Electrochemical devices and electronic devices

Abstract

The application provides an electrochemical device and an electronic device. The electrochemical device comprises a positive electrode sheet. The positive electrode sheet comprises a positive electrode active material layer, and the positive electrode active material layer comprises a first positive electrode active material and a second positive electrode active material. The first positive electrode active material contains Ni elements and Mn elements, and the second positive electrode active material contains Co elements. The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharging comprises a peak a with a peak position of 4.1 V to 4.3 V and a peak b with a peak position of 3.6 V to 4.0 V. Thus, the electrochemical device has good cycle performance and safety performance while the energy density of the electrochemical device is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrochemical energy storage, in particular to an electrochemical device and an electronic device. BACKGROUND

[0002] With the wide application of portable electronic devices, electric tools and the like, the market puts forward higher and higher requirements on the comprehensive performance of electrochemical devices (e.g., lithium ion batteries) therein, for example, not only the electrochemical devices need to have higher energy density to meet the requirement of long endurance, but also the electrochemical devices need to have longer cycle life and higher safety. Therefore, it is urgent to develop electrochemical devices with better comprehensive performance. SUMMARY

[0003] In view of this, the present application provides an electrochemical device and an electronic device to improve the energy density of the electrochemical device while making it have good cycle life and safety.

[0004] In a first aspect, the present application provides an electrochemical device, which comprises a positive electrode sheet. The positive electrode sheet comprises a positive electrode active material layer, and the positive electrode active material layer comprises a first positive electrode active material and a second positive electrode active material. The first positive electrode active material contains Ni element and Mn element, and the second positive electrode active material contains Co element. The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge comprises a peak a with a peak position of 4.1 V to 4.3 V and a peak b with a peak position of 3.6 V to 4.0 V. In the electrochemical device of the present application, the peak a is mainly dependent on the first positive electrode active material, the peak b is mainly dependent on the second positive electrode active material, and the first positive electrode active material has a characteristic peak a in the high voltage region of the electrochemical device during discharge relative to the second positive electrode active material, which can make the electrochemical device have higher reversible discharge capacity in the high voltage region, thereby greatly improving the energy density of the electrochemical device. At the same time, since the first positive electrode active material has a higher delithiation amount in the high voltage region, it can reduce the risk of excessive delithiation of the second positive electrode active material in the high voltage region, thereby improving the structural stability of the second positive electrode active material at high temperature and high voltage, and since the first positive electrode active material itself has surface oxygen vacancies, it can inhibit the release of oxygen from its surface, so that it still has higher structural stability at high temperature and high voltage, thereby making the electrochemical device have good cycle performance and safety performance.

[0005] In some embodiments, the peak area of the peak a is S a , the peak area of the peak b is S b , and satisfies: 0.4≤S a / S b ≤1. By adjusting the relative content of the first positive electrode active material and the second positive electrode active material, the area ratio S a / Sb Satisfying the above range, the electrochemical device can have excellent cycle performance and safety performance while greatly improving the energy density of the electrochemical device. Further, in some embodiments, 0.45≤S a b ≤0.65.

[0006] In some embodiments, when the electrochemical device is in a full discharge state, the positive electrode tab is characterized by X-ray diffraction, and has two diffraction peaks in the range of 18° to 19.5°, diffraction peak A and diffraction peak B in order from low angle to high angle, the peak intensity of diffraction peak A is I A , and the peak intensity of diffraction peak B is I B , satisfying: 1.2≤I B / I A ≤5. Wherein, diffraction peak A corresponds to the first positive electrode active material, and diffraction peak B corresponds to the second positive electrode active material, I B / I A Satisfying the above range, the electrochemical device can have excellent cycle performance and safety performance while greatly improving the energy density of the electrochemical device. Further, in some embodiments, 2≤I B / I A ≤5.

[0007] In some embodiments, the first positive electrode active material includes lithium nickel manganese composite oxide, and the second positive electrode active material includes lithium cobalt composite oxide.

[0008] In some embodiments, the first positive electrode active material is secondary particles formed by aggregation of primary particles.

[0009] In some embodiments, the average diameter of the first positive electrode active material is 6 μm to 14 μm.

[0010] In some embodiments, the second positive electrode active material is a primary particle.

[0011] In some embodiments, the average diameter of the second positive electrode active material is 10 μm to 25 μm.

[0012] In some embodiments, in the cross section in the thickness direction of the positive electrode active material layer, the area of the first positive electrode active material in a 127 μm x 34 μm region is S1, and the area of the second positive electrode active material is S2, satisfying: 1≤S2 / S1≤7. S2 / S1 satisfies the above range, the electrochemical device can have excellent cycle performance and safety performance while greatly improving the energy density of the electrochemical device. Further, in some embodiments, 3.4≤S2 / S1≤6.8.

[0013] ​In some embodiments, the first positive electrode active material further contains an M1 element, the M1 element includes at least one of Na or K, and a molar percentage content of the Ni element in the first positive electrode active material is a1, a molar percentage content of the Mn element is b1, and a molar percentage content of the M1 element is c1, based on metal elements other than the Li element and the M1 element in the first positive electrode active material, and the following conditions are satisfied: 30%≤a1≤70%, 30%≤b1≤70%, and 0.1%≤c1≤15%. The doping of the M1 element enables the first positive electrode active material to have good structural stability at high temperature and high voltage, thereby improving the cycle performance and safety performance of the electrochemical device.

[0014] In some embodiments, the first positive electrode active material includes a matrix and a coating layer on the surface of the matrix, and the coating layer contains a B element. The coating layer containing a boron-containing compound can neutralize free lithium on the surface of the matrix on the one hand, and can eliminate HF generated by decomposition of lithium salt or other side reactions in the electrochemical device on the other hand, thereby reducing the corrosion of the HF to the electrode material and the current collector, and improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0015] In some embodiments, the coating layer contains a B-containing oxide.

[0016] In some embodiments, a molar percentage content of the B element in the first positive electrode active material is d1, based on metal elements other than the Li element and the M1 element in the first positive electrode active material, and the following condition is satisfied: 0.5%≤d1≤10%.

[0017] In some embodiments, the first positive electrode active material has a layered crystal structure belonging to the R-3m space group. In some embodiments, the first positive electrode active material includes Li x1 M1 m Ni y1 Mn z1 B k R1 q1 O 2±n1 T1 n1 , 0.6≤x1≤1.2, 0.001≤m≤0.15, 0.3≤y1≤0.7, 0.3≤z1≤0.7, 0≤k≤0.1, 0≤q1≤0.2, and 0≤n1≤0.2, wherein the element M1 includes at least one of Na or K, the element R1 includes at least one of Co, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Al, Mg, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga, or Ge, and the element T1 includes at least one of F, Cl, Br, I, or N.

[0018] In some embodiments, the molar percentage content of Co element in the second positive electrode active material is a2 based on metal elements other than Li element in the second positive electrode active material, and a2 satisfies: 80%≤a2≤100%.

[0019] In some embodiments, the second positive electrode active material further comprises M2 element, M2 comprises at least one of Ni, Mn, Al, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Na, K, Mg, B, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga or Ge, the molar percentage content of M2 element in the second positive electrode active material is b2 based on metal elements other than Li element in the second positive electrode active material, and b2 satisfies: 0.1%≤b2≤20%.

[0020] In some embodiments, the second positive electrode active material has a layered crystal structure belonging to R-3m space group. In some embodiments, the second positive electrode active material comprises Li x2 Co y2 M2 z2 O 2±n2 T2 n2 , 0.6≤x2≤1.2, 0.8≤y2≤1, 0≤z2≤0.2, 0≤n2≤0.2, wherein element M2 comprises at least one of Ni, Mn, Al, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Na, K, Mg, B, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga or Ge, and element T2 comprises at least one of F, Cl, Br, I or N.

[0021] The second aspect of the present application further provides an electronic device comprising the above-mentioned electrochemical device. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0023] Figure 1 The discharge curve of the electrochemical device of Embodiment 1 of the present application;

[0024] Figure 2 The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the discharge of the electrochemical device of Embodiment 1 of the present application;

[0025] Figure 3The X-ray diffraction (XRD) pattern of the positive electrode tab of the full state of the electrochemical device of Embodiment 1. DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments.

[0027] In a first aspect, the present application provides an electrochemical device, which comprises a positive electrode tab. The positive electrode tab comprises a positive electrode active material layer. The positive electrode active material layer comprises a first positive electrode active material and a second positive electrode active material. The first positive electrode active material contains Ni and Mn elements, and the second positive electrode active material contains Co elements. The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge comprises a peak a with a peak position of 4.1 V to 4.3 V and a peak b with a peak position of 3.6 V to 4.0 V. The peak a is mainly dependent on the first positive electrode active material, and the peak b is mainly dependent on the second positive electrode active material. The first positive electrode active material has a characteristic peak a in the high-voltage region of the electrochemical device during discharge relative to the second positive electrode active material, which can enable the electrochemical device to have a higher reversible discharge capacity in the high-voltage region, thereby greatly improving the energy density of the electrochemical device. At the same time, since the first positive electrode active material has a higher delithiation amount in the high-voltage region, it can reduce the risk of excessive delithiation of the second positive electrode active material in the high-voltage region, thereby improving the structural stability of the second positive electrode active material at high temperature and high voltage. In addition, due to the existence of surface oxygen vacancies in the first positive electrode active material itself, the release of oxygen from its surface can be inhibited, so that it still has a higher structural stability at high temperature and high voltage, thereby enabling the electrochemical device to have good cycle performance and safety performance.

[0028] In some embodiments, the peak area of the peak a is S a , and the peak area of the peak b is S b , satisfying: 0.4≤S a / S b ≤1. S a / S b may be used to indirectly represent the mixing ratio of the first positive electrode active material and the second positive electrode active material. By adjusting the relative content of the first positive electrode active material and the second positive electrode active material, the area ratio S a / S b of the peak a and the peak b satisfies the above range, which can greatly improve the energy density of the electrochemical device while enabling the electrochemical device to have excellent cycle performance and safety performance. Alternatively, S a / S b0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, or a range consisting of any two of the foregoing. Further, in some embodiments, 0.45≤S a / S b ≤0.65.

[0029] In some embodiments, when the electrochemical device is in a full discharge state, the positive electrode sheet is characterized by X-ray diffraction, and has two diffraction peaks in the range of 18° to 19.5°, in order from low angle to high angle, diffraction peak A and diffraction peak B, the peak intensity of diffraction peak A is I A , and the peak intensity of diffraction peak B is I B , satisfying: 1.2≤I B / I A ≤5. Wherein, diffraction peak A corresponds to the first positive electrode active material, and diffraction peak B corresponds to the second positive electrode active material, when I B / I A satisfies the above range, the electrochemical device can greatly improve the energy density while having excellent cycle performance and safety performance. Further, in some embodiments, 2≤I B / I A ≤5. At this time, the cycle performance and safety performance of the electrochemical device are better.

[0030] In some embodiments, the first positive electrode active material includes lithium nickel manganese composite oxide, and the second positive electrode active material includes lithium cobalt composite oxide.

[0031] In some embodiments, the first positive electrode active material is secondary particles formed by aggregation of primary particles. In this way, the kinetic performance of the electrochemical device can be improved.

[0032] In some embodiments, the average diameter of the first positive electrode active material is 6 μm to 14 μm.

[0033] In some embodiments, the second positive electrode active material is a primary particle.

[0034] In some embodiments, the average diameter of the second positive electrode active material is 10 μm to 25 μm.

[0035] In some embodiments, in a cross section in the thickness direction of the positive electrode active material layer, the area of the first positive electrode active material in a 127 μm x 34 μm region is S1, the area of the second positive electrode active material is S2, and the following is satisfied: 1≤S2 / S1≤7. When S2 / S1 satisfies the above range, the electrochemical device can have excellent cycle performance and safety performance while greatly improving the energy density of the electrochemical device. Alternatively, S2 / S1 is 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, or a range defined by any two of the above. Further, in some embodiments, 3.4≤S2 / S1≤6.8. In this case, the cycle performance and safety performance of the electrochemical device are better.

[0036] In some embodiments, the first positive electrode active material further contains an M1 element, and M1 includes at least one of Na or K. The doping of the M1 element enables the first positive electrode active material to have good structural stability at high temperature and high voltage, thereby improving the cycle performance and safety performance of the electrochemical device. In some embodiments, based on the metal elements other than the Li element and the M1 element in the first positive electrode active material, the molar percentage content of the Ni element in the first positive electrode active material is a1, the molar percentage content of the Mn element is b1, and the molar percentage content of the M1 element is c1, and the following is satisfied: 30%≤a1≤70%, 30%≤b1≤70%, and 0.1%≤c1≤15%.

[0037] In some embodiments, the first positive electrode active material includes a matrix and a coating layer on the surface of the matrix, and the coating layer contains a B element. The coating layer containing a boron compound can neutralize free lithium on the surface of the matrix on the one hand, and can eliminate HF generated by the decomposition of lithium salt or other side reactions in the electrochemical device, thereby reducing the corrosion of the electrode material and the current collector by HF, and improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0038] In some embodiments, the coating layer contains a B-containing oxide.

[0039] In some embodiments, based on the metal elements other than the Li element and the M1 element in the first positive electrode active material, the molar percentage content of the B element in the first positive electrode active material is d1, and 0.5%≤d1≤10%. Alternatively, d1 is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 8%, 9%, 10%, or a range defined by any two of the above. Further, in some embodiments, 1.5%≤d1≤7%.

[0040] In some embodiments, the first positive electrode active material has a layered crystal structure belonging to the R-3m space group. In some embodiments, the first positive electrode active material includes Li x1 M1m Ni y1 Mn z1 B k R1 q1 O 2±n1 T1 n1 0.6≤x1≤1.2, 0.001≤m≤0.15, 0.3≤y1≤0.7, 0.3≤z1≤0.7, 0≤k≤0.1, 0≤q1≤0.2, 0≤n1≤0.2, wherein the element M1 includes at least one of Na or K, the element R1 includes at least one of Co, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Al, Mg, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga, or Ge, and the element T1 includes at least one of F, Cl, Br, I, or N.

[0041] In some embodiments, based on metal elements other than the Li element in the second positive electrode active material, the molar percentage content of the Co element in the second positive electrode active material is a2, which satisfies: 80%≤a2≤100%.

[0042] In some embodiments, the second positive electrode active material further comprises an M2 element, M2 including at least one of Ni, Mn, Al, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Na, K, Mg, B, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga, or Ge, based on metal elements other than the Li element in the second positive electrode active material, the molar percentage content of the M2 element in the second positive electrode active material is b2, which satisfies: 0.1%≤b2≤20%.

[0043] In some embodiments, the second positive electrode active material has a layered crystal structure belonging to the R-3m space group. In some embodiments, the second positive electrode active material includes Li x2 Co y2 M2 z2 O 2±n2 T2 n2 0.6≤x2≤1.2, 0.8≤y2≤1, 0≤z2≤0.2, 0≤n2≤0.2, wherein the element M2 includes at least one of Ni, Mn, Al, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Na, K, Mg, B, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga, or Ge, and the element T2 includes at least one of F, Cl, Br, I, or N.

[0044] In some embodiments, the positive electrode tab can further include a positive current collector, in which case the positive active material layer can be disposed on one or both sides of the positive current collector. In some embodiments, the positive current collector can include an aluminum foil, although other positive current collectors commonly used in the art can also be employed. In some embodiments, the positive current collector has a thickness of 7 pm to 20 pm.

[0045] In some embodiments, the positive active material layer can further include a conductive agent and a binder. In some embodiments, the conductive agent includes at least one of acetylene black, carbon black, ketjen black, carbon nanotubes, graphene, or carbon nanofibers. In some embodiments, the binder can include at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, a styrene-acrylate copolymer, polyamide, polyacrylonitrile, an acrylate polymer, polyacrylic acid, a polyacrylic acid salt, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, or polyhexafluoropropylene. In some embodiments, the mass ratio of the positive active material, the conductive agent, and the binder in the positive active material layer is (90-99):(0.1-10):(0.1-10), although this is merely exemplary, and any other suitable mass ratio can be employed.

[0046] In some embodiments, the electrochemical device further includes a negative electrode tab and a separator membrane, the positive electrode tab and the negative electrode tab being spaced apart by the separator membrane disposed therebetween. In some embodiments, the negative electrode tab includes a negative current collector and a negative active material layer. In some embodiments, the negative active material layer can be disposed on one or both sides of the negative current collector.

[0047] In some embodiments, the negative active material layer can include a negative active material, a binder, and optionally a conductive agent. In some embodiments, the negative active material can include at least one of a carbon material or a silicon-based material. In some embodiments, the carbon material includes at least one of graphite, hard carbon, soft carbon, or mesocarbon microbeads. In some embodiments, the silicon-based material includes at least one of silicon, a silicon-oxygen material, a silicon-carbon material, or a silicon-oxygen-carbon material.

[0048] In some embodiments, the binder in the negative active material layer can include at least one of sodium carboxymethyl cellulose (CMC), polyacrylic acid, polyacrylate, polyvinyl pyrrolidone, polyimide, polysiloxane, butadiene-styrene rubber, polyurethane resin, or acrylate polymer. In some embodiments, the conductive agent in the negative active material layer can include at least one of conductive carbon black, acetylene black, ketjen black, carbon nanotube, graphene, or carbon nanofiber. In some embodiments, the mass ratio of the negative active material, the conductive agent, and the binder in the negative active material layer can be (78 to 98.5):(0.1 to 10):(0.1 to 10). It should be understood that the above is merely an example, and any other suitable materials and mass ratios can be employed. In some embodiments, the negative current collector can employ at least one of copper foil, nickel foil, or carbon-based current collector.

[0049] In some embodiments, the separator film includes a porous substrate layer and a heat-resistant layer on a surface of the porous substrate layer. In some embodiments, the porous substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, or aramid. In some embodiments, the thickness of the separator film is in a range of 3 μm to 20 μm. In some embodiments, the pores of the porous substrate layer have a diameter in a range of 0.01 μm to 1 μm.

[0050] In some embodiments, the heat-resistant layer includes inorganic particles selected from at least one of aluminum oxide (AI2O3), silicon oxide (SiO2), magnesium oxide (MgO), titanium oxide (TiO2), hafnium dioxide (HfO2), tin oxide (SnO2), cerium dioxide (CeO2), nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO2), yttrium oxide (Y2O3), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate, and a binder. The binder of the heat-resistant layer is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, sodium carboxymethyl cellulose, polyvinyl pyrrolidone, polyvinyl ether, polytetrafluoroethylene, or polyhexafluoropropylene.

[0051] In some embodiments, the separator film further includes a bonding layer on a surface of the porous substrate layer and / or the heat-resistant layer. In some embodiments, the bonding layer includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, sodium carboxymethyl cellulose, polyvinyl pyrrolidone, polyvinyl ether, polytetrafluoroethylene, or polyhexafluoropropylene.

[0052] In some embodiments, the electrochemical device includes a lithium ion battery, but the present application is not limited thereto.

[0053] In some embodiments, the electrochemical device further comprises an electrolyte, the electrolyte comprising a lithium salt and a non-aqueous solvent. In some embodiments, the lithium salt comprises lithium hexafluorophosphate. In some embodiments, the concentration of the lithium salt is 1 mol / L to 2 mol / L. In some embodiments, the non-aqueous solvent can be a carbonate compound, a carboxylate compound, an ether compound, or a combination thereof.

[0054] In some embodiments, the carbonate compound can be a chain carbonate compound, a cyclic carbonate compound, or a combination thereof. Examples of the chain carbonate compound are diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), and a combination thereof. Examples of the cyclic carbonate compound are ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or a combination thereof. Examples of the carboxylate compound are methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, caprolactone, methyl formate, or a combination thereof. Examples of the ether compound are dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, or a combination thereof.

[0055] In some embodiments, the electronic device can include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a drone, a lighting appliance, a toy, a game machine, a clock, an electric tool, a flash, a camera, a household large storage battery, a lithium ion capacitor, and the like.

[0056] The following examples and comparative examples are provided to better illustrate the present application, with lithium ion batteries as examples.

[0057] Example 1

[0058] Preparation of the negative electrode sheet: the artificial graphite, styrene-butadiene rubber and sodium carboxymethyl cellulose were mixed in a mass ratio of 96:2:2, and then dispersed in deionized water to prepare a negative electrode slurry with a solid content of 0.8. After uniform stirring, the negative electrode slurry was uniformly coated on one side surface of the negative current collector copper foil, and then dried at 80°C to obtain a negative electrode sheet coated with a negative electrode active material layer on one side. The above steps were repeated on the other side surface of the copper foil to obtain a negative electrode sheet coated with a negative electrode active material layer on both sides. After cold pressing, slitting and welding of the tabs, the negative electrode sheet was obtained.

[0059] Preparation of the positive electrode sheet: the positive electrode active material (including the first positive electrode active material and the second positive electrode active material, and the mixing mass ratio is shown in Table 1), conductive carbon black, carbon nanotube and polyvinylidene fluoride were mixed in a mass ratio of 93.7:2.8:1.2:2.3, and then dispersed in N-methyl pyrrolidone to prepare a positive electrode slurry with a solid content of 0.7. After uniform stirring and mixing, the positive electrode slurry was uniformly coated on one side surface of the positive current collector aluminum foil, and then dried at 85°C to obtain a positive electrode sheet coated with a positive electrode active material layer on one side. The above steps were repeated on the other side surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides. After cold pressing, slitting and welding of the tabs, the positive electrode sheet was obtained.

[0060] The preparation steps of the first positive electrode active material are as follows:

[0061] 1) A mixed solution containing NiSO4 and MnSO4 was prepared according to the element molar ratio Ni:Mn=50:50, and then mixed with a precipitating agent (NaOH solution) and a complexing agent (ammonia water) for reaction, with the reaction time being controlled to 60 hours, the ammonia water concentration being 1 mol / L, and the pH value being 12.2, to obtain a nickel-manganese precursor TM(OH)2(TM=Ni / Mn) with an average particle size Dv50 of 11 μm;

[0062] 2) The above nickel-manganese precursor and sodium carbonate were ground and mixed uniformly in a Na:(Ni+Mn) molar ratio of 1.05:1. The product one was obtained after calcination at 800°C in an air atmosphere for 20 hours, followed by crushing, sieving and demagnetization.

[0063] 3) The mixture of the product one and lithium hydroxide and lithium nitrate (molar ratio of 1:1) was mixed, and the mass ratio of the mixture of lithium hydroxide and lithium nitrate to the product one was 10:1. The temperature was increased to 400°C at a rate of 10°C / min, and then maintained for 6 hours. After that, the temperature was decreased to room temperature at a rate of 50°C / min in a mixed gas of Ar and air (volume ratio of Ar to air was 1:2) to obtain the product two.

[0064] 4) The product II is washed and soaked in deionized water, and dried; 5) Finally, the first positive active material is obtained by crushing and sieving, and the Na / (Ni+Mn) molar ratio thereof is 10.7%, and the Dv50 thereof is 9.4 μm.

[0065] The second positive active material is lithium cobaltate with a Dv50 of 21.7 μm.

[0066] Preparation of the separator film: A porous polyethylene (PE) film with a thickness of 8 μm is selected as the separator film.

[0067] Preparation of the electrolyte: In an argon glove box with a water content of less than 10 ppm, lithium hexafluorophosphate is uniformly mixed with a non-aqueous organic solvent (ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC): propyl propionate (PP): ethyl propionate (EP) = 1:1:1:1:1, weight ratio) to prepare the electrolyte, wherein the mass percentage of lithium hexafluorophosphate in the electrolyte is 12.5%.

[0068] Preparation of the lithium ion battery: The positive electrode sheet, the separator film, and the negative electrode sheet are sequentially stacked in order, with the separator film between the positive electrode sheet and the negative electrode sheet to play a separating role, and the electrode assembly is obtained by winding. The electrode assembly is placed in an outer packaging aluminum plastic film, and after removing the water at 80°C, the above-mentioned electrolyte is injected and packaged, and the lithium ion battery is obtained after processes such as formation, degassing, and shaping.

[0069] Examples 2 to 9 differ from Example 1 in that the composition of the positive active material is adjusted according to the mass mixing ratio of the first positive active material and the second positive active material shown in Table 1.

[0070] Comparative Example 1 differs from Example 1 in that the positive active material only uses the first positive active material.

[0071] Comparative Example 2 differs from Example 1 in that the positive active material only uses the second positive active material.

[0072] Comparative Example 3 differs from Example 1 in that the first positive active material is selected to be LiNi 0.92 Co 0.06 Mn 0.02 O2 secondary particles with a Dv50 of 12.9 μm, wherein the average diameter of the primary particles is 0.6 μm.

[0073] Examples 10 to 15 differ from Example 1 in that the first positive active material is prepared according to the following method, and the first positive active material is B-coated.

[0074] 1) A mixed solution containing NiSO4 and MnSO4 is prepared according to the molar ratio of elements Ni:Mn = 50:50, and is mixed with a precipitant (NaOH solution) and a complexing agent (ammonia water) to react, to obtain a nickel-manganese precursor TM(OH)2 (TM = Ni / Mn) with an average particle size Dv50 of 11 μm by controlling the reaction time to be 60 hours, the ammonia water concentration to be 1 mol / L, and the pH value to be 12.2;

[0075] 2) The nickel-manganese precursor and sodium carbonate are ground and mixed uniformly according to a molar ratio of Na:(Ni+Mn) = 1.05:1. The mixture is calcined at 800°C in an air atmosphere for 20 hours, and then is crushed, sieved, and demagnetized to obtain a product one;

[0076] 3) The product one is mixed with a mixture of lithium hydroxide and lithium nitrate (molar ratio 1:1), and the mass ratio of the mixture of lithium hydroxide and lithium nitrate to the product one is 10:1. The mixture is heated at a rate of 10°C / min to 400°C, and is kept at 400°C for 6 hours. Then, the mixture is cooled to room temperature in a mixed gas of Ar and air (volume ratio of Ar to air is 1:2) at a cooling rate of 50°C / min to obtain a product two;

[0077] 4) The product two is washed and soaked in deionized water, and is dried. The product two is crushed and sieved to obtain a product three;

[0078] 5) The product three is ground and mixed uniformly with boric acid according to the ratio d1 shown in Table 2, and is calcined at 200°C in an air atmosphere for 12 hours to obtain a first positive electrode active material.

[0079] In addition, in the present application, the corresponding parameters are measured by the following methods.

[0080] 1) Peak area:

[0081] The lithium ion battery is charged and discharged at a current of 0.1C in a voltage range of 3V to 4.45V to obtain a relationship curve of the capacity voltage differential dQ / dV and the voltage V. The peak a and the peak b in the curve are integrated to obtain the peak area of each peak.

[0082] 2) Particle morphology, average diameter, and area ratio S2 / S1:

[0083] The positive electrode sheet is cut along the longitudinal direction by using ion polishing, and then the morphology of the positive electrode active material particles in the cross section of the positive electrode sheet is observed by using a scanning electron microscope (instrument model: ZEISS SEM, acceleration voltage: 0.1KV-30KV). In the cross section photograph, 50 first or second positive electrode active material particles are randomly selected, and the average value of the longest diameters thereof is taken as the average diameter of the first or second positive electrode active material.

[0084] In the cross-section photo, a 127 μm x 34 μm area is randomly selected, and the image software is used to count the area of the first positive electrode active material as S1 and the area of the second positive electrode active material as S2, so as to calculate the area ratio S2 / S1.

[0085] 3) Element content:

[0086] For the initial positive electrode active material, the first and second positive electrode active materials are dissolved using a mixed solvent (for example, 0.4 g of positive electrode active material is dissolved using 10 ml of aqua regia (nitric acid and hydrochloric acid mixed at 1:1) and 2 ml of HF mixed solvent), and the volume is made up to 100 ml, and then an ICP analyzer is used to test the content of each element in the solution.

[0087] For the first positive electrode active material and the second positive electrode active material in the positive electrode sheet, the positive electrode sheet is cut longitudinally using ion polishing, and then the cross-section of the positive electrode sheet is observed using a scanning electron microscope (instrument model: ZEISS SEM), and the first and second positive electrode active materials are tested using an energy dispersive spectrometer (EDS) to determine the element content in the first and second positive electrode active materials.

[0088] 4) X-ray diffraction:

[0089] The lithium ion battery is fully discharged, and the positive electrode sheet is obtained by disassembly, and X-ray diffraction analysis of the positive electrode sheet is performed using an X-ray diffractometer (instrument model: Bruker D8 ADVANCE, target material: Cu Kα, scanning angle: 5-80°).

[0090] 5) Discharge gram capacity test:

[0091] The lithium ion battery is first charged at a current of 0.5 C under constant temperature conditions at 25°C, and then charged at a constant voltage of 4.45 V until the current is 0.05 C, and then discharged at a current of 0.2 C, and discharged to 3 V. The discharge gram capacity = the first discharge capacity at 0.2 C / the mass of the positive electrode active material.

[0092] 6) Cycle performance test:

[0093] First, the first charge and discharge are performed in an environment of 45°C, and then the constant current charging is performed at a current of 0.5 C, and then the constant voltage charging is performed at 4.45 V until the current is 0.05 C, and then the constant current discharging is performed at a current of 0.5 C, and then the discharging is performed to 3 V. The above charging and discharging cycles are repeated, and the discharge capacity of the 3rd cycle and the discharge capacity of the 500th cycle are recorded.

[0094] 45°C cycle capacity retention rate = (discharge capacity of the 500th cycle / discharge capacity of the 3rd cycle) x 100%.

[0095] 7) High temperature storage performance test:

[0096] First, the lithium ion battery is charged at 25℃ by using a current of 0.5C, and after charging to 4.45V, 4.45V constant voltage charging is performed until the current is 0.05C. The thickness of the lithium ion battery is measured by using a micrometer, and the thickness of the lithium ion battery at this time is defined as the initial thickness H0 of the lithium ion battery. Then the lithium ion battery is placed in an oven at 85℃ for 24h, and the thickness of the lithium ion battery is measured in the same way, and the thickness of the lithium ion battery after storage is defined as H1.

[0097] 85℃ thickness expansion rate = (H1-H0) / H0x100%.

[0098] Table 1 shows the parameters and evaluation results of Examples 1 to 9 and Comparative Examples 1 to 3.

[0099] Table 1

[0100]

[0101]

[0102] By comparing Examples 1 to 9 and Comparative Example 2, it can be seen that the discharge gram capacity of the lithium ion battery of Examples 1 to 9 is improved compared with Comparative Example 2 which does not use the first positive electrode active material.

[0103] By comparing Examples 1 to 9 and Comparative Example 1, it can be seen that the high temperature cycle capacity retention rate and high temperature thickness expansion rate of the lithium ion battery of Examples 1 to 9 are significantly improved compared with Comparative Example 1 which does not use the second positive electrode active material.

[0104] By comparing Example 1 and Comparative Example 3, it can be seen that the first positive electrode active material mixed with lithium cobaltate in the present application can significantly improve the high temperature cycle performance and high temperature storage performance of the lithium ion battery on the basis of greatly improving the discharge gram capacity compared with the conventional mixed system of high-nickel ternary material and lithium cobaltate.

[0105] In addition, from Examples 1 to 9, it can be seen that when 0.4≤S a / S b ≤1, 1.2≤I B / I A ≤5 or 1≤S2 / S1≤7, the lithium ion battery can have good cycle capacity retention rate and low thickness expansion rate while having high discharge gram capacity. When 0.45≤S a / S b ≤0.65, 2≤I B / I AWhen S2 / S1 is less than or equal to 5 or 3.4 and greater than or equal to 6.8, the high-temperature cycle performance and high-temperature storage performance of the lithium ion battery are better.

[0106] Table 2 shows various parameters and evaluation results of Example 1 and Examples 10 to 15.

[0107] Table 2

[0108]

[0109] As can be seen from the comparison between Example 1 and Examples 10 to 15, by B-coating the first positive electrode active material, the high-temperature cycle capacity retention of the lithium ion battery is significantly improved, and the high-temperature storage thickness expansion rate is significantly reduced. It can be seen that by B-coating the first positive electrode active material, the high-temperature cycle performance and safety performance of the lithium ion battery can be greatly improved. The possible reason is that the first positive electrode active material is coated with a boron-containing compound on the surface, which neutralizes the free lithium on the surface of the material on the one hand, and can eliminate HF generated by the decomposition of lithium salt and other side reactions in the battery, thereby improving the high-temperature cycle performance and high-temperature storage performance of the battery.

[0110] The above only describes the preferred embodiments of the present application and is not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An electrochemical device, characterized in that, The invention includes a positive electrode sheet, wherein the positive electrode sheet includes a positive active material layer, the positive active material layer includes a first positive active material and a second positive active material, the first positive active material contains Ni and Mn elements, and the second positive active material contains Co element; The relationship curve between the capacity voltage differential dQ / dV of the electrochemical device and the voltage V includes peak a with a peak position between 4.1V and 4.3V and peak b with a peak position between 3.6V and 4.0V. The first positive electrode active material includes a lithium nickel manganese composite oxide, and the second positive electrode active material includes a lithium cobalt composite oxide; the first positive electrode active material has a layered crystal structure belonging to the R-3m space group; the second positive electrode active material has a layered crystal structure belonging to the R-3m space group.

2. The electrochemical device according to claim 1, characterized in that, The peak area of ​​peak a is S a The peak area of ​​peak b is S. b Satisfying: 0.4≤S a / S b ≤1.

3. The electrochemical device according to claim 2, characterized in that, 0.45≤S a / S b ≤0.65。 4. The electrochemical device according to claim 1, characterized in that, When the electrochemical device is in a fully discharged state, X-ray diffraction characterization of the positive electrode plate reveals two diffraction peaks in the range of 18° to 19.5°, namely diffraction peak A and diffraction peak B from low angle to high angle. The peak intensity of diffraction peak A is I. A The peak intensity of diffraction peak B is I. B , satisfying: 1.2≤I B / I A ≤5.

5. The electrochemical device according to claim 4, characterized in that, 2≤I B / I A ≤5。 6. The electrochemical device according to claim 1, characterized in that, The electrochemical device satisfies at least one of the following conditions: (1) The first positive electrode active material is a secondary particle formed by the aggregation of primary particles; (2) The average diameter of the first positive electrode active material is 6 μm to 14 μm; (3) The second positive electrode active material is a primary particle; (4) The average diameter of the second positive electrode active material is 10 μm to 25 μm.

7. The electrochemical device according to claim 1, characterized in that, In the cross section along the thickness direction of the positive electrode active material layer, the area of ​​the first positive electrode active material in the 127μm×34μm region is S1, and the area of ​​the second positive electrode active material is S2, satisfying: 1≤S2 / S1≤7.

8. The electrochemical device according to claim 1, characterized in that, The electrochemical device satisfies at least one of the following conditions: (1) The first positive electrode active material also contains element M1, which includes at least one of Na or K. Based on the metal elements other than Li and M1 in the first positive electrode active material, the molar percentage of Ni in the first positive electrode active material is a1, the molar percentage of Mn is b1, and the molar percentage of M1 is c1, satisfying: 30%≤a1≤70%, 30%≤b1≤70%, 0.1%≤c1≤15%; (2) The first positive electrode active material includes Li x1 M1 m Ni y1 Mn z1 B k R1 q1 O 2±n1 T1 n1 , 0.6≤x1≤1.2, 0.001≤m≤0.15, 0.3≤y1≤0.7, 0.3≤z1≤0.7, 0≤k≤0.1, 0≤q1≤0.2, 0≤n1≤0.2, wherein element M1 includes at least one of Na or K, element R1 includes at least one of Co, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Al, Mg, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga or Ge, and element T1 includes at least one of F, Cl, Br, I or N; (3) Based on the metal elements other than Li in the second positive electrode active material, the molar percentage of Co in the second positive electrode active material is a2, which satisfies: 80%≤a2≤100%; (4) The second positive electrode active material also contains the element M2, which includes at least one of Ni, Mn, Al, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Na, K, Mg, B, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga or Ge. Based on the metal elements other than Li in the second positive electrode active material, the molar percentage of the element M2 in the second positive electrode active material is b2, which satisfies: 0.1%≤b2≤20%; (5) The second positive electrode active material includes Li x2 Co y2 M2 z2 O 2±n2 T2 n2 , 0.6≤x2≤1.2, 0.8≤y2≤1, 0≤z2≤0.2, 0≤n2≤0.2, wherein element M2 includes at least one of Ni, Mn, Al, Ca, Sr, Y, Zr, Nb, Mo, In, Sn, Sb, Ba, La, Ce, Hf, Ta, W, Pb, Na, K, Mg, B, Si, P, S, Ti, V, Cr, Fe, Cu, Zn, Ga or Ge, and element T2 includes at least one of F, Cl, Br, I or N.

9. The electrochemical device according to claim 1, characterized in that, The first positive electrode active material includes a matrix and a coating layer located on the surface of the matrix, the coating layer comprising B, and the electrochemical device satisfies at least one of the following conditions: (1) The coating layer contains B oxide; (2) Based on the metal elements other than Li and M in the first positive electrode active material, the molar percentage of B in the first positive electrode active material is d1, 0.5%≤d1≤10%.

10. An electronic device comprising an electrochemical device according to any one of claims 1 to 9.

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