Secondary battery and electronic device

Abstract

The invention discloses a secondary battery and an electronic device. The secondary battery includes a positive electrode including a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, the positive electrode material layer including a positive electrode active material and a lithium pyrochlore type oxyfluoride, and an electrolyte. The positive electrode active material comprises lithium cobalt oxide and / or nickel cobalt lithium manganate, the average particle size of the positive electrode active material is L1 [mu] m, and L1 is greater than or equal to 3 and less than or equal to 20; the average particle diameter of the lithium pyrochlore type oxyfluoride is L2 [mu] m, and L2 is greater than or equal to 0.1 and less than or equal to 1. The lithium pyrochlore oxyfluoride is a composite anionic compound with a pyrochlore structure, and in an X-ray diffraction pattern of the pyrochlore structure, characteristic diffraction peaks appear at positions of diffraction angles 2 theta at (29.5 + / -1) degrees, (34.5 + / -1) degrees and (49.5 + / -1) degrees. The secondary battery provided by the invention has relatively excellent low-temperature load performance.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and in particular to a secondary battery and electronic device. Background Technology

[0002] Lithium-ion batteries, containing cathode materials such as lithium cobalt oxide and / or lithium nickel cobalt manganese oxide, are widely used in portable electronic devices, power tools, and new energy vehicles due to their high operating voltage and energy density. However, under low-temperature conditions, the diffusion rate of lithium ions in lithium cobalt oxide and lithium nickel cobalt manganese oxide materials decreases sharply, and the electrode-electrolyte interface impedance increases dramatically, resulting in severe battery polarization and insufficient capacity release. Therefore, improving the power output capability of cathode materials such as lithium cobalt oxide and lithium nickel cobalt manganese oxide at low temperatures is of great significance for improving the reliability of lithium-ion batteries in low-temperature environments. Summary of the Invention

[0003] In view of this, the purpose of this application is to provide a secondary battery to solve the problem of insufficient low-temperature power output of lithium-ion batteries containing positive electrode materials such as lithium cobalt oxide and / or lithium nickel cobalt manganese oxide, thereby improving the reliability of lithium-ion batteries in low-temperature environments.

[0004] To achieve the above and other related objectives, a first aspect of this application provides a secondary battery, comprising a positive electrode and an electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode active material and lithium pyrochlore-type fluorine oxide. The positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide, and the average particle size of the positive electrode active material is L1 μm, 3 ≤ L1 ≤ 20; the average particle size of the lithium pyrochlore-type fluorine oxide is L2 μm, 0.1 ≤ L2 ≤ 1.

[0005] The lithium pyrochlore-type fluorine oxides of this application refer to a class of complex anionic compounds with a pyrochlore structure (space group generally Fd-3m). They possess an open framework structure, facilitating rapid lithium-ion migration. Oxygen and fluoride ions jointly occupy the anion sites, and their chemical composition can also be represented as Li... (2-x) La (1+x) / 3 M₂O₆F, where 0 ≤ x ≤ 1.99, and M is one or more transition metal elements selected from Ti, V, Nb, Ta, Mo, or W. The above-mentioned pyrochlore structure can be confirmed by characteristic diffraction peaks located at approximately (29.5 ± 1)°, (34.5 ± 1)°, and (49.5 ± 1)° (2θ, Cu Kα radiation) in the X-ray diffraction pattern.

[0006] In some embodiments of the first aspect of this application, the aforementioned lithium pyrochlore-type fluorine oxide may include at least one of lithium lanthanum niobium oxyfluoride, lithium lanthanum tantalum oxyfluoride, or other doped or coated modified materials. For example, the lithium pyrochlore-type fluorine oxide may include Li₂La 1 / 3 Ta2O6F, Li 1.5 La 0.5 Ta2O6F, Li 0.9 La 0.7 Ta2O6F, Li 0.01 La 2.99 / 3 Ta2O6F, Li2La 1 / 3Nb2O6F, Li 1.2 La 0.6 Nb2O6F, Li 0.6 La 0.8 Nb₂O₆F or Li 0.01 La 2.99 / 3 At least one of Nb2O6F.

[0007] In this application, when measured by scanning electron microscopy, the aforementioned "average particle size" refers to the arithmetic mean of the equivalent circle diameter based on the particle's projected area.

[0008] Based on the above embodiments, this application introduces the aforementioned lithium pyrochlore-type fluoride into the cathode material layer. Since lithium pyrochlore-type fluoride possesses high lithium-ion conductivity and low lithium migration activation energy, dispersing it around the cathode active particles can construct an efficient ion transport network within and on the surface of the cathode material layer, reducing the cathode / electrolyte interface impedance and improving the low-temperature load-bearing performance of the secondary battery. Furthermore, by controlling the average particle size of the cathode active material and the lithium pyrochlore-type fluoride within the aforementioned range, a denser and more efficient ion transport network is formed around the cathode active material. This improves the ion transport efficiency within the cathode material layer at low temperatures, reduces the cathode active material / electrolyte interface impedance, and thus enhances the low-temperature load-bearing performance of the secondary battery. Additionally, it optimizes the ion insertion / extraction reaction efficiency on the cathode side, thereby facilitating the capacity utilization of the cathode active material and improving the cycle capacity retention rate of the secondary battery.

[0009] In some embodiments of the first aspect of this application, the secondary battery satisfies at least one of the following conditions: 12≤L1≤20; 0.3≤L2≤0.8; 21.3≤L1 / L2≤56.7.

[0010] For example, a secondary battery satisfies the condition: 0.3 ≤ L2 ≤ 0.8.

[0011] For example, a secondary battery satisfies the following conditions: 0.3≤L2≤0.8; 21.3≤L1 / L2≤56.7.

[0012] For example, a secondary battery satisfies the following conditions: 12≤L1≤20; 0.3≤L2≤0.8.

[0013] For example, a secondary battery satisfies: 21.3≤L1 / L2≤56.7.

[0014] Based on the above embodiments, by controlling the average particle size or ratio of the positive electrode active material and lithium pyrochlore fluoride within the above range, it helps the lithium pyrochlore fluoride to form a uniform and continuous contact network around the positive electrode active material. On the one hand, this improves the ion transport efficiency inside the positive electrode material layer at low temperatures and reduces the interface impedance between the positive electrode and the electrolyte, thereby further improving the low-temperature load-bearing performance of the secondary battery. On the other hand, it optimizes the ion insertion / extraction reaction efficiency on the positive electrode side, which is beneficial to the capacity utilization of the positive electrode active material (lithium cobalt oxide and / or lithium nickel cobalt manganese oxide) and the structural stability of the positive electrode active material, thereby improving the cycle capacity retention rate of the secondary battery.

[0015] In some embodiments of the first aspect of this application, the lithium pyrochlore-type fluorine oxide includes lanthanum and M, wherein M is selected from at least one of niobium or tantalum. Based on the above embodiments, when the lithium pyrochlore-type fluorine oxide includes lanthanum, niobium, or tantalum, these elements can also be micro-doped into the lattice of lithium cobalt oxide and / or lithium nickel cobalt manganese oxide to help stabilize the layered structure of the positive electrode active material (lithium cobalt oxide / lithium nickel cobalt manganese oxide material), further supporting the ion exchange reaction process of the positive electrode active material, thereby improving the low-temperature load-bearing performance and cycle capacity retention of the secondary battery.

[0016] In some embodiments of the first aspect of this application, based on the mass of the lithium pyrochlore-type fluoride oxide, the mass percentage of lanthanum is A%, the mass percentage of M is B%, and 1.34 ≤ B / A ≤ 7.84. Based on the above embodiments, when B / A is within the above range, the lithium pyrochlore-type fluoride oxide not only provides better ionic conductivity and lower lithium migration activation energy to maintain smooth ion transport channels, but also further helps lithium cobalt oxide and / or lithium nickel cobalt manganese oxide optimize their crystal structure stability, thereby further improving the low-temperature load-bearing performance and cycle capacity retention of the secondary battery.

[0017] In some embodiments of the first aspect of this application, based on the mass of the lithium pyrochlore-type fluorine oxide, the lithium pyrochlore-type fluorine oxide satisfies at least one of the following conditions: the mass percentage of lanthanum is A%, 8.6≤A≤31.5; the mass percentage of M is B%, 42.3≤B≤67.4; the mass percentage of oxygen is C%, 15.6≤C≤26.6; and the mass percentage of fluorine is D%, 3.1≤D≤5.3.

[0018] For example: 8.6≤A≤31.5; 42.3≤B≤67.4.

[0019] For example: 8.6≤A≤31.5; 15.6≤C≤26.6; 3.1≤D≤5.3.

[0020] For example: 8.6≤A≤31.5; 42.3≤B≤67.4; 15.6≤C≤26.6; 3.1≤D≤5.3.

[0021] Based on the above embodiments, this application selects lithium pyrochlore-type fluoride oxides with the above composition, which have better ionic conductivity, lower lithium migration barrier and oxidation stability (stable above 4.35V), thereby constructing a more efficient and stable ion transport network inside and on the surface of the cathode material layer, further improving the ion transport efficiency inside the cathode material layer, alleviating concentration polarization on the cathode side, reducing the ion migration barrier at the cathode / electrolyte interface, and thus further improving the low-temperature load-bearing performance of the secondary battery.

[0022] In some embodiments of the first aspect of this application, the mass percentage of lithium pyrochlore fluoride oxide is E%, 0.4 ≤ E ≤ 5, based on the mass of the cathode material layer. Based on the above embodiments, this application, by controlling the mass percentage of lithium pyrochlore fluoride oxide in the cathode material layer, enables the secondary battery to maintain a higher energy density while maximizing its low-temperature tensile performance.

[0023] In some embodiments of the first aspect of this application, the maximum particle size of the lithium pyrochlore fluoride is 1 μm, and L2≤1≤15. Based on the above embodiments, when the maximum particle size of the lithium pyrochlore fluoride in this application is within the above range, the risk of potential puncture of the separator leading to a short circuit in the secondary battery can be reduced. At the same time, a suitable particle size distribution facilitates a more uniform ion diffusion process in the positive electrode, thereby maximizing the improvement of low-temperature tensile performance.

[0024] In some embodiments of the first aspect of this application, the ionic conductivity of the lithium pyrochlore-type fluorine oxide is greater than or equal to 3 mS / cm.

[0025] In some embodiments of the first aspect of this application, the XRD diffraction pattern of lithium pyrochlore-type fluoride oxide has a characteristic peak A at a 2θ angle of 14.5°±1° and a characteristic peak B at 30°±1°.

[0026] In some embodiments of the first aspect of this application, the electrolyte includes lithium difluorophosphate. Based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is G%, 0.005 ≤ G ≤ 5. Based on the above embodiments, the electrolyte further includes lithium difluorophosphate, which can preferentially oxidize and decompose on the surface of the positive electrode active particles during formation and early cycling to form a dense CEI film with low impedance and high ionic conductivity. The CEI film formed by the lithium difluorophosphate has good interfacial compatibility with lithium pyrochlore oxide, facilitating the transport of lithium ions in the interface / phase between the lithium pyrochlore oxide and the positive electrode active particles, thereby further improving the low-temperature load-bearing performance of the secondary battery.

[0027] In some embodiments of the first aspect of this application, the electrolyte comprises a nitrile compound, including at least one selected from succinic anhydride, adiponitrile, ethylene glycol di(propionitrile) ether, 1,3,5-pentanetricarbonyl anhydride, 1,2,3-propanetricarbonyl anhydride, 1,3,6-hexanetricarbonyl anhydride, 1,2,6-hexanetricarbonyl anhydride, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, 1,2,5-tris(cyanoethoxy)pentane, 1,2,3-tris(2-cyanoethoxy)propane, 1,2-bis(cyanoethoxy)ethane, or ethylene glycol bis(propionitrile) ether. Based on the mass of the electrolyte, the mass percentage of nitrile compounds is H%, 2≤H≤8. Based on the above embodiments, the electrolyte further includes the aforementioned nitrile compounds. On the one hand, these compounds can form stable coordination bonds with transition metal ions on the surface of the positive electrode active material, coating the surface of the positive electrode active material and inhibiting the dissolution and migration of transition metal ions to the negative electrode, thus preventing damage to the SEI film. On the other hand, the nitrile compounds can also form an effective protective layer adsorbed on the surface of lithium pyrochlore fluoride particles, reducing the corrosion of lithium pyrochlore fluoride oxides by HF generated from the decomposition of lithium hexafluorophosphate or fluoroethylene carbonate in the electrolyte, thereby maintaining the long-term stability of the ion-conducting network of lithium pyrochlore fluoride oxides and improving the low-temperature load-bearing performance of the secondary battery.

[0028] A second aspect of this application also provides an electronic device comprising a secondary battery according to any of the above claims.

[0029] In some embodiments of the second aspect of this application, the above-described electronic device has superior reliability for use at low temperatures. Detailed Implementation

[0030] The following specific embodiments illustrate the implementation of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification.

[0031] The present application will now be described in further detail with reference to specific embodiments. The technical solutions in the embodiments of the present application are clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, and not all of them. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present application.

[0032] As described in the background section, under low-temperature conditions, lithium-ion batteries experience increased electrolyte viscosity, reduced lithium-ion diffusion rate, and a slowed desolvation process at the electrode-electrolyte interface. This leads to a surge in interfacial impedance, resulting in severe polarization during high-rate discharge at low temperatures. Consequently, the battery reaches its cutoff voltage prematurely, preventing full capacity release. Therefore, improving the power output capability of widely used cathode materials such as lithium cobalt oxide and lithium nickel cobalt manganese oxide at low temperatures is crucial for enhancing the reliability of lithium-ion batteries in cold environments.

[0033] In view of this, the purpose of this application is to provide a secondary battery that, by introducing a specific type of lithium pyrochlore-type fluoride into the cathode material layer including lithium cobalt oxide and / or lithium nickel cobalt manganese oxide cathode active material, and by controlling the average particle size of the cathode active material and the lithium pyrochlore-type fluoride, can improve the ionic conductivity of the cathode and reduce the cathode / electrolyte interface impedance, thereby enabling the secondary battery to achieve better low-temperature load-bearing performance.

[0034] An embodiment of the first aspect of this application provides a secondary battery, including a positive electrode, an electrolyte, a negative electrode, and a separator.

[0035] positive electrode The positive electrode includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode active material and a lithium pyrochlore-type fluorine oxide. The positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide, and the average particle size of the positive electrode active material is L1 μm, where 3 ≤ L1 ≤ 20. For example, the average particle size of the positive electrode active material can be 3 μm, 4 μm, 5 μm, 7 μm, 10 μm, 12 μm, 14 μm, 16 μm, 19 μm, 20 μm, or any combination of these values. The average particle size of the lithium pyrochlore-type fluorine oxide is L2 μm, where 0.1 ≤ L2 ≤ 1. For example, the average particle size of the lithium pyrochlore-type fluorine oxide can be 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.6 μm, 0.7 μm, 0.8 μm, 1 μm, or any combination of these values. In this application, when measured by scanning electron microscopy, the aforementioned "average particle size" refers to the arithmetic mean of the equivalent circle diameter based on the particle's projected area.

[0036] The lithium pyrochlore-type fluorine oxides of this application refer to a class of complex anionic compounds with a pyrochlore structure (space group generally Fd-3m). They possess an open framework structure, facilitating rapid lithium-ion migration. Oxygen and fluoride ions jointly occupy the anion sites, and their chemical composition can also be represented as Li... (2-x) La (1+x) / 3 M₂O₆F, where 0 ≤ x ≤ 1.99, and M is one or more transition metal elements selected from Ti, V, Nb, Ta, Mo, or W. The above-mentioned pyrochlore structure can be confirmed by characteristic diffraction peaks located at approximately (29.5 ± 1)°, (34.5 ± 1)°, and (49.5 ± 1)° (2θ, Cu Kα radiation) in the X-ray diffraction pattern.

[0037] In some embodiments, the chemical formula of the lithium pyrochlore-type fluorine oxide is Li. (2-x) La (1+x) / 3 M2O6F, where 0 ≤ x ≤ 1.99. In some embodiments, the lithium pyrochlore-type fluorine oxide may include at least one of lithium lanthanum niobium oxyfluoride, lithium lanthanum niobium tantalum oxyfluoride, or other doped or coated modified materials. In some embodiments, the lithium pyrochlore-type fluorine oxide may be selected from Li2La. 1 / 3 Ta2O6F, Li 1.5 La 0.5 Ta2O6F, Li 0.9 La 0.7 Ta2O6F, Li 0.01 La 2.99 / 3 Ta2O6F, Li2La 1 / 3Nb2O6F, Li 1.2 La 0.6 Nb2O6F, Li 0.6 La 0.8 Nb₂O₆F or Li 0.01 La 2.99 / 3 At least one of Nb2O6F.

[0038] This application introduces the aforementioned lithium pyrochlore-type fluorine oxide into a cathode material layer comprising lithium cobalt oxide and / or lithium nickel cobalt manganese oxide. This fluorine oxide possesses high lithium-ion conductivity and low lithium migration activation energy. Dispersing it around the cathode active particles enables the construction of a highly efficient ion transport network within and on the surface of the cathode material layer. This improves ion transport efficiency within the cathode material layer, alleviating concentration polarization on the cathode side. Furthermore, it helps some lithium ions bypass the desolvation process at the cathode-electrolyte interface, directly inserting and extracting through the lithium pyrochlore-type fluorine oxide, thereby reducing the cathode / electrolyte interface impedance and improving the low-temperature load-bearing performance of the secondary battery. Based on this, by controlling the average particle size of the positive electrode active material and lithium pyrochlore-type fluoride oxide within the above-mentioned range, it is beneficial to form a denser and more efficient ion transport network around the positive electrode active material. On the one hand, this improves the ion transport efficiency inside the positive electrode material layer at low temperatures and reduces the interfacial impedance of the positive electrode active material / electrolyte, thereby improving the low-temperature load-bearing performance of the secondary battery. On the other hand, it can also optimize the ion insertion / extraction reaction efficiency on the positive electrode side, thereby facilitating the capacity utilization of the positive electrode active material and improving the cycle capacity retention rate of the secondary battery.

[0039] In some embodiments, the secondary battery satisfies at least one of the following conditions: 12 ≤ L1 ≤ 20; 0.3 ≤ L2 ≤ 0.8; 21.3 ≤ L1 / L2 ≤ 56.7. For example, the secondary battery satisfies: 0.3 ≤ L2 ≤ 0.8. For example, the secondary battery satisfies: 0.3 ≤ L2 ≤ 0.8; 21.3 ≤ L1 / L2 ≤ 56.7. For example, the secondary battery satisfies: 12 ≤ L1 ≤ 20; 0.3 ≤ L2 ≤ 0.8. For example, the secondary battery satisfies: 21.3 ≤ L1 / L2 ≤ 56.7. For example, the ratio of the average particle size of the positive electrode active material and the lithium pyrochlore-type fluoride oxide can be 21.3, 25.4, 28.3, 34.0, 39.6, 42.1, 46.8, 50.9, 56.7, or a range of any two of these values. This application controls the average particle size ratio of the positive electrode active material and lithium pyrochlore fluoride within the aforementioned range. This facilitates the more uniform filling of the gaps between the large-particle-size positive electrode active material by the small-particle-size lithium pyrochlore fluoride particles. Simultaneously, it helps the lithium pyrochlore fluoride form a uniform and continuous contact network around the positive electrode active material, thereby further improving the ion transport efficiency inside the positive electrode material layer and reducing the interface impedance between the positive electrode and the electrolyte. Furthermore, it benefits the capacity utilization and structural stability of the positive electrode active material (lithium cobalt oxide or lithium nickel cobalt manganese oxide), further improving the low-temperature load-bearing performance of the secondary battery.

[0040] In some embodiments, the lithium pyrochlore-type fluorine oxide includes lanthanum and M, where M is selected from at least one of niobium or tantalum. The lanthanum, niobium, or tantalum elements included in the lithium pyrochlore-type fluorine oxide can be micro-doped into the lattice of lithium cobalt oxide and / or lithium nickel cobalt manganese oxide during co-sintering, passivating highly reactive oxygen vacancies on the material surface, helping to stabilize the layered structure of the cathode active material (lithium cobalt oxide / lithium nickel cobalt manganese oxide material), further supporting the ion exchange reaction process of the cathode active material, maintaining smooth ion transport channels, thereby improving the low-temperature load-bearing performance and cycle capacity retention of the secondary battery.

[0041] In some embodiments, based on the mass of the lithium pyrochlore-type fluoride oxide, the mass percentage of lanthanum is A%, the mass percentage of M is B%, and 1.34 ≤ B / A ≤ 7.84. For example, the ratio of the mass percentage of lanthanum to the mass percentage of M in the lithium pyrochlore-type fluoride oxide can be 1.34, 1.67, 2.23, 2.61, 3.71, 4.02, 5.2, 5.88, 6.41, 7.84, or any combination of these values. When the ratio of the mass percentage of lanthanum to the mass percentage of M in the lithium pyrochlore-type fluoride oxide of this application is within the above range, the lithium pyrochlore-type fluoride oxide not only gives it better ionic conductivity and lower lithium migration activation energy to maintain smooth ion transport channels, but also further helps lithium cobalt oxide and / or lithium nickel cobalt manganese oxide to optimize their crystal structure stability, thereby further improving the low-temperature load-bearing performance and cycle capacity retention of the secondary battery.

[0042] In some embodiments, the mass percentage of lanthanum in the lithium pyrochlore-type fluoride oxide is A%, where 8.6 ≤ A ≤ 31.5%. For example, the mass percentage of lanthanum in the lithium pyrochlore-type fluoride oxide can be 8.6%, 10.2%, 12.5%, 12.8%, 16.8%, 21.2%, 22.5%, 26.7%, 31.5%, or any combination of these values. In some embodiments, the mass percentage of element M in the lithium pyrochlore-type fluoride oxide is B%, where 42.3 ≤ B ≤ 67.4%. For example, the mass percentage of element M in the lithium pyrochlore-type fluoride oxide can be 42.3%, 44.7%, 47.3%, 51.5%, 58.8%, 62.4%, 65%, 67.4%, or any combination of these values. In some embodiments, the mass percentage of oxygen in the lithium pyrochlore-type fluoride oxide is C%, where 15.6 ≤ C ≤ 26.6%. For example, the mass percentage of oxygen in lithium pyrochlore-type fluorine oxides can be 15.6%, 16.5%, 17.2%, 17.9%, 21.9%, 23.1%, 24.5%, 26.6%, or any combination of these values. In some embodiments, the mass percentage of fluorine, based on the mass of the lithium pyrochlore-type fluorine oxide, is D%, where 3.1 ≤ D ≤ 5.3. For example, the mass percentage of fluorine in lithium pyrochlore-type fluorine oxides can be 3.1%, 3.3%, 3.4%, 3.5%, 4.3%, 4.6%, 4.8%, 5.3%, or any combination of these values. In some embodiments, for example: 8.6 ≤ A ≤ 31.5; 42.3 ≤ B ≤ 67.4. In some embodiments, for example: 8.6 ≤ A ≤ 31.5; 15.6 ≤ C ≤ 26.6; 3.1 ≤ D ≤ 5.3. In some of these embodiments, for example: 8.6≤A≤31.5; 42.3≤B≤67.4; 15.6≤C≤26.6; 3.1≤D≤5.3.

[0043] This application selects the aforementioned lithium pyrochlore-type fluorine oxide, which has better ionic conductivity, lower lithium migration barrier, and oxidation stability (stable above 4.35V). It can construct a more efficient ion transport network inside and on the surface of the cathode material layer, further improve the ion transport efficiency inside the cathode material layer, alleviate concentration polarization on the cathode side, and reduce the ion migration barrier at the cathode / electrolyte interface, thereby further improving the low-temperature load-bearing performance of the secondary battery.

[0044] In some embodiments, the mass percentage of lithium pyrochlore fluoride in the cathode material layer is E%, 0.4 ≤ E ≤ 5. For example, the mass percentage of lithium pyrochlore fluoride in the cathode material layer can be 0.4%, 0.8%, 1.3%, 1.6%, 2.5%, 3.3%, 3.8%, 4.2%, 4.5%, 5%, or any combination of these values. By controlling the mass percentage of lithium pyrochlore fluoride in the cathode material layer, this application enables the secondary battery to achieve higher energy density while maximizing its low-temperature tensile performance.

[0045] In some embodiments, the maximum particle size of the lithium pyrochlore fluoride is 1 μm, where 0.3 ≤ 1 ≤ 15. For example, the maximum particle size of the lithium pyrochlore fluoride can be 0.3 μm, 1 μm, 3 μm, 4 μm, 6 μm, 8 μm, 10 μm, 13 μm, 15 μm, or any combination of these values. When the maximum particle size of the lithium pyrochlore fluoride in this application is within the above range, the risk of potential short circuit in the secondary battery due to puncture of the separator can be reduced. At the same time, a suitable particle size distribution facilitates a more uniform ion diffusion process in the positive electrode, thereby maximizing the improvement of low-temperature tensile performance.

[0046] In some embodiments, the ionic conductivity of the lithium pyrochlore-type fluorine oxide is greater than or equal to 3 mS / cm. For example, the ionic conductivity of the lithium pyrochlore-type fluorine oxide of this application can be 3 mS / cm, 3.4 mS / cm, 4 mS / cm, 4.6 mS / cm, 5.1 mS / cm, 6 mS / cm, or any combination of these values. In some embodiments, the XRD diffraction pattern of the lithium pyrochlore-type fluorine oxide shows a characteristic peak A near 14.5° and a characteristic peak B near 30°.

[0047] In some embodiments, the positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide (LiN). i0.90 Co 0.05 Mn 0.05 O2 (NCM955, NCM811, NCM622, NCM523, NCM111), and may also include at least one of lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium-rich manganese-based materials, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganese oxide, spinel-type lithium nickel manganese oxide, and lithium titanate. Among these, lithium nickel cobalt manganese oxide refers to a lithium transition metal oxide containing nickel, cobalt, manganese, and other metal ions. Based on the total molar number of transition metals, the nickel content can be 60 mol% or greater, for example, 75 mol% or greater, 80 mol% or greater, 85 mol% or greater, or 90 mol% or greater.

[0048] In some embodiments, at least a portion of the surface of the positive electrode active material may have a coating layer, which may be amorphous or crystalline. Coating elements included in the coating layer may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or mixtures thereof.

[0049] In some embodiments, the positive electrode material layer includes a positive electrode conductive material. There is no limitation on the type of positive electrode conductive material; any known conductive material can be used. Examples of positive electrode conductive materials include, but are not limited to, acetylene black, Super-P carbon black, amorphous carbon such as needle coke, carbon nanotubes, and graphene. The above-mentioned positive electrode conductive materials can be used alone or in any combination.

[0050] In some embodiments, the positive electrode material layer includes a positive electrode binder. There are no particular limitations on the type of positive electrode binder; in the case of a coating method, any material that is soluble or dispersible in the liquid medium used during electrode manufacturing is acceptable. Examples of positive electrode binders may include, but are not limited to, one or more of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber-like polymers such as styrene-butadiene rubber, nitrile rubber, fluororubber, isoprene rubber, polybutadiene rubber, and ethylene-propylene rubber; thermoplastic elastomer polymers such as styrene-butadiene-styrene block copolymers or their hydrides, ethylene-propylene-diene terpolymers, styrene-ethylene-butadiene-ethylene copolymers, and styrene-isoprene-styrene block copolymers or their hydrides; soft resin-like polymers such as polyvinyl acetate, ethylene-vinyl acetate copolymers, and propylene-α-olefin copolymers; polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-hexafluoropropylene copolymers, acrylate polymers, acrylate polymers, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymers; and polymer compositions with alkali metal ion conductivity. The above-mentioned positive electrode binder can be used alone or in any combination.

[0051] There are no restrictions on the type of solvent used to form the positive electrode slurry, as long as it can dissolve or disperse the positive electrode active material, conductive material, positive electrode binder, and thickener used as needed. Examples of solvents used to form the positive electrode slurry can include any of aqueous solvents and organic solvents. Examples of aqueous media include, but are not limited to, mixtures of alcohol and water or water. Examples of organic media include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran; amides such as N-methylpyrrolidone, dimethylformamide, and dimethylacetamide; and aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide.

[0052] Thickeners are typically used to adjust the viscosity of slurries. In the case of aqueous media, thickeners and styrene-butadiene rubber latex can be used for slurry preparation. There are no particular limitations on the types of thickeners, but examples include, but are not limited to, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts. The above thickeners can be used alone or in any combination.

[0053] There are no particular limitations on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. Examples of positive electrode current collectors include, but are not limited to, metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; and materials such as carbon cloth and carbon paper. In some embodiments, the positive electrode current collector is aluminum foil.

[0054] To reduce the electronic contact resistance between the positive current collector and the positive electrode material layer, the surface of the positive current collector may include a conductive additive or a conductive coating. Examples of conductive additives include, but are not limited to, carbon, gold, platinum, and silver.

[0055] electrolyte This application does not impose any particular restrictions on the type of electrolyte, as long as it is a medium capable of transporting ions between the positive and negative electrodes. In this application, the electrolyte includes a non-aqueous organic solvent, a lithium salt dissolved in the non-aqueous organic solvent, and functional electrolyte additives.

[0056] This application does not impose any particular limitation on non-aqueous organic solvents, as long as they can serve as a medium for the movement of ions participating in the electrochemical reactions of the battery cell. For example, non-aqueous organic solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. These non-aqueous organic solvents may be used alone or in combination of two or more. Preferably, the combination of two or more non-aqueous organic solvents is selected from cyclic carbonate compounds, chain carbonate compounds, and carboxylic acid ester compounds. Preferably, the non-aqueous organic solvent is selected from at least one of ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), ethyl propionate (PE), propyl propionate (PP), ethyl methyl-2,2-difluoroacetate, ethyl 2,2,2-trifluoroacetate, ethyl 2,2-difluoroacetate (DFEA), methyl 2,2-difluoroacetate, difluoroethyl acetate, difluoropropyl acetate, difluoroethyl propionate, and fluoroethers.

[0057] In some embodiments, the total mass of the non-aqueous organic solvent accounts for 50% to 90% of the total mass of the electrolyte.

[0058] In some embodiments, the electrolyte comprises lithium difluorophosphate. Based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is G%, 0.005 ≤ G ≤ 5. For example, the mass percentage of lithium difluorophosphate in the electrolyte can be 0.005%, 0.01%, 0.3%, 1.0%, 1.6%, 2%, 2.7%, 3.8%, 5%, or any combination of these values. The electrolyte of this application further comprises lithium difluorophosphate, which can preferentially oxidize and decompose on the surface of the positive electrode active particles during formation and early cycling to form a dense CEI film with low impedance and high ionic conductivity. This CEI film has good interfacial compatibility with lithium pyrochlore oxide, facilitating the transport of lithium ions in the interface / phase between the lithium pyrochlore oxide and the positive electrode active particles, thereby further improving the low-temperature load-bearing performance of the secondary battery.

[0059] In some embodiments, the lithium salt includes at least one selected from lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonylimide (LiFSI), lithium bistrifluoromethanesulfonylimide (LiTFSI), lithium perchlorate (LiClO4), and lithium bis(pentafluoroethylsulfonyl)imide (LiN(SO2C2F5)2). In some embodiments, the lithium salt is selected from lithium hexafluorophosphate (LiPF6). In some embodiments, to achieve stability of lithium hexafluorophosphate (LiPF6) in the electrolyte, a binary or even multi-component lithium salt scheme can be adopted. Specifically, lithium hexafluorophosphate (LiPF6) can be used as the main salt, combined with other auxiliary lithium salts, such as lithium bisfluorosulfonylimide (LiFSI), lithium bistrifluoromethanesulfonylimide (LiTFSI), etc. These auxiliary lithium salts can improve the thermal and chemical stability of the electrolyte, thereby enhancing the overall performance of the secondary battery.

[0060] In some embodiments, the total mass of the lithium salt accounts for 8% to 15% of the total mass of the electrolyte.

[0061] In some embodiments, the electrolyte comprises a nitrile compound, including at least one selected from succinic anhydride, adiponitrile, ethylene glycol di(propionitrile) ether, 1,3,5-pentanetricarbonyl anhydride, 1,2,3-propanetricarbonyl anhydride, 1,3,6-hexanetricarbonyl anhydride, 1,2,6-hexanetricarbonyl anhydride, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, 1,2,5-tris(cyanoethoxy)pentane, 1,2,3-tris(2-cyanoethoxy)propane, 1,2-bis(cyanoethoxy)ethane, or ethylene glycol bis(propionitrile) ether. Based on the mass of the electrolyte, the mass percentage of nitrile compounds is H%, 2≤H≤8. This ranges from 2.0%, 3.5%, 3.8%, 4.5%, 5.3%, 6.3%, 6.7%, 7.1%, 8%, or any combination of these values. The electrolyte of this application further includes the aforementioned nitrile compounds, wherein the cyano groups possess strong coordination ability. On one hand, they can form stable coordination bonds with transition metal ions on the surface of the positive electrode active material, coating the surface of the positive electrode active material and inhibiting the dissolution and migration of transition metal ions to the negative electrode, thus damaging the SEI film. On the other hand, the nitrile compounds can also form an effective protective layer adsorbed on the surface of lithium pyrochlore fluoride particles, reducing the corrosion of lithium pyrochlore fluoride oxides by HF from the decomposition of lithium hexafluorophosphate or fluoroethylene carbonate in the electrolyte, thereby maintaining the long-term stability of the ion-conducting network of lithium pyrochlore fluoride oxides and improving the low-temperature load-bearing performance of the secondary battery.

[0062] In some embodiments, the electrolyte may further include other functional additives, which may be cyclic carbonate compounds, boron-containing compounds, nitrogen-containing lithium salt compounds, sulfonates, sulfates, phosphates, silanes, ethers, pyridines, or other compounds.

[0063] Optionally, the functional additives include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), trans-difluoroethylene carbonate (DFEC), vinyl sulfite (ES), 1,3-propanesulfonyl lactone (PS), propenyl-1,3-sulfonyl lactone (PST), vinyl sulfate (DTD), methyl vinyl sulfate, pentaerythritol bicyclic sulfate, lithium tris(oxalato) phosphate (LiTOP), tris(trimethylsilane) phosphate (TMSP), tris(trimethylsilane) phosphite, tris(2,2,2-trifluoroethyl) phosphite (TFEP), trimethyl phosphite (TMP), triethyl phosphate (TEP), tris(pentafluorophenyl)phosphine (TPFPP), and pentafluoroethoxycyclotriphosphazene (PFPN). The following are included in the list of at least one of the following: fluorotris(trimethylsilane) phosphate, dimethyldimethoxysilane (DODSi), tetravinylsilane (TVS), lithium tetrafluoroborate (LiBF4), lithium difluorooxalate borate (LiDFOB), lithium bis(oxalate borate) (LiBOB), lithium monofluorophosphate, 1,2-bis(difluorophosphoxy)ethane, lithium fluorosulfonate, tris(trimethylsilane)borate (TMSB), hexamethyldisilazane (HMDS), fluorobenzene (FB), cyclohexylbenzene, biphenyl (BP), maleic anhydride (MA), acetamide (EA), polyamide (PA), γ-butyrolactone (GBL), 1,3-dioxopentane (DOL), ethylene glycol dimethyl ether (DME), pyridine, 2-methylpyridine, 2-fluoropyridine, 2-cyanopyridine, or 3-vinylpyridine.

[0064] There are no particular restrictions on the selection and content of the aforementioned functional additives in the electrolyte; they can be chosen according to actual needs. For example, specific components can be selected as film-forming additives, flame-retardant additives, overcharge protection additives, water / acid removal additives, passivating agents for protecting Al current collectors or battery casings, and wetting agents to improve wetting properties to meet the battery's needs for different application scenarios and positive and negative electrode matching.

[0065] negative electrode The negative electrode includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, the negative electrode material layer comprising a negative electrode active material. In some embodiments, the rechargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material, in order to reduce the problem of lithium metal deposition at the negative electrode during charging.

[0066] Negative electrode active materials may include natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), silicon, silicon-carbon composites, silicon-oxygen composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithium titanate Li4Ti5O. 12 At least one of Li-Al alloy or lithium metal. Optionally, the negative electrode active material may further include amorphous carbon material, which may be soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, or calcined coke, etc.

[0067] In some embodiments, the negative electrode active material comprises silicon-carbon material. In some embodiments, the silicon particles in the silicon-carbon material are selected to have a spherical morphology ≥0.66, which is beneficial for reducing side reactions. In some embodiments, the silicon particles in the silicon-carbon material are selected to have a bulk silicon morphology, which is beneficial for capacity utilization. In some embodiments, the Dv10 of the silicon-carbon material ranges from 2 μm to 7 μm. In some embodiments, the Dv50 of the silicon-carbon material ranges from 6 μm to 12 μm. In some embodiments, the Dv90 of the silicon-carbon material ranges from 10 μm to 20 μm. In some embodiments, the Dv99 of the silicon-carbon material ranges from 15 μm to 30 μm.

[0068] The negative electrode material layer also includes a negative electrode binder. The negative electrode binder improves the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector. This application does not particularly limit the type of negative electrode binder, as long as it is a material stable to the electrolyte or the solvent used in electrode manufacturing. In some embodiments, the negative electrode binder includes a resin binder. Examples of resin binders include, but are not limited to, fluoropolymers, polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, etc. When using an aqueous solvent to prepare the negative electrode slurry, the negative electrode binder includes, but is not limited to, carboxymethyl cellulose (CMC) or its salts, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salts, polyvinyl alcohol, etc.

[0069] In some embodiments, the negative electrode material layer may further include a negative electrode conductive agent. This application does not impose any particular limitation on the type of negative electrode conductive agent, as long as it can achieve the purpose of this application. For example, the negative electrode conductive agent may be at least one of acetylene black, Ketjen black, carbon nanotubes, carbon fibers, carbon dots, or graphene, etc., and the aforementioned carbon nanotubes may include, but are not limited to, at least one of single-walled carbon nanotubes or multi-walled carbon nanotubes.

[0070] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may comprise copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a polymer substrate coated with a conductive metal. The conductive metal includes, but is not limited to, copper, nickel, or titanium, and the polymer substrate material includes, but is not limited to, at least one of polyethylene, polypropylene, ethylene-propylene copolymer, polyethylene terephthalate, polyethylene terephthalate, or poly(p-phenylene terephthalate). In this application, there are no particular limitations on the thickness of the negative electrode current collector and the negative electrode material layer, as long as it achieves the purpose of this application. For example, the thickness of the negative electrode current collector is 4 μm to 30 μm, and the thickness of the single-sided negative electrode material layer is 30 μm to 160 μm. In this application, the negative electrode material layer may be disposed on one surface or on two surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector surface or a part of the negative electrode current collector surface. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0071] This application does not impose any particular limitation on the compaction density of the negative electrode sheet, as long as it achieves the purpose of this application. For example, the compaction density of the negative electrode sheet can be 1.0 g / cm³. 3 Up to 1.85 g / cm 3 This application does not impose any particular limitation on the cold pressing pressure of the negative electrode sheet, as long as the purpose of this application can be achieved. For example, the cold pressing pressure of the negative electrode sheet can be from 3 tons to 30 tons.

[0072] Optionally, the negative electrode sheet may further include a conductive layer, which is located between the negative electrode current collector and the negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, and it can be a conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer, and they can be at least one of the aforementioned negative electrode conductive agents and negative electrode binders. This application does not impose any particular limitation on the mass ratio of the conductive agent to the binder in the conductive layer; those skilled in the art can choose according to actual needs, as long as the purpose of this application is achieved. This application does not impose any particular limitation on the thickness of the conductive layer, as long as the purpose of this application is achieved; for example, the thickness of the conductive layer is 1 μm to 10 μm.

[0073] diaphragm The secondary battery of this application typically has a separator between the positive and negative electrodes. The separator is used to separate the positive and negative electrodes, reduce the problem of internal short circuits in the secondary battery, allow electrolyte ions to pass freely, and does not affect the electrochemical charging and discharging process.

[0074] This application does not impose any particular limitation on the diaphragm, as long as it can achieve the purpose of this application. For example, the material of the diaphragm may include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid; the type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane or spun membrane.

[0075] In this application, the diaphragm may include a substrate and a surface treatment layer. The substrate may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. For example, the inorganic layer includes inorganic particles and a binder. This application does not have any particular limitation on the aforementioned inorganic particles, and may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not have any particular limitation on the aforementioned binders, and may include at least one of the aforementioned binders. The polymer layer contains a polymer, the polymer material of which includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0076] In this application, the pore size of the separator is from 0.01 μm to 1 μm, and the thickness is from 5 μm to 50 μm. In some embodiments, the separator thickness is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the separator thickness is less than 50 μm, less than 40 μm, or less than 30 μm. When the separator thickness is within the above ranges, insulation and mechanical strength can be balanced, and the rate characteristics and energy density of the secondary battery can be optimized.

[0077] Electronic devices The second aspect of this application provides an electronic device that includes the secondary battery of the first aspect of this application. The electronic device includes, but is not limited to, mobile phones, laptops, tablets, electronic watches, Bluetooth headsets, drones, e-book players, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, electric vehicles, motorcycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0078] Example The following examples, using lithium-ion batteries as an example, provide more specific illustrations of the implementation methods of the secondary battery of this application. Those skilled in the art will understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" refer to mass measurements.

[0079] Example 1-1 1. Preparation of the positive electrode Lithium nickel cobalt manganese oxide (LiNi) is used as the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2 (average particle size 3μm), lithium pyrochlore-type fluoride oxide Li2La 1 / 3 Ta₂O₆F (average particle size 1μm), conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 94.6:0.4:2:3. N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 70wt%. The positive electrode slurry was uniformly coated onto one surface of a positive electrode current collector aluminum foil and dried to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The above steps were repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After cold pressing, slitting, and welding of tabs, the sheet was dried to obtain a positive electrode sheet with dimensions of 74mm × 867mm.

[0080] 2. Preparation of electrolyte In a dry argon atmosphere glove box, ethylene carbonate, propylene carbonate, ethyl acetate, and propyl propionate were mixed in a mass ratio of 1:1:1:1 as a base solvent. Lithium hexafluorophosphate (LiPF6) was then dissolved in this base solvent, and vinylene carbonate and vinyl sulfate were added as additives to obtain the electrolyte. Based on the mass of the electrolyte, LiPF6 accounted for 12.5% ​​by mass, vinylene carbonate for 2% by mass, vinyl sulfate for 0.5% by mass, and the remainder was the base solvent.

[0081] 3. Preparation of the negative electrode A silicon-carbon composite material (silicon:carbon mass ratio = 1:1), graphite, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), carbon nanotubes (CNT), and sodium carboxymethyl cellulose (CMC-Na) were mixed in a mass ratio of 10:85.8:2.4:0.5:0.5:0.8. Deionized water was then added as a solvent and the mixture was stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 45 wt%. The negative electrode slurry was uniformly coated onto one surface of a copper foil current collector and dried to obtain a negative electrode sheet with a single-sided coating of negative electrode material. The above steps were repeated on the other surface of the copper foil current collector to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After cold pressing, slitting, and welding of tabs, the sheet was dried to obtain a negative electrode sheet with dimensions of 76.6 mm × 875 mm.

[0082] 4. Preparation of the diaphragm Porous polyethylene (PE) with a thickness of 8 μm and a porosity of 50% was selected as the base membrane. A slurry containing PVDF and inorganic particles (plate boehmite and Al2O3 in a mass ratio of 70:30) was coated on both surfaces of the base membrane, with the PVDF to inorganic particles in a mass ratio of 40:60. After drying, a separation membrane was obtained. The coating thickness on each surface of the separation membrane was 3 μm.

[0083] 5. Preparation of lithium-ion batteries The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes for isolation. The electrode assembly is then wound to form the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dehydrated at 80°C, injected with the electrolyte, and sealed. After hot-pressing formation, degassing, edge trimming, and capacity testing, a lithium-ion battery is obtained.

[0084] 6. Testing Methods (1) Average particle size measurement Remove the positive electrode from the lithium-ion battery, then cut the positive electrode using ion polishing, and observe and measure the morphology of the positive active material particles and lithium pyrochlore-type fluorine oxide particles in the positive electrode using a scanning electron microscope (instrument model: ZEISS SEM, accelerating voltage: 0.1kV~30kV).

[0085] In scanning electron microscope images, at least 100 independent positive electrode active material particles and lithium pyrochlore fluoride particles were selected respectively. The projected area of ​​each particle was measured using image analysis software, and its corresponding equivalent circle diameter was calculated. The arithmetic mean of all the measurements was then taken to obtain the average particle size of the positive electrode active material particles and lithium pyrochlore fluoride particles.

[0086] The maximum particle size is determined by the maximum projected area of ​​the lithium pyrochlore-type fluoride particles corresponding to the diameter of the corresponding circle in the above measurements.

[0087] (2) Element content measurement Take an appropriate amount of lithium pyrochlore-type fluoride powder sample and use an ICP analyzer to test the mass percentage of each element in the lithium pyrochlore-type fluoride powder.

[0088] (3) Low-temperature tensile performance test Take a lithium-ion battery and place it in a constant temperature environment of 25℃ for 30 minutes to allow the lithium-ion battery to reach a constant temperature. 1) Charge at a constant current of 0.2C to 4.53V, then charge at a constant voltage of 4.53V to a current of 0.025C, let stand for 5 minutes, and discharge at a constant current of 0.7C to 2.8V; repeat this charge and discharge cycle 5 times. 2) Charge at a constant current of 0.2C to 4.53V, then charge at a constant voltage of 4.53V to a current of 0.025C, let stand for 5 minutes, discharge at a constant current of 0.5C for 1.6 hours, and let stand for 5 minutes. 3) Transfer the lithium-ion battery to a constant temperature environment of -20℃ and let it stand for 60 minutes to allow the lithium-ion battery to reach a constant temperature. 4) Discharge at a constant current of 1C for 100ms, then discharge at a constant current of 0.4C for 10s; 5) Discharge at a constant current of 1.2C for 100ms, then discharge at a constant current of 0.4C for 2s, and measure; repeat this discharge condition 100 times, and record the number of repetitions and the end voltage after each discharge. 6) Plot the number of repetitions on the horizontal axis and the corresponding end voltage on the vertical axis to obtain the low-temperature load voltage drop, which is used as an indicator to evaluate the low-temperature load performance of lithium-ion batteries.

[0089] The lithium-ion batteries in the following examples or comparative examples differ from those in Examples 1-1 only in that: according to Table 1, the types and proportions of raw materials used in the positive electrode preparation steps are adjusted to modify the positive electrode active material and the lithium calcined chlorite-type fluoride, selecting positive electrode active materials and lithium calcined chlorite-type fluoride with different average particle sizes. Specifically, when the positive electrode active material is lithium cobalt oxide (LiCoO2), the compaction density of the positive electrode material layer is 4.15 g / cm³. 3 When lithium nickel cobalt manganese oxide is selected as the positive electrode active material, the compaction density of the positive electrode material layer is 3.6 g / cm³. 3 In Comparative Example 1, no lithium pyrochlore-type fluorine oxide was added during the preparation of the positive electrode. In Comparative Examples 6, 7, and 8, the lithium pyrochlore-type fluorine oxide was replaced with an equal mass of Li7La3Zr2O, respectively. 12 Li 0.5 La 0.5 TiO3 and Li 1.3 Al 0.3 Ti 1.7 (PO4)3. The performance test results of the lithium-ion batteries in each embodiment and comparative example are shown in Table 1 below. The low-temperature load voltage drop of the lithium-ion batteries in Table 1 can directly reflect the energy release capability of the battery under low capacity. The smaller the value of the low-temperature load voltage drop, the better the battery can maintain a higher voltage platform and has a longer low-temperature cycle life.

[0090] Table 1

[0091] In the table above, the mass ratio of lithium cobalt oxide to lithium nickel cobalt manganese oxide in Examples 1-2 is 1:1.

[0092] As shown in Table 1, compared to Comparative Example 1, which did not add lithium pyrochlore-type fluoride to the positive electrode, the lithium-ion batteries prepared in each embodiment have a lower low-temperature load voltage drop; compared to Comparative Example 2, which has an excessively small average particle size of the positive electrode active material, the lithium-ion batteries prepared in each embodiment have a lower low-temperature load voltage drop; compared to Comparative Example 3, which has an excessively large average particle size of the positive electrode active material, the lithium-ion batteries prepared in each embodiment have a lower low-temperature load voltage drop; compared to Comparative Example 4, which has an excessively small average particle size of the lithium pyrochlore-type fluoride, the lithium-ion batteries prepared in each embodiment have a lower low-temperature load voltage drop; compared to Comparative Example 5, which has an excessively large average particle size of the positive electrode active material, the lithium-ion batteries prepared in each embodiment have a lower low-temperature load voltage drop; compared to replacing the lithium pyrochlore-type fluoride with Li7La3Zr2O respectively... 12 Li 0.5 La 0.5 TiO3 and Li 1.3 Al0.3 Ti 1.7 (PO4)3 Comparative Examples 6, 7 and 8: The lithium-ion batteries prepared in each example have low low-temperature load voltage drop.

[0093] Specifically, in this embodiment, the lithium-ion battery exhibits a lower low-temperature load voltage drop when the average particle size of the positive electrode active material is adjusted to satisfy 3≤L1≤20; a lower low-temperature load voltage drop when the average particle size of lithium pyrochlore fluoride oxide is adjusted to satisfy 0.1≤L2≤1; and a lower low-temperature load voltage drop when the ratio of the average particle size of the positive electrode active material to the average particle size of the lithium pyrochlore fluoride oxide is adjusted to satisfy 21.3≤L1 / L2≤56.7.

[0094] The lithium-ion batteries in Examples 2-1 to 2-20 differ from those in Examples 1-18 only in that the mass ratio of lithium pyrochlore fluoride in the positive electrode material layer is adjusted according to Table 2, lithium pyrochlore fluoride with different maximum particle sizes is selected, the mass ratio of lithium difluorophosphate in the electrolyte is adjusted, and the type and mass ratio of nitrile compounds in the electrolyte are adjusted.

[0095] Table 2

[0096] In the table above, the mass ratio of succinate to 1,3,6-hexanetrionitrile in the electrolytes of Examples 2-16 to 2-20 is 1:1.

[0097] As shown in Table 2, the lithium-ion battery prepared in the embodiments of this application exhibits lower low-temperature load voltage drop when the mass ratio of lithium pyrochlore-type fluoride oxide in the positive electrode material layer is controlled to satisfy 0.4≤E≤5; lower low-temperature load voltage drop when the maximum particle size of lithium pyrochlore-type fluoride oxide is controlled to satisfy 0.3≤I≤15; lower low-temperature load voltage drop when the electrolyte further includes lithium difluorophosphate with a mass ratio of 0.005≤G≤5; and lower low-temperature load voltage drop when the electrolyte further includes a specific type of nitrile compound with a mass ratio of 2≤H≤8.

[0098] In summary, the secondary battery and electronic device disclosed in this application include a positive electrode and an electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode active material and lithium pyrochlore-type fluorine oxide. The positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide, and the average particle size of the positive electrode active material is L1 μm, 3 ≤ L1 ≤ 20. The lithium pyrochlore-type fluorine oxide includes lanthanum and M, where M is selected from at least one of niobium or tantalum, and the average particle size of the lithium pyrochlore-type fluorine oxide is L2 μm, 0.1 ≤ L2 ≤ 1. By introducing a specific type of lithium pyrochlore-type fluorine oxide into the positive electrode material layer including lithium cobalt oxide and / or lithium nickel cobalt manganese oxide positive electrode active material, and by controlling the average particle size of the positive electrode active material and the lithium pyrochlore-type fluorine oxide, the secondary battery provided in this application has superior low-temperature tensile strength.

[0099] The above embodiments are merely illustrative of the principles and effects of this application and are not intended to limit this application. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of this application. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in this application should still be covered by the claims of this application.

Claims

1. A secondary battery, characterized in that, The device includes a positive electrode and an electrolyte. The positive electrode includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode active material and a lithium pyrochlore-type fluorine oxide. The positive electrode active material includes lithium cobalt oxide and / or lithium nickel cobalt manganese oxide, and the average particle size of the positive electrode active material is L1μm, 3≤L1≤20; The average particle size of the lithium pyrochlore-type fluoride oxide is L2μm, 0.1≤L2≤1; The lithium pyrochlore-type fluoride oxide is a composite anionic compound with a pyrochlore structure. The X-ray diffraction pattern of the pyrochlore structure shows characteristic diffraction peaks at diffraction angles 2θ at (29.5±1)°, (34.5±1)° and (49.5±1)°.

2. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies at least one of the following conditions: （1）12≤L1≤20； （2）0.3≤L2≤0.8； (3) 21.3≤L1 / L2≤56.

7.

3. The secondary battery according to claim 1, characterized in that, The lithium pyrochlore-type fluoride oxide includes lanthanum and M, wherein the M element is selected from at least one of niobium or tantalum.

4. The secondary battery according to claim 3, characterized in that, Based on the mass of the lithium pyrochlore-type fluorine oxide, the mass percentage of lanthanum is A, the mass percentage of M is B, and the lithium pyrochlore-type fluorine oxide satisfies 1.34≤B / A≤7.

84.

5. The secondary battery according to claim 4, characterized in that, Based on the mass of the lithium pyrochlore-type fluoride, the lithium pyrochlore-type fluoride satisfies at least one of the following conditions: （1）8.6≤A≤31.5； （2）42.3≤B≤67.4； (3) The mass percentage of oxygen is C%, 15.6 ≤ C ≤ 26.6; (4) The mass percentage of fluorine is D%, 3.1≤D≤5.

3.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, Based on the mass of the cathode material layer, the mass percentage of the lithium pyrochlore-type fluorine oxide is E%, 0.4≤E≤5.

7. The secondary battery according to any one of claims 1 to 5, characterized in that, The lithium pyrochlore-type fluoride oxide satisfies at least one of the following conditions: (1) The maximum particle size of the lithium pyrochlore type fluoride oxide is 1 μm, and 0.3 ≤ 1 ≤ 15; (2) The X-ray diffraction pattern of the lithium pyrochlore type fluoride oxide has a characteristic peak A at a 2θ angle of 14.5°±1° and a characteristic peak B at 30.0°±1°.

8. The secondary battery according to any one of claims 1 to 5, characterized in that, The electrolyte includes lithium difluorophosphate; Based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is G%, 0.005≤G≤5.

9. The secondary battery according to any one of claims 1 to 5, characterized in that, The electrolyte comprises nitrile compounds, including at least one selected from the following: succinate, adiponitrile, ethylene glycol di(propionitrile) ether, 1,3,5-pentanetricarbonitrile, 1,2,3-propanetricarbonitrile, 1,3,6-hexanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, 1,2,5-tris(cyanoethoxy)pentane, 1,2,3-tris(2-cyanoethoxy)propane, 1,2-bis(cyanoethoxy)ethane, or ethylene glycol bis(propionitrile) ether. Based on the mass of the electrolyte, the mass percentage of the nitrile compound is H%, 2≤H≤8.

10. An electronic device, characterized in that, Includes the secondary battery as described in any one of claims 1 to 9.

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