Secondary battery and electrochemical device

Abstract

Disclosed in the present disclosure are a secondary battery and an electrochemical device. The secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, the positive electrode active material layer comprises a positive electrode active material, and the particle size distribution curve of the positive electrode active material comprises a first peak and a second peak. The secondary battery satisfies: 0.22≤K≤2.13, and K=Dn50×(Dv90-Dv10) / Dv50. On the basis that the proportion of the area of the first peak and the proportion of the area of the second peak sum to 100%, the proportion of the area of the first peak is S1%, the proportion of the area of the second peak is S2%, R=S1 / S2, and 0.25≤R≤2.33. In the present application, the volume distribution curve of the positive electrode active material is limited to be in a bimodal form by optimizing the particle size distribution thereof; and by means of particle size grading, the compaction density is improved, and lithium-ion transport pathways are reduced, thereby improving the energy density of the secondary battery.

Description

Secondary batteries and electrochemical devices

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on July 14, 2023, with application number 202310864495.1 and application name “Secondary Batteries and Electrochemical Devices”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The embodiments of the present application relate to, but are not limited to, the field of battery technology, and specifically to a secondary battery and an electrochemical device. Background Art

[0003] Currently, the main method to increase the compaction density is to increase the primary particle size. However, due to the poor electrical conductivity of the lithium iron phosphate cathode material itself, the increase in primary particle size will lead to a longer lithium ion transmission path, deterioration of kinetic performance, and a significant decrease in gram capacity and rate performance, thus limiting the application of lithium iron phosphate cathode materials in lithium-ion secondary batteries.

[0004] Summary of the Invention

[0005] The present application provides a secondary battery and an electrochemical device to solve the problems of single particle size distribution, low compaction density and low gram capacity of lithium iron phosphate.

[0006] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0007] In a first aspect, the present application provides a secondary battery comprising: a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material, and a particle size distribution curve of the positive electrode active material comprising a first peak and a second peak;

[0008] The secondary battery satisfies: 0.22≤K≤2.13, and K=Dn50×(Dv90-Dv10) / Dv50;

[0009] Wherein, Dn50 μm is the particle size corresponding to when the cumulative number percentage of the positive electrode active material reaches 50%; Dv10 μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 10%; Dv50 μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 50%; Dv90 μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 90%;

[0010] And taking the sum of the area proportion of the first peak and the area proportion of the second peak as 100%, the area proportion of the first peak is S1%, the area proportion of the second peak is S2%, R=S1 / S2, 0.25≤R≤2.33.

[0011] Optionally, the first peak and the second peak satisfy at least one of the following characteristics: (a) 0.43≤R≤1.50;

[0012] (b) the particle size corresponding to the peak value of the first peak is D1 μm; the particle size corresponding to the peak value of the second peak is D2 μm; and the following conditions are satisfied: 0.4≤D1≤0.8, 2.0≤D2≤3.0;

[0013] (c) The height of the first peak is H1 μm; the height of the second peak is H2 μm; and the following conditions are satisfied: H1<H2.

[0014] Optionally, the Dn50 is 0.3 to 0.45.

[0015] Optionally, the Dv10 is 0.3 to 0.55.

[0016] Optionally, the Dv50 is 1.1 to 2.0.

[0017] Optionally, the Dv90 is 2.0 to 6.0.

[0018] Optionally, the Dn50 is 0.3 to 0.35.

[0019] Optionally, the Dv10 is 0.33 to 0.5.

[0020] Optionally, the Dv50 is 1.2 to 1.5.

[0021] Optionally, the Dv90 is 3.0 to 5.5.

[0022] Optionally, the secondary battery meets the following requirements:

[0023] 0.13≤Z≤13.4, and Z=K×R×PD;

[0024] Where, PD g / cm 3 is the compaction density of the positive electrode sheet.

[0025] In some embodiments, the compaction density of the positive electrode sheet is PD g / cm 3 Meets: 2.45≤PD≤2.7.

[0026] In some embodiments, the positive electrode active material includes a lithium-containing compound, the lithium-containing compound further includes an M element, and the M element includes one or more of Ti, V, Mn, Co, Ni, Zr, Nb, Mo, and Y.

[0027] In some embodiments, the mass percentage of the M element in the positive electrode active material is e%, satisfying: 0.1≤e≤0.6.

[0028] In some embodiments, the positive electrode active material includes Li a M x Fe (1-x) PO4; wherein, M is selected from one or more of Ti, V, Mn, Co, Ni, Zr, Nb, Mo, and Y, 0.8≤a≤1.2, and 0≤x<1.

[0029] In a second aspect, the present application also provides an electrochemical device comprising the secondary battery.

[0030] Compared to the prior art, the present application provides a secondary battery comprising: a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; the positive electrode sheet comprises a positive current collector and a positive active material disposed on the positive current collector; the particle size distribution curve of the positive active material comprises a first peak and a second peak; the secondary battery satisfies: 0.22≤K≤2.13, and K=Dn50×(Dv90-Dv10) / Dv50, and also satisfies 0.25≤R≤2.33. The present application optimizes the particle size distribution of the positive electrode active material, restricts the volume distribution curve to a bimodal form, and achieves an increase in compaction density and a reduction in lithium ion transmission paths by grading the particle size and strictly controlling the particle size, thereby ensuring the full performance of the secondary battery's kinetic performance and achieving an increase in the energy density of the secondary battery, making the secondary battery easy to control and suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG1 is a volume particle size distribution diagram of the positive electrode active materials of Example 1 and Comparative Example 1 provided in the examples of the present application.

[0032] Implementation Methods of the Application

[0033] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. The described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative efforts are within the scope of protection of this application.

[0034] The disclosure below provides many different embodiments or examples to realize the different structures of the present application. In order to simplify the disclosure of the present application, the components and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present application.

[0035] As a core component of new energy vehicles, lithium-ion power battery technology has become the core of the development of the new energy vehicle industry. Lithium iron phosphate materials have the advantages of wide availability, low cost, low toxicity, easy recycling, good safety performance, and long service life. It is currently one of the most promising positive electrode materials for secondary batteries. Due to the low specific capacity of lithium iron phosphate materials, the energy density of lithium iron phosphate batteries is low. In order to further improve the energy density, increasing the compaction density of lithium iron phosphate electrodes is an effective way. Currently, the main method to increase the compaction density is to increase the primary particle size. However, due to the poor electrical conductivity of lithium iron phosphate, the increase in primary particle size will lead to a longer lithium ion transmission path, deterioration of kinetic performance, and a significant decrease in specific capacity and rate performance. Therefore, the application of high-density lithium iron phosphate in lithium-ion batteries is limited.

[0036] Therefore, it is necessary to develop a lithium iron phosphate material ion battery that takes into account both high compaction density and dynamic performance to solve the above technical problems.

[0037] secondary batteries

[0038] The present application provides a secondary battery comprising: a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material, and a particle size distribution curve of the positive electrode active material comprising a first peak and a second peak;

[0039] The secondary battery satisfies: 0.22≤K≤2.13, and K=Dn50×(Dv90-Dv10) / Dv50;

[0040] Wherein, Dn50μm is the particle size corresponding to when the cumulative percentage of the positive electrode active material reaches 50%;

[0041] Dv10μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 10%;

[0042] Dv50μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 50%;

[0043] Dv90μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 90%; and taking the sum of the area proportion of the first peak and the area proportion of the second peak as 100%, the area proportion of the first peak is S1%, the area proportion of the second peak is S2%, R=S1 / S2, 0.25≤R≤2.33.

[0044] In some embodiments, due to the bimodal structure of the positive electrode active material, the positive electrode active material has large and small particle sizes coexisting, so that the positive electrode active material particles with different particle size ranges can reduce the gaps between the materials by moving with each other; among them, the small particle size particles can determine the electrical properties such as gram capacity and rate performance, while the large particle size particles determine the material compaction density. This is because the movement between the particles allows the small particles to fill the gaps between the large particles, and the particles are more densely packed, resulting in a higher compaction density. When the sum of the area proportion of the first peak and the area proportion of the second peak is 100%, and when K ranges from 0.22 to 2.13 and R ranges from 0.25 to 2.33, a positive electrode active material that takes into account both compaction density and gram capacity and rate performance can be obtained, thereby improving the performance, lifespan, and safety of the secondary battery.

[0045] In some embodiments, the particle size distribution curve is obtained by testing using a laser particle size analyzer. The particle size distribution curve uses particle size as the horizontal axis and percentage content as the vertical axis. After plotting the corresponding points according to the percentage content of each particle size, the points of each particle size percentage content are connected to form a wavy and smooth frequency curve.

[0046] In some embodiments, the value of K can be any value among 0.22, 0.24, 0.26, 0.28, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00, 1.10, 1.20, 1.30, 1.40, 1.50, 1.60, 1.70, 1.80, 1.90, 2.00, 2.05, 2.10 and 2.13, or a range value between any two values; it is worth noting that the specific numerical value of K is only given as an example, and any value within the range of 0.22 to 2.13 or a range value between any two values ​​is within the scope of protection of this application.

[0047] In some embodiments, the value of R can be any value among 0.25, 0.28, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00, 1.10, 1.20, 1.30, 1.40, 1.50, 1.60, 1.70, 1.80, 1.90, 2.00, 2.05, 2.10, 2.20, 2.30 and 2.33, or a range value between any two values; it is worth noting that the specific numerical value of R is only given by way of example, and any value within the range of 0.25 to 2.33 or a range value between any two values ​​is within the scope of protection of this application.

[0048] In some embodiments, 0.33≤R≤2.0.

[0049] In some embodiments, 0.55≤R≤1.78.

[0050] In some embodiments, 0.43≤R≤1.50. When R is within this range, the morphology and particle size distribution of the positive electrode active material are both in a relatively good state, thereby improving the overall performance of the secondary battery.

[0051] In some embodiments, the area percentage of the first peak (S1%) and the area percentage of the second peak (S2%) are determined by dividing the volume particle size distribution curve perpendicular to the horizontal axis at the lowest point between the two peaks, and taking the sum of the volume distribution percentage values ​​on both sides of the dividing point as the area ratio. The R value quantifies the ratio of large and small particles. When the R value is within this range, the compaction density of the positive electrode active material and the kinetic performance of the secondary battery are both excellent.

[0052] In some embodiments, the particle size corresponding to the peak value of the first peak is D1 μm, satisfying the following: 0.4 ≤ D1 ≤ 0.8. The peak value D1 μm of the first peak specifically represents the position of the apex of the first peak in the particle size distribution curve. When the peak value D1 μm of the first peak is within the above range, the distribution of the positive electrode active material in the positive electrode active material layer can be optimized, thereby improving the dynamic performance of the secondary battery.

[0053] In some embodiments, the particle size corresponding to the peak value of the second peak is D2μm, and satisfies the following relationship: 2.0≤D2≤3.0. The peak value D2μm of the second peak specifically indicates the position of the apex of the second peak in the particle size distribution curve. The peak value D2μm of the second peak affects the compaction density of the positive electrode active material and the kinetic performance of the secondary battery. Therefore, D2μm should be controlled within a certain range.

[0054] In some embodiments, the particle size corresponding to the peak value of the first peak is D1 μm, and the particle size corresponding to the peak value of the second peak is D2 μm, satisfying 1.5≤D2-D1≤2.4. For example, D2-D1 can be any value among 1.5, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, and 2.4, or a range between any two values.

[0055] In some embodiments, 1.6≤D2-D1≤2.37.

[0056] In some embodiments, 1.75≤D2-D1≤2.3.

[0057] When D2-D1 is within the above range, the distribution of the positive electrode active material particles on the positive electrode active material layer can be made more reasonable, the pore structure in the positive electrode active material layer can be further optimized, the lithium ion transmission path can be further shortened, the contact between the positive electrode active materials can be in a better state, the internal resistance of the secondary battery can be smaller, and the overall performance of the secondary battery can be better.

[0058] In some embodiments, the value of D1 μm is any one of 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm and 0.8 μm, or a range between any two of the values.

[0059] In some embodiments, 0.45≤D1≤0.75.

[0060] When the value of D1 μm is within the above range, the distribution of the positive electrode active material particles can be improved, and the overall performance of the secondary battery can be further improved.

[0061] In some embodiments, the value of D2μm is any one of 2.0μm, 2.1μm, 2.2μm, 2.3μm, 2.4μm, 2.5μm, 2.6μm, 2.7μm, 2.8μm, 2.9μm and 3.0μm, or a range between any two values.

[0062] In some embodiments, 2.15≤D2≤2.9.

[0063] In some embodiments, 2.25≤D2≤2.9.

[0064] When the value of D2 μm is within the above range, the distribution of the positive electrode active material particles can be improved, and the overall performance of the secondary battery can be further improved.

[0065] In some embodiments, the height of the first peak is H1 μm; the height of the second peak is H2 μm; and the relationship H1<H2 is satisfied.

[0066] In some embodiments, when the height H1 μm of the first peak is less than the height H2 μm of the second peak, it is possible to take into account both compaction density and gram capacity, rate performance and improve lithium ion transmission capacity, while improving the cycle life and energy efficiency of the secondary battery.

[0067] In some embodiments, Dn50μm is any value selected from the group consisting of 0.3μm, 0.31μm, 0.32μm, 0.33μm, 0.34μm, 0.35μm, 0.36μm, 0.37μm, 0.38μm, 0.39μm, 0.40μm, 0.41μm, 0.42μm, 0.43μm, 0.44μm, and 0.45μm, or a range between any two of these values. Dn50 reflects the particle size of small particles in the positive electrode active material, which directly affects the battery's gram capacity and rate performance. Controlling the value of Dn50μm can affect the gaps between small and large particles, thereby affecting the material's compaction density. Controlling Dn50μm within the above range can effectively improve the overall performance of the secondary battery.

[0068] In some embodiments, Dn50 μm ranges from 0.3 μm to 0.45 μm.

[0069] In some embodiments, Dn50 μm ranges from 0.31 μm to 0.42 μm.

[0070] In some embodiments, Dv10μm is any value of 0.3μm, 0.31μm, 0.32μm, 0.33μm, 0.34μm, 0.35μm, 0.36μm, 0.37μm, 0.38μm, 0.39μm, 0.40μm, 0.41μm, 0.42μm, 0.43μm, 0.44μm, 0.45μm, 0.46μm, 0.47μm, 0.48μm, 0.49μm, 0.50μm, 0.51μm, 0.52μm, 0.53μm, 0.54μm and 0.55μm, or a range value between any two values.

[0071] In some embodiments, Dv10 μm ranges from 0.31 μm to 0.5 μm.

[0072] In some embodiments, Dv10 μm ranges from 0.32 μm to 0.42 μm.

[0073] In some embodiments, Dv10 μm ranges from 0.33 μm to 0.41 μm. When the value of Dv10 μm is within the above range, the distribution of the positive electrode active material particles in the positive electrode active material layer can be further improved, and the overall performance of the secondary battery can be further improved.

[0074] In some embodiments, Dv50μm is any value selected from the group consisting of 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm, and 2.0μm, or a range between any two of these values. When Dv50μm is within the above range, the proportion of large particles in the positive electrode active particles can be controlled, the distribution of the positive electrode active material particles in the positive electrode active material layer can be optimized, and the overall performance of the secondary battery can be improved.

[0075] In some embodiments, Dv50 μm ranges from 1.2 μm to 1.8 μm.

[0076] In some embodiments, Dv50μm ranges from 1.2μm to 1.7μm. When Dv50μm is within the above range, the distribution of the positive electrode active material particles in the positive electrode active material layer can be further improved, the contact between the positive electrode active material particles can be affected, and the overall performance of the secondary battery can be further improved.

[0077] In some embodiments, Dv90 μm is any one of 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm and 6.0 μm, or a range between any two of the values.

[0078] In some embodiments, Dv90 ranges from 3.3 μm to 5.6 μm.

[0079] In some embodiments, Dv10μm, Dv50μm and Dv90μm can reflect the particle size of large particles in the positive electrode active material, and are used to directly affect the compaction density of the positive electrode active material. When Dv10μm, Dv50μm and Dv90μm are too small, the compaction density of the positive electrode active material is reduced, and the positive electrode active material layer is prone to cracking. If the compaction density is increased by external pressure, the large particles will become brittle after being subjected to force, causing the positive electrode active material to be destroyed, affecting the cycle performance of the secondary battery; when Dv10μm, Dv50μm and Dv90μm are too large, the large particles have more edges and corners, which will affect the strength of the current collector in the subsequent preparation process of the electrode, resulting in damage to the performance of the electrode, affecting the energy efficiency and cycle performance of the secondary battery.

[0080] In some embodiments, the secondary battery satisfies:

[0081] 0.13≤Z≤13.4, and Z=K×R×PD; where PD g / cm 3 is the compaction density of the positive electrode sheet.

[0082] In some embodiments, the compaction density of the positive electrode sheet is calculated by the following formula:

[0083] Positive electrode sheet compaction density = positive electrode sheet surface density / (positive electrode sheet thickness after rolling - positive electrode current collector thickness), unit is g / cm 3 .

[0084] In some embodiments, the compaction density is tested using a compaction density meter, and the testing process may refer to the national standard GB / T 24533-2019.

[0085] In some embodiments, Z is any value of 0.13, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, 1.50, 2.00, 2.50, 3.00, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00, 10.00, 11.00, 12.00, 13.00, 13.10, 13.20, 13.30, and 13.40, or a range between any two of the values. By further limiting the range of the Z value, and on the premise of regulating the compaction density by optimizing the particle size distribution and optimizing the ratio of large and small particles, the compaction density range of the positive electrode plate is further controlled so that the positive electrode active material with this particle size distribution has an optimized large and small particle size gradation and an appropriate large particle primary particle size, thereby achieving a high compaction effect. The secondary battery prepared from this plate has lower kinetic polarization and higher gram capacity and rate performance, thereby achieving better kinetic performance of the secondary battery.

[0086] In some embodiments, 0.53≤Z≤11.4.

[0087] In some embodiments, 0.53≤Z≤9.8.

[0088] In some embodiments, 0.65 ≤ Z ≤ 8.9. When Z is within the above range, the positive electrode active material particles in the positive electrode active material layer have a more optimal size distribution, resulting in better contact between the positive electrode active material particles, shortening the electrolyte infiltration path, reducing kinetic polarization problems, and improving the overall performance of the secondary battery.

[0089] In some embodiments, the compaction density of the positive electrode sheet is PD g / cm 3 Satisfies: 2.45≤PD≤2.7. When the compaction density is within this range, the energy density of the secondary battery is higher, and the kinetics of the positive electrode active material layer and the intensity of the side reactions can be balanced, thereby improving the life of the secondary battery.

[0090] In some embodiments, the compaction density of the positive electrode sheet is PD g / cm 3 The value can be 2.45g / cm 3 , 2.50g / cm 3 , 2.55g / cm 3, 2.60g / cm 3 , 2.65g / cm 3 and 2.70g / cm 3 Any value in , or a range of values ​​between any two values.

[0091] In some embodiments, the positive electrode active material layer can be one or more layers. Each layer in the multilayer positive electrode active material layer can contain the same or different positive electrode active materials. The positive electrode active material is any substance that can reversibly intercalate and deintercalate metal ions such as lithium ions.

[0092] The positive electrode sheet can be single-sided or double-sided. In single-sided cases, the positive electrode active material layer is located on one surface of the negative electrode current collector. In double-sided cases, the positive electrode active material layer is located on both surfaces of the positive electrode current collector. A single-sided positive electrode sheet and a double-sided positive electrode sheet region can coexist on the positive electrode sheet.

[0093] In some embodiments, the positive electrode active material comprises a doping element and / or a coating element.

[0094] In some embodiments, the positive electrode active material includes a lithium-containing compound, which further includes an M element, and the M element includes one or more of Ti, V, Mn, Co, Ni, Zr, Nb, Mo, and Y; wherein the mass percentage of the M element in the positive electrode active material is e%, and satisfies: 0.1≤e≤0.6; when the content of the M element satisfies the above range, it can broaden the one-dimensional lithium ion channel of the positive electrode active material and improve the kinetic performance of large particles.

[0095] In some embodiments, the positive active material includes at least one of lithium iron phosphate (LFP) or lithium manganese iron phosphate.

[0096] In some embodiments, the positive electrode active material includes Li a M x Fe (1-x) PO4; wherein the M element includes one or more of Ti, V, Mn, Co, Ni, Zr, Nb, Mo, and Y, 0.8≤a≤1.2, and 0≤x<1.

[0097] In some embodiments, the M element comprises Mn.

[0098] In some embodiments, the M element includes Mn, and one or more of Ti, V, Co, Ni, Zr, Nb, Mo, and Y.

[0099] In some embodiments, the present application uses a carbon composite lithium iron phosphate material, which can be prepared by a solid phase process, or by mixing materials prepared by solid phase and liquid phase methods. The carbon content of the prepared carbon composite lithium iron phosphate is between 0.8% and 2.0%.

[0100] In some embodiments, the preparation method of the positive electrode active material may include: high temperature solid phase method, carbon thermal reduction method, spray drying method, liquid phase method, sol-gel method, template method or hydrothermal synthesis method.

[0101] In some embodiments, the intermediate product obtained during the preparation of the positive electrode active material can also be crushed and sieved to obtain a positive electrode active material with an optimized particle size distribution and specific surface area. There is no particular limitation on the crushing method, and it can be selected according to actual needs, such as using a particle crusher or a jet mill. The preparation method of the positive electrode active material of the present application is not limited to the above preparation method, as long as the formed positive electrode active material has the characteristics shown in this application.

[0102] In addition, the positive electrode active material layer further includes a positive electrode conductor and a positive electrode binder.

[0103] In some embodiments, the type of positive electrode conductive agent is not limited, and any known conductive agent can be used. Examples of positive electrode conductive agents include, but are not limited to, natural graphite, artificial graphite, acetylene black, amorphous carbon such as needle coke, carbon nanotubes, graphene, and the like. The above-mentioned positive electrode conductive agents can be used alone or in any combination.

[0104] In some embodiments, the type of positive electrode binder used in the manufacture of the positive electrode active material layer is not particularly limited. In the case of a coating method, any material can be dissolved or dispersed in the liquid medium used in electrode manufacture. Examples of positive electrode binders may include, but are not limited to, one or more of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber-like polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, polybutadiene rubber, and ethylene-propylene rubber; thermoplastic elastomer-like polymers such as styrene-butadiene-styrene block copolymers or their hydrides, ethylene-propylene-diene terpolymers (EPDM), styrene-ethylene-butadiene-ethylene copolymers, styrene-isoprene-styrene block copolymers or their hydrides; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers; polymer compositions having ionic conductivity of alkali metal ions (especially lithium ions), etc. The above-mentioned positive electrode binders can be used alone or in any combination.

[0105] In some embodiments, the type of solvent used to form the positive electrode slurry is not limited, as long as it is a solvent that can dissolve or disperse the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder. Examples of solvents used to form the positive electrode slurry may include any of aqueous solvents and organic solvents. Examples of aqueous media may include, but are not limited to, water and mixed media of alcohol and water. Examples of organic media may include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide, etc.

[0106] The type of positive electrode current collector is not particularly limited and can be any known material suitable for use as a positive electrode current collector. Examples of positive electrode current collectors include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; metal composites with a carbon coating; and composites of polymers and metal layers. In some embodiments, the positive electrode current collector is a carbon-coated metal material. In some embodiments, the positive electrode current collector is aluminum.

[0107] In some embodiments, the preparation process of the positive electrode sheet may include stirring, coating, drying, cold pressing, slitting, and cutting. The preparation of the positive electrode sheet includes dispersing the positive electrode active material, positive electrode conductive agent, and positive electrode binder in N-methylpyrrolidone (NMP) in a certain proportion, coating the resulting slurry on aluminum foil, drying, and then cold pressing and slitting to obtain the positive electrode sheet.

[0108] Negative electrode

[0109] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material, and the negative electrode active material contains graphite.

[0110] The negative electrode sheet can be single-sided or double-sided. In single-sided applications, the negative electrode active material layer is located on one surface of the negative electrode current collector. In double-sided applications, the negative electrode active material layer is located on both surfaces of the negative electrode current collector. A negative electrode sheet can also have both single-sided and double-sided negative electrode sheet regions.

[0111] diaphragm

[0112] To prevent short circuits, a separator is typically placed between the positive and negative electrodes. The type of separator is not particularly limited and can be selected based on actual needs. Separators can be polypropylene, polyethylene, polyvinylidene fluoride, spandex, aramid, or a multilayer composite film modified with a coating.

[0113] electrolyte

[0114] In some embodiments, the main components of the electrolyte include a lithium salt, an organic solvent, and an additive. The types and compositions of the lithium salt and organic solvent are not particularly limited and can be selected based on actual needs. The lithium salt can include lithium hexafluorophosphate, and the solvent can include ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, or propyl propionate.

[0115] In some embodiments, the preparation of a secondary battery includes: stacking the positive electrode sheet, the separator, and the negative electrode sheet in order, so that the separator is located between the positive and negative electrode sheets to play an isolating role, and then winding them into a square bare battery cell, placing them in a battery shell, and then baking them at 65 to 120°C to remove water, injecting electrolyte, sealing, and obtaining a secondary battery after standing, hot and cold pressing, formation, aging, and capacity separation.

[0116] In some embodiments, preparing a secondary battery includes:

[0117] Mixing lithium iron phosphate, a positive electrode conductive agent, a positive electrode binder and a positive electrode solvent to prepare a positive electrode slurry, and coating the positive electrode slurry on the surface of the positive electrode current collector to obtain a positive electrode sheet;

[0118] Mixing graphite, a negative electrode dispersant, a negative electrode conductive agent, a negative electrode binder and a negative electrode solvent to prepare a negative electrode slurry, and coating the negative electrode slurry on the surface of the negative electrode current collector to obtain a negative electrode sheet;

[0119] The positive electrode sheet, negative electrode sheet, separator and other battery components prepared in this application are assembled, and after processes such as shaping, baking, packaging, liquid injection, formation, and capacity division, a secondary battery is obtained. The battery types include soft-pack, cylindrical, aluminum shell, etc. The above only takes soft-pack lithium-ion batteries as an example. This application is not limited to the application of soft-pack batteries, but also includes the application of common lithium-ion battery forms such as aluminum shell batteries and cylindrical batteries.

[0120] electrochemical devices

[0121] In some embodiments, the present application provides an electrochemical device, which includes the above-mentioned secondary battery. The electrochemical device can be used for but is not limited to backup power supplies, motors, electric vehicles, electric motorcycles, power-assisted bicycles, bicycles, power tools, large household batteries, etc.

[0122] The present invention has been tested many times, and some of the test results are now cited as reference to further describe the present embodiment in detail, and the following is a detailed description of the specific examples. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0123] Example 1

[0124] Lithium carbonate and iron phosphate are mixed in a molar ratio of Li:Fe=1.05:1, and then 1.0% by weight of iron oxide, 5% of glucose, and 0.3% of Ti-doped metal oxide are added to the mixture of lithium carbonate and iron phosphate, and pure water is added and ground. Wherein, the Dv50μm of the iron phosphate is ≤5μm, and the mixture is ground until the Dv50μm is between 100-400nm; the ground mixture is spray-dried; sintered in a nitrogen protective atmosphere at a sintering temperature of 800℃ and a holding time of 10h; after sintering, it is crushed to obtain a carbon composite lithium iron phosphate material with a large and small particle size distribution, wherein the mass percentage of carbon in the carbon composite lithium iron phosphate material is 1.35wt%, the doping element is Ti, and the mass percentage of doping is 0.3%; its volume particle size is 1.35wt%. The particle size distribution curve is shown in Figure 1, showing a bimodal shape, wherein Dn50μm is 0.35μm, Dv10μm is 0.4μm, Dv50μm is 1.22μm, and Dv90μm is 3.38μm, so K=0.85; the peak value D1μm of the first peak of the volume particle size distribution is 0.63μm, the peak value D2μm of the second peak is 2.51μm, and the area of ​​the first peak is R to the area of ​​the second peak of 46:54=0.85; the height of the first peak is smaller than the height of the second peak.

[0125] Lithium iron phosphate, conductive carbon black (SP) and PVDF are mixed in a weight ratio of 97:0.7:2.3, then added into NMP and fully mixed. After mixing evenly, they are coated on both sides on carbon-coated aluminum foil. The electrode is then dried, rolled, slit and cut to obtain the positive electrode.

[0126] Graphite, conductive carbon black (SP), CMC, and SBR are mixed in a mass ratio of 96.3:0.7:1.1:1.9, then fully mixed in water, and coated on both surfaces of the copper foil after mixing evenly. The electrode is then dried, rolled, slit, and cut to obtain a negative electrode.

[0127] A polyethylene film was used as a separator.

[0128] Vinylene carbonate, ethyl methyl carbonate, and dimethyl carbonate are mixed in a mass ratio of 1:1:1, and then lithium hexafluorophosphate is added. After mixing evenly, an additive is added. The content of lithium hexafluorophosphate is 12% and the content of the vinylene carbonate additive is 0.5% based on the mass of the electrolyte, thereby obtaining an electrolyte.

[0129] The prepared positive electrode sheet, negative electrode sheet, separator, electrolyte and other battery components are assembled, and a secondary battery is obtained through processes such as shaping, baking, packaging, liquid injection, formation, and capacity division.

[0130] Examples 2-7

[0131] The particle size of the positive electrode active material was screened to obtain carbon-containing lithium iron phosphate of different particle sizes. The compaction density was controlled by adjusting the rolling thickness of the positive electrode active material layer. Secondary batteries were prepared according to the method of Example 1. The parameters of Examples 2-7 are shown in Tables 1 and 2.

[0132] Example 8

[0133] The specific preparation is the same as that of Example 1, except that the doping element is V and the doping content is 0.25%. The parameters of Example 8 are shown in Tables 1 and 2.

[0134] Example 9

[0135] The specific preparation is the same as that of Example 1, except that the doping element is Nb and the doping content is 0.2%. The parameters of Example 9 are shown in Tables 1 and 2.

[0136] Examples 10-32

[0137] The specific preparation is the same as that of Example 1, except that the parameters of Examples 10-32 are as shown in Table 1 and Table 2 by adjusting the particle size distribution and the roller pressure.

[0138] Comparative Example 1

[0139] A conventional high-gram capacity lithium iron phosphate material was used, as shown in Figure 1. Its particle size distribution is unimodal, as shown in Table 1. Its compacted density is lower than that of the embodiments of the present invention. A secondary battery directly prepared from this conventional lithium iron phosphate material has electrical properties, as shown in Table 3.

[0140] Comparative Example 2

[0141] High-quality lithium iron phosphate material was prepared using conventional methods. No flux was added during the preparation process. A higher sintering temperature (≥880°C) was required to fuse small particles into larger particles, resulting in a lithium iron phosphate material. The particle size distribution data is shown in Table 1. This lithium iron phosphate material was directly used to prepare a secondary battery, and its electrical properties are shown in Table 3.

[0142] Comparative Example 3

[0143] High-density lithium iron phosphate (LIFP) materials were prepared using conventional methods, without the addition of flux. 0.15% Ti was added as the doping element and sintered at a relatively high temperature to yield LFP materials. The parameters are shown in Tables 1 and 2. This LFP material was then directly fabricated into secondary batteries, and its electrical properties are shown in Table 3.

[0144] Battery performance test

[0145] The battery capacity (mAh / g) is measured at a 0.1C discharge rate. Charge and discharge are performed at a constant temperature of 25°C with a voltage range from 2.0V to 3.7V and a current rate of 0.1C.

[0146] Fully charged 0.33C discharge capacity (mAh / g), charging and discharging are carried out in a constant temperature environment of 25℃, the voltage range is from 2.5V to 3.65V, and the current rate is 0.33C.

[0147] Fully charged 4C discharge capacity (mAh / g), charging and discharging in a constant temperature environment of 25℃, voltage range from 2.5V to 3.65V, current rate of 4C.

[0148] DC internal resistance: In a constant temperature environment of 25°C, use a current of 1C to adjust the battery's state of charge (SOC) to 50%. Let it stand for 30 minutes. Then use a current of 5C to discharge for 10 seconds. Record the voltage difference △V before and after discharge. DC internal resistance DCR = △V / I.

[0149] Normal temperature cycle life: At 25°C, charge the secondary battery at a constant current of 1C to a voltage of 3.65V, then charge at a constant voltage of 3.65V to a current of 0.05C, let it stand for 5 minutes, and then discharge it at a constant current of 1C to 2.5V. This is the first cycle charge / discharge process of the secondary battery. The discharge capacity at this time is recorded as the discharge capacity of the first cycle of the secondary battery. Perform a cycle test on the secondary battery according to the above process and record the discharge capacity of the battery. When the discharge capacity of the battery is 80% of the discharge capacity of the first cycle, record the number of cycles of the secondary battery.

[0150] The battery performance parameters of the examples and comparative examples are shown in Table 3.

[0151] Table 1

[0152] Table 2

[0153] Table 3

[0154] As can be seen from Tables 1, 2, and 3, the present application optimizes the particle size distribution of the positive electrode active material, limits the volume distribution curve to a bimodal form, and achieves an increase in compaction density and a reduction in lithium ion transmission path through large and small particle size grading and strict control of particle size, thereby ensuring the kinetic performance of the secondary battery and achieving an increase in the energy density of the secondary battery.

[0155] The secondary battery and electrochemical device provided by the present application are introduced in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and core idea of ​​the present application. At the same time, for those skilled in the art, according to the idea of ​​the present application, there may be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present application.

Claims

1. A secondary battery, wherein: include: A positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the positive electrode active material layer comprises a positive electrode active material, and a particle size distribution curve of the positive electrode active material comprises a first peak and a second peak; The secondary battery satisfies: 0.22≤K≤2.13, and K=Dn50×(Dv90-Dv10) / Dv50; Wherein, Dn50μm is the particle size corresponding to when the cumulative amount percentage of the positive electrode active material reaches 50%; Dv10μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 10%; Dv50μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 50%; Dv90μm is the particle size corresponding to when the cumulative volume percentage of the positive electrode active material reaches 90%; And taking the sum of the area proportion of the first peak and the area proportion of the second peak as 100%, the area proportion of the first peak is S1%, the area proportion of the second peak is S2%, R=S1 / S2, and satisfying 0.25≤R≤2.

33.

2. The secondary battery according to claim 1, wherein The first peak and the second peak satisfy: 0.43≤R≤1.

50.

3. The secondary battery according to claim 1, wherein The particle size corresponding to the peak value of the first peak is D1 μm; the particle size corresponding to the peak value of the second peak is D2 μm; and the following conditions are satisfied: 0.4≤D1≤0.8, 2.0≤D2≤3.

0.

4. The secondary battery according to claim 1, wherein The height of the first peak is H1 μm; the height of the second peak is H2 μm; and the following conditions are satisfied: H1<H2.

5. The secondary battery according to claim 1, wherein The Dn50 is 0.3 to 0.

45.

6. The secondary battery according to claim 5, wherein The Dn50 is 0.3 to 0.

35.

7. The secondary battery according to claim 1, wherein The Dv10 is 0.3 to 0.

55.

8. The secondary battery according to claim 7, wherein The Dv10 is 0.33 to 0.

5.

9. The secondary battery according to claim 1, wherein The Dv50 is 1.1 to 2.

0.

10. The secondary battery according to claim 9, wherein The Dv50 is 1.2 to 1.

5.

11. The secondary battery according to claim 1, wherein The Dv90 is 2.0 to 6.

0.

12. The secondary battery according to claim 11, wherein The Dv90 is 3.0 to 5.

5.

13. The secondary battery according to claim 1, wherein The secondary battery satisfies: 0.13≤Z≤13.4, and Z=K×R×PD; wherein PD g / cm 3 is the compaction density of the positive electrode sheet.

14. The secondary battery according to claim 13, wherein The compaction density of the positive electrode sheet is PD g / cm 3 Satisfies: 2.45≤PD≤2.

7.

15. The secondary battery according to claim 1, wherein The positive electrode active material includes a lithium-containing compound, and the lithium-containing compound further includes an M element, and the M element includes one or more of Ti, V, Mn, Co, Ni, Zr, Nb, Mo, and Y.

16. The secondary battery according to claim 15, wherein The mass percentage of the M element in the positive electrode active material is e%, and the following is satisfied: 0.1≤e≤0.

6.

17. The secondary battery according to claim 1, wherein: The positive electrode active material includes Li a M x Fe (1-x) PO4; wherein M is selected from one or more of Ti, V, Mn, Co, Ni, Zr, Nb, Mo, and Y, 0.8≤a≤1.2, and 0≤x<1.

18. An electrochemical device, wherein: The invention comprises the secondary battery according to any one of claims 1 to 17.

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