Preparation method of graphite negative electrode material with artificial SEI film

Abstract

The invention discloses a preparation method of a graphite negative electrode material with an artificial SEI (solid electrolyte interface) film, which comprises the following steps: 1) pretreating graphite at high temperature, and drying to obtain spherical graphite; step 2) adding 50-200 parts by weight of PF5, 50-200 parts by weight of Li3PO4 and 50-200 parts by weight of graphite into 500 parts by weight of an NMP solvent in a nitrogen atmosphere, and stirring to obtain a reaction solution; 3) placing the reaction solution in a normal pressure environment, and reacting to obtain lithium salt coated graphite; 4) after the reaction is finished, heating to remove part of NMP; (5) carrying out carbonization reaction on the graphite which is coated with the lithium salt and contains a certain amount of NMP in a nitrogen atmosphere; the lithium salt-coated and N-doped graphite negative electrode material is obtained. The lithium salt is used as the artificial SEI film, so that the rate capability and the charge-discharge efficiency of the battery are improved; and by adopting an N doping technical method, side reaction and electrolyte decomposition are reduced, so that the coulombic efficiency of the first circle is improved.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion batteries, and in particular to a method for preparing a graphite negative electrode material with an artificial SEI film. Background Art

[0002] With the rapid development of portable devices and electric vehicles, lithium-ion batteries have become one of the most important energy storage technologies in today's era. Their high energy density, long cycle life and relatively stable discharge characteristics make lithium-ion batteries very popular. One of the core components of batteries is the negative electrode material, and graphite has become an excellent negative electrode material for lithium-ion batteries due to its low cost, low lithium potential and high reversible lithium ion insertion and extraction.

[0003] Graphite has many advantages as a negative electrode material for lithium-ion batteries, which makes it a very popular negative electrode material in today's lithium-ion batteries. As a negative electrode material for lithium-ion batteries, graphite has the advantages of high specific capacity (theoretical specific capacity is 372mAh g-1), good cycle stability (supporting the battery to perform thousands of discharge cycles without obvious loss of capacity), low cost, mature processing technology, good safety performance and good low-temperature performance. However, there are still some limitations of graphite negative electrode materials. The peeling of graphite sheets and the continuous formation of SEI during the charge and discharge cycle lead to serious capacity loss and lithium deposition, which reduces the performance of graphite-based batteries and increases safety risks. Therefore, it is necessary to modify its surface, that is, to realize an artificial SEI film on the graphite surface, which can enhance the long-cycle performance of graphite.

[0004] The naturally formed SEI film is not uniform or stable enough, so researchers have developed a variety of methods to artificially synthesize a better SEI film to improve battery performance. For example, the electrolyte additive method (adding specific additives to the electrolyte can form a more stable SEI film with good lithium conductivity at the beginning of the cycle), pre-lithiation treatment (introducing a small amount of lithium into the negative electrode before battery assembly can pre-build a part of the SEI film), surface coating technology (coating a protective layer on the surface of the electrode material by physical or chemical methods, which can help form a stable SEI film when the battery is working), direct deposition of SEI film (using chemical vapor deposition, electrochemical deposition and other technologies to directly deposit a layer of material similar to the ideal SEI structure on the electrochemical surface. This method allows the composition and thickness of the SEI film to be controlled).

[0005] Patent CN117361519A uses polymer as an artificial SEI film to prevent direct contact between graphite and electrolyte, effectively inhibiting graphite peeling. Another example is the invention application with application number CN201510137268.4, which discloses a method for preparing a negative electrode for a lithium-ion battery, in which lithium salt is added to the negative electrode slurry to form a metastable SEI film on the negative electrode surface. However, the graphite negative electrode material obtained in the above prior art still has defects such as unstable performance and unsatisfactory electrochemical performance. Summary of the invention

[0006] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a method for preparing a graphite negative electrode material with an artificial SEI film. The artificial SEI film is made of lithium salt, which not only prevents direct contact between graphite and electrolyte, but also provides more lithium ions, improves the transmission efficiency of electrons and lithium ions, thereby improving the rate performance and charge and discharge efficiency of the battery; and the present invention adopts N-doping technology to provide active sites for the embedding and de-embedding of lithium ions, thereby improving the specific capacity of the graphite negative electrode material; nitrogen doping can promote the formation of a more stable and uniform SEI film on the graphite surface, reduce the occurrence of side reactions and the decomposition of the electrolyte, thereby improving the first-cycle coulomb efficiency.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a method for preparing a graphite negative electrode material with an artificial SEI film, the preparation method comprising the following steps:

[0008] Step 1), pre-treating graphite at high temperature, and then drying it to obtain spherical graphite;

[0009] Step 2), under a nitrogen atmosphere, by weight, 50 to 200 parts of PF5, 50 to 200 parts of Li3PO4, and 50 to 200 parts of graphite are added to 500 parts of NMP solvent, and stirred to obtain a reaction solution;

[0010] Step 3), placing the reaction solution in a normal pressure environment to react to obtain lithium salt-coated graphite;

[0011] Step 4), after the reaction is completed, heating is performed to remove part of the NMP;

[0012] Step 5), subjecting the graphite coated with lithium salt and containing a certain amount of NMP to carbonization reaction in a nitrogen atmosphere to obtain a graphite negative electrode material coated with lithium salt and containing N doping.

[0013] Furthermore, in step 2), the stirring time is 30 to 90 minutes; in step 3), the reaction temperature is 80 to 200°C, and the reaction time is 10 to 30 hours; in step 4), the heating temperature is 80 to 200°C; in step 5), the temperature of the carbonization reaction is 400 to 700°C, and the carbonization reaction time is 1 to 5 hours.

[0014] Furthermore, in step 1), the particle size range D50 of spherical graphite is 6 to 20 μm. Graphite pretreatment is mainly carried out using a spray dryer. By selecting a suitable nozzle, adjusting the spray pressure and spray rate, the size and shape of the droplets are accurately controlled to produce more uniform droplets, thereby forming more regular spherical particles during the drying process. The advantages of spherical graphite are small specific surface area, high tap density, and uniform particle size distribution. It can provide higher first-cycle coulomb efficiency and reversible specific capacity, and has better cycle stability.

[0015] Furthermore, in step 4), the NMP content is measured by gas chromatograph to ensure that the mass fraction of the remaining NMP in the total NMP is 5 to 20%.

[0016] Furthermore, in the lithium salt-coated spherical graphite, the thickness of the lithium salt coating layer ranges from 3 to 6 nm.

[0017] The beneficial effects of the present invention are as follows: compared with the prior art, the method for preparing a graphite negative electrode material with an artificial SEI film provided by the present invention has the following advantages:

[0018] 1) LiPF2O2 coating can improve the high temperature performance and cycle stability of the battery.

[0019] First, a stable SEI film is formed. The lithium salt coating can form a uniform and stable SEI film on the surface of the negative electrode. This film can effectively isolate the electrolyte from the negative electrode material, prevent the decomposition of the electrolyte and the occurrence of side reactions, thereby improving the cycle stability and life of the battery. The lithium salt coating can promote the uniform formation of the SEI film and reduce unnecessary electrochemical reactions. Secondly, after adding lithium difluorophosphate, the surface resistance can be reduced and the electron and ion transmission can be enhanced. The lithium salt coating can improve the surface conductivity of the negative electrode material and improve the transmission efficiency of electrons and lithium ions, thereby improving the rate performance and charge and discharge efficiency of the battery. It can reduce the adverse reaction between the negative electrode material and the electrolyte, reduce the risk of thermal runaway of the battery under high temperature conditions, and improve the safety performance of the battery. It can reduce the structural changes of the negative electrode material during the charge and discharge process and improve the cycle stability and life of the battery.

[0020] 2) Nitrogen doping can significantly improve the electrochemical properties of graphite anode materials through various mechanisms.

[0021] The introduction of nitrogen atom pairs provides active sites for the embedding and de-embedding of lithium ions, thereby improving the specific capacity of graphite negative electrode materials; nitrogen doping can promote the formation of a more stable and uniform SEI film on the graphite surface, reduce the occurrence of side reactions and the decomposition of the electrolyte, and thus improve the first-cycle coulombic efficiency. DETAILED DESCRIPTION

[0022] The present invention is further described below by specific examples, but these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0023] Example 1

[0024] A method for preparing a graphite negative electrode material with an artificial SEI film:

[0025] (1) Graphite is pretreated at high temperature and then treated with a spray dryer to obtain spherical graphite.

[0026] (2) Under a nitrogen atmosphere, add 126 g PF5, 116 g Li3PO4, and 109 g spherical graphite into NMP solvent and stir for 30 min.

[0027] (3) Place in a normal pressure environment at 100°C and react for 24 hours.

[0028] (4) After the reaction is completed, the mixture is placed in an oven and heated at 120° C. to remove part of the NMP. The NMP content is measured using a gas chromatograph to ensure that the remaining NMP content is 5%.

[0029] (5) The graphite coated with LiPF2O2 and containing 5% NMP was placed in a tube furnace, nitrogen was introduced, and carbonization reaction was carried out at 600°C for 3 hours to obtain a graphite negative electrode material coated with LiPF2O2 and containing N doping.

[0030] The above graphite negative electrode material was made into an electrode sheet, installed with a lithium sheet into a button cell, and tested.

[0031] The test method refers to GB / T 24533-2019, graphite negative electrode materials for lithium-ion batteries.

[0032] The test results are: first cycle coulombic efficiency: 96%, first cycle specific capacity: 356mAh g -1 .

[0033] Example 2

[0034] A method for preparing a graphite negative electrode material with an artificial SEI film:

[0035] (1) Graphite is pretreated at high temperature and then treated with a spray dryer to obtain spherical graphite.

[0036] (2) Under a nitrogen atmosphere, add 112 g PF5, 125 g Li3PO4, and 100 g spherical graphite into NMP solvent and stir for 60 min.

[0037] (3) Place in a normal pressure environment at 90°C and react for 24 hours.

[0038] (4) After the reaction is completed, the mixture is placed in an oven and heated at 100° C. to remove part of the NMP. The NMP content is measured using a gas chromatograph to ensure that 10% of the NMP content remains.

[0039] (5) The graphite coated with LiPF2O2 and containing 10% NMP was placed in a tube furnace, nitrogen was introduced, and carbonization reaction was carried out at 500°C for 4 hours to obtain a graphite negative electrode material coated with LiPF2O2 and containing N doping.

[0040] The above graphite negative electrode material was made into an electrode sheet, installed with a lithium sheet into a button cell, and tested.

[0041] The test method refers to GB / T 24533-2019, graphite negative electrode materials for lithium-ion batteries.

[0042] The test results are: first cycle coulombic efficiency: 95%, first cycle specific capacity: 357mAh g -1 .

[0043] Example 3

[0044] A method for preparing a graphite negative electrode material with an artificial SEI film:

[0045] (1) Graphite is pretreated at high temperature and then treated with a spray dryer to obtain spherical graphite.

[0046] (2) Under a nitrogen atmosphere, add 120 g PF5, 100 g Li3PO4, and 110 g spherical graphite into NMP solvent and stir for 60 min.

[0047] (3) Place in a normal pressure environment at 80°C and react for 12 hours.

[0048] (4) After the reaction is completed, the mixture is placed in an oven and heated at 120° C. to remove part of the NMP. The NMP content is measured using a gas chromatograph to ensure that the remaining NMP content is 5%.

[0049] (5) The graphite coated with LiPF2O2 and containing 5% NMP was placed in a tube furnace, nitrogen was introduced, and carbonization reaction was carried out at 500°C for 4 hours to obtain a graphite negative electrode material coated with LiPF2O2 and containing N doping.

[0050] The above graphite negative electrode material was made into an electrode sheet, installed with a lithium sheet into a button cell, and tested.

[0051] The test method refers to GB / T 24533-2019, graphite negative electrode materials for lithium-ion batteries.

[0052] The test results are: first cycle coulombic efficiency: 95%, first cycle specific capacity: 355mAh g-1 .

[0053] The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also belong to the scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A method for preparing a graphite negative electrode material having an artificial SEI film, characterized in that: The preparation method comprises the following steps: Step 1), pre-treating graphite at high temperature, and then drying it to obtain spherical graphite; Step 2), under a nitrogen atmosphere, by weight, 50 to 200 parts of PF5, 50 to 200 parts of Li3PO4, and 50 to 200 parts of graphite are added to 500 parts of NMP solvent, and stirred to obtain a reaction solution; Step 3), placing the reaction solution in a normal pressure environment to react to obtain lithium salt-coated graphite; Step 4), after the reaction is completed, heating is performed to remove part of the NMP; Step 5), subjecting the graphite coated with lithium salt and containing a certain amount of NMP to carbonization reaction in a nitrogen atmosphere to obtain a graphite negative electrode material coated with lithium salt and containing N doping.

2. The method for preparing a graphite negative electrode material having an artificial SEI film according to claim 1, characterized in that: In step 2), the stirring time is 30 to 90 minutes; in step 3), the reaction temperature is 80 to 200°C and the reaction time is 10 to 30 hours; in step 4), the heating temperature is 80 to 200°C; in step 5), the temperature of the carbonization reaction is 400 to 700°C and the carbonization reaction time is 1 to 5 hours.

3. The method for preparing a graphite negative electrode material having an artificial SEI film according to claim 1, characterized in that: In step 1), the particle size range D50 of the spherical graphite is 6 to 20 μm.

4. The method for preparing a graphite negative electrode material having an artificial SEI film according to claim 1, characterized in that: In step 4), the NMP content is measured by gas chromatograph to ensure that the mass fraction of the remaining NMP in the total NMP is 5 to 20%.

5. The method for preparing a graphite negative electrode material having an artificial SEI film according to claim 1, characterized in that: In the lithium salt coated spherical graphite, the thickness of the lithium salt coating layer ranges from 3 to 6 nm.