Formation process for improving electrolyte interfacial film through cooperation of multiple physical fields and lithium ion battery

Abstract

The invention provides a formation process for improving an electrolyte interfacial film through cooperation of multiple physical fields and a lithium ion battery. The formation process for improving the electrolyte interfacial film through multi-physical field cooperation comprises the following steps: acquiring a battery cell after liquid injection; carrying out first-time charging operation on the battery cell by adopting low-temperature small current; carrying out second charging operation on the battery cell subjected to the first charging operation by adopting medium-temperature asymmetric pulse current; carrying out third-time charging operation on the battery cell subjected to the second-time charging operation by adopting high-temperature large current; and carrying out continuous ultrasonic vibration operation on the battery cell. According to the method, under the combined action of various physical fields of temperature rise, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation, on the premise of reducing electrolyte volatilization, residual tiny bubbles in the SEI membrane can be comprehensively extracted, lithium dendrites on the surface of the SEI membrane can be removed, and the service life of the SEI membrane is prolonged. The SEI film which is good in continuity of the inner layer and the outer layer, free of accumulation, good in connectivity, uniform, compact and free of microcracks can be formed.

Description

Technical Field

[0001] The present disclosure relates to the technical field of lithium-ion batteries, and in particular to a multi-physical field synergistically improved formation process of an electrolyte interface film and a lithium-ion battery. Background Art

[0002] The formation process of lithium-ion batteries is a key step in battery manufacturing, which aims to activate the electrode material and form a stable SEI film solid electrolyte interface on the negative electrode surface to improve the cycle life and safety of lithium-ion batteries.

[0003] At present, early stage formation processes often only consider the single factor of low-rate constant current charge-discharge formation process, but adopting low-rate constant current charge-discharge formation process has the problem of long formation time. Therefore, a method for constructing a lithium-ion battery electrolyte interface film by double pulse excitation disclosed in Chinese patent document number CN 109755682 A has appeared on the market. By performing double pulse excitation on a lithium-ion battery to which an electrolyte has been added and coordinating repeated operations of charge and discharge, it is conducive to constructing an electrolyte interface film with uniform composition, thickness and structure, high consistency and process controllable, and shortening the formation time of the lithium-ion battery.

[0004] Although the above-mentioned dual-pulse excitation effectively shortens the formation time of lithium-ion batteries and is also conducive to constructing an electrolyte interface film with uniform composition, thickness, structure and high consistency, the amplitude and width of the pulses used in the dual-pulse excitation are symmetrical. As a result, lithium-ion batteries are prone to induce local lithium dendrite formation in the SEI film area with higher surface roughness on the negative electrode under symmetrical charge and discharge conditions, thereby reducing the uniformity and stability of the SEI film.

[0005] To this end, some scholars have developed a method for in-situ removal of lithium dendrites on the surfaces of the positive and negative electrodes of lithium-ion batteries, such as Chinese patent document No. CN 114388909 A. The method locates the SEI film after formation by X-rays, and then uses electromagnetic shock waves to shock the SEI film, thereby removing the lithium dendrites generated on the surfaces of the positive and negative electrodes during the formation process.

[0006] However, the above-mentioned traditional methods can only remove lithium dendrites on the surface of the positive and negative electrodes, but cannot effectively remove lithium dendrites in the inner layer of the SEI film. In actual applications, however, lithium dendrites located in the inner layer of the SEI film will destroy the continuity of the SEI film, causing the SEI film to easily form microcracks or holes during the cycling of lithium-ion batteries, thereby destroying the SEI film structure and causing SEI film interface failure. Summary of the Invention

[0007] The purpose of the present disclosure is to overcome the shortcomings of the prior art and provide a method for effectively removing lithium dendrites in the inner and outer layers of the SEI film during the formation process, so as to ensure that the battery cell, under the joint action of multiple physical fields such as temperature increase, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation, can fully extract the tiny bubbles remaining inside the SEI film while reducing the volatilization of the electrolyte, and can also remove the lithium dendrites on the surface of the SEI film, and also help to form a SEI film with good inner and outer layer continuity, good connectivity, uniformity, density and no microcracks, so as to improve the formation process of the electrolyte interface membrane and the lithium-ion battery.

[0008] The purpose of this disclosure is achieved through the following technical solutions:

[0009] A multi-physical field synergistic improvement process for forming an electrolyte interface film comprises the following steps:

[0010] Get the battery cell after filling with liquid;

[0011] Performing a first charging operation on the battery cell using low temperature and low current;

[0012] Performing a second charging operation on the battery cell after the first charging operation using a medium-temperature asymmetric pulse current;

[0013] Performing a third charging operation on the battery cell after the second charging operation using high temperature and high current; and

[0014] The battery core is subjected to continuous ultrasonic vibration operation to complete the formation process of the battery core.

[0015] In one embodiment, the temperature of the low temperature and low current is 10°C to 25°C, and the current is 0.1C to 0.2C; and / or,

[0016] The voltage during the first charging operation is 3.0V to 3.7V.

[0017] In one embodiment, the temperature of the medium-temperature asymmetric pulse current is 40°C to 45°C, and the asymmetric pulse current is: forward pulse: 1°C to 2°C, pulse width 5s to 30s; reverse pulse: 0.1°C to 0.2°C, pulse width 2s to 5s; and / or,

[0018] The voltage during the second charging operation is 3.8V to 4.1V.

[0019] In one embodiment, the temperature of the high temperature and high current is 60°C to 65°C, and the current is 0.5°C to 1.0°C; and / or,

[0020] The voltage during the third charging operation is 4.2V to 4.35V.

[0021] In one embodiment, the step of performing continuous ultrasonic vibration operation on the battery cell includes the following specific steps:

[0022] After the battery cell is charged to a voltage of 4.2V to 4.35V, the battery cell is continuously ultrasonically vibrated and simultaneously charged with a small current for 20 minutes to 30 minutes.

[0023] In one embodiment, the vibration frequency of the continuous ultrasonic vibration operation is 20 kHz to 30 kHz.

[0024] In one embodiment, the step of obtaining the battery cell after liquid injection includes the following specific steps:

[0025] baking the battery cell;

[0026] The pre-wetting operation is performed on the baked battery cell by adopting alternating pressure.

[0027] In one embodiment, the alternating pressure is 0.1 MPa to 0.6 MPa; and / or,

[0028] The alternating pressure is applied ≥2 times.

[0029] In one embodiment, before the step of performing a first charging operation on the battery cell using a low temperature and low current, the method further includes the following steps:

[0030] Allowing the battery cell to stand for 12 to 24 hours;

[0031] The battery cell is pre-pressed for 20 minutes to 30 minutes under a high temperature condition of 70° C. to 85° C.

[0032] A lithium-ion battery is prepared by using the multi-physical field synergistic improvement of the electrolyte interface film formation process described in any of the above embodiments.

[0033] Compared with the prior art, the present disclosure has at least the following advantages:

[0034] 1) Since the battery cell is charged for the first time at low temperature and low current, the battery cell can better inhibit the decomposition of the solvent in the electrolyte under low temperature conditions, thereby promoting the inorganic components in the electrolyte, such as LiF and Li2CO3, so that the inorganic components preferentially form uniform and dense inorganic nuclei on the surface of the negative electrode, which better compensates for the roughness of the negative electrode surface and is beneficial to reducing the current distribution difference on the negative electrode surface. When a medium-temperature asymmetric pulse current is used to charge the battery cell for the second time after the first charging operation; since the amplitude and width of the pulse of the asymmetric pulse current are asymmetric, the current distribution difference on the negative electrode surface during the charge and discharge process is further reduced, thereby breaking the concentration polarization of lithium ions during the battery cell formation process; effectively inhibiting the generation of lithium dendrites in the inner layer of the SEI membrane, thereby ensuring the continuity of the inner and outer layers of the SEI membrane, and effectively avoiding the problem of lithium-ion batteries being prone to forming microcracks or holes in the SEI membrane during the cycle due to the presence of lithium dendrites in the inner layer of the SEI membrane.

[0035] 2) In addition, since the Li+ mobility of the battery cell is more suitable under medium temperature conditions, that is, the Li+ mobility is faster than at low temperature and slower than at high temperature, thereby ensuring that the organic polymer layer in the battery cell, such as ROCO2Li, can be more evenly covered on the inorganic nucleation body, which not only helps to form a continuous, uniform and dense SEI film inner layer, but also shortens the battery cell formation process; then a high temperature and high current are used to perform a third charging operation on the battery cell after the second charging operation; the local positive and negative electrode materials of the battery cell can generate Joule heat under high temperature conditions, and the locally generated Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the SEI film microcracks, thereby ensuring the generation of a dense, uniform and microcrack-free SEI film; and because the continuous ultrasonic vibration operation is added in the third charging operation, on the one hand, the microvibration of the continuous ultrasonic vibration will make the surface of the SEI film The lithium dendrites are broken, thereby effectively removing the lithium dendrites on the surface of the SEI film; on the other hand, continuous ultrasonic vibration can effectively remove the tiny bubbles inside the SEI film inside the battery cell, that is, the tiny bubbles inside the SEI film inside the battery cell will repeatedly expand and contract under the action of continuous ultrasonic vibration, so that the adjacent tiny bubbles are merged into larger bubbles to accelerate the floating and removal of the bubbles, thereby effectively removing the tiny bubbles remaining inside the SEI film, so as to ensure that the inner and outer layers of the SEI film formed in the multi-physical field synergistic improvement of the electrolyte interface film in the battery cell under the multi-physical field of temperature increase, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation are continuous and free of accumulation, good connectivity, uniformity, density and no microcracks, thereby ensuring that the SEI film is not easy to form microcracks or holes during the cycle of the lithium-ion battery, thereby effectively ensuring the cycle performance of the lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0037] Figure 1 A flow chart of a multi-physical field synergistic improvement process for forming an electrolyte interface film according to an embodiment of the present invention;

[0038] Figure 2 This is a structural diagram of a multifunctional forming cabinet in one direction according to an embodiment of the present invention. DETAILED DESCRIPTION

[0039] To facilitate understanding of the present disclosure, a more comprehensive description of the present disclosure will be provided below with reference to the accompanying drawings. The accompanying drawings illustrate preferred embodiments of the present disclosure. However, the present disclosure can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the disclosure.

[0040] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly attached to the other element or there may be an intermediate element. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an intermediate element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only implementation methods.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this disclosure pertains. The terms used herein in the specification of this disclosure are intended only to describe specific embodiments and are not intended to limit this disclosure. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0042] The present disclosure provides a multi-physical field collaboratively improved electrolyte interface membrane formation process, obtaining a battery cell after liquid injection; using a low-temperature small current to perform a first charging operation on the battery cell; using a medium-temperature asymmetric pulse current to perform a second charging operation on the battery cell after the first charging operation; using a high-temperature large current to perform a third charging operation on the battery cell after the second charging operation; and performing a continuous ultrasonic vibration operation on the battery cell to complete the battery cell formation process.

[0043] The above method uses a low temperature and low current to perform the first charging operation on the battery cell, so that the battery cell can better suppress the decomposition of the solvent in the electrolyte under low temperature conditions, thereby promoting the inorganic components in the electrolyte, such as LiF and Li2CO3, so that the inorganic components preferentially form uniform and dense inorganic nuclei on the surface of the negative electrode, which better compensates for the roughness of the negative electrode surface and is beneficial to reducing the current distribution difference on the negative electrode surface. When a medium temperature asymmetric pulse current is used to perform a second charging operation on the battery cell after the first charging operation; since the amplitude and width of the pulse of the asymmetric pulse current are asymmetric, the current distribution difference on the negative electrode surface during the charging and discharging process is further reduced. , thereby breaking the concentration polarization of lithium ions during the battery cell formation process; effectively inhibiting the formation of lithium dendrites in the inner layer of the SEI film, thereby ensuring the continuity of the inner and outer layers of the SEI film, and effectively avoiding the problem of lithium-ion batteries being prone to microcracks or holes in the SEI film during the cycle due to the presence of lithium dendrites in the inner layer of the SEI film; in addition, since the Li+ mobility of the battery cell is more suitable under medium temperature conditions, that is, the Li+ mobility is faster than at low temperature and slower than at high temperature, thereby ensuring that the organic polymer layer in the battery cell, such as ROCO2Li, can be more evenly covered on the inorganic nucleus, which not only helps to form a continuous, uniform and dense inner layer of the SEI film, but also shortens the battery cell formation time. process; then a high temperature and high current are used to perform a third charging operation on the battery cell after the second charging operation; the local positive and negative electrode materials of the battery cell can generate Joule heat under high temperature conditions, and the locally generated Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the SEI film microcracks, thereby ensuring the formation of a dense, uniform and microcrack-free SEI film; and because the continuous ultrasonic vibration operation is added in the third charging operation, on the one hand, the microvibration of the continuous ultrasonic vibration can cause the lithium dendrites on the surface of the SEI film to break, thereby effectively removing the lithium dendrites on the surface of the SEI film; on the other hand, the continuous ultrasonic vibration can effectively remove the tiny gas inside the SEI film inside the battery cell. Bubbles, that is, tiny bubbles inside the SEI film inside the battery cell will repeatedly expand and contract under the action of continuous ultrasonic vibration, so that adjacent tiny bubbles will merge into larger bubbles to accelerate the floating and removal of bubbles, thereby effectively removing tiny bubbles remaining inside the SEI film. In this way, it is ensured that the inner and outer layers of the SEI film formed in the formation process of the electrolyte interface film of the battery cell under the multi-physical field synergistic improvement of temperature, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation are continuous and free of accumulation, good connectivity, uniformity, density and no microcracks, thereby ensuring that the SEI film of the lithium-ion battery is not easy to form microcracks or holes during the cycle, thereby effectively ensuring the cycle performance of the lithium-ion battery.

[0044] See also Figure 1To better understand the technical solutions and beneficial effects of the present disclosure, the present disclosure is further described in detail below in conjunction with specific embodiments. In one embodiment, the multi-physical field synergistic improvement of the electrolyte interface film formation process includes some or all of the following steps:

[0045] S101. Obtain the battery cell after liquid injection so that the electrolyte can penetrate the electrode sheets and separators inside the battery cell, which is beneficial to the uniform distribution of Li+ during battery cell formation, reduces the excessive local current density, promotes the uniform deposition of lithium, and inhibits the nucleation of lithium dendrites.

[0046] In one embodiment, the step of obtaining the battery cell after liquid injection includes the following specific steps: baking the battery cell to effectively remove moisture in the battery cell; then, using alternating pressure to pre-wet the baked battery cell to ensure that the electrolyte can fully and quickly infiltrate the electrode sheets and diaphragms inside the battery cell.

[0047] It is understandable that if the alternating pressure is less than 0.1 MPa, it will not be possible to accelerate the electrolyte from infiltrating the electrode sheets and diaphragms inside the battery cell. If the alternating pressure is greater than 0.6 MPa, the viscosity of the electrolyte will increase significantly under higher pressure, which will be detrimental to the electrolyte's penetration into the pores of the electrode sheets, resulting in incomplete infiltration. Therefore, in the present disclosure, by controlling the alternating pressure to 0.1 MPa to 0.6 MPa, while ensuring that the electrolyte infiltrates the battery cell at an accelerated rate, the problem of the electrolyte's viscosity increasing significantly due to higher pressure, which is detrimental to the electrolyte's penetration into the pores of the electrode sheets, is avoided.

[0048] In one embodiment, the alternating pressure is applied ≥2 times to ensure that the electrolyte can completely infiltrate the electrode sheets and the diaphragm inside the battery cell.

[0049] In a preferred embodiment, the pressure is alternated 2 to 5 times.

[0050] In one embodiment, after the step of obtaining the battery cell after liquid injection and before the step of performing the first charging operation on the battery cell using low temperature and low current, the following steps are also included: first, the battery cell is allowed to stand for 12 hours to 24 hours. On the one hand, it is ensured that the electrolyte can completely infiltrate the electrode sheet to avoid the electrolyte from being partially non-infiltrated due to insufficient standing time, thereby inducing the formation of lithium dendrites; on the other hand, standing can effectively eliminate small bubbles remaining during the injection process, which is helpful for the subsequent formation of an SEI film with good continuity, no accumulation, good connectivity, uniformity, density and no microcracks in the inner and outer layers; then, the battery cell is pre-pressed for 20 minutes to 30 minutes under high temperature conditions of 70°C to 85°C to ensure that the solvent is not easily volatilized during the pre-pressing process, and to ensure that the electrolyte completely infiltrates the electrode sheet and diaphragm inside the battery cell.

[0051] It should be noted that the pre-pressing temperature of the electrolyte of conventional lithium-ion batteries is generally 20℃~30℃, but for some high-temperature stable liquid electrolyte lithium-ion batteries, if a high-temperature stable solvent is used, such as lithium-ion batteries containing lithium hexafluorophosphate, vinylene carbonate, fluorocarbonate or silicon-based negative electrode lithium-ion batteries, if the conventional 20℃~30℃ is used for pre-pressing, it is impossible to ensure that the electrolyte can completely infiltrate the electrode sheets and diaphragms inside the battery cell. Therefore, in the present disclosure, the battery cell is pre-pressed for 20min~30min under high temperature conditions of 70℃~85℃ to ensure that the solvent of the electrolyte of the high-temperature stable liquid electrolyte lithium-ion battery is not easily volatilized during the pre-pressing process, and to ensure that the electrolyte completely infiltrates the electrode sheets and diaphragms inside the battery cell to reduce the infiltration dead corners.

[0052] In one embodiment, the pre-pressing pressure is 0.3 MPa to 0.6 MPa to accelerate the comprehensive and rapid infiltration of the electrolyte of the high-temperature stable liquid electrolyte lithium-ion battery.

[0053] S102. The battery cell is charged for the first time at low temperature and low current, so that the battery cell can better inhibit the decomposition of the solvent in the electrolyte under low temperature conditions, so as to promote the inorganic components in the electrolyte, such as LiF and Li2CO3, to preferentially form uniform and dense inorganic nuclei on the surface of the negative electrode, thereby compensating for the roughness of the negative electrode surface and helping to reduce the current distribution difference on the negative electrode surface during the second or third charging operation, thereby helping to form inner and outer layers of the SEI film with good continuity, no accumulation, good connectivity, uniformity, density and no microcracks.

[0054] In one embodiment, the temperature of the low temperature and low current is 10°C to 25°C, and the current is 0.1C to 0.2C; especially when the voltage during the first charging operation is 3.0V to 3.7V, it ensures that the battery cell can better suppress the decomposition of the solvent in the electrolyte under low temperature conditions, while also promoting the inorganic components in the electrolyte to preferentially form uniform and dense inorganic nuclei on the surface of the negative electrode, so as to ensure the subsequent formation of an SEI film with good continuity, no accumulation, good connectivity, uniformity, density and no microcracks in the inner and outer layers.

[0055] It should be noted that since the high-temperature stable liquid electrolyte lithium-ion battery has a relatively high thermal stability due to the addition of a high-temperature stabilizer, if the soaked battery cell is directly subjected to a low-temperature small battery operation at 10°C to 25°C, it is not conducive to the inorganic components preferentially forming a uniform and dense inorganic nucleation body on the negative electrode surface. Therefore, in the present disclosure, after the battery cell is pre-pressed, the battery cell is placed in a formation cabinet while it is hot for a low-temperature small battery operation, so as to ensure that the migration and diffusion of the inorganic ions of the inorganic components are more appropriate, which is conducive to the preferential formation of a uniform and dense inorganic nucleation body on the negative electrode surface.

[0056] It should also be noted that when the pre-pressed battery cell is placed into the forming cabinet while hot, the temperature of the forming cabinet is 10°C ~ 25°C, so that the inorganic nucleation body of the inorganic component at this stage gradually transitions from high temperature to low temperature, thereby ensuring the density and uniformity of the initial inorganic nucleation body, that is, the density and uniformity of the surface of the inorganic nucleation body formed in the later stage are slightly worse than the density and uniformity of the inorganic nucleation body in the previous stage, so that the surface of the inorganic nucleation body has a certain roughness, which can provide better adhesion for the organic polymer layer after the second charging operation, thereby improving the firmness of the connection between the organic polymer layer and the inorganic nucleation body.

[0057] S103 , performing a second charging operation on the battery cell after the first charging operation using a medium-temperature asymmetric pulse current.

[0058] It can be understood that when a medium-temperature asymmetric pulse current is used to perform a second charging operation on the battery cell after the first charging operation; since the amplitude and width of the pulse of the asymmetric pulse current are asymmetric, the current distribution difference on the negative electrode surface during the charging and discharging process is further reduced, thereby breaking the concentration polarization of lithium ions during the battery cell formation process; the generation of lithium dendrites in the inner layer of the SEI film is effectively suppressed, thereby ensuring the continuity of the inner and outer layers of the SEI film, and effectively avoiding the problem of lithium-ion batteries being prone to microcracks or holes in the SEI film during the cycle due to the presence of lithium dendrites in the inner layer of the SEI film; in addition, since the Li+ mobility of the battery cell is more suitable under medium temperature conditions, that is, the Li+ mobility is faster than at low temperature and slower than at high temperature, thereby ensuring that the organic polymer layer in the battery cell, such as ROCO2Li, can be more evenly covered on the inorganic nucleation body, which not only helps to form a continuous, uniform and dense inner layer of the SEI film, but also shortens the battery cell formation process.

[0059] It is worth mentioning that although the traditional dual-pulse excitation method of constructing the electrolyte interface film of lithium-ion batteries uses a symmetrical and uniform current, due to the influence of the roughness of the negative electrode material itself, the actual current has an uneven distribution problem, which easily leads to the formation of lithium dendrites at this stage, resulting in the presence of lithium dendrites in the inner and outer layers of the final SEI film.

[0060] Therefore, in the present disclosure, an asymmetric pulse current is adopted, that is, the high current density of the positive pulse is used to briefly break the lithium concentration polarization to promote uniform deposition, and at the same time, the lithium is redistributed through the reverse low current density to eliminate local accumulation and reduce the nucleation points of lithium dendrites, thereby facilitating the formation of an SEI film with good continuity, no accumulation, good connectivity, uniformity, density and no microcracks in the inner and outer layers.

[0061] In one embodiment, the temperature of the medium-temperature asymmetric pulse current is 40°C to 45°C, which provides a relatively suitable mobility for Li+ and helps the SEI film grow uniformly and stably; because the asymmetric pulse current is: forward pulse: 1C to 2C, pulse width 5s to 30s, to ensure that the high current density of the forward pulse briefly breaks the lithium concentration polarization to promote uniform deposition of Li+; and because the reverse pulse is: 0.1C to 0.2C, pulse width 2s to 5s, the small current used in the reverse pulse can redistribute lithium and eliminate local accumulation to reduce the nucleation points of lithium dendrites, especially in conjunction with the use of a voltage of 3.8V to 4.1V during the second charging operation, to ensure that the organic polymer layer in the battery cell can cover the inorganic nucleus more evenly, while also inhibiting the growth of lithium dendrites, helping to form a continuous, uniform and dense SEI film inner layer, and shortening the battery cell formation process.

[0062] In one embodiment, a medium-temperature asymmetric pulse current is used to perform a second charging operation on the battery cell after the first charging operation, which means that the battery cell is charged with an asymmetric pulse current at a temperature of 40°C to 45°C, and the charging standard is: forward pulse: 1C to 2C, pulse width 5s to 30s; reverse pulse: 0.1C to 0.2C, pulse width 2s to 5s.

[0063] In one embodiment, when a medium-temperature asymmetric pulse current is used to perform a second charging operation on the battery cell after the first charging operation, the temperature is raised to a medium temperature of 40°C to 45°C at a heating rate of 0.5°C / min to achieve reliable and smooth heating during the second charging operation, so that the organic polymer layer can be uniformly deposited and embedded on the inorganic nucleation body, thereby improving the firmness of the connection between the organic polymer layer and the inorganic nucleation body.

[0064] It should be noted that, since the second charging operation of the lithium-ion battery mainly generates an organic polymer layer, and by controlling the temperature of the second charging operation to 40°C ~ 45°C, on the one hand, the segment movement of the organic polymer layer is improved to form more free volume, to provide more channels for Li+ migration; on the other hand, the organic polymer layer will soften at 40°C ~ 45°C, so that the organic polymer layer can be well deposited and embedded in the inorganic nucleus, so that the organic polymer layer can fit closely with the negative electrode surface, reduce the interface gap, reduce the contact impedance, and help to form an SEI film with good continuity, no accumulation, good connectivity, uniformity, density and no microcracks in the inner and outer layers; on the other hand, it reduces the volatilization of low-boiling point solvents, such as carbonate solvents (EC, DEC), which leads to less electrolyte and affects the cycle performance of the lithium-ion battery.

[0065] S104, performing a third charging operation on the battery cell after the second charging operation using high temperature and high current; and performing a continuous ultrasonic vibration operation on the battery cell; to complete the formation process of the battery cell.

[0066] It can be understood that the battery cell after the second charging operation is charged for the third time using high temperature and high current; the local positive and negative electrode materials of the battery cell can generate Joule heat under high temperature conditions, and the locally generated Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the microcracks of the SEI film, thereby ensuring the generation of a dense, uniform and microcrack-free SEI film; and because the continuous ultrasonic vibration operation is added in the third charging operation, on the one hand, the microvibration of the continuous ultrasonic vibration will cause the lithium dendrites on the surface of the SEI film to break, thereby effectively removing the lithium dendrites on the surface of the SEI film; on the other hand, the continuous ultrasonic vibration can effectively eliminate the tiny bubbles inside the SEI film inside the battery cell. That is, the tiny bubbles inside the SEI film inside the battery cell will repeatedly expand and contract under the action of continuous ultrasonic vibration, so that adjacent tiny bubbles will merge into larger bubbles to accelerate the floating and removal of the bubbles, thereby effectively removing the tiny bubbles remaining inside the SEI film. In this way, it is ensured that the inner and outer layers of the SEI film formed in the formation process of the electrolyte interface film of the battery cell under the multi-physical field synergistic improvement of temperature, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation are continuous and free of accumulation, good connectivity, uniformity, density and no microcracks, thereby ensuring that the SEI film of the lithium-ion battery is not easy to form microcracks or holes during the cycle, thereby effectively ensuring the cycle performance of the lithium-ion battery.

[0067] In one embodiment, the temperature of the high temperature and high current is 60°C to 65°C, and the current is 0.5C to 1.0C; especially when the voltage during the third charging operation is 4.2V to 4.35V, it is ensured that the local positive and negative electrode materials of the battery cell can generate Joule heat under high temperature conditions, and the locally generated Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the SEI film microcracks, thereby ensuring the formation of a dense, uniform and microcrack-free SEI film.

[0068] In one embodiment, in the step of performing a third charging operation on the battery cell after the second charging operation using high temperature and high current, the temperature is raised to a high temperature of 60°C to 65°C at a heating rate of 0.5°C / min to achieve reliable and smooth heating during the third charging operation, so as to ensure that the locally generated Joule heat energy causes the positive and negative electrode materials to recrystallize to repair the SEI film microcracks, thereby ensuring the generation of a dense, uniform and microcrack-free SEI film.

[0069] It should be noted that some power batteries currently use ultrasonic technology to remove impurities or gases inside the battery during the formation process. For example, Chinese patent document No. CN 105742742 A discloses that vacuum extraction combined with ultrasound is used to remove tiny bubbles adhering to the electrode, diaphragm, and electrolyte, thereby improving the quality of the SEI film in lithium-ion batteries. However, because the ultrasound in this document is applied after formation, and the porosity of the dense SEI film composed of inorganic and organic polymer layers after formation is generally lower than that of the electrode, diaphragm, and electrolyte, if ultrasound technology is used to remove tiny bubbles in lithium-ion batteries after formation, it will make it difficult to ultrasonically extract the tiny bubbles in the SEI film.

[0070] Of course, some scholars may think that: since Chinese patent document No. CN 105742742 A discloses the introduction of ultrasonic technology in the formation process to effectively remove tiny bubbles in lithium-ion batteries, those skilled in the art can make a choice according to actual requirements.

[0071] However, in actual applications, since the inorganic nuclei formed in the initial stage of formation (the first charging operation) are relatively few and have poor adhesion and connectivity to the negative electrode surface, if ultrasonic technology is introduced in the initial stage of formation, it is easy to vibrate the inorganic nuclei just deposited on the negative electrode surface. If ultrasonic technology is introduced in the intermediate stage of formation (the second charging operation), and the SEI film at this time has not formed a relatively complete structure, the introduced ultrasonic technology will still vibrate the unformed SEI film. At this time, some scholars are well aware that the structure of the SEI film after the third charging operation is relatively stable, so they will directly introduce ultrasonic technology into the third charging operation. As disclosed in the present invention, by setting the continuous ultrasonic vibration operation in the third charging operation, the organic polymer layer in the electrolyte at this time can more completely cover and be embedded in the inorganic nuclei, making it difficult for continuous ultrasonic vibration to vibrate the SEI film. In addition, the microcracks of the SEI film at this stage have not been fully repaired, that is, the porosity of the SEI film at this time is larger than the porosity of the SEI film after chemical formation, which is conducive to the escape of tiny bubbles in the SEI film.

[0072] It is understandable that although the reasonable introduction of continuous ultrasonic vibration in the third charging operation can reduce the probability of ultrasonic vibration shedding the SEI film and enhance the escape of tiny bubbles in the SEI film to a certain extent, since the temperature during the formation process of traditional conventional lithium-ion batteries is usually 25°C to 35°C, of course, the formation temperature of some high-temperature stable liquid electrolyte lithium-ion batteries can be as high as 30°C to 60°C, there is still a probability of repeated continuous ultrasonic vibration shedding the SEI film.

[0073] Therefore, in the present disclosure, by further adjusting the temperature of the third charging operation to a high temperature environment of 60°C to 65°C, on the one hand, the organic polymer layer of the SEI film is ensured to be well softened at 60°C to 65°C, thereby ensuring that the SEI film cannot be shaken off by repeated continuous ultrasonic vibration for a long time; on the other hand, the porosity of the SEI film in the high temperature environment of 60°C to 65°C is higher than that of 30°C to 60°C, which is more conducive to the escape of tiny bubbles in the inner layer of the SEI film; on the other hand, the micro-vibration of the SEI film in the high temperature environment of 60°C to 65°C during continuous ultrasonic vibration helps to level the SEI film, so that the micro-vibration of the continuous ultrasonic vibration causes the softened organic polymer layer of the SEI film to fill the concave part of the SEI film, thereby achieving the effect of leveling the SEI film; and the micro-vibration of the continuous ultrasonic vibration causes the lithium dendrites on the surface of the SEI film to break, thereby effectively removing the lithium dendrites on the surface of the SEI film, which is conducive to obtaining an SEI film with good continuity, no accumulation, good connectivity, uniformity, density and no microcracks in the inner and outer layers.

[0074] It can be understood that although the introduction of continuous ultrasonic vibration during the third charging operation can reduce the problem of SEI film shedding caused by continuous ultrasonic vibration during the formation stage to a certain extent, after the battery cell completes the second charging operation, that is, when the charging voltage of the lithium-ion battery reaches 3.8V~4.1V, the organic polymer layer of the SEI film has not yet fully covered the inorganic nuclei. If continuous ultrasonic vibration is started at this time, there is still a probability of SEI film shedding.

[0075] Therefore, in the present disclosure, the step of performing continuous ultrasonic vibration operation on the battery cell includes the following specific steps: first, after the battery cell is charged to a voltage of 4.2V~4.35V, the organic polymer layer of the SEI film can fully cover the inorganic nucleus, that is, the organic polymer layer fully covers the inorganic inorganic nucleus, ensuring the connection strength between the organic polymer layer of the SEI film and the negative electrode surface, and further reducing the probability of SEI film falling off due to continuous ultrasonic vibration; then immediately perform continuous ultrasonic vibration on the battery cell, effectively reducing the probability of SEI film falling off due to continuous ultrasonic vibration; at the same time, use a small current to charge the battery cell for 20min~30min to avoid concentration polarization caused by high current density, so that the SEI film is promoted to be uniformly formed under the synergistic action of multiple physical fields, so as to ensure that the inner and outer layers are continuous with good accumulation, good connectivity, uniformity, density, no microcracks and very few lithium dendrites.

[0076] It should also be noted that the use of continuous ultrasonic vibration can effectively shake off and remove lithium dendrites, thereby ensuring the formation of a SEI film with very few lithium dendrites.

[0077] In one embodiment, the battery cell is charged with a low current of 0.01C to 0.02C for 20 to 30 minutes.

[0078] It should be noted that the current traditional method for removing tiny bubbles in lithium-ion batteries usually uses intermittent ultrasonic vibration, such as that described in Chinese patent document CN 105742742 A. However, intermittent ultrasonic vibration cannot comprehensively remove tiny bubbles inside the SEI membrane. This is mainly because after a certain period of vibration, some tiny bubbles that repeatedly expand and contract still cannot float up and be removed.

[0079] Therefore, in the present disclosure, continuous ultrasonic vibration operation is adopted, especially after the battery cell is charged to a voltage of 4.2V~4.35V, the battery cell is immediately subjected to continuous ultrasonic vibration, and a small current is used to charge the battery cell for 20min~30min, so as to ensure that the continuous micro-vibration force can comprehensively and effectively remove the tiny bubbles remaining inside the SEI film, and also ensure that the continuous micro-vibration force can break the lithium dendrites on the surface of the SEI film, so as to effectively remove the lithium dendrites on the surface of the SEI film.

[0080] It is understood that if the vibration frequency of the continuous ultrasonic vibration is less than 20kHz, it cannot be ensured that the continuous ultrasonic vibration force can completely remove the tiny bubbles remaining inside the SEI film and shake off the lithium dendrites on the surface of the SEI film; if the vibration frequency of the continuous ultrasonic vibration is greater than 30kHz, the continuous ultrasonic vibration force will damage the SEI film. Therefore, in one embodiment, the vibration frequency of the continuous ultrasonic vibration operation is 20kHz to 30kHz to ensure that the vibration frequency of the continuous ultrasonic vibration is more appropriate, while ensuring that the long-term repeated continuous ultrasonic vibration force will not damage the SEI film, and also ensure that the long-term repeated continuous ultrasonic vibration force can completely remove the tiny bubbles remaining inside the SEI film and completely shake off the lithium dendrites on the surface of the SEI film.

[0081] The above method uses a low temperature and low current to perform the first charging operation on the battery cell, so that the battery cell can better suppress the decomposition of the solvent in the electrolyte under low temperature conditions, thereby promoting the inorganic components in the electrolyte, such as LiF and Li2CO3, so that the inorganic components preferentially form uniform and dense inorganic nuclei on the surface of the negative electrode, which better compensates for the roughness of the negative electrode surface and is beneficial to reducing the current distribution difference on the negative electrode surface. When a medium temperature asymmetric pulse current is used to perform a second charging operation on the battery cell after the first charging operation; since the amplitude and width of the pulse of the asymmetric pulse current are asymmetric, the current distribution difference on the negative electrode surface during the charging and discharging process is further reduced. , thereby breaking the concentration polarization of lithium ions during the battery cell formation process; effectively inhibiting the formation of lithium dendrites in the inner layer of the SEI film, thereby ensuring the continuity of the inner and outer layers of the SEI film, and effectively avoiding the problem of lithium-ion batteries being prone to microcracks or holes in the SEI film during the cycle due to the presence of lithium dendrites in the inner layer of the SEI film; in addition, since the Li+ mobility of the battery cell is more suitable under medium temperature conditions, that is, the Li+ mobility is faster than at low temperature and slower than at high temperature, thereby ensuring that the organic polymer layer in the battery cell, such as ROCO2Li, can be more evenly covered on the inorganic nucleus, which not only helps to form a continuous, uniform and dense inner layer of the SEI film, but also shortens the battery cell formation time. process; then a high temperature and high current are used to perform a third charging operation on the battery cell after the second charging operation; the local positive and negative electrode materials of the battery cell can generate Joule heat under high temperature conditions, and the locally generated Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the SEI film microcracks, thereby ensuring the formation of a dense, uniform and microcrack-free SEI film; and because the continuous ultrasonic vibration operation is added in the third charging operation, on the one hand, the microvibration of the continuous ultrasonic vibration can cause the lithium dendrites on the surface of the SEI film to break, thereby effectively removing the lithium dendrites on the surface of the SEI film; on the other hand, the continuous ultrasonic vibration can effectively remove the tiny gas inside the SEI film inside the battery cell. Bubbles, that is, tiny bubbles inside the SEI film inside the battery cell will repeatedly expand and contract under the action of continuous ultrasonic vibration, so that adjacent tiny bubbles will merge into larger bubbles to accelerate the floating and removal of bubbles, thereby effectively removing tiny bubbles remaining inside the SEI film. In this way, it is ensured that the inner and outer layers of the SEI film formed in the formation process of the electrolyte interface film of the battery cell under the multi-physical field synergistic improvement of temperature, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation are continuous and free of accumulation, good connectivity, uniformity, density and no microcracks, thereby ensuring that the SEI film of the lithium-ion battery is not easy to form microcracks or holes during the cycle, thereby effectively ensuring the cycle performance of the lithium-ion battery.

[0082] It should be noted that the traditional operation of removing bubbles usually involves ultrasonic vibration under vacuum conditions. Coupled with the effect of intermittent ultrasonic vibration, the SEI film inside the battery cell is prone to deformation or rupture under vacuum negative pressure conditions. In addition, certain solvents in the battery cell's electrolyte are prone to volatilization under low pressure and high temperature, resulting in a decrease in the amount of electrolyte in the battery cell after formation, which affects the cycle performance of the lithium-ion battery.

[0083] Therefore, in one embodiment, when the battery cell is subjected to the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation, pressure is also applied to the battery cell, especially when the pressure is at 3 kgf / cm 2 ~8kgf / cm 2 Adjustment is made to ensure that the battery cell is in a state of dynamic pressurization during the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation, thereby ensuring that the battery cell is in a positive pressure state during the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation, which not only avoids the problem of SEI film deformation or rupture due to negative pressure, but also reduces the problem of easy volatility of electrolyte solvent at high temperature, thereby ensuring a large amount of electrolyte in the battery cell after formation, thereby ensuring the cycle performance of the lithium ion battery; it also ensures that the ultrasonic vibration wave can be better distributed inside the battery cell to make the residual The tiny bubbles remaining in the SEI membrane will repeatedly expand and contract under the action of continuous ultrasonic vibration, causing adjacent tiny bubbles to merge into larger bubbles to accelerate the floating and removal of the bubbles, thereby achieving a more comprehensive removal of the residual tiny bubbles in the SEI membrane, and effectively avoiding the inability of ultrasonic vibration waves to fully remove the residual tiny bubbles in the SEI membrane due to the difficulty of vibrating under vacuum conditions; at the same time, the micro-vibration of continuous ultrasonic vibration will shake off the lithium dendrites on the surface of the SEI membrane, so as to effectively remove the lithium dendrites on the surface of the SEI membrane; at the same time, applying pressure to the battery cell is also conducive to close contact between the positive and negative electrodes, avoiding lithium deposition caused by poor contact.

[0084] In one embodiment, when the battery cell is subjected to the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation, the step of applying pressure to the battery cell includes the following specific steps: dynamically adjusting the pressure as the SOC increases so that the pressure is at 3 kgf / cm 2 ~8kgf / cm 2 This is conducive to the complete escape of tiny bubbles remaining inside the SEI film, and avoids the problem of SEI film rupture or deformation due to excessive pressure during the first charging operation, the second charging operation, the third charging operation and continuous ultrasonic vibration operation of the battery cell.

[0085] In one embodiment, the pressure of the battery cell during the first charging operation is 2 kgf / cm2 ~3kgf / cm 2 The pressure of the battery cell during the second charging operation is 3kgf / cm 2 ~4kgf / cm 2 The pressure of the battery cell during the third charging operation is 3kgf / cm 2 ~5kgf / cm 2 The pressure of the battery cell in continuous ultrasonic vibration operation is 5kgf / cm 2 ~8kgf / cm 2 In this way, while achieving the comprehensive escape of tiny bubbles remaining inside the SEI film, it also avoids the problem of SEI film rupture or deformation due to excessive pressure during continuous ultrasonic vibration operation, and reduces the problem of easy volatilization of electrolyte solvents at high temperatures.

[0086] In one embodiment, the step of baking the battery cell includes the following specific steps: placing the wound battery cell in a vacuum oven at 80°C to 95°C and baking it cyclically for 5 to 10 times to obtain a water content of the entire battery cell of less than 150ppm, wherein the positive electrode sheet is less than 120ppm, and the negative electrode sheet + diaphragm is less than 100ppm, which is conducive to the rapid infiltration of the subsequent electrolyte.

[0087] In one embodiment, the battery cell includes a positive electrode sheet, a negative electrode sheet and a separator, which are stacked and wound in sequence. The positive electrode sheet is coated with a positive electrode material, and the negative electrode sheet is coated with a negative electrode material. The separator includes at least one of a double-sided oily PVDF separator, a single-sided oily PVDF separator, a double-sided water-based PVDF separator, and a single-sided water-based PVDF separator.

[0088] It can be understood that since the double-sided oily PVDF membrane, the single-sided oily PVDF membrane, the double-sided water-based PVDF membrane, and the single-sided water-based PVDF membrane are coated with PVDF adhesive, the heated PVDF adhesive can be more tightly adhered between the positive electrode sheet and the negative electrode sheet during the hot pressing process of the battery cell, so as to effectively exclude the air between the membrane and the gap between the positive and negative electrode sheets, which is beneficial for the battery cell to form an SEI film with good continuity, no accumulation, good connectivity, uniformity, density, no microcracks and very few lithium dendrites in the subsequent first charging operation, second charging operation, third charging operation and continuous ultrasonic vibration operation.

[0089] The present disclosure also provides a lithium-ion battery prepared using the multi-physics field synergistic improvement of the electrolyte interface film formation process described in any of the above embodiments. Specifically, the lithium-ion battery of the present disclosure is a high-temperature stable liquid electrolyte lithium-ion battery, and the multi-physics field synergistic improvement of the SEI film of the high-temperature stable liquid electrolyte lithium-ion battery ensures that the SEI film prepared has good inner and outer layer continuity without accumulation, good connectivity, uniformity, density, no microcracks, and minimal lithium dendrites.

[0090] like Figure 2 As shown, in one embodiment, a multifunctional formation cabinet is used to perform formation operation on a high-temperature stable liquid electrolyte lithium ion battery. The multifunctional formation cabinet includes a cabinet body, a control operation module, an asymmetric pulse power supply module, a heating module, a pressurizing module and an ultrasonic generator. The control operation module is arranged on the cabinet body and is electrically connected to the asymmetric pulse power supply module, the heating module, the pressurizing module and the ultrasonic generator respectively to realize intelligent control of the asymmetric pulse power supply module, the heating module, the pressurizing module and the ultrasonic generator, which is beneficial for the operator to adjust the parameters of the multifunctional formation cabinet through the control panel of the control operation module to improve the formation efficiency; and because a placement cavity is formed in the cabinet body, the placement cavity is used to place the high-temperature stable liquid electrolyte lithium ion battery, the heating module The block and the pressurizing module are arranged on the inner wall and the bottom of the cabinet to realize the rapid heating and pressurizing operation of the high-temperature stable liquid electrolyte lithium-ion battery; and because the ultrasonic generator is arranged at the bottom of the cabinet, it is ensured that the ultrasonic generator located at the bottom can provide more comprehensive and continuous ultrasonic vibration for the placement cavity, which helps to quickly discharge the tiny bubbles remaining in the SEI film and shake off the lithium dendrites of the SEI film; and the exhaust hole is arranged at the top of the cabinet to ensure that the multifunctional formation cabinet can quickly and comprehensively discharge the tiny bubbles remaining in the SEI film of the high-temperature stable liquid electrolyte lithium-ion battery under the action of the pressurizing module, which helps to prepare an SEI film with good continuity and no accumulation of inner and outer layers, good connectivity, uniformity, density, no microcracks and very few lithium dendrites.

[0091] Compared with the prior art, the present disclosure has at least the following advantages:

[0092] 1) Since the battery cell is charged for the first time at low temperature and low current, the battery cell can better inhibit the decomposition of the solvent in the electrolyte under low temperature conditions, thereby promoting the inorganic components in the electrolyte, such as LiF and Li2CO3, so that the inorganic components preferentially form uniform and dense inorganic nuclei on the surface of the negative electrode, which better compensates for the roughness of the negative electrode surface and is beneficial to reducing the current distribution difference on the negative electrode surface. When a medium-temperature asymmetric pulse current is used to charge the battery cell for the second time after the first charging operation; since the amplitude and width of the pulse of the asymmetric pulse current are asymmetric, the current distribution difference on the negative electrode surface during the charge and discharge process is further reduced, thereby breaking the concentration polarization of lithium ions during the battery cell formation process; effectively inhibiting the generation of lithium dendrites in the inner layer of the SEI membrane, thereby ensuring the continuity of the inner and outer layers of the SEI membrane, and effectively avoiding the problem of lithium-ion batteries being prone to forming microcracks or holes in the SEI membrane during the cycle due to the presence of lithium dendrites in the inner layer of the SEI membrane.

[0093] 2) In addition, since the Li+ mobility of the battery cell is more suitable under medium temperature conditions, that is, the Li+ mobility is faster than at low temperature and slower than at high temperature, thereby ensuring that the organic polymer layer in the battery cell, such as ROCO2Li, can be more evenly covered on the inorganic nucleation body, which not only helps to form a continuous, uniform and dense SEI film inner layer, but also shortens the battery cell formation process; then a high temperature and high current are used to perform a third charging operation on the battery cell after the second charging operation; the local positive and negative electrode materials of the battery cell can generate Joule heat under high temperature conditions, and the locally generated Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the SEI film microcracks, thereby ensuring the generation of a dense, uniform and microcrack-free SEI film; and because the continuous ultrasonic vibration operation is added in the third charging operation, on the one hand, the microvibration of the continuous ultrasonic vibration will make the surface of the SEI film The lithium dendrites are broken, thereby effectively removing the lithium dendrites on the surface of the SEI film; on the other hand, continuous ultrasonic vibration can effectively remove the tiny bubbles inside the SEI film inside the battery cell, that is, the tiny bubbles inside the SEI film inside the battery cell will repeatedly expand and contract under the action of continuous ultrasonic vibration, so that the adjacent tiny bubbles are merged into larger bubbles to accelerate the floating and removal of the bubbles, thereby effectively removing the tiny bubbles remaining inside the SEI film, so as to ensure that the inner and outer layers of the SEI film formed in the multi-physical field synergistic improvement of the electrolyte interface film in the battery cell under the multi-physical field of temperature increase, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation are continuous and free of accumulation, good connectivity, uniformity, density and no microcracks, thereby ensuring that the SEI film is not easy to form microcracks or holes during the cycle of the lithium-ion battery, thereby effectively ensuring the cycle performance of the lithium-ion battery.

[0094] Compared with the prior art, the present disclosure has at least the following advantages:

[0095] The following examples illustrate some specific embodiments, where percentages are expressed by weight. It should be noted that the following examples do not exhaust all possible situations, and that the materials used in the following examples, unless otherwise specified, can be obtained from commercial sources.

[0096] Example 1

[0097] S1 Cell Dehydration: Place the wound cell (the cell is made by stacking and winding the silicon-based negative electrode sheet with double-sided oily PVDF separator and the positive electrode sheet in sequence) in an 85°C vacuum oven and bake it 10 times to remove the moisture inside the cell. The overall cell moisture is less than 150ppm, the positive electrode sheet moisture is less than 120ppm, and the negative electrode sheet + separator moisture is less than 100ppm.

[0098] S2 pre-soaking: Place the battery cell baked in S1 in a vacuum chamber and add an electrolyte containing a high-temperature stable solvent of fluorocarbonate. Then apply an alternating pressure of 0.1MPa-0.6MPa to the battery cell after the injection. The alternating pressure is applied twice. After the alternating pressure is applied, the battery cell is placed at room temperature for 24 hours.

[0099] S3 pre-pressing: pre-press the battery cell for 30 minutes at 85°C and a pre-pressure of 0.4 MPa;

[0100] S4 formation stage: Place the battery cells after S3 completion in the multifunctional formation cabinet while they are still hot. Then, use the control operation module to set the parameters of the asymmetric pulse power module, heating module, pressurizing module and ultrasonic generator, so that the battery cells enter the following steps respectively:

[0101] S41, at a low temperature of 25 ° C, the S3 battery cell is charged to 3.7V at a current of 0.2C, and the dynamic pressure of the battery cell during the first charging operation is controlled to 3kgf / cm 2 ;

[0102] S42, heat up to a medium temperature of 40°C at a heating rate of 0.5°C / min, and simultaneously use an asymmetric pulse current (forward pulse: 1C~2C, pulse width 5s~30s; reverse pulse: 0.1C~0.2C, pulse width 2s~5s) to charge the cell of S41 to 4.0V at a constant current; and control the dynamic pressure of the cell in the second charging operation to 5kgf / cm 2 ;

[0103] S43, heat up to 65℃ at a rate of 0.5℃ / min, and charge the cell of S42 to 4.2V at a constant current of 0.5C, then charge the cell at a low current of 0.02C to the cut-off voltage; and control the dynamic pressure of the cell in the third charging operation to 8kgf / cm 2 ; Immediately perform continuous ultrasonic vibration operation on the battery cell, the vibration frequency of the ultrasonic vibration is 25kHz, and the battery cell is continuously charged for 30 minutes with a current of 0.02C, and the dynamic pressure of the battery cell during ultrasonic vibration is controlled to 8kgf / cm 2 ;

[0104] Example 2

[0105] The difference from Example 1 is that the 40° C. in S42 is replaced by 60° C., and the rest remain unchanged.

[0106] Example 3

[0107] The difference from Example 1 is that the 25kHz in S43 is replaced by 30kHz, and the dynamic pressure of the battery cell under ultrasonic vibration is 7kgf / cm 2, the rest remain unchanged.

[0108] Comparative Example 1

[0109] The difference from Example 1 is that S3 is omitted and the formation stage of S4 is different: the formation stage of S4 in Comparative Example 1: charging to 3.7V with a constant current of 0.1C, then charging to 3.9V with a constant current of 0.2C, and then charging to 4.2V with a constant current and constant voltage of 0.5C, the cut-off current is 0.02C, the formation temperature is 45°C, and the rest remains unchanged.

[0110] Comparative Example 2

[0111] The difference from Example 1 is that the formation stage of S4 is different, the asymmetric pulse current of S42 in Example 1 is directly replaced by a constant current of 0.5C; and the continuous ultrasonic vibration operation in S43 in Example 1 is omitted, and the rest remains unchanged.

[0112] Comparative Example 3

[0113] The difference from Example 1 is that the formation stage of S4 is different, and step S43 in Example 1 is omitted, while the rest remains unchanged.

[0114] Comparative Example 4

[0115] The difference from Example 1 is that the formation stage of S4 is different. The asymmetric pulse current of S42 in Example 1 is directly replaced by a constant current of 0.5C, and the rest remain unchanged.

[0116] Comparative Example 5

[0117] The difference from Example 1 is that the formation stage of S4 is different. The dynamic pressure of the battery cell of S4 in the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration is directly replaced by the vacuum exhaust operation, and the rest remains unchanged.

[0118] Comparative Example 6

[0119] The difference from Example 1 lies in step S43. The continuous ultrasonic vibration operation of S43 of Example 1 is placed between the steps of "heating to a high temperature of 65°C at a heating rate of 0.5°C / min" and "heating to a high temperature of 65°C at a heating rate of 0.5°C / min", and the rest remain unchanged.

[0120] Comparative Example 7

[0121] The difference from Example 1 is that the dynamic pressure of the ultrasonic vibration in S43 of Example 1 is set to 8 kgf / cm 2 Replaced with 10kgf / cm 2 , the rest remain unchanged.

[0122] Comparative Example 8

[0123] The difference from Example 1 lies in step S43, where the temperature of S43 in Example 1 is replaced by 70°C, and the rest remain unchanged.

[0124] Comparative Example 9

[0125] The difference from Example 1 lies in step S43, where the 65°C in S43 of Example 1 is replaced with 50°C, and the rest remain unchanged.

[0126] The lithium-ion batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 9 were subjected to electrical performance tests to obtain the experimental data shown in Table 1:

[0127] Among them, the SEI film appearance method: the appearance morphology of the SEI film is tested using a transmission electron microscope (TEM).

[0128] SEI film internal resistance detection method: Test the battery internal resistance using an electrochemical workstation.

[0129] Electrolyte capacity detection method: electrolyte injection amount; weigh the battery cell before injection, weigh the battery cell after injection, and weigh the battery cell by subtracting the weight before injection from the second weighing weight.

[0130] First efficiency calculation: The first coulombic efficiency is defined as the ratio of the first discharge capacity to the first charge capacity.

[0131] Calculation of cycle (1000 times) capacity retention rate: (end capacity / starting capacity)*100%.

[0132] Table 1

[0133]

[0134]

[0135] It can be seen from Examples 1 to 3 and Comparative Examples 1 to 9 in the above table that, since Examples 1 to 3 adopt the combined effects of heating, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation, while reducing the volatilization of the electrolyte, they can also fully extract the tiny bubbles remaining inside the SEI film and remove the lithium dendrites on the surface of the SEI film. Combined with the use of asymmetric pulse current, it is helpful to form an SEI film with good continuity, no accumulation, good connectivity, uniformity, density and no microcracks in the inner and outer layers. Among them, the comprehensive indicators of Example 1 are the best.

[0136] The above-described embodiments merely represent several implementation methods of the present disclosure. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the disclosed patent. It should be noted that a person of ordinary skill in the art could make various modifications and improvements without departing from the scope of the present disclosure, all of which fall within the scope of protection of the present disclosure. Therefore, the scope of protection of the disclosed patent shall be determined by the appended claims.

Claims

1. A multi-physical field synergistic improvement process for the formation of electrolyte interface films, characterized in that: The steps include: Get the battery cell after filling with liquid; Performing a first charging operation on the battery cell using low temperature and low current; Performing a second charging operation on the battery cell after the first charging operation using a medium-temperature asymmetric pulse current; Performing a third charging operation on the battery cell after the second charging operation using high temperature and high current; and The battery core is subjected to continuous ultrasonic vibration operation to complete the formation process of the battery core.

2. The multi-physical field synergistic improvement of the electrolyte interface film formation process according to claim 1, characterized in that: The temperature of the low temperature and low current is 10°C to 25°C, and the current is 0.1C to 0.2C; and / or, The voltage during the first charging operation is 3.0V to 3.7V.

3. The multi-physical field synergistic improvement of the electrolyte interface film formation process according to claim 1, characterized in that: The temperature of the medium-temperature asymmetric pulse current is 40°C to 45°C, and the asymmetric pulse current is: forward pulse: 1°C to 2°C, pulse width 5s to 30s; reverse pulse: 0.1°C to 0.2°C, pulse width 2s to 5s; and / or, The voltage during the second charging operation is 3.8V to 4.1V.

4. The multi-physical field synergistic improvement of the electrolyte interface film formation process according to claim 1, characterized in that: The temperature of the high temperature and high current is 60°C to 65°C, and the current is 0.5C to 1.0C; and / or, The voltage during the third charging operation is 4.2V to 4.35V.

5. The multi-physical field synergistic improvement of the electrolyte interface film formation process according to claim 4, characterized in that: The step of performing continuous ultrasonic vibration operation on the battery cell includes the following specific steps: After the battery cell is charged to a voltage of 4.2V to 4.35V, the battery cell is continuously ultrasonically vibrated and simultaneously charged with a small current for 20 minutes to 30 minutes.

6. The multi-physical field synergistic improvement of the electrolyte interface film formation process according to claim 5, characterized in that: The vibration frequency of the continuous ultrasonic vibration operation is 20kHz to 30kHz.

7. The multi-physical field synergistic improvement of the electrolyte interface film formation process according to claim 1, characterized in that: The steps of obtaining the battery cell after liquid injection include the following specific steps: baking the battery cell; The pre-wetting operation is performed on the baked battery cell by adopting alternating pressure.

8. The multi-physical field synergistic improvement of the electrolyte interface film formation process according to claim 7, characterized in that: The alternating pressure is 0.1 MPa to 0.6 MPa; and / or, The alternating pressure is applied ≥2 times.

9. The multi-physical field synergistic improvement of the electrolyte interface film formation process according to claim 1, characterized in that: After the step of obtaining the battery cell after liquid injection and before the step of performing the first charging operation on the battery cell using low temperature and low current, the following steps are also included: Allowing the battery cell to stand for 12 to 24 hours; The battery cell is pre-pressed for 20 minutes to 30 minutes under a high temperature condition of 70° C. to 85° C.

10. A lithium ion battery, characterized in that: The electrolyte interface film is prepared by adopting the multi-physical field synergistic improvement chemical formation process according to any one of claims 1 to 9.

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