Secondary battery and electronic device

Abstract

The invention discloses a secondary battery and electronic equipment, the secondary battery comprises a first bag body, an electrode assembly and a first electrolyte, the first bag body encloses a first accommodating cavity, the electrode assembly and the first electrolyte are arranged in the first accommodating cavity, and the secondary battery further comprises a second bag body and a second electrolyte. A second containing cavity is defined by the second bag body, and the second electrolyte is arranged in the second containing cavity. The second electrolyte comprises ethylene carbonate, the mass percentage content of the ethylene carbonate in the second electrolyte is C1, and C1 is larger than or equal to 60% and smaller than or equal to 90%. A first wall part is arranged between the first bag body and the second bag body, the first containing cavity and the second containing cavity are separated through the first wall part, the melting point of the first wall part is T, and T is larger than or equal to 70 DEG C and smaller than or equal to 120 DEG C. The lithium precipitation interface of the electrode assembly can be rapidly passivated, the risk of thermal runaway of the secondary battery is reduced, and the safety performance of the secondary battery is improved.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and particularly to a secondary battery and an electronic device. Background Art

[0002] With the rapid development of modern technology, the demand for high-performance energy storage devices in fields such as portable electronic devices and electric vehicles is increasing day by day. As an efficient and environmentally friendly energy storage device, lithium-ion batteries have been widely used in many fields due to their high energy density, long cycle life, low self-discharge rate, etc. In the continuous development of lithium-ion batteries, the requirements for the safety performance of the batteries are getting higher and higher. Summary of the Invention

[0003] The present application provides a secondary battery and an electronic device, aiming to improve the safety performance of the secondary battery.

[0004] In a first aspect, the present application provides a secondary battery, including a first bag body, an electrode assembly, and a first electrolyte. The first bag body encloses a first accommodation cavity, and the electrode assembly and the first electrolyte are disposed in the first accommodation cavity. The secondary battery further includes a second bag body and a second electrolyte. The second bag body encloses a second accommodation cavity, and the second electrolyte is disposed in the second accommodation cavity. The second electrolyte includes ethylene carbonate, and the mass percentage content of ethylene carbonate in the second electrolyte is C1, where 60% ≤ C1 ≤ 90%. There is a first wall portion between the first bag body and the second bag body. The first accommodation cavity and the second accommodation cavity are separated by the first wall portion, and the melting point of the first wall portion is T, where 70°C ≤ T ≤ 120°C.

[0005] In the above technical solution, the first bag body and the second bag body are separated by the first wall portion, and the melting point T of the first wall portion is between 70°C and 120°C. When the temperature inside the secondary battery rises to near or reach this melting point range due to abnormal conditions such as lithium deposition, the first wall portion melts. At this time, the high-content ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) originally in the second bag body can quickly flow into the first bag body. Ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) can quickly passivate the lithium deposition interface of the electrode assembly, effectively reducing the further reaction between lithium metal and the electrolyte, reducing the risk of thermal runaway of the secondary battery, reducing the possibility of the secondary battery catching fire or exploding, and improving the safety performance of the secondary battery.

[0006] In some embodiments, the second electrolyte further includes fluoroethylene carbonate, and the mass percentage content of fluoroethylene carbonate in the second electrolyte is C2, where 5% ≤ C2 ≤ 20%. This can further slow down the occurrence of side reactions and passivate the lithium deposition interface. Moreover, by limiting 5% ≤ C2 ≤ 20%, the lithium deposition can be further slowed down, reducing the possibility of thermal runaway of the secondary battery and minimizing the impact on the viscosity of the second electrolyte.

[0007] In some embodiments, 75°C ≤ T ≤ 100°C, which can reduce the premature melting of the first wall due to slight temperature fluctuations and reduce the unnecessary premature release of the second electrolyte. Moreover, during the rapid temperature rise stage caused by lithium plating, additives such as ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) can be released more promptly, which is beneficial to more precisely reduce the risk of thermal runaway.

[0008] In some embodiments, 80°C ≤ T ≤ 90°C. In a high-temperature environment, additives such as ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) can be quickly released in response, thereby protecting the secondary battery; in a low-temperature environment, the possibility of prematurely releasing additives and affecting the normal electrolyte performance can be reduced, which can further enhance the adaptability and reliability of the secondary battery in different temperature environments.

[0009] In some embodiments, the second electrolyte further includes dimethyl carbonate, and the mass percentage content of dimethyl carbonate in the second electrolyte is C3, where 5% ≤ C3 ≤ 20%. Dimethyl carbonate (DMC) can reduce the viscosity of the second electrolyte, improve the fluidity of the second electrolyte, enable the second electrolyte to more quickly enter the first bag through the melted first wall, shorten the time interval from lithium plating to the start of passivation, so as to facilitate timely treatment when lithium plating just occurs or is still in a relatively mild stage, thereby reducing the risk of thermal runaway of the secondary battery and reducing the impact of lithium plating on the performance and safety of the secondary battery.

[0010] In some embodiments, the material of the first wall includes polyethylene, polyvinyl chloride, Wood's alloy or beeswax. Each of the above materials can melt at about 70°C to 120°C. On the one hand, it can absorb a large amount of heat and relieve the temperature rise of the secondary battery; on the other hand, it can release ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) in the second bag into the first bag, reducing the occurrence of thermal runaway.

[0011] In some embodiments, the area of the first wall is S, 2 cm 2 ≤ S ≤ 5 cm 2 which is beneficial to promptly passivate the lithium plating interface, improve the safety performance of the secondary battery, and is beneficial to reducing the impact on the overall structural strength of the secondary battery.

[0012] In some embodiments, the volume of the second pouch is G, where 2 mL ≤ G ≤ 4 mL. When the volume of the second pouch is more than 2 mL, it can provide a sufficient amount of high-concentration ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC), etc., so that after the first wall portion melts, there is enough second electrolyte flowing into the first pouch to sufficiently and effectively passivate the lithium plating interface and reduce the further deterioration of the lithium plating phenomenon. Setting the upper limit of the volume of the second pouch to 4 mL can reduce the excess of the second electrolyte and the loss of the energy density of the secondary battery.

[0013] In some embodiments, the viscosity of the second electrolyte is P, where 0.3 mPa·s ≤ P ≤ 1 mPa·s.

[0014] In some embodiments, the electrode assembly includes a positive electrode tab. The positive electrode tab has a positive electrode active material layer, and the positive electrode active material layer includes a ternary material. The ternary material includes nickel element, cobalt element, and manganese element. Based on the total mass of the nickel element, cobalt element, and manganese element, the mass percentage content of the nickel element is A1, where 50% ≤ A1 ≤ 90%. The mass percentage content of the cobalt element is A2, where 2% ≤ A2 ≤ 20%. The mass percentage content of the manganese element is A3, where 8% ≤ A3 ≤ 30%. The secondary battery with a ternary system has advantages such as high energy density, good cycling performance, and high charge and discharge efficiency.

[0015] In some embodiments, the secondary battery further includes a first tab. The first pouch includes a top sealing edge. One end of the first tab is connected to the electrode assembly, and the other end extends out of the first pouch from the top sealing edge. The second pouch includes a first sealing edge. Along the thickness direction of the first sealing edge, the first sealing edge and the top sealing edge are stacked. Encapsulating the first pouch and the second pouch simultaneously can reduce the encapsulation steps and improve the production efficiency. Compared with encapsulating the two pouches separately, it can save time and labor costs and is beneficial to mass production. Moreover, the first sealing edge and the top sealing edge are stacked, making the encapsulation structure of the entire secondary battery more compact, enhancing the encapsulation stability to a certain extent, and being able to better protect the internal electrode assembly and electrolyte.

[0016] In some embodiments, the secondary battery further includes a second tab. One end of the second tab is connected to the electrode assembly, and the other end extends out of the first pouch from the top sealing edge. At least a part of the second pouch is located between the first tab and the second tab, which is beneficial to making full use of the space outside the secondary battery, making the overall structure of the secondary battery more compact. Reasonably arranging each component within a limited space improves the space utilization rate and facilitates the realization of the miniaturization and lightweight design of the secondary battery.

[0017] In some embodiments, along the thickness direction of the secondary battery, the first pouch includes a first side wall and a second side wall that are oppositely arranged, and the second pouch is disposed on the first side wall or the second side wall. The first pouch includes a side seal, and the second pouch includes a first seal. Along the thickness direction of the side seal, the first seal is stacked with the side seal. The first side wall and the second side wall have a larger area, that is, the heat dissipation area of the first side wall and the second side wall is larger, which is conducive to more rapid transfer of temperature to the second pouch, and thus timely response to the temperature change inside the first pouch. Moreover, disposing the second pouch on the first side wall or the second side wall with a larger area means that the distance between the second electrolyte and the electrode assembly is relatively shorter, and ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) in the second electrolyte can reach the electrode assembly along a shorter path, thereby reducing the transmission time and accelerating the speed of ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) entering the electrode assembly.

[0018] In some embodiments, the second pouch is disposed in the first accommodation cavity. The second pouch includes a first wall portion. Placing the second pouch in the first accommodation cavity is conducive to utilizing the internal space of the secondary battery, making the external dimensions of the secondary battery more compact. Moreover, since the second pouch is directly in the first accommodation cavity, the distance from the electrode assembly is further reduced. When abnormal conditions such as lithium plating occur in the secondary battery and cause the temperature to rise, after the first wall portion melts, the high-concentration electrolyte in the second pouch can quickly and directly contact the lithium plating interface, and passivation treatment can be carried out almost without delay, improving the response speed and effect of the protection mechanism, and being able to more effectively inhibit the further deterioration of the lithium plating reaction and improve the safety performance of the secondary battery.

[0019] In some embodiments, the first electrolyte includes ethylene carbonate, and the mass percentage content of ethylene carbonate in the first electrolyte is B1, where 20% ≤ B1 ≤ 30%. The first electrolyte includes fluoroethylene carbonate, and the mass percentage content of fluoroethylene carbonate in the first electrolyte is B2, where 0.1% ≤ B2 ≤ 3%.

[0020] In a second aspect, the present application also proposes an electronic device including the secondary battery according to any one of the embodiments in the first aspect above.

[0021] Additional aspects and advantages of the embodiments of the present application will be described, shown, or elucidated in part in the subsequent description, or through the implementation of the embodiments of the present application. Description of the Drawings

[0022] One or more embodiments are illustrated by way of example in the accompanying drawings, and these exemplary illustrations are not intended to limit the embodiments. Elements with the same reference numerals in the drawings are represented as similar elements.

[0023] Figure 1Schematic structural diagram of a secondary battery according to some embodiments of the present application;

[0024] Figure 2 Schematic connection diagram (cross-sectional view) of the first bag body and the second bag body according to some embodiments of the present application;

[0025] Figure 3 Explosion schematic diagram of a secondary battery according to some embodiments of the present application;

[0026] Figure 4 Schematic diagram of the stacked structure of the positive electrode sheet and the negative electrode sheet according to some embodiments of the present application;

[0027] Figure 5 Schematic structural diagram of a secondary battery according to some embodiments of the present application;

[0028] Figure 6 Schematic connection diagram (cross-sectional view, the first tab and the second tab are not shown) of the first bag body and the second bag body according to some embodiments of the present application;

[0029] Figure 7 Schematic connection diagram (cross-sectional view) of the first bag body and the second bag body according to some embodiments of the present application;

[0030] Figure 8 Schematic connection diagram (cross-sectional view) of the first bag body and the second bag body according to some embodiments of the present application.

[0031] Explanation of reference numerals:

[0032] 100, secondary battery;

[0033] 10, first bag body; 10a, first accommodation cavity; 11, first part; 111, first pit cavity; 12, second part; 13, top sealing edge; 14, side sealing edge; 15, first side wall; 16, second side wall;

[0034] 20, electrode assembly; 21, positive electrode sheet; 21a, positive current collector; 21b, positive active material layer; 22, negative electrode sheet; 22a, negative current collector; 22b, negative active material layer; 23, separator;

[0035] 30, first electrolyte;

[0036] 40, second bag body; 40a, second accommodation cavity; 41, first sealing edge;

[0037] 50, second electrolyte;

[0038] 60, first wall portion;

[0039] 70, first tab;

[0040] 80, second tab;

[0041] X, the first direction. Specific embodiments

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application.

[0043] Referring to "embodiments" in the present application means that the specific features, structures, or characteristics described in connection with the embodiments may be included in at least one embodiment of the present application. The phrase appears in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments.

[0044] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, "a plurality of" means two or more unless otherwise specifically defined.

[0045] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this article generally represents an "or" relationship between the front and back associated objects.

[0046] The technical features involved in different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0047] In a first aspect, the present application provides a secondary battery 100. Please refer to Figure 1 , the secondary battery 100 includes a first bag 10, an electrode assembly 20, and a first electrolyte 30. The electrode assembly 20 is disposed in the first bag 10, and the first electrolyte 30 infiltrates the electrode assembly 20 in the first bag 10.

[0048] Regarding the above-mentioned first bag 10, please refer to Figure 1 and Figure 2 , the first bag 10 encloses a first accommodation cavity 10a, and the electrode assembly 20 and the first electrolyte 30 are disposed in the first accommodation cavity 10a. For example, please further refer to Figure 3, the first bag body 10 includes a first part 11 and a second part 12. The first part 11 is provided with a first cavity 111, and the electrode assembly 20 is disposed in the first cavity 111. The second bag body 40 is provided with a second cavity (not labeled in the figure) or not provided with a cavity. Then, the second part 12 covers the first cavity 111, and the edges of the first part 11 and the second part 12 are hot-pressed and sealed, so that the first part 11 and the second part 12 form a complete first packaging bag 10. Among them, the hot-pressed edges of the first part 11 and the second part 12 form a sealing structure, which can improve the sealing performance of the secondary battery 100.

[0049] For the above electrode assembly 20, please refer to Figure 1 and Figure 4 , the electrode assembly 20 is housed in the first bag body 10. The electrode assembly 20 includes a positive electrode plate 21, a negative electrode plate 22, and a separator 23. The separator 23 is disposed between the positive electrode plate 21 and the negative electrode plate 22 for insulating and separating the positive electrode plate 21 and the negative electrode plate 22. The electrode assembly 20 can be in a wound shape, that is, the positive electrode plate 21, the separator 23, and the negative electrode plate 22 are stacked and wound. The electrode assembly 20 can also be in a stacked shape, that is, a plurality of positive electrode plates 21 and a plurality of negative electrode plates 22 are alternately stacked, and a separator 23 is disposed between adjacent positive electrode plates 21 and negative electrode plates 22.

[0050] Please refer to Figure 4 , the negative electrode plate 22 includes a negative electrode current collector 22a and a negative electrode active material layer 22b. The negative electrode active material layer 22b is stacked on at least one surface of the negative electrode current collector 22a. The negative electrode current collector 22a can be made of a conductive metal material such as copper foil, which can make the negative electrode current collector 22a have higher strength and higher conductivity, reduce the risk of tearing of the negative electrode current collector 22a, and is beneficial to improving the charge and discharge rate of the secondary battery 100. The negative electrode active material layer 22b includes a negative electrode active material, a conductive agent, an adhesive, etc. These materials are mixed and stirred evenly and then coated on the negative electrode current collector 22a to obtain the negative electrode active material layer 22b. Among them, the negative electrode active material includes one or more of graphite, soft carbon, hard carbon, elemental silicon, silicon oxide compound, silicon alloy, etc.

[0051] The positive electrode plate 21 includes a positive electrode current collector 21a and a positive electrode active material layer 21b, and the positive electrode active material layer 21b is stacked on at least one surface of the positive electrode current collector 21a. The positive electrode current collector 21a can be made of a conductive metal material such as aluminum foil, which can endow the positive electrode current collector 21a with relatively high strength and relatively high electrical conductivity, reduce the tearing of the positive electrode current collector 21a, and is beneficial to improving the charge-discharge rate of the secondary battery 100. The positive electrode active material layer 21b includes a positive electrode active material, a conductive agent, a binder, etc. After the above-mentioned various material components are mixed and stirred evenly, they are coated on the positive electrode current collector 21a to obtain the positive electrode active material layer 21b. Among them, the positive electrode active material includes one or more of lithium nickel cobalt manganate, lithium cobaltate, lithium manganate, etc.

[0052] In the embodiments of the present application, the positive electrode active material preferably includes a lithium nickel cobalt manganate ternary material, that is, it includes nickel element, cobalt element and manganese element. Based on the total mass of nickel element, cobalt element and manganese element, the mass percentage content of nickel element is A1, 50% ≤ A1 ≤ 90%; the mass percentage content of cobalt element is A2, 2% ≤ A2 ≤ 20%; the mass percentage content of manganese element is A3, 8% ≤ A3 ≤ 30%. The secondary battery 100 of the ternary system has the advantages of high energy density, good cycle performance, high charge-discharge efficiency, etc.

[0053] The proportion of each element in the ternary material can be measured by inductively coupled plasma optical emission spectrometry (ICP-OES). By measuring the emission intensity of the test sample and combining with the standard curve method, the nickel, cobalt and manganese contents in the sample can be calculated simultaneously.

[0054] However, in the ternary system, there are problems such as lithium / nickel mixing, transition metal dissolution, oxygen release, etc. In a high-temperature environment, the reaction will be accelerated, which is likely to lead to the aggravation of side reactions and serious gas generation. Adding negative electrode protection additives such as ethylene carbonate (EC) and fluoroethylene carbonate (FEC) to the first electrolyte 30 can play a protective role on the negative electrode. The inventors of the present application have found that if the content of the above-mentioned negative electrode protection additives such as ethylene carbonate (EC) and fluoroethylene carbonate (FEC) is too high, the side reaction may be further aggravated, resulting in more serious gas generation phenomenon. Therefore, in the secondary battery 100 of the ternary system, by controlling the content of the negative electrode protection additive in the first electrolyte 30 to a relatively low level, for example, the mass percentage content of ethylene carbonate (EC) is B1, 20% ≤ B1 ≤ 30%, and the mass percentage content of fluoroethylene carbonate (FEC) is B2, 0.1% ≤ B2 ≤ 3%, it is beneficial to reduce the occurrence of gas generation phenomenon and is beneficial to reducing problems such as cycle attenuation.

[0055] It can be understood that the first electrolyte 30 further includes a lithium salt, and the addition amount of the lithium salt is 0.8 mol / L - 1.2 mol / L. As long as it can achieve the function of transporting lithium ions in the secondary battery 100 of the present application, it may include but is not limited to at least one of substances such as lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), etc. The first electrolyte 30 may further include at least one of the solvents propylene carbonate (DC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC). This is beneficial to reducing the viscosity of the first electrolyte 30, reducing the decomposition of the electrolyte at high temperatures, and is beneficial to improving the safety performance of the secondary battery 100.

[0056] However, the inventors of the present application further studied and found that when lithium plating occurs in the ternary system secondary battery 100, due to the relatively low content of anode protection additives such as ethylene carbonate (EC) and fluoroethylene carbonate (FEC) in the electrolyte, it is difficult to effectively passivate and protect the lithium plating interface. The secondary battery 100 will continuously generate heat, which may then lead to fire or even explosion.

[0057] To reduce the above problems, in the embodiments of the present application, please refer to Figure 1 and Figure 2 , the secondary battery 100 further includes a second bag body 40 and a second electrolyte 50, and the second electrolyte 50 is disposed inside the second bag body 40. The second electrolyte 50 includes ethylene carbonate (EC), and the content of ethylene carbonate (EC) is relatively high. For example, the mass percentage content of ethylene carbonate (EC) in the second electrolyte 50 is C1, and 60% ≤ C1 ≤ 90%. There is a first wall portion 60 between the first bag body 10 and the second bag body 40. The first accommodation cavity 10a and the second accommodation cavity 40a are separated by the first wall portion 60, and the melting point of the first wall portion 60 is T, and 70°C ≤ T ≤ 120°C.

[0058] In the embodiments of the present application, the first bag body 10 and the second bag body 40 are separated by the first wall portion 60, and the melting point T of the first wall portion 60 is 70°C to 120°C. When the temperature inside the secondary battery 100 rises to near or reach this melting point range due to abnormal conditions such as lithium plating, the first wall portion 60 melts. At this time, the high-content ethylene carbonate (EC) originally located in the second bag body 40 can quickly flow into the first bag body 10. Ethylene carbonate (EC) can quickly passivate the lithium plating interface of the electrode assembly 20, effectively reducing the further reaction between the lithium metal at the lithium plating site and the electrolyte, reducing the risk of thermal runaway of the secondary battery 100, reducing the possibility of fire or explosion of the secondary battery 100, and improving the safety performance of the secondary battery 100. The secondary battery 100 of the embodiments of the present application can automatically adjust the release of additives according to the internal temperature change, enhancing the adaptability and reliability of the secondary battery 100 in different temperature environments.

[0059] Moreover, in the embodiments of the present application, it is defined that 70°C ≤ T ≤ 120°C. During the normal charge and discharge process of the secondary battery 100, the temperature usually remains within a relatively low range. If the melting point is less than 70°C, it may cause premature melting of the first wall portion 60, affecting the normal use of the secondary battery 100, and the first wall portion 60 may melt during the conventional charge and discharge cycle. When the temperature rises to 70°C and above, it is very likely that there are problems inside the secondary battery 100, such as the temperature rise caused by lithium plating. At this time, the first wall portion 60 begins to melt to initiate the subsequent protection mechanism. If the temperature exceeds 120°C, it may cause the intervention time of ethylene carbonate (EC) for lithium plating to be too late. At this time, relatively serious lithium plating may have occurred inside the secondary battery 100, and it is difficult for the second electrolyte 50 to play a role and effectively prevent safety accidents of the secondary battery 100. Defining 70°C ≤ T ≤ 120°C can facilitate the secondary battery 100 to respond in time before the temperature reaches an overly dangerous level and release the second electrolyte 50 for protection.

[0060] In some other embodiments, 75°C ≤ T ≤ 100°C can reduce the premature melting of the first wall portion 60 caused by slight temperature fluctuations and reduce the unnecessary premature release of the second electrolyte 50. Moreover, during the stage of rapid temperature rise caused by lithium plating, it can respond more timely and release additives such as ethylene carbonate (EC), which is beneficial to more precisely reduce the risk of thermal runaway.

[0061] Furthermore, 80°C ≤ T ≤ 90°C can quickly respond and release additives such as ethylene carbonate (EC) in a high-temperature environment to protect the secondary battery 100; in a low-temperature environment, it can reduce the influence on the performance of the first electrolyte 30 due to the premature release of the second electrolyte 50, and can further enhance the adaptability and reliability of the secondary battery 100 in different temperature environments.

[0062] In addition, in the embodiments of the present application, it is defined that 60% ≤ C1 ≤ 90%. On the one hand, it makes the second electrolyte 50 have better fluidity, facilitating the rapid arrival of ethylene carbonate (EC) at the lithium plating interface; moreover, it can facilitate the formation of a stable solid electrolyte interface film at the interface, effectively passivating the lithium plating interface and reducing side reactions. If it is less than 60%, it may be difficult to effectively passivate the lithium plating interface due to the low content of ethylene carbonate (EC). If it is greater than 90%, it may increase the viscosity of the second electrolyte 50 and make it difficult to quickly reach the lithium plating interface.

[0063] In some other embodiments, the second electrolyte 50 includes fluoroethylene carbonate (FEC), and the mass percentage content of fluoroethylene carbonate (FEC) in the second electrolyte 50 is C2, where 5% ≤ C2 ≤ 20%. Fluoroethylene carbonate (FEC) can also participate in the formation of the solid electrolyte interface film. Compared with the solid electrolyte interface film formed solely by ethylene carbonate (EC), the solid electrolyte interface film containing fluoroethylene carbonate (FEC) has higher stability, which helps to reduce side reactions and passivate the lithium deposition interface. Moreover, by limiting 5% ≤ C2 ≤ 20%, the lithium deposition can be further alleviated, the thermal runaway of the secondary battery 100 can be reduced, and the influence on the viscosity of the second electrolyte 50 can be reduced. If it is less than 5%, the effect on passivating lithium deposition is small; while if it is greater than 20%, it will also cause an increase in the viscosity of the second electrolyte 50, making it difficult to quickly reach the lithium deposition interface.

[0064] For the measurement methods of the mass content C1 of ethylene carbonate (EC) and the mass content C2 of fluoroethylene carbonate (FEC), gas chromatography (GC) testing can be used. Inject the electrolyte to be tested into the GC system and detect it using a flame ionization detector (FID) or a thermal conductivity detector (TCD). By utilizing the differences in the volatility of different components in the gas phase and the retention times on the chromatographic column, the mass percentage contents of EC and FEC are calculated through the standard curve method.

[0065] In the embodiments of the present application, the material of the first wall portion 60 can be polyethylene, polyvinyl chloride, Wood's alloy, beeswax, etc. Ethylene has good flexibility and processability, which is beneficial to meeting the design requirements of packaging bags with different shapes and sizes. Wood's alloy has good thermal conductivity and can quickly conduct the heat in the surrounding environment to itself, accelerating its melting process. And at room temperature, Wood's alloy has a certain mechanical strength, which can provide reliable structural support for the packaging bag and improve the stability and sealing performance of the packaging during normal use. Beeswax has good sealing performance and can effectively reduce the risk of liquid leakage. And at room temperature, it has a certain hardness and toughness, and can form a tight sealing layer, which can isolate the first bag body 10 and the second bag body 40 from each other under normal circumstances. The above-mentioned various materials can be melted at about 70°C to 120°C. On the one hand, they can absorb a large amount of heat and relieve the temperature rise of the secondary battery 100. On the other hand, they can release ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) in the second bag body 40 into the first bag body 10, reducing the occurrence of thermal runaway.

[0066] Regarding the adjustment of the melting points of the materials of the above-mentioned first wall portion 60, for polyethylene, its melting point can be adjusted by adjusting the degree of polymerization. Generally, the higher the degree of polymerization of ethylene polymers, the longer the molecular chains, the stronger the intermolecular forces, and the correspondingly higher the melting point. By controlling the conditions of the polymerization reaction, such as reaction temperature, pressure, types and dosages of catalysts, etc., the degree of polymerization of ethylene can be adjusted, thereby adjusting its melting point. For example, under lower reaction temperatures and the action of appropriate catalysts, polyethylene with a relatively low degree of polymerization and a relatively low melting point may be obtained; while under higher temperatures and a specific catalyst system, polyethylene with a higher degree of polymerization and a higher melting point can be obtained. The same applies to polyvinyl chloride. For Wood's alloy, Wood's alloy is usually composed of metals such as bismuth, lead, tin, and cadmium. Changing the proportion of each metal component can significantly affect its melting point. Generally, increasing the content of bismuth will increase the melting point of Wood's alloy, while increasing the content of tin will lower the melting point. For beeswax, adding some substances with different melting points to beeswax can change its melting point. For example, adding paraffin wax can lower the melting point of beeswax. The melting point of paraffin wax is generally lower than that of beeswax, and it has good compatibility with beeswax. After mixing the two, a eutectic system will be formed, reducing the overall melting point. The higher the proportion of added paraffin wax, the more obvious the decrease in the melting point of beeswax. On the contrary, adding some substances with high melting points, such as stearic acid, etc., can increase the melting point of beeswax.

[0067] The viscosity of the second electrolyte 50 affects the rate at which it flows into the first bag body 10. The faster the rate, the more timely it can passivate the lithium deposition interface. The inventors of the present application have found that after dimethyl carbonate (DMC) is mixed with ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC), through intermolecular interactions, the originally relatively regular arrangement of ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) can be disrupted, weakening the strong intermolecular forces between them, thereby reducing the overall viscosity of the mixed solvent and effectively improving the fluidity of the second electrolyte 50.

[0068] In the embodiments of the present application, the second electrolyte 50 further includes dimethyl carbonate (DMC), and the mass percentage content of dimethyl carbonate (DMC) in the second electrolyte 50 is C3, where 5% ≤ C3 ≤ 20%. When lithium deposition occurs in the secondary battery 100 and the temperature rises, causing the first wall portion 60 to melt, dimethyl carbonate (DMC) can reduce the viscosity of the second electrolyte 50, improve the fluidity of the second electrolyte 50, enable the second electrolyte 50 to more quickly enter the first bag body 10 through the melted first wall portion 60, shorten the time interval from lithium deposition to the start of passivation, thereby facilitating timely treatment when lithium deposition just occurs or is still in a relatively mild stage, effectively reducing the further development of lithium deposition, and reducing the impact of lithium deposition on the performance and safety of the secondary battery 100. For the mass content C3 of dimethyl carbonate (DMC), it can be obtained by the same gas chromatography (GC) test.

[0069] It is understandable that the second electrolyte 50 may further include at least one of the solvents propylene carbonate (DC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC). This is beneficial for reducing the viscosity of the second electrolyte 50, reducing the decomposition of the electrolyte at high temperatures, and is beneficial for improving the safety performance of the secondary battery 100.

[0070] Among them, the viscosity of the second electrolyte 50 is P, where 0.3 mPa·s ≤ P ≤ 1 mPa·s. This is beneficial because after the second electrolyte 50 melts on the first wall portion 60, it can quickly flow into the first bag body 10. When problems such as lithium plating occur in the secondary battery 100 and cause the temperature to rise, this rapid flow characteristic enables the second electrolyte 50 to quickly reach the lithium plating interface, achieve timely passivation, effectively inhibit the further development of lithium plating, and reduce the damage to the performance of the secondary battery 100. And it can make the second electrolyte 50 more evenly cover the lithium plating interface after flowing into the first bag body 10, which is beneficial for providing comprehensive passivation for the lithium plating interface and reducing the risk of failures caused by local non-passivation.

[0071] Regarding the measurement method of the electrolyte viscosity, a suitable viscometer can be used for testing. Pour the electrolyte to be measured into the sample cup of the viscometer to ensure that the sample cup is full and free of bubbles. Then place the sample cup in a constant temperature water bath to stabilize its temperature at the required test temperature (usually 25 °C), select a suitable rotor according to the expected viscosity of the electrolyte, start the viscometer, and record the reading.

[0072] In some embodiments, the area of the first wall portion 60 is S, 2 cm 2 ≤ S ≤ 5 cm 2 , and the size of the area of the first wall portion 60 directly affects the speed at which additives such as high-content ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) in the second bag body 40 are released into the first bag body 10. The inventors of the present application have found that when the area is small (less than 2 cm 2 ), the release channel is relatively narrow and the release speed is slow. When lithium plating occurs in the secondary battery 100 and causes the temperature to rise, it may be difficult to provide sufficient additives in a timely manner to effectively passivate the lithium plating, affecting the safety performance of the secondary battery 100. And when the area is large (greater than 5 cm 2 ), it may weaken the overall structural strength of the secondary battery 100, resulting in the secondary battery 100 being more likely to deform, rupture, etc. when subjected to external force impacts or vibrations. In the embodiments of the present application, it is defined that 2 cm 2 ≤ S ≤ 5 cm 2 , which is beneficial for timely passivating the lithium plating interface, improving the safety performance of the secondary battery 100, and is beneficial for reducing the impact on the overall structural strength of the secondary battery 100.

[0073] For the measurement method of the area S of the first wall portion 60, when the first wall portion 60 is of a regular shape, a ruler or vernier caliper can be used to measure the length L and width W of the first wall portion 60, and the area can be calculated by S = L * W. When the first wall portion 60 is of an irregular shape, an optical microscope or an electron microscope can be used to observe the first wall portion 60. After magnifying the first wall portion 60 to an appropriate magnification, an image is taken, and then image analysis software is used to measure the area of the first wall portion 60.

[0074] In the embodiments of the present application, the volume of the second bag body 40 is G, and 2 mL ≤ G ≤ 4 mL. When lithium deposition or other conditions occur in the secondary battery 100, the second electrolyte 50 is required to passivate the lithium deposition interface. When the volume of the second bag body 40 is more than 2 mL, a sufficient amount of high-concentration ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) can be provided, so that after the first wall portion 60 melts, there is enough second electrolyte 50 flowing into the first bag body 10 to fully and effectively passivate the lithium deposition interface and reduce the further deterioration of the lithium deposition phenomenon. Setting the upper limit of the volume of the second bag body 40 to 4 mL can reduce the excess of the second electrolyte 50. For example, when it exceeds 4 mL, it may cause local accumulation after flowing into the first bag body 10, which not only makes it difficult to better play the passivation role, but also causes waste of cost and resources. And when the volume of the second bag body 40 is relatively large, the installation of the second bag body 40 will be more inconvenient, and it will also lead to a loss of the energy density of the secondary battery 100.

[0075] For the measurement method of the volume G of the second bag body 40, a vernier caliper can be used to measure the thickness D of the secondary packaging bag, and the area S of the second bag body 40 can be measured in a similar way to the method of measuring the area of the first wall portion 60, and the volume of the second bag body 40 can be calculated by G = S * D.

[0076] For the installation of the second bag body 40, in some embodiments, please refer to Figure 1 and Figure 2 , along the thickness direction (the first direction X) of the secondary battery 100, the first bag body 10 includes a first side wall 15 and a second side wall 16 that are oppositely arranged, and the second bag body 40 is arranged on the first side wall 15 or the second side wall 16. The first side wall 15 and the second side wall 16 have a larger area, that is, the heat dissipation area of the first side wall 15 and the second side wall 16 is larger, which is beneficial to the more rapid transfer of temperature to the second bag body 40, and then timely respond to the temperature change inside the first bag body 10. And, arranging the second bag body 40 on the first side wall 15 or the second side wall 16 with a larger area means that the distance between the second electrolyte 50 and the electrode assembly 20 is relatively shorter, and ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) in the second electrolyte 50 can reach the electrode assembly 20 along a shorter path, thereby reducing the transmission time and accelerating the speed of ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) entering the electrode assembly 20.

[0077] The first bag body 10 includes a side seal 14, and the second bag body 40 includes a first seal 41. Along the thickness direction of the side seal 14 ( Figure 1 The structure in which the side seal 14 is not folded is shown, and the thickness direction of the side seal 14 is the first direction X), the first seal 41 and the side seal 14 are stacked. For example, after injecting electrolyte into the first bag body 10 and the second bag body 40 respectively, a hot pressing and encapsulation operation is performed on the first bag body 10 and the second bag body 40 simultaneously, so that the first bag body 10 forms a side seal 14, the second bag body 40 forms a first seal 41, and the first seal 41 and the side seal 14 are stacked. The first seal 41 of the first bag body 10 and the side seal 14 of the second bag body 40 support each other, increasing the stability of the overall structure of the secondary battery 100. At the same time, the stacked seal method helps to improve the sealing performance of the secondary battery 100. By superimposing the two seals, a thicker sealing layer can be formed, effectively blocking the entry of external air, moisture, etc. into the secondary battery 100, and at the same time reducing the leakage of the electrolyte inside the secondary battery 100.

[0078] In some other embodiments, please refer to Figure 5 , the secondary battery 100 further includes a first tab 70. The first bag body 10 includes a top seal 13. One end of the first tab 70 is connected to the electrode assembly 20, for example, connected to the positive electrode tab 21 of the electrode assembly 20, and the other end extends out of the first bag body 10 from the top seal 13. The first tab 70 is used to lead out the positive electrode of the secondary battery 100. The second bag body 40 includes a first seal 41. Along the thickness direction of the first seal 41, the first seal 41 and the top seal 13 are stacked. For example, after injecting electrolyte into the first bag body 10 and the second bag body 40 respectively, a hot pressing and encapsulation operation is performed on the first bag body 10 and the second bag body 40 simultaneously, so that the first bag body 10 forms a top seal 13, the second bag body 40 forms a first seal 41, and the first seal 41 and the top seal 13 are stacked. Encapsulating simultaneously can reduce the encapsulation steps and improve production efficiency. Compared with encapsulating the two bag bodies separately, time and labor costs can be saved, which is beneficial to mass production. And the first seal 41 and the top seal 13 are stacked, making the encapsulation structure of the entire secondary battery 100 more compact, enhancing the stability of the encapsulation to a certain extent, and being able to better protect the internal electrode assembly 20 and electrolyte.

[0079] Among them, part of the top seal 13 and part of the first seal 41 can form the first wall portion 60 together, for example, the part of the top seal 13 facing the first seal 41 and the part of the first seal 41 facing the top seal 13 together form the first wall portion 60. The part of the top seal 13 away from the first seal 41 and the part of the first seal 41 away from the top seal are made of conventional packaging materials. After the first wall portion 60 is melted, a channel connecting the first bag body 10 and the second bag body 40 is formed, so that the second electrolyte enters the first bag body, thereby passivating the lithium deposition interface.

[0080] In some other embodiments, please refer to Figure 6 , before packaging, the first edge seal 41 can also be clamped in the top edge seal 13, and then the top edge seal 13 and the first edge seal 41 are packaged together, wherein the first edge seal 41 can be melted, and when the temperature rises to the melting point of the first edge seal 41 due to lithium precipitation, the top edge seal 13 continues to maintain its original state, and the first edge seal 41 located in the top edge seal 13 melts to form a channel connecting the first bag body 10 and the second bag body 40 in the top edge seal 13, that is, the first wall portion 60 that can be melted at this time is the first edge seal 41. In order to form a larger channel to facilitate the transmission of the second electrolyte 50, the thickness of the first edge seal 41 can also be appropriately increased, and a larger channel can be formed after the first edge seal 41 melts. Optionally, the temperature of the hot-press packaging of the first edge seal 41 and the top edge seal 13 is lower than the melting point of the first edge seal 41 to reduce the melting of the first edge seal 41 during packaging; or the top edge seal 13 and the first edge seal 41 are directly sealed by gluing.

[0081] In some embodiments, please refer to Figure 5 The secondary battery 100 also includes a second pole ear 80, one end of which is connected to the electrode assembly 20, and the second pole ear 80 can be connected to the negative electrode sheet 22 of the electrode assembly 20, and the other end extends from the top seal 13 outside the first bag body 10 to lead out the negative electrode. The first pole ear 70 and the second pole ear 80 extend from the top seal 13 to facilitate the connection of the secondary battery 100 with the external circuit. The second bag body 40 is at least partially located between the first pole ear 70 and the second pole ear 80, which is conducive to making full use of the space outside the secondary battery 100, making the overall structure of the secondary battery 100 more compact. Reasonable arrangement of various components in a limited space improves space utilization, reduces the energy density loss of the secondary battery 100, and facilitates the miniaturization and lightweight design of the secondary battery 100.

[0082] In some other embodiments, please refer to Figure 7, the first wall portion 60 can also be disposed at the top of the first bag body 10. Heat-sealing the top sealing edge 13 and the first sealing edge 41 can seal the first bag body 10 and the second bag body 40. And when lithium plating occurs, the first wall portion 60 at the top of the first bag body 10 melts, allowing the second electrolyte 50 to enter the first bag body 10, thereby passivating the lithium plating interface. In some embodiments, please refer to Figure 8 , the second bag body 40 can also be disposed in the first accommodation cavity 10a. At this time, the second bag body 40 includes the first wall portion 60. Placing the second bag body 40 in the first accommodation cavity 10a is conducive to making full use of the internal space of the secondary battery 100, making the external dimensions of the secondary battery 100 more compact. And since the second bag body 40 is directly in the first accommodation cavity 10a, the distance from the electrode assembly 20 is further reduced. When abnormal conditions such as lithium plating occur in the secondary battery 100 and cause the temperature to rise, after the first wall portion 60 melts, the high-concentration electrolyte in the second bag body 40 can quickly and directly contact the lithium plating interface, and can perform passivation treatment almost without delay, improving the response speed and effect of the protection mechanism, and can more effectively inhibit the further deterioration of the lithium plating reaction. And the first bag body 10 plays a certain physical protection role for the internal second bag body 40, can effectively block the influence of external mechanical shocks, vibrations, etc. on the second bag body 40, and is conducive to reducing the melting of the first wall portion 60 due to external temperature changes, improving the stability of the secondary battery 100 under normal working conditions.

[0083] In a second aspect, the present application also proposes an electronic device, including the secondary battery 100 described in any of the embodiments of the first aspect above. The electronic device of the embodiments of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, the electronic device includes but is not limited to Bluetooth headsets, mobile phones, tablets, laptop computers, electric toys, power tools, battery cars, electric vehicles, ships, spacecraft, etc. Among them, the electric toy can include fixed or mobile electric toys, such as game consoles, electric vehicle toys, electric ship toys, and electric aircraft toys, etc., and the spacecraft can include airplanes, rockets, space shuttles, and spaceships, etc.

[0084] Example 1

[0085] <Preparation of the positive electrode plate>

[0086] Mix the positive electrode active material lithium nickel cobalt manganese oxide (based on the total mass of nickel, cobalt, and manganese elements, the mass percentage content A1 of nickel element is 70%, the mass percentage content A2 of cobalt element is 10%, and the mass percentage content A3 of manganese element is 20%), the positive electrode conductive agent acetylene black, and the positive electrode binder polyvinylidene fluoride (PVDF, weight average molecular weight is 5×10 5)Mix in a mass ratio of 94:3:3, add N-methylpyrrolidone (NMP) as a solvent, and formulate a positive electrode slurry with a solid content of 75 wt%, and stir evenly under a vacuum mixer. Use an aluminum foil with a thickness of 9 μm, a length of 1540 mm, and a width of 67.9 mm as the positive electrode current collector, and the width of the positive electrode tab is 20 mm. Coat the positive electrode slurry evenly on one surface of the positive electrode current collector aluminum foil, and dry it at 100 °C to obtain a positive electrode sheet with a single-sided coated positive electrode active material layer. Then, repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coated positive electrode active material layer. Cold press the coated sheet to obtain a positive electrode sheet with a double-sided positive electrode active material layer after cold pressing. The single-sided coating weight of the positive electrode sheet is 21 mg / cm 2 , the thickness after single-sided cold pressing is 62.25 μm, and the thickness after double-sided cold pressing is 112.5 μm.

[0087] <Preparation of negative electrode sheet>

[0088] Mix the negative electrode active material graphite powder, silicon powder, conductive agent conductive carbon black (Super P), and binder styrene-butadiene rubber (SBR) in a weight ratio of 87.5:10:1:1.5, then add deionized water as a solvent, formulate a negative electrode slurry with a solid content of 50 wt%, and stir evenly. Select a copper foil with a thickness of 6 μm, a length of 1538 mm, and a width of 69.1 mm as the negative electrode current collector, and the width of the negative electrode tab is 20 mm. Coat the negative electrode slurry evenly on one surface of the negative electrode current collector copper foil, and dry it at 90 °C to obtain a single-sided negative electrode sheet. After the above steps are completed, the single-sided coating of the negative electrode sheet is completed. Then, repeat the above steps on the other surface of the negative electrode sheet to obtain a negative electrode sheet with a double-sided coated negative electrode active material layer. Cold press the coated sheet to obtain a negative electrode sheet with a double-sided negative electrode active material layer after cold pressing. The single-sided coating weight of the negative electrode sheet is 8.6 mg / cm 2 , the thickness after single-sided cold pressing is 61.4 μm, and the thickness after double-sided cold pressing is 112.8 μm.

[0089] <Preparation of separator>

[0090] Use a polyethylene (PE) with a thickness of 7 μm and a porous film with a 2-μm alumina (Al2O3) coating as the separator.

[0091] <Preparation of lithium-ion secondary battery>

[0092] Stack the above-prepared separator, positive electrode sheet, separator, and negative electrode sheet in sequence and wind them to obtain an electrode assembly. Among them, the positive electrode sheet is connected to the positive electrode tab, and the negative electrode tab is connected to the negative electrode tab. Use the first and second bags of the aluminum-plastic film. The second bag is arranged on one wall surface in the thickness direction of the first bag. The first bag encloses a first accommodation cavity (the size of the first bag is 83mm×70mm×5.5mm). The second bag encloses a second accommodation cavity, and the first accommodation cavity and the second accommodation cavity are separated by a first wall portion made of polyvinyl chloride. The melting point T of the first wall portion is 100°C, and the area S of the first wall portion is 3cm 2 . The electrode assembly is arranged in the first bag and filled with a first electrolyte. The first electrolyte includes 25% ethylene carbonate (EC), 2% fluoroethylene carbonate (FEC), the concentration of lithium hexafluorophosphate is 1.2mol / L, and the remaining component is ethyl methyl carbonate (EMC). The thickness D of the second bag in the direction perpendicular to the first wall portion is 1mm, and the overall volume G is 3mL. A second electrolyte is injected into the second bag. Among them, the second electrolyte includes 60% ethylene carbonate (EC), and the remaining component is ethyl methyl carbonate (EMC), that is, ethyl methyl carbonate (EMC) is 40%. After the positive and negative electrode tabs are led out, the first bag and the second bag are hot-pressed and sealed together, so that the side seal of the first bag and the first seal of the second bag are stacked.

[0093] Different from Example 1, the relevant parameters in Comparative Examples 1 to 7 and Examples 2 to 62 are shown in Table 1 below. Among them, Comparative Example 1 only includes the first bag and does not have the second bag. In Examples 14 to 22, the second electrolyte in the second bag also includes fluoroethylene carbonate (FEC), and the relevant parameters are shown in Table 1. In Examples 23 to 40, the second electrolyte also includes dimethyl carbonate (DMC), and the relevant parameters are shown in Table 1. In Example 49, the second bag is arranged on the top of the first bag, and the first wall portion is located on the top wall of the first bag. In Example 50, the second bag is directly arranged in the first bag. In Example 51, the first seal of the second bag is clamped in the top seal of the second bag, and the second bag is located between the positive and negative electrode tabs. In each example, the content of ethyl methyl carbonate (EMC) is adjusted according to the content of ethylene carbonate (EC), fluoroethylene carbonate (FEC), and dimethyl carbonate (DMC).

[0094] The thermal shock test process is as follows:

[0095] First, set the test environment temperature to 23°C±2°C and let the test sample stand for 5 minutes. Then perform a charging operation on the lithium-ion secondary battery sample, charging it to a voltage of 3.0V with a constant current (DC) of 0.2C. After completing the above charging operation, let the sample stand for 5 minutes again to allow for a short stable adjustment period inside the lithium-ion secondary battery after charging.

[0096] Adjust the furnace temperature for testing to 37°C (±2°C), and let the lithium-ion battery sample stand still in this high-temperature environment for 2 hours to allow the sample to fully adapt to the high-temperature environment, so as to observe the performance of the sample under this high-temperature condition and subsequent operations.

[0097] Then charge at a constant current of 1.65C to a voltage of 4.1V, and then switch to the constant voltage (CV) charging mode until the current drops to 1.55C. Then charge at a constant current of 1.55C to a voltage of 4.2V, and then switch to constant voltage charging until the current drops to 1.4C. Subsequently, charge at a constant current of 1.4C to a voltage of 4.24V, then switch to constant voltage charging until the current drops to 1.1C. Then charge at a constant current of 1.1C to a voltage of 4.27V, and then switch to constant voltage charging until the current drops to 0.7C. Finally, charge at a constant current of 0.7C to a voltage of 4.3V, then switch to constant voltage charging until the current drops to 0.4C, and charge at a constant current of 0.4C to a voltage of 4.505V, then switch to constant voltage charging until the current drops to 0.025C.

[0098] When the above series of complex charging operations are completed, observe whether the first wall of the lithium-ion secondary battery is burst open. If not, let the lithium-ion secondary battery sample stand still for another 5 minutes and conduct the following tests:

[0099] Test process: ① Check the appearance and take pictures before and after the test; ② The position where the temperature sensor wire is attached is near the negative electrode tab; ③ Place the lithium-ion battery sample horizontally in the box and heat it up at a heating rate of 5±2°C to 130±2°C and maintain for 60 min; ④ Measurement frequency: The voltage and internal resistance are measured using a 1KHz specification, and are measured after pretreatment and after the test; ⑤ Judgment criteria: No explosion, no smoke, no fire.

[0100] It can be understood that the charge and discharge cut-off voltages of the lithium-ion secondary battery shall be subject to the markings on the outer packaging of the battery when it leaves the factory. The charging cut-off voltage of the lithium-ion battery as an example in this application is 4.5V, and the discharging cut-off voltage is 3.0V.

[0101] Table 1

[0102]

[0103]

[0104]

[0105]

[0106] According to Table 1 above, in combination with Examples 1 to 6 and Comparative Examples 1 to 7, it can be seen that by setting the second packaging bag, with ethylene carbonate (EC) provided in the second packaging bag and the mass percentage content of ethylene carbonate (EC) in the second electrolyte being 60% ≤ C1 ≤ 90%, and the melting point of the first wall portion being 70°C ≤ T ≤ 120°C, the test failure rate can be effectively reduced. This is because when the temperature inside the lithium-ion secondary battery rises to near or reach this melting point range due to abnormal conditions such as lithium deposition, the first wall portion melts. At this time, the high-content ethylene carbonate (EC) originally located in the second bag body can quickly flow into the first bag body. Ethylene carbonate (EC) can quickly passivate the lithium deposition interface of the electrode assembly, effectively reducing the further reaction between lithium metal and the electrolyte, reducing the risk of thermal runaway of the secondary battery, reducing the occurrence of fire or explosion in the secondary battery, and improving the safety performance of the secondary battery.

[0107] In Comparative Example 3, the content of ethylene carbonate (EC) is relatively low. It may be due to the small amount of ethylene carbonate (EC) that it is difficult to effectively passivate the lithium deposition interface. In Comparative Example 6, the content of ethylene carbonate (EC) is relatively high, which may increase the viscosity of the second electrolyte, making it difficult for ethylene carbonate (EC) to quickly reach the lithium deposition interface and thus difficult to passivate lithium deposition in a timely manner. In combination with Examples 1 to 6, with 60% ≤ C1 ≤ 90% being defined, the content of ethylene carbonate (EC) is more suitable, which is beneficial to making the second electrolyte have better fluidity, facilitating the quick arrival of ethylene carbonate (EC) at the lithium deposition interface; and it can also facilitate the formation of a stable solid electrolyte interface film at the interface, effectively passivating lithium deposition, and thus reducing the failure rate.

[0108] In Comparative Examples 2 and 5, the melting point of the first wall portion is relatively low, which may cause the first wall portion to melt prematurely, affecting the normal use of the secondary battery, and the first wall portion may melt during the normal charge and discharge cycle process. In Comparative Examples 4 and 7, the melting point of the first wall portion is relatively high, which will cause the time for ethylene carbonate (EC) to interfere with lithium deposition to be too late. At this time, relatively serious lithium deposition may have occurred inside the secondary battery, and it is difficult for the second electrolyte to play a role and effectively prevent safety accidents from occurring in the secondary battery. In combination with Examples 7 to 13 and Example 4, their test failure rates are relatively low. By selecting 70°C ≤ T ≤ 120°C, it is convenient for the secondary battery to respond in a timely manner before the temperature reaches an overly dangerous level and release the second electrolyte for protection.

[0109] In Examples 8 to 11 and Example 4, the failure rate is further reduced. In the embodiments of the present application, it is possible to select 75°C ≤ T ≤ 100°C, which can reduce the premature melting of the first wall due to slight temperature fluctuations and reduce the unnecessary premature release of the second electrolyte 50. Moreover, in the stage of rapid temperature rise caused by lithium plating, additives such as ethylene carbonate (EC) and / or fluoroethylene carbonate (FEC) can be released more timely, which is beneficial to more precisely reduce the risk of thermal runaway. In Examples 9 to 11, the failure rate is further reduced. In the embodiments of the present application, it is possible to further select 80°C ≤ T ≤ 90°C. In a high-temperature environment, ethylene carbonate (EC) can be quickly released in response to protect the secondary battery; in a low-temperature environment, the premature release of additives can be reduced to affect the normal electrolyte performance, and the adaptability and reliability of the secondary battery in different temperature environments can be further enhanced.

[0110] Combined with Examples 14 to 22, in Example 14, the content of fluoroethylene carbonate (FEC) is relatively low, and its effect on passivating lithium plating is small. In Example 22, the content is relatively high, which will also cause an increase in the viscosity of the second electrolyte and make it difficult to quickly reach the lithium plating interface. Combined with Examples 13 to 21, it is possible to select 5% ≤ C2 ≤ 20%, which is beneficial to stably passivate the lithium plating interface and further reduce the occurrence of lithium plating.

[0111] Combined with Examples 23 to 31, dimethyl carbonate (DMC) is added to the second electrolyte. Dimethyl carbonate (DMC) can reduce the viscosity of the second electrolyte, improve the fluidity of the second electrolyte, enable the second electrolyte to enter the first bag more quickly through the melted first wall, shorten the time interval from lithium plating to the start of passivation, so as to facilitate timely treatment when lithium plating just occurs or is still in a relatively mild stage, effectively reduce the further development of lithium plating, and reduce the impact of lithium plating on the performance and safety of the secondary battery.

[0112] In Example 23, the content of dimethyl carbonate (DMC) is relatively low, and its impact on improving the fluidity of ethylene carbonate (EC) is relatively small. In Example 31, the content of dimethyl carbonate (DMC) is relatively high. Although it improves the fluidity of ethylene carbonate (EC), the content of ethylene carbonate (EC) is relatively low. Moreover, the failure rate of Example 31 is similar to that of Example 30. Therefore, when the content of dimethyl carbonate (DMC) exceeds 20%, further increasing its content has no obvious effect on reducing the failure rate. Combining Examples 32 to 40, in Example 32, the content of dimethyl carbonate (DMC) is relatively low, and in Example 40, the content of dimethyl carbonate (DMC) is relatively high. In Examples 33 to 39, the failure rate is less than that of Example 32 and Example 40. Therefore, in the examples of this application, 5% ≤ C2 ≤ 20% can be selected. In the corresponding Example 22, the viscosity is relatively high, resulting in weakened fluidity of the second electrolyte, making it difficult to passivate the lithium deposition interface in a timely manner, and there is also a risk of thermal runaway. In Example 31, the content of dimethyl carbonate (DMC) is relatively high, but the viscosity is similar to that of Example 30. Considering cost issues, further increasing the content of dimethyl carbonate (DMC) has little benefit in reducing viscosity, and at the same time, it will also lead to a decrease in the content of ethylene carbonate (EC). Therefore, the viscosity of the second electrolyte can be selected as 0.3 mPa·s ≤ P ≤ 1 mPa·s.

[0113] Combining Examples 41 to 44 and Example 35, in Example 41, when the area of the first wall portion is relatively small, the release channel is relatively narrow and the release speed is slow. When lithium deposition in the secondary battery causes the temperature to rise, it may be difficult to provide sufficient additives in a timely manner to effectively passivate lithium deposition, affecting the safety performance of the secondary battery. In Example 44, the area is relatively large, which may weaken the overall structural strength of the secondary battery, resulting in the secondary battery being more likely to deform, rupture, etc. when subjected to external impact or vibration. Therefore, in the examples of this application, combining Examples 42 to 43 and Example 35, 2 cm 2 ≤ S ≤ 5 cm 2 is beneficial to timely passivate the lithium deposition interface, improve the safety performance of the secondary battery, and is beneficial to reducing the impact on the overall structural strength of the secondary battery.

[0114] Combining Examples 45 to 48 and Example 35, in Example 45, the volume G of the second bag is relatively small, so that the amount of ethylene carbonate (EC) is relatively small, making it difficult to effectively passivate the lithium deposition interface. In Example 48, the volume G of the second bag is relatively large, which will cause serious loss of the energy density of the secondary battery, and its failure rate is similar to that of Example 47. Therefore, in the examples of this application, 2 mL ≤ G ≤ 4 mL can be selected, so that after the first wall portion melts, there is enough second electrolyte flowing into the first bag to fully and effectively passivate the lithium deposition interface and reduce the further deterioration of the lithium deposition phenomenon.

[0115] In Example 49, the second bag body is disposed on the top of the first bag body, and the first wall portion is located at the top of the first bag body, which can reduce the thickness occupied by the battery, thereby reducing the energy density loss. When lithium plating occurs and the temperature rises to the melting point of the first wall portion, the second electrolyte in the second bag body can also be transmitted to the first bag body, thereby passivating the lithium plating interface, and the test failure rate is also significantly lower than that of Comparative Examples 1 to 7.

[0116] In Example 50, the second bag body is disposed inside the first bag body, such that the distance between the second bag body and the electrode assembly is smaller. After the first wall portion melts, the second electrolyte can quickly reach and passivate the lithium plating interface, and the failure rate is further reduced.

[0117] In Example 51, the first sealing edge of the second bag body is clamped inside the top sealing edge of the first bag body, and the second bag body is located between the positive and negative electrode tabs, which can further reduce the energy density loss. After the first sealing edge melts, a channel can be formed, and the second electrolyte in the second bag body can also enter the first bag body, thereby passivating the lithium plating interface, and its failure rate is also less than that of Comparative Examples 1 to 7.

[0118] Combined with Examples 52 to 62, in Example 52, the failure rate is similar to that of Example 53. However, compared with Example 52, in Example 53, the mass percentage content of nickel in the ternary material is larger, such that the secondary battery has a higher energy density. In contrast, Example 52 will result in a certain loss of energy density. In Example 62, the mass percentage content of nickel in the ternary material is larger, so that the mass percentage content of cobalt in the ternary material is smaller, and the effect of improving the thermal stability of the positive electrode active material layer is not obvious. Therefore, combined with Examples 53 to 61, in the examples of the present application, considering improving the energy density and the thermal stability, the mass percentage content of nickel in the ternary material can be selected as 50% ≤ A1 ≤ 90%.

[0119] In Example 60, the mass percentage content of cobalt in the ternary material is smaller, and it is difficult to stabilize the structure of the positive electrode active material layer, and the effect of improving the thermal stability of the positive electrode active material layer is not obvious. In Example 52, the mass percentage content of cobalt in the ternary material is larger, so that the mass percentage content of nickel in the ternary material is smaller, resulting in a certain loss of energy density, and the effect of testing the failure rate is similar to that of Example 53. Combined with Examples 53 to 59, in the examples of the present application, the mass percentage content of cobalt in the ternary material can be selected as 2% ≤ A2 ≤ 20%.

[0120] In Example 61, the mass percentage of manganese in the ternary material is relatively small, making it difficult to stabilize the structure of the positive electrode active material layer, and the effect on improving the thermal stability of the positive electrode active material layer is not obvious. Combining with Example 59, the lower limit of manganese can be selected as 8%. Combining with Examples 55 to 57, in Examples 56 to 57, the mass percentage of manganese in the ternary material is relatively large, but compared with Example 55, the effect of Examples 56 to 57 on improving thermal stability is not obvious, and it will also cause a decrease in the mass content of cobalt, which may also affect the stability of the positive electrode active material layer. Therefore, the upper limit of manganese can be selected as 30%, that is, the mass percentage of manganese in the ternary material that can be selected in this application is 8% ≤ A3 ≤ 30%.

[0121] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; under the idea of the present application, the technical features in the above embodiments or different embodiments can also be combined, and the steps can be implemented in any order, and there are many other changes in different aspects of the present application as described above. For the sake of brevity, they are not provided in detail; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A secondary battery, comprising a first bag body, an electrode assembly and a first electrolyte, wherein the first bag body encloses a first accommodation cavity, the electrode assembly and the first electrolyte are arranged in the first accommodation cavity, characterized in that: The secondary battery further includes a second bag body and a second electrolyte; The second bag body encloses a second containing cavity, and the second electrolyte is arranged in the second containing cavity; The second electrolyte includes ethylene carbonate, and the mass percentage of the ethylene carbonate in the second electrolyte is C1, 60%≤C1≤90%; A first wall portion is provided between the first bag body and the second bag body, the first accommodating cavity and the second accommodating cavity are separated by the first wall portion, and the melting point of the first wall portion is T, 70°C≤T≤120°C.

2. The secondary battery according to claim 1, characterized in that: The second electrolyte also includes fluoroethylene carbonate, and the mass percentage of the fluoroethylene carbonate in the second electrolyte is C2, 5%≤C2≤20%.

3. The secondary battery according to claim 1 or 2, characterized in that: The second electrolyte also includes dimethyl carbonate, and the mass percentage of the dimethyl carbonate in the second electrolyte is C3, 5%≤C3≤20%.

4. The secondary battery according to claim 1, characterized in that: 75℃≤T≤100℃。 5. The secondary battery according to claim 4, characterized in that: 80℃≤T≤90℃。 6. The secondary battery according to claim 1, characterized in that: The material of the first wall portion includes polyethylene, polyvinyl chloride, Wood's metal or beeswax.

7. The secondary battery according to claim 1, characterized in that: The area of ​​the first wall is S, 2 cm 2 ≤S≤5cm 2 .

8. The secondary battery according to claim 1, characterized in that: The volume of the second bag body is G, 2mL≤G≤4mL.

9. The secondary battery according to claim 1, characterized in that: The viscosity of the second electrolyte is P, 0.3 mPa·s≤P≤1 mPa·s.

10. The secondary battery according to claim 1, characterized in that: The electrode assembly includes a positive electrode sheet, the positive electrode sheet has a positive electrode active material layer, the positive electrode active material layer includes a ternary material, the ternary material includes a nickel element, a cobalt element and a manganese element, based on the total mass of the nickel element, the cobalt element and the manganese element, The mass percentage of the nickel element is A1, 50%≤A1≤90%; The mass percentage of the cobalt element is A2, 2%≤A2≤20%; The mass percentage of the manganese element is A3, 8%≤A3≤30%.

11. The secondary battery according to claim 1, characterized in that: The secondary battery further comprises a first pole ear, the first bag body comprises a top sealing edge, one end of the first pole ear is connected to the electrode assembly, and the other end of the first pole ear extends out of the first bag body from the top sealing edge; The second bag body comprises a first edge seal, and along a thickness direction of the first edge seal, the first edge seal and the top edge seal are stacked.

12. The secondary battery according to claim 11, characterized in that: The secondary battery further includes a second pole lug, one end of which is connected to the electrode assembly, and the other end of which extends out of the first bag body from the top seal edge, and the second bag body is at least partially located between the first pole lug and the second pole lug.

13. The secondary battery according to claim 1, characterized in that: Along the thickness direction of the secondary battery, the first bag body includes a first side wall and a second side wall that are oppositely arranged, and the second bag body is arranged on the first side wall or the second side wall; The first bag body includes a side seal, and the second bag body includes a first seal. Along a thickness direction of the side seal, the first seal and the side seal are stacked.

14. The secondary battery according to claim 1, characterized in that: The second bag body is disposed in the first accommodating cavity, and the second bag body includes the first wall portion.

15. The secondary battery according to claim 1, characterized in that: The first electrolyte includes ethylene carbonate, and the mass percentage of the ethylene carbonate in the first electrolyte is B1, 20%≤B1≤30%; and / or, The first electrolyte includes fluoroethylene carbonate, and the mass percentage of the fluoroethylene carbonate in the first electrolyte is B2, 0.1%≤B2≤3%.

16. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 15.

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