A cathode lithium supplement, a lithium-ion battery, and a formation method for pre-lithiation of the cathode

Abstract

The present invention provides a cathode lithium supplement, a lithium-ion battery and a formation method for pre-lithiation of the cathode. By using a core-shell structure of Li2NiO2 coated on Li5FeO4 as the cathode lithium supplement and performing formation treatment on the pre-lithiated lithium-ion battery of the cathode by using a determined formation pressure, charge-discharge current and cut-off voltage, the polarization phenomenon of the lithium-ion battery during the formation process can be effectively reduced, thereby effectively improving the density and uniformity of the internal current of the lithium-ion battery during formation, the matching of temperature and pressure, avoiding the polarization and swelling phenomena generated during charge-discharge of the formation, making the generated SEI film denser, improving the integrity and stability of the SEI film, and improving the cycle performance and safety performance of the lithium-ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a cathode lithium supplement agent, a lithium ion battery and a formation method for pre-lithiation of the cathode thereof. Background Art

[0002] During the first charge and discharge process of a lithium ion battery, a large amount of active lithium is consumed to form a SEI film. The loss of active lithium during the initial charge and discharge process will significantly reduce the capacity and energy density of the lithium ion battery.

[0003] The relevant technical means to solve the above problems is to pre-lithiate the positive and negative electrodes of the lithium ion battery, and introduce active lithium into the material to supplement the loss of lithium ions during the cycle. Pre-lithiation can be divided into positive electrode pre-lithiation and negative electrode pre-lithiation. Negative electrode pre-lithiation mainly includes lithium foil pre-lithiation, lithium powder pre-lithiation, lithium silicide pre-lithiation and pre-lithiation with an electrolytic lithium salt solution. In the negative electrode pre-lithiation route, the lithium reagent has a high capacity, but the operation is complex; in addition, due to the relatively active lithium metal, the requirements for the storage and manufacturing environment are high, and the production cost is high. Compared with negative electrode pre-lithiation, positive electrode pre-lithiation is more convenient. Typical positive electrode pre-lithiation is to add a small amount of high-capacity material during the mixing process of the positive electrode slurry. During charging, lithium is separated from the high-capacity material to supplement the irreversible lithium loss caused by the first charge and discharge. Positive electrode pre-lithiation has received extensive attention in the lithium battery industry due to its high safety and stability and simple production process.

[0004] For a conventional lithium iron phosphate battery cell, after adding a pre-lithiation additive, the active lithium consumed to form the SEI film during the first charge and discharge can be supplemented, and the capacity, energy density and cycle performance of the lithium ion battery can all be improved to a certain extent. However, as a key process for activating the lithium ion battery, the formation process has a great influence on the capacity performance and cycle ability improvement of the lithium ion battery in the later stage. The traditional formation process is difficult to exert its pre-lithiation effect, so that after the lithium ion battery is pre-lithiated, the specific capacity performance is low and the first efficiency improvement is not obvious. Summary of the Invention

[0005] Embodiments of the present invention provide a cathode lithium supplement agent, a lithium ion battery and a formation method for pre-lithiation of the cathode thereof, which can improve the integrity and stability of the SEI film, and improve the cycle performance and safety performance of the lithium ion battery.

[0006] In a first aspect, embodiments of the present invention provide a cathode lithium supplement agent, the cathode lithium supplement agent having a core-shell structure, the core of the core-shell structure including Li5FeO4, and the shell including Li2NiO2.

[0007] In a second aspect, embodiments of the present invention provide a formation method for pre-lithiation of a lithium ion battery cathode, the formation method comprising:

[0008] Prepare a positive electrode pre-lithiated lithium-ion battery; wherein, the positive electrode pre-lithiated lithium-ion battery includes the positive electrode lithium supplement agent described in the embodiments of the present invention;

[0009] Perform formation treatment on the positive electrode pre-lithiated lithium-ion battery using a predetermined formation pressure, formation current, and cut-off voltage; wherein, the formation pressure is a constant value within the range of 0.1 MPa - 0.3 MPa, the range of the formation current is 0.01C - 0.5C, and the range of the cut-off voltage is 2.0V - 4.5V.

[0010] In one embodiment, the step of performing formation treatment on the positive electrode pre-lithiated lithium-ion battery using a predetermined formation pressure, formation current, and cut-off voltage includes:

[0011] In the first stage, charge the positive electrode pre-lithiated lithium-ion battery using a first predetermined current, the charge amount is the first charge amount, and the cut-off voltage is the first cut-off voltage;

[0012] In the second stage, charge the positive electrode pre-lithiated lithium-ion battery using a second predetermined current, the charge amount is the second charge amount, and the cut-off voltage is the second cut-off voltage;

[0013] In the third stage, charge the positive electrode pre-lithiated lithium-ion battery using a third predetermined current, the charge amount is the third charge amount, and the cut-off voltage is the third cut-off voltage;

[0014] In the fourth stage, discharge the positive electrode pre-lithiated lithium-ion battery using a fourth predetermined current, the discharge amount is the fourth discharge capacity, and the cut-off voltage is the fourth cut-off voltage;

[0015] In the fifth stage, charge the positive electrode pre-lithiated lithium-ion battery using a fifth predetermined current, the charge amount is the fifth charge amount, and the cut-off voltage is the fifth cut-off voltage;

[0016] Wherein, the first predetermined current is greater than or equal to the third predetermined current, and less than or equal to the second predetermined current, the fourth predetermined current, and the fifth predetermined current; the first charge amount is equal to the fifth charge amount, and less than the second charge amount, the fourth discharge amount is greater than the second charge amount and less than or equal to the design capacity of the positive electrode pre-lithiated lithium-ion battery, the third charge amount is greater than the design capacity of the positive electrode pre-lithiated lithium-ion battery; the first cut-off voltage, the second cut-off voltage, and the fifth cut-off voltage are equal, the fourth cut-off voltage is less than the first cut-off voltage, and the third cut-off voltage is greater than the first cut-off voltage.

[0017] In one embodiment, the step of performing formation treatment on the positive electrode pre-lithiated lithium-ion battery using a predetermined formation pressure, formation current, and cut-off voltage includes:

[0018] In the first stage, the pre-lithiated lithium-ion battery with a positive electrode is charged with a current of 0.1C - 0.2C, the charging time is controlled to be 1.5 hours - 2 hours, the charging amount is 30% of the designed capacity of the pre-lithiated lithium-ion battery with a positive electrode, and the cut-off voltage is 3.65V;

[0019] In the second stage, the pre-lithiated lithium-ion battery with a positive electrode is charged with a current of 0.2C - 0.5C, the charging time is controlled to be 1.5 hours - 3.5 hours, the charging amount is 70% of the designed capacity of the pre-lithiated lithium-ion battery with a positive electrode, and the cut-off voltage is 3.65V;

[0020] In the third stage, the pre-lithiated lithium-ion battery with a positive electrode is charged with a current of 0.01C - 0.1C, the charging time is controlled to be 1.5 hours - 2 hours, the charging amount is 110% - 120% of the designed capacity of the pre-lithiated lithium-ion battery with a positive electrode, and the cut-off voltage is 4.3V;

[0021] In the fourth stage, the pre-lithiated lithium-ion battery with a positive electrode is discharged with a current of 0.2C - 0.5C, the discharging time is controlled to be 2 hours - 5 hours, the discharging amount is 100% of the designed capacity of the pre-lithiated lithium-ion battery with a positive electrode, and the cut-off voltage is 2.5V or 2.0V;

[0022] In the fifth stage, the pre-lithiated lithium-ion battery with a positive electrode is charged with a current of 0.2C - 0.5C, the charging time is controlled to be 0.6 hours - 1.5 hours, the charging amount is 30% of the designed capacity of the pre-lithiated lithium-ion battery with a positive electrode, and the cut-off voltage is 3.65V.

[0023] In one embodiment, after the formation treatment, the formation method further includes:

[0024] The pre-lithiated lithium-ion battery with a positive electrode after formation is aged, degassed, and secondarily packaged; wherein, the temperature range for aging is 30°C - 60°C, the time range is 24 hours - 48 hours, and the vacuum degree for degassing and secondary packaging is less than -90 kPa.

[0025] In one embodiment, after the secondary packaging, the formation method further includes:

[0026] The pre-lithiated lithium-ion battery with a positive electrode is charged and discharged to complete capacitance grading; wherein, the charging process is constant current and constant voltage charging, the current in the constant current stage is 0.05C - 0.2C, the cut-off current is 0.01C, and the current in the discharging process is 0.2C - 0.5C.

[0027] In one embodiment, after the preparation of the pre-lithiated lithium-ion battery with a positive electrode and before the formation treatment, the formation method further includes:

[0028] The pre-lithiated positive electrode lithium-ion battery is lithiated for 8 to 15 hours under the conditions of a temperature of 45°C to 60°C and a pressure of 0.03 MPa to 0.6 MPa.

[0029] In one embodiment, after preparing the pre-lithiated positive electrode lithium-ion battery and before the lithiation treatment, the formation method further includes:

[0030] Inject electrolyte and vacuum seal the pre-lithiated positive electrode lithium-ion battery, and then let it stand; wherein, the ambient temperature for standing is 25°C to 60°C, and the standing time range is 24 hours to 72 hours.

[0031] In one embodiment, the steps for preparing the pre-lithiated positive electrode lithium-ion battery include:

[0032] Mix lithium iron phosphate with a mass ratio of 90% - 98%, a binder with a mass ratio of 1% - 2%, a conductive agent with a mass ratio of 0.5% - 4%, a dispersant with a mass ratio of 0.2% - 0.8%, and a lithium supplement agent with a mass ratio of 1% - 4.5% uniformly, and add a solvent to prepare a positive electrode slurry.

[0033] Coat the positive electrode slurry evenly on aluminum foil, dry it, then cold press and die cut and slit it to prepare a positive electrode sheet.

[0034] In a third aspect, an embodiment of the present invention provides a lithium-ion battery, which is prepared by using the formation method described in any one of the embodiments of the present invention.

[0035] Beneficial effects of the embodiments of the present invention:

[0036] In the embodiments of the present invention, by using a core-shell structure of Li2NiO2 coating Li5FeO4 as the positive electrode lithium supplement agent, and by using a determined formation pressure, charge-discharge current, and cut-off voltage to perform formation treatment on the pre-lithiated positive electrode lithium-ion battery, it is possible to effectively reduce the polarization phenomenon of the lithium-ion battery during the formation process, thereby effectively improving the density and uniformity of the internal current of the lithium-ion battery during formation, the matching of temperature and pressure, avoiding polarization and bulging phenomena during charge and discharge of formation, making the generated SEI film denser, improving the integrity and stability of the SEI film, and improving the cycle performance and safety performance of the lithium-ion battery. Description of the Drawings

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for description in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those skilled in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0038] Figure 1 is a flowchart of the formation method for pre-lithiation of the positive electrode of a lithium-ion battery provided by an embodiment of the present invention;

[0039] Figure 2 is a flowchart of the preparation stage of the formation method provided by an embodiment of the present invention

[0040] Figure 3 is a flowchart of the formation stage of the formation method provided by an embodiment of the present invention. Specific Embodiments

[0041] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present invention. In addition, it should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention. In the present invention, unless otherwise stated, the orientation terms such as "upper" and "lower" usually refer to the upper and lower in the actual use or working state of the device, specifically the drawing direction in the drawings; and "inner" and "outer" refer to the outline of the device.

[0042] In the related art, the main materials of the pre-lithiation additive for the positive electrode of a lithium-ion battery include lithium-rich compounds (Li2NiO2, Li5FeO4, Li6CoO4), organolithium compounds (Li2OHBM, Li2C2O4), and binary lithium compounds (Li3N, Li2S, Li2O2, Li2O). When pre-lithiating the positive electrode of a lithium-ion battery, usually one of the above compounds is selected as the lithiating agent, which easily causes problems such as polarization and bulging of the lithium-ion battery during the formation charge and discharge process.

[0043] An embodiment of the present invention provides a lithium supplement agent for the positive electrode of a lithium-ion battery. The lithium supplement agent has a core-shell structure, wherein the core includes Li5FeO4 and the shell includes Li2NiO2.

[0044] The cathode lithium supplement agent with a core-shell structure provided by the embodiments of the present invention coats Li5FeO4 with Li2NiO2, avoiding the contact of Li5FeO4 with air, enhancing the environmental stability of Li5FeO4, and enabling the capacity of Li5FeO4 to be maximally exerted. On the other hand, Li2NiO2 as the shell can also provide active lithium, further improving the lithium supplement effect of the cathode lithium supplement agent and further enhancing the energy density and cycle stability of the lithium-ion battery. In addition, the core-shell structure reduces the residual alkali amount on the material surface, avoids the occurrence of agglomeration in the cathode slurry, and improves the safety performance of the lithium-ion battery.

[0045] Meanwhile, the embodiments of the present invention also provide a formation method for pre-lithiation of the lithium-ion battery cathode, using the cathode lithium supplement agent provided by the embodiments of the present invention for pre-lithiation and formation of the lithium-ion battery.

[0046] Please refer to Figure 1 , and the formation method includes the following steps B1 and B2.

[0047] Step B1: Prepare a pre-lithiated lithium-ion battery cathode.

[0048] It should be noted that step B1 is not a formation step in the strict sense. The embodiments of the present invention classify it under the formation method to illustrate that the formation method described in the embodiments of the present invention utilizes the cathode lithium agent described in the embodiments of the present invention, and the addition of the cathode lithium agent belongs to the step content of preparing a pre-lithiated lithium-ion battery cathode.

[0049] Please refer to Figure 2 , and the specific steps for preparing a pre-lithiated lithium-ion battery cathode in step B1 include:

[0050] Step B11: Mix lithium iron phosphate (LFP) with a mass ratio of 90% - 96% (such as 90%, 91%, 92%, 93%, 94%, 95%, 96%), a binder with a mass ratio of 1% - 2% (such as 1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2%), a conductive agent with a mass ratio of 0.5% - 4% (such as 0.5%, 1%, 2%, 3%, 4%), a dispersant with a mass ratio of 0.2% - 0.8% (such as 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%), and a lithium supplement agent with a mass ratio of 1% - 4.5% (such as 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%) evenly, and add a solvent to prepare a positive electrode slurry; coat the positive electrode slurry evenly on aluminum foil, dry it, then cold press and die cut it into strips to prepare a positive electrode plate. In this step, the lithium supplement agent is the positive electrode lithium supplement agent provided in the embodiments of the present invention; the instrument and equipment used for evenly mixing each substance component is a double planetary mixer; during the coating process of the positive electrode slurry, it is necessary to control the discharge viscosity and solid content of the positive electrode slurry to avoid problems such as agglomeration or uneven coating of the positive electrode slurry.

[0051] Step B12: Mix graphite with a mass ratio of 90% - 98% (such as 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%), a conductive agent with a mass ratio of 0.5% - 2% (such as 0.5%, 1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 2%), and a binder with a mass ratio of 2.5% - 4% (such as 2.5%, 3%, 3.1%, 3.3%, 3.4%, 3.6%, 3.8%, 4%) evenly, and add deionized water to prepare a negative electrode slurry; coat the negative electrode slurry evenly on copper foil, dry it, then cold press and die cut it into strips to prepare a negative electrode plate. In this step, the instrument and equipment used for evenly mixing each substance component is a double planetary mixer; during the coating process of the negative electrode slurry, it is also necessary to control the discharge viscosity and solid content of the negative electrode slurry to avoid problems such as agglomeration or uneven coating of the negative electrode slurry.

[0052] Step B13: Wind the positive electrode plate, separator, and negative electrode plate under a certain pressure to prepare an electric core, and place the electric core in an aluminum shell. Among them, the separator is located between the positive electrode plate and the negative electrode plate, and the separator is made of a polymer material with good air permeability, such as polypropylene (PP) or polyethylene (PE).

[0053] Step B14: Prepare a predetermined amount of electrolyte solution. Among them, the electrolyte solution uses 1mol / L LiPF6 as the lithium salt and EC / DC / EMC (volume ratio 1:1:1) as the basic solvent.

[0054] Step B15: Inject the electrolyte into the battery cell under negative pressure, and encapsulate it after standing fully.

[0055] Step B2: Perform formation on the pre-lithiated positive electrode lithium-ion battery.

[0056] Please refer to Figure 3 , the steps for performing formation on the pre-lithiated positive electrode lithium-ion battery specifically include:

[0057] Step B21: Inject liquid and perform vacuum sealing on the encapsulated pre-lithiated positive electrode lithium-ion battery, and then set it aside. Herein, the setting aside specifically means: Set aside the pre-lithiated positive electrode lithium-ion battery in an environment with a temperature of 25°C - 60°C (such as 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C) for 24 hours - 72 hours (such as 24h, 30h, 36h, 42h, 48h, 54h, 60h, 66h, 72h).

[0058] Step B22: Perform lithiation treatment on the pre-lithiated positive electrode lithium-ion battery under the conditions of a temperature of 45°C - 60°C (such as 45°C, 48°C, 50°C, 52°C, 55°C, 58°C, 60°C) and a pressure of 0.03 MPa - 0.6 MPa (such as 0.03 MPa, 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa) for 8 hours - 15 hours (such as 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h). It should be noted that pressure is always applied to the pre-lithiated positive electrode lithium-ion battery during the lithiation treatment, specifically: Use a clamping member to apply pressure to the pre-lithiated positive electrode lithium-ion battery, and the pressure is maintained between 0.03 MPa - 0.6 MPa (such as 0.03 MPa, 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa).

[0059] Step B23: Perform formation treatment on the pre-lithiated positive electrode lithium-ion battery with a formation pressure of 0.1 MPa - 0.3 MPa (such as 0.1 MPa, 0.15 MPa, 0.2 MPa, 0.25 MPa, 0.3 MPa) and a formation current of 0.01C - 0.5C (such as 0.01C, 0.1C, 0.2C, 0.3C, 0.4C, 0.5C). Herein, the formation pressure is a constant value, and the formation charge is 10% - 120% (such as 10%, 20%, 30%, 50%, 80%, 100%, 120%) of the designed capacity of the lithium-ion battery.

[0060] The specific steps of the formation treatment are as follows: In the first stage, the pre-lithiated lithium-ion battery of the positive electrode is charged with a current of 0.1C - 0.2C (such as 0.1C, 0.12C, 0.14C, 0.16C, 0.18C, 0.2C), the charging time is controlled to be 1.5 hours - 2 hours (such as 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h), the charging amount is 30% of the designed capacity of the lithium-ion battery, and the cut-off voltage is 3.65V; In the second stage, the pre-lithiated lithium-ion battery of the positive electrode is charged with a current of 0.2C - 0.5C (such as 0.2C, 0.3C, 0.4C, 0.5C), the charging time is controlled to be 1.5 hours - 3.5 hours (such as 1.5h, 2h, 2.5h, 3h, 3.5h), the charging amount is 70% of the designed capacity of the lithium-ion battery, and the cut-off voltage is 3.65V; In the third stage, the pre-lithiated lithium-ion battery of the positive electrode is charged with a current of 0.01C - 0.1C (such as 0.01C, 0.02C, 0.04C, 0.06C, 0.08C, 0.1C), the charging time is controlled to be 1.5 hours - 2 hours (such as 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h), the charging amount is 110% - 120% of the designed capacity of the lithium-ion battery (such as 110%, 112%, 114%, 116%, 118%, 120%), and the cut-off voltage is 4.3V; In the fourth stage, the pre-lithiated lithium-ion battery of the positive electrode is discharged with a current of 0.2C - 0.5C (such as 0.2C, 0.3C, 0.4C, 5C), the discharge time is controlled to be 2 hours - 5 hours (such as 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h), the discharge amount is 100% of the designed capacity of the lithium-ion battery, and the cut-off voltage is 2.5V or 2.0V (such as 2.0V, 2.1V, 2.2V, 2.3V, 2.4V, 2.5V); In the fifth stage, the pre-lithiated lithium-ion battery of the positive electrode is charged with a current of 0.2C - 0.5C (such as 0.2C, 0.3C, 0.4C, 5C), the charging time is controlled to be 0.6 hours - 1.5 hours (such as 0.6h, 0.8h, 1h, 1.2h, 1.4h, 1.5h), the charging amount is 30% of the designed capacity of the lithium-ion battery, and the cut-off voltage is 3.65V.

[0061] In the first and second stages, the pre-lithiated lithium-ion battery with the positive electrode is charged and discharged at a high rate of 0.1C - 0.5C using 30% - 100% of the designed capacity of the lithium-ion battery, which is beneficial to the formation of a stable SEI film, promotes the transmission of lithium ions, and ensures good contact at the electrode / electrolyte interface. In the third stage, with a small current of 0.01C - 0.1C and 110% - 120% of the designed capacity of the lithium-ion battery, the pre-lithiated lithium-ion battery with the positive electrode is fully charged to 4.3V, which can fully release the active lithium in the positive electrode pre-lithiation reagent. A part of the active lithium is embedded in the graphite negative electrode to improve the cycle performance of the lithium-ion battery, and the other part of the active lithium can increase the first discharge specific capacity; meanwhile, this process can make the active components in the positive electrode pre-lithiation reagent generate more oxygen, reduce the risk of the binder deactivation caused by gas production, resulting in pole piece powdering, and reduce the impact of the gas generated during storage on the safety performance of the lithium-ion battery, thus improving the overall stability of the lithium-ion battery. In the fourth and fifth stages, high-rate charging and discharging at 0.2C - 0.5C are carried out, which improves the ion transmission rate, effectively shortens the formation time, is easy to achieve large-scale production in factories, and improves economic benefits.

[0062] Step B24: Aging, degassing, and secondary encapsulation are performed on the pre-lithiated lithium-ion battery with the positive electrode after formation. Among them, the specific process of the aging is: the pre-lithiated lithium-ion battery with the positive electrode is aged for 24 hours - 48 hours (such as 24h, 30h, 36h, 42h, 48h) in an environment with a temperature of 30°C - 60°C (such as 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C); both the degassing and the secondary encapsulation are carried out under the condition that the vacuum degree is less than -90 kPa.

[0063] Performing long-term aging and static placement on the pre-lithiated lithium-ion battery with the positive electrode after formation is beneficial to promoting the full gas production of the active components in the pre-lithiated lithium-ion battery with the positive electrode, ensuring good contact at the electrode / electrolyte interface, and promoting the transmission of lithium ions; reducing the risk of the SEI film being damaged by subsequent storage gas production, so that both the rate performance and the cycle performance of the pre-lithiated lithium-ion battery with the positive electrode can be improved to a certain extent.

[0064] Step B25: Charging and discharging are performed on the pre-lithiated lithium-ion battery with the positive electrode to complete the capacity grading. Among them, the specific process of the capacity grading is: the charging process is constant current and constant voltage charging. The current in the constant current stage is 0.05C - 0.2C (such as 0.05C, 0.08C, 0.1C, 0.12C, 0.15C, 0.18C, 0.2C). After constant current charging to the cut-off voltage, it is switched to constant voltage charging, and the cut-off current is 0.01C; the discharging process is discharging at 0.2C - 0.5C (such as 0.2C, 0.25C, 0.3C, 0.35C, 0.4C, 0.45C, 0.5C) to the cut-off voltage.

[0065] In this embodiment, by using a determined formation pressure, charge-discharge current, and cut-off voltage to perform formation treatment on the positive pre-lithiated lithium-ion battery, the polarization phenomenon of the lithium-ion battery during formation can be effectively reduced, thereby effectively improving the density and uniformity of the internal current of the lithium-ion battery during formation, as well as the matching of temperature and pressure, avoiding polarization and bulging phenomena during formation charge-discharge, and making the generated SEI film denser. Since a determined formation pressure is adopted, it can prevent the gas in the airbag from infiltrating into the battery cell when the pressure is too small and excessive gas is generated at high temperature; it can avoid damage to the lithium-ion battery caused by the traditional continuous large pressure. Generally speaking, by using the formation method described in the embodiment of the present invention, the formation time can be shortened, the integrity and stability of the SEI film can be improved, and the cycle performance and safety performance of the lithium-ion battery can be improved.

[0066] Embodiment

[0067] Use a double planetary mixer to uniformly mix lithium iron phosphate (LFP) with a mass ratio of 95.2%, polyvinylidene fluoride (PVDF) with a mass ratio of 1.7%, conductive carbon black (SP) with a mass ratio of 0.9%, YT dispersant with a mass ratio of 0.2%, and the lithium supplement agent with a mass ratio of 2%, and add NMP solvent to prepare a positive electrode slurry; uniformly coat the positive electrode slurry on the aluminum foil, dry it, cold press it, and die-cut and slit it to prepare a positive electrode plate.

[0068] Use a double planetary mixer to uniformly mix graphite with a mass ratio of 96.7%, carbon nanotubes (CNT) with a mass ratio of 0.6%, and styrene-butadiene rubber (SBR) with a mass ratio of 2.7%, and add deionized water to prepare a negative electrode slurry; uniformly coat the negative electrode slurry on the copper foil, dry it, cold press it, and die-cut and slit it to prepare a negative electrode plate.

[0069] Wind the positive electrode plate, separator, and the negative electrode plate under a certain pressure to prepare a battery cell, and place the battery cell in an aluminum shell.

[0070] Prepare an electrolyte solution with 1mol / L LiPF6 as the lithium salt and EC / DC / EMC (volume ratio 1:1:1) as the base solvent.

[0071] Under negative pressure conditions, inject the electrolyte into the battery cell, and perform encapsulation after sufficient rest.

[0072] Perform liquid injection and vacuum sealing on the encapsulated positive pre-lithiated lithium-ion battery, and leave it in an environment at 45°C for 48h.

[0073] Lithiate the positive pre-lithiated lithium-ion battery at a temperature of 45°C and a pressure of 0.3MPa for 8h.

[0074] Charge the pre-lithiated lithium-ion battery of the positive electrode with a current of 0.2C, control the charging time to be 1.5 hours, the charging amount to be 30% of the designed capacity of the lithium-ion battery, and the cut-off voltage to be 3.65V; in the second stage, charge the pre-lithiated lithium-ion battery of the positive electrode with a current of 0.5C, control the charging time to be 3.5 hours, the charging amount to be 70% of the designed capacity of the lithium-ion battery, and the cut-off voltage to be 3.65V; in the third stage, charge the pre-lithiated lithium-ion battery of the positive electrode with a current of 0.1C, control the charging time to be 2 hours, the charging amount to be 120% of the designed capacity of the lithium-ion battery, and the cut-off voltage to be 4.3V; in the fourth stage, discharge the pre-lithiated lithium-ion battery of the positive electrode with a current of 0.5C, control the discharge time to be 2 hours, the discharge amount to be 100% of the designed capacity of the lithium-ion battery, and the cut-off voltage to be 2.5V or 2.0V; in the fifth stage, charge the pre-lithiated lithium-ion battery of the positive electrode with a current of 0.5C, control the charging time to be 0.6 hours, the charging amount to be 30% of the designed capacity of the lithium-ion battery, and the cut-off voltage to be 3.65V.

[0075] Age the pre-lithiated lithium-ion battery of the positive electrode in an environment with a temperature of 45°C for 48 hours; perform degassing and secondary encapsulation under the condition that the vacuum degree is less than -90 kPa.

[0076] Perform charge and discharge on the pre-lithiated lithium-ion battery of the positive electrode. The charging process is constant current and constant voltage charging. The current in the constant current stage is 0.2C. After charging to the cut-off voltage in the constant current charging, it turns to constant voltage charging, and the cut-off current is 0.01C; the discharging process is discharging at 0.5C to the cut-off voltage.

[0077] Reference example

[0078] The formation steps in the reference example are basically the same as those in the above-mentioned embodiment, except for step B11. In the reference example, lithium iron phosphate (LFP) with a mass ratio of 97.2%, polyvinylidene fluoride (PVDF) with a mass ratio of 1.7%, conductive carbon black (SP) with a mass ratio of 0.9%, and YT dispersant with a mass ratio of 0.2% are mixed evenly and a solvent is added to prepare a positive electrode slurry; the positive electrode slurry is evenly coated on aluminum foil, dried, cold-pressed, die-cut and slit to prepare a positive electrode plate. That is, the positive electrode lithium supplement agent described in the embodiment of the present invention is not used in the reference example.

[0079] Correspondingly, the embodiment of the present invention also provides a pre-lithiated lithium-ion battery, and the pre-lithiated lithium-ion battery is prepared by using the formation method described in the embodiment of the present invention.

[0080] The lithium-ion batteries prepared in the above examples and reference examples were subjected to electrochemical tests to obtain the electrochemical parameters shown in Table 1 and Table 2. Among them, Table 1 shows the comparison data of the charge and discharge gram capacities of the lithium-ion batteries in the examples and reference examples, and Table 2 shows the comparison data of the internal resistances and cycle performances of the two lithium-ion batteries. Among them, the internal resistance was measured using a lithium-ion battery internal resistance tester. The test method for the capacity retention rate was to place the lithium-ion battery under the corresponding temperature conditions, conduct charge and discharge cycle tests on it, and compare the capacity retention rates of the two lithium-ion batteries when cycling to 100 weeks / 500 weeks / 2000 weeks. Among them, charging: 0.33C - 0.5C (such as 0.33C, 0.35C, 0.38C, 0.4C, 0.43C, 0.45C, 0.48C, 0.5C) current, the charging current corresponding to Table 1 and Table 2 was 0.5C, constant current and constant voltage charging to 3.65V, and the cut-off current was 0.05C; discharging: 0.33C - 0.5C (such as 0.33C, 0.35C, 0.38C, 0.4C, 0.43C, 0.45C, 0.48C, 0.5C) current, the discharging current corresponding to Table 1 and Table 2 was 0.5C, constant current discharging to 2.5V.

[0081] Table 1

[0082] Example - Capacity per gram (mAh / g) Reference Example - Capacity per gram (mAh / g) First charge to 3.65 V 152 152 First charge to 4.3 V 168 185 First discharge to 2.5 V / 2.0 V 151 / 151 155 / 159

[0083] As can be seen from Table 1, compared with the conventional lithium iron phosphate battery prepared using the above reference example, the positive electrode pre-lithiated lithium-ion battery prepared using the formation method described in the embodiments of the present invention has the following excellent charge and discharge gram capacities: the full charge gram capacity is increased by about 5 mAh / g - 17 mAh / g, the gram capacity when discharging to 2.5V is increased by 2 mAh / g - 5 mAh / g, and the gram capacity when discharging to 2.0V is increased by 7 mAh / g - 9 mAh / g. This is mainly because: a high-capacity core-shell structured positive electrode lithiating agent is added to the positive electrode sheet. When charging to 4.3V with a small current in the third stage of formation, lithium is easily separated from the positive electrode lithiating agent material, supplementing the irreversible capacity loss caused by the first charge. On the other hand, it will also increase the discharge gram capacity of the positive electrode pre-lithiated lithium-ion battery and improve the Coulomb efficiency of the positive electrode pre-lithiated lithium-ion battery.

[0084] Table 2

[0085] Example Reference Example Internal resistance / mΩ 14 8 Capacity retention rate after 100 cycles / % 99 100 Capacity retention rate after 500 cycles / % 96 99 Capacity retention rate after 2000 cycles / % 91 97

[0086] As can be seen from Table 2, compared with the conventional lithium iron phosphate battery prepared by the above reference example, the internal resistance of the pre-lithiated lithium-ion battery prepared by the formation method described in the embodiment of the present invention is reduced by about 40%, and the cycle performance is also significantly improved, especially the capacity retention rate after long cycling is significantly improved. This is mainly because: after the pre-lithiated lithium-ion battery is charged to 4.3V with a small current, it can promote a part of the active lithium to embed into the negative electrode of the lithium-ion battery, supplement the irreversible active lithium consumed to form the SEI film, and improve the rate and cycle performance of the battery cell to a certain extent.

[0087] In the embodiment of the present invention, a method of charging to 4.3V with a small current is used to perform formation treatment on the pre-lithiated lithium-ion battery, so as to maximize the performance of the pre-lithiated lithium-ion battery, and significantly improve the Coulomb efficiency of the pre-lithiated lithium-ion battery and the cycle ability of the lithium-ion battery.

[0088] The embodiments of the present invention have been introduced in detail above. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those skilled in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A formation method for pre-lithiation of the positive electrode of a lithium-ion battery, characterized in that, Including: Preparing a positive electrode pre-lithiated lithium-ion battery; wherein, the positive electrode pre-lithiated lithium-ion battery includes a positive electrode lithium supplement agent, the positive electrode lithium supplement agent has a core-shell structure, the core of the core-shell structure includes Li5FeO4, and the shell includes Li2NiO2; Performing formation treatment on the positive electrode pre-lithiated lithium-ion battery using a predetermined formation pressure, formation current, and cut-off voltage; wherein, the formation pressure is a constant value within the range of 0.1 MPa - 0.3 MPa, the range of the formation current is 0.01C - 0.5C, and the range of the cut-off voltage is 2.0V - 4.5V; The step of performing formation treatment on the positive electrode pre-lithiated lithium-ion battery using a predetermined formation pressure, formation current, and cut-off voltage includes: In the first stage, charging the positive electrode pre-lithiated lithium-ion battery using a first predetermined current, the charge amount is the first charge amount, and the cut-off voltage is the first cut-off voltage; In the second stage, charging the positive electrode pre-lithiated lithium-ion battery using a second predetermined current, the charge amount is the second charge amount, and the cut-off voltage is the second cut-off voltage; In the third stage, charging the positive electrode pre-lithiated lithium-ion battery using a third predetermined current, the charge amount is the third charge amount, and the cut-off voltage is the third cut-off voltage; In the fourth stage, discharging the positive electrode pre-lithiated lithium-ion battery using a fourth predetermined current, the discharge amount is the fourth discharge amount, and the cut-off voltage is the fourth cut-off voltage; In the fifth stage, charging the positive electrode pre-lithiated lithium-ion battery using a fifth predetermined current, the charge amount is the fifth charge amount, and the cut-off voltage is the fifth cut-off voltage; Wherein, the first predetermined current is greater than or equal to the third predetermined current and less than or equal to the second predetermined current, the fourth predetermined current, and the fifth predetermined current; the first charge amount is equal to the fifth charge amount and less than the second charge amount, the fourth discharge amount is greater than the second charge amount and less than or equal to the design capacity of the positive electrode pre-lithiated lithium-ion battery, and the third charge amount is greater than the design capacity of the positive electrode pre-lithiated lithium-ion battery; the first cut-off voltage, the second cut-off voltage, and the fifth cut-off voltage are equal, the fourth cut-off voltage is less than the first cut-off voltage, and the third cut-off voltage is greater than the first cut-off voltage; The first predetermined current is 0.1C - 0.2C, the second predetermined current is 0.2C - 0.5C, the third predetermined current is 0.01C - 0.1C, the fourth predetermined current is 0.2C - 0.5C, and the fifth predetermined current is 0.2C - 0.5C; The first charge amount is 30% - 100% of the design capacity of the positive electrode pre-lithiated lithium-ion battery, the second charge amount is 30% - 100% of the design capacity of the positive electrode pre-lithiated lithium-ion battery, the third charge amount is 110% - 120% of the design capacity of the positive electrode pre-lithiated lithium-ion battery, the fourth discharge amount is 100% of the design capacity of the positive electrode pre-lithiated lithium-ion battery, and the fifth charge amount is 30% - 100% of the design capacity of the positive electrode pre-lithiated lithium-ion battery; The first cut-off voltage, the second cut-off voltage, and the fifth cut-off voltage are 3.65 V, the third cut-off voltage is 4.2 V, and the fourth cut-off voltage is 2.5 V or 2.0 V.

2. The formation method according to claim 1, characterized in that, The step of forming the pre-lithiated lithium-ion battery of the positive electrode by using a predetermined forming pressure, forming current, and cut-off voltage includes: In the first stage, the pre-lithiated lithium-ion battery of the positive electrode is charged with a current of 0.1C - 0.2C, the charging time is controlled to be 1.5 hours - 2 hours, the charging amount is 30% of the designed capacity of the pre-lithiated lithium-ion battery of the positive electrode, and the cut-off voltage is 3.65 V; In the second stage, the pre-lithiated lithium-ion battery of the positive electrode is charged with a current of 0.2C - 0.5C, the charging time is controlled to be 1.5 hours - 3.5 hours, the charging amount is 70% of the designed capacity of the pre-lithiated lithium-ion battery of the positive electrode, and the cut-off voltage is 3.65 V; In the third stage, the pre-lithiated lithium-ion battery of the positive electrode is charged with a current of 0.01C - 0.1C, the charging time is controlled to be 1.5 hours - 2 hours, the charging amount is 110% - 120% of the designed capacity of the pre-lithiated lithium-ion battery of the positive electrode, and the cut-off voltage is 4.3 V; In the fourth stage, the pre-lithiated lithium-ion battery of the positive electrode is discharged with a current of 0.2C - 0.5C, the discharge time is controlled to be 2 hours - 5 hours, the discharge amount is 100% of the designed capacity of the pre-lithiated lithium-ion battery of the positive electrode, and the cut-off voltage is 2.5 V or 2.0 V; In the fifth stage, the pre-lithiated lithium-ion battery of the positive electrode is charged with a current of 0.2C - 0.5C, the charging time is controlled to be 0.6 hours - 1.5 hours, the charging amount is 30% of the designed capacity of the pre-lithiated lithium-ion battery of the positive electrode, and the cut-off voltage is 3.65 V.

3. The formation method according to claim 2, characterized in that, After the forming treatment, the forming method further includes: Aging, degassing, and secondary packaging are performed on the formed pre-lithiated lithium-ion battery of the positive electrode; wherein, the temperature range for aging is 30°C - 60°C, the time range is 24 hours - 48 hours, and the vacuum degree for degassing and secondary packaging is less than -90 kPa.

4. The formation method according to claim 3, characterized in that, After the secondary packaging, the forming method further includes: Charging and discharging are performed on the pre-lithiated lithium-ion battery of the positive electrode to complete grading; wherein, the charging process is constant current and constant voltage charging, the current in the constant current stage is 0.05C - 0.2C, the cut-off current is 0.01C, and the current in the discharging process is 0.2C - 0.5C.

5. The formation method according to claim 4, characterized in that Before the forming treatment and after the preparation of the pre-lithiated lithium-ion battery of the positive electrode, the forming method further includes: The pre-lithiated lithium-ion battery of the positive electrode is subjected to a lithiation treatment for 8 hours - 15 hours under the conditions of a temperature of 45°C - 60°C and a pressure of 0.03 MPa - 0.6 MPa.

6. The formation method according to claim 5, characterized in that, Before the lithiation treatment and after the preparation of the pre-lithiated lithium-ion battery of the positive electrode, the forming method further includes: Inject electrolyte and vacuum seal the pre-lithiated positive electrode lithium-ion battery, and then let it stand; wherein, the ambient temperature during standing is 25°C - 60°C, and the standing time ranges from 24 hours to 72 hours.

7. The formation method according to any one of claims 1 to 6, characterized in that, The steps for preparing a pre-lithiated positive electrode lithium-ion battery include: Mix lithium iron phosphate with a mass ratio of 90% - 98%, a binder with a mass ratio of 1% - 2%, a conductive agent with a mass ratio of 0.5% - 4%, a dispersant with a mass ratio of 0.2% - 0.8%, and a lithium supplement agent with a mass ratio of 1% - 4.5% evenly, and add a solvent to prepare a positive electrode slurry. Coat the positive electrode slurry evenly on aluminum foil, dry it, cold press it, and die cut it into strips to prepare a positive electrode sheet.

8. A lithium-ion battery, characterized in that, The lithium-ion battery is prepared by using the formation method according to any one of claims 1 to 7.

Images