Low-temperature-resistant graphite negative electrode material and preparation method thereof

Abstract

According to the low-temperature-resistant graphite negative electrode material disclosed by the invention, the characteristic that expanded graphite has a large interlayer spacing is adopted, and meanwhile, the interlayer spacing is synchronously expanded by utilizing P doping, so that the migration rate of low-temperature lithium ions in the graphite is favorably improved due to the expansion of the interlayer spacing; the surface metal Ag coating can further improve the conductivity and preferentially occupy the surface defect position, so that the situation that an electrolyte and negative electrode reaction graphite fall off to damage an SEI film is reduced, and the stability of the SEI film is improved; according to the low-temperature-resistant graphite negative electrode material, the diffusion rate and the SEI film stability are improved, and the low-temperature-resistant performance is good.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery negative electrode materials, and in particular to a low-temperature resistant graphite negative electrode material and a preparation method thereof. Background Art

[0002] Carbon materials have the advantages of high lithium storage capacity, abundant raw materials, low electrode potential, low cost, non-toxicity and stability, and have been rapidly developed. The carbon negative electrode materials of lithium-ion batteries are mainly natural graphite and artificial graphite. At present, the modification technology of carbon negative electrode materials is mainly through multi-phase coating technology to construct core-shell structure, doping, surface oxidation, etc., so as to improve the electrochemical performance. However, there are still many problems, such as poor contact between electrode material particles and between electrode material particles and current collector, large surface resistance, poor peel strength, cyclic expansion during charge and discharge, etc. The above problems seriously affect the cycle life and stability of lithium-ion batteries.

[0003] High energy density and high power density lithium-ion battery technology has attracted widespread attention in the past few years because of its potential application in hybrid vehicles and electric vehicles. As a commercial graphite negative electrode material, when charging, Li+ is embedded in the graphite material, it must first be desolvated. This process consumes a certain amount of energy and hinders the diffusion of Li+ into the graphite. In addition, at low temperatures, the SEI film impedance is large, the electrolyte ion conductivity is low in a low temperature environment (-40°C), the interface impedance between the positive and negative electrodes and the electrolyte increases, the charge transfer impedance on the surface of the positive and negative electrodes increases, the diffusion rate of lithium ions in the negative electrode decreases, and the kinetic characteristics of the graphite negative electrode deteriorate. Especially during the charging process, the electrochemical polarization of the negative electrode is significantly aggravated, and it is easy to precipitate metallic lithium to form lithium dendrites, penetrate the diaphragm and cause a short circuit between the positive and negative electrodes, causing safety hazards.

[0004] At present, the research on low-temperature graphite modification is mainly surface coating, which is to coat the surface of graphite powder with a layer of soft carbon or hard carbon, or materials such as lithium titanate to improve its interface properties. However, these methods do not improve the diffusion capacity of lithium ions at low temperatures. Summary of the invention

[0005] The main purpose of the present invention is to provide a low-temperature resistant graphite negative electrode material and a preparation method thereof, so that the graphite negative electrode material has better SEI film stability.

[0006] To achieve the above object, the present invention provides a method for preparing a low-temperature resistant graphite negative electrode material, comprising the steps of:

[0007] S1, providing an expanded graphite substrate;

[0008] S2, mixing sodium phosphate and expanded graphite substrate according to a P:C ratio of 1:8 to 12, calcining at 500-700° C. for 3-6 hours, and grinding and crushing after cooling to obtain P-doped expanded graphite with enlarged interlayer spacing;

[0009] S3, using a deposition process to deposit Ag metal on the surface of the P-doped expanded graphite with enlarged interlayer spacing obtained in step S2, to obtain a low-temperature resistant graphite negative electrode material of Ag-coated graphite with large interlayer spacing.

[0010] Furthermore, the heating rate during calcination in step S2 is 1.5-3°C / min.

[0011] Furthermore, the product obtained later in the middle of step S2 is cooled and ground, washed with distilled water and then dried to obtain the P-doped expanded graphite with enlarged interlayer spacing.

[0012] Furthermore, the deposition process in step S3 is atomic layer deposition.

[0013] Furthermore, the mixed material of the expanded graphite substrate and the sodium phosphate in step S2 is placed in a semi-enclosed crucible and calcined in a furnace.

[0014] Furthermore, the calcination temperature in step S2 is 600° C. for 4 hours.

[0015] Furthermore, the expanded graphite substrate and the sodium phosphate have a P:C ratio of 1:10.

[0016] Furthermore, the present invention also provides a low-temperature resistant graphite negative electrode material, which is prepared by the preparation method as described in any one of the above claims.

[0017] The present invention also provides a negative electrode current collector for a lithium ion battery, the negative electrode current collector comprising: a conductive substrate and a negative electrode slurry layer coated on one or both sides of the conductive substrate, the negative electrode slurry layer being prepared by any of the preparation methods described above.

[0018] The present invention further provides a lithium ion battery, comprising a positive electrode, an electrolyte and a negative electrode, characterized in that the negative electrode comprises the lithium ion battery negative electrode collector as described above.

[0019] The low-temperature resistant graphite negative electrode material of the present invention adopts expanded graphite which has the characteristic of large interlayer spacing, and simultaneously expands the interlayer spacing by P doping. Since the expansion of the interlayer spacing is conducive to improving the migration rate of low-temperature lithium ions in graphite, the surface metal Ag coating improves the conductivity and preferentially occupies the surface defect positions, reduces the graphite shedding and damage to the SEI film caused by the reaction between the electrolyte and the negative electrode, and improves the stability of the SEI film. Therefore, the low-temperature resistant graphite negative electrode material of the present invention has the advantages of improved diffusion rate and SEI film stability, and has good low-temperature resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0021] Figure 1 This is the SEM image of the undoped expanded graphite obtained in Comparative Example 1.

[0022] Figure 2 This is the SEM+EDS surface scan of the P-doped expanded graphite of Comparative Example 2.

[0023] Figure 3 SEM+EDS scan of the expanded graphite doped with P and coated with Ag in Example 1. DETAILED DESCRIPTION

[0024] The technical solution in this embodiment will be described clearly and completely below in conjunction with the drawings in this embodiment. Obviously, the described embodiment is only a part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0026] It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0027] Comparative Example 1

[0028] Expanded graphite material before doping;

[0029] The expanded graphite without introducing other impurities is obtained by explosively expanding natural graphite flakes sold on the market using HCLO as an expanding agent.

[0030] Comparative Example 2

[0031] Selection of expanded graphite material (10 g): Expanded graphite without other impurities obtained by explosively expanding natural graphite flakes using HCLO as an expanding agent is selected.

[0032] Expanded graphite material (10 g), red phosphorus (2 g) and 200 ml of anhydrous ethanol were subjected to high-energy ball milling at 1200 rpm for 8 h, and dried in a drying oven at 80° C. for 24 h to obtain P-doped expanded graphite.

[0033] Example 1

[0034] Selection of expanded graphite material (2 g): Expanded graphite sold on the market, which is obtained by explosively expanding natural graphite flakes using HCLO as an expanding agent and which does not introduce other impurities, is selected.

[0035] The expanded graphite substrate was mixed with sodium phosphate according to a P:C ratio of 1:10, and then transferred to a semi-closed crucible and calcined at 600°C in a muffle furnace for 4 hours at a heating rate of 2°C / min. The obtained product was cooled to room temperature and then ground, washed with distilled water for multiple times, and dried to obtain P-doped expanded graphite with enlarged interlayer spacing. Then, Ag metal (1 g) was deposited on the surface of the P-doped expanded graphite with enlarged interlayer spacing by ALD deposition to obtain Ag-coated large interlayer spacing graphite (i.e., low-temperature resistant graphite negative electrode material).

[0036] Example 2

[0037] Selection of expanded graphite material (2 g): Expanded graphite sold on the market, which is obtained by explosively expanding natural graphite flakes using HCLO as an expanding agent and which does not introduce other impurities, is selected.

[0038] The expanded graphite substrate was mixed with sodium phosphate according to the ratio of P:C of 1:12, and then transferred to a semi-closed crucible and calcined at 650°C in a muffle furnace for 4 hours at a heating rate of 2°C / min. The obtained product was cooled to room temperature and then ground, washed with distilled water for multiple times, and dried to obtain P-doped expanded graphite with enlarged interlayer spacing. Then, Ag metal (1 g) was deposited on the surface of the P-doped expanded graphite with enlarged interlayer spacing by ALD deposition to obtain Ag-coated large interlayer spacing graphite (i.e., low-temperature resistant graphite negative electrode material).

[0039] Example 3

[0040] Selection of expanded graphite material (2 g): Expanded graphite sold on the market, which is obtained by explosively expanding natural graphite flakes using HCLO as an expanding agent and which does not introduce other impurities, is selected.

[0041] The expanded graphite substrate was mixed with sodium phosphate according to a P:C ratio of 1:8, and then transferred to a semi-closed crucible and calcined at 550°C in a muffle furnace for 4 hours at a heating rate of 2°C / min. The obtained product was cooled to room temperature and then ground, washed with distilled water for multiple times, and dried to obtain P-doped expanded graphite with enlarged interlayer spacing. Then, Ag metal (1 g) was deposited on the surface of the P-doped expanded graphite with enlarged interlayer spacing by ALD deposition to obtain Ag-coated large interlayer spacing graphite (i.e., low-temperature resistant graphite negative electrode material).

[0042] Please refer to Figure 1-3 It can be seen that P and Ag are uniformly doped into the surface of graphite, and the changes in the interlayer spacing are compared by HRTEM+XRD diffraction of the peak position, and it is found that the interlayer spacing of graphite becomes larger; Figure 1-3 Combined with EDS energy spectrum analysis, it can be seen that Ag is completely coated.

[0043] By conducting electrical performance tests on Comparative Examples 1-2 and Example 1, the test results are as follows:

[0044]

[0045] The capacity retention rate difference before and after modification was compared through low temperature environment cycle test, which showed that the graphite negative electrode material prepared in Example 1 had good low temperature resistance.

[0046] The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for preparing a low-temperature resistant graphite negative electrode material, characterized in that: Includes steps: S1, providing an expanded graphite substrate; S2, mixing sodium phosphate and expanded graphite substrate according to a P:C ratio of 1:8 to 12, calcining at 500-700° C. for 3-6 hours, and grinding and crushing after cooling to obtain P-doped expanded graphite with enlarged interlayer spacing; S3, using a deposition process to deposit Ag metal on the surface of the P-doped expanded graphite with enlarged interlayer spacing obtained in step S2, to obtain a low-temperature resistant graphite negative electrode material of Ag-coated graphite with large interlayer spacing.

2. The method according to claim 1, characterized in that The heating rate during calcination in step S2 is 1.5-3°C / min.

3. The method according to claim 1, characterized in that The product calcined in step S2 is cooled and ground, washed with distilled water and then dried to obtain the P-doped expanded graphite with enlarged interlayer spacing.

4. The method according to claim 1, characterized in that: The deposition process in step S3 is atomic layer deposition.

5. The method according to claim 1, characterized in that The mixed material of the expanded graphite substrate and the sodium phosphate in the step S2 is placed in a semi-closed crucible and calcined in a furnace.

6. The method according to claim 1, characterized in that The calcination temperature in step S2 is 600° C. for 4 hours.

7. The method according to claim 1, characterized in that The expanded graphite substrate and sodium phosphate have a P:C ratio of 1:

10.

8. A low temperature resistant graphite negative electrode material, characterized in that: The method is prepared according to any one of claims 1 to 8.

9. A negative electrode current collector for a lithium ion battery, characterized in that: The negative electrode current collector comprises: a conductive substrate and a negative electrode slurry layer coated on one side or both sides of the conductive substrate, wherein the negative electrode slurry layer is prepared by the preparation method according to any one of claims 1 to 7.

10. A lithium ion battery comprising a positive electrode, an electrolyte and a negative electrode, characterized in that: The negative electrode comprises the negative electrode current collector of the lithium ion battery as claimed in claim 9.

Images