Secondary battery and electric device

Abstract

The embodiment of the application discloses a secondary battery and an electric device. The secondary battery defines the roundness alpha of the positive active material particles by limiting the relationship between the radius r1 of the maximum inscribed circle of the positive active material particles and the radius r2 of the circumscribed circle concentric with the maximum inscribed circle, and limits the range value of the roundness alpha, so that the positive active material particles have a specific morphology, effectively reduces the gap between the positive active material particles, improves the compaction ability of the positive active material particle powder, the prepared positive electrode plate has high compaction density, can load more positive active material per unit area, improves the energy density of the secondary battery, the crushing of the positive material is obviously improved in the high pressure state, the exposure of the new interface and the generation of the small particles are reduced, thereby the generation of the side reaction is reduced, the safety problems such as gas production are improved, the structure of the positive material is stabilized, and the cycle stability of the secondary battery is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a secondary battery and an electric equipment. BACKGROUND

[0002] Secondary batteries, such as lithium ion batteries, have the advantages of high energy density, good cycle performance, and high charging efficiency, and have been widely used in digital products, electric tools, electric vehicles, and other fields. A secondary battery includes a positive electrode sheet, and the positive electrode sheet includes a positive electrode material. Among a plurality of positive electrode materials constituting the positive electrode sheet, a ternary positive electrode material has been widely concerned due to its high energy density. However, the safety performance has become a major bottleneck restricting the development of the ternary positive electrode material. During the production process and the sheet rolling process, the ternary positive electrode material may be broken into fine powder particles. During the cycle process of the secondary battery, the surface of the fine powder particles and the particles broken and cracked may have a serious side reaction with the electrolyte, causing serious safety problems such as gas production of the secondary battery. SUMMARY

[0003] Embodiments of the present application provide a secondary battery and an electric equipment, which can improve the safety problems of the existing secondary battery caused by the particle breakage of the positive electrode material.

[0004] A first aspect of the present application provides a secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material, the positive electrode active material particles being cut along a direction perpendicular to the surface of the positive electrode active material layer, the cut positive electrode active material particles having a cross section; defining the roundness of the positive electrode active material particles as a, the radius of the maximum inscribed circle in the cross section as r1, and the radius of the circumscribed circle concentric with the maximum inscribed circle as r2; then a = (r2-r1) / r1, in the positive electrode sheet, the proportion of the positive electrode active material particles with 0≤a<0.5 is β1, the proportion of the positive electrode active material particles with 0.5≤a<0.8 is β2, the proportion of the positive electrode active material particles with 0.8≤a<1.2 is β3, and the proportion of the positive electrode active material particles with 1.2≤a≤5 is β4, the β1, β2, β3, and β4 satisfy: 0.1≤2(β1-β4) / (β2+β3)≤1.9.

[0005] Optionally, the β1, β2, β3, and β4 satisfy: 0.2≤2(β1-β4) / (β2+β3)≤1.7.

[0006] Optionally, the β1, β2, β3, and β4 satisfy: 0.4≤2(β1-β4) / (β2+β3)≤1.0.

[0007] Optionally, the β1 satisfies: 20%≤β1≤50%; and / or, the β2 satisfies: 20%≤β2≤40%; and / or, the β3 satisfies: 10%≤β3≤30%; and / or, the β4 satisfies: 2%≤β4≤25%.

[0008] Optionally, the powder compaction density P1 of the positive electrode active material particles under a pressure of 30KN satisfies: 3.0g / cm 3 ≤P1≤3.3g / cm 3 .

[0009] Optionally, the compaction density P2 of the positive electrode plate satisfies: 3.3g / cm 3 ≤P2≤3.5g / cm 3 .

[0010] Optionally, the porosity P3 of the positive electrode plate is 20%~30%.

[0011] Optionally, the general formula of the positive electrode active material includes Li x (Ni a Co b Mn c ) 1-y A y O2, wherein 0.95≤x≤1.2, a+b+c=1, 0≤y≤0.2, A includes at least one of Zr, Sr, W, Al, Ti, Mg, Ce, Y, B elements.

[0012] Optionally, at least a part of the positive electrode active material is a single crystal particle.

[0013] The second aspect of the application further provides a power consuming device comprising the secondary battery as described above.

[0014] The application has the beneficial effect of providing a secondary battery and a power consuming device, in the positive electrode plate of the secondary battery, the proportion of positive electrode active material particles with different roundness is different, the roundness can reflect the morphology of the positive electrode active material particles, the gap between the particles with high roundness is larger, the particles with low roundness have more edges and corners, by limiting the proportion β1, β2, β3, β4 of different roundness in the positive electrode plate satisfies: 0.1≤2(β1-β4) / (β2+β3)≤1.9, the gap between the positive electrode active material particles in the positive electrode plate can be effectively reduced, thereby improving the compaction density of the positive electrode plate, the positive electrode active material with high compaction density can be loaded in unit area, the energy density of the secondary battery is improved, and under high pressure, the crushing of the positive electrode active material is obviously improved, the exposure of new interface and the generation of small particles are reduced, thereby reducing the side reaction, improving the safety problems such as gas production, stabilizing the structure of the positive electrode material, and improving the cycle stability of the secondary battery. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments description will be briefly introduced. Obviously, the drawings in the following description only represent some of the embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0016] Figure 1 is a scanning electron microscope image of a cross section of a positive electrode sheet in a prior art secondary battery;

[0017] Figure 2 is a scanning electron microscope image of a cross section of a positive electrode sheet in a secondary battery prepared by the present application. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present application will be described clearly and completely in the following description with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments only represent some 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 any creative effort fall within the scope of the present application. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present application, and are not used to limit the present application.

[0019] In the detailed description and in the claims, a list of items connected by the term “at least one of’ can mean any combination of the listed items. For example, if a list specifies items A and B, the phrase “at least one of A and B” can mean A alone; B alone; or A and B together. In another example, if a list specifies items A, B, and C, the phrase “at least one of A, B, and C” can mean A alone; B alone; C alone; A and B together; A and C together; B and C together; or A, B, and C together. The term “at least one of’ can have the same meaning as the term “one or more of’.

[0020] In the present specification, a numerical range indicated by “~” shows a range including a minimum value and a maximum value indicated by the numbers written before and after “~” respectively.

[0021] The embodiment of the present application provides a secondary battery and an electric device, in the positive electrode sheet of the secondary battery, the proportion of positive electrode active material particles with different roundness is different, the roundness can reflect the morphology of the positive electrode active material particles, the gap between the particles with high roundness is larger, and the particles with low roundness have more edges and corners, by limiting the proportion of different roundness β1, β2, β3 and β4 in the positive electrode sheet to meet 0.1 <= 2(β1-β4) / (β2+β3) <= 1.9, the gap between the positive electrode active material particles in the positive electrode sheet can be effectively reduced, so that the compaction density of the positive electrode sheet is improved, the positive electrode active material with high compaction density can be loaded in unit area, the energy density of the secondary battery is improved, and in the high pressure state, the crushing of the positive electrode active material is obviously improved, the exposure of the new interface and the generation of the small particles are reduced, so that the side reaction is reduced, the safety problems such as gas production are improved, the structure of the positive electrode material is stabilized, and the cycle stability of the secondary battery is improved.

[0022] In the embodiment of the present application, a secondary battery is provided, which comprises a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a shell.

[0023] I. Positive electrode tab

[0024] The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, and the positive electrode active material layer comprises a positive electrode active material.

[0025] The positive electrode sheet is a single-sided electrode sheet or a double-sided electrode sheet, when the positive electrode sheet is a single-sided electrode sheet, the positive electrode active material layer is arranged on one surface of the positive electrode current collector, and when the positive electrode sheet is a double-sided electrode sheet, the positive electrode active material layer is arranged on both surfaces of the positive electrode current collector. There can also be a single-sided positive electrode sheet region and a double-sided positive electrode sheet region on the positive electrode sheet.

[0026] Positive active material layer

[0027] The positive electrode active material layer can be one layer or multiple layers. Each layer of the multiple-layer positive electrode active material can comprise the same or different positive electrode active material. The positive electrode active material is any substance capable of reversibly intercalating and deintercalating metal ions such as lithium ions.

[0028] The positive electrode active material layer can be one layer or multiple layers. Each layer of the multiple-layer positive electrode active material can comprise the same or different positive electrode active material. The positive electrode active material is any substance capable of reversibly intercalating and deintercalating metal ions such as lithium ions.

[0029] Roundness α

[0030] The morphology of conventional ternary single-crystal positive electrode active materials is generally flat, ellipsoidal and agglomerate-like. The surface of the flat positive electrode active material particles mostly has sharp corners or concave surfaces, which leads to less space between the positive electrode active material particles. However, during the preparation of the positive electrode sheet, the positive electrode active material particles are prone to cracking, breaking and other situations after rolling, exposing new surfaces and causing side reactions with the electrolyte. The structure of the positive electrode active material particles is destroyed, resulting in poor cycle performance and gas production of the secondary battery. In addition, the ellipsoidal positive electrode active material particles form larger gaps between them, resulting in low compaction of the positive electrode sheet, low energy density of the formed secondary battery, limiting the capacity of the secondary battery, and forcibly increasing the compaction density of the positive electrode sheet also causes the positive electrode active material particles to crack, leading to poor cycle and gas production performance.

[0031] Therefore, the present application defines the concept of roundness α of the positive electrode active material particles. The measurement method of roundness α is as follows: cut the positive electrode sheet along the direction perpendicular to the surface of the positive electrode active material layer. During the cutting process, the positive electrode active material particles in the positive electrode active material layer will be cut. The cut positive electrode active material particles have a cross-section as shown in Figure 2 after being cut. As shown in Figure 2 , an arbitrary cross-section of one positive electrode active material particle is selected. As shown in Figure 2 , two cross-sections of positive electrode active material particles are selected. The inscribed circle is defined in the cross-section of the positive electrode active material particle. The inscribed circle is located inside the cross-section of the positive electrode active material particle and is tangent to the cross-section of the positive electrode active material particle. Therefore, the inscribed circle can also be called the tangent circle. Several inscribed circles can be defined in each cross-section. The radius of the largest inscribed circle is defined as r1. The largest inscribed circle is the inscribed circle with the largest radius (or the inscribed circle with the largest area). Then, the center of the largest inscribed circle is taken as the base point to define the circumscribed circle with the same center as the largest inscribed circle. The circumscribed circle encompasses the cross-section of the positive electrode active material particle, and the circumscribed circle is tangent to the edge of the cross-section of the positive electrode active material particle. Therefore, the circumscribed circle can also be called the tangent circle. The radius of the circumscribed circle is defined as r2. Then α=(r2-r1) / r1.

[0032] The smaller the α value, the higher the roundness of the positive electrode active material particles, and the more regular the surface of the positive electrode active material particles. However, it does not mean that the smaller the roundness α, the better. If the α value of all positive electrode active material particles is small, it means that the roundness of the positive electrode active material particles is high, which will cause the positive electrode sheet to have low compaction density during the preparation of the positive electrode sheet, which is not conducive to the improvement of the overall capacity and energy density of the secondary battery. Therefore, it is necessary to control it within a certain range, and match other positive electrode active material particles with different α ranges to effectively improve the powder, sheet compaction capacity and overall capacity and energy density of the secondary battery.

[0033] Specifically, in some embodiments, the roundness α of the positive electrode active material particles in the positive electrode active material layer ranges from 0 to 5, specifically, the proportion of the number of positive electrode active material particles with 0≤α<0.5 in the total number of positive electrode active material particles is β1, that is, the proportion of the number of positive electrode active material particles with 0≤α<0.5 in the total number of positive electrode active material particles in the positive electrode active material layer is β1, specifically, in the range of 0≤α<0.5, the value of the roundness α can be 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.49, or a range formed by any two of them.

[0034] The proportion of positive electrode active material particles with 0.5≤α<0.8 is β2, that is, the proportion of the number of positive electrode active material particles with 0.5≤α<0.8 in the total number of positive electrode active material particles is β2, specifically, in the range of 0.5≤α<0.8, the value of the roundness α can be 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.79, or a range formed by any two of them.

[0035] The proportion of positive electrode active material particles with 0.8≤α<1.2 is β3, that is, the proportion of the number of positive electrode active material particles with 0.8≤α<1.2 in the total number of positive electrode active material particles is β3, specifically, in the range of 0.8≤α<1.2, the value of the roundness α can be 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.19, or a range formed by any two of them.

[0036] The proportion of positive electrode active material particles with 1.2≤α≤5 is β4, that is, the proportion of the number of positive electrode active material particles with 1.2≤α≤5 in the total number of positive electrode active material particles is β4, specifically, in the range of 1.2≤α≤5, the value of the roundness α can be 1.2, 1.5, 1.7, 2, 2.2, 2.5, 2.7, 3, 3.2, 3.5, 3.7, 4, 4.2, 4.5, 4.7, 4.9, 5.0, or a range formed by any two of them.

[0037] Moreover, β1, β2, β3, β4 satisfy: 0.1≤2(β1-β4) / (β2+β3)≤1.9, specifically, the value of 2(β1-β4) / (β2+β3) can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or a range formed by any two of them.

[0038] In some embodiments, β1, β2, β3, β4 satisfy: 0.2≤2(β1-β4) / (β2+β3)≤1.7.

[0039] In some embodiments, β1, β2, β3, β4 satisfy: 0.4≤2(β1-β4) / (β2+β3)≤1.0.

[0040] In some embodiments, β1 satisfies: 20%≤β1≤50%, specifically, β1 can be 20%, 22%, 25%, 27%, 30%, 32%, 35%, 37%, 40%, 42%, 45%, 47%, 50% or a range between any two of them.

[0041] In some embodiments, β2 satisfies: 20%≤β2≤40%, specifically, β2 can be 20%, 22%, 25%, 27%, 30%, 32%, 35%, 37%, 40% or a range between any two of them.

[0042] In some embodiments, β3 satisfies: 10%≤β3≤30%, specifically, β3 can be 10%, 12%, 15%, 17%, 20%, 22%, 25%, 27%, 30% or a range between any two of them.

[0043] In some embodiments, β4 satisfies: 2%≤β4≤25%, specifically, β4 can be 2%, 5%, 7%, 10%, 12%, 15%, 17%, 20%, 22%, 25% or a range between any two of them.

[0044] Therefore, the positive electrode active material layer, due to the positive electrode active material particles with different roundness α proportions, the combination of positive electrode active material particles with different roundness α, can effectively reduce the voids between positive electrode active material particles of different sizes, so that the positive electrode active material has a high compaction density as a whole. The positive electrode sheet prepared from the positive electrode active material has a high compaction density and can load more positive electrode active material per unit area, so that the secondary battery has a high energy density. In addition, under high compaction density, the situation of positive electrode active material particle breakage during the rolling process is significantly improved, the risk of exposure of the new interface formed by particle breakage and the risk of formation of micro-particles by particle breakage are reduced, the micro-powder particles and cracked particles generated during the particle breakage process are avoided to have serious side reactions with electrolyte during the secondary battery cycle process, the structural stability of the positive electrode active material layer is improved, the side reactions of the secondary battery during high-temperature storage and cycle process are reduced, the gas production of the secondary battery is reduced, and the safety performance is improved. Therefore, the secondary battery has high energy density and high stability at the same time, and the cycle performance of the secondary battery is improved.

[0045] In some embodiments, the general formula of the positive electrode active material includes Li x (Ni a Co b Mn c ) 1-y A y O2, wherein 0.95≤x≤1.2, a+b+c=1, 0≤y≤0.2, A includes at least one of Zr, Sr, W, Al, Ti, Mg, Ce, Y, B elements.

[0046] In some embodiments, at least a portion of the positive electrode active material is a single-crystal particle morphology.

[0047] In some embodiments, the powder compaction density P1 of the positive electrode active material particles under a pressure of 30KN satisfies: 3.0g / cm 3 ≤P1≤3.3g / cm 3 , in particular, the value of P1 can be 3.0g / cm 3 , 3.02g / cm 3 , 3.04g / cm 3 , 3.06g / cm 3 , 3.08g / cm 3 , 3.1g / cm 3 , 3.12g / cm 3 , 3.14g / cm 3 , 3.18g / cm 3 , 3.20g / cm 3 , 3.22g / cm 3 , 3.24g / cm 3 , 3.26g / cm 3 , 3.28g / cm 3 , 3.30g / cm 3 or a range formed by any two of them.

[0048] In some embodiments, the test method of the powder compaction density includes the following steps: weigh 1.5g of the finished positive electrode active material and place it in the mold of the test instrument, the pressing speed is 1mm / min, the pressure points are 5KN, 10KN, 15KN, 20KN, 25KN, 30KN (continuous pressing), the pressure holding time after pressing is 60s, the pressure after pressure removal is 30N, the pressure removal speed is 4mm / min, and the holding time after pressure removal is 10s.

[0049] In some embodiments, the compaction density P2 of the positive electrode sheet satisfies: 3.3g / cm 3 ≤P2≤3.5g / cm 3 , in particular, the value of P2 can be 3.3g / cm 3 , 3.32g / cm3 3.34 g / cm3 3 3.36 g / cm3 3 3.38 g / cm3 3 3.4 g / cm3 3 3.42 g / cm3 3 3.44 g / cm3 3 3.46 g / cm3 3 3.48 g / cm3 3 3.5 g / cm3 3 or a range consisting of any two of them.

[0050] In some embodiments, the test method of the positive electrode tab compaction density comprises the following steps: measuring the thickness of the positive electrode tab after rolling with a micrometer, punching out 10 small discs with an area of 1540.25 mm2in the middle area of the positive electrode tab, weighing the total mass of the small discs, obtaining the thickness and mass of the positive current collector (aluminum foil) in the same way, obtaining the compaction density P2 of the positive electrode tab using the formula p = m / V, m is the weight of the tab excluding the mass of the positive current collector (aluminum foil), V is the volume of the positive electrode tab excluding the positive current collector (aluminum foil). 2

[0051] In some embodiments, the porosity P3 of the positive electrode tab is 20% to 30%, and specifically, P3 can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30% or a range consisting of any two of them.

[0052] When the compaction density P1 of the positive active material particles, the compaction density P2 of the positive electrode tab, and the porosity P3 of the positive electrode tab are distributed within the above ranges, the situation of positive active material particle breakage during the rolling process is significantly improved, and the risk of exposure of the new interface formed by positive active material particle breakage and the risk of formation of micro-particles by positive active material particle breakage are effectively reduced, the gas production of the secondary battery is reduced, the safety performance is improved, the positive active material layer structure stability is improved, the energy density of the secondary battery is improved, and the cycle performance of the secondary battery is enhanced.

[0053] Preparation of positive active material

[0054] The main synthesis process of the positive active material is to prepare a precursor by co-precipitation, the precursor is a polycrystalline material, the precursor is mixed with doping elements and a proper proportion of lithium salt, and then sintered, lithium ions enter the precursor, the crystal grains grow, a semi-finished product is obtained, and single crystal particles are obtained after crushing, different morphologies and roundness of the particles can be obtained by adjusting the grain growth conditions and crushing process, and after mixing and coating elements, secondary sintering is performed, and the finished positive active material particles have high powder compaction density and excellent electrochemical performance.​

[0055] During the preparation of the precursor, the primary crystal nucleation of the precursor is controlled by regulating the pH value. A lower pH value is conducive to the growth of the crystal nucleus, and a thicker primary crystal. A higher pH value is conducive to the formation of the crystal nucleus, and the primary crystal is in the form of a thin sheet, which appears small. The morphology of the primary crystal of the precursor will affect the growth process of the precursor during the first firing, resulting in different morphologies of the primary crystal. The thicker the primary crystal, the more rounded the crystal obtained by the first firing. The thinner the primary crystal, the more flat the crystal obtained by the first firing. The oxygen concentration during the first firing also affects the morphology of the primary crystal. A higher oxygen concentration is conducive to the growth of the crystal cell along the c-axis, making the crystal more rounded. A lower oxygen concentration is conducive to the growth of the crystal cell along the a and b axes, making the particle flat. Therefore, by optimizing the sintering process, an appropriate size and content ratio of the positive electrode active material product with proper roundness can be obtained.

[0056] Specifically, the preparation of the positive electrode active material includes the following steps:

[0057] Preparation of the precursor: disperse the nickel source, cobalt source and manganese source in deionized water to obtain a mixed solution; adopt a continuous and parallel flow reaction mode, pump the mixed solution, NaOH solution and ammonia complexing agent solution into a stirred reaction kettle at the same time, control the pH value of the reaction solution to be 10-13, and the temperature in the reaction kettle to be 25-90°C, and protect the reaction process with inert gas; after the reaction is completed, perform processes such as washing, filtration, vacuum drying, iron removal by sieving, etc., to obtain a nickel-cobalt-manganese hydroxide precursor.

[0058] Perform first high-temperature sintering (i.e., first firing) on the nickel-cobalt-manganese hydroxide precursor prepared by the coprecipitation method and the lithium source, the sintering temperature is between 600-1100°C, the sintering time is 8-24h, and the sintering atmosphere is a pure oxygen atmosphere or a mixed gas atmosphere of air and oxygen. By regulating the morphology of the precursor, the temperature and time of the first sintering, and the oxygen concentration, a semi-finished product is obtained, which is coated to obtain a positive electrode active material product with different roundness and ratio.

[0059] In some embodiments, the precursor includes spherical secondary particles formed by the accumulation of primary particles, the average particle size Dv50 is 4μm, the length of the primary particles is 200-800nm, the width is 100-400nm, and the aspect ratio is 2-8.

[0060] In some embodiments, the lithium source includes at least one of lithium carbonate and lithium hydroxide.

[0061] In some embodiments, the volume ratio of air to oxygen in the mixed gas atmosphere of air and oxygen is selected from any one of the following ratios: air:oxygen = 6:1, air:oxygen = 5:1, air:oxygen = 4:1, air:oxygen = 3:1, air:oxygen = 2:1, and air:oxygen = 1:1.

[0062] In addition, the positive electrode active material further includes a positive electrode conductive agent and a positive electrode binder.

[0063] Positive electrode conductive agent

[0064] The kind of the positive electrode conductive agent is not limited, and any known conductive agent can be used. Examples of the positive electrode conductive agent can include, but are not limited to, carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotube, graphene, and the like. The above-mentioned positive electrode conductive agent can be used alone or in any combination.

[0065] Positive electrode binder

[0066] The kind of the positive electrode binder used in the manufacturing of the positive electrode active material layer is not particularly limited, and in the case of a coating method, it is only required to be a material that is soluble or dispersible in a liquid medium used at the time of manufacturing the electrode. Examples of the positive electrode binder can include, but are not limited to, one or more of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, and the like; rubber-like polymers such as styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), fluorine rubber, isoprene rubber, ethylene-propylene rubber, and the like; thermoplastic elastomer-like polymers such as styrene-butadiene-styrene block copolymer or its hydrogenated product, ethylene-propylene-diene terpolymer (EPDM), styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer or its hydrogenated product, and the like; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, propylene-a-olefin copolymer, and the like; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene-ethylene copolymer, and the like; and polymer compositions having ionic conductivity of alkali metal ions (particularly, lithium ions), and the like. The above-mentioned positive electrode binder can be used alone or in any combination.

[0067] The kind of the solvent used for forming the positive electrode slurry is not limited, and it is only required to be a solvent capable of dissolving or dispersing the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder. Examples of the solvent used for forming the positive electrode slurry can include any one of an aqueous solvent and an organic solvent. Examples of the aqueous medium can include, but are not limited to, water, a mixed medium of alcohol and water, and the like. Examples of the organic medium can include, but are not limited to, diethylenetriamine, N,N-dimethylaminopropylamine, diethyl ether, propylene oxide, tetrahydrofuran (THF), N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, hexamethylphosphoramide, dimethyl sulfoxide, and the like.

[0068] Positive electrode current collector

[0069] The type of positive current collector is not particularly limited and can be any material known to be suitable for use as a positive current collector. Examples of the positive current collector can include, but are not limited to, metallic materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, and the like; carbon materials such as carbon cloth, carbon paper, and the like; and composite materials formed of a polymer and a metal layer. In some embodiments, the positive current collector is a metallic material. In some embodiments, the positive current collector is aluminum.

[0070] The form of the positive current collector is not particularly limited. The positive current collector can be a metallic material. The positive current collector can be a carbon material. In some embodiments, the positive current collector is a metal foil. In some embodiments, the metal foil is meshed. The thickness of the metal foil is not particularly limited. In some embodiments, the thickness of the metal foil is greater than 1 pm, greater than 3 pm, or greater than 5 pm. In some embodiments, the thickness of the metal foil is less than 1 mm, less than 50 pm, or less than 20 pm. In some embodiments, the thickness of the metal foil is within a range between any two of the above values.

[0071] II. Negative electrode tab

[0072] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer including a negative active material, the negative active material including graphite.

[0073] The negative electrode sheet is a single-sided electrode sheet or a double-sided electrode sheet. When the negative electrode sheet is a single-sided electrode sheet, the negative active material layer is disposed on one surface of the negative current collector. When the negative electrode sheet is a double-sided electrode sheet, the negative active material layer is disposed on both surfaces of the negative current collector. There can also be both single-sided negative electrode sheet regions and double-sided negative electrode sheet regions on the negative electrode sheet.

[0074] Negative electrode current collector

[0075] In some embodiments, the negative current collector is a metal foil. In some embodiments, the negative current collector is an aluminum foil or a copper foil. As used herein, the term “copper foil” includes copper alloy foils.

[0076] In some embodiments, the negative current collector is an electrically conductive resin. In some embodiments, the electrically conductive resin includes a film obtained by vapor depositing copper on a polypropylene film.

[0077] Negative active material layer

[0078] The negative active material layer can be one layer or multiple layers, and each layer of the multiple negative active material layers can include the same or different negative active material. The negative active material is any material capable of reversibly intercalating and deintercalating metal ions such as lithium ions. In some embodiments, the chargeable capacity of the negative active material is greater than the discharge capacity of the positive active material to prevent lithium metal from being deposited on the negative electrode sheet during charging.

[0079] In some embodiments, the negative active material layer includes a negative active material, a conductive agent, a binder, and a dispersant.

[0080] Negative active material

[0081] In some embodiments, the negative active material is selected from at least one of natural graphite, artificial graphite, hard carbon, soft carbon, mesocarbon microbeads, silicon, silicon-based alloys, silicon monoxide, silicon / carbon composites, silicon oxide / carbon composites.

[0082] Conductive agent

[0083] In some embodiments, the conductive agent includes one or more of carbon black, graphite, carbon fiber, carbon nanotube, or graphene, preferably carbon black.

[0084] Binder

[0085] The binder can improve the adhesion between the negative active materials. The kind of the binder is not particularly limited as long as it is a material stable to an electrolyte or a solvent used at the time of manufacturing an electrode. In some embodiments, the binder includes sodium carboxymethyl cellulose and styrene butadiene rubber. In some embodiments, the binder includes sodium carboxymethyl cellulose, oxidized starch, and styrene butadiene rubber.

[0086] III. Electrolyte

[0087] The electrolyte includes a lithium salt, an organic solvent, and an additive.

[0088] Lithium salt

[0089] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium organoborate, lithium perchlorate, lithium sulfonimide. The content of the lithium salt is not particularly limited as long as it does not impair the effects of the present application.

[0090] Organic solvent

[0091] In some embodiments, the organic solvent includes a cyclic carbonate and a chain carbonate.

[0092] Specifically, the organic solvent is one or more mixed solvents among EC (ethylene carbonate), DEC (diethyl carbonate), DMC (dimethyl carbonate), PC (polycarbonate), and EMC (ethyl methyl carbonate). Among them, EC (ethylene carbonate) and PC (polycarbonate) are cyclic carbonates. DEC (diethyl carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate) are chain carbonates.

[0093] Additive

[0094] In some embodiments, the additive includes at least one of vinylene carbonate, 1,3-propane sultone, lithium difluorophosphate, fluoroethylene carbonate (FEC), lithium difluoro(oxalato)borate, tripropargyl phosphate, triallyl phosphate, and triallyl isocyanurate. In some embodiments, fluoroethylene carbonate (FEC) is preferred.

[0095] IV. Separator

[0096] To prevent short circuit, a separator is usually arranged between the positive electrode tab and the negative electrode tab. In this case, the electrolyte of the present application is usually used by penetrating into the separator.

[0097] V. Application

[0098] The embodiments of the present application also provide a power consuming device including the secondary battery described above. As typical applications, the power consuming device can be used in, but is not limited to, electric toys, electric tools, electric vehicles, electric cars, energy storage devices, ships, spacecraft, etc.

[0099] The preparation method of the secondary battery provided by the present application is described as follows in combination with specific embodiments:

[0100] Embodiment 1

[0101] 1. Preparation of the precursor: the nickel-cobalt-manganese hydroxide precursor [Ni 0.6 Co 0.1 Mn 0.3 ](OH)2, wherein the pH value is 12, the precursor is a spherical secondary particle formed by accumulation of primary particles, the average particle size Dv50 is 4 μm, the length of the primary particle is 200 nm to 800 nm, the width is 100 nm to 400 nm, and the length-width ratio is 2 to 8.

[0102] 2. Preparation of the positive electrode active material: the doping element Zr, lithium carbonate, and the obtained precursor are added to a high-speed mixer and mixed for 40 min, followed by sintering, with a sintering temperature of 950°C, a sintering time of 20 h, and a sintering atmosphere of air:oxygen at a ratio of 2:1, to obtain an intermediate product. After crushing, secondary mixing is performed, and then the coating element Al is added to the intermediate product and mixed in a high-speed mixer for 20 min, followed by secondary sintering, with a sintering temperature of 500°C and a sintering time of 8 h. After sieving and demagnetization, the finished positive electrode active material is obtained. The powder compaction density P1 of the finished positive electrode active material is 3.24 g / cm3 under a pressure of 30 KN. 3 .

[0103] 3. Preparation of the positive electrode sheet: the positive electrode active material, acetylene black, and polyvinylidene fluoride are dispersed in N-methylpyrrolidone (NMP) at a mass ratio of 96:3:1, and the obtained slurry is coated on a 12 μm aluminum foil, dried in an oven at 110°C, and then rolled to obtain a positive electrode sheet with a compaction density P2 of 3.45 g / cm3. 3 .

[0104] 4. Preparation of the negative electrode sheet: graphite, conductive carbon black SP, thickening agent CMC, and binder SBR are dispersed in deionized water at a mass ratio of 96.5:0.8:0.9:1.8 to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on a negative electrode current collector copper foil, and after baking, cold pressing, and sheet cutting, a negative electrode sheet is obtained.

[0105] 5. Preparation of the electrolyte: ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1:1 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0106] 6. The above positive electrode sheet, PP separator, and negative electrode sheet are sequentially stacked with the separator between the positive and negative electrode sheets to serve as a separator, and after winding, baking, liquid injection, sealing, standing, hot and cold pressing, formation, and capacity distribution, a secondary battery is obtained.

[0107] Examples 2-8

[0108] The secondary battery is prepared according to the method of Example 1, except for the following differences.

[0109] The pH value in step 1 and the sintering atmosphere in step 2.

[0110] Comparative Example 1

[0111] The precursor of the comparative example 1 has a pH value of 13.5, and the primary grains are fine. The sintering process uses pure air atmosphere, so that the sintered product has a flat and low roundness. The preparation method of the secondary battery is the same as that of the example 1.

[0112] The cross section of the positive active material particles cut in the positive electrode tab prepared in the comparative example 1 is shown in FIG. 2. Figure 1

[0113] The statistical area of the proportion of the positive active material particles with different roundness in the positive electrode tab of the examples 1-8 and the comparative example 1 is in the 5K field of view of the scanning electron microscope.

[0114] Performance test:

[0115] The secondary batteries prepared in the examples 1-8 and the comparative example 1 were respectively tested for cycle life at 25°C with a charge-discharge rate of 1C / 1C in the range of (2.8-4.35) V. The cycle number when the cycle capacity retention rate of the secondary battery was reduced to 80% was recorded in Table 1. The secondary batteries were stored in an oven at 80°C after being fully charged, and the volume expansion rate was tested every day. The number of days when the volume expansion rate reached 50% was recorded in Table 1.

[0116] Table 1

[0117]

[0118]

[0119] As can be seen from Table 1, when β1, β2, β3, and β4 satisfy 0.1≤2(β1-β4) / (β2+β3)≤1.9, the prepared secondary battery has better cycle performance and high-temperature storage performance.

[0120] Unlike the examples, the value of 2(β1-β4) / (β2+β3) in the comparative example 1 is -0.33, which does not satisfy 0.1≤2(β1-β4) / (β2+β3)≤1.9, resulting in poor gas production of the secondary battery at high temperature and reduced cycle performance. The reason may be that the rolling process causes some positive active material particles to crack, and the cracked positive active material particles form a new surface, which reacts with the electrolyte in the secondary battery.

[0121] ​The above describes in detail the secondary battery and the electric device provided by the embodiment of the application. The principle and implementation mode of the application are described by applying specific examples. The above description of the embodiments is only used to help understand the method of the application and its core idea. Meanwhile, for those skilled in the art, according to the idea of the application, the specific implementation mode and application range can be changed. In conclusion, the content of the specification should not be understood as a limitation of the application.

Claims

1. A secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer comprising a positive active material, characterized in that, The positive electrode sheet is cut along a direction perpendicular to the surface of the positive electrode active material layer, and the cut positive electrode active material particles have a cross section; The roundness of the positive electrode active material particles is defined as α, the radius of the largest inscribed circle in the cross section is r1, and the radius of the circumscribed circle concentric with the largest inscribed circle is r2. Then, α = (r2-r1) / r1. In the positive electrode sheet, the proportion of positive active material particles with 0 ≤ α < 0.5 is β1, the proportion of positive active material particles with 0.5 ≤ α < 0.8 is β2, the proportion of positive active material particles with 0.8 ≤ α < 1.2 is β3, and the proportion of positive active material particles with 1.2 ≤ α ≤ 5 is β4. β1, β2, β3, and β4 satisfy: 0.1 ≤ 2(β1-β4) / (β2+β3) ≤ 1.9, and β1 satisfies: 20% ≤ β1 ≤ 50%.

2. The secondary battery as described in claim 1, characterized in that, The β1, β2, β3 and β4 satisfy: 0.2≤2(β1-β4) / (β2+β3)≤1.

7.

3. The secondary battery as described in claim 2, characterized in that, The β1, β2, β3 and β4 satisfy: 0.4≤2(β1-β4) / (β2+β3)≤1.

0.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The β2 satisfies: 20% ≤ β2 ≤ 40%; And / or, the β3 satisfies: 10% ≤ β3 ≤ 30%; And / or, the β4 satisfies: 2% ≤ β4 ≤ 25%.

5. The secondary battery as described in claim 1, characterized in that, The compacted density P1 of the positive electrode active material particles under a pressure of 30 kN satisfies: 3.0 g / cm³. 3 ≤P1≤3.3g / cm 3 .

6. The secondary battery as described in claim 1, characterized in that, The compaction density P2 of the positive electrode sheet satisfies: 3.3 g / cm³ 3 ≤P2≤3.5g / cm 3 .

7. The secondary battery as described in claim 1, characterized in that, The porosity P3 of the positive electrode sheet is 20% to 30%.

8. The secondary battery as described in claim 1, characterized in that, The general formula of the positive electrode active material includes Li x (Ni a Co b Mn c ) 1-y A y O2, where 0.95≤x≤1.2, a+b+c=1, 0≤y≤0.2, and A is selected from at least one of the elements Zr, Sr, W, Al, Ti, Mg, Ce, Y, and B.

9. The secondary battery as described in claim 1, characterized in that, At least a portion of the positive electrode active material has a single-crystal particle morphology.

10. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 9.

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