Positive electrode sheet, electrochemical device, and electric device

Abstract

The application provides a positive electrode sheet, an electrochemical device and a power utilization device comprising the positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer on the surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode material, the positive electrode material comprising first lithium cobalt oxide particles having a P63mc crystal structure, and in a cross section of the positive electrode active material layer perpendicular to the surface of the positive electrode current collector, the aspect ratio a / b of the cross section of the first lithium cobalt oxide particles satisfies: 3≤a / b≤9. The positive electrode sheet of the application can reduce the side reaction of electrolyte on its surface at high voltage, so that the electrochemical device has higher discharge specific capacity and cycle stability at high voltage.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive electrode sheet, an electrochemical device and a power utilization device. BACKGROUND

[0002] Currently, the crystal structure of commercial lithium cobalt oxide positive electrode material is extremely unstable at high voltage, accompanied by serious capacity decay, which leads to the inability to fully exert its theoretical capacity. Although the lithium cobalt oxide positive electrode material is modified by means such as doping (Al, Mg, Ti, etc.) and coating (Al2O3, MgO, etc.), which can alleviate the problem of rapid capacity decay at high voltage to some extent, the improvement effect is not good, and stable cycle performance at high voltage cannot be achieved. SUMMARY

[0003] In view of the above problems existing in the prior art, the present application provides a positive electrode sheet, an electrochemical device and a power utilization device comprising the same, to improve the cycle performance of the electrochemical device at high voltage.

[0004] In a first aspect, the present application provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode active material layer located on the surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode material, the positive electrode material comprising first lithium cobalt oxide particles having a P63mc crystal structure, in a cross section of the positive electrode active material layer perpendicular to the surface of the positive electrode current collector, the aspect ratio a / b of the cross section of the first lithium cobalt oxide particles satisfies: 3≤a / b≤9, wherein a is the longest diameter of the cross section of the first lithium cobalt oxide particles, and b is the length of the short diameter of the cross section of the first lithium cobalt oxide particles which is perpendicular to and bisects the longest diameter.

[0005] At high voltage, lithium cobalt oxide with a P63mc crystal structure is not prone to spinel phase transition, and has a higher discharge specific capacity, but electrochemical devices using such positive electrode materials also face rapid cycle decay problems. The inventors of the present application found through research that the cycle decay failure of electrochemical devices using lithium cobalt oxide positive electrode materials with a P63mc crystal structure is largely due to the lack of spinel phase protection on the active surface, which leads to the failure of electrolyte to occur side reactions on the active surface. By controlling the aspect ratio of the lithium cobalt oxide particles with a P63mc crystal structure to be within the range of 3 to 9, on the one hand, the lithium cobalt oxide particles have a relatively small active surface area, reducing the direct contact area with the electrolyte, thereby inhibiting the occurrence of interface side reactions with the electrolyte; on the other hand, the morphology of the lithium cobalt oxide particles with an aspect ratio within the above range tends to be lamellar, and after cold pressing of the positive electrode sheet, the grains have an orientation with the grain plane being a non-active surface, which tends to be parallel to the current collector, which is also conducive to reducing the cycle decay caused by side reactions. When the value of a / b is too high, the migration barrier of lithium ions increases, which is also not conducive to the performance of the electrochemical device.

[0006] In some embodiments, 4≤a / b≤8. In this way, the electrochemical device can have good cycle performance and kinetic performance at high voltage.

[0007] In some embodiments, the average particle size of the first lithium cobalt oxide particles is 5 μm to 20 μm.

[0008] In some embodiments, the first lithium cobalt oxide particles have a morphology in the form of a sheet layer.

[0009] In some embodiments, the first lithium cobalt oxide particles have cracks in the cross section. The lithium cobalt oxide with a P63mc crystal structure has about 10% expansion and contraction between layers during charging and discharging. The presence of the cracks can release the lattice stress during charging and discharging and reduce further rupture of the material during cycling.

[0010] In some embodiments, the first lithium cobalt oxide includes Mn elements and Ni elements.

[0011] In some embodiments, the first lithium cobalt oxide includes Mn elements and Ni elements.

[0012] In some embodiments, the first lithium cobalt oxide includes Mn elements and Ni elements.

[0013] In some embodiments, the first lithium cobalt oxide further includes M elements, the M elements including at least one of Na, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba, or Cu. The introduction of the M elements can further improve the structural stability of the material.

[0014] In some embodiments, the first lithium cobalt oxide includes Mn elements and Ni elements.

[0015] In some embodiments, the first lithium cobalt oxide includes Li x1 Co y1 Ni z1 Mn q M s O 2±n T nwherein 0.6≤x1≤1.1, 0.8≤y1<1, 0

[0016] In some embodiments, the positive electrode material further comprises second lithium cobalt oxide particles having an R-3m crystal structure, and a ratio of a long diameter c to a short diameter d of a cross section of the second lithium cobalt oxide particles is 1 to 1.2 in a cross section of the positive electrode active material layer perpendicular to a surface of the positive electrode current collector, wherein the long diameter c is a longest diameter of the cross section of the second lithium cobalt oxide particles, and the short diameter d is a length of a short diameter perpendicular to and bisecting the longest diameter of the cross section of the second lithium cobalt oxide particles. By further including the second lithium cobalt oxide particles having the R-3m crystal structure with a smaller long diameter c to short diameter d ratio c / d in the positive electrode active material layer, the capacity of the positive electrode sheet can be promoted, and meanwhile, the first lithium cobalt oxide particles with a sheet-like morphology can protect the active surface of the second lithium cobalt oxide particles, inhibit side reactions of the electrolyte on the surface thereof, and thus improve the energy density of the electrochemical device while improving the cycle performance of the electrochemical device at high voltage.

[0017] In some embodiments, a ratio S1 / S2 between a cross-sectional area S1 of the first lithium cobalt oxide particles and a cross-sectional area S2 of the second lithium cobalt oxide particles satisfies 0.05≤S1 / S2≤0.45 in a cross section of the positive electrode active material layer perpendicular to a surface of the positive electrode current collector. By controlling S1 / S2 in the above range, the electrochemical device can have both good cycle performance and higher capacity.

[0018] In some embodiments, 0.1≤S1 / S2≤0.35.

[0019] In some embodiments, the average particle size of the second lithium cobalt oxide particles is 10 μm to 25 μm.

[0020] In some embodiments, a mole percentage content of Co in the second lithium cobalt oxide is 90% to 100% based on the mole amount of metal elements other than Li in the second lithium cobalt oxide.

[0021] In some embodiments, the second lithium cobalt oxide further comprises an R element, the R element comprising at least one of Ni, Mn, Na, K, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba or Cu. In some embodiments, the molar percentage content of the R element in the second lithium cobalt oxide is 0.01% to 10% based on the molar amount of the metal elements other than the Li element in the second lithium cobalt oxide. The introduction of the R element can improve the structural stability of the second lithium cobalt oxide at high voltage, thereby improving the cycle performance of the electrochemical device at high voltage.

[0022] In some embodiments, the second lithium cobalt oxide comprises Li x2 Co y2 R z2 O 2±m T’ m , wherein 0.6≤x2≤1.1, 0.9≤y2≤1, 0≤z2≤0.1, 0≤m≤0.05, the R element comprising at least one of Ni, Mn, Na, K, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba, Pr, Hf, Lu or Cu, and the T’ is a halogen.

[0023] In some embodiments, the mass ratio of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles is 5:95 to 40:60.

[0024] In a second aspect, the present application provides an electrochemical device comprising the positive electrode sheet of the first aspect.

[0025] In a third aspect, the present application provides a power utilization device comprising the electrochemical device of the second aspect.

[0026] The present application can effectively reduce the interface side reaction between the positive electrode material and the electrolyte while improving the capacity of the electrochemical device by controlling the aspect ratio of the first lithium cobalt oxide particles having a P63mc crystal structure within a certain range, thereby improving the cycle performance of the electrochemical device at high voltage. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The XRD pattern of the positive electrode material of Example 1.

[0028] Figure 2 The SEM pattern of the positive electrode material of Example 1.

[0029] Figure 3 The cross-sectional SEM pattern of the positive electrode sheet of Example 1. DETAILED DESCRIPTION

[0030] Embodiments of the present application will be described in detail below. Embodiments of the present application should not be construed as limiting the present application.

[0031] In addition, quantities, ratios, and other numerical values in the present application are sometimes presented in a range format. It is to be understood that such range format is used for convenience and brevity and should be construed as having been followed had each distinct value contended in the range been individually listed. All numerical values in the detailed description are modified in all instances by the term "about" and / or "approximately" unless otherwise specified.

[0032] In the present application, a list of items joined by the term "at least one of" or "one or more of" can mean any combination of the listed terms. For example, if a list of items includes A, B, and C, then "at least one of A, B, and C" or "one or more 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 item A can include a single element or multiple elements. The item B can include a single element or multiple elements. The item C can include a single element or multiple elements.

[0033] I. Positive electrode tab

[0034] The positive electrode tab provided by the present application includes a positive electrode current collector and a positive electrode active material layer located on the surface of the positive electrode current collector, and the positive electrode active material layer contains a positive electrode material, and the positive electrode material includes first lithium cobalt oxide particles having a P63mc crystal structure, and in a cross section of the positive electrode active material layer perpendicular to the surface of the positive electrode current collector, the aspect ratio a / b of the cross section of the first lithium cobalt oxide particles satisfies: 3≤a / b≤9, wherein a is the longest diameter of the cross section of the first lithium cobalt oxide particles, and b is the length of the short diameter of the cross section of the first lithium cobalt oxide particles which is perpendicular to and bisects the longest diameter.

[0035] At high voltage, lithium cobalt oxide with P63mc crystal structure is not prone to spinel phase transition, and has higher discharge capacity per gram. However, the electrochemical device using such positive electrode material also faces the problem of rapid cycle decay. The inventors of the present application found through research that the cycle decay failure of the electrochemical device using lithium cobalt oxide positive electrode material with P63mc crystal structure is largely due to the lack of spinel phase protection on the active surface, which leads to side reactions of the electrolyte on the active surface and failure. By controlling the aspect ratio of the lithium cobalt oxide particles with P63mc crystal structure to be within the range of 3 to 9, on the one hand, the lithium cobalt oxide particles have a relatively small active surface area, reducing the direct contact area with the electrolyte, thereby inhibiting the occurrence of interface side reactions with the electrolyte; on the other hand, the lithium cobalt oxide particles with an aspect ratio within the above range tend to be lamellar in shape, and after cold pressing of the positive electrode sheet, they have an orientation with the grain plane being the non-active surface and tending to be parallel to the current collector, which is also conducive to reducing the cycle decay caused by side reactions.

[0036] In some embodiments, a / b is 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, or a range consisting of any two of these values. The lithium ion channel of the lithium cobalt oxide with lamellar morphology is a two-dimensional channel parallel to the transition metal layer, and the larger the value of a / b, the smaller the proportion of the active surface area for lithium ion deintercalation. When the value of a / b is too low, the grain tends to be spherical, the proportion of the active surface is large, the contact area with the electrolyte is also larger, and side reactions are more likely to occur. When the value of a / b is too high, the grain tends to be flattened, and although the proportion of the active surface is smaller, the migration barrier of lithium ions increases, which is also not conducive to the performance of the electrochemical device. In some embodiments, 3≤a / b≤8. In some embodiments, 4≤a / b≤8. In some embodiments, 4≤a / b≤6. In this way, the electrochemical device can have good cycle performance and kinetic performance at high voltage.

[0037] In the present application, the aspect ratio a / b of the cross section of the first lithium cobalt oxide particle is obtained by obtaining the cross section of the positive electrode active material layer perpendicular to the surface of the positive electrode current collector using an ion polisher (CP), and measuring the maximum length and the length of the short diameter perpendicular and bisecting the maximum length of the first lithium cobalt oxide particle in the cross section using a scanning electron microscope (SEM). The length of the short diameter perpendicular and bisecting the maximum length is the distance between the two intersection points of the median of the maximum length and the surface of the first lithium cobalt oxide particle in the cross section. For specific statistical test methods, please refer to the test methods in the specific examples below.

[0038] In some embodiments, the average particle size of the first lithium cobalt oxide particles is 5 μm to 20 μm. In some embodiments, the average particle size of the first lithium cobalt oxide particles is 5 μm, 8 μm, 10 μm, 15 μm, 20 μm, or a range between any two of these values. In the present application, the average particle size of the first lithium cobalt oxide particles is the average value of the longest diameter a of the cross section of the first lithium cobalt oxide particles.

[0039] In some embodiments, the first lithium cobalt oxide particles have a morphology of laminar shape.

[0040] In some embodiments, there are cracks in the cross section of the first lithium cobalt oxide particles. The lithium cobalt oxide with P63mc crystal structure has about 10% expansion and contraction between layers during charging and discharging, and the presence of these cracks can release the lattice stress during charging and discharging, and reduce further cracking of the material during cycling. In some embodiments, the morphology of the cracks includes at least one of both ends being closed, one end being closed, one end being open, or penetrating through the entire particle.

[0041] In some embodiments, the first lithium cobalt oxide includes Mn elements and Ni elements.

[0042] In some embodiments, the molar percentage content of Mn elements in the first lithium cobalt oxide is 0.05% to 10%, based on the molar amount of metal elements other than Li elements in the first lithium cobalt oxide. In some embodiments, the molar percentage content of Mn elements is 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range between any two of these values.

[0043] In some embodiments, the molar percentage content of Ni elements in the first lithium cobalt oxide is 0.05% to 10%, based on the molar amount of metal elements other than Li elements in the first lithium cobalt oxide. In some embodiments, the molar percentage content of Ni elements is 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range between any two of these values.

[0044] In some embodiments, the first lithium cobalt oxide further includes M elements, which include at least one of Na, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba, or Cu. The introduction of M elements can further improve the structural stability of the material.

[0045] In some embodiments, the first lithium cobalt oxide has a molar percentage of M elements of 0.01% to 10% based on the molar amount of the metal elements other than Li element in the first lithium cobalt oxide. In some embodiments, the molar percentage of M elements is 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range between any two of these values.

[0046] In some embodiments, the first lithium cobalt oxide includes Li x1 Co y1 Ni z1 Mn q M s O 2±n T n , wherein 0.6≤x1≤1.1, 0.8≤y1<1, 0

[0047] In some embodiments, the positive electrode material further includes second lithium cobalt oxide particles having R-3m crystal structure, and the aspect ratio c / d of the cross section of the second lithium cobalt oxide particles is 1 to 1.2, where c is the longest diameter of the cross section of the second lithium cobalt oxide particles, and d is the length of the short diameter of the cross section of the second lithium cobalt oxide particles that is perpendicular to and bisects the longest diameter, in a cross section of the positive electrode active material layer perpendicular to the surface of the positive electrode current collector. By further including second lithium cobalt oxide particles having R-3m crystal structure with a smaller aspect ratio c / d in the positive electrode active material layer, the capacity of the positive electrode sheet can be promoted, and meanwhile, the first lithium cobalt oxide particles with sheet-like morphology can protect the active surface of the second lithium cobalt oxide particles, inhibit the side reactions of the electrolyte on the surface thereof, and thus improve the energy density of the electrochemical device while improving the cycle performance of the electrochemical device at high voltage. In some embodiments, c / d is 1, 1.05, 1.08, 1.1, 1.13, 1.15, 1.16, 1.18, 1.2, or a range between any two of these values. In this application, the test method of the aspect ratio c / d of the cross section of the second lithium cobalt oxide particles is the same as the test method of the aspect ratio a / b of the cross section of the first lithium cobalt oxide particles.

[0048] In some embodiments, in a cross section of the positive electrode active material layer perpendicular to the surface of the positive electrode current collector, the ratio S1 / S2 between the cross-sectional area S1 of the first lithium cobalt oxide particles and the cross-sectional area S2 of the second lithium cobalt oxide particles satisfies: 0.05≤S1 / S2≤0.44. By controlling S1 / S2 within the above range, the electrochemical device can have both good cycle performance and high capacity.

[0049] In some embodiments, S1 / S2 is 0.05, 0.1, 0.13, 0.15, 0.17, 0.2, 0.23, 0.25, 0.27, 0.3, 0.33, 0.35, 0.37, 0.4, 0.45, or a range between any two of these values. In some embodiments, 0.1≤S1 / S2≤0.35.

[0050] In some embodiments, the average particle size of the second lithium cobalt oxide particles is 10 μm to 25 μm. In some embodiments, the average particle size of the second lithium cobalt oxide particles is 10 μm, 12 μm, 14 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, or a range between any two of these values. In the present application, the average particle size of the second lithium cobalt oxide particles is the average value of the longest diameter c of the cross section of the second lithium cobalt oxide particles.

[0051] In some embodiments, the molar percentage content of Co element in the second lithium cobalt oxide is 90% to 100% based on the molar amount of metal elements other than Li element in the second lithium cobalt oxide.

[0052] In some embodiments, the second lithium cobalt oxide further comprises an R element, and the R element comprises at least one of Ni, Mn, Na, K, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba, or Cu. In some embodiments, the molar percentage content of R element in the second lithium cobalt oxide is 0.01% to 10% based on the molar amount of metal elements other than Li element in the second lithium cobalt oxide. The introduction of the R element can improve the structural stability of the second lithium cobalt oxide at high voltage, thereby improving the cycle performance of the electrochemical device at high voltage. In some embodiments, the molar percentage content of R element in the second lithium cobalt oxide is 0.01%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range between any two of these values.

[0053] In some embodiments, the second lithium cobalt oxide comprises Li x2 Co y2 R z2 O 2±m T’m at least one of Ni, Mn, Na, K, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba, Pr, Hf, Lu, or Cu, T' is halogen.

[0054] In some embodiments, the mass ratio of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles is 5:95 to 40:60. In some embodiments, the mass ratio of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles is 5:95, 10:90, 15:85, 20:80, 30:70, 40:60, or a range consisting of any two of these values.

[0055] In some embodiments, the positive electrode active material layer further includes a binder and a conductive agent. In some embodiments, the binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, a vinylidene fluoride-hexafluoropropylene copolymer, an acrylate polymer, a polyacrylate, a polyamide, or a polyurethane. In some embodiments, the conductive agent includes a carbon-based material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, graphene, or carbon fibers; a metal-based material, such as metal powder or metal fibers of copper, nickel, aluminum, silver, and the like; a conductive polymer, such as polyphenylene derivatives; or a mixture thereof.

[0056] In some embodiments, the positive electrode current collector can employ a metal foil or a composite current collector. For example, an aluminum foil can be used. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, and the like) on a polymer substrate.

[0057] The present application also provides a method of preparing a first lithium cobalt oxide having a P63mc crystal structure, which includes subjecting a sodium-containing cobalt oxide and a lithium-containing compound to a solid phase reaction, wherein the solid phase reaction temperature is 180°C to 320°C, and the solid phase reaction time is 3h to 12h. In some embodiments, the solid phase reaction temperature is 180°C, 190°C, 200°C, 210°C, 250°C, 260°C, 270°C, 300°C, 320°C, or a range consisting of any two of these values. In some embodiments, the solid phase reaction time is 3h, 5h, 6h, 8h, 10h, 12h, or a range consisting of any two of these values.

[0058] According to some embodiments of the present application, the sodium-containing cobalt oxide has a P63 / mmc structure. According to some embodiments of the present application, the method for preparing the sodium-containing cobalt oxide comprises the following steps: S1: providing an oxide precursor comprising cobalt element, manganese element and nickel element, and optionally M element; S2: mixing the oxide precursor in S1 with a sodium salt and a fluxing agent, and calcining to obtain the sodium-containing cobalt oxide.

[0059] In some embodiments, in S1, the molar percentage of cobalt element in the total molar content of metal elements in the oxide is 90% to 97%.

[0060] In some embodiments, in S2, the fluxing agent comprises ammonium chloride and / or boric acid. In some embodiments, the mass percentage of the fluxing agent is 0.5% to 2%, for example 1% or 1.5%, based on the total mass of the oxide precursor in S1, the sodium salt and the fluxing agent. The a / b ratio of the first lithium cobalt oxide particles having a P63mc crystal structure is related to whether the fluxing agent is added and the amount of the fluxing agent added, and the fluxing agent is helpful for the fusion between the particles during calcination. The more the amount of the fluxing agent added, the more conducive to the fusion between the particles, and therefore, the a / b value can be adjusted by adjusting the amount of the fluxing agent added.

[0061] According to some embodiments of the present application, in S2, the temperature for calcination is 700°C to 880°C. In the present application, the a / b ratio of the first lithium cobalt oxide particles is related to the sintering conditions of the sodium-containing cobalt oxide in S2, and the higher the sintering temperature, the smaller the a / b ratio. This may be due to that increasing the sintering temperature can promote the grain growth, especially the growth in the vertical direction, so as to increase the thickness of the material grain, thereby reducing the a / b ratio. Therefore, the a / b ratio can be adjusted by adjusting the sintering temperature.

[0062] In some embodiments, in S2, the time for calcination is 24h to 72h. In some embodiments, the sodium salt comprises sodium carbonate. In some embodiments, the molar ratio of the oxide precursor in S1 to the sodium salt is 0.3:1 to 0.5:1.

[0063] According to some embodiments of the present application, the preparation of the oxide precursor in S1 can comprise the following steps: S11: mixing a cobalt salt, a manganese salt, a nickel salt and optionally a salt of M element with a solvent to obtain a first mixed solution; S12: adding a precipitant to the first mixed solution to perform a precipitation reaction under the condition that the pH is 9 to 12 to obtain a precipitate; S13: calcining the precipitate.

[0064] According to some embodiments of the present application, the preparation of the oxide precursor in S1 comprises the following steps: S11': mixing a cobalt salt, a manganese salt and a nickel salt with a solvent to obtain a first mixed solution; S12': adding a precipitant to the first mixed solution to perform a precipitation reaction under the condition that the pH is 9-12 to obtain a precipitate; S13': mixing the precipitate with an optional M salt and then performing calcination.

[0065] According to some embodiments of the present application, the cobalt salt comprises at least one of cobalt sulfate, cobalt nitrate, cobalt chloride or cobalt acetate. According to some embodiments of the present application, the manganese salt comprises at least one of manganese sulfate, manganese nitrate, manganese chloride or manganese acetate. According to some embodiments of the present application, the nickel salt comprises at least one of nickel sulfate, nickel nitrate, nickel chloride or nickel acetate.

[0066] According to some embodiments of the present application, the M element salt comprises at least one of a chloride salt, an acetate salt, a sulfate salt or a nitrate salt containing the M element. In some embodiments, the M element comprises at least one of K, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba, Pr, Hf, Lu or Cu.

[0067] According to some embodiments of the present application, in S13 and S13', the temperature of calcination is 500-750°C and the time of calcination is 8-24h.

[0068] According to some embodiments of the present application, the solvent comprises at least one of water, methanol, ethanol, acetone, isopropanol or n-hexanol.

[0069] According to some embodiments of the present application, the precipitant comprises at least one of ammonia, sodium hydroxide, potassium hydroxide, sodium bicarbonate, potassium bicarbonate, sodium carbonate or potassium carbonate.

[0070] According to some embodiments of the present application, the lithium-containing compound comprises at least one of lithium sulfate, lithium carbonate, lithium nitrate, lithium halide, lithium carboxylate, lithium squarate, lithium alcoholate or lithium hydroxide. In some embodiments, the lithium-containing compound comprises at least one of lithium sulfate, lithium carbonate, lithium nitrate, lithium chloride, lithium bromide, lithium formate, lithium acetate or lithium hydroxide.

[0071] II. Electrochemical device

[0072] The electrochemical device provided by the present application comprises the positive electrode sheet of the first aspect.

[0073] The electrochemical device provided herein further includes a negative electrode tab including a negative electrode current collector and a negative electrode active material layer on a surface of the negative electrode current collector. In some embodiments, the negative electrode active material layer includes a negative electrode active material and a binder, and optionally, a conductive agent.

[0074] In some embodiments, the negative electrode active material can include a material that reversibly intercalates / deintercalates lithium ions, lithium metal, lithium metal alloy, or transition metal oxide. In some embodiments, the negative electrode active material includes at least one of a carbon material including at least one of graphite, hard carbon, or soft carbon, or a silicon-based material including at least one of silicon, silicon-oxygen composite, silicon-carbon composite, or silicon alloy. In some embodiments, the binder includes at least one of styrene-butadiene rubber, polyacrylic acid, polyacrylate, acrylate polymer, polyamide, or styrene-acrylate copolymer. In some embodiments, the conductive agent includes at least one of conductive carbon black, acetylene black, ketjen black, carbon nanotube, or graphene.

[0075] In some embodiments, the negative electrode current collector includes a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0076] The electrochemical device of the present application further includes a separator disposed between the positive electrode tab and the negative electrode tab. The material and shape of the separator used in the electrochemical device of the present application are not particularly limited and can be any of the techniques disclosed in the prior art. In some embodiments, the separator can include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, a film, or a composite film having a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

[0077] The surface treatment layer provided on at least one surface of the substrate layer can be an adhesive layer or a heat-resistant layer. The heat-resistant layer includes inorganic particles selected from at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate, and a binder. The binder includes at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polytetrafluoroethylene, or polyhexafluoropropylene. The adhesive layer includes a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer.

[0078] The electrochemical device of the present application further comprises an electrolyte. The electrolyte used in the present application can be an electrolyte known in the prior art.

[0079] In some embodiments, the electrolyte comprises an organic solvent, a lithium salt, and optionally an additive. In some embodiments, the organic solvent comprises, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), ethyl propionate, or propyl propionate. In some embodiments, the lithium salt comprises at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt comprises, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis-trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalato)borate LiB(C2O4)2 (LiBOB), or lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive comprises at least one of fluoroethylene carbonate, ethylene vinyl carbonate, or adiponitrile.

[0080] In some embodiments, the electrochemical device of the present application comprises, but is not limited to, a primary battery or a secondary battery. In some embodiments, the electrochemical device comprises a lithium ion battery.

[0081] III. Electrical device

[0082] The present application further provides an electrical device comprising the electrochemical device as described above.

[0083] The electrical device of the present application is not particularly limited. In some embodiments, the electrical device of the present application comprises, but is not limited to, a mobile phone, a notebook computer, a tablet computer, a drone, an electric vehicle, an electric bicycle, an electric tool, a Bluetooth headset, etc.

[0084] In the following examples and comparative examples, the reagents, materials, and instruments used are commercially available unless otherwise specified.

[0085] Examples and comparative examples

[0086] Example 1

[0087] 1. Preparation of positive electrode material

[0088] Step 1: Nickel nitrate (Ni(NO3)2), cobalt nitrate (Co(NO3)2) and manganese nitrate (Mn(NO3)2) were weighed according to the molar ratio of 2:90:8, then deionized water was added for rapid stirring and dissolution, followed by the addition of ammonia water to adjust the pH to 9-12 until the reaction was complete to form a homogeneous hydroxide precipitate. The precipitate was sintered at 550°C for 12h, and then subjected to crushing and sieving processes to obtain a metal oxide precursor material.

[0089] Step 2: Sodium carbonate (Na2CO3) and ammonium chloride (NH4Cl) were mixed with the above metal oxide precursor material to obtain a mixture, wherein the molar ratio of sodium carbonate to metal oxide precursor material was 0.45:1, and the mass fraction of ammonium chloride in the mixture was 1%. The above mixture was heated at 850°C for 48h, and then subjected to post-treatment to obtain a corresponding sodium cobalt oxide material.

[0090] Step 3: The above sodium cobalt oxide material, lithium nitrate and lithium hydroxide were mixed uniformly according to the molar ratio of 1:2:3, and then loaded into a corundum crucible and stirred at 250°C for 6h to obtain a mixture containing a first lithium cobalt oxide positive electrode material.

[0091] Step 4: The above mixture material was subjected to crushing treatment, and then washed with deionized water multiple times to remove soluble sodium and lithium salts until the conductivity of the supernatant was less than 200uS / cm. Then, the residual powder was subjected to centrifugation, drying and classification to finally obtain a first lithium cobalt oxide.

[0092] 2. Preparation of positive electrode sheet

[0093] The positive electrode active material (the above first lithium cobalt oxide), conductive agent conductive carbon (SP) and binder polyvinylidene fluoride (PVDF) were mixed according to the mass ratio of 90:5:5, and then a solvent N-methyl-2-pyrrolidone (NMP) was added to prepare a slurry, which was coated on an aluminum foil with a thickness of 12μm. After drying, cold pressing, punching and weighing processes were performed to obtain a positive electrode sheet.

[0094] 3. Preparation of button cell

[0095] Under an inert atmosphere, the above positive electrode sheet, separator film and lithium sheet were sequentially placed in a button cell steel shell, the separator film was a porous polyethylene film with a thickness of 7μm, and an appropriate amount of electrolyte (ethylene carbonate (EC): methyl ethyl carbonate (EMC): diethyl carbonate (DEC) mass ratio of 1:1:1, mass concentration of LiPF6 in the electrolyte was 12.5%) was added, and then the button cell was sealed.

[0096] Examples 2-3, Comparative Examples 1 and 2

[0097] Examples 2-3 differ from Example 1 in that the molar ratio of nickel nitrate, cobalt nitrate and manganese nitrate in Step 1 of the preparation of the positive electrode material is adjusted to 5:92:3 and 3:95:2, respectively.

[0098] Comparative Example 1 differs from Example 1 in that no ammonium chloride is added in Step 2 of the preparation of the positive electrode material.

[0099] Comparative Example 2 differs from Example 1 in that the molar ratio of nickel nitrate, cobalt nitrate and manganese nitrate in Step 1 of the preparation of the positive electrode material is adjusted to 10:85:5.

[0100] Examples 4-6, Comparative Example 3

[0101] Examples 4-6 and Comparative Example 3 differ from Example 2 in that the holding temperature of the mixture in Step 2 is adjusted to 830℃, 800℃, 750℃ and 950℃, respectively.

[0102] Examples 7-10

[0103] Examples 7-10 differ from Example 3 in that lithium hydroxide (LiOH) in Step 3 is replaced by LiCl, LiBr, HCOOLi and CH3COOLi, respectively.

[0104] Examples 11-16

[0105] Examples 11-16 differ from Example 1 in that other metal sources are used to replace the manganese source in Step 1. The specific adjustment measures are shown in Table a.

[0106] Table a

[0107]

[0108] Comparative Example 4

[0109] 1. Preparation of the positive electrode active material

[0110] Step: 0.5 mol of cobalt sulfate (CoSO4) raw material is weighed, added to deionized water and stirred quickly to dissolve, then ammonium carbonate is added to adjust the pH to 8, until the reaction is complete and a homogeneous cobalt carbonate precipitate is formed. The precipitate is sintered at 650℃ for 12h, and then subjected to crushing and sieving processes to obtain a Co3O4 metal oxide material.

[0111] Step 2: Lithium carbonate (Li2CO3), aluminum oxide (Al2O3) and the above Co3O4 were weighed and mixed uniformly according to a molar ratio of Li:Co:Al of 1.05:0.97:0.03, and then heat-treated at 900°C for 12h. After crushing, sieving and other post-treatment, a corresponding second lithium cobalt oxide material was obtained. XRD test analysis showed that the crystal structure of the obtained second lithium cobalt oxide material was R-3m. Particle size distribution test analysis showed that the Dv50 of the obtained second lithium cobalt oxide material was 17.8μm.

[0112] 2. Preparation of the positive electrode sheet

[0113] The above positive electrode active material, conductive carbon (SP) and adhesive polyvinylidene fluoride (PVDF) were mixed according to a mass ratio of 96:2.5:1.5, a solvent N-methyl-2-pyrrolidone (NMP) was added to prepare a slurry, which was coated on an aluminum foil with a thickness of 12μm, and then dried. After cold pressing, punching, weighing and other processes under a pressure of about 15 tons, a positive electrode sheet was obtained.

[0114] 3. Preparation of the button cell

[0115] The same as Example 1.

[0116] Example 17

[0117] Example 17 is different from Comparative Example 4 in that the positive electrode active material is a mixture of the first lithium cobalt oxide of Example 3 and the second lithium cobalt oxide of Comparative Example 4 according to a mass ratio of 10:90.

[0118] Examples 18 to 24 and Comparative Examples 5 to 6

[0119] Examples 18 to 24 and Comparative Examples 5 to 6 are different from Example 17 in that the ratio of the first lithium cobalt oxide and the second lithium cobalt oxide and the types of the first lithium cobalt oxide and the second lithium cobalt oxide are adjusted, wherein the c / d value of the second lithium cobalt oxide particles is controlled by crushing and sieving conditions. For specific adjustment measures, see Table B.

[0120] Table B

[0121]

[0122] Test method

[0123] 1. Test of the aspect ratio and average particle size of the lithium cobalt oxide particles

[0124] Take the positive electrode sheet, use the slicer to process, obtain the cross section perpendicular to the positive electrode current collector surface, use the scanning electron microscope (SEM) to take the cross section, obtain the cross section image, in the cross section image, randomly select 50 first lithium cobalt oxide particles, respectively test the maximum length diameter of the first lithium cobalt oxide particle and the distance between the two intersection points of the median line of the maximum length diameter and the surface of the first lithium cobalt oxide particle as the short diameter, and then obtain the ratio of the maximum length diameter / short diameter, and take the average value as the length diameter ratio a / b of the first lithium cobalt oxide. The average value of the maximum length diameter of the first lithium cobalt oxide particle is the average particle size of the first lithium cobalt oxide particle.

[0125] The second lithium cobalt oxide particle length diameter ratio c / d is tested as the first lithium cobalt oxide particle length diameter ratio a / b. The average value of the maximum length diameter of the second lithium cobalt oxide particle is the average particle size of the second lithium cobalt oxide particle.

[0126] 2. Test of cross-sectional area ratio of first lithium cobalt oxide and second lithium cobalt oxide in positive electrode sheet

[0127] In the above cross section image, randomly select a positive electrode active material layer region of 100 μm x 80 μm, respectively count the cross-sectional areas of the first lithium cobalt oxide particles and the second lithium cobalt oxide particles in the region, and calculate the corresponding cross-sectional area ratio S1 / S2.

[0128] 3. Test of compaction density of positive electrode active material layer

[0129] Take a positive electrode sheet coated with a positive electrode active material layer on one side, punch to obtain a positive electrode sheet sample with an area of S, weigh the sample using an analytical balance and record the weight as m1, and measure the thickness of the sample using a micrometer and record the thickness as H. Soak the positive electrode sheet sample in NMP to remove the surface positive electrode active material layer to obtain the positive electrode current collector, dry, weigh the positive electrode current collector using an analytical balance and record the weight as m2, and measure the thickness using a micrometer and record the thickness as h.

[0130] The compaction density of the positive electrode active material layer = (m1-m2) / ((H-h) x S).

[0131] 4. Test of electrochemical performance

[0132] Charge and discharge the button cell in the voltage range of 3V-4.6V (vs. Li / Li + ) to test the electrochemical performance. 2

[0133] Specifically, at 25°C, the current size of the test is set to 0.3mA / cm 2 , first constant current charging to 4.6V, then constant voltage charging to a current of 50 μA; thereafter, 0.3mA / cm 2 ​The current is discharged at a constant current to 3.0 V. This is one charge-discharge cycle. The above charge-discharge cycle is repeated for 100 cycles.

[0134] First charge gram capacity = first charge capacity / positive electrode material mass;

[0135] First discharge gram capacity = first discharge capacity / positive electrode material mass;

[0136] Capacity retention rate (%) after 100 cycles = [discharge capacity of the 100th cycle / discharge capacity of the first cycle] x 100%.

[0137] Test results

[0138] Table 1 shows the influence of the a / b value of the first lithium cobalt oxide particles on the performance of the coin cell.

[0139] Table 1

[0140]

[0141] The positive electrode material is subjected to XRD testing, Figure 1 The XRD spectrum of the positive electrode material powder provided in Example 1 shows that the material obtained has a crystal structure of P63mc. Figure 2 The SEM image of the positive electrode material powder provided in Example 1 shows that the positive electrode material provided in Example 1 has a sheet-like morphology. Further, Figure 3 The cross-sectional SEM image of the positive electrode material provided in Example 1 in the electrode sheet is given, further illustrating the sheet-like morphology of the material.

[0142] The positive electrode materials provided in Comparative Examples 1 to 3 and Examples 1 to 16 are subjected to coin cell testing, and the electrical performance test results are shown in Table 1. As can be seen from the data in Table 1, the first charge gram capacity of the positive electrode materials with different a / b ratios differs little, but the first discharge gram capacity and cycle stability differ greatly. When the a / b value is in the range of 3 to 9, the positive electrode material has both high discharge gram capacity and cycle stability.

[0143] As can be seen by comparing Comparative Example 1 and Example 1, although the material components of the two are the same, whether or not an ammonium chloride fluxing agent is added in the preparation step 2 will cause the materials to have different a / b values.

[0144] Table 2 further studies the influence of the ratio of the first cobalt lithium oxide and the second cobalt lithium oxide in the positive electrode sheet on the performance of the coin cell.

[0145] Table 2

[0146]

[0147] As can be seen from the data in Table 2, the positive electrode sheet in which the aspect ratio a / b of the first lithium cobalt oxide is in the range of 3 to 9 and the aspect ratio c / d of the second lithium cobalt oxide B particles is in the range of 1 to 1.2 can better improve the discharge capacity of the button cell and improve its cycle stability compared to Comparative Examples 5 and 6 in which the aspect ratio a / b of the first lithium cobalt oxide is not in the range of 3 to 9. The possible reason is that when the aspect ratio a / b of the first lithium cobalt composite oxide is less than 3, it is difficult to inhibit the side reaction of the electrolyte on the active surface thereof, and the second lithium cobalt composite oxide cannot be well protected by covering. When the aspect ratio a / b of the first lithium cobalt composite oxide is greater than 9, the first lithium cobalt composite oxide and the second lithium cobalt composite oxide are difficult to match during the compaction process, and the first lithium cobalt composite oxide is prone to breakage, thereby causing the cycle capacity retention rate to be unable to be effectively improved.

[0148] In addition, as can be seen from the comparison of Example 17 to Example 21, when the cross-sectional area ratio S1 / S2 of the first lithium cobalt oxide to the second lithium cobalt oxide is in the range of 0.1 to 0.35, the first lithium cobalt oxide and the second lithium cobalt oxide can better match, so that the button cell can have both a high discharge capacity and excellent cycle stability.

[0149] Although the illustrative embodiments have been demonstrated and described, those skilled in the art should understand that the above embodiments cannot be interpreted as a limitation of the present application, and changes, substitutions and modifications can be made to the embodiments without departing from the spirit, principles and scope of the present application.

Claims

1. A positive electrode sheet, comprising a positive current collector and a positive active material layer located on the surface of the positive current collector, the positive active material layer comprising a positive electrode material, the positive electrode material comprising first lithium cobalt oxide particles having a P63mc crystal structure, wherein in a cross-section of the positive active material layer perpendicular to the surface of the positive current collector, the aspect ratio a / b of the cross-section of the first lithium cobalt oxide particles satisfies: 3 ≤ a / b ≤ 9. in, a is the longest diameter of the cross-section of the first lithium cobalt oxide particle, and b is the length of the minor axis of the cross-section of the first lithium cobalt oxide particle that is perpendicular to and bisects its longest diameter.

2. The positive electrode sheet according to claim 1, wherein, The positive electrode plate satisfies at least one of the following conditions (1) to (2): (1) 4 ≤ a / b ≤ 8; (2) The average particle size of the first lithium cobalt oxide particles is 5 μm to 20 μm.

3. The positive electrode sheet according to claim 1, wherein, The positive electrode plate satisfies at least one of the following conditions (1) to (2): (1) The morphology of the first lithium cobalt oxide particles is lamellar; (2) Cracks exist in the cross section of the first lithium cobalt oxide particle.

4. The positive electrode sheet according to claim 1, wherein, The positive electrode plate satisfies at least one of the following conditions (1) to (3): (1) The first lithium cobalt oxide includes Mn and Ni elements. Based on the molar amount of metal elements other than Li in the first lithium cobalt oxide, the molar percentage of Mn in the first lithium cobalt oxide is 0.05% to 10%, and the molar percentage of Ni in the first lithium cobalt oxide is 0.05% to 10%. (2) The first lithium cobalt oxide includes element M, which includes at least one of Na, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba or Cu. Based on the molar amount of the metal elements other than Li in the first lithium cobalt oxide, the molar percentage of element M in the first lithium cobalt oxide is 0.01% to 10%. (3) The first lithium cobalt oxide includes Li x1 Co y1 Ni z1 Mn q M s O 2±n T n Where 0.6≤x1≤1.1, 0.8≤y1<1, 0<z1≤0.1, 0<q≤0.1, 0≤s≤0.1, 0≤n≤0.05, M includes at least one of Na, K, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba, Pr, Hf, Lu or Cu, and T is a halogen.

5. The positive electrode sheet according to claim 1, wherein, The cathode material further includes second lithium cobalt oxide particles with an R-3m crystal structure. In the cross-section of the cathode active material layer perpendicular to the surface of the cathode current collector, the aspect ratio c / d of the cross-section of the second lithium cobalt oxide particles is 1 to 1.2, where c is the longest diameter of the cross-section of the second lithium cobalt oxide particles, and d is the length of the short axis of the cross-section of the second lithium cobalt oxide particles that is perpendicular to and bisects its longest diameter.

6. The positive electrode sheet according to claim 5, wherein, In the cross-section of the positive electrode active material layer perpendicular to the surface of the positive electrode current collector, the ratio S1 / S2 between the cross-sectional area S1 of the first lithium cobalt oxide particle and the cross-sectional area S2 of the second lithium cobalt oxide particle satisfies: 0.05≤S1 / S2≤0.

45.

7. The positive electrode sheet according to claim 6, wherein, 0.1≤S1 / S2≤0.

35.

8. The positive electrode sheet according to claim 5, wherein, The positive electrode plate satisfies at least one of the following conditions (1) to (5): (1) The average particle size of the second lithium cobalt oxide particles is 10 μm to 25 μm; (2) Based on the molar amount of metal elements other than Li in the second lithium cobalt oxide, the molar percentage of Co in the second lithium cobalt oxide is 90% to 100%. (3) The second lithium cobalt oxide includes element R, which includes at least one of Ni, Mn, Na, K, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba or Cu. Based on the molar amount of the metal elements other than Li in the second lithium cobalt oxide, the molar percentage of element R in the second lithium cobalt oxide is 0.01% to 10%. (4) The second lithium cobalt oxide includes Li x2 Co y2 R z2 O 2±m T' m At least one of the following, wherein 0.6≤x2≤1.1, 0.9≤y2≤1, 0≤z2≤0.1, 0≤m≤0.05, R includes at least one of Ni, Mn, Na, K, Al, Mg, Ti, Zr, Fe, Zn, Nb, Cr, Ca, La, Y, Mo, W, Ta, Sn, V, Ce, Sr, Ge, Ga, Pb, Ba, Pr, Hf, Lu or Cu, and T' is a halogen; (5) The mass ratio of the first lithium cobalt oxide particles to the second lithium cobalt oxide particles is 5:95 to 40:

60.

9. An electrochemical device comprising a positive electrode sheet according to any one of claims 1 to 8.

10. An electrical device comprising the electrochemical device of claim 9.

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