Lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, and provides a lithium ion battery. The battery comprises a negative plate; the negative plate comprises a current collector and a negative active material layer arranged on at least one side surface of the current collector; the negative electrode active material layer comprises a first binder and a negative electrode active material; the first binder comprises a lithiated first polymer; monomers forming the lithiated first polymer comprise a first monomer, and the first monomer comprises at least one of acrylic acid or maleic anhydride; the negative electrode active material comprises a graphite matrix, the surface of the graphite matrix is provided with a coating layer, and the coating layer comprises an amorphous carbon material. The negative plate with the structure can accelerate the transmission of lithium ions, reduce the embedding impedance of the lithium ions and improve the low-temperature charging and discharging capability of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a lithium-ion battery. Background Art

[0002] Lithium-ion batteries have advantages such as high energy density and long cycle life, and have been widely used in fields such as mobile phones, laptop computers, new energy vehicles, and energy storage. At present, lithium-ion batteries perform well at room temperature (25°C) and high temperatures (35°C and 45°C), but when used under low-temperature conditions (below 0°C), their charge-discharge performance will deteriorate significantly, especially the battery charging performance under low-temperature conditions. At low temperatures, ion transport is slow, the polarization of the battery system is large, and during charging, the negative electrode potential is likely to be relatively low, even lower than the potential for lithium metal precipitation, inducing the precipitation of lithium metal dendrites on the surface of the negative electrode, damaging the structure of the negative electrode sheet, exacerbating the swelling of the negative electrode sheet, and more seriously, the lithium metal dendrites will also pierce the separator, inducing battery short circuit and causing safety problems such as battery fire and explosion. Therefore, how to achieve normal charge and discharge of lithium-ion batteries without lithium precipitation in low-temperature application scenarios, optimize the low-temperature performance of the batteries, and broaden the application scenarios of the batteries is an urgent problem to be solved in this field. Summary of the Invention

[0003] The purpose of the present invention is to overcome the above problems existing in the prior art and provide a lithium-ion battery. In the negative electrode sheet of the present invention, by coating an amorphous carbon material on the surface of the graphite matrix and simultaneously cooperating with a first binder having a lithiated first polymer, the transmission of lithium ions can be accelerated, the lithium ion insertion impedance can be reduced, and the low-temperature charge-discharge capacity of the battery can be improved.

[0004] To achieve the above purpose, the present invention provides a lithium-ion battery, the battery comprising a negative electrode sheet; the negative electrode sheet comprising a current collector and a negative electrode active material layer provided on at least one surface of the current collector; the negative electrode active material layer comprising a first binder and a negative electrode active material; the first binder comprising a lithiated first polymer; forming the lithiated first polymer comprising a first monomer, the first monomer comprising at least one of acrylic acid or maleic anhydride;

[0005] The negative electrode active material comprises a graphite matrix, and the surface of the graphite matrix has a coating layer, the coating layer comprising an amorphous carbon material.

[0006] The present invention adopts the above technical solutions and has the following beneficial effects:

[0007] The lithium-ion battery provided by the present invention can improve the isotropy of the graphite matrix, reduce the lithium-ion insertion impedance, and enhance the low-temperature charging performance of the battery by coating an amorphous carbon material on the surface of the graphite matrix in the negative electrode sheet. Further adding a first binder containing lithiated first polymer in the negative electrode sheet can alleviate the high-temperature side reactions caused by the relatively large number of active sites of the amorphous carbon material and take into account the high-temperature performance of the battery. At the same time, the first binder also provides an additional lithium-ion source, forming a locally high-concentration lithium-ion distribution around the negative electrode active material, accelerating the transmission of lithium ions, and cooperating with coating the amorphous carbon material on the surface of the graphite matrix, which can further enhance the low-temperature charge-discharge capacity of the battery.

[0008] The endpoints and any values disclosed in this text for a range are not limited to the exact range or value. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in this text. In this text, unless otherwise specified, data ranges include endpoints. Brief Description of the Drawings

[0009] Figure 1 The figure shows a thermogravimetric analysis curve of a negative electrode paste in a nitrogen atmosphere in an example of the present invention.

[0010] Figure 2 The figure shows the capacity retention rate curve of the battery prepared in Example 6-1 of the present invention during 300 cycles at -20°C.

[0011] Figure 3 The figure shows the 1C cycle capacity retention rate curve of the battery prepared in Example 6-1 of the present invention at 45°C. Detailed Description of the Invention

[0012] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for explaining and illustrating the present invention and are not used to limit the present invention.

[0013] Unless otherwise defined, all scientific and technical terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention pertains.

[0014] In the present invention, the terms "battery", "lithium battery", "lithium-ion battery", and "lithium-ion secondary battery" all have the same meaning, referring to a lithium-ion secondary battery, which generally includes an electrode assembly (such as a positive electrode sheet, a negative electrode sheet, and a separator), a container (housing) for accommodating the electrode assembly, and an electrolyte.

[0015] In the present invention, the term "Dv10" refers to the particle size corresponding to when the cumulative volume particle size distribution percentage of a sample reaches 10%.

[0016] In the present invention, the term "Dv50" refers to the particle size corresponding to when the cumulative volume particle size distribution percentage of a sample reaches 50%.

[0017] In the present invention, the term "Dv90" refers to the particle size corresponding to when the cumulative volume particle size distribution percentage of a sample reaches 90%.

[0018] The present invention provides a lithium-ion battery, which includes a negative electrode sheet; the negative electrode sheet includes a current collector and a negative electrode active material layer disposed on at least one surface of the current collector; the negative electrode active material layer includes a first binder and a negative electrode active material; the first binder includes a lithiated first polymer; forming the lithiated first polymer includes a first monomer, and the first monomer includes at least one of acrylic acid or maleic anhydride; the negative electrode active material includes a graphite matrix, and the surface of the graphite matrix has a coating layer, and the coating layer includes an amorphous carbon material.

[0019] Due to anisotropy, there are significant differences in strength and modulus of graphite materials in different directions. When lithium ions are inserted into and extracted from the negative electrode at low temperature, graphite materials are prone to uneven stress, resulting in deformation or rupture, increasing the battery impedance. In addition, anisotropy affects the ion diffusion path and rate. Especially when the viscosity of the electrolyte increases and the kinetic performance decreases at low temperature, the diffusion of lithium ions is limited. If further hindered by the anisotropy of graphite materials, the transmission rate will be further reduced, exacerbating battery polarization.

[0020] In the present invention, by coating an amorphous carbon material on the surface of the graphite matrix, the amorphous carbon material provides more active sites, enabling lithium ions to be uniformly inserted into the negative electrode in all directions, improving the isotropy of the graphite matrix, reducing the lithium ion insertion impedance, and thus enhancing the low-temperature charging performance of the battery; a first binder is further added to the negative electrode sheet, and the first binder has a lithiated first polymer. In the negative electrode sheet, the lithiated first polymer provides an additional source of lithium ions, which can form a locally high-concentration lithium ion distribution around the negative electrode active material, accelerating ion transport, and thus further improving the low-temperature charge and discharge capacity of the battery.

[0021] In addition, although coating an amorphous carbon material on the surface of the graphite matrix will improve the electrochemistry reaction activity, the amorphous carbon material will exacerbate the side reactions at high temperature due to its many active sites. In the first binder, the first monomer forming the lithiated first polymer includes acrylic acid and / or maleic anhydride, which can make the first binder exhibit strong adhesion, improve the structural stability of the negative electrode sheet, and can alleviate the high-temperature side reactions caused by the many active sites of the amorphous carbon material in the coating layer, thereby extending the battery cycle life.

[0022] In the present invention, the method for lithiating the first polymer may be as follows: carboxyl groups are present in the first polymer formed by polymerizing acrylic acid and / or maleic anhydride. The first polymer is titrated and neutralized with a lithium salt such as lithium hydroxide or lithium carbonate. Lithium replaces the hydrogen on the carboxyl group, and thus the lithiated first polymer can be obtained.

[0023] In the present invention, the lithiated first polymer includes at least one -COOLi structure formed by the combination of a carboxylate ion formed by deprotonation of a carboxyl group and a lithium ion. The -COOLi structure exists as part of the side chain in the polymer.

[0024] In some embodiments, the amorphous carbon material includes at least one of a soft carbon material or a hard carbon material. Based on the total mass of the negative electrode active material, the mass ratio of the amorphous carbon material is 1% to 5%, for example, it can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or any value within the range formed by any two of the above values, preferably 2% to 4%. When the mass ratio of the amorphous carbon material in the coating layer further satisfies the above range, on the one hand, it can avoid the mass ratio of the amorphous carbon material being too small, resulting in an insignificant improvement in the anisotropy of the graphite matrix and an ineffective improvement in the low-temperature charging performance of the battery; on the other hand, it can avoid the mass ratio of the amorphous carbon material being too large, resulting in too many active sites provided by the amorphous carbon material, leading to too high an electro-chemical reaction activity, which will exacerbate the side reactions of the battery at high temperatures and reduce the high-temperature cycle life of the battery.

[0025] In some embodiments, in the lithiated first polymer, the lithium element content is 1.5% to 4%, for example, it can be 1.5%, 2%, 2.5%, 3%, 3.5%, 4% or any value within the range formed by any two of the above values, preferably 2% to 3.5%. An appropriate lithium content helps to improve the conductivity of the first binder itself, and at the same time, it can form a locally high-concentration lithium ion distribution around the negative electrode active material, further accelerating the lithium ion transport rate at low temperatures. At the same time, it can avoid the improvement of the low-temperature charge and discharge performance of the battery being insignificant when the lithium element content is too low; and it can avoid too many negative electrode active lithium ions when the lithium element content is too high, which are prone to side reactions with the electrolyte to generate gas at high temperatures, deteriorating the high-temperature performance of the battery.

[0026] Exemplarily, in the lithiated first polymer, the lithium element content can be tested on the first binder using a Thermo Scientific TM iCAP TM PRO XP ICP-OES inductively coupled plasma optical emission spectrometer.

[0027] In some embodiments, the glass transition temperature of the lithiated first polymer is 60°C to 100°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C or any value within the range composed of any two of the above values. When the glass transition temperature of the lithiated first polymer is within this range, it helps to maintain the structural stability of the negative electrode sheet, construct a smooth lithium ion transmission channel, and improve the low-temperature charge-discharge performance and high-temperature cycle performance of the battery.

[0028] In some embodiments, the monomer for forming the lithiated first polymer further includes a second monomer, and the second monomer includes one or more of acrylonitrile, acrylamide, vinyl alcohol, and ethylene glycol, wherein acrylonitrile monomer is preferably included. Selecting the above second monomer can further optimize the performance of the first binder, and can improve the binding force of the first binder and the suspension stability of the negative electrode slurry. Among them, when the second monomer includes acrylonitrile monomer, it can add a strongly electronegative element with lone pair electrons to the molecular chain of the lithiated first polymer. Under the action of an electric field, the first polymer will continuously undergo complexation / dissociation reactions with lithium ions, which is beneficial to the diffusion of lithium ions, thereby further improving the low-temperature performance of the battery.

[0029] In some embodiments, based on the total mass of the first monomer and the second monomer for forming the lithiated first polymer, the mass ratio of the acrylonitrile monomer is 10% to 60%, for example, it can be 10%, 20%, 30%, 40%, 50%, 60% or any value within the range composed of any two of the above values, and preferably 20% to 40%. Limiting the mass ratio of the acrylonitrile monomer within the above range can, on the one hand, avoid the situation where when the mass ratio of the acrylonitrile monomer is too low, the further improvement effect on the low-temperature performance of the battery is not obvious; on the other hand, avoid too much acrylonitrile monomer and too little proportion of other monomers that mainly play a binding role, resulting in a reduction in the binding performance of the first binder, which is likely to cause powder dropping during die-cutting and winding of the negative electrode sheet, and peeling or fracture of the negative electrode active material during charge and discharge.

[0030] When there is acrylonitrile monomer in the lithiated first polymer, in the infrared spectrum obtained by infrared testing, there is a characteristic peak attributed to the stretching vibration of -C≡N nitrile group at the position of 2000 cm -1 ~2500 cm -1 . In the present invention, infrared testing is carried out using a Thermo Fisher Nicolet iS20 Fourier transform infrared spectrometer, and the scanning range is 400 cm -1 ~4000 cm -1 .

[0031] In some embodiments, the negative electrode sheet further includes a second binder, the second binder includes a second polymer, and the glass transition temperature of the second polymer is -40°C to 40°C. The glass transition temperature of the second polymer can be, for example, 40°C, 30°C, 20°C, 10°C, 0°C, -10°C, -20°C, -30°C, -40°C or any value within the range composed of any two of the above values. When only the first binder is used in the negative electrode sheet, since the glass transition temperature of the lithiated first polymer in the first binder is relatively high, the negative electrode sheet will be relatively hard and brittle, and cracking or edge powdering is likely to occur during the electrode coating and slitting processes. Based on the use of the first binder, the present invention further introduces a second binder with a lower glass transition temperature, which helps to improve the flexibility of the negative electrode sheet and improve the edge powdering situation that may occur during the slitting of the negative electrode sheet. Further, the second binder can also build an elastic binding network for the negative electrode sheet, effectively absorbing and buffering the stress generated due to the volume change of the active material, thereby further suppressing the swelling that occurs during the cycling of the battery cell and improving the high and low temperature cycling performance of the battery.

[0032] In the present invention, the glass transition temperature of the lithiated first polymer in the first binder and the glass transition temperature of the second polymer in the second binder can be obtained, for example, by a differential scanning calorimeter (DSC) with the model of 910s (manufactured by TA Instruments, USA). The specific test method may include the following steps: Take a 5 - 10 mg dry film sample of the first binder / second binder, under a nitrogen atmosphere with a flow rate of 50 ± 5 ml / min, heat it to 150°C at a rate of 10 ± 1°C / min and keep it stable for 3 - 5 min; then cool it down to -60°C and keep it stable for 5 - 7 min; then heat it to 200°C at a rate of 10 ± 1°C / min, and the software calibrates the midpoint value of the glass transition temperature. After taking the average of the three test results (at least three tests, for example, it can also be four times, five times, etc.), the glass transition temperature of the lithiated first polymer in the first binder or the glass transition temperature of the second polymer in the second binder is obtained.

[0033] In some embodiments, the monomer forming the second polymer includes a third monomer, and the third monomer includes one or more of propylene, butadiene, styrene, vinylidene fluoride, dopamine, vinyl alkyl ether, ethylene oxide, dimethyl siloxane.

[0034] By selecting the type of the above third monomer, the formed second polymer helps to better improve the flexibility of the negative electrode sheet, improve the edge powdering situation during the slitting of the negative electrode sheet, and at the same time build a better elastic binding network to inhibit the swelling that occurs during the battery cycling process, thereby further improving the high and low temperature cycling performance of the battery.

[0035] In some embodiments, the monomers forming the second polymer further include ester monomers. Based on the total mass of the third monomer and the ester monomers forming the second polymer, the mass proportion of the ester monomers is 5% to 70%, for example, it can be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70% or any point value within the range composed of any two of the above values, preferably 10% to 50%. When there are ester monomers in the second binder, in the spectrogram obtained by infrared testing, at the position of 1650 cm -1 ~1750 cm -1 and at the position of 1030 cm -1 ~1300 cm -1 there are characteristic peaks attributed to -C=O and -C-O respectively. The second binder further includes ester monomers, and there are ester functional groups in the structure of the second polymer. The ester-modified second polymer helps to enhance the affinity of the negative electrode side for the electrolyte, improve the liquid absorption and liquid retention capabilities of the negative electrode side, and further extend the cycle life of the battery. The affinity of the ester monomers for the electrolyte can also accelerate the lithium ion transmission, thereby further improving the low-temperature charge and discharge performance of the battery. At the same time, controlling the mass proportion of the ester monomers in the second polymer to meet the above range can avoid the poor affinity of the second binder for the electrolyte when the mass proportion of the ester monomers is less than 5%, which is not conducive to further improving the low-temperature performance of the battery; and avoid the excessive liquid absorption capacity of the second binder and the excessive absorption of the electrolyte when the content of the ester monomers is greater than 70%, which reduces the bonding performance of the second binder itself.

[0036] In some embodiments, the ester monomers include one or two of (meth)acrylic acid alkyl esters and (meth)acrylic acid hydroxyalkyl esters.

[0037] Exemplarily, the (meth)acrylic acid alkyl esters can be selected from at least one of butyl methacrylate, butyl acrylate, methyl methacrylate, methyl acrylate, ethyl methacrylate, ethyl acrylate, n-octyl methacrylate, n-octyl acrylate, isooctyl methacrylate, isooctyl acrylate, and dodecyl methacrylate.

[0038] Exemplarily, the (meth)acrylic acid hydroxyalkyl esters can be selected from at least one of 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl methacrylate, and 2-hydroxypropyl acrylate.

[0039] Selecting the above types of ester monomers to form the ester-modified second polymer can enable the second binder to better improve the liquid absorption and liquid retention capabilities of the negative electrode side, extend the cycle life of the battery, and at the same time accelerate the lithium ion transmission and improve the low-temperature charge and discharge performance of the battery.

[0040] In some embodiments, the compaction density range of the negative electrode sheet is 1.2 g / cm 3-1.65 g / cm 3 , the tap density of the negative electrode sheet can be, for example, 1.2 g / cm 3 , 1.3 g / cm 3 , 1.4 g / cm 3 , 1.5 g / cm 3 , 1.6 g / cm 3 , 1.65 g / cm 3 or any value within the range formed by any two of the above values. Controlling an appropriate tap density can endow the negative electrode sheet with a suitable porosity, which helps the electrolyte to infiltrate, improves the ion transport efficiency, and further improves the low-temperature charge and discharge performance of the battery. At the same time, in the present invention, due to the coating of amorphous carbon material on the surface of the graphite matrix, the isotropy of the graphite matrix is improved. During the battery cycling process, the graphite matrix will expand uniformly in all directions, thereby enhancing the structural stability of the negative electrode sheet. By further adjusting the tap density of the negative electrode sheet to be lower than 1.65 g / cm 3 , the expansion stress can be further dispersed, the negative electrode expansion phenomenon can be alleviated, and the long-cycle stability of the battery can be improved.

[0041] In some embodiments, the Dv50 of the second binder is 150 nm to 600 nm. The Dv50 of the second binder can be, for example, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm or any value within the range formed by any two of the above values. In the present invention, to improve the low-temperature performance of the battery, the tap density of the negative electrode is relatively low, and the pores of the negative electrode sheet are relatively large. By controlling the particle size of the second binder within the above range, the appropriate particle size range can provide sufficient bonding sites, further stabilize the structure of the negative electrode sheet, and at the same time improve the fluidity and coating performance of the negative electrode slurry; it can be avoided that when the Dv50 of the second binder is less than 150 nm, due to the enhanced interaction between particles, agglomeration is likely to occur, reducing the stability of the negative electrode sheet; it can be avoided that when the Dv50 of the second binder is greater than 600 nm, the bonding effect of the second binder is poor.

[0042] In some embodiments, based on the total mass of the negative electrode active material layer, the mass ratio of the first binder is denoted as n%, and the mass ratio of the second binder is denoted as m%. n and m satisfy: 1 ≤ n / m ≤ 3. The value of n / m can be, for example, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3 or any value within the range formed by any two of the above values.

[0043] In some embodiments, the mass ratio (n) of the first binder satisfies: 0.8 ≤ n ≤ 5. The mass ratio of the first binder can be, for example, 0.8%, 1%, 2%, 3%, 4%, 5% or any value within the range composed of any two of the above values. Preferably, 1 ≤ n ≤ 2.

[0044] In some embodiments, the mass ratio (m) of the second binder satisfies: 0.1 ≤ m ≤ 5. The mass ratio of the second binder can be, for example, 0.1%, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5% or any value within the range composed of any two of the above values. Preferably, 0.5 ≤ m ≤ 1.5.

[0045] In some embodiments, n and m satisfy: 1.5 ≤ n + m ≤ 5. The value of n + m can be, for example, 1.5, 2, 3, 4, 5 or any value within the range composed of any two of the above values. Preferably, 1.5 ≤ n + m ≤ 3.

[0046] Figure 1 Shown is the thermogravimetric test of a negative electrode paste in a nitrogen atmosphere in an example of the present invention. From the test results, it can be seen that the negative electrode paste has a thermogravimetric weight loss peak at 350°C to 600°C, corresponding to the decomposition weight loss of the first binder and the second binder. Through the thermogravimetric test results, it can also be measured that, based on the total mass of the negative electrode active material layer, the range of the total mass of the first binder and the second binder (i.e., 1.5 ≤ n + m ≤ 5). Specifically, the thermal decomposition of the negative electrode paste between 350°C and 600°C corresponds to the decomposition of the first binder and the second binder, and the weight loss rate in this temperature range is the mass ratio of the first binder and the second binder in the negative electrode active material layer.

[0047] In the present invention, the glass transition temperature of the first binder is relatively high, having stronger bonding performance, and the lithiated first polymer in the first binder gives the first binder better kinetic performance. By controlling the mass ratio of the first binder at a relatively high level, the low-temperature transmission rate of lithium ions can be effectively improved, and the low-temperature charge and discharge capacity of the battery can be better improved. However, if the proportion of the first binder in the negative electrode active material layer is too high, the negative electrode sheet will be too hard and brittle, easily causing the negative electrode sheet to break and powder off, reducing the structural stability of the negative electrode sheet, and thus deteriorating the battery performance. Therefore, by further controlling the ratio relationship between the first binder and the second binder, as well as controlling the total mass ratio and the respective mass ratios of the first binder and the second binder in the negative electrode active material layer, while further stabilizing the structure of the negative electrode sheet, the negative electrode expansion can be inhibited, the transmission rate of lithium ions can be increased, the low-temperature charge and discharge performance of the battery can be taken into account, and the cycle performance of the battery can be improved.

[0048] In some embodiments, the battery includes an electrolyte, the electrolyte includes an organic solvent, the organic solvent includes a linear organic solvent and a cyclic organic solvent; the cyclic organic solvent includes cyclic carbonates, and the linear organic solvent includes linear carboxylates and linear carbonates.

[0049] As the main solvent of the electrolyte, cyclic carbonates have a high dielectric constant, can dissolve lithium salts well, improve the conductivity of the electrolyte, and help improve the comprehensive performance of the battery.

[0050] As a co-solvent in the electrolyte, linear carboxylates can reduce the viscosity of the electrolyte, improve kinetics, and at the same time, linear carboxylates also have a lower solvation energy, which helps improve the charge and discharge performance of the battery at low temperatures.

[0051] As a co-solvent in the electrolyte, linear carbonates can also dissolve lithium salts, and at the same time, can significantly reduce the viscosity of the electrolyte and increase the migration speed of lithium ions.

[0052] In some embodiments, based on the total mass of the electrolyte, the mass percentage of cyclic carbonates is denoted as A%, the mass percentage of linear carboxylates is denoted as B%, and the mass percentage of linear carbonates is denoted as C%; wherein, B and C satisfy: 0.6 ≤ B / C ≤ 7, and the value of B / C can be, for example, 0.6, 1, 2, 3, 4, 5, 6, 7 or any value within the range composed of any two of the above values. By further controlling the ratio of the mass percentage of cyclic carbonates to the mass percentage of linear carboxylates within the above range, on the one hand, the conductivity of the electrolyte can be effectively improved while reducing the viscosity of the electrolyte; the desolvation energy of lithium ions can also be reduced, the desolvation of lithium ions can be accelerated, and the migration kinetics of lithium ions can be improved, thereby improving the low-temperature charging performance of the battery. On the other hand, it can also ensure that the second binder exhibits appropriate swelling in the electrolyte, enabling the second binder to have good affinity for the electrolyte, accelerating the transport of lithium ions in the negative electrode while not significantly losing the binding force of the second binder to the negative electrode, improving the low-temperature charge and discharge capacity of the battery while slowing down the side reactions caused by negative electrode expansion, and improving the high-temperature cycle performance and high-temperature storage performance of the battery.

[0053] In the present invention, the mass swelling range of the second binder in the electrolyte is 35% to 80%, for example, it can be 35%, 40%, 50%, 60%, 70%, 80% or any value within the range composed of any two of the above values, and preferably 40% to 70%. In the present invention, the second binder and the electrolyte cooperate with each other, so that the mass swelling of the second binder in the electrolyte is within this range, which can ensure that the second binder has good bonding ability and helps to maintain the structural stability of the negative electrode side electrode sheet. At the same time, when the mass swelling of the second binder is greater than 80%, the liquid absorption capacity of the second binder is too strong, and too much electrolyte is absorbed, the bonding ability of the second binder will be significantly weakened, which is not conducive to the structural stability of the negative electrode sheet, and even will exacerbate the side reactions on the negative electrode side at high temperatures and deteriorate the cycle performance.

[0054] In the present invention, the method for measuring the mass swelling of the second binder may include the following steps: Immerse the dry second binder film (weight denoted as m1) in the corresponding electrolyte solvent used in the preparation of the battery. After soaking at 60 °C for 72 h, take out the second binder film and blot the solvent on the surface of the second binder film with lint-free paper. The weight of the swollen film in the solvent is denoted as m2. The mass swelling of the second binder = (m2 - m1) / m1 × 100%.

[0055] In some embodiments, the sum of the mass fraction (B) of the linear carboxylic acid ester and the mass fraction (C) of the linear carbonate satisfies: 30 ≤ B + C ≤ 80, and the value of B + C can be, for example, 30, 40, 50, 60, 70, 80 or any value within the range composed of any two of the above values. By further controlling the sum of the mass fraction of the linear carboxylic acid ester and the mass fraction of the linear carbonate to satisfy the above range, it can be avoided that when B + C > 80, too little lithium salt is dissolved, resulting in too low number of migratable lithium ions and deterioration of the overall performance of the battery; and it can be avoided that when B + C < 30, the viscosity of the electrolyte increases and the migration speed of lithium ions decreases, which is not conducive to the improvement of the low-temperature performance of the battery.

[0056] In some embodiments, the mass fraction (A) of the cyclic carbonate satisfies: 1 ≤ A ≤ 50, and the mass fraction of the cyclic carbonate can be, for example, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or any value within the range composed of any two of the above values, and preferably 15 ≤ A ≤ 40.

[0057] In some embodiments, the mass fraction (B) of the linear carboxylic acid ester satisfies: 5 ≤ B ≤ 70, and the mass fraction of the linear carboxylic acid ester can be, for example, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70% or any value within the range composed of any two of the above values.

[0058] In some embodiments, the mass proportion (C) of the linear carbonate satisfies: 10≤C≤80, and the mass proportion of the linear carbonate can be, for example, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or any point value in the range consisting of any two of the above point values.

[0059] In some embodiments, the cyclic carbonate includes at least one of ethylene carbonate, propylene carbonate, and butylene carbonate. Preferably, the cyclic carbonate is ethylene carbonate.

[0060] In some embodiments, the linear carboxylic acid ester includes at least one of propyl acetate, ethyl propionate, ethyl acetate, propyl propionate, methyl acetate, and methyl propionate. Preferably, the linear carboxylic acid ester includes one or both of propyl acetate and ethyl propionate.

[0061] In some embodiments, the linear carbonate comprises at least one of ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, and methyl propyl carbonate. Preferably, the linear carbonate comprises one or both of ethyl methyl carbonate and dimethyl carbonate.

[0062] Selecting the above-mentioned cyclic carbonate, linear carboxylic acid ester and linear carbonate type can more effectively improve the low-temperature charge and discharge performance of the battery, taking into account the high-temperature cycle performance and high-temperature storage performance of the battery. When the cyclic carbonate is ethylene carbonate, the linear carbonate is one or two of ethyl methyl carbonate and dimethyl carbonate, and the linear carbonate is one or two of ethyl methyl carbonate and dimethyl carbonate, the improvement effect on the high and low temperature performance of the battery is better.

[0063] In some embodiments, the negative electrode active material is measured by Raman spectrometer at 100 cm -1 ~4000cm -1 Peak intensity in the range I D With 1300cm -1 ~1400cm -1 Peak intensity in the range I G The strength ratio satisfies: 0.1≤I D / I G ≤0.8, I D / I G The value of can be, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or any point in the range consisting of any two of the above points, and is preferably 0.3-0.6.

[0064] In Raman spectroscopy, I D Represents the degree of defect, I G Represents the degree of graphitization, controlling I D / I GWhen the value is within the above range, it can ensure that the amorphous carbon material achieves good coating on the graphite substrate. When the mass ratio of the amorphous carbon material in the negative electrode active material is too small, the polarization of the graphite substrate during low-temperature charge and discharge of the battery is too large, and the low-temperature performance of the battery cannot be effectively improved. When the mass ratio of the amorphous carbon material is too large, due to the excessive active sites provided by the amorphous carbon material, the side reactions of the battery at high temperature will be aggravated, and the high-temperature cycle life of the battery will be reduced.

[0065] In some embodiments, I D and I G satisfy: 0.3 ≤ I D / I G ≤ 0.6, and the numerical distribution ≥ 90%, preferably 0.3 ≤ I D / I G ≤ 0.6, and the numerical distribution ≥ 95%. In the Raman spectrum, the more concentrated the numerical distribution of 0.3 ≤ I D / I G ≤ 0.6, the higher the uniformity of the amorphous carbon material coating on the graphite matrix. When the numerical distribution of 0.3 ≤ I D / I G ≤ 0.6 further satisfies the above range, it can make the graphite matrix coated with amorphous carbon material have no excessive lattice defects, and the high stability and consistency of the graphite matrix structure are beneficial to improving the low-temperature cycle performance of the battery.

[0066] In the present invention, the Raman test method uses an in-situ microscopic laser confocal Raman spectrometer, model HORIBA HR800, and is equipped with four lasers with wavelengths of 325 nm, 532 nm, 633 nm, and 785 nm respectively. The Raman shift range is 100 cm -1 ~4000 cm -1 , the peak position of the D peak is 1300 cm -1 ~1400 cm -1 , the peak position of the G peak is 1550 cm -1 ~1650 cm -1 , the diffraction intensity of the D peak is I D , and the diffraction intensity of the G peak is I G .

[0067] In some embodiments, in the XRD pattern of the negative electrode sheet, the intensity of the diffraction peak of the (004) crystal plane where the diffraction angle 2θ is located at 54 ± 1° is D004, and the intensity of the diffraction peak of the (110) crystal plane where the diffraction angle 2θ is located at 78 ± 1° is D110. D004 and D110 satisfy: 1.0 ≤ D004 / D110 ≤ 10.0. The value of D004 / D110 can be, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or any value within the range composed of any two of the above values, preferably 3 - 7. When the value of D004 / D110 is within the above range, it indicates that the degree of isotropy of the negative electrode sheet is relatively high, the performance in all directions is relatively consistent, which is beneficial to accelerating the lithium ion transmission speed, improving the low-temperature charge and discharge performance of the battery, reducing the expansion rate of the negative electrode, and extending the cycle life of the battery.

[0068] Exemplarily, the method for measuring the value of D004 / D110 may include the following steps: Cut the negative electrode sheet into a suitable size, attach it to the sample plate, and use an X-ray diffractometer (such as Thermo Scientific TM ARL TM EQUINOX Pro vertical X-ray diffractometer) to test and obtain the X-ray diffraction spectrum (XRD pattern) of the negative electrode. The excitation source of the X-ray diffractometer is CuKα, the scanning angle range is 10° - 90°, and the scanning speed is 2° / min.

[0069] In some embodiments, the negative electrode active material satisfies at least one of the following conditions:

[0070] (i) Dv50 is 5 μm - 15 μm;

[0071] (ii) Dv90 / Dv10 is 2.3 - 2.5;

[0072] (iii) Dn10 ≥ 1.0 μm;

[0073] (iv) The specific surface area is 0.5 m 2 / g - 1.5 m 2 / g;

[0074] (v) The specific capacity is 320 mAh / g - 355 mAh / g.

[0075] Specifically, Dv50 of the negative electrode active material can be, for example, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, or any value within the range composed of any two of the above values. By controlling Dv50 of the negative electrode active material to further satisfy the above range, the particle size of the negative electrode active material can be optimized, the path of lithium ion insertion and extraction can be shortened, and the volume change of the negative electrode material during charge and discharge can be reduced, thereby improving the high and low temperature performance of the battery.

[0076] Specifically, the Dv90 / Dv10 value of the negative electrode active material can be, for example, 2.3, 2.35, 2.4, 2.45, 2.5, or any value within the range formed by any two of the above values. Since coating the surface of the graphite matrix with an amorphous carbon material will improve the electrochemical reaction activity of the negative electrode sheet, but the large number of active sites of the amorphous carbon material will exacerbate the side reactions at high temperatures. In order to balance the high and low temperature performance of the lithium-ion battery, the present invention controls the Dv90 / Dv10 value of the negative electrode active material to further meet the above range, optimizes the particle size distribution, makes the particle size distribution more uniform, and can improve the high temperature performance of the battery.

[0077] Dn10 represents the minimum value of the particle size of the negative electrode active material, which is equivalent to Dv00, and the minimum value of the particle size in the particles needs to be greater than 1 μm.

[0078] In the present invention, the test method for the particle size of the negative electrode active material: the particle sizes Dn10, Dv10, Dv50, and Dv90 can be measured by a laser particle size analysis method. For example, a particle size analyzer with the instrument model Mastersizer 3000 is used for measurement.

[0079] Specifically, the specific surface area of the negative electrode active material can be, for example, 0.5 m 2 / g, 0.7 m 2 / g, 0.9 m 2 / g, 1.1 m 2 / g, 1.3 m 2 / g, 1.5 m 2 / g, or any value within the range formed by any two of the above values. When controlling the specific surface area of the negative electrode active material to further meet the above range, the lithium-ion transmission path can be optimized, the charge and discharge capacity of the battery at low temperatures can be improved, and at the same time, the occurrence of side reactions can be reduced, and the high temperature performance of the battery can be further improved.

[0080] Exemplarily, the specific surface area is tested using a Micromeritics TriStar II 3020Plus high-throughput specific surface area and pore size analyzer in the United States.

[0081] Specifically, the specific capacity of the negative electrode active material can be, for example, 320 mAh / g, 325 mAh / g, 330 mAh / g, 335 mAh / g, 340 mAh / g, 345 mAh / g, 350 mAh / g, 355 mAh / g, or any value within the range formed by any two of the above values. By controlling the specific capacity of the negative electrode active material within this range, the corresponding lithium-ion battery can ensure a high energy density and a long service life to meet the actual requirements.

[0082] In some embodiments, the electrolyte further includes an electrolyte salt, and the type of the electrolyte salt is not specifically limited. For example, it may include at least one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium hexafluorophosphate (LiPF6).

[0083] In some embodiments, the electrolyte further includes an additive. The type of the additive is not specifically limited. For example, it includes at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), methylene methanedisulfonate (MMDS), tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB), trimethylfluorosilane, lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiODFB), lithium difluoro(oxalato)phosphate (LiODFP), lithium difluorophosphate (LiDFP), and lithium tetrafluoroborate (LiBF4).

[0084] In some embodiments, the battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes lithium iron phosphate particles.

[0085] In some embodiments, the particle size Dv50 of the lithium iron phosphate particles is 0.5 μm to 1.5 μm. For example, it may be 0.5 μm, 0.7 μm, 0.9 μm, 1.1 μm, 1.3 μm, 1.5 μm, or any value within the range formed by any two of the above values.

[0086] In some embodiments, the specific surface area range of the lithium iron phosphate particles is 5 m 2 / g to 20 m 2 / g. For example, it may be 5 m 2 / g, 8 m 2 / g, 10 m 2 / g, 12 m 2 / g, 14 m 2 / g, 16 m 2 / g, 18 m 2 / g, 20 m 2 / g or any value within the range formed by any two of the above values.

[0087] In some embodiments, in the lithium iron phosphate particles, the carbon element (C) content is 1.0% to 1.5%, for example, it can be 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or any value within the range composed of any two of the above values; the lithium element (Li) content is 1% to 10%, for example, it can be 1%, 2%, 4%, 6%, 8%, 10% or any value within the range composed of any two of the above values; the iron element (Fe) content is 25% to 40%, for example, it can be 25%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40% or any value within the range composed of any two of the above values; the phosphorus element (P) content is 10% to 20%, for example, it can be 10%, 12%, 14%, 16%, 18%, 20% or any value within the range composed of any two of the above values.

[0088] Exemplarily, the content tests of C, Li, Fe, and P in the lithium iron phosphate particles are carried out using a Thermo Scientific TM iCAP TM PRO XP ICP-OES inductively coupled plasma optical emission spectrometer for testing.

[0089] In some embodiments, based on the total mass of the negative electrode active material layer, the mass ratio of the negative electrode active material is 85% to 99.5%, for example, it can be 85%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 99.5% or any value within the range composed of any two of the above values.

[0090] In some embodiments, the negative electrode sheet further includes a conductive agent. Based on the total mass of the negative electrode active material layer, the weight content of the negative electrode conductive agent is 0.1% to 5%, for example, it can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or any value within the range composed of any two of the above values.

[0091] In some embodiments, the battery includes one or more of a square aluminum shell battery, a soft package battery, and a cylindrical battery.

[0092] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the protection scope of the present invention.

[0093] The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels without special instructions.

[0094] The present invention will be described in detail below in conjunction with specific embodiments, which are used for understanding rather than limiting the present invention.

[0095] Example 1-1

[0096] (1) Preparation of the positive electrode sheet

[0097] The positive electrode active material lithium iron phosphate particles, polyvinylidene fluoride, and acetylene black are put into a vacuum mixer according to a mass ratio of 96:2:2, and N-methylpyrrolidone (NMP) is added. Under the action of the vacuum mixer, they are fully mixed until a uniform and good-flowing positive electrode paste is formed, with a solid content of 60 wt%. The above positive electrode paste is evenly coated on an aluminum foil with a thickness of 12 μm, dried, rolled, slit, and punched to obtain the positive electrode sheet.

[0098] (2) Preparation of the negative electrode sheet

[0099] The negative electrode active material (graphite coated with soft carbon), the first binder, sodium carboxymethyl cellulose, and acetylene black are put into a vacuum mixer according to a mass ratio of 96:1.8:1.2:1, and deionized water is added. Under the action of the vacuum mixer, they are fully mixed to finally form a uniform and good-flowing negative electrode paste, with a solid content of 50 wt%. The above negative electrode paste is evenly coated on a copper foil with a thickness of 6 μm, dried, rolled, and die-cut to obtain the negative electrode sheet.

[0100] The preparation method of the first binder is as follows: The measured mass parts of acrylic acid and acrylamide are sequentially added to a reaction kettle, and soap-free emulsion polymerization is carried out; it is neutralized with an aqueous solution of lithium carbonate (Li2CO3) to obtain a lithiated first polymer, and then the first binder is obtained. Among them, the mass ratio of acrylic acid to acrylamide is 7:3.

[0101] (3) Preparation of the electrolyte

[0102] In an argon glove box with a water content of <0.1 ppm and an oxygen content of <0.1 ppm, a cyclic carbonate, a linear carbonate, and a linear carboxylic acid ester are mixed evenly in proportion. Lithium hexafluorophosphate (LiPF6) that has been fully dried is added thereto and stirred until dissolved. Subsequently, an additive is added and stirred evenly. After passing the physical property test, an electrolyte is obtained. Among them, the cyclic carbonate is ethylene carbonate (EC), the linear carbonates are ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), and the mass ratio of EMC to DMC is 3:2. The linear carboxylic acid ester is ethyl acetate (EA). Based on the total mass of the electrolyte, the mass fraction of the cyclic carbonate is 30%, the mass fraction of the linear carbonate is 25%, the mass fraction of the linear carboxylic acid ester is 30%, the mass fraction of LiPF6 is 12.5%, and the mass fraction of the additive is 2.5%. The composition of the additive is 1.55% VC, 0.25% FEC, 0.25% DTD, 0.25% LiODFB, and 0.2% TMSP.

[0103] (4) Preparation of the battery

[0104] The positive electrode sheet obtained in step (1), the negative electrode sheet obtained in step (2), and a separator (an 8-μm-thick polyethylene separator provided by Asahi Kasei Corporation) are used to obtain a bare battery cell by winding; the bare battery cell is welded with tabs and placed in a battery case. The electrolyte prepared in step (3) is injected into the dried and qualified battery cell. After processes such as standing, aging, formation, degassing, aging, and sorting, a battery is obtained.

[0105] (5) Testing of the battery

[0106] i) Low-temperature performance test (cycle retention rate and thickness expansion rate at -20°C)

[0107] 1) The batteries prepared in the above-mentioned examples and comparative examples are left standing in an environment at 25°C for 1 h; 2) The battery is charged at a current density of 0.33C to 3.65V, and then charged at a constant voltage with a cut-off current of 0.05C; 3) Stand for 30 min; 4) The battery is discharged at a current density of 0.33C to 2.2V; 5) Stand for 30 min; 6) The battery is moved to an environment at -20°C and left standing for 4 h; 7) The battery is charged at a current of 0.2C to 3.65V, and then charged at a constant voltage with a cut-off current of 0.05C; 8) Stand for 30 min; 9) The battery is discharged at a current of 0.2C to 2.2V; 10) Stand for 30 min; 11) Repeat the test steps of 7) to 10) until 300 charge-discharge cycles.

[0108] Taking the discharge capacity of the first cycle as C1 and the discharge capacity of the 300th cycle as C2, the capacity retention rate of the battery for 300 cycles at -20°C with 0.2C / 0.2C is: C2 / C1×100%. Taking the thickness of the first cell in the fully charged state as H1 and the fully charged thickness of the cell after 300 cycles as H2, the thickness change rate of the battery for 300 cycles at -20°C with 0.2C / 0.2C is: (H2 - H1) / H1×100%.

[0109] ii) High-temperature performance test (cycle retention rate at 45°C, thickness expansion rate)

[0110] 1) Let the batteries prepared in the above examples and comparative examples stand in a 25°C environment for 1 h; 2) Charge the battery at a current density of 0.33C to 3.65V, and then charge at a constant voltage with a cut-off current of 0.05C;

[0111] 3) Stand for 30 min; 4) Discharge the battery at a current density of 0.33C to 2.2V; 5) Stand for 30 min; 6) Move the battery to a 45°C environment and stand for 4 h; 7) Charge the battery at a current of 1C to 3.65V, and then charge at a constant voltage with a cut-off current of 0.05C; 8) Stand for 30 min; 9) Discharge the battery at a current of 1C to 2.2V; 10) Stand for 30 min; 11) Repeat the test steps of 7) - 10) until 1000 charge-discharge cycles.

[0112] Taking the discharge capacity of the first cycle as C3 and the discharge capacity of the 1000th cycle as C4, the capacity retention rate of the battery for 1000 cycles at 45°C with 1C / 1C is: C4 / C3×100%. Taking the thickness of the first cell in the fully charged state as H3 and the fully charged thickness of the cell after 1000 cycles as H4, the thickness change rate of the battery for 1000 cycles at 45°C with 1C / 1C is: (H4 - H3) / H3×100%.

[0113] iii) High-temperature storage performance test (residual capacity retention rate of storage performance / %)

[0114] 1) Let the batteries prepared in the above examples and comparative examples stand still in an environment of 25 °C for 1 h; 2) Charge the batteries at a current density of 0.33C until 3.65V, then perform constant voltage charging, and the cut-off current is 0.05C; 3) Stand still for 30 min; 4) Discharge the batteries at a current density of 0.33C until 2.2V; 5) Stand still for 30 min; 6) Repeat the test steps of 2) - 5) three times, and the average value of the discharge capacity for the three times is denoted as C5; 7) Charge the batteries at a current density of 0.33C until 3.65V, then perform constant voltage charging, and the cut-off current is 0.05C; Record the thickness of the fully charged battery cell as H5; 8) Storage: Place the battery cells with 100% SOC at 60 °C for 30 days; 9) After storage, cool the battery cells to room temperature, record the thickness of the battery cells, denoted as H6; 10) Discharge the batteries at a current density of 0.33C until 2.2V; The discharge capacity is the residual capacity of the battery cell, denoted as C6. The residual capacity retention rate of the battery is: C6 / C5×100%. The battery thickness change rate is: (H6 - H5) / H5×100%.

[0115] iv) Lithium deposition test of the negative electrode at low temperature (-20 °C)

[0116] 1) Let the batteries prepared in the above examples and comparative examples stand still in an environment of 25 °C for 1 h; 2) Charge the batteries at a current density of 0.33C until 3.65V, then perform constant voltage charging, and the cut-off current is 0.05C; 3) Stand still for 30 min; 4) Discharge the batteries at a current density of 0.33C until 2.2V; 5) Stand still for 30 min; 6) Move the batteries to an environment of -20 °C and stand still for 4 h; 7) Charge the batteries at a current of 0.2C until 3.65V, then perform constant voltage charging, and the cut-off current is 0.05C; 8) Stand still for 30 min; 9) Discharge the batteries at a current of 0.2C until 2.2V; 10) Stand still for 30 min; 11) Repeat the test steps of 7) - 10) until 50 charge-discharge cycles; 12) Take out the batteries that have undergone 50 low-temperature cycles of the above tests after being fully charged, perform full-charge disassembly, and observe the degree of lithium deposition on the negative electrode (classified as no lithium deposition, slight lithium deposition, and severe lithium deposition from small to large according to the degree of lithium deposition).

[0117] The Example 1 group and Comparative Examples 1 - 3 were carried out with reference to Example 1 - 1, and the main differences are shown in Table 1. Among them, the proportion of lithium element in the first binder was changed in the Example 1 group. The first binder in Comparative Example 1 does not contain lithium element. The negative electrode graphite matrix in Comparative Example 2 is not coated with amorphous carbon material. The first binder in Comparative Example 3 does not contain lithium element and the negative electrode graphite matrix is not coated with amorphous carbon material.

[0118] Table 1

[0119]

[0120] Note: " / " indicates that the corresponding parameter has not been tested.

[0121] In the present invention, by having an amorphous carbon material coated on the surface of the negative electrode graphite matrix and adding a first binder with lithiated first polymer, the low-temperature charge and discharge capacity of the battery can be improved while taking into account the high-temperature performance of the battery. At the same time, by adjusting the proportion of lithium element in the first binder within a suitable range, the present invention can avoid an excessive proportion of lithium element, which leads to an increase in high-temperature side reactions of the battery; and avoid too low a proportion of lithium element, which has an insignificant improvement on the low-temperature performance of the battery.

[0122] The second group of examples was carried out with reference to Example 1-1, and the main differences are shown in Table 2. Among them, the second group of examples changed the mass proportion of the amorphous carbon material coated in the negative electrode active material.

[0123] Table 2

[0124]

[0125] As can be seen from Table 2, in the present invention, by further controlling the mass proportion of the amorphous carbon material within a suitable range based on the total mass of the negative electrode active material, it is possible to avoid the situation where when the coating of the amorphous carbon material is too little, the ID / IG value is small and the OI value is large, resulting in an insignificant improvement in the low-temperature performance of the battery; and avoid the situation where when the coating of the amorphous carbon material is too much, the ID / IG value is large and the OI value is small, which deteriorates the high-temperature performance of the battery.

[0126] The third group of examples was carried out with reference to Example 1-1, and the main differences are shown in Table 3. Among them, the preparation method of the first binder in the third group of examples was referred to the preparation method of the first binder in Example 1-1, and the third group of examples changed the monomer types forming the lithiated first polymer in the first binder.

[0127] Table 3

[0128]

[0129] As can be seen from Table 3, by selecting different monomer types defined in the present invention to synthesize the first binder with lithiated first polymer, similar effect can be achieved, and the high and low temperature performance of the battery can be improved.

[0130] The fourth group of examples was carried out with reference to Example 1-1. Specifically, on the basis of Example 1-1, acrylonitrile monomer was further added to the lithiated first polymer, and the mass proportion of acrylonitrile monomer was changed based on the total mass of the first monomer and the second monomer forming the lithiated first polymer. The specific method was that acrylonitrile, acrylic acid, and acrylamide were subjected to soap-free emulsion polymerization in a reaction kettle; and neutralized with an aqueous solution of lithium carbonate (Li2CO3) to obtain the lithiated first polymer, and then the first binder was obtained. Among them, the mass fraction ratio of acrylic acid and acrylamide was the same as that in Example 1-1, and the main differences are shown in Table 4.

[0131] Table 4

[0132]

[0133] In the present invention, when the monomer forming the lithiated first polymer includes an acrylonitrile monomer, the low-temperature performance of the battery can be further improved. By controlling the mass ratio of the acrylonitrile monomer within a suitable range based on the total mass of the first monomer and the second monomer for forming the lithiated first polymer, it is possible to avoid the situation where the mass ratio of the acrylonitrile monomer is too low and the effect of further improving the low-temperature performance is not obvious; and to avoid the situation where the mass ratio of the acrylonitrile monomer is too high, which may reduce the binding performance of the first binder and lead to a decrease in the high-temperature performance of the battery.

[0134] In the fifth set of examples, on the basis of Example 4-1, a second binder was further added to the negative electrode sheet, and the mass ratio of the first binder in the negative electrode active material layer was adjusted. The main differences are shown in Table 5. Specifically, the preparation method of the second binder is as follows: 60 parts (parts by mass, the same hereinafter) of a third monomer (styrene and butadiene, with a mass ratio of 1:1), 200 parts of water, 4.5 parts of sodium stearate, and 0.5 part of a molecular weight regulator dodecyl mercaptan were sequentially added to a reaction kettle, protected by nitrogen, stirred at 300 rpm, and heated to 65°C. After continuing to stir for 20 min, 0.31 part of potassium persulfate was added, and the mixture was kept at 60°C for condensation, continuously stirred at 300 rpm, and reacted for 7 h. After the reaction ended, the pH value was adjusted with ammonia water, and the gel in it was filtered through a 200-mesh screen to obtain the second binder.

[0135] Table 5

[0136]

[0137] On the basis of using the first binder, the present invention further introduces a second binder with a lower glass transition temperature, which helps to improve the flexibility of the negative electrode sheet and improve the edge powder loss situation that may occur during the slitting of the negative electrode sheet. Further, the second binder can also construct an elastic binding network for the negative electrode sheet, effectively absorbing and buffering the stress generated due to the volume change of the active material, thereby further suppressing the swelling of the battery cell during the cycling process and improving the high and low temperature cycling performance of the battery.

[0138] In the sixth set of examples, on the basis of Example 5-1, an ester monomer was further added to the second binder, and the mass ratio of the ester monomer was changed based on the total mass of the third monomer and the ester monomer for forming the second polymer. The main differences are shown in Table 6. The preparation method of the second binder in the sixth set of examples refers to the preparation method of the second binder in Example 5-1. An ester monomer (butyl acrylate) was further added to the reaction kettle, and the second binder was obtained after polymerization. Among them, the mass ratio of styrene and butadiene is the same as that in Example 5-1.

[0139] Table 6

[0140]

[0141] From Table 6 and Figure 2 and Figure 3 As can be seen from the results shown, the present invention further includes an ester monomer in the second binder, which can improve the liquid absorption and retention capacity on the negative electrode side, further extend the cycle life of the battery, and can also accelerate the lithium ion transmission, thereby further improving the low-temperature charge and discharge performance of the battery. By further controlling the mass ratio of the ester monomer in the second polymer to meet a suitable range, the present invention can avoid that when the mass ratio of the ester monomer is too small, the affinity of the second binder for the electrolyte is poor, which is not conducive to further improving the low-temperature performance of the battery; and avoid that when the content of the ester monomer is too high, the liquid absorption capacity of the second binder is too strong, and the excessive absorbed electrolyte reduces the bonding performance of the second binder itself.

[0142] Group 7 of the examples was carried out with reference to Example 6-1, and the main difference is shown in Table 7. Among them, in Group 7 of the examples, the mass ratio of each solvent type in the electrolyte was changed.

[0143] Table 7

[0144]

[0145] By adjusting the respective proportions of the cyclic carbonate, linear carboxylate, and linear carbonate in the electrolyte, and at the same time, when the linear carboxylate and linear carbonate satisfy a specific relationship, the second binder can have a suitable swelling in the electrolyte, and the overall viscosity of the electrolyte can be reduced, further improving the high and low temperature performance of the battery.

[0146] It should be noted that in this article, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including a..." does not exclude the existence of additional identical elements in the process, method, article or device including the element. In addition, it should be pointed out that the scope of the methods and devices in the embodiments of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in a reverse order according to the functions involved. For example, the described methods may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0147] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A lithium-ion battery, characterized in that, The battery includes a negative electrode sheet; the negative electrode sheet includes a current collector and a negative electrode active material layer provided on at least one surface of the current collector; the negative electrode active material layer includes a first binder and a negative electrode active material; the first binder includes a lithiated first polymer; the monomer forming the lithiated first polymer includes a first monomer, and the first monomer includes at least one of acrylic acid or maleic anhydride; The negative electrode active material includes a graphite matrix, and a coating layer is provided on the surface of the graphite matrix, and the coating layer includes an amorphous carbon material.

2. The lithium ion battery according to claim 1, wherein The amorphous carbon material includes at least one of soft carbon material or hard carbon material, and based on the total mass of the negative electrode active material, the mass ratio of the amorphous carbon material is 1% to 5%; and / or, the glass transition temperature of the lithiated first polymer is 60°C to 100°C; and / or, in the lithiated first polymer, the lithium element content is 1.5% to 4%.

3. The lithium-ion battery according to claim 1, characterized in that, The monomer forming the lithiated first polymer further includes a second monomer, and the second monomer includes one or more of acrylonitrile, acrylamide, vinyl alcohol, and ethylene glycol, and preferably includes acrylonitrile monomer; Preferably, based on the total mass of the first monomer and the second monomer forming the lithiated first polymer, the mass ratio of the acrylonitrile monomer is 10% to 60%.

4. The lithium ion battery according to claim 1, wherein, The negative electrode sheet further includes a second binder, the second binder includes a second polymer, and the glass transition temperature of the second polymer is -40°C to 40°C; and / or, the particle size Dv50 of the second binder is 150 nm to 600 nm; and / or, the compaction density of the negative electrode sheet ranges from 1.2 g / cm 3 - 1.65 g / cm 3 .

5. The lithium-ion battery according to claim 4, characterized in that, The monomer forming the second polymer includes a third monomer, and the third monomer includes one or more of propylene, butadiene, styrene, vinylidene fluoride, dopamine, vinyl alkyl ether, ethylene oxide, and dimethyl siloxane; Preferably, the monomer forming the second polymer further includes an ester monomer, and based on the total mass of the third monomer and the ester monomer forming the second polymer, the mass ratio of the ester monomer is 5% to 70%, preferably 10% to 50%; More preferably, the ester monomer includes one or two of (meth)acrylic acid alkyl ester and (meth)acrylic acid hydroxyalkyl ester.

6. The lithium-ion battery according to any one of claims 1 to 5, characterized in that, Based on the total mass of the negative electrode active material layer, the mass ratio of the first binder is denoted as n%, and the mass ratio of the second binder is denoted as m%, and n and m satisfy: 1 ≤ n / m ≤ 3; Preferably, n satisfies: 0.8 ≤ n ≤ 5; Preferably, m satisfies: 0.1 ≤ m ≤ 5; Preferably, n and m satisfy: 1.5 ≤ n + m ≤ 5.

7. The lithium ion battery according to any one of claims 1 to 5, characterized in that, The battery includes an electrolyte, the electrolyte includes an organic solvent, the organic solvent includes a linear organic solvent and a cyclic organic solvent; the cyclic organic solvent includes a cyclic carbonate, and the linear organic solvent includes a linear carboxylic acid ester and a linear carbonate; Preferably, the cyclic carbonate includes at least one of ethylene carbonate, propylene carbonate, and butylene carbonate; Preferably, the linear carboxylic acid ester includes at least one of propyl acetate, ethyl propionate, ethyl acetate, propyl propionate, methyl acetate, and methyl propionate; Preferably, the linear carbonate includes at least one of ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, and methyl propyl carbonate.

8. The lithium ion battery according to claim 7, characterized in that, Based on the total mass of the electrolyte, the mass percentage of the cyclic carbonate is denoted as A%, the mass percentage of the linear carboxylic acid ester is denoted as B%, and the mass percentage of the linear carbonate is denoted as C%; wherein, B and C satisfy: 0.6 ≤ B / C ≤ 7, and / or, 30 ≤ B + C ≤ 80; Preferably, A satisfies: 1 ≤ A ≤ 50; Preferably, B satisfies: 5 ≤ B ≤ 70; Preferably, C satisfies: 10 ≤ C ≤ 80.

9. The lithium-ion battery according to any one of claims 1 to 5, characterized in that, The negative electrode active material is measured by a Raman spectrometer, and the peak intensity I within the range of 1300 cm -1 to 1400 cm -1 and the peak intensity I D within the range of 1550 cm -1 to 1650 cm -1 satisfy the following intensity ratio: 0.1 ≤ I G / I D / I G ≤ 0.8; Preferably, I D and I G satisfy: 0.3 ≤ I D / I G ≤ 0.6 with a numerical distribution ≥ 90; And / or, in the XRD pattern of the negative electrode sheet, the intensity of the diffraction peak of the (004) crystal plane with a diffraction angle 2θ located at 54 ± 1° is D004, and the intensity of the diffraction peak of the (110) crystal plane with a diffraction angle 2θ located at 78 ± 1° is D110. D004 and D110 satisfy: 1.0 ≤ D004 / D110 ≤ 10.

0.

10. The lithium-ion battery according to any one of claims 1 to 5, characterized in that, The negative electrode active material satisfies at least one of the following conditions: (i) Dv50 is 5 μm to 15 μm; (ii) Dv90 / Dv10 is 2.3 to 2.5; (iii) Dn10 ≥ 1.0 μm; (iv) The specific surface area is 0.5 m 2 / g to 1.5 m 2 / g; (v) The specific capacity is 320 mAh / g to 355 mAh / g.

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