Secondary battery and electronic device

Abstract

The present application provides a secondary battery and an electronic device. Negative electrode material layers of the secondary battery comprise a first negative electrode material layer and a second negative electrode material layer which are stacked together. The second negative electrode material layer is located between the first negative electrode material layer and a negative electrode current collector. The thickness of the first negative electrode material layer is H1 μm, and the thickness of the second negative electrode material layer is H2 μm, where 20 ≤ H1 ≤ 40, and 60 ≤ H2 ≤ 90. The Dv50 of a first negative electrode active material in the first negative electrode material layer is greater than the Dv50 of a second negative electrode active material in the second negative electrode material layer. An electrolyte comprises lithium difluorophosphate and a propionate ester compound. The propionate ester compound comprises propyl propionate. Based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is W1%, the mass percentage of propyl propionate is W2%, 0.1 ≤ W1 ≤ 1, 10 ≤ W2 ≤ 60, and 4 ≤ W1 × H1 ≤ 35. The secondary battery of the present application has good kinetic performance and good high-temperature floating charge performance.

Description

A secondary battery and electronic device

[0001] This application claims priority to Chinese Patent Application No. 202410707511.0, filed on May 31, 2024, entitled "A Secondary Battery and Electronic Device", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of electrochemical technology, and in particular to a secondary battery and electronic device. Background Technology

[0003] Secondary batteries, such as lithium-ion batteries, are widely used in smartphones, wearable devices, consumer drones, and electric vehicles due to their advantages such as high energy density, long cycle life, and no memory effect. With the widespread application of lithium-ion batteries in these fields, the market demands increasingly higher levels of performance in terms of kinetics and high-temperature float charging.

[0004] However, existing lithium-ion batteries have many side reactions between the positive electrode and the electrolyte, and between the negative electrode and the electrolyte during cycling, resulting in a large initial impedance and a significant increase in impedance during cycling, which affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. Summary of the Invention

[0005] The purpose of this application is to provide a secondary battery and electronic device to improve the kinetic performance and high-temperature float charging performance of the secondary battery. The specific technical solution is as follows:

[0006] The first aspect of this application provides a secondary battery, which includes a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer stacked together. The second negative electrode material layer is located between the first negative electrode material layer and the negative current collector. The thickness of the first negative electrode material layer is H1 μm, the thickness of the second negative electrode material layer is H2 μm, 20 ≤ H1 ≤ 40, and 60 ≤ H2 ≤ 90. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode material layer includes a second negative electrode active material. The Dv50 of the first negative electrode active material is greater than that of the second negative electrode active material. The electrolyte includes lithium difluorophosphate and a propionate compound, and the propionate compound includes propyl propionate. Based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is W1%, the mass percentage of propyl propionate is W2%, 0.1≤W1≤1, 10≤W2≤60, and 4≤W1×H1≤35. This application designs the negative electrode structure and electrolyte composition, and controls the values ​​of H1, H2, W1, W2, and W1×H1 within the aforementioned ranges, and ensures that the Dv50 of the first and second negative electrode active materials satisfies the aforementioned relationship. This improves the wettability of the electrolyte on the negative electrode, reduces side reactions between the positive electrode and the electrolyte, and between the negative electrode and the electrolyte during cycling, increases the solubility of lithium salt in the electrolyte and the conductivity of the electrolyte, and facilitates the transport of lithium ions in the negative electrode and the electrolyte. This reduces the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance and high-temperature float charging performance of the secondary battery.

[0007] In some embodiments of this application, 2 ≤ H2 / H1 ≤ 4.5. By adjusting the value of H2 / H1 within the above range, the wettability of the electrolyte to the negative electrode can be further improved, and the side reactions between the negative electrode and the electrolyte during cycling can be reduced. This helps to reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance and high-temperature float charge performance of the secondary battery.

[0008] In some embodiments of this application, 0.005 ≤ W1 / H1 ≤ 0.05. By adjusting the value of W1 / H1 within the above range, it is beneficial to leverage the synergistic effect between the negative electrode and the electrolyte, further improving the dynamic performance of the first negative electrode material layer. This, in turn, helps to further reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby further improving the dynamic performance and high-temperature float charge performance of the secondary battery.

[0009] In some embodiments of this application, 250 ≤ W2 × H1 ≤ 2400. By adjusting the value of W2 × H1 within the above range, it is beneficial to leverage the synergistic effect between the negative electrode and the electrolyte, improve the dynamic performance of the first negative electrode material layer, thereby further reducing the initial impedance of the secondary battery and the impedance growth during cycling, and further improving the dynamic performance and high-temperature float charge performance of the secondary battery.

[0010] In some embodiments of this application, the electrolyte includes a polynitrile compound, and the mass percentage of the polynitrile compound is W3% based on the mass of the electrolyte, where 0.1 ≤ W3 ≤ 5. The polynitrile compound includes at least one selected from succinic anhydride, glutaronitrile, adiponitrile, heptanilide, octanoic anhydride, methylglutaronitrile, 1,3,5-pentanetricarbonyl, 1,2,3-propanetricarbonyl, 1,3,6-hexanetrionitrile, or 1,2,3-tris(2-cyanoethoxy)propane. By further introducing the aforementioned polynitrile compound and controlling its mass percentage W3% within the above range, based on an electrolyte comprising lithium difluorophosphate and propyl propionate, the stability of both the positive and negative electrode plates can be simultaneously improved, as well as the electrochemical and thermal stability of the electrolyte. This reduces the initial impedance and impedance growth during cycling of the secondary battery, improves the kinetic performance of the secondary battery, and also enhances the high-temperature float charge performance of the secondary battery.

[0011] In some embodiments of this application, 50 ≤ W3 × H1 ≤ 200. By adjusting the value of W3 × H1 within the above range, it is possible to reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the dynamic performance of the secondary battery and further enhancing its high-temperature float charge performance.

[0012] In some embodiments of this application, the mass percentage of the propionate ester compound is W4% based on the mass of the electrolyte, with 10 ≤ W4 ≤ 65% and 0.154 ≤ W2 / W4 ≤ 1. By controlling the mass percentage of the propionate ester compound W4% and the W2 / W4 value within the above ranges, based on the electrolyte comprising lithium difluorophosphate and propyl propionate, it is beneficial to further reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance and high-temperature float charge performance of the secondary battery.

[0013] In some embodiments of this application, the propionate compound further includes at least one of methyl propionate, ethyl propionate, butyl acrylate, or butyl propionate. In addition to the electrolyte comprising lithium difluorophosphate and propyl propionate, the inclusion of the aforementioned propionate compound in the electrolyte further reduces the initial impedance of the secondary battery and the impedance increase during cycling, thereby further improving the kinetic performance and high-temperature float charge performance of the secondary battery.

[0014] In some embodiments of this application, the Dv50 of the first negative electrode active material is 12 μm to 14.9 μm, and the Dv90 of the first negative electrode active material is 20.5 μm to 25.9 μm. By controlling the Dv50 and Dv90 of the first negative electrode active material within the above ranges, the wettability of the electrolyte to the negative electrode sheet can be improved, the side reactions between the negative electrode sheet and the electrolyte during cycling can be reduced, and the stability of the negative electrode sheet can be improved. This is beneficial to further reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance of the secondary battery and also improving the high-temperature float charge performance of the secondary battery.

[0015] In some embodiments of this application, the Dv50 of the second negative electrode active material is 10 μm to 11.5 μm, and the Dv90 of the second negative electrode active material is 19.4 μm to 24.8 μm. By controlling the Dv50 and Dv90 of the second negative electrode active material within the above ranges, the wettability of the electrolyte to the negative electrode sheet can be further improved, and the side reactions between the negative electrode sheet and the electrolyte during cycling can be reduced. This is beneficial to reducing the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance of the secondary battery and also improving the high-temperature float charge performance of the secondary battery.

[0016] In some embodiments of this application, the first negative electrode active material and the second negative electrode active material each independently include at least one of natural graphite, artificial graphite, soft carbon, hard carbon, mesophase carbon microspheres, silicon carbide, silicon oxide, or lithium titanate. Based on the fact that the negative electrode sheet includes a first negative electrode material layer and a second negative electrode material layer, and the values ​​of H1 and H2 are controlled within the scope of this application, and the relationship between the Dv50 of the first and second negative electrode active materials is controlled within the scope of this application, and the electrolyte includes lithium difluorophosphate and propyl propionate, and the values ​​of W1, W2, and W1×H1 are controlled within the scope of this application, the selection of the above-mentioned first and second negative electrode active materials can enable the secondary battery to have good kinetic performance and high-temperature float charging performance.

[0017] The second aspect of this application provides an electronic device that includes the secondary battery provided in the first aspect of this application. The secondary battery provided in this application has good kinetic performance and high-temperature float charging performance, thereby giving the electronic device a long service life and good performance.

[0018] The beneficial effects of this application are:

[0019] This application provides a secondary battery and an electronic device. The secondary battery includes a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer stacked together, with the second negative electrode material layer located between the first negative electrode material layer and the negative current collector. The thickness of the first negative electrode material layer is H1 μm, and the thickness of the second negative electrode material layer is H2 μm, where 20 ≤ H1 ≤ 40 and 60 ≤ H2 ≤ 90. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode layer includes a second negative electrode active material. The Dv50 of the first negative electrode active material is greater than that of the second negative electrode active material. The electrolyte includes lithium difluorophosphate and a propionate ester compound, where the propionate ester compound includes propyl propionate. Based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is W1%, and the mass percentage of propyl propionate is W2%, where 0.1 ≤ W1 ≤ 1, 10 ≤ W2 ≤ 60, and 4 ≤ W1 × H1 ≤ 35. This application designs the negative electrode structure and electrolyte composition, and controls the values ​​of H1, H2, W1, W2, and W1×H1 within the aforementioned ranges, and ensures that the Dv50 of the first and second negative electrode active materials satisfies the aforementioned relationship. This improves the wettability of the electrolyte on the negative electrode, reduces side reactions between the positive electrode and the electrolyte, and between the negative electrode and the electrolyte during cycling, increases the solubility of lithium salt in the electrolyte and the conductivity of the electrolyte, and facilitates the transport of lithium ions in the negative electrode and the electrolyte. This reduces the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance and high-temperature float charging performance of the secondary battery.

[0020] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0022] Figure 1 is a schematic diagram of the negative electrode sheet along its own thickness direction according to one embodiment of this application;

[0023] Figure 2 shows the impedance of the lithium-ion batteries of Examples 1-1 and Comparative Example 1 of this application as a function of the number of cycles.

[0024] Figure 3 shows the change rate of thickness of the lithium-ion batteries in Examples 1-1 and Comparative Example 1 of this application as a function of charging time.

[0025] Reference numerals: negative electrode 10, negative current collector 11, first negative electrode material layer 12, second negative electrode material layer 13. Detailed Implementation

[0026] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0027] It should be noted that, in the following explanation, lithium-ion batteries are used as an example of secondary batteries to illustrate this application; however, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0028] The first aspect of this application provides a secondary battery, comprising a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The negative electrode material layer includes a first negative electrode material layer and a second negative electrode material layer stacked thereon, with the second negative electrode material layer located between the first negative electrode material layer and the negative current collector. The thickness of the first negative electrode material layer is H1 μm, and the thickness of the second negative electrode material layer is H2 μm, where 20 ≤ H1 ≤ 40 and 60 ≤ H2 ≤ 90. For example, the value of H1 can be 20, 23, 25, 28, 30, 32, 36, 38, 40, or a range of any two of these values, and the value of H2 can be 60, 63, 65, 68, 70, 72, 76, 78, 80, 83, 85, 87, 90, or a range of any two of these values. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode material layer includes a second negative electrode active material. The Dv50 of the first negative electrode active material is greater than that of the second negative electrode active material. The electrolyte includes lithium difluorophosphate (LiPO2F2) and a propionate compound, which includes propyl propionate. Based on the mass of the electrolyte, the mass percentage of lithium difluorophosphate is W1%, and the mass percentage of propyl propionate is W2%, where 0.1 ≤ W1 ≤ 1, 10 ≤ W2 ≤ 60, and 4 ≤ W1 × H1 ≤ 35. For example, the value of W1 can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any range of any two values; the value of W2 can be 10, 15, 20, 24, 30, 35, 40, 46, 50, 54, 60, or any range of any two values; and the value of W1×H1 can be 4, 8, 10, 12, 15, 17, 20, 23, 25, 28, 30, 35, or any range of any two values.

[0029] The negative electrode sheet includes a first negative electrode material layer and a second negative electrode material layer. By adjusting the values ​​of H1 and H2 within the aforementioned range, electrolyte consumption can be reduced, and the electrolyte can have good wettability to the first and second negative electrode material layers. This improves the wettability of the electrolyte to the negative electrode sheet, which is beneficial for the transport of lithium ions in the negative electrode sheet. It can also reduce side reactions between the negative electrode sheet and the electrolyte during cycling, thereby reducing the initial impedance of the secondary battery and the impedance growth during cycling, and thus improving the kinetic performance and high-temperature float charge performance of the secondary battery. The first negative electrode material layer includes a first negative electrode active material, and the second negative electrode material layer includes a second negative electrode active material. The Dv50 of the first negative electrode active material is greater than that of the second negative electrode active material. The first negative electrode active material has a smaller specific surface area, which can reduce side reactions between the negative electrode sheet and the electrolyte and reduce the initial impedance of the secondary battery. The second negative electrode active material has a smaller particle size, which can improve the wettability of the electrolyte to the negative electrode sheet, increase the diffusion path of lithium ions, and reduce the impedance growth during the cycling process of the secondary battery, thereby improving the kinetic performance and high-temperature float charge performance of the secondary battery. The electrolyte includes lithium difluorophosphate and propyl propionate, and the values ​​of W1 and W2 are adjusted within the above range. Lithium difluorophosphate can participate in the film formation process at both the positive and negative electrode interfaces, forming a denser and more stable positive electrode solid electrolyte interphase (CEI) film and negative electrode solid electrolyte interphase (SEI) film, reducing side reactions between the positive electrode and the electrolyte, and between the negative electrode and the electrolyte. Propyl propionate has a large dielectric constant and better solvent kinetics, which can improve the solubility of lithium salt in the electrolyte and the conductivity of the electrolyte, which is beneficial to the transport of lithium ions in the electrolyte. This can reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance and high-temperature float charge performance of the secondary battery. By adjusting the values ​​of W1×H1 within the aforementioned range, it is beneficial to leverage the synergistic effect between the negative electrode and the electrolyte, forming a denser and more stable SEI film at the negative electrode interface. This further reduces side reactions between the negative electrode and the electrolyte, improves the wettability of the electrolyte on the negative electrode, facilitates lithium-ion transport within the negative electrode, and enhances the kinetic performance of the first negative electrode material layer. Consequently, it can further reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby further improving the kinetic performance and high-temperature float charge performance of the secondary battery.

[0030] When the value of H1 is too small, for example, less than 20, it is difficult to improve the impedance growth of the secondary battery during cycling, thus affecting the kinetic performance and high-temperature float charging performance of the secondary battery. When the value of H1 is too large, for example, greater than 40, the kinetic performance of the first negative electrode material layer is too poor, and the wettability of the electrolyte to the second negative electrode material layer is too poor, resulting in a slow lithium-ion transport speed on the negative electrode sheet. This leads to excessive initial impedance and impedance growth during cycling, thus affecting the kinetic performance and high-temperature float charging performance of the secondary battery. When the value of H2 is too small, for example, less than 60, it is difficult to improve the initial impedance of the secondary battery, thus affecting the kinetic performance and high-temperature float charging performance of the secondary battery. When the value of H2 is too large, for example, greater than 90, the wettability of the electrolyte to the second negative electrode material layer is too poor, and the side reactions between the negative electrode sheet and the electrolyte increase during cycling, leading to excessive initial impedance and impedance growth during cycling, thus affecting the kinetic performance and high-temperature float charging performance of the secondary battery.

[0031] When the Dv50 of the first negative electrode active material is less than that of the second negative electrode active material, the wettability of the electrolyte to the negative electrode sheet is too poor, the transport speed of lithium ions on the negative electrode sheet is too slow, and there are too many side reactions between the negative electrode sheet and the electrolyte, resulting in excessive growth of the initial impedance and the impedance during cycling of the secondary battery, thereby affecting the kinetic performance and high-temperature float charging performance of the secondary battery.

[0032] When the value of W1 is too small, for example, less than 0.1, the mass percentage of lithium difluorophosphate is too low, making it difficult to form dense and stable CEI and SEI films at the positive and negative electrode interfaces. This leads to excessive side reactions between the positive and negative electrode plates and the electrolyte, resulting in excessive initial impedance and impedance growth during cycling, thus affecting the kinetic performance and high-temperature float charge performance of the secondary battery. When the value of W1 is too large, for example, greater than 1, the mass percentage of lithium difluorophosphate is too high, resulting in poor solubility in the electrolyte and difficulty in complete dissolution. This causes turbidity in the electrolyte, making it unsuitable for use in secondary batteries. When the value of W2 is too small, for example, less than 10, the mass percentage of propyl propionate is too low, resulting in low solubility of the lithium salt in the electrolyte. This affects the conductivity of the electrolyte, hindering lithium ion transport and leading to excessive initial impedance and impedance growth during cycling, thus affecting the kinetic performance and high-temperature float charge performance of the secondary battery. When the value of W2 is too large, such as greater than 60, the mass percentage of propyl propionate is too high, which is not conducive to the dissolution of lithium ions and affects the transport of lithium ions in the electrolyte, thereby affecting the kinetic performance and high-temperature float charging performance of the secondary battery.

[0033] When the value of W1×H1 is too small, for example, less than 4, it is not conducive to the synergistic effect between the negative electrode and the electrolyte. A dense and stable SEI film is difficult to form at the negative electrode interface, and there are too many side reactions between the negative electrode and the electrolyte. This leads to excessive initial impedance and impedance growth during cycling of the secondary battery, thus affecting the kinetic performance and high-temperature float charge performance of the secondary battery. When the value of W1×H1 is too large, for example, greater than 35, it is not conducive to the synergistic effect between the negative electrode and the electrolyte. The kinetic performance of the first negative electrode material layer is too poor, and the wettability of the electrolyte to the second negative electrode material layer is also too poor. The transport speed of lithium ions on the negative electrode is too slow, and the solubility of lithium difluorophosphate in the electrolyte is too poor. This leads to excessive initial impedance and impedance growth during cycling of the secondary battery, thus affecting the kinetic performance and high-temperature float charge performance of the secondary battery.

[0034] Therefore, the negative electrode sheet includes a first negative electrode material layer and a second negative electrode material layer, and by controlling the values ​​of H1 and H2 within the above range, the Dv50 of the first negative electrode active material and the second negative electrode active material satisfies the above relationship, and the electrolyte includes lithium difluorophosphate and propyl propionate, and by controlling the values ​​of W1, W2, and W1×H1 within the above range, the initial impedance of the secondary battery and the impedance growth during the cycling process can be reduced, thereby improving the dynamic performance and high-temperature float charging performance of the secondary battery.

[0035] For ease of understanding, in this application, the length direction of the negative electrode sheet is defined as Y, and its thickness direction as Z. The negative electrode sheet typically has a long side and a short side, and the aforementioned length direction refers to the extension direction of the long side of the negative electrode sheet. It should be understood that the above definition of direction is for the convenience of describing the purpose of this application. As shown in Figure 1, the negative electrode sheet 10 includes a negative current collector 11 and a second negative electrode material layer 13 and a first negative electrode material layer 12 sequentially disposed on one surface of the negative current collector 11. H1 is the thickness of the first negative electrode material layer, and H2 is the thickness of the second negative electrode material layer. In this application, the negative electrode material layers can be disposed on one surface of the negative current collector along its thickness direction, or on two surfaces of the negative current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative current collector, or it can be a partial surface area of ​​the negative current collector; this application has no particular limitation, as long as the purpose of this application is achieved.

[0036] In some embodiments of this application, 2 ≤ H2 / H1 ≤ 4.5. For example, the value of H2 / H1 can be 2, 2.3, 2.5, 2.8, 3, 3.2, 3.6, 3.8, 4, 4.2, 4.5, or a range of any two of these values. By adjusting the value of H2 / H1 within the above range, the electrolyte can have good wettability to the first and second negative electrode material layers, improving the wettability of the electrolyte to the negative electrode sheet, which is beneficial for the transport of lithium ions in the negative electrode sheet. At the same time, it can also reduce the side reactions between the negative electrode sheet and the electrolyte during cycling, thereby reducing the initial impedance of the secondary battery and the impedance growth during cycling, and thus improving the kinetic performance and high-temperature float charge performance of the secondary battery.

[0037] In some embodiments of this application, 0.005 ≤ W1 / H1 ≤ 0.05. For example, the value of W1 / H1 can be 0.005, 0.01, 0.015, 0.02, 0.024, 0.03, 0.036, 0.04, 0.045, 0.05, or a range consisting of any two of these values. By adjusting the value of W1 / H1 within the above range, it is beneficial to leverage the synergistic effect between the negative electrode and the electrolyte, forming a denser and more stable SEI film at the negative electrode interface, further reducing side reactions between the negative electrode interface and the electrolyte, and further improving the wettability of the electrolyte on the negative electrode, which is beneficial for the transport of lithium ions in the negative electrode, further improving the kinetic performance of the first negative electrode material layer, thereby further reducing the initial impedance of the secondary battery and the impedance growth during cycling, and thus further improving the kinetic performance and high-temperature float charge performance of the secondary battery.

[0038] In some embodiments of this application, 250 ≤ W2 × H1 ≤ 2400. For example, the value of W2 × H1 can be 250, 500, 800, 1000, 1200, 1500, 1800, 2000, 2400, or a range of any two of these values. By adjusting the value of W2 × H1 within the above range, it is beneficial to leverage the synergistic effect between the negative electrode and the electrolyte, improve the solubility of lithium salt in the electrolyte and the conductivity of the electrolyte, and also improve the wettability of the electrolyte on the negative electrode. This facilitates the transport of lithium ions in the electrolyte and the negative electrode, improves the kinetic performance of the first negative electrode material layer, and thus helps to further reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby further improving the kinetic performance and high-temperature float charge performance of the secondary battery.

[0039] In some embodiments of this application, the electrolyte comprises a polynitrile compound, and the mass percentage of the polynitrile compound is W3% based on the mass of the electrolyte, where 0.1 ≤ W3 ≤ 5. The polynitrile compound includes at least one selected from succinic anionyl nitrile, glutaronitrile, adiponitrile, heptanonitrile, octanoic anionyl nitrile, methylglutaronitrile, 1,3,5-pentanetricarbonyl nitrile, 1,2,3-propanetricarbonyl nitrile, 1,3,6-hexanetrionitrile, or 1,2,3-tris(2-cyanoethoxy)propane. For example, the value of W3 can be 0.1, 0.6, 1, 1.5, 2, 2.4, 3, 3.6, 4, 4.5, 5, or a range consisting of any two of these values. Based on an electrolyte comprising lithium difluorophosphate and propyl propionate, the aforementioned polynitrile compound is further introduced, and its mass percentage W is controlled within the aforementioned range (W 3%). The strong coordination ability of the polynitrile compound allows it to complex with high-valence metal ions (e.g., cobalt ions) at the positive electrode interface, reducing the phase transition reaction of the positive electrode active material caused by metal ion dissolution and forming a more stable CEI film. At the negative electrode interface, it can undergo a reduction reaction to form a stable SEI film, simultaneously improving the stability of both the positive and negative electrode plates. Furthermore, the high carbon-nitrogen triple bond energy in the polynitrile compound makes it less susceptible to oxidation, reducing oxidative decomposition and gas production in the electrolyte, and improving the electrochemical and thermal stability of the electrolyte. This reduces the initial impedance and impedance growth during cycling of the secondary battery, improving its kinetic performance and high-temperature float charge performance. In this application, high temperature refers to a temperature greater than or equal to 40°C.

[0040] In some embodiments of this application, 50 ≤ W3 × H1 ≤ 200. For example, the value of W3 × H1 can be 50, 80, 100, 120, 150, 180, 200, or a range of any two of these values. By adjusting the value of W3 × H1 within the above range, it is beneficial to leverage the synergistic effect between the negative electrode and the electrolyte, enabling a reduction reaction at the negative electrode interface to form a stable SEI film, thereby improving the stability of the negative electrode. It also improves the wettability of the electrolyte on the negative electrode, facilitating lithium-ion transport within the negative electrode, enhancing the kinetic performance of the first negative electrode material layer, reducing the oxidative decomposition and gas production of the electrolyte, and improving the electrochemical and thermal stability of the electrolyte. This reduces the initial impedance of the secondary battery and the impedance increase during cycling, improving the kinetic performance of the secondary battery, and further enhancing its high-temperature float charge performance.

[0041] In some embodiments of this application, the mass percentage of the propionate ester compound is W4% based on the mass of the electrolyte, where 10 ≤ W4 ≤ 65 and 0.154 ≤ W2 / W4 ≤ 1. For example, the value of W4 can be 10, 15, 20, 24, 30, 35, 40, 46, 50, 54, 60, 65, or a range of any two of these values, and the value of W2 / W4 can be 0.154, 0.3, 0.35, 0.4, 0.5, 0.62, 0.7, 0.85, 0.9, 1, or a range of any two of these values. Based on an electrolyte comprising lithium difluorophosphate and propyl propionate, adjusting the mass percentage content W4% and the W2 / W4 value of the propionate compound within the aforementioned range can improve the solubility of lithium salts in the electrolyte and the conductivity of the electrolyte, which is beneficial for the transport of lithium ions in the electrolyte. This helps to reduce the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance and high-temperature float charging performance of the secondary battery.

[0042] In some embodiments of this application, the propionate compound further includes at least one of methyl propionate, ethyl propionate, butyl acrylate, or butyl propionate. In addition to the electrolyte comprising lithium difluorophosphate and propyl propionate, the inclusion of the aforementioned propionate compound can improve the solubility of lithium salts in the electrolyte and the conductivity of the electrolyte, which is beneficial for lithium ion transport in the electrolyte. This further reduces the initial impedance of the secondary battery and the impedance increase during cycling, thereby further improving the kinetic performance and high-temperature float charge performance of the secondary battery.

[0043] In this application, the electrolyte also includes other organic solvents. This application does not particularly limit the types of other organic solvents, as long as they achieve the purpose of this application. For example, other 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. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). The aforementioned fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. Based on the mass of the electrolyte, the mass percentage of other organic solvents is from 14% to 81%. For example, the mass percentage of other organic solvents may be 14%, 19%, 24%, 30%, 40%, 50%, 62%, 70%, 78%, 81%, or a range consisting of any two of these values.

[0044] In this application, the electrolyte also includes lithium salts. There are no particular limitations on the lithium salts used, as long as they achieve the purpose of this application. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. Based on the mass of the electrolyte, the mass percentage of lithium salt is 8% to 15%.

[0045] In some embodiments, the electrolyte may include lithium salts, lithium difluorophosphate, propyl propionate, and other organic solvents. The mass percentages of lithium salts, lithium difluorophosphate, and propyl propionate are as described above, and the mass percentages of other organic solvents range from 24% to 81%. Secondary batteries comprising the above-described electrolytes exhibit good kinetic performance and high-temperature float charging performance.

[0046] In some embodiments, the electrolyte may include lithium salts, lithium difluorophosphate, propyl propionate, polynitrile compounds, and other organic solvents. The mass percentages of lithium salts, lithium difluorophosphate, propyl propionate, and polynitrile compounds are as described above, while the mass percentages of other organic solvents range from 19% to 81%. Secondary batteries comprising the above-described electrolytes exhibit good kinetic performance and high-temperature float-charge performance.

[0047] In some embodiments, the electrolyte may include lithium salts, lithium difluorophosphate, propionate compounds, and other organic solvents. The propionate compounds include propyl propionate and at least one of methyl propionate, ethyl propionate, butyl acrylate, or butyl propionate. The mass percentages of lithium salts, lithium difluorophosphate, and propionate compounds are as described above, and the mass percentages of other organic solvents are 19% to 81%. Secondary batteries comprising the above electrolytes exhibit good kinetic performance and high-temperature float-charge performance.

[0048] In some embodiments, the electrolyte may include lithium salts, lithium difluorophosphate, propionate compounds, polynitrile compounds, and other organic solvents. The propionate compounds include propyl propionate and at least one of methyl propionate, ethyl propionate, butyl acrylate, or butyl propionate. The mass percentages of lithium salts, lithium difluorophosphate, propionate compounds, and polynitrile compounds are as described above, and the mass percentages of other organic solvents are 14% to 81%. Secondary batteries comprising the above electrolytes exhibit better kinetic performance and high-temperature float-charge performance.

[0049] In some embodiments of this application, the Dv50 of the first negative electrode active material is from 12 μm to 14.9 μm, and the Dv90 of the first negative electrode active material is from 20.5 μm to 25.9 μm. For example, the Dv50 of the first negative electrode active material can be 12 μm, 12.3 μm, 12.7 μm, 13 μm, 13.4 μm, 13.7 μm, 14 μm, 14.5 μm, 14.9 μm, or a range of any two of these values, and the Dv90 of the first negative electrode active material can be 20.5 μm, 21 μm, 21.6 μm, 22 μm, 22.4 μm, 23 μm, 23.5 μm, 24 μm, 24.6 μm, 25 μm, 25.9 μm, or a range of any two of these values. By adjusting the Dv50 and Dv90 of the first negative electrode active material within the aforementioned range, the consumption of electrolyte by the negative electrode sheet can be reduced, the wettability of the electrolyte on the negative electrode sheet can be improved, which is beneficial for the transport of lithium ions in the negative electrode sheet. It can also reduce the side reactions between the negative electrode sheet and the electrolyte during cycling, and improve the stability of the negative electrode sheet. This is beneficial for reducing the initial impedance of the secondary battery and the impedance growth during cycling, thereby improving the kinetic performance of the secondary battery and the high-temperature float charge performance of the secondary battery.

[0050] In some embodiments of this application, the Dv50 of the second negative electrode active material is from 10 μm to 11.5 μm, and the Dv90 of the second negative electrode active material is from 19.4 μm to 24.8 μm. For example, the Dv50 of the second negative electrode active material can be 10 μm, 10.3 μm, 10.5 μm, 10.7 μm, 11 μm, 11.2 μm, 11.5 μm, or a range consisting of any two of these values, and the Dv90 of the second negative electrode active material can be 19.4 μm, 20 μm, 20.5 μm, 21 μm, 21.6 μm, 22 μm, 22.4 μm, 23 μm, 23.5 μm, 24 μm, 24.8 μm, or a range consisting of any two of these values. By adjusting the Dv50 and Dv90 of the second negative electrode active material within the above range, the wettability of the electrolyte to the negative electrode sheet can be improved, and the side reactions between the negative electrode sheet and the electrolyte during cycling can be reduced. This is beneficial to reducing the initial impedance of the secondary battery and the impedance growth during cycling, which in turn is beneficial to improving the dynamic performance of the secondary battery and also beneficial to improving the high-temperature float charging performance of the secondary battery.

[0051] In this application, Dv50 refers to the particle size that, in the volumetric particle size distribution of the material, reaches 50% of the cumulative volume from the smallest particle size. Dv90 refers to the particle size that, in the volumetric particle size distribution of the material, reaches 90% of the cumulative volume from the smallest particle size. In some embodiments, the Dv90 of the first negative electrode active material is smaller than the thickness H1 of the first negative electrode material layer.

[0052] Typically, first and second anode active materials with different Dv50 and Dv90 values ​​can be obtained through mechanical crushing (e.g., ball milling). For example, the Dv50 and Dv90 values ​​of both the first and second anode active materials can be controlled by adjusting the ball milling time. When other conditions remain constant, extending the ball milling time decreases the Dv50 of the first anode active material, while shortening it increases it. Similarly, extending the ball milling time decreases the Dv90 of the first anode active material, while shortening it increases it. Likewise, extending the ball milling time decreases the Dv50 of the second anode active material, while shortening it increases it.

[0053] In some embodiments of this application, the first negative electrode active material and the second negative electrode active material each independently include natural graphite, artificial graphite, soft carbon, hard carbon, mesophase carbon microspheres, silicon carbide (SiC), and silicon oxide (SiO). x (0 < x ≤ 2) or at least one of lithium titanate. Based on the following: the negative electrode includes a first negative electrode material layer and a second negative electrode material layer, and the values ​​of H1 and H2 are controlled, the relationship between the Dv50 of the first and second negative electrode active materials is controlled within the scope of this application; and the electrolyte includes lithium difluorophosphate and propyl propionate, and the values ​​of W1, W2, and W1 × H1 are controlled within the scope of this application, the selection of the above-mentioned first and second negative electrode active materials can enable the secondary battery to have good kinetic performance and high-temperature float charge performance. In this application, the types of the first and second negative electrode active materials can be the same or different.

[0054] In this application, the first negative electrode material layer includes a first negative electrode active material, and may also include a first negative electrode binder and a first negative electrode conductive agent; alternatively, the first negative electrode material layer may also include a first negative electrode binder, a first negative electrode conductive agent, or a first thickener. Based on the mass of the first negative electrode material layer, the mass percentage content of the first negative electrode active material may be 86% to 99%, the mass percentage content of the first negative electrode binder may be 0.5% to 5%, the mass percentage content of the first negative electrode conductive agent may be 0.5% to 5%, and the mass percentage content of the first thickener may be 0% to 4%.

[0055] In this application, the second negative electrode material layer includes a second negative electrode active material, and may also include a second negative electrode binder and a second negative electrode conductive agent. Alternatively, the second negative electrode material layer may also include a second negative electrode binder, a second negative electrode conductive agent, or a second thickener. Based on the mass of the second negative electrode material layer, the mass percentage content of the second negative electrode active material may be 86% to 99%, the mass percentage content of the second negative electrode binder may be 0.5% to 5%, the mass percentage content of the second negative electrode conductive agent may be 0.5% to 5%, and the mass percentage content of the second thickener may be 0% to 4%.

[0056] This application does not impose any particular limitation on the types of the first negative electrode binder, the second negative electrode binder, the first negative electrode conductive agent, and the second negative electrode conductive agent, as long as they can achieve the purpose of this application. For example, the first negative electrode binder and the second negative electrode binder may each independently include, but are not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, polyamide-imide, styrene-butadiene rubber, or polyvinylidene fluoride. For example, the first negative electrode conductive agent and the second negative electrode conductive agent may each independently include, but are not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. Conductive carbon black may include, but is not limited to, at least one of acetylene black or Ketjen black. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. The aforementioned metallic materials may include, but are not limited to, metal powders and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not impose any particular limitation on the types of the first and second thickeners, as long as they achieve the purpose of this application. For example, the first and second thickeners may each independently include, but are not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose.

[0057] This application does not impose any particular limitation on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. For example, the composite current collector may be a lithium copper composite current collector, a carbon copper composite current collector, a nickel copper composite current collector, a titanium copper composite current collector, etc. This application does not impose any particular limitation on the thickness of the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the thickness of the negative electrode current collector may be from 4 μm to 15 μm.

[0058] Optionally, the negative electrode material layer may further include a conductive layer located between the negative electrode current collector and the second negative electrode material layer. This application does not impose any particular limitation on the composition of the conductive layer, which can be a conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. This application does not impose any particular limitation on the conductive agent and binder in the conductive layer, which may be at least one of the aforementioned first negative electrode conductive agent and first negative electrode binder.

[0059] This application does not impose any particular limitation on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, the preparation method of the negative electrode sheet may include, but is not limited to, the following steps: (1) preparing a first negative electrode slurry and a second negative electrode slurry; (2) coating the second negative electrode slurry onto one surface of the negative electrode current collector, drying it, and forming a second negative electrode material layer on one surface of the negative electrode current collector; then coating the first negative electrode slurry onto the surface of the second negative electrode material layer, drying it, and forming the second negative electrode material layer and the first negative electrode material layer sequentially on one surface of the negative electrode current collector; (3) repeating the above steps on the other surface of the negative electrode current collector, forming the first negative electrode material layer and the second negative electrode material layer on both surfaces of the negative electrode current collector; (4) obtaining the negative electrode sheet by cold pressing, cutting, and welding the negative electrode tabs.

[0060] Typically, the thickness H1 of the first negative electrode material layer can be controlled by adjusting the coating surface density of the first negative electrode material layer. For example, when other conditions remain unchanged, increasing the coating surface density of the first negative electrode material layer increases H1; decreasing the coating surface density of the first negative electrode material layer decreases H1.

[0061] Typically, the thickness H2 of the second negative electrode material layer can be controlled by adjusting the coating surface density of the second negative electrode material layer. For example, when other conditions remain unchanged, increasing the coating surface density of the second negative electrode material layer increases H2; decreasing the coating surface density of the second negative electrode material layer decreases H2.

[0062] In this application, the secondary battery further includes a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The phrase "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that the term "surface" here can refer to the entire surface area of ​​the positive current collector, or only a portion thereof; this application does not impose any particular limitation, as long as the purpose of this application is achieved.

[0063] This application does not impose any particular limitation on the positive electrode current collector, as long as it achieves the purpose of this application. For example, the positive electrode current collector may contain aluminum foil, aluminum alloy foil, or composite current collectors (such as aluminum-carbon composite current collectors). This application also does not impose any particular limitation on the thickness of the positive electrode current collector, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector may be from 5 μm to 20 μm.

[0064] The positive electrode material layer includes a positive electrode active material. This application does not impose any particular limitation on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include, but is not limited to, lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate.

[0065] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they achieve the purpose of this application. For example, they may be at least one of the aforementioned first negative electrode conductive agent and first negative electrode binder. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, positive electrode conductive agent, and positive electrode binder in the positive electrode material layer. Those skilled in the art can select them according to actual needs, as long as the purpose of this application is achieved.

[0066] This application does not impose any particular limitation on the thickness of the cathode material layer, as long as it achieves the purpose of this application. For example, the thickness of the cathode material layer can be from 30 μm to 120 μm. This application also does not impose any particular limitation on the compaction density of the cathode material layer, as long as it achieves the purpose of this application. For example, the compaction density of the cathode material layer can be 3 g / cm³. 3 Up to 5g / cm 3 .

[0067] Optionally, the positive electrode may further include a conductive layer located between the positive current collector and the positive electrode material layer. The composition of the conductive layer is not particularly limited and can be any 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; for example, it can be at least one of the aforementioned first negative electrode conductive agent and first negative electrode binder.

[0068] In this application, the secondary battery also includes a separator. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the material of the separator may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of separator may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0069] In some embodiments, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer 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.

[0070] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic substances.

[0071] In some embodiments, the inorganic layer comprises inorganic particles and a binder. This application does not particularly limit the inorganic particles; for example, the inorganic particles may include at least one selected from 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 particularly limit the binder in the inorganic layer; for example, the binder in the inorganic layer may be at least one of the aforementioned first negative electrode binders. In some embodiments, the polymer layer comprises a polymer, the polymer material of which includes at least one selected from polyamide, polyacrylonitrile, acrylate polymers, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0072] In this application, there is no particular limitation on the thickness of the separator, as long as it can achieve the purpose of this application. For example, the thickness of the separator can be from 3 μm to 30 μm.

[0073] In this application, the secondary battery also includes a casing for housing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the field of secondary batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0074] The fabrication process of the secondary battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the fabrication process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain the secondary battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent pressure rise and overcharging / discharging inside the secondary battery.

[0075] The second aspect of this application provides an electronic device that includes the secondary battery provided in the first aspect of this application. The secondary battery provided in this application has good kinetic performance and high-temperature float charging performance, thereby giving the electronic device a long service life and good performance.

[0076] This application does not specifically limit the type of electronic device, which can be any electronic device known in the prior art. For example, electronic devices may include, but are not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, 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, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0077] Example

[0078] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0079] Test methods and equipment:

[0080] Thickness test

[0081] The lithium-ion batteries of each embodiment and comparative example were discharged to 3V at 0.2C, and the negative electrode sheets were obtained by disassembly. The negative electrode sheets were cleaned with dimethyl carbonate (DMC), dried at 60°C, and the cross-section of the negative electrode sheet along the thickness direction was polished with argon ions. Then, the cross-section of the negative electrode sheet was observed using a scanning electron microscope (SEM), and the thickness H1 of the first negative electrode material layer, the thickness H2 of the second negative electrode material layer, and the H2 / H1 ratio were measured.

[0082] Testing of Dv50 and Dv90 of the first and second negative electrode active materials

[0083] According to the national standard "Particle Size Distribution by Laser Diffraction" (GB / T19077-2016), the Dv50 and Dv90 of the first and second negative electrode active materials were tested using a laser particle size analyzer (model MS3000).

[0084] Electrolyte composition testing:

[0085] The lithium-ion battery was discharged to 3V at a constant current of 0.2C and then disassembled. The electrolyte was collected by centrifugation, squeezing or other methods. Then, gas chromatography-mass spectrometry (GC-MS, Agilent 8890 instrument) and ion chromatography (IC, AQUION ion chromatograph) were used to test the individual components in the electrolyte and determine their content.

[0086] Initial impedance test

[0087] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 4 hours. Then, it was charged at a constant current of 0.7C to the rated voltage (4.48V in this embodiment), charged at a constant voltage of 4.48V to a current of 0.05C, left to stand for 2 hours, and then discharged at a constant current of 0.2C to a voltage of 3.0V. The capacity of the lithium-ion battery measured at this time was recorded as the reference capacity C1, that is, the capacity at 100% SOC (State of Charge). The lithium-ion battery was charged at a constant current of 0.7C to a voltage of 4.48V, then charged at a constant voltage of 4.48V to a current of 0.05C. After resting for 10 minutes, it was sequentially discharged at a constant current of 0.1C to 0.3C1 (70% SOC), 0.8C1 (20% SOC), and 0.9C1 (10% SOC). The voltage and current of the lithium-ion battery were measured at each of these three different SOCs. The voltage-to-current ratio was calculated to obtain the impedance at 70% SOC (denoted as R1), 20% SOC (denoted as R2), and 10% SOC (denoted as R3). The initial impedance of the lithium-ion battery was evaluated by the values ​​of R1, R2, and R3. The smaller the values ​​of R1, R2, and R3, the smaller the initial impedance; the larger the values ​​of R1, R2, and R3, the larger the initial impedance.

[0088] Impedance growth test

[0089] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 4 hours. Then, it was sequentially discharged at a constant current of 0.02C to 0.3C1 (70% SOC), 0.8C1 (20% SOC), and 0.9C1 (10% SOC). The voltage of the lithium-ion battery at 70% SOC is denoted as U'. 70％ The current is denoted as I' 70％ The voltage of a lithium-ion battery at 20% SOC is denoted as U'. 20％ The current is denoted as I' 20％ The voltage of a lithium-ion battery at 10% SOC is denoted as U'. 10％ The current is denoted as I' 10％ The lithium-ion battery was charged at a constant current of 0.7C to a voltage of 4.48V, then charged at a constant voltage of 4.48V to a current of 0.05C, allowed to rest for 2 hours, and then discharged at a constant current of 0.5C to a voltage of 3.0V. This constitutes one charge-discharge cycle. The lithium-ion battery was then subjected to charge-discharge cycles as described above, with a discharge capacity recovery test performed every 50 charge-discharge cycles. The lithium-ion battery was charged at a constant current of 0.7C to a voltage of 4.48V, then charged at a constant voltage of 4.48V to a current of 0.05C, allowed to rest for 2 hours, and then discharged sequentially at a constant current of 0.2C to 0.3C1 (70% SOC), 0.8C1 (20% SOC), and 0.9C1 (10% SOC). The voltage of the lithium-ion battery at 70% SOC is denoted as U. 70％ The current is denoted as I. 70％The voltage of a lithium-ion battery at 20% SOC is denoted as U. 20％ The current is denoted as I. 20％ The voltage of a lithium-ion battery at 10% SOC is denoted as U. 10％ The current is denoted as I. 10％ R is calculated using the following formula. 70％ R 20％ R 10％ :R 70％ =(U 70％ -U' 70％ ) / (I 70％ -I' 70％ ); R 20％ =(U 20％ -U' 20％ ) / (I 20％ -I' 20％ ); R 10％ =(U 10％ -U' 10％ ) / (I 10％ -I' 10％ ). Calculate R 70％ R 20％ R 10％ The average value was used as the impedance value of the lithium-ion battery at the corresponding number of cycles. The charging and discharging method described above was followed until 1000 cycles were completed. Three lithium-ion batteries were tested for each embodiment and comparative example, and the average impedance of the three lithium-ion batteries was calculated as the impedance of the lithium-ion battery. The impedance of the lithium-ion battery at the 300th cycle was denoted as R. 300 The impedance at the 1000th cycle is denoted as R. 1000 The impedance growth of a lithium-ion battery is evaluated using the following formula, K, calculated from the resistance of the battery: K = (R... 1000 -R 300 () / (1000-300). The smaller the value of K, the smaller the impedance increase; the larger the value of K, the greater the impedance increase.

[0090] High-temperature float charging performance testing

[0091] The lithium-ion battery was placed in a 25°C constant temperature chamber and left to stand for 4 hours. Then, it was charged at a constant current of 0.7C to 4.48V, and then charged at a constant voltage of 4.48V to a current of 0.05C. After standing for 2 hours, it was discharged at a constant current of 0.2C to a voltage of 3.0V. The lithium-ion battery was then charged again at a constant current of 0.7C to 4.48V, and then charged at a constant voltage of 4.48V to a current of 0.05C. The thickness of the lithium-ion battery at this point was measured and recorded as the initial thickness T0. The lithium-ion battery was then placed in a 55°C constant temperature chamber and left to stand for 1.5 hours. It was then charged at a constant current of 0.4C to 4.48V, and then charged at a constant voltage of 4.48V to a current of 60mA. The battery was continuously charged at a constant current (100mA) and constant voltage (4.48V), with the thickness of the lithium-ion battery measured every 96 hours and recorded as T. i Let T be the thickness change rate of the lithium-ion battery. i / T0, when T i When the / T0 value reaches 108.5%, the charging time of the lithium-ion battery at constant current (100mA) and constant voltage (4.48V) is recorded. Three lithium-ion batteries are tested for each embodiment and comparative example, and the average charging time of the three batteries is calculated and denoted as D. The high-temperature float charging performance is evaluated by the value of D. The larger the D value of the lithium-ion battery, the better the high-temperature float charging performance; the smaller the D value, the worse the high-temperature float charging performance.

[0092] Example 1-1

[0093] <Preparation of Negative Electrode Sheets>

[0094] The first negative electrode active material is artificial graphite, and the first negative electrode binder is styrene-butadiene rubber (SBR, weight average molecular weight Mw = 5 × 10⁻⁶). 6 The first thickener, sodium carboxymethyl cellulose (CMC-Na, Mw = 7 × 10⁻⁶), is... 5 The first negative electrode conductive agent, conductive carbon black (Super P), was mixed at a mass ratio of 96:1.5:1.5:1, and then deionized water was added as a solvent to prepare a slurry with a solid content of 50wt%. After vacuum stirring, the first negative electrode slurry was obtained.

[0095] The second negative electrode active material is artificial graphite, and the second negative electrode binder is styrene-butadiene rubber (SBR, Mw = 5 × 10⁻⁶). 6 The second thickener is sodium carboxymethyl cellulose (CMC-Na, Mw = 7 × 10⁻⁶). 5 The second negative electrode conductive agent, conductive carbon black (Super P), was mixed at a mass ratio of 96:1.5:1.5:1, and then deionized water was added as a solvent to prepare a slurry with a solid content of 50wt%. After vacuum stirring, the second negative electrode slurry was obtained.

[0096] The second negative electrode slurry was uniformly coated onto one surface of a 10 μm thick copper foil current collector, with a coating density of 5.81 mg / cm³. 2 The material is dried at 85℃ to obtain a negative electrode sheet with a second negative electrode material layer coated on one side. Then, the first negative electrode slurry is uniformly coated on the surface of the second negative electrode material layer away from the negative electrode current collector, with a coating surface density of 2.33 mg / cm³. 2 The copper foil was dried at 85°C to obtain a negative electrode sheet with a single-sided coating of the second and first negative electrode material layers. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of the second and first negative electrode material layers. After cold pressing, cutting, and welding of negative electrode tabs and nickel tabs, a negative electrode sheet with dimensions of 76mm × 867mm was obtained for later use. The thicknesses H1 of the first and second negative electrode material layers are shown in Table 1, and the Dv50 and Dv90 of the first and second negative electrode active materials are shown in Table 3.

[0097] <Preparation of Electrolyte>

[0098] In an argon-atmospheric glove box with a water content of less than 10 ppm, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed uniformly at a mass ratio of 1:1:1 to form a base solvent. Then, lithium hexafluorophosphate (LiPF6), lithium difluorophosphate, and propyl propionate were added to the base solvent and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass percentage of lithium salt LiPF6 was 12.5%, the mass percentage of lithium difluorophosphate (W1%) and the mass percentage of propyl propionate (W2%) are shown in Table 1, with the remainder being the base solvent.

[0099] <Preparation of the positive electrode>

[0100] The positive electrode active material is lithium cobalt oxide, the positive electrode conductive agent is conductive carbon black (Super P), the positive electrode conductive agent is carbon nanotubes (CNT), and the positive electrode binder is polyvinylidene fluoride (PVDF, Mw = 7 × 10⁻⁶). 6The materials were mixed in a mass ratio of 97:0.8:0.7:1.5, with N-methylpyrrolidone (NMP) added as a solvent to prepare a slurry with a solid content of 70 wt%. After vacuum stirring, a positive electrode slurry was obtained. This slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil current collector and dried at 120°C to obtain a single-sided coated positive electrode sheet. The above steps were repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode sheet. After drying at 120°C, the sheet was cold-pressed, then cut and welded with positive electrode tabs to obtain a 74 mm × 851 mm positive electrode sheet for use. The thickness of the positive electrode material layer was 92 μm, and the compaction density was 4.15 g / cm³. 3 .

[0101] <Preparation of the separating membrane>

[0102] A porous polyethylene film (supplied by Celgard) with a thickness of 7 μm was used as the substrate layer. Inorganic alumina particles and an inorganic layer binder, polyvinylidene fluoride (Mw = 7 × 10⁻⁶), were then bonded together. 6 The mixture was prepared at a mass ratio of 4:1, and N-methylpyrrolidone (NMP) was added. The mixture was stirred evenly under vacuum to obtain an inorganic slurry with a solid content of 30 wt%. This inorganic slurry was then uniformly coated onto one surface of a substrate layer to form an inorganic layer with a thickness of 2 μm. Finally, the polymer polyvinylidene fluoride (Mw = 7 × 10⁻⁶) was applied... 6 The polymer layer slurry was dissolved in deionized water to obtain a solid content of 25 wt%. This slurry was then uniformly coated onto the surfaces of an inorganic layer on one side and a substrate layer on the other, with a coating density of 2.5 mg / cm³. 2 A polymer layer is formed, resulting in a separator membrane with an inorganic layer and a polymer layer on one side and only a polymer layer on the other side.

[0103] <Preparation of Lithium-ion Batteries>

[0104] The positive electrode, separator, and negative electrode prepared above are stacked and wound sequentially to obtain a wound electrode assembly. The side of the separator containing the inorganic layer and the polymer layer faces the positive electrode, and the side containing only the polymer layer faces the negative electrode. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. After vacuum sealing, standing, formation (0.3C constant current charging to 3.5V, then 1C constant current charging to 3.9V), capacity testing, degassing, and edge trimming, a lithium-ion battery is obtained.

[0105] Examples 1-2 to Examples 1-16

[0106] Except for adjusting the relevant preparation parameters according to Table 1 in <Preparation of Negative Electrode> and <Preparation of Electrolyte>, the rest is the same as in Example 1-1. Specifically, H1 is controlled by adjusting the coating surface density of the first negative electrode material layer, and H2 is controlled by adjusting the coating surface density of the second negative electrode material layer. When the mass percentage of lithium difluorophosphate W1% or the mass percentage of propyl propionate W2% changes, the mass percentage of the base solvent changes accordingly, while the mass ratio of each component in the base solvent and the mass percentage of lithium salt remain unchanged.

[0107] Examples 2-1 to 2-7

[0108] Except for adjusting the coating density of the first negative electrode material layer in the <Preparation of the Negative Electrode Sheet> to achieve the H1 value as shown in Table 2, and adding the polynitrile compound 1,2,3-tris(2-cyanoethoxy)propane in the <Preparation of the Electrolyte> as shown in Table 2, the rest is the same as in Examples 1-1. Specifically, when the mass percentage of the polynitrile compound W3% changes, the mass percentage of the base solvent changes accordingly, while the mass ratio of each component in the base solvent, and the mass percentages of lithium salt, lithium difluorophosphate, and propyl propionate remain unchanged.

[0109] Examples 2-8 to 2-10

[0110] Except for the addition of methyl propionate, a propionate ester compound, as shown in Table 2 in the <Preparation of Electrolyte>, the rest is the same as in Examples 1-1. When the mass percentage of propyl propionate (W2%) and the mass percentage of the propionate ester compound (W4%) change, the mass percentage of the base solvent changes accordingly, while the mass ratio of each component in the base solvent and the mass percentages of lithium salt and lithium difluorophosphate remain unchanged.

[0111] Example 2-11

[0112] Except for the addition of polynitrile compound 1,2,3-tris(2-cyanoethoxy)propane and propionate compound methyl propionate as shown in Table 2 in the <Preparation of Electrolyte>, the mass percentage of the base solvent is changed accordingly, and the mass ratio of each component in the base solvent, the mass percentage of lithium salt and lithium difluorophosphate remain unchanged, the rest is the same as in Example 1-1.

[0113] Examples 3-1 to 3-8

[0114] Except for adjusting the relevant preparation parameters according to Table 3 in the <Preparation of Negative Electrode Sheet> section, the rest is the same as in Examples 1-1. Specifically, the Dv50 and Dv90 of the first negative electrode active material and the second negative electrode active material are adjusted by controlling the ball milling time.

[0115] Comparative Example 1

[0116] Except for the preparation method of the negative electrode sheet, the rest is the same as in Example 1-1.

[0117] <Preparation of Negative Electrode Sheets>

[0118] Artificial graphite as the negative electrode active material and styrene-butadiene rubber (SBR, Mw = 5 × 10⁻⁶) as the negative electrode binder are used. 6 Thickener sodium carboxymethyl cellulose (CMC-Na, Mw = 7 × 10⁻⁶) 5 Conductive carbon black (Super P), the negative electrode conductive agent, was mixed at a mass ratio of 96:1.5:1.5:1, and then deionized water was added as a solvent to prepare a slurry with a solid content of 50wt%. After vacuum stirring, the slurry was obtained. The negative electrode slurry was then uniformly coated onto one surface of a 10μm thick copper foil negative electrode current collector, with a coating surface density of 8.14 mg / cm². 2 The copper foil is dried at 85℃ to obtain a negative electrode sheet with a single-sided coating of negative electrode material. Then, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After cold pressing, cutting, and welding of negative electrode tabs and nickel tabs, a negative electrode sheet with dimensions of 76mm × 867mm is obtained for use. The thickness of the negative electrode material layer is 105μm, and the Dv50 and Dv90 of the negative electrode active material are 11μm and 22μm respectively.

[0119] Comparative Examples 2 to 5

[0120] Except for adjusting the relevant preparation parameters according to Table 1 in the <Preparation of Negative Electrode Sheet> section, the rest is the same as in Example 1-1. Specifically, H1 is controlled by adjusting the coating surface density of the first negative electrode material layer, and H2 is controlled by adjusting the coating surface density of the second negative electrode material layer.

[0121] Comparative Example 6

[0122] Except that lithium difluorophosphate is not added in the <Preparation of Electrolyte>, the mass percentage of the base solvent is changed accordingly, and the mass ratio of each component in the base solvent, the mass percentage of lithium salt and propyl propionate remain unchanged, the rest is the same as in Example 1-1.

[0123] Comparative Examples 7 to 8

[0124] Except for adjusting the relevant preparation parameters according to Table 1 in the <Preparation of Electrolyte> section, the rest is the same as in Example 1-1. Specifically, when the mass percentage of lithium difluorophosphate W1% changes, the mass percentage of the base solvent changes accordingly, while the mass ratio of each component in the base solvent and the mass percentages of lithium salt and propyl propionate remain unchanged.

[0125] Comparative Example 9

[0126] Except that propyl propionate is not added in the <Preparation of Electrolyte>, the mass percentage of the base solvent is changed accordingly, and the mass ratio of each component in the base solvent, the mass percentage of lithium salt and lithium difluorophosphate remain unchanged, the rest is the same as in Example 1-1.

[0127] Comparative Examples 10 to 11

[0128] Except for adjusting the relevant preparation parameters according to Table 1 in the <Preparation of Electrolyte> section, the rest is the same as in Example 1-1. Specifically, when the mass percentage of propyl propionate W2% changes, the mass percentage of the base solvent changes accordingly, while the mass ratio of each component in the base solvent and the mass percentages of lithium salt and lithium difluorophosphate remain unchanged.

[0129] Comparative Example 12

[0130] Except that lithium difluorophosphate and propyl propionate are not added in the <Preparation of Electrolyte>, the mass percentage of the base solvent is changed accordingly, and the mass ratio of each component in the base solvent and the mass percentage of lithium salt remain unchanged, the rest is the same as in Example 1-1.

[0131] Comparative Example 13

[0132] Except for adjusting the relevant preparation parameters according to Table 1 in the <Preparation of Negative Electrode Sheet>, where the Dv50 of the first negative electrode active material is 11 μm and the Dv90 is 22 μm, and the Dv50 of the second negative electrode active material is 13 μm and the Dv90 is 23 μm, the rest are the same as in Example 1-1.

[0133] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0134] Table 1

[0135] Note: " / " in Table 1 indicates that there is no corresponding parameter.

[0136] As can be seen from Examples 1-1 to 1-16 and Comparative Examples 1 to 13, this application designs the negative electrode structure and electrolyte composition, and controls the values ​​of H1, H2, W1, W2, and W1×H1 within the range of this application, and controls the Dv50 of the first negative electrode active material to be greater than the Dv50 of the second negative electrode active material. As a result, the R1, R2, R3, and K of the lithium-ion battery are smaller, that is, the initial impedance and the impedance growth during the cycling process are smaller, and D is larger, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance. The lithium-ion battery in Comparative Example 1 does not include a first negative electrode material layer and a second negative electrode material layer in its negative electrode sheet; the lithium-ion batteries in Comparative Examples 2 and 3 have H1 values ​​that are not within the scope of this application; the lithium-ion batteries in Comparative Examples 4 and 5 have H2 values ​​that are not within the scope of this application; the lithium-ion battery in Comparative Example 6 does not contain lithium difluorophosphate in its electrolyte and the value of W1×H1 is not within the scope of this application; the lithium-ion batteries in Comparative Examples 7 and 8 have W1 and W1×H1 values ​​that are not within the scope of this application; the lithium-ion battery in Comparative Example 9 does not contain lithium difluorophosphate in its electrolyte. The lithium-ion batteries in Comparative Examples 10 and 11 have W2 values ​​outside the scope of this application. The lithium-ion battery in Comparative Example 12 does not contain lithium difluorophosphate and propyl propionate in its electrolyte. The lithium-ion battery in Comparative Example 13 has a Dv50 of the first negative electrode active material that is smaller than that of the second negative electrode active material. The lithium-ion batteries in Comparative Examples 1 to 13 have larger R1, R2, R3, and K, meaning larger initial impedance and greater impedance growth during cycling. At the same time, D is smaller, indicating poor kinetic performance and high-temperature float charging performance of the lithium-ion batteries.

[0137] The value of H1 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-3 and Comparative Examples 2 to 3, when the value of H1 is too small, such as in Comparative Example 2, the R1, R2, R3, and K values ​​of the lithium-ion battery are larger, meaning the initial impedance and impedance growth during cycling are greater, while D is smaller. When the value of H1 is too large, such as in Comparative Example 3, the R1, R2, R3, and K values ​​of the lithium-ion battery are larger, meaning the initial impedance and impedance growth during cycling are greater, while D is smaller, indicating poor kinetic performance and high-temperature float charging performance of the lithium-ion battery. When the value of H1 is within the range specified in this application, the R1, R2, R3, and K values ​​of the lithium-ion battery are smaller, meaning the initial impedance and impedance growth during cycling are smaller, while D is larger, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance.

[0138] The value of H2 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-4 to 1-5, and Comparative Examples 4 to 5, when the value of H2 is too small, such as in Comparative Example 4, the R1, R2, R3, and K values ​​of the lithium-ion battery are larger, meaning the initial impedance and impedance growth during cycling are greater, while D is smaller. When the value of H2 is too large, such as in Comparative Example 5, the R1, R2, R3, and K values ​​of the lithium-ion battery are larger, meaning the initial impedance and impedance growth during cycling are greater, while D is smaller, indicating poor kinetic performance and high-temperature float charging performance of the lithium-ion battery. When the value of H2 is within the range specified in this application, the R1, R2, R3, and K values ​​of the lithium-ion battery are smaller, meaning the initial impedance and impedance growth during cycling are smaller, while D is larger, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance.

[0139] The relationship between the Dv50 values ​​of the first and second negative electrode active materials typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As seen in Examples 1-1 and Comparative Example 13, when the Dv50 of the first negative electrode active material is smaller than that of the second negative electrode active material (e.g., in Comparative Example 13), the lithium-ion battery exhibits larger R1, R2, R3, and K values, meaning a greater initial impedance and greater impedance increase during cycling, while D is smaller, indicating poor kinetic performance and high-temperature float charging performance. Conversely, when the Dv50 of the first negative electrode active material is greater than that of the second negative electrode active material, the lithium-ion battery exhibits smaller R1, R2, R3, and K values, meaning a smaller initial impedance and less impedance increase during cycling, while D is larger, indicating good kinetic performance and high-temperature float charging performance.

[0140] The value of W1 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-7 to 1-10, and Comparative Examples 6 to 8, when the value of W1 is too small, such as in Comparative Examples 6 and 7, the R1, R2, R3, and K values ​​of the lithium-ion battery are larger, meaning the initial impedance and impedance growth during cycling are greater, while D is smaller. When the value of W1 is too large, such as in Comparative Example 8, the R1, R2, R3, and K values ​​of the lithium-ion battery are larger, meaning the initial impedance and impedance growth during cycling are greater, while D is smaller, indicating poor kinetic performance and high-temperature float charging performance of the lithium-ion battery. When the value of W1 is within the range of this application, the R1, R2, R3, and K values ​​of the lithium-ion battery are smaller, meaning the initial impedance and impedance growth during cycling are smaller, while D is larger, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance.

[0141] The value of W2 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. From Examples 1-1 and 1-13 to...

[0142] Examples 1-14 and Comparative Examples 9 to 11 show that when the value of W2 is too small, such as in Comparative Examples 9 and 10, the R1, R2, R3, and K of the lithium-ion battery are larger, meaning the initial impedance and the impedance increase during cycling are greater, while D is smaller. When the value of W2 is too large, such as in Comparative Example 11, the R1, R2, R3, and K of the lithium-ion battery are larger, meaning the initial impedance and the impedance increase during cycling are greater, while D is smaller, indicating poor kinetic performance and high-temperature float charging performance of the lithium-ion battery. When the value of W2 is within the range of this application, the R1, R2, R3, and K of the lithium-ion battery are smaller, meaning the initial impedance and the impedance increase during cycling are smaller, while D is larger, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance.

[0143] The value of W1×H1 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-7 to 1-10, and Comparative Examples 6 to 8, when the value of W1×H1 is too small, such as in Comparative Examples 6 and 7, the lithium-ion battery exhibits larger values ​​for R1, R2, R3, and K, meaning a greater initial impedance and a larger impedance increase during cycling, while D is smaller. Conversely, when the value of W1×H1 is too large, such as in Comparative Example 8, the lithium-ion battery exhibits larger values ​​for R1, R2, R3, and K, meaning a greater initial impedance and a larger impedance increase during cycling, while D is smaller, indicating poor kinetic performance and high-temperature float charging performance. When the value of W1×H1 falls within the range specified in this application, the lithium-ion battery exhibits smaller values ​​for R1, R2, R3, and K, meaning a smaller initial impedance and a smaller impedance increase during cycling, while D is larger, indicating good kinetic performance and high-temperature float charging performance.

[0144] The H2 / H1 ratio typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-6, when the H2 / H1 ratio falls within the range specified in this application, the lithium-ion battery exhibits smaller R1, R2, R3, and K values, meaning smaller initial impedance and smaller impedance growth during cycling. Simultaneously, a larger D value indicates that the lithium-ion battery possesses excellent kinetic performance and high-temperature float charging performance.

[0145] The value of W1 / H1 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-12, when the value of W1 / H1 is within the range of this application, the R1, R2, R3, and K of the lithium-ion battery are relatively small, that is, the initial impedance and the impedance growth during cycling are relatively small, while D is relatively large, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance.

[0146] The value of W2×H1 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-13 to 1-16, when the value of W2×H1 is within the range of this application, the lithium-ion battery has smaller R1, R2, R3, and K values, meaning smaller initial impedance and smaller impedance growth during cycling, while D is larger, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance.

[0147] As shown in Figure 2, the lithium-ion battery of Example 1-1 has an impedance of 46.1 mΩ after 300 cycles and an impedance of 50.4 mΩ after 1000 cycles, with a K value of 6.14 × 10⁻⁶. -3 mΩ / cycle. The lithium-ion battery in Comparative Example 1 has an impedance of 59.0 mΩ after 300 cycles and 66.2 mΩ after 1000 cycles, with a K value of 10.29 × 10⁻⁶. -3 mΩ / cycle. The K value of Example 1-1 is smaller than that of Comparative Example 1, indicating that the lithium-ion battery of Example 1-1 has less impedance growth and better kinetic performance during cycling.

[0148] As shown in Figure 3, under the same charging time, the lithium-ion battery of Example 1-1 has a smaller thickness change rate. When the thickness change rate of the lithium-ion battery reaches 108.5%, the D value of Example 1-1 is 1248h, while the D value of Comparative Example 1 is 932h. The larger D value of Example 1-1 compared to Comparative Example 1 indicates that the lithium-ion battery of Example 1-1 has better high-temperature float charging performance.

[0149] Table 2

[0150] Note: " / " in Table 2 indicates that there is no corresponding parameter.

[0151] The presence of polynitrile compounds in the electrolyte, and the mass percentage W3% of these compounds, typically affects the kinetic performance and high-temperature float charge performance of lithium-ion batteries. As seen in Examples 1-1, 2-1 to 2-7, and 2-11, when the electrolyte includes polynitrile compounds and the mass percentage W3% of these compounds is controlled within the scope of this application, the lithium-ion battery exhibits smaller R1, R2, R3, and K values, meaning smaller initial impedance and smaller impedance growth during cycling. Simultaneously, D is larger, indicating that the lithium-ion battery possesses good kinetic performance and high-temperature float charge performance.

[0152] The value of W3×H1 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-7, and 2-11, when the value of W3×H1 is within the range of this application, the lithium-ion battery exhibits smaller R1, R2, R3, and K values, meaning smaller initial impedance and smaller impedance growth during cycling, while D is larger, indicating that the lithium-ion battery possesses good kinetic performance and high-temperature float charging performance.

[0153] The presence of propionate ester compounds in the electrolyte, and the mass percentage W4% of these compounds, typically affects the kinetic performance and high-temperature float charge performance of lithium-ion batteries. As seen in Examples 1-1, 2-8 to 2-10, when the electrolyte includes propionate ester compounds and the mass percentage W4% of these compounds is controlled within the scope of this application, the lithium-ion battery exhibits smaller R1, R2, R3, and K values, meaning smaller initial impedance and smaller impedance growth during cycling. Simultaneously, D is larger, indicating that the lithium-ion battery possesses good kinetic performance and high-temperature float charge performance.

[0154] The value of W2 / W4 typically affects the kinetic performance and high-temperature float charging performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-8 to 2-10, when the value of W2 / W4 is within the range of this application, the lithium-ion battery exhibits smaller R1, R2, R3, and K values, meaning smaller initial impedance and smaller impedance growth during cycling, while D is larger, indicating that the lithium-ion battery possesses good kinetic performance and high-temperature float charging performance.

[0155] Table 3

[0156] The Dv50 and Dv90 of the first negative electrode active material typically affect the kinetic performance and high-temperature float charge performance of lithium-ion batteries. From...

[0157] As can be seen from Examples 1-1, 3-1 to 3-4, when the Dv50 and Dv90 of the first negative electrode active material are within the scope of this application, the R1, R2, R3, and K of the lithium-ion battery are relatively small, that is, the initial impedance and the impedance growth during the cycle process are relatively small, while D is relatively large, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance.

[0158] The Dv50 and Dv90 of the second negative electrode active material typically affect the kinetic performance and high-temperature float charge performance of lithium-ion batteries. From...

[0159] As can be seen from Examples 1-1, 3-5 to 3-7, when the Dv50 and Dv90 of the second negative electrode active material are within the scope of this application, the R1, R2, R3, and K of the lithium-ion battery are relatively small, that is, the initial impedance and the impedance growth during the cycle process are relatively small, while D is relatively large, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charging performance.

[0160] The type of first negative electrode active material typically affects the kinetic performance and high-temperature float charge performance of lithium-ion batteries. As can be seen from Examples 1-1 and 3-8, when the first negative electrode active material within the scope of this application is selected, the lithium-ion battery has smaller R1, R2, R3, and K values, meaning smaller initial impedance and smaller impedance growth during cycling, while D is larger, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charge performance.

[0161] The type of second negative electrode active material typically affects the kinetic performance and high-temperature float charge performance of lithium-ion batteries. As can be seen from Examples 1-1 and 3-8, when the second negative electrode active material within the scope of this application is selected, the lithium-ion battery has smaller R1, R2, R3, and K values, meaning smaller initial impedance and smaller impedance growth during cycling, while D is larger, indicating that the lithium-ion battery has good kinetic performance and high-temperature float charge performance.

[0162] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0163] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0164] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery comprising a negative electrode tab and an electrolyte, the negative electrode tab comprising 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 first negative electrode material layer and a second negative electrode material layer disposed in layers, the second negative electrode material layer being located between the first negative electrode material layer and the negative electrode current collector, the first negative electrode material layer having a thickness of H1 μm, the second negative electrode material layer having a thickness of H2 μm, 20 ≤ H1 ≤ 40, 60 ≤ H2 ≤ 90; the first negative electrode material layer comprising a first negative electrode active material, the second negative electrode material layer comprising a second negative electrode active material, the first negative electrode active material having a Dv50 greater than a Dv50 of the second negative electrode active material; the electrolyte comprising lithium difluorophosphate and a propionate compound, the propionate compound comprising propyl propionate, the lithium difluorophosphate having a mass percentage of W1% and the propyl propionate having a mass percentage of W2% based on a mass of the electrolyte, 0.1 ≤ W1 ≤ 1, 10 ≤ W2 ≤ 60, 4 ≤ W1 × H1 ≤ 35.

2. The secondary battery according to claim 1, wherein 2 ≤ H2 / H1 ≤ 4.

5.

3. The secondary battery according to claim 1, wherein 0.005 ≤ W1 / H1 ≤ 0.

05.

4. The secondary battery according to claim 1, wherein 250 ≤ W2 × H1 ≤ 2400.

5. The secondary battery according to any one of claims 1 to 4, wherein the electrolyte comprising a polycarbonitrile compound, the polycarbonitrile compound having a mass percentage of W3% based on a mass of the electrolyte, 0.1 ≤ W3 ≤ 5; the polycarbonitrile compound comprising at least one of butanedinitrile, pentanedinitrile, hexanedinitrile, heptanedinitrile, octanedinitrile, methyl pentanedinitrile, 1,3,5-pentanetricarbonitrile, 1,2,3-propanetricarbonitrile, 1,3,6-hexanetricarbonitrile, or 1,2,3-tris(2-cyanoethoxy)propane.

6. The secondary battery according to claim 5, wherein 50 ≤ W3 × H1 ≤ 200.

7. The secondary battery according to any one of claims 1 to 4, wherein the propionate compound having a mass percentage of W4% based on a mass of the electrolyte, 10 ≤ W4 ≤ 65, 0.154 ≤ W2 / W4 ≤ 1.

8. The secondary battery according to claim 7, wherein the propionate compound further comprising at least one of methyl propionate, ethyl propionate, butyl acrylate, or butyl propionate.

9. The secondary battery according to any one of claims 1 to 4, wherein the first negative electrode active material having a Dv50 of 12 μm to 14.9 μm and a Dv90 of 20.5 μm to 25.9 μm.

10. The secondary battery according to any one of claims 1 to 4, wherein the second negative electrode active material having a Dv50 of 10 μm to 11.5 μm and a Dv90 of 19.4 μm to 24.8 μm.

11. The secondary battery according to any one of claims 1 to 4, wherein the first negative electrode active material and the second negative electrode active material each independently comprising at least one of natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon microbeads, silicon-carbon compounds, silicon-oxide compounds, or lithium titanate. 12.An electronic device comprising the secondary battery of any one of claims 1 to 11.

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