Secondary battery

Abstract

The invention relates to the field of secondary batteries, and discloses a secondary battery which comprises a battery cell, and a negative active material layer of the battery cell comprises a silicon-based material; an accommodating cavity for accommodating the battery cell is formed in the packaging film; the packaging film comprises a metal layer and a heat sealing layer which are arranged in a stacked mode. The heat sealing layer is arranged on the side, facing the containing cavity, of the metal layer. The outer surface of the battery cell is provided with an edge region close to at least one side in the length direction of the battery cell; the first bonding piece is attached to the edge area; the heat sealing layer is provided with a first groove which is sunken towards the metal layer, and at least part of the first bonding piece is arranged in the first groove in the thickness direction of the battery cell. The groove structure is constructed to provide an accommodating space for the first bonding piece, so that the risk that the outermost ring of pole piece is overturned to scratch the heat sealing layer is reduced; the thickness fluctuation caused by expansion is relieved, the thickness consistency and the cycling stability of the battery are improved, and the energy density of the battery can also be improved.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and specifically to a secondary battery. Background Technology

[0002] Silicon-based materials are becoming an important direction for battery technology development due to their significant high energy density. However, the dramatic volume expansion of silicon-containing anodes during charging and discharging is a detrimental factor affecting the long-term performance of batteries. This problem is particularly prominent in pouch batteries: due to the continuous effect of expansion stress, the electrode sheets located at the edges of the outer surface of the cell are prone to folding or warping, scratching, abrasion, or even puncturing the inner wall of the encapsulation film.

[0003] Adhesives are typically attached to the edge areas of the outer surface of the battery cell to prevent the electrode sheets from folding or warping and puncturing the encapsulation film due to cell expansion. However, the installation of adhesives not only occupies space and affects the overall energy density of the battery, but also affects the consistency of adhesion between the cell and the encapsulation film, as well as the consistency of battery thickness, accelerating interface degradation, increasing the risk of internal short circuits, and ultimately impairing the long-term cycle performance and safety of the battery. Summary of the Invention

[0004] The purpose of this application is to solve at least some of the technical problems mentioned above, and this purpose is achieved through the following technical solutions: This invention provides a secondary battery, comprising: a battery cell, the battery cell including a positive electrode, a separator, and a negative electrode, the negative electrode including a negative electrode active material layer disposed on at least one side surface of a negative electrode current collector, the negative electrode active material layer including a silicon-based material; an encapsulation film, the encapsulation film forming a receiving cavity for accommodating the battery cell; the encapsulation film including a stacked metal layer and a heat-sealing layer, the heat-sealing layer being disposed on the side of the metal layer facing the receiving cavity; the battery cell including an outer surface, the outer surface having a first surface and a second surface disposed at intervals along the thickness direction of the battery cell, the first surface and / or the second surface having an edge region near at least one side along the length direction of the battery cell; a first adhesive member, the first adhesive member being attached to the edge region; the heat-sealing layer having a first groove recessed towards the metal layer, the first adhesive member being at least partially embedded in the first groove along the thickness direction of the battery cell.

[0005] In some embodiments, along the cell thickness direction, the size of the first adhesive is T1 μm and the size of the first groove is D1 μm, satisfying: 0.7≤D1 / T1≤1.5.

[0006] In some embodiments, along the width direction of the battery cell, the size of the first groove is L1 mm and the size of the first adhesive is L2 mm; along the length direction of the battery cell, the size of the first groove is W1 mm ​​and the size of the first adhesive is W2 mm; satisfying: L2+0.5≤L1≤L2+2, W2+0.5≤W1≤W2+2.

[0007] In some embodiments, the size of the first groove along the cell thickness direction is D1 μm, satisfying: 5≤D1≤54.

[0008] In some embodiments, the size of the first adhesive member along the cell thickness direction is T1 μm, satisfying: 4.5≤T1≤45.

[0009] In some embodiments, along the cell thickness direction, the size of the first groove is D1 μm and the size of the heat-sealing layer is X1 μm, satisfying: 30%≤D1 / X1≤90%.

[0010] In some embodiments, the orthographic projection of the first groove onto the first surface completely covers the orthographic projection of the first adhesive onto the first surface.

[0011] In some embodiments, the roughness of the inner surface of the first groove is R1, and the roughness of the side of the first adhesive member facing away from the battery cell is R2, satisfying: 2 < R1 / R2 < 15.

[0012] In some embodiments, the first surface and / or the second surface have a central region located between the edge regions along the length direction of the cell; the secondary battery includes a second adhesive member attached to the central region; the heat-sealing layer is provided with a second groove recessed toward the metal layer; along the thickness direction of the cell, the second adhesive member is at least partially embedded in the second groove.

[0013] In some embodiments, the orthographic projection of the second groove on the first surface completely covers the orthographic projection of the second adhesive on the first surface.

[0014] In some embodiments, along the width direction of the battery cell, the size of the second groove is L3 mm and the size of the second adhesive is L4 mm; along the length direction of the battery cell, the size of the second groove is W3 mm and the size of the second adhesive is W4 mm; satisfying: L4+0.5≤L3≤L4+2, W4+0.5≤W3≤W4+2.

[0015] In some embodiments, along the thickness direction of the battery cell, the size of the second adhesive is T2 μm and the size of the second groove is D2 μm, satisfying: 0.9≤D2 / T2≤1.2.

[0016] In some embodiments, the size of the second adhesive member along the cell thickness direction is T2 μm, satisfying: 4.5≤T2≤45.

[0017] In some embodiments, the size of the second groove along the cell thickness direction is D2 μm, satisfying: 5≤D2≤54.

[0018] In some embodiments, the roughness of the inner surface of the second groove is R3, and the roughness of the side of the second adhesive member facing away from the battery cell is R4, satisfying: 2 < R3 / R4 < 15.

[0019] In some embodiments, the encapsulation film includes a first encapsulation film and a second encapsulation film, the first encapsulation film and the first surface being disposed opposite to each other, and the second encapsulation film and the second surface being disposed opposite to each other; the battery cell further has a third surface connecting the first surface and the second surface, the third surface being located on one side of the first surface and the second surface along the length direction of the battery cell, the battery cell including a first tab and a second tab with opposite polarities, the first tab and the second tab extending out from the third surface along the length direction of the battery cell; in the first encapsulation film or the second encapsulation film, two second grooves are spaced apart along the width direction of the battery cell, and a non-groove area extending along the length direction of the battery cell is between the two second grooves, the non-groove area extending at least to the edge region near the first tab and the second tab; along the width direction of the battery cell, the minimum distance between the first tab and the second tab is W6 mm, and the minimum distance between the two second grooves is W5 mm, satisfying: W5 < W6.

[0020] In some embodiments, both the first encapsulation film and the second encapsulation film are provided with a first groove; the first groove of the first encapsulation film and the first groove of the second encapsulation film are disposed opposite to each other along the thickness direction of the battery cell, and the orthographic projections of the first groove of the first encapsulation film and the first groove of the second encapsulation film onto the first surface have a first projection overlap area, the total area of ​​the first projection overlap area is Q1 mm. 2 The total projected area of ​​the first groove of the first encapsulation film on the first surface is P1 mm. 2 The total projected area of ​​the first groove of the second encapsulation film on the first surface is P2 mm. 2 The following conditions must be met: Q1 / P1 > 90%, Q1 / P2 > 90%.

[0021] In some embodiments, both the first encapsulation film and the second encapsulation film are provided with a second groove; the second grooves of the first encapsulation film and the second encapsulation film are disposed opposite to each other along the cell thickness direction, and the orthographic projections of the second grooves of the first encapsulation film and the second encapsulation film onto the first surface have a second projection overlap area, the total area of ​​the second projection overlap area is Q2 mm. 2 The total projected area of ​​the second groove of the first encapsulation film on the first surface is P3 mm. 2 The total projected area of ​​the second groove of the second encapsulation film on the first surface is P4 mm. 2 The following conditions must be met: Q2 / P3 > 90%, Q2 / P4 > 90%.

[0022] In some embodiments, in the heat-sealing layer opposite to the first surface or the second surface along the cell thickness direction, the first groove and the second groove have an overlapping area, the dimension of the overlapping area along the cell thickness direction is greater than the dimension of the portion of the first groove excluding the overlapping area along the cell thickness direction, and the dimension of the overlapping area along the cell thickness direction is greater than the dimension of the portion of the second groove excluding the overlapping area along the cell thickness direction; the orthographic projection area of ​​the first groove on the first surface is S1, the orthographic projection area of ​​the second groove on the first surface is S2, and the orthographic projection area of ​​the overlapping area on the first surface is S3, satisfying: S3 / S1≤20%, S3 / S2≤20%.

[0023] In some embodiments, the battery cell includes a tab, the orthographic projection of the tab onto the first surface at least partially coinciding with the orthographic projection of the first groove onto the first surface.

[0024] In some embodiments, the battery cell includes tabs, each tab including an outer tab and an inner tab, the inner tab being welded to the outer tab; the inner tab includes a positively charged inner tab and a negatively charged inner tab, wherein at least two stacked positively charged inner tabs are integrally connected to the positive electrode sheet, and at least two stacked negatively charged inner tabs are integrally connected to the negative electrode sheet; the inner tab is bent to form a bent portion, along the thickness direction of the battery cell, the first surface is closer to the bent portion than the second surface, and a first groove is provided at least on the heat-sealing layer opposite to the first surface, the orthographic projection of the bent portion on the first surface is at least partially located in the first groove; a third adhesive is provided on at least one side surface of the welding area between the inner tab and the outer tab, the third adhesive is integrally formed with or partially overlaps the first adhesive, and the third adhesive is at least partially embedded in the first groove.

[0025] In some embodiments, the encapsulation film includes a sealing area extending from the periphery of the first and second encapsulation films. The two layers of the encapsulation film are sealed and bonded together in the sealing area. The first encapsulation film, the second encapsulation film, and the sealing area together form a receiving cavity for accommodating the wound battery cell. The sum of the projected areas of the first and second grooves is (S1 + S2 - S3), where S1 is the projected area of ​​the first groove on the first surface, S2 is the projected area of ​​the second groove on the first surface, and S3 is the projected area of ​​the overlapping area of ​​the first and second grooves on the first surface. The area of ​​the first encapsulation film is S4 mm. 2 , (S1+S2-S3) / S4 is denoted as λ; the secondary battery also includes an electrolyte, the electrolyte includes fluoroethylene carbonate, and the mass content of fluoroethylene carbonate is f% based on the total mass of the electrolyte, satisfying: 0.5≤λ / f≤12, and / or, 0<λ≤95, and / or, 5≤f≤30.

[0026] In some embodiments, the electrolyte comprises a lithium salt, wherein the lithium salt comprises lithium hexafluorophosphate, and the mass content of the lithium hexafluorophosphate is 8%-20% based on the total mass of the electrolyte.

[0027] In some embodiments, the electrolyte comprises nitrile compounds and fluorocarboxylic acid ester compounds. Based on the total mass of the electrolyte, the mass content of the nitrile compounds is 0.5%-5%, and the mass content of the fluorocarboxylic acid ester compounds is 5%-65%. The nitrile compounds include benzonitrile, p-toluenenitrile, 3,5-difluorobenzonitrile, adiponitrile, succinic acidnitrile, ethylene glycol bis(propionitrile) ether, 1,4-dicyano-2-butene, 1,3,6-hexanetrionitrile, 1,2,6-hexanetrionitrile, and 1,2,3-tris(2-cyano)nitrile. The fluorocarboxylic acid ester compound includes at least one of the following: 1,2,3,4,5-penta(2-cyanoethoxy)pentane, ethylene glycol di(2-cyanoethyl) ether, diethylene glycol di(2-cyanoethyl) ether, triethylene glycol di(2-cyanoethyl) ether, tetraethylene glycol di(2-cyanoethyl) ether, ethylene glycol di(4-cyanobutyl) ether, ethylene glycol (bis)propionitrile ether, and 1,2,3-tris(2-cyanoethoxy)propane; the fluorocarboxylic acid ester compound includes at least one of 2,2-difluoroethyl acetate, ethyl trifluoroacetate, ethyl monofluoroacetate, and propyl difluoroacetate.

[0028] The technical solution proposed in this application has at least the following technical effects: The secondary battery provided by this invention thins the area corresponding to the first adhesive in the heat-sealing layer of the encapsulation film to create a groove structure, providing a space for the first adhesive. The presence of the first groove can also effectively reduce the risk of the outermost electrode sheet scraping against the heat-sealing layer after folding. It also alleviates the problem of inconsistent battery thickness in local areas caused by battery cycle expansion after the first adhesive is installed, thereby improving the thickness consistency and cycle stability of the battery. In addition, it also helps to reduce the overall weight and volume and increase the battery energy density. Attached Figure Description

[0029] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0030] Figure 1 This is one of the structural schematic diagrams of a secondary battery cell according to an embodiment of the present invention; Figure 2 This is a second schematic diagram of the structure of a secondary battery cell according to an embodiment of the present invention; Figure 3 This is one of the structural schematic diagrams of the encapsulation film after it has been unfolded (containing the battery cell) according to an embodiment of the present invention; Figure 4 This is a third schematic diagram of the structure of a secondary battery cell according to an embodiment of the present invention; Figure 5 for Figure 4 A sectional view along section AA; Figure 6 for Figure 4 A sectional view along section BB. Figure 7 This is a schematic diagram of the encapsulation film according to an embodiment of the present invention; Figure 8 This is a second schematic diagram of the structure of the encapsulation film after it has been unfolded (containing the battery cell) according to an embodiment of the present invention; Figure 9 This is one of the structural schematic diagrams of the encapsulation film after it has been unfolded (without containing the battery cell) according to an embodiment of the present invention; Figure 10 This is the third schematic diagram of the structure of the encapsulation film after it has been unfolded (containing the battery cell) according to an embodiment of the present invention; Figure 11 This is a schematic diagram of the structure of the region near the tab of a secondary battery according to an embodiment of the present invention; Figure 12 This is the fourth schematic diagram of the structure of a secondary battery cell according to an embodiment of the present invention; Figure 13 This is the fifth schematic diagram of the structure of a secondary battery cell according to an embodiment of the present invention.

[0031] Explanation of reference numerals in the attached figures: 1. Battery cell; 1a. First surface; 1b. Second surface; 1c. Third surface; 11. Edge region; 12. Central region; 3. Encapsulation film; 3a. First encapsulation film; 3b. Second encapsulation film; 3c. Sealing area; 31. Protective layer; 32. Metal layer; 33. Heat-sealing layer; 331. First groove; 332. Second groove; 333. Non-groove area; 334. Overlapping area; 4. First adhesive component; 5. Second adhesive component; 6. Third adhesive component; 7. First tab; 8. Second tab; 9. Inner tab; 10. Outer tab; 91. Bending portion; 92. Solder mark. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Silicon-based materials are becoming an important direction for battery technology development due to their significant high energy density. However, the dramatic volume expansion of silicon-containing anodes during charging and discharging is a detrimental factor affecting the long-term reliability of batteries. This problem is particularly prominent in pouch cells: due to the continuous effect of expansion stress, the electrodes located on the outer surface of the cell are prone to folding or warping, scratching, abrasion, or even puncturing the inner wall of the encapsulation film.

[0034] The encapsulation film 3 typically consists of a protective layer 31, a metal layer 32, and a heat-sealing layer 33, from the outside in. The heat-sealing layer 33 not only performs the crucial sealing function but also provides internal insulation. When the electrode is folded, its sharp edges may pierce the heat-sealing layer 33, causing electrolyte leakage and inducing electrochemical corrosion between the metal layer 32 and the electrode, thus accelerating battery failure. Currently, a common approach is to attach adhesive components to the outermost layer of the cell 1 to physically isolate the electrode from the encapsulation film and constrain and suppress electrode folding or warping deformation, thereby reducing the risk of scratches. However, this solution also introduces new problems: First, the thickness of the adhesive itself increases the overall volume of the battery and reduces the battery energy density; second, the adhesive affects the bonding consistency between cell 1 and the encapsulation film; during cycling, due to the presence of the adhesive, there is a gap between the cell and the encapsulation film in areas where the adhesive is not installed. Therefore, the expansion of the silicon anode will repeatedly squeeze the areas where the adhesive is installed, increasing the risk of the adhesive falling off, causing local deformation of the outermost electrode or encapsulation film, which in turn affects the thickness consistency of the battery. Under the action of external clamping force, the inconsistent battery thickness will cause local stress concentration, which will accelerate interface degradation, increase the risk of internal short circuit, and thus damage the long-term cycle performance and safety of the battery.

[0035] The following is combined Figures 1 to 13 The following describes embodiments of the present invention.

[0036] According to an embodiment of the present invention, a secondary battery is provided, including a battery cell 1. The battery cell 1 includes a positive electrode, a separator, and a negative electrode. The negative electrode includes a negative electrode active material layer disposed on at least one side surface of a negative electrode current collector, and the negative electrode active material layer includes a silicon-based material. It also includes an encapsulation film 3, which can be an aluminum-plastic film, a steel-plastic film, etc., used to encapsulate the battery cell 1 and form a receiving cavity for accommodating the battery cell 1. The encapsulation film 3 includes a stacked metal layer 32 and a heat-sealing layer 33, with the heat-sealing layer 33 disposed on the side of the metal layer 32 facing the receiving cavity. In some embodiments, the encapsulation film 3 may further include a protective layer 31, such as a nylon layer, disposed on the side of the metal layer 32 away from the heat-sealing layer 33. The metal layer 32, such as an aluminum layer, mainly serves to block moisture and oxygen. The heat-sealing layer 33 is the inner layer of the encapsulation film 3, and during encapsulation, it is melted by heating and adhered to the opposing encapsulation film 3 or itself, thereby achieving battery sealing. The material of the heat-sealing layer 33 is, for example, cast polypropylene.

[0037] like Figures 1-2 As shown, the battery cell 1 has an outer surface, which includes a first surface 1a and a second surface 1b that are spaced apart from each other along the cell thickness direction Z. The first surface 1a and / or the second surface 1b have an edge region 11 near at least one side along the cell length direction Y. In this embodiment, the cell thickness direction Z refers to the direction of the normals of the two large surfaces of the battery cell 1, that is... Figure 1 , Figure 2 , Figures 5-7 , Figure 11 The Z direction is shown. The first surface 1a and / or the second surface 1b have an edge region 11 near at least one side along the length direction Y of the cell. The length direction Y of the cell is the direction in which the tab extends from the cell 1, that is... Figures 1-4 , Figures 8-13 The Y direction is shown. The edge region 11 is located on the surface of the outermost electrode / separator of the wound cell 1, or on the surface of the outermost electrode / separator of the stacked cell 1. During the charging and discharging of the battery, the edge region 11 is more likely to fold outward or warp due to the expansion of the internal material of the electrode.

[0038] The first adhesive element 4 is affixed to the aforementioned edge region 11. The first adhesive element 4 can be high-temperature tape, polyimide tape, or other tapes with insulating and cushioning properties. The purpose of setting the first adhesive element 4 is to physically isolate the edge of the outermost electrode of the battery cell 1 from the heat-sealing layer 33 of the encapsulation film 3, reducing the risk of the electrode directly scraping against the heat-sealing layer 33 when it is folded, while providing a certain mechanical constraint to suppress excessive displacement of the electrode.

[0039] The heat-sealing layer 33 of the encapsulation film 3 is provided with a first groove 331 recessed towards the metal layer 32. The position of the first groove 331 corresponds to the first adhesive member 4 on the edge region 11 of the cell 1. Along the thickness direction Z of the cell 1, the first adhesive member 4 is at least partially embedded in the first groove 331. Here, "embedded" means that at least a portion of the first adhesive member 4 is accommodated within the space formed by the first groove 331 in the thickness direction Z of the cell 1. By at least partially placing the first adhesive member 4 within the first groove 331, the protective function of the first adhesive member 4 can be achieved without increasing the overall thickness of the battery. The first groove 331 provides a accommodating space for the first adhesive member 4, preventing the first adhesive member 4 from protruding additionally in the thickness direction. This helps maintain the consistency of adhesion between the cell 1 and the encapsulation film 3 and avoids stress concentration points caused by local protrusions of the first adhesive member 4 when the battery is subjected to external clamping force. Meanwhile, even if the electrode sheet is folded to a certain extent, due to the presence of the first groove 331, the displacement required for the edge of the electrode sheet to contact the bottom of the groove is greater, thereby further reducing the risk of the heat seal layer 33 being repeatedly scratched or punctured due to the folding or warping of the electrode sheet, improving the long-term packaging reliability and safety of the battery, and also improving the energy density of the battery.

[0040] In some embodiments, such as Figure 5As shown, along the cell thickness direction Z, the size of the first adhesive 4 is T1 μm, and the size of the first groove 331 is D1 μm, satisfying: 0.7≤D1 / T1≤1.5. By limiting this ratio range, the first groove 331 can more effectively accommodate the first adhesive 4. As an example, D1 / T1 can be 0.7, 0.72, 0.77, 0.8, 0.89, 0.95, 0.97, 1.05, 1.08, 1.12, 1.19, 1.23, 1.27, 1.32, 1.4, 1.45, 1.48, 1.5, etc., or values ​​within the range of any two of the above values.

[0041] Specifically, if D1 / T1 < 0.7, it means that the depth of the first groove 331 is too shallow, and most of the first adhesive 4 still protrudes outside the groove, which has limited contribution to reducing the overall thickness of the battery and improving the fit. If D1 / T1 > 1.5, it means that the depth of the first groove 331 is much greater than the thickness of the first adhesive 4, which may lead to excessive thinning of the heat-sealing layer 33 in this area, weakening its sealing and insulation performance. In addition, during the cycle, the excessively deep space not filled by the first adhesive 4 may become a weak point due to stress, resulting in local damage to the encapsulation film and a decrease in battery cycle performance.

[0042] In some embodiments, such as Figure 3 As shown, along the cell width direction X, the size of the first groove 331 is L1 mm, and the size of the first adhesive component 4 is L2 mm; along the cell length direction Y, the size of the first groove 331 is W1 mm, and the size of the first adhesive component 4 is W2 mm; satisfying: L2+0.5≤L1≤L2+2, W2+0.5≤W1≤W2+2, that is, L1 is 0.5mm to 2.0mm larger than L2, and W1 is 0.5mm to 2.0mm larger than W2. Here, the cell width direction X refers to the direction perpendicular to the cell thickness direction Z and the length direction, that is... Figures 1-10 , Figure 12 , Figure 13 The X direction is shown.

[0043] The first groove 331 is designed to be slightly larger than the first adhesive member 4 on the plane. This provides a certain installation tolerance for the first adhesive member 4 and prevents the edges of the first adhesive member 4 from bulging due to misalignment. If the area of ​​the first groove 331 is too large, the heat-sealing layer 33 will be excessively thinned, thus affecting the overall strength. If the area of ​​the first groove 331 is too small, it cannot fully accommodate the first adhesive member 4, and the edges of the first adhesive member 4 may still bulge, causing local stress and contributing little to the flatness of the battery surface.

[0044] In some embodiments, the dimension of the first groove 331 along the cell thickness direction Z is D1 μm, satisfying: 5 ≤ D1 ≤ 54. For example, D1 can be 5, 6, 8, 12, 13, 17, 22, 24, 25, 29, 32, 36, 39, 41, 43, 47, 50, 54, etc., or a value within the range of any two of the above values. This depth range can effectively accommodate the first adhesive 4 while also taking into account the remaining thickness of the heat-sealing layer 33, which is beneficial for maintaining the reliability of the seal.

[0045] In some embodiments, such as Figure 5 As shown, along the thickness direction Z of the battery cell, the dimension of the first adhesive component 4 is T1 μm, satisfying: 4.5 ≤ T1 ≤ 45. For example, T1 can be 4.5, 5, 7, 8, 11, 14, 18, 22, 24, 27, 31, 33, 36, 43, 46, 47, 51, 52, 54, etc., or values ​​within any two of the above ranges. This thickness range of the first adhesive component 4 provides sufficient buffering and insulation without imposing excessive requirements on the thinning design due to its own thickness.

[0046] In some embodiments, such as Figure 7 As shown, along the thickness direction Z of the battery cell, the size of the first groove 331 is D1 μm, and the size of the heat-sealing layer 33 is X1 μm, satisfying: 30% ≤ D1 / X1 ≤ 90%. Specifically, D1 / X1 can be 30%, 35%, 38%, 42%, 45%, 48%, 53%, 57%, 59%, 61%, 65%, 68%, 71%, 74%, 82%, 86%, 89%, 90%, or a value within any two of the above ranges. If the thinning ratio is too low (<30%), the effect of accommodating the bonded component will be insignificant. If the thinning ratio is too high (>90%), it may lead to insufficient strength of the heat-sealing layer 33 at that location.

[0047] In some embodiments, the orthographic projection of the first groove 331 onto the first surface 1a completely covers the orthographic projection of the first adhesive 4 onto the first surface 1a. This means that, viewed from a direction perpendicular to the first surface 1a (Z direction), the first adhesive 4 is completely within the contour of the first groove 331. That is, no part of the first adhesive 4 is directly pressed onto the unthinned heat-sealing layer 33, thereby further enhancing the effects of thickness compensation and uniform stress distribution.

[0048] In some embodiments, the roughness of the inner surface of the first groove 331 is R1, and the roughness of the side of the first adhesive 4 facing away from the battery cell 1 is R2, satisfying: 2 < R1 / R2 < 15. Specifically, the roughness R1 of the inner surface of the first groove 331 can be controlled within the range of 1.5 μm to 10 μm by chemical etching or physical polishing processes. The roughness R2 of the surface of the first adhesive 4 is typically within the range of 0.1 μm to 5 μm. Exemplarily, R1 / R2 can be 2.3, 2.8, 3.1, 3.6, 4.2, 4.7, 5.3, 5.9, 6.5, 7.2, 7.9, 8.6, 9.4, 10.3, 11.1, 12, 13.2, 14.1, etc., or values ​​within the range of any two of the above values.

[0049] By making the inner surface of the groove rougher, the friction and mechanical interlocking between the first adhesive component 4 and the inner wall of the groove can be increased, thereby enhancing the ability of the first adhesive component 4 to resist displacement or detachment during battery cycling and improving the long-term stability of the structure. Of course, the difference in roughness should not be too large. If R1 / R2 > 15, an excessively rough inner surface of the groove may accelerate the wear of the first adhesive component 4 during long-term contact.

[0050] In some embodiments, the first surface 1a and / or the second surface 1b of the battery cell 1 further have a central region 12 located between the edge regions 11 along the length Y direction of the battery cell. The central region 12 generally refers to the central portion of the large surface of the battery cell 1 excluding the edge regions 11 near the tab lead-out side and the edge regions 11 away from the tab lead-out side. This region is attached with a second adhesive 5, which may be a hot melt adhesive for fixing the electrode or providing additional insulation, a double-sided adhesive for bonding the battery cell 1 and the encapsulation film 3, a finishing adhesive for winding the battery cell 1, or other types of insulating tape. The heat-sealing layer 33 of the encapsulation film 3 is correspondingly provided with a second groove 332 recessed toward the metal layer 32. Along the thickness Z direction of the battery cell 1, the second adhesive 5 is at least partially embedded within the second groove 332. Here, "embedded" means that along the thickness Z direction of the battery cell 1, at least a portion of the second adhesive 5 is accommodated within the space formed by the second groove 332.

[0051] By providing a second groove 332 in the heat-sealing layer 33 corresponding to the encapsulation film 3 in the central region 12, a space is also provided for the second adhesive 5, which can further optimize the overall thickness of the battery. If the second adhesive 5 in the central region 12 is directly attached to the flat heat-sealing layer 33, its thickness will be completely added to the total thickness of the battery. The central region 12 is also the area with a relatively large thickness expansion of the cell 1, especially for silicon-based batteries, the thickness expansion of the central region 12 is even greater. By providing a matching second groove 332, the second adhesive 5 can be embedded, thereby offsetting or reducing the thickness increase caused by this part. This design helps to achieve comprehensive thickness consistency control of the battery at multiple adhesive application points of the cell 1, making the battery thickness more uniform after encapsulation. The improvement in thickness uniformity helps to reduce the assembly stress caused by surface unevenness during battery assembly, and also helps to improve the uniformity of interface pressure distribution during battery cycle expansion and contraction, further improving the cycle performance and safety of the battery.

[0052] In some embodiments, the orthographic projection of the second groove 332 on the first surface 1a completely covers the orthographic projection of the second adhesive member 5 on the first surface 1a. This maximizes the thickness compensation effect of the second groove 332 and reduces the probability of local stress concentration at the edge of the second adhesive member 5.

[0053] In some embodiments, such as Figure 3 As shown, along the width direction X of the battery cell, the size of the second groove 332 is L3 mm, and the size of the second adhesive component 5 is L4 mm; along the length direction Y of the battery cell, the size of the second groove 332 is W3 mm, and the size of the second adhesive component 5 is W4 mm; satisfying: L4+0.5≤L3≤L4+2, W4+0.5≤W3≤W4+2, that is, L3 is 0.5mm to 2.0mm larger than L4, and W3 is 0.5mm to 2.0mm larger than W4. This setting allows for a certain alignment error margin for the attachment of the second adhesive component 5, preventing the edge of the second adhesive component 5 from being unable to be accommodated by the groove due to misalignment and thus lifting up. However, if the area of ​​the second groove 332 is too large, it will cause unnecessary removal of the heat-sealing layer 33 material, affecting the overall mechanical strength and sealing reliability of this area.

[0054] In some embodiments, such as Figure 6 As shown, along the cell thickness direction Z, the size of the second adhesive 5 is T2 μm, and the size of the second groove 332 is D2 μm, satisfying: 0.9≤D2 / T2≤1.2. As an example, D2 / T2 can be 0.9, 0.94, 0.98, 1, 1.05, 1.09, 1.12, 1.16, 1.18, 1.2, or a value within the range of any two of the above values.

[0055] By controlling D2 / T2 between 0.9 and 1.2, the second adhesive component 5 can be more tightly accommodated by the second groove 332. If D2 / T2 < 0.9, a significant portion of the second adhesive component 5 will protrude, resulting in poor thickness compensation. If D2 / T2 > 1.2, an excessively deep second groove 332 may lead to insufficient remaining thickness of the heat-sealing layer 33 at that location, becoming a weak point in the seal. Furthermore, during the cycle, the second adhesive component 5 may undergo micro-displacement within the groove.

[0056] In some embodiments, along the cell thickness direction Z, the dimension of the second adhesive 5 is T2 μm, satisfying: 4.5 ≤ T2 ≤ 45. For example, T2 can be 4, 5, 8, 12, 17, 23, 27, 31, 36, 40, or a value within any two of the above ranges. This thickness range of the second adhesive 5 satisfies the functional requirements of fixing, insulation, or buffering, and also matches a reasonable depth of the second groove 332, avoiding excessive impact on the battery thickness.

[0057] In some embodiments, the dimension of the second groove 332 along the cell thickness direction Z is D2 μm, satisfying: 5 ≤ D2 ≤ 54. For example, D2 can be 5, 8, 12, 17, 22, 26, 30, 34, 38, 42, 45, or a value within any two of the above ranges. This depth range of the second groove 332 enables it to achieve its accommodating function while ensuring that the heat-sealing layer 33 retains sufficient thickness to maintain its basic sealing and insulating properties.

[0058] In some embodiments, the roughness of the inner surface of the second groove 332 is R3, and the roughness of the side of the second adhesive 5 facing away from the battery cell 1 is R4, satisfying: 2 < R3 / R4 < 15. For example, the roughness R3 of the inner surface of the second groove 332 can be controlled in the range of 1.5 μm to 10 μm by a chemical etching or physical polishing process similar to that used to form the first groove 331; while the original roughness R4 of the surface of the second adhesive 5 is typically in the range of 0.1 μm to 5 μm. Exemplarily, R3 / R4 can be 3, 5, 6, 8, 10, 11, 13, 14, etc., or values ​​within the range of any two of the above values.

[0059] By making the inner surface of the second groove 332 rougher, the mechanical engagement and friction between the two can be enhanced. This enhanced interfacial effect is particularly important for the second adhesive 5 in the central region 12, as this region experiences relatively high shear stress during battery cycle expansion and contraction. Appropriate roughness variation can effectively prevent the second adhesive 5 from shifting or detaching from the heat-sealing layer 33 during long-term use, thus helping to maintain its fixation and insulation functions. However, the roughness variation should not be too large; an excessively large R3 / R4 (>15) may mean that the inner surface of the second groove 332 is too rough, which may not only increase the manufacturing difficulty but also accelerate the wear of the second adhesive 5 during long-term contact, leading to a weakening of its fixation and insulation functions.

[0060] In some embodiments, such as Figure 8 , Figure 9 , Figure 10 As shown, the encapsulation film 3 specifically includes a first encapsulation film 3a and a second encapsulation film 3b. The first encapsulation film 3a is disposed opposite to the first surface 1a of the battery cell 1, and the second encapsulation film 3b is disposed opposite to the second surface 1b of the battery cell 1. The battery cell 1 also has a third surface 1c connecting the first surface 1a and the second surface 1b. The third surface 1c is located on one side of the first surface 1a and the second surface 1b along the length Y direction of the battery cell. The battery cell 1 includes a first tab 7 and a second tab 8 with opposite polarities. The first tab 7 and the second tab 8 are led out from the third surface 1c along the length Y direction of the battery cell. The third surface 1c generally refers to the side surface of the battery cell 1, from which the first tab 7 and the second tab 8 extend outward.

[0061] It is important to note that the tab location within the sealing area 3c of the encapsulation film 3 is a pre-designed and controllable weak point. Specifically, at the sealing edge where the tab extends, the tab adhesive covering the outer periphery of the tab is thermally bonded to the heat-sealing layer 33 (such as a PP layer) of the encapsulation film 3. Due to the presence of the metal tab itself, complete fusion cannot be achieved between it and the heat-sealing layer 33. This results in a relatively weak overall bond strength between the sealing area 3c where the tab is located and the sealing area 3c between the two tabs, making it a relatively weak area. During high-stress safety tests such as furnace temperature testing, the rapidly generated internal gas pressure needs a timely release outlet. The aforementioned weak area is designed to be preferentially opened under this pressure, thereby achieving controllable pressure relief, ensuring the overall safety of the battery, and preventing dangerous bulging or rupture.

[0062] In the first encapsulation film 3a or the second encapsulation film 3b, there may be two second grooves 332 spaced apart along the width direction X of the battery cell. For example, on the large surface (the first surface 1a or the second surface 1b) of the battery cell 1, two second bonding members 5 may be adhered symmetrically or asymmetrically, and two independent second grooves 332 are formed on the heat-sealing layer 33 of the corresponding encapsulation film 3 to respectively accommodate the two second bonding members 5. Between the two second grooves 332, a non-groove area 333 extending along the length direction Y of the battery cell is provided. The non-groove area 333 refers to the area where the heat-sealing layer 33 is not thinned and remains at its original thickness. The non-groove area 333 at least extends to the edge area 11 near the first tab 7 and the second tab 8, thereby forming a structurally continuous non-groove area 333 channel between the two second grooves 332.

[0063] In the groove area, the heat-sealing layer is locally thinned to form a depression, and its inner surface is not a continuous and flat plane. Such a depression structure is likely to form dead corners for gas retention microscopically, increasing the interfacial resistance and diffusion difficulty when gas passes through. In contrast, the non-groove area retains the original complete thickness and flat polymer surface of the heat-sealing layer, and its structure is continuous and uniform. This continuous and flat surface provides a barrier-free moving plane for gas molecules, enabling gas to migrate efficiently along this path with lower energy consumption and fewer direction changes. Therefore, due to its structural continuity and surface flatness, the non-groove area becomes a preferential path with less resistance and better connectivity for gas migration inside the battery.

[0064] As Figure 8 shown, along the width direction X of the battery cell, the minimum distance between the first tab 7 and the second tab 8 is W6 mm, and the minimum distance between the two second grooves 332 (i.e., the width of the non-groove area 333) is W5 mm, satisfying: W5 < W6. W5 < W6 means that this non-groove area 333 channel is limited in the area between the first tab 7 and the second tab 8 in the width direction. That is, the range of the non-groove area along the X direction is included within the width area defined by the first tab 7 and the second tab 8, avoiding the non-groove area channel being too wide (W5 ≥ W6) and extending to or covering the outside of the tabs, thereby ensuring that when gas migrates along this channel, it is effectively guided and restricted in the direction towards the specific sealing area between the first tab and the second tab. This non-groove area 333 channel provides a preferential diffusion path with less resistance and better connectivity for gas. When the battery cycles to generate gas or experiences abuse conditions such as high temperature, gas can more easily migrate along the surface of this complete non-groove area 333 channel, causing the gas to be guided to converge on the tab lead-out side of the battery cell 1, ensuring that when the battery cycles abnormally to generate gas or experiences abuse conditions such as high temperature, the gas inside the battery can be discharged to the outside in time, avoiding safety problems such as fire and explosion of the battery.

[0065] In some embodiments, as Figure 9 As shown, in order to ensure the overall symmetry of the battery structure and the balance of stress distribution, the alignment accuracy of the grooves provided on the two encapsulation films 3 is limited. Specifically, the first encapsulation film 3a and the second encapsulation film 3b can both be provided with the first groove 331, or both can be provided with the second groove 332, or both can be provided simultaneously.

[0066] When both sides are provided with the first groove 331, the first groove 331 of the first encapsulation film 3a and the first groove 331 of the second encapsulation film 3b are arranged opposite each other along the thickness direction Z of the battery cell. The orthogonal projections of the two on the first surface 1a have a first projection overlap area, the total area of ​​which is Q1 mm. 2 The total projected area of ​​the first groove 331 of the first encapsulation film 3a on the first surface 1a is P1 mm. 2 The total projected area of ​​the first groove 331 of the second encapsulation film 3b on the first surface 1a is P2 mm. 2 The following conditions must be met: Q1 / P1 > 90%, Q1 / P2 > 90%.

[0067] Correspondingly, when both sides are provided with a second groove 332, the second groove 332 of the first encapsulation film 3a and the second groove 332 of the second encapsulation film 3b are arranged opposite each other along the cell thickness direction Z. Their orthogonal projections on the first surface 1a have a second projection overlap area, the total area of ​​which is Q2 mm. 2 The total projected area of ​​the second groove 332 of the first encapsulation film 3a on the first surface 1a is P3 mm. 2 The total projected area of ​​the second groove 332 of the second encapsulation film 3b on the first surface 1a is P4mm. 2 The following conditions must be met: Q2 / P3 > 90%, Q2 / P4 > 90%.

[0068] The requirement that the overlapping area of ​​the projected area exceeds 90% means that the corresponding grooves on the two sides of the encapsulation film 3 must be precisely aligned in terms of planar position and shape. This symmetrical design helps to balance the clamping force on both sides of the cell 1 in its thickness direction, thereby reducing the additional local stress concentration that may be caused by the asymmetry of forces on both sides of the battery. This is beneficial to improving the thickness consistency, appearance flatness, and assembly uniformity during integration of the battery.

[0069] In some embodiments, within the same heat-sealing layer 33 disposed opposite to the first surface 1a or the second surface 1b along the cell thickness direction Z, the first groove 331 and the second groove 332 may not be completely independent, and an overlapping region 334 may exist between them. The overlapping region 334 refers to a local area where the recessed portion of the first groove 331 and the recessed portion of the second groove 332 overlap each other in the thickness direction. The dimension (i.e., depth) of this overlapping region 334 along the cell thickness direction Z is greater than the depth of the remaining portion of the first groove 331 excluding the overlapping region 334, and also greater than the depth of the remaining portion of the second groove 332 excluding the overlapping region 334. That is, at the location where the two overlap, the heat-sealing layer 33 is thinned to the greatest depth, forming a composite pit that is deeper than the surrounding single groove area, in order to better accommodate the overlapping area of ​​the two adhesives.

[0070] The projected area of ​​the first groove 331 on the first surface 1a is S1, the projected area of ​​the second groove 332 on the first surface 1a is S2, and the projected area of ​​the overlapping region 334 on the plane of the first surface 1a is S3. They satisfy the following relationships: S3 / S1≤20%, S3 / S2≤20%.

[0071] S3 is limited to no more than 20% of the area of ​​any groove itself, which limits the proportion of the thinnest area on the heat-sealing layer 33 due to double thinning. If the area of ​​the overlapping region 334 is too large (i.e., the value of S3 / S1 or S3 / S2 is too large), it means that there is an excessively large, significantly thinned weak area on the heat-sealing layer 33, which will weaken the mechanical strength, sealing reliability and puncture resistance of the encapsulation film 3 at that location, and may become a potential risk point for electrolyte leakage or external moisture intrusion, thus hindering the further improvement of the long-term safety and reliability of the battery.

[0072] In some embodiments, such as Figure 11 , Figure 12 , Figure 13As shown, the battery cell 1 includes tabs, the orthographic projection of which on the first surface 1a at least partially coincides with the orthographic projection of the first groove 331 on the first surface 1a. When the tab is bent to adjust the lead-out direction, a local bulge is formed at the root of the bend. To fix and insulate this area, a third adhesive 6 is typically attached to at least one side surface. Since the bent area of ​​the tab is spatially close to the edge region 11 of the outer surface of the battery cell 1, the projection position of the third adhesive 6 is adjacent to or partially overlaps with the projection area of ​​the first groove 331 used to prevent the edge of the outermost electrode from folding over. Therefore, the first groove 331 not only provides a receiving space for the first adhesive 4, but its design also takes into account the possibility of receiving the third adhesive 6. By at least partially embedding the third adhesive 6 in the first groove 331, the significant increase in battery thickness at this local location due to additional adhesive application in the bent area of ​​the tab can be avoided, which helps to maintain the overall flatness and thickness consistency of the outer surface of the package.

[0073] In some embodiments, such as Figure 11 As shown, the battery cell 1 includes tabs, which include an outer tab 10 and an inner tab 9. The inner tab 9 is welded to the outer tab 10. The inner tab 9 includes a positive inner tab 9 and a negative inner tab 9, wherein at least two stacked positive inner tabs 9 are integrally connected to the positive electrode plate, and at least two stacked negative inner tabs 9 are integrally connected to the negative electrode plate. The inner tab 9 is bent to form a bending portion 91. Along the thickness direction Z of the battery cell, the first surface 1a is closer to the bending portion 91 than the second surface 1b. At least on the heat-sealing layer 33 opposite to the first surface 1a, a first groove 331 is provided. The orthogonal projection of the bending portion 91 on the first surface 1a is at least partially located in the first groove 331. A third adhesive 6 is provided on at least one side surface of the welding area between the inner tab 9 and the outer tab 10. The third adhesive 6 may be, for example, tape for fixing, insulating, or protecting the welding area. The third adhesive 6 and the first adhesive 4, which are attached to the edge region 11, can be a single integral component or two separate components that partially overlap in space. The third adhesive 6 is at least partially embedded in the first groove 331.

[0074] The layout of the first groove 331 not only considers the protection of the electrode in the edge region 11, but also reserves space for the bending portion 91 of the inner tab 9. Including the projection of the bending portion 91 within the range of the first groove 331 reduces the possibility of the bending portion 91 directly pressing against the flat heat-sealing layer 33, causing localized bulges and helping to maintain the flatness of the outer surface of the package. Furthermore, by also accommodating the third adhesive 6 within the first groove 331, the increase in thickness in the welding area due to additional adhesive application can be reduced, maintaining battery thickness consistency, improving space utilization and structural compactness, and further increasing battery energy density.

[0075] In some embodiments, such as Figure 8 , Figure 9 , Figure 10 As shown, the encapsulation film 3 includes a sealing area 3c, which extends from the periphery of the first encapsulation film 3a and the second encapsulation film 3b. The two encapsulation films 3 are sealed and bonded within the sealing area 3c. The first encapsulation film 3a, the second encapsulation film 3b, and the sealing area 3c together form a cavity for accommodating the wound battery cell 1. The sum of the projected areas of the first groove 331 and the second groove 332 is (S1 + S2 - S3), where S1 is the projected area of ​​the first groove 331 on the first surface 1a, S2 is the projected area of ​​the second groove 332 on the first surface 1a, and S3 is the projected area of ​​the overlapping region 334 of the first groove 331 and the second groove 332 on the plane of the first surface 1a. The area of ​​the first encapsulation film 3a is S4 mm. 2 (S1+S2-S3) / S4 is denoted as λ%, where λ% defines the total area percentage of the groove region within the first encapsulation film 3a. The secondary battery also includes an electrolyte comprising fluoroethylene carbonate (FEC), a key additive used to form a stable solid electrolyte interphase (SEI) film on the silicon-based anode surface, inhibit electrolyte decomposition, and reduce gas production. Based on the total mass of the electrolyte, the FEC mass content is f%, satisfying: 0.5≤λ / f≤12, and / or, 0<λ≤95, and / or, 5≤f≤30.

[0076] It should be noted that area S4 refers to the area of ​​the large area on the first encapsulation film 3a, facing the cavity, which is actually opposite to the battery cell 1 after encapsulation. Specifically, its measurement boundary is the heat-sealing fusion line on the inner side of the encapsulation film. This fusion line is the boundary between the large area of ​​the encapsulation film and the surrounding sealing area 3c, located at the inner edge of the sealing area 3c. Therefore, S4 is the area of ​​the effectively flat area inside the first encapsulation film 3a after excluding the area of ​​the surrounding sealing area 3c. S4 can be measured by conventional testing methods in the art, for example: dissecting the encapsulated battery along a direction parallel to the large surface, peeling off and flattening the first encapsulation film. On the inner side of the unfolded encapsulation film, the heat-sealing fusion line will appear as a clear indentation or a color / texture change band. Using a high-precision ruler or optical measuring instrument, the actual size of the area surrounded by the fusion line can be directly measured and the area calculated.

[0077] The area of ​​the groove region (S1, S2, S3) can be measured using the following method: After obtaining the first encapsulation film sample as described above, the groove region is easily identified due to its surface depression. A surface morphology measurement device such as a three-dimensional surface profilometer or a laser confocal microscope can be used to scan the surface of the heat-sealing layer. During scanning, the original, flat surface of the heat-sealing layer (unthinned) is used as a reference plane. The device can automatically identify and calculate the contours of all depression regions below this reference plane. By setting a depth threshold (e.g., depth greater than 1 μm), the device distinguishes between the actual grooves and surface micro-roughness, and the software can directly output the projected area of ​​a single groove. The total area S1 of the first groove 331 is obtained by summing the projected areas of all identified first grooves 331; similarly, the total area S2 of the second groove is obtained. For the overlapping region 334 of the first groove 331 and the second groove 332, the measuring device can identify the common depression portion belonging to both grooves on the morphology map based on the change in depression depth (the overlapping area is deeper), and calculate its projected area, i.e., S3.

[0078] The area ratio of the recessed region serves two purposes. First, it provides space for the first, second, and third adhesive components on the battery cell, reducing the need for additional adhesives and improving the adhesion and thickness consistency between the cell and the encapsulation film. This enhances the battery's cycle performance and safety. Second, a properly proportioned recessed region provides space for gases generated during battery cycling, reducing localized deformation of the encapsulation film 3. If the total area ratio of the recessed region is too small, it will not achieve the above effects; if it is too large, it may cause damage to the heat-sealing layer 33, creating electrolyte leakage channels or electrochemical corrosion between the metal layer 32 and the electrodes. It also increases the accumulation of gas within the recessed region of the battery encapsulation film, increasing the risk of battery swelling and explosion, and raising the probability of battery failure. For example, λ can be 2, 5, 8, 11, 16, 22, 27, 32, 38, 44, 50, 56, 62, 68, 74, 81, 89, 95, or a value within any two of the above ranges.

[0079] A low FEC content in the electrolyte is detrimental to the formation of a stable fluorinated SEI film on the surface of the negative electrode, especially on silicon-based negative electrodes, thus reducing side reactions, gas generation, and heat generation in the battery. Excessive FEC, under high-temperature conditions, may increase the risk of hydrofluoric acid formation, leading to the dissolution of transition metals from the positive electrode active material, which then migrate and deposit on the negative electrode surface, damaging the SEI film, increasing the probability of self-discharge, further exacerbating side reactions in the battery, and reducing battery cycle life and safety performance. As an example, f can be 5, 6, 7, 9, 11, 13, 15, 17, 18, 20, 21, 23, 24, 25, 26, 28, 29, 30, or a value within any two of the above ranges.

[0080] If the area of ​​the groove region is large and the FEC content is too low, relatively more side reaction gases may be generated in the groove region due to interface instability. These gases are more likely to remain in the groove, thus exacerbating local pressure and deformation of the encapsulation film. Conversely, if the FEC content is high but the area of ​​the groove region is small, the groove cannot improve the battery's energy density while also failing to adequately accommodate the gases generated by the internal reactions. This leads to localized bulging and deformation of the battery, reducing its thickness flatness, which is detrimental to energy density and cycle performance. Excessive FEC may also introduce other side reactions (such as high-temperature acid production), exacerbating side reactions and gas generation risks, similarly hindering the improvement of surface flatness and thickness uniformity. For example, λ / f can be 0.5, 0.7, 0.9, 1.2, 1.6, 2.1, 2.7, 3.3, 4.0, 4.8, 5.7, 6.6, 7.6, 8.7, 9.9, 11.2, 11.8, 12, or values ​​within any two of the above ranges.

[0081] During long-term battery cycling, the content of lithium salts such as lithium hexafluorophosphate in the electrolyte affects the number of migratable lithium ions in the solution. Maintaining suitable ionic conductivity prevents increased internal resistance and capacity and cycle performance degradation. However, lithium hexafluorophosphate itself has poor thermal stability and may produce hydrofluoric acid (HF) due to trace amounts of moisture or its own decomposition. Hydrofluoric acid, as a strong acid, releases free protons (H+). + This can catalyze the breakage of polymer molecular chains in the adhesive components of the battery (such as the first adhesive component 4, the second adhesive component 5, etc.), accelerating the aging, loss of adhesion, and even detachment of the adhesive components. This not only weakens the buffering, fixing, or insulating functions of the aforementioned adhesive components, but its degradation products may also damage the SEI film, triggering additional side reactions or internal short circuit risks, thereby seriously affecting the long-term reliability and safety of the battery. Therefore, controlling the content of lithium hexafluorophosphate in the electrolyte is crucial.

[0082] In some embodiments, the electrolyte comprises nitrile compounds and fluorocarboxylic acid ester compounds. Based on the total mass of the electrolyte, the mass content of the nitrile compounds is 0.5%-5%, wherein the nitrile compounds include benzonitrile, p-toluenenitrile, 3,5-difluorobenzonitrile, adiponitrile, succinic acidnitrile, ethylene glycol bis(propionitrile) ether, 1,4-dicyano-2-butene, 1,3,6-hexanetrionitrile, 1,2,6-hexanetrionitrile, 1, At least one of 2,3-tris(2-cyanoethoxy)propane, 1,2,3,4,5-penta(2-cyanoethoxy)pentane, ethylene glycol di(2-cyanoethyl) ether, diethylene glycol di(2-cyanoethyl) ether, triethylene glycol di(2-cyanoethyl) ether, tetraethylene glycol di(2-cyanoethyl) ether, ethylene glycol di(4-cyanobutyl) ether, ethylene glycol (bis)propionitrile ether, and 1,2,3-tris(2-cyanoethoxy)propane; On the one hand, nitrile compounds, due to their strong oxidation resistance and high voltage resistance, help enhance the stability of the electrolyte at high potentials and reduce the occurrence of side reactions such as oxidative decomposition. On the other hand, the free protons H in the electrolyte... + It can coordinate or react with the lone pair electrons in the cyano group (-CN) of nitrile compounds, thereby reducing the production of hydrofluoric acid and lowering the probability of aging and adhesive failure of the first adhesive component 4 and the second adhesive component 5 caused by hydrofluoric acid. If the content of nitrile compounds is too low (e.g., below 0.5%), it cannot fully play its role in stabilizing the electrolyte and consuming protonated hydrogen; if the content is too high (e.g., above 5%), it may erode the formed negative electrode solid electrolyte interface film, thereby damaging the cycle performance of the battery.

[0083] Furthermore, in some embodiments, the mass content of lithium hexafluorophosphate is controlled to be 8%-20%, which means that while ensuring the ionic conductivity of the battery, the risk of lithium hexafluorophosphate decomposition to produce hydrofluoric acid is reduced, the problems of aging, catalysis and detachment of adhesive components are alleviated, and the battery safety performance is further improved.

[0084] In some embodiments, the mass content of the fluorocarboxylic acid ester compound is 5%-65%; the fluorocarboxylic acid ester compound includes at least one of 2,2-difluoroethyl acetate, ethyl trifluoroacetate, ethyl monofluoroacetate, and propyl difluoroacetate.

[0085] Fluorinated carboxylic acid esters are also key additives used to form a stable SEI film on the surface of silicon-based anodes, inhibit electrolyte decomposition, and reduce gas generation. Fluorinated carboxylic acid esters reduce the generation of side reactions in the battery, lower the negative impacts of gas generation within the battery, and reduce excessive gas accumulation at the groove.

[0086] If the mass content of fluorocarboxylic acid esters is too low (<5%), it will be difficult to form a sufficiently stable and effective SEI film on the surface of the silicon-based anode, which is not conducive to suppressing the side reactions between the electrolyte and the anode material, and thus hinders further improvement in battery cycle performance. If the mass content is too high (>65%), it will increase the overall viscosity of the electrolyte system. Electrolytes with excessively high viscosity are not conducive to the rapid migration of lithium ions during charging and discharging, and will increase the lithium ion transport impedance, thus hindering further improvement in the battery's rate performance and cycle performance.

[0087] The present application will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Reagents or instruments used, unless otherwise specified, are all commercially available conventional reagent products.

[0088] It should be noted that, as a unit of measurement, part by mass refers to the mass proportion of a certain component in a mixture. For example, 1 part by mass can be 0.1g, 1g, 8g, 25g, 60g, 100g, or any other positive number.

[0089] Example 1 Step 1: Preparation of the positive electrode sheet Lithium cobalt oxide (LiCoO2), polyvinylidene fluoride (PVDF), carbon black (super P), and carbon nanotubes (CNT) were mixed in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone (NMP) was added, and the mixture was stirred under vacuum until it formed a uniform and fluid positive electrode slurry. The positive electrode slurry was then uniformly coated onto both surfaces of an aluminum foil. The coated aluminum foil was dried, then rolled and slit. The slit positive electrode sheets were then processed with special rollers containing convex parts to obtain the desired positive electrode sheets.

[0090] Step 2: Preparation of the negative electrode The negative electrode active materials—artificial graphite, silicon carbide, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber, conductive carbon black (SP), and single-walled carbon nanotubes (SWCNTs)—were mixed in a mass ratio of 70:24.5:2.5:1.5:1:0.5. Deionized water was added, and the mixture was stirred in a vacuum mixer to obtain a negative electrode active slurry. The negative electrode active slurry was uniformly coated onto both surfaces of a copper foil. The coated copper foil was dried at room temperature and then transferred to an 80°C oven for drying for 10 hours. After cold pressing, slitting, cleaning, and sheet forming, the negative electrode sheet was obtained.

[0091] Step 3: Preparation of electrolyte In an argon-filled glove box (moisture content < 1 ppm, oxygen content < 1 ppm), fully dried lithium hexafluorophosphate (LiPF6), q wt% nitrile compound (see Table 2 for specific types of substances, where HTCN represents 1,3,6-hexanetrionitrile), and r wt% fluorocarboxylic acid ester compound, specifically 2,2-difluoroethyl acetate, are slowly added to a fluoroethylene carbonate solvent based on f wt% of the total electrolyte mass. The specific amounts added are shown in Table 2. The remainder is a mixed solvent composed of propylene carbonate and ethyl propionate in a mass ratio of 1:1. After stirring evenly, and after passing the moisture and free acid tests, the desired electrolyte is obtained.

[0092] It should be noted that the amount of fluorocarboxylic acid esters, lithium hexafluorophosphate (LiPF6), and nitrile compounds added to the electrolyte can be adjusted by adjusting the content of the mixed solvent composed of propylene carbonate and ethyl propionate in the electrolyte.

[0093] The electrolyte sample was separated and quantitatively analyzed using gas chromatography-mass spectrometry (GC-MS). The chromatograms were compared with those of standard samples or the mass spectrometry library was searched. Then, the contents of the above-mentioned fluorocarboxylic acid esters, lithium hexafluorophosphate (LiPF6), nitrile compounds, and fluoroethylene carbonates in the electrolyte were calculated by plotting a standard curve using the external standard method or the internal standard method.

[0094] Step 4: Preparation of lithium-ion batteries The positive electrode sheet from step one, the negative electrode sheet from step two, and a commercially available separator are stacked in the order of positive electrode sheet, separator, and negative electrode sheet, and then wound to obtain a cell 1 with a theoretical capacity of 6000 mAh. Cell 1 is placed in an outer packaging aluminum foil, and the electrolyte from step three is injected into the outer packaging. After vacuum sealing, settling, formation, shaping, and sorting processes, a lithium-ion battery is obtained. The charge / discharge range of this invention's battery is 3.0-4.55V.

[0095] The preparation methods of Examples 2-22 and Comparative Example 1 are basically the same as those of Example 1. The differences are shown in Tables 1, 2 and 3. In the table, p wt% represents the mass content of lithium hexafluorophosphate (LiPF6) in the electrolyte, q wt% represents the mass content of nitrile compounds in the electrolyte, r wt% represents the mass content of fluorocarboxylic acid esters in the electrolyte, and " / " indicates that the item is not present.

[0096] Table 1

[0097] Table 2

[0098] Table 3

[0099] Test case 1. Energy density improvement test The batteries obtained in the examples and comparative examples were charged at 25°C with a constant current of 3C to a voltage of 4.55V, then charged at 4.55V with a constant voltage to a current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 1.5C to a voltage of 3.0V. The discharge capacity was measured as x mAh, and the energy density was calculated as x mAh × average discharge voltage / battery volume. The energy densities of the batteries in Examples 1-22 and Comparative Examples 1-3 were tested and calculated. Using the heat seal thickness as a standard, the batteries in the examples were paired and compared with those in the comparative examples using the same heat seal thickness. That is, Examples 1, 4, 5, 8, and 10... Compare 22 with Comparative Example 1, compare Examples 2, 7, and 9 with Comparative Example 2, and compare Examples 3 and 6 with Comparative Example 3 to calculate the percentage increase in energy density.

[0100] 2. Cyclic performance test: The batteries obtained in the examples and comparative examples were charged at 25°C with a constant current of 1C to a voltage of 4.55V, then charged at 4.55V with a constant voltage to a current of 0.05C, allowed to stand for 5 minutes, and then discharged with a constant current of 1C to a voltage of 3.0V. This constitutes one charge-discharge cycle. The discharge capacity of the first week was measured as x mAh, and the discharge capacity of the 500th week was measured as y mAh. The capacity of the 500th week was divided by the capacity of the first week to obtain the cycle capacity retention rate R = y / x.

[0101] 3. Hot box test Test Step 1: Discharge the batteries obtained in the examples and comparative examples at 23°C with a constant current of 0.2C to 3.0V, and then fully charge them according to the following steps: charge with a constant current of 2.4A to 4.15V, charge with a constant current and constant voltage of 10A to 4.25V, cut-off current 8A, charge with a constant current and constant voltage of 8A to 4.35V, cut-off current 6A, charge with a constant current and constant voltage of 6A to 4.45V, cut-off current 4.675A, charge with a constant current and constant voltage of 4.675A to 4.53V, cut-off current 234mA.

[0102] Test step 2: Measure the battery's full-charge voltage, internal resistance, and thickness at 25℃.

[0103] Test Step 3: Suspend the fully charged battery in a gravity convection or circulating air oven at 25℃ (heat transfer is not allowed for non-integral battery modules). Ensure that the voltage and temperature leads are properly insulated to avoid short circuits. Monitor the battery surface temperature and the ambient temperature of the oven. Starting from room temperature, increase the temperature at 5±2℃ / min until the battery experiences thermal runaway. Thermal runaway is characterized by a sudden and rapid increase in battery surface temperature (e.g., a temperature rise rate ≥1℃ / s), a voltage drop of ≥25% of the initial voltage within a short period (e.g., within 3 seconds), or the battery exhibiting smoke, fire, explosion, leakage, gas leakage, bulging, or mesh penetration. Record the battery surface temperature at the time of thermal runaway; this temperature is the critical thermal runaway temperature for the battery under the test conditions.

[0104] Three samples were tested for each embodiment or comparative example, and the critical temperature of thermal runaway was recorded.

[0105] Please see Table 4 for the test results.

[0106] Table 4

[0107] As can be seen from Tables 1-4, compared with Comparative Examples 1-3, the energy density of the batteries in Examples 1-22 is improved to varying degrees while maintaining the same heat-sealing layer thickness, and the cycle performance is also improved to varying degrees. This indicates that constructing the groove structure provides a space for the first adhesive component, which can effectively reduce the risk of the outermost electrode sheet scratching the heat-sealing layer when folded, alleviate thickness fluctuations caused by expansion, and improve the thickness consistency and cycle stability of the battery; in addition, it also helps to improve the battery energy density.

[0108] It is worth noting that, compared with Example 1, the thermal runaway critical temperature during the hot box test of Example 10 (which does not satisfy W5 < W6) has decreased significantly. This indicates that when W5 is greater than W6, it is not conducive to the gas concentration and diffusion to the area between the positive and negative electrodes, resulting in the gas not being able to diffuse to the vicinity of the electrodes in time, thus reducing the timeliness of pressure relief during the battery hot box test.

[0109] Compared to Example 1, Example 16 has a low λ / f and a high FEC content, but the area of ​​the groove region is very small. Therefore, the groove cannot effectively accommodate the gas generated by the film-forming reaction, affecting the further participation of FEC in the film-forming reaction. Excessive FEC may also bring other side reactions, which is detrimental to improving the surface smoothness and thickness uniformity of the battery. Consequently, the battery of Example 16 exhibits poor cycle performance and thermal safety performance. Conversely, Example 17 has a high λ / f and a large groove region area, but a low FEC content. This may lead to relatively more side reaction gases being generated in the groove region due to interface instability. These gases are more likely to remain in the groove, causing deformation of the encapsulation film. Therefore, the battery of Example 17 also exhibits poor cycle performance and thermal safety performance.

[0110] Furthermore, since no nitrile compounds were added in Example 20, the water in the electrolyte could not be effectively removed, and the acidity in the electrolyte could not be inhibited, thereby accelerating the aging of the adhesive components. As a result, the battery of Example 20 exhibited poorer cycle performance compared to Example 1.

[0111] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A secondary battery, characterized in that, include: The battery cell (1) includes a positive electrode, a separator and a negative electrode, wherein the negative electrode includes a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, and the negative electrode active material layer includes a silicon-based material. The encapsulation film (3) forms a receiving cavity for accommodating the battery cell (1); the encapsulation film (3) includes a metal layer (32) and a heat-sealing layer (33) stacked together, the heat-sealing layer (33) being disposed on the side of the metal layer (32) facing the receiving cavity; The cell (1) includes an outer surface, the outer surface having a first surface (1a) and a second surface (1b) disposed at a distance from each other along the thickness direction (Z) of the cell (1), the first surface (1a) and / or the second surface (1b) having a side region (11) near at least one side along the length direction (Y) of the cell (1). The first adhesive (4) is attached to the edge region (11). The heat-sealing layer (33) is provided with a first groove (331) recessed toward the metal layer (32), and the first adhesive (4) is at least partially embedded in the first groove (331) along the thickness direction (Z) of the cell (1).

2. The secondary battery according to claim 1, characterized in that, Along the thickness direction (Z) of the cell (1), the size of the first adhesive (4) is T1 μm, and the size of the first groove (331) is D1 μm, satisfying: 0.7≤D1 / T1≤1.5; And / or, along the width direction (X) of the cell (1), the size of the first groove (331) is L1 mm, and the size of the first adhesive (4) is L2 mm; along the length direction (Y) of the cell (1), the size of the first groove (331) is W1 mm, and the size of the first adhesive (4) is W2 mm; satisfying: L2+0.5≤L1≤L2+2, W2+0.5≤W1≤W2+2.

3. The secondary battery according to claim 2, characterized in that, Along the thickness direction (Z) of the cell (1), the size of the first groove (331) is D1 μm, which satisfies: 5≤D1≤54; And / or, along the thickness direction (Z) of the cell (1), the size of the first adhesive (4) is T1 μm, satisfying: 4.5≤T1≤45; And / or, along the thickness direction (Z) of the cell (1), the size of the first groove (331) is D1 μm, and the size of the heat-sealing layer (33) is X1 μm, satisfying: 30%≤D1 / X1≤90%; And / or, the orthographic projection of the first groove (331) on the first surface (1a) completely covers the orthographic projection of the first adhesive (4) on the first surface (1a); And / or, the roughness of the inner surface of the first groove (331) is R1, and the roughness of the side of the first adhesive (4) facing away from the battery cell (1) is R2, satisfying: 2 < R1 / R2 < 15.

4. The secondary battery according to any one of claims 1-3, characterized in that, The first surface (1a) and / or the second surface (1b) have a central region (12) located between the edge regions (11) along the length direction (Y) of the cell (1); the secondary battery includes a second adhesive (5) attached to the central region (12); the heat-sealing layer (33) is provided with a second groove (332) recessed toward the metal layer (32); along the thickness direction (Z) of the cell (1), the second adhesive (5) is at least partially embedded in the second groove (332).

5. The secondary battery according to claim 4, characterized in that, The orthographic projection of the second groove (332) on the first surface (1a) completely covers the orthographic projection of the second adhesive (5) on the first surface (1a); And / or, along the width direction (X) of the cell (1), the size of the second groove (332) is L3 mm, and the size of the second adhesive (5) is L4 mm; along the length direction (Y) of the cell (1), the size of the second groove (332) is W3 mm, and the size of the second adhesive (5) is W4 mm; satisfying: L4+0.5≤L3≤L4+2, W4+0.5≤W3≤W4+2; And / or, along the thickness direction (Z) of the cell (1), the size of the second adhesive (5) is T2 μm, and the size of the second groove (332) is D2 μm, satisfying: 0.9≤D2 / T2≤1.2; And / or, along the thickness direction (Z) of the cell (1), the size of the second adhesive (5) is T2 μm, satisfying: 4.5≤T2≤45; And / or, along the thickness direction (Z) of the cell (1), the size of the second groove (332) is D2 μm, satisfying: 5≤D2≤54; And / or, the roughness of the inner surface of the second groove (332) is R3, and the roughness of the side of the second adhesive (5) facing away from the battery cell (1) is R4, satisfying: 2 < R3 / R4 < 15.

6. The secondary battery according to claim 4 or 5, characterized in that, The encapsulation film (3) includes a first encapsulation film (3a) and a second encapsulation film (3b), wherein the first encapsulation film (3a) and the first surface (1a) are disposed opposite to each other, and the second encapsulation film (3b) and the second surface (1b) are disposed opposite to each other; The battery cell (1) also has a third surface (1c) connecting the first surface (1a) and the second surface (1b), the third surface (1c) being located on one side of the first surface (1a) and the second surface (1b) along the length direction (Y) of the battery cell (1), the battery cell (1) including a first tab (7) and a second tab (8) with opposite polarities, the first tab (7) and the second tab (8) being led out from the third surface (1c) along the length direction (Y) of the battery cell (1); In the first encapsulation film (3a) or the second encapsulation film (3b), two second grooves (332) are provided at intervals along the width direction (X) of the cell (1), and a non-groove area (333) extending along the length direction (Y) of the cell (1) is provided between the two second grooves (332), and the non-groove area (333) extends at least to the edge region (11) near the first tab (7) and the second tab (8). Along the width direction (X) of the cell (1), the minimum distance between the first tab (7) and the second tab (8) is W6 mm, and the minimum distance between the two second grooves (332) is W5 mm, satisfying: W5 < W6; And / or, both the first encapsulation film (3a) and the second encapsulation film (3b) are provided with a first groove (331); the first groove (331) of the first encapsulation film (3a) and the first groove (331) of the second encapsulation film (3b) are arranged opposite to each other along the thickness direction (Z) of the cell (1), and the orthogonal projections of the first groove (331) of the first encapsulation film (3a) and the first groove (331) of the second encapsulation film (3b) on the first surface (1a) have a first projection overlap area, and the total area of ​​the first projection overlap area is Q1 mm. 2 The total projected area of ​​the first groove (331) of the first encapsulation film (3a) on the first surface (1a) is P1 mm. 2 The total projected area of ​​the first groove (331) of the second encapsulation film (3b) on the first surface (1a) is P2 mm. 2 The following conditions must be met: Q1 / P1 > 90%, Q1 / P2 > 90%; And / or, both the first encapsulation film (3a) and the second encapsulation film (3b) are provided with a second groove (332); the second groove (332) of the first encapsulation film (3a) and the second groove (332) of the second encapsulation film (3b) are arranged opposite to each other along the thickness direction (Z) of the cell (1), and the orthogonal projections of the second groove (332) of the first encapsulation film (3a) and the second groove (332) of the second encapsulation film (3b) on the first surface (1a) have a second projection overlap area, and the total area of ​​the second projection overlap area is Q2 mm. 2 The total projected area of ​​the second groove (332) of the first encapsulation film (3a) on the first surface (1a) is P3 mm. 2 The total projected area of ​​the second groove (332) of the second encapsulation film (3b) on the first surface (1a) is P4 mm. 2 The following conditions must be met: Q2 / P3 > 90%, Q2 / P4 > 90%.

7. The secondary battery according to claim 4 or 5, characterized in that, In the heat-sealing layer (33) opposite to the first surface (1a) or the second surface (1b) along the thickness direction (Z) of the cell (1), the first groove (331) and the second groove (332) have an overlapping region (334), the dimension of the overlapping region (334) along the thickness direction (Z) of the cell (1) is greater than the dimension of the portion of the first groove (331) excluding the overlapping region (334) along the thickness direction (Z) of the cell (1), and the overlapping region (334) along the thickness direction (Z) of the cell (1) The dimension in the thickness direction (Z) of the second groove (332) is greater than the dimension in the thickness direction (Z) of the cell (1) excluding the overlapping area (334); the orthographic projection area of ​​the first groove (331) on the first surface (1a) is S1, the orthographic projection area of ​​the second groove (332) on the first surface (1a) is S2, and the orthographic projection area of ​​the overlapping area (334) on the first surface (1a) is S3, satisfying: S3 / S1≤20%, S3 / S2≤20%.

8. The secondary battery according to any one of claims 1-3, characterized in that, The battery cell (1) includes a tab, the orthographic projection of the tab on the first surface (1a) at least partially coincides with the orthographic projection of the first groove (331) on the first surface (1a); Alternatively, the battery cell (1) includes tabs, the tabs include an outer tab (10) and an inner tab (9), the inner tab (9) is welded to the outer tab (10); the inner tab (9) includes an inner tab (9) with positive charge and an inner tab (9) with negative charge, wherein at least two stacked inner tabs (9) with positive charge are integrally connected to the positive electrode plate, and at least two stacked inner tabs (9) with negative charge are integrally connected to the negative electrode plate; the inner tab (9) is bent to form a bent portion (91), along the thickness direction (Z) of the battery cell (1), the first surface (1a) is closer to the bent portion (91) than the second surface (1b), and at least on the heat-sealing layer (33) disposed opposite to the first surface (1a), the first groove (331) is provided, and the orthogonal projection of the bent portion (91) on the first surface (1a) is at least partially located in the first groove (331); A third adhesive (6) is provided on at least one side surface of the welding area between the inner electrode (9) and the outer electrode (10). The third adhesive (6) is integrally formed with or partially overlaps with the first adhesive (4). The third adhesive (6) is at least partially embedded in the first groove (331).

9. The secondary battery according to claim 6, characterized in that, The encapsulation film (3) includes a sealing area (3c), which extends from the periphery of the first encapsulation film (3a) and the second encapsulation film (3b). The two layers of the encapsulation film (3) are sealed and bonded in the sealing area (3c). The first encapsulation film (3a), the second encapsulation film (3b) and the sealing area (3c) together form a cavity for accommodating the battery cell (1). The sum of the projected areas of the first groove (331) and the second groove (332) is (S1+S2-S3), where S1 is the projected area of ​​the first groove (331) on the first surface (1a), S2 is the projected area of ​​the second groove (332) on the first surface (1a), and S3 is the projected area of ​​the overlapping region (334) of the first groove (331) and the second groove (332) on the first surface (1a); the area of ​​the first encapsulation film (3a) is S4 mm. 2 (S1+S2-S3) / S4 is denoted as λ% The secondary battery further includes an electrolyte comprising fluoroethylene carbonate, wherein the mass content of the fluoroethylene carbonate is f% based on the total mass of the electrolyte, satisfying: 0.5≤λ / f≤12, and / or, 0<λ≤95, and / or, 5≤f≤30.

10. The secondary battery according to claim 9, characterized in that, The electrolyte includes a lithium salt, which includes lithium hexafluorophosphate, and the lithium hexafluorophosphate content is 8%-20% based on the total mass of the electrolyte. And / or, the electrolyte comprises nitrile compounds and fluorocarboxylic acid ester compounds, wherein, based on the total mass of the electrolyte, the mass content of the nitrile compounds is 0.5%-5%, and the mass content of the fluorocarboxylic acid ester compounds is 5%-65%; The nitrile compounds include at least one of benzonitrile, p-toluenenitrile, 3,5-difluorobenzonitrile, adiponitrile, succinic acid, ethylene glycol bis(propionitrile) ether, 1,4-dicyano-2-butene, 1,3,6-hexanetrionitrile, 1,2,6-hexanetrionitrile, 1,2,3-tris(2-cyanoethoxy)propane, 1,2,3,4,5-penta(2-cyanoethoxy)pentane, ethylene glycol di(2-cyanoethyl) ether, diethylene glycol di(2-cyanoethyl) ether, triethylene glycol di(2-cyanoethyl) ether, tetraethylene glycol di(2-cyanoethyl) ether, ethylene glycol di(4-cyanobutyl) ether, ethylene glycol (bis)propionitrile ether, and 1,2,3-tris(2-cyanooxy)propane. The fluorocarboxylic acid esters include at least one of 2,2-difluoroethyl acetate, ethyl trifluoroacetate, ethyl monofluoroacetate, and propyl difluoroacetate.

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