Secondary batteries

Abstract

To provide a secondary battery which uses a three-dimensional material as a positive electrode material and can improve the safety performance.SOLUTION: The secondary battery includes a positive electrode sheet, a separator, and a negative electrode sheet. The positive electrode sheet has a positive electrode material including a first active material and a second active material. The first active material is a layered structural material, and the second active material is an olivine structural material. The mass ratio of the second active material to the sum of masses of the first active material and the second active material is in the range of 5 wt.%-30 wt.%. The discharge curve of the secondary battery has a first voltage platform and a second voltage platform.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the technical field of secondary batteries, and more particularly to secondary batteries. [Background technology]

[0002] With advances in science and technology and the growing need for environmental protection, electrochemical devices (such as lithium-ion batteries) are becoming more prevalent in people's daily lives. As lithium-ion batteries become more widespread, their safety performance is receiving increased attention, especially due to fires and explosions involving electric vehicles. As a result, users, after-sales personnel, and lithium-ion battery manufacturers are all raising new requirements for the safety performance of lithium-ion batteries. In response, various countries are also introducing stricter laws and regulations. Based on the current safety situation of lithium batteries, several design improvements are needed to enhance their safety.

[0003] Currently, ternary materials are used as cathode materials for lithium-ion batteries. They have excellent advantages, such as high energy density, a high voltage platform, an ideal crystal structure, low self-discharge, and no memory effect. Ternary materials are currently commonly used in lithium-ion batteries. However, their high-temperature performance is deficient, and their rate performance and structural stability are not as favorable as those of lithium iron phosphate. Therefore, the resulting ternary-based lithium-ion batteries have inferior high-temperature performance, cycle performance, and structural stability compared to lithium iron phosphate batteries.

[0004] Therefore, in order to solve the above problems, it is necessary to design a secondary battery that can improve the safety performance of batteries that use ternary materials as the main positive electrode material. Summary of the Invention [Problem to be solved by the invention]

[0005] In view of the above-mentioned shortcomings of the related art, the present invention provides a secondary battery that solves the technical problem of the poor safety performance of currently available ternary secondary batteries. [Means for solving the problem]

[0006] To achieve the above and other related objects, the present invention provides a secondary battery including a positive electrode sheet, a separator, and a negative electrode sheet. The positive electrode sheet includes a positive electrode material including a first active material and a second active material.

[0007] Here, the first active material is a layered structure material. The second active material is an olivine structure material. The mass ratio of the second active material to the total mass of the first active material and the second active material is 5% by weight to 30% by weight. The discharge curve of the secondary battery has a first voltage platform and a second voltage platform. The measured voltage y of the secondary battery is y1 min <y<y1 max or y2 min <y<y2 max and y1 and y2 satisfy the formulas (1) and (2). y1=0.5265a+d (1) y2=-0.5265b+c (2) In the formula, a is the mass ratio of the first active material to the total mass of the first active material and the second active material, b is the mass ratio of the second active material to the total mass of the first active material and the second active material, c is a first voltage platform on which the first active material is involved in discharge, the first voltage platform being 3.7143V to 3.7743V, and d is a second voltage platform on which the second active material is involved in discharge, the second voltage platform being 3.1878V to 3.2478V.

[0008] In one embodiment of the present invention, the curve proportion ratio value of the second active material involved in discharge in the discharge curve is 80% to 87% of the mass proportion b.

[0009] In one embodiment of the present invention, the curve ratio of the second active material involved in the discharge curve is 4% to 25%.

[0010] In one embodiment of the present invention, the layered structure material is LiNi x Co y M 1-x-y O2, where 0 < x < 1, 0 < y < 1, and the element M includes one or more of Mn, Ti, Zr, Al, Sn, Zn, Mg, Cu, In, Ga, Ta.

[0011] In one embodiment of the present invention, the olivine structure material is LiFe z G 1-z PO4, where 0 < z < 1, and the element G includes one or more of Mn, Ti, Zr, Al, Sn, Zn, Mg, Cu, In, Ga, Ta.

[0012] In one embodiment of the present invention, the average particle diameter D50 of the first active material is 0.5 μm to 5 μm, and the average particle diameter D50 of the second active material is 0.3 μm to 3 μm.

[0013] In one embodiment of the present invention, the first active material is single crystal and / or polycrystal. <0​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

[0017] In the secondary battery provided by the present invention, a two-stage discharge voltage platform is introduced during the discharge process of the secondary battery by mixing an olivine structure material with a positive electrode material mainly composed of a layered structure material. In this way, the secondary battery can transition from discharging at a higher first voltage platform to discharging at a lower second voltage platform during discharge. As a result, the voltage and capacity of the secondary battery are improved, while the discharge depth of the layered structure material is reduced, thereby improving the thermal stability of the secondary battery. Thermal runaway of the battery during overcharge and overdischarge is prevented, improving the safety performance of the secondary battery. Therefore, the present invention can effectively solve several practical problems in the prior art and demonstrate great utility and significance. [Brief explanation of the drawings]

[0018] In order to more clearly illustrate the technical solutions provided in the embodiments of the present invention or related technologies, some accompanying drawings necessary for the description in the embodiments or related technologies are briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative efforts. [Figure 1] 1 is a graph of discharge curve testing of secondary batteries according to some embodiments of the present invention. [Figure 2] 4 is a graph showing the relationship between the measured voltage and the mass fraction of the first active material of secondary batteries according to some embodiments of the present invention. [Figure 3] 4 is a graph showing the relationship between the measured voltage and the mass fraction of the second active material of a secondary battery according to an embodiment of the present invention. [Figure 4] 1 is an XRD test graph of a positive electrode sheet of a secondary battery according to an embodiment of the present invention. [Figure 5] 1 is an XRD test graph of a negative electrode sheet of a secondary battery according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] The implementation of the present invention will be described below through specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the disclosure of this specification. The present invention can also be implemented or applied through other different specific implementation methods. The details of this specification can be modified or changed based on different perspectives and applications without departing from the spirit of the present invention. The following embodiments and features of each embodiment can be combined if not inconsistent. It should also be understood that the terms used in the embodiments of the present invention are intended to describe specific implementations and are not intended to limit the scope of protection of the present invention. Test methods without specific conditions in the following embodiments usually follow conventional conditions or conditions provided by each manufacturer.

[0020] In addition, terms such as "upper," "lower," "left," "right," "middle," and "one side" used in this specification are used solely for the convenience of explanation and are not used to limit the scope of application of the present invention. Changes or adjustments of the relative relationships without substantially changing the technical content are also included in the scope of application of the present invention.

[0021] Through research, the inventors have found that although secondary batteries using ternary laminated materials as positive electrode materials have the advantages of high energy density and high discharge voltage platform, there are hidden risks to the safety performance of secondary batteries due to the poor thermal stability of ternary materials. Specifically, during the over-discharge process at high voltage, Ni in the ternary materials 4+ Ions are Ni 2+ At the same time, to maintain charge neutrality, O 2- is oxidized and escapes from the crystal lattice. The escape of a large amount of oxygen creates a large number of oxygen vacancies, which reduces the activation energy barrier for the migration of transition metal cations and accelerates the phase transition process of the material from a layered structure to a spinel structure and a rock salt structure. The release of oxygen and the structural degradation of the cathode material form a vicious cycle, which affects the thermal runaway temperature of the battery. As a result, the thermal stability of the battery is reduced, leading to safety risks.

[0022] Therefore, when the cathode material is subjected to a higher discharge voltage for a long period of time during the discharge process, the irreversible structural collapse of the layered structure material worsens. The layered structure material generates a large amount of gas as the structure collapses, lowering the activation energy barrier. This causes the layered structure of the cathode material to continue to collapse during the discharge process, resulting in a rapid decrease in the thermal runaway temperature of the battery.

[0023] Therefore, the present invention provides a secondary battery that solves the above-mentioned problems. In the secondary battery, an olivine structure material is mixed with a positive electrode material mainly composed of a layered structure material. By mixing the olivine structure material with the layered structure material, the storage voltage capacity of the secondary battery is improved, while at the same time introducing a two-stage discharge voltage platform during the discharge process. During discharge, the secondary battery can transition from discharging at a higher first voltage platform to discharging at a lower second voltage platform. In this way, the discharge depth of the layered structure material is effectively reduced under the high voltage platform, improving the structural thermal stability of the positive electrode material and preventing a rapid drop in the battery's thermal runaway temperature due to overdischarge.

[0024] The present invention provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, a separator, corresponding communication accessories, and a circuit. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the positive electrode active material layer comprising a positive electrode material. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer comprising a negative electrode material. The negative electrode material may be a carbon-based material (e.g., graphite, hard carbon, soft carbon), a silicon-based material (e.g., silicon oxide), or a carbon-silicon composite material. In the secondary battery, the positive electrode material and the negative electrode material can desorb lithium ions to achieve energy storage and release. The electrolyte is a medium for transporting lithium ions between the positive electrode sheet and the negative electrode sheet. The separator is permeable to lithium ions but not to electricity, separating the positive and negative electrode sheets to prevent short circuits. Generally, the positive and negative electrode materials are crucial for the energy storage function of lithium batteries, as well as for the cell's energy density, cycle performance, and safety.

[0025] In the secondary battery, the positive electrode material includes a first active material and a second active material. The first active material is a layered structure material such as lithium cobalt oxide, lithium nickel cobalt manganese oxide, or lithium nickel cobalt aluminate. The second active material is an olivine structure material such as lithium iron phosphate or lithium iron manganese phosphate. In the positive electrode material, the mass ratio of the first active material to the total mass of the first and second active materials is 70% to 95% by weight. The mass ratio of the second active material to the total mass of the first and second active materials is 5% to 30% by weight. For example, in some embodiments, the mass percentage of the first active material is 95%, 90%, 85%, 80%, 75%, or 70% by weight, and correspondingly, the mass percentage of the second active material is 95%, 90%, 85%, 80%, 75%, or 70% by weight, and 5%, 10%, 15%, 20%, 25%, or 30% by weight.

[0026] In the present invention, the first active material and the second active material may further contain a nonmetallic element, such as at least one of fluorine, phosphorus, boron, chlorine, silicon, and sulfur, which can further improve the stability of the positive electrode active material.

[0027] The material form of the first active material is not limited. For example, the first active material may be single crystal, polycrystalline, or a mixture of single crystal and polycrystalline. In some embodiments, the first active material is polycrystalline.

[0028] As shown in Figures 1 to 3, the positive electrode material of a secondary battery includes a layered structure material and an olivine structure material, so the discharge curve of the secondary battery has a first voltage platform with a higher voltage and a second voltage platform with a lower voltage. The measured voltage y during the discharge process of the secondary battery is between the first voltage platform and the second voltage platform. The measured voltage is the operating voltage of the secondary battery during discharge, and usually refers to the potential difference between the positive and negative electrodes of the battery under normal operating conditions. Specifically, the measured voltage y of the secondary battery is y1 min <y<y1 max or y2 min <y<y2 max and y1 and y2 satisfy the formulas (1) and (2). y1=0.5265a+d (1) y2=-0.5265b+c (2) In the formula, a is the mass ratio of the first active material to the total mass of the first active material and the second active material, b is the mass ratio of the second active material to the total mass of the first active material and the second active material, c is a first voltage platform on which the first active material is involved in discharge, the first voltage platform being 3.7143V to 3.7743V, and d is a second voltage platform on which the second active material is involved in discharge, the second voltage platform being 3.1878V to 3.2478V.

[0029] Specifically, as shown in FIGS. 2 and 3, the measured voltage y of the secondary battery is y1 min <y<y1 max where y1 min =0.5265a+3.1878 and y1 max =0.5265a+3.2478. Alternatively, the measured voltage y of the secondary battery is y2 min <y<y2 max where y2 min =-0.5265b+3.7143 and y2 max =-0.5265b+3.7743.

[0030] As shown in Figures 1-3, the layered structure material and olivine structure material mixed in the positive electrode material not only establish a first voltage platform and a second voltage platform during the discharge process of the secondary battery, but also ensure that the measured voltage of the secondary battery does not decay excessively. Based on the change in the mass fraction b of the second active material, the measured voltage of the secondary battery during discharge is maintained between 3.55 V and 3.75 V. For example, when the mass fraction b of the second active material relative to the total mass of the first and second active materials in the positive electrode material is 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, or 30 wt%, the corresponding measured voltage of the secondary battery during discharge is 3.72 V, 3.70 V, 3.68 V, 3.66 V, 3.63 V, or 3.61 V, respectively. Within this operating voltage range, the secondary battery can maintain high discharge efficiency.

[0031] As shown in FIGS. 1 to 3, the proportion of the curve occupied by the introduced second voltage platform in the discharge curve of a secondary battery is positively correlated with the mass content of the olivine structure material mixed with the positive electrode material. In some embodiments, the proportion of the curve occupied by the second active material involved in the discharge in the discharge curve is 80% to 87% of the mass proportion b of the second active material. For example, in some examples, when the mass proportion b of the second active material in the positive electrode material is 5% to 30% by weight, the proportion of the curve occupied by the second active material involved in the discharge is 4% to 25%. In a secondary battery, as the mass proportion of the positive electrode material mixed with the olivine structure material increases, the discharge depth of the second voltage platform in the discharge curve also increases, thereby increasing the thermal runaway temperature of the secondary battery and significantly improving the thermal stability of the battery.

[0032] In some embodiments, the layered structure material is LiNi x Co y M 1-x-yO2, where 0 < x < 1 and 0 < y < 1. In some embodiments, element M is selected from one or more combinations of Mn, Ti, Zr, Al, Sn, Zn, Mg, Cu, In, Ga, and Ta. That is, element M may be any one of the elements listed above, such as Mn, Ti, Zr, Al, Sn, Zn, Mg, Cu, In, Ga, or Ta. Element M may also be any combination of two or more of the elements listed above. For example, element M may be a combination of Mn and Zr, a combination of Mn and Mg, a combination of Al and Ti, a combination of Al and Zn, a combination of Mn, Al, and Ga, or a combination of Mn, Al, Cu, Mg, etc., which are not enumerated one by one here.

[0033] The olivine structure material is LiFe z G 1-z PO4, where 0 < z < 1. In some embodiments, element G is selected from one or more combinations of Mn, Ti, Zr, Al, Sn, Zn, Mg, Cu, In, Ga, and Ta. That is, element G may be any one of the elements listed above, such as Mn, Ti, Zr, Al, Sn, Zn, Mg, Cu, In, Ga, or Ta. Element G may also be any combination of two or more of the elements listed above. For example, element G may be a combination of Mn and Zr, a combination of Mn and Mg, a combination of Al and Ti, a combination of Al and Zn, a combination of Mn, Al, and Ga, or a combination of Mn, Al, Cu, Mg, etc., which are not enumerated one by one here.

[0034] Also, when element M and element G are in a combination of two or more, the combination ratio of each element is not particularly limited, and the elements can be mixed in any ratio. In other embodiments, element M and element G may be types of elements not listed above.

[0035] In some embodiments, the average particle size D50 of the first active material is 0.5 μm to 5 μm, for example, the average particle size D50 of the first active material can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm. The average particle size D50 of the second active material is 0.3 μm to 3 μm, for example, the average particle size D50 of the second active material can be 0.3 μm, 0.5 μm, 0.7 μm, 1 μm, 1.3 μm, 1.5 μm, 1.7 μm, 2 μm, 2.3 μm, 2.5 μm, 2.7 μm, or 3 μm.

[0036] Within the particle size range, the particle size distributions of the first and second active materials overlap. The resulting positive electrode material has good uniformity after mixing and rolling the first and second active materials, and the contact interface between the first and second active materials has good affinity. This ensures the conductive efficiency of the positive electrode material and improves its structural stability, further improving the thermal safety of the secondary battery.

[0037] In some embodiments, the degree of orientation of the positive electrode sheet is 70 to 99, for example, the degree of orientation of the positive electrode sheet may be 70, 75, 80, 85, 90, 95, or 99. Here, as shown in Fig. 4, the degree of orientation of the positive electrode sheet is Qc = C003 / C110, where C003 is the peak area of ​​the 003 characteristic diffraction peak in the X-ray diffraction spectrum of the positive electrode sheet, and C110 is the peak area of ​​the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the positive electrode sheet. The diffraction angle of the 003 characteristic peak is in the range of 17° to 19.5°, and the diffraction angle of the 110 characteristic diffraction peak is in the range of 65° to 66.5°.

[0038] In some embodiments, the degree of orientation of the negative electrode sheet may be 10 to 60. For example, the degree of orientation of the negative electrode sheet may be 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60. Here, as shown in FIG. 5 , the degree of orientation of the negative electrode sheet is Qc=C004 / C110. C004 is the peak area of ​​the 004 characteristic diffraction peak in the X-ray diffraction spectrum of the negative electrode sheet, and C110 is the peak area of ​​the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the negative electrode sheet. The diffraction angle of the 004 characteristic peak is in the range of 54° to 56°, and the diffraction angle of the 110 characteristic diffraction peak is in the range of 76° to 78°.

[0039] The inventors have found that the mechanical performance of the positive electrode sheet and the negative electrode sheet is best when the orientation degree of the positive electrode sheet is 70 to 99 and the orientation degree of the negative electrode sheet is 10 to 60. Because lithium ions are released smoothly from the positive and negative electrode sheets, the charge / discharge performance, cycle performance, and safety performance of the lithium-ion battery are improved.

[0040] In the present invention, the positive electrode sheet is subjected to an XRD scan test in accordance with JIS K 0131-1996, and the negative electrode sheet is subjected to an XRD scan test in accordance with JB / T4220-2011. For example, the installed electrode sheet is scanned with X-rays at a scanning speed of (1 / 4)° / min, and X-ray diffracted light is scanned and received at different diffraction angles using a 0.2 mm wide beam to obtain an X-ray spectrum of the test electrode sheet. Here, for the positive electrode sheet, the diffraction angle of the 003 diffraction characteristic peak is in the range of 17° to 19.5°, and the diffraction angle of the 110 diffraction characteristic peak is in the range of 65° to 66.5°. For the negative electrode sheet, the diffraction angle of the 004 diffraction characteristic peak is in the range of 54° to 56°. For the negative electrode sheet, the scanning range of the diffraction angle of the 110 diffraction characteristic peak is 76° to 78°.

[0041] In some embodiments, the positive electrode sheet has a compressed density of 3.0 g / cm 3 ~3.6g / cm 3 For example, 3.0 g / cm 3 , 3.1g / cm 3 , 3.2g / cm3 , 3.3g / cm 3 , 3.4g / cm 3 , 3.5g / cm 3 , or 3.6 g / cm 3 The surface density of the coating on one side of the positive electrode sheet may be 0.014 g / cm 2 ~0.02g / cm 2 For example, 0.014 g / cm 2 , 0.015g / cm 2 , 0.016g / cm 2 , 0.017g / cm 2 , 0.018g / cm 2 , 0.019g / cm 2 , or 0.020 g / cm 2 That's fine.

[0042] In some embodiments, the compressed density of the negative electrode sheet is 1.1 g / cm 3 ~1.7g / cm 3 For example, 1.1 g / cm 3 , 1.2g / cm 3 , 1.3g / cm 3 , 1.4g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , or 1.7 g / cm 3 The surface density of the coating on one side of the negative electrode sheet may be, for example, 0.008 g / cm 2 , 0.009g / cm 2 , 0.010g / cm 2 , 0.011g / cm 2 , 0.012g / cm 2 , or 0.013 g / cm 2 That's fine.

[0043] In the present invention, the compression density and areal density of the positive electrode sheet and the negative electrode sheet are one-sided values, and the test method can refer to conventional test methods used by those skilled in the art. For example, the areal density of the electrode sheet is first determined by testing, and then the compression density of the electrode sheet is calculated by dividing the areal density by the thickness of the electrode sheet.

[0044] Furthermore, through research, the inventors have found that when a lithium-ion battery including a composite positive electrode material (i.e., a positive electrode material including both a first active material and a second active material) satisfies the above-mentioned multiple parameter constraints, the resulting lithium-ion battery will have optimal charge / discharge performance, cycle performance, and safety performance, thereby meeting actual usage needs and reducing usage costs.

[0045] The composition and manufacturing method of the secondary battery provided by the present invention will be described in detail below.

[0046] To prepare the positive electrode sheet, the positive electrode material, conductive agent, and binder were mixed in a mass ratio of (90-99):1.5:1.5, with a preferred mass ratio of 97:1.5:1.5. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was thoroughly stirred to obtain a positive electrode slurry. The positive electrode slurry was applied to an aluminum foil positive electrode current collector, followed by drying, cold pressing, cutting, and other processes to prepare a positive electrode sheet. The conductive agent can be selected from at least one conductive material, such as Super P (SP) (carbon black), acetylene black, carbon nanotubes (CNT), graphene, and nanocarbon fiber (VGCF). For example, when the conductive agent is SP and CNT, the mass ratio of SP to CNT is 1:0.5. The binder can be selected from at least one of PVDF, PTFE, and the like.

[0047] Here, the method for producing the positive electrode material includes the following steps.

[0048] The first active material and the second active material were mixed uniformly in a mass ratio of (70-95):(5-30) to obtain a mixture. The mixture was rolled to obtain a positive electrode material. Here, the first active material is a layered structure material such as lithium cobalt oxide, nickel cobalt lithium manganate, or nickel cobalt lithium aluminate. The second active material is an olivine structure material such as lithium iron phosphate or lithium iron manganese phosphate.

[0049] To prepare the negative electrode sheet, negative electrode materials such as graphite, conductive agent acetylene black, thickener CMC (sodium carboxymethyl cellulose), and binder SBR (styrene butadiene rubber) were mixed in a mass ratio of 96.4:1:1.2:1.4, deionized water was added, and the mixture was thoroughly stirred to obtain negative electrode slurry. The negative electrode slurry was uniformly applied to a negative electrode current collector copper foil, and then subjected to processes such as drying, cold pressing, and slitting to prepare the negative electrode sheet.

[0050] In producing the separator, the material is a PE or PP porous membrane as the separator, preferably a PP / PE / PP porous membrane having a thickness of 9 μm to 18 μm (e.g., 9 μm, 12 μm, 16 μm, or 18 μm), an air permeability of 180 sec / 100 mL to 380 sec / 100 mL (e.g., 180 sec / 100 mL, 280 sec / 100 mL, or 380 sec / 100 mL), and a porosity of 30% to 50% (e.g., 30%, 40%, or 50%).

[0051] To prepare the electrolyte, EC (ethylene carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate) were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Next, thoroughly dried lithium salt LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0052] To assemble the battery, the fabricated positive electrode sheet, separator, and negative electrode sheet were stacked in this order, with a separator placed between the positive and negative electrode sheets to provide insulation. The stack was then covered with aluminum plastic film and dried, after which the electrolyte prepared above was poured in. After sealing, leaving it, chemical conversion, and other processes, a pouch battery (lithium-ion battery) with a capacity of 1 Ah was finally obtained.

[0053] The specific process and conditions for forming the electrolyte are provided below. After the electrolyte was injected, the battery was maintained in a high-temperature and pressure environment of 0.1 MPa, and then charged at 45°C and 0.02°C for 17 minutes in a static state. After leaving it for 5 minutes, it was charged at 0.02°C to 0.3 Ah. The airbag was then removed and vacuum-sealed, and the electrolyte was left at room temperature for 48 hours to complete the formation of the electrolyte.

[0054] The technical solutions of the present invention will be described in detail below through some specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art.

[0055] Example 1

[0056] In this embodiment, a secondary battery is provided, and the steps of preparing the secondary battery are provided as follows.

[0057] (1) To prepare the positive electrode sheet, the positive electrode material, conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97:1.5:1.5. N-methylpyrrolidone (NMP) was added as a solvent to the mixture and thoroughly stirred to obtain a positive electrode slurry. The mixture was stirred using a vacuum mixer until it became homogeneous and transparent, yielding a positive electrode slurry. The positive electrode slurry was uniformly applied to a 16 μm aluminum foil current collector. The aluminum foil current collector was dried at room temperature and then transferred to an oven where it was dried at 80°C to 120°C for 6 hours. Subsequently, cold pressing and slitting were performed to obtain a positive electrode sheet.

[0058] Here, the positive electrode material is lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 The cathode material contained a first active material (NCM) and a second active material (LFP) at a mass ratio of 95:5, and the first active material (NCM) and the second active material (LFP) were uniformly mixed at a mass ratio of 95:5 to obtain a mixture, which was then rolled to obtain the cathode material.

[0059] (2) To prepare the negative electrode sheet, graphite as the negative electrode material, acetylene black as the conductive agent, sodium carboxymethylcellulose (CMC) as the thickener, and styrene-butadiene rubber (SBR) as the binder were mixed in a mass ratio of 96.4:1:1.2:1.4, followed by the addition of deionized water. The solids concentration of the slurry was adjusted to 55%, and the mixture was thoroughly stirred and mixed using a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly applied to both sides of an 8 μm negative electrode current collector copper foil, dried at room temperature, and then transferred to an oven for drying. Thereafter, cold pressing, slitting, etc. were performed to obtain a negative electrode sheet.

[0060] (3) For the preparation of the electrolyte, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 in an argon atmosphere glove box with a water content of less than 10 ppm to obtain an organic solvent. Next, the lithium salt LiPF6 that had been thoroughly dried was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0061] (4) In the preparation of the separator, a 9 μm polyethylene separator with a 2 μm ceramic coating on both sides was used.

[0062] (5) To assemble the battery, the resulting positive electrode sheet, separator, and negative electrode sheet were stacked in this order, with a separator placed between the positive and negative electrode sheets to provide insulation. The assembly was then covered with aluminum plastic film, transferred to a vacuum oven at 120°C to dry, and then sealed after being filled with 3.0 g / Ah of electrolyte. After undergoing processes such as standing, hot pressing, cold pressing, chemical formation, clamping, and grading, a pouch battery (i.e., a lithium-ion battery) with a capacity of 1 Ah was finally obtained.

[0063] The specific process and conditions for forming the electrolyte are provided below. After the electrolyte was injected, the battery was maintained in a high-temperature and pressure environment of 0.1 MPa, and then charged at 45°C and 0.02°C for 17 minutes in a static state. After leaving it for 5 minutes, it was charged to 0.3 Ah at 0.02°C. The airbag was then separated and vacuum-sealed, and the electrolyte was left at room temperature for 48 hours to complete the formation of the electrolyte.

[0064] Example 2

[0065] In this example, a secondary battery of the same system as in Example 1 is provided. The difference between this example and Example 1 is that the positive electrode material of the positive electrode sheet is lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2, NCM) and lithium iron phosphate (LiFePO4, LFP) with a mass fraction b of 10 wt %.

[0066] Example 3

[0067] In this example, a secondary battery of the same type as in Example 1 is provided. The difference between this example and Example 1 is that the positive electrode material of the positive electrode sheet is lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2, NCM) and lithium iron phosphate (LiFePO4, LFP) with a mass fraction b of 15 wt %.

[0068] Example 4

[0069] In this example, a secondary battery of the same type as in Example 1 is provided. The difference between this example and Example 1 is that the positive electrode material of the positive electrode sheet is lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2, NCM) and lithium iron phosphate (LiFePO4, LFP) with a mass fraction b of 20 wt %.

[0070] Example 5

[0071] In this example, a secondary battery of the same type as in Example 1 is provided. The difference between this example and Example 1 is that the positive electrode material of the positive electrode sheet is lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2, NCM) and lithium iron phosphate (LiFePO4, LFP) with a mass fraction b of 25 wt %.

[0072] Example 6

[0073] In this example, a secondary battery of the same type as in Example 1 is provided. The difference between this example and Example 1 is that the positive electrode material of the positive electrode sheet is lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2, NCM) and lithium iron phosphate (LiFePO4, LFP) with a mass fraction b of 30 wt %.

[0074] Comparative Example 1

[0075] In this comparative example, a secondary battery of the same system as in Example 1 is provided. The difference between this comparative example and Example 1 is that the positive electrode material of the positive electrode sheet is not mixed with the second active material, lithium iron phosphate (LiFePO4, LFP), but with the first active material, lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2, NCM).

[0076] The secondary batteries produced in Examples 1 to 6 and Comparative Example 1 were subjected to a discharge test and a nail penetration test to verify the difference in safety performance between the secondary batteries of Examples and Comparative Example.

[0077] Discharge curve test:

[0078] At 25°C, the secondary battery was charged at a constant current of 0.1 C to the upper cutoff voltage, then charged at a constant voltage until the current became less than 0.05 C, left for 10 minutes, and then discharged at a constant current of 0.1 C to the lower cutoff voltage. The discharge capacity curve at this time is the discharge curve.

[0079] In the nail penetration test, the secondary battery was fully charged to 100% SOC. An 8mm diameter steel needle was used to pierce the secondary battery at a speed of 25mm / s. After one hour of observation, the maximum temperature of the secondary battery cell was collected. If the secondary battery did not ignite, it was considered to have passed the nail penetration test. If the secondary battery ignited, the elapsed time from ignition to ignition was recorded.

[0080] Measurement voltage test:

[0081] The secondary battery was discharged at a standard rate of 1 C. The energy released by discharging the secondary battery was divided by its capacity to obtain the actual measured voltage.

[0082] The test results of the secondary batteries of Examples 1 to 6 and Comparative Example 1 are shown in Table 1.

[0083] Table 1: Discharge performance and safety performance tests of secondary batteries of Examples 1 to 6 and Comparative Example 1

[0084] [Table 1]

[0085] Comparing the test results of Examples 1-6 and Comparative Example 1, it can be seen that adding LFP to an NCM-based cathode material effectively improves the thermal stability of the cathode material and significantly improves the safety performance of secondary batteries. Here, by adding LFP to the cathode material, the LFP participates in the second voltage platform of discharge during the discharge process of the secondary battery. This effectively reduces the discharge depth of the layered structure material under high voltage platforms, improves the structural thermal stability of the cathode material, and prevents a rapid drop in the battery's thermal runaway temperature due to overdischarge. This significantly improves the safety performance of secondary batteries.

[0086] Comparing the test results for Examples 1 to 6, it can be seen that as the mass fraction b of the LFP material added to the positive electrode material increases, the proportion of the curve in which NCM contributes to the discharge of the secondary battery decreases. Accordingly, the proportion of the curve in which LFP contributes to the discharge increases, and the maximum temperature of the secondary battery during the nail penetration process also decreases significantly. For example, when the mass fraction b of the LFP material in the positive electrode material increases from 5 wt% to 20 wt%, the maximum temperature of the secondary battery during the nail penetration process decreases from 210°C to 30°C.

[0087] Furthermore, the test results of Examples 3 to 6 indicate that when the mass fraction b of LFP exceeds 20 wt%, increasing the mass content of LFP in the positive electrode material does not significantly improve the thermal safety performance of the secondary battery. Instead, the measured voltage of the secondary battery further decreases during discharge. For example, when the mass fraction b of the LFP material in the positive electrode material increases from 20 wt% to 30 wt%, the maximum temperature of the secondary battery during the nail penetration test only decreases from 30°C to 28.5°C, while the measured voltage of the secondary battery continues to decrease from 3.66 V to 3.61 V. It is clear that the mass fraction b of the second active material LFP in the positive electrode material is preferably 20 wt%. A positive electrode material with this mass fraction can achieve the best improvement in the thermal safety performance of the secondary battery, provided that the measured voltage of the secondary battery does not significantly decrease. [Industrial Applicability]

[0088] Furthermore, the secondary battery of the present invention can be used in the field of electronics and has high industrial applicability.

[0089] The above-described embodiments are illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Those skilled in the art may modify or change the above-described embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical ideas disclosed in the present invention are also intended to be included in the scope of the claims of the present invention.

Claims

1. A secondary battery including a positive electrode sheet, a separator, an electrolyte, and a negative electrode sheet, the positive electrode sheet includes a positive electrode material including a first active material that is a layered structure material and a second active material that is an olivine structure material; a mass ratio of the second active material to the total mass of the first active material and the second active material is 5 wt % to 30 wt %, a discharge curve of the secondary battery includes a first voltage platform and a second voltage platform, a measured voltage y of the secondary battery satisfies the relationship y1min<y<y1max or y2min<y<y2max, and y1 and y2 satisfy formula (1) and formula (2). y1=0.5265a+d (1) y2=-0.5265b+c (2) wherein a is the mass fraction of the first active material relative to the sum of the masses of the first active material and the second active material; b is the mass fraction of the second active material relative to the sum of the masses of the first active material and the second active material; c is the first voltage platform on which the first active material participates in discharge, the first voltage platform being 3.7143V to 3.7743V; d is the second voltage platform on which the second active material participates in discharge, the second voltage platform being 3.1878V to 3.2478V; The orientation degree of the positive electrode sheet is 70 to 99, and the orientation degree of the negative electrode sheet is 10 to 60, The layered structure material is LiNi0.8Co0.1Mn0.1O2.

2. 2. The secondary battery according to claim 1, wherein the curve ratio value of the second active material involved in discharge in the discharge curve is 80% to 87% of the mass ratio b.

3. 3. The secondary battery according to claim 2, wherein the curve ratio of the second active material involved in the discharge curve is 4% to 25%.

4. 2. The secondary battery of claim 1, wherein the olivine structure material is LiFezG1-zPO4, where 0<z<1, and element G includes one or more of Mn, Ti, Zr, Al, Sn, Zn, Mg, Cu, In, Ga, and Ta.

5. 2. The secondary battery according to claim 1, wherein the first active material has an average particle size D50 of 0.5 μm to 5 μm, and the second active material has an average particle size D50 of 0.3 μm to 3 μm.

6. The secondary battery according to claim 1 , wherein the first active material is single crystalline and / or polycrystalline.

7. 2. The secondary battery of claim 1, wherein the positive electrode sheet has a compressed density of 3.0 g / cm to 3.6 g / cm, and an areal density of the coating on one side of the positive electrode sheet is 0.014 g / cm to 0.02 g / cm.

8. 2. The secondary battery of claim 1, wherein the negative electrode sheet has a compressed density of 1.1 g / cm to 1.7 g / cm, and an areal density of the coating on one side of the negative electrode sheet is 0.008 g / cm to 0.013 g / cm.

9. The negative electrode sheet includes a negative electrode material, 2. The secondary battery according to claim 1, wherein the negative electrode material is a carbon-based material, a silicon-based material, or a carbon-silicon composite material.

10. The secondary battery according to claim 9 , wherein the negative electrode material is graphite, hard carbon, soft carbon, or silicon oxide.

11. 2. The secondary battery according to claim 1, wherein the electrolyte comprises EC, DMC, or EMC and a lithium salt comprising LiPF6, and the separator comprises PE, PP, or a PP / PE / PP composite film.

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