A lithium ion secondary battery

Abstract

The application relates to the technical field of lithium ion batteries, and discloses a lithium ion secondary battery. The lithium ion secondary battery comprises a negative electrode sheet and an electrolyte, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer, the content of chromium elements in the negative electrode current collector is within 10-300 ppm, and a groove with a depth of H microns is arranged on the surface of the negative electrode active layer away from the negative electrode current collector; the electrolyte comprises 1,4-dicyano-2-butene with a content of 0.01-5%, and satisfies 0.0005<=E*H<=1.75. By controlling the content of chromium elements in the negative electrode current collector within the range, the current collector can have good conductivity and corrosion resistance; on the basis, the value of E*H is regulated, which can slow down the oxidation corrosion rate of the negative electrode current collector, improve the tensile strength of the negative electrode current collector, prevent the negative electrode current collector from being broken, and also ensure that the battery has high rate and high-temperature cycle performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a lithium ion secondary battery. BACKGROUND

[0002] In the lithium ion secondary battery, in order to improve the energy density of the lithium ion secondary battery, one of the methods commonly used in the industry is to improve the gram capacity of the negative active material. Silicon material is often used as the negative active material because of its high theoretical gram capacity. However, the silicon particles are prone to volume expansion during the lithium ion charging and discharging process, which leads to the destruction of the internal structure of the negative electrode material, seriously affecting the service life and stability of the battery. At present, the stability of the negative electrode sheet can be improved by adding 1,4-dicyano-2-butene in the electrolyte, and the presence of 1,4-dicyano-2-butene can also improve the high-temperature cycle and storage performance of the battery. However, 1,4-dicyano-2-butene will have a certain corrosive effect on the negative electrode current collector, not only reducing the conductivity of the current collector, making the overall internal resistance of the battery larger, affecting the rate performance of the battery, but also leading to the decrease of the mechanical properties of the copper foil, and even the fracture of the copper foil, thereby affecting the performance of the battery. SUMMARY

[0003] Therefore, the present application provides a lithium ion secondary battery, which aims to solve the problem that the existing lithium ion secondary battery cannot improve the high-temperature cycle and rate performance while improving the corrosion of the negative electrode current collector.

[0004] According to an embodiment of the present application, a lithium ion secondary battery is provided, which comprises a negative electrode sheet, a positive electrode sheet, a separator and an electrolyte.

[0005] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector; the negative electrode current collector comprises chromium elements, and the content of the chromium elements is Cppm based on the mass of the negative electrode current collector, 10≤C≤300; a groove is arranged on the surface of the negative electrode active layer away from the negative electrode current collector, and the depth of the groove is Hμm.

[0006] The electrolyte comprises 1,4-dicyano-2-butene, and the content of the 1,4-dicyano-2-butene is E based on the mass of the electrolyte, 0.01%≤E≤5%; and 0.0005≤E*H≤1.75 is satisfied.

[0007] In some optional embodiments, 0.005≤E*H≤1.5.

[0008] In some optional embodiments, 5≤H≤35.

[0009] In some optional embodiments, 200≤C / E≤3×10 6 .

[0010] In some optional embodiments, at least one of the following conditions is met:

[0011] a, 10≤C≤100;

[0012] b, 0.1%≤E≤4.3%;

[0013] c, 233≤C / E≤10 5 .

[0014] In some optional embodiments, the negative electrode sheet is provided with a recessed area with an opening facing the separator, the recessed area comprising a tab slot and a tab adhesive paper slot; along the thickness direction of the negative electrode sheet, the tab adhesive paper slot is higher than the tab slot to form a stepped surface, and the tab adhesive paper is arranged on the stepped surface, and the tab adhesive paper covers the tab slot.

[0015] In the thickness direction of the negative electrode sheet, the thickness T3 of the negative electrode active layer is greater than the depth T1 of the tab adhesive paper slot, and the depth T1 of the tab adhesive paper slot is greater than the thickness T2 of the tab protective adhesive paper.

[0016] And / or, in the length direction of the negative electrode sheet, the width W1 of the tab adhesive paper slot is greater than the width W2 of the tab adhesive paper, and the width W2 of the tab adhesive paper is greater than the width S of the tab slot.

[0017] In some optional embodiments, the length direction of the groove is parallel to the width direction of the negative electrode sheet, the number of the grooves is at least 2, and the distance between two adjacent grooves is L, 0.5mm≤L≤2mm.

[0018] The width S of the tab slot satisfies: 6mm≤S≤15mm, and 3≤S / L≤30.

[0019] In some optional embodiments, the distance between the intersection of the tab slot and the negative electrode active layer and the groove is G1, 0

[0020] In some optional embodiments, the distance between the end of the groove and the edge of the negative electrode active layer is G2, 0

[0021] In some optional embodiments, the negative electrode active layer comprises a first active layer and a second active layer arranged in a stack, the first active layer is located between the negative electrode current collector and the second active layer, and the ratio of the thickness of the second active layer to the total thickness of the negative electrode active layer is x, 0.1≤x≤0.5.

[0022] The first active layer comprises a first negative active material, the first negative active material comprises a first graphite, and a median particle size Dv501 of the first graphite is 12-20 μm.

[0023] The second active layer comprises a second negative active material, the second negative active material comprises a second graphite, and a median particle size Dv502 of the second graphite is 5-12 μm.

[0024] Dv501>Dv502 is satisfied.

[0025] In some optional embodiments, the first negative active material and / or the second negative active material further comprises a silicon-based material, and a content of the silicon-based material is D based on a mass of the first negative active material or the second negative active material, and 0.2≤D / E≤3000 and 0.01%≤E≤5% are satisfied.

[0026] In some optional embodiments, the D is 1%-30%.

[0027] In some optional embodiments, the D is 3%-30%, the E is 0.1%-4.3%, and 0.7≤D / E≤300.

[0028] In some optional embodiments, a charge cut-off voltage of the lithium ion secondary battery is ≥4.48 V.

[0029] The technical scheme has the following advantages:

[0030] The lithium ion secondary battery provided in the application comprises a negative electrode sheet, a positive electrode sheet, a diaphragm and an electrolyte, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector; the negative electrode current collector comprises a chromium element, the content of the chromium element is Cppm, 10≤C≤300, based on the mass of the negative electrode current collector; a groove is arranged on the surface of the negative electrode active layer away from the negative electrode current collector, the depth of the groove is Hμm; the electrolyte comprises 1,4-dicyano-2-butene, the content of the 1,4-dicyano-2-butene is E, 0.01%≤E≤5%, based on the mass of the electrolyte; and 0.0005≤E*H≤1.75 is satisfied. The research of the application shows that when the content of the chromium element in the negative electrode current collector is in the range of 10-300ppm, the negative electrode current collector can have good conductivity and corrosion resistance; on this basis, by regulating the groove depth H on the negative electrode sheet and the content E of the electrolyte additive 1,4-dicyano-2-butene to satisfy E*H in the range of 0.0005-1.75, on one hand, the rate of oxidation corrosion of the negative electrode current collector can be slowed down, the tensile strength of the negative electrode current collector can be improved, and the negative electrode current collector can be prevented from being broken, on the other hand, the battery can also have high rate and high temperature cycle performance.

[0031] Additional aspects and advantages of the embodiments of the application will be described in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following description and drawings. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0033] Figure 1 is a structural schematic diagram of a negative electrode sheet in one embodiment of the application.

[0034] Figure 2 is a partial cross-sectional schematic diagram of a negative electrode sheet in one embodiment of the application.

[0035] Figure 3 is Figure 1 a partial enlarged view of

[0036] In the drawings, the reference signs are explained as follows:

[0037] 1, negative electrode current collector; 2, negative electrode active layer; 3, groove; 4, tab slot; 5, tab; 6, tab rubber slot; 7, step surface. DETAILED DESCRIPTION

[0038] The following examples are provided to better enable those skilled in the art to further understand the application, and are not intended to limit the content and scope of the application. Any product that is the same as or similar to the present application, which is obtained by the disclosure of the present application or by combining the present application with other prior art features, falls within the scope of the present application.

[0039] It should be noted that the terms "inner", "outer", etc. indicate the position or location relationship based on the position or location relationship shown in the drawings, and are only for the purpose of describing the present application and simplifying the description, and do not indicate or imply that the device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0040] The specific experimental steps or conditions not specified in the examples can be performed according to the conventional experimental steps described in the literature in the art. The reagents or instruments not specified by the manufacturer are all conventional reagent products that can be obtained by purchase.

[0041] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as there is no conflict between them.

[0042] First of all, it should be noted that at high potential, the 1,4-dicyanobutene-2 additive in the electrolyte can participate in the formation of the SEI film on the negative electrode surface, stabilize the electrode-electrolyte interface, and effectively improve the high-temperature cycle and high-temperature storage performance of the battery. Unfortunately, the present application found that the process of 1,4-dicyanobutene-2 participating in the construction of the SEI film would cause oxidation corrosion to the negative electrode current collector, resulting in a decrease in the mechanical properties of the negative electrode current collector and even fracture.

[0043] To improve the corrosion resistance of the negative electrode current collector, the usual practice is to plate chromium on the surface of the negative electrode current collector to form a passivation layer. The higher the chromium content on the negative electrode current collector, the better the corrosion resistance, but it will also affect the conductivity of the negative electrode current collector.

[0044] Using a laser to process a certain depth and width of scribe (or "groove") on the surface of the negative electrode sheet can increase the contact area of the negative electrode sheet and the electrolyte, which is beneficial to the infiltration of the electrolyte, so that the electrochemical reaction is more sufficient, thereby improving the capacity and energy density of the battery. At the same time, it can also shorten the migration distance of lithium ions in the negative electrode sheet and improve the charge and discharge rate of the battery.

[0045] The present application researches and finds that, in the laser scribing process, the energy emitted by the laser is not only absorbed by the negative active layer, but also absorbed by the negative current collector, causing the passivation layer on the surface of the negative current collector (such as copper foil) to be damaged. With the increase of laser intensity, the groove depth increases, at the same time, the negative current collector absorbs more laser energy, and accordingly, the degree of damage to the passivation layer is greater, and even the copper foil is also affected. The damaged negative current collector is oxidized during the charging and discharging process, which reduces the conductivity of the negative current collector, increases the overall internal resistance of the battery, and on the other hand, reduces the thickness of the negative current collector, affecting its tensile strength.

[0046] To solve the above problems existing in the related art, the present application provides a lithium ion secondary battery, comprising a negative electrode sheet, a positive electrode sheet, a separator and an electrolyte;

[0047] The negative electrode sheet comprises a negative current collector and a negative active layer arranged on at least one side surface of the negative current collector;

[0048] The negative current collector comprises a chromium element, and the content of the chromium element is Cppm, 10≤C≤300, based on the mass of the negative current collector;

[0049] A groove is arranged on the surface of the negative active layer away from the negative current collector, and the depth of the groove is Hμm;

[0050] The electrolyte comprises 1,4-dicyano-2-butene, and the content of the 1,4-dicyano-2-butene is E, 0.01%≤E≤5%, based on the mass of the electrolyte; and 0.0005≤E*H≤1.75 is satisfied.

[0051] The present application researches and finds that when the content of the chromium element in the negative current collector is in the range of 10-300ppm, the negative current collector can have good conductivity and corrosion resistance; on this basis, by regulating the groove depth H on the negative electrode sheet and the content E of the electrolyte additive 1,4-dicyano-2-butene to satisfy E*H in the range of 0.0005-1.75, on the one hand, the rate of oxidation and corrosion of the negative current collector can be slowed down, the tensile strength of the negative current collector can be improved, and the negative current collector can be prevented from breaking, and on the other hand, the battery can have high rate and high temperature cycle performance.

[0052] If the content of 1,4-dicyanobutene-2 in the electrolyte is too high, the corrosion effect on the negative current collector is greater, at which time the groove on the negative plate needs to be shallower, i.e. the H value is reduced. On the contrary, if the content of 1,4-dicyanobutene-2 in the electrolyte is too low, although it is not conducive to improving the high-temperature cycle performance of the battery, the corrosion effect on the negative current collector is smaller, at which time the groove on the negative plate can be deeper, i.e. the H value needs to be increased, thereby facilitating the electrolyte to be soaked and improving the rate performance of the battery. It can be understood that the content of 1,4-dicyanobutene-2 in the electrolyte and the depth of the groove on the negative plate cannot be too large at the same time, otherwise the corrosion of the negative current collector will be accelerated, affecting the performance of the battery. That is, the depth of the groove H on the negative plate and the content E of the electrolyte additive 1,4-dicyanobutene-2 are negatively correlated, and the value of E*H needs to be controlled in the range of 0.0005-1.75, so as to balance the tensile strength of the negative current collector and the high-temperature cycle and rate performance of the battery.

[0053] As an example, the content C of chromium element in the negative current collector may be, for example, 10 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, etc. or within a range formed by any two of the above values. The content E of 1,4-dicyanobutene-2 in the electrolyte may be 0.01%, 0.1%, 0.25%, 0.5%, 0.75%, 1%, 2.5%, 4.3%, 5%, etc. or within a range formed by any two of the above values. The value of E*H may be 0.0005, 0.005, 0.05, 0.1, 0.5, 1, 1.5, 1.75, etc. or within a range formed by any two of the above values.

[0054] Further research found that when the product E*H of the depth H of the groove on the negative plate and the content E of 1,4-dicyanobutene-2 in the electrolyte is in the range of 0.005-1.5, the rate of oxidation and corrosion of the negative current collector can be more effectively slowed down, the negative current collector is prevented from being broken, and the rate and high-temperature cycle performance of the battery are ensured. In particular, when the content E of 1,4-dicyanobutene-2 in the electrolyte is in the range of 0.1%-4.3%, it can not only play a role in improving the lattice stability of the positive material, but also will not cause too much corrosion to the negative current collector, which is conducive to maintaining the strength of the negative current collector and thus ensuring the performance of the battery.

[0055] In some embodiments, the groove depth H on the negative electrode sheet is 5-35 μm, which may, for example, be 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, or within a range between any two of the above values. This can on the one hand ensure the wettability of the electrolyte on the negative electrode sheet, reduce the negative electrode polarization, reduce the battery internal resistance, and ensure good rate performance; on the other hand, it can also not cause excessive damage to the negative current collector, which is conducive to maintaining a high tensile strength of the negative current collector, thereby ensuring the performance of the battery.

[0056] The present application has found that when the ratio C / E of the content C of chromium element in the negative current collector to the content E of 1,4-dicyano-2-butene in the electrolyte falls within the range of 200-3x10 6 , the conductivity of the negative current collector can be ensured while preventing the electrolyte from corroding the negative current collector, thereby ensuring the rate and high-temperature cycle performance of the battery.

[0057] If the content of chromium element in the negative current collector is high, it indicates that the corrosion resistance of the negative current collector is good, in which case the content of 1,4-dicyano-2-butene additive in the electrolyte can be appropriately increased to improve the stability of the negative electrode sheet and the high-temperature cycle performance of the battery; conversely, if the content of chromium element in the negative current collector is low, it indicates that the corrosion resistance of the negative current collector is poor, in which case the content of 1,4-dicyano-2-butene additive in the electrolyte should be appropriately reduced to ensure that the negative current collector meets the strength requirement, thereby ensuring the performance of the battery.

[0058] For example, the value of C / E can be 200, 233, 600, 1000, 5000, 10000, 50000, 100000, 500000, 1000000, 3000000, or within a range between any two of the above values. In particular, when C / E falls within the range of 233-10 5 , the negative current collector can have a higher tensile strength, and the battery can have better rate and high-temperature cycle performance.

[0059] In a wound lithium ion secondary battery, the winding core is formed by winding the positive electrode sheet, the negative electrode sheet, and the separator between the positive and negative electrode sheets. In some embodiments, the planar view and the partial cross-sectional view of the negative electrode sheet are shown in Figure 1 , Figure 2 , respectively. Referring to Figure 1 and Figure 2At the middle position of the negative electrode sheet, a recessed area is provided with an opening facing the separator, the recessed area includes a tab slot 4 and a tab adhesive paper slot 6; along the thickness direction (z direction) of the negative electrode sheet, the tab adhesive paper slot 6 is higher than the tab slot 4 to form a stepped surface 7, on which the tab adhesive paper (not shown in the figure) is provided, which covers the tab slot 4. In the thickness direction of the negative electrode sheet, the thickness T3 of the negative electrode active layer is greater than the depth T1 of the tab adhesive paper slot, and the depth T1 of the tab adhesive paper slot is greater than the thickness T2 of the tab protective adhesive paper.

[0060] It can be understood that the tab 5 is welded in the tab slot 4, so that a welding point protrusion is formed at the tab welding position. In order to prevent the tab protrusion from piercing the separator and causing short circuit, it is common practice to paste tab adhesive paper at the opening of the tab slot to isolate the welding point protrusion from the separator. However, due to the thickness of the tab adhesive paper itself, the tab protrusion will form a bridging mode with the separator after winding, which will cause the lithium ion conduction interface path to be longer at this position, the impedance to increase, and also easily cause interface defects such as bubbles, pores, etc., causing lithium precipitation during battery charging. Therefore, the present application adopts a stepped adhesive embedding technology to embed the tab adhesive paper in the negative electrode active layer to offset the thickness sacrifice of the battery cell caused by the thickness of the adhesive paper, so as to achieve the purpose of improving the energy density of the battery cell.

[0061] Further, in some embodiments, in the length direction (x direction) of the negative electrode sheet, the width W1 of the tab adhesive paper slot is greater than the width W2 of the tab adhesive paper, and the width W2 of the tab adhesive paper is greater than the width S of the tab slot. Thereby the flatness of the tab adhesive paper can be ensured, so that the tab adhesive paper can completely cover the exposed copper foil in the tab slot to prevent the copper foil current collector from being oxidized and corroded by the electrolyte.

[0062] In order to prevent the exposed copper foil current collector in the tab slot from reducing in strength after absorbing heat due to the large heat radiation range of the laser when laser scribing, in some embodiments of the present application, please refer to Figure 3 , it is necessary to ensure that the distance G1 between the groove and the edge of the tab slot (i.e. the intersection of the tab slot and the negative electrode active layer) is greater than 0 and less than or equal to 5mm, which can be 0.1mm, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, etc. or within the range composed of any two of the above values.

[0063] It can be understood that the width of the negative electrode active layer is less than the width of the negative electrode current collector, thereby forming a blank foil area, i.e. the exposed copper foil current collector, in the width direction (y direction) of the negative electrode current collector. In order to prevent the exposed copper foil current collector in the blank foil area from reducing in strength after absorbing heat due to the large heat radiation range of the laser when laser scribing, in some embodiments of the present application, it is necessary to ensure that the length of the groove is less than the width of the negative electrode active layer. Please refer to Figure 3For example, the distance between the two ends of the groove (the length of the groove) and the edge of the negative active layer adjacent to the end is G2, which is greater than 0 and less than or equal to 5 mm, and can be 0.1 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, etc. or within a range defined by any two of the above values.

[0064] In some embodiments, the negative active layer is combined with Figure 1 and Figure 3 The groove on the negative tab can be linear or curved. For ease of processing, linear grooves are often used. The number of grooves can be one, two, three or more. To balance battery performance, multiple grooves are usually required, which can be evenly spaced or irregularly spaced. To facilitate processing, multiple grooves are evenly spaced in this application, and the distance between two adjacent grooves is L, which satisfies 0.5 mm≤L≤2 mm. For example, the distance between grooves L can be 0.5 mm, 1 mm, 1.5 mm, 2 mm, etc. or within a range defined by any two of the above values. Each groove can extend along the width of the negative tab.

[0065] The present application found that when the width S of the tab slot (in the length direction of the negative tab) and the distance L between the grooves satisfy 3≤S / L≤30, the exposed copper foil in the tab slot can be reduced. The effect of laser radiation can prevent the tab from being insufficiently welded or the negative current collector from being damaged after welding. In some embodiments, the width S of the tab slot is in the range of 6-15 mm, for example, it can be 6 mm, 8 mm, 10 mm, 12 mm, 15 mm, etc. or within a range defined by any two of the above values. The ratio S / L between the tab slot width and the groove spacing can be 3, 5, 8, 10, 13, 15, 18, 20, 23, 25, 28, 30, etc. or within a range defined by any two of the above values.

[0066] To optimize battery performance, the negative paste can use a double-layer coating technique, so in some embodiments of the present application, the negative active layer includes a first active layer and a second active layer stacked, and the first active layer is located between the negative current collector and the second active layer.

[0067] The first active layer comprises a first negative active material, the first negative active material comprises a first graphite, a median particle size Dv501 of the first graphite is 12-20 μm, for example, can be 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, etc. or in the range formed by any two of the above values. The second active layer comprises a second negative active material, the second negative active material comprises a second graphite, a median particle size Dv502 of the second graphite is 5-12 μm, for example, can be 5 μm, 7 μm, 9 μm, 10 μm, 11 μm, 12 μm, etc. or in the range formed by any two of the above values, and the median particle size Dv501 of the first graphite is greater than the median particle size Dv502 of the second graphite.

[0068] It should be noted that the median particle size Dv501 of the first graphite and the median particle size Dv502 of the second graphite can be obtained by laser particle size instrument test.

[0069] The present application researches and finds that the second active layer located on the surface of the negative electrode sheet comprises small particle graphite, which is beneficial to the infiltration of electrolyte and the deintercalation of lithium ions, which not only improves the charge and discharge rate of the battery, but also increases the risk of corrosion of the negative electrode current collector. Therefore, on the one hand, the present application controls the ratio x of the thickness of the second active layer to the total thickness of the negative active layer to satisfy 0.1≤x≤0.5, so as to balance the rate performance of the battery and the tensile strength of the copper foil, and on the other hand, a first active layer comprising large particle graphite is arranged under the second active layer. Compared with small particle graphite, the large particle graphite has better structural stability, which is beneficial to the cycle performance of the battery, and through the particle size grading of large / small particle graphite, the compaction density of the negative electrode sheet can also be improved, further improving the problem that the negative electrode current collector is easily corroded, thereby ensuring the performance of the battery.

[0070] For example, the value of the ratio x of the thickness of the second active layer to the total thickness of the negative active layer, for example, can be 0.1, 0.2, 0.2, 0.4, 0.5, etc. or in the range formed by any two of the above values.

[0071] Further, when the ratio x of the thickness of the second active layer to the total thickness of the negative active layer, the content E of the 1,4-dicyano-2-butene additive in the electrolyte, the median particle size Dv501 of the first graphite and the median particle size Dv502 of the second graphite satisfy: 200≤(x*Dv502+(1-x)*Dv501) / E≤190000, the tensile strength of the copper foil can be ensured while the rate and cycle performance of the battery are balanced, thereby ensuring the performance of the battery.

[0072] As an example, the value of (x*Dv502+(1-x)*Dv501) / E can be 200, 1000, 5000, 10000, 25000, 50000, 75000, 100000, 150000, 190000, or within a range between any two of the above values.

[0073] It can be understood that the silicon negative electrode has the advantages of high specific capacity and high energy density, and when applied to the lithium ion secondary battery of the present application, the energy density of the battery cell can be significantly improved. Specifically, in some embodiments of the present application, the first negative electrode active material and / or the second negative electrode active material further comprises a silicon-based material, and the content of the silicon-based material is D, 1%≤D≤30%, based on the mass of the first negative electrode active material or the second negative electrode active material. As an example, the incorporation amount of the silicon-based material may, for example, be 1%, 3%, 7%, 10%, 15%, 20%, 25%, 30%, or within a range between any two of the above values.

[0074] The present application research found that when the content D of the silicon-based material and the content E of the 1,4-dicyano-2-butene additive in the electrolyte satisfy 0.2≤D / E≤3000, especially 0.7≤D / E≤300, the stability of the negative electrode material can be further ensured on the basis of preventing the corrosion of the copper foil, thereby being beneficial to improving the cycle and safety performance of the battery. As an example, the value of D / E can be 0.2, 0.7, 2, 10, 50, 100, 200, 300, 500, 1000, 3000, or within a range between any two of the above values.

[0075] It should be noted that the charging cutoff voltage of the lithium ion secondary battery provided by the present application is ≥4.48V, which is higher than the charging upper limit voltage of the conventional lithium ion secondary battery, so that the lithium ion secondary battery of the present application has more advantages in energy density, use time and power performance.

[0076] The present application will be further described in detail below in conjunction with specific examples, which cannot be understood as limiting the scope of the present application. If the specific experimental steps or conditions are not specified in the examples and comparative examples, the operation or conditions can be carried out according to the conventional experimental steps described in the literature in the art. If the reagents or instruments used are not specified by the manufacturer, they are all conventional reagent products that can be obtained by market purchase. In all examples and comparative examples of the present application, the unit % represents the mass percentage.

[0077] Example 1

[0078] First step: preparation of positive electrode sheet

[0079] Lithium cobalt oxide, conductive carbon black (SP), polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 97.6:1.4:1, N-methyl pyrrolidone (NMP) is added and stirred uniformly to prepare a positive electrode slurry.

[0080] The positive electrode slurry is coated on both surfaces of the aluminum foil, and after baking and rolling, a positive electrode sheet with a thickness of 80 μm is obtained. The positive electrode sheet has one fixed-size slot, and the aluminum tab is laser-welded in the slot to obtain a positive electrode sheet containing an aluminum tab.

[0081] Second step: negative electrode sheet preparation

[0082] The first negative electrode active material is mixed with SP, lithium carboxymethyl cellulose (CMC-Li), and styrene-butadiene rubber (SBR) in a mass ratio of 97:0.4:0.1:2.5, and deionized water is added to prepare a first negative electrode slurry. The first negative electrode active material is silicon-carbon-doped graphite, the graphite particle size Dv501 is 13 μm, the weight of silicon-carbon accounts for 5% of the total weight of the first negative electrode active material, and the silicon content of the silicon-carbon particles is 50%.

[0083] The second negative electrode active material is mixed with SP, CMC-Li, and polyacrylic acid (PAA) in a mass ratio of 97:0.4:0.1:2.5, and deionized water is added to prepare a second negative electrode slurry. The second negative electrode active material is silicon-carbon-doped graphite, the graphite particle size Dv502 is 8 μm, the weight of silicon-carbon accounts for 5% of the total weight of the second negative electrode active material, and the silicon content of the silicon-carbon particles is 50%.

[0084] The first negative electrode slurry is coated on both surfaces of the chromium-plated copper foil, and the copper foil has a Cr element content of 100 ppm. The second negative electrode slurry is then coated on the surface of the first negative electrode slurry, and after baking and rolling, a negative electrode sheet with a thickness of 95 μm is obtained. The negative electrode sheet has a tab slot with a width S = 9 mm, and the depth of the negative electrode active layer is T3 = 45 μm. A nickel-plated copper tab with a width of 6 mm is laser-welded in the slot, and a protective adhesive is attached to the surface of the tab. The protective adhesive has a width W2 = 12 mm and a thickness T2 = 12 μm. The protective adhesive is housed in the tab slot, and the projection of the tab slot on the copper foil falls within the projection range of the tab slot on the copper foil. The tab slot has a depth T1 = 15 μm and a width W1 = 14 mm.

[0085] A plurality of equally spaced grooves are formed on the surface of the negative electrode sheet using a laser. The groove spacing L = 1.2 mm, the groove depth H = 15 μm, the groove end distance G1 = 0.5 mm from the edge of the tab slot, and the groove end distance G2 = 0.5 mm from the edge of the negative electrode active layer.

[0086] Third step: after the positive and negative electrode sheets are cut and sheeted, the separator is wound to obtain a core.

[0087] The fourth step: the core is packaged, baked, injected, formed, sealed, sorted and OCV to obtain a lithium ion battery; wherein the electrolyte is a commercially available conventional electrolyte, the lithium salt in the electrolyte is LiFP6, and the additive amount of the additive 1,4-dicyano-2-butene is 1%.

[0088] The preparation methods of Examples 2-6 and Examples 1-1 to 1-26 are basically the same as those of Example 1, except for the differences shown in Tables 1 and 2. Among them, E is the content of 1,4-dicyano-2-butene in the electrolyte, H is the depth of the groove on the negative plate, C is the content of chromium element in the chromium-plated copper foil, L is the pitch of the groove on the negative plate, S is the width of the tab groove on the negative plate, D is the silicon content in the negative active material, Dv501 is the median particle size of graphite in the first active layer, Dv502 is the median particle size of graphite in the second active layer, and x is the ratio of the thickness of the second active layer to the total thickness of the negative active layer.

[0089] Table 1

[0090]

[0091]

[0092] Table 2

[0093]

[0094]

[0095] Test Example

[0096] 1. Particle size test

[0097] After disassembling the lithium ion secondary battery to 0% SOC, the negative plate is taken out, soaked in dimethyl carbonate (DMC) solvent for 12h, then washed with DMC to remove the lithium salt attached to the plate, then the negative active layer is washed off from the plate with deionized water, after ultrasonic centrifugation, the filtrate is removed and dried, and the Malvern particle size tester is used for measurement.

[0098] The test steps are as follows: the negative active layer material is dispersed in deionized water containing a dispersant nonylphenol polyoxyethylene ether (content 0.02-0.03wt%), to form a mixture, the mixture is ultrasonicated for 2 minutes, and then put into the Malvern particle size tester for testing.

[0099] Among them, the median particle size of the second graphite is obtained by taking the negative powder within 10μm thickness on the upper layer of the plate; the median particle size of the first graphite is obtained by taking the negative powder within 20μm thickness on the lower layer of the plate.

[0100] 2. Silicon-based material content test

[0101] After disassembling the lithium ion secondary battery to 0% SOC, the negative electrode sheet is taken out, soaked in DMC for 12h, then rinsed with DMC to remove the lithium salt attached to the sheet, and dried. The active layer of the negative electrode is peeled off from the current collector after high-temperature treatment of the sheet in an inert atmosphere at 400°C for 2h, and the negative electrode active material is collected.

[0102] In the silicon content test, a TGA 550 thermal gravimetric analyzer is used, the sample amount for testing is 5-15mg, the temperature is raised from room temperature to 900°C at a rate of 10°C / min in air or oxygen atmosphere, and the temperature is kept at 900°C for 40min, so that the non-silicon components in the negative electrode active material are volatilized while the silicon is fully oxidized to silicon dioxide. The weight percentage at the end of the entire test process is the ash content of the negative electrode active material. The ash value is divided by the molar mass of silicon dioxide (60) and then multiplied by the molar mass of silicon (28) to obtain the percentage of silicon in the negative electrode active material.

[0103] 3. Chromium content test on negative electrode current collector

[0104] The Cr element concentration in the standard solution measured according to the test method in GBT 30902-2014 is in ppm, which is the chromium content on the negative electrode current collector.

[0105] 4. Groove depth and groove spacing test

[0106] After disassembling the lithium ion secondary battery to 0% SOC, the negative electrode sheet is taken out, soaked in dimethyl carbonate (DMC) solvent for 12h, then rinsed with DMC to remove the lithium salt attached to the sheet, and tested using a 3D profilometer.

[0107] 5. Battery internal resistance test

[0108] Charge the battery to 50% SOC at 0.5C rate, and test using an AC resistance meter or a multimeter.

[0109] 6. Rate performance test

[0110] After keeping in a 25°C constant temperature room for 2h, discharge at 0.2C constant current to 3.0V, stand for 5min, then charge at 3C constant current to 4.2V, convert to 2.5C constant current to 4.25V, convert to 2C constant current to 4.48V (1.2C cut-off), record the time of charging at 3C constant current to 4.2V, and set the sampling recording time every 1s on the blue light tester.

[0111] 7. Copper foil tensile strength test

[0112] After discharging the lithium ion secondary battery to 0% SOC, the negative electrode sheet was taken out, the head part of the copper foil was taken, and after soaking in dimethyl carbonate (DMC) solvent for 12 h, the lithium salt attached to the electrode sheet was removed by washing with DMC, and after drying, the negative electrode current collector was cut into a sample to be tested with a width of 15 mm and a length of more than 50 mm using a knife; using a WD-D3 electronic universal testing machine (precision of 0.5 level, accuracy of ±1% of the indicated value), setting the gauge length to 50 mm and the speed to 50 mm / min, the tensile strength of the copper foil was measured by tensile testing of the sample to be tested.

[0113] 8. 45℃ high temperature cycle capacity retention rate test

[0114] After standing in a 45℃ constant temperature room for 2h, the sample was charged at 3C constant current to 4.2V, then charged at 2C constant current and constant voltage to 4.48V, cut off at 0.05C, and after standing for 10min, discharged at 0.7C to 3.0V, and this cycle was repeated for 500T. The discharge capacity was C1, the first cycle discharge capacity was C0, and the C1 / C0 was the capacity retention rate after 500T cycles.

[0115] The above test results are shown in Tables 1-3.

[0116] Table 3

[0117]

[0118]

[0119] It can be seen from Tables 1-3 that compared with Comparative Examples 1-2, the battery internal resistance of all the examples is not more than 34mΩ, the time required for 3C constant current charging to 4.2V is not less than 1.2min, the 45℃ high temperature cycle capacity retention rate is more than 70%, and the tensile strength of the copper foil is not less than 490MPa, which shows that the lithium ion secondary battery of the present application can improve the high temperature cycle and rate performance while effectively improving the tensile strength of the negative electrode current collector. It should be noted that the smaller the battery internal resistance, the longer the large rate charging time, i.e. the faster the full charge, and the better the rate performance.

[0120] Compared with Example 1, the content of 1,4-dicyanobutene-2 in the electrolyte of Example 1-3 is low, which leads to poor effect of improving the lattice stability of the positive electrode material, and lattice collapse is prone to occur during high-temperature cycling, causing irreversible damage to the positive electrode, so the high-temperature cycling performance is relatively poor; at the same time, the groove depth of the negative electrode sheet of Example 1-3 is small, which makes the negative electrode polarization larger, resulting in an increase in the overall internal resistance of the battery, and the rate performance is poor; but at the same time, it also reduces the contact between the electrolyte and the copper foil, thereby reducing the corrosion of the copper foil and ensuring the tensile strength of the copper foil. The case of Example 1-6 is just the opposite of Example 1-3, so the rate and high-temperature cycling performance of Example 1-6 are good, and the tensile strength of the copper foil is significantly reduced but can still meet the requirements.

[0121] It should be noted that the tensile strength test result of the copper foil is above 480 MPa, indicating that the negative current collector has good corrosion resistance and is not prone to belt breakage during battery cycling.

[0122] Compared with Example 1, the chromium content in the negative current collector of Example 1-7 is low, which is beneficial to reduce the current collector resistance and thus the overall internal resistance of the battery, thereby optimizing the rate performance, and the content of 1,4-dicyanobutene-2 in the electrolyte is high, which is beneficial to the high-temperature cycling performance of the battery, but due to the small ratio of chromium content to 1,4-dicyanobutene-2 content, although it is not conducive to maintaining a high tensile strength of the copper foil, it can still meet the requirements through practical testing. The case of Example 1-11 is just the opposite of Example 1-7, so the rate and high-temperature cycling performance of Example 1-11 are poor, but the tensile strength of the copper foil is high.

[0123] Compared with Example 1, the groove spacing L on the negative electrode sheet of Example 1-17 is large, and the negative electrode tab slot width S is small, so that the ratio of the negative electrode tab slot width S to L is too small, the grooves are sparse, the improvement effect on the negative electrode polarization is poor, and the negative electrode conductivity is reduced, resulting in an increase in the internal resistance of the battery and affecting the rate performance. The case of Example 1-18 is just the opposite of Example 1-17, so the internal resistance of the battery of Example 1-18 is small and the rate performance is good, but due to the dense grooves, the tensile strength of the copper foil is reduced.

[0124] Compared with Example 1, the silicon content of the negative electrode sheet of Example 1-22 is very small, which is beneficial to reduce the internal resistance of the battery and optimize the rate performance, and due to fewer side reactions, the high-temperature capacity retention rate is high, and the content of 1,4-dicyanobutene-2 in the electrolyte is low, so it is beneficial to ensure the tensile strength of the copper foil. The silicon content of the negative electrode sheet of Example 1-23 is high, and due to the high strength and expansion of silicon-based materials, the copper foil will be damaged, resulting in a decrease in the tensile strength of the copper foil, and it is also not conducive to the high-temperature cycling performance.

[0125] Compared with Example 1, the graphite particle size in the negative electrode sheet of Example 1-26 is smaller, the conductivity is good, the battery internal resistance is smaller, the rate performance is better, and the content of 1,4-dicyan-2-butene in the electrolyte is high, so the high-temperature cycle performance is good but the tensile strength of the copper foil is low. The graphite particle size in the negative electrode sheet of Example 1-27 is larger, which affects the conductivity, so the battery internal resistance is larger, which affects the rate performance, and the content of 1,4-dicyan-2-butene in the electrolyte is low, which is beneficial to ensure the tensile strength of the copper foil but is not conducive to the high-temperature cycle performance.

[0126] Compared with Example 1, the content of 1,4-dicyan-2-butene in the electrolyte of the battery of Comparative Example 1 is too low, so the value of E*H is too small, which ensures the tensile strength of the copper foil but seriously affects the high-temperature cycle performance of the battery, and since the groove depth on the negative electrode sheet is small, the electrolyte infiltration effect is poor, which deteriorates the rate performance. On the contrary, the content of 1,4-dicyan-2-butene in the electrolyte of Comparative Example 2 is too high, so the value of E*H is too large, the copper foil is severely corroded, which leads to the tensile strength of the copper foil being too low to meet the requirements, and it is also not conducive to the performance of the battery, which leads to the rate and high-temperature cycle performance of the battery being poor.

[0127] Obviously, the above examples are only examples for clearly illustrating but not limiting the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, it is not necessary and impossible to enumerate all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A lithium ion secondary battery comprising a negative electrode sheet, a positive electrode sheet, a separator, and an electrolyte; characterized by, The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector; The negative electrode current collector comprises a chromium element, and the content of the chromium element is Cppm based on the mass of the negative electrode current collector, 10≤C≤300; A groove is arranged on the surface of the negative electrode active layer away from the negative electrode current collector, and the depth of the groove is Hμm, 5≤H≤35; The electrolyte comprises 1,4-dicyano-2-butene, and the content of the 1,4-dicyano-2-butene is E based on the mass of the electrolyte, 0.01%≤E≤5%; satisfies: 0.0005 < E*H < 1.75, 200 < C / E < 3 x 10 6 .

2. The lithium-ion secondary battery according to claim 1, characterized by 0.005≤E*H≤1.5 is met.

3. The lithium-ion secondary battery according to claim 1, characterized by At least one of the following conditions is met: a, 10≤C≤100; b, 0.1%≤E≤4.3%; c. 233 < C / E < 10 5 .

4. The lithium-ion secondary battery according to claim 1, 2 or 3, characterized by The negative electrode sheet is provided with a recessed area with an opening facing the separator, and the recessed area comprises a tab slot and a tab adhesive paper slot; along the thickness direction of the negative electrode sheet, the tab adhesive paper slot is higher than the tab slot to form a stepped surface, and the tab adhesive paper is arranged on the stepped surface and covers the tab slot; In the thickness direction of the negative electrode sheet, the thickness T3 of the negative electrode active layer is greater than the depth T1 of the tab adhesive paper slot, and the depth T1 of the tab adhesive paper slot is greater than the thickness T2 of the tab adhesive paper; And / or, in the length direction of the negative electrode sheet, the width W1 of the tab adhesive paper slot is greater than the width W2 of the tab adhesive paper, and the width W2 of the tab adhesive paper is greater than the width S of the tab slot.

5. The lithium-ion secondary battery according to claim 4, characterized by The length direction of the groove is parallel to the width direction of the negative electrode sheet, the number of the grooves is at least two, and the distance between two adjacent grooves is L, 0.5mm≤L≤2mm; The width S of the tab slot satisfies 6mm≤S≤15mm, and 3≤S / L≤30.

6. The lithium-ion secondary battery according to claim 4, characterized by The distance between the intersection of the tab slot and the negative electrode active layer and the groove is G1, 0<G1≤5mm; And / or, the distance between the end of the groove and the edge of the negative electrode active layer is G2, 0<G2≤5mm.

7. The lithium-ion secondary battery according to claim 1, 2 or 3, characterized by The negative electrode active layer comprises a first active layer and a second active layer arranged in a stack, the first active layer is located between the negative electrode current collector and the second active layer, and the ratio of the thickness of the second active layer to the total thickness of the negative electrode active layer is x, 0.1≤x≤0.5; The first active layer comprises a first negative electrode active material, and the first negative electrode active material comprises first graphite, and the median particle size Dv501 of the first graphite is 12μm-20μm; The second active layer comprises a second negative electrode active material, and the second negative electrode active material comprises second graphite, and the median particle size Dv502 of the second graphite is 5μm-12μm; Dv501>Dv502 is met.

8. The lithium-ion secondary battery according to claim 7, characterized by The first negative electrode active material and / or the second negative electrode active material further comprises a silicon-based material, and the content of the silicon-based material is D based on the mass of the first negative electrode active material or the second negative electrode active material, and 0.2≤D / E≤3000 is met.

9. The lithium-ion secondary battery according to claim 8, characterized by The D is 1%-30%.

10. The lithium-ion secondary battery according to claim 8, characterized by The D is 3%-30%, the E is 0.1%-4.3%, 0.7≤D / E≤300.

11. The lithium-ion secondary battery according to any one of claims 1, 2, 3, 5, 6, 8, 9, or 10, characterized by, The charging cut-off voltage of the lithium ion secondary battery is ≥4.48V.

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