Silicon-carbon material and preparation method thereof, negative pole piece and secondary battery

Abstract

The invention discloses a silicon-carbon material and a preparation method thereof, a negative pole piece and a secondary battery, and relates to the technical field of secondary batteries. Silicon particles are deposited on porous carbon to form a silicon-carbon core, and a lithium silicate coating layer is formed on the silicon-carbon core, so that uniform'passivation 'is realized, meanwhile, a lithium element can be introduced, the surface pre-lithiation process of the silicon-carbon material is realized, and the first efficiency and gram volume of the material can be improved to a great extent.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a silicon-carbon material and a preparation method thereof, a negative electrode sheet and a secondary battery. Background Art

[0002] The new silicon-carbon material prepared by depositing silane on porous carbon is mainly used in lithium-ion battery negative electrode materials. Compared with traditional carbon-based negative electrode materials, it has a higher capacity. Benefiting from the "confinement" effect of porous carbon, compared with traditional silicon-carbon and silicon-oxygen negative electrodes, its volume expansion is controllable and has good cycle performance.

[0003] Generally speaking, the main processes for the preparation of silicon-carbon materials include: porous carbon preparation and classification, silane deposition of porous carbon and "passivation" treatment. After the porous carbon is graded to remove fine powder, it is put into the reactor, and silane is introduced to deposit nano-silicon in the pores of the porous carbon, and then "passivation" treatment is performed. The reason for the need for "passivation" treatment is that silane is deposited in the pores of porous carbon in the form of nano-silicon, which has high reactivity and is very easy to react with various components in the air. After the "passivation" treatment, it can effectively prevent the excessive activity of nano-silicon from causing the material to spontaneously combust.

[0004] There are two main types of existing "passivation" treatment processes:

[0005] (1) Micro-oxidation

[0006] The process of micro-oxidation is as follows: use nitrogen to fluidize the silicon-carbon product after silicon deposition in a fluidized bed to make it dispersed, and then introduce a small amount of carbon dioxide or water vapor into the system to oxidize the active nano-silicon into silicon dioxide. However, the micro-oxidation process will cause the first efficiency of the battery material to decrease after passivation treatment. There is also lithium loss caused by the formation of SEI film during the first charge and discharge process of the battery, which will further cause the first efficiency to decrease. Therefore, it is very important to replenish lithium for the negative electrode material.

[0007] (2) Carbon coating

[0008] The carbon coating process is as follows: use nitrogen to fluidize the silicon-carbon product after silicon deposition in a fluidized bed to make it dispersed, and then introduce acetylene / methane or other carbon-containing gas sources into the system to deposit carbon on the surface of silicon-carbon. The carbon coating process is limited by the uneven distribution of carbon source gas concentration, temperature, and pressure in the reactor. Some nano-silicon may not be coated, which will cause the nano-silicon to react with air and even spontaneous combustion.

[0009] Therefore, it is urgent to develop a new passivation process that can achieve effective passivation without affecting the electrochemical performance.

[0010] In view of this, the present invention is proposed. Summary of the invention

[0011] The purpose of the present invention is to provide a silicon-carbon material and a preparation method thereof, aiming to realize the "passivation" process of the carbon-silicon material under the premise of improving the material capacity and the initial efficiency.

[0012] Another object of the present invention is to provide a negative electrode plate and a secondary battery, aiming to improve the capacity and initial efficiency of the battery.

[0013] The present invention is achieved in that:

[0014] In a first aspect, the present invention provides a silicon-carbon material, comprising a silicon-carbon core and a lithium silicate coating layer coated on the silicon-carbon core, wherein the silicon-carbon core comprises porous carbon and silicon particles, and the silicon particles are deposited on the porous carbon.

[0015] In an optional embodiment, the lithium silicate salt in the lithium silicate coating layer is selected from Li 2 SiO 3 , Li 2 Si 2 O 5 and Li 4 SiO 4 At least one of .

[0016] In an optional embodiment, in the silicon-carbon material, the carbon content is 40%-50%, the silicon content is 45%-55%, and the lithium content is 3%-8%, by mass fraction;

[0017] Preferably, the specific surface area of ​​the porous carbon before deposition is 1000 g / cm 3 -2000g / cm 3 , the volume fraction of micropores is greater than 80%.

[0018] In a second aspect, the present invention provides a method for preparing a silicon-carbon material, comprising: depositing silicon particles on porous carbon to form a silicon-carbon core, and utilizing the silicon-carbon core to react with a lithium-containing compound to form a lithium silicate coating layer.

[0019] In an optional embodiment, the method comprises: utilizing porous carbon to react with a silicon source gas to deposit silicon particles in the pores of the porous carbon to form a silicon-carbon core; then, replacing the silicon source gas with a secondary reaction gas, and adding a lithium-containing compound into the reactor to react to form a lithium silicate salt coating layer;

[0020] Wherein, the secondary reaction gas is selected from at least one of carbon dioxide and water vapor;

[0021] Preferably, the lithium-containing compound is added after the introduction of the secondary reaction gas is stopped.

[0022] In an optional embodiment, during the reaction of the porous carbon and the silicon source gas, the reaction temperature is controlled to be 350° C.-750° C., and the reaction time is 1 h-10 h.

[0023] In an alternative embodiment, the silicon source gas is silane.

[0024] In an optional embodiment, during the reaction of adding the lithium-containing compound, the reaction temperature is controlled to be 400° C.-700° C., and the reaction time is 0.5 h-10 h;

[0025] Preferably, the lithium-containing compound is selected from at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium hydride and lithium metal; more preferably lithium hydride;

[0026] Preferably, the mass ratio of the lithium-containing compound to the porous carbon is controlled to be (0.1-10):100.

[0027] In a third aspect, the present invention provides a negative electrode plate, comprising the silicon-carbon material of any one of the aforementioned embodiments or the silicon-carbon material prepared by the preparation method of any one of the aforementioned embodiments.

[0028] In a fourth aspect, the present invention provides a secondary battery comprising the negative electrode sheet of the aforementioned embodiment.

[0029] The present invention has the following beneficial effects: silicon particles are deposited on porous carbon to form a silicon-carbon core, and a lithium silicate coating layer is formed on the silicon-carbon core, thereby achieving uniform "passivation" and introducing lithium elements, thereby realizing the process of pre-lithiation of the surface of the silicon-carbon material, which can greatly improve the initial efficiency and gram capacity of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0031] Figure 1 This is the XRD pattern of the carbon silicon material prepared in Example 1.

[0032] Figure 2 This is the SEM image of the carbon silicon material prepared in Example 1. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0034] An embodiment of the present invention provides a method for preparing a silicon-carbon material, comprising the following steps:

[0035] S1. Preparation of silicon carbon core

[0036] Silicon particles are deposited on the porous carbon to form a silicon-carbon core. Through this step, a silicon-carbon negative electrode material can be prepared. However, since the silicon on the surface is too active, it needs to be "passivated" through step S2.

[0037] In actual operation, the process of preparing the silicon-carbon core includes: utilizing porous carbon to react with silicon source gas to deposit silicon particles in the pores of the porous carbon to form a silicon-carbon core. This reaction process can be carried out in a fluidized bed reactor, and the silicon source gas introduced can be silane, but is not limited to this.

[0038] In some embodiments, during the reaction of porous carbon and silicon source gas, the reaction temperature is controlled to be 350°C-750°C, and the reaction time is 1h-10h. The reaction temperature and time are regulated to achieve uniform deposition of silicon particles. During the reaction, the deposition amount of silicon particles corresponding to each gram of porous carbon is controlled by regulating the introduction rate of the silicon source gas, so that the silicon ratio and carbon ratio meet the requirements of the product.

[0039] Specifically, during the reaction of porous carbon and silicon source gas, the reaction temperature can be 350°C, 400°C, 500°C, 600°C, 700°C, 750°C, etc.; the reaction time can be 1h, 3h, 5h, 8h, 10h, etc.

[0040] S2. Passivation and pre-lithiation

[0041] The silicon-carbon core is reacted with lithium-containing compounds to form a lithium silicate coating layer, which not only achieves "passivation" but also pre-lithiation.

[0042] In the actual operation process, after the porous carbon reacts with the silicon source gas, the silicon source gas is replaced by the secondary reaction gas, and a lithium-containing compound is added to the reactor to react to form a lithium silicate coating layer. The lithium silicate can be Li 2 SiO 3 , Li 2 Si 2 O 5 , Li 4 SiO 4Etc. Wherein, the secondary reaction gas is selected from at least one of carbon dioxide and water vapor, and can be any one or more of the above.

[0043] In some embodiments, the lithium-containing compound is selected from at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium hydride and lithium metal, and may be any one or more of the above; more preferably, lithium hydride. Taking lithium hydride as the lithium-containing compound and carbon dioxide as the secondary reaction gas, the reaction principle is explained, and the overall reaction equation is:

[0044] 2LiH+Si+3CO 2 =Li 2 SiO 3 +3CO+H 2 ;

[0045] It is divided into the following two steps:

[0046] Si+CO 2 =SiO+CO;

[0047] 2LiH+3SiO=Li 2 SiO 3 +2Si+H 2 ;

[0048] In some embodiments, the lithium-containing compound is added after the secondary reaction gas is stopped to prevent lithium hydride from reacting with carbon dioxide to form lithium carbonate. The amount of the secondary reaction gas introduced is determined according to the content of the silicon particles and the lithium-containing compound, and the secondary reaction gas is slightly excessive, such as about 5%-10% excess relative to the lithium-containing compound.

[0049] Furthermore, during the reaction of adding the lithium-containing compound, the reaction temperature is controlled to be 400°C-700°C, the reaction time is 0.5h-10h, and the mass ratio of the lithium-containing compound to the porous carbon is controlled to be (0.1-10): 100. By controlling the amount of the lithium-containing compound, the reaction temperature and time and other parameters, the amount of lithium introduced is more appropriate to ensure the electrochemical properties of the material.

[0050] Specifically, in the process of adding lithium-containing compounds, the reaction temperature can be controlled to 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, etc., and the reaction time can be 0.5h, 1.0h, 3.0h, 5.0h, 8.0h, 10.0h, etc.

[0051] An embodiment of the present invention also provides a silicon-carbon material, including a silicon-carbon core and a lithium silicate coating layer coated on the silicon-carbon core. The silicon-carbon core includes porous carbon and silicon particles. The silicon particles are deposited on the porous carbon. By forming a core-shell structure, the lithium silicate is coated on the surface of the material, thereby achieving surface passivation and surface pre-lithiation.

[0052] In some embodiments, the lithium silicate salt in the lithium silicate salt coating layer is selected from Li 2 SiO 3 , Li 2 Si 2 O 5 and Li 4 SiO 4 At least one of the above, can be any one or more of the above.

[0053] In some embodiments, in the silicon-carbon material, the carbon content is 40%-50%, the silicon content is 45%-55%, and the lithium content is 3%-8%, by mass fraction. By optimizing the content of each element, the first efficiency and gram capacity of the material are significantly improved while ensuring the "passivation" effect.

[0054] Specifically, in terms of mass fraction, in silicon-carbon materials, the carbon content can be 40%, 45%, 50%, etc., the silicon content can be 45%, 50%, 55%, etc., and the lithium content can be 3%, 4%, 5%, 6%, 7%, 8%, etc., and there is no lithium element distributed inside the particles.

[0055] In some embodiments, the specific surface area of ​​the porous carbon is 1000 g / cm 3 -2000g / cm(such as 1000g / cm 3 , 1200g / cm 3 , 1500g / cm 3 , 1800g / cm 3 , 2000g / cm 3 The particle size of the deposited silicon particles is 0.01nm-1μm, and the size of the silicon particles depends on the porous carbon.

[0056] An embodiment of the present invention further provides a negative electrode plate, comprising the above-mentioned silicon-carbon material, and may also include a negative electrode current collector, etc. The silicon-carbon material provided by the embodiment of the present invention can be used as the negative electrode active material, and a negative electrode slurry can be prepared by using a binder, a conductive agent and a dispersant, etc. The negative electrode slurry is coated on the negative electrode current collector, and the negative electrode plate is obtained after drying.

[0057] An embodiment of the present invention provides a secondary battery, including the above-mentioned negative electrode plate, and may also include a positive electrode plate, an electrolyte, a separator, etc., to form a complete battery structure. The active material on the positive electrode plate can be a lithium battery positive electrode material, and the specific type is not limited.

[0058] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0059] It should be noted that the specific surface area of ​​the porous carbon used in the following examples is 2500 m2 / g, the purity of silane gas is 90%; lithium hydride is a powder with a particle size of 3-10 μm, purchased from MacLean.

[0060] Example 1

[0061] This embodiment provides a method for preparing a silicon-carbon material, comprising the following steps:

[0062] 1 kg of porous carbon was placed in a fluidized bed reactor with argon as the protective atmosphere, and silane gas was introduced at 500°C for 5 hours for silicon deposition. The introduction rate of silane gas was 3 L / min. After the silicon deposition was completed, a silicon-carbon core was obtained. The argon atmosphere in the fluidized bed reactor was maintained, and the silane gas was replaced with carbon dioxide gas at a rate of 1 L / min for 1 hour. After that, the introduction of carbon dioxide gas was stopped, and 60 g of lithium hydride was input into the reactor. The reaction temperature was controlled to 700°C and the reaction time was 1 hour.

[0063] Example 2

[0064] The only difference from Example 1 is that after the introduction of carbon dioxide gas is stopped, 120 g of lithium oxide is introduced into the reactor for reaction, and other conditions remain unchanged.

[0065] Example 3

[0066] The only difference from Example 1 is that after the introduction of carbon dioxide gas is stopped, 30 g of lithium oxide is introduced into the reactor for reaction, and other conditions remain unchanged.

[0067] Comparative Example 1

[0068] This embodiment provides a method for preparing a silicon-carbon material, comprising the following steps: taking 1 kg of porous carbon and placing it in a fluidized bed reactor with argon as a protective atmosphere, introducing silane gas at 500° C. for silicon deposition for 5 hours, and the introduction rate of silane gas is 3 L / min.

[0069] Test example

[0070] (1) The XRD results of the carbon silicon material prepared in Example 1 are as follows: Figure 1 shown.

[0071] Figure 1 The characteristic peaks of 2θ at 18.91°, 26.92°, 33.01°, and 38.44° are attributed to Li 2 SiO 3 ; 2θ characteristic peaks of 22.23°, 22.59°, and 33.92° are attributed to Li 4 SiO 4 ; 2θ characteristic peaks of 28.52°, 47.41°, and 56.25° are attributed to Li2 Si 2 O 5 , proving the formation of silicates.

[0072] (2) The SEM image of the carbon silicon material prepared in Test Example 1 is as follows: Figure 2 shown.

[0073] from Figure 2 It can be seen that the prepared carbon silicon material is distributed in blocks, and the particle size is relatively uniform, approximately 8-15 μm.

[0074] (3) The specific composition of the carbon silicon material prepared in the embodiment and the comparative example was tested by the ICP and carbon-sulfur tester. The results are shown in Table 1.

[0075] Table 1 Specific element composition of carbon silicon materials

[0076] Carbon content wt / % Silicon content wt / % Lithium content wt / % Oxygen content wt / % Example 1 40 50 5 5 Example 2 35 50 10 5 Example 3 45 50 2.5 2.5 Comparative Example 1 50 50 0 0

[0077] (4) The carbon silicon materials prepared in the examples and comparative examples were used to assemble batteries, and the capacity and initial efficiency were tested. The results are shown in Table 2.

[0078] Battery assembly: Mix silicon-carbon material: binder: conductive agent = 8:1:1 (mass ratio) to obtain negative electrode slurry, apply the negative electrode slurry on the surface of nickel foam, and obtain the negative electrode plate after drying; assemble the negative electrode plate, positive electrode plate (active material is ternary NCM material), electrolyte (lithium hexafluorophosphate), and separator (PE film) into button batteries.

[0079] Test conditions: ①Discharge: 0.1C-0.0005V, the discharge capacity is recorded as Q1, Q1 is the first discharge capacity in grams; ②Charge: 0.1C-0.8V, the charge capacity is recorded as Q2, Q2 is the first charge capacity in grams; the first charge efficiency is abbreviated as ICE, ICE=Q2 / Q1.

[0080] Table 2 Gram capacity and first effect test results

[0081] 0.8v capacity / mAh / g 0.8V first coulomb efficiency / % Example 1 1700 80％ Example 2 1680 79％ Example 3 1670 75％ Comparative Example 1 1650 73％

[0082] Combining Table 1 and Table 2, and comparing Examples 1-3 with Comparative Example 1, it can be seen that appropriate lithium addition is helpful to improve the capacity and initial efficiency of the battery material.

[0083] (5) The gas production of the carbon silicon material was obtained by testing the embodiment and the comparative example. The results are shown in Table 3.

[0084] Test method: Take the prepared carbon silicon material and test it in a constant temperature and humidity chamber at 25℃. Take out the material after 1 hour to test the gas production.

[0085] Table 3 Gas production test results

[0086] Dipping time Gas output / L Example 1 1h 0 Comparative Example 1 1h 0.3L

[0087] It can be seen that the gas production of Example 1 is substantially 0, which proves the uniformity of its passivation.

[0088] The above are only preferred embodiments of the present invention and are 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 silicon-carbon material, characterized in that: It comprises a silicon-carbon core and a lithium silicate coating layer coated on the silicon-carbon core. The silicon-carbon core comprises porous carbon and silicon particles. The silicon particles are deposited on the porous carbon.

2. The silicon-carbon material according to claim 1, characterized in that: The lithium silicate salt in the lithium silicate coating layer is selected from at least one of Li2SiO3, Li2Si2O5 and Li4SiO4.

3. The silicon-carbon material according to claim 1 or 2, characterized in that: In terms of mass fraction, in the silicon-carbon material, the carbon content is 40%-50%, the silicon content is 45%-55%, and the lithium content is 3%-8%; Preferably, the specific surface area of ​​the porous carbon before deposition is 1000 g / cm 3 -2000g / cm 3 , the volume fraction of micropores is greater than 80%.

4. A method for preparing a silicon-carbon material, characterized in that: include: Silicon particles are deposited on porous carbon to form a silicon-carbon core, and the silicon-carbon core is reacted with a lithium-containing compound to form a lithium silicate coating layer.

5. The preparation method according to claim 4, characterized in that: include: Using the porous carbon to react with a silicon source gas, the silicon particles are deposited in the pores of the porous carbon to form the silicon-carbon core; Afterwards, the silicon source gas is replaced by a secondary reaction gas, and the lithium-containing compound is added into the reactor for reaction to form the lithium silicate coating layer; Wherein, the secondary reaction gas is selected from at least one of carbon dioxide and water vapor; Preferably, the lithium-containing compound is added after the introduction of the secondary reaction gas is stopped.

6. The preparation method according to claim 4, characterized in that: During the reaction of the porous carbon and the silicon source gas, the reaction temperature is controlled to be 350° C.-750° C., and the reaction time is 1 h-10 h.

7. The preparation method according to claim 6, characterized in that: The silicon source gas is silane.

8. The preparation method according to claim 4, characterized in that: During the reaction of adding the lithium-containing compound, the reaction temperature is controlled to be 400° C.-700° C., and the reaction time is 0.5 h-10 h; Preferably, the lithium-containing compound is selected from at least one of lithium hydroxide, lithium oxide, lithium carbonate, lithium hydride and lithium metal; more preferably lithium hydride; Preferably, the mass ratio of the lithium-containing compound to the porous carbon is controlled to be (0.1-10):

100.

9. A negative electrode plate, characterized in that: The invention comprises the silicon-carbon material according to any one of claims 1 to 3 or the silicon-carbon material prepared by the preparation method according to any one of claims 4 to 8.

10. A secondary battery, characterized in that: Including the negative electrode sheet as described in claim 9.

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