Negative electrode sheet and secondary battery

Abstract

To overcome the problem that the insufficient stability of the SEI film in existing lithium-ion batteries leads to the deterioration of the battery cycle performance and capacity attenuation, the present invention provides a negative electrode sheet, which includes a negative electrode material layer. The negative electrode material layer includes a negative electrode active material containing a carbon-based material and a compound represented by Formula I: #imgabs0# wherein, R1, R2, and R3 are each independently selected from an alkyl group having 1-5 carbon atoms, a fluoroalkyl group having 1-5 carbon atoms, an ether group having 1-5 carbon atoms, a fluoroether group having 1-5 carbon atoms, and an unsaturated hydrocarbon group having 2-5 carbon atoms, and at least one of R1, R2, and R3 is an unsaturated hydrocarbon group having 2-5 carbon atoms; the negative electrode sheet satisfies the following conditions: 0.2 ≤ 10*c*a / b ≤ 40; and 0.005 ≤ a ≤ 1, 10 ≤ b ≤ 50, 92 ≤ c ≤ 98. Meanwhile, the present invention also discloses a secondary battery including the above negative electrode sheet. The negative electrode sheet provided by the present invention has a lower impedance, can improve the penetration performance of the non-aqueous electrolyte into the negative electrode material layer, and effectively improves the cycle performance of the battery.

Description

Technical Field

[0001] The invention belongs to the technical field of energy storage electronic components, and in particular relates to a negative electrode sheet and a secondary battery. Background Art

[0002] At present, compared with traditional batteries such as nickel-cadmium, lead-zinc, etc., which are more polluting chemical power sources, lithium-ion batteries have become an important research topic for scientific researchers and entrepreneurs due to their advantages such as low environmental pollution, high battery capacity, and long service life. With the rapid development of the electric vehicle and portable electronic device industries, the market currently has higher requirements for the safety, energy density, cycle life and other performance of existing lithium-ion batteries.

[0003] Lithium-ion batteries usually use graphite as the negative electrode, which can be further divided into natural graphite and artificial graphite. Both natural graphite and artificial graphite have a crystalline layered structure, which enables the reversible insertion and removal of lithium ions during the charge and discharge process. The graphite material has a distinct charge and discharge platform, and the platform potential is low (0.01-0.2V vs Li), so it can provide a high and stable operating voltage. In addition, graphite has a high specific capacity for lithium insertion, and its lithium insertion compound can reach its theoretical composition LiC6, corresponding to a theoretical specific capacity of 372mAh / g. In practical applications, the specific capacity is mostly more than 300mAh / g, and some are even close to its theoretical specific capacity. However, graphite is sensitive to solvents. Its high crystallinity and highly oriented layered structure make it less compatible with electrolytes. Specifically, some solvents will decompose on the graphite surface, and some solvents will co-embed with lithium ions into the graphite layered structure to cause graphite exfoliation, resulting in reduced lithium insertion performance. With the study of electrolytes, people found that the electrolyte will undergo a reduction reaction on the surface of the graphite negative electrode during the first charge, which will form a passivation film (SEI film) composed of inorganic and organic products of electrolyte decomposition. Under ideal conditions, this passivation film can inhibit the continued decomposition of the electrolyte because it can prevent the transmission of electrons during the battery cycle while allowing lithium ions to pass through. In some solvents such as cycloalkyl carbonates, an effective SEI film can be formed to ensure good cycle stability of the negative electrode. In addition, a certain proportion of Si is often added to the current negative electrode to increase the battery capacity, but the silicon-carbon negative electrode will have a greater expansion and contraction rate during the cycle, resulting in more serious SEI film damage. Therefore, battery performance, irreversible capacity loss, rate performance, cycle performance, graphite exfoliation and battery safety performance are closely related to the quality of the SEI film.

[0004] In order to form a high-performance SEI film on the surface of the negative electrode of the battery, it is currently common to introduce film-forming additives and other functional additives into the electrolyte to form a dense SEI film with good ion conductivity, thereby inhibiting the decomposition of the electrolyte and extending the cycle life. However, the introduction of film-forming additives and functional additives into the electrolyte generally increases the impedance of lithium-ion batteries and reduces power and low-temperature performance. Summary of the invention

[0005] In view of the problem that the SEI film stability of existing lithium-ion batteries is insufficient, resulting in degradation of battery cycle performance and capacity decay, the present invention provides a negative electrode sheet and a secondary battery.

[0006] The technical solution adopted by the present invention to solve the above technical problems is as follows:

[0007] In one aspect, the present invention provides a negative electrode sheet, comprising a negative electrode material layer, wherein the negative electrode material layer comprises a negative electrode active material comprising a carbon-based material and a compound represented by formula I;

[0008]

[0009] Among them, R 1 , R 2 , R 3 are each independently selected from an alkyl group of 1 to 5 carbon atoms, a fluoroalkyl group of 1 to 5 carbon atoms, an ether group of 1 to 5 carbon atoms, a fluoroether group of 1 to 5 carbon atoms, and an unsaturated hydrocarbon group of 2 to 5 carbon atoms, and R 1 , R 2 , R 3 At least one of them is an unsaturated hydrocarbon group of 2 to 5 carbon atoms;

[0010] The negative electrode sheet meets the following conditions:

[0011] 0.2≤10*c*a / b≤40;

[0012] And 0.005≤a≤1, 10≤b≤50, 92≤c≤98;

[0013] Wherein, c is the mass percentage of the negative electrode active material in the negative electrode material layer, in %;

[0014] a is the mass percentage of the compound represented by formula I in the negative electrode material layer, in %;

[0015] b is the porosity of the negative electrode material layer, in %;

[0016] The resistivity of the negative electrode sheet is ≤200Ω·m. The solution obtained after ultrasonic oscillation of the negative electrode sheet in a solvent is subjected to liquid chromatography-mass spectrometry (LC-MS) analysis, and a characteristic peak appears in the region of retention time of 6.5min to 7.5min.

[0017] Optionally, the negative electrode sheet meets the following conditions:

[0018] 2≤10*c*a / b≤10.

[0019] Optionally, the mass percentage c of the negative electrode active material in the negative electrode material layer is 94% to 96%.

[0020] Optionally, the mass percentage a of the compound represented by formula I in the negative electrode material layer is 0.05% to 0.3%.

[0021] Optionally, the porosity b of the negative electrode material layer is 20% to 40%.

[0022] Optionally, the carbon-based material includes one or more of graphite, hard carbon, soft carbon, graphene, and mesophase carbon microbeads.

[0023] Optionally, the negative electrode active material further includes a silicon-based material, and the negative electrode sheet meets the following conditions:

[0024] 0.05≤m / b≤2;

[0025] And 10≤b≤50, 1≤m≤30;

[0026] Wherein, m is the mass percentage of the silicon-based material in the negative electrode material layer, in %;

[0027] b is the porosity of the negative electrode material layer, in %.

[0028] Optionally, the alkyl group of 1 to 5 carbon atoms is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, sec-pentyl or neopentyl; the fluoroalkyl group of 1 to 5 carbon atoms is selected from a group obtained by replacing one or more hydrogen elements in the alkyl group of 1 to 5 carbon atoms with fluorine elements;

[0029] The unsaturated hydrocarbon group of 2 to 5 carbon atoms is selected from vinyl, propenyl, allyl, butenyl, pentenyl, methylvinyl, methylallyl, ethynyl, propynyl, propargyl, butynyl or pentynyl;

[0030] The ether group of 1 to 5 carbon atoms is selected from methyl ether, ethyl ether, methyl ethyl ether, propyl ether, methyl propyl ether or ethyl propyl ether;

[0031] The fluorinated ether group of 1 to 5 carbon atoms is selected from fluoromethyl ether, fluoroethyl ether, fluoromethylethyl ether, fluoropropyl ether, fluoromethylpropyl ether or fluoroethylpropyl ether.

[0032] Optionally, the compound represented by formula I is selected from at least one of tripropargyl phosphate, dipropargyl methyl phosphate, dipropargyl fluoromethyl phosphate, dipropargyl methoxymethyl phosphate, dipropargyl ethyl phosphate, dipropargyl propyl phosphate, trifluoromethyl dipropargyl phosphate, dipropargyl 2,2,2-trifluoroethyl phosphate, dipropargyl 3,3,3-trifluoropropyl phosphate, hexafluoroisopropyl dipropargyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, trifluoromethyl diallyl phosphate, dipropargyl methyl ether phosphate, dipropargyl fluoromethyl ether phosphate, 2,2,2-trifluoroethyl diallyl phosphate, diallyl 3,3,3-trifluoropropyl phosphate or diallyl hexafluoroisopropyl phosphate.

[0033] In another aspect, the present invention provides a secondary battery comprising a positive electrode sheet, a non-aqueous electrolyte, and the negative electrode sheet as described above.

[0034] According to the negative electrode sheet provided by the present invention, the compound represented by formula I is innovatively dispersed into the negative electrode slurry, and after coating and drying, the compound represented by formula I and the negative electrode active material are uniformly mixed together. The inventors have found through a large number of studies that by reasonably designing the mass percentage a of the compound represented by formula I in the negative electrode material layer, the porosity b of the negative electrode material layer, and the mass percentage c of the negative electrode active material in the negative electrode material layer, so that the condition 0.2≤10*c*a / b≤40 is satisfied, the compound represented by formula I can fully play the role of in-situ participation in the film formation on the surface of the negative electrode material, so that the battery can quickly repair and reform the damaged SEI film during the cycle process at a high energy density, significantly inhibit the decomposition reaction of the non-aqueous electrolyte on the negative electrode, and at the same time reduce the impedance of the negative electrode sheet, so that the resistivity of the negative electrode sheet is controlled below 200Ω·m, improve the permeability of the non-aqueous electrolyte to the negative electrode material layer, and effectively improve the cycle performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The negative electrode sheet provided by the present invention is tested by a liquid chromatography-mass spectrometer (LC-MS). DETAILED DESCRIPTION

[0036] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0037] An embodiment of the present invention provides a negative electrode sheet, comprising a negative electrode material layer, wherein the negative electrode material layer comprises a negative electrode active material comprising a carbon-based material and a compound shown in Formula I:

[0038]

[0039] Among them, R 1 , R 2 , R 3 are each independently selected from an alkyl group of 1 to 5 carbon atoms, a fluoroalkyl group of 1 to 5 carbon atoms, an ether group of 1 to 5 carbon atoms, a fluoroether group of 1 to 5 carbon atoms, and an unsaturated hydrocarbon group of 2 to 5 carbon atoms, and R 1 , R 2 , R 3 At least one of them is an unsaturated hydrocarbon group of 2 to 5 carbon atoms;

[0040] The negative electrode sheet meets the following conditions:

[0041] 0.2≤10*c*a / b≤40;

[0042] And 0.005≤a≤1, 10≤b≤50, 92≤c≤98;

[0043] Wherein, c is the mass percentage of the negative electrode active material in the negative electrode material layer, in %;

[0044] a is the mass percentage of the compound represented by formula I in the negative electrode material layer, in %;

[0045] b is the porosity of the negative electrode material layer, in %;

[0046] The resistivity of the negative electrode sheet is ≤200Ω·m. The solution obtained after ultrasonic oscillation of the negative electrode sheet in a solvent is subjected to liquid chromatography-mass spectrometry (LC-MS) analysis, and a characteristic peak appears in the region of retention time of 6.5min to 7.5min.

[0047] The inventors have found through extensive research that by reasonably designing the mass percentage a of the compound represented by formula I in the negative electrode material layer, the porosity b of the negative electrode material layer, and the mass percentage c of the negative electrode active material in the negative electrode material layer, so that the condition 0.2≤10*c*a / b≤40 is satisfied, the compound represented by formula I can fully participate in the film formation on the surface of the negative electrode material in situ, so that the battery can quickly repair and reform the damaged SEI film during the cycle at a high energy density, significantly inhibit the decomposition reaction of the non-aqueous electrolyte on the negative electrode, and at the same time reduce the impedance of the negative electrode sheet, so that the resistivity of the negative electrode sheet is controlled below 200Ω·m, improve the permeability of the non-aqueous electrolyte to the negative electrode material layer, and effectively improve the cycle performance of the battery.

[0048] It should be noted that in terms of improving the electrochemical performance of the negative electrode, the mass percentage a of the compound represented by formula I in the negative electrode material layer has a certain correlation with the mass percentage c of the negative electrode active material in the negative electrode material layer and the porosity b of the negative electrode material layer. By adding the compound represented by formula I to the negative electrode material layer, the dispersibility of the negative electrode active material and the negative electrode conductive agent can be improved, thereby further improving the mass percentage c of the negative electrode active material in the negative electrode material layer while ensuring the performance of the electrode sheet, thereby improving the energy density and reducing the impedance of the negative electrode sheet at the battery level; at the same time, during the first charging process of the battery, the compound represented by formula I can participate in the in-situ formation of a chemical layer on the surface of the negative electrode material. The polymer film is stable, electrochemically stable and thermally stable. The SEI film formed is dense and stable, which can reduce the irreversible lithium consumption when forming the SEI film. In the subsequent cycle process, the SEI film can be quickly repaired and reformed again after being damaged, thereby improving the cycle performance of the battery. The SEI film formed by the compound is also denser, thereby reducing the gas production caused by the reaction between the electrolyte and the negative electrode; in particular, the compound shown in formula I can effectively improve the compatibility of the non-aqueous electrolyte and the negative electrode, which is beneficial to the penetration of the non-aqueous electrolyte into the negative electrode material layer, and then by further adjusting the porosity b of the negative electrode material layer and cooperating with the SEI film formed in situ with the compound shown in formula I, the battery can take into account the performance of impedance, cycle and energy density.

[0049] When 10*c*a / b>40, it means that the mass percentage of the compound represented by formula I or the negative electrode active material in the negative electrode material layer is too high or the porosity of the negative electrode sheet is too low. The excessive amount of the compound represented by formula I in the negative electrode material layer will reduce the mass percentage of the active material, resulting in a decrease in the battery energy density and capacity, and the formed SEI film will be too thick, resulting in increased impedance and deterioration of the cycle performance. The excessively high mass percentage of the negative electrode active material will lead to too low content of other auxiliary materials such as adhesives and conductive agents, so that the negative electrode sheet is easy to fall off, break and have too large impedance; the low porosity of the negative electrode sheet will reduce The electrolyte wettability leads to increased battery impedance and deteriorated cycle performance. In addition, too low a porosity of the negative electrode sheet will also affect the in-situ film-forming effect of the compound of formula I, thereby reducing its impedance improvement effect. When 10*c*a / b<0.2, it means that the mass percentage of the compound of formula I or the negative electrode active material in the negative electrode material layer is too low or the porosity of the negative electrode sheet is too high. Too low a content of the compound of formula I cannot exert the improvement effect on the negative electrode sheet. Too low a content of the negative electrode active material or too high a porosity of the negative electrode sheet will lead to too low an energy density, which is not conducive to commercial applications.

[0050] In addition, after the compound represented by formula I is reduced, the free radicals generated by the rupture of PO diffuse to the surface of the positive electrode and are oxidized, and a CEI film with excellent performance can also be formed.

[0051] The method for performing chromatographic analysis of the negative electrode sheet by liquid chromatography-mass spectrometry is as follows: disassembling the battery in a glove box to take out the negative electrode sheet, then immersing the cut negative electrode sheet in a suitable solvent (e.g., DMC, acetonitrile), and performing ultrasonic vibration for a suitable time to dissolve the substance in the negative electrode material layer of the negative electrode sheet into the solvent. Then, the solution is detected by liquid chromatography-mass spectrometry (LC-MS), and a characteristic peak is found in the region with a retention time of 6.5 min to 7.5 min, such as Figure 1 As shown, the model of the liquid chromatography-mass spectrometer is Waters ACQUITY UPLC / Xevo G2-XS Qtof MS, and the chromatographic conditions are: using a Waters T3 chromatographic column, a column temperature of 35-40°C, a mobile phase of a mixture of 40% water and 60% acetonitrile, and a mobile phase flow rate of 0.2-0.3 ml / min.

[0052] In a preferred embodiment, the negative electrode sheet meets the following conditions:

[0053] 2≤10*c*a / b≤10.

[0054] When the mass percentage a of the compound represented by formula I in the negative electrode material layer, the porosity b of the negative electrode material layer and the mass percentage c of the negative electrode active material in the negative electrode material layer meet the above conditions, the energy density of the battery can be further improved while taking into account the cycle performance of the battery.

[0055] In a specific embodiment, the mass percentage c of the negative electrode active material in the negative electrode material layer can be 92%, 92.5%, 93%, 93.5%, 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5% or 98%.

[0056] In a preferred embodiment, the mass percentage c of the negative electrode active material in the negative electrode material layer is 94% to 96%.

[0057] The mass percentage c of the negative electrode active material in the negative electrode material layer is a key parameter that determines the battery capacity. Too little active material content will seriously affect the battery capacity and lead to low energy density, but too high content will make the proportion of auxiliary materials such as binders and conductive agents too small, and the negative electrode material layer will easily fall off, break and distribute unevenly, affecting the processing performance of the negative electrode sheet and increasing the impedance. At the same time, during the battery cycle, the negative electrode is prone to pulverization, resulting in a shortened cycle life.

[0058] In a specific embodiment, the mass percentage a of the compound represented by Formula I in the negative electrode material layer can be 0.005%, 0.008%, 0.01%, 0.02%, 0.04%, 0.08%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95% or 1%.

[0059] In a preferred embodiment, the mass percentage a of the compound represented by formula I in the negative electrode material layer is 0.05% to 0.3%.

[0060] Adding the compound shown in Formula I to the negative electrode material layer can improve the dispersibility of the negative electrode active material and the negative electrode conductive agent, reduce the interface contact impedance of the negative electrode active material, and improve the electronic conductivity; and the dense SEI film formed in situ by the compound shown in Formula I on the surface of the negative electrode active material can effectively inhibit the electrochemical reduction reaction between the non-aqueous electrolyte and the negative electrode, reduce the decomposition products and gases generated by the decomposition of the non-aqueous electrolyte, and improve the reduction resistance and structural stability of the negative electrode active material. At the same time, due to its own phosphorus-containing functional group, the film formation on the surface of the negative electrode material will improve the high temperature resistance and electrochemical stability of the SEI film. At the same time, the compound can improve the dispersibility of the binder and the conductive agent, improve the electronic conductivity of the negative electrode material layer, and help reduce the internal resistance of the negative electrode sheet.

[0061] When the mass percentage a of the compound represented by formula I in the negative electrode material layer is too large, the electrode resistance will increase due to its own low conductivity, and an excessively thick SEI film will be formed in the battery, the membrane impedance will increase and affect the battery's cycle and power performance. When the mass percentage a of the compound represented by formula I in the negative electrode material layer is too small, the compound will not be able to exert its improvement effect on the electrode and the battery as a whole.

[0062] In a specific embodiment, the porosity b of the negative electrode material layer may be 10%, 13%, 15%, 18%, 20%, 23%, 25%, 28%, 30%, 33%, 35%, 38% or 40%.

[0063] In a preferred embodiment, the porosity b of the negative electrode material layer is 20% to 40%.

[0064] The porosity b of the negative electrode material layer represents the percentage of the pore volume in the negative electrode material layer to the total volume of the negative electrode material layer. The porosity of the negative electrode material layer can be obtained by testing with a porosity measuring instrument. As an example, a certain mass of negative electrode sheet sample is weighed, and the thickness of the sample is measured using a micrometer. The apparent volume V1 of the sample is obtained according to the surface area and thickness of the sample. The sample is placed in an AccuPycⅡ1340 fully automatic true density tester, a closed test system, and nitrogen is introduced according to the program. The gas pressure in the sample chamber and the expansion chamber is detected, and then the real volume V2 is calculated according to Boyle's law (PV=C, where P is the pressure of the gas, V is the volume of the gas, and C is a constant). Then, the porosity b of the negative electrode active material layer is obtained according to b=(V1-V2) / V1×100%.

[0065] The porosity of the electrode is the main factor affecting the wettability of the electrolyte. The higher the porosity, the more electrolyte will be retained in the negative electrode, thereby improving the interfacial conduction efficiency of lithium ions inside the negative electrode. Therefore, high porosity can reduce the initial impedance of the battery and the irreversible capacity loss of the battery during the cycle. In addition, the battery will have continuous SEI damage and repair during the cycle. Keeping enough electrolyte in the electrode is conducive to the repair of the SEI membrane and extend the battery cycle life. However, the excessive porosity of the negative electrode will obviously lead to a decrease in the content of negative active materials in the negative electrode material layer per unit volume, thereby affecting the energy density of the battery. Therefore, if the porosity b of the negative electrode material layer is too high, the energy density of the battery will be reduced at the battery level. In addition, it will cause the contact between the negative electrode conductive agent and the negative electrode active material in the negative electrode sheet to deteriorate, thereby increasing the sheet resistance. Although the porosity b of the negative electrode material layer that is too low can improve the contact between the active material and the conductive agent, thereby increasing the electronic conductivity of the electrode sheet and reducing the sheet resistance, it will cause difficulty in electrolyte infiltration and reduce the ion conductivity in the negative electrode material layer, thereby increasing the battery impedance, increasing polarization and deteriorating the cycle performance.

[0066] In some embodiments, the carbon-based material includes one or more of graphite, hard carbon, soft carbon, graphene, and mesocarbon microbeads.

[0067] The graphite includes artificial graphite or natural graphite.

[0068] In some embodiments, the negative electrode active material further comprises a silicon-based material, and the negative electrode sheet satisfies the following conditions:

[0069] 0.05≤m / b≤2;

[0070] And 10≤b≤50, 1≤m≤30;

[0071] Wherein, m is the mass percentage of the silicon-based material in the negative electrode material layer, in %;

[0072] b is the porosity of the negative electrode material layer, in %.

[0073] As negative electrode active materials, silicon-based materials have higher gram capacity than conventional carbon-based materials. Therefore, by doping a certain amount of silicon-based materials in the negative electrode active materials, the capacity and energy density of the battery can be significantly improved. However, silicon-based materials have a greater volume change rate during the cycle, resulting in more serious SEI film damage and electrolyte side reactions on the negative electrode. Excessive silicon-based materials in the negative electrode material layer will seriously deteriorate the cycle performance and easily produce gas. By adjusting the porosity of the negative electrode material layer to provide the buffer space required for the volume change of the silicon-based material, and adding the compound shown in Formula I to the negative electrode material layer, the problems of SEI film damage and side reactions caused by the addition of silicon-based materials can be alleviated to a certain extent. At the same time, through the restriction of the above relationship 0.05≤m / b≤2, the energy density advantage and the improvement of battery cycle stability can be effectively combined to obtain a negative electrode sheet with high energy density and excellent cycle performance. When m / b<0.05, the content of silicon-based materials in the negative electrode active material layer is too small or the porosity of the negative electrode sheet is too large. Too little silicon-based materials cannot play their advantages, and too large porosity will seriously affect the initial capacity and energy density of the battery; when m / b>2, the content of silicon-based materials in the negative electrode active material layer is too large or the porosity of the negative electrode sheet is too small. The more silicon-based materials there are, the greater the volume change rate of the battery during the cycle, which makes the damage and reforming of the negative electrode SEI film more serious, the side reactions are more, and the capacity decay is faster. If the porosity is too small, the battery impedance and polarization will increase, the electrolyte infiltration will be difficult, and the in-situ film-forming effect of the compound of formula I cannot be fully exerted, resulting in a significant deterioration in the cycle performance. At the same time, too low a porosity will make the negative electrode sheet more sensitive to the high volume change rate of the silicon-based material and the performance will deteriorate rapidly during the cycle.

[0074] In a specific embodiment, the mass percentage m of the silicon-based material in the negative electrode material layer can be 1%, 2%, 5%, 7%, 8%, 10%, 13%, 15%, 18%, 20%, 23%, 25%, 28% or 30%.

[0075] In a preferred embodiment, the mass percentage m of the silicon-based material in the negative electrode material layer is 3% to 15%.

[0076] In some implementations, the silicon-based material includes one or more of a silicon material, a silicon oxide, a silicon-carbon composite material, and a silicon alloy material.

[0077] In a preferred embodiment, the silicon material includes one or more of silicon nanoparticles, silicon nanowires, silicon nanotubes, silicon thin films, 3D porous silicon, and hollow porous silicon.

[0078] The silicon oxide includes silicon monoxide.

[0079] In some embodiments, the negative electrode active material further comprises at least one of a lithium-based material and a tin-based material.

[0080] The lithium-based material includes one or more of metallic lithium or lithium alloys. The lithium alloy may be at least one of lithium silicon alloy, lithium sodium alloy, lithium potassium alloy, lithium aluminum alloy, lithium tin alloy and lithium indium alloy. The tin-based material includes one or more of tin, tin carbon, tin oxygen and tin metal compounds.

[0081] In some embodiments, the negative electrode material layer further includes a negative electrode binder and a negative electrode conductive agent, and the negative electrode active material, the negative electrode binder and the negative electrode conductive agent are blended to obtain the negative electrode material layer.

[0082] The negative electrode binder includes polyvinylidene fluoride, copolymers of vinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride-hexafluoropropylene, copolymers of tetrafluoroethylene-hexafluoropropylene, copolymers of tetrafluoroethylene-perfluoroalkyl vinyl ether, copolymers of ethylene-tetrafluoroethylene, copolymers of vinylidene fluoride-tetrafluoroethylene, copolymers of vinylidene fluoride-trifluoroethylene, copolymers of vinylidene fluoride-trichloroethylene, copolymers of vinylidene fluoride-fluoroethylene, copolymers of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, thermoplastic resins such as polyethylene and polypropylene; acrylic resin; sodium hydroxymethyl cellulose; polyvinyl butyral; ethylene-vinyl acetate copolymer; polyvinyl alcohol; and one or more of styrene butadiene rubber.

[0083] The negative electrode conductive agent includes one or more of conductive carbon black, conductive carbon balls, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene or reduced graphene oxide.

[0084] In some embodiments, the compound represented by Formula I is formed on the surface of the negative electrode material layer, or the compound represented by Formula I is mixed in the interior of the negative electrode material layer.

[0085] When the compound represented by formula I is formed on the surface of the negative electrode material layer, the preparation method thereof can refer to the following method:

[0086] A coating containing the compound shown in Formula I is formed on the surface of the negative electrode material layer by surface coating. Specifically, the negative electrode active material, the negative electrode conductive agent and the negative electrode binder are first dispersed in an organic solvent to prepare a negative electrode slurry. After the negative electrode slurry is coated and dried to form a negative electrode material layer, the compound shown in Formula I is dispersed in an organic solvent, and the obtained solution of the compound shown in Formula I is sprayed on the surface of the negative electrode material layer. After drying and removing the solvent, the negative electrode material layer including the compound shown in Formula I is obtained.

[0087] When the compound represented by formula I is mixed in the negative electrode material layer, its preparation method can refer to the following method:

[0088] Method 1: The negative electrode slurry for preparing the negative electrode material layer contains the compound represented by formula I. Specifically, the compound represented by formula I, the negative electrode active material, the negative electrode conductive agent and the negative electrode binder are dispersed in an organic solvent to prepare the negative electrode slurry, and then the negative electrode slurry is coated and dried to form the negative electrode material layer;

[0089] Method 2: After preparing the negative electrode material layer, immerse the negative electrode material layer in a solution containing the compound represented by formula I to allow the compound represented by formula I to penetrate into the interior of the negative electrode material layer, and dry and remove the solvent to obtain a negative electrode material layer containing the compound represented by formula I.

[0090] In some embodiments, the negative electrode sheet further includes a negative electrode current collector, and the negative electrode material layer is formed on a surface of the negative electrode current collector.

[0091] The negative electrode current collector is selected from metal materials that can conduct electrons. Preferably, the negative electrode current collector includes one or more of Al, Ni, tin, copper, and stainless steel. In a more preferred embodiment, the negative electrode current collector is selected from copper foil.

[0092] In the present invention, the alkyl group of 1 to 5 carbon atoms is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, sec-pentyl or neopentyl; the fluoroalkyl group of 1 to 5 carbon atoms is selected from the group obtained by replacing one or more hydrogen elements in the alkyl group of 1 to 5 carbon atoms with fluorine elements;

[0093] The unsaturated hydrocarbon group of 2 to 5 carbon atoms is selected from vinyl, propenyl, allyl, butenyl, pentenyl, methylvinyl, methylallyl, ethynyl, propynyl, propargyl, butynyl or pentynyl;

[0094] The ether group of 1 to 5 carbon atoms is selected from methyl ether, ethyl ether, methyl ethyl ether, propyl ether, methyl propyl ether or ethyl propyl ether;

[0095] The fluorinated ether group of 1 to 5 carbon atoms is selected from fluoromethyl ether, fluoroethyl ether, fluoromethylethyl ether, fluoropropyl ether, fluoromethylpropyl ether or fluoroethylpropyl ether.

[0096] In a preferred embodiment, the compound represented by formula I is selected from at least one of tripropargyl phosphate, dipropargyl methyl phosphate, dipropargyl fluoromethyl phosphate, dipropargyl methoxymethyl phosphate, dipropargyl ethyl phosphate, dipropargyl propyl phosphate, trifluoromethyl dipropargyl phosphate, dipropargyl 2,2,2-trifluoroethyl phosphate, dipropargyl 3,3,3-trifluoropropyl phosphate, hexafluoroisopropyl dipropargyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, trifluoromethyl diallyl phosphate, dipropargyl methyl ether phosphate, dipropargyl fluoromethyl ether phosphate, 2,2,2-trifluoroethyl diallyl phosphate, diallyl 3,3,3-trifluoropropyl phosphate or diallyl hexafluoroisopropyl phosphate.

[0097] In a preferred embodiment, the compound represented by formula I is selected from one or more of the following compounds:

[0098]

[0099] It should be noted that the above are only some of the compounds claimed to be protected by the present invention and should not be construed as limiting the present invention.

[0100] Another embodiment of the present invention provides a secondary battery, including a positive electrode sheet, a non-aqueous electrolyte, and the negative electrode sheet as described above.

[0101] In some embodiments, the nonaqueous electrolyte includes a nonaqueous organic solvent and an electrolyte salt.

[0102] In some embodiments, the non-aqueous organic solvent includes one or more of an ether solvent, a nitrile solvent, a carbonate solvent, and a carboxylate solvent.

[0103] In some embodiments, the ether solvent includes a cyclic ether or a chain ether, preferably a chain ether with 3 to 10 carbon atoms and a cyclic ether with 3 to 6 carbon atoms. The cyclic ether can be specifically but not limited to 1,3-dioxolane (DOL), 1,4-dioxane (DX), crown ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH 3 -THF), 2-trifluoromethyltetrahydrofuran (2-CF 3-THF); the chain ether may specifically be, but is not limited to, dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether. Since the chain ether has a high solvation ability with lithium ions and can improve ion dissociation, dimethoxymethane, diethoxymethane, and ethoxymethoxymethane, which have low viscosity and can impart high ionic conductivity, are particularly preferred. The ether compound can be used alone or in any combination and ratio. There is no special restriction on the amount of ether compound added, which is arbitrary within the range that does not significantly damage the effect of the high-density lithium-ion battery of the present invention. In the non-aqueous solvent volume ratio of 100%, the volume ratio is usually 1% or more, preferably 2% or more, and more preferably 3% or more. In addition, the volume ratio is usually 30% or less, preferably 25% or less, and more preferably 20% or less. When two or more ether compounds are used in combination, the total amount of the ether compounds can be within the above range. When the amount of the ether compound added is within the above preferred range, it is easy to ensure the improvement of ion conductivity brought about by the increase in lithium ion dissociation degree of the chain ether and the decrease in viscosity. In addition, when the negative electrode active material is a carbon-based material, the phenomenon of co-embedding of the chain ether and lithium ions can be suppressed, so that the input-output characteristics and the charge-discharge rate characteristics can reach an appropriate range.

[0104] In some embodiments, the nitrile solvent may specifically be, but is not limited to, one or more of acetonitrile, glutaronitrile, and malononitrile.

[0105] In some embodiments, the carbonate solvent includes a cyclic carbonate or a chain carbonate, and the cyclic carbonate can be specifically but not limited to one or more of ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), and butylene carbonate (BC); the chain carbonate can be specifically but not limited to one or more of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC). There is no special restriction on the content of the cyclic carbonate, and it is arbitrary within the range that does not significantly damage the effect of the lithium ion battery of the present invention, but when one is used alone, the lower limit of its content is usually 3% or more by volume, preferably 5% or more by volume, relative to the total amount of solvent in the non-aqueous electrolyte. By setting this range, the reduction in conductivity due to the reduction in the dielectric constant of the non-aqueous electrolyte can be avoided, and it is easy to make the large current discharge characteristics, stability relative to the negative electrode, and cycle characteristics of the non-aqueous electrolyte battery reach a good range. In addition, the upper limit is usually less than 90% by volume, preferably less than 85% by volume, and more preferably less than 80% by volume. By setting this range, the oxidation / reduction tolerance of the nonaqueous electrolyte can be improved, thereby helping to improve the stability during high temperature storage. The content of the linear carbonate is not particularly limited, and relative to the total amount of solvent of the nonaqueous electrolyte, it is usually more than 15% by volume, preferably more than 20% by volume, and more preferably more than 25% by volume. In addition, usually the volume ratio is less than 90%, preferably less than 85% by volume, and more preferably less than 80% by volume. By making the content of the linear carbonate in the above-mentioned range, it is easy to make the viscosity of the nonaqueous electrolyte reach an appropriate range, suppress the reduction of ionic conductivity, and then help to make the output characteristics of the nonaqueous electrolyte battery reach a good range. When two or more linear carbonates are used in combination, the total amount of the linear carbonate is made to meet the above-mentioned range.

[0106] In certain embodiments, it is also possible to preferably use chain carbonates with fluorine atoms (hereinafter referred to as "fluorinated chain carbonates"). The number of fluorine atoms possessed by the fluorinated chain carbonate is not particularly limited as long as it is more than 1, but is generally less than 6, preferably less than 4. When the fluorinated chain carbonate has a plurality of fluorine atoms, these fluorine atoms can be bonded to the same carbon or to different carbons. As the fluorinated chain carbonate, fluorinated dimethyl carbonate derivatives, fluorinated ethyl methyl carbonate derivatives, fluorinated diethyl carbonate derivatives, etc. can be listed.

[0107] Carboxylate solvents include cyclic carboxylate and / or chain carbonate. Examples of cyclic carboxylate include one or more of γ-butyrolactone, γ-valerolactone, and δ-valerolactone. Examples of chain carbonate include one or more of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, propyl propionate (PP), and butyl propionate.

[0108] In some embodiments, the sulfone solvent includes a cyclic sulfone and a chain sulfone. Preferably, in the case of a cyclic sulfone, it is usually a compound with 3 to 6 carbon atoms, preferably 3 to 5 carbon atoms, and in the case of a chain sulfone, it is usually a compound with 2 to 6 carbon atoms, preferably 2 to 5 carbon atoms. There is no special restriction on the amount of sulfone solvent added, and it is arbitrary within the range that does not significantly damage the effect of the lithium ion battery of the present invention. Relative to the total amount of solvent in the non-aqueous electrolyte, the volume ratio is usually 0.3% or more, preferably 0.5% or more, and more preferably 1% or more. In addition, the volume ratio is usually 40% or less, preferably 35% or less, and more preferably 30% or less. In the case of using two or more sulfone solvents in combination, the total amount of sulfone solvents can meet the above range. When the amount of sulfone solvent added is within the above range, an electrolyte with excellent high temperature storage stability tends to be obtained.

[0109] In a preferred embodiment, the solvent is a mixture of cyclic carbonate and linear carbonate.

[0110] In some embodiments, the electrolyte salt is selected from a lithium salt, and the lithium salt includes LiPF 6 、LiBOB、LiDFOB、LiPO 2 F 2 , LiBF 4 、LiSbF 6 、LiAsF 6 、LiN(SO 2 CF 3 ) 2 、LiN(SO 2 C 2 F 5 ) 2 、LiC(SO 2 CF 3 ) 3 、LiN(SO 2 F) 2 、LiClO 4 、LiAlCl 4 、LiCF 3 SO 3 , Li 2 B 10 Cl 10 , one or more of lower aliphatic carboxylic acid lithium salts.

[0111] In a preferred embodiment, the lithium salt comprises LiPF 6 and auxiliary lithium salts, the auxiliary lithium salts comprising LiBOB, LiDFOB, LiPO 2 F 2 , LiBF4 、LiSbF 6 、LiAsF 6 、LiN(SO 2 CF 3 ) 2 、LiN(SO 2 C 2 F 5 ) 2 、LiC(SO 2 CF 3 ) 3 、LiN(SO 2 F) 2 、LiClO 4 、LiAlCl 4 、LiCF 3 SO 3 , Li 2 B 10 Cl 10 , one or more of lower aliphatic carboxylic acid lithium salts.

[0112] In some embodiments, the concentration of the lithium salt in the non-aqueous electrolyte is 0.1 mol / L-8 mol / L. In a preferred embodiment, the concentration of the electrolyte salt in the non-aqueous electrolyte is 0.5 mol / L-4 mol / L. Specifically, the concentration of the lithium salt can be 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L or 4 mol / L.

[0113] In some embodiments, in the non-aqueous electrolyte, the LiPF 6 The mass percentage of is 5% to 20%, and the mass percentage of the auxiliary lithium salt is 0.05% to 5%.

[0114] In some embodiments, the non-aqueous electrolyte further comprises an additive, wherein the additive comprises at least one of a cyclic sulfate compound, a sultone compound, a cyclic carbonate compound, an unsaturated phosphate compound, a borate compound, and a nitrile compound;

[0115] Preferably, based on the total mass of the non-aqueous electrolyte being 100%, the additive amount is 0.01% to 30%.

[0116] Preferably, the cyclic sulfate ester compound is selected from at least one of vinyl sulfate, 4-methyl vinyl sulfate, propylene sulfate, and methyl vinyl sulfate;

[0117] The sultone compound is selected from at least one of 1,3-propane sultone, 1,4-butane sultone, and 1,3-propylene sultone;

[0118] The cyclic carbonate compound is selected from at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate or a compound represented by formula II:

[0119]

[0120] In the formula II, R 21 , R 22 , R 23 , R 24 , R 25 , R 26 Each is independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group. Specifically, the compound represented by formula II includes At least one of .

[0121] The unsaturated phosphate compound is selected from at least one of the compounds shown in formula III:

[0122]

[0123] In the formula III, R 31 , R 32 , R 32 Each independently selected from a C1-C5 saturated hydrocarbon group, an unsaturated hydrocarbon group, a halogenated hydrocarbon group, -Si(C m H 2m+1 ) 3 , m is a natural number from 1 to 3, and R 31 , R 32 , R 33 At least one of them is an unsaturated hydrocarbon group;

[0124] In a preferred embodiment, the unsaturated phosphate compound may be at least one of tripropargyl phosphate, dipropargyl methyl phosphate, dipropargyl ethyl phosphate, dipropargyl propyl phosphate, dipropargyl trifluoromethyl phosphate, dipropargyl-2,2,2-trifluoroethyl phosphate, dipropargyl-3,3,3-trifluoropropyl phosphate, dipropargyl hexafluoroisopropyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, and diallyl hexafluoroisopropyl phosphate;

[0125] The borate compound is selected from at least one of tris(trimethylsilyl)borate and tris(triethylsilyl)borate;

[0126] The nitrile compound is selected from one or more of succinonitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrinitrile, adiponitrile, pimelonitrile, suberonitrile, azelaic acid dinitrile and sebacononitrile.

[0127] In other embodiments, the additives may also include other additives that can improve battery performance: for example, additives that enhance battery safety performance, such as flame retardant additives such as fluorophosphates and cyclophosphazenes, or overcharge prevention additives such as tert-amylbenzene and tert-butylbenzene.

[0128] It should be noted that, unless otherwise specified, in general, the amount of any one of the optional substances in the additives added to the non-aqueous electrolyte is less than 10%, preferably, the amount added is 0.1-5%, and more preferably, the amount added is 0.1% to 2%. Specifically, the amount of any one of the optional substances in the additives can be 0.05%, 0.08%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 7.8%, 8%, 8.5%, 9%, 9.5%, 10%.

[0129] In some embodiments, when the additive is selected from fluoroethylene carbonate, the amount of the fluoroethylene carbonate added is 0.05% to 30% based on the total mass of the non-aqueous electrolyte as 100%.

[0130] In some embodiments, the positive electrode sheet includes a positive electrode material layer and a positive electrode current collector, and the positive electrode material layer is formed on a surface of the positive electrode current collector.

[0131] The positive electrode current collector is selected from a metal material that can conduct electrons. Preferably, the positive electrode current collector includes one or more of Al, Ni, tin, copper, and stainless steel. In a more preferred embodiment, the positive electrode current collector is selected from aluminum foil.

[0132] The positive electrode material layer includes a positive electrode active material, a positive electrode binder and a positive electrode conductor.

[0133] The positive electrode binder includes polyvinylidene fluoride, copolymers of vinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride-hexafluoropropylene, copolymers of tetrafluoroethylene-hexafluoropropylene, copolymers of tetrafluoroethylene-perfluoroalkyl vinyl ether, copolymers of ethylene-tetrafluoroethylene, copolymers of vinylidene fluoride-tetrafluoroethylene, copolymers of vinylidene fluoride-trifluoroethylene, copolymers of vinylidene fluoride-trichloroethylene, copolymers of vinylidene fluoride-fluoroethylene, copolymers of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, thermoplastic resins such as polyethylene and polypropylene; acrylic resin; sodium hydroxymethyl cellulose; and one or more of styrene butadiene rubber.

[0134] The positive electrode conductive agent includes one or more of conductive carbon black, conductive carbon balls, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene or reduced graphene oxide.

[0135] The type of the positive electrode active material is not particularly limited and can be selected according to actual needs, as long as it is a positive electrode active material or a conversion positive electrode material that can reversibly embed / de-embed lithium ions.

[0136] In a preferred embodiment, the battery is a lithium-ion battery, and its positive electrode active material can be selected from LiFe 1-x’ M' x’ PO 4 、LiMn 2-y’ M y’ O 4 and LiNi x Co y Mn z M 1-x-y-z O 2 One or more of, wherein M' is selected from one or more of Mn, Mg, Co, Ni, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V or Ti, M is selected from one or more of Fe, Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V or Ti, and 0≤x'<1, 0≤y'≤1, 0≤y≤1, 0≤x≤1, 0≤z≤1, x+y+z≤1, and the positive electrode active material can also be selected from one or more of sulfide, selenide, and halide. More preferably, the positive electrode active material can be selected from LiCoO 2 、LiFePO 4 、LiFe 0.8 Mn 0.2 PO 4 、LiNi 0.5 Co 0.2 Mn 0.3 O 2 、LiNi 0.6 Co 0.2 Mn0.2 O 2 、LiNi 0.8 Co 0.1 Mn 0.1 O 2 、LiNi 0.5 Co 0.2 Mn 0.2 Al 0.1 O 2 、LiMn 2 O 4 、LiNi 0.5 Co 0.2 Al 0.3 O 2 One or more of .

[0137] In some embodiments, the secondary battery further includes a separator, and the separator is located between the positive electrode sheet and the negative electrode sheet.

[0138] The diaphragm may be an existing conventional diaphragm, and may be a polymer diaphragm, a non-woven fabric, etc., including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP and triple-layer PP / PE / PP diaphragms.

[0139] The present invention is further described below by way of examples.

[0140] The compounds involved in the following examples and comparative examples are shown in Table 1 below:

[0141] Table 1

[0142]

[0143] Table 2 Parameter design of embodiments and comparative examples

[0144]

[0145]

[0146]

[0147]

[0148] Example 1

[0149] This embodiment is used to illustrate the lithium ion battery and the preparation method thereof disclosed in the present invention, and includes the following steps:

[0150] 1) Preparation of positive electrode

[0151] Step 1: Add PVDF as a binder to NMP solvent and stir well to obtain PVDF glue.

[0152] Step 2: Add the conductive agent (super P+CNT) into the PVDF glue and stir thoroughly.

[0153] Step 3: Continue to add positive electrode active materials and stir thoroughly to finally obtain the required positive electrode slurry. The selection of positive electrode active materials is shown in Table 2.

[0154] Step 4: evenly coat the prepared positive electrode slurry on the positive electrode current collector (such as aluminum foil), and obtain the positive electrode sheet by drying, rolling, die-cutting or striping.

[0155] 2) Preparation of negative electrode

[0156] Step 1: First, add CMC into pure water at a solid content of 1.5%, stir well (for example, stirring time 120 minutes) to prepare a transparent CMC glue solution.

[0157] Step 3: Add conductive carbon (super P) to the CMC glue solution and stir well (for example, stirring time 90 minutes) to prepare a conductive glue, then add the compound represented by formula I and stir well.

[0158] Step 4: Continue to add negative electrode active materials and stir thoroughly to obtain the required negative electrode slurry.

[0159] Step 5: The prepared negative electrode slurry is evenly coated on the copper foil, and the negative electrode material layer is obtained by drying, and the negative electrode sheet is obtained by rolling, die-cutting or slitting. The selection of negative electrode active material, the mass percentage of negative electrode active material in the negative electrode material layer, the mass percentage of the compound represented by formula I and the porosity of the negative electrode material layer are shown in Table 2.

[0160] 3) Preparation of non-aqueous electrolyte

[0161] Ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of EC:DEC:EMC=1:1:1, and additives in the mass percentages shown in Table 2 were added, and then lithium hexafluorophosphate (LiPF 6 ) to a molar concentration of 1 mol / L, and then add 1% LiPO 2 F 2 .

[0162] 4) Lithium-ion battery preparation

[0163] The prepared positive electrode sheet and the negative electrode sheet are assembled into a laminated soft-pack battery cell.

[0164] 5) Battery filling and formation

[0165] In a glove box with a dew point controlled below -40°C, the prepared electrolyte was injected into the battery cell, vacuum-sealed, and left to stand for 24 hours. Then, the conventional formation of the first charge was carried out according to the following steps: 0.05C constant current charging for 180 minutes, 0.2C constant current charging to 3.7V, secondary vacuum sealing, and then further 0.2C constant current charging to 4.2V, after standing at room temperature for 24 hours, 0.2C constant current discharge to 3.0V.

[0166] Embodiments 2 to 39

[0167] Examples 2 to 39 are used to illustrate the lithium ion battery and the preparation method thereof disclosed in the present invention, and include most of the operation steps in Example 1, except that:

[0168] The positive electrode active material, negative electrode sheet components and electrolyte additive components shown in Table 2 were used.

[0169] Comparative Examples 1 to 17

[0170] Comparative Examples 1 to 17 are used to compare and illustrate the lithium ion battery and the preparation method thereof disclosed in the present invention, and include most of the operation steps in Example 1, except that:

[0171] The positive electrode active material, negative electrode sheet components and electrolyte additive components shown in Table 2 were used.

[0172] Performance Testing

[0173] The lithium-ion battery prepared above was subjected to the following performance tests:

[0174] High temperature cycle performance test:

[0175] At 45°C, the lithium ion batteries prepared in the examples and comparative examples were charged at a 1C rate and discharged at a 1C rate, and full charge and discharge cycle tests were performed within a charge and discharge cut-off voltage range of 3V to 4.2V until the capacity of the lithium ion battery decayed to 80% of the initial capacity, and the number of cycles was recorded.

[0176] Negative electrode resistivity test:

[0177] Using the Hioki BT3562 internal resistance tester, clamp the upper and lower sides of the negative electrode sheet between the two conductive terminals of the internal resistance tester and apply a certain pressure to fix it. Test the resistance R of the negative electrode sheet and calculate the resistivity b of the negative electrode sheet according to the formula b=R·p / h, where p is the contact area between the negative electrode sheet and the conductive terminal of the internal resistance tester, and h is the thickness of the negative electrode sheet. The diameter of the conductive terminal is 14mm, that is, the contact area p=49πmm 2 The applied pressure is 15MPa~27MPa, and the sampling time ranges from 5s to 17s.

[0178] (1) The test results obtained in Examples 1 to 20 and Comparative Examples 1 to 14 are entered in Table 3.

[0179] Table 3

[0180]

[0181]

[0182] It can be seen from the test results of Examples 1 to 9 and Comparative Examples 1 to 4 and 11 to 14 that when the positive electrode active material and the negative electrode active material of the lithium ion battery are the same, the compound represented by formula I is added to the negative electrode material layer, and at the same time, the mass percentage a of the compound represented by formula I in the negative electrode material layer, the porosity b of the negative electrode material layer, and the mass percentage c of the negative electrode active material in the negative electrode material layer meet the preset condition of 0.2≤10*c*a / b≤40. The lithium ion battery has excellent initial capacity, high temperature cycle performance and low impedance, indicating that by adjusting the mass percentage a of the compound represented by formula I in the negative electrode material layer, the porosity b of the negative electrode material layer, and the mass percentage c of the negative electrode active material in the negative electrode material layer, the negative electrode active material can be improved. Conductive bonding between the negative electrode active material and the negative electrode The dispersibility of the agent can be improved, so that the mass percentage c of the negative electrode active material in the negative electrode material layer can be further increased under the premise of ensuring the performance of the electrode sheet, thereby improving the energy density and reducing the impedance of the negative electrode sheet at the battery level; at the same time, the SEI film formed in situ by the compound represented by Formula I on the surface of the negative electrode material is dense and stable, and can be quickly repaired and reformed again after the SEI film is damaged in the subsequent cycle process, thereby improving the cycle performance of the battery; in particular, the compound represented by Formula I can effectively improve the compatibility of the non-aqueous electrolyte and the negative electrode, which is beneficial to the penetration of the non-aqueous electrolyte into the negative electrode material layer, and then by further adjusting the porosity b of the negative electrode material layer and cooperating with the SEI film formed in situ with the participation of the compound represented by Formula I, the battery can take into account the performance of impedance, cycle and energy density.

[0183] From the test results of Comparative Examples 2 and 3, it can be seen that by adding the compound shown in Formula I or VC (vinylene carbonate) into the non-aqueous electrolyte as an additive, the improvement in the performance of the lithium ion battery is far less than adding the compound shown in Formula I into the negative electrode material layer.

[0184] It can be seen from the test results of Examples 1 to 9 that when the condition 2≤10*c*a / b≤10 is further met, the lithium ion battery obtained at this time has the best electrochemical performance. It is speculated that the SEI film formed by the compound shown in Formula I at this time has lower impedance and higher ionic conductivity, so that the battery has the best initial capacity and high temperature cycle performance.

[0185] It can be seen from the test results of Examples 10 to 20 and Comparative Examples 5 to 10 that when lithium-ion batteries use different positive electrode active materials, they also meet the condition of 0.2≤10*c*a / b≤40. When the mass percentage a of the compound represented by Formula I in the negative electrode material layer, the porosity b of the negative electrode material layer and the mass percentage c of the negative electrode active material in the negative electrode material layer meet the preset condition of 0.2≤10*c*a / b≤40, the impedance of the lithium-ion battery is reduced to a certain extent, and the high-temperature cycle life of the lithium-ion battery is extended to a certain extent, indicating that satisfying the relationship 0.2≤10*c*a / b≤40 has a universal improvement for lithium-ion batteries with different positive electrode types.

[0186] It can be seen from the test results of Comparative Examples 4 to 8 that even if the mass percentage a of the compound represented by Formula I in the negative electrode material layer, the porosity b of the negative electrode material layer and the mass percentage c of the negative electrode active material in the negative electrode material layer meet the preset condition of 0.2≤10*c*a / b≤40, when the a value, b value and c value do not meet their range limits, the lithium ion battery still does not have good electrochemical performance, indicating that the a value, b value and c value have a strong correlation in improving the performance of lithium ion batteries.

[0187] (2) The test results obtained in Examples 1 and 21 to 24 are entered in Table 4.

[0188] Table 4

[0189]

[0190]

[0191] It can be seen from the test results of Examples 1 and 21 to 24 that in a lithium ion battery containing the negative electrode provided by the present invention, adding additives VC (ethylene carbonate), PS (1,3-propane sultone), tripropargyl phosphate or FEC (fluoroethylene carbonate) to the non-aqueous electrolyte can further improve the initial capacity and high-temperature cycle performance of the battery. It is speculated that this is because the compound represented by formula I in the negative electrode and the above-mentioned additives jointly participate in the formation of the passivation film on the electrode surface, thereby obtaining a passivation film with excellent thermal stability, thereby effectively reducing the reaction of the electrolyte on the electrode surface and improving the electrochemical performance of the battery.

[0192] (3) The test results obtained in Examples 1 and 25 to 28 are entered in Table 5.

[0193] Table 5

[0194]

[0195] It can be seen from the test results of Examples 1 and 25 to 28 that for different compounds represented by formula I, when the mass percentage a of the compound represented by formula I in the negative electrode material layer, the porosity b of the negative electrode material layer and the mass percentage c of the negative electrode active material in the negative electrode material layer meet the preset condition 0.2≤10*c*a / b≤40, they play similar roles and have a certain improvement effect on the impedance, initial capacity and high temperature cycle performance of the lithium-ion battery, indicating that the relationship provided by the present invention is applicable to different compounds represented by formula I.

[0196] (4) The test results obtained from Examples 29 to 39 and Comparative Examples 15 to 17 are entered in Table 6.

[0197] Table 6

[0198]

[0199]

[0200] From the test results obtained in Examples 29 to 39 and Comparative Examples 15 to 17, it can be seen that adding a certain amount of silicon-based material to the negative electrode active material can greatly increase the initial capacity of the battery, while its cycle performance will decrease. The reason is that the silicon-based material has a larger volume change rate during the cycle, resulting in more serious SEI film damage and side reactions of the electrolyte on the negative electrode. By limiting the condition of 0.2≤10*c*a / b≤40 and the further condition of 0.05≤m / b≤2, the high temperature cycle performance of the battery can be improved while effectively maintaining the energy density. It is speculated that by controlling the content of the compound shown in Formula I, the porosity of the negative electrode material layer, the mass percentage of the negative electrode active material, and the proportion of the silicon-based material, the density and flexibility of the SEI film formed by the compound shown in Formula I can be affected, and then when the silicon-based material produces a volume change under charge and discharge conditions, it can adapt to the change of the silicon-based material, reduce the rupture of the SEI film, and improve its stability, thereby improving the high temperature cycle life while retaining a certain amount of silicon-based material.

[0201] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode material layer includes a negative electrode active material including a carbon-based material and a compound shown in Formula I: Formula I wherein R1, R2, and R3 are each independently selected from an alkyl group of 1 to 5 carbon atoms, a fluoroalkyl group of 1 to 5 carbon atoms, an ether group of 1 to 5 carbon atoms, a fluoroether group of 1 to 5 carbon atoms, and an unsaturated hydrocarbon group of 2 to 5 carbon atoms, and at least one of R1, R2, and R3 is an unsaturated hydrocarbon group of 2 to 5 carbon atoms; The negative electrode sheet meets the following conditions: 0.2≤10*c*a / b≤40; And 0.005≤a≤1, 10≤b≤50, 92≤c≤98; Wherein, c is the mass percentage of the negative electrode active material in the negative electrode material layer, and the unit is %; a is the mass percentage of the compound represented by formula I in the negative electrode material layer, in units of %; b is the porosity of the negative electrode material layer, unit is %; The resistivity of the negative electrode sheet is ≤200Ω·m, and a solution obtained after ultrasonic oscillation of the negative electrode sheet in a solvent is subjected to liquid chromatography-mass spectrometry (LC-MS) analysis, and a characteristic peak appears in the region of retention time of 6.5 min to 7.5 min; The porosity of the negative electrode material layer is tested by the following method: Weigh the negative electrode sheet sample, and obtain the apparent volume V1 of the sample based on the surface area and thickness of the sample. Place the sample in the tester, seal the test system, introduce nitrogen according to the procedure, and detect the gas pressure in the sample chamber and the expansion chamber. Calculate the true volume V2 according to Boyle's law (PV=C, where P is the gas pressure, V is the gas volume, and C is a constant). Then, the porosity b of the negative electrode material layer is obtained according to b=(V1-V2) / V1×100%.

2. The negative electrode sheet according to claim 1, characterized in that: The negative electrode sheet meets the following conditions: 2≤10*c*a / b≤10.

3. The negative electrode sheet according to claim 1, characterized in that: The mass percentage c of the negative electrode active material in the negative electrode material layer is 94% to 96%.

4. The negative electrode sheet according to claim 1, characterized in that: The mass percentage a of the compound represented by formula I in the negative electrode material layer is 0.05% to 0.3%.

5. The negative electrode sheet according to claim 1, characterized in that: The porosity b of the negative electrode material layer is 20% to 40%.

6. The negative electrode sheet according to claim 1, characterized in that: The carbon-based material includes one or more of graphite, hard carbon, soft carbon, graphene, and mesophase carbon microspheres.

7. The negative electrode sheet according to claim 1, characterized in that: The negative electrode active material further comprises a silicon-based material, and the negative electrode sheet meets the following conditions: 0.05≤m / b≤2; And 10≤b≤50, 1≤m≤30; Wherein, m is the mass percentage of silicon-based material in the negative electrode material layer, unit is %; b is the porosity of the negative electrode material layer, in %.

8. The negative electrode sheet according to claim 1, characterized in that: The alkyl group of 1 to 5 carbon atoms is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, sec-pentyl or neopentyl; the fluoroalkyl group of 1 to 5 carbon atoms is selected from the group obtained by replacing one or more hydrogen elements in the alkyl group of 1 to 5 carbon atoms with fluorine elements; The unsaturated hydrocarbon group of 2 to 5 carbon atoms is selected from vinyl, propenyl, allyl, butenyl, pentenyl, methylvinyl, methylallyl, ethynyl, propynyl, propargyl, butynyl or pentynyl; The ether group of 1 to 5 carbon atoms is selected from methyl ether, ethyl ether, methyl ethyl ether, propyl ether, methyl propyl ether or ethyl propyl ether; The fluorinated ether group of 1 to 5 carbon atoms is selected from fluoromethyl ether, fluoroethyl ether, fluoromethylethyl ether, fluoropropyl ether, fluoromethylpropyl ether or fluoroethylpropyl ether.

9. The negative electrode sheet according to claim 8, characterized in that: The compound represented by formula I is selected from at least one of tripropargyl phosphate, dipropargyl methyl phosphate, dipropargyl fluoromethyl phosphate, dipropargyl methoxymethyl phosphate, dipropargyl ethyl phosphate, dipropargyl propyl phosphate, trifluoromethyl dipropargyl phosphate, dipropargyl 2,2,2-trifluoroethyl phosphate, dipropargyl 3,3,3-trifluoropropyl phosphate, hexafluoroisopropyl dipropargyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, trifluoromethyl diallyl phosphate, dipropargyl methyl ether phosphate, dipropargyl fluoromethyl ether phosphate, 2,2,2-trifluoroethyl diallyl phosphate, diallyl 3,3,3-trifluoropropyl phosphate or diallyl hexafluoroisopropyl phosphate.

10. A secondary battery, characterized in that: The invention comprises a positive electrode sheet, a non-aqueous electrolyte and a negative electrode sheet as claimed in any one of claims 1 to 9.

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