Electrochemical devices and electronic devices

Abstract

The application discloses 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, the positive electrode active material layer comprises a positive electrode active material, the positive electrode active material comprises a lithium transition metal composite oxide, the lithium transition metal composite oxide contains Ni elements, Mn elements, B elements and M elements, wherein the M elements comprise at least one of Na elements or K elements. The electrochemical device is charged and discharged at a current of 0.2 C, and a relationship curve between a capacity voltage differential dQ / dV of the electrochemical device and a voltage V has a characteristic peak above 4.1 V. The electrochemical device of the application can have high energy density and excellent cycle performance and high-temperature storage performance.

Description

TECHNICAL FIELD

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

[0002] With the increasing demand for long-lasting electronic devices, it is urgent to improve the energy density of electrochemical devices (e.g., lithium ion batteries). Increasing the operating voltage is an effective way to improve the energy density of electrochemical devices. However, increasing the operating voltage may cause oxygen release from the positive electrode material, decomposition of the electrolyte, gas generation in the battery cell, and cycle diving, etc. Therefore, it is urgent to develop an electrochemical device with high energy density, long cycle performance, and high safety. SUMMARY

[0003] Therefore, the present application provides an electrochemical device and an electronic device to improve the cycle performance and safety performance of the electrochemical device while maintaining high energy density.

[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 positive electrode active material. The positive electrode active material comprises a lithium transition metal composite oxide, and the lithium transition metal composite oxide contains Ni, Mn, B, and M elements. The M element comprises at least one of Na or K. The electrochemical device is charged and discharged at a current of 0.2C, and the relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device has a characteristic peak above 4.1V. The electrochemical device of the present application, on the one hand, since the positive electrode active material is doped with M elements, the structural stability of the positive electrode active material at high voltage can be improved. On the other hand, due to the existence of oxygen vacancies on the surface of the positive electrode active material, the surface oxygen release of the positive electrode active material is inhibited, so that the electrochemical device has reversible discharge capacity in the high voltage range, thereby greatly improving the energy density of the electrochemical device. Furthermore, the B element contained in the positive electrode active material is coated on the surface of the positive electrode active material, which can further inhibit the surface oxygen release of the positive electrode active material, reduce the direct contact between the electrolyte and the surface of the positive electrode active material, inhibit the decomposition of the electrolyte and the occurrence of side reactions, thereby improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0005] In some embodiments, the molar content of B element in the lithium transition metal composite oxide is a, the molar content of Ni element is b, the molar content of Mn element is c, and the molar content of M element is d. In some embodiments, 0.01≤a / (b+c)≤0.1. Further, in some embodiments, 0.02≤a / (b+c)≤0.07. In this way, the surface oxygen release of the positive electrode active material can be better inhibited, and the cycle performance and high-temperature storage performance of the electrochemical device can be improved.

[0006] In some embodiments, 0.01≤d / (b+c)≤0.15. Further, in some embodiments, 0.03≤d / (b+c)≤0.08. In this way, the structural stability of the positive electrode active material at high voltage can be further improved, and the impact on the capacity of the electrochemical device can be reduced.

[0007] In some embodiments, 0.3≤b / (b+c)≤0.7. In this way, the discharge capacity of the positive electrode active material can be improved, thereby improving the energy density of the electrochemical device.

[0008] In some embodiments, the electrochemical device further comprises a negative electrode tab, and when the electrochemical device is in a full discharge state, the molar content of the M element in the negative electrode tab is e, and the molar content of the Li element is f, which satisfies: 0.03≤e / f≤0.09. In the electrochemical device of the present application, the M element introduced into the positive electrode active material is dissolved out from the positive electrode during formation and subsequent charging and discharging processes, and is deposited on the negative electrode through the electrolyte, so that the M element carbonate is contained in the solid electrolyte interface (SEI) film composition on the surface of the negative electrode. The M element carbonate has high electron affinity, promotes the continuous growth of the SEI film, optimizes the film forming process of the SEI film on the surface of the negative electrode, makes the formed SEI film more dense and stable, and at the same time improves the viscoelasticity and flexibility of the SEI film, reduces the surface cracks. In addition, the M element carbonate can improve the Li + migration rate, and can also reduce the decomposition of the electrolyte on the negative electrode side during the cycle and high-temperature storage processes, thereby improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0009] In some embodiments, when the electrochemical device is in a full discharge state, the molar content of the M element in the lithium transition metal composite oxide is g, and the molar content of the Li element is h, which satisfies: 0.02≤g / h≤0.1. In this way, the electrochemical device can have both high energy density and good cycle and high-temperature storage performance.

[0010] In some embodiments, when the electrochemical device is in a full charge state, the positive electrode tab is characterized by X-ray diffraction, and has a diffraction peak A in the range of 36° to 37.5°, and the peak intensity of the diffraction peak A is I A , and has a diffraction peak B in the range of 43.5° to 45°, and the peak intensity of the diffraction peak B is I B , which satisfies: 1.7≤I A / I B ≤2. In this way, the positive electrode active material has good structural stability in the full charge state, thereby improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0011] In some embodiments, the lithium transition metal composite oxide comprises a matrix and a coating layer, the matrix comprising a Ni element, a Mn element and an M element, and the coating layer comprising a B element. The doping of the M element in the matrix improves the structural stability of the positive electrode active material at high voltage; at the same time, the coating layer containing the B element can inhibit the surface oxygen release of the positive electrode active material, reduce the direct contact of the electrolyte with the surface of the positive electrode active material, inhibit the decomposition of the electrolyte and the occurrence of side reactions, thereby improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0012] In some embodiments, the coating layer comprises a B-containing oxide.

[0013] In some embodiments, the lithium transition metal composite oxide comprises secondary particles composed of primary particles. In this way, the kinetic performance of the electrochemical device can be improved.

[0014] In some embodiments, the average diameter of the secondary particles is 6 μm to 14 μm.

[0015] In some embodiments, the interior of the secondary particles comprises pores. In this way, a buffer space can be provided for the strain during the charging and discharging process of the secondary particles, the risk of rupture of the secondary particles is reduced, and the cycle performance and high-temperature storage performance of the electrochemical device are improved.

[0016] In some embodiments, the secondary particles have cracks extending from the interior to the surface. In this way, the deintercalation of lithium ions in the interior of the secondary particles can be promoted, and the kinetic performance of the electrochemical device is improved.

[0017] In some embodiments, the lithium transition metal composite oxide has a layered crystal structure belonging to the R-3m space group.

[0018] In some embodiments, the lithium transition metal composite oxide comprises Li x1 M x2 Ni y1 Co y2 Mn y3 R y4 B y5 O z1 T z20.6≤x1≤1.2, 0.01≤x2≤0.15, 0.3≤y1≤0.7, 0≤y2≤0.3, 0.3≤y3≤0.7, 0≤y4≤0.2, 0

[0019] The second aspect of the present application further provides an electronic device comprising the above-mentioned electrochemical device. BRIEF DESCRIPTION OF DRAWINGS

[0020] 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 according to these drawings.

[0021] Figure 1 A scanning electron microscope (SEM) photo of a cross section of the positive electrode tab of Embodiment 1 of the present application;

[0022] Figure 2 An X-ray diffraction (XRD) spectrum of the positive electrode tab of Embodiment 1 of the present application in a full charge state. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present application will be described clearly and completely in the following with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, but not all the embodiments.

[0024] The first aspect of 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 positive electrode active material. The positive electrode active material comprises a lithium transition metal composite oxide containing Ni elements, Mn elements, B elements and M elements, wherein the M elements comprise at least one of Na elements or K elements. The electrochemical device is charged and discharged at a current of 0.2C, and the relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device discharged has a characteristic peak above 4.1V.

[0025] The electrochemical device of the present application, by using the above positive electrode active material in the positive electrode active material layer, on the one hand, due to the M element doping of the positive electrode active material, the structural stability of the positive electrode active material at high voltage can be improved, and at the same time, due to the existence of oxygen vacancies on the surface of the positive electrode active material, the surface oxygen release of the positive electrode active material is inhibited, so that the electrochemical device has reversible discharge capacity in the high voltage interval, thereby greatly improving the energy density of the electrochemical device. On the other hand, the B element contained in the positive electrode active material is coated on the surface of the positive electrode active material, which can further inhibit the surface oxygen release of the positive electrode active material, reduce the direct contact between the electrolyte and the surface of the positive electrode active material, and inhibit the decomposition of the electrolyte and the occurrence of side reactions, thereby improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0026] In some embodiments, in the lithium transition metal composite oxide of the present application, the molar content of B is a, the molar content of Ni is b, the molar content of Mn is c, and the molar content of M is d. In some embodiments, 0.01≤a / (b+c)≤0.1. Alternatively, a / (b+c) is 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or a range consisting of any two of the above. Further, in some embodiments, 0.02≤a / (b+c)≤0.07. In this way, the surface oxygen release of the positive electrode active material can be better inhibited, and the cycle performance and high-temperature storage performance of the electrochemical device can be improved.

[0027] In some embodiments, 0.01≤d / (b+c)≤0.15. Alternatively, d / (b+c) is 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.15, or a range consisting of any two of the above. Further, in some embodiments, 0.03≤d / (b+c)≤0.08. In this way, the structural stability of the positive electrode active material at high voltage can be further improved, and the impact on the capacity of the electrochemical device can be reduced.

[0028] In some embodiments, 0.3≤b / (b+c)≤0.7. In this way, the discharge gram capacity of the positive electrode active material can be improved, thereby improving the energy density of the electrochemical device.

[0029] In some embodiments, the electrochemical device further comprises a negative electrode tab, and when the electrochemical device is in a full discharge state, the molar content of the element M in the negative electrode tab is e, and the molar content of the element Li is f, and 0.03≤e / f≤0.09 is satisfied. In the electrochemical device of the present application, the element M introduced into the positive electrode active material is dissolved out from the positive electrode during formation and subsequent charging and discharging processes, and is deposited on the negative electrode through the electrolyte, so that the carbonate of the element M is contained in the composition of the solid electrolyte interface (SEI) film on the surface of the negative electrode. The carbonate of the element M has high electron affinity, promotes the continuous growth of the SEI film, optimizes the film formation process of the SEI film on the surface of the negative electrode, makes the formed SEI film more dense and stable, and at the same time improves the viscoelasticity and flexibility of the SEI film, and reduces surface cracks. In addition, the carbonate of the element M can improve the Li + migration rate, and can also reduce the decomposition of the electrolyte on the negative electrode side during the cycle and high-temperature storage processes, thereby improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0030] In some embodiments, when the electrochemical device is in a full discharge state, the molar content of the element M in the lithium transition metal composite oxide is g, and the molar content of the element Li is h, and 0.02≤g / h≤0.1 is satisfied. In this way, the electrochemical device can have both high energy density and good cycle and high-temperature storage performance.

[0031] In some embodiments, when the electrochemical device is in a full charge state, the positive electrode tab is characterized by X-ray diffraction, and has a diffraction peak A in the range of 36° to 37.5°, and the peak intensity of the diffraction peak A is I A , and has a diffraction peak B in the range of 43.5° to 45°, and the peak intensity of the diffraction peak B is I B , and 1.7≤I A / I B ≤2 is satisfied. In this way, the positive electrode active material has good structural stability in the full charge state, thereby improving the cycle performance and high-temperature storage performance of the electrochemical device. Optionally, I A / I B is 1.7, 1.75, 1.8, 1.85, 1.9, 1.95, 2, or a range consisting of any two of the above.

[0032] In some embodiments, the lithium transition metal composite oxide includes a matrix and a coating layer, the matrix comprising a Ni element, a Mn element and an M element, and the coating layer comprising a B element. The doping of the M element in the matrix improves the structural stability of the positive electrode active material at high voltage; at the same time, the coating layer containing the B element can inhibit the surface oxygen release of the positive electrode active material, reduce the direct contact of the electrolyte with the surface of the positive electrode active material, inhibit the decomposition of the electrolyte and the occurrence of side reactions, thereby improving the cycle performance and high-temperature storage performance of the electrochemical device. In some embodiments, the coating layer comprises a B-containing oxide.

[0033] In some embodiments, the lithium transition metal composite oxide includes secondary particles composed of primary particles. In this way, the kinetic performance of the electrochemical device can be improved.

[0034] In some embodiments, the BET specific surface area of the lithium transition metal composite oxide is greater than 1 m 2 / g.

[0035] In some embodiments, the average diameter of the secondary particles is 6 μm to 14 μm.

[0036] In some embodiments, the interior of the secondary particles includes pores. In this way, a buffer space can be provided for the strain during the charging and discharging process of the secondary particles, reducing the risk of rupture of the secondary particles, and thereby improving the cycle performance and high-temperature storage performance of the electrochemical device.

[0037] In some embodiments, the secondary particles have cracks extending from the interior to the surface. In this way, the deintercalation of lithium ions in the interior of the secondary particles can be promoted, and the kinetic performance of the electrochemical device can be improved.

[0038] In some embodiments, the lithium transition metal composite oxide has a layered crystal structure belonging to the R-3m space group. In some embodiments, the lithium transition metal composite oxide includes Li x1 M x2 Ni y1 Co y2 Mn y3 R y4 B y5 O z1 T z20.6 < x1 < 1.2, 0.01 < x2 < 0.15, 0.3 < y1 < 0.7, 0 < y2 < 0.3, 0.3 < y3 < 0.7, 0 < y4 < 0.2, 0 < y5 < 0.1, 1.8 < z1 < 2.2, 0 < z2 < 0.2, wherein element M includes at least one of Na or K, element R includes at least one of Ca, Sr, Ba, Al, Fe, Mg, Si, P, S, Ti, V, Cr, Cu, Zn, Ga, Ge, Zr, Mo, W, Y, Nb, In, Sn, Pb, Sb, Ce, La, Ta, or Hf, and element T includes at least one of F, Cl, Br, I, or N.

[0039] 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 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.

[0040] 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 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, a styrene-butadiene copolymer, a polyamide, a polyacrylonitrile, an acrylate polymer, a polyacrylic acid, a polyacrylate, carboxymethylcellulose-Na, polyvinyl acetate, polyvinylpyrrolidone, a 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 an example, and any other suitable mass ratio can be employed.

[0041] 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.

[0042] 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.

[0043] In some embodiments, the binder in the negative active material layer can include at least one of sodium carboxymethyl cellulose (CMC), polyacrylic acid, polyacrylate, polyvinylpyrrolidone, 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.

[0044] In some embodiments, the electrochemical device further includes a separator disposed between the positive electrode tab and the negative electrode tab. In some embodiments, the thickness of the separator is in a range of 3 pm to 20 pm. In some embodiments, the separator includes a base layer and a heat-resistant layer. In some embodiments, the base layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, or aramid. In some embodiments, the pores of the base layer have a diameter in a range of 0.01 pm to 1 pm.

[0045] 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, polyvinylpyrrolidone, polyvinyl ether, polytetrafluoroethylene, or polyhexafluoropropylene.

[0046] In some embodiments, the electrochemical device includes a lithium ion battery, but the present application is not limited thereto.

[0047] In some embodiments, the electrochemical device further includes an 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.

[0048] 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 carboxylic acid ester compound are methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, gamma-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.

[0049] 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 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 flashlight, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0050] Some specific examples and comparative examples are listed below to better illustrate the present application, taking lithium ion batteries as examples.

[0051] Example 1

[0052] 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, which was 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.

[0053] Preparation of the positive electrode sheet: the positive electrode active material, 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-methylpyrrolidone to prepare a positive electrode slurry with a solid content of 0.7. After being uniformly mixed by stirring, 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 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.

[0054] The preparation steps of the positive electrode active material are as follows:

[0055] 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. The reaction time was controlled to be 60 hours, the ammonia water concentration was 1 mol / L, and the pH value was 12.2. A nickel-manganese precursor TM(OH)2(TM=Ni / Mn) with an average particle size Dv50 of 11 μm was obtained.

[0056] 2) The nickel-manganese precursor and sodium carbonate were uniformly ground and mixed in a Na:(Ni+Mn) molar ratio of 1.05:1. The product one was obtained after calcination at 800°C in air for 20h, followed by crushing, sieving and demagnetization.

[0057] 3) The mixture of product one, lithium hydroxide and lithium nitrate (molar ratio 1:1) was mixed, and the mass ratio of the mixture of lithium hydroxide and lithium nitrate to 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. 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 product two.

[0058] 4) The product two was washed and soaked in deionized water, and then dried. The product three was obtained after crushing and sieving.

[0059] 5) The product three and boric acid were uniformly ground and mixed in a B:(Ni+Mn) molar ratio a / (b+c) of 0.034:1. The positive electrode active material was obtained after calcination at 200°C in air for 12h.

[0060] Preparation of the separator film: a porous polyethylene (PE) film was selected as the separator film.

[0061] Preparation of electrolyte: In an argon atmosphere glove box with water content less than 10 ppm, ethylene carbonate (EC): propylene carbonate (PC): diethyl carbonate (DEC): propyl propionate (PP): ethyl propionate (EP) were mixed uniformly according to the mass ratio of 1:1:1:1:1, and lithium hexafluorophosphate was added and mixed uniformly to prepare the electrolyte, wherein the mass percentage of lithium hexafluorophosphate in the electrolyte was 12.5%.

[0062] Preparation of lithium ion battery: the positive electrode sheet, the separator film and the negative electrode sheet were sequentially stacked in order, the separator film was between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and the electrode assembly was obtained by winding. The electrode assembly was placed in an outer packaging aluminum plastic film, water was removed at 80°C, the above electrolyte was injected and packaged, and the lithium ion battery was obtained after processes such as formation, degassing and shaping.

[0063] Examples 2 to 5

[0064] The difference from Example 1 is that the addition amount of B source in step 5) is adjusted according to a / (b+c) shown in Table 1, and the rest is the same as Example 1.

[0065] Examples 6 to 9

[0066] The difference from Example 1 is that the holding time in step 3) is adjusted to 10 hours, 8 hours, 5 hours and 4 hours respectively, so as to adjust d / (b+c) in the positive electrode active material, and the rest is the same as Example 1.

[0067] The difference from Example 1 is that the B coating treatment in step 5) is not performed.

[0068] The difference from Example 1 is that the nickel-manganese precursor and lithium carbonate in step 1) are mixed uniformly according to the molar ratio of Li:(Ni+Mn) of 1.02, calcined at 800°C in air for 20h, cooled to room temperature at a rate of 10°C / min, crushed and sieved to obtain product one; product one and boric acid are mixed uniformly according to the molar ratio of B:(Ni+Mn) a / (b+c) of 0.034:1, calcined at 200°C in air for 12h to obtain the positive electrode active material.

[0069] In addition, in the present application, the corresponding parameters are measured by the following method.

[0070] 1) Particle morphology and average diameter:

[0071] The positive electrode sheet was cut in the longitudinal direction 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 section, 50 positive electrode active material particles were randomly selected, and the average value of their longest diameters was taken as the average diameter of the positive electrode active material.

[0072] 2) Element content:

[0073] The positive electrode active material or negative electrode sheet sample was dissolved using a mixed solvent (for example, 0.4g sample was dissolved using 10ml aqua regia (nitric acid and hydrochloric acid mixed at 1:1) and 2ml HF mixed solvent), diluted to 100ml, and then the content of elements such as Li, Ni, Mn, B, Na and K in the solution was tested using an ICP analyzer.

[0074] The lithium ion battery was charged to full charge state, 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: 15-70°).

[0075] 4) Discharge gram capacity test:

[0076] The lithium ion battery was first charged at a constant current of 0.5C to 4.35V, then charged at a constant voltage until the current was 0.05C, and then discharged at a constant current of 0.2C to 2.8V under the condition of constant temperature at 25°C. The discharge gram capacity = discharge capacity at 0.2C / mass of positive electrode active material.

[0077] 5) Cycle performance test:

[0078] First, the first charge and discharge were performed in an environment of 45°C. The battery was first charged at a constant current of 0.5C to 4.35V, then charged at a constant voltage until the current was 0.05C, and then discharged at a constant current of 0.5C to 2.8V. 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.

[0079] 45°C cycle capacity retention rate = (discharge capacity at the 500th cycle / discharge capacity at the 3rd cycle) x 100%.

[0080] 6) High temperature storage performance test:

[0081] First, the lithium ion battery was charged at 25°C using a current of 0.5C first, and after charging to 4.35V, it was charged at constant voltage until the current was 0.05C. The thickness of the lithium ion battery was measured by a micrometer, and the thickness of the lithium ion battery at this time was defined as the initial thickness H0 of the lithium ion battery. Then the lithium ion battery was stored in an 85°C constant temperature box for 24h, and the thickness of the lithium ion battery was measured in the same way, and the thickness of the lithium ion battery after storage was defined as H1.

[0082] 85°C thickness expansion rate = (H1-H0) / H0x100%.

[0083] Table 1 shows the parameters and evaluation results of Examples 1 to 9 and Comparative Examples 1 to 2.

[0084] Table 1

[0085]

[0086]

[0087] By comparing Examples 1 to 5 and Comparative Example 1, it can be seen that the high-temperature cycle capacity retention rate and high-temperature storage thickness expansion rate of the lithium ion battery of Examples 1 to 5 are significantly improved compared with the positive electrode active material without B.

[0088] By comparing Examples 1 to 9 and Comparative Example 2, it can be seen that the lithium ion battery of Examples 1 to 9 has higher discharge gram capacity, while at the same time, it can have higher cycle capacity retention rate and lower high-temperature storage thickness expansion rate compared with the positive electrode active material without M. The possible reasons are as follows: on the one hand, the M element doped in the lithium layer of the positive electrode active material can greatly improve the structural stability of the positive electrode active material in the high delithiation state; on the other hand, the M element introduced into the positive electrode active material will dissolve out of the positive electrode during formation and subsequent charging and discharging, and will be deposited on the negative electrode through the electrolyte, so that the M element carbonate is contained in the composition of the solid electrolyte interface (SEI) film on the surface of the negative electrode. The M element carbonate has high electron affinity, which promotes the continuous growth of the SEI film, optimizes the film forming process of the SEI film on the surface of the negative electrode, makes the formed SEI film more dense and stable, and at the same time improves the viscoelasticity and flexibility of the SEI film, reduces the surface cracks. In addition, the M element carbonate can improve the Li + migration rate, and can also reduce the decomposition of the electrolyte on the negative electrode side during the cycle and high-temperature storage process, thereby improving the cycle performance and high-temperature storage performance of the lithium ion battery.

[0089] In addition, it can be seen from the comparison of Examples 1 to 5 that the lithium ion battery has both high discharge gram capacity and excellent high-temperature cycle performance and high-temperature storage performance when the content of element B in the positive electrode active material satisfies 0.01≤a / (b+c)≤0.1. A / I B ≤2, the lithium ion battery has high discharge gram capacity, and the high-temperature cycle performance and high-temperature storage performance are more excellent.

[0090] The above merely describes preferred embodiments of the present application, but is 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 including a positive electrode active material including a lithium transition metal composite oxide containing a Ni element, a Mn element, a B element, and an M element, wherein the M element includes at least one of a Na element or a K element. The relationship curve between the capacity voltage differential dQ / dV and the voltage V of the electrochemical device has a characteristic peak above 4.1 V when the electrochemical device is charged and discharged at a current of 0.2 C.

2. The electrochemical device of claim 1, wherein The molar content of the B element in the lithium transition metal composite oxide is a, the molar content of the Ni element is b, the molar content of the Mn element is c, and the molar content of the M element is d, and at least one of the following conditions is satisfied: (1) 0.01≤a / (b+c)≤0.1; (2) 0.01≤d / (b+c)≤0.15; (3) 0.3≤b / (b+c)≤0.

7.

3. The electrochemical device of claim 2, wherein At least one of the following conditions is satisfied: (1) 0.02≤a / (b+c)≤0.07; (2) 0.03≤d / (b+c)≤0.

08.

4. The electrochemical device of claim 1, wherein The electrochemical device satisfies at least one of the following conditions: (1) The electrochemical device further includes a negative electrode sheet, and the molar content of the M element in the negative electrode sheet is e and the molar content of the Li element is f when the electrochemical device is in a full discharge state, and 0.03≤e / f≤0.09 is satisfied; (2) The molar content of the M element in the lithium transition metal composite oxide is g and the molar content of the Li element is h when the electrochemical device is in a full discharge state, and 0.02≤g / h≤0.1 is satisfied.

5. The electrochemical device of claim 1, wherein The positive electrode sheet is characterized by X-ray diffraction when the electrochemical device is in a full charge state, having a diffraction peak A in the range of 36° to 37.5°, the peak intensity of the diffraction peak A being I A , and having a diffraction peak B in the range of 43.5° to 45°, the peak intensity of the diffraction peak B being I B , satisfying: 1.7 ≤ I A / I B ≤ 2.

6. The electrochemical device of claim 1, wherein The lithium transition metal composite oxide includes a base layer containing a Ni element, a Mn element, and an M element, and a coating layer containing a B element.

7. The electrochemical device of claim 6, wherein, The coating layer contains a B-containing oxide.

8. The electrochemical device of claim 1, wherein, The lithium transition metal composite oxide includes secondary particles composed of primary particles, and at least one of the following conditions is satisfied: (1) The average diameter of the secondary particles is 6 μm to 14 μm; (2) The inside of the secondary particles includes pores; (3) The secondary particles have a gap extending from the inside to the surface.

9. The electrochemical device of claim 1, wherein, At least one of the following conditions is satisfied: (1) The lithium transition metal composite oxide has a layered crystal structure belonging to the R-3m space group; (2) the lithium transition metal complex oxide includes Li x1 M x2 Ni y1 Co y2 Mn y3 R y4 B y5 O z1 T z2 , 0.6≤x1≤1.2, 0.01≤x2≤0.15, 0.3≤y1≤0.7, 0≤y2≤0.3, 0.3≤y3≤0.7, 0≤y4≤0.2, 0<y5≤0.1, 1.8≤z1≤2.2, 0≤z2≤0.2, wherein the element M includes at least one of Na or K, the element R includes at least one of Ca, Sr, Ba, Al, Fe, Mg, Si, P, S, Ti, V, Cr, Cu, Zn, Ga, Ge, Zr, Mo, W, Y, Nb, In, Sn, Pb, Sb, Ce, La, Ta or Hf, and the element T includes at least one of F, Cl, Br, I or N.

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

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