A positive electrode sheet, an electrochemical device and an electronic device comprising the positive electrode sheet

Abstract

The present invention discloses a positive electrode sheet, an electrochemical device and an electronic device comprising the positive electrode sheet, belonging to the technical field of batteries. The positive electrode sheet of the present invention includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer contains a ternary positive electrode active material; the positive electrode sheet satisfies the following relational expression: 1.2 ≤ D / (R / 5 + P / 2) ≤ 3.5; wherein, the D is the particle size change rate of the positive electrode active material before and after holding pressure for 30 s under a pressure of 35 KN, R is the roughness of the positive electrode sheet, and P is the porosity of the positive electrode sheet. By controlling the roughness of the positive electrode sheet, the porosity of the positive electrode sheet and the particle size change rate of the positive electrode active material, the positive electrode sheet of the present invention can significantly improve the low-temperature capacity retention rate and the low-temperature high-rate discharge performance of the electrochemical device after being applied to the electrochemical device. V The present invention can significantly improve the low-temperature capacity retention rate and the low-temperature high-rate discharge performance of the electrochemical device after being applied to the electrochemical device by controlling the roughness of the positive electrode sheet, the porosity of the positive electrode sheet and the particle size change rate of the positive electrode active material.

Description

[0001] This application is a divisional application of CN116885098A (with an application date of September 7, 2023, an application number of 202311148794.1, and an invention title of a positive electrode sheet, an electrochemical device, and an electronic device including the positive electrode sheet). Technical Field

[0002] The present invention relates to the field of electrochemistry technology, and specifically relates to a positive electrode sheet, an electrochemical device, and an electronic device including the positive electrode sheet. Background Art

[0003] Lithium-ion batteries have advantages such as high specific energy, light weight, long life, and no memory effect, and are widely used in various civilian electronic devices, as well as in fields such as electric vehicles, energy storage, and mobile power supplies. With the increasingly wide application range of lithium-ion batteries, there are higher requirements for the discharge performance of lithium-ion batteries in low-temperature environments.

[0004] At low temperatures, the decrease in the transmission rate of lithium ions is the main reason for the reduction in the discharge efficiency of lithium-ion batteries. The properties of the positive electrode material, negative electrode material, electrolyte, and current collector may all cause a decrease in the transmission rate of lithium ions. Among them, the influence of the positive electrode material is relatively more important. At low temperatures, the decrease in the conductivity of the electrolyte will lead to a decrease in the transmission rate of lithium ions in the positive electrode material, reducing the discharge capacity of the lithium-ion battery under low-temperature conditions. For high-nickel ternary positive electrode materials, the decrease in lithium-ion conduction ability at low temperatures is also likely to cause an increase in resistance, resulting in a decrease in the battery discharge performance, and more obvious problems will occur during high-rate discharge.

[0005] Therefore, developing low-temperature lithium-ion batteries that can be applied in the cold regions of the north or in the wild is of great significance for expanding the application fields and scope of lithium-ion batteries. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a positive electrode sheet, an electrochemical device, and an electronic device including the positive electrode sheet. After the positive electrode sheet is applied to the electrochemical device, the low-temperature cycle capacity retention rate and low-temperature high-rate discharge performance of the electrochemical device can be significantly improved.

[0007] To achieve the above purpose, in the first aspect of the present invention, the present invention provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material, and the positive electrode active material includes a ternary positive electrode material;

[0008] The particle size change rate of the positive electrode active material, the roughness of the positive electrode sheet, and the porosity of the positive electrode sheet satisfy the following relational formula:

[0009] 1.2 ≤ D / (R / 5 + P / 2) ≤ 3.5;

[0010] Wherein, D is the D V 50 particle size change rate of the positive electrode active material before and after holding pressure for 30 s under a pressure of 35 KN, in %,

[0011] R is the roughness of the positive electrode sheet, in nm,

[0012] P is the porosity of the positive electrode sheet, in %.

[0013] As a preferred embodiment of the present invention, the particle size change rate of the positive electrode active material, the roughness of the positive electrode sheet, and the porosity of the positive electrode sheet satisfy the following relational expression: 1.7 ≤ D / (R / 5 + P / 2) ≤ 2.5.

[0014] As a preferred embodiment of the present invention, the range of the particle size change rate (D) of the positive electrode active material is 60 - 90%.

[0015] As a more preferred embodiment of the present invention, the range of the particle size change rate (D) of the positive electrode active material is 65 - 80%.

[0016] As a preferred embodiment of the present invention, the range of the roughness (R) of the positive electrode sheet is 80 - 200 nm.

[0017] As a more preferred embodiment of the present invention, the range of the roughness (R) of the positive electrode sheet is 100 - 150 nm.

[0018] As a preferred embodiment of the present invention, the range of the porosity (P) of the positive electrode sheet is 15 - 35%.

[0019] As a more preferred embodiment of the present invention, the range of the porosity (P) of the positive electrode sheet is 18 - 25%.

[0020] As a preferred embodiment of the present invention, the Dv50 of the positive electrode active material is 3 - 8 μm.

[0021] As a preferred embodiment of the present invention, the chemical formula of the ternary positive electrode material is LiNi x Co y Mn (1-x-y) O2, where 0.7 ≤ x < 1, 0 < y ≤ 0.3, 0 < x + y < 1.

[0022] In the second aspect of the present invention, the present invention provides an electrochemical device comprising the above-mentioned positive electrode sheet.

[0023] In a third aspect of the present invention, the present invention provides an electronic device comprising the above-described electrochemical device.

[0024] The beneficial effects of the present invention are as follows:

[0025] The present invention provides a positive electrode sheet, an electrochemical device comprising the positive electrode sheet, and an electronic device. By controlling the roughness of the positive electrode sheet, the porosity of the positive electrode sheet, and the particle size change rate of the positive active material, after the positive electrode sheet is applied to the electrochemical device, the low-temperature capacity retention rate and the low-temperature high-rate discharge performance of the electrochemical device can be significantly improved. Detailed Embodiments

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be described clearly and completely below. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] In the present invention, among the technically characterized described in an open-ended manner, a closed technical solution composed of the listed features is included, and an open technical solution including the listed features is also included.

[0028] In the present invention, regarding numerical ranges, unless otherwise specified, the above numerical ranges are considered continuous and include the minimum and maximum values of the range, as well as each value between such minimum and maximum values. Further, when the range refers to integers, each integer between the minimum and maximum values of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0029] In the present invention, there are no particular limitations on the specific dispersion and stirring treatment methods.

[0030] The reagents or instruments used in the present invention that are not specified by the manufacturer can all be conventional products obtained through commercial purchase.

[0031] It should be noted that in the content of the present application, a lithium-ion secondary battery is used as an example of the electrochemical device to explain the present application, but the electrochemical device of the present application is not limited to lithium-ion secondary batteries.

[0032] Positive Electrode Sheet

[0033] An embodiment of the present invention provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material, and the positive electrode active material includes a ternary positive electrode material;

[0034] The particle size change rate D of the positive electrode active material, the roughness R of the positive electrode sheet, and the porosity P of the positive electrode sheet satisfy the following relational formula:

[0035] 1.2 ≤ D / (R / 5 + P / 2) ≤ 3.5;

[0036] Wherein, the D is the particle size change rate of the positive electrode active material before and after holding pressure for 30 s under a pressure of 35 KN V 50, and the unit is %,

[0037] The R is the roughness of the positive electrode sheet, and the unit is nm,

[0038] The P is the porosity of the positive electrode sheet, and the unit is %.

[0039] In the present invention, the particle size change rate (D) of the positive electrode active material is calculated by the D V 50 change situation of the positive electrode active material before and after holding pressure for 30 s under a pressure of 35 KN. In this application, the positive electrode active material includes primary particles and / or secondary particles. Among them, "primary particles" represent the primary structure of a single particle, that is, single crystal type particles; "secondary particles" represent an aggregate in which primary particles are aggregated through physical or chemical bonding between primary particles, that is, secondary structure.

[0040] The particle size change rate of the positive electrode active material is related to the particle strength of the positive electrode active material and the proportion of secondary particles (which is also equivalent to the degree of single crystallization). As the positive electrode active material is subjected to pressure, the secondary particles of the positive electrode active material may break into multiple smaller particles, or the primary particles and / or secondary particles may undergo certain deformation or fragmentation. Generally speaking, when the particle size change rate of the positive electrode active material is relatively low, it often means that the particle strength is higher, the layered structure is more stable, or the proportion of secondary particles is lower and the degree of single crystallization is higher; while a higher particle size change rate means relatively lower particle strength or relatively higher proportion of secondary particles.

[0041] The particle size change rate of the positive electrode active material should be within a suitable range. For the positive electrode active material, a higher particle size change rate often means that there are more pores and voids inside the positive electrode active material layer. These pores and voids may be generated during the rolling step in the process of preparing the positive electrode sheet. The pores and voids inside the positive electrode active material layer can be microscopic cracks, voids or cavities inside the positive electrode active material particles, or the gaps between particles. Due to the existence of pores and voids, the diffusion path of ions inside the particles is shortened, the ion intercalation / deintercalation rate is increased, and thus the high-rate discharge performance of the lithium-ion battery at low temperature is improved. In addition, during the charge and discharge process of the lithium-ion battery, the positive electrode active material will experience volume expansion and contraction. A higher particle size change rate can provide more space to accommodate the volume change, thereby reducing the risk of stress concentration and damage, and further improving the electrochemical performance of the positive electrode sheet after long-term cycling at low temperature.

[0042] The particle size change rate of the positive electrode active material should not be too high. The positive electrode active material requires appropriate particle strength to ensure good structural strength and stability. At low temperature, the materials inside the battery tend to become brittle. The positive electrode active material with too high a particle size change rate may not be able to provide sufficient mechanical support due to its low structural strength and stability, resulting in structural damage to the positive electrode sheet, and thus the electrochemical performance of the battery at low temperature deteriorates. In addition, in a low-temperature environment, the diffusion rate of ions slows down, resulting in a decrease in the rate of the electrochemical reaction of the battery. When the particle size change rate of the positive electrode active material is too high, the proportion of secondary particles may be too large, reducing the density of the active material and affecting the charge and ion transport in the positive electrode sheet, and thus reducing the capacity of the battery at low temperature. The particle size change rate of the positive electrode active material should not be too low either. A too small particle size change rate is often related to the heterogeneity of the internal structure of the particles. The bonding between transition metals and oxygen is too strong, resulting in an increase in the migration potential energy of lithium ions. Moreover, a too low particle size change rate may also mean that there are too few pores and voids inside the positive electrode active material layer, and the diffusion path of ions inside the particles is too long, which is not conducive to the high-rate discharge performance of the lithium-ion battery at low temperature. In addition, when the particle size change rate is too low, the stress generated during the repeated charge and discharge (lattice expansion / contraction) process of the positive electrode sheet is likely to be locally concentrated, which is not conducive to the long-term storage and use of the battery cell at low temperature.

[0043] The roughness and porosity of the positive electrode sheet also have a great influence on the electrochemical performance of the battery at low temperature.

[0044] The roughness of the positive electrode plate affects the contact performance between the electrode and the electrolyte. In a low-temperature environment, the rate of the electrochemical reaction is slow. Appropriate roughness means that the contact area between the surface of the positive active material layer in the positive electrode plate and the electrolyte is relatively larger, which is conducive to the transport of ions within the positive electrode plate, promotes the progress of the electrochemical reaction at low temperatures, and thus improves the low-temperature capacity retention rate of the battery. The roughness of the positive electrode plate should not be too low, otherwise it is difficult to construct a uniform and complete contact interface between the active particles and the electrolyte to ensure the normal migration of lithium ions. However, too high roughness may cause excessive accumulation of the electrolyte on the surface of the strongly oxidizing positive active material, resulting in too many side reactions of the electrolyte on the surface of the positive active material, thus reducing the capacity retention rate of the battery. Too high roughness may also lead to an uneven electric field distribution on the surface of the positive electrode plate, exacerbating the electrode polarization phenomenon. Especially at low temperatures, the degree of electrode polarization usually increases, further affecting the electrochemical performance of the battery. In addition, too high roughness may also mean that there are too many pores in the positive electrode plate, resulting in weakened electrical connectivity and increased resistance, affecting the power output ability of the battery at low temperatures.

[0045] The porosity of the positive electrode plate mainly affects the diffusion space of the electrolyte and the contact area with the positive active material. Appropriate porosity can increase the contact area between the electrolyte and the positive active material, promote ion transport and reaction occurrence, and improve the efficiency of the electrochemical reaction of the battery at low temperatures. However, too high porosity is not conducive to the improvement of the energy density of the battery cell, and there is also the problem of excessive consumption of the electrolyte. And too high porosity may lead to weakened electrical connectivity inside the positive electrode plate, resulting in a decrease in the conductivity of the positive electrode plate, an increase in resistance, and further affecting the high-rate discharge performance of the battery at low temperatures. The porosity should not be too low either. When the porosity is low, it is difficult for the electrolyte to flow inside the electrode plate, resulting in uneven wetting. The region with excessive enrichment of the electrolyte reacts excessively, and lithium precipitation is likely to occur, causing potential safety hazards; while in the region with insufficient wetting, the migration of lithium ions is blocked, and the specific capacity performance is abnormal.

[0046] For the positive electrode plate, the roughness and porosity of the positive electrode plate can be adjusted by means of adjusting the particle size and shape, coating design, particle size distribution and packing mode of the positive active material, or adjusting the coating and rolling conditions of the positive electrode paste during the preparation process of the positive electrode plate, or adjusting the addition of components such as binders and dispersants in the positive active material layer, or performing surface treatment and filling treatment on the positive electrode plate.

[0047] In addition, there is also a certain mutual influence among the particle size change rate of the positive electrode active material, the roughness of the positive electrode sheet, and the porosity of the positive electrode sheet. For example, when the porosity of the positive electrode sheet is high, the surface of the positive electrode sheet may exhibit a certain uneven structure, increasing the roughness of the positive electrode sheet; the particle size change rate of the positive electrode active material reflects the particle strength of the positive electrode active material and the proportion of secondary particles. The degree of the proportion of secondary particles affects the porosity size and roughness level, and the particle strength affects the generation of microcracks, voids or cavities inside the particles of the positive electrode active material, thereby also affecting the porosity of the positive electrode sheet.

[0048] Considering that the particle size change rate of the positive electrode active material, the roughness of the positive electrode sheet, and the porosity of the positive electrode sheet all affect the electrical performance of the battery at low temperatures to varying degrees and have a certain mutual influence, it is difficult to achieve both a high low-temperature capacity retention rate and good low-temperature high-rate discharge performance of the battery by controlling a single variable. The present invention reasonably controls the particle size change rate D of the positive electrode active material, the roughness R of the positive electrode sheet, and the porosity P of the positive electrode sheet so that they satisfy the relational expression: 1.2 ≤ D / (R / 5 + P / 2) ≤ 3.5, enabling the battery containing the positive electrode sheet to have a high capacity retention rate at low temperatures and good low-temperature high-rate discharge performance.

[0049] Exemplarily, in the present invention, the value of D / (R / 5 + P / 2) can be 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.5, 2.8, 3.0, 3.2, 3.4, 3.5, or it can also be an interval range formed by any two of the above numerical values.

[0050] In one of the embodiments, the particle size change rate D of the positive electrode active material, the roughness R of the positive electrode sheet, and the porosity P of the positive electrode sheet satisfy the following relational expression: 1.7 ≤ D / (R / 5 + P / 2) ≤ 2.5.

[0051] In one of the embodiments, the range of the particle size change rate (D) of the positive electrode active material is 60% to 90%. For example, the D is 60%, 62%, 65%, 70%, 75%, 80%, 85%, 88%, 90%.

[0052] In one of the preferred embodiments, the range of the particle size change rate (D) of the positive electrode active material is 65% to 80%.

[0053] Within the above range of the particle size change rate, the positive electrode active material has appropriate particle strength and proportion of secondary particles, which can not only ensure good structural strength and stability of the positive electrode sheet, but also enable the positive electrode sheet to have appropriate pores and voids, contributing to the improvement of the high-rate discharge performance and capacity retention rate of the battery at low temperatures.

[0054] Regarding the detection method of the particle size change rate (D), the present invention does not make any limitations. Those skilled in the art can detect the particle size change rate of the positive electrode active material according to conventional technical means. Exemplarily, D can be detected by the following method:

[0055] Disassemble the lithium-ion battery to obtain the positive electrode sheet. Immerse the positive electrode sheet in DMC (dimethyl carbonate) at room temperature for 60 minutes, take it out, and dry it at room temperature with a humidity ≤ 15%; scrape off the positive electrode active material layer on the surface of the current collector, and calcine it at 500 °C for 3 hours to remove the conductive agent, binder, surface side reaction products, and residual electrolyte. After post-treatment, obtain the positive electrode active material;

[0056] Disperse the positive electrode active material in an aqueous solution containing 3% sodium hexametaphosphate dispersant. After continuously stirring 10 circles with a glass rod, quickly pour all the samples into the sample pool of the particle size distribution analyzer for D V 50 measurement to obtain D V 50 before pressure holding, denoted as D V 50 压前 ;

[0057] Then, perform solid-liquid separation and drying on the mixture containing the positive electrode active material to obtain the positive electrode active material again. Use a powder compactor to hold the pressure on the positive electrode active material at 35 KN for 30 s; after pressure holding, take out the positive electrode active material and perform D V 50 measurement again using the particle size tester as described above to obtain D V 50 after pressure holding, denoted as D V 50 压后 ;

[0058] The particle size change rate (D) of the positive electrode active material can be calculated as follows:

[0059] D = |(D V 50 压前 - D V 50 压后 )| / D V 50 压前 × 100%;

[0060] Among them, D V 50 is the particle size corresponding to when the volume cumulative distribution percentage of the positive electrode active material reaches 50%, with the unit of μm.

[0061] The particle size change rate of the positive electrode active material is related to various factors and can be controlled by changing the precursor of the positive electrode active material, the pulverization conditions, and the heat treatment conditions during the preparation process; it is also possible to obtain positive electrode active materials with a specific particle size change rate through extensive screening.

[0062] In one embodiment, the roughness (R) of the positive electrode plate ranges from 80 to 200 nm. For example, the R can be 80 nm, 90 nm, 100 nm, 120 nm, 150 nm, 180 nm, 190 nm, 200 nm.

[0063] In one preferred embodiment, the roughness (R) of the positive electrode plate ranges from 100 to 150 nm.

[0064] When the roughness is within the above preferred range, the positive electrode plate has good contact performance with the electrolyte, the battery has a high electro-chemical reaction efficiency, and the electrode polarization phenomenon will not be aggravated. The battery has a high low-temperature capacity retention rate and low-temperature high-rate discharge performance.

[0065] Regarding the detection method of the roughness (R), the present invention does not make any limitation. Those skilled in the art can detect the roughness of the positive electrode plate according to conventional technical means. Exemplarily, R can be detected by the following method:

[0066] Disassemble the lithium-ion battery to obtain the positive electrode plate. Immerse the disassembled positive electrode plate in a solvent (DMC) to clean the residual electrolyte, change the solvent every 4 hours, and continuously clean 3 times. Take out the positive electrode plate and dry it to obtain a sample of the electrode plate.

[0067] Select a clean and flat 40×40 mm sample of the electrode plate, fix it on the sample stage of the atomic force microscope, contact the surface of the positive active material layer of the sample of the electrode plate with the atomic force microscope probe and automatically scan. After the test is completed, obtain the roughness of the sample of the electrode plate by testing the "image Ra" value in the "result" of the software; select at least three different places on the same sample of the electrode plate for parallel testing, and calculate the average value, which is the roughness of the positive electrode plate.

[0068] In one embodiment, the porosity (P) of the positive electrode plate ranges from 15 to 30%. For example, the P can be 15%, 16%, 18%, 20%, 23%, 25%, 28%, 29%, 30%.

[0069] In one preferred embodiment, the porosity (P) of the positive electrode plate ranges from 18 to 25%.

[0070] Regarding the detection method of the porosity (P), the present invention does not make any limitation. Those skilled in the art can detect the porosity of the positive electrode plate according to conventional technical means. Exemplarily, P can be detected by the following method:

[0071] Disassemble the lithium-ion battery to obtain the positive electrode sheet. Immerse the obtained positive electrode sheet in a solvent (DMC) to clean the residual electrolyte, and change the solvent every 4 hours. Continuously clean 3 times, take out the positive electrode sheet and dry it. Use a punching machine to cut it into circular pieces with a diameter of 12 mm to obtain the electrode sheet samples.

[0072] Weigh the weight of the circular electrode sheet sample with a diameter of 12 mm. Immerse the electrode sheet sample in the cetane solution for 1 h and then take it out. After blotting the solution on the surface with filter paper, weigh the electrode sheet sample again. Calculate the change rate of the mass of the electrode sheet sample before and after immersion in cetane, which is the porosity of the positive electrode sheet.

[0073] In one of the embodiments, the particle size Dv50 of the positive electrode active material is 3 - 8 μm.

[0074] In one of the preferred embodiments, the particle size Dv50 of the positive electrode active material is 5 - 7.5 μm.

[0075] When the particle size Dv50 of the positive electrode active material is within the above preferred range, the positive electrode sheet has higher capacity performance and energy density at low temperature, and better high-rate discharge ability.

[0076] In one of the embodiments, the chemical formula of the ternary positive electrode material is LiNi x Co y Mn (1-x-y) O2, where 0.7 ≤ x < 1, 0 < y ≤ 0.3, 0 < x + y < 1.

[0077] In the present invention, the preparation method of LiNi x Co y Mn (1-x-y) O2 is not limited. Those skilled in the art can prepare the positive electrode active material according to conventional technical means. Exemplarily, mix the positive electrode active material precursor and the lithium source, and perform a sintering treatment to obtain the positive electrode active material.

[0078] The positive electrode active material precursor can be one or more of oxides, hydroxides, and carbonates containing Ni, Co, and Mn in a stoichiometric ratio, for example, a hydroxide containing Ni, Co, and Mn in a stoichiometric ratio. The positive electrode active material precursor can be obtained by methods known in the art, such as by co-precipitation method, gel method, or solid-phase method.

[0079] As an example, a Ni source, a Co source, and a Mn source are dispersed in a solvent to obtain a mixed solution; in a continuous countercurrent reaction manner, the mixed solution, a strong base solution, and a complexing agent solution are simultaneously pumped into a stirred reaction kettle, the pH value of the reaction solution is controlled to be 10-13, the temperature in the reaction kettle is 25°C-90°C, and an inert gas is introduced for protection during the reaction process; after the reaction is completed, through aging, filtration, washing, and vacuum drying, a hydroxide containing Ni, Co, and Mn is obtained.

[0080] In some embodiments of the present invention, the Ni source includes at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, or nickel acetate; and / or the Co source includes at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, or cobalt acetate; and / or the Mn source includes at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, or manganese acetate; and / or the Li source includes at least one of lithium oxide (Li2O), lithium phosphate (Li3PO4), lithium dihydrogen phosphate (LiH2PO4), lithium acetate (CH3COOLi), lithium hydroxide (LiOH), lithium carbonate (Li2CO3), or lithium nitrate (LiNO3).

[0081] The precursor of the positive electrode active material and the lithium source can be mixed by a ball mill mixer or a high-speed mixer. The mixed material is added to an atmosphere sintering furnace for sintering. The sintering atmosphere is an oxygen-containing atmosphere, such as an air atmosphere or an oxygen atmosphere.

[0082] In addition, a coating process can also be performed on the positive electrode active material. Specifically, a coating material is coated on the surface of the positive electrode active material by a dry coating method (high-temperature solid-phase method), and a coating layer formed by partially or completely coating the surface of the positive electrode active material with the coating material. The coating layer contains at least one element selected from the following (hereinafter referred to as "coating element"): aluminum (Al), titanium (Ti), tungsten (W), boron (B), phosphorus (P), cobalt (Co), yttrium (Y), and silicon (Si).

[0083] In addition to the above positive electrode active material, the positive electrode active material layer may further include a conductive agent and a binder.

[0084] The conductive agent is used to provide conductivity in the electrode. Any conductive agent can be used without particular limitation as long as it has suitable electron conductivity and does not cause adverse chemical changes in the battery. Carbon materials such as carbon nanofibers and other carbon fibers, acetylene black, and Ketjen black, activated carbon, graphite, mesoporous carbon, fullerenes, and carbon nanotubes are preferred.

[0085] The binder improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Therefore, the binder suitably used in the embodiments is a fluorinated polyolefin binder, and the fluorinated polyolefin binder may include, but is not limited to, polyvinylidene fluoride (PVDF), vinylidene fluoride copolymer, or their modified (e.g., carboxylic acid, acrylic acid, acrylonitrile, etc. modified) derivatives, etc.

[0086] The present invention places no particular limitation on the positive electrode current collector, as long as it has conductivity and will not cause adverse chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, fired carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used.

[0087] In the present invention, the positive electrode sheet can be prepared according to the conventional methods in the art. For example, the positive electrode active material, the conductive agent, and the binder are dispersed in a solvent, and the solvent can be N-methylpyrrolidone (NMP) or deionized water to form a uniform positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector, and after processes such as drying and rolling, the positive electrode sheet is obtained.

[0088] Electrochemical device

[0089] An embodiment of the present invention provides an electrochemical device, comprising the above-mentioned positive electrode sheet, negative electrode sheet, and electrolyte.

[0090] The negative electrode sheet of the present invention includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, and may further include a conductive agent and / or a binder.

[0091] The present invention places no particular limitation on the negative electrode current collector, as long as it has high conductivity and will not cause adverse chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, or silver, or an aluminum-cadmium alloy can be used.

[0092] Regarding the negative electrode active material, the embodiments of the present invention do not specifically limit the type of the negative electrode active material, and it can be selected according to actual needs. As an example, the negative electrode active material can be natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiOm (0 < m < 2, such as m = 1), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 , Li-Al alloy, and metallic lithium, or one or more of them.

[0093] In the embodiments of the present invention, there are no specific restrictions on the types of conductive agents and binders in the negative electrode active material layer, and they can be selected according to actual needs. As an example, the conductive agent is one or more of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder is one or more of styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, water-based acrylic resin, and carboxymethyl cellulose. Optionally, the negative electrode active material layer may further include a thickener, such as carboxymethyl cellulose.

[0094] The electrolyte of the present invention can be various electrolytes applicable to electrochemical energy storage devices in the art. The electrolyte includes an electrolyte and a solvent, and the electrolyte usually includes a lithium salt.

[0095] Specifically, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluoro bis(oxalato)phosphate (LiDFOP), and lithium tetrafluoro bis(oxalato)phosphate (LiTFOP). The concentration of the electrolyte in the electrolyte can be 0.5 to 5 mol / L.

[0096] Specifically, the solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), and diethyl sulfone (ESE). Based on the weight of the electrolyte, the weight content of the solvent can be 70 to 98%.

[0097] In addition, the electrolyte may further include additives. Specifically, the additives may include negative electrode film-forming additives, may also include positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve the overcharge performance of the battery, additives that improve the high-temperature performance of the battery, additives that improve the low-temperature performance of the battery, and the like.

[0098] The electrochemical device may further include a separator located between the positive electrode plate and the negative electrode plate, for spacing the positive electrode plate and the negative electrode plate to prevent contact short circuit between the positive electrode plate and the negative electrode plate. The separator may be a material of various isolation membranes applicable to electrochemical energy storage devices in the art. Specifically, the separator includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fiber.

[0099] Electronic device

[0100] An embodiment of the present invention provides an electronic device including the above-mentioned electrochemical device. The electrochemical device serves as a power supply for the electronic device.

[0101] The electronic device refers to any device that can utilize electrical energy and convert it into other forms of energy such as mechanical energy, thermal energy, and light energy, such as electric motors, electrothermal machines, and electric light sources. Specifically, it may include but is not limited to mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc. Mobile devices may be mobile phones, laptops, drones, floor cleaning robots, electronic cigarettes, etc.; electric vehicles may be pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.

[0102] The present invention will be further elaborated with specific embodiments as follows:

[0103] Example 1

[0104] Example 1 provides a lithium-ion battery, and the preparation method is as follows:

[0105] (1) Preparation of the positive electrode plate

[0106] (1.1) Weigh nickel sulfate, cobalt sulfate, manganese sulfate, and lithium carbonate according to the molar ratios of Li, Ni, Co, and Mn elements in the chemical formula LiNi 0.9 Co 0.05 Mn 0.05 O2 respectively, with lithium carbonate being slightly in excess. Specifically, the molar ratio of lithium carbonate to the total molar amounts of nickel sulfate, cobalt sulfate, and manganese sulfate (Li / Me) is shown in Table 1;

[0107] (1.2) Dissolve nickel sulfate, cobalt sulfate, and manganese sulfate in deionized water respectively. Transport each metal solution through pipelines to a reaction kettle to form a mixed metal solution, and introduce nitrogen as a protective gas. Add an aqueous NaOH solution as a precipitant and ammonia water as a complexing agent to the mixed metal solution. Adjust the ammonia concentration and addition amount of the solution to control the pH of the solution (i.e., the pH of the precursor reaction). React for 10 hours, and the product is filtered and dried to obtain a ternary precursor. The pH of the precursor reaction and the drying temperature (i.e., the precursor drying temperature) are shown in Table 1;

[0108] (1.3) Mix the ternary precursor with a part of lithium carbonate (20 wt.% lithium carbonate) and carry out pre-sintering. The pre-sintering atmosphere is an oxygen-containing atmosphere (oxygen flow rate is 120 m 3 / h), and the pre-sintering time is 3 h. The pre-sintering temperature is shown in Table 1; After the pre-sintered product is cooled, it is mixed with the remaining lithium carbonate (80 wt.% lithium carbonate) and subjected to high-temperature sintering. The high-temperature sintering atmosphere is an oxygen-containing atmosphere. The control of the oxygen flow rate, the high-temperature sintering temperature, and the time are shown in Table 1;

[0109] (1.4) Grind and crush the sintered material. After sieving, obtain the ternary cathode active material;

[0110] (1.5) Mix the cathode active material, binder (polyvinylidene fluoride), and conductive agent (carbon black) according to a mass ratio of 97:1:2, add N-methylpyrrolidone (NMP), and stir under a vacuum mixer until the mixed system becomes a homogeneous and flowable cathode slurry; Uniformly coat the cathode slurry on the cathode current collector (aluminum foil); Transfer the cathode current collector coated with the cathode slurry to an oven for drying, and then through rolling and slitting, obtain the cathode plate.

[0111] (2) Preparation of the anode plate

[0112] Mix the anode active material (artificial graphite), conductive agent (CNT), thickening agent (carboxymethyl cellulose, CMC), and binder (styrene-butadiene rubber, SBR) according to a mass ratio of 96:2:1:1, and use a vacuum mixer to make an anode slurry by a wet process. Uniformly coat the anode slurry on the anode current collector (copper foil), transfer the anode current collector coated with the anode slurry to an oven for drying, and then through rolling and slitting, obtain the anode plate.

[0113] (3) Preparation of the electrolyte

[0114] Mix ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a weight ratio of 1:1 to obtain an organic solvent. Then dissolve the fully dried lithium salt LiPF6 in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0115] (4) Preparation of separator

[0116] Use a polyethylene (PE) separator coated with ceramic and polyvinylidene fluoride.

[0117] (5) Preparation of battery

[0118] Wind the above-prepared positive electrode sheet, separator, and negative electrode sheet to obtain an un-injected bare battery cell; place the bare battery cell in an outer packaging foil, inject the above-prepared electrolyte into the dried bare battery cell, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, formation, shaping, and sorting.

[0119] Examples 2 to 13 and Comparative Examples 1 to 3

[0120] Examples 2 to 13 and Comparative Examples 1 to 3 each provide a lithium-ion battery, and the preparation method is similar to that of Example 1. The difference is that when preparing the positive electrode sheet, the pH of the precursor reaction, the drying temperature of the precursor, Li / Me, the oxygen flow rate, temperature, and time of high-temperature sintering are shown in Table 1.

[0121] Examples 14, 16

[0122] Examples 14 and 16 each provide a lithium-ion battery, and the preparation method is similar to that of Example 1. The differences are as follows:

[0123] (a) In steps (1.1) and (1.2), the pH of the precursor reaction, the drying temperature of the precursor, and Li / Me are shown in Table 1;

[0124] (b) In step (1.3): Mix the ternary precursor with all of the lithium carbonate, without pre-sintering, and directly perform high-temperature sintering. The high-temperature sintering atmosphere is an oxygen-containing atmosphere, and the controlled oxygen flow rate, temperature, and time of high-temperature sintering are shown in Table 1.

[0125] Example 15

[0126] Example 15 provides a lithium-ion battery, and the preparation method is similar to that of Example 1. The differences are as follows:

[0127] (a) In steps (1.1) and (1.2), the pH of the precursor reaction, the drying temperature of the precursor, and Li / Me are shown in Table 1;

[0128] (b) In step (1.3): Mix the ternary precursor with a part of the lithium carbonate (30 wt.% of the lithium carbonate), perform pre-sintering, and the pre-sintering atmosphere is an oxygen-containing atmosphere (the oxygen flow rate is 120 m 3 / h), the pre-sintering time was 3 h, and the pre-sintering temperature is shown in Table 1; after the pre-sintered product was cooled, it was mixed with the remaining lithium carbonate (70 wt.% lithium carbonate) and then subjected to high-temperature sintering. The high-temperature sintering atmosphere was an oxygen-containing atmosphere, and the oxygen flow rate, high-temperature sintering temperature, and time are shown in Table 1.

[0129] Examples 17 - 18

[0130] Examples 17 and 18 respectively provide a lithium-ion battery, and the preparation method is similar to that of Example 1, except that:

[0131] In Example 17, according to the molar ratio of each element of Li, Ni, 0.8 Co, 0.1 Mn, 0.1 and O2 in the chemical formula, nickel sulfate, cobalt sulfate, manganese sulfate, and lithium carbonate were weighed respectively, with lithium carbonate being slightly in excess; the pH of the precursor reaction, the precursor drying temperature, Li / Me, the oxygen flow rate, temperature, and time of high-temperature sintering are shown in Table 1;

[0132] In Example 18, according to the molar ratio of each element of Li, Ni, 0.7 Co, 0.2 Mn, 0.1 and O2 in the chemical formula, nickel sulfate, cobalt sulfate, manganese sulfate, and lithium carbonate were weighed respectively, with lithium carbonate being slightly in excess; the pH of the precursor reaction, the precursor drying temperature, Li / Me, the oxygen flow rate, temperature, and time of high-temperature sintering are shown in Table 1.

[0133] In addition, in each example and comparative example of the present invention, by controlling the conditions of positive electrode slurry coating and rolling, and by using different positive electrode active materials in combination, the roughness and porosity of the positive electrode sheet were adjusted.

[0134] Table 1

[0135]

[0136]

[0137] For each example and comparative example, the roughness and porosity of the positive electrode sheet, as well as the Dv50 and particle size change rate of the positive electrode active material, were detected as shown in Table 2. The detection methods for the above items are as follows:

[0138] Dv50 and particle size change rate of the positive electrode active material:

[0139] Disassemble the lithium-ion battery to obtain the positive electrode sheet. Immerse the positive electrode sheet in DMC (dimethyl carbonate) at room temperature for 60 minutes, take it out, and dry it at room temperature with a humidity ≤ 15%; scrape off the positive electrode active material layer on the surface of the current collector, and calcine it at 500 °C for 3 hours to remove the conductive agent, binder, surface side reaction products, and residual electrolyte. After post-treatment, obtain the positive electrode active material;

[0140] Disperse the positive electrode active material in an aqueous solution containing 3% sodium hexametaphosphate dispersant. After continuously stirring with a glass rod for 10 circles, quickly pour all the samples into the sample cell of the particle size distribution analyzer for D V 50 measurement to obtain the D before pressure holding V 50, denoted as D V 50 压前 , that is, the Dv50 of the positive electrode active material;

[0141] Then, perform solid-liquid separation and drying on the mixture containing the positive electrode active material to obtain the positive electrode active material again. Use a powder compactor to hold the pressure on the positive electrode active material at 35 KN for 30 s; after pressure holding, take out the positive electrode active material and perform D V 50 measurement again according to the above method in the particle size tester to obtain the D after pressure holding V 50, denoted as D V 50 压后 ;

[0142] The particle size change rate (D) of the positive electrode active material can be calculated as follows: D = |(D V 50 压前 - D V 50 压后 )| / D V 50 压前 ×100%;

[0143] Among them, D V 50 is the particle size corresponding to when the volume cumulative distribution percentage of the positive electrode active material reaches 50%, and the unit is μm.

[0144] Roughness of the positive electrode sheet:

[0145] Disassemble the lithium-ion battery to obtain the positive electrode sheet. Immerse the obtained positive electrode sheet in a solvent (DMC) to wash the residual electrolyte, change the solvent every 4 hours, and wash continuously for 3 times. Take out the positive electrode sheet and dry it to obtain the electrode sheet sample;

[0146] Select a clean and flat 40×40mm electrode sample, fix it on the atomic force microscope sample stage, contact the surface of the electrode sample with the atomic force microscope probe and automatically scan. After the test is completed, obtain the roughness of the electrode sample by testing the "imageRa" value in the "Results" of the software; select at least three different places on the same electrode sample for parallel testing and calculate the average value, which is the roughness of the positive electrode.

[0147] Porosity of the positive electrode:

[0148] Disassemble the lithium-ion battery to obtain the positive electrode. Immerse the disassembled positive electrode in a solvent (DMC) to clean the residual electrolyte, change the solvent every 4 hours, and clean continuously for 3 times. Take out the positive electrode and dry it, and use a punching machine to cut it into a 12mm diameter round piece to obtain an electrode sample;

[0149] Weigh the weight of the 12mm diameter round electrode sample; place the electrode sample in a hexadecane solution and soak it for 1h, then take it out, dry the surface solution on the filter paper, and weigh the electrode sample again; calculate the mass change rate of the electrode sample before and after soaking in hexadecane, which is the porosity of the positive electrode.

[0150] Table 2

[0151] Dv50 (μm) D(％) R(°) P(％) D / (R / 5 + P / 2) Example 1 6.5 68.1 114 22.77 1.99 Example 2 6.9 66 135 18.21 1.83 Example 3 5.8 71.5 103 20.35 2.32 Example 4 6.1 77.8 110 19.3 2.46 Example 5 7.2 72.2 147 24.2 1.74 Example 6 4.8 62.8 113 28.5 1.70 Example 7 4.4 83.4 95 33.29 2.34 Example 8 6.8 76.7 174 16.52 1.78 Example 9 5.5 79.6 102 18.74 2.67 Example 10 7.5 65.1 145 24.94 1.57 Example 11 7.7 83 191 33.7 1.51 Example 12 7.3 62.1 172 34.53 1.20 Example 13 4.0 78 83 21 2.88 Example 14 4.1 88.9 86 17.17 3.45 Example 15 4.6 93.6 121 13.3 3.03 Example 16 7.4 86.1 230 39 1.31 Example 17 6.2 63 97 21.1 2.10 Example 18 6.4 60.5 83 18.9 2.32 Comparative Example 1 7.1 61 198 33 1.09 Comparative Example 2 5.1 89.6 81.3 15.2 3.76 Comparative Example 3 6.2 46 127 32.1 1.11

[0152] Perform performance tests on the lithium-ion batteries prepared in the above examples and comparative examples. The specific items and methods are as follows:

[0153] (1) -10°C cycle capacity retention rate:

[0154] Place the lithium-ion battery in an environment of -10°C and let it stand until the lithium-ion battery reaches a constant temperature; at -10°C, charge it at a constant current of 0.2C to 4.3V, and then charge it at a constant voltage until the current is 0.05C; discharge it at 0.33C to 2.75V, and take this capacity as the initial capacity C0; repeat this step 500 times and record the capacity of 500 cycles as C1; calculate the low-temperature cycle capacity retention rate: low-temperature cycle capacity retention rate = C1 / C0×100%.

[0155] (2) 5°C rate discharge performance:

[0156] The lithium-ion battery is placed in an environment of 5°C and left to stand until it reaches a constant temperature. At 5°C, it is charged at a constant current / constant voltage of 0.33C to 4.3V, left to stand for 10 minutes, and then discharged at a constant current of 0.33C to the cut-off voltage of 2.8V. The discharge capacity is recorded and marked as C3. At 5°C, it is charged at a constant current / constant voltage of 0.33C to 4.3V, left to stand for 10 minutes, and then discharged at a constant current of 2C to the cut-off voltage of 2.8V. The discharge capacity is recorded and marked as C4. The rate discharge capacity retention rate is calculated according to the following formula: Rate discharge capacity retention rate = C4 / C3 × 100%.

[0157] The test results are shown in Table 3.

[0158] Table 3

[0159]

[0160]

[0161] For the lithium-ion batteries prepared in the embodiments of the present invention, the capacity retention rate after 500 cycles at -10°C is ≥83%, and the capacity retention rate at a 2C rate discharge under the condition of 5°C is ≥91%. It can be seen that the electrochemical performance of the lithium-ion batteries containing the positive electrode sheets of the present invention at low temperature is significantly improved, especially having excellent low-temperature cycle capacity retention rate and low-temperature high-rate discharge performance.

[0162] From Examples 6 to 8 compared with Examples 1 to 5, and Examples 11 to 14 compared with Comparative Examples 9 to 10, it can be seen that when the particle size change rate of the positive electrode active material, the roughness and porosity of the positive electrode sheet meet the preferred ranges described in the present invention, the low-temperature cycle capacity retention rate and low-temperature high-rate discharge performance of the lithium-ion battery are relatively better.

[0163] From Examples 9 to 10 in combination with Examples 1 to 5, it can be seen that when the value of D / (R / 5 + P / 2) of the positive electrode sheet is 1.7 to 2.5, the low-temperature cycle capacity retention rate and low-temperature high-rate discharge performance of the lithium-ion battery are relatively higher.

[0164] According to the test results of Comparative Examples 1 to 3, even if the roughness, porosity of the positive electrode sheet or the particle size change rate of the positive electrode active material are respectively in the appropriate ranges, but when the value of D / (R / 5 + P / 2) of the positive electrode sheet exceeds the range of 1.2 to 3.5, the electrochemical performance of the lithium-ion battery containing the positive electrode sheet at low temperature is still poor, the low-temperature cycle capacity retention rate does not exceed 72.7%, and the low-temperature rate discharge capacity retention rate does not exceed 84.5%.

[0165] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. An electrochemical device, characterized in that, including a positive electrode plate, the positive electrode plate including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, The positive electrode active material layer contains a positive electrode active material, and the positive electrode active material includes a ternary positive electrode material. The chemical formula of the ternary positive electrode material is LiNi x Co y Mn (1-x-y) O2, where 0.7 ≤ x < 1, 0 < y ≤ 0.3, 0 < x + y < 1, the particle size change rate of the positive active material, the roughness of the positive electrode plate, and the porosity of the positive electrode plate satisfy the following relational expression: 1.2 ≤ D / (R / 5 + P / 2) ≤ 3.5; Wherein, D is the particle size change rate of the positive electrode active material before and after maintaining pressure at 35 KN for 30 s, the unit of D is %, and the range of D is 60-90%. V 50, and the range of D is 60 to 90%, wherein R is the roughness of the positive electrode plate, with the unit of nm, and the range of R is 80 - 200 nm; wherein P is the porosity of the positive electrode plate, with the unit of %, and the range of P is 15 - 35%; 2. The electrochemical device according to claim 1, wherein the particle size change rate of the positive active material, the roughness of the positive electrode plate, and the porosity of the positive electrode plate satisfy the following relational expression: 1.7 ≤ D / (R / 5 + P / 2) ≤ 2.5; 3. The electrochemical device according to claim 1, characterized in that, the range of D is 65 - 80%; 4. The electrochemical device according to claim 1, characterized in that the range of R is 100 - 150 nm; 5. The electrochemical device according to claim 1, wherein, the range of P is 18 - 25%; 6. The electrochemical device according to claim 1, characterized in that, the particle size Dv50 of the positive active material is 3 - 8 μm; 7. An electronic device, characterized in that, comprising the electrochemical device according to any one of claims 1 - 6.

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