Secondary battery with asymmetric positive and negative electrode capacities and preparation method and application thereof

Abstract

The invention relates to a secondary battery with asymmetric positive and negative electrode capacities and a preparation method and application thereof. The secondary battery comprises a positive electrode and a negative electrode, the negative electrode capacity of the secondary battery is recorded as N, the positive electrode capacity is recorded as P, and N / P is smaller than or equal to 0.2. The secondary battery can overcome strict requirements on positive and negative pole capacities in the traditional battery field, can avoid a complex positive and negative pole capacity matching process, improves the energy density, safety and cycle life of the battery, provides high design flexibility and adaptability, simplifies the battery and reduces the production cost.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium batteries, and relates to a secondary battery, in particular to a secondary battery with asymmetric positive and negative electrode capacities and a preparation method and application thereof. BACKGROUND

[0002] With the rapid development of renewable energy and electric vehicles, the requirements for battery performance are also increasing. In the field of traditional batteries, the matching degree of positive and negative electrode capacities is one of the important factors affecting battery performance. By reasonably designing and regulating the capacity ratio (N / P) of positive and negative electrode materials, the optimization of battery capacity, energy density, cycle life and safety can be achieved, thereby promoting the progress of battery technology. The calculation formula of N / P is:

[0003] N / P = [(negative electrode surface density x negative electrode active material ratio x negative electrode active material specific discharge capacity) / (positive electrode surface density x positive electrode active material ratio x positive electrode active material specific discharge capacity)]

[0004] Ideally, the positive and negative electrodes of a battery are usually designed with symmetric capacity, i.e. positive electrode capacity = negative electrode capacity, N / P = 1.

[0005] The symmetric design of positive and negative electrode capacities is affected by the following factors: ① Positive and negative electrode material system: the charge and discharge platform, structural stability of the positive and negative electrode materials need to be considered; ② First efficiency of positive and negative electrode materials: all existing reactions need to be considered, including conductive agents, binders, current collectors, separators and electrolytes; ③ Coating precision of the equipment; ④ Decay rate of positive and negative electrodes: if the positive electrode decays quickly, the N / P ratio needs to be designed to be low to keep the positive electrode in a shallow charge and discharge state, and vice versa, if the negative electrode decays quickly, the N / P needs to be designed to be high to keep the negative electrode in a shallow charge and discharge state; ⑤ Required rate performance of the battery.

[0006] For traditional lithium-ion batteries using graphite negative electrodes and sodium-ion batteries using hard carbon negative electrodes, if the positive electrode capacity is excessive, the excess lithium ions in the positive electrode cannot enter the negative electrode during charging, and will deposit on the surface of the negative electrode to form dendrites, which will cause poor cycle performance of the battery, and even cause internal short circuit of the battery, leading to safety problems of the battery. Therefore, the negative electrode is usually designed to be excessive, in which case the capacity of the battery is limited by the positive electrode capacity, and the N / P is usually greater than 1. However, the negative electrode capacity cannot be excessive, otherwise an SEI film will be formed on the surface of the negative electrode, consuming active lithium in the positive electrode, resulting in a decrease in the first coulombic efficiency; in addition, it will also cause waste of negative electrode materials, reduce the energy density of the battery, and increase the cost of the battery. Therefore, the N / P is generally between 1.03 and 1.5.

[0007] For the battery with lithium titanate negative electrode, since the lithium titanate negative electrode has stable structure, high voltage platform, excellent cycle performance and no lithium precipitation phenomenon, the reason for cycle failure mainly occurs at the positive electrode end, and the battery system adopts the scheme that the positive electrode capacity is slightly excessive, and the N / P is generally between 0.85-0.9. Therefore, the N / P ratio in the traditional battery is generally 1±0.15, and generally does not exceed 2, and the positive and negative electrode capacities are symmetrically designed.

[0008] However, due to the complex battery material system, the symmetric matching design of the positive and negative electrode capacities is a complex systematic engineering. In addition, in the actual application process of the battery, the charging and discharging mode and the complex and changeable use environment, the existing symmetric positive and negative electrode capacity design is difficult to meet the requirements and lacks flexibility.

[0009] Therefore, it is necessary to overcome the strict symmetric design requirements of the positive and negative electrode capacities in the traditional battery field and the complex positive and negative electrode capacity matching design process, so as to improve the energy density, safety and cycle life of the battery, improve the design flexibility and adaptability, simplify the battery and reduce the production cost. SUMMARY

[0010] The purpose of the present application is to provide a kind of positive and negative electrode capacity asymmetric secondary battery and its preparation method and application, the secondary battery can overcome the strict requirements of the positive and negative electrode capacity in the traditional battery field, also can avoid complex positive and negative electrode capacity matching process, in the improvement of the energy density, safety and cycle life of the battery, provide the high degree of design flexibility and adaptability, simplify the battery and reduce the production cost.

[0011] In order to achieve this purpose of the application, the following technical solutions are adopted in the present application:

[0012] In a first aspect, the present application provides a kind of positive and negative electrode capacity asymmetric secondary battery, the secondary battery includes positive and negative electrode;

[0013] The negative electrode capacity of the secondary battery is denoted as N, and the positive electrode capacity is denoted as P, then N / P≤0.2.

[0014] The negative electrode capacity of the secondary battery provided by the present application is far less than the positive electrode capacity, so that the negative electrode sheet thickness and the load are far less than the positive electrode thickness and the load, which is beneficial to the improvement of the battery coulomb efficiency and mass / volume energy density.

[0015] The secondary battery provided by the present application can overcome the strict requirements of the positive and negative electrode capacity in the traditional battery field, also can avoid complex positive and negative electrode capacity matching process, in the improvement of the energy density, safety and cycle life of the battery, provide the high degree of design flexibility and adaptability, simplify the battery and reduce the production cost.

[0016] In the present application, the negative electrode capacity is: negative electrode surface density x negative electrode active material ratio x negative electrode active material specific discharge capacity; and the positive electrode capacity is: positive electrode surface density x positive electrode active material ratio x positive electrode active material specific discharge capacity.

[0017] The positive electrode of the secondary battery according to the present application comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; and the negative electrode of the secondary battery comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector.

[0018] In the present application, the negative electrode capacity of the secondary battery is denoted as N, and the positive electrode capacity is denoted as P, then N / P≤0.2, for example, it can be 0.001, 0.005, 0.01, 0.03, 0.05, 0.08, 0.1, 0.15 or 0.2, but is not limited to the listed values, other values not listed within the value range are also applicable, and preferably 0.001≤N / P≤0.1. When N / P is higher than 0.2, the battery coulomb efficiency and volume / mass energy density are reduced.

[0019] Preferably, the mass ratio of the positive electrode active material in the positive electrode to the negative electrode active material in the negative electrode is (5-200):1, for example, it can be 5:1, 30:1, 50:1, 80:1, 100:1, 120:1, 150:1, 180:1 or 200:1, but is not limited to the listed values, other values not listed within the value range are also applicable, and preferably (50-150):1.

[0020] In order to make the ratio of the negative electrode capacity to the positive electrode capacity in the secondary battery satisfy N / P≤0.2, the present application makes the mass ratio of the positive electrode active material in the positive electrode to the negative electrode active material in the negative electrode be (5-200):1.

[0021] Preferably, the thickness ratio of the positive electrode active material layer in the positive electrode to the negative electrode active material layer in the negative electrode is (2.5-20):1, for example, it can be 2.5:1, 5:1, 7.5:1, 10:1, 15:1 or 20:1, but is not limited to the listed values, other values not listed within the value range are also applicable, and preferably (5-10):1.

[0022] In order to make the ratio of the negative electrode capacity to the positive electrode capacity in the secondary battery satisfy N / P≤0.2, the present application makes the thickness ratio of the positive electrode active material layer in the positive electrode to the negative electrode active material layer in the negative electrode be (2.5-20):1, when the ratio is less than 2.5:1, the battery coulomb efficiency and volume / mass energy density are reduced; and when the ratio exceeds 20:1, the battery cycle life is shortened.

[0023] Preferably, the thickness of the positive electrode active material layer is 50-500 μm, for example, it can be 50 μm, 60 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 400 μm or 500 μm, but is not limited to the listed values, other values not listed within the range of values are also applicable, preferably 60-250 μm.

[0024] The thickness of the positive electrode active material layer according to the present application refers to the thickness of the active material layer on one side of the positive electrode current collector; when the positive electrode current collector has positive electrode active material layers on both sides, the thickness of the positive electrode active material layers on both sides is independently 50-500 μm.

[0025] Preferably, the areal density of the positive electrode is 5-50 mg / cm 2 , for example, it can be 5 mg / cm 2 , 7 mg / cm 2 , 10 mg / cm 2 , 15 mg / cm 2 , 20 mg / cm 2 , 25 mg / cm 2 , 30 mg / cm 2 , 40 mg / cm 2 or 50 mg / cm 2 , but is not limited to the listed values, other values not listed within the range of values are also applicable, preferably 7-25 mg / cm 2 .

[0026] The present application does not specifically limit the areal density of the negative electrode, as long as it can make the N / P of the secondary battery ≤0.2.

[0027] Preferably, the positive electrode active material in the positive electrode active material layer comprises any one or a combination of at least two of sodium vanadium phosphate (Na3V2(PO4)3), sodium iron phosphate (NaFePO4), sodium iron pyrophosphate, sodium iron phosphate pyrophosphate, sodium manganese oxide, sodium cobalt oxide, sodium iron oxide, fluorinated sodium vanadium phosphate (Na3V2(PO4)3F), sodium iron sulfate (Na2Fe(SO4)2), sodium nickel manganese oxide, sodium iron cobalt oxide or sodium nickel cobalt manganese oxide.

[0028] Preferably, the negative electrode active material in the negative electrode active material layer comprises any one or a combination of at least two of carbon material, metal or metal compound, typically but not limitedly, the combination includes a combination of carbon material and metal, a combination of metal and metal compound, a combination of carbon material and metal compound, or a combination of carbon material, metal and metal compound.

[0029] Preferably, the carbon material comprises any one or a combination of at least two of hard carbon, soft carbon, amorphous carbon, carbon nanotube, carbon fiber, graphene, graphite, mesocarbon microbead, carbon molecular sieve, or activated carbon, typically but not limitedly including a combination of hard carbon and soft carbon, a combination of amorphous carbon and carbon nanotube, a combination of carbon fiber and graphene, a combination of graphite, mesocarbon microbead, and carbon molecular sieve, a combination of graphite, carbon molecular sieve, and activated carbon, or a combination of hard carbon, soft carbon, amorphous carbon, carbon nanotube, carbon fiber, graphene, graphite, mesocarbon microbead, carbon molecular sieve, and activated carbon.

[0030] Preferably, the metal comprises any one or a combination of at least two of Sn, Ge, Bi, Si, Zn, Ca, Mg, Ti, Fe, Sb, or Pb, typically but not limitedly including a combination of Sn and Ge, a combination of Bi and Si, a combination of Zn and Ca, a combination of Mg and Ti, a combination of Fe, Sb, and Pb, a combination of Sn, Bi, Zn, and Ca, a combination of Ca, Mg, Ti, and Fe, or a combination of Sn, Ge, Bi, Si, Zn, Ca, Mg, Ti, Fe, Sb, and Pb.

[0031] Preferably, the metal compound comprises any one or a combination of at least two of a halide, a sulfide, an oxide, a nitride, a selenide, a phosphide, a boride, or a hydride of the metal, typically but not limitedly including a combination of a halide and a sulfide, a combination of an oxide and a nitride, a combination of a selenide and a phosphide, a combination of a boride and a cyanide, or a combination of a halide, a sulfide, an oxide, a nitride, a selenide, a phosphide, a boride, and a hydride.

[0032] Preferably, the negative active material comprises any one or a combination of at least two of activated carbon, hard carbon, carbon nanotube, Sn, a halide of Sn, a phosphide of Sn, Zn, a halide of Zn, a phosphide of Zn, Bi, a halide of Bi, or a phosphide of Bi.

[0033] Preferably, the positive current collector used by the positive electrode of the present application comprises any one or a combination of at least two of a metal foil, a foam current collector, a metal mesh current collector, a carbon felt current collector, a carbon cloth current collector, or a carbon paper current collector; further preferably, the metal foil used by the positive current collector is an aluminum foil.

[0034] Preferably, the negative current collector used by the negative electrode of the present application comprises any one or a combination of at least two of a metal foil, a foam current collector, a metal mesh current collector, a carbon felt current collector, a carbon cloth current collector, or a carbon paper current collector; further preferably, the metal foil used by the negative current collector is a copper foil.

[0035] The secondary battery of the present application further comprises a separator and an electrolyte.

[0036] Preferably, the material of the diaphragm comprises any one or a combination of at least two of polyethylene, polypropylene or polyimide, and a typical but non-limiting combination includes a combination of polyethylene and polypropylene, a combination of polypropylene and polyimide, a combination of polyethylene and polyimide, or a combination of polyethylene, polypropylene and polyimide.

[0037] Preferably, the solute in the electrolyte solution comprises sodium hexafluorophosphate.

[0038] Preferably, the electrolyte solution further comprises an additive fluoroethylene carbonate (FEC).

[0039] Preferably, the solvent of the electrolyte solution comprises ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

[0040] In a second aspect, the present application provides a preparation method of the secondary battery as described in the first aspect, and the preparation method comprises the following steps:

[0041] (1) coating a positive electrode slurry on a positive electrode current collector, drying and then performing roll forming to obtain a positive electrode;

[0042] The obtained positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector;

[0043] (2) coating a negative electrode slurry on a negative electrode current collector, and drying to obtain a negative electrode;

[0044] The obtained negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector;

[0045] (3) assembling the positive electrode obtained in step (1) and the negative electrode obtained in step (2) to obtain the secondary battery;

[0046] Step (1) and step (2) are not in a specific order.

[0047] The preparation method provided by the present application does not need to perform roll forming operation when preparing the negative electrode, which guarantees the porous characteristics of the negative electrode. On the one hand, it is beneficial to the penetration and infiltration of the electrodeposition liquid in the negative electrode, promotes ion transmission and improves the rate performance of the battery; on the other hand, it provides more sodium metal storage space, promotes the uniform deposition of sodium metal and improves the capacity and cycle life of the secondary battery.

[0048] The positive electrode slurry in step (1) of the present application comprises positive electrode active material, conductive agent, binder and solvent.

[0049] Illustratively, the conductive agent in the positive electrode slurry comprises Super P; the binder in the positive electrode slurry comprises polyvinylidene fluoride (PVDF); and the solvent in the positive electrode slurry comprises N-methyl pyrrolidone (NMP).

[0050] Exemplarily, the mass ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode slurry is 97.5:1:1.5;

[0051] Exemplarily, the solid content of the positive electrode slurry is 70-80wt%, for example, can be 70wt%, 72wt%, 75wt%, 78wt% or 80wt%, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0052] The negative electrode slurry in step (2) of the application comprises a negative electrode active material, a binder and a solvent.

[0053] Exemplarily, the binder in the negative electrode slurry comprises sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR); and the solvent in the negative electrode slurry comprises water;

[0054] Exemplarily, the mass ratio of the negative electrode active material, CMC and SBR in the negative electrode slurry is 95:1.5:3.5;

[0055] Exemplarily, the solid content of the negative electrode slurry is 40-50wt%, for example, can be 40wt%, 42wt%, 45wt%, 48wt% or 50wt%, but is not limited to the listed values, and other values not listed in the value range are also applicable.

[0056] Exemplarily, the assembling in step (3) of the application comprises: stacking the positive electrode, the separator and the negative electrode in order, with the separator between the positive electrode and the negative electrode to play a separating role, and then winding to obtain an electrode assembly; placing the electrode assembly into a packaging shell formed by an aluminum-plastic composite film (provided by DNP Company), then removing the moisture, injecting the electrolyte, and going through processes such as vacuum packaging, standing, formation, shaping and the like to obtain a secondary battery.

[0057] In a third aspect, the application provides an electrochemical device, which comprises the secondary battery of the first aspect.

[0058] Compared with the prior art, the application has the following beneficial effects:

[0059] (1) The secondary battery of the application can overcome the strict requirements for the positive and negative electrode capacities in the field of conventional batteries, and can also avoid the complex positive and negative electrode capacity matching process, thereby providing high design flexibility and adaptability while improving the energy density, safety and cycle life of the battery, simplifying the battery and reducing the production cost;

[0060] (2) The preparation method provided by the application does not need to perform a rolling operation when preparing the negative electrode, thereby ensuring the porous characteristics of the negative electrode, which is beneficial to the penetration and infiltration of the electrodeposition liquid in the negative electrode, promotes ion transmission, improves the rate performance of the battery, and provides more sodium metal storage space, promotes the uniform deposition of sodium metal, and improves the capacity and cycle life of the secondary battery. DETAILED DESCRIPTION

[0061] The technical solutions of the application are further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the application and should not be regarded as specific limitations on the application.

[0062] In order to clearly illustrate the technical solutions of the application, the separator in the specific embodiment of the application is a polyethylene film with a thickness of 15 μm provided by Celgard Company; the solvent of the electrolyte is ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether in a volume ratio of 1:1:2, the solute of the electrolyte is sodium hexafluorophosphate, the additive is FEC, the concentration of sodium hexafluorophosphate in the electrolyte is 1 mol / L, and the concentration of FEC is 1 vol%.

[0063] In the specific embodiment of the application, the conductive agent in the positive electrode slurry is Super P, the binder is polyvinylidene fluoride (PVDF), and the solvent is N-methyl pyrrolidone (NMP); and in the positive electrode slurry, the mass ratio of the positive electrode active material, the conductive agent and the binder is 98:1:1.

[0064] In the specific embodiment of the application, the negative electrode slurry comprises a negative electrode active material, a binder and a solvent.

[0065] In the specific embodiment of the application, the binder in the negative electrode slurry is sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR), and the solvent is water; the mass ratio of the negative electrode active material, CMC and SBR is 90:3:7.

[0066] In the specific embodiment of the application, the assembling comprises: stacking the positive electrode, the separator and the negative electrode in order, so that the separator is between the positive electrode and the negative electrode to play a separating role, and then winding to obtain an electrode assembly; the electrode assembly is loaded into a packaging shell formed by an aluminum-plastic composite film (provided by DNP Company) with a thickness of 113 μm, then water is removed at 80℃, electrolyte is injected, and the secondary battery is obtained through processes such as vacuum packaging, standing, formation and shaping.

[0067] Example 1

[0068] The embodiment provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte.

[0069] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 14 mg / cm 2 ;

[0070] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 0.15 mg / cm 2 ;

[0071] The thickness of the single-sided positive electrode active material layer in the positive electrode is 120 μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 20 μm;

[0072] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided in the embodiment comprises the following steps:

[0073] (1) A positive electrode slurry with a solid content of 75 wt% is coated on one side surface of a positive electrode current collector, and dried at 100 ℃ to obtain a positive electrode with single-side positive electrode slurry coating; then the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode with double-side positive electrode slurry coating; after the coating is completed, roll forming and cutting into a specification of 56 mm×43 mm are performed to obtain the positive electrode;

[0074] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0075] (2) A negative electrode slurry with a solid content of 45 wt% is coated on one side surface of a negative electrode current collector, and dried at 80 ℃ to obtain a negative electrode with single-side negative electrode slurry coating; then the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode with double-side negative electrode slurry coating; after the coating is completed, cutting into a specification of 58 mm×45 mm is performed to obtain the negative electrode;

[0076] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0077] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled to obtain the secondary battery.

[0078] Embodiment 2

[0079] The embodiment provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0080] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 14 mg / cm 2 ;

[0081] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 0.21 mg / cm 2 ;

[0082] The thickness of the single-sided positive electrode active material layer in the positive electrode is 120 μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 18 μm;

[0083] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided in the embodiment comprises the following steps:

[0084] (1) A positive electrode slurry with a solid content of 75 wt% is coated on one side surface of a positive electrode current collector, and dried at 100 ℃ to obtain a positive electrode with single-side positive electrode slurry coating; then, the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode with double-side positive electrode slurry coating; after the coating is completed, roll forming and cutting into a specification of 56 mm×43 mm are performed to obtain the positive electrode;

[0085] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0086] (2) A negative electrode slurry with a solid content of 45 wt% is coated on one side surface of a negative electrode current collector, and dried at 80 ℃ to obtain a negative electrode with single-side negative electrode slurry coating; then, the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode with double-side negative electrode slurry coating; after the coating is completed, cutting into a specification of 58 mm×45 mm is performed to obtain the negative electrode;

[0087] The negative electrode active material in the negative electrode slurry is coconut shell active carbon and magnesium chloride with a mass ratio of 90:5;

[0088] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled to obtain the secondary battery.

[0089] Embodiment 3

[0090] The embodiment provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0091] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 28 mg / cm 2 ;

[0092] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 0.15 mg / cm 2 ;

[0093] The thickness of the single-sided positive electrode active material layer in the positive electrode is 240 μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 20 μm;

[0094] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided in this embodiment comprises the following steps:

[0095] (1) The positive electrode slurry with a solid content of 75 wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode with single-side coated positive electrode slurry; then the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode with double-side coated positive electrode slurry; after the coating is completed, roll forming and cutting into a specification of 56 mm x 43 mm are performed for standby, to obtain the positive electrode;

[0096] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0097] (2) The negative electrode slurry with a solid content of 45 wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode with single-side coated negative electrode slurry; then the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode with double-side coated negative electrode slurry; after the coating is completed, cutting into a specification of 58 mm x 45 mm is performed for standby, to obtain the negative electrode;

[0098] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0099] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled, to obtain the secondary battery.

[0100] Embodiment 4

[0101] The secondary battery with asymmetric positive and negative electrode capacities provided in this embodiment comprises a positive electrode, a negative electrode, a separator, and an electrolyte;

[0102] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 7 mg / cm 2 ;

[0103] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 0.15 mg / cm 2 ;

[0104] The thickness of the single-sided positive electrode active material layer in the positive electrode is 60 μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 20 μm;

[0105] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided in this embodiment comprises the following steps:

[0106] (1) The positive electrode slurry with a solid content of 75 wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode with single-side coated positive electrode slurry; then the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode with double-side coated positive electrode slurry; after the coating is completed, roll forming and cutting into a specification of 56 mm x 43 mm are performed for standby, to obtain a positive electrode;

[0107] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0108] (2) The negative electrode slurry with a solid content of 45 wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode with single-side coated negative electrode slurry; then the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode with double-side coated negative electrode slurry; after the coating is completed, cutting into a specification of 58 mm x 45 mm is performed for standby, to obtain a negative electrode;

[0109] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0110] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled, to obtain the secondary battery.

[0111] Example 5

[0112] The present embodiment provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0113] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 14 mg / cm 2 ;

[0114] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 0.02 mg / cm 2 ;

[0115] The thickness of the single-side positive electrode active material layer in the positive electrode is 120 μm, and the thickness of the single-side negative electrode active material layer in the negative electrode is 5 μm;

[0116] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided by the present embodiment comprises the following steps:

[0117] (1) The positive electrode slurry with a solid content of 75wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode coated with the positive electrode slurry on one side; then, the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode coated with the positive electrode slurry on both sides; after the coating is completed, roll forming and cutting into a specification of 56mmx43mm are performed to obtain the positive electrode;

[0118] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0119] (2) The negative electrode slurry with a solid content of 15wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode coated with the negative electrode slurry on one side; then, the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode coated with the negative electrode slurry on both sides; after the coating is completed, cutting into a specification of 58mmx45mm is performed to obtain the negative electrode;

[0120] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0121] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled to obtain the secondary battery.

[0122] Example 6

[0123] The present embodiment provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0124] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12μm; the mass of the positive electrode active material in the positive electrode is 14mg / cm 2 ;

[0125] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12μm; the mass of the negative electrode active material in the negative electrode is 0.2mg / cm 2 ;

[0126] The thickness of the single-sided positive electrode active material layer in the positive electrode is 120μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 25μm;

[0127] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided by the present embodiment comprises the following steps:

[0128] (1) The positive electrode slurry with a solid content of 75 wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode coated with the positive electrode slurry on one side; then, the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode coated with the positive electrode slurry on both sides; after the coating is completed, roll forming and cutting into a specification of 56 mm x 43 mm are performed, to obtain the positive electrode;

[0129] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0130] (2) The negative electrode slurry with a solid content of 25 wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode coated with the negative electrode slurry on one side; then, the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode coated with the negative electrode slurry on both sides; after the coating is completed, cutting into a specification of 58 mm x 45 mm is performed, to obtain the negative electrode;

[0131] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0132] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled, to obtain the secondary battery.

[0133] Example 7

[0134] The present embodiment provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0135] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 14 mg / cm 2 ;

[0136] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 4.0 mg / cm 2 ;

[0137] The thickness of the single-sided positive electrode active material layer in the positive electrode is 120 μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 45 μm;

[0138] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided in the present embodiment comprises the following steps:

[0139] (1) The positive electrode slurry with a solid content of 75 wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode coated with the positive electrode slurry on one side; then, the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode coated with the positive electrode slurry on both sides; after the coating is completed, roll forming and cutting into a specification of 56 mm x 43 mm are performed to obtain the positive electrode;

[0140] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0141] (2) The negative electrode slurry with a solid content of 70 wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode coated with the negative electrode slurry on one side; then, the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode coated with the negative electrode slurry on both sides; after the coating is completed, cutting into a specification of 58 mm x 45 mm is performed to obtain the negative electrode;

[0142] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0143] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled to obtain the secondary battery.

[0144] Comparative Example 1

[0145] The present comparative example provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0146] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 14 mg / cm 2 ;

[0147] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 21 mg / cm 2 ;

[0148] The thickness of the single-sided positive electrode active material layer in the positive electrode is 120 μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 300 μm;

[0149] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided by the present comparative example comprises the following steps:

[0150] (1) The positive electrode slurry with a solid content of 75 wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode coated with the positive electrode slurry on one side; then, the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode coated with the positive electrode slurry on both sides; after the coating is completed, roll forming and cutting into a specification of 56 mm x 43 mm are performed to obtain the positive electrode;

[0151] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0152] (2) The negative electrode slurry with a solid content of 70 wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode coated with the negative electrode slurry on one side; then, the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode coated with the negative electrode slurry on both sides; after the coating is completed, cutting into a specification of 58 mm x 45 mm is performed to obtain the negative electrode;

[0153] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0154] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled to obtain the secondary battery.

[0155] Comparative Example 2

[0156] The present comparative example provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0157] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 1 mg / cm 2 ;

[0158] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 18 mg / cm 2 ;

[0159] The thickness of the single-sided positive electrode active material layer in the positive electrode is 10 μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 200 μm;

[0160] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided by the present comparative example comprises the following steps:

[0161] (1) The positive electrode slurry with a solid content of 40wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode coated with the positive electrode slurry on one side; then, the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode coated with the positive electrode slurry on both sides; after the coating is completed, roll forming and cutting into a specification of 56mmx43mm are performed to obtain the positive electrode;

[0162] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0163] (2) The negative electrode slurry with a solid content of 70wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode coated with the negative electrode slurry on one side; then, the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode coated with the negative electrode slurry on both sides; after the coating is completed, cutting into a specification of 58mmx45mm is performed to obtain the negative electrode;

[0164] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0165] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled to obtain the secondary battery.

[0166] Comparative Example 3

[0167] The present embodiment provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0168] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12μm; the mass of the positive electrode active material in the positive electrode is 14mg / cm 2 ;

[0169] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12μm; the mass of the negative electrode active material in the negative electrode is 5.4mg / cm 2 ;

[0170] The thickness of the single-sided positive electrode active material layer in the positive electrode is 120μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 70μm;

[0171] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided by the present embodiment comprises the following steps:

[0172] (1) The positive electrode slurry with a solid content of 75 wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode coated with the positive electrode slurry on one side; then, the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode coated with the positive electrode slurry on both sides; after the coating is completed, roll forming and cutting into a specification of 56 mm x 43 mm are performed, to obtain the positive electrode;

[0173] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0174] (2) The negative electrode slurry with a solid content of 70 wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode coated with the negative electrode slurry on one side; then, the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode coated with the negative electrode slurry on both sides; after the coating is completed, cutting into a specification of 58 mm x 45 mm is performed, to obtain the negative electrode;

[0175] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0176] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled, to obtain the secondary battery.

[0177] Comparative Example 4

[0178] The present embodiment provides a secondary battery with asymmetric positive and negative electrode capacities, which comprises a positive electrode, a negative electrode, a separator and an electrolyte;

[0179] The positive electrode comprises a positive electrode current collector and positive electrode active material layers arranged on both sides of the positive electrode current collector; the positive electrode current collector is an aluminum foil with a thickness of 12 μm; the mass of the positive electrode active material in the positive electrode is 14 mg / cm 2 ;

[0180] The negative electrode comprises a negative electrode current collector and negative electrode active material layers arranged on both sides of the negative electrode current collector; the negative electrode current collector is a copper foil with a thickness of 12 μm; the mass of the negative electrode active material in the negative electrode is 9 mg / cm 2 ;

[0181] The thickness of the single-sided positive electrode active material layer in the positive electrode is 120 μm, and the thickness of the single-sided negative electrode active material layer in the negative electrode is 100 μm;

[0182] The preparation method of the secondary battery with asymmetric positive and negative electrode capacities provided in the present embodiment comprises the following steps:

[0183] (1) The positive electrode slurry with a solid content of 75 wt% is coated on one side surface of the positive electrode current collector, dried at 100°C, to obtain a positive electrode coated with the positive electrode slurry on one side; then the above operation is repeated on the other surface of the positive electrode current collector to obtain a positive electrode coated with the positive electrode slurry on both sides; after the coating is completed, roll forming and cutting into a specification of 56 mm x 43 mm are performed to obtain the positive electrode;

[0184] The positive electrode active material in the positive electrode slurry is sodium vanadium phosphate;

[0185] (2) The negative electrode slurry with a solid content of 70 wt% is coated on one side surface of the negative electrode current collector, dried at 80°C, to obtain a negative electrode coated with the negative electrode slurry on one side; then the above operation is repeated on the other surface of the negative electrode current collector to obtain a negative electrode coated with the negative electrode slurry on both sides; after the coating is completed, cutting into a specification of 58 mm x 45 mm is performed to obtain the negative electrode;

[0186] The negative electrode active material in the negative electrode slurry is coconut shell activated carbon;

[0187] (3) The positive electrode obtained in step (1) and the negative electrode obtained in step (2) are assembled to obtain the secondary battery.

[0188] The positive electrode capacity, negative electrode capacity, initial efficiency and cycle life of the secondary battery with asymmetrically arranged positive and negative electrode capacities provided in Examples 1-7 and Comparative Examples 1-4 are determined, wherein the determination method of the positive electrode capacity is that the positive electrode tab and sodium metal are assembled into a button-type half battery, at room temperature (25°C), charged at a current of 0.2C to 3.6V, then left for 5 minutes, discharged at a current of 0.2C to 2.0V, to obtain the discharge capacity of the positive electrode; the determination method of the negative electrode capacity is that the negative electrode tab and sodium metal are assembled into a button-type half battery, at room temperature (25°C), discharged at a current of 0.2C to 0.005V, then left for 5 minutes, charged at a current of 0.2C to 1V, to obtain the capacity of the negative electrode; the determination method of the initial efficiency is that the full battery assembled with the positive electrode and the negative electrode is charged at a current of 0.2C to 3.6V at room temperature (25°C), to obtain the charge capacity of the battery, then left for 5 minutes, discharged at a current of 0.2C to 1.5V, to obtain the discharge capacity of the battery, the initial discharge capacity is divided by the initial charge capacity to obtain the initial efficiency of the battery; the determination method of the cycle life is that the full battery is tested for pre-cycling at a current of 0.2C in the voltage range of 1.5-3.6V for 3 times at room temperature (25°C), then tested for cycling at a current of 0.5C in the voltage range of 1.5-3.6V, when the battery capacity is lower than 80% of the initial discharge capacity (0.5C), the cycle life of the battery is considered to be terminated; the results are shown in Table 1.

[0189] Table 1

[0190]

[0191] Examples 5-7 and Comparative Examples 3-4 are the same as Example 1, except that the thickness and areal density of the negative electrode are changed to vary the negative electrode capacity as shown in Table 2.

[0192] Table 2

[0193]

[0194]

[0195] In summary, the secondary battery described in the present application can overcome the strict requirements for positive and negative electrode capacity in the field of conventional batteries, and can also avoid the complex positive and negative electrode capacity matching process, while improving the energy density, safety and cycle life of the battery, providing high design flexibility and adaptability, simplifying the battery and reducing production costs; The preparation method provided in the present application does not need to perform a rolling operation when preparing the negative electrode, which guarantees the porous characteristics of the negative electrode, on the one hand, it is beneficial to the penetration and infiltration of the electrolyte in the negative electrode, promotes ion transmission, and improves the rate performance of the battery; on the other hand, it provides more sodium metal storage space, promotes the uniform deposition of sodium metal, and improves the capacity and cycle life of the secondary battery.

[0196] The above merely provides a specific implementation of the present application, but the protection scope of the present application is not limited thereto, and those skilled in the art should understand that any changes or replacements within the technical scope disclosed in the present application, which can be easily thought of by any person skilled in the art, falls within the protection scope and disclosure scope of the present application.

Claims

1. A secondary battery with asymmetric positive and negative electrode capacities, characterized in that: The secondary battery includes a positive electrode and a negative electrode; The negative electrode capacity of the secondary battery is recorded as N, and the positive electrode capacity is recorded as P, then N / P≤0.

2.

2. The secondary battery according to claim 1, characterized by The 0.001≤N / P≤0.

1.

3. The secondary battery according to claim 1 or 2, characterized by The mass ratio of the positive electrode active material in the positive electrode to the negative electrode active material in the negative electrode is (5-200):1, preferably (50-150):

1.

4. The secondary battery according to any one of claims 1 to 3, characterized by The ratio of the thickness of the positive electrode active material layer in the positive electrode to the thickness of the negative electrode active material layer in the negative electrode is (2.5-20):1, preferably (5-10):

1.

5. The secondary battery according to claim 4, characterized by The thickness of the positive electrode active material layer is 50-500 μm, preferably 60-250 μm; Preferably, the face density of the positive electrode is 5-50 mg / cm 2 , preferably 7-25 mg / cm 2 .

6. The secondary battery according to claim 4 or 5, characterized by The positive electrode active material in the positive electrode active material layer includes any one of sodium vanadium phosphate, sodium iron phosphate, sodium iron pyrophosphate, sodium iron pyrophosphate, sodium manganese oxide, sodium cobalt oxide, sodium iron oxide, sodium vanadium fluoride phosphate, sodium iron sulfate, sodium nickel manganese oxide, sodium iron cobalt oxide or sodium nickel cobalt manganese oxide, or a combination of at least two thereof.

7. The secondary battery according to any one of claims 4 to 6, characterized by The negative electrode active material in the negative electrode active material layer includes any one of carbon materials, metals or metal compounds, or a combination of at least two thereof; Preferably, the carbon material comprises any one or a combination of at least two of hard carbon, soft carbon, amorphous carbon, carbon nanotubes, carbon fibers, graphene, graphite, mesocarbon microbeads, carbon molecular sieves or activated carbon; Preferably, the metal includes any one or a combination of at least two of Sn, Ge, Bi, Si, Zn, Ca, Mg, Ti, Fe, Sb or Pb; Preferably, the metal compound includes any one of metal halides, sulfides, oxides, nitrides, selenides, phosphides, borides or hydrides, or a combination of at least two of the metals.

8. The secondary battery according to claim 7, characterized by The negative electrode active material includes any one or a combination of at least two of activated carbon, hard carbon, carbon nanotubes, Sn, Sn halide, Sn phosphide, Zn, Zn halide, Zn phosphide, Bi, Bi halide or Bi phosphide.

9. A method of producing the secondary battery according to any one of claims 1 to 8, characterized by, The preparation method comprises the following steps: (1) The positive electrode slurry is applied to the positive electrode current collector, and after drying, the positive electrode is roll-formed to obtain the positive electrode; The obtained positive electrode includes a positive electrode current collector and a positive electrode active material layer arranged on both sides of the positive electrode current collector; (2) coating the negative electrode slurry on the negative electrode current collector and drying to obtain the negative electrode; The obtained negative electrode includes a negative electrode current collector and a negative electrode active material layer arranged on both sides of the negative electrode current collector; (3) assembling the positive electrode obtained in step (1) and the negative electrode obtained in step (2) to obtain the secondary battery; Step (1) and step (2) are performed in no particular order.

10. An electrochemical device, characterized by, The electrochemical device comprises the secondary battery according to any one of claims 1 to 8.

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