Electrolyte suitable for working electrode made of graphite-based material, lithium ion battery, and use

Abstract

The present application relates to the technical field of secondary batteries, and specifically to an electrolyte suitable for a working electrode made of a graphite-based material, a lithium ion battery, and a use. The electrolyte is composed of a non-aqueous organic solvent, a lithium salt, and an additive, wherein the additive is fluoroethylene carbonate. The electrolyte provided by the present application has a simple formula and does not need to compound and use multiple additives, and use of only one additive can enable the electrolyte to form a thinner and more stable solid electrolyte interface film on the surface of a material, thereby achieving better stability, effectively reducing the diffusion impedance of Li+, improving the 0.1C discharge capacity, shortening the total discharge time, reducing the rebound of a 0.01C discharge voltage, reducing the polarization of a charge-discharge curve, and ensuring that a discharge curve fully embodies a typical discharge platform of graphite. The electrolyte can accurately and effectively evaluate the capacity per gram and the first efficiency of the graphite-based material, and has great guiding significance for negative electrode material selection, raw material evaluation, electrode sheet formula optimization, battery design and the like.

Description

An electrolyte suitable for working electrodes with graphite-based materials, a lithium-ion battery, and applications thereof

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on June 30, 2023, with application number 202310797236.1 and invention name “An electrolyte, lithium-ion battery and application suitable for working electrodes with graphite-based materials”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application belongs to the technical field of secondary batteries, and specifically relates to an electrolyte, a lithium-ion battery and applications suitable for working electrodes made of graphite-based materials. Background Art

[0004] Lithium-ion batteries, the primary power source for new energy vehicles, face increasingly stringent raw material evaluation requirements to improve battery quality. For commonly used mass-produced batteries, the mainstream cathode materials are lithium iron phosphate and ternary materials, while the mainstream anode material is graphite. Silicon-based materials are often blended or composited with graphite.

[0005] Both raw material manufacturers and battery manufacturers will conduct evaluations on the gram capacity and initial efficiency of their products or raw materials. However, large battery polarization will significantly affect the gram capacity of graphite-based materials, resulting in an unclear typical discharge platform for graphite. The 0.1C discharge capacity in the four-step discharge is relatively small, and a large portion of the capacity is mainly achieved by discharging at very low currents of 0.01C and 0.01mA. This results in a very long total discharge time, a very long test cycle, and inaccurate test results. Therefore, whether it is front-end material inspection and evaluation of material gram capacity, initial efficiency, constant current charging ratio, etc., or input of gram capacity of positive and negative electrode materials at the battery design end, the goal is to more accurately evaluate the material gram capacity and initial efficiency, minimize the polarization of the charge and discharge curve, and ensure that the curve fully reflects the charge and discharge platform.

[0006] The internal factors that influence battery polarization primarily include the electrolyte, positive and negative electrode materials, conductive agents, SEI films, electrode thickness, and compaction density. Under the same conditions, the electrolyte is the primary cause of lithium-ion battery polarization. One way to improve the electrolyte's high-rate discharge capability is to increase its conductivity and improve SEI film formation. Under the same solvent and lithium salt conditions, the primary factors influencing electrolyte conductivity, electrode surface film formation, graphite material discharge capacity, and discharge curve polarization are the type and amount of lithium salt, solvent, and additives in the electrolyte.

[0007] Therefore, the importance of accurately and objectively evaluating the specific capacity and initial efficiency of graphite-based materials is self-evident. To address the issues of unclear typical discharge platforms in graphite half-cells, reducing discharge curve polarization, reducing 0.01C discharge voltage rebound, and addressing the long total four-step discharge time, existing technologies often use a combination of multiple additives. However, these additives are ineffective, and the electrolyte composition is complex and costly.

[0008] In view of this, it is urgent to develop an electrolyte with a simple formula suitable for working electrodes made of graphite-based materials, which can reduce the polarization of the discharge curve, increase the 0.1C discharge capacity, reduce the 0.01C discharge voltage rebound, and shorten the total discharge time.

[0009] Summary of the Invention

[0010] Therefore, the technical problem to be solved by the present application is to overcome the above-mentioned defects of the electrolyte in the prior art, thereby providing an electrolyte suitable for working electrodes made of graphite-based materials and its application.

[0011] To this end, this application provides the following technical solutions:

[0012] The present application provides an electrolyte suitable for a working electrode made of a graphite-based material, which is composed of a non-aqueous organic solvent, a lithium salt and an additive, wherein the additive is fluoroethylene carbonate.

[0013] Optionally, based on the total mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is 0.1% to 10%.

[0014] Optionally, the mass percentage of the fluoroethylene carbonate is 5% to 9%.

[0015] Optionally, the lithium salt includes at least one of LiPF6, LiBF4, LiClO4, LiPO2F2, LiTFSI, and LiFSI;

[0016] Optionally, the lithium salt is LiPF6.

[0017] Optionally, the concentration of the lithium salt is 0.8 to 1.3 mol / L based on the total volume of the electrolyte;

[0018] Optionally, the concentration of the lithium salt is 1.0 to 1.2 mol / L.

[0019] Optionally, the non-aqueous organic solvent includes at least two of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0020] Optionally, the non-aqueous organic solvent is a combination of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate.

[0021] Optionally, the volume ratio of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate is 1.5-3.5:1-5:1-5.

[0022] The present application also provides a lithium-ion battery, comprising the above-mentioned electrolyte suitable for a working electrode made of a graphite-based material;

[0023] Optionally, the negative electrode sheet of the lithium-ion battery is made of graphite-based material.

[0024] In this application, the graphite-based material refers to graphite or a material in which graphite is mixed or composited with other materials. The materials in which graphite is mixed or composited with other materials are conventional in the field.

[0025] In the present application, other components and preparation methods of the lithium-ion battery are conventional in the art. Typically, but not limiting, the positive electrode sheet can be a lithium sheet, and the separator can be a polypropylene single-layer separator.

[0026] The method for preparing the negative electrode sheet may include the following steps:

[0027] (1) Weigh the materials and NMP according to the ratio of graphite-based material: PVDF: conductive carbon black = 91-92: 6-7: 1.8-2, with a solid content of 30% to 40%;

[0028] (2) placing a homogenization tank containing weighed graphite-based material, conductive carbon black, PVDF glue and solvent NMP into a homogenizer;

[0029] (3) Set the homogenization system and start homogenization;

[0030] (4) Lay the cut 6-8 μm copper foil flat on the foil carrier platform of the coater and adjust the coater gap;

[0031] (5) Pour the evenly mixed slurry onto one end of the copper foil and evenly coat the slurry onto the copper foil using an applicator with the gap adjusted.

[0032] (6) Fix the copper foil coated with slurry on a glass plate and transfer it to a blast oven for drying at 80-100°C for 30-120 minutes;

[0033] (7) Punch the electrode into discs of φ12mm~φ16m with a manual slicer, clamp the cut discs with 6μm or 8μm copper foil, and use a laboratory roller machine to roll each disc to the specified compaction density (generally 1.15-1.65g / cm 3 );

[0034] (8) The rolled electrode is transferred to a vacuum oven and vacuum dried at 105-110°C for 3-15 hours to obtain the negative electrode for later use.

[0035] The lithium-ion battery assembly method may include the following steps:

[0036] Transfer the vacuum-dried electrodes into the glove box and assemble the battery in the following stacking order:

[0037] Positive electrode shell → negative electrode sheet → electrolyte → diaphragm → electrolyte → lithium sheet → nickel foam → negative electrode shell → packaging.

[0038] The present application also provides an application of the above-mentioned electrolyte suitable for working electrodes with graphite-based materials in electrode material research and development, raw material evaluation or battery design.

[0039] The technical solution of this application has the following advantages:

[0040] The electrolyte provided by the present application is suitable for working electrodes made of graphite-based materials, and is composed of a non-aqueous organic solvent, a lithium salt, and an additive, wherein the additive is fluoroethylene carbonate. The electrolyte provided by the present application has a simple formula and does not require the use of multiple additives. Only one additive is used to form a thinner and more stable solid electrolyte interface film on the surface of the material, which has better stability and effectively reduces Li + Diffusion impedance, improves 0.1C discharge capacity, shortens total discharge time, reduces 0.01C discharge voltage rebound, reduces charge and discharge curve polarization, ensures that the discharge curve fully reflects the typical discharge platform of graphite, and thus can accurately and effectively evaluate the specific capacity and first efficiency of graphite-based materials.

[0041] The electrolyte provided in this application, which is suitable for working electrodes made of graphite-based materials, is used in electrode material research and development, raw material evaluation or battery design. It can accurately and effectively evaluate the gram capacity and initial efficiency of graphite-based materials, and has great guiding significance for negative electrode material screening, raw material evaluation and electrode formulation optimization. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0043] FIG1 is a comparison diagram of the first charge and discharge curves of Example 1 and Examples 2-5 of the present application;

[0044] FIG2 is a comparison diagram of the first charge and discharge curves of Example 1 of the present application and Comparative Examples 1-3. DETAILED DESCRIPTION

[0045] The following examples are provided to further better understand the present application, but are not limited to the best implementation mode described herein, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the scope of protection of the present application.

[0046] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0047] Example 1

[0048] This embodiment provides an electrolyte suitable for a working electrode made of a graphite-based material, comprising a non-aqueous organic solvent, a lithium salt, and an additive, wherein the additive is fluoroethylene carbonate, and the mass percentage of the fluoroethylene carbonate is 7% based on the total mass of the electrolyte; the lithium salt is LiPF6; the concentration of the lithium salt is 1.15 mol / L based on the total volume of the electrolyte; and the non-aqueous organic solvent is ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:4:4.

[0049] This embodiment also provides a lithium-ion battery, the composition and specific preparation steps of which are as follows:

[0050] The method for preparing the negative electrode sheet may include the following steps:

[0051] (1) Weigh the materials and NMP according to the ratio of graphite-based material: PVDF: conductive carbon black = 91.6:6.6:1.8, with a solid content of 40%;

[0052] (2) placing a homogenization tank containing weighed graphite-based material, conductive carbon black, PVDF glue and solvent NMP into a homogenizer;

[0053] (3) Set the homogenization system and start homogenization;

[0054] (4) Lay the cut 6μm copper foil flat on the foil carrier platform of the coater and adjust the coater gap;

[0055] (5) Apply the homogenized slurry evenly on the copper foil, and control the surface density to 8 mg / cm 2 ;

[0056] (6) Fix the copper foil coated with slurry on a glass plate and transfer it to a blast oven for drying at 80°C for 20 minutes;

[0057] (7) Punch the electrode into φ14 mm discs using a manual slicer, clamp the cut discs with 6 μm copper foil, and roll them one by one using a laboratory roller machine to the specified compaction density (1.6 g / cm in this embodiment). 3 );

[0058] (8) The rolled electrode was transferred to a vacuum oven and dried at 110°C for 12 h to obtain the negative electrode for later use.

[0059] The lithium-ion battery assembly method may include the following steps:

[0060] Transfer the vacuum-dried electrode into the glove box and assemble the CR2032 button half-cell according to the following stacking sequence:

[0061] Positive electrode shell → negative electrode sheet → electrolyte (200μL) → diaphragm (φ19mm) → electrolyte (20μL) → lithium sheet (φ16mm×0.6mm) → nickel foam (φ16mm) → negative electrode shell → packaging.

[0062] Example 2

[0063] This embodiment provides an electrolyte suitable for a working electrode made of a graphite-based material, comprising a non-aqueous organic solvent, a lithium salt, and an additive, wherein the additive is fluoroethylene carbonate, and the mass percentage of the fluoroethylene carbonate is 7% based on the total mass of the electrolyte; the lithium salt is LiPF6; the concentration of the lithium salt is 1.15 mol / L based on the total volume of the electrolyte; and the non-aqueous organic solvent is ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:3:5.

[0064] This embodiment also provides a lithium-ion battery, whose composition and specific preparation steps are the same as those of Example 1.

[0065] Example 3

[0066] This embodiment provides an electrolyte suitable for a working electrode made of a graphite-based material, comprising a non-aqueous organic solvent, a lithium salt, and an additive, wherein the additive is fluoroethylene carbonate, and the mass percentage of the fluoroethylene carbonate is 5% based on the total mass of the electrolyte; the lithium salt is LiPF6; the concentration of the lithium salt is 1.2 mol / L based on the total volume of the electrolyte; and the non-aqueous organic solvent is ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 3:3:4.

[0067] This embodiment also provides a lithium-ion battery, whose composition and specific preparation steps are the same as those of Example 1.

[0068] Example 4

[0069] This embodiment provides an electrolyte suitable for a working electrode made of graphite-based material. The difference from Example 1 is that the solvent is composed of dimethyl carbonate and ethylene carbonate in a volume ratio of 2:1.

[0070] This embodiment also provides a lithium-ion battery, whose composition and specific preparation steps are the same as those of Example 1.

[0071] Example 5

[0072] This embodiment provides an electrolyte suitable for a working electrode made of graphite-based materials. The difference from Example 1 is that an equimolar amount of LiTFSI is used instead of LiPF6.

[0073] This embodiment also provides a lithium-ion battery, whose composition and specific preparation steps are the same as those of Example 1.

[0074] Comparative Example 1

[0075] This comparative example provides an electrolyte suitable for a working electrode made of graphite-based materials, which differs from Example 1 in that no additives are included.

[0076] This comparative example also provides a lithium ion battery, the composition and specific preparation steps of which are the same as those of Example 1.

[0077] Comparative Example 2

[0078] This comparative example provides an electrolyte suitable for a working electrode made of a graphite-based material. The difference from Example 1 is that an equal mass of vinylene carbonate is used instead of fluoroethylene carbonate.

[0079] This comparative example also provides a lithium ion battery, the composition and specific preparation steps of which are the same as those of Example 1.

[0080] Comparative Example 3

[0081] This comparative example provides an electrolyte suitable for a working electrode made of a graphite-based material. The difference from Example 1 is that, based on the total mass of the electrolyte, it further includes 0.5% by mass of vinylene carbonate.

[0082] This comparative example also provides a lithium ion battery, the composition and specific preparation steps of which are the same as those of Example 1.

[0083] Test Case

[0084] The assembled batteries of each embodiment and comparative example were tested according to the following steps:

[0085] Table 1

[0086] The test results are shown in the table below:

[0087] Table 2

[0088] As can be seen from the data in the above table, the use of simple fluoroethylene carbonate as an additive in the electrolyte can increase the 0.1C discharge capacity, increase the 0.1C discharge time, ensure that the discharge curve fully reflects the typical discharge platform of graphite, shorten the total discharge time, reduce the 0.01C discharge voltage rebound, and reduce the polarization of the charge and discharge curve. Specifically, compared with Comparative Example 1 and Comparative Example 2, and compared with Example 1 and Comparative Example 3: the addition of vinylene carbonate additive leads to a shortening of the 0.1C discharge time (time taken for the first step), indicating that polarization is increased; The maximum value of the 0.01C discharge voltage increases, indicating that the voltage rebound increases. Compared with Example 1 and Example 1: after adding a certain amount of fluoroethylene carbonate additive, the 0.1C discharge time increases and the maximum value of the 0.01C discharge voltage decreases, indicating that polarization is reduced and the voltage rebound is reduced. Comparing Example 1 with Comparative Example 3, adding a certain amount of vinylene carbonate to an electrolyte containing a certain amount of fluoroethylene carbonate additive shortened the 0.1C discharge time and increased the maximum 0.01C discharge voltage, indicating increased polarization and voltage rebound. Comparing Examples 2-5 with Example 1, it was shown that, while maintaining the same electrolyte additive composition, appropriate changes in the lithium salt type and content, as well as the solvent ratio, produced similar 0.1C discharge times and 0.01C discharge voltages.

[0089] Figures 1 and 2 are comparative diagrams of the first charge and discharge curves of Examples 1 and 2-5 of the present application, and Examples 1 and Comparative Examples 1-3 of the present application, respectively. It can be seen from the figure that the discharge curves under different electrolytes are divided into four sections. The first section is from the starting point of the curve to the first lowest voltage point (i.e., the end point of the first section), the second section is from the end point of the first section to the second lowest voltage point (i.e., the end point of the second section), the third section is from the end point of the second section to the third lowest voltage point (i.e., the end point of the third section), and the fourth section is from the end point of the third section to the end of the discharge, corresponding to different discharge currents (0.1C, 0.05C, 0.01C, and 0.01mA). As can be seen from Figure 1, under the condition of ensuring that the composition of the electrolyte additive remains unchanged, appropriately changing the type and content of the lithium salt and the solvent ratio has little effect on the charge and discharge curve of the graphite-based material. As can be seen from Figure 2, compared with other electrolyte additives, when the electrolyte additive includes only fluoroethylene carbonate (Example 1), the first discharge step (0.1C discharge, first segment of the curve) is the longest, indicating that the 0.1C discharge duration is the longest and the first discharge specific capacity is the highest. The third discharge step (0.01C discharge, third segment of the curve) has the smallest peak value, indicating that the 0.01C discharge voltage maximum is the smallest, indicating reduced polarization and reduced voltage rebound.

[0090] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. An electrolyte suitable for a working electrode made of graphite-based material, characterized in that: The invention is composed of a non-aqueous organic solvent, a lithium salt and an additive, wherein the additive is fluoroethylene carbonate.

2. The electrolyte suitable for a working electrode made of graphite-based materials according to claim 1, characterized in that: Based on the total mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is 0.1% to 10%.

3. The electrolyte suitable for a working electrode made of graphite-based materials according to claim 2, characterized in that: The mass percentage of the fluoroethylene carbonate is 5% to 9%.

4. The electrolyte suitable for a working electrode made of graphite-based materials according to any one of claims 1 to 3, characterized in that: The lithium salt includes at least one of LiPF6, LiBF4, LiClO4, LiPO2F2, LiTFSI, and LiFSI; Optionally, the lithium salt is LiPF6.

5. The electrolyte suitable for working electrode of graphite-based material according to claim 4, characterized in that: The concentration of the lithium salt is 0.8 to 1.3 mol / L based on the total volume of the electrolyte; Optionally, the concentration of the lithium salt is 1.0-1.2 mol / L.

6. The electrolyte suitable for a working electrode made of graphite-based materials according to any one of claims 1 to 3, characterized in that: The non-aqueous organic solvent includes at least two of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

7. The electrolyte suitable for a working electrode made of graphite-based material according to claim 6, characterized in that: The non-aqueous organic solvent is a combination of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate.

8. The electrolyte suitable for working electrodes made of graphite-based materials according to claim 7, characterized in that: The volume ratio of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate is 1.5-3.5:1-5:1-5.

9. A lithium ion battery, characterized in that: An electrolyte suitable for a working electrode of a graphite-based material as claimed in any one of claims 1 to 8; Optionally, the negative electrode of the lithium-ion battery is a graphite-based material.

10. Use of the electrolyte according to any one of claims 1 to 8, which is suitable for working electrodes made of graphite-based materials, in electrode material research and development, raw material evaluation or battery design.

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