performance degradation caused by volume expansion.

[0070] Furthermore, when the depth d of the recess is equal to the thickness h of the nth positive electrode active layer... n Satisfying 0.01h n ≤d≤h nAt this time, the depth of the recessed part is appropriate, which can make the positive plate have higher liquid retention capacity, improve the rate performance and cycle performance of the battery, and will not affect the protection of the inner side active material by the n-th positive active layer, and will not hinder the temperature from the outside to the inside of the electrode, so that the battery has higher safety performance.

[0071] Further, the depth d of the recessed part is 1-20 μm. In this range, the battery can have higher cycle performance, rate performance and safety performance.

[0072] Exemplarily, d is 1 μm, 5 μm, 10 μm, 15 μm or 20 μm.

[0073] It should be noted that the recessed part is arranged in the area other than the adhesive paper area. Figure 2 The schematic view of the positive plate including the positive tab is shown in FIG. 1, Figure 2 In the figure, the positive tab 5 is arranged on the surface of the positive current collector 1 (the area where the positive tab is arranged is not arranged with the positive active layer), and the positive tab adhesive paper 6 exists on the surface of the positive tab 5, and the negative tab adhesive paper 7 also exists at the position corresponding to the negative tab, which plays a role in preventing the burr from piercing the diaphragm and short circuit.

[0074] The shape of the recessed part is not specifically limited in the present application, for example, it can be at least one of circular, oval, polygonal.

[0075] The implementation of the recessed part is not specifically limited in the present application, for example, the embossing roller can be used to form a rough pattern on the surface of the pole piece, that is, the recessed part.

[0076] In one specific embodiment, the maximum size s of the recessed part is 5-200 μm. In this range, the battery can accommodate more free electrolyte, and the overall wettability of the cell is better, which is beneficial to the long cycle and high safety of the cell; at the same time, during winding, the phenomenon of "wavy sheet" will not occur, which reduces the risk of lithium precipitation and even fire in the later cycle.

[0077] The maximum size s of the recessed part in the present application refers to the circumscribed circle of the outer contour of the recessed part, and the diameter of the circumscribed circle is the maximum size s of the recessed part.

[0078] Exemplarily, s is 5 μm, 50 μm, 100 μm, 150 μm or 200 μm.

[0079] In one specific embodiment, the pitch t of the recessed part is 100-2000 μm. In this range, the integrity of the pole piece can be improved under the premise of ensuring the wettability of the electrolyte to the pole piece and the liquid retention capacity of the pole piece, so that the surface roughness of the pole piece is moderate, and the problem of poor formation and lithium precipitation at the sharp end can be avoided.

[0080] The interval of the recesses in the present application refers to the distance between the centers of two adjacent recesses.

[0081] Exemplarily, t is 100 μm, 500 μm, 1000 μm, 1500 μm or 2000 μm.

[0082] It can be understood that although the recesses can improve the cycle performance of the battery, the recesses form protrusions, i.e. the junctions of the recesses and the flat parts, causing the problem of sharp protrusion lithium precipitation. In order to improve this problem, the pressure can be reduced during the formation process of the battery, specifically, the pressure is reduced by d / 2.5H MPa compared to the theoretical formation pressure.

[0083] In a specific embodiment, the median particle size Dv50 of the ternary cathode material is 5-15 μm, and / or the median particle size Dv50 of the lithium iron phosphate is 1-2.5 μm. Within this range, on the one hand, the consumption of electrolyte can be reduced, and the problem of black spots caused by electrolyte drying at the later stage of the cycle can be alleviated; on the other hand, the smooth transmission of lithium ions is facilitated, and the transmission impedance is reduced, thereby further improving the cycle performance and safety performance of the battery.

[0084] The "median particle size Dv50" in the present application refers to the particle size corresponding to the cumulative volume distribution of 50% of the ternary cathode material particles or lithium iron phosphate particles, which can be obtained by a laser particle size analyzer.

[0085] Exemplarily, the median particle size Dv50 of the ternary cathode material is 5 μm, 7 μm, 9 μm, 11 μm, 13 μm or 15 μm; and / or the median particle size Dv50 of the lithium iron phosphate is 1 μm, 1.3 μm, 1.6 μm, 1.9 μm, 2.2 μm or 2.5 μm.

[0086] The second aspect of the present application provides a lithium ion battery comprising the positive electrode sheet of the first aspect. Therefore, the lithium ion battery has high cycle performance and safety performance.

[0087] The lithium ion battery in the present application can be a wound battery or a stacked battery.

[0088] The method for manufacturing the lithium ion battery in the present application is not specifically limited, and can be prepared by a method commonly used by those skilled in the art.

[0089] In a specific embodiment, the lithium ion battery further comprises an electrolyte, and the electrolyte comprises a nitrile additive and an ether nitrile additive.

[0090] The nitrile additive includes at least one of succinonitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluorohexanedinitrile, 2,2-difluorosuccinonitrile, tricyanobenzene, acrylonitrile, and 1,2,3-tris(cyanoethyloxy)propane.

[0091] The ether nitrile additive includes at least one of 1,2,3-tris(cyanoethyloxy)propane, ethylene glycol bis(2-cyanoethyl) ether, and diethylene glycol bis(2-cyanoethyl) ether.

[0092] When the electrolyte includes the nitrile additive and the ether nitrile additive, the two additives are synergistic with each other. The nitrile additive can coordinate with cobalt ions in the positive electrode to form a complex, which can inhibit the dissolution of cobalt ions to some extent and maintain the stability of the positive electrode. However, there are still exposed active sites in the positive electrode. The C-O bond in the ether nitrile additive can be oxidized during the discharge process to participate in the formation of the interface film. The cyan group generated after oxidation can continue to react with cobalt ions to form a stable and firm interface film, further inhibiting the dissolution of transition metal ions in the positive electrode and improving the cycle performance of the battery. At the same time, although the recessed part on the surface of the positive active layer can improve the wettability of the electrolyte and improve the volume expansion of the negative electrode, a protrusion (i.e., the junction of the recessed part and the flat part) is formed at the recessed part, which is prone to lithium precipitation. The nitrile additive and the ether nitrile additive, especially the ether nitrile additive, can preferentially form a film at the protrusion, thereby reducing the surface roughness of the electrode sheet, reducing the impedance, reducing the risk of lithium precipitation and capacity collapse, and effectively improving the safety performance of the battery. Therefore, when the electrolyte includes both the nitrile additive and the ether nitrile additive, the cycle performance and safety performance of the battery can be further improved.

[0093] When the nitrile additive and the ether nitrile additive are mixtures of the specific compounds described above, the present application does not specifically limit the ratio between the specific compounds.

[0094] The present application does not specifically limit the source of the nitrile additive and the ether nitrile additive. Commercially available products or products prepared by conventional methods known to those skilled in the art can be used.

[0095] In one specific embodiment, the sum of the mass percentage content i of the nitrile additive and the mass percentage content j of the ether nitrile additive in the electrolyte is 2-18%;

[0096] Preferably, 2i≤j;

[0097] More preferably, i is 0.1-10%, and / or j is 0.1-8%.

[0098] When the sum of the mass content of the nitrile compound and the ether nitrile compound in the electrolyte is in the aforementioned range, further satisfying 2i≤j, and more further satisfying i is 0.1-10%, and / or, j is 0.1-8%, the two can better synergize, further improving the cycle performance and safety performance of the battery.

[0099] Illustratively, the sum of i and j is 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, or 18%.

[0100] Illustratively, i is 0.1%, 1%, 2%, 4%, 6%, 8%, or 10%; j is 0.1%, 1%, 3%, 4%, 5%, 6%, 7%, or 8%.

[0101] It can be understood that the electrolyte further includes a lithium salt and an organic solvent; wherein the lithium salt is preferably at least one of lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluorophosphate (LiPF6); the organic solvent is preferably at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC).

[0102] In a specific embodiment, the lithium ion battery satisfies: 0.5h n / H≤i≤5h n / H+8, 0.5h n / H≤j≤5h n / H+8. When the lithium ion battery satisfies the aforementioned relationship, not only can a sufficient protective film be formed on the protrusions to ensure the safety of the battery cell, but also the excessive ether nitrile additive can be avoided to cause excessive electrolyte impedance, thereby further reducing the risk of lithium precipitation and capacity collapse.

[0103] It can be understood that the lithium ion battery further includes a negative electrode sheet, the negative electrode sheet including a negative electrode current collector and a negative electrode active layer disposed on at least part of the surface of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material.

[0104] The negative electrode current collector in the application can be a material commonly used by those skilled in the art, such as a copper foil.

[0105] The negative electrode active material in the application is preferably at least one of artificial graphite, natural graphite, tin-based negative electrode material, lithium-containing transition metal nitride negative electrode material, alloy-based negative electrode material, and nanoscale negative electrode material.

[0106] Hereinafter, the lithium ion battery including the positive electrode sheet of the application is described in detail through specific embodiments.

[0107] Example 1

[0108] 1) The positive electrode active layer in this embodiment is 4 layers, the mass content of ternary positive electrode material NCM613 (Dv50 is 10 pm) in the first layer of positive electrode active material is 100%, the mass content of ternary positive electrode material NCM613 in the fourth layer of positive electrode active material is 0, substituting y = p * ln (m) + q, it is calculated that p = -0.721, q = 1, then y = -0.721 * ln (m) + 1, the mass content of ternary positive electrode material in the second layer of positive electrode active material is 50%, and the mass content of ternary positive electrode material in the third layer of positive electrode active material is 20.8%;

[0109] The thickness of the first layer of positive electrode active layer and the fourth layer of positive electrode active layer is set to 100 pm, h (4+1) / 2 = 0, substituting h = am 2 + bm + c, it is obtained that a = 44.444, b = -222.22, c = 277.78, then the relationship is h = 44.444m 2 -222.22m+277.78, the thickness of the second layer of positive electrode active layer is 11.12 pm, and the thickness of the third layer of positive electrode active layer is 11.12 pm;

[0110] The ternary positive electrode material NCM613 (Dv50 is 10 pm), lithium iron phosphate LiFePO4 (Dv50 is 1.5 pm), conductive agent carbon black and binder PVDF are mixed according to the mass ratio of 96.5:0:2:1.5, N-methyl pyrrolidone (NMP) is added, and the first layer of positive electrode active layer slurry is obtained after stirring uniformly;

[0111] The ternary positive electrode material NCM613 (Dv50 is 10 pm), lithium iron phosphate LiFePO4 (Dv50 is 1.5 pm), conductive agent carbon black and binder PVDF are mixed according to the mass ratio of 96.5:0:2:1.5, N-methyl pyrrolidone (NMP) is added, and the first layer of positive electrode active layer slurry is obtained after stirring uniformly;

[0112] The ternary positive electrode material NCM613 (Dv50 is 10 pm), lithium iron phosphate LiFePO4 (Dv50 is 1.5 pm), conductive agent carbon black and binder PVDF are mixed according to the mass ratio of 96.5:0:2:1.5, N-methyl pyrrolidone (NMP) is added, and the first layer of positive electrode active layer slurry is obtained after stirring uniformly;

[0113] The ternary positive electrode material NCM613 (Dv50 of 10 μm), lithium iron phosphate LiFePO4 (Dv50 of 1.5 μm), conductive agent carbon black, and binder PVDF were mixed in a mass ratio of 0:96.5:2:1.5, N-methyl pyrrolidone (NMP) was added, and the fourth layer of positive electrode active layer slurry was obtained after uniform stirring;

[0114] The first layer of positive electrode active layer slurry, the second layer of positive electrode active layer slurry, the third layer of positive electrode active layer slurry, and the fourth layer of positive electrode active layer slurry were sequentially coated on the surface of the positive electrode current collector aluminum foil, and after drying, rolling, and slitting, the positive electrode sheet was obtained, wherein the thickness of the first layer of positive electrode active layer was 100 μm, the thickness of the second layer of positive electrode active layer was 11.12 μm, the thickness of the third layer of positive electrode active layer was 11.12 μm, and the thickness of the fourth layer of positive electrode active layer was 100 μm;

[0115] Finally, embossing treatment was performed using an embossing roller to form recessed portions on the surface of the positive electrode active layer, the pitch of the recessed portions was 500 μm, the depth was 5 μm, and the maximum size was 50 μm; Figure 3 SEM image of the recessed portions on the surface of the positive electrode active layer in Example 1;

[0116] 2) The negative electrode active material artificial graphite, conductive agent carbon fiber, and adhesive styrene butadiene rubber (SBR) were added to the stirring tank in a mass ratio of 98.2:0.6:1.2, deionized water was added, and after sufficient stirring, the mixture was passed through a 150-mesh sieve to prepare the negative electrode slurry. The negative electrode slurry was coated onto the surface of the copper foil using a coating machine, and after drying (100°C), rolling, and slitting, the negative electrode sheet was obtained;

[0117] 3) In an argon-filled glove box (H2O <0.1 ppm, O2 <0.1 ppm), organic solvents ethylene carbonate (EC), propylene carbonate (PC), and other solvents (propionic acid ethyl ester (EP), ethyl butyrate (EB), ethyl acetate (EA), etc.) were mixed in a volume ratio of 20:20:60; then 15 wt% of fully dried lithium salt lithium hexafluorophosphate (LiPF6) based on the total mass of the electrolyte was quickly added, and after dissolution, 2.0 wt% of a nitrile additive (including butanedinitrile, tricyanobenzene, acrylonitrile, and 1,2,3-tris(cyanoethoxy)propane in a mass ratio of 1:1:1:1) based on the total mass of the electrolyte was added, followed by the addition of 4 wt% of an ether nitrile additive (including 1,2,3-tris(cyanoethoxy)propane and ethylene glycol bis(2-cyanoethyl) ether in a mass ratio of 1:1) based on the total mass of the electrolyte, and after uniform stirring, the electrolyte was obtained after passing the water and free acid detection;

[0118] 4) sequentially place the above positive electrode sheet, separator, and negative electrode sheet, and roll to form an electrode core, then package with an aluminum plastic film, remove moisture by baking, inject electrolyte, then go through formation (theoretical formation pressure is 1.1 MPa, actual formation pressure is 1.09 MPa), two-sealing, and other treatments to obtain a lithium ion battery.

[0119] Example 2

[0120] The preparation method of the lithium ion battery in this example is basically the same as that in Example 1, except that in step 1), the Dv50 of the ternary positive electrode material NCM613 is adjusted to 5 μm, the Dv50 of lithium iron phosphate LiFePO4 is adjusted to 1 μm, and the others remain unchanged.

[0121] Example 3

[0122] The preparation method of the lithium ion battery in this example is basically the same as that in Example 1, except that in step 1), the Dv50 of the ternary positive electrode material NCM613 is adjusted to 15 μm, the Dv50 of lithium iron phosphate LiFePO4 is adjusted to 2.5 μm, and the others remain unchanged.

[0123] Example 4

[0124] The preparation method of the lithium ion battery in this example is basically the same as that in Example 1, except that in step 1), the pitch of the recessed part is adjusted to 100 μm, the depth is adjusted to 1 μm, and the maximum size is adjusted to 5 μm.

[0125] Example 5

[0126] The preparation method of the lithium ion battery in this example is basically the same as that in Example 1, except that in step 1), the pitch of the recessed part is adjusted to 2000 μm, the depth is adjusted to 20 μm, and the maximum size is adjusted to 200 μm.

[0127] Example 6

[0128] The preparation method of the lithium ion battery in this example is basically the same as that in Example 1, except that in step 3), the mass percentage of the nitrile additive in the electrolyte is adjusted to 4%, and the mass percentage of the ether nitrile additive in the electrolyte is adjusted to 8%.

[0129] Example 7

[0130] The preparation method of the lithium ion battery in this example is basically the same as that in Example 1, except that in step 3), the mass percentage of the nitrile additive in the electrolyte is adjusted to 1%, and the mass percentage of the ether nitrile additive in the electrolyte is adjusted to 2%.

[0131] Example 8

[0132] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 3), the mass percentage of the nitrile additive in the electrolyte is adjusted to 2%, and the mass percentage of the ether nitrile additive in the electrolyte is adjusted to 6%.

[0133] Embodiment 9

[0134] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 1), the positive active layer is 7 layers, the mass content of the ternary positive material NCM613 in the positive active material in the first layer of the positive active layer is 100%, and the mass content of the ternary positive material NCM613 in the positive active material in the seventh layer of the positive active layer is 0. Substituting y = p * ln(m) + q, it is calculated that p = -0.514, q = 1, then y = -0.514 * ln(m) + 1, and the mass content of the ternary positive material in the positive active material in the second layer, the third layer, the fourth layer, the fifth layer, and the sixth layer of the positive active layer is 64.37%, 43.53%, 28.74%, 17.27%, and 7.90%, respectively.

[0135] The thickness of the first layer of the positive active layer and the seventh layer of the positive active layer is set to 100 μm, h (7+1) / 2 = 0, and substituting h = am 2 + bm + c, it is obtained that a = 11.111, b = -88.889, and c = 177.78, then the relationship is h = 11.111m 2 - 88.889m + 177.78, and the thickness of the second layer, the third layer, the fourth layer, the fifth layer, and the sixth layer of the positive active layer is 44.45 μm, 11.11 μm, 0, 11.11 μm, and 44.44 μm, respectively.

[0136] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent, and binder in the second layer of the positive active layer slurry is 62.12:34.38:2:1.5.

[0137] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent, and binder in the third layer of the positive active layer slurry is 42.01:54.49:2:1.5.

[0138] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent, and binder in the fifth layer of the positive active layer slurry is 42.01:54.49:2:1.5.

[0139] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent, and binder in the sixth layer of the positive active layer slurry is 7.63:88.87:2:1.5.

[0140] Embodiment 10

[0141] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 1), the positive active layer is 10 layers, the mass content of the ternary positive material NCM613 in the positive active material in the first layer of the positive active layer is 100%, and the mass content of the ternary positive material NCM613 in the positive active material in the tenth layer of the positive active layer is 0. Substituting y = p * ln(m) + q, it is calculated that p = -0.434, q = 1, then y = -0.434 * ln(m) + 1, and the mass content of the ternary positive material in the positive active material in the second layer, the third layer, the fourth layer, the fifth layer, the sixth layer, the seventh layer, the eighth layer and the ninth layer of the positive active layer is 69.92%, 52.32%, 39.83%, 30.15%, 22.24%, 15.55%, 9.75% and 4.64%, respectively.

[0142] The thickness of the first layer of the positive active layer and the tenth layer of the positive active layer is set to 100 μm, h (10+1) / 2 = 0, and substituting h = am 2 + bm + c, it is obtained that a = 1.4815, b = -16.296, and c = 44.815, then the relationship is h = 1.4815m 2 - 16.296m + 44.815, and the thickness of the second layer, the third layer, the fourth layer, the fifth layer, the sixth layer, the seventh layer, the eighth layer and the ninth layer of the positive active layer is 18.15 μm, 9.26 μm, 3.34 μm, 0.37 μm, 0.37 μm, 3.34 μm, 9.26 μm and 18.15 μm, respectively.

[0143] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent and binder in the second layer of the positive active layer slurry is 67.47:29.03:2:1.5.

[0144] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent and binder in the third layer of the positive active layer slurry is 50.49:46.01:2:1.5.

[0145] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent and binder in the fourth layer of the positive active layer slurry is 38.44:58.06:2:1.5.

[0146] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent and binder in the fifth layer of the positive active layer slurry is 29.10:67.40:2:1.5.

[0147] The mass ratio of the ternary positive material, lithium iron phosphate, conductive agent and binder in the sixth layer of the positive active layer slurry is 21.46:75.04:2:1.5

[0148] The mass ratio of ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the 7th layer positive electrode active layer slurry is 15.00:81.50:2:1.5

[0149] The mass ratio of ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the 8th layer positive electrode active layer slurry is 9.41:87.09:2:1.5

[0150] The mass ratio of ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the 9th layer positive electrode active layer slurry is 4.48:92.02:2:1.5.

[0151] Example 11

[0152] The preparation method of the lithium ion battery in the present example is basically the same as that in Example 1, except that in step 1), the mass content of ternary positive electrode material in the 2nd layer positive electrode active layer is adjusted to 67%, and the mass content of ternary positive electrode material in the 3rd layer positive electrode active layer is adjusted to 34%.

[0153] The mass ratio of ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the 2nd layer positive electrode active layer slurry is 64.65:31.85:2:1.5;

[0154] The mass ratio of ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the 3rd layer positive electrode active layer slurry is 32.81:63.69:2:1.5.

[0155] Example 12

[0156] The preparation method of the lithium ion battery in the present example is basically the same as that in Example 1, except that in step 1), the thickness of the 2nd layer positive electrode active layer is adjusted to 60 μm, the thickness of the 3rd layer positive electrode active layer is adjusted to 40 μm, and the thickness of the 4th layer positive electrode active layer is adjusted to 22 μm.

[0157] Example 13

[0158] The preparation method of the lithium ion battery in the present example is basically the same as that in Example 1, except that in step 1), the thickness of the 1st layer positive electrode active layer and the 4th layer positive electrode active layer are both set to 200 μm, then a=88.889, b=-444.44, c=555.56, and the relationship is h=88.889m 2 -444.44m+555.56, the thickness of the 2nd layer positive electrode active layer is 22.24 μm, and the thickness of the 3rd layer positive electrode active layer is 22.24 μm.

[0159] Example 14

[0160] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 1), the positive plate is not embossed, and the positive active layer surface has no recessed part.

[0161] Embodiment 15

[0162] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 1), the depth of the recessed part is adjusted to 0.5 μm.

[0163] Embodiment 16

[0164] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 1), the depth of the recessed part is adjusted to 25 μm.

[0165] Embodiment 17

[0166] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 1), the maximum size of the recessed part is adjusted to 400 μm.

[0167] Embodiment 18

[0168] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 1), the pitch of the recessed part is adjusted to 10 μm.

[0169] Embodiment 19

[0170] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 1), the Dv50 of the ternary positive material NCM613 is adjusted to 3.5 μm, and the Dv50 of the lithium iron phosphate LiFePO4 is adjusted to 0.8 μm, and the others remain unchanged.

[0171] Embodiment 20

[0172] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 3), the nitrile additive and the ether nitrile additive are not included in the electrolyte, and accordingly, the mass percentage content of the organic solvent in the electrolyte is changed to 85%.

[0173] Embodiment 21

[0174] The preparation method of the lithium ion battery in this embodiment is basically the same as that in Embodiment 1, except that in step 3), the nitrile additive is not included in the electrolyte, and accordingly, the mass percentage content of the organic solvent in the electrolyte is changed to 82.8%.

[0175] Embodiment 22

[0176] The preparation method of the lithium ion battery in the embodiment is basically the same as that in Embodiment 1, except that in step 3), the ether nitrile additive is not included in the electrolyte, and accordingly, the mass percentage content of the organic solvent in the electrolyte is changed to 83%.

[0177] Embodiment 23

[0178] The preparation method of the lithium ion battery in the embodiment is basically the same as that in Embodiment 1, except that in step 3), the mass percentage content of the nitrile additive in the electrolyte is adjusted to 0.5%, and the mass percentage content of the ether nitrile additive in the electrolyte is adjusted to 1%.

[0179] Embodiment 24

[0180] The preparation method of the lithium ion battery in the embodiment is basically the same as that in Embodiment 1, except that in step 3), the mass percentage content of the nitrile additive in the electrolyte is adjusted to 13%.

[0181] Embodiment 25

[0182] The preparation method of the lithium ion battery in the embodiment is basically the same as that in Embodiment 1, except that in step 3), the mass percentage content of the ether nitrile additive in the electrolyte is adjusted to 10%.

[0183] Embodiment 26

[0184] The preparation method of the lithium ion battery in the embodiment is basically the same as that in Embodiment 1, except that in step 3), the mass percentage content of the nitrile additive in the electrolyte is adjusted to 0.1%.

[0185] Embodiment 27

[0186] The preparation method of the lithium ion battery in the embodiment is basically the same as that in Embodiment 1, except that in step 4), the formation pressure is 1.1 MPa.

[0187] Embodiment 28

[0188] The preparation method of the lithium ion battery in the embodiment is basically the same as that in Embodiment 1, except that in step 1), the mass content of the ternary cathode material NCM613 in the positive active material in the fourth layer of the positive active layer is set to 50%; and in y = p*ln(m) + q, p = -0.361, q = 1, y = -0.361*ln(m) + 1 is calculated; the mass content of the ternary cathode material in the positive active material in the second layer of the positive active layer is 74.98%, and the mass content of the ternary cathode material in the positive active material in the third layer of the positive active layer is 60.34%.

[0189] The mass ratio of the ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the second layer positive electrode active layer slurry is 72.35:24.15:2:1.5.

[0190] The mass ratio of the ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the third layer positive electrode active layer slurry is 58.23:38.27:2:1.5.

[0191] Example 29

[0192] The preparation method of the lithium ion battery in the example is basically the same as that in example 1, except that in step 1), the mass content of the ternary positive electrode material NCM613 in the positive electrode active material in the first layer positive electrode active layer is 90%; substituting y=p*ln(m)+q, it is calculated that p=-0.649, q=0.9, then y=-0.649*ln(m)+0.9; the mass content of the ternary positive electrode material in the positive electrode active material in the second layer positive electrode active layer is 45.01%, and the mass content of the ternary positive electrode material in the positive electrode active material in the third layer positive electrode active layer is 18.70%.

[0193] The mass ratio of the ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the second layer positive electrode active layer slurry is 43.44:53.06:2:1.5.

[0194] The mass ratio of the ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the third layer positive electrode active layer slurry is 18.05:78.45:2:1.5.

[0195] Comparative Example 1

[0196] The preparation method of the lithium ion battery in the example is basically the same as that in example 1, except that in step 1), the first layer positive electrode active layer slurry, the second layer positive electrode active layer slurry, the third layer positive electrode active layer slurry and the fourth layer positive electrode active layer slurry are mixed uniformly to obtain a positive electrode slurry, and the positive electrode slurry is coated on the surface of an aluminum foil, and after drying, rolling, and cutting, a positive electrode sheet is obtained, and the thickness of the positive electrode active layer is 222.32 μm, and the other steps are the same as those in example 1.

[0197] Comparative Example 2

[0198] The preparation method of the lithium ion battery in the example is basically the same as that in example 1, except that in step 1), the mass content of the ternary positive electrode material in the positive electrode active material in the first layer positive electrode active layer is 0, the mass content of the ternary positive electrode material in the positive electrode active material in the second layer positive electrode active layer is 20.8%, the mass content of the ternary positive electrode material in the positive electrode active material in the third layer positive electrode active layer is 50%, and the mass content of the ternary positive electrode material in the positive electrode active material in the fourth layer positive electrode active layer is 100%.

[0199] The mass ratio of the ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the first layer positive electrode active layer slurry is 0:96.5:2:1.5;

[0200] The mass ratio of the ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the second layer positive electrode active layer slurry is 20.06:76.44:2:1.5;

[0201] The mass ratio of the ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the third layer positive electrode active layer slurry is 48.27:48.23:2:1.5;

[0202] The mass ratio of the ternary positive electrode material, lithium iron phosphate, conductive agent and binder in the fourth layer positive electrode active layer slurry is 96.5:0:2:1.5.

[0203] Comparative Example 3

[0204] The preparation method of the lithium ion battery in the present comparative example is basically the same as that in Example 1, except that in step 1), the positive electrode active layer is 2 layers, the mass content of the ternary positive electrode material in the positive electrode active material in the first layer positive electrode active layer is 100%, the mass content of lithium iron phosphate in the positive electrode active material is 0, the mass content of the ternary positive electrode material in the positive electrode active material in the second layer positive electrode active layer is 0, and the mass content of lithium iron phosphate in the positive electrode active material is 100%.

[0205] The thickness of the first layer positive electrode active layer and the second layer positive electrode active layer is both set to 100 μm.

[0206] Test Example

[0207] 1. The energy density, cycle performance, fast charging performance, oven temperature safety performance and lithium precipitation of the lithium ion batteries prepared in the above examples and comparative examples were tested, and the test results are shown in Table 1.

[0208] 1) Cycle performance

[0209] At an ambient temperature of 25°C, charge at 2C to 3.5V, the cutoff current is 1C, then charge at 1C to 3.65V, the cutoff current is 0.05C, then discharge at 0.5C to 2.5V, and record the discharge capacity as C0; cycle according to the foregoing charge and discharge mechanism, cycle 600 times, and record the discharge capacities at 300 times and 600 times as C1 and C2, respectively, then the cycle capacity retention rate at 300 times (%) = C1 / C0 x 100%, and the cycle capacity retention rate at 600 times (%) = C2 / C0 x 100%, and the test results are shown in Table 1.

[0210] The lithium ion batteries cycled to 300 times and 600 times were disassembled, respectively, to observe whether there were black spots on the anode interface, and the results were recorded.

[0211] 2) Battery rate performance

[0212] The rate performance of the battery was determined by rate charging test of the battery at 25°C, and the details were as follows:

[0213] At an ambient temperature of 25°C, the state voltage, internal resistance and thickness of the battery were tested, and the battery was rested for 10 min; discharged at 0.2C to 3.0V, and rested for 10 min; charged to the upper limit voltage (charged to 4.45V in the present application) at 0.2C, 0.3C and 0.4C in turn, with a cutoff current of 0.05C, and rested for 10 min; discharged at 0.5C to 3.0V, and rested for 10 min. After 30 cycles according to the foregoing charge and discharge mechanism, the battery was disassembled at full charge.

[0214] The anode interface was observed. If the anode tab was golden yellow without abnormality and lithium was not precipitated, the battery supported the corresponding rate performance; if there were gray / brown / white particles covering on the interface, the battery precipitated lithium, and at this time the battery did not support the corresponding rate performance. The maximum rate of the battery without lithium precipitation was the rate performance of the battery.

[0215] 3) Oven temperature safety performance

[0216] Five lithium ion batteries were taken as a group for each example and comparative example, discharged at 0.2C to the cutoff voltage 3.0V at an ambient temperature of 25°C, and rested for 5 min; then charged to the upper limit voltage 4.45V at 0.5C constant current and constant voltage, with a cutoff current of 0.05C, at this time the battery was in full charge state, and a photo was taken; then the full charge battery was placed in a test chamber, and the temperature was raised at 5°C / min, when the temperature in the chamber reached 140°C±2°C, the temperature was kept constant for 60 min; after the end, a photo was taken, and the battery did not catch fire or explode, which was passed, and the number of passed groups was recorded. The test results are shown in Table 1.

[0217] Table 1

[0218]

[0219]

[0220] From Table 1, it can be seen that:

[0221] Compared with Comparative Examples 1-3, the lithium ion batteries in Examples 1-29 have higher cycle performance, fast charging performance and oven temperature safety performance, and the lithium precipitation problem during the cycle process is effectively improved. Among them, the capacity retention rate of the lithium ion battery in Example 10 after 300 cycles can reach 95.45%, the capacity retention rate after 600 cycles is still 90.51%, and the pole piece does not precipitate lithium; In addition, it can also pass the oven temperature safety performance test. It can be seen that the positive pole piece in the application can make the lithium ion battery have high energy density and safety performance.

[0222] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A positive electrode plate, characterized in that, The positive electrode sheet includes a positive current collector and n positive active layers disposed on at least a portion of the surface of the positive current collector, where n is an integer greater than or equal to 3; The n positive electrode active layers are stacked sequentially on a surface perpendicular to the positive electrode current collector. Each positive electrode active layer includes a positive electrode active material, which includes lithium iron phosphate and ternary positive electrode materials. In the thickness direction of the positive electrode active layer, from the side closer to the surface of the positive electrode current collector to the side farther away from the surface of the positive electrode current collector, the mass content of ternary positive electrode material in each positive electrode active layer gradually decreases, and the mass content of lithium iron phosphate in each positive electrode active layer gradually increases.

2. The positive electrode sheet according to claim 1, characterized in that, In the positive electrode active material of the first positive electrode active layer, the mass content of the ternary positive electrode material is not less than 99%; And / or, in the positive electrode active material of the nth positive electrode active layer, the mass content of the lithium iron phosphate is not less than 99%.

3. The positive electrode sheet according to claim 2, characterized in that, The thickness of the first positive electrode active layer is equal to the thickness of the nth positive electrode active layer; The total thickness H on one side of the n-layer positive electrode active layer is ≤400μm.

4. The positive electrode sheet according to claim 3, characterized in that, The outer surface of the positive electrode active layer is provided with a recessed portion; Wherein, the depth d of the recessed portion is equal to the single-sided thickness h of the nth positive electrode active layer. n Satisfying: 0.01h n ≤d≤h n ; The depth d of the recess is 1~20μm.

5. The positive electrode sheet according to claim 4, characterized in that, The maximum size s of the recess is 5~200μm, and / or the spacing t of the recesses is 100~2000μm.

6. The positive electrode sheet according to any one of claims 1-5, characterized in that, The median particle size Dv50 of the ternary cathode material is 5~15μm, and / or the median particle size Dv50 of the lithium iron phosphate is 1~2.5μm.

7. A lithium-ion battery, characterized in that, Includes the positive electrode sheet as described in any one of claims 1-6.

8. The lithium-ion battery according to claim 7, characterized in that, The lithium-ion battery also includes an electrolyte, which includes nitrile additives and ether nitrile additives; The nitrile additives include at least one of butadionitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, p-fluorobenzonitrile, p-methylbenzonitrile, 2-fluoroadiponitrile, 2,2-difluorobutadionitrile, tricyanobenzene, acrylonitrile, and 1,2,3-tris(cyanoethoxy)propane. The ether nitrile additives include at least one of 1,2,3-tris(cyanoethoxy)propane, ethylene glycol di(2-cyanoethyl) ether, and diethylene glycol di(2-cyanoethyl) ether.

9. The lithium-ion battery according to claim 8, characterized in that, The sum of the mass percentage i of nitrile additives and the mass percentage j of ether nitrile additives in the electrolyte is 2-18%. Where 2i≤j; i is 0.1% to 10%, and / or j is 0.1% to 8%.

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