Formation process for improving electrolyte interface film by multi-physical field cooperation and lithium ion battery

Abstract

The present disclosure provides a multi-physical field synergistic improvement of electrolyte interface film formation process and lithium ion battery. The multi-physical field synergistic improvement of electrolyte interface film formation process: obtaining the battery after injection; using low temperature small current to charge the battery for the first time; using medium temperature asymmetric pulse current to charge the battery after the first charging operation for the second time; using high temperature large current to charge the battery after the second charging operation for the third time; continuously ultrasonic vibration operation is carried out on the battery. The method, under the combined action of multiple physical fields such as temperature rise, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation, can reduce the volatilization of electrolyte, extract the micro bubbles in the residual SEI film, and remove the lithium dendrite on the surface of SEI film, which helps to form SEI film with good continuity, good connection, uniformity, density and no micro crack.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of lithium ion batteries, in particular to a formation process for improving electrolyte interface film in coordination with multiple physical fields and a lithium ion battery. BACKGROUND

[0002] The formation process of lithium ion batteries is a key step in battery manufacturing, aiming 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] Currently, the early formation process often only considers the single factor of low-rate constant current charge-discharge formation process, but the low-rate constant current charge-discharge formation process has the problem of long formation time. Therefore, a method for constructing lithium ion battery electrolyte interface film by double pulse excitation is disclosed in Chinese patent document No. CN 109755682 A, which is beneficial to construct electrolyte interface film with uniform composition, thickness, structure, high consistency and controllable process by double pulse excitation on lithium ion battery after adding electrolyte and repeated operation of charging and discharging, and the formation time of lithium ion battery is shortened.

[0004] Although the above-mentioned double pulse excitation effectively shortens the formation time of lithium ion battery and is beneficial to construct electrolyte interface film with uniform composition, thickness, structure and high consistency, but due to the symmetry of the amplitude and width of the pulse used by the double pulse excitation, the lithium ion battery is prone to generate local lithium dendrites in the area of SEI film with high negative electrode surface roughness under the condition of symmetrical charge and discharge, which reduces the uniformity and stability of SEI film.

[0005] Therefore, some scholars have developed a method for in-situ removing lithium dendrites on the positive and negative electrode surfaces of lithium ion batteries as disclosed in Chinese patent document No. CN 114388909 A, and positioning the SEI film after formation by X-ray, and then using electromagnetic shock wave to impact the SEI film, thereby removing the lithium dendrites generated on the positive and negative electrode surfaces during the formation process.

[0006] However, the above-mentioned traditional method can only remove the lithium dendrites on the positive and negative electrode surfaces, but cannot well remove the lithium dendrites in the inner layer of the SEI film. However, in actual application, the lithium dendrites located in the inner layer of the SEI film can destroy the continuity of the SEI film, leading to the problem that the SEI film is prone to form micro-cracks or holes during the cycle process of the lithium ion battery, thereby causing the problem of SEI film interface failure due to the destruction of the SEI film structure. SUMMARY

[0007] The purpose of the present disclosure is to overcome the deficiencies in the prior art, provide a multi-physical field synergistic improvement electrolyte interface film formation process and a lithium ion battery, which realizes effective removal of lithium dendrites in the inner and outer layers of SEI film in the formation process, ensures that the battery is under the combined action of multiple physical fields such as temperature rise, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation, reduces electrolyte evaporation, extracts micro bubbles in the residual SEI film, removes lithium dendrites on the surface of the SEI film, and helps to form a SEI film with good continuity, good connectivity, uniformity, density and no micro-cracks.

[0008] The purpose of the present disclosure is achieved by the following technical solutions:

[0009] A multi-physical field synergistic improvement electrolyte interface film formation process, comprising the following steps:

[0010] Obtaining the battery after injection of the liquid;

[0011] Performing a first charging operation on the battery by using a low-temperature small current;

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

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

[0014] Performing a continuous ultrasonic vibration operation on the battery; to complete the formation process of the battery.

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

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

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

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

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

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

[0021] In one of the embodiments, the step of continuously ultrasonic vibrating the battery cell comprises the following specific steps:

[0022] After the battery cell is charged to a voltage of 4.2V-4.35V, continuously ultrasonic vibrating the battery cell while charging the battery cell with a small current for 20-30 minutes.

[0023] In one of the embodiments, the vibration frequency of the continuous ultrasonic vibrating operation is 20-30 kHz.

[0024] In one of the embodiments, the step of obtaining the battery cell after injection comprises the following specific steps:

[0025] Baking the battery cell;

[0026] Pre-soaking the baked battery cell with alternating pressure.

[0027] In one of the embodiments, the alternating pressure is 0.1-0.6 MPa; and / or,

[0028] The number of times of applying the alternating pressure is ≥2.

[0029] In one of the embodiments, before the step of charging the battery cell with a low-temperature small current for the first time, the method further comprises the following steps:

[0030] Letting the battery cell stand for 12-24 hours;

[0031] Pre-pressing the battery cell at a high temperature of 70-85°C for 20-30 minutes.

[0032] A lithium ion battery prepared by using the formation process for improving the electrolyte interface film in multiple physical fields according to any one of the above embodiments.

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

[0034] 1) Since the first charging operation of the battery cell is carried out at low temperature and low current, the decomposition of the solvent in the electrolyte can be well inhibited at low temperature, so as to promote the inorganic components in the electrolyte, such as LiF and Li2CO3, to form uniform and dense inorganic nucleation on the negative electrode surface, which can well compensate for the roughness of the negative electrode surface and reduce the current distribution difference on the negative electrode surface. When the second charging operation of the battery cell is carried out at medium temperature and asymmetric pulse current, the amplitude and width of the asymmetric pulse current can further reduce the current distribution difference on the negative electrode surface during charging and discharging, thereby breaking the concentration polarization of lithium ions during the formation process of the battery cell. The generation of lithium dendrites in the inner layer of the SEI film is effectively inhibited, thereby ensuring the continuity of the inner and outer layers of the SEI film and effectively avoiding the problem that the SEI film is prone to form micro-cracks or holes due to the presence of lithium dendrites in the inner layer of the SEI film during the cycle process of the lithium ion battery.

[0035] 2) In addition, since the Li+ migration rate of the battery cell at medium temperature is appropriate, i.e. faster than at low temperature and slower than at high temperature, the organic polymer layer in the battery cell, such as ROCO2Li, can uniformly cover the inorganic nucleation, which not only helps to form a continuous, uniform and dense inner layer of the SEI film, but also shortens the process of the battery cell formation. Then, the third charging operation of the battery cell is carried out at high temperature and large current. The local positive and negative electrode materials of the battery cell can generate Joule heat at high temperature, and the local Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the micro-cracks of the SEI film, thereby ensuring the formation of a dense, uniform and micro-crack-free SEI film. In addition, the continuous ultrasonic vibration operation is added in the third charging operation. On the one hand, the micro-vibration 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 micro-bubbles inside the SEI film inside the battery cell. That is, the micro-bubbles inside the SEI film inside the battery cell will repeatedly expand and contract under the action of the continuous ultrasonic vibration, so as to merge adjacent micro-bubbles into larger bubbles to accelerate the floating and removal of the bubbles, thereby effectively removing the micro-bubbles remaining in the SEI film. In this way, the continuity of the inner and outer layers of the SEI film formed in the formation process of the electrolyte interface film under the synergistic effect of the multiple physical fields of temperature rise, external pressure, asymmetric pulse current and continuous ultrasonic vibration is good, without accumulation, good connectivity, uniformity, density and micro-cracks, thereby ensuring that the SEI film is not prone to form micro-cracks or holes during the cycle process of the lithium ion battery, and thereby effectively ensuring the cycle performance of the lithium ion battery. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some of the embodiments of the present disclosure, and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor.

[0037] Figure 1 The flow chart of the formation process of the multi-physical field synergistic improved electrolyte interface film for an embodiment of the present application;

[0038] Figure 2 The structural schematic diagram of one direction of the multifunctional formation cabinet for an embodiment of the present application. DETAILED DESCRIPTION

[0039] In order to facilitate the understanding of the present disclosure, the following will make a more comprehensive description of the present disclosure with reference to the related drawings. The preferred embodiments of the present disclosure are shown in the drawings. However, the present disclosure can be realized in many different forms, and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present disclosure more thorough and comprehensive.

[0040] It should be noted that when an element is referred to as being "fixed" to another element, it can be directly on the other element or there can be an intervening element. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or there can be an intervening element. The terms "vertical", "horizontal", "left", "right", and similar expressions used herein are for purposes of illustration only and are not intended to limit the embodiments of the present disclosure.

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

[0042] The present disclosure provides a formation process of a multi-physical field synergistically improved electrolyte interface film, obtains a battery cell after injection; adopts a low-temperature small current to perform a first charging operation on the battery cell; adopts a medium-temperature asymmetric pulse current to perform a second charging operation on the battery cell after the first charging operation; adopts a high-temperature large current to perform a third charging operation on the battery cell after the second charging operation; and performs a continuous ultrasonic vibration operation on the battery cell; to complete the formation process of the battery cell.

[0043] The above method, since the first charging operation of the battery cell is performed at low temperature and low current, the decomposition of the solvent in the electrolyte can be inhibited at low temperature, so as to promote the inorganic components in the electrolyte, such as LiF and Li2CO3, to form uniform and dense inorganic nucleation bodies on the negative electrode surface, which can compensate for the roughness of the negative electrode surface and reduce the current distribution difference on the negative electrode surface. When the second charging operation of the battery cell is performed at medium temperature and asymmetric pulse current, the amplitude and width of the asymmetric pulse current can further reduce the current distribution difference on the negative electrode surface during charging and discharging, thereby breaking the concentration polarization of lithium ions during the formation process of the battery cell. The generation of lithium dendrites in the inner layer of the SEI film is effectively inhibited, thereby ensuring the continuity of the inner and outer layers of the SEI film and effectively avoiding the problem that the SEI film is prone to form micro-cracks or pores due to the presence of lithium dendrites in the inner layer of the SEI film during the cycle process of the lithium ion battery. In addition, the Li+ migration rate of the battery cell at medium temperature is appropriate, i.e., the Li+ migration rate is faster than at low temperature and slower than at high temperature, thereby ensuring that the organic polymer layer, such as ROCO2Li, in the battery cell can uniformly cover 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 process of the battery cell formation. Then, the third charging operation of the battery cell is performed at high temperature and high current. The local positive and negative electrode materials of the battery cell can generate Joule heat at high temperature, and the local Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the micro-cracks of the SEI film, thereby ensuring the formation of a dense, uniform and micro-crack-free SEI film. In addition, the continuous ultrasonic vibration operation is added in the third charging operation. On the one hand, the micro-vibration 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 micro-bubbles inside the SEI film inside the battery cell. That is, the micro-bubbles inside the SEI film inside the battery cell will repeatedly expand and contract under the action of the continuous ultrasonic vibration, causing adjacent micro-bubbles to merge into larger bubbles to accelerate the floating and removal of the bubbles, thereby effectively removing the micro-bubbles remaining in the SEI film. In this way, the inner and outer layers of the SEI film formed in the formation process of the electrolyte interface film under the synergistic effect of the multiple physical fields of temperature rise, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation are continuous, have good connectivity, are uniform, dense and free of micro-cracks, thereby ensuring that the SEI film is not prone to form micro-cracks or pores during the cycle process of the lithium ion battery, and thereby effectively ensuring the cycle performance of the lithium ion battery.

[0044] Please refer to Figure 1In order to better understand the technical solutions and beneficial effects of the present disclosure, the present disclosure will be further described in detail below in combination with specific embodiments. The formation process of the multi-physical field synergistic improved electrolyte interface film of one embodiment includes part or all of the following steps:

[0045] In step S101, the liquid-injected battery cell is obtained, so that the electrolyte can infiltrate the electrode sheets and the separator inside the battery cell. This is conducive to the uniform distribution of Li+ during the formation of the battery cell, reduces the local current density, promotes the uniform deposition of lithium, and inhibits the nucleation of lithium dendrites.

[0046] In one of the embodiments, in the step of obtaining the liquid-injected battery cell, the following specific steps are included: baking the battery cell to effectively remove the moisture in the battery cell; then, pre-infiltrating the baked battery cell using alternating pressure to ensure that the electrolyte can fully and quickly infiltrate the electrode sheets and the separator inside the battery cell.

[0047] It can be understood that if the alternating pressure is less than 0.1 MPa, the effect of accelerating the infiltration of the electrolyte into the electrode sheets and the separator inside the battery cell cannot be achieved; if the alternating pressure is greater than 0.6 MPa, the viscosity of the electrolyte will significantly increase under high pressure, which is not conducive to the penetration of the electrolyte into the electrode sheet pores, causing incomplete infiltration. Therefore, in the present disclosure, by controlling the alternating pressure to be 0.1 MPa to 0.6 MPa, the electrolyte can be accelerated to infiltrate the battery cell under the premise of ensuring that the electrolyte can be fully infiltrated, and the problem of the viscosity of the electrolyte significantly increasing due to high pressure and being not conducive to the penetration of the electrolyte into the electrode sheet pores is avoided.

[0048] In one of the embodiments, the number of times of applying the alternating pressure is greater than or equal to 2, so as to ensure that the electrolyte can fully infiltrate the electrode sheets and the separator inside the battery cell.

[0049] In a more preferred embodiment, the number of times of applying the alternating pressure is 2 to 5.

[0050] In one of the embodiments, after the step of obtaining the liquid-injected battery cell and before the step of charging the battery cell with low temperature and small current for the first time, the following steps are included: first, the battery cell is allowed to stand for 12 to 24 hours. On the one hand, this ensures that the electrolyte can fully infiltrate the electrode sheets, so as to avoid the generation of lithium dendrites due to insufficient standing time and incomplete infiltration of the electrolyte; on the other hand, standing can effectively remove small air bubbles remaining during the liquid injection process, which is helpful for the subsequent formation of a SEI film that is continuous, has no accumulation, has good connectivity, is uniform, dense and has no micro-cracks; then, the battery cell is pre-pressed for 20 to 30 minutes under high temperature conditions of 70 to 85 degrees Celsius, so as to ensure that the electrolyte does not easily volatilize during pre-pressing, and also ensures that the electrolyte fully infiltrates the electrode sheets and the separator inside the battery cell.

[0051] It should be noted that the pre-pressing temperature of the electrolyte of the conventional general lithium ion battery is generally 20-30°C, but for some high-temperature stable liquid electrolyte lithium ion batteries, such as lithium ion batteries using high-temperature stable solvents, such as lithium hexafluorophosphate, vinylene carbonate, fluorinated carbonate, or silicon-based negative electrode lithium ion batteries, if the conventional 20-30°C is used for pre-pressing, the electrolyte cannot be completely soaked into the electrode sheets and the separator inside the battery cell. Therefore, in the present disclosure, the battery cell is pre-pressed at a high temperature of 70-85°C for 20-30 min to ensure that the electrolyte of the high-temperature stable liquid electrolyte lithium ion battery does not easily volatilize during pre-pressing, and also ensures that the electrolyte completely soaks into the electrode sheets and the separator inside the battery cell to reduce the soaking dead angle.

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

[0053] S102, the first charging operation is performed on the battery cell using a low-temperature small current, so that the battery cell can better inhibit the decomposition of the solvent in the electrolyte under low-temperature conditions, to promote the inorganic components in the electrolyte, such as LiF and Li2CO3, to preferentially form uniform and dense inorganic nucleation bodies on the negative electrode surface, which better compensates for the roughness of the negative electrode surface, which is conducive to reducing the current distribution difference on the negative electrode surface during the second charging operation or the third charging operation, thereby facilitating the formation of a SEI film with good internal and external layer continuity, no accumulation, good connectivity, uniformity, density and no micro-cracks.

[0054] In one embodiment, the low-temperature small current has a temperature of 10-25°C and a current of 0.1-0.2C; especially in combination with the use of a voltage of 3.0-3.7V during the first charging operation, to ensure that the battery cell can better inhibit 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 nucleation bodies on the negative electrode surface, to ensure the formation of a SEI film with good internal and external layer continuity, no accumulation, good connectivity, uniformity, density and no micro-cracks.

[0055] It should be noted that, since the high-temperature stable liquid electrolyte lithium ion battery adds a high-temperature stabilizer, the thermal stability is relatively high, and if the infiltrated battery is directly operated at 10-25℃ low temperature small battery, it is not conducive to the inorganic component to form uniform and dense inorganic nucleation body on the negative electrode surface. Therefore, in the present disclosure, after the battery is pre-pressed, the battery is placed in the formation cabinet for low temperature small battery operation while hot, so as to ensure the migration and diffusion of inorganic ions of inorganic components, which is conducive to the formation of uniform and dense inorganic nucleation body on the negative electrode surface.

[0056] It should also be noted that when the pre-pressed battery is placed in the formation cabinet while hot, and the temperature of the formation cabinet is 10-25℃, 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 first inorganic nucleation body, i.e. the density and uniformity of the surface of the inorganic nucleation body formed in the later stage is slightly worse than that 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, using a medium temperature asymmetric pulse current to perform a second charging operation on the battery after the first charging operation.

[0058] It can be understood that when a medium temperature asymmetric pulse current is used to perform a second charging operation on the battery after the first charging operation; due to the asymmetry of the amplitude and width of the pulse of the asymmetric pulse current, the difference in current distribution on the negative electrode surface during charging and discharging is further reduced, thereby breaking the concentration polarization of lithium ions in the battery formation process; effectively inhibiting the generation 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, effectively avoiding the problem that the SEI film is prone to form micro-cracks or holes in the cycle process due to the presence of lithium dendrites in the inner layer of the SEI film; in addition, since the Li+ migration rate of the battery under medium temperature conditions is relatively appropriate, i.e. the Li+ migration rate is faster than that at low temperature and slower than that at high temperature, thereby ensuring that the organic polymer layer in the battery, such as ROCO2Li, can uniformly cover 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 process of battery formation.

[0059] It is worth mentioning that although the traditional double-pulse excitation method for constructing lithium ion battery electrolyte interface film uses symmetric and uniform current, due to the influence of the roughness of the negative electrode sheet itself, the actual current exists the problem of uneven distribution, which is easy to form lithium dendrites at this stage, thereby causing the problem of lithium dendrites in the inner and outer layers of the finally prepared SEI film.

[0060] Therefore, in the present disclosure, by adopting the asymmetric pulse current, i.e. using the high current density of the positive pulse to break the lithium concentration polarization temporarily to promote uniform deposition, and by using the low current density of the reverse pulse to redistribute lithium to eliminate local accumulation to reduce the nucleation points of lithium dendrites, thereby facilitating the formation of the SEI film with good continuity between the inner and outer layers, no accumulation, good connectivity, uniformity, density and no micro-cracks.

[0061] In one embodiment, the temperature of the medium-temperature asymmetric pulse current is 40-45℃, which provides a relatively suitable mobility for Li+ to facilitate the uniform and stable growth of the SEI film; since the asymmetric pulse current is: forward pulse: 1C-2C, pulse width 5-30s, to ensure that the high current density of the forward pulse breaks the lithium concentration polarization temporarily to promote the uniform deposition of Li+; and since the reverse pulse is: 0.1C-0.2C, pulse width 2-5s, to make the small current used in the reverse pulse redistribute lithium to eliminate local accumulation to reduce the nucleation points of lithium dendrites, especially in combination with the use of the voltage of 3.8-4.1V during the second charging operation, to ensure that the organic polymer layer in the battery can be evenly covered on the inorganic nucleating body, while also inhibiting the growth of lithium dendrites, which helps to form a continuous, uniform and dense inner layer of the SEI film, and also shortens the process of battery formation.

[0062] In one embodiment, the second charging operation of the battery after the first charging operation is performed by using a medium-temperature asymmetric pulse current, which means that the charging of the battery is performed at a temperature of 40-45℃ by using an asymmetric pulse current, and the charging standard is: forward pulse: 1C-2C, pulse width 5-30s; reverse pulse: 0.1C-0.2C, pulse width 2-5s.

[0063] In one embodiment, when the second charging operation of the battery after the first charging operation is performed by using a medium-temperature asymmetric pulse current, the temperature is raised to a medium temperature of 40-45℃ at a temperature rise rate of 0.5℃ / min to achieve reliable and smooth temperature rise during the second charging operation, so that the organic polymer layer can be uniformly deposited and embedded in the inorganic nucleating body, thereby improving the firmness of the connection between the organic polymer layer and the inorganic nucleating body.

[0064] It should be noted that since the lithium ion battery is mainly the generation of organic polymer layer during the second charging operation, and by controlling the temperature of the second charging operation to 40-45℃, on the one hand, the chain 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 40-45℃, so that the organic polymer layer can be well deposited and embedded on the inorganic nucleator, so that the organic polymer layer can be closely attached to the negative electrode surface, reducing the interface gap and reducing the contact impedance, which helps to form a SEI film with good continuity of inner and outer layers, good connectivity, uniformity, density and no micro-cracks; on the other hand, it reduces the volatilization of low-boiling-point solvents such as carbonate solvents (EC, DEC), which reduces the amount of electrolyte and affects the cycle performance of the lithium ion battery.

[0065] S104, using high-temperature large current to perform third charging operation on the battery after the second charging operation; and continuously ultrasonic vibration operation is performed on the battery; to complete the formation process of the battery.

[0066] It can be understood that the third charging operation is performed on the battery after the second charging operation by using high-temperature large current; the local positive and negative electrode materials of the battery can generate Joule heat under high temperature conditions, and the local Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the micro-cracks of the SEI film, thereby ensuring the formation of a dense, uniform and micro-crack-free SEI film; and because the continuous ultrasonic vibration operation is added in the third charging operation, on the one hand, the micro-vibration 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 small bubbles inside the SEI film inside the battery, that is, the small bubbles inside the SEI film inside the battery will repeatedly expand and contract under the action of continuous ultrasonic vibration, so that adjacent small bubbles merge into larger bubbles to accelerate the floating and removal of the bubbles, thereby effectively removing the small bubbles remaining in the SEI film. In this way, the formation of the SEI film in the formation process of the lithium ion battery in the multi-physical field of temperature rise, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation is ensured to have good continuity of inner and outer layers, good connectivity, uniformity, density and no micro-cracks, thereby ensuring that the SEI film is not easy to form micro-cracks or holes during the cycle process of the lithium ion battery, thereby effectively ensuring the cycle performance of the lithium ion battery.

[0067] In one embodiment, the high temperature and large current is 0.5C-1.0C at 60-65℃, and the voltage during the third charging operation is 4.2-4.35V, which ensures that the local positive and negative materials of the battery cell generate Joule heat under high temperature conditions, and the local Joule heat promotes recrystallization of the positive and negative materials to repair the micro-cracks of the SEI film, thereby ensuring the formation of a dense, uniform and micro-crack-free SEI film.

[0068] In one embodiment, the high temperature and large current is 0.5C-1.0C at 60-65℃, and the voltage during the third charging operation is 4.2-4.35V, which ensures that the local positive and negative materials of the battery cell generate Joule heat under high temperature conditions, and the local Joule heat promotes recrystallization of the positive and negative materials to repair the micro-cracks of the SEI film, thereby ensuring the formation of a dense, uniform and micro-crack-free SEI film.

[0069] It should be noted that some power batteries also use ultrasonic technology to remove impurities or gases inside the battery during formation, such as disclosed in Chinese Patent Document No. CN 105742742 A, which uses vacuum and ultrasonic technology to remove micro-bubbles adhering to the electrode sheet, separator and electrolyte, thereby improving the quality of the SEI film of the lithium ion battery. However, since the ultrasonic technology in this document is placed after formation, the porosity of the dense SEI film composed of inorganic and organic polymer layers after formation is usually lower than that of the electrode sheet, separator and electrolyte. If ultrasonic technology is used to remove micro-bubbles in the lithium ion battery after formation, it will be difficult to remove micro-bubbles in the SEI film.

[0070] Of course, some scholars will think that since Chinese Patent Document No. CN 105742742 A discloses the introduction of ultrasonic technology during formation to effectively remove micro-bubbles in the lithium ion battery, those skilled in the art can choose according to actual requirements.

[0071] However, in actual application, due to the relatively small amount of inorganic nucleation formed in the initial stage of formation (first charging operation) and the poor adhesion to the negative electrode surface, if the ultrasonic technology is introduced in the initial stage of formation, the inorganic nucleation just deposited on the negative electrode surface is easily vibrated down. If the ultrasonic technology is introduced in the intermediate stage of formation (second charging operation), and the SEI film at this time has not formed a relatively complete structure, the introduced ultrasonic technology will still cause the unformed SEI film to vibrate down. At this time, some scholars know that the structure of the SEI film after the third charging operation is relatively stable, so they will directly introduce the ultrasonic technology into the third charging operation. As in the present disclosure, by setting the continuous ultrasonic vibration operation in the third charging operation, the organic polymer layer in the electrolyte at this time can completely cover the intercalation on the inorganic nucleation, so that it is difficult for the continuous ultrasonic vibration to vibrate the SEI film down. In addition, the microcracks in the SEI film at this stage are not fully repaired, that is, the porosity of the SEI film at this time is relatively large compared to the porosity of the SEI film after formation, which is beneficial to the escape of the microbubbles in the SEI film.

[0072] It can be understood that, although the reasonable introduction of continuous ultrasonic vibration operation in the third charging operation can reduce the probability of ultrasonic vibration of the SEI film and improve the escape of microbubbles in the SEI film to a certain extent, due to the fact that the temperature during the formation process of the conventional lithium-ion battery is usually 25-35°C, and the formation temperature of some high-temperature stable liquid electrolyte lithium-ion batteries can be as high as 30-60°C, the long-term repeated continuous ultrasonic vibration still has the probability of vibrating the SEI film.

[0073] Therefore, in the present disclosure, the temperature of the third charging operation is further adjusted to reach a high-temperature environment of 60-65°C. In this way, on the one hand, it ensures that the organic polymer layer of the SEI film can be well softened at 60-65°C, so that the long-term repeated continuous ultrasonic vibration cannot vibrate the SEI film down; on the other hand, the porosity of the SEI film under the high-temperature environment of 60-65°C is relatively high compared to 30-60°C, which is more beneficial to the escape of microbubbles in the inner layer of the SEI film; on the other hand, the micro-vibration of the continuous ultrasonic vibration under the high-temperature environment of 60-65°C helps to flatten the SEI film, so that the micro-vibration of the continuous ultrasonic vibration can fill the softened organic polymer layer of the SEI film in the recessed part of the SEI film to achieve the effect of flattening the SEI film; and the micro-vibration of the continuous ultrasonic vibration can cause the lithium dendrites on the surface of the SEI film to break, so as to effectively remove the lithium dendrites on the surface of the SEI film, which is beneficial to obtaining the SEI film with good continuity, no accumulation, good connectivity, uniformity, density and no microcracks.

[0074] It can be understood that although the continuous ultrasonic vibration is introduced at the third charging operation, the problem of SEI film peeling caused by continuous ultrasonic vibration during the formation stage can be reduced to a certain extent, but since the organic polymer layer of the SEI film does not fully cover the inorganic nucleating agent after the second charging operation is completed, that is, when the voltage of the lithium ion battery reaches 3.8V-4.1V, the probability of SEI film peeling still exists if the continuous ultrasonic vibration is started at this time.

[0075] Therefore, in the present disclosure, the step of continuously ultrasonic vibrating the battery includes the following specific steps: first, after the battery is charged to a voltage of 4.2V-4.35V, the organic polymer layer of the SEI film can fully cover the inorganic nucleating agent, that is, the full coverage of the organic polymer layer on the inorganic nucleating agent is achieved, ensuring the connection strength of the organic polymer layer of the SEI film to the negative electrode surface, and further reducing the probability of SEI film peeling caused by continuous ultrasonic vibration; then immediately continuously ultrasonic vibrating the battery, effectively reducing the probability of SEI film peeling caused by continuous ultrasonic vibration; at the same time, charging the battery with a small current for 20-30min to avoid concentration polarization caused by high current density, and promoting the uniform formation of SEI film under the synergistic action of multiple physical fields, so as to ensure the preparation of SEI film with good continuity, no accumulation, good connectivity, uniformity, density, no micro-cracks and few lithium dendrites.

[0076] It should be noted that the continuous ultrasonic vibration can effectively shake off the lithium dendrites to ensure the formation of SEI film with few lithium dendrites.

[0077] In one embodiment, the current for charging the battery with a small current for 20-30min is 0.01C-0.02C.

[0078] It should be noted that the current conventional method for removing micro-bubbles in lithium ion batteries is interval ultrasonic vibration, such as Chinese patent document No. CN 105742742 A, but interval ultrasonic vibration cannot completely remove the micro-bubbles in the SEI film. The main reason is that part of the repeatedly expanded and contracted micro-bubbles still cannot be removed by floating and removing after a certain time of vibration.

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

[0080] It can be understood that if the vibration frequency of the continuous ultrasonic vibration is less than 20 kHz, the continuous ultrasonic vibration force cannot ensure to completely remove the micro-bubbles remaining in the SEI film and the lithium dendrites falling on the surface of the SEI film; if the vibration frequency of the continuous ultrasonic vibration is greater than 30 kHz, the continuous ultrasonic vibration force will damage the SEI film. Therefore, in one of the embodiments, the vibration frequency of the continuous ultrasonic vibration operation is 20 kHz-30 kHz, so as to ensure that the vibration frequency of the continuous ultrasonic vibration is appropriate, and on the premise that the long-time repeated continuous ultrasonic vibration force does not damage the SEI film, it is also ensured that the long-time repeated continuous ultrasonic vibration force can completely remove the micro-bubbles remaining in the SEI film and completely remove the lithium dendrites falling on the surface of the SEI film.

[0081] The method above, since the first charging operation of the battery cell is performed by low temperature and small current, the decomposition of the solvent in the electrolyte can be inhibited under low temperature condition, so as to promote the inorganic components in the electrolyte, such as LiF and Li2CO3, to form uniform and dense inorganic nucleation on the negative electrode surface, which can compensate the roughness of the negative electrode surface, and is beneficial to reduce the current distribution difference of the negative electrode surface. When the second charging operation of the battery cell after the first charging operation is performed by using the medium temperature and asymmetric pulse current, the amplitude and width of the pulse current are asymmetric, which further reduces the current distribution difference of the negative electrode surface in the charging and discharging process, so as to break the concentration polarization of lithium ions in the formation process of the battery cell. The generation of lithium dendrites in the inner layer of the SEI film is effectively inhibited, which ensures the continuity of the inner and outer layers of the SEI film, and effectively avoids the problem that the SEI film is easy to form micro-cracks or holes in the cycle process due to the existence of lithium dendrites in the inner layer of the SEI film. In addition, the Li+ migration rate of the battery cell under medium temperature condition is appropriate, that is, the Li+ migration rate is faster than that under low temperature condition, and slower than that under high temperature condition, so as to ensure that the organic polymer layer, such as ROCO2Li, in the battery cell can uniformly cover on the inorganic nucleation, which not only helps to form a continuous, uniform and dense inner layer of the SEI film, but also shortens the process of the battery cell formation. Then, the third charging operation of the battery cell after the second charging operation is performed by using high temperature and large current. The local positive and negative electrode materials of the battery cell can generate Joule heat under high temperature condition, and the local Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the micro-cracks of the SEI film, so as to ensure the formation of the SEI film with dense, uniform and no micro-cracks. In addition, the continuous ultrasonic vibration operation is added in the third charging operation. On the one hand, the micro-vibration of the continuous ultrasonic vibration 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. On the other hand, the continuous ultrasonic vibration can effectively remove the micro-bubbles in the SEI film inside the battery cell. That is, the micro-bubbles in the SEI film inside the battery cell will repeatedly expand and contract under the action of the continuous ultrasonic vibration, so as to combine adjacent micro-bubbles into larger bubbles to accelerate the floating and removal of the bubbles, so as to effectively remove the micro-bubbles remaining in the SEI film. In this way, the continuity of the inner and outer layers of the SEI film formed in the formation process of the electrolyte interface film under the condition of the multi-physical field of temperature rising, external pressure, asymmetric pulse current and continuous ultrasonic vibration is good without accumulation, connection, uniformity, density and micro-cracks, so as to ensure that the SEI film is not easy to form micro-cracks or holes in the cycle process of the lithium ion battery, and the cycle performance of the lithium ion battery is effectively ensured.

[0082] It should be noted that the conventional operation of removing bubbles is usually ultrasonic vibration under vacuum, plus the role of interval ultrasonic vibration, resulting in the SEI film inside the battery under the condition of vacuum negative pressure is prone to deformation or rupture problem, and some solvents of the electrolyte of the battery are prone to volatilize under low pressure, high temperature, resulting in the amount of electrolyte in the battery after formation is less to affect the cycle performance of lithium ion battery.

[0083] Therefore, in one embodiment, when the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation are performed on the battery, pressure is also applied to the battery, especially in combination with the pressure at 3kgf / cm 2 ~ 8kgf / cm 2 adjustment to ensure that the battery is in a dynamic pressure state during the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation, so as to ensure that the battery 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, not only avoids the problem of deformation or rupture of the SEI film due to negative pressure, but also reduces the problem of easy volatilization of electrolyte solvent at high temperature, thereby ensuring that the amount of electrolyte in the battery after formation is more, and further ensuring the cycle performance of the lithium ion battery; also ensure that the ultrasonic vibration wave can be well distributed inside the battery, so that the micro-bubbles remaining in the SEI film will repeatedly expand and shrink under the action of continuous ultrasonic vibration, and the adjacent micro-bubbles will merge into larger bubbles, so as to accelerate the floating and exclusion of the bubbles, thereby realizing more comprehensive removal of the micro-bubbles remaining in the SEI film, effectively avoiding the problem that the ultrasonic vibration wave is difficult to vibrate under vacuum conditions and cannot realize comprehensive removal of the micro-bubbles remaining in the SEI film; At the same time, the micro-vibration of continuous ultrasonic vibration can shake off 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; At the same time, applying pressure to the battery is also beneficial to the close contact between the positive and negative plates, avoiding the lithium deposition caused by poor contact.

[0084] In one embodiment, when the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation are performed on the battery, the step of applying pressure to the battery includes the following specific steps: dynamically adjusting the pressure as the SOC rises, so that the pressure is at 3kgf / cm 2 ~ 8kgf / cm 2 , so as to facilitate the comprehensive escape of the micro-bubbles remaining in the SEI film, and avoid the problem of rupture or deformation of the SEI film caused by excessive pressure during the first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation.

[0085] In one embodiment, the pressure of the battery during the first charging operation is 2kgf / cm2 ~3kgf / cm 2 , the pressure of the battery cell in the second charging operation is 3kgf / cm 2 ~4kgf / cm 2 , the pressure of the battery cell in the third charging operation is 3kgf / cm 2 ~5kgf / cm 2 , the pressure of the battery cell in the continuous ultrasonic vibration operation is 5kgf / cm 2 ~8kgf / cm 2 , in this way, while achieving the comprehensive escape of the micro-bubbles remaining in the SEI film, the problem of the rupture or deformation of the SEI film due to excessive pressure during the continuous ultrasonic vibration operation is avoided, and the problem of the easy evaporation of the electrolyte solvent at a high temperature is reduced.

[0086] In one of the embodiments, in the step of baking the battery cell, the following specific steps are included: the wound battery cell is placed in a vacuum oven at 80-95℃ for 5-10 cycles of baking to obtain the water content of the battery cell as a whole < 150ppm, wherein the water content of the positive plate < 120ppm, and the water content of the negative plate + separator < 100ppm, which is conducive to the rapid infiltration of the subsequent electrolyte.

[0087] In one of the embodiments, the battery cell includes a positive plate, a negative plate and a separator, the positive plate, the separator and the negative plate are sequentially and woundly arranged, the positive plate is coated with a positive material, the negative plate is coated with a negative material, and the separator includes at least one of a double-sided oily PVDF separator, a single-sided oily PVDF separator, a double-sided aqueous PVDF separator and a single-sided aqueous PVDF separator.

[0088] It can be understood that, due to the PVDF adhesive coated on the double-sided oily PVDF separator, the single-sided oily PVDF separator, the double-sided aqueous PVDF separator and the single-sided aqueous PVDF separator, the heated PVDF adhesive can be more closely attached between the positive plate and the negative plate in the hot-pressing process, so as to well exclude the air between the gap of the separator and the positive and negative plates, which is conducive to the formation of the SEI film with good inner-outer layer continuity, no accumulation, good connectivity, uniformity, density, no micro-cracks and few lithium dendrites during the subsequent first charging operation, the second charging operation, the third charging operation and the continuous ultrasonic vibration operation.

[0089] The present disclosure also provides a lithium ion battery prepared by the formation process of the SEI film improved by the multi-physical field. Specifically, the lithium ion battery of the present disclosure is a high-temperature stable liquid electrolyte lithium ion battery, and the SEI film of the high-temperature stable liquid electrolyte lithium ion battery is improved by the multi-physical field to ensure the preparation of the SEI film with good inner-outer layer continuity, no accumulation, good connectivity, uniformity, density, no micro-cracks and few lithium dendrites.

[0090] As Figure 2 shown, in one embodiment, the high-temperature stable liquid electrolyte lithium ion battery is subjected to formation operation by using a multifunctional formation cabinet, the multifunctional formation cabinet comprises a cabinet body, a control operation module, an asymmetric pulse power 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 with the asymmetric pulse power module, the heating module, the pressurizing module and the ultrasonic generator, so as to realize intelligent control of the asymmetric pulse power module, the heating module, the pressurizing module and the ultrasonic generator, which is conducive to adjusting the parameters of the multifunctional formation cabinet by the control panel of the control operation module, so as to improve the formation efficiency; and since a placing cavity is formed in the cabinet body, the placing cavity is used for placing the high-temperature stable liquid electrolyte lithium ion battery, the heating module and the pressurizing module are arranged on the inner side wall of the cabinet body and the bottom of the cabinet body, so as to realize rapid heating and pressurizing operation of the high-temperature stable liquid electrolyte lithium ion battery; and since the ultrasonic generator is arranged at the bottom of the cabinet body, it is ensured that the ultrasonic generator at the bottom can provide comprehensive and continuous ultrasonic vibration for the placing cavity, which is helpful to quickly discharge the micro 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 body, so as to ensure that the multifunctional formation cabinet can quickly and comprehensively discharge the micro bubbles remaining in the SEI film of the high-temperature stable liquid electrolyte lithium ion battery under the action of the pressurizing module, which is helpful to prepare the SEI film with good continuity, no accumulation, good connectivity, uniformity, density, no micro cracks and few lithium dendrites.

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

[0092] 1) Since the first charging operation is performed on the battery by using low-temperature small current, the decomposition of the solvent in the electrolyte can be better inhibited under low-temperature conditions, so as to promote the inorganic components such as LiF and Li2CO3 in the electrolyte to preferentially form uniform and dense inorganic nucleation bodies on the negative electrode surface, which better compensates for the roughness of the negative electrode surface, and is conducive to reducing the current distribution difference of the negative electrode surface; when the second charging operation is performed on the battery after the first charging operation by using medium-temperature asymmetric pulse current, the current distribution difference of the negative electrode surface in the charging and discharging process is further reduced due to the asymmetric amplitude and width of the pulse of the asymmetric pulse current, so as to break the concentration polarization of lithium ions in the battery formation process; the generation of lithium dendrites in the inner layer of the SEI film is effectively inhibited, thereby ensuring the continuity of the inner and outer layers of the SEI film and effectively avoiding the problem that the SEI film is prone to form micro cracks or holes in the cycle process due to the existence of lithium dendrites in the inner layer of the SEI film.

[0093] 2) In addition, since the Li+ mobility of the battery cell is suitable at medium temperature, i.e. the Li+ mobility is faster than at low temperature and slower than at high temperature, it ensures that the organic polymer layer in the battery cell, such as ROCO2Li, can uniformly cover the inorganic nucleating body, which not only helps to form a continuous, uniform and dense inner layer of the SEI film, but also shortens the process of battery formation; then the battery cell after the second charging operation is subjected to a third charging operation with high temperature and large current; the local positive and negative electrode materials of the battery cell can generate Joule heat under high temperature conditions, and the local Joule heat can promote the recrystallization of the positive and negative electrode materials to repair the micro-cracks of the SEI film, thereby ensuring the formation of a dense, uniform and micro-crack-free SEI film; in addition, the continuous ultrasonic vibration operation is added in the third charging operation, on the one hand, the micro-vibration 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 micro-bubbles inside the SEI film inside the battery cell, i.e. the micro-bubbles inside the SEI film inside the battery cell will repeatedly expand and contract under the action of continuous ultrasonic vibration, so that adjacent micro-bubbles merge into larger bubbles to accelerate the floating and removal of the bubbles, thereby effectively removing the micro-bubbles remaining in the SEI film. In this way, the inner and outer layers of the SEI film formed in the formation process of the electrolyte interface film under the synergistic improvement of the multi-physical fields of temperature rise, external pressure, asymmetric pulse current and continuous ultrasonic vibration operation are continuous, have good connectivity, are uniform, dense and free of micro-cracks, thereby ensuring that the SEI film is not prone to form micro-cracks or holes during the cycle process of the lithium ion battery, and 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. If % is mentioned, it means percentage by weight. It should be noted that the following examples do not exhaust all possible cases, and the materials used in the following examples, unless otherwise specified, can be obtained from commercial channels.

[0096] Example 1

[0097] S1 battery cell water removal: the wound battery cell (the battery cell is obtained by winding the silicon-based negative electrode sheet, double-sided oil PVDF separator and positive electrode sheet in sequence) is placed in a 85℃ vacuum oven for 10 cycles to remove the water in the battery cell, and the water content of the battery cell is <150ppm, the water content of the positive electrode sheet is <120ppm, and the water content of the negative electrode sheet + separator is <100ppm;

[0098] S2 pre-infiltration: the roasted battery cell of S1 is placed in a vacuum cavity, and an electrolyte containing fluorocarbonate high-temperature stable solvent is added, then the battery cell after injection is subjected to 0.1 MPa-0.6 MPa alternating pressure, the number of times of applying alternating pressure is 2, after applying alternating pressure, the battery cell is placed at room temperature for 24 h;

[0099] S3 pre-pressing: the battery cell is pre-pressed at 85℃ under a pre-pressing pressure of 0.4 MPa for 30 min;

[0100] S4 formation stage: the battery cell after S3 is placed in a multifunctional formation cabinet while hot, then the parameters of the asymmetric pulse power module, the heating module, the pressurizing module and the ultrasonic generator are set through the control operation module, so that the battery cell enters the following steps respectively:

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

[0102] S42, the battery cell of S41 is charged at a constant current to 4.0 V by using asymmetric pulse current (forward pulse: 1 C-2 C, pulse width 5 s-30 s; reverse pulse: 0.1 C-0.2 C, pulse width 2 s-5 s) at a temperature rising rate of 0.5℃ / min to 40℃, and the dynamic pressure of the battery cell in the second charging operation is controlled to be 5 kgf / cm 2 ;

[0103] S43, the battery cell of S42 is charged to 4.2 V at a constant current of 0.5 C at a temperature rising rate of 0.5℃ / min to 65℃, and then charged to the cut-off voltage at a constant voltage with a small current of 0.02 C, and the dynamic pressure of the battery cell in the third charging operation is controlled to be 8 kgf / cm 2 ; immediately perform continuous ultrasonic vibration operation on the battery cell, the vibration frequency of the ultrasonic vibration is 25 kHz, and the battery cell is continuously charged for 30 min by using a current of 0.02 C, and the dynamic pressure of the battery cell in the ultrasonic vibration is controlled to be 8 kgf / cm 2 ;

[0104] Example 2

[0105] The difference from example 1 is that 40℃ in S42 is replaced by 60℃, and the rest remains unchanged.

[0106] Example 3

[0107] The difference from example 1 is that 25 kHz in S43 is replaced by 30 kHz, and the dynamic pressure of the battery cell in the ultrasonic vibration is 7 kgf / cm 2The rest is 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 of Comparative Example 1 is constant current charging to 3.7V at a current of 0.1C, then constant current charging to 3.9V at a current of 0.2C, followed by constant current constant voltage charging to 4.2V at a current of 0.5C, the cutoff current is 0.02C, the formation temperature is 45℃, and the rest is 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 is unchanged.

[0112] Comparative Example 3

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

[0116] Comparative Example 5

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

[0118] Comparative Example 6

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

[0120] Comparative Example 7

[0121] The difference from Example 1 is the step of S43, the dynamic pressure of the ultrasonic vibration of S43 in Example 1 is 8kgf / cm 2 replaced by 10kgf / cm 2 , and the rest is unchanged.

[0122] Comparative Example 8

[0123] The difference from Example 1 is in step S43, where 65°C in S43 of Example 1 is replaced with 70°C, while the rest remains the same.

[0124] Comparative Example 9

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

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

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

[0128] SEI film internal resistance detection method: The battery internal resistance is tested using an electrochemical workstation.

[0129] Electrolyte capacity testing method: Electrolyte injection amount; weigh the cell before injection and weigh the cell after injection, and subtract the cell weight before injection from the second weighing weight.

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

[0131] Calculation of capacity retention rate after 1000 cycles: (Termination capacity / Initial capacity) * 100%.

[0132] Table 1

[0133]

[0134]

[0135] As can be seen from Examples 1-3 and Comparative Examples 1-9 above, because Examples 1-3 adopted the combined effects of heating, external pressure, asymmetric pulsed current and continuous ultrasonic vibration, while reducing electrolyte evaporation, they also completely extracted the microbubbles inside the residual SEI film and removed the lithium dendrites on the surface of the SEI film. With the use of asymmetric pulsed current, it helps to form an SEI film with good continuity between the inner and outer layers, no accumulation, good connectivity, uniformity, density and no microcracks. Among them, Example 1 has the best comprehensive index.

[0136] The above-described embodiments are merely illustrative of several embodiments of the present disclosure, which are described in a more specific and detailed manner, but should not be construed as limiting the scope of the patent disclosure. It should be noted that for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present disclosure, and these all belong to the protection scope of the present disclosure. Therefore, the protection scope of the patent of the present disclosure should be subject to the appended claims.

Claims

1. A formation process for improving a solid electrolyte interface film in a multi-physical field, characterized by, The method comprises the following steps: obtaining the battery cell after liquid injection; performing first charging operation on the battery cell by using low-temperature small current; wherein the temperature of the low-temperature small current is 10-25℃, and the current is 0.1C-0.2C; the voltage during the first charging operation is 3.0V-3.7V; performing second charging operation on the battery cell after the first charging operation by using medium-temperature asymmetric pulse current; wherein the temperature of the medium-temperature asymmetric pulse current is 40-45℃, and the asymmetric pulse current is as follows: positive pulse: 1C-2C, pulse width 5-30s; negative pulse: 0.1C-0.2C, pulse width 2-5s; the voltage during the second charging operation is 3.8V-4.1V; performing third charging operation on the battery cell after the second charging operation by using high-temperature large current; wherein the temperature of the high-temperature large current is 60-65℃, and the current is 0.5C-1.0C; the voltage during the third charging operation is 4.2V-4.35V; performing continuous ultrasonic vibration operation on the battery cell to complete the formation process of the battery cell; In the step of performing continuous ultrasonic vibration operation on the battery cell, the following specific steps are included: after the voltage of the battery cell is charged to 4.2V-4.35V, continuously perform ultrasonic vibration on the battery cell, and charge the battery cell by using small current for 20-30min; the vibration frequency of the continuous ultrasonic vibration operation is 20-30kHz; The pressure of the first charging operation is 2 kgf / cm 2 ~3 kgf / cm 2 The pressure of the second charging operation is 3 kgf / cm 2 ~4 kgf / cm 2 The pressure of the third charging operation is 3 kgf / cm 2 ~5 kgf / cm 2 The pressure of the continuous ultrasonic vibration operation is 5 kgf / cm 2 ~8 kgf / cm 2 .

2. The process for co-improving the formation of the electrolyte interface film according to claim 1, wherein In the step of obtaining the battery cell after liquid injection, the following specific steps are included: baking the battery cell; performing pre-wetting operation on the battery cell after baking by using alternating pressure.

3. The multi-physics synergistic formation process of the solid electrolyte interphase film according to claim 2, wherein, The alternating pressure is 0.1-0.6MPa; and / or, the number of times of applying the alternating pressure is greater than or equal to 2.

4. The process for co-improving the formation of the electrolyte interface film according to claim 1, wherein After the step of obtaining the battery cell after liquid injection, and before the step of performing first charging operation on the battery cell by using low-temperature small current, the following steps are further included: standing the battery cell for 12-24h; pre-pressing the battery cell at high temperature of 70-85℃ for 20-30min.

5. A lithium-ion battery, characterized by The battery cell is prepared by using the formation process of improving electrolyte interface film by multi-physical field cooperation according to any one of claims 1-4.

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