Lithium ion secondary battery and electric equipment

Abstract

The invention belongs to the technical field of secondary batteries, and particularly relates to a lithium ion secondary battery and electric equipment. The lithium ion secondary battery provided by the invention comprises a positive plate, a negative plate and an electrolyte, the positive plate comprises a positive active material, the positive active material is doped with a sodium element, the mass percentage content of the sodium element is m2% based on the total mass of the positive active material, and 0.001 < = m2 < = 1; the electrolyte comprises a fluorobenzene compound, the mass percentage content of the fluorobenzene compound is m1% based on the total mass of the electrolyte, and m1 is larger than or equal to 2 and smaller than or equal to 30. According to the lithium ion secondary battery provided by the invention, the sodium element is doped in the positive electrode active material, and the fluorobenzene compound is added in the electrolyte, so that the diffusion rate of lithium ions in the positive electrode active material under high voltage can be improved, and the battery has relatively good charge-discharge performance and cycle performance, especially low-temperature and normal-temperature cycle performance.

Description

Technical Field

[0001] This application belongs to the technical field of secondary batteries, and particularly relates to a lithium-ion secondary battery and an electrical device. Background Art

[0002] With the development of technology and the improvement of market demand, the application fields of lithium-ion batteries have been expanding day by day. From consumer electronics to electric vehicles, energy storage systems and other fields, the requirements for battery performance are also getting higher and higher. The development background of high-voltage (≥4.55V) lithium-ion battery electrolytes mainly stems from the pursuit of higher energy density and safety performance.

[0003] High voltage has a great impact on battery materials, especially cathode materials. Excessive voltage may cause the cathode material to undergo delithiation (i.e., the release of Li + ), or the structure may collapse, making the diffusion path of lithium ions become narrower or more unstable. The damage to the crystal structure will lead to a decrease in ionic conductivity, thereby reducing the diffusion rate of lithium ions; in addition, high voltage may also cause the decomposition of the electrolyte to generate by-products. These by-products will not only lead to a decline in battery performance, but may also form a solid electrolyte interface (CEI) film or a film that hinders ion conduction on the surface of the cathode, thereby slowing down the diffusion of lithium ions.

[0004] However, the diffusion rate of lithium ions is one of the key factors affecting battery performance. Therefore, it is urgent to develop a lithium-ion secondary battery that can improve the diffusion rate of lithium ions under high voltage. Summary of the Invention

[0005] Therefore, the technical problem to be solved by this application is that the diffusion rate of lithium ions in the cathode active material under high voltage in the prior art is low, which will in turn affect the charge and discharge performance and cycle performance of the battery, etc. Thus, a lithium-ion secondary battery and an electrical device are provided.

[0006] To this end, this application provides the following technical solutions:

[0007] According to one aspect of this application, a lithium-ion secondary battery is provided, including a positive electrode sheet, a negative electrode sheet and an electrolyte,

[0008] wherein, the positive electrode sheet includes a positive electrode active material, and sodium element is doped in the positive electrode active material. Based on the total mass of the positive electrode active material, the mass percentage content of the sodium element is m2%, and 0.001 ≤ m2 ≤ 1;

[0009] The electrolyte includes fluorobenzene compounds. Based on the total mass of the electrolyte, the mass percentage content of the fluorobenzene compounds is m1%, and 2 ≤ m1 ≤ 30.

[0010] In some optional embodiments, the mass percentage content of the fluorobenzene compound in the electrolyte and the mass percentage content of sodium element in the positive electrode active material satisfy: 100 ≤ m1 / m2 ≤ 3000.

[0011] In some optional embodiments, the fluorobenzene compound has a structure represented by the following general formula:

[0012]

[0013] Wherein, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 are each independently selected from any one of hydrogen, halogen, C1-C20 alkyl group which may or may not be substituted by halogen, C1-C20 unsaturated olefin group which may or may not be substituted by halogen, C3-C20 cycloalkyl group which may or may not be substituted by halogen, phenyl group which may or may not be substituted by halogen, biphenyl group which may or may not be substituted by halogen, C6-C26 phenylalkyl group which may or may not be substituted by halogen, and C6-C26 polycyclic aromatic hydrocarbon group which may or may not be substituted by halogen, and at least one of R 1 , R 2 , R 3 , R 4 , R 5 , R 6 is F or a group containing F element;

[0014] Preferably, the R 1 , R 2 , R 3 , R 4 , R 5 , R 6 are each independently selected from any one of hydrogen, F, C1-C5 fluoroalkyl group, and C1-C5 fluoro unsaturated olefin group;

[0015] Preferably, the fluorobenzene compound has any one of the following structures:

[0016]

[0017] In some optional embodiments, the positive electrode active material includes at least one of lithium cobaltate material and ternary single crystal material;

[0018] Preferably, the positive electrode active material includes lithium cobaltate material and ternary single crystal material. Based on the total mass of the positive electrode active material, the mass percentage content of the ternary single crystal material is 20-40%;

[0019] And / or, the Dv50 of the positive electrode active material is 10 - 30 μm. Preferably, the ratio of the Dv50 of the ternary single crystal material to that of the lithium cobaltate material is 0.1 - 10;

[0020] And / or, the positive electrode active material further contains a first element, and the first element includes at least one of Al, Mg, Ti, Zr, Nb, and Mo;

[0021] Preferably, based on the total mass of the positive electrode active material, the mass percentage content of the first element is 0.3% - 3.5%.

[0022] In some alternative embodiments, the electrolyte further includes a fluorinated ethylene carbonate compound. Based on the total mass of the electrolyte, the mass percentage content of the fluorinated ethylene carbonate compound is m3%, and 5 ≤ m3 ≤ 25;

[0023] Preferably, the mass percentage contents of the fluorobenzene compound and the fluorinated ethylene carbonate compound satisfy: 12 ≤ m1 + m3 ≤ 50;

[0024] Preferably, the fluorinated ethylene carbonate compound includes at least one of fluorinated ethylene carbonate, difluorinated ethylene carbonate, trifluoromethyl carbonate, and 4,4,5,5 - tetrafluoro - 1,3 - dioxolane - 2 - one.

[0025] In some alternative embodiments, the negative electrode sheet includes a negative electrode active material. The mass percentage content of silicon element in the negative electrode active material is M%. The mass percentage content of silicon element in the negative electrode active material and the mass percentage content of the fluorobenzene compound in the electrolyte satisfy: 0.3 ≤ M / m1 ≤ 5, and 3 ≤ M ≤ 30;

[0026] Preferably, the negative electrode active material includes artificial graphite and silicon - carbon material;

[0027] Preferably, the particle size Dv50 of the silicon - carbon material is d μm, and 5 ≤ d ≤ 30.

[0028] In some alternative embodiments, a number of holes are provided on the surface of the negative electrode sheet. The aperture of the holes is d1 μm, and 50 ≤ d1 ≤ 500, and / or the depth of the holes is d2 μm, and 5 ≤ d2 ≤ 45; and / or the center - to - center distance between two adjacent holes is d3 μm, and 100 ≤ d3 ≤ 2000.

[0029] In some alternative embodiments, the CB value of the battery is denoted as N / P, and 1.05 ≤ N / P ≤ 1.5;

[0030] Preferably, N / P and m1 satisfy: N / P ≥ m1 / 10.

[0031] According to another aspect of the present application, there is provided an electrical device including the above-mentioned lithium-ion secondary battery.

[0032] The technical solution of the present application has the following advantages:

[0033] The lithium-ion secondary battery provided by the present application includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. Among them, the positive electrode sheet includes a positive electrode active material, and sodium element is doped in the positive electrode active material. Based on the total mass of the positive electrode active material, the mass percentage content of the sodium element is m2%, where 0.001 ≤ m2 ≤ 1; the electrolyte includes fluorobenzene compounds, and based on the total mass of the electrolyte, the mass percentage content of the fluorobenzene compounds is m1%, where 2 ≤ m1 ≤ 30. The lithium-ion secondary battery provided by the present application can improve the diffusion rate of lithium ions in the positive electrode material at high voltages by doping sodium element in the positive electrode active material and adding fluorobenzene compounds in the electrolyte, so that the lithium-ion secondary battery has good charge and discharge performance and cycle performance, especially low-temperature and normal-temperature cycle performance. Specifically, the fluorine atoms in the fluorobenzene compound molecules have strong electronegativity, and the fluorobenzene compound can reduce the surface tension of the electrolyte. The complex formed by the fluorobenzene compound and lithium ions helps to accelerate the diffusion process of lithium ions, which makes the electrolyte easier to penetrate into the pores and grain boundaries of the positive electrode active material, thereby accelerating the diffusion and migration of lithium ions; in addition, the fluorobenzene compound has better stability in the electrolyte than carboxylic ester solvents, and can maintain the long-term stability of the battery, which helps to reduce the performance attenuation of the battery during charge and discharge and extend the service life of the battery. When sodium element (Na + ) is doped into the lattice of the positive electrode active material, it will replace part of the position of lithium element (Li + ). Due to the differences in radius and charge between sodium ions and lithium ions, this substitution will cause local adjustment of the lattice structure of the doped material. This adjustment makes the diffusion channels of the complex formed by the fluorobenzene compound and lithium ions in the lattice of the positive electrode become wider and smoother, thereby reducing the activation energy of lithium ion diffusion and enabling lithium ions to better pass through the pores and grain boundaries of the positive electrode active material, effectively improving the diffusion and migration of lithium ions in the positive electrode active material; the complex of the fluorobenzene compound and lithium ions also contains a part of anionic groups (such as bis(fluorosulfonyl)imide anion (FSI - ). When the complex structure contacts the sodium ions doped on the surface of the positive electrode active material, this solvation layer further migrates the lithium ion-containing group into the positive electrode active material through electrostatic attraction, effectively improving the wetting of the electrolyte on the surface of the positive electrode active material, accelerating the diffusion rate and kinetic transport performance of lithium ions in the positive electrode active material, and improving the charge and discharge performance and low-temperature / normal-temperature cycle performance of the battery.

[0034] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the following description, or will be elucidated through the implementation of the embodiments of the present application. Detailed Description of the Embodiments

[0035] The following embodiments are provided to better further understand the present application. They are not limited to the best mode of implementation, and do not limit the content and protection scope of the present application. Any product that is the same or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior art features falls within the protection scope of the present application.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the text of this application are intended to cover non-exclusive inclusion.

[0037] Referring to "embodiments" herein means that a particular feature, structure, or characteristic described in connection with the embodiments can be included in at least one embodiment of this application. The phrase appears in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0038] The "range" disclosed in this application is defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of the specific range. The range defined in this way can include or not include the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. In this application, unless otherwise stated, the numerical range "a - b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0 - 5" means that all real numbers between "0 - 5" have been fully listed herein, and "0 - 5" is only an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥2, it is equivalent to disclosing that the parameter can be, for example, the integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0039] In the description of the embodiments of this application, the term "and / or" is merely a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Additionally, the character " / " in this text generally represents an "or" relationship between the associated objects before and after.

[0040] In the description of the embodiments of the present application, the term "at least one" refers to one or more than two (including two).

[0041] For those embodiments where specific experimental steps or conditions are not indicated, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For reagents or instruments whose manufacturers are not indicated, they are all conventional reagent products that can be obtained through commercial purchase.

[0042] As described in the background art, in order to solve the defects in the prior art that the diffusion rate of lithium ions in the positive electrode material is low under high voltage, which will in turn affect the charge-discharge performance and cycle performance of the battery, the present application provides the following technical solutions:

[0043] According to one aspect of the present application, a lithium-ion secondary battery is provided, including a positive electrode sheet, a negative electrode sheet, and an electrolyte,

[0044] wherein, the positive electrode sheet includes a positive electrode active material, and sodium element is doped in the positive electrode active material. Based on the total mass of the positive electrode active material, the mass percentage content of the sodium element is m2%, and 0.001 ≤ m2 ≤ 1;

[0045] The electrolyte includes a fluorobenzene compound. Based on the total mass of the electrolyte, the mass percentage content of the fluorobenzene compound is m1%, and 2 ≤ m1 ≤ 30.

[0046] As an example, based on the total mass of the positive electrode active material, the mass percentage content of the sodium element can be 0.001%, 0.005%, 0.01%, 0.03%, 0.08%, 0.1%, 0.2%, 0.3%, 0.5%, 0.6%, 0.8%, 0.9%, 1%, or within the range composed of any of the above values. As an example, based on the total mass of the electrolyte, the mass percentage content of the fluorobenzene compound can be 2%, 5%, 10%, 15%, 20%, 25%, 30%, or within the range composed of any of the above values.

[0047] The lithium-ion secondary battery provided by this application can improve the diffusion rate of lithium ions in the cathode active material at high voltages by doping sodium elements into the cathode active material and adding fluorobenzene compounds to the electrolyte, enabling the lithium-ion secondary battery to have good charge and discharge performance and cycling performance, especially low-temperature and normal-temperature cycling performance. Specifically, the fluorine atoms in the fluorobenzene compound molecules have strong electronegativity, and the fluorobenzene compounds can reduce the surface tension of the electrolyte. The complex formed by the fluorobenzene compounds and lithium ions helps to accelerate the diffusion process of lithium ions, making it easier for the electrolyte to penetrate into the pores and grain boundaries of the cathode active material, thereby accelerating the diffusion and migration of lithium ions. In addition, the fluorobenzene compounds have better stability in the electrolyte compared to carboxylic ester solvents, which can maintain the long-term stability of the battery, helping to reduce the performance degradation during the charge and discharge process of the battery and extend the service life of the battery. When sodium element (Na + ) is doped into the lattice of the cathode active material, it will replace part of the lithium element (Li + ). Due to the differences in radius and charge between sodium ions and lithium ions, this substitution will cause local adjustment of the lattice structure of the doped material. This adjustment makes the diffusion channels of the complex formed by the fluorobenzene compounds and lithium ions in the lattice of the cathode more spacious and unobstructed, thereby reducing the activation energy of lithium ion diffusion and enabling lithium ions to better pass through the pores and grain boundaries of the cathode active material, effectively improving the diffusion and migration of lithium ions in the cathode active material. The complex of the fluorobenzene compound and lithium ions also contains a part of anionic groups (such as bis(fluorosulfonyl)imide anion (FSI - ). When the complex structure contacts the sodium ions doped on the surface of the cathode active material, this solvation layer migrates the lithium-ion-containing groups further into the cathode active material through electrostatic attraction, effectively improving the wetting of the electrolyte on the surface of the cathode active material, accelerating the diffusion rate and kinetic transport performance of lithium ions in the cathode active material, and improving the charge and discharge performance and low-temperature / normal-temperature cycling performance of the battery. If the content of the fluorobenzene compound is too high, it will dilute the concentration of the overall electrolyte, and at the same time reduce the concentration of lithium ions, lowering the conductivity and resulting in poor kinetic performance. If the content of the fluorobenzene compound is too low, the improvement of the charge and discharge performance and low-temperature / normal-temperature cycling performance of the battery is not obvious. If the sodium element is too much, it will cause continuous accumulation of sodium in the lithium-ion secondary battery, and the internal structure of the battery will gradually be damaged, including the crystal structure of the cathode active material, the stability of the electrolyte, and the integrity of the battery separator, etc. The damage of these structures will accelerate the aging process of the battery and shorten its cycle life. If the sodium element is too little, the local adjustment effect on the lattice structure of the cathode material is not obvious, and it cannot improve the diffusion rate of lithium ions in the cathode material. In this application, the sodium element content of the cathode active material meets the above range, which can achieve the balance between the lithium ion diffusion rate and the battery cycling performance.

[0048] In some optional embodiments, the mass percentage content of the fluorobenzene compound in the electrolyte and the mass percentage content of sodium element in the cathode active material satisfy: 100 ≤ m1 / m2 ≤ 3000. As an example, the ratio m1 / m2 of the mass percentage content of the fluorobenzene compound in the electrolyte to the mass percentage content of sodium element in the cathode active material can be 100, 300, 500, 800, 1000, 1500, 1700, 2000, 2500, 3000, or within the range composed of any of the above values.

[0049] In this application, when m1 / m2 ≥ 3000, it means that there is too much fluorobenzene solvent and too little sodium element, and the local adjustment effect of sodium element on the lattice is insufficient to match the diffusion of the complex formed by the fluorobenzene compound and lithium ions in the lattice of the cathode active material, and lithium deposition may occur; when m1 / m2 ≤ 100, there is too little fluorobenzene solvent and too much sodium element, and the overall oxidation resistance and kinetic performance of the electrolyte are average, which is not conducive to further improving the stability of high-temperature storage and high-temperature cycling.

[0050] In some optional embodiments, the fluorobenzene compound has a structure represented by the following general formula:

[0051]

[0052] Wherein, R 1 、R 2 、R 3 、R 4 、R 5 、R 6 are each independently selected from hydrogen, halogen, a C1-C20 alkyl group substituted or unsubstituted by halogen, a C1-C20 unsaturated olefin group substituted or unsubstituted by halogen, a C3-C20 cycloalkyl group substituted or unsubstituted by halogen, a phenyl group substituted or unsubstituted by halogen, a biphenyl group substituted or unsubstituted by halogen, a C6-C26 phenylalkyl group substituted or unsubstituted by halogen, and a C6-C26 polycyclic aromatic hydrocarbon group substituted or unsubstituted by halogen, and at least one of R 1 、R 2 、R 3 、R 4 、R 5 、R 6 is F or a group containing F element;

[0053] Preferably, the R 1 、R 2 、R 3 、R 4 、R 5 、R 6independently selected from any one of hydrogen, F, C1-C5 fluoroalkyl groups, and C1-C5 fluoro unsaturated olefin groups;

[0054] Preferably, the fluorobenzene compound has any one of the following structures:

[0055]

[0056]

[0057] Those skilled in the art can understand that other halogens Cl and Br have stronger acidity than the F element. By further limiting the fluorobenzene compound in this application, preferably a fluorobenzene compound without other halogens, the cycle life of the lithium-ion secondary battery can be further improved; in addition, the fluorobenzene compound preferably without other halogens in this application has a high conductivity, which helps the charge transfer inside the battery, thereby improving the charge and discharge rate of the battery. The preferably disubstituted fluorobenzene compound in this application has a higher oxidation decomposition voltage, can effectively match the high-voltage cathode material, and further broaden the electrochemical window of the electrolyte.

[0058] In some alternative embodiments, the positive electrode active material includes at least one of lithium cobaltate material and ternary single crystal material;

[0059] Preferably, the positive electrode active material includes lithium cobaltate material and ternary single crystal material. Based on the total mass of the positive electrode active material, the mass percentage content of the ternary single crystal material is 20-40%; as an example, the mass percentage content of the ternary single crystal material in the positive electrode active material can be 20%, 23%, 25%, 28%, 30%, 32%, 35%, 37%, 40%, or within the range composed of any of the above values.

[0060] Those skilled in the art can understand that lithium cobaltate material is a commonly used cathode active material in high-voltage platform lithium-ion secondary batteries. In this application, by using lithium cobaltate material (LCO) in combination with a certain proportion of ternary single-crystal material (NCM), the diffusion coefficient of lithium ions in the cathode active material can be further improved, and the high-temperature performance of the battery can be enhanced. The reason is as follows: The channel width and connectivity in the crystal structure affect the migration speed of lithium ions. Since the internal arrangement orientation of the ternary single-crystal material is consistent and there are no grain boundaries, its structural stability is stronger. Blending a part of the ternary single-crystal material in the lithium cobaltate material can increase the diffusion coefficient of lithium ions. Specifically, when a part of the ternary single-crystal material is blended, the structural stability of the cathode active material is enhanced, with fewer structural defects and dislocations, and these defects and dislocations are often obstacles in the lithium-ion diffusion process because they interrupt the migration path of lithium ions and increase the diffusion resistance. When the structural stability is improved, the number of these defects and dislocations decreases, thereby reducing the obstacles encountered by lithium ions during diffusion and increasing the diffusion coefficient. In addition, fluorobenzene compounds and lithium-ion complexes together reduce the surface tension of the electrolyte on the surface of the cathode active material (LCO / NCM). After the fluorobenzene compounds are complexed with lithium ions, they are more likely to penetrate into the pores in the cathode active material (LCO / NCM). The path of the diffusion passage in the lattice of the ternary single-crystal material becomes shorter, which helps the rapid migration of lithium ions, thereby reducing the activation energy of lithium-ion diffusion and increasing the diffusion coefficient of lithium ions. When the content ratio of the ternary single-crystal material is higher than 40%, due to the relatively high nickel content in the ternary single-crystal material, the catalytic oxidation activity of the electrolyte increases, which easily causes the oxidation and decomposition of the electrolyte and gas generation, having a destructive impact on the cycle life of the battery and also bringing safety problems; when the content of the ternary single-crystal material is lower than 20%, the diffusion rate of lithium ions will decrease, which is not conducive to the fast charge and discharge performance of lithium-ion batteries and causes battery capacity attenuation.

[0061] And / or, the Dv50 of the cathode active material is 10 - 30 μm. Preferably, the ratio of the Dv50 of the ternary single-crystal material to that of the lithium cobaltate material is 0.1 - 10. As an example, the Dv50 of the cathode active material can be 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, or within the range composed of any of the above values; the ratio of the Dv50 of the ternary single-crystal material to that of the lithium cobaltate material can be 0.1, 0.2, 0.3, 0.4, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or within the range composed of any of the above values.

[0062] Those skilled in the art can understand that a relatively large particle size of the positive electrode active material may lead to a longer diffusion path for lithium ions during the insertion and extraction processes, an increase in internal resistance, thereby reducing the energy density of the battery. In addition, the large-particle positive electrode active material is prone to particle breakage during charge and discharge, damaging the electrode structure and affecting the cycle life of the battery. However, an overly small particle size of the positive electrode active material will result in an increase in the specific surface area of the material and a corresponding increase in surface energy, making the composite material prone to agglomeration and uneven distribution. At the same time, the side reactions on the surface of the positive electrode active material with an overly small particle size will increase correspondingly, ultimately leading to a decline in the comprehensive performance of the positive electrode active material, resulting in a decrease in battery capacity and a deterioration of the battery cycle life. By adjusting the particle size of the positive electrode active material within the above range in this application, a balance between energy density and cycle performance can be achieved.

[0063] In this application, by defining the ratio of Dv50 of the ternary single crystal material to the lithium cobaltate material, the particle size distribution of the positive electrode active material can be optimized, the diffusion rate of lithium ions and the insertion / extraction efficiency can be improved, thereby enhancing the energy density, high-temperature cycle performance and rate performance of the battery. If the ratio of the particle sizes of the two is less than 0.1, it may lead to uneven mixing, affecting the uniformity of electrode coating, and further causing local stress concentration or a decrease in the utilization rate of the active material. It will also make the lithium ion diffusion path more complex, especially under high-voltage or high-rate conditions, increasing the polarization phenomenon and reducing the rate performance. When the ratio of the particle sizes of the two is greater than 10, the particles of the ternary single crystal material are relatively large, and there are more catalytic elements such as Ni contained therein, and there are more by-products for the battery cycle. The volume changes are inconsistent during charge and discharge, which may lead to local stress concentration, causing particle breakage or electrode cracking.

[0064] In some optional embodiments, the positive electrode active material further contains a first element, and the first element includes at least one of Al, Mg, Ti, Zr, Nb, and Mo. Preferably, based on the total mass of the positive electrode active material, the mass percentage content of the first element is 0.3%-3.5%. As an example, based on the total mass of the positive electrode active material, the mass percentage content of the first element can be 0.3%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.7%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8%, 3.0%, 3.3%, 3.5%, or within the range composed of any of the above values.

[0065] It should be noted that the first element can be doped in the lithium cobaltate material, or in the ternary single crystal material, or simultaneously doped in both the lithium cobaltate material and the ternary single crystal material. In this application, in the positive electrode active material, by doping the above-mentioned first element, the charge transfer resistance inside the positive electrode active material is further reduced, the resistance encountered by lithium ions during diffusion is decreased, the diffusion rate of lithium ions is increased, and the polarization degree during charge and discharge is reduced, which means that the voltage loss inside the battery is reduced. If the voltage loss is large (big voltage drop), it will cause the overall performance of the battery pack to decline, such as a decrease in capacity and an increase in internal resistance, thereby affecting the cycle capacity retention rate and service life of the battery, and it may also cause some single cells to be overcharged or over-discharged, thus triggering serious safety accidents such as fires or explosions. When the mass percentage content of the doped element > 3.5%, when there are too many metal elements doped in the positive electrode active material, it may hinder the lithium ion transmission channel, resulting in a decline in the rate performance of the battery. This is because too many doped metal elements may occupy the positions of lithium ions, making the diffusion of lithium ions in the positive electrode material difficult. The doping of metal elements may also change the electrochemical properties of the positive electrode active material, causing the battery to generate more heat during charge and discharge. If the heat cannot be dissipated in time, it will cause the battery temperature to rise, increasing the risk of thermal runaway and explosion. When the mass percentage content of the doping content < 0.3%, when there are too few metal elements doped in the positive electrode active material, the improvement effect of the metal elements on the battery performance may not be fully exerted. For example, the doping of some metal elements (such as aluminum, titanium, etc.) can improve the conductivity and rate performance of the positive electrode material, but if the doping amount is insufficient, these improvement effects may not be obvious.

[0066] In addition, it should be noted that the sodium element and the first element in the positive electrode active material can be doped by a solid-phase doping method. Through a high-temperature solid-phase reaction, the doped element is mixed and calcined with the raw materials, so that the doped atoms diffuse into the lattice.

[0067] As an example, the general chemical formula of the lithium cobaltate material is: Li b Na 1-b Co a A 1-a O 2 ; where A is selected from at least one of Al, Mg, Ti, Zr, Nb, Mo, 0 ≤ a < 1, 0 < b < 1.

[0068] As an example, the chemical formula of the ternary single crystal material can be LiNi x Co y Mn z N (1-x-y-z) O 2, where each N is independently selected from at least one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, V, and Ti, 0 < x < 1, 0 < y < 1, 0 < z < 1, and x + y + z ≤ 1; specifically, the composition of the ternary single crystal material can be LiNi 0.5 Co 0.2 Mn 0.3 O 2 、LiNi 0.6 Co 0.2 Mn 0.2 O 2 etc.

[0069] In some alternative embodiments, the electrolyte further includes a fluorinated ethylene carbonate compound. Based on the total mass of the electrolyte, the mass percentage of the fluorinated ethylene carbonate compound is m3%, and 5 ≤ m3 ≤ 25; as an example, based on the total mass of the electrolyte, the mass percentage of the fluorinated ethylene carbonate compound can be 5%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20%, 23%, 25%, or within the range composed of any of the above values.

[0070] Those skilled in the art can understand that the fluorinated ethylene carbonate compound is a film-forming protection additive on the negative electrode, which can effectively prevent the direct contact between the electrolyte and the negative electrode, thereby reducing the occurrence of side reactions. At the same time, the fluorinated ethylene carbonate compound can form a stable coordination bond with lithium ions, thereby enhancing the dissolution stability of lithium ions, which helps to reduce the migration and drift of lithium ions and improve the conduction efficiency of lithium ions; while the fluorobenzene compound can be used as a solvent to dilute the high-concentration electrolyte, which can effectively reduce the viscosity of the electrolyte and ensure the low-impedance migration of lithium ions between the positive and negative electrodes. The combined use of the two can jointly improve the conduction of lithium ions on the negative electrode side and improve the charge and discharge performance and cycle performance at low temperatures.

[0071] Preferably, the mass percentage content of the fluorobenzene compound and the fluorinated ethylene carbonate compound satisfies: 12 ≤ m1 + m3 ≤ 50; preferably, the fluorinated ethylene carbonate compound includes at least one of fluorinated ethylene carbonate, difluorinated ethylene carbonate, trifluoromethyl carbonate, 4,4,5,5-tetrafluoro-1,3-dioxolane-2-one (183301-46-4). As an example, the sum of the mass percentage content of the fluorobenzene compound and the fluorinated ethylene carbonate compound m1 + m3 can be 12, 14, 15, 18, 20, 23, 25, 28, 30, 35, 40, 45, 50, or within the range composed of any of the above values. In this application, when the value of m1 + m3 is within the above range, it can ensure that the electrolyte has an appropriate viscosity, can form an SEI film with sufficient thickness and quality, and further improve the diffusion rate and cycling performance of lithium ions. When the value of m1 + m3 is higher than 50, it will cause an increase in the viscosity of the electrolyte, affecting the further improvement of the diffusion rate of lithium ions; in addition, under abnormal conditions such as high temperature or short circuit, excessive fluorobenzene compounds and fluorinated ethylene carbonate compounds may cause thermal runaway and combustion of the electrolyte, increasing the safety hazards of the battery. When the value of m1 + m3 is lower than 12, it will affect the thickness and quality of the formed SEI film, limiting the further improvement of the battery cycling stability and the capacity retention rate at high temperature.

[0072] In some alternative embodiments, the negative electrode sheet includes a negative electrode active material, and the mass percentage content of silicon element in the negative electrode active material is M%, and the mass percentage content of silicon element in the negative electrode active material and the mass percentage content of the fluorobenzene compound in the electrolyte satisfy: 0.3 ≤ M / m1 ≤ 5, 3 ≤ M ≤ 30; as an example, the mass percentage content of silicon element in the negative electrode active material can be 3%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, or within the range composed of any of the above values; the ratio of the mass percentage content of silicon element in the negative electrode active material to the mass percentage content of the fluorobenzene compound in the electrolyte M / m1 can be 0.3, 0.5, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, or within the range composed of any of the above values.

[0073] It should be noted that in the silicon-doped system, fluorobenzene compounds can reduce the activation energy of lithium-ion diffusion in silicon-containing anode materials. The reduction of the activation energy means that the energy barrier that lithium ions need to overcome during diffusion decreases. Fluorobenzene compounds can form a solvent shell around silicon atoms or other solute molecules. This solvent shell can stabilize the charge distribution and geometric configuration of solute molecules, thereby reducing the energy of the system and increasing the diffusion rate of lithium ions. Fluorobenzene compounds can also form a stable interfacial protective film on the anode. The stable interface helps to reduce the charge transfer resistance and concentration polarization at the interface, thereby improving the diffusion efficiency of lithium ions.

[0074] When M / m1 is greater than 5, it indicates that the silicon element content is relatively too high, and the side reaction between the interface on the anode side and the electrolyte increases, which will exacerbate this volume change effect, resulting in battery structure damage, accelerated capacity decay, and shortened battery life. When M / m1 is less than 0.3, it indicates that the content of fluorobenzene substances is relatively too high, which may have a negative impact on the electrolyte content of the battery and the wetting ability of the electrolyte on the electrode plate, thereby affecting the charge-discharge performance and life of the battery. In addition, too high a fluorine content in the electrolyte may also cause the internal reaction of the battery to get out of control, resulting in a violent reaction and even safety problems such as battery explosion.

[0075] In this application, the selection of the anode active material can be conventional in the field, including but not limited to at least one of artificial graphite and silicon-carbon materials, etc.

[0076] Preferably, the anode active material includes artificial graphite and silicon-carbon materials, and the Dv50 particle size of the silicon-carbon material is d μm, where 5 ≤ d ≤ 30. As an example, the particle size of the silicon-carbon material can be 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, or within the range composed of any of the above values.

[0077] Those skilled in the art can understand that the particle size of the silicon-carbon material will also affect the diffusion coefficient of lithium ions in the anode. Smaller particle size particles have a larger specific surface area, making it easier for lithium ions to contact the inside of the particles, thereby shortening the diffusion path and increasing the diffusion coefficient. However, too small a particle size of the silicon-carbon material will cause an increase in the specific surface area of the material and a corresponding increase in surface energy, making the silicon-carbon material more likely to agglomerate. The side reactions on the surface of the silicon-carbon material with too small a particle size will increase accordingly, ultimately resulting in a decline in the comprehensive performance of the anode material, leading to a decrease in battery capacity and a deterioration of the battery cycle life. A larger particle size of the silicon-carbon material may lead to a longer diffusion path and an increase in internal resistance during the insertion and extraction of lithium ions, thereby reducing the energy density of the battery. Large particle silicon-carbon materials are prone to particle breakage during charge and discharge, damaging the electrode structure and affecting the battery cycle life. In this application, the particle size of the silicon-carbon material is limited to the above range, which can better balance the diffusion coefficient and cycle performance.

[0078] In some optional embodiments, a plurality of holes are provided on the surface of the negative electrode sheet. The aperture of the holes is d1 μm, where 50 ≤ d1 ≤ 500, and the depth of the holes is d2 μm, where 5 ≤ d2 ≤ 45. The center distance between two adjacent holes is d3 μm, where 100 ≤ d3 ≤ 2000. As an example, the aperture of the holes can be 50 μm, 80 μm, 100 μm, 120 μm, 150 μm, 170 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm, or within the range composed of any of the above values; the depth of the holes can be 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 17 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, or within the range composed of any of the above values. As an example, the center distance between the adjacent holes can be 100 μm, 150 μm, 170 μm, 190 μm, 210 μm, 230 μm, 250 μm, 300 μm, 350 μm, 400 μm, 500 μm, 600 μm, 800 μm, 1000 μm, 1200 μm, 1500 μm, 1700 μm, 1900 μm, 2000 μm, or within the range composed of any of the above values.

[0079] Those skilled in the art can understand that by providing a plurality of holes on the surface of the negative electrode sheet, the penetration rate of the electrolyte in the electrode sheet with holes will also increase accordingly, which is beneficial to the conduction of lithium ions and the improvement of battery performance. If the aperture and depth of the holes are too large or the center distance between two adjacent holes is too small, it may lead to insufficient compaction of the electrode material and prone to the problem of electrode loosening. This will not only affect the service life of the battery, but also exacerbate the polarization effect of the battery and further reduce the cycle life of the battery. In addition, during the charge and discharge process, too large pores will cause large volume changes in the electrode material, resulting in the destruction of the electrode structure and rapid attenuation of the capacity. However, if the aperture and depth of the holes are too small or the center distance between two adjacent holes is too large, the improvement of the wetting performance of the electrolyte is not obvious, thereby affecting the further improvement of the lithium ion transport efficiency.

[0080] In some optional embodiments, the CB value of the battery is denoted as N / P, where 1.05 ≤ N / P ≤ 1.5; as an example, the N / P can be 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, or within the range composed of any of the above values.

[0081] Preferably, N / P and m1 satisfy: N / P ≥ m1 / 10.

[0082] Those skilled in the art can understand that in the design of lithium batteries, the CB value (Cell Balance) refers to the margin by which the negative electrode capacity exceeds the positive electrode capacity within the same stage, also known as N / P (Negative / Positive). The calculation formula for the CB value is as follows:

[0083] CB value = (gram capacity of negative electrode active material × negative electrode surface density × negative electrode active material content ratio) / (gram capacity of positive electrode active material × positive electrode surface density × positive electrode active material content ratio).

[0084] It should be noted that in this application, the gram capacity of the positive and negative electrode active materials is calculated through experiments. For example, for the prepared electrode material, using the active material electrode as the working electrode, metallic lithium as the counter electrode, and an electrolyte (such as LiPF / EC-DMC) and a separator to form a half-cell (button cell), under a constant current charge-discharge mode (such as 0.1C rate), record the first charge-discharge curve:

[0085] Gram capacity (mAh / g) = {discharge capacity (mAh)} / {mass of active material in the electrode (g)}.

[0086] By controlling the CB value within the above range, this application can effectively improve the fast charging performance of the battery at room temperature, prevent lithium plating, and further improve the safety performance. If the CB value is too small, lithium plating is likely to occur, resulting in hindered lithium ion transmission. However, if the CB value is too large, the cycle life attenuation is relatively obvious. This is because during the charge-discharge process, the negative electrode may overreact, leading to the loss of active material and the destruction of the structure. This overreaction will accelerate the capacity attenuation of the negative electrode, thereby affecting the cycle life of the entire battery.

[0087] In this application, by regulating the N / P and m1 to satisfy the above relationship, the complex formed by the fluorobenzene compound and lithium ions helps to accelerate the diffusion process of lithium ions, improve the transmission rate of lithium ions between the positive and negative electrodes, and thus reduce the safety risk; the negative electrode has sufficient capacity to accept lithium ions, and the fluorobenzene compound can be more evenly reduced on the surface of the negative electrode to form a stable SEI film, avoiding overcharging of the positive electrode and lithium plating on the negative electrode. Moreover, the fluorobenzene compound further removes the active substances in the electrolyte, reducing the occurrence of side reactions. It reduces the stress concentration and local overheating risk inside the battery, and the fluorobenzene compound improves the thermal stability of the electrolyte. The combined effect of the two significantly reduces the possibility of thermal runaway.

[0088] Those skilled in the art can understand that during the charge-discharge process of the battery, lithium ions are embedded and extracted back and forth between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The separator is arranged between the positive electrode plate and the negative electrode plate, mainly to prevent short circuit between the positive and negative electrodes, and at the same time allows lithium ions to pass through.

[0089] As an example, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode active material layer is provided on either one or both of the two opposite surfaces of the positive electrode current collector. The materials, compositions, and manufacturing methods of the positive electrode sheet used in the lithium-ion secondary battery of the present application may include any techniques disclosed in the prior art.

[0090] As an example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is provided on either one or both of the two opposite surfaces of the negative electrode current collector. The materials, compositions, and manufacturing methods of the negative electrode sheet used in the lithium-ion secondary battery of the present application may include any techniques disclosed in the prior art.

[0091] There are no particular limitations on the materials and shapes of the separator used in the lithium-ion secondary battery of the present application, and it may include any techniques disclosed in the prior art.

[0092] The electrolyte used in the lithium-ion secondary battery of the present application may include any techniques disclosed in the prior art.

[0093] As an example, other organic solvents in the electrolyte of the present application are selected from one or more of unsubstituted carbonate solvents, unsubstituted carboxylate solvents, fluorinated carboxylates, and fluorinated ethers;

[0094] Preferably, the unsubstituted carbonate solvents are selected from one or more of the following solvents: ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate;

[0095] Preferably, the unsubstituted carboxylate solvents are selected from one or more of the following solvents: propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, ethyl propionate, n-propyl propionate, methyl butyrate, ethyl butyrate (EB);

[0096] As an example, the fluorinated carboxylate includes at least one of fluorinated ethyl methyl carbonate, fluorinated diethyl carbonate, fluorinated dimethyl carbonate, fluorinated ethyl acetate, fluorinated methyl propionate, fluorinated ethyl propionate, and fluorinated propyl propionate; the fluorinated ethers include at least one of bis(2,2,2-trifluoroethyl) ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, methyl nonafluorobutyl ether, tris(trifluoroethoxy)methane, and ethyl nonafluorobutyl ether.

[0097] The lithium salt in the electrolyte of the present application is selected from lithium hexafluorophosphate (LiPF 6 ) and lithium difluorophosphate (LiPO 2 F2 ) one or more of lithium difluoro(oxalato)borate (LiODFB), lithium bis(trifluoromethanesulfonyl)imide, lithium difluorobis(oxalato)phosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methyl or lithium bis(trifluoromethylsulfonyl)imide. Based on the total mass of the electrolyte, the concentration of the lithium salt is 12-18%.

[0098] The electrolyte further includes other additives selected from one or more of vinylene carbonate, vinyl ethylene carbonate, ethylene sulfate, succinonitrile (SN), glutaronitrile, adiponitrile (ADN), pimelonitrile, suberonitrile, sebaconitrile, 1,3,6-hexanetricarbonitrile, glycerol trinitrile, 1,2-bis(2-cyanoethoxy)ethane, 1,3-propane sultone (PS), allyl-1,3-sultone, etc.

[0099] In the present application, the preparation method of the lithium ion secondary battery is conventional in the art. As an example, the preparation method may include:

[0100] Stack the positive electrode sheet, separator, and negative electrode sheet in sequence, ensure that the separator is between the positive and negative electrode sheets to play an isolation role, and then obtain an un-injected bare battery cell by winding; place the bare battery cell in an outer packaging foil, inject the prepared electrolyte into the dried bare battery cell, and obtain the required lithium ion secondary battery through processes such as vacuum packaging, standing, formation, shaping, sorting, etc.

[0101] According to another aspect of the present application, there is also provided an electrical device including the above lithium ion secondary battery.

[0102] As an example, the lithium ion secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device can be, but is not limited to, a mobile device (such as a mobile phone, laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0103] The following further describes the present application in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present application.

[0104] Example 1

[0105] This example provides a lithium ion secondary battery, and its specific composition and preparation method are as follows:

[0106] Composition of the electrolyte:

[0107] Base organic solvents: The mass ratio of ethylene carbonate (EC): propylene carbonate (PC): ethyl butyrate (EB) is 1:1:8, which is used to adjust the total amount of the electrolyte to 100%;

[0108] Based on the total mass of the electrolyte, it includes 10% of fluorobenzene compounds (the structure shown in Formula 1); 15% of fluoroethylene carbonate (FEC), 3% of 1,3 - propane sultone, 2.5% of 1,3,6 - hexanetricarbonitrile, 1% of adiponitrile (ADN), 1.5% of succinonitrile (SN);

[0109] Lithium salt: Based on the total mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is 13%.

[0110] Composition and preparation of the positive electrode sheet:

[0111] (1) Preparation of the positive electrode sheet

[0112] Mix the positive electrode active material with a Dv50 of 18 μm (including lithium cobaltate material and ternary single - crystal material NCM622, the mass percentage of the ternary single - crystal material in the positive electrode active material is 30%, and Na element and the first element are doped in the lithium cobaltate material), polyvinylidene fluoride (molecular weight 20000 - 100000), conductive carbon black, and carbon nanotubes according to a mass ratio of 96:2:1.5:0.5, add N - methylpyrrolidone (NMP), and stir under the action of a vacuum mixer until the mixed system becomes a homogeneous and fluid positive electrode active slurry; uniformly coat the two surfaces of the aluminum foil with the positive electrode active slurry; dry the coated aluminum foil, and obtain the required positive electrode sheet through rolling and slitting. The areal density of the positive electrode sheet is 0.015 g / cm 2 。

[0113] (2) Preparation of the negative electrode sheet

[0114] Mix the negative electrode active material (composed of 85 wt% graphite + 15 wt% silicon - carbon material, and the mass percentage of silicon element in the silicon - carbon material is 20%), styrene - butadiene rubber (SBR), lithium polyacrylate (molecular weight 20000 - 100000), conductive carbon black (SP), and carbon nanotubes (CNTs) according to a mass ratio of 96.5:1.5:0.5:1.0:0.5, then add deionized water step by step, and make it fully mixed under the action of a vacuum mixer to finally form a homogeneous and fluid negative electrode slurry with a solid content of 45 wt%; uniformly coat the above - mentioned negative electrode slurry on a copper foil with a thickness of 6 μm, dry, roll, die - cut, and punch holes to obtain the negative electrode sheet. The areal density of the negative electrode sheet is 0.005 g / cm 2 。The particle size Dv50 of the silicon - carbon material is 8.5 μm.

[0115] Separator:

[0116] An 8-μm thick polyethylene separator (provided by Asahi Kasei Corporation) was selected.

[0117] Battery assembly:

[0118] The prepared positive electrode sheet, separator, and negative electrode sheet were stacked in sequence to ensure that the separator was between the positive and negative electrode sheets to play an isolation role, and then an un-injected bare battery cell was obtained by winding; the bare battery cell was placed in an outer packaging foil, and the prepared electrolyte was injected into the dried bare battery cell. After processes such as vacuum packaging, standing, forming, shaping, and sorting, the required lithium-ion secondary battery was obtained. The total capacity of the battery was 3600 mAh, the thickness was 3.8 mm, the width was 62 mm, and the length was 80 mm.

[0119] The CB value of the battery was designed to be 1.06.

[0120] The calculation method of the CB value was carried out as follows:

[0121] CB value = (gram capacity of negative electrode active material × negative electrode surface density × negative electrode active material content ratio) ÷ (gram capacity of positive electrode active material × positive electrode surface density × positive electrode active material content ratio);

[0122] In this embodiment, the gram capacity of the positive electrode active material was 187 mAh / g, and the gram capacity of the negative electrode active material was 460 mAh / g.

[0123] Examples 2 - 17

[0124] The differences between Examples 2 - 17 and Example 1 were that the composition of the electrolyte was different, the sodium element content in the lithium cobaltate material was different, or the silicon element content in the negative electrode active material was different. See the following table for details:

[0125] Table 1

[0126]

[0127] Examples 18 - 40

[0128] The differences between Examples 18 - 40 and Example 1 were that the composition and particle size of the positive electrode active material were different, the particle size of the silicon-carbon material was different, the pore diameter and pore depth were different, or the CB value was different. See the following table for details:

[0129] Table 2

[0130]

[0131]

[0132] Example 41

[0133] Example 41 is different from Example 1 in that the positive electrode active material does not contain ternary single crystal material, that is, lithium cobaltate material with the same mass is used to replace the ternary single crystal material.

[0134] Example 42

[0135] Example 42 is different from Example 1 in that the surface of the negative electrode sheet is not perforated.

[0136] Example 43

[0137] Example 43 is different from Example 1 in that the first elements are Al and Ti, where the mass percentage of Al is 0.3% and the mass percentage of Ti is 0.2%.

[0138] Comparative Example 1

[0139] Compared with Example 1, the difference is only that the lithium cobaltate material is not doped with sodium element.

[0140] Comparative Example 2

[0141] Compared with Example 1, the difference is only that the content of fluorobenzene compounds is 0%.

[0142] Comparative Example 3

[0143] Compared with Example 1, the difference is only that the content of fluorobenzene compounds is 40%.

[0144] Comparative Example 4

[0145] Compared with Example 1, the difference is only that the doping amount of sodium element in the lithium cobaltate material is 2%.

[0146] Test Example

[0147] Perform performance tests on the lithium-ion secondary batteries prepared in each example and comparative example. The specific test methods are as follows:

[0148] 1. Ambient temperature cycling

[0149] Perform a 25°C cycling test on the batteries prepared in the examples and comparative examples. The specific test methods are as follows:

[0150] Test the incoming samples and let them stand for 10 min at 25°C; discharge at 1C to the lower limit voltage of 3.0V and let it stand for 10 min; charge at 1C to the upper limit voltage of 4.55V at 25°C, with a cut-off current of 0.025C, and let it stand for 10 min; in a 25°C constant temperature oven environment, discharge at a rate of 1.2C to the cut-off voltage of 3.0V and let it stand for 10 min; cycle 600 times, record the discharge capacity Q1 after 600 ambient temperature cycles and the initial discharge capacity Q0, and calculate the ambient temperature cycle capacity retention rate.

[0151] 2. Low-temperature cycling

[0152] The batteries prepared in the examples and comparative examples were subjected to a cycling test at 0°C. The specific test method is as follows:

[0153] The samples to be tested were left standing for 10 min in an environment at 0°C; discharged at 1C to the lower limit voltage of 3.0V, and left standing for 10 min; charged at 1C to the upper limit voltage of 4.55V at 0°C, with a cut-off current of 0.025C, and left standing for 10 min; in an environment of a 0°C constant temperature oven, discharged at a rate of 1.2C to the cut-off voltage of 3.0V, and left standing for 10 min; cycled 600 times, and the discharge capacity Q2 after 600 low-temperature cycles and the initial discharge capacity Q0 were recorded, and the low-temperature cycling capacity retention rate was calculated.

[0154] 3. High-temperature cycling

[0155] The batteries prepared in the examples and comparative examples were subjected to a cycling test at 45°C. The specific test method is as follows:

[0156] The samples to be tested were left standing for 10 min in an environment at 45°C; discharged at 1C to the lower limit voltage of 3.0V, and left standing for 10 min; charged at 1C to the upper limit voltage of 4.55V at 45°C, with a cut-off current of 0.025C, and left standing for 10 min; in an environment of a 45°C constant temperature oven, discharged at a rate of 1.2C to the cut-off voltage of 3.0V, and left standing for 10 min; cycled 600 times, and the discharge capacity Q3 after 600 high-temperature cycles and the initial discharge capacity Q0 were recorded, and the high-temperature cycling capacity retention rate was calculated.

[0157] 4. Normal-temperature 2C charging performance

[0158] The batteries prepared in the examples and comparative examples were subjected to a cycling test at 25°C. The specific test method is as follows:

[0159] The samples to be tested were left standing for 10 min in an environment at 25°C; discharged at 2C to the lower limit voltage of 3.0V, and left standing for 10 min; charged at 2C to the upper limit voltage of 4.55V at 25°C, with a cut-off current of 0.025C, and left standing for 10 min; in an environment of a 25°C constant temperature oven, discharged at a rate of 1.2C to the cut-off voltage of 3.0V, and left standing for 10 min; the battery was subjected to charge and discharge cycles 50 times, and the negative electrode sheet was disassembled to observe whether there was lithium deposition.

[0160] The specific test results are shown in the following table:

[0161] Table 3

[0162]

[0163]

[0164]

[0165] As can be seen from the data in the above table, for the lithium-ion secondary battery provided by the embodiments of the present application, by doping sodium element in the positive electrode active material and cooperating with adding fluorobenzene compounds in the electrolyte, the diffusion rate of lithium ions in the positive electrode active material at high voltage can be increased, so that the battery has good charge and discharge performance and cycle performance, especially low-temperature and normal-temperature cycle performance.

[0166] Obviously, the above embodiments are merely examples given for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or alterations can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. And the obvious changes or alterations derived therefrom still fall within the protection scope of the present invention.

Claims

1. A lithium ion secondary battery, characterized in that: Including positive electrode, negative electrode and electrolyte, The positive electrode sheet includes a positive electrode active material, the positive electrode active material is doped with a sodium element, and the mass percentage of the sodium element is m2% based on the total mass of the positive electrode active material, 0.001≤m2≤1; The electrolyte includes fluorobenzene compounds, and the mass percentage of the fluorobenzene compounds is m1% based on the total mass of the electrolyte, 2≤m1≤30.

2. The lithium ion secondary battery according to claim 1, characterized in that: The mass percentage of the fluorobenzene compound in the electrolyte and the mass percentage of the sodium element in the positive electrode active material satisfy: 100≤m1 / m2≤3000.

3. The lithium ion secondary battery according to claim 1, characterized in that: The fluorobenzene compound has a structure shown in the following general formula: Wherein, R1, R2, R3, R4, R5, and R6 are independently selected from any one of hydrogen, halogen, C1-C20 alkyl groups substituted or not substituted by halogen, C1-C20 unsaturated olefin groups substituted or not substituted by halogen, C3-C20 cycloalkyl groups substituted or not substituted by halogen, phenyl groups substituted or not substituted by halogen, biphenyl groups substituted or not substituted by halogen, C6-C26 phenylalkyl groups substituted or not substituted by halogen, and C6-C26 condensed aromatic hydrocarbon groups substituted or not substituted by halogen, and at least one of R1, R2, R3, R4, R5, and R6 is F or a group containing the element F; Preferably, R1, R2, R3, R4, R5 and R6 are independently selected from any one of hydrogen, F, C1-C5 fluorinated alkyl group and C1-C5 fluorinated unsaturated olefin group.

4. The lithium ion secondary battery according to claim 3, characterized in that: The fluorobenzene compound has any of the following structures:

5. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material includes at least one of a lithium cobalt oxide material and a ternary single crystal material; Preferably, the positive electrode active material comprises a lithium cobalt oxide material and a ternary single crystal material, and the mass percentage of the ternary single crystal material is 20-40% based on the total mass of the positive electrode active material; and / or, the Dv50 of the positive electrode active material is 10-30 μm, preferably, the ratio of the Dv50 of the ternary single crystal material to the Dv50 of the lithium cobalt oxide material is 0.1-10; And / or, the positive electrode active material further contains a first element, and the first element includes at least one of Al, Mg, Ti, Zr, Nb, and Mo; Preferably, based on the total mass of the positive electrode active material, the mass percentage of the first element is 0.3%-3.5%.

6. The lithium ion secondary battery according to claim 1, characterized in that: The electrolyte further comprises a fluoroethylene carbonate compound, and the mass percentage of the fluoroethylene carbonate compound is m3% based on the total mass of the electrolyte, 5≤m3≤25; Preferably, the mass percentage of the fluorobenzene compound and the fluoroethylene carbonate compound satisfies: 12≤m1+m3≤50; Preferably, the fluoroethylene carbonate compound includes at least one of fluoroethylene carbonate, bisfluoroethylene carbonate, trifluoromethylethylene carbonate, and 4,4,5,5-tetrafluoro-1,3-dioxolane-2-one.

7. The lithium ion secondary battery according to claim 1, characterized in that: The negative electrode sheet comprises a negative electrode active material, the mass percentage of silicon in the negative electrode active material is M%, and the mass percentage of silicon in the negative electrode active material and the mass percentage of fluorobenzene compounds in the electrolyte satisfy: 0.3≤M / m1≤5, 3≤M≤30; Preferably, the negative electrode active material comprises artificial graphite and silicon-carbon material; Preferably, the particle size Dv50 of the silicon-carbon material is d μm, 5≤d≤30.

8. The lithium ion secondary battery according to claim 1, characterized in that: A plurality of holes are arranged on the surface of the negative electrode sheet, wherein the pore diameter of the holes is d1 μm, 50≤d1≤500, and / or the depth of the holes is d2 μm, 5≤d2≤45; and / or the center distance between two adjacent holes is d3 μm, 100≤d3≤2000.

9. The lithium ion secondary battery according to any one of claims 1 to 8, characterized in that: The CB value of the battery is recorded as N / P, 1.05≤N / P≤1.5; Preferably, N / P and m1 satisfy: N / P ≥ m1 / 10.

10. An electrical device, characterized in that: A lithium ion secondary battery comprising any one of claims 1 to 9.

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