Secondary battery

Abstract

The invention 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 cooperatively regulating and controlling the tortuosity of a positive pole piece and the transverse sagging length of a negative pole piece, and meanwhile, can reduce the impact influence of the lithium ions on the negative pole piece in the fast charging process, so that the battery performance is improved. And the rapid charging capacity is improved, and finally, relatively excellent dynamic performance and rapid charging cycle performance are shown.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery. Background Art

[0002] The ternary positive electrode material in the secondary battery has a high energy density and good cycle stability. However, during the fast charging process, it is easy to cause a large impact on the negative electrode during lithium ion transmission, which in turn causes damage to the negative electrode. At the same time, during the charging and discharging process, the ternary material will cause a certain strain effect when lithium ions are inserted and removed, causing the electrode size to expand / contract, reducing the lithium ion transmission efficiency, and therefore cannot 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 coordinating the tortuosity of the positive electrode plate and the lateral droop length of the negative electrode plate, the transmission efficiency of lithium ions can be effectively improved. At the same time, the impact of lithium ions on the negative electrode plate during fast charging can be reduced, the fast charging capability can be improved, and ultimately better dynamic performance and fast charging cycle performance can be exhibited.

[0004] To achieve the above-mentioned object, in a first aspect of the present application, the present application provides a secondary battery, comprising a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode material layer, and the positive electrode material layer comprises lithium nickel cobalt manganese oxide particles; The secondary battery satisfies: a / b=1~300; Said a is the tortuosity of the positive electrode sheet; The b=(L2 / L1)-1, wherein L2cm is the lateral length of the negative electrode when the electrode is drooped by 4cm, and L1cm is the lateral length of the negative electrode when the electrode is drooped by 2cm.

[0005] The beneficial effects of this application are: The present application provides a secondary battery, which can effectively improve the transmission efficiency of lithium ions by coordinating the tortuosity of the positive electrode plate and the lateral droop length of the negative electrode plate, while reducing the impact of lithium ions on the negative electrode plate during fast charging, improving the fast charging capability, and ultimately showing better dynamic performance and fast charging cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 Schematic diagram of the tortuosity test of the positive electrode sheet in the secondary battery described in this application.

[0007] Figure 2 Schematic diagram of L1 and L2 tests of the negative electrode sheet in the secondary battery described in this application.

[0008] Figure 3Schematic diagram of the L1 and L2 tests of the negative electrode sheets in the secondary battery described in this application. DETAILED DESCRIPTION

[0009] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0010] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0011] In this application, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0012] The present application is further described below with specific examples: A secondary battery comprises a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode material layer, and the positive electrode material layer comprises lithium nickel cobalt manganese oxide particles; The secondary battery satisfies: a / b=1~300; Said a is the tortuosity of the positive electrode sheet; The b=(L2 / L1)-1, wherein L2cm is the lateral length of the negative electrode when the electrode is drooped by 4cm, and L1cm is the lateral length of the negative electrode when the electrode is drooped by 2cm.

[0013] In order to improve the cycle performance of the ternary secondary battery under fast charging state, while taking into account its dynamic performance and ensuring the fast charging rate, in the present application scheme, the secondary battery is coordinated with the tortuosity of the positive electrode sheet and the drooping transverse length of the negative electrode sheet to regulate. When the tortuosity of the positive electrode sheet in the secondary battery is large, although the dynamic performance is better, the impact of lithium ions on the negative electrode sheet when conducting between the positive and negative electrodes is greater. Therefore, it is necessary to adapt the drooping transverse length of the negative electrode sheet, that is, the flexibility of the negative electrode sheet; similarly, if the tortuosity is small, through the negative electrode sheet Coordinated regulation of the flexibility of the pole pieces can ensure that the dynamic performance is at a high level. Through the coordinated matching of the two, not only can the transmission efficiency of lithium ions during charging and discharging, especially fast charging, be improved by regulating the tortuosity of the lithium ion transmission path in the electrode, and the volume effect of the ternary material during lithium ion deintercalation can be reduced, but also the negative pole piece can be coordinated to have sufficient flexibility to alleviate the impact of a large number of lithium ions on the pole piece end, thereby 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.

[0014] In some embodiments, a / b is a range of one or any two of 1, 2, 5, 8, 10, 20, 50, 80, 100, 120, 150, 180, 200, 220, 250, 280, 300.

[0015] More preferably, a / b=5~150.

[0016] As mentioned above, the synchronous regulation of the tortuosity of the positive electrode sheet and the flexibility of the negative electrode sheet can not only ensure the lithium ion transmission efficiency of the secondary battery during charging and discharging, but also avoid the influence of ion impact on the integrity of the overall secondary battery electrode structure. When the a / b is preferably in the range of 5~155, the secondary battery has more ideal dynamic performance, and has better cycle stability in the cycle stage, especially in the cycle stage under fast charging conditions, and the fast charging cycle capacity retention rate is higher.

[0017] In some embodiments, b=0.01~1.3.

[0018] Further preferably, b is in the range of one or any two of 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, and 1.3.

[0019] More preferably, b=0.015~0.5.

[0020] In the present application, the ratio of the lateral droop lengths of the negative electrode plate is related to the stiffness and toughness of the plate. As a plate structure, the negative electrode plate requires a certain rigidity to maintain its 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 plate when lithium ions impact during the secondary battery charging. When the ratio of the different lateral droop lengths of the negative electrode plate is preferably within the above range, the negative electrode plate can be controlled to have a better stiffness and toughness balance to cope with external stress and lithium ion impact, and the secondary battery can achieve better fast charging performance, shorter fast charging time and better fast charging cycle stability.

[0021] In some embodiments, L1=3~5cm.

[0022] In some embodiments, L2=4~7cm.

[0023] The size of L1 and / or L2 is related to the basic flexibility of the negative electrode. When both are controlled within the above-mentioned preferred range, the elasticity of the negative electrode can be effectively improved while ensuring sufficient bearing capacity, thereby avoiding adverse effects such as breakage and crushing of the negative electrode when facing rapid lithium ion embedding.

[0024] It should be noted that in the present application, L1 and L2 of the negative electrode sheet can be regulated by variables such as the current collector during the preparation of the negative electrode sheet, the type and particle size of the negative electrode active material, the weight and addition amount of the binder, the surface density and compaction density of the negative electrode sheet, but is not limited to this. Those skilled in the art can also regulate by other means according to actual conditions.

[0025] It should be noted that the lateral length of the negative electrode plate when it droops 4 cm and the lateral length when it droops 2 cm described in the present application can be tested and confirmed by, but not limited to, the following method: the secondary battery in the empty state is disassembled, and the obtained negative electrode plate is pre-soaked in dimethyl carbonate DMC for 4 hours, and then cut into 4*25 cm test samples, which are placed on a platform with a certain height, and then one side of the sample is fixed, and the other side is suspended in the air to droop naturally, and the lateral length when it droops 4 cm and 2 cm is measured respectively (mainly the lateral length of the drooping end of the plate), as shown in the following figure: Figure 2 and 3 As shown, Figure 2 As shown in the figure, when the suspended part of the sample naturally sags 4 cm, the horizontal length of the sample corresponding to the suspended part is L2, as shown in the figure. Figure 3 As shown in the figure, after sagging 4 cm, the horizontal length indicated by the red arrow is L2.

[0026] In some embodiments, a=1~5.

[0027] Further preferably, a is in the range of one or any two of 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5.

[0028] More preferably, a=1.5~3.

[0029] The tortuosity of the positive electrode sheet in the secondary battery is not only related to the length of the lithium ion diffusion path, but also related to the degree of stress it withstands due to the deintercalation of lithium ions during charging and discharging. When the relative flexibility of the negative electrode sheet is optimized, the tortuosity of the positive electrode sheet also needs to be matched and regulated accordingly to ensure the overall lithium ion deintercalation stability of the secondary battery and improve its deintercalation rate. When the tortuosity of the positive electrode sheet is preferably within the above range, the stability of the positive electrode sheet is better, it is more matched with the negative electrode sheet, and the lithium ion transmission path is shorter, and the fast charging performance and cycle performance are better.

[0030] It should be noted that in the present application, the tortuosity of the positive electrode sheet can be regulated by adjusting the size and size distribution of the positive electrode active material particles and the surface density and compaction density of the positive electrode sheet, but is not limited to this. Those skilled in the art may also use other means to regulate the tortuosity according to actual conditions.

[0031] It should be noted that the tortuosity of the positive electrode sheet described in the present application can be tested and confirmed by, but not limited to, the following methods. The specific steps are: disassembling the secondary battery in an empty state, and then immersing the obtained positive electrode sheet in a dimethyl carbonate (DMC) solution for 2 hours, and then using FIB (instrument model) to slice the electrode area, with a cutting voltage of 30kV, a cutting beam of 7nA, a cutting range of 60μm*65μm*30μm, a shooting voltage of 5kV, and a shooting beam of 0.8nA; the number of sections depends on the z-axis cutting range and the interlayer spacing. For example, when the cut sample is 30μm thick and the interlayer spacing is 60nm, the number of cuts is about 500. The slices are three-dimensionally reconstructed, and the pores on the cross section are segmented and extracted. The centroid of the connecting pores in each cross section is calculated by software, and the tortuosity of the two adjacent surfaces is calculated by the ratio of the distance between the centroids to the straight-line distance between the cross sections. Finally, the tortuosity of the entire positive electrode sheet is calculated, such as Figure 1 As shown, the tortuosity of the positive electrode sheet 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 double-beam electron microscope, model: SCIOS 2 HiVac.

[0032] In some embodiments, the particle size D of the lithium nickel cobalt manganese oxide particles is v50 2~20μm.

[0033] More preferably, the particle size D of the lithium nickel cobalt manganese oxide particles is v50 The range value is one or any two of 2μm, 4μm, 5μm, 8μm, 10μm, 12μm, 14μm, 15μm, 18μm, and 20μm.

[0034] In some embodiments, the SPAN value of the lithium nickel cobalt manganese oxide particles is 1.1 to 1.7, where SPAN = (D v90 -D v10 ) / D v50 , where D v90 is the particle size D of lithium nickel cobalt manganese oxide particles v90 , D v10 is the particle size D of lithium nickel cobalt manganese oxide particles v10 .

[0035] More preferably, the SPAN value of the lithium nickel cobalt manganese oxide particles is in the range of one or any two of 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, and 1.7.

[0036] Particle size D of lithium nickel cobalt manganese oxide particles v50 The change in particle size distribution uniformity will cause the volume effect of the particles during lithium ion deintercalation and the overall lithium ion deintercalation rate to be different. When the particle size D of the lithium nickel cobalt manganese oxide particles in the positive electrode sheet is v50 When the SPAN value is preferably within the above range, the volume effect of the lithium nickel cobalt manganese oxide particles during lithium ion deintercalation has a lower impact on the stability of the electrode, and is more adapted to the relative flexibility of the negative electrode, and the lithium ion transmission rate is faster.

[0037] 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 for confirmation, but are not limited to them. The specific steps are as follows: the secondary battery in the empty state is disassembled, the obtained positive electrode plate is soaked in DMC for 2 hours, and then the positive electrode material layer on the plate is scraped off. The obtained powder is measured by laser particle size distribution measuring instrument (Mastersizer3000) according to the particle size distribution laser diffraction method (specific steps refer to GB / T19077-2016), and the corresponding particle size of 10%, 50%, and 90% of the volume distribution is used to represent D respectively. v10 、D v50 and D v90 .

[0038] In some embodiments, the surface density of the positive electrode sheet is 180-700 g / m 2 .

[0039] Further preferably, the surface density of the positive electrode sheet is 180g / m 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 values ​​for one or both of .

[0040] In some embodiments, the compaction density of the positive electrode sheet is 3-3.8 g / cm 3 .

[0041] Further preferably, the compaction density of the positive electrode sheet is 3g / cm 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 values ​​for one or both of .

[0042] When the surface 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 deintercalation is more evenly dispersed, which has less impact on the stability of the electrode, and is more compatible with the negative electrode. The impact of the impact force of lithium ions on the negative electrode can be further reduced without reducing the lithium ion transmission rate.

[0043] It should be noted that the compaction density and surface density of the positive electrode sheet in the present application can be confirmed by, but not limited to, the following test method. The specific steps are: disassemble the secondary battery, soak the obtained positive electrode sheet in DMC for 4 hours, dry it, and then use a punching machine to punch the positive electrode sheet into a fixed area disc, the area is recorded as S0, and the unit is mm 2; Take three discs as parallel samples, and then use an electronic balance to weigh the mass of the three discs respectively. The mass of each disc is recorded as M1, in g. Finally, add appropriate amount of deionized water on the three discs, gently wipe off the coating on the disc with dust-free paper to expose the copper foil, and let it stand (dry) at room temperature for 10 minutes. After the copper foil is dry, weigh the mass of the three copper foils respectively and record it as M0. The surface density is calculated by surface density = (M1-M0) / S0.

[0044] 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.

[0045] In some embodiments, the particle size D of the negative electrode active material is v50 10~30μm.

[0046] In some embodiments, the surface density of the negative electrode sheet is 120-300 g / m 2 .

[0047] In some embodiments, the compacted density of the negative electrode sheet is 1.3-1.8 g / cm 3 .

[0048] By regulating the size of the negative electrode active material in the negative electrode sheet, the surface density and the compaction density of the negative electrode sheet, the flexibility of the negative electrode sheet can be further improved, while at the same time, the structure between the sheets will not be weak and the active particles will not fall off.

[0049] In some embodiments, the negative electrode active material includes at least one of graphite and a silicon-carbon composite material.

[0050] More preferably, the negative electrode active material includes graphite, and b=0.015~0.6.

[0051] When the active material in the negative electrode plate is graphite, the material itself has high kinetic properties, which can effectively increase the embedding rate of lithium ions on the surface of the negative electrode plate, thereby improving the kinetic performance of the secondary battery. At this time, it is preferred to match a lower negative electrode flexibility, which can reduce the impact of lithium ions on the overall structural stability of the negative electrode plate and improve the cycle stability of the secondary battery.

[0052] More preferably, the negative electrode active material includes graphite and a silicon-carbon composite material, and b=0.02-0.8.

[0053] When the active material in the negative electrode plate contains both graphite and silicon-carbon composite material, the flexibility of the negative electrode plate cannot be too high due to the volume expansion effect of the silicon-carbon composite material to avoid the plate from breaking due to the expansion effect. At the same time, the kinetic performance of the material is low, so the lithium ion transmission rate is low. Therefore, when the flexibility range of the above-mentioned negative electrode plate is preferred, the secondary battery can ensure better fast charging cycle stability and kinetic performance.

[0054] In some embodiments, the negative electrode plate further includes a current collector, and the current collector includes at least one of a copper layer and a composite layer.

[0055] More preferably, the current collector includes a composite layer, the composite layer includes a polymer layer and a conductive layer, and the conductive layers are arranged on both sides of the polymer layer; and b=0.015~0.04.

[0056] 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).

[0057] More preferably, the thickness of the polymer layer is 2 to 8 μm, and can specifically be a polymer polyethylene terephthalate layer, a polymer 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 (PTFE) 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, a derivative thereof, a cross-linked product thereof, and a copolymer layer thereof; More preferably, the thickness of the conductive layer is 1-3 μm.

[0058] More preferably, the current collector includes a copper layer, and b=0.02~0.08.

[0059] In view of the current collector composition of different systems, the secondary battery described in the present application adopts the setting of a current collector composite layer in the negative electrode plate, which can effectively improve the flexibility of the overall plate through the middle polymer layer, enhance the fast charging cycle stability of the negative electrode plate, and further enhance the stability and efficiency of the negative electrode plate when lithium ions are transmitted between the positive and negative electrodes.

[0060] In some embodiments, the lithium nickel cobalt manganese oxide particles include single crystal particles, and b=0.03-1.2.

[0061] It should be noted that the single crystal particles refer to single particles with a complete crystal appearance or aggregated particles with less than 3 primary particles.

[0062] In some embodiments, the lithium nickel cobalt manganese oxide particles include polycrystalline particles, and b=0.01-0.5.

[0063] It should be noted that the polycrystalline particles refer to particles containing at least three or more crystal grains.

[0064] When the nickel cobalt manganese oxide particles contain polycrystalline particles, the relative path of lithium ions during transmission is shorter and the transmission rate is faster. Further preferred regulation of the flexibility of the negative electrode plate can avoid excessive impact of lithium ions on the negative electrode plate during transmission. When the nickel cobalt manganese oxide particles contain single crystal particles, the relative lithium ion transmission distance is longer, and the transmission efficiency is lower. Therefore, when the flexibility of the negative electrode plate is preferably regulated, the dynamic performance of the overall plate can be further reduced.

[0065] In some embodiments, the structural formula of the lithium nickel cobalt manganese oxide is 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.

[0066] 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.

[0067] 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.

[0068] When the nickel content in the lithium nickel cobalt manganese oxide particles is relatively high, the trivalent nickel ions contained in the particles will reduce the migration energy of lithium ions, thereby increasing the activity of lithium ion deintercalation and the kinetic performance. It requires higher flexibility of the negative electrode during lithium ion deintercalation and transmission. Therefore, when the droop length ratio of the negative electrode is preferably within the above range, the impact of lithium ion transmission impact can be further reduced. When the nickel content in the particles is further increased, the migration energy of lithium ions is lower and the transmission rate is faster, further improving the flexibility of the negative electrode. That is, further optimizing b within the above range can take into account better kinetic performance and fast charging cycle stability performance of the secondary battery.

[0069] In some embodiments, the lithium nickel cobalt manganese oxide particles may be commercially available products or may be obtained by a homemade method. Specifically, the lithium nickel cobalt manganese oxide particles may be obtained by the following preparation method: A nickel source, a cobalt source and a manganese source are mixed in a solvent, a precipitant is added for precipitation reaction, and the mixture is allowed to stand. After filtering, washing and drying, the obtained mixed precursor is mixed with a lithium source and a flux, and calcined at 400-600° C. in an air atmosphere. The mixture is then heated to 880-930° C. for a second calcination, and the temperature is adjusted to 710-890° C. for heat preservation treatment. The mixture is cooled and crushed to obtain the lithium nickel cobalt manganese oxide particles.

[0070] 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° C., and the finally obtained lithium nickel cobalt manganese oxide particles contain polycrystalline particles; 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° C., and the finally obtained lithium nickel cobalt manganese oxide particles contain single crystal particles; In some embodiments, the nickel source includes at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate; In some embodiments, the cobalt source used includes at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate; In some embodiments, the manganese source includes at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate; 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; Further preferably, the lithium source is molten salt; In some embodiments, the precipitant includes at least one of sodium hydroxide, sodium carbonate, oxalic acid, and ammonia water.

[0071] In some embodiments, the solvent comprises water.

[0072] In some embodiments, the precipitation reaction time is 0.5 to 24 hours.

[0073] In some embodiments, the flux includes a compound containing at least one of Sr, Na, and K. Furthermore, the compound may be, but is not limited to, oxides and / or carbonates of the above elements.

[0074] Further preferably, the amount of flux added is 0.08-0.42% by mass of the total mass of the mixed precursor during the first calcination.

[0075] In some embodiments, the heating rate during the primary calcination is 1-15° C. / min, the calcination temperature is 400-600° C., and the calcination time is 2-4 h.

[0076] In some embodiments, the heating rate during the secondary calcination is 1-15° C. / min, the calcination temperature is 880-930° C., and the time is 3-10 min.

[0077] In some embodiments, the heat preservation treatment time is 8 to 12 hours.

[0078] Sintering under the above conditions is conducive to the formation of crystal nuclei of lithium nickel cobalt manganese oxide particles. Those skilled in the art can adjust the particle size by adjusting the condition parameters during the above sintering, and can also use other methods to adjust, without specific limitation.

[0079] In some embodiments, the crushing is performed by air jet mill crushing, and the air induced frequency of the air jet mill crushing is 10-50 Hz.

[0080] In some embodiments, the lithium nickel cobalt manganese oxide particles may further contain doping elements, and the doping elements include at least one of Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Al, W, Sr, V, Y, Mg, F, Cl, and S.

[0081] More preferably, the content of the doping element is 100-10000 ppm.

[0082] In some embodiments, the lithium nickel cobalt manganese oxide particles are provided with a coating layer.

[0083] Specifically, the coating layer may be, but is not limited to, an aluminum oxide coating layer.

[0084] 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%.

[0085] In some embodiments, the binder is used to improve the adhesion between the 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 binding properties and does not significantly cause adverse chemical changes in the battery. For example, the binder includes a fluorinated polyolefin binder, which includes but is not limited to polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers, or modified derivatives thereof (e.g., modified with carboxylic acid, acrylic acid, acrylonitrile, etc.).

[0086] Specifically, the binder is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0087] In some embodiments, the mass percentage of the binder in the positive electrode material layer is 0.5%~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 an interval formed by any two of the above values.

[0088] In some embodiments, the conductive agent is used to provide electrical 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 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 include carbon nanofibers, and carbon black includes SP (Super P, the same below), acetylene black, and Ketjen black.

[0089] 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 an interval formed by any two of the above values.

[0090] In some embodiments, the positive electrode plate can be prepared by, but not limited to, the following method: The positive electrode active material, the conductive agent and the binder are mixed in a solvent and then stirred to prepare a slurry. The slurry is coated on a current collector in a single layer or double layer, dried, rolled and cut to obtain the positive electrode sheet.

[0091] In some embodiments, the solvent includes N-methylpyrrolidone (NMP).

[0092] In the technical solution of the present application, the tortuosity of the positive electrode sheet can be regulated by the rolling pressure of the positive electrode sheet during preparation or the size of the rolling gap during rolling, but it is not limited to this. Those skilled in the art can also use other methods to regulate the tortuosity according to actual conditions, 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.

[0093] In some embodiments, the secondary battery further includes an electrolyte.

[0094] In some embodiments, the electrolyte includes an additive, a solvent, and a lithium salt.

[0095] In some embodiments, the solvent includes at least one of a carbonate solvent, a carboxylate solvent, an ether solvent, a sulfone solvent, a nitrile solvent, and a phosphate solvent.

[0096] Illustratively, the carbonate solvent includes but is not limited to at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); the carboxylate solvent includes but is not limited to at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; the ether solvent includes at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; the sulfone solvent includes at least one of methyl sulfone and dimethyl sulfoxide; the nitrile solvent includes at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrinitrile; and the phosphate solvent includes at least one of trimethyl triphosphate and triethyl phosphate.

[0097] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium trifluoromethanesulfonate, lithium bisfluoromethanesulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium difluorobisoxalatophosphate, lithium tetrafluorooxalatophosphate, and the like.

[0098] In some embodiments, the additive includes, but is not limited to, vinylene carbonate.

[0099] In some embodiments, the negative electrode plate includes a negative electrode material layer, the negative electrode material layer includes a negative electrode material, and the negative electrode material includes at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, elemental silicon, silicon oxide, silicon-carbon composite material, and lithium titanate.

[0100] The negative electrode material layer may further include a conductive agent, a thickener, and a binder.

[0101] The conductive agent in the negative electrode active material layer is used to provide electrical 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 conductive agents include, but are not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes. Examples of carbon fibers include carbon nanofibers; and examples of carbon black include SP, acetylene black, and Ketjen black.

[0102] 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 an interval formed by any two of the above values.

[0103] 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 binding properties and does not significantly cause adverse chemical changes in the battery. Exemplary thickeners and / or binders include, but are not limited to, at least one of carboxymethyl cellulose (CMC), styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and water-based acrylic resins.

[0104] In some embodiments, the weight 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 an interval formed by any two of the above values. The weight 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 an interval 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 weight percentage of the negative electrode material in the negative electrode material layer is 70% to 99%.

[0105] The present invention is further described below with reference to specific examples, which are not to be construed as limiting the scope of the present invention. Example 1 A secondary battery, the preparation method comprising the following steps: (1) Preparation of lithium nickel cobalt manganese oxide particles: nickel sulfate, cobalt sulfate and manganese sulfate are mixed in water according to a stoichiometric ratio, and then sodium hydroxide is added as a precipitant. The pH is regulated within the range of 12.1 and the temperature is reacted within the range of 55°C. After filtration, washing and drying, the obtained precursor is mixed with LiOH-LiNO3 molten salt in a molar ratio of lithium atoms to the total atoms of nickel, cobalt and manganese atoms in the precursor of 1.04:1, and 0.36wt% of strontium oxide as a flux is mixed and ball milled based on the total mass of the molten salt and the precursor. The mixture is heated to 500°C at a rate of 5°C / min in an air atmosphere and calcined for 3h. The mixture is then heated to 850°C at a rate of 5°C / min and calcined for 5min. The temperature was adjusted to 780°C and kept for 10 hours. The cooled material was crushed by a jet mill with an induced draft frequency of 30 Hz. After crushing and screening, lithium nickel cobalt manganese oxide particles were obtained. (2) Preparation of positive electrode sheet: lithium nickel cobalt manganese oxide particles are used as positive electrode material, and 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, and vacuum stirred to prepare a slurry, which is then coated on a current collector aluminum foil. After drying, rolling and stripping, the positive electrode sheet is obtained; (3) Preparation of negative electrode sheet: artificial graphite as negative electrode material, 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 vacuum stirred to prepare a slurry, which is then coated on a current collector copper foil. After drying, cold pressing, and stripping, the negative electrode sheet is obtained. (4) Preparation of electrolyte: EC (ethylene carbonate), EMC (ethyl methyl carbonate) and DEC (diethyl carbonate) were mixed in a volume ratio of 1:1:1 as a solvent, and then lithium hexafluorophosphate was added based on the total mass of the electrolyte to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L; (5) The positive electrode sheet, the commercially available PP separator, and the negative electrode sheet are stacked in order to form a battery cell, and the battery cell is placed in an outer packaging shell. After drying, the electrolyte is injected, and after vacuum packaging, standing, forming, and constant capacity, the lithium-ion secondary battery is obtained.

[0106] Examples 2 to 38, Comparative Examples 1 to 6 A positive electrode sheet and a lithium-ion battery prepared therefrom differ from Example 1 only in the preparation processes of the positive and negative electrode sheets. The parameters of the positive and negative electrode sheets, and the secondary battery during preparation and the test results and parameters of the products are shown in Tables 1 to 3, and the test methods for the parameters of each product are as described above. When the negative electrode sheet is prepared, the labeled current collector is a composite current collector comprising a metal layer and a polymer layer disposed on both sides of the metal layer. When the binder content in the negative electrode sheet slurry changes, the ratio of the other materials remains unchanged, but this is not limited to this. Those skilled in the art can also adjust the content of the other materials individually or as a whole according to actual conditions.

[0107] Table 1 Table 2 Table 3 Effect Examples The lithium ion batteries obtained in each embodiment and comparative example were tested as follows: Fast charging test: (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 at 0.02C to 4.1V, then charge it at 0.33C constant current to 4.25V, and then charge it at constant voltage to 0.05C. After standing for 10 minutes, discharge at a constant current of 0.33C to 2.5V to complete formation, then charge at a constant current rate of 0.33C to 4.25V, and then charge at a constant voltage to 0.05C. After standing for 10 minutes, discharge at a constant current rate of 0.33C to 2.5V, which is regarded as one cycle constant capacity. After the cycle constant capacity is repeated twice, the copper wire is first lithium-plated at a rate of 0.02C for 4 hours, and then charged to 10% of the second capacity of the cycle constant capacity at a rate of 0.33C, followed by constant current charging at a rate of 4C until the negative reference potential is 0 or the terminal voltage is 4.25V, and then the battery is charged at a gradient of 0.2C, and the cutoff condition remains unchanged until the charge rate is reduced to 0.4C / 0.33C and the battery reaches 100% of the second week charge capacity; the test temperature is 25°C, the equipment signal acquisition frequency is 100ms, and after the test is completed, the charging time t required for the secondary battery to go from 10% to 80% SOC is calculated; (2) Each secondary battery was charged at 0.33C constant current to 4.25V at 25℃, and then charged at constant voltage to 0.05C. After standing for 10 minutes, it was discharged at 0.33C constant current to 2.5V. This was regarded as one cycle constant capacity. After two cycles of constant capacity, the second cycle capacity was used as the constant capacity, and 10% of the second capacity of the cycle constant capacity was charged at 0.33C. Then, it was charged at 4C constant current, and then charged at 0.4C until the charging rate dropped to 0.33C. The negative reference potential of each rate was 0mV or 4.25V. Then, it was charged at 0.33C constant current and constant voltage to 100% SOC, and then discharged at 0.33C to 2.5V. One cycle was regarded as one cycle. The cycle was repeated 300 times, and the discharge capacity of the 300th cycle was divided by the constant capacity. The test results are shown in Table 4.

[0108] Table 4 According to Table 4, we can see that: (1) The secondary battery described in this application uses lithium nickel cobalt manganese oxide particles as the positive electrode active material, and at the same time coordinates the tortuosity of the positive electrode sheet and the flexibility of the negative electrode sheet, which can not only effectively improve the fast charging rate of the secondary battery, but also avoid the impact of a large number of lithium ions on the integrity of the electrode sheet when it quickly impacts the electrode sheet, thereby improving its stability under fast charging conditions. The secondary battery is highly efficient under fast charging conditions, and the time from 10% to 80% SOC is 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 cannot match the a / b ratio of 1 to 300 in the range of the present application solution because the tortuosity of the positive electrode sheet and the flexibility of the negative electrode sheet cannot match the a / b ratio of 1 to 300 in the range of the present application solution. Therefore, it is difficult to take into account both fast charging efficiency and fast charging stability when conducting fast charging tests.

[0109] (2) The ratio regulation of the tortuosity of the positive electrode sheet and the flexibility of the negative electrode sheet of the secondary battery can ensure the lithium ion transmission efficiency of the secondary battery during charging and discharging, and can also avoid the influence of ion impact on the structural integrity of the overall secondary battery sheet. As mentioned above, whether the ratio is too high or too low, the fast charging performance of the secondary battery will be greatly reduced. When the a / b is further preferably controlled within the range of 5~150, the secondary battery can have more ideal dynamic performance and fast charging cycle capacity retention rate; on the other hand, the greater the flexibility of the negative electrode sheet, the better. While providing a buffering effect for fast ion impact, 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 too high, the better the performance. When the toughness b is preferably in the range of 0.015~0.5, the negative electrode plate can better take into account both rigidity and buffering effects; similarly, the tortuosity of the positive electrode plate will also affect the fast charging performance of the secondary battery, which is related to the diffusion path length of lithium ions and the stress tolerance during lithium ion deintercalation. When the tortuosity of the positive electrode plate is preferably in the range of 1.5~3, the tortuosity of the positive electrode plate can match the negative electrode plate to the maximum extent, ensuring the stability of lithium ion deintercalation, and at the same time, the lithium ion deintercalation path is short, and the fast charging performance is better; when the secondary battery is subjected to fast charging test, as shown in Examples 1~36, the fast charging time can be as short as less than 10 minutes, and the capacity retention rate after 300 fast charges can reach more than 96%.

Claims

1. A secondary battery, characterized in that: It includes a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet includes a positive electrode material layer, and the positive electrode material layer includes lithium nickel cobalt manganese oxide particles; The secondary battery satisfies: a / b=1~300; Said a is the tortuosity of the positive electrode sheet; The b=(L2 / L1)-1, wherein L2cm is the lateral length of the negative electrode when the electrode is drooped by 4cm, and L1cm is the lateral length of the negative electrode when the electrode is drooped by 2cm.

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

3. The secondary battery according to claim 1, wherein: The b=0.01~1.

3.

4. The secondary battery according to claim 3, wherein: The lithium nickel cobalt manganese oxide particles include single crystal particles, and b=0.03~1.

2.

5. The secondary battery according to claim 3, wherein: The lithium nickel cobalt manganese oxide particles include polycrystalline particles, and b=0.01~0.

5.

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

7. The secondary battery according to claim 1, wherein: Said a=1~5.

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

9. The secondary battery according to claim 1, wherein: The surface density of the positive electrode sheet is 180-700 g / m 2 , and / or, the compaction density of the positive electrode sheet is 3.2~3.8g / cm 3 .

10. The secondary battery according to claim 1, wherein: The negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode active material, and the particle size D of the negative electrode active material is v50 10~30μm.

11. The secondary battery according to claim 1, wherein: The surface density of the negative electrode sheet is 120~455g / m 2 , and / or, the compaction density of the negative electrode sheet is 1.2-1.8 g / cm 3 .

12. The secondary battery according to claim 8, wherein: The negative electrode active material includes at least one of graphite and a silicon-carbon composite material.

13. The secondary battery according to claim 10, wherein: The negative electrode active material includes graphite, and b=0.015~0.

6.

14. The secondary battery according to claim 10, wherein: The negative electrode active material includes graphite and a silicon-carbon composite material, and b=0.02-0.

8.

15. The secondary battery according to claim 1, wherein: The negative electrode plate further includes a current collector, and the current collector includes at least one of a copper layer and a composite layer.

16. The secondary battery according to claim 15, wherein: The current collector includes a composite layer, the composite layer includes a polymer layer and a conductive layer, and the conductive layers are arranged on both sides of the polymer layer; b=0.015~1.

17. The secondary battery according to claim 16, wherein: 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).

18. The secondary battery according to claim 16, wherein: The thickness of the polymer layer is 2-8 μm, and / or the thickness of the conductive layer is 1-3 μm.

19. The secondary battery according to claim 1, wherein: The structural formula of the lithium nickel cobalt manganese oxide is 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.

20. The secondary battery according to claim 19, wherein: The o is greater than or equal to 0.5 and less than or equal to 0.7, and the b is 0.02~1.

21. The secondary battery according to claim 19, wherein: The o is greater than 0.7 and less than 0.95, and the b is 0.015~0.7.

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