Formation method of long-life lithium ion battery and obtained long-life lithium ion battery

Abstract

The invention discloses a formation method of a long-life lithium ion battery and the obtained long-life lithium ion battery, and belongs to the technical field of lithium ion batteries. The formation method comprises the following steps: carrying out constant-current charging with a first preset current C1 until a first preset charge state SOC1 is reached, and standing; carrying out constant current charging at a second preset current C2 until a second preset charge state SOC2 is reached, and standing; constant-current charging is carried out with a third preset current C3 to a first preset voltage U1, and standing is carried out; constant-current charging is carried out with a fourth preset current C4 to a second preset voltage U2, and standing is carried out; and carrying out constant-current charging at a fifth preset current C5 to a third preset voltage U3, standing, and ending. According to the invention, the lithium iron phosphate long-life lithium ion battery containing the lithium supplement agent is subjected to stepped current step-by-step formation, and the obtained battery has relatively low direct-current internal resistance and good stability and cycle performance at the same time.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a formation method of a long-life lithium ion battery and the obtained long-life lithium ion battery. Background Art

[0002] The information disclosed in the background of the invention is only intended to enhance the understanding of the overall background of the invention and should not be necessarily regarded as an acknowledgment or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] In the production process of lithium-ion batteries, formation is an indispensable process as the first charging process of lithium-ion batteries. After the battery is manufactured, the internal positive and negative electrode materials are activated through a certain charging and discharging method, and a solid electrolyte interface film (SEI) is generated on the negative electrode side of the lithium battery, thereby improving the battery's charging and discharging performance and the overall performance of self-discharge, circulation, storage, etc. The formation process directly affects the film quality of the SEI film.

[0004] The formation method currently used in lithium iron phosphate batteries is the traditional low-current pre-charging method. However, long-term low-current charging will increase the impedance of the formed SEI film and cause defects in the SEI film. The DC internal resistance of the battery is high, which affects the battery's cycle performance. The addition of a lithium supplement can significantly improve the cycle performance of lithium iron phosphate batteries, giving them a longer service life. The formation methods used for different battery systems are not the same. Most of the existing lithium-ion battery formation methods cannot be directly applied to the formation of lithium iron phosphate batteries with lithium supplements. Therefore, how to provide a formation method for lithium iron phosphate batteries with lithium supplements to improve the stability of the SEI film, reduce the DC internal resistance of the battery, and improve the battery's cycle performance is an urgent problem to be solved. Summary of the invention

[0005] In view of this, the present invention provides a formation method for a long-life lithium-ion battery and the resulting long-life lithium-ion battery. The formation method provided by the present invention greatly improves the stability of the SEI film compared to the traditional formation method, and the SEI film has fewer defects and higher uniformity, which is beneficial to improving the interface performance of the negative electrode, improving the energy density and cycle performance of the battery, and improving the stability of the battery.

[0006] In a first aspect, the present invention provides a formation method for a long-life lithium-ion battery, comprising the following steps:

[0007] S1, charging with a first preset current C1 until a first preset state of charge SOC1 is reached, and then standing;

[0008] S2, charging with a second preset current C2 until a second preset state of charge SOC2 is reached, and then leaving it at rest;

[0009] S3, charging with a third preset current C3 to a first preset voltage U1, and leaving it at rest;

[0010] S4, charging with a fourth preset current C4 to a second preset voltage U2, and leaving it at rest;

[0011] S5, charging with a fifth preset current C5 to a third preset voltage U3, and then terminating after standing still;

[0012] Among them, 3%≤SOC1≤5%, 38%≤SOC2≤42%, U1<U2<U3, C5<C1<C2<C3; the positive electrode material of the long-life lithium-ion battery includes lithium iron phosphate and a lithium supplement.

[0013] In a second aspect, the present invention provides a long-life lithium-ion battery manufactured by the above-mentioned formation method.

[0014] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0015] (1) The present invention adopts a step-by-step formation process for the lithium iron phosphate long-life lithium-ion battery containing a lithium supplement to control the state of charge and the charging voltage. Compared with the traditional formation process using a constant small current, the stability of the SEI film is greatly improved, and it has fewer defects and higher uniformity, which is beneficial to improving the interface performance of the negative electrode and improving the cycle performance of the battery.

[0016] (2) The lithium-ion battery obtained by the formation method provided by the present invention has a low DC internal resistance of less than 25 mΩ; at the same time, the cycle stability is good. At 25°C, 2.5-3.65V, step charging, and 1C discharge conditions, the capacity retention rate can still be maintained at more than 88% after 4000 cycles; and even if stored at a high temperature of 55°C for 28 days, the capacity recovery rate can still reach more than 98%, and the battery has good stability. DETAILED DESCRIPTION

[0017] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0018] The present invention provides a formation method for a long-life lithium-ion battery, comprising the following steps:

[0019] S1, charging with a first preset current C1 until a first preset state of charge SOC1 is reached, and then standing;

[0020] S2, charging with a second preset current C2 until a second preset state of charge SOC2 is reached, and then leaving it at rest;

[0021] S3, charging with a third preset current C3 to a first preset voltage U1, and leaving it at rest;

[0022] S4, charging with a fourth preset current C4 to a second preset voltage U2, and leaving it at rest;

[0023] S5, charging with a fifth preset current C5 to a third preset voltage U3, and then terminating after standing still;

[0024] Among them, 3%≤SOC1≤5%, 38%≤SOC2≤42%, U1<U2<U3, C5<C1<C2<C3; the positive electrode material of the long-life lithium-ion battery includes lithium iron phosphate and a lithium supplement.

[0025] The present invention adopts multi-step ladder formation for the long-life lithium-ion battery to be formed. In the S1 stage, the initial film is formed mainly by charging with a small current to a low SOC. In the S2 stage, the current and SOC are further increased to make the side reaction more complete and the film formation more dense. S3, S4 and S5 are mainly based on the voltage window of the lithium supplement agent, so that the lithium is fully embedded in the negative electrode to supplement the irreversible capacity of the first charge of lithium iron phosphate, thereby increasing the capacity of the battery cell. In general, the state of charge and the charging voltage are controlled. Compared with the traditional formation process using a constant small current, the stability of the SEI film is greatly improved, and there are fewer defects and higher uniformity, which is conducive to improving the interface performance of the negative electrode and improving the energy density and cycle performance of the battery.

[0026] In the present invention, the first preset current C1 is 0.05C to 0.12C, for example, it can be 0.05C, 0.06C, 0.07C, 0.08C, 0.09C, 0.1C, etc.; the second preset current C2 is 0.12C to 0.16C, for example, it can be 0.12C, 0.13C, 0.14C, 0.15C, etc.; the third preset current C3 is 0.16C to 0.25C; the fourth preset current C4 is 0.15C to 0.3C; the fifth preset current C5 is 0.02C to 0.05C. The fifth preset current C5 of the present invention is the smallest, mainly to eliminate the polarization of the battery and make the side reaction more thorough.

[0027] In the present invention, the first preset voltage U1 is 3.6-3.7V, more preferably 3.65V; the second preset voltage U2 is 3.75-3.85V, more preferably 3.8V; the third preset voltage U3 is 4.15-4.25V, more preferably 4.2V.

[0028] In the present invention, the standing time of steps S1 to S5 is 2 to 10 minutes, more preferably 3 to 5 minutes. The standing time is to allow the gas generated during the formation process to be fully discharged from the battery.

[0029] In the present invention, steps S1 to S5 are performed under vacuum conditions, the temperature during charging is 40 to 50°C, and the environmental dew point is ≤-30°C. High temperature and negative pressure conditions are conducive to more thorough battery side reactions and more thorough exhaust. Further, the vacuum conditions are: negative pressure -85 to -75 kPa; the temperature during charging is 42 to 48°C; the environmental dew point is -45 to -35°C. The above environmental conditions are provided by the formation equipment, and the present invention does not impose any special restrictions on the model of the formation equipment.

[0030] In the present invention, the mass ratio of lithium iron phosphate and lithium supplement agent is (95-99):(1-5), for example, it can be 95:5, 96:4, 96:4, 97:3, 98:2, 99:1, etc. The formation method of the present invention plays a role of pre-lithiation on the one hand, so that the lithium in the lithium supplement agent is released therefrom, and lithium source is added to the inside of the electrode before the lithium ion battery works normally, thereby replenishing lithium ions, compensating for the irreversible lithium loss caused by the formation of SEI film, and improving the battery performance. On the other hand, it is important that the excess lithium source is embedded in the negative electrode material, which increases the capacity of the battery and reduces the attenuation rate of the capacity of the lithium ion battery, thereby increasing the life of the lithium ion battery.

[0031] In the present invention, the lithium supplement is preferably lithium ferrite. The introduction of lithium ferrite can significantly improve the cycle life of lithium iron phosphate batteries.

[0032] In the present invention, the negative electrode material of the long-life lithium-ion battery is selected from one or more of artificial graphite, natural graphite, soft carbon or hard carbon. The present invention does not impose any special restrictions on the preparation method of the long-life lithium-ion battery, and the preparation method of the long-life lithium-ion battery commonly used in the art can be used.

[0033] The present invention also provides a long-life lithium-ion battery, which is made by the above-mentioned formation method. The lithium-ion battery obtained by the formation method provided by the present invention has a low DC internal resistance, which is below 25mΩ; at the same time, the cycle stability is good, and the capacity retention rate can still be maintained at more than 88% under the conditions of step charging at 25°C, 2.0-3.65V, and 1C discharge for 4000 cycles; and even if stored at a high temperature of 55°C for 28 days, its capacity recovery rate can still reach more than 98%, and the battery has good stability.

[0034] The technical solution of the present invention is further described below in conjunction with specific embodiments.

[0035] The positive electrode material of the long-life lithium-ion battery (battery to be formed) used in the following examples is lithium iron phosphate (LFP) and lithium ferrite (LFO) in a mass ratio of 98:2, and the negative electrode material is artificial graphite (C). The positive electrode is mixed and stirred in a ratio of (LFP+LFO):SP:CNT:PVDF=97:0.5:0.5:2, and NMP is used as a solvent, and coated on aluminum foil to make a positive electrode sheet with a surface density of 375g / m 2 The negative electrode was mixed with C:SP:CMC:SBR in a ratio of 95.6:1.0:1.4:2.0, and coated on copper foil with water as solvent to make a negative electrode sheet with a surface density of 242g / m 2 The electrolyte used was a solution of 1 mol / L LiPF6 in EC, DMC and EMC (volume ratio was 1:1:1).

[0036] Example 1

[0037] This embodiment provides a formation method for a long-life lithium-ion battery.

[0038] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0039] (2) Charge at a constant current of C1 = 0.05C to SOC1 = 4%, which takes a total of 48 minutes, and then let it stand for 4 minutes.

[0040] (3) Charge at a constant current of C2 = 0.12C to SOC2 = 40%, which takes a total of 180 min, and then let it stand for 4 min.

[0041] (4) Charge at a constant current of C3 = 0.2C until U1 = 3.65V, and then let stand for 4 minutes.

[0042] (5) Charge at a constant current of C4 = 0.25C until U2 = 3.8V, and then let stand for 4 minutes.

[0043] (6) Charge at a constant current of C5 = 0.035C until U3 = 4.2V, and then let stand for 4 minutes.

[0044] Example 2

[0045] This embodiment provides a formation method for a long-life lithium-ion battery.

[0046] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0047] (2) Charge at a constant current of C1 = 0.05C to SOC1 = 4%, which takes a total of 48 minutes, and then let it stand for 4 minutes.

[0048] (3) Charge at a constant current of C2 = 0.14C to SOC2 = 40%, which takes a total of 154.28 min, and then let it stand for 4 min.

[0049] (4) Charge at a constant current of C3 = 0.2C until U1 = 3.65V, and then let stand for 4 minutes.

[0050] (5) Charge at a constant current of C4 = 0.25C until U2 = 3.8V, and then let stand for 4 minutes.

[0051] (6) Charge at a constant current of C5 = 0.035C until U3 = 4.2V, and then let stand for 4 minutes.

[0052] Example 3

[0053] This embodiment provides a formation method for a long-life lithium-ion battery.

[0054] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0055] (2) Charge at a constant current of C1 = 0.05C to SOC1 = 4%, which takes a total of 48 minutes, and then let it stand for 4 minutes.

[0056] (3) Charge at a constant current of C2 = 0.16C to SOC2 = 40%, which takes a total of 135 min, and then let it stand for 4 min.

[0057] (4) Charge at a constant current of C3 = 0.2C until U1 = 3.65V, and then let stand for 4 minutes.

[0058] (5) Charge at a constant current of C4 = 0.25C until U2 = 3.8V, and then let stand for 4 minutes.

[0059] (6) Charge at a constant current of C5 = 0.035C until U3 = 4.2V, and then let stand for 4 minutes.

[0060] Example 4

[0061] This embodiment provides a formation method for a long-life lithium-ion battery.

[0062] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0063] (2) Charge at a constant current of C1 = 0.1C to SOC1 = 4%, which takes a total of 24 min, and then let it stand for 4 min.

[0064] (3) Charge at a constant current of C2 = 0.12C to SOC2 = 40%, which takes a total of 180 min, and then let it stand for 4 min.

[0065] (4) Charge at a constant current of C3 = 0.2C until U1 = 3.65V, and then let stand for 4 minutes.

[0066] (5) Charge at a constant current of C4 = 0.25C until U2 = 3.8V, and then let stand for 4 minutes.

[0067] (6) Charge at a constant current of C5 = 0.035C until U3 = 4.2V, and then let stand for 4 minutes.

[0068] Example 5

[0069] This embodiment provides a formation method for a long-life lithium-ion battery.

[0070] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0071] (2) Charge at a constant current of C1 = 0.1C to SOC1 = 4%, which takes a total of 24 min, and then let it stand for 4 min.

[0072] (3) Charge at a constant current of C2 = 0.14C to SOC2 = 40%, which takes a total of 154.28 min, and then let it stand for 4 min.

[0073] (4) Charge at a constant current of C3 = 0.2C until U1 = 3.65V, and then let stand for 4 minutes.

[0074] (5) Charge at a constant current of C4 = 0.25C until U2 = 3.8V, and then let stand for 4 minutes.

[0075] (6) Charge at a constant current of C5 = 0.035C until U3 = 4.2V, and then let stand for 4 minutes.

[0076] Example 6

[0077] This embodiment provides a formation method for a long-life lithium-ion battery.

[0078] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0079] (2) Charge at a constant current of C1 = 0.1C to SOC1 = 4%, which takes a total of 24 min, and then let it stand for 4 min.

[0080] (3) Charge at a constant current of C2 = 0.16C to SOC2 = 40%, which takes a total of 135 min, and then let it stand for 4 min.

[0081] (4) Charge at a constant current of C3 = 0.2C until U1 = 3.65V, and then let stand for 4 minutes.

[0082] (5) Charge at a constant current of C4 = 0.25C until U2 = 3.8V, and then let stand for 4 minutes.

[0083] (6) Charge at a constant current of C5 = 0.035C until U3 = 4.2V, and then let stand for 4 minutes.

[0084] Comparative Example 1

[0085] The difference between this comparative example and Example 1 is that the formation method is to adopt 0.02C constant current charging for 20h.

[0086] Comparative Example 2

[0087] The difference between this comparative example and Example 1 is that this comparative example does not include step (2). The specific steps are as follows:

[0088] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0089] (2) Charge at a constant current of C1 = 0.12C to SOC = 40%, which takes a total of 200 min, and then let it stand for 4 min.

[0090] (3) Charge at a constant current of C2 = 0.2C until U1 = 3.65V, and then let stand for 4 minutes.

[0091] (4) Charge at a constant current of C3 = 0.25C until U2 = 3.8V, and then let stand for 4 minutes.

[0092] (5) Charge at a constant current of C4 = 0.035C until U3 = 4.2V, and then let stand for 4 minutes.

[0093] Comparative Example 3

[0094] The difference between this comparative example and Example 1 is that in step (1) of this comparative example, constant current charging is performed until SOC1=10%. The specific steps are as follows:

[0095] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0096] (2) Charge at a constant current of C1 = 0.05C to SOC1 = 10%, which takes a total of 120 min, and then let it stand for 4 min.

[0097] (3) Charge at a constant current of C2 = 0.12C to SOC2 = 46%, which takes a total of 180 min, and then let it stand for 4 min.

[0098] (4) Charge at a constant current of C3 = 0.2C until U1 = 3.65V, and then let stand for 4 minutes.

[0099] (5) Charge at a constant current of C4 = 0.25C until U2 = 3.8V, and then let stand for 4 minutes.

[0100] (6) Charge at a constant current of C5 = 0.035C until U3 = 4.2V, and then let stand for 4 minutes.

[0101] Comparative Example 4

[0102] The difference between this comparative example and Example 1 is that in step (6) of this comparative example, C5=0.1C.

[0103] Comparative Example 5

[0104] The formation method of this comparative example is as follows:

[0105] (1) Place the battery to be formed into the formation equipment, evacuate to -80 kPa, set the ambient dew point to -40 °C, and the ambient temperature to 45 °C.

[0106] (2) Charge at a constant current of C1 = 0.05C until U1 = 3.65V, then let stand for 4 minutes.

[0107] (3) Charge at a constant current of C2 = 0.1C until U2 = 3.8V, and then let stand for 4 minutes.

[0108] (4) Charge at a constant current of C3 = 0.2C until U3 = 4.2V, and then let stand for 4 minutes.

[0109] Test example

[0110] 1. Stability determination at different temperatures:

[0111] The lithium-ion batteries formed in Examples 1 to 6 and Comparative Examples 1 to 5 were stored in different temperature environments (25° C., 45° C., and 55° C.) for 28 days, and a charge and discharge test (1 / 3C charge and discharge) was performed. The capacity of the battery was measured and the capacity recovery rate was calculated, thereby comparing the storage performance of the battery. The results are shown in Table 1.

[0112] Table 1 Battery capacity recovery rate after formation of Examples 1 to 6 and Comparative Examples 1 to 5

[0113]

[0114]

[0115] It can be seen from Table 1 that the battery storage performance of the embodiment is better than that of the comparative example. The storage performance of different embodiments is not much different. The capacity recovery rate decreases slightly with increasing temperature. However, after storage at 55°C for 28 days, the battery capacity recovery rate of the embodiment can still be maintained at more than 98%, indicating that the battery formed by the formation method provided in the embodiment has good storage performance, slow decay, and good stability.

[0116] 2. DC internal resistance measurement

[0117] The DC internal resistance of the lithium ion batteries of Examples 1 to 6 and Comparative Examples 1 to 5 was measured (50% SOC, 5C10s). The results are shown in Table 2.

[0118] Table 2 DC internal resistance of batteries after formation of Examples 1 to 6 and Comparative Examples 1 to 5

[0119]

[0120]

[0121] It can be seen that the DC internal resistance of the battery in the embodiment is lower than that in the comparative example, both lower than 25 mΩ, and the storage performance of different embodiments is not much different, indicating that the battery formed by the formation method provided in the embodiment has lower DC internal resistance, slow decay, and good cycle stability.

[0122] 3. Cycle performance measurement

[0123] The cycle performance of the lithium-ion batteries formed in Examples 1 to 6 and Comparative Examples 1 to 5 was measured under the following test conditions (25° C., step charging in the range of 2.5-3.65 V, and 1C discharge); the results are shown in Table 3.

[0124] Table 3 Cycle performance of lithium ion batteries after formation of Examples 1 to 6 and Comparative Examples 1 to 5

[0125]

[0126]

[0127] It can be seen from Table 3 that the battery of the embodiment shows good cycle stability compared with the battery of the comparative example. After 4000 cycles, the capacity retention rate can be maintained at more than 88%.

[0128] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A formation method for a long-life lithium-ion battery, characterized in that: The steps include: S1, charging with a first preset current C1 until a first preset state of charge SOC1 is reached, and then standing; S2, charging with a second preset current C2 until a second preset state of charge SOC2 is reached, and then leaving it at rest; S3, charging with a third preset current C3 to a first preset voltage U1, and leaving it at rest; S4, charging with a fourth preset current C4 to a second preset voltage U2, and leaving it at rest; S5, charging with a fifth preset current C5 to a third preset voltage U3, and then terminating after standing still; Among them, 3%≤SOC1≤5%, 38%≤SOC2≤42%, U1<U2<U3, C5<C1<C2<C3; the positive electrode material of the long-life lithium-ion battery includes lithium iron phosphate and a lithium supplement.

2. The chemical formation method according to claim 1, characterized in that The first preset current C1 is 0.05C to 0.12C, the second preset current C2 is 0.12C to 0.16C, the third preset current C3 is 0.16C to 0.25C, the fourth preset current C4 is 0.15C to 0.3C, and the fifth preset current C5 is 0.02C to 0.05C.

3. The chemical formation method according to claim 1, characterized in that: The first preset voltage U1 is 3.6-3.7V, the second preset voltage U2 is 3.75-3.85V, and the third preset voltage U3 is 4.15-4.25V.

4. The chemical formation method according to claim 1, characterized in that: The standing time of steps S1 to S5 is 2 to 10 minutes.

5. The chemical formation method according to claim 1, characterized in that: Steps S1 to S5 are performed under vacuum conditions, the temperature during charging is 40 to 50° C., and the ambient dew point is ≤-30° C.

6. The chemical formation method according to claim 5, characterized in that: The vacuum conditions are: negative pressure of -85 to -75 kPa; temperature during charging of 42 to 48°C; and ambient dew point of -45 to -35°C.

7. The chemical formation method according to claim 1, characterized in that: The mass ratio of lithium iron phosphate to lithium supplement is (95-99):(1-5).

8. The chemical formation method according to claim 1, characterized in that: The lithium supplement is lithium ferrite.

9. The chemical formation method according to claim 1, characterized in that: The negative electrode material of the long-life lithium-ion battery is selected from one or more of artificial graphite, natural graphite, soft carbon or hard carbon.

10. A long-life lithium-ion battery, characterized in that: The method is prepared by the chemical conversion method according to any one of claims 1 to 9.

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