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 a Ni element and a Mn element, and the second positive electrode active material contains a Ni element and a Co element. A relationship curve between a capacity voltage differential dQ / dV and a voltage V of the electrochemical device during discharging comprises at least two peaks above 3.8 V. The electrochemical device provided by the application can meet the demand for high energy density, and can also have good cycle performance and safety performance.

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 development of electrochemical energy storage technology, higher and higher requirements are put forward for the energy density and safety performance of electrochemical devices (for example, lithium ion batteries). For example, with the increasing demand for long driving range in the power battery market, high-energy-density high-nickel ternary materials have gradually become one of the development hotspots. However, with the increase of nickel content, the thermal stability of ternary materials becomes poor, which reduces the safety of high-nickel ternary materials and limits the wide application of high-nickel ternary materials. SUMMARY

[0003] Therefore, the present application provides an electrochemical device and an electronic device to meet the demand for high energy density of the electrochemical device while improving the safety of the electrochemical device.

[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 and Mn elements, and the second positive electrode active material contains Ni and Co elements. The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge contains at least two peaks above 3.8 V. In the electrochemical device of the present application, the first positive electrode active material has a characteristic peak in the high-voltage region in the relationship curve between the capacity voltage differential dQ / dV and the voltage V 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 meeting the demand for high energy density of the electrochemical device. At the same time, since the first positive electrode active material has a higher amount of lithium extraction in the high-voltage region, it can reduce the risk of excessive lithium extraction of the second positive electrode active material, 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 oxygen vacancies on the surface of the first positive electrode active material itself, it can inhibit the release of oxygen from its surface, so that it still has a higher structural stability at high temperature and high voltage, thereby greatly improving the cycle performance and safety performance of the electrochemical device.

[0005] In some embodiments, the relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge contains peak A and peak B above 3.8 V, wherein the peak position of peak A is in the range of 3.8 V to 4.1 V, the peak position of peak B is in the range of 4.1 V to 4.3 V, the peak area of peak A is S A , and the peak area of peak B is S B , satisfying: 0.5≤S B / S A≤ 1. Wherein, peak A is mainly dependent on the second positive electrode active material, peak B is mainly dependent on the first positive electrode active material, by adjusting the relative content of the first positive electrode active material and the second positive electrode active material, peak A and peak B meet 0.5 ≤ S B / S A ≤ 1, which can make the electrochemical device have excellent cycle performance and safety performance. Further, in some embodiments, 0.5 ≤ S B / S A ≤ 0.85.

[0006] In some embodiments, the relationship curve of the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge further includes peak C in the range of 3.5V to 3.7V. Wherein, the presence of peak C can further enable the electrochemical device to have a higher discharge capacity, thereby further improving the energy density of the electrochemical device.

[0007] In some embodiments, the relationship curve of the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge includes at least one peak above 4.1V. This can indicate that the first positive electrode active material has good reversible capacity in the high voltage region, thereby improving the cycle performance and safety performance of the electrochemical device.

[0008] In some embodiments, when the electrochemical device is in a full discharge state, the positive electrode sheet is characterized by X-ray diffraction, having a diffraction peak 1 in the range of 43.7° to 44.1°, the peak intensity of the diffraction peak 1 being P1, having a diffraction peak 2 in the range of 44.2° to 44.6°, the peak intensity of the diffraction peak 2 being P2, satisfying: 1 ≤ P2 / P1 ≤ 6.5. Wherein, the diffraction peak 1 corresponds to the first positive electrode active material, the diffraction peak 2 corresponds to the second positive electrode active material, P2 / P1 satisfies the above range, which can make the electrochemical device have excellent cycle performance and safety performance. Further, in some embodiments, 2.3 ≤ P2 / P1 ≤ 6.5.

[0009] In some embodiments, the first positive electrode active material further contains M1 elements, M1 includes at least one of Na or K, based on the metal elements in the first positive electrode active material except Li elements and M1 elements, the molar percentage content of Ni elements in the first positive electrode active material is a1, the molar percentage content of Mn elements is b1, and the molar percentage content of M1 elements is c1, satisfying: 30% ≤ a1 ≤ 70%, 30% ≤ b1 ≤ 70%, 0.1% ≤ c1 ≤ 15%. The doping of M1 elements 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.

[0010] In some embodiments, the second positive electrode active material further contains an M2 element, M2 includes at least one of Mn or Al, the molar percentage content of the Ni element in the second positive electrode active material is a2, the molar percentage content of the Co element is b2, and the molar percentage content of the M2 element is c2, based on the metal elements other than the Li element in the second positive electrode active material, and 80%≤a2≤98%, 1%≤b2≤19%, and 1%≤c2≤19% are satisfied. The presence of the M2 element can improve the structural stability of the second positive electrode active material at high temperature and high voltage, thereby improving the cycle performance and safety performance of the electrochemical device.

[0011] In some embodiments, the first positive electrode active material is a secondary particle formed by aggregation of primary particles. In this way, the kinetic performance of the electrochemical device can be improved.

[0012] In some embodiments, the average diameter of the first positive electrode active material is 6 μm to 14 μm.

[0013] In some embodiments, the second positive electrode active material is a primary particle. In this way, the structural stability of the second positive electrode active material at high temperature and high voltage can be improved, thereby improving the cycle performance and safety performance of the electrochemical device.

[0014] In some embodiments, the average diameter of the second positive electrode active material is 3 μm to 10 μm.

[0015] 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, and the area of the second positive electrode active material is S2, and 0.5≤S2 / S1≤1.5 is satisfied. S2 / S1 satisfies the above range, and the electrochemical device can have excellent cycle performance and safety performance. Further, in some embodiments, 0.7≤S2 / S1≤1.3.

[0016] 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 R1 q1 O 2±n1 T1 n10.6≤x1≤1.2, 0.001≤m≤0.15, 0.3≤y1≤0.7, 0.3≤z1≤0.7, 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, B, 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.

[0017] 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 Ni y2 Co z2 M2 p R2 q2 O 2±n2 T2 n2 0.6≤x2≤1.2, 0.8≤y2≤0.98, 0.01≤z2≤0.19, 0.01≤p≤0.19, 0≤q2≤0.1, 0≤n2≤0.2, wherein element M2 includes at least one of Mn or Al, element R2 includes at least one of 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.

[0018] The second aspect of the present application further provides an electronic device comprising the above-mentioned electrochemical device. BRIEF DESCRIPTION OF DRAWINGS

[0019] 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.

[0020] Figure 1 Discharge curve of the electrochemical device of Embodiment 1 of the present application;

[0021] Figure 2 Capacity-voltage differential dQ / dV versus voltage V relationship curve of the discharge of the electrochemical device of Embodiment 1 of the present application;

[0022] Figure 3A scanning electron microscope (SEM) photo of a cross section of the positive electrode tab of Example 1 of the present application;

[0023] Figure 4 An X-ray diffraction (XRD) spectrum of the positive electrode tab of the full-discharge state of the electrochemical device of Example 1 of the present application. DETAILED DESCRIPTION

[0024] 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, but not all the embodiments.

[0025] 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 Ni and Co elements. The relationship curve of the capacity voltage differential dQ / dV of the electrochemical device during discharge and the voltage V contains at least two peaks above 3.8 V.

[0026] In the electrochemical device of the present application, the first positive electrode active material has a characteristic peak in the high-voltage region in the relationship curve of the capacity voltage differential dQ / dV of the electrochemical device during discharge and the voltage V, 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 meeting the demand for high energy density of the electrochemical device. At the same time, since the first positive electrode active material has a higher amount of lithium extraction in the high-voltage region, it can reduce the risk of excessive lithium extraction of the second positive electrode active material, thereby improving the structural stability of the second positive electrode active material at high temperature and high voltage. Moreover, due to the presence of oxygen vacancies on the surface of the first positive electrode active material itself, it can inhibit the release of oxygen from its surface, so that it still has a higher structural stability at high temperature and high voltage, thereby greatly improving the cycle performance and safety performance of the electrochemical device.

[0027] In some embodiments, the relationship curve of the capacity voltage differential dQ / dV of the electrochemical device during discharge and the voltage V contains peak A and peak B above 3.8 V, wherein the peak position of peak A is in the range of 3.8 V to 4.1 V, the peak position of peak B is in the range of 4.1 V to 4.3 V, the peak area of peak A is S A , and the peak area of peak B is S B , satisfying: 0.5≤S B / S A ≤1. Wherein, peak A is mainly dependent on the second positive electrode active material, and peak B is mainly dependent on the first positive electrode active material, S B / S Amay 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, peak A and peak B satisfy 0.5≤S B / A ≤1, the electrochemical device can have excellent cycle performance and safety performance. Alternatively, S B / A is 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, 0.5≤S B / A ≤0.85.

[0028] In some embodiments, the relationship curve of the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge further includes peak C in the range of 3.5V to 3.7V. The presence of peak C can further enable the electrochemical device to have a higher discharge capacity, thereby further improving the energy density of the electrochemical device.

[0029] In some embodiments, the relationship curve of the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge includes at least one peak above 4.1V. This can indicate that the first positive electrode active material has good reversible capacity in the high voltage region, thereby improving the cycle performance and safety performance of the electrochemical device.

[0030] In some embodiments, when the electrochemical device is in a full discharge state, the positive electrode sheet is characterized by X-ray diffraction, has a diffraction peak 1 in the range of 43.7° to 44.1°, the peak intensity of the diffraction peak 1 is P1, has a diffraction peak 2 in the range of 44.2° to 44.6°, the peak intensity of the diffraction peak 2 is P2, and satisfies: 1≤P2 / P1≤6.5. The diffraction peak 1 corresponds to the first positive electrode active material, and the diffraction peak 2 corresponds to the second positive electrode active material. When P2 / P1 satisfies the above range, the electrochemical device can have excellent cycle performance and safety performance. Alternatively, P2 / P1 is 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, or a range consisting of any two of the foregoing. Further, in some embodiments, 2.3≤P2 / P1≤6.5. In this case, the cycle performance and safety performance of the electrochemical device are better.

[0031] 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.

[0032] In some embodiments, the M1 element is doped in the lithium layer. In this way, the crystal structure of the first positive electrode active material in a high delithiation state can be better stabilized, and the cycle performance and safety performance of the electrochemical device can be improved.

[0033] In some embodiments, based on the metal elements in the first positive electrode active material other than the Li element and the M1 element, 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, which satisfy: 30%≤a1≤70%, 30%≤b1≤70%, and 0.1%≤c1≤15%.

[0034] In some embodiments, the second positive electrode active material further contains an M2 element, the M2 element including at least one of Mn or Al, based on the metal elements in the second positive electrode active material other than the Li element, the molar percentage content of the Ni element in the second positive electrode active material is a2, the molar percentage content of the Co element is b2, and the molar percentage content of the M2 element is c2, which satisfy: 80%≤a2≤98%, 1%≤b2≤19%, and 1%≤c2≤19%. The presence of the M2 element can improve the structural stability of the second positive electrode active material at high temperature and high voltage, thereby improving the cycle performance and safety performance of the electrochemical device.

[0035] In some embodiments, the first positive electrode active material is a secondary particle formed by aggregation of primary particles. In this way, the kinetic performance of the electrochemical device can be improved.

[0036] In some embodiments, the average diameter of the first positive electrode active material is 6 μm to 14 μm.

[0037] In some embodiments, the second positive electrode active material is a primary particle. In this way, the structural stability of the second positive electrode active material at high temperature and high voltage can be improved, thereby improving the cycle performance and safety performance of the electrochemical device.

[0038] In some embodiments, the average diameter of the second positive electrode active material is 3 μm to 10 μm.

[0039] 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, and the area of the second positive electrode active material is S2, which satisfy: 0.5≤S2 / S1≤1.5. In this way, the electrochemical device can have excellent cycle performance and safety performance. Optionally, S2 / S1 is 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, or a range defined by any two of the above. Further, in some embodiments, 0.7≤S2 / S1≤1.3. In this case, the cycle performance and safety performance of the electrochemical device are better.

[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 M1 m Ni y1 Mn z1 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 < 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, B, 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.

[0041] 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 Ni y2 Co z2 M2 p R2 q2 O 2±n2 T2 n2 , 0.6 < x2 < 1.2, 0.8 < y2 < 0.98, 0.01 < z2 < 0.19, 0.01 < p < 0.19, 0 < q2 < 0.1, 0 < n2 < 0.2, wherein element M2 includes at least one of Mn or Al, element R2 includes at least one of 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.

[0042] In some embodiments, the positive electrode sheet can further include a positive electrode current collector, in which case the positive electrode active material layer can be located on one or both sides of the positive electrode current collector. In some embodiments, the positive electrode current collector can include an aluminum foil, although other positive electrode current collectors commonly used in the art can also be used. In some embodiments, the positive electrode current collector has a thickness of 7 pm to 20 pm.

[0043] 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 conductive carbon black, acetylene black, ketjen black, carbon nanotube, graphene, or carbon nanofiber. 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 can be (90-99):(0.1-10):(0.1-10), although this is merely an example, and any other suitable mass ratio can be employed.

[0044] 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.

[0045] 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, silicon-oxygen material, silicon-carbon material, or silicon-oxygen-carbon material.

[0046] In some embodiments, the binder in the negative active material layer can include at least one of sodium carboxymethyl cellulose (CMC), polyacrylic acid, a polyacrylic acid salt, polyvinylpyrrolidone, polyimide, polysiloxane, styrene butadiene rubber, polyurethane resin, or an 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 a copper foil, a nickel foil, or a carbon-based current collector.

[0047] In some embodiments, the separator film includes a porous substrate layer and a heat-resistant 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.

[0048] In some embodiments, the heat-resistant layer includes inorganic particles and a binder, the inorganic particles being 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. 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, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, or polyhexafluoropropylene.

[0049] In some embodiments, the electrochemical device includes a lithium ion battery, but the present application is not limited thereto.

[0050] In some embodiments, the electrochemical device further includes an electrolyte, the electrolyte including a lithium salt and a non-aqueous solvent. In some embodiments, the lithium salt includes 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.

[0051] 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), or 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.

[0052] The second aspect of the present application also provides an electronic device comprising the above-mentioned electrochemical device. The electronic device of the embodiments of the present application is not particularly limited, and can be any electronic device known in the prior art. 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, a portable copier, a portable printer, a head-mounted stereo headset, 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, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0053] Some specific examples and comparative examples are listed below to better illustrate the present application, taking lithium ion batteries as examples.

[0054] Example 1

[0055] 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 electrode 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-mentioned step was 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.

[0056] 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 electrode 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-mentioned step was 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.

[0057] The preparation steps of the first positive electrode active material are as follows:

[0058] 1) A mixed solution containing NiSO4 and MnSO4 is prepared according to the element molar ratio Ni:Mn=50:50, and is mixed with a precipitant (NaOH solution) and a complexing agent (ammonia water) to react, with the reaction time controlled to 60 hours, the ammonia water concentration controlled to 1 mol / L, and the pH value controlled to 12.2, to obtain a nickel-manganese precursor TM(OH)2 (TM=Ni / Mn) with an average particle size Dv50 of 11 μm;

[0059] 2) The nickel-manganese precursor and sodium carbonate are ground and mixed uniformly according to a Na:(Ni+Mn) molar ratio of 1.05:1. The mixture is calcined at 800°C in an air atmosphere for 20 hours, and then is subjected to crushing, screening, and magnetic removal to obtain a product I;

[0060] 3) The product I 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 I is 10:1. The mixture is heated at a rate of 10°C / min to 400°C, maintained for 6 hours, and then is cooled to room temperature at a rate of 50°C / min in a mixed gas of Ar and air (volume ratio of Ar to air is 1:2) to obtain a product II;

[0061] 4) The product II is washed and soaked in deionized water, and is dried;

[0062] 5) Finally, the first positive active material is obtained after crushing and screening, with a Na / (Ni+Mn) molar ratio of 10.7% and a Dv50 of 9.4 μm.

[0063] The second positive active material is a single-crystal high-nickel ternary material LiNi 0.9 Co 0.06 Mn 0.04 O2.

[0064] Preparation of the separator film: A porous polyethylene (PE) film with a thickness of 8 μm is selected as the separator film.

[0065] Preparation of the electrolyte: In an argon glove box with a water content of less than 10 ppm, lithium hexafluorophosphate is mixed uniformly with non-aqueous organic solvents (ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC): propyl propionate (PP): ethyl propionate (EP)=1:1:1:1:1, weight ratio) to prepare an electrolyte, wherein the mass percentage of lithium hexafluorophosphate in the electrolyte is 12.5%.

[0066] Preparation of the lithium ion battery: the positive electrode sheet, the separator, and the negative electrode sheet were stacked in order, with the separator between the positive electrode sheet and the negative electrode sheet to play a separating role, and were wound to obtain an electrode assembly. The electrode assembly was placed in an outer packaging aluminum plastic film, and after water was removed at 80°C, the above-mentioned electrolyte was injected and packaged, and after processes such as formation, degassing, and shaping, a lithium ion battery was obtained.

[0067] Examples 2 to 13 differ from Example 1 in that the composition of the positive electrode active material is adjusted according to the mixed mass ratio of the first positive electrode active material and the second positive electrode active material shown in Table 1.

[0068] Comparative Example 1 differs from Example 1 in that the positive electrode active material only uses the first positive electrode active material.

[0069] Comparative Example 2 differs from Example 1 in that the positive electrode active material only uses the second positive electrode active material.

[0070] In addition, in the present application, the corresponding parameters are measured by the following method.

[0071] 1) Peak area:

[0072] In the voltage range of 2.8V to 4.35V, the lithium ion battery was charged and discharged with a current of 0.1C, and the relationship curve of the capacity voltage differential dQ / dV and the voltage V was obtained. The peak area of each peak in the curve was obtained by integrating the curve. For two peaks partially overlapping, the inflection point at the connection of the two peaks was divided.

[0073] 2) Particle morphology, average diameter, and area ratio S2 / S1:

[0074] The positive electrode sheet was cut longitudinally using ion polishing, and then the morphology of the positive electrode active material particles in the cross section of the positive electrode sheet was observed using a scanning electron microscope (instrument model: ZEISS SEM, acceleration voltage: 0.1KV-30KV). In the cross-sectional photo, 50 first or second positive electrode active material particles were randomly selected, and the average value of their longest diameters was taken as the average diameter of the first or second positive electrode active material. In the cross-sectional photo, a region of 127μm×34μm was randomly selected, and the area of the first positive electrode active material therein was S1, and the area of the second positive electrode active material was S2, so that the area ratio S2 / S1 was calculated.

[0075] 3) Element content:

[0076] For the initial positive active material, the first and second positive active materials were dissolved separately using a mixed solvent (for example, 0.4 g of positive active material was dissolved using a mixed solvent of 10 ml of aqua regia (nitric acid and hydrochloric acid mixed at 1:1) and 2 ml of HF), the volume was made up to 100 mL, and then the content of each element in the solution was tested using an ICP analyzer.

[0077] For the first and second positive active materials in the positive electrode sheet, the positive electrode sheet was cut in the longitudinal direction using ion polishing, and then the cross section of the positive electrode sheet was observed using a scanning electron microscope (instrument model: ZEISS SEM), and the first and second positive active materials were tested using an energy dispersive spectrometer (EDS) to determine the content of elements in the first and second positive active materials.

[0078] 4) X-ray diffraction:

[0079] The lithium ion battery was fully discharged, and the positive electrode sheet was obtained by disassembly. X-ray diffraction analysis of the positive electrode sheet was performed using an X-ray diffractometer (instrument model: Bruker D8 ADVANCE, target material: Cu Kα, scanning angle: 5-80°).

[0080] 5) Discharge gram capacity test:

[0081] The lithium ion battery was first charged at a constant current of 0.5 C to 4.35 V, then charged at a constant voltage until the current was 0.05 C, and then discharged at a constant current of 0.2 C to 2.8 V under constant temperature conditions at 25°C. The discharge gram capacity = discharge capacity at 0.2 C / mass of positive active material.

[0082] 6) Cycle performance test:

[0083] First, the first charge and discharge were performed in an environment of 45°C. The battery was first charged at a constant current of 0.5 C to 4.35 V, then charged at a constant voltage until the current was 0.05 C, and then discharged at a constant current of 0.5 C to 2.8 V. The above charge and discharge cycle was repeated, and the discharge capacity at the 3rd cycle and the discharge capacity at the 500th cycle were recorded.

[0084] 45°C cycle capacity retention rate = (discharge capacity at the 500th cycle / discharge capacity at the 3rd cycle) x 100%.

[0085] 7) Needle penetration rate:

[0086] The lithium ion battery is charged at a rate of 0.5C to a voltage of 4.35V, and then charged at a constant voltage of 4.35V to a current of 0.05C, so that the lithium ion battery reaches a full charge state, and the appearance of the lithium ion battery before testing is recorded. The nail penetration test is performed on the battery in an environment of 25±3℃, the steel nail has a diameter of 4mm, the penetration speed is 30mm / s, the penetration position is located at the geometric center of the lithium ion battery plane, the test is performed for 3.5min or the battery surface temperature is reduced to 50℃, the test is stopped, 10 lithium ion batteries are taken as a group, the state of the lithium ion battery during the test is observed, the lithium ion battery does not burn or explode, and the test is passed, and the penetration rate = the number of passes / 10.

[0087] 8) Hot box test pass rate:

[0088] 10 lithium ion batteries are taken for the hot box test, the lithium ion battery is charged at a constant current of 0.5C to a voltage of 4.35V, and then charged at a constant voltage of 4.35V to a current of 0.05C, so that the lithium ion battery reaches a full charge state, and the full charge state lithium ion battery is placed in the experimental box, the experimental box is heated at a temperature rise rate of 5℃ / min, when the temperature in the box reaches (150±2)℃, the temperature is kept constant, and the test is stopped for 60min, the lithium ion battery does not catch fire or explode, and the test is passed. The hot box test pass rate = the number of passes / 10.

[0089] Table 1 shows the parameters and evaluation results of examples 1 to 13 and comparative examples 1 to 2.

[0090] Table 1

[0091]

[0092] By comparing examples 1 to 13 and comparative example 2, it can be seen that, compared with comparative example 2 which does not use the first positive electrode active material, the penetration pass rate and the hot box pass rate of the lithium ion battery of examples 1 to 13 are significantly improved, which is beneficial to improve the safety performance of the lithium ion battery.

[0093] By comparing examples 1 to 13 and comparative example 1, it can be seen that, compared with comparative example 1 which does not use the second positive electrode active material, the discharge capacity and the cycle capacity retention rate of the lithium ion battery of examples 1 to 13 are significantly improved.

[0094] In addition, by comparing examples 1 to 13, it can be seen that when 0.5≤S B / S A ≤0.85 or 2.3≤P2 / P1≤6.5, the cycle capacity retention rate, the penetration pass rate and the hot box pass rate of the lithium ion battery are better. In addition, when 0.75≤S2 / S1≤1.3, the cycle capacity retention rate, the penetration pass rate and the hot box pass rate of the lithium ion battery are better.

[0095] The above merely provides the preferred embodiment of the present application, and not intended 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 by, The positive electrode sheet includes a positive electrode active material layer, and the positive electrode active material layer includes 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 Ni and Co elements. The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge includes at least two peaks above 3.8 V. The relationship curve of the capacity voltage differential dQ / dV of the electrochemical device during discharging and the voltage V contains a peak A and a peak B above 3.8 V, wherein the peak position of the peak A is in the range of 3.8 V to 4.1 V, the peak position of the peak B is in the range of 4.1 V to 4.3 V, the peak area of the peak A is S A , and the peak area of the peak B is S B , and the following condition is met: 0.5 ≤ S B / S A ≤ 1.

2. The electrochemical device of claim 1, wherein 0.5 < S B / S A ≤ 0.

85.

3. The electrochemical device of claim 1, wherein At least one of the following conditions is met: (1) The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge further includes a peak C in the range of 3.5 V to 3.7 V. (2) The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device during discharge includes at least one peak above 4.1 V.

4. The electrochemical device of claim 1, wherein When the electrochemical device is in a full discharge state, the positive electrode sheet is characterized by X-ray diffraction, and has a diffraction peak 1 in the range of 43.7° to 44.1°, the peak intensity of the diffraction peak 1 is P1, and has a diffraction peak 2 in the range of 44.2° to 44.6°, the peak intensity of the diffraction peak 2 is P2, and satisfies: 1≤P2 / P1≤6.

5.

5. The electrochemical device of claim 4, wherein 2.3≤P2 / P1≤6.

5.

6. The electrochemical device of claim 1, wherein At least one of the following conditions is met: (1) The first positive electrode active material further contains M1 elements, M1 includes at least one of Na or K, based on the metal elements in the first positive electrode active material excluding Li and M1 elements, the molar percentage content of Ni elements in the first positive electrode active material is a1, the molar percentage content of Mn elements is b1, and the molar percentage content of M1 elements is c1, which satisfies: 30%≤a1≤70%, 30%≤b1≤70%, 0.1%≤c1≤15%; (2) The second positive electrode active material further contains M2 elements, M2 includes at least one of Mn or Al, based on the metal elements in the second positive electrode active material excluding Li, the molar percentage content of Ni elements in the second positive electrode active material is a2, the molar percentage content of Co elements is b2, and the molar percentage content of M2 elements is c2, which satisfies: 80%≤a2≤98%, 1%≤b2≤19%, 1%≤c2≤19%; (3) the first positive electrode active material includes Li x1 M1 m Ni y1 Mn z1 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≤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, B, 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; (4) the second positive electrode active material includes Li x2 Ni y2 Co z2 M2 p R2 q2 O 2±n2 T2 n2 , 0.6≤x2≤1.2, 0.8≤y2≤0.98, 0.01≤z2≤0.19, 0.01≤p≤0.19, 0≤q2≤0.1, 0≤n2≤0.2, wherein the element M2 includes at least one of Mn or Al, the element R2 includes at least one of 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.

7. The electrochemical device of claim 1, wherein At least one of the following conditions is met: (1) The first positive electrode active material is a secondary particle formed by 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 3 μm to 10 μm.

8. The electrochemical device of claim 1, wherein 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, which satisfies: 0.5≤S2 / S1≤1.

5.

9. The electrochemical device of claim 8, wherein, 0.75≤S2 / S1≤1.

3.

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

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