A secondary battery

Abstract

The application discloses a secondary battery, and belongs to the technical field of batteries. The secondary battery introduces carbon nanotubes as a conductive agent on the basis of active materials of spinel-type lithium nickel manganese oxide as a positive electrode material, and simultaneously regulates the parameter relationship of the size of the spinel-type lithium nickel manganese oxide and the carbon nanotubes, so that the electronic conduction performance of the positive electrode material is significantly improved. In the case that the rate performance of the secondary battery is improved, the high-temperature stability and the cycle performance also reach a high level.

Description

Technical Field

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

[0002] Spinel-type lithium nickel manganese oxide, as a high-voltage cathode material, possesses a high theoretical capacity (146 mAh / g), strong structural and thermal stability, resulting in long cycle life and high safety performance. Therefore, it is theoretically very suitable for new energy vehicles, energy storage, and other fields. However, the large band gap of spinel-type lithium nickel manganese oxide results in relatively low electronic conductivity. Consequently, when applied as a cathode material for secondary batteries, its rate performance still cannot effectively meet market demands. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery. Based on the active material of spinel-type lithium nickel manganese oxide as the positive electrode material, carbon nanotubes are introduced as a conductive agent. At the same time, the parameter relationship between the size of spinel-type lithium nickel manganese oxide and carbon nanotubes is controlled, so that the electronic conduction performance of the positive electrode material is significantly improved. While improving the rate performance of the secondary battery, the high-temperature stability and cycle performance also reach a high level.

[0004] To achieve the above objectives, in a first aspect of this application, a secondary battery is provided, including a positive electrode sheet, wherein the positive electrode sheet includes a positive active material and a conductive agent;

[0005] The positive electrode active material includes spinel-type lithium nickel manganese oxide;

[0006] After 100 charge-discharge cycles, the characteristic peak at 630 nm in the Raman spectrum of the spinel-type lithium nickel manganese oxide shifted by a% compared to before the charge-discharge cycles. The charge-discharge cycle temperature was 45℃, the charge-discharge rate was 1C / 1C, and the charge-discharge cutoff voltage was 3.5V~4.8V.

[0007] The conductive agent includes carbon nanotubes;

[0008] The secondary battery satisfies: a / b = 0.0005~0.05;

[0009] Where b is the average aspect ratio of the carbon nanotube.

[0010] In a second aspect, this application provides an electrical device including the secondary battery described in this application, wherein the secondary battery serves as the power supply for the electrical device.

[0011] The beneficial effects of this application are as follows:

[0012] This application provides a secondary battery that, based on spinel-type lithium nickel manganese oxide as the active material of the positive electrode, introduces carbon nanotubes as a conductive agent, and simultaneously controls the parameter relationship between the size of the spinel-type lithium nickel manganese oxide and the carbon nanotubes, thereby significantly improving the electronic conduction performance of the positive electrode material. While improving the rate performance of the secondary battery, high-temperature stability and cycle performance also reach a high level. 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 sheet, wherein the positive electrode sheet includes a positive active material and a conductive agent;

[0018] The positive electrode active material includes spinel-type lithium nickel manganese oxide;

[0019] After 100 charge-discharge cycles, the characteristic peak at 630 nm in the Raman spectrum of the spinel-type lithium nickel manganese oxide shifted by a% compared to before the charge-discharge cycles. The charge-discharge cycle temperature was 45℃, the charge-discharge rate was 1C / 1C, and the charge-discharge cutoff voltage was 3.5V~4.8V.

[0020] The conductive agent includes carbon nanotubes;

[0021] The secondary battery satisfies: a / b = 0.0005~0.05;

[0022] Where b is the average aspect ratio of the carbon nanotube, that is, the length of the carbon nanotube / the diameter of the carbon nanotube.

[0023] Spinel-type lithium nickel manganese oxide has a high theoretical capacity but poor conductivity, limiting its application in new energy vehicles and energy storage. Therefore, this application uses carbon nanotubes, which have high conductivity, as a conductive agent to prepare the positive electrode of a secondary battery. Unlike traditional graphite particles, carbon nanotubes can form a three-dimensional conductive contact network after being combined with the positive electrode active material, resulting in superior conductivity. However, their tubular structure means that their dispersion in the positive electrode material layer is inevitably less than that of particulate conductive agents. Less dispersion leads to an incomplete conductive contact network structure and insufficient conductivity improvement; excessive dispersion, on the other hand, can easily cause material entanglement and difficulty in dispersion. Furthermore, the introduction of carbon nanotubes provides a large number of electron transport sites and pathways for the positive electrode active material in the secondary battery. However, this also increases the probability of side reactions between the positive electrode active material and the electrolyte, especially at high temperatures, reducing the stability and cycle performance of the secondary battery at high temperatures. Therefore, this application further combines and controls the size of spinel-type lithium nickel manganese oxide and carbon nanotubes to explore the influence of high-temperature cycling on the bond energy of Mn-O bonds (i.e., the chemical bonds to which the characteristic peak at 630 nm in the Raman spectrum of the material belongs) in spinel lithium nickel manganese oxide. At the same time, by controlling the ratio of this factor to the aspect ratio of carbon nanotubes within a specific range, the carbon nanotubes and positive electrode active materials in the positive electrode material layer of the secondary battery can have high conductivity and high dispersion during use. Meanwhile, the probability of side reactions between the positive electrode active material and the electrolyte is kept at a low level under high temperature conditions, thus taking into account ideal rate performance, high-temperature stability and cycle performance.

[0024] In some implementations, a / b = a range of one or any two of the following: 0.0005, 0.0008, 0.001, 0.0013, 0.0015, 0.002, 0.0025, 0.0028, 0.003, 0.0035, 0.004, 0.0045, 0.005, 0.0055, 0.006, 0.0065, 0.007, 0.01, 0.02, 0.025, 0.03, 0.04, and 0.05.

[0025] More preferably, a / b = 0.0013~0.0028.

[0026] In the secondary battery described in this application, the characteristic peak at 630 nm in the Raman spectrum of spinel-type lithium nickel manganese oxide represents its Mn-O bond. The magnitude of the shift after cycling indicates the strength of the Mn-O bond in the material, that is, the structural stability of the material. When the structural stability is poor, the material is prone to side reactions at high temperatures. However, if the structural stability is too good, the efficiency of ion insertion and extraction during cycling will decrease. Therefore, it is necessary to maintain an appropriate range. The aspect ratio of carbon nanotubes affects the integrity and dispersion uniformity of the conductive network formed when they are combined with the positive electrode active material. When the relationship between the two is constructed, it is found that when the ratio of the two falls within the above-mentioned preferred range, the secondary battery can achieve the effects of good conductivity and high stability of the positive electrode material, as well as better dispersion of the conductive agent carbon nanotubes in the positive electrode material, resulting in better electrochemical performance of the secondary battery.

[0027] In some implementations, a = 0.5~5.

[0028] More preferably, a is a range of one or any two of the following: 0.5, 1, 1.01, 1.2, 1.5, 1.8, 1.99, 2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5.

[0029] More preferably, a = 1.5~2.

[0030] As mentioned above, the shift of the Raman characteristic peak at 630 nm after cycling of the spinel lithium nickel manganese oxide is related to its structural stability and ion / electron conduction efficiency when combined with carbon nanotubes as a positive electrode active material. By combining carbon nanotubes with different average aspect ratios at different shifts to meet the a / b range requirements, the secondary battery prepared by the spinel lithium nickel manganese oxide can exhibit excellent rate performance and high-temperature stability. When preferably within the above range, it can be further combined with carbon nanotubes to achieve good high electrolyte wettability and ion insertion / extraction activity, while the probability of side reactions with the electrolyte can also be kept at a low level, resulting in better cycling performance of the secondary battery at high temperatures.

[0031] It should be noted that the test method for a is as follows: The secondary battery is disassembled, 10 positive electrode sheets (6cm × 7.5cm) are taken, and the positive active material layer powder from the surface of 5 positive electrode sheets is scraped off and soaked in DMC for 30 minutes. Then, it is dried at 45℃ for 30 minutes to obtain treated powder I. The obtained treated powder I is then subjected to configuration analysis using a confocal micro Raman spectrometer (RM2000) manufactured by Renishaw (UK). The test conditions are: laser wavelength of 532 nm, spectral scanning range of 100 cm⁻¹. -1 ~1000cm -1The initial Raman spectrum of the untreated powder was obtained, and the plot was marked at 630 cm⁻¹ based on the test results. -1 The specific position of the peak characterizing the Mn-O bond is L1. The remaining 5 positive electrode sheets were assembled into a lithium half-cell for cycle testing. The disassembled positive electrode sheet was used as the positive electrode, the lithium sheet as the negative electrode, and a 15μm thick PE film as the separator, placed between the positive and negative electrodes to form a single cell. The single cell was placed in an aluminum-plastic film and injected with electrolyte (3mL). After vacuum sealing, it was left to stand for 24 hours to obtain a single battery cell. The electrolyte formulation consisted of ethylene carbonate, dimethyl carbonate, and diethyl carbonate mixed in a volume ratio of 5:3:2, with the addition of lithium hexafluorophosphate to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1mol / L. The single battery cell was charged to 4.8V at 0.33C, left to stand for 10 minutes, and then discharged at 0.33C. The battery was charged to 3.5V and cycled for two full cycles. At 45℃, the single-cell battery was charged to 4.8V at a 1C rate. After standing for 10 minutes, it was discharged to 3.5V at a 1C rate, followed by 100 charge-discharge cycles. The single-cell battery was then disassembled, and the cycled positive electrode was removed. The positive electrode active material powder layer on the surface of the cycled positive electrode was scraped off and soaked in DMC for 30 minutes, then dried at 45℃ for 30 minutes to obtain processed powder II. The obtained processed powder II was analyzed using a confocal Raman micro-spectroscopy system (RM2000, Renishaw, UK). The test conditions were: laser wavelength 532 nm, spectral scanning range 100 cm⁻¹. -1 ~1000cm -1 The initial Raman spectrum of powder II was obtained, and the plot was marked at 630 cm⁻¹ based on the test results. -1 The specific location of the peak representing the Mn-O bond is L2, and a is calculated using the formula 100%×|L2-L1| / L1.

[0032] In some embodiments, 'a' is controlled by the sintering temperature during the preparation of spinel-type lithium nickel manganese oxide. However, it is not limited to this. Those skilled in the art can further control 'a' by adjusting other methods such as raw material selection and ball milling process conditions, depending on the actual situation.

[0033] In some embodiments, the spinel-type lithium nickel manganese oxide includes LiNi x Mn y O4, where 0 < x ≤ 0.5, y = 2 - x.

[0034] In some embodiments, the carbon nanotubes include at least one of single-walled carbon nanotubes, modified single-walled carbon nanotubes, multi-walled carbon nanotubes, and modified multi-walled carbon nanotubes.

[0035] It should be noted that the carbon nanotubes described in this application can be commercially available or self-made. Those skilled in the art should know that carbon nanotubes obtained from different sources or preparation methods have different aspect ratios. Therefore, appropriate types can be selected and compounded with spinel lithium nickel manganese oxide according to actual needs. As long as the secondary battery meets the range of a / b=0.0005~0.05 and achieves the expected technical effect, there are no restrictions.

[0036] In some implementations, b = 100~1000.

[0037] More preferably, b is a range of one or any two of the following: 100, 200, 250, 300, 400, 500, 600, 700, 710, 750, 780, 800, 850, 900, and 1000.

[0038] More preferably, b = 700~800.

[0039] It should be noted that the test method for the average aspect ratio b of the carbon nanotubes described in this application is as follows: The secondary battery is disassembled, and then the positive electrode material layer on the obtained positive electrode sheet is scraped off. The resulting mixed powder is soaked in DMC for 30 minutes and then dried at 45°C for 30 minutes. Subsequently, a FEI TF20 transmission electron microscope energy dispersive spectroscopy instrument is used to identify the carbon nanotubes in the material at 5000 magnification. 3-5 carbon nanotubes are randomly selected from the top, bottom, left, right and middle areas of the image, for a total of 25 nanotubes. Then, the image is magnified to 50000x, and the diameter and length of the carbon nanotubes are measured using mapping software at this magnification. The test sites are the front end, middle and rear end of a single carbon nanotube, and the average value is taken. The aspect ratio of a single carbon nanotube is calculated by the diameter and length. The aspect ratios of all carbon nanotubes are counted and the average value is calculated, which is b.

[0040] More preferably, the carbon nanotubes have a diameter of 2~20nm and a length of 200~2000nm.

[0041] The average aspect ratio of carbon nanotubes reflects the morphology of the material to a certain extent. The smaller the aspect ratio, the closer the morphology of the carbon nanotubes is to particles, and the higher the dispersibility. However, the integrity of the conductive network built with the positive electrode active material is lower, and the number of transport paths and sites provided for ions / electrons is fewer. On the other hand, the larger the aspect ratio, the better the overall three-dimensional structure of the material after being compounded with the positive electrode active material, and the better the conductivity. However, the dispersibility is lower, and it also provides sites for side reactions between the electrolyte and the positive electrode material. Therefore, it is necessary to balance the conductivity, electrode material dispersibility, and chemical stability of carbon nanotubes after they are introduced into the secondary battery. When the average aspect ratio of the carbon nanotubes is preferably within the above range, the overall electrochemical performance of the secondary battery is better.

[0042] In some embodiments, the positive electrode sheet includes a positive electrode material layer, which includes a positive electrode active material and carbon nanotubes, wherein the mass percentage of carbon nanotubes in the positive electrode material layer is 0.05~1%.

[0043] More preferably, the mass percentage of the carbon nanotubes in the cathode material layer is one or any two of the following values: 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, and 1%.

[0044] In some embodiments, the positive electrode active material further includes a doping element, which includes at least one of Al, Zr, P, and W.

[0045] When spinel lithium nickel manganese oxide, a 5V material, is used as the positive electrode active material for secondary batteries, the introduction of certain doping elements can further improve its balance between structural stability and conductivity, enabling the secondary battery to have both better cycle stability and charge / discharge efficiency during cycling.

[0046] In some embodiments, the positive electrode active material further includes a coating layer, which includes at least one of Al2O3, Li3P, ZrO2, and TaO2.

[0047] By using the above-mentioned coating layer to coat the positive electrode active material, the side reaction activity between spinel lithium nickel manganese oxide and electrolyte can be further effectively suppressed, and the stability of secondary batteries at high temperatures can be improved.

[0048] In some embodiments, the secondary battery includes an electrolyte comprising a solvent and a lithium salt.

[0049] 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.

[0050] 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.

[0051] More preferably, the solvent may also include, but is not limited to, at least one of the following: carbonate solvent fluorinated derivatives, carboxylic acid ester solvent fluorinated derivatives, ether solvent fluorinated derivatives, sulfone solvent fluorinated derivatives, nitrile solvent fluorinated derivatives, and phosphate ester solvent fluorinated derivatives.

[0052] 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.

[0053] More preferably, the concentration of lithium salt in the electrolyte is 0.8~2.5 mol / L.

[0054] More preferably, the concentration of lithium salt in the electrolyte is one or any two of the following: 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 1.8 mol / L, 2 mol / L, 2.2 mol / L, and 2.5 mol / L.

[0055] The secondary battery described in this application is formulated by compounding various suitable solvents and lithium salts according to actual needs, and the concentration of lithium salts is adjusted as long as normal use effect can be achieved, and is not limited to the above-described scheme.

[0056] In some embodiments, the positive electrode material layer further includes a binder.

[0057] More preferably, the adhesive comprises polyvinylidene fluoride.

[0058] In some embodiments, the secondary battery includes a negative electrode sheet, which includes a negative electrode active material.

[0059] In some embodiments, the negative electrode active material includes at least one of carbon-based materials, silicon-based materials, and silicon-carbon composite materials.

[0060] In some embodiments, the positive electrode sheet further includes a current collector, and at least one side of the current collector is provided with a positive electrode material layer, the positive electrode material layer including a positive electrode active material.

[0061] In some embodiments, the negative electrode sheet further includes a current collector, and at least one side of the current collector is provided with a negative electrode material layer, the negative electrode material layer including a negative electrode active material;

[0062] More preferably, the negative electrode material layer includes a negative electrode material, a binder, a thickener, and a conductive agent.

[0063] 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:

[0064] The method for preparing the positive electrode sheet in this invention is as follows:

[0065] Lithium, nickel, and manganese sources are mixed at a target molar ratio and then mixed with zirconia balls with a diameter of 0.6-1.0 mm at a ball-to-material ratio of 5:1-8:1. The mixture is ball-milled at a speed of 1700-2300 rpm for 5-12 hours. The ball-milled mixture is then calcined at a temperature of 700-1200℃ for 6-24 hours and cooled to room temperature to obtain spinel-type lithium nickel manganese oxide.

[0066] The lithium source options are: lithium carbonate and lithium hydroxide.

[0067] Nickel source selection: nickel hydroxide, nickel carbonate, nickel oxide;

[0068] Manganese source selection: manganese hydroxide, manganese carbonate, manganese oxide;

[0069] The positive electrode active material, conductive agent, and binder are dispersed in N-methylpyrrolidone (NMP) at a mass ratio of (94.0~99.85):(0.05~1.0):(0.1~5.0) to obtain a positive electrode slurry. The positive electrode slurry is coated on aluminum foil to obtain an aluminum foil with the positive electrode slurry coated on its surface. After drying, it is rolled and cut to obtain a positive electrode sheet.

[0070] The adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0071] The conductive agent is selected from one or more of conductive carbon black, acetylene black, and carbon nanotubes. Method for preparing the negative electrode sheet.

[0072] The negative electrode material, binder, and conductive agent are dispersed in deionized water at a mass ratio of (93.0~99.8): (0.1~2.0): (0.1~5.0) to obtain a negative electrode slurry. The negative electrode slurry is coated on copper foil. After drying, cold pressing, and slitting, the negative electrode sheet is obtained.

[0073] The negative electrode material is selected from one or more of natural graphite, artificial graphite, soft carbon, and hard carbon.

[0074] The adhesive is selected from one or more of polyacrylic acid (PAA), styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC);

[0075] The conductive agent is selected from conductive carbon black and / or carbon nanotubes.

[0076] Electrolyte preparation:

[0077] The solvent and lithium salt are mixed at a mass ratio of (98~99.99):(0.01:2) to obtain the electrolyte.

[0078] Solvent selection: at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0079] The carbonate solvent is selected from at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC); the carboxylic acid ester solvent is selected from at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; the ether solvent is selected from at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; the sulfone solvent is selected from at least one of methyl sulfone and dimethyl sulfoxide; the nitrile solvent is selected from at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile; and the phosphate ester solvent is selected from at least one of trimethyl triphosphate and triethyl phosphate.

[0080] The lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0081] Example 1

[0082] A secondary battery, the preparation method comprising the following steps:

[0083] (1) Preparation of positive electrode active material: Li2CO3, NiCO3 and MnO2 were ball-milled mechanically at 2000 rpm for 8 h according to the stoichiometric ratio, and then calcined at T℃ in air atmosphere for H h and naturally cooled to room temperature to obtain spinel-type lithium nickel manganese oxide LiNi 0.5 Mn 1.5 O4;

[0084] (2) Preparation of positive electrode sheet: The positive electrode active material, conductive carbon nanotubes and binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 98:0.65:1.35, and a slurry is prepared by vacuum stirring. The slurry is then coated on both sides of the current collector aluminum foil, and after drying, cold pressing and cutting, the positive electrode sheet is obtained. The average length of the carbon nanotubes is cm and the average diameter is dnm. The carbon nanotubes are multi-walled carbon nanotubes.

[0085] (3) Preparation of negative electrode sheet: The negative electrode active material, conductive agent acetylene black, 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 both sides of the current collector copper foil, and after drying, cold pressing and cutting, the negative electrode sheet is obtained; the negative electrode material is graphite.

[0086] (4) Selection of diaphragm: Select commercially available polyethylene diaphragm with a thickness of 15μm;

[0087] (5) Preparation of electrolyte: Ethylene carbonate and dimethyl carbonate are mixed in a volume ratio of 1:1 to obtain an organic solvent. Lithium salt LiPF6 is added and dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0088] (6) The positive electrode, separator and negative electrode are stacked and wound 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 for 24 hours, formation and volume adjustment, the secondary battery is obtained.

[0089] The calibration step is as follows: charge the secondary battery at 0.33C to 4.6V, and then charge it at PC rate to the upper limit cutoff voltage of 4.8V.

[0090] The specific process parameters are shown in Table 1;

[0091] The size parameters of the carbon nanotubes in the positive electrode and the characteristic parameters of the spinel-type lithium nickel manganese oxide are shown in Table 1.

[0092] Examples 2-18

[0093] A secondary battery differs from Example 1 only in that the preparation process parameters of the positive electrode active material and the characteristic parameters of the obtained positive electrode material are different, as shown in Table 1.

[0094] Examples 19-22

[0095] A secondary battery differs from Example 1 only in that the preparation process parameters of the positive electrode active material and the characteristic parameters of the obtained positive electrode material are different. Specifically, the positive electrode active material, the conductive agent carbon nanotubes, and the binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 98:1:1, and the rest are shown in Table 1.

[0096] Example 23

[0097] A secondary battery, the preparation method comprising the following steps:

[0098] (1) Preparation of positive electrode active material: Li2CO3, NiCO3 and MnO2 were ball-milled mechanically at 2000 rpm for 8 h according to the stoichiometric ratio, and then calcined at T℃ in air atmosphere for H h and naturally cooled to room temperature to obtain spinel-type lithium nickel manganese oxide LiNi 0.8 Mn 1.2 O4;

[0099] The specific process parameters are shown in Table 1;

[0100] (2) Preparation of positive electrode sheet: The positive electrode active material, conductive carbon nanotubes and binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 98:0.65:1.35, and a slurry is prepared by vacuum stirring. The slurry is then coated on both sides of the current collector aluminum foil, and after drying, cold pressing and cutting, the positive electrode sheet is obtained. The average length of the carbon nanotubes is cm and the average diameter is dnm. The carbon nanotubes are multi-walled carbon nanotubes.

[0101] (3) Preparation of negative electrode sheet: The negative electrode active material, conductive agent acetylene black, 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 both sides of the current collector copper foil, and after drying, cold pressing and cutting, the negative electrode sheet is obtained; the negative electrode material is graphite.

[0102] (4) Selection of diaphragm: Select commercially available polyethylene diaphragm with a thickness of 15μm;

[0103] (5) Preparation of electrolyte: Ethylene carbonate and dimethyl carbonate are mixed in a volume ratio of 1:1 to obtain an organic solvent. Lithium salt LiPF6 is added and dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0104] (6) The positive electrode, separator and negative electrode are stacked and wound 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 for 24 hours, formation and volume adjustment, the secondary battery is obtained.

[0105] The calibration step is as follows: charge the secondary battery at 0.33C to 4.6V, and then charge it at PC rate to the upper limit cutoff voltage of 4.8V.

[0106] The size parameters of the carbon nanotubes in the positive electrode and the characteristic parameters of the spinel-type lithium nickel manganese oxide are shown in Table 1.

[0107] Comparative Examples 1-6

[0108] A secondary battery differs from Example 1 only in that the preparation process parameters of the positive electrode active material and the characteristic parameters of the obtained positive electrode material are different, as shown in Table 1.

[0109] Comparative Examples 7-8

[0110] A secondary battery differs from Example 23 only in that the preparation process parameters of the positive electrode active material and the characteristic parameters of the obtained positive electrode material are different, as shown in Table 1.

[0111] Table 1

[0112]

[0113] Example of effect

[0114] The cathode materials and secondary batteries obtained in each embodiment and comparative example were tested as follows:

[0115] (1) High temperature cycling performance test: The secondary batteries obtained in each example and comparative example were pre-charged to 4.8V with constant current using the LAND charge-discharge system at 25℃, with a charging rate of 0.33C; then discharged to 3.5V with constant current, with a rate of 0.33C, and cycled twice. Then, they were placed in a 45℃ environment and charged and discharged 100 times at the same system and the same working voltage, i.e., the cutoff voltage was 3.5V and 4.8V, with a rate of 1C / 1C. The discharge capacity at each cycle was counted. Finally, the high temperature cycling capacity retention rate (%) was calculated as: discharge capacity of the secondary battery after the 100th 1C / 1C cycle / discharge capacity of the secondary battery after the first 1C / 1C cycle.

[0116] (2) High-temperature storage gas generation performance test: The secondary batteries obtained from each embodiment and comparative example were pre-charged and discharged at room temperature using the LAND charge-discharge system.

[0117] Charge the battery at a constant current of 0.33C to the upper limit voltage of 4.8V, then charge at a constant voltage until the current is less than or equal to 0.05C, followed by constant current discharge to 3.5V at a rate of 0.33C. After two cycles, test the battery volume using the water displacement method and record it as V0. Then, place the battery in a 45℃ oven for a period of time, remove it, and wait for the battery temperature to drop to room temperature (25℃). Test the battery volume again using the water displacement method and record it as V1. Calculate the gas production at 45℃ using the following formula:

[0118] Gas production at 45℃ = (V1-V0) / battery capacity.

[0119] The specific method for testing battery volume using the water displacement method is as follows:

[0120] 1) Add an appropriate amount of pure water to the container and test its density ρ with a hydrometer and record the result;

[0121] 2) Place the aforementioned container on a balance and tare it (tare the container before testing each battery cell).

[0122] 3) Submerge the battery cell body along with the tabs in pure water, ensuring that the battery cell does not contact the container wall. After stabilization, take a reading and record the data. Before the battery is placed in the oven for storage, this data is recorded as T0. After the battery is placed in the oven for storage, this data is recorded as T0.

[0123] 4) Turn off the balance and seal the container to prevent the reagent from evaporating.

[0124] Calculate V1-V0 using the following formula: V1-V0 = T0 / ρ - T1 / ρ.

[0125] The test results are shown in Table 2.

[0126] Table 2

[0127]

[0128] As can be seen from Table 2:

[0129] (1) In the secondary battery described in this application, by setting spinel-type lithium nickel manganese oxide and conductive carbon nanotubes on the positive electrode, and simultaneously coordinating the Mn-O bond energy of spinel-type lithium nickel manganese oxide at high temperature and the aspect ratio of carbon nanotubes, not only can the compatibility and dispersibility of the positive electrode active material and the conductive agent be improved, but the probability of side reactions between the positive electrode active material and the electrolyte under high temperature environment can also be reduced to a low level. The capacity retention rate of the secondary battery after 100 cycles at high temperature can reach 80% or more, and the gas production at high temperature can be reduced to less than 25 mL. The electrochemical performance under high temperature environment is excellent. In contrast, the secondary batteries or carbon nanotubes obtained in each comparative example were either too large or too small, or the Mn-O bond energy of spinel-type lithium nickel manganese oxide at high temperatures was either too high or too low, or the degree of control of both was inappropriate, resulting in the a / b ratio failing to meet the range of 0.0005~0.05. Their high-temperature cycle capacity retention rate was basically maintained at around 75%, and the high-temperature gas production was high, reaching a maximum of 42mL. Their high-temperature performance and even safety could not meet the standards.

[0130] (2) As can be seen from Examples 1 to 18, when the strength of the Mn-O bond of spinel-type lithium nickel manganese oxide in the positive electrode of the secondary battery changes, that is, when the structural stability of the material changes, the probability of side reactions at high temperature and the efficiency of ion cycling insertion and extraction will also change. On the other hand, the aspect ratio of carbon nanotubes affects the integrity and dispersion uniformity of the conductive network formed when it is compounded with the positive electrode active material. When the two are constructed, the change in their ratio will also directly affect the high-temperature electrochemical performance of the secondary battery. When a / b is further preferably in the range of 0.0013 to 0.0028, the high-temperature cycle capacity retention rate of the secondary battery can be maintained above 85%, and the gas production can be maintained below 15 mL. When a is preferably 1.5 to 2% and b is preferably 700 to 800, the electrochemical performance of the secondary battery can be further improved, the cycle capacity retention rate at high temperature reaches above 90%, and the gas production is less than 10 mL.

Claims

1. A secondary battery, characterized in that, Includes a positive electrode sheet, wherein the positive electrode sheet comprises a positive active material and a conductive agent; The positive electrode active material includes spinel-type lithium nickel manganese oxide; After 100 charge-discharge cycles, the characteristic peak at 630 nm in the Raman spectrum of the spinel-type lithium nickel manganese oxide shifts by a% compared to before the charge-discharge cycles. The charge-discharge cycle temperature is 45℃, the charge-discharge rate is 1C, and the charge-discharge cutoff voltage is 3.5V~4.8V; where a = 0.5~5. The conductive agent includes carbon nanotubes; The secondary battery satisfies: a / b = 0.0005~0.05; Where b is the average aspect ratio of the carbon nanotube, and b = 100~1000.

2. The secondary battery as described in claim 1, characterized in that, The a / b ratio is 0.0013 to 0.0028.

3. The secondary battery as described in claim 1, characterized in that, The carbon nanotubes have a diameter of 2-20 nm and a length of 200-2000 nm.

4. The secondary battery as described in claim 1, characterized in that, The positive electrode sheet includes a positive electrode material layer, which includes a positive electrode active material and carbon nanotubes. The mass percentage of carbon nanotubes in the positive electrode material layer is 0.05~1%.

5. The secondary battery as described in claim 1, characterized in that, The carbon nanotubes include at least one of single-walled carbon nanotubes, modified single-walled carbon nanotubes, multi-walled carbon nanotubes, and modified multi-walled carbon nanotubes.

6. The secondary battery as described in claim 1, characterized in that, The positive electrode active material further includes doping elements, which include at least one of Al, Zr, P, and W.

7. The secondary battery as described in claim 1, characterized in that, The positive electrode active material further includes a coating layer, which includes at least one of Al2O3, Li3P, ZrO2, and TaO2.

8. An electrical device, characterized in that, Includes the secondary battery as described in any one of claims 1 to 7, wherein the secondary battery serves as the power supply for the electrical device.

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