A secondary battery and an electrochemical device

Abstract

The application provides a secondary battery and an electrochemical device, and relates to the field of electrochemical energy storage. The secondary battery comprises an electrolyte and a negative electrode sheet, the electrolyte comprises a first substance, the first substance comprises at least one of a compound with a structure shown in formula I; the negative electrode sheet comprises a negative electrode material layer, the negative electrode material layer comprises carbon black, single-walled carbon nanotubes, carbon fibers and a silicon-based material; the silicon-based material comprises at least one of a silicon-carbon material, a silicon-oxygen material, a silicon alloy material and a pure silicon material. The secondary battery contains the compound with the structure shown in formula I with a specific structure in the electrolyte, and the mass content of the compound is controlled to be between 0.5% and 30%, and the negative electrode material layer comprises carbon black, single-walled carbon nanotubes and carbon fibers, so that the high-temperature storage performance, the cycle performance and the high and low temperature fast charging performance of the electrochemical device with the silicon-containing negative electrode are significantly improved.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, specifically to a secondary battery and an electrochemical device. Background Technology

[0002] As an energy storage device that can convert electrical energy into chemical energy, secondary batteries have been widely used in consumer electronics, electric vehicles, energy storage systems and other fields due to their advantages such as high energy density and long service life.

[0003] Currently, phosphate esters are added to the electrolytes of secondary batteries. On the one hand, the addition of phosphate esters reduces the ionic conductivity of the electrolyte, leading to a decrease in the internal charge transport efficiency of the secondary battery. This severely affects the fast-charging performance of electrochemical devices, making it difficult to meet the rapid energy replenishment requirements of electric vehicles and other scenarios. On the other hand, phosphate esters have poor compatibility with silicon-based anode materials, causing adverse side reactions on the surface of the silicon-based anode. This makes it difficult to maintain a stable solid electrolyte interface film at the electrode interface, significantly deteriorating the cycle performance of the secondary battery and electrochemical device. Furthermore, during charge-discharge cycles, the silicon-based anode material undergoes significant volume expansion and contraction due to lithium ion insertion and extraction, further exacerbating the side reactions between the electrode and the electrolyte. This results in rapid capacity decay and increased internal resistance, hindering the performance improvement and industrial application of secondary batteries and electrochemical devices. Summary of the Invention

[0004] This application provides a secondary battery and an electrochemical device. The secondary battery is designed to contain a compound with a specific structure as shown in Formula I in the electrolyte, and the mass content of the compound is controlled between 0.5% and 30%. At the same time, the negative electrode material layer contains carbon black, single-walled carbon nanotubes, and carbon fibers, which significantly improves the high-temperature storage performance, cycle performance, and high and low temperature fast charging performance of the silicon-containing negative electrode electrochemical device.

[0005] In a first aspect, this application provides a secondary battery, including an electrolyte and a negative electrode, wherein the electrolyte includes a first substance, and the first substance includes at least one of compounds having the structure shown in Formula I.

[0006]

[0007] R1, R2, and R3 are each independently one of the following groups:

[0008] Fluorinated alkyl groups containing 1 to 4 carbons, fluorinated alkenyl groups containing 2 to 3 carbons, fluorinated alkynyl groups containing 2 to 3 carbons, or non-fluorinated alkyl groups containing 1 to 4 carbons, non-fluorinated alkenyl groups containing 2 to 3 carbons, or non-fluorinated alkynyl groups containing 2 to 3 carbons.

[0009] Furthermore, among R1, R2, and R3, at least one is a fluorinated alkyl group containing 1 to 4 carbon atoms;

[0010] Based on the total mass of the electrolyte, the mass content of the first substance is a%, satisfying: 0.5≤a≤30;

[0011] The negative electrode sheet includes a negative electrode material layer, which includes carbon black, single-walled carbon nanotubes, carbon fibers, and silicon-based materials.

[0012] The silicon-based material includes at least one of silicon-carbon materials, silicon-oxygen materials, silicon alloy materials, and pure silicon materials.

[0013] Based on the above technical solution, the compound with the structure shown in Formula I contains at least one fluorinated alkyl group with 1 to 4 carbons. In the early stage of cycling, the compound with the structure shown in Formula I undergoes oxidative decomposition at the electrode interface to form a stable CEI / SEI film rich in lithium fluoride, which inhibits the continuous decomposition of the electrolyte, reduces side reactions, and improves the interfacial ionic conductivity, thereby improving fast charging performance. Furthermore, during high-temperature storage / charge-discharge process, the compound with Formula I releases P-containing free radicals with flame-retardant properties when heated, which can capture H· free radicals in the chain combustion reaction of organic free radicals, thereby improving the high-temperature storage / charge-discharge performance of the secondary battery. Furthermore, when carbon black, single-walled carbon nanotubes, and carbon fibers are added to the negative electrode as conductive agents, the conductivity and structural stability of the silicon-based negative electrode can be improved simultaneously. On the one hand, this can alleviate the problem of aggravated side reactions caused by the expansion and contraction of the silicon-based negative electrode material during cycling, thereby improving high-temperature storage and cycling performance. On the other hand, it provides better conductivity, which can optimize the reaction uniformity of the silicon-based negative electrode and further support the ion diffusion process in the silicon-based negative electrode, significantly improving the high-temperature storage performance, cycling performance, and high and low temperature fast charging performance of the electrochemical device.

[0014] In some embodiments of this application, based on the mass of the negative electrode material layer, the mass percentage of carbon black is H%, the mass percentage of single-walled carbon nanotubes is I%, the mass percentage of carbon fibers is J%, and the mass percentage of silicon in the silicon-based material is K, satisfying at least one of the following conditions:

[0015] (1) 0.1≤H+I+J≤2;

[0016] (2)0.006≤(H+I+J) / K≤0.04.

[0017] In some embodiments of this application, 1 ≤ a ≤ 15.

[0018] In some embodiments of this application, the secondary battery satisfies at least one of the following conditions:

[0019] (1) 0.2 ≤ H ≤ 1.0;

[0020] (2) 0.1 ≤ I ≤ 0.5;

[0021] (3) 0.01≤J≤0.2;

[0022] (4) 3≤K≤20.

[0023] In some embodiments of this application, the compound with the structure shown in Formula I is selected from at least one of the following compounds:

[0024]

[0025]

[0026]

[0027] In some embodiments of this application, the electrolyte further comprises fluoroethylene carbonate; based on the total mass of the electrolyte, the mass content of the fluoroethylene carbonate is b%, satisfying 5≤b≤25 and 1≤b / a≤40.

[0028] In some embodiments of this application, 7.5 ≤ b ≤ 25.

[0029] In some embodiments of this application, the electrolyte includes a first lithium salt, which includes lithium hexafluorophosphate and a sulfur-containing lithium salt, wherein the sulfur-containing lithium salt includes at least one of lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethylsulfonyl)imide; based on the total mass of the electrolyte, the mass content of the lithium hexafluorophosphate is e%, and the mass content of the sulfur-containing lithium salt is f%, 1≤e / f≤5.

[0030] In some embodiments of this application, the electrolyte further includes a boron-containing lithium salt, which includes at least one of lithium tetrafluoroborate, lithium difluorooxalate borate, or lithium dioxalate borate; the total mass content of the boron-containing lithium salt is 0.1% to 5% based on the total mass of the electrolyte.

[0031] Secondly, this application provides an electrochemical device including the secondary battery described in the first aspect. Therefore, the electrochemical device provided by this application has good high-temperature storage performance, cycle performance, and high-temperature and low-temperature fast-charging performance. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0033] It should be noted that, in the specific embodiments of this application, lithium-ion secondary batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion secondary batteries.

[0034] Lithium-ion secondary batteries

[0035] It includes an electrolyte and a negative electrode sheet, wherein the electrolyte includes a first substance, and the first substance includes at least one of the compounds having the structure shown in Formula I;

[0036]

[0037] R1, R2, and R3 are each independently one of the following groups:

[0038] Fluorinated alkyl groups containing 1 to 4 carbons, fluorinated alkenyl groups containing 2 to 3 carbons, fluorinated alkynyl groups containing 2 to 3 carbons, or non-fluorinated alkyl groups containing 1 to 4 carbons, non-fluorinated alkenyl groups containing 2 to 3 carbons, or non-fluorinated alkynyl groups containing 2 to 3 carbons.

[0039] Furthermore, among R1, R2, and R3, at least one is a fluorinated alkyl group containing 1 to 4 carbon atoms;

[0040] Based on the total mass of the electrolyte, the mass content of the first substance is a%, satisfying: 0.5≤a≤30;

[0041] The negative electrode sheet includes a negative electrode material layer, which includes carbon black, single-walled carbon nanotubes, carbon fibers, and silicon-based materials.

[0042] The silicon-based material includes at least one of silicon-carbon materials, silicon-oxygen materials, silicon alloy materials, and pure silicon materials.

[0043] For example, the fluorinated alkyl group containing 1 to 4 carbons can be, for example, -CF3, -CH2CF3, -CF2CF3, -CHFCF3, -CH2CH2CF3, -CF(CF3)2, or -CH2CH2CH2CF3;

[0044] Fluorinated alkenyl groups with 2 to 3 carbons, for example, can be CHF=CH-, CF2=CF-, CF3-CH=CH- or CF3-CF=CH-;

[0045] Fluorinated alkynyl groups with 2 to 3 carbons, for example, can be FC≡C-, CF3-C≡C-, CF3-C≡C- or CH2F-C≡C-;

[0046] A non-fluorinated alkyl group with 1 to 4 carbons, for example, it can be -CH3, -CH2CH3, -CH2CH2CH3, -CH(CF3)2 or -CH2CH2CH2CH3;

[0047] A non-fluorinated alkenyl group with 2 to 3 carbons, for example, can be -CH=CH2, -CH=CH-CH3 or -CH2-CH=CH2;

[0048] A non-fluorinated alkynyl group with 2 to 3 carbons, such as -C≡CH, -C≡C-CH3 or -CH2-C≡CH.

[0049] The inventors discovered that adding phosphate esters to the electrolyte can improve high-temperature storage performance. However, adding phosphate esters to the electrolyte reduces electrolyte conductivity, affecting the fast-charging performance of the electrochemical device. Furthermore, phosphate esters have poor compatibility with silicon-based anodes, which deteriorates cycle performance. Therefore, in the above technical solution, a compound with the structure shown in Formula I is added to the electrolyte. This compound contains at least 1-4 carbon fluorinated alkyl groups and phosphate groups. Part of the Formula I compound is oxidized and decomposed in the early stages of cycling to form a lithium fluoride-rich CEI / SEI film, which can reduce the continuous consumption reaction of the electrolyte. The Formula I compound also indirectly improves the conductivity of the electrolyte. In addition, during high-temperature storage / charge-discharge processes, the Formula I compound releases P-containing free radicals with flame-retardant properties when heated. These free radicals can capture H· free radicals in the chain combustion reaction of organic free radicals, inhibiting the chain exothermic reaction and improving the high-temperature storage, cycle performance, and high-temperature and low-temperature fast-charging performance of the secondary battery.

[0050] Furthermore, the addition of carbon black, single-walled carbon nanotubes, and carbon fibers to the negative electrode can simultaneously improve the conductivity and structural stability of the silicon-based negative electrode. On the one hand, it can alleviate the problem of aggravated side reactions caused by the expansion and contraction of the silicon-based negative electrode material during cycling, thereby improving high-temperature storage and cycling performance. On the other hand, it provides better conductivity, further supporting the ion diffusion process in the silicon-based negative electrode, optimizing the reaction uniformity of the silicon-based negative electrode, and thus improving the high-temperature storage performance, cycling performance, and high and low temperature fast charging performance of the secondary battery.

[0051] Based on the above technical solution, this application significantly improves the high-temperature storage performance, cycle performance, and high / low temperature fast charging performance of the silicon-containing anode electrochemical device by controlling the content of a compound with a specific structure of Formula I in the electrolyte and keeping its mass content between 0.5% and 30%, while the anode material layer includes carbon black, single-walled carbon nanotubes, and carbon fibers. The compound with the structure of Formula I contains at least one fluorinated alkyl group with 1 to 4 carbons, which can oxidize and decompose at the electrode interface in the early stage of cycling to form a stable CEI / SEI film rich in lithium fluoride, inhibiting the continuous decomposition of the electrolyte, reducing side reactions, and improving the interfacial ionic conductivity and fast charging performance. Furthermore, during high-temperature storage / charge-discharge, the compound of Formula I releases P-containing free radicals with flame-retardant properties when heated, which can capture H· free radicals in the free radical chain combustion reaction of organic matter, thereby improving the high-temperature storage / charge-discharge performance of the secondary battery. Furthermore, when carbon black, single-walled carbon nanotubes, and carbon fibers are added to the negative electrode as conductive agents, the conductivity and structural stability of the silicon-based negative electrode can be improved simultaneously. On the one hand, this can alleviate the problem of aggravated side reactions caused by the expansion and contraction of the silicon-based negative electrode material during cycling, thereby improving high-temperature storage and cycling performance. On the other hand, it provides better conductivity, which can optimize the reaction uniformity of the silicon-based negative electrode and further support the ion diffusion process in the silicon-based negative electrode, significantly improving the high-temperature storage performance, cycling performance, and high and low temperature fast charging performance of the electrochemical device.

[0052] Understandably, carbon nanotubes are hollow tubular structures with diameters of up to several hundred nanometers, formed by rolling single or multiple layers of graphene sheets at a certain helical angle. In this application, single-walled carbon nanotubes (SWCNTs) are tubular structures formed by rolling single layers of graphene sheets, which can enhance the conductivity, mechanical toughness, and interfacial reaction efficiency of the negative electrode material layer, effectively buffer the volume expansion of silicon-based materials, and maintain interfacial integrity. The single-walled carbon nanotubes in the embodiments or comparative examples are commercially available.

[0053] In this application, the type of carbon black is not specifically limited, as long as it can achieve the purpose of this application; specifically, the carbon black can be acetylene black, Super P or other highly conductive carbon black materials, which have high specific surface area and excellent conductivity, can significantly enhance the electronic conductivity of the negative electrode material layer, and at the same time provide support through its physical structure, alleviate the volume expansion of silicon-based materials during charging and discharging, thereby maintaining the overall structural stability and interface integrity of the electrode.

[0054] In this application, the type of carbon fiber is not specifically limited, as long as it can achieve the purpose of this application; specifically, the carbon fiber can be polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, or other high-modulus carbon fiber materials; carbon fiber has high conductivity, excellent mechanical strength and low density characteristics, and can provide additional physical support through its fiber network structure, effectively alleviate the volume expansion of silicon-based materials during charging and discharging, thereby enhancing the structural stability of the negative electrode material layer, maintaining interface integrity, and promoting uniform electron conduction and lithium ion diffusion.

[0055] As is understandable, silicon-based materials refer to anode materials with silicon or silicon-based composite materials as the main active components, possessing high theoretical specific capacity. Silicon-based materials can include silicon-carbon materials, silicon-oxygen materials, silicon alloy materials, pure silicon materials, etc.

[0056] In some embodiments, based on the total mass of the electrolyte, the mass content of the first substance is a%, satisfying: 0.5≤a≤30; preferably, 1≤a≤15; for example, a can be 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, or a range consisting of any two of these values; when the mass content of the compound with the structure shown in Formula I is within the above range, a stable CEI / SEI film can be formed, reducing electrolyte consumption, improving compatibility with silicon-based anodes, and improving high-temperature storage, cycling, and fast-charging performance. The inventors speculate that if the mass content of the first substance is below this range, a complete protective film cannot be formed, side reactions are aggravated, and cycle performance deteriorates; if the mass content of the first substance is above this range, excess compounds may increase interfacial impedance, reduce electrolyte conductivity, hinder fast charging, and may also trigger new side reactions, affecting battery performance. In some embodiments, a can range from 0.5 to 5. In some embodiments, a can range from 3 to 10. In some embodiments, a can range from 5 to 15. In some embodiments, a can range from 8 to 20.

[0057] In some embodiments, based on the mass of the negative electrode material layer, the mass percentage of carbon black is H%, the mass percentage of single-walled carbon nanotubes is I%, the mass percentage of carbon fibers is J%, and the mass percentage of silicon in the silicon-based material is K%, satisfying at least one of the following conditions:

[0058] (1) 0.1≤H+I+J≤2;

[0059] (2)0.006≤(H+I+J) / K≤0.04.

[0060] Based on the above technical solution, by controlling the silicon content and the contents of carbon black, single-walled carbon nanotubes, and carbon fibers in the negative electrode material layer within the appropriate range, it is possible to improve the conductivity and electrode structure stability of the silicon-based negative electrode while ensuring a better energy density of the electrochemical device. On the one hand, it can alleviate the problem of aggravated side reactions caused by the expansion and contraction of the silicon-based negative electrode material during cycling, thereby improving high-temperature storage and cycling performance. On the other hand, it provides better conductivity, supports the ion diffusion process in the silicon-based negative electrode sheet, optimizes the reaction uniformity of the silicon-based negative electrode, and thus improves the high-temperature fast charging performance and low-temperature discharge performance after high-temperature storage of the battery.

[0061] In some examples, the values ​​of H, I, and J satisfy 0.1 ≤ H + I + J ≤ 2, preferably 0.5 ≤ H + I + J ≤ 1.5; for example, H + I + J can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, or any two of these values. Adjusting the values ​​of the negative electrode material layer (H + I + J) within the range of 0.1 to 2 can synergistically enhance the conductive network and electrode structure stability of the silicon-based negative electrode, effectively mitigating interfacial side reactions caused by the volume expansion and contraction of the silicon-based material during charging and discharging, thereby significantly improving the high-temperature storage performance, cycle performance, and high / low temperature fast charging performance of the electrochemical device. Within this range, carbon black, single-walled carbon nanotubes, and carbon fibers can form a highly efficient three-dimensional conductive pathway, supporting uniform ion / electron transport, improving the reaction uniformity of silicon-based materials, and suppressing electrode structure damage. If (H+I+J) is below 0.1, the conductive network is incomplete, electrode impedance increases, and fast-charging performance and cycle life decrease; if (H+I+J) exceeds 2, excess will reduce the proportion of electrode active material, impair battery energy density, and may hinder lithium-ion diffusion, which is detrimental to the optimization of electrochemical performance.

[0062] In some examples, the values ​​of H, I, J, and K satisfy 0.006 ≤ (H+I+J) / K ≤ 0.04, preferably 0.01 ≤ (H+I+J) / K ≤ 0.03. For example, (H+I+J) / K can be 0.006, 0.007, 0.008, 0.009, 0.010, 0.012, 0.015, 0.018, 0.020, 0.022, 0.025, 0.027, 0.030, or any combination of two of these values. Adjusting the ratio of the total content of carbon black, single-walled carbon nanotubes, and carbon fibers to the silicon content (H+I+J) / K within the range of 0.006 to 0.04 can optimize the ratio between the conductive network and the active material in the silicon-based anode, significantly improving the structural stability and interfacial reaction uniformity of the electrode while ensuring energy density. This ratio range ensures sufficient carbon black, single-walled carbon nanotubes, and carbon fibers to support the volume changes of silicon materials during charge and discharge, suppressing electrode cracking and interfacial side reactions. If the (H+I+J) / K ratio is below 0.006, there is a relative deficiency of carbon black, single-walled carbon nanotubes, and carbon fibers, resulting in poor electrode conductivity and decreased fast-charging performance and cycle life. If the (H+I+J) / K ratio is above 0.04, there is an excess of carbon black, single-walled carbon nanotubes, and carbon fibers, which not only reduces battery energy density but may also hinder lithium-ion diffusion, thus being detrimental to the optimization of electrochemical performance.

[0063] In some embodiments, the secondary battery satisfies 0.2 ≤ H ≤ 1.0; preferably 0.4 ≤ H ≤ 0.8, for example, H + I + J can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, or any two of these values. Adjusting the mass ratio H of carbon black in the negative electrode material layer to the range of 0.2 to 1.0 can optimize the conductive network construction and mechanical strength of the electrode, effectively maintaining the stability of the electrode structure while ensuring good electron conduction. Carbon black within this content range can provide sufficient point contact conductive pathways, forming a synergistic conductive network with single-walled carbon nanotubes and carbon fibers, enhancing the electron transport efficiency of silicon-based materials during charging and discharging, while mitigating stress concentration on the electrode caused by volume changes. If the H value is below 0.2, the carbon black content is insufficient, the electrode conductivity decreases, and the fast charging performance and cycle life are affected. If the H value is above 1.0, the excessive carbon black will occupy too much electrode volume, reduce the proportion of active material, damage the battery energy density, and may hinder lithium-ion diffusion, which is not conducive to the overall optimization of electrochemical performance.

[0064] In some embodiments, 0.1 ≤ I ≤ 0.5; preferably 0.2 ≤ I ≤ 0.4, for example, I can be 0.1, 0.12, 0.15, 0.2, 0.23, 0.27, 0.3, 0.35, 0.4, 0.45, 0.5, or a range of any two of these values. Adjusting the mass percentage I of single-walled carbon nanotubes in the anode material layer to the range of 0.1 to 0.5 can effectively construct a long-range conductive network and enhance the mechanical toughness of the electrode, significantly improving the electronic conductivity and structural integrity of the silicon-based anode. Single-walled carbon nanotubes within this content range can form highly efficient conductive pathways in the electrode, synergistically working with carbon black and carbon fibers to promote rapid charge transport, while simultaneously buffering the stress caused by volume changes in the silicon material through their unique one-dimensional fiber structure, suppressing electrode crack formation. If the I value is below 0.1, the content of single-walled carbon nanotubes is insufficient, making it difficult to form a continuous conductive network, increasing electrode impedance, and reducing fast charging performance and cycle stability. If the I value is above 0.5, excessive single-walled carbon nanotubes are prone to entanglement and aggregation, which hinders ion diffusion paths, increases electrode internal resistance, and reduces battery energy density, which is not conducive to the overall optimization of electrochemical performance.

[0065] In some embodiments, 0.01 ≤ J ≤ 0.2; preferably 0.05 ≤ J ≤ 0.15, for example, J can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, or a range of any two of these values. Adjusting the mass percentage J of carbon fiber in the anode material layer to the range of 0.01 to 0.2 can effectively enhance the three-dimensional skeletal support and longitudinal conductivity of the electrode, significantly improving the structural stability and interfacial charge transport efficiency of the silicon-based anode. Carbon fiber within this content range can construct a stable mechanical support network in the electrode, forming a multi-dimensional synergistic conductive system with carbon black and single-walled carbon nanotubes, effectively suppressing electrode pulverization and cracking caused by the volume effect of silicon materials, while providing a rapid electron transport path. If the J value is below 0.01, the carbon fiber content is insufficient, the electrode structure has limited support, and the electrode is prone to damage during cycling, leading to rapid capacity decay. If the J value is above 0.2, excessive carbon fiber will occupy too much electrode space, significantly reducing the content of active material and damaging the battery energy density. At the same time, it may hinder electrolyte wetting and lithium ion migration, which will degrade the battery's fast charging performance and cycle life.

[0066] In some embodiments, 3 ≤ K ≤ 20. Preferably, 5 ≤ K ≤ 15, for example, K can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range of any two of these values. Adjusting the mass percentage K of silicon in the silicon-based material to the range of 3 to 20 can effectively balance the energy density and cycle stability of the battery, significantly improving battery capacity while ensuring the integrity of the electrode structure. Silicon within this content range can fully leverage its high specific capacity advantage, working synergistically with carbon black, single-walled carbon nanotubes, and carbon fibers to achieve higher energy density through appropriate silicon participation in electrode reactions, while controlling volume expansion within an acceptable range. If the K value is below 3, the battery energy density is insufficient to meet design requirements; if the K value is above 20, excessive silicon will cause excessive volume changes during charging and discharging, leading to electrode structure damage, intensified interfacial side reactions, significantly deteriorating battery cycle life and fast-charging performance, and potentially causing safety issues.

[0067] Furthermore, this application does not specifically limit the content of silicon-based materials in the negative electrode material layer. That is, the mass percentage of silicon element based on the mass of the negative electrode material layer is K%, which satisfies 0.006≤(H+I+J) / K≤0.04 and / or 3≤K≤20.

[0068] In some embodiments, the compound with the structure shown in Formula I is selected from at least one of the following compounds:

[0069]

[0070]

[0071]

[0072] The specific types of compounds with the structure shown in Formula I in the electrolyte are selected from the above-mentioned specific fluorinated phosphate compounds. These specific compounds preferentially oxidize and decompose in the early stage of cycling to form a dense CEI / SEI interface film rich in lithium fluoride, which effectively inhibits the continuous decomposition of the electrolyte and the side reactions of the silicon-based anode. At the same time, they enhance the interfacial ion conduction efficiency, optimize the composition and stability of the electrode interface film, and significantly improve the high-temperature storage performance, cycle life and fast charging capability of the electrochemical device.

[0073] In some embodiments, the electrolyte further comprises fluoroethylene carbonate; based on the total mass of the electrolyte, the mass content of the fluoroethylene carbonate is b%, satisfying 5≤b≤25, 1≤b / a≤40; preferably, 1≤b≤10; for example, b can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 or a range of any two of these values; for example, the value of b / a can be 1, 2, 3, 4, 5, 6, 7, 7.5, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 1 The range of values ​​9, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, and 40, or any two of these values, is considered. Controlling the mass content of fluoroethylene carbonate within this suitable range can improve the compatibility of the electrolyte containing the compound shown in Formula I with the silicon-based anode. Fluoroethylene carbonate can promote the dissociation of lithium salts, thereby improving the electrolyte kinetics. Its decomposition products can form a stable and dense SEI film on the surface of the anode active material, improving interfacial impedance and thus enhancing the battery's high-temperature fast-charging performance and low-temperature discharge performance after high-temperature storage. When the content of fluoroethylene carbonate is below the range, the protection of the anode interface is insufficient, deteriorating the high-temperature fast-charging performance and low-temperature discharge performance after high-temperature storage. When the content of fluoroethylene carbonate is above the range, the electrolyte viscosity increases, deteriorating the high-temperature fast-charging performance.

[0074] In some embodiments, the electrolyte comprises a first lithium salt, which comprises lithium hexafluorophosphate and a sulfur-containing lithium salt, wherein the sulfur-containing lithium salt comprises at least one of lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethylsulfonyl)imide; based on the total mass of the electrolyte, the mass content of lithium hexafluorophosphate is e%, and the mass content of the sulfur-containing lithium salt is f%, 1≤e / f≤5; preferably 2≤e / f≤4, for example, the e / f ratio can be 1.0, 1.2, 1.5, 1.8, 2.0, 2.5, 3.0, 3.3, 3.7, 4.0, 4.3, 4.6, 5.0 or a range of any two values ​​therein; adjusting the mass ratio e / f of lithium hexafluorophosphate to sulfur-containing lithium salt in the electrolyte to be in the range of 1 to 5 can optimize the synergistic effect of the lithium salt system and significantly improve the ionic conductivity, interfacial stability and thermal stability of the electrolyte. Within this ratio range, lithium hexafluorophosphate provides good basic conductivity, while sulfur-containing lithium salts (such as lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide) can promote the formation of a more stable electrode interface film, inhibit electrolyte decomposition, and improve low-temperature performance and high-rate charge / discharge capability. If the e / f value is less than 1, there is a relatively excessive amount of sulfur-containing lithium salt, which may exacerbate corrosion of the current collector and increase costs; if the e / f value is greater than 5, the proportion of lithium hexafluorophosphate is too high, resulting in decreased thermal stability and high-temperature cycling performance of the electrolyte, weakened interface film protection, and thus deterioration of the battery's high-temperature storage and long-cycle performance.

[0075] Furthermore, based on the total mass of the electrolyte, the mass content of lithium hexafluorophosphate is e%, 8≤e≤15, preferably 10≤e≤14. For example, e can be 8, 9, 10, 11, 12, 13, 14, 15, or a range of any two of these values. When the lithium hexafluorophosphate in the electrolyte is controlled within the above range, the requirements for ionic conductivity and chemical stability can be effectively balanced.

[0076] Furthermore, based on the total mass of the electrolyte, the mass content of the sulfur-containing lithium salt is f%, 3≤e≤6, for example, e can be 3, 4, 5, 6 or any two of these values; when the sulfur-containing lithium salt in the electrolyte is controlled within the above range, the oxidative decomposition of the electrolyte under high voltage can be effectively suppressed, gas production can be reduced, and the transmission efficiency of lithium ions between the positive and negative electrodes can be enhanced.

[0077] In some embodiments, the electrolyte further includes a boron-containing lithium salt, which includes at least one of lithium tetrafluoroborate, lithium difluorooxalate borate, or lithium dioxalate borate; based on the total mass of the electrolyte, the mass content of the boron-containing lithium salt is 0.1% to 5%, for example, the total mass content of the boron-containing lithium salt can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.5%, 2.6%, 3%. The range is 0%, 3.3%, 3.8%, 4.0%, 4.5%, 5.0%, or any two of these values. The inventors discovered that adding phosphate ester additives can improve the heat box problem in high-energy-density batteries. However, phosphate ester additives cannot be reduced to form an effective SEI on the negative electrode side to improve the stability of the negative electrode interface. In this application, by introducing boron-containing lithium salt and controlling it within the above range, a stable SEI can be formed on the negative electrode side, improving the interface impedance, thereby improving the battery's high-temperature fast charging performance and low-temperature discharge performance after high-temperature storage.

[0078] This application does not impose any particular restrictions on the type of electrolyte, as long as it is a medium capable of transporting ions between the positive and negative electrodes. In this application, the electrolyte includes non-organic solvents, lithium salts dissolved in non-aqueous organic solvents, and functional electrolyte additives.

[0079] This application does not impose any particular limitation on non-aqueous organic solvents, as long as they can serve as a medium for the movement of ions participating in the electrochemical reactions of the battery cell. For example, non-aqueous organic solvents may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. These non-aqueous organic solvents may be used alone or in combination of two or more. Preferably, a combination of two or more non-aqueous organic solvents is selected from saturated cyclic carbonate compounds, chain carbonate compounds, and carboxylic acid ester compounds. Preferably, the non-aqueous organic solvent is selected from at least one of ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), ethyl propionate (PE), propyl propionate (PP), ethyl methyl-2,2-difluoroacetate, ethyl 2,2,2-trifluoroacetate, ethyl 2,2-difluoroacetate (DFEA), methyl 2,2-difluoroacetate, and fluoroethers.

[0080] In some embodiments, the total mass of the non-aqueous organic solvent accounts for 50% to 90% of the total mass of the electrolyte.

[0081] Optionally, the functional additive may be a cyclic carbonate compound, a nitrile compound, a boron-containing compound, a nitrogen-containing lithium salt compound, a sulfonate compound, a sulfate compound, a silane compound, an ether compound, a pyridine compound, or other compounds.

[0082] Preferably, the functional additives include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), trans-difluoroethylene carbonate (DFEC), vinyl sulfite (ES), succinate (SN), glutaronitrile, adiponitrile (ADN), heptanonitrile, caprylate, decanonitrile, triacrylonitrile (TCP), 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetricarbonyl (HTCN), ethylene glycol bis(propionitrile) ether (DENE), 1,2,3-tris(2-cyanoethoxy)propane, 1,4-dicyano-2-butene, 1,3-propanesulfonate lactone (PS), propenyl-1,3-sulfonate lactone (PST), vinyl sulfate (DTD), methyl vinyl sulfate, and quaternary ammonium chloride. The following is a list of at least one of the following: pentafluoroethoxycyclotriphosphazene (PFPN), fluorotris(trimethylsilane) phosphate, dimethyldimethoxysilane (DODSi), tetravinylsilane (TVS), lithium monofluorophosphate, 1,2-bis(difluorophospho)ethane, lithium fluorosulfonate, tris(trimethylsilane)borate (TMSB), hexamethyldisilazane (HMDS), fluorobenzene (FB), cyclohexylbenzene, biphenyl (BP), maleic anhydride (MA), acetamide (EA), polyamide (PA), γ-butyrolactone (GBL), 1,3-dioxapentane (DOL), ethylene glycol dimethyl ether (DME), pyridine, 2-methylpyridine, 2-fluoropyridine, 2-cyanopyridine, or 3-vinylpyridine.

[0083] There are no particular restrictions on the selection and content of the aforementioned functional additives in the electrolyte; they can be chosen according to actual needs. For example, specific components can be selected as film-forming additives, flame-retardant additives, overcharge protection additives, water / acid removal additives, passivating agents for protecting Al current collectors or battery casings, and wetting agents to improve wetting properties to meet the battery's needs for different application scenarios and positive and negative electrode matching.

[0084] In the embodiments of this application, the types and contents of inorganic components / electrolyte lithium salts in the electrolyte are known in the art and can be detected using equipment and methods known in the art. For example, the inorganic components / electrolyte lithium salts in the electrolyte can be qualitatively or quantitatively analyzed by ion chromatography analysis method with reference to standard JY / T 05752020 "General Rules for Ion Chromatography Analysis". In the embodiments of this application, freshly prepared electrolyte can be used as a sample, the free electrolyte of a fresh battery can be used as a sample, or a fully discharged (discharged to the discharge cutoff voltage) battery cell can be disassembled in reverse and the free electrolyte obtained from the battery cell can be used as a sample for detection by ion chromatography analysis method.

[0085] In the embodiments of this application, the types and contents of organic components in the electrolyte have meanings known in the art and can be detected using equipment and methods known in the art, such as GB / T9722. The 2006 "General Rules for Gas Chromatography of Chemical Reagents" uses gas chromatography to perform qualitative and quantitative analysis of the organic components in electrolytes.

[0086] The negative electrode sheet of this application also includes a negative electrode current collector. In this application, the negative electrode material layer can be disposed on one surface in the thickness direction of the negative electrode current collector, or it can be disposed on both surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a part of the negative electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0087] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collectors (e.g., carbon-copper composite current collector, nickel-copper composite current collector, 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 achieves the purpose of this application. For example, the thickness of the negative electrode current collector may be from 4 μm to 15 μm.

[0088] In this application, the negative electrode material layer may also include a negative electrode binder. This application does not have any particular limitation on the negative electrode binder, as long as it can achieve the purpose of this application. For example, it may include, but is not limited to, at least one of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyacrylic acid, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon.

[0089] In this application, there are no particular limitations on the preparation method of the negative electrode sheet, as long as the purpose of this application can be achieved. For example, it can be prepared by the following method: mixing the raw materials and binder of the negative electrode material layer, adding deionized water and stirring evenly to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on one surface of the negative electrode current collector, and after drying, a negative electrode sheet with a single-sided negative electrode material layer is obtained. Then, the above coating steps are repeated on the other surface of the negative electrode current collector, and after drying, a negative electrode sheet with a double-sided negative electrode material layer is obtained. After coating, the negative electrode sheet is obtained by cold pressing and cutting. This application does not have particular limitations on the thickness of the negative electrode material layer after cold pressing, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided negative electrode material layer after cold pressing is 10 μm to 50 μm.

[0090] other

[0091] The lithium-ion secondary battery also includes a positive electrode sheet, which includes a positive current collector and a positive active material layer disposed on the surface of the positive current collector. This application does not impose any particular limitation on the positive current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or a composite current collector (e.g., a composite current collector with a metal layer disposed on the surface of a polymer layer).

[0092] This application 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 can be from 5 μm to 40 μm.

[0093] The positive electrode active 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 achieves the purpose of this application. For example, the positive electrode active material may contain one or more lithium-containing composite metal oxides selected from the group consisting of cobalt, manganese, and nickel, or one or more lithium-containing olivine-type phosphates selected from iron, cobalt, nickel, and manganese. These positive electrode active materials can be used individually or in combination.

[0094] Suitable examples of such lithium composite metal oxides include, for instance, those selected from LiCoO2, LiMn2O4, LiNiO2, and LiCo. 1-x Ni x O2 (0.01) <x<1)、LiNi x Mn y Co z O2 (x+y+z=1), solid solutions of Li2MnO3 and LiMO2 (M is a transition metal such as Co, Ni, Mn, Fe, etc.), and LiNi 1 / 2 Mn 3 / 2 O4, LiFePO4, LiMnPO4, and LiMn 1-x Fe xOne or more of PO4 (0.01 < x < 1), more preferably two or more. A part of these composite metal oxides with lithium or olivine-type phosphates containing lithium can be replaced by other elements, or a part of cobalt, nickel, manganese, and iron can be replaced by one or two or more elements selected from Co, Mn, Ni, Mg, Al, B, Ti, V, Nb, Cu, Zn, Mo, Ca, Sr, W, and Zr, or coated with a compound containing these other elements or a carbon material. The present application does not particularly limit the thickness of the positive electrode active material layer, as long as the object of the present application can be achieved. For example, the thickness of the positive electrode active material layer is 30 μm to 120 μm.

[0095] The positive electrode active material layer may further include a conductive agent and a binder. The present application does not particularly limit the types of the conductive agent and the binder, as long as the object of the present application can be achieved. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNT), carbon fibers, Ketjen black, graphene, metal materials, or conductive polymers. The binder may include, but is not limited to, at least one of polyacrylic acid, polyacrylates, acrylate polymers, polyvinyl alcohol, polyvinylidene fluoride, polytetrafluoroethylene, or vinylidene fluoride-hexafluoropropylene copolymer.

[0096] The present application does not particularly limit the mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode active material layer. Those skilled in the art can select according to actual needs, as long as the object of the present application can be achieved.

[0097] The lithium ion secondary battery further includes a separator, which is used to separate the positive electrode sheet and the negative electrode sheet, prevent internal short circuit of the secondary battery, allow electrolyte ions to pass freely, and does not affect the progress of the electrochemical charge and discharge process. The present application does not particularly limit the separator, as long as the object of the present application can be achieved. For example, the material of the separator may include, but is not limited to, at least one of polyolefins (PO) mainly composed of polyethylene (PE) and polypropylene (PP), polyesters (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid; the type of the separator may include at least one of woven films, non-woven films, microporous films, composite films, rolled films, or spun films.

[0098] In this application, the diaphragm may include a substrate and a surface treatment layer. The substrate may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. For example, the inorganic layer includes inorganic particles and a binder. This application does not have any particular limitation on the aforementioned inorganic particles, and may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not have any particular limitation on the aforementioned binders, and may include at least one of the aforementioned binders. The polymer layer contains a polymer, the polymer material of which includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0099] In this application, the pore size of the separator is from 0.01 μm to 1 μm, and the thickness is from 5 μm to 50 μm. In some embodiments, the thickness of the separator is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the separator is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the separator is within the above ranges, insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the secondary battery can be ensured.

[0100] Lithium-ion secondary batteries can be prepared according to conventional methods in the art. Exemplarily, the above-mentioned positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes, to obtain an electrode assembly. Alternatively, the electrode assembly can be obtained by winding. The electrode assembly is placed in a packaging shell, electrolyte is injected, and the shell is sealed to obtain a secondary battery. The structure of the lithium-ion secondary battery is not particularly limited, and coin-shaped batteries, cylindrical batteries, prismatic batteries, or pouch batteries with single or multiple separators are applicable.

[0101] Secondly, this application provides an electronic device including any of the aforementioned secondary batteries.

[0102] In some embodiments, the electronic devices of this application include, but are not limited to, laptop computers, 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, household large-capacity batteries, and lithium-ion capacitors, etc.

[0103] Example

[0104] The following uses a lithium-ion secondary battery as an example to illustrate the implementation of the secondary battery of this application in more detail through embodiments and comparative examples. Those skilled in the art will understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application.

[0105] Test methods and equipment

[0106] 45℃ Cyclic Performance Test

[0107] The lithium-ion battery was placed in a constant temperature environment of 45℃ and allowed to stand for 30 minutes to reach a constant temperature. It was then charged at a constant current of 0.5C to 4.53V, followed by constant voltage charging at 4.53V to a current of 0.025C. After standing for 5 minutes, it was discharged at a constant current of 0.5C to 3.0V. The discharge capacity of the first cycle was recorded. The above steps were then repeated for charge and discharge cycles, and the number of cycles N when the cycle capacity retention was ≤80% was recorded.

[0108] Cycle capacity retention = (Discharge capacity in the Nth cycle / Discharge capacity in the first cycle) × 100%.

[0109] High-temperature storage performance test

[0110] At 25°C, a lithium-ion secondary battery was charged at a constant current rate of 0.33C to 4.5V, then charged at a constant voltage rate until the current was less than or equal to 0.05C, and finally discharged at a constant current rate of 0.33C to 2.8V. The initial discharge capacity of the lithium-ion secondary battery was measured. At 25°C, the lithium-ion secondary battery was charged at a constant current rate of 0.33C to 4.5V, then charged at a constant voltage rate until the current was less than or equal to 0.05C. The fully charged lithium-ion secondary battery was then placed in a 60°C oven. After 15 days, the lithium-ion secondary battery was removed from the high-temperature storage and allowed to cool naturally to 25°C. It was then discharged at a constant current rate of 0.33C to 2.8V, then charged at a constant current rate of 0.33C to 4.5V, then charged at a constant voltage rate until the current was less than or equal to 0.05C, and finally discharged at a constant current rate of 0.33C to 2.8V. The discharge capacity of the lithium-ion secondary battery after 15 days of high-temperature storage was measured. Then place it in a 60℃ oven until the capacity retention is below 80%. Repeat the measurement every 15 days.

[0111] Battery capacity retention rate (%) after 60 days of high-temperature storage = Discharge capacity after 60 days of high-temperature storage / Initial discharge capacity × 100%. Draw a line based on the test data to obtain the number of days it takes for the capacity to decay to 80%.

[0112] Fast charging performance test at high and low temperatures

[0113] Sixteen identical lithium-ion batteries were randomly and evenly divided into two groups: a low-temperature group and a high-temperature group. The low-temperature group was placed in a -5°C environment and left to stand for 1 hour to allow the lithium-ion batteries to reach a constant temperature. The high-temperature group was placed in a 65°C environment and left to stand for 1 hour to allow the lithium-ion batteries to reach a constant temperature. Then, the lithium-ion batteries in both groups were cycled at 3C. After the cycle was completed, the lithium batteries were disassembled, and the lithium deposition window of the lithium-ion batteries was determined by comparing the lithium deposition on the negative electrode (i.e., the anode).

[0114] In the test results, the less severe the lithium plating and the fewer the lithium plating areas, the stronger the fast charging capability at high or low temperatures. Specifically, the fast charging capability is ranked from worst to best as follows: severe lithium plating on the entire anode surface < severe lithium plating on the entire anode surface < moderate lithium plating on the entire anode surface < slight lithium plating on the entire anode surface < very slight lithium plating on the entire anode surface < severe local lithium plating on the anode < moderate local lithium plating on the anode < slight local lithium plating on the anode < no lithium plating on the anode, corresponding to "9", "8", "7", "6", "5", "4", "3", "2", "1", and "0" respectively.

[0115] Example 1-1

[0116] (I) Preparation of lithium-ion batteries

[0117] <Preparation of Electrolyte>

[0118] In an argon-atmosphere glove box with a water content of less than 10 ppm, methyl ethyl carbonate, propyl propionate, and propylene carbonate were used as base solvents. Lithium hexafluorophosphate and a compound with the structure shown in Formula I were added to the base solvents and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass content of lithium hexafluorophosphate was 12.5%, the mass percentage of the compound with the structure shown in Formula I was a%, and the remainder was the base solvent. The mass ratio of methyl ethyl carbonate, propyl propionate, and propylene carbonate was 1:1:1. The mass percentage of the compound with the structure shown in Formula I (a%) and the types of compounds with the structure shown in Formula I are shown in Table 1.

[0119] <Preparation of Negative Electrode Sheets>

[0120] Silicon-based particles (using silicon-carbon composite material), artificial graphite, single-walled carbon nanotubes, carbon black, carbon fiber, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), and carboxymethyl cellulose (CMC) were mixed, and deionized water was added as a solvent to prepare a slurry with a solid content of 45 wt%. After being stirred evenly in a vacuum mixer, the negative electrode slurry was obtained. The negative electrode slurry contained, per 100 parts by weight of solid phase, 32 parts of silicon-based material (silicon-carbon composite material), 1 part of single-walled carbon nanotubes, H parts of carbon black, J parts of carbon fiber, 1.2 parts of styrene-butadiene rubber, 0.5 parts of polyacrylic acid, 0.5 parts of carboxymethyl cellulose, and the balance being artificial graphite. The mass percentage of silicon was K%. The specific values ​​of I, H, J, and K are shown in Table 1. The value of K was controlled by adjusting the amount of silicon-carbon composite material added, with the silicon content in the silicon-carbon composite material being 50% by weight.

[0121] The negative electrode slurry was uniformly coated onto one surface of a 6μm thick negative electrode current collector and dried at 120℃ to obtain a negative electrode sheet with a single-sided negative electrode material layer. The above steps were then repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided negative electrode material layer. After drying at 120℃, the sheet was cold-pressed, then cut and had tabs welded to obtain a negative electrode sheet with dimensions of 78mm × 875mm for later use. The coating weight of the single-sided negative electrode material layer was 142mg / 1540mm. 2 <Preparation of the Positive Electrode>

[0122] Lithium cobalt oxide (LiCoO2), a positive electrode active material, Super P, and polyvinylidene fluoride (PVDF), a binder, were mixed in a mass ratio of 97:1.4:1.6. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 75 wt%. The mixture was stirred evenly in a vacuum mixer to obtain the positive electrode slurry. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil current collector and dried at 85°C to obtain a single-sided coated positive electrode sheet with a coating thickness of 110 μm. The above steps were then repeated on the other surface of the aluminum foil to obtain a double-sided coated positive electrode sheet. After coating, the positive electrode sheet was cold-pressed and cut into 74 mm × 867 mm dimensions for later use. The compaction density of the positive electrode layer after cold pressing was 4.15 g / cm³. 3 .

[0123] <Septum>

[0124] A 5 μm thick porous polyethylene (PE) membrane was used as the base membrane. PVDF slurry and inorganic particle slurry (a mass ratio of lamellar boehmite and Al2O3 of 70:30) were coated onto both surfaces of the base membrane and dried to obtain the separator membrane. The coating thickness on each surface of the separator membrane was 3 μm.

[0125] <Preparation of Lithium-ion Batteries>

[0126] The prepared positive electrode, separator, negative electrode, and separator are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator. The electrodes are then wound to obtain the electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 85°C vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, shaping, and capacity testing.

[0127] The following describes the testing methods for various parameters of various embodiments of this application.

[0128] Examples 1-2 to Examples 1-49

[0129] Except for adjusting the values ​​of a, H, I, J, K, H+I+J, (H+I+J) / K according to Table 1, and the specific types of compounds with the structure shown in Formula I, the rest are basically the same as in Examples 1-1.

[0130] Furthermore, the following adjustments were made in the following embodiments:

[0131] In Examples 1-19, the silicon-based particles are made of silicon-oxygen composite material, and the value of K is controlled by adjusting the amount of silicon-oxygen composite material added. The mass percentage of silicon in the silicon-oxygen composite material is 50%.

[0132] In Examples 1-20, the silicon-based particles are made of silicon alloy material, and the value of K is controlled by adjusting the amount of silicon alloy material added. The mass percentage of silicon element in the silicon alloy material is 50%.

[0133] In Examples 1-21, the silicon-based particles used were pure silicon materials, and the value of K was controlled by adjusting the amount of pure silicon material added. The mass percentage of silicon in the pure silicon material was 99.9%.

[0134] In Examples 1-22, the silicon-based particles are made of silicon-carbon composite material and silicon-oxygen composite material, and the mass ratio of silicon-carbon composite material to silicon-oxygen composite material is 1:1. The value of K is controlled by adjusting the amount of silicon-carbon composite material and silicon-oxygen composite material added. The mass percentage of silicon element in silicon-carbon composite material is 50%, and the mass percentage of silicon element in silicon-oxygen composite material is 50%.

[0135] In Examples 1-23, the silicon-based particles are made of silicon-carbon composite material, silicon-oxygen composite material, and silicon alloy material, and the mass ratio of the silicon-carbon composite material, silicon-oxygen composite material, and silicon alloy material is 1:1:1. The value of K is controlled by adjusting the amount of silicon-carbon composite material, silicon-oxygen composite material, and silicon alloy material added. The mass percentage of silicon element in the silicon-carbon composite material is 50%, the mass percentage of silicon element in the silicon-oxygen composite material is 50%, and the mass percentage of silicon element in the silicon alloy material is 50%.

[0136] In Examples 1-24, the silicon-based particles were made of silicon-carbon composite material, silicon-oxygen composite material, silicon alloy material, and pure silicon material, with the mass ratio of the added silicon-carbon composite material, silicon-oxygen composite material, silicon alloy material, and pure silicon material being 1:1:1:1. The value of K was controlled by adjusting the added amounts of silicon-carbon composite material, silicon-oxygen composite material, silicon alloy material, and pure silicon material. The mass percentage of silicon in the silicon-carbon composite material was 50%, the mass percentage of silicon in the silicon-oxygen composite material was 50%, the mass percentage of silicon in the silicon alloy material was 50%, and the mass percentage of silicon in the pure silicon material was 99.9%.

[0137] In Examples 1-49, compounds with the structure shown in Formula I-1 and compounds with the structure shown in Formula I-25 were added. Based on the mass of the electrolyte, the total mass content of compounds with the structure shown in Formula I-1 and compounds with the structure shown in Formula I-25 was 10%, and the mass ratio of compounds with the structure shown in Formula I-1 to compounds with the structure shown in Formula I-25 was 1:1.

[0138] Comparative Examples 1 to 5

[0139] Except for adjusting the values ​​of a, H, I, J, K, H+I+J, and the ratio of (H+I+J) / K according to Table 1, the rest are basically the same as in Example 1-1.

[0140] In Comparative Example 5, no compound with the structure shown in Formula I was added, but a dialkyl phosphate (i.e., trimethyl phosphate) was added; the mass content of trimethyl phosphate was 10% based on the mass of the electrolyte.

[0141] Table 1

[0142]

[0143] Note: In Table 1, " / " indicates that the corresponding substance or parameter does not exist.

[0144] Referring to Table 1, it can be seen from Examples 1-1 to 1-49 and Comparative Examples 1 to 5 that the electrolyte includes the compound with the structure shown in Formula I. By adjusting the values ​​of a (0.5≤a≤30), H+I+J (0.1≤H+I+J≤2) and (H+I+J) / K (0.006≤(H+I+J) / K≤0.04) in the negative electrode material layer within the scope of this application, the secondary battery can have a higher cycle capacity retention rate (≥581 cycles when capacity is below 80%), a higher capacity retention rate after 60 days of high-temperature storage (≥91.1%), and a lower degree of high and low temperature lithium plating (≤6 levels). This indicates that the secondary battery is superior in terms of high-temperature storage, cycle performance, and high and low temperature fast charging performance. The a value of Comparative Example 1 is too small (0.01%), the a value of Comparative Example 2 is too large (40%), the negative electrode material layer of Comparative Example 3 does not contain carbon black, single-walled carbon nanotubes, or carbon fibers (H+I+J=0), the H+I+J value of Comparative Example 4 is too large (3.5) and the (H+I+J) / K value is too large (0.117), and Comparative Example 5 uses traditional phosphate ester additives. Its secondary battery capacity is less than 80%, the number of cycles is reduced, the capacity retention rate after 60 days of high-temperature storage is reduced, and the degree of high and low temperature lithium plating (>6 levels) is poor. This shows that when at least one of a, H+I+J, and (H+I+J) / K is not within the scope of this application, or when Formula I compound is not used, the high-temperature storage performance, cycle performance, and high and low temperature fast charging performance of the secondary battery are significantly deteriorated.

[0145] The content 'a' of the compound with the structure shown in Formula I typically affects the high-temperature storage performance, cycle performance, and high / low temperature fast charging performance of secondary batteries. As can be seen from Examples 1-1 to 1-9, by controlling the value of 'a' within the range of 0.5% to 30%, the secondary batteries exhibit higher cycle capacity retention (≥581 cycles when capacity is below 80%), higher capacity retention after 60 days of high-temperature storage (≥91.1%), and lower lithium plating levels at high and low temperatures (≤6 levels). This indicates that the secondary batteries are superior in terms of high-temperature storage, cycle performance, and high / low temperature fast charging performance.

[0146] The total mass percentage (H+I+J) of carbon black, single-walled carbon nanotubes, and carbon fibers in the negative electrode material layer, and their ratio to the mass percentage of silicon ((H+I+J) / K), typically affect the high-temperature storage performance, cycle performance, and high / low temperature fast-charging performance of the secondary battery. As seen in Examples 1-10 to 1-18, adjusting the value of (H+I+J)% to 0.1% to 2% and the value of (H+I+J) / K to 0.006 to 0.04 results in a secondary battery exhibiting higher cycle capacity retention (≥586 cycles with capacity below 80%), higher capacity retention after 60 days of high-temperature storage (≥92.0%), and lower lithium plating levels at high and low temperatures (≤6 levels). This indicates that the secondary battery is superior in terms of high-temperature storage, cycle performance, and high / low temperature fast-charging performance.

[0147] The type of compound with the structure shown in Formula I typically affects the high-temperature storage performance, cycle performance, and high / low temperature fast charging performance of secondary batteries. As can be seen from Examples 1-25 to 1-48, by selecting different types of compounds with the structure shown in Formula I (such as Formulas I-1 to I-25), the secondary batteries exhibit higher cycle capacity retention (≥590 cycles with capacity below 80%), higher capacity retention after 60 days of high-temperature storage (≥92.7%), and lower lithium plating levels at high and low temperatures (≤5 levels). This demonstrates that when the type of compound with the structure shown in Formula I is within the scope of this application, the secondary battery exhibits superior performance in high-temperature storage, cycle performance, and high / low temperature fast charging performance.

[0148] Examples 2-1 to 2-16

[0149] <Preparation of Electrolyte>

[0150] In an argon atmosphere glove box with a water content of less than 10 ppm, methyl ethyl carbonate, propyl propionate, and propylene carbonate are used as base solvents. Lithium hexafluorophosphate, a compound with the structure shown in Formula I-1, and fluoroethylene carbonate are added to the base solvents and mixed evenly to obtain an electrolyte.

[0151] Based on the mass of the electrolyte, the mass content of lithium hexafluorophosphate is 12.5%, the mass percentage of the compound with the structure shown in Formula I-1 is a%, the mass content of fluoroethylene carbonate is b%, and the remainder is the base solvent, with the mass ratio of methyl ethyl carbonate, propyl propionate, and propylene carbonate being 1:1:1.

[0152] Wherein, the mass percentage of the compound with the structure shown in Formula I-1 is a%, the mass content of fluoroethylene carbonate is b%, and the ratio of b / a is adjusted according to Table 2.

[0153] The rest is basically the same as in Example 1-1.

[0154] Table 2

[0155]

[0156] Note: In Table 2, " / " indicates that the corresponding substance or parameter does not exist.

[0157] The electrolyte comprises a compound with the structure shown in Formula I and fluoroethylene carbonate. Adjusting the mass content b% (5 ≤ b ≤ 25) of the fluoroethylene carbonate and the mass ratio b / a (1 ≤ b / a ≤ 40) of the fluoroethylene carbonate to the first substance (the compound with the structure shown in Formula I) typically affects the high-temperature storage performance, cycle performance, and high / low temperature fast charging performance of the secondary battery. Referring to Table 2, from Examples 2-1 to 2-14, it can be seen that when the electrolyte comprises fluoroethylene carbonate and the values ​​of b (5 ≤ b ≤ 25) and b / a (1 ≤ b / a ≤ 40) are adjusted within the scope of this application, the secondary battery exhibits higher cycle capacity retention (≥600 cycles when capacity is below 80%), higher capacity retention after 60 days of high-temperature storage (≥91.5%), and lower high / low temperature lithium plating degree (≤6 levels). This indicates that the secondary battery is superior in terms of high-temperature storage, cycle performance, and high / low temperature fast charging performance. In Examples 2-15, b / a = 0.1 (less than 1), and in Examples 2-16, b / a = 100 (more than 40). The number of cycles decreased (≤586), the capacity retention rate decreased after 60 days of high-temperature storage (≤89%), and the degree of lithium plating at high and low temperatures increased (≥8 levels). This further verifies the necessity of b and b / a within the scope of this application.

[0158] The mass content (b%) of fluoroethylene carbonate typically affects the high-temperature storage performance, cycle performance, and high / low temperature fast-charging performance of secondary batteries. Examples 2-1, 2-3, 2-5, 2-6, and 2-11 show that when the value of b% is controlled within the range of 5 ≤ b ≤ 25, the secondary batteries exhibit higher cycle capacity retention (≥608 cycles with capacity below 80%), higher capacity retention after 60 days of high-temperature storage (≥91.5%), and lower lithium plating levels at high and low temperatures (≤6 levels). This indicates that the secondary batteries are superior in terms of high-temperature storage, cycle performance, and high / low temperature fast-charging performance.

[0159] The mass ratio b / a of fluoroethylene carbonate to the first substance typically affects the high-temperature storage performance, cycle performance, and high / low temperature fast charging performance of secondary batteries. Examples 2-1, 2-2, 2-4, 2-7, and 2-8 show that by adjusting the b / a value within the range of 1 ≤ b / a ≤ 40, the secondary batteries exhibit higher cycle capacity retention (≥600 cycles when capacity is below 80%), higher capacity retention after 60 days of high-temperature storage (≥91.5%), and lower lithium plating levels at high and low temperatures (≤6 levels). This indicates that the secondary batteries are superior in terms of high-temperature storage, cycle performance, and high / low temperature fast charging performance. When b / a is less than 1 (e.g., b / a = 0.1 in Examples 2-15), there is a relative deficiency of fluoroethylene carbonate, resulting in poor interfacial film stability, deteriorated cycle performance, and a significant reduction in the number of cycles when the capacity is below 80%, and a high degree of lithium plating at high and low temperatures. When b / a is greater than 40 (e.g., b / a = 100 in Examples 2-16), an excess of fluoroethylene carbonate leads to an increase in electrolyte viscosity, hindered ion diffusion, and a significant decrease in capacity retention after 60 days of high-temperature storage.

[0160] Examples 3-1 to 3-11

[0161] <Preparation of Electrolyte>

[0162] In an argon-atmosphere glove box with a water content of less than 10 ppm, methyl ethyl carbonate, propyl propionate, and propylene carbonate were used as base solvents. Lithium hexafluorophosphate, a compound with the structure shown in Formula I-1, fluoroethylene carbonate, and a sulfur-containing lithium salt were added to the base solvents and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass content of lithium hexafluorophosphate was e%, the mass percentage of the compound with the structure shown in Formula I-1 was 10%, the mass content of fluoroethylene carbonate was 15%, the mass content of the sulfur-containing lithium salt was f%, and the remainder consisted of methyl ethyl carbonate, propyl propionate, and propylene carbonate, with a mass ratio of methyl ethyl carbonate, propyl propionate, and propylene carbonate of 1:1:1. The values ​​of e, f, and e / f, as well as the types of sulfur-containing lithium salts, were adjusted according to Table 3.

[0163] The rest is basically the same as in Example 2-1.

[0164] Table 3

[0165]

[0166] Note: In Table 3, " / " indicates that the corresponding substance or parameter does not exist.

[0167] Referring to Table 3, it can be seen from Examples 3-1 to 3-8 and Comparative Examples 3-9 that the electrolyte includes lithium hexafluorophosphate and sulfur-containing lithium salts, and by adjusting the e / f value (1≤e / f≤5) within the range of this application, the secondary battery can exhibit higher cycle capacity retention (≥605 cycles with capacity below 80%), higher capacity retention after 60 days of high-temperature storage (≥93.4%), and lower lithium plating at high and low temperatures (≤5 levels). This indicates that the secondary battery is superior in terms of high-temperature storage performance, cycle performance, and high and low temperature fast charging performance. The e / f value of Example 3-10 is too small (0.50), resulting in a significant decrease in the number of cycles with capacity below 80% and a significantly reduced capacity retention after 60 days of high-temperature storage. The e / f value of Example 3-11 is too large (10.00), resulting in a significant decrease in the number of cycles with capacity below 80% and poor performance in both high and low temperature lithium plating (≥7 levels). This indicates that when e / f is not within the scope of this application, at least some of the high-temperature storage performance, cycle performance, and high and low temperature fast charging performance of the secondary battery are significantly degraded.

[0168] The mass ratio (e / f) of lithium hexafluorophosphate to sulfur-containing lithium salt typically affects the high-temperature storage performance, cycle performance, and high / low temperature fast-charging performance of secondary batteries. Examples 3-1 to 3-8 show that by adjusting the e / f value within the range of 1 to 5, the secondary batteries exhibit higher cycle capacity retention (≥605 cycles with capacity below 80%), higher capacity retention after 60 days of high-temperature storage (≥93.4%), and lower lithium plating levels at high and low temperatures (≤5 levels). This indicates that the secondary batteries are superior in terms of high-temperature storage performance, cycle performance, and high / low temperature fast-charging performance.

[0169] The type of sulfur-containing lithium salt generally has little impact on the high-temperature storage performance, cycle performance, and high / low temperature fast charging performance of secondary batteries. As can be seen from Examples 3-8 to 3-9, when lithium bis(trifluoromethanesulfonyl)imide or a mixture of lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide are used as the sulfur-containing lithium salt, the cycle capacity retention rate (≥610 cycles when capacity is below 80%), the capacity retention rate after 60 days of high-temperature storage (≥93.5%), and the degree of lithium plating at high and low temperatures (≤4 levels) of the secondary battery are basically consistent. This indicates that when the type of sulfur-containing lithium salt is varied within the scope of this application, the high-temperature storage performance, cycle performance, and high / low temperature fast charging performance of the secondary battery remain stable.

[0170] Examples 4-1 to 4-14

[0171] <Preparation of Electrolyte>

[0172] In an argon-atmospheric glove box with a water content of less than 10 ppm, methyl ethyl carbonate, propyl propionate, and propylene carbonate were used as base solvents. Lithium hexafluorophosphate, a compound with the structure shown in Formula I-1, fluoroethylene carbonate, lithium difluorosulfonyl imide, and boron-containing lithium salts were added to the base solvents and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass content of lithium hexafluorophosphate was e%, the mass percentage of the compound with the structure shown in Formula I-1 was 10%, the mass content of fluoroethylene carbonate was 15%, the mass content of lithium difluorosulfonyl imide was f%, the mass content of boron-containing lithium salts was g%, and the remainder consisted of methyl ethyl carbonate, propyl propionate, and propylene carbonate, with a mass ratio of methyl ethyl carbonate, propyl propionate, and propylene carbonate of 1:1:1. The values ​​of e, f, e / f, g, and the type of boron-containing lithium salt were adjusted according to Table 4.

[0173] The rest is basically the same as in Example 2-1.

[0174] Table 4

[0175]

[0176] Note: In Table 4, " / " indicates that the corresponding substance or parameter does not exist.

[0177] The electrolyte, including boron-containing lithium salts, and the values ​​of g and e / f typically affect the cycle performance, high-temperature storage performance, and high / low temperature fast charging performance of secondary batteries. Referring to Table 4, from Examples 4-1 to 4-14, it can be seen that when the electrolyte includes boron-containing lithium salts, further adjusting the values ​​of g (0.1≤g≤5) and e / f (1≤e / f≤5) within the range of this application results in secondary batteries exhibiting higher cycle counts below 80% capacity (≥605), higher capacity retention after 60 days of high-temperature storage (≥93.5%), and lower lithium plating levels at high and low temperatures (≤5 levels). This indicates that the secondary batteries are superior in terms of cycle performance, high-temperature storage stability, and high / low temperature fast charging capability.

[0178] The type of boron-containing lithium salt typically affects the cycle performance, high-temperature storage performance, and high / low temperature fast charging performance of secondary batteries. As can be seen from Examples 4-1 to 4-10, secondary batteries using boron-containing lithium salts (lithium tetrafluoroborate, lithium difluorooxalate borate, and lithium dioxalate borate) within the scope of this application exhibit higher cycle counts with less than 80% capacity (≥605), higher capacity retention after 60 days of high-temperature storage (≥93.5%), and lower lithium plating levels at high and low temperatures (≤5 levels). This indicates that the secondary batteries are superior in terms of cycle performance, high-temperature storage stability, and high / low temperature fast charging capability.

[0179] When the electrolyte simultaneously contains boron-containing lithium salt, lithium hexafluorophosphate, and sulfur-containing lithium salt, and the mass ratio of lithium hexafluorophosphate to sulfur-containing lithium salt is adjusted to satisfy 1≤e / f≤5 and the mass content of boron-containing lithium salt is satisfied to satisfy 0.1≤g≤5, the secondary battery exhibits superior overall performance.

[0180] In terms of cycle performance, the number of cycles with capacity below 80% was significantly improved to 615-616, which is higher than Example 2-1 without boron-containing lithium salt and sulfur-containing lithium salt;

[0181] Regarding high-temperature storage stability, the capacity retention rate reached 93.5%-93.7% after 60 days of high-temperature storage, which is slightly better than Example 2-1;

[0182] In terms of high and low temperature fast charging performance, the degree of lithium plating at low temperature was level 3, which was significantly better than that of Example 2-1, indicating that the interface ion conduction efficiency and reaction uniformity were further improved.

[0183] It should be noted that, in this document, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0184] 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.

[0185] The above are merely embodiments of this application and are not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. 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 an electrolyte and a negative electrode, characterized in that, The electrolyte includes a first substance, which includes at least one of compounds having the structure shown in Formula I; R1, R2, and R3 are each independently one of the following groups: Fluorinated alkyl groups containing 1 to 4 carbons, fluorinated alkenyl groups containing 2 to 3 carbons, fluorinated alkynyl groups containing 2 to 3 carbons, or non-fluorinated alkyl groups containing 1 to 4 carbons, non-fluorinated alkenyl groups containing 2 to 3 carbons, or non-fluorinated alkynyl groups containing 2 to 3 carbons. Furthermore, among R1, R2, and R3, at least one is a fluorinated alkyl group containing 1 to 4 carbon atoms; Based on the total mass of the electrolyte, the total mass content of the first substance is a%, satisfying: 0.5≤a≤30; The electrolyte comprises a first lithium salt, which includes lithium hexafluorophosphate and a sulfur-containing lithium salt, wherein the sulfur-containing lithium salt includes at least one of lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide; based on the total mass of the electrolyte, the mass content of the lithium hexafluorophosphate is e%, and the mass content of the sulfur-containing lithium salt is f%, 1.33≤e / f≤5; The electrolyte further includes a boron-containing lithium salt, which includes at least one of lithium tetrafluoroborate, lithium difluorooxalate borate, or lithium dioxalate borate; the mass content of the boron-containing lithium salt is 0.1% to 5% based on the total mass of the electrolyte. The negative electrode sheet includes a negative electrode material layer, which includes carbon black, single-walled carbon nanotubes, carbon fibers, and silicon-based materials. The silicon-based material includes at least one of silicon-carbon materials, silicon-oxygen materials, silicon alloy materials, and pure silicon materials.

2. The secondary battery according to claim 1, characterized in that, Based on the mass of the negative electrode material layer, the mass percentage of carbon black is H%, the mass percentage of single-walled carbon nanotubes is I%, the mass percentage of carbon fiber is J%, and the mass percentage of silicon in the negative electrode material layer is K, satisfying at least one of the following conditions: (1) 0.1≤H+I+J≤2; (2)0.006≤(H+I+J) / K≤0.

04.

3. The secondary battery according to claim 1, characterized in that, 1≤a≤15。 4. The secondary battery according to claim 2, characterized in that, The secondary battery satisfies at least one of the following conditions: （1）0.2≤H≤1.0； （2）0.1≤I≤0.5； （3）0.01≤J≤0.2； （4）3≤K≤20。 5. The secondary battery according to any one of claims 1 to 4, characterized in that, The compound with the structure shown in Formula I is selected from at least one of the following compounds: 。 6. The secondary battery according to any one of claims 1 to 3, characterized in that, The electrolyte also contains fluoroethylene carbonate; based on the total mass of the electrolyte, the mass content of the fluoroethylene carbonate is b%, satisfying 5≤b≤25 and 1≤b / a≤40.

7. The secondary battery according to claim 6, characterized in that, 7.5≤b≤25。 8. An electrochemical device, characterized in that, It includes the secondary battery as described in any one of claims 1 to 7.

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