A secondary battery

Abstract

The application discloses a secondary battery and belongs to the technical field of batteries. The secondary battery can effectively improve the transmission efficiency of lithium ions by synergistically controlling the tortuosity of a positive electrode sheet and the transverse sag length of a negative electrode sheet, can reduce the impact of lithium ions on the negative electrode sheet during fast charging, can improve the fast charging capacity, and finally can exhibit relatively optimal kinetic performance and fast charging cycle performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a secondary battery. Background Technology

[0002] Ternary cathode materials in secondary batteries have high energy density and good cycle stability. However, during fast charging, they are prone to causing significant impact on the negative electrode during lithium-ion transmission, which can lead to damage to the negative electrode. At the same time, during charging and discharging, the ternary material will cause a certain strain effect when lithium-ion insertion and extraction occur, resulting in electrode size expansion / contraction and reducing lithium-ion transmission efficiency. Therefore, it is impossible to achieve good fast charging efficiency and fast charging stability. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery. By synergistically controlling the tortuosity of the positive electrode and the lateral drooping length of the negative electrode, the lithium-ion transport efficiency can be effectively improved. At the same time, the impact of lithium-ion shock on the negative electrode during fast charging can be reduced, thus improving the fast charging capability and ultimately exhibiting superior kinetic performance and fast charging cycle performance.

[0004] To achieve the above objectives, in a first aspect of this application, this application provides a secondary battery, including a positive electrode and a negative electrode, wherein the positive electrode includes a positive electrode material layer, and the positive electrode material layer includes lithium nickel cobalt manganese oxide particles;

[0005] The secondary battery satisfies: a / b = 1~300;

[0006] 'a' refers to the tortuosity of the positive electrode plate;

[0007] The value of b = (L2 / L1) - 1 is given, where L2cm is the lateral length of the negative electrode plate when it droops by 4cm, and L1cm is the lateral length of the negative electrode plate when it droops by 2cm.

[0008] The beneficial effects of this application are as follows:

[0009] This application provides a secondary battery that can effectively improve the lithium-ion transport efficiency by synergistically controlling the tortuosity of the positive electrode and the lateral droop length of the negative electrode. At the same time, it can reduce the impact of lithium-ion shock on the negative electrode during fast charging, improve fast charging capability, and ultimately exhibit superior kinetic performance and fast charging cycle performance. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the tortuosity test of the positive electrode sheet in the secondary battery described in this application.

[0011] Figure 2This is a schematic diagram of the L1 and L2 tests of the negative electrode in the secondary battery described in this application.

[0012] Figure 3 This is a schematic diagram of the L1 and L2 test specimens of the negative electrode in the secondary battery described in this application. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0014] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0015] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0016] The present application is further illustrated below with specific embodiments:

[0017] A secondary battery includes a positive electrode and a negative electrode, wherein the positive electrode includes a positive electrode material layer, and the positive electrode material layer includes lithium nickel cobalt manganese oxide particles;

[0018] The secondary battery satisfies: a / b = 1~300;

[0019] 'a' refers to the tortuosity of the positive electrode plate;

[0020] The value of b = (L2 / L1) - 1 is given, where L2cm is the lateral length of the negative electrode plate when it droops by 4cm, and L1cm is the lateral length of the negative electrode plate when it droops by 2cm.

[0021] To improve the cycle performance of ternary secondary batteries under fast charging conditions while maintaining their kinetic performance and ensuring fast charging speed, this application proposes a method that coordinates the tortuosity of the positive electrode and the lateral length of the negative electrode. When the tortuosity of the positive electrode is large, although the kinetic performance is better, the impact of lithium ions on the negative electrode is greater during conduction between the positive and negative electrodes. Therefore, it is necessary to adapt the lateral length of the negative electrode, i.e., the flexibility of the negative electrode. Similarly, if the tortuosity is small, the negative electrode... By synergistically regulating the flexibility of the electrodes, high-level dynamic performance can be ensured. Through the synergistic matching of the two, not only can the tortuosity of the lithium-ion transport path in the electrode be adjusted to improve the transport efficiency of lithium ions during charging and discharging, especially during fast charging, and reduce the volume effect of ternary materials during lithium-ion insertion and extraction, but the negative electrode can also be coordinated so that it has sufficient flexibility to mitigate the impact of a large number of lithium ions on the electrode end, avoiding affecting the cycle stability of the secondary battery under fast charging conditions, and improving the fast charging efficiency and fast charging stability of the secondary battery.

[0022] In some implementations, the a / b range is one or any two of the following: 1, 2, 5, 8, 10, 20, 50, 80, 100, 120, 150, 180, 200, 220, 250, 280, 300.

[0023] More preferably, a / b = 5~150.

[0024] As mentioned above, simultaneously controlling the tortuosity of the positive electrode and the flexibility of the negative electrode can ensure the lithium-ion transport efficiency of the secondary battery during charging and discharging, and also avoid the impact of ion impact on the overall integrity of the secondary battery electrode structure. When the a / b ratio is preferably in the range of 5 to 155, the secondary battery has more ideal dynamic performance and better cycle stability, especially in the cycle stage under fast charging conditions, with a higher fast charging cycle capacity retention rate.

[0025] In some implementations, b = 0.01~1.3.

[0026] More preferably, b is a range of one or any two of the following: 0.01, 0.011, 0.015, 0.02, 0.03, 0.045, 0.05, 0.08, 0.10, 0.12, 0.15, 0.5, 0.8, 1, 1.3.

[0027] More preferably, b = 0.015~0.5.

[0028] In this application, the ratio of the lateral drooping length of the negative electrode sheet is related to the stiffness and toughness of the electrode sheet. As an electrode sheet structure, the negative electrode sheet needs a certain rigidity to maintain its load-bearing capacity and ensure the basic mechanical stability of the secondary battery during the cycle. At the same time, it also needs to have a certain relative flexibility to maintain the integrity of the negative electrode sheet when lithium ions impact during the charging of the secondary battery. When the ratio of the different lateral drooping lengths of the negative electrode sheet is preferably within the above range, the negative electrode sheet can be controlled to have a better balance of stiffness and toughness to cope with external stress and lithium ion impact. The secondary battery can achieve better fast charging performance, shorter fast charging time and better fast charging cycle stability.

[0029] In some implementations, L1 = 3~5cm.

[0030] In some implementations, L2 = 4~7cm.

[0031] The size of L1 and / or L2 is related to the basic flexibility of the negative electrode sheet. When both are controlled within the above-mentioned preferred range, the negative electrode sheet can effectively improve its elasticity while ensuring sufficient load-bearing capacity, thus avoiding adverse effects such as breakage and fragmentation when the negative electrode sheet faces rapid lithium ion insertion.

[0032] It should be noted that, in the present application, the L1 and L2 of the negative electrode sheet can be controlled by variables such as the current collector, the type and particle size of the negative electrode active material, the weight and amount of binder, and the areal density and compaction density of the negative electrode sheet during the preparation of the negative electrode sheet. However, it is not limited to these variables. Those skilled in the art can also control them by other means according to the actual situation.

[0033] It should be noted that the lateral lengths of the negative electrode sheet when it droops by 4cm and 2cm, as described in this application, can be tested and confirmed using, but not limited to, the following method: Disassemble the secondary battery in an empty state, soak the obtained negative electrode sheet in dimethyl carbonate (DMC) for 4 hours, then cut it into 4*25cm test samples, place them on a platform of a certain height, fix one side of the sample, and let the other side hang naturally in the air. Measure the lateral lengths (mainly the lateral length of the drooping end of the electrode sheet) when it droops by 4cm and 2cm, respectively. Specifically, as shown below... Figure 2 and 3 As shown, Figure 2 As shown, when the suspended portion of the sample hangs down naturally by 4cm, the horizontal length of the corresponding suspended portion of the sample in the horizontal direction is L2. Figure 3 As shown, after a 4cm drop, the horizontal length in the horizontal direction indicated by the red arrow is L2.

[0034] In some implementations, a = 1 to 5.

[0035] More preferably, a is a range of one or any two of the following: 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5.

[0036] More preferably, a = 1.5~3.

[0037] In a secondary battery, the tortuosity of the positive electrode is related not only to the length of the lithium-ion diffusion path but also to the stress it withstands during charging and discharging due to lithium-ion insertion / extraction. When the relative flexibility of the negative electrode is optimized, the tortuosity of the positive electrode also needs to be matched and controlled accordingly to ensure the overall stability of lithium-ion insertion / extraction in the secondary battery and improve its insertion / extraction rate. When the tortuosity of the positive electrode is preferably within the above range, the stability of the positive electrode is better, it is more matched with the negative electrode, and the lithium-ion transport path is shorter, resulting in better fast charging performance and cycle performance.

[0038] It should be noted that, in the present application, the tortuosity of the positive electrode sheet can be controlled by adjusting the size and size distribution of the positive active material particles, as well as the areal density and compaction density of the positive electrode sheet. However, it is not limited to this. Those skilled in the art can also use other means to control the tortuosity according to the actual situation.

[0039] It should be noted that the tortuosity of the positive electrode sheet described in this application can be tested and confirmed using, but is not limited to, the following method. The specific steps are as follows: disassemble the secondary battery in its empty state, then immerse the obtained positive electrode sheet in dimethyl carbonate (DMC) solution for 2 hours. Subsequently, use FIB (instrument model) to slice the electrode area. The cutting voltage is 30kV, the cutting beam current is 7nA, and the cutting range is 60μm*65μm*30μm. The imaging voltage is 5kV, and the imaging beam current is 0.8nA. The number of cross-sections depends on the z-axis cutting range and the interlayer spacing. For example, when the sample is 30μm thick and the interlayer spacing is 60nm, the number of cuts is approximately 500. The slices are then reconstructed in three dimensions, and the pores on the cross-sections are extracted. The centroid of the connecting pores in each cross-section is calculated using software. The tortuosity of adjacent surfaces is calculated by the ratio of the distance between centroids to the straight-line distance between cross-sections. Finally, the tortuosity of the entire positive electrode sheet is calculated. Figure 1 As shown, the tortuosity of the positive electrode plate is a = ∑d(i) / H, d = C. n -C n-1 d represents the path of the pore between two adjacent surfaces, and H represents the cross-sectional distance of the entire slice; the test was conducted using a dual-beam electron microscope, model: SCIOS 2 HiVac.

[0040] In some embodiments, the particle size D of the lithium nickel cobalt manganese oxide particles v50 The value is 2~20μm.

[0041] More preferably, the particle size D of the lithium nickel cobalt manganese oxide particles is... v50 The range is one or any two of the following values: 2μm, 4μm, 5μm, 8μm, 10μm, 12μm, 14μm, 15μm, 18μm, and 20μm.

[0042] In some embodiments, the SPAN value of the lithium nickel cobalt manganese oxide particles is 1.1~1.7, where SPAN = (D v90 -D v10 ) / D v50 D v90 The particle size D of lithium nickel cobalt manganese oxide particles v90 D v10 The particle size D of lithium nickel cobalt manganese oxide particles v10 .

[0043] More preferably, the SPAN value of the lithium nickel cobalt manganese oxide particles is a range of one or both of 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, and 1.7.

[0044] Particle size D of lithium nickel cobalt manganese oxide particles v50 Variations in particle size distribution uniformity will affect the degree of volume effect and the overall lithium-ion intercalation / deintercalation rate during lithium-ion insertion / extraction. When the particle size D of the lithium nickel cobalt manganese oxide particles in the positive electrode sheet... v50 When the SPAN value is preferably within the above range, the volume effect generated by the lithium nickel cobalt manganese oxide particles during lithium ion intercalation and deintercalation has a lower impact on the stability of the electrode sheet, and is more compatible with the relative flexibility of the negative electrode sheet, resulting in a faster lithium ion transport rate.

[0045] It should be noted that the D of the lithium nickel cobalt manganese oxide particles in this application is... v10 D v50 and D v90 The following test methods can be used, but are not limited to, to confirm the results. The specific steps are as follows: Disassemble the secondary battery in its empty state. Immerse the obtained positive electrode sheet in DMC for 2 hours. Then scrape off the positive electrode material layer on the electrode sheet. Measure the resulting powder using a laser particle size distribution analyzer (Mastersizer3000). Measure the particle size distribution using the laser diffraction method (specific steps refer to GB / T19077-2016). Use the particle sizes corresponding to 10%, 50%, and 90% of the volume distribution to represent DMC, respectively. v10 D v50 and D v90 .

[0046] In some embodiments, the areal density of the positive electrode sheet is 180~700 g / m³. 2 .

[0047] More preferably, the areal density of the positive electrode sheet is 180 g / m³. 2 200g / m 2 220g / m 2 250g / m 2 280g / m 2 300g / m 2 350g / m 2 400g / m 2 450g / m 2 500g / m 2 550g / m 2 600g / m 2 650g / m 2 700g / m 2 The range of one or any two of them.

[0048] In some embodiments, the compaction density of the positive electrode sheet is 3~3.8 g / cm³. 3 .

[0049] More preferably, the compaction density of the positive electrode sheet is 3 g / cm³. 3 3.1g / cm 3 3.2g / cm 3 3.3g / cm 3 3.4g / cm 3 3.5g / cm 3 3.6g / cm 3 3.7g / cm 3 3.8g / cm 3 The range of one or any two of them.

[0050] When the areal density and compaction density of the positive electrode are preferably within the above range, the volume stress generated by the lithium nickel cobalt manganese oxide particles during rapid lithium ion intercalation and deintercalation is more uniformly dispersed, which has less impact on the stability of the electrode and is more compatible with the negative electrode. Without reducing the lithium ion transport rate, the impact of lithium ion impact on the negative electrode can be further reduced.

[0051] It should be noted that the compaction density and areal density of the positive electrode sheet in this application can be confirmed by, but is not limited to, the following test methods. The specific steps are as follows: the secondary battery is disassembled, the obtained positive electrode sheet is soaked in DMC for 4 hours, dried, and then the positive electrode sheet is punched into a circular sheet of a fixed area using a punching machine. The area is denoted as S0, and the unit is mm. 2Take three circular pieces as parallel samples, and weigh the three circular pieces separately using an electronic balance. Record the mass of each circular piece as M1 (in grams). Finally, add an appropriate amount of deionized water to each of the three circular pieces, gently wipe off the coating on the circular pieces with lint-free paper to expose the copper foil, and let them stand (dry) at room temperature for 10 minutes. After the copper foil is dry, weigh the three copper foil pieces separately and record the mass as M0. Calculate the surface density using the surface density = (M1-M0) / S0.

[0052] In some embodiments, the negative electrode sheet includes a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material.

[0053] In some embodiments, the particle size D of the negative electrode active material v50 The value is 10~30μm.

[0054] In some embodiments, the areal density of the negative electrode sheet is 120~300 g / m³. 2 .

[0055] In some embodiments, the compaction density of the negative electrode sheet is 1.3~1.8 g / cm³. 3 .

[0056] By controlling the size of the negative electrode active material, the areal density and compaction density of the negative electrode sheet, the flexibility of the negative electrode sheet can be further improved, while avoiding the situation of weak structure between the electrodes and the shedding of active particles.

[0057] In some embodiments, the negative electrode active material includes at least one of graphite and silicon-carbon composite materials.

[0058] More preferably, the negative electrode active material includes graphite, and b = 0.015~0.6.

[0059] When the active material in the negative electrode is graphite, the material itself has high kinetic performance, which can effectively improve the insertion rate of lithium ions on the surface of the negative electrode, thereby improving the kinetic performance of the secondary battery. In this case, it is preferable to match a lower negative electrode flexibility, so that the impact of lithium ion impact on the overall structural stability of the negative electrode is lower, and the cycle stability of the secondary battery is better.

[0060] More preferably, the negative electrode active material includes a graphite and silicon-carbon composite material, wherein b = 0.02~0.8.

[0061] When the active material in the negative electrode contains both graphite and silicon-carbon composite materials, the flexibility of the negative electrode cannot be too high due to the volume expansion effect of the silicon-carbon composite material, so as to avoid the electrode breaking due to the expansion effect. At the same time, the kinetic performance of this material is low, so the lithium-ion transport rate is low. Therefore, when the above-mentioned flexibility range of the negative electrode is optimized, the secondary battery can ensure both better fast-charging cycle stability and kinetic performance.

[0062] In some embodiments, the negative electrode sheet further includes a current collector, which includes at least one of a copper layer and a composite layer.

[0063] More preferably, the current collector includes a composite layer, the composite layer includes a polymer layer and a conductive layer, the conductive layer is disposed on both sides of the polymer layer; and b = 0.015~0.04.

[0064] More preferably, the ratio of the thickness of the current collector to the thickness of the polymer layer in the current collector is 1:(0.1~0.8).

[0065] More preferably, the thickness of the polymer layer is 2-8 μm, and specifically it can be at least one of the following: a polyethylene terephthalate layer, a polypropylene layer, a polyamide (PA) layer, a polyterephthalate layer, a polyimide (PI) layer, a polyethylene (PE) layer, a polystyrene (PPE) layer, a polyvinyl chloride (PVC) layer, an aramid layer, an acrylonitrile-butadiene-styrene copolymer (ABS) layer, a polybutylene terephthalate (PET) layer, a poly(p-phenylene terephthalamide) (PPTA) layer, a polypropylene (PPE) layer, a polyoxymethylene (POM) layer, an epoxy resin layer, a phenolic resin layer, a polytetrafluoroethylene (PTEE) layer, a polyvinylidene fluoride (PVDF) layer, a silicone rubber layer, a polycarbonate (PC) layer, a polyvinyl alcohol (PVA) layer, a polyethylene glycol (PEG) layer, a cellulose layer, a starch layer, a protein layer, their derivatives, their crosslinks, and their copolymers.

[0066] More preferably, the thickness of the conductive layer is 1~3μm.

[0067] More preferably, the current collector comprises a copper layer, wherein b = 0.02~0.08.

[0068] For different current collector compositions, the secondary battery of this application adopts a current collector composite layer in the negative electrode sheet, which can effectively improve the overall flexibility of the electrode sheet through the intermediate polymer layer, enhance the fast charging cycle stability of the negative electrode sheet, and further improve the stability and efficiency of the negative electrode sheet when lithium ions are transported between the positive and negative electrodes.

[0069] In some embodiments, the lithium nickel cobalt manganese oxide particles comprise single-crystal particles, where b = 0.03~1.2.

[0070] It should be noted that the single crystal particle refers to a single particle with a complete crystal shape or an aggregate of less than three primary particles.

[0071] In some embodiments, the lithium nickel cobalt manganese oxide particles comprise polycrystalline particles, where b = 0.01~0.5.

[0072] It should be noted that the polycrystalline particles refer to a number of particles containing at least three or more crystal grains.

[0073] When lithium nickel cobalt manganese oxide particles contain polycrystalline particles, lithium ions travel a shorter path and travel at a faster rate. By further optimizing the flexibility of the negative electrode sheet as described above, excessive impact on the negative electrode sheet during lithium ion transport can be avoided. However, when lithium nickel cobalt manganese oxide particles contain monocrystalline particles, the lithium ion transport distance is relatively longer and the transport efficiency is lower. Therefore, by optimizing the flexibility of the negative electrode sheet as described above, the overall dynamic performance of the electrode sheet can be further reduced.

[0074] In some embodiments, the lithium nickel cobalt manganese oxide has the structural formula Li x Ni o Co p Mn q O2, where x is greater than or equal to 0.9 and less than or equal to 1.1; o is greater than or equal to 0.5 and less than 1; p is greater than 0 and less than 1; q is greater than 0 and less than 1; o+p+q=1.

[0075] More preferably, o is greater than or equal to 0.5 and less than or equal to 0.7, and the secondary battery satisfies: b = 0.02~0.5.

[0076] More preferably, o is greater than or equal to 0.7 and less than or equal to 0.95, and the secondary battery satisfies: b = 0.015~0.4.

[0077] When the nickel content in lithium nickel cobalt manganese oxide particles is relatively high, the trivalent nickel ions in the particles reduce the migration energy of lithium ions, thereby improving the activity of lithium ion insertion / extraction and enhancing the kinetic performance. This places higher demands on the flexibility of the negative electrode sheet during lithium ion insertion / extraction transport. Therefore, when the droop length ratio of the negative electrode sheet is preferably within the above range, the impact caused by lithium ion transport shock can be further reduced. When the nickel content in the particles is further increased, the migration energy of lithium ions is lower and the transport rate is faster, further improving the flexibility of the negative electrode sheet. In other words, further optimizing b within the above range can balance better kinetic performance and fast charging cycle stability of the secondary battery.

[0078] In some embodiments, the lithium nickel cobalt manganese oxide particles can be commercially available products or can be obtained by a self-made method. Specifically, the lithium nickel cobalt manganese oxide particles can be obtained by the following preparation method:

[0079] Nickel, cobalt, and manganese sources are mixed in a solvent, a precipitant is added to precipitate the reaction, the mixture is allowed to stand, filtered, washed, and dried. The resulting mixed precursor is then mixed with a lithium source and a flux, and calcined once at 400-600°C in air. Subsequently, the temperature is raised to 880-930°C for a second calcination, and the temperature is adjusted to 710-890°C for heat treatment. After cooling and crushing, the lithium nickel cobalt manganese oxide particles are obtained.

[0080] In some embodiments, the reaction time of the mixed precursor during preparation is greater than 3 hours and less than or equal to 24 hours, the temperature during the heat preservation treatment is 710~820℃, and the final lithium nickel cobalt manganese oxide particles contain polycrystalline particles.

[0081] In some embodiments, the reaction time of the mixed precursor during preparation is less than or equal to 3 hours, the temperature during the heat preservation treatment is 810~890℃, and the final lithium nickel cobalt manganese oxide particles contain single crystal particles.

[0082] In some embodiments, the nickel source includes at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate;

[0083] In some embodiments, the cobalt source used includes at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate;

[0084] In some embodiments, the manganese source includes at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate;

[0085] In some embodiments, the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, and lithium acetate.

[0086] More preferably, the lithium source is molten salt;

[0087] In some embodiments, the precipitant includes at least one of sodium hydroxide, sodium carbonate, oxalic acid, and ammonia.

[0088] In some embodiments, the solvent includes water.

[0089] In some embodiments, the precipitation reaction takes 0.5 to 24 hours.

[0090] In some embodiments, the flux comprises a compound containing at least one of the elements Sr, Na, and K. Further, the compound may be, but is not limited to, oxides and / or carbonates containing the aforementioned elements.

[0091] More preferably, the amount of flux added is 0.08 to 0.42% of the total mass of the mixed precursors during a single calcination.

[0092] In some embodiments, the heating rate during the first calcination is 1~15℃ / min, the calcination temperature is 400~600℃, and the time is 2~4h.

[0093] In some embodiments, the heating rate during the secondary calcination is 1~15℃ / min, the calcination temperature is 880~930℃, and the time is 3~10min.

[0094] In some embodiments, the heat preservation treatment lasts for 8 to 12 hours.

[0095] Sintering under the above conditions is conducive to the formation of crystal nuclei in lithium nickel cobalt manganese oxide particles. Those skilled in the art can adjust the particle size using the above sintering conditions and parameters, or other methods, without making specific limitations.

[0096] In some embodiments, the crushing is carried out using an air jet mill, with the air jet mill having an induced draft frequency of 10~50Hz.

[0097] In some embodiments, the lithium nickel cobalt manganese oxide particles may further contain doping elements, including at least one of Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Al, W, Sr, V, Y, Mg, F, Cl, and S.

[0098] More preferably, the content of the dopant element is 100~10000ppm.

[0099] In some embodiments, the lithium nickel cobalt manganese oxide particles are provided with a coating layer.

[0100] Specifically, the coating layer may be, but is not limited to, an alumina coating layer.

[0101] In some embodiments, the positive electrode material layer in the positive electrode sheet includes a positive electrode material, a binder, and a conductive agent. The positive electrode material includes lithium nickel cobalt manganese oxide particles, and the mass percentage of the positive electrode material in the positive electrode material layer is 92-99%.

[0102] In some embodiments, the binder is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesive properties and does not significantly cause adverse chemical changes in the battery. For example, the binder includes fluorinated polyolefin binders, including but not limited to polyvinylidene fluoride (PVDF), PVDF copolymers, or their modified derivatives (e.g., modified with carboxylic acids, acrylic acid, acrylonitrile, etc.).

[0103] Specifically, the adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0104] In some embodiments, the mass percentage of the binder in the positive electrode material layer is 0.5% to 4.0%, such as 0.5%, 1%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, or any range formed by any two of the above values.

[0105] In some embodiments, the conductive agent is used to provide conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Exemplary examples of conductive agents in the positive electrode active material layer include, but are not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP (Super P), acetylene black, Ketjen black, etc.

[0106] In some embodiments, the mass percentage of the conductive agent in the positive electrode material layer is 0.5% to 4%, such as 0.5%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.5%, 3.0%, 4.0%, or any range formed by any two of the above values.

[0107] In some embodiments, the positive electrode sheet can be prepared by, but is not limited to, the following methods:

[0108] The positive electrode active material, conductive agent, and binder are mixed in a solvent and then stirred to prepare a slurry. The slurry is coated onto a current collector in one or two layers, dried, rolled, and cut to obtain the positive electrode sheet.

[0109] In some embodiments, the solvent includes N-methylpyrrolidone (NMP).

[0110] In the technical solution of this application, the tortuosity of the positive electrode sheet can be controlled by the rolling pressure or the size of the roller gap during the preparation of the positive electrode sheet, but it is not limited to this. Those skilled in the art can also use other methods to control the tortuosity according to the actual situation, such as by adjusting the particle size and particle distribution of the slurry before rolling, mainly the particle size and particle size distribution of lithium nickel cobalt manganese oxide particles.

[0111] In some embodiments, the secondary battery further includes an electrolyte.

[0112] In some embodiments, the electrolyte includes additives, solvents, and lithium salts.

[0113] In some embodiments, the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0114] Exemplary examples include, but are not limited to, at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); carboxylic acid ester solvents include, but are not limited to, at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; ether solvents include, at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; sulfone solvents include, at least one of methyl sulfone and dimethyl sulfoxide; nitrile solvents include, at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile; and phosphate ester solvents include, at least one of trimethyl triphosphate and triethyl phosphate.

[0115] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0116] In some embodiments, the additives include, but are not limited to, vinylene carbonate.

[0117] In some embodiments, the negative electrode sheet includes a negative electrode material layer, the negative electrode material layer including a negative electrode material, the negative electrode material including at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, elemental silicon, silicon suboxide, silicon-carbon composite material, and lithium titanate.

[0118] The negative electrode material layer may also contain conductive agents, thickeners, and binders.

[0119] The conductive agent in the negative electrode active material layer is used to provide conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. For example, the conductive agent includes, but is not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP, acetylene black, Ketjen black, etc.

[0120] In some embodiments, the mass percentage of the conductive agent in the negative electrode material layer is 0.4% to 2%, such as 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or any range formed by any two of the above values.

[0121] The thickener and binder in the negative electrode active layer are used to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesive properties and does not significantly cause adverse chemical changes in the battery. For example, the thickener and / or binder includes, but is not limited to, at least one of carboxymethyl cellulose (CMC), styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and waterborne acrylic resin.

[0122] In some embodiments, the mass percentage of the binder in the negative electrode material layer is 1.0% to 4.5%, such as 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, or any range formed by any two of the above values. The mass percentage of the thickener in the negative electrode material layer is 1.0% to 4.5%, such as 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, or any range formed by any two of the above values. In some embodiments, the negative electrode material layer in the negative electrode sheet includes a negative electrode material, a binder, and a conductive agent, and the mass percentage of the negative electrode material in the negative electrode material layer is 70% to 99%.

[0123] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention:

[0124] Example 1

[0125] A secondary battery, the preparation method comprising the following steps:

[0126] (1) Preparation of lithium nickel cobalt manganese oxide particles: Nickel sulfate, cobalt sulfate, and manganese sulfate were mixed in water according to the stoichiometric ratio, and then sodium hydroxide precipitant was added. The pH was adjusted to within the range of 12.1, and the temperature was adjusted to within the range of 55℃. After filtration, washing, and drying, the resulting precursor was mixed with LiOH-LiNO3 molten salt at a molar ratio of lithium atoms to the total atoms of nickel, cobalt, and manganese in the precursor of 1.04:1. Strontium oxide flux was added at a mass of 0.36 wt% of the total mass of the molten salt and the precursor and ball-milled. The mixture was calcined once at 500℃ for 3 h at a rate of 5℃ / min under air atmosphere, and then calcined again at 850℃ for 5 min at a rate of 5℃ / min.

[0127] The temperature was adjusted to 780℃ and kept for 10 hours. The material obtained after cooling was crushed by an air jet mill with an induced draft frequency of 30Hz. After crushing and sieving, lithium nickel cobalt manganese oxide particles were obtained.

[0128] (2) Preparation of positive electrode sheet: Lithium nickel cobalt manganese oxide particles are used as positive electrode material. Then, the positive electrode material, conductive agent acetylene black and binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 98:1:1. The slurry is prepared by vacuum stirring and then coated on the current collector aluminum foil. After drying, rolling and slitting, the positive electrode sheet is obtained.

[0129] (3) Preparation of negative electrode sheet: The negative electrode material artificial graphite, conductive agent SP, thickener sodium carboxymethyl cellulose and binder styrene-butadiene rubber are dispersed in water at a mass ratio of 96.4:1:1.2:1.4, and a slurry is prepared by vacuum stirring. The slurry is then coated on the current collector copper foil, and after drying, cold pressing and slitting, the negative electrode sheet is obtained.

[0130] (4) Preparation of electrolyte: EC (ethylene carbonate), EMC (ethyl methyl carbonate) and DEC (diethyl carbonate) are mixed in a volume ratio of 1:1:1 as solvent. Then, lithium hexafluorophosphate is added based on the total mass of the electrolyte to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L.

[0131] (5) The positive electrode, commercially available PP separator and negative electrode are stacked in sequence to form a battery cell. The battery cell is placed in the outer packaging shell, dried and injected with electrolyte. After vacuum sealing, standing, formation and volume adjustment, the lithium-ion secondary battery is obtained.

[0132] Examples 2-38, Comparative Examples 1-6

[0133] A positive electrode sheet and the lithium-ion battery prepared therefrom differ from Example 1 only in the preparation process of the positive and negative electrode sheets. The parameters and test results of the positive and negative electrode sheets and the secondary battery during preparation are shown in Tables 1-3. The test methods for the parameters of each product are as described above. In the preparation of the negative electrode sheet, the current collector is a composite current collector consisting of a metal layer and polymer layers disposed on both sides of the metal layer. When the binder content in the negative electrode sheet slurry changes, the proportions of other materials remain constant, but this is not a limitation. Those skilled in the art can adjust the individual or overall content of other materials according to actual conditions.

[0134] Table 1

[0135]

[0136]

[0137] Table 2

[0138]

[0139]

[0140] Table 3

[0141]

[0142] Example of effect

[0143] The lithium-ion batteries obtained in each embodiment and comparative example were tested as follows:

[0144] Fast charging test:

[0145] (1) When assembling the secondary battery, add a copper wire to the negative electrode side. After assembling the secondary battery, let it stand for 24 hours, then charge it to 4.1V at 0.02C, then charge it to 4.25V at a constant current of 0.33C, and then charge it to 0.05C at a constant voltage. After standing for 10 minutes, the battery was discharged at a constant current of 0.33C to 2.5V to complete formation. Then, it was charged at a constant current of 0.33C to 4.25V, followed by constant voltage charging to 0.05C. After standing for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V. This constituted one cycle of capacity determination. After two cycles of capacity determination, the copper wire was first lithium-plated at a rate of 0.02C for 4 hours. Then, it was charged at a rate of 0.33C to 10% of the capacity from the second cycle of capacity determination. Immediately afterwards, it was charged at a constant current of 4C until the negative parameter potential reached 0 or the terminal voltage reached 4.25V. Then, it was charged at a decreasing rate of 0.2C, with the cutoff conditions remaining unchanged, until the charging rate decreased to 0.4C / 0.33C, at which point the battery reached 100% of its second-cycle charging capacity. The test temperature was 25℃, and the equipment signal acquisition frequency was 100ms. After the test, the charging time t required for the secondary battery to go from 10% to 80% SOC was calculated.

[0146] (2) Charge each secondary battery at 25°C with a constant current of 0.33C to 4.25V, then charge with a constant voltage to 0.05C. After standing for 10 minutes, discharge with a constant current of 0.33C to 2.5V. This is one cycle of fixed capacity. After two cycles of fixed capacity, use the capacity of the second cycle as the fixed capacity. Charge 10% of the capacity of the second cycle of fixed capacity at a rate of 0.33C. Then charge with a constant current of 4C, and then charge at a rate of 0.4C in a step-down manner until the charging rate drops to 0.33C. The cutoff negative parameter potential of each rate is 0mV or 4.25V. Then charge with a constant current and constant voltage of 0.33C to 100% SOC, and then discharge with a rate of 0.33C to 2.5V. One cycle is one cycle. Cycle 300 times. Use the discharge capacity of the 300th cycle / the fixed capacity.

[0147] The test results are shown in Table 4.

[0148] Table 4

[0149]

[0150]

[0151] As can be seen from Table 4:

[0152] (1) The secondary battery described in this application uses lithium nickel cobalt manganese oxide particles as the positive electrode active material, and simultaneously matches the tortuosity of the positive electrode sheet and the flexibility of the negative electrode sheet. This not only effectively improves the fast charging rate of the secondary battery, but also avoids the impact of a large number of lithium ions on the integrity of the electrode sheet when rapidly impacting it, thus improving its stability under fast charging conditions. The secondary battery has high efficiency under fast charging conditions, charging from 10% to 80% SOC in less than 12 minutes, and the capacity retention rate after 300 cycles under fast charging conditions can reach more than 91%. In contrast, the secondary batteries described in Comparative Examples 1 to 6 do not match the tortuosity of the positive electrode sheet and the flexibility of the negative electrode sheet in the a / b range of 1 to 300 of the solution in this application. Therefore, it is difficult to balance fast charging efficiency and fast charging stability when conducting fast charging tests.

[0153] (2) The ratio of the tortuosity of the positive electrode sheet and the flexibility of the negative electrode sheet in the secondary battery can ensure the lithium-ion transport efficiency of the secondary battery during charging and discharging, and can also avoid the impact of ion shock on the overall integrity of the secondary battery electrode structure. As mentioned above, if the ratio is too high or too low, the fast charging performance of the secondary battery will be greatly reduced. When the a / b ratio is further optimized to be controlled within the range of 5 to 150, the secondary battery can have more ideal dynamic performance and fast charging cycle capacity retention rate. On the other hand, the flexibility of the negative electrode sheet is not necessarily better the greater it is. While providing a buffering effect for fast ion shock, the negative electrode sheet also needs to provide a certain rigidity to maintain the load-bearing capacity. When the flexibility of the negative electrode sheet is greater than that of the positive electrode sheet, the negative electrode sheet can be more flexible than that of the negative electrode sheet. When the toughness b is preferably in the range of 0.015 to 0.5, the negative electrode sheet can better balance rigidity and buffering effect. Similarly, the tortuosity of the positive electrode sheet also affects the fast charging performance of the secondary battery. It is related to the length of the lithium-ion diffusion path and the stress resistance during lithium-ion insertion and extraction. When the tortuosity of the positive electrode sheet is preferably in the range of 1.5 to 3, the tortuosity of the positive electrode sheet can match the negative electrode sheet to the maximum extent, ensuring the stability of lithium-ion insertion and extraction. At the same time, the lithium-ion insertion and extraction path is short, resulting in better fast charging performance. When the secondary battery is tested for fast charging, as shown in Examples 1 to 36, the shortest fast charging time can be less than 10 minutes, and the capacity retention rate after 300 fast charging cycles can reach more than 96%.

Claims

1. A secondary battery characterized by comprising: The secondary battery comprises a positive electrode tab and a negative electrode tab, the positive electrode tab comprises a positive electrode material layer, the positive electrode material layer comprises lithium nickel cobalt manganese oxide particles; The secondary battery satisfies a / b=1-300; The a is the tortuosity of the positive electrode tab; the a=1-5; The b=(L2 / L1)-1, wherein L2 cm is the transverse length of the negative electrode tab when it is sagged by 4 cm, and L1 cm is the transverse length of the negative electrode tab when it is sagged by 2 cm; the b=0.01-1.

3.

2. The secondary battery according to claim 1, wherein The a / b=5-150.

3. The secondary battery according to claim 1, wherein The lithium nickel cobalt manganese oxide particles comprise single-crystal particles, and the b=0.03-1.

2.

4. The secondary battery according to claim 1, wherein The lithium nickel cobalt manganese oxide particles comprise polycrystal particles, and the b=0.01-0.

5.

5. The secondary battery according to claim 1, wherein The L1=3-5 cm, and / or the L2=4-7 cm.

6. The secondary battery of claim 1, wherein The particle size D of the lithium nickel cobalt manganese oxide particles v50 The particle size is 2~20 μm, and / or the SPAN value of the lithium nickel cobalt manganese oxide particles is 1.1~1.7, where SPAN = (D v90 -D v10 ) / D v50 .

7. The secondary battery as described in claim 1, characterized in that, The areal density of the positive electrode tab is 180-700 g / m 2 , and / or the compaction density of the positive electrode tab is 3.2-3.8 g / cm 3 .

8. The secondary battery of claim 1, wherein The negative electrode sheet includes a negative electrode active material layer including a negative electrode active material, the particle diameter D v50 is 10 to 30 pm.

9. The secondary battery as described in claim 1, characterized in that, The areal density of the negative electrode sheet is 120 to 455 g / m 2 , and / or the compaction density of the negative electrode sheet is 1.2 to 1.8 g / cm 3 .

10. The secondary battery as described in claim 8, characterized in that, The negative electrode active material comprises at least one of graphite, silicon-carbon composite material.

11. The secondary battery as described in claim 8, characterized in that, The negative electrode active material comprises graphite, and the b=0.015-0.

6.

12. The secondary battery as described in claim 8, characterized in that, The negative electrode active material comprises graphite and silicon-carbon composite material, and the b=0.02-0.

8.

13. The secondary battery as described in claim 1, characterized in that, The negative electrode tab further comprises a current collector, the current collector comprises at least one of a copper layer and a composite layer.

14. The secondary battery as described in claim 13, characterized in that, The current collector comprises a composite layer, the composite layer comprises a polymer layer and a conductive layer, the conductive layer is arranged on both sides of the polymer layer; the b=0.015-1.

15. The secondary battery of claim 14, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The ratio of the total thickness of the current collector to the thickness of the polymer layer in the current collector is 1:(0.1-0.8).

16. The secondary battery of claim 14, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The thickness of the polymer layer is 2-8 μm, and / or the thickness of the conductive layer is 1-3 μm.

17. The secondary battery of claim 1, wherein Li x Ni o Co p Mn q O2, wherein x is greater than or equal to 0.9 and less than or equal to 1.1; o is greater than or equal to 0.5 and less than 1; p is greater than 0 and less than 1; q is greater than 0 and less than 1; o + p + q = 1.

18. The secondary battery of claim 17, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The o is greater than or equal to 0.5 and less than or equal to 0.7, and the b=0.02-1.

19. The secondary battery of claim 17, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The o is greater than 0.7 and less than 0.95, and the b=0.015-0.7.

Images