A lithium ion battery formation process

Abstract

The application discloses a lithium ion battery formation process, comprising the following steps: step one, injecting electrolyte into the lithium ion battery at room temperature, standing, and immersing the solvent of the electrolyte into the interior of the negative electrode material; wherein the electrolyte comprises a first organic solvent, a second organic solvent, a lithium salt and a film-forming additive; the melting point of the first organic solvent is lower than that of the second organic solvent; the first organic solvent and the second organic solvent are uniformly dispersed with each other at room temperature; step two, reducing the ambient temperature to the second organic solvent to be solidified on the surface of the negative electrode material, while the first organic solvent remains liquid to continue to infiltrate the negative electrode material; standing; forming SEI film at low temperature for the first time; step three, heating to room temperature, standing, and melting the second organic solvent; forming SEI film for the second time at room temperature; the application prevents the negative electrode from being solvated by combining the temperature with the different melting point solvents in the electrolyte, thereby prolonging the cycle life of the battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery formation technology, and in particular to a lithium-ion battery formation process that utilizes temperature and electrolyte to control the density of SEI film formation. Background Technology

[0002] During the first charge and discharge of a lithium-ion battery, a small amount of polar aprotic solvent in the electrolyte undergoes a reduction reaction after gaining some electrons. This solvent combines with lithium ions to form an interface film approximately 100-120 nm thick, known as the SEI (Sediment Intercalation). The SEI typically forms at the solid-liquid interface between the electrode material and the electrolyte. The main factor affecting the cycle life of lithium-ion batteries is the stability of the SEI film on the negative electrode surface. The primary function of the SEI film is to coat the negative electrode surface, preventing solvation between the negative electrode and the electrolyte solvent. Solvation causes the graphite structure of the negative electrode to collapse, reducing its capacity and preventing the normal insertion and extraction of lithium ions, a phenomenon known as "dead lithium." The SEI film only forms during formation. However, there is a 24-hour processing time between electrolyte injection and formation. During this time, some of the electrolyte solvent can penetrate the surface of the negative electrode material, causing solvation and affecting its electrochemical performance. Summary of the Invention

[0003] The purpose of this invention is to provide a lithium-ion battery formation process. This invention utilizes the combination of temperature and solvents with different melting points in the electrolyte to prevent the negative electrode from becoming solvated and to extend the battery's cycle life.

[0004] To solve this technical problem, the technical solution of the present invention is: a lithium-ion battery formation process.

[0005] Includes the following steps:

[0006] Step 1: At room temperature, inject electrolyte into the lithium-ion battery and let it stand. The solvent of the electrolyte will penetrate into the negative electrode material.

[0007] The electrolyte includes a first organic solvent, a second organic solvent, a lithium salt, and a film-forming additive;

[0008] The melting point of the first organic solvent is lower than that of the second organic solvent;

[0009] At room temperature, the first and second organic solvents are uniformly dispersed in each other.

[0010] Step 2: Lower the ambient temperature until the second organic solvent solidifies on the surface of the negative electrode material, while the first organic solvent remains liquid and continues to wet the negative electrode material;

[0011] Let it stand;

[0012] The SEI film is formed during the first formation at low temperature.

[0013] Step 3: Heat to room temperature, let stand, and the second organic solvent will melt;

[0014] The SEI film continues to form after a second formation at room temperature.

[0015] The preferred electrolyte contains ethyl butyrate (EB) and methyl formate (MF) as the first organic solvent and dimethyl carbonate (DMC) as the second organic solvent. The volume ratio of methyl formate, ethyl butyrate, and dimethyl carbonate is 2:2:1, 3:5:2, or 4:5:1. Ethyl butyrate (EB), with a melting point of -101℃, is used as the main solvent. It is a linear carbonate with an extremely low melting point. At a low temperature of -40℃, its viscosity does not increase, and it does not cause an increase in electrolyte impedance, allowing it to function normally. Therefore, EB improves the low-temperature conductivity of the electrolyte. EB has a wide operating voltage range and can participate in the formation of the SEI throughout the entire formation process.

[0016] Methyl formate (MF), with a melting point of -99℃, is also a linear carbonate with an extremely low melting point. It maintains fluidity at low temperatures, sustaining the liquid environment for lithium-ion conduction. During low-temperature formation, it can ensure the normal conduction of lithium-ions. It can decompose to form an SEI film under low voltage. Therefore, MF improves the low-temperature conductivity of the electrolyte and initially forms an SEI film during the first formation.

[0017] Dimethyl carbonate (DMC) has a melting point of 2°C. DMC has good electrochemical stability and low viscosity, which is beneficial for improving conductivity. In the electrolyte solvent, it is in a solidified state during the first formation and does not participate in the formation of the SEI film. After the system is heated, DMC melts. During the second formation, DMC, as a long-chain alkyl carbonate, becomes the main film-forming solvent when the voltage is higher than 3.0V, further forming a dense SEI film on the basis of the SEI film formed in the first formation.

[0018] In the preferred step two, the low temperature is -10℃ to 0℃. This invention effectively ensures the solidification of the second organic solvent during the first formation process, resulting in batch film formation. The purpose of batch film formation is that the first formed film is incomplete; the second organic solvent, solidified on the surface of the negative electrode material, protects the covered negative electrode material, preventing solvation of the negative electrode material, reducing the adverse effects of the organic solvent on the negative electrode material during the formation process, and improving the density and uniformity of the SEI film.

[0019] The preferred process conditions for the first formation in step two are: charging at 0.03C to 0.05C until reaching 3.0V. This invention controls the charging current and voltage during the first formation. The current primarily affects the film density, preventing excessive current at low temperatures from causing lithium plating and ensuring the battery's performance after formation.

[0020] The preferred settling time in step two is 20 hours. This ensures that the first organic solvent fully wets the negative electrode material, which is beneficial for the complete coverage of the SEI film.

[0021] The preferred process conditions for the second formation in step two are: 0.08C to 0.15C, charging to 4.2V. This second formation is at room temperature, and the current primarily affects the density of the SEI film; the denser the SEI film, the better the cycle performance. In the second formation, it is important to avoid using too low a current, which would result in a slow SEI film formation rate and prevent the organic solvent from forming a solvation layer with the negative electrode material, thus damaging the material's cycle life.

[0022] The preferred lithium salt in the electrolyte is lithium hexafluorophosphate, with a concentration of 1.5 mol / L.

[0023] Preferably, the film-forming additive in the electrolyte is trimethyl borate, and the trimethyl borate accounts for 2% to 4% of the electrolyte mass.

[0024] The preferred lithium-ion battery anode material is graphite. The SEI film formed in this invention coats the surface of the anode, preventing solvation between the anode and the electrolyte solvent. This avoids the collapse of the graphite structure of the anode due to solvation, maintains the anode capacity, and prevents lithium ions embedded in the anode from being unable to properly intercalate or deintercalate. In other words, this invention effectively suppresses the phenomenon of dead lithium.

[0025] By adopting the above technical solution, the beneficial effects of the present invention are:

[0026] The lithium-ion battery formation process proposed in this invention utilizes a first formation at low temperature and a second formation at room temperature, combined with a first organic solvent with a low melting point and a second organic solvent with a high melting point (i.e., solidified at low temperature) in the electrolyte, to form a more dense SEI film than existing formation processes under two different charging currents and charging voltages.

[0027] This invention involves adding a second organic solvent with a high melting point to the electrolyte. After injection, the electrolyte is rapidly cooled, and the second organic solvent solidifies on the electrode surface as it wets the electrode material, preventing solvation of the negative electrode. At the same time, the first organic solvent in the solution can function normally at this temperature. During this initial first formation, an SEI film is formed covering the solidified second organic solvent and the exposed electrode surface. Then, the temperature is raised to room temperature, the second organic solvent melts, and a second formation is performed at room temperature. The second formation further densifies the SEI film, ensuring a complete coverage of the negative electrode surface.

[0028] The electrolyte solvent formulation and formation process proposed in this invention can effectively prevent solvation between the negative electrode material and the electrolyte, forming a stable SEI film. By maintaining the stability of the negative electrode material structure, the cycle life of the resulting battery can be extended. Attached Figure Description

[0029] Figure 1 The cycle life test results are obtained from the lithium-ion batteries of Examples 1 to 4 and Comparative Examples 1 and 2 of this invention. Detailed Implementation

[0030] To further explain the technical solution of the present invention, the present invention will be described in detail below through specific embodiments.

[0031] Example 1

[0032] This embodiment discloses a lithium-ion battery formation process, including the following steps:

[0033] Step 1: At room temperature, inject electrolyte into the lithium-ion battery and let it stand for 2 hours to allow the electrolyte solvent to penetrate into the negative electrode material.

[0034] The electrolyte includes a first organic solvent, a second organic solvent, a lithium salt, and a film-forming additive;

[0035] In this embodiment, the first organic solvent is MF and EB, and the second organic solvent is DMC;

[0036] The volume ratio of MF, EB, and DMC is 2:2:1;

[0037] The amount of lithium hexafluorophosphate added was 1.5 mol / L;

[0038] The film-forming additive is 2% of the electrolyte mass by weight (TB).

[0039] The melting point of the first organic solvent is lower than that of the second organic solvent;

[0040] At room temperature, the first and second organic solvents are uniformly dispersed in each other.

[0041] Step 2: Lower the ambient temperature until the second organic solvent solidifies on the surface of the negative electrode material, while the first organic solvent remains liquid and continues to wet the negative electrode material;

[0042] Let stand for 20 hours;

[0043] The first formation at -10℃ forms an SEI film;

[0044] First transformation: 0.03C charging, charging to 3.0V;

[0045] Step 3: Heat to room temperature (25°C), let stand for 2 hours, and the second organic solvent will melt.

[0046] A second formation at room temperature continues to form an SEI film.

[0047] Second formation: 0.08C, charged to 4.2V.

[0048] Example 2

[0049] This embodiment discloses a lithium-ion battery formation process, including the following steps:

[0050] Step 1: At room temperature, inject electrolyte into the lithium-ion battery and let it stand for 2 hours to allow the electrolyte solvent to penetrate into the negative electrode material.

[0051] The electrolyte includes a first organic solvent, a second organic solvent, a lithium salt, and a film-forming additive;

[0052] In this embodiment, the first organic solvent is MF and EB, and the second organic solvent is DMC;

[0053] The volume ratio of MF, EB, and DMC is 3:5:2;

[0054] The amount of lithium hexafluorophosphate added was 1.5 mol / L;

[0055] The film-forming additive TB is used at a rate of 3 wt% of the electrolyte.

[0056] The melting point of the first organic solvent is lower than that of the second organic solvent;

[0057] At room temperature, the first and second organic solvents are uniformly dispersed in each other.

[0058] Step 2: Lower the ambient temperature until the second organic solvent solidifies on the surface of the negative electrode material, while the first organic solvent remains liquid and continues to wet the negative electrode material;

[0059] Let stand for 20 hours;

[0060] The first formation of the SEI film occurs at -7℃.

[0061] First transformation: 0.04C charging, charging to 3.0V;

[0062] Step 3: Heat to room temperature (25°C), let stand for 2 hours, and the second organic solvent will melt.

[0063] A second formation at room temperature continues to form an SEI film.

[0064] Second formation: 0.1C, charging to 4.2V.

[0065] Example 3

[0066] This embodiment discloses a lithium-ion battery formation process, including the following steps:

[0067] Step 1: At room temperature, inject electrolyte into the lithium-ion battery and let it stand for 2 hours to allow the electrolyte solvent to penetrate into the negative electrode material.

[0068] The electrolyte includes a first organic solvent, a second organic solvent, a lithium salt, and a film-forming additive;

[0069] In this embodiment, the first organic solvent is MF and EB, and the second organic solvent is DMC;

[0070] The volume ratio of MF, EB, and DMC is 4:5:1;

[0071] The amount of lithium hexafluorophosphate added was 1.5 mol / L;

[0072] The film-forming additive TB is used at a rate of 4 wt% of the electrolyte.

[0073] The melting point of the first organic solvent is lower than that of the second organic solvent;

[0074] At room temperature, the first and second organic solvents are uniformly dispersed in each other.

[0075] Step 2: Lower the ambient temperature until the second organic solvent solidifies on the surface of the negative electrode material, while the first organic solvent remains liquid and continues to wet the negative electrode material;

[0076] Let stand for 20 hours;

[0077] The first formation of the SEI film occurs at -4℃.

[0078] First transformation: 0.05C charging, charging to 3.0V;

[0079] Step 3: Heat to room temperature (25°C), let stand for 2 hours, and the second organic solvent will melt.

[0080] A second formation at room temperature continues to form an SEI film.

[0081] Second formation: 0.12C, charged to 4.2V.

[0082] Example 4

[0083] This embodiment discloses a lithium-ion battery formation process, including the following steps:

[0084] Step 1: At room temperature, inject electrolyte into the lithium-ion battery and let it stand for 2 hours to allow the electrolyte solvent to penetrate into the negative electrode material.

[0085] The electrolyte includes a first organic solvent, a second organic solvent, a lithium salt, and a film-forming additive;

[0086] In this embodiment, the first organic solvent is MF and EB, and the second organic solvent is DMC;

[0087] The volume ratio of MF, EB, and DMC is 4:5:1;

[0088] The amount of lithium hexafluorophosphate added was 1.5 mol / L;

[0089] The film-forming additive TB is used at a rate of 4 wt% of the electrolyte.

[0090] The melting point of the first organic solvent is lower than that of the second organic solvent;

[0091] At room temperature, the first and second organic solvents are uniformly dispersed in each other.

[0092] Step 2: Lower the ambient temperature until the second organic solvent solidifies on the surface of the negative electrode material, while the first organic solvent remains liquid and continues to wet the negative electrode material;

[0093] Let stand for 20 hours;

[0094] The first formation at -0℃ forms an SEI film;

[0095] First transformation: 0.05C charging, charging to 3.0V;

[0096] Step 3: Heat to room temperature (25°C), let stand for 2 hours, and the second organic solvent will melt.

[0097] A second formation at room temperature continues to form an SEI film.

[0098] Second transformation: 0.15C, charged to 4.2V.

[0099] Comparative Example 1

[0100] The lithium-ion battery formation process used in this comparative example:

[0101] At room temperature, the lithium-ion battery is injected with electrolyte and left to stand, allowing the electrolyte solvent to penetrate into the negative electrode material.

[0102] In this example, the conventional electrolyte used has a volume ratio of EC, DMC, and EMC of 1:1:1, lithium hexafluorophosphate of 1.5 mol / L, and VC as the film-forming additive, with a VC mass of 2% of the electrolyte mass.

[0103] The formation process conditions at room temperature are: 0.05C to 3.0V, and 0.1C to 4.2V.

[0104] Comparative Example 2

[0105] The lithium-ion battery formation process used in this comparative example:

[0106] The solvents in the electrolyte are MF, EB and DMC, with a volume ratio of 2:2:1.

[0107] Lithium hexafluorophosphate, addition amount 1.5 mol / L;

[0108] The film-forming additive is TB, and the mass of TB is 2% of the mass of the electrolyte.

[0109] Formation: At room temperature: 0.05C, charge to 3.0V, 0.1C to 4.2V.

[0110] The formation processes of Examples 1 to 4, as well as Comparative Examples 1 and 2, were applied to a 2000mAh cylindrical battery, with the positive electrode made of ternary material NCM622, to obtain the finished battery.

[0111] The batteries prepared according to Examples 1 to 4, Comparative Examples 1 and 2 were subjected to internal resistance and cycle performance tests, as detailed in Table 1 and [other tables]. Figure 1 As shown, the cycle performance test conditions are: 0.5C charging and 1C discharging.

[0112] Table 1. Internal resistance data of batteries after formation in Examples 1 to 4 and Comparative Examples 1 and 2.

[0113] Group Internal resistance / mΩ Example 1 20.3 Example 2 20.0 Example 3 19.1 Example 4 19.3 Comparative Example 1 23.2 Comparative Example 2 22.2

[0114] The proposed solution achieves SEI film formation in stages from the perspective of protecting the negative electrode material, effectively reducing the battery's internal resistance. This is because the first and second organic solvents, used sequentially at different formation temperatures during the first and second formation processes, prevent all organic solvents from contacting the negative electrode during the first formation. Simultaneously, solidification on the negative electrode material surface during the first formation further reduces contact between the first and second organic solvents covering the negative electrode material, thus reducing negative electrode solvation from two aspects. Furthermore, the film is formed in batches during the two formation processes, with the first organic solvent forming the film during the first formation. The second organic solvent protects the negative electrode material. During the second formation, a portion of the SEI film has already formed. The second organic solvent further works with the first organic solvent to continue forming an SEI film on the previously exposed electrode surface. In other words, during the two formations, a portion of the organic solvent comes into contact with a portion of the electrode material, forming a stable, dense, and self-perfecting SEI film on the negative electrode surface. This protects the integrity of the negative electrode material and effectively prevents the solvent from forming a solvation layer with the negative electrode material. The solvation layer is a structurally collapsed negative electrode material that does not have conductivity. Therefore, the internal resistance of the comparative example is greater than that of the embodiment.

[0115] Further combining Table 1 and Figure 1Analysis shows that Examples 1 to 4 of this invention effectively improve the cycle life of the battery compared to Comparative Examples 1 and 2, achieving the objective of this invention. In Comparative Example 1, using a conventional electrolyte and conventional formation steps, the resulting battery had a remaining capacity of only 1600mAh after approximately 320 cycles, representing 80% of the original capacity. Comparative Example 2 used the electrolyte proposed in this invention combined with conventional formation steps, showing some improvement in cycle life compared to Comparative Example 1, but after 400 cycles, the remaining capacity was only 1600mAh, decreasing to 80%, which was also unsatisfactory. The cycle curves of Examples 1-4 proposed in this invention showed slow capacity decay, with Example 3 exhibiting the flattest curve. After approximately 500 cycles, the capacity remained at 1773mAh, representing 89% of the original capacity, significantly better than the comparative examples. The electrolyte proposed in this invention, combined with temperature-controlled formation, effectively improves the cycle life of the battery.

Claims

1. A lithium-ion battery formation process, characterized in that: Includes the following steps: Step 1: At room temperature, inject electrolyte into the lithium-ion battery and let it stand. The solvent of the electrolyte will penetrate into the negative electrode material. The electrolyte includes a first organic solvent, a second organic solvent, a lithium salt, and a film-forming additive; The melting point of the first organic solvent is lower than that of the second organic solvent; At room temperature, the first and second organic solvents are uniformly dispersed in each other. Step 2: Lower the ambient temperature until the second organic solvent solidifies on the surface of the negative electrode material, while the first organic solvent remains liquid and continues to wet the negative electrode material; Let it stand; The SEI film is formed during the first formation at low temperature. In step two, the low temperature ranges from -10°C to 0°C. The process conditions for the first formation in step two are: charging at 0.03C to 0.05C until reaching 3.0V; Step 3: Heat to room temperature, let stand, and the second organic solvent will melt; A second formation at room temperature continues to form an SEI film. The process conditions for the second formation in step three are: 0.08C to 0.15C, charging to 4.2V; The first organic solvent in the electrolyte includes ethyl butyrate (EB) and methyl formate (MF), and the second organic solvent is dimethyl carbonate (DMC); wherein the volume ratio of methyl formate, ethyl butyrate and dimethyl carbonate is 2:2:1 or 3:5:2 or 4:5:

1.

2. The formation process as described in claim 1, characterized in that: The settling time in step two is 20 hours.

3. The formation process as described in claim 1, characterized in that: The lithium salt in the electrolyte is lithium hexafluorophosphate, and the concentration of the lithium salt is 1.5 mol / L.

4. The formation process as described in claim 1, characterized in that: The film-forming additive in the electrolyte is trimethyl borate, which accounts for 2% to 4% of the electrolyte mass.

5. The formation process as described in claim 1, characterized in that: The negative electrode of the lithium-ion battery is a graphite material.

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