Modified silicon negative electrode material and preparation method and application thereof

Abstract

The invention discloses a modified silicon negative electrode material and a preparation method and application thereof, belongs to the technical field of all-solid-state batteries, and can compensate the active lithium loss of the whole battery, improve the cycle performance of the battery and relieve the volume change through negative electrode pre-lithiation. The method comprises the following steps: pressing silicon powder into blocks under a certain pressure; coating the blocky silicon and the cut lithium foil according to a certain proportion, grinding and mixing to obtain a precursor I; under the protection of inert gas, the precursor I is heated for the first time, and a powder precursor II is obtained; and grinding the precursor II, pressing the precursor II into blocks, heating the blocks in a vacuum or inert atmosphere to a preset temperature, and grinding the blocks to obtain the lithium-silicon alloy. The preparation process is simple, the silicon negative electrodes with different pre-lithiation degrees can be prepared by controlling the lithium addition amount, the cycling stability of the battery is improved, and the volume expansion of the electrode is relieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of all-solid-state batteries, and particularly relates to a modified silicon negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] With the increasing requirements of energy density and safety of rechargeable batteries, the development of all-solid-state batteries has become an important direction for the development of next-generation batteries. Silicon has a low lithium intercalation platform and is environmentally friendly and has abundant reserves, and is considered to be one of the most promising new-generation lithium-ion battery negative electrode materials.

[0003] At present, silicon as a negative electrode material still faces many challenges in practical application. First, silicon is a semiconductor material with low electronic conductivity, which leads to poor rate capability and cycle performance. During the lithium ion intercalation and deintercalation process, the volume of silicon changes significantly, and the huge volume expansion easily causes problems such as electrode structure pulverization and active material shedding, thereby resulting in a first coulomb efficiency of the silicon negative electrode usually between 50% and 80%, which is far below the level required for commercialization. The low first coulomb efficiency irreversibly consumes active lithium ions in the positive electrode, which means that additional positive electrode capacity is needed to compensate, resulting in a decrease in the energy density of the full battery, which is not conducive to its practical application.

[0004] At present, composite modification strategies are mainly used to improve the conductivity and mechanical stability of silicon materials. Due to the weak conductivity of silicon materials and poor ion transport, the rate capability is poor, which cannot meet the demand of large power devices for large rate charging and discharging in actual working conditions. Therefore, silicon materials often need to be compounded with materials with good conductivity to improve their conductivity. The silicon / carbon composite structure is one of the common modification measures for silicon negative electrode materials. However, the severe volume expansion of silicon in practical application will still be transmitted to the carbon-based skeleton, resulting in cracking of the composite structure and loss of electrical contact, thereby affecting the cycle stability of the electrode, which still cannot well solve the problem of large volume expansion of the silicon negative electrode. SUMMARY

[0005] The application provides a modified silicon negative electrode material and a preparation method and application thereof. The lithium intercalation of the electrode material provides an additional lithium source, the negative electrode prelithiation can compensate for the loss of active lithium in the full battery, improve the cycle performance of the battery, and improve the volume expansion of the all-solid-state battery, solving the problems of low first coulomb efficiency and large volume expansion of the silicon negative electrode.

[0006] To achieve the above object, the application adopts the following technical scheme: A preparation method of a modified silicon negative electrode material, comprising the following steps: S1, pressing silicon powder into a block under a certain pressure; S2, coating the bulk silicon with the cut lithium foil in a certain proportion, grinding and mixing to obtain a precursor I; S3, under the protection of inert gas, primary heating of the precursor I to obtain a powder precursor II; S4, grinding the precursor II, pressing into a block, heating to a predetermined temperature in a vacuum or inert atmosphere, and grinding to obtain a lithium-silicon alloy.

[0007] In the above steps, the particle size of the silicon powder is 10-250 nm; the silicon powder pressing pressure is 300-500 MPa; the molar ratio of lithium to silicon is 0.88-4.4.

[0008] Under the protection of inert gas, the precursor I is heated for the first time to obtain a powder precursor II, the precursor II is ground and pressed into a block, heated to a predetermined temperature in a vacuum or inert atmosphere, and heat treated in an inert gas atmosphere to ensure complete alloying reaction through multiple grinding; the first heating is to allow lithium and silicon to react initially, which is not uniform at this time; the second heating is to mix the components uniformly after grinding, and then heat again to make the reaction more uniform and obtain a more complete alloy phase.

[0009] The first heating temperature is 200-800℃, and the time is 15-60min; the second heating temperature is 200-800℃, and the time is 1-10h.

[0010] The inert gas is N2, He or Ar.

[0011] The pressing pressure of the precursor II is 300-800MPa.

[0012] A modified silicon negative electrode material is a lithium-silicon alloy formed by uniformly embedding lithium ions into a silicon negative electrode.

[0013] The above modified silicon negative electrode material can be used in a full solid-state battery, by pre-embedding lithium ions into the silicon negative electrode to compensate for the lithium consumed due to side reactions, and improving the volume expansion of the full solid-state battery, thereby improving the cycle life of the electrode, and improving the total capacity and energy density of the battery. When pre-lithiation reaches the maximum silicon capacity, highly uniform lithium distribution will lead to the formation of a uniform alloy phase, and the potential level of the silicon negative electrode is close to that of lithium.

[0014] The full solid-state battery includes a lithium-silicon alloy prepared by the preparation method as the negative electrode, a sulfide solid-state electrolyte as the solid-state electrolyte, and LiNi 0.33 Co 0.33 Mn 0.33 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi0.92 Co 0.05 Mn 0.03 O2, and the positive electrode comprises a positive electrode active material, a conductive agent, and a halide solid-state electrolyte.

[0015] Beneficial effects: The present application provides a modified silicon negative electrode material and its preparation method and application, compared with the prior art, the present application has the following advantages: The present application provides a silicon negative electrode pre-lithiation method based on thermal alloying reaction, and constructs a corresponding full solid-state battery system. The method introduces lithium source into the electrode through thermal alloying method, effectively realizes the pre-lithiation treatment of silicon material, adjusts the degree of silicon pre-lithiation, reduces the volume expansion of silicon in the battery charging and discharging process, reduces the damage to the battery, and improves the battery performance. Moreover, it provides additional active lithium to compensate for the capacity loss during the cycle process, improves the coulomb efficiency, and at the same time shows a lower volume expansion rate, which helps to alleviate the damage of volume expansion to the electrode structure, and improves the electrochemical performance and cycle stability of the silicon negative electrode in the full solid-state battery. The lithium silicon alloy prepared by the present application is directly used as the negative electrode of the full solid-state battery without adding conductive carbon and solid-state electrolyte. This is because the embedding of lithium ions significantly improves the electronic conductivity of lithium silicon alloy compared with silicon. At the same time, the interface contact area between the solid-state electrolyte and the silicon negative electrode is reduced to a two-dimensional plane, which helps to form a stable passivation SEI layer and avoids continuous interface side reactions between the solid-state electrolyte and the silicon negative electrode.

[0016] During the cycle process of the full solid-state battery, the formed lithium silicon alloy not only has significantly better ion and electron conductivity than silicon, but also shows a lower volume expansion rate, which helps to alleviate the stress concentration and structural damage inside the full battery. It is suitable for the cold pressing forming process commonly used in full solid-state batteries, and is beneficial to improve the overall structural stability and electrochemical performance of the full battery. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 X-ray diffraction patterns of the negative electrode materials in Example 1, Example 2 and Example 3 and Comparative Example 1, wherein "Si" represents the battery prepared in Comparative Example 1, "Li 0.88 Si" represents the battery prepared in Example 1, "Li 1.76 Si" represents the battery prepared in Example 2, "Li 2.33 Si" represents the battery prepared in Example 3.

[0018] Figure 2 Discharge specific capacity vs. cycle number for the batteries prepared in Example 1-Example 3 and Comparative Example 1, wherein "Si" represents the battery prepared in Comparative Example 1, "Li 0.88Si” represents the battery prepared in Example 1, “Li 1.76 Si” represents the battery prepared in Example 2, “Li 2.33 Si” represents the battery prepared in Example 3.

[0019] Figure 3 The pressure trend of the battery prepared in Comparative Example 1 with the change of cycle time.

[0020] Figure 4 The pressure trend of the battery prepared in Example 1 with the change of cycle time.

[0021] Figure 5 The pressure trend of the battery prepared in Example 2 with the change of cycle time.

[0022] Figure 6 The pressure trend of the battery prepared in Example 3 with the change of cycle time. DETAILED DESCRIPTION

[0023] The application will be described in detail below with specific examples and accompanying drawings: Example 1

[0024] The lithium silicon alloy negative electrode and the all-solid-state battery of the present example are prepared by the following steps: S1: Preparation of lithium silicon alloy negative electrode material The nanosilicon powder is placed in a 10 mm pressing mold and pressed into a sheet at a pressure of 300 MPa; the cut lithium sheet is wrapped around the silicon sheet, and the precursor I is obtained by grinding and mixing according to the molar ratio of lithium sheet to silicon sheet of 0.88:1; the precursor I is heat treated at 250°C for 30 min to obtain the powder precursor II; the precursor II is ground and placed in a 10 mm pressing mold and pressed into a sheet at a pressure of 300 MPa; the precursor II is heated to 300°C in a vacuum or inert atmosphere, and the lithium silicon alloy is obtained by grinding for 10 h of heat treatment time; the obtained lithium silicon alloy is directly used as a negative electrode material, assembled into a simulated half-cell, and subjected to electrochemical test.

[0025] S2: Preparation of all-solid-state battery LiNi 0.8 Co 0.1 Mn 0.1 O, carbon fiber (VGCF) and Li3InCl6 are mixed according to a mass ratio of 80:3:27 to obtain a positive electrode material. The sulfide electrolyte Li6SP5Cl is pressed into a sheet under a pressure of 300 MPa to obtain a solid-state electrolyte sheet. The lithium silicon alloy negative electrode material is pressed into a sheet on one side of the solid-state electrolyte sheet at a pressure of 50 MPa. The positive electrode material is pressed into a sheet on the other side of the solid-state electrolyte sheet at a pressure of 50 MPa to form a positive electrode, and an all-solid-state battery is obtained. Example 2

[0026] The difference between this embodiment and embodiment 1 is only that the content of lithium foil added in the pre-lithiation process is different, resulting in different pre-lithiation degrees of the silicon negative electrode. Specifically, the lithium-silicon alloy negative electrode and the all-solid-state battery of this embodiment are prepared by the following steps: S1: Preparation of lithium-silicon alloy negative electrode material The nano-silicon powder was placed in a 10 mm pressing mold to press into a sheet with a pressure of 300 MPa; the cut lithium sheet was wrapped with the silicon sheet, and the precursor I was obtained by grinding and mixing according to the molar ratio of lithium sheet to silicon sheet of 1.76:1; the precursor I was heat-treated at 250°C for 30 min to obtain powder precursor II; the precursor II was ground and placed in a 10 mm pressing mold to press into a sheet with a pressure of 300 MPa; the precursor II was heated to 300°C in a vacuum or inert atmosphere, and the heat treatment time was 10 h; the lithium-silicon alloy was obtained by grinding; the obtained lithium-silicon alloy was directly used as a negative electrode material to assemble a simulated half-cell for electrochemical test.

[0027] S2: Preparation of all-solid-state battery LiNi 0.8 Co 0.1 Mn 0.1 O, carbon fiber (VGCF) and Li3InCl6 were mixed according to a mass ratio of 80:3:27 to obtain a positive electrode material. The sulfide electrolyte Li6SP5Cl was pressed into a sheet under a pressure of 300 MPa to obtain a solid-state electrolyte sheet. The lithium-silicon alloy negative electrode material was pressed into a sheet on one side of the solid-state electrolyte sheet with a pressure of 50 MPa. The positive electrode material was pressed into a sheet on the other side of the solid-state electrolyte sheet with a pressure of 50 MPa to form a positive electrode, thereby obtaining an all-solid-state battery. Embodiment 3

[0028] The difference between this embodiment and embodiment 1 is only that the content of lithium foil added in the pre-lithiation process is different, resulting in different pre-lithiation degrees of the silicon negative electrode. Specifically, the lithium-silicon alloy negative electrode and the all-solid-state battery of this embodiment are prepared by the following steps: S1: Preparation of lithium-silicon alloy negative electrode material The nano-silicon powder was placed in a 10 mm pressing mold to press into a sheet with a pressure of 300 MPa; the cut lithium sheet was wrapped with the silicon sheet, and the precursor I was obtained by grinding and mixing according to the molar ratio of lithium sheet to silicon sheet of 1.76:1; the precursor I was heat-treated at 250°C for 30 min to obtain powder precursor II; the precursor II was ground and placed in a 10 mm pressing mold to press into a sheet with a pressure of 300 MPa; the precursor II was heated to 300°C in a vacuum or inert atmosphere, and the heat treatment time was 10 h; the lithium-silicon alloy was obtained by grinding; the obtained lithium-silicon alloy was directly used as a negative electrode material to assemble a simulated half-cell for electrochemical test.

[0029] S2: Preparation of all-solid-state battery LiNi0.8 Co 0.1 Mn 0.1 O, carbon fiber (VGCF) and Li3InCl6 were mixed in a mass ratio of 80:3:27 to obtain a positive electrode material. The sulfide electrolyte Li6SP5Cl was pressed into a sheet under a pressure of 300 MPa to obtain a solid electrolyte sheet. The silicon negative electrode material was pressed into a sheet on one side of the solid electrolyte sheet at a pressure of 50 MPa. The positive electrode material was pressed into a sheet on the other side of the solid electrolyte sheet at a pressure of 50 MPa to form a positive electrode, thereby obtaining a full solid-state battery.

[0030] Comparative Example 1 The difference between this comparative example and Example 1 is that silicon is directly used as the negative electrode of the full solid-state battery. Specifically, the full solid-state battery of this comparative example is prepared by the following steps: S1: Preparation of a full solid-state battery LiNi 0.8 Co 0.1 Mn 0.1 O, carbon fiber (VGCF) and Li3InCl6 were mixed in a mass ratio of 80:3:27 to obtain a positive electrode material. The sulfide electrolyte Li6SP5Cl was pressed into a sheet under a pressure of 300 MPa to obtain a solid electrolyte sheet. The silicon negative electrode material was pressed into a sheet on one side of the solid electrolyte sheet at a pressure of 50 MPa. The positive electrode material was pressed into a sheet on the other side of the solid electrolyte sheet at a pressure of 50 MPa to form a positive electrode, thereby obtaining a full solid-state battery. Test

[0031] Negative electrode thermal alloy prelithiation: The negative electrode materials prepared in Example 1, Example 2, Example 3, and Comparative Example 1 were taken to detect the X-ray diffraction (XRD) pattern, and the test results are shown in Figure 1 From the Figure 1 , it can be seen that from Si to Li 2.33 Si, the characteristic diffraction peak of Si gradually weakens, widens, and eventually disappears, indicating that the crystal structure of silicon is gradually destroyed during lithium intercalation. At the same time, at a low degree of lithiation, amorphous lithium-silicon alloy is formed, so no clear crystal phase peak is formed. With the increase of the degree of lithiation, the crystal phase Li 2.33The peak of Si is more and more obvious, which can better explain that with the increase of the degree of lithiation, the amorphous phase will gradually transform into the crystal phase; pure silicon needs the most lithium to be inserted, and the total volume expansion will be greater, although it is a crystal phase; therefore, although there is stress relaxation, the stress accumulated by the huge volume change is the largest; the phase of early lithiation, the volume has been partially expanded when pre-inserting lithium, when the lithium insertion starts, the amorphous phase is formed, and the Young's modulus of the amorphous phase is lower than that of the crystal phase, which means that under the same pressure, the deformation of the amorphous phase is greater, and the intermediate pores are smaller, at this time the lithium ion insertion during the charging process will cause the volume expansion of silicon, thereby causing a greater degree of volume change; and with the increase of the degree of lithiation, the amorphous phase gradually transforms into the crystal phase, the volume change will decrease, and the pressure will also decrease.

[0032] The all-solid-state batteries prepared in each example and comparative example were tested according to the following method: Battery performance: The cycle performance of the all-solid-state battery was detected under the conditions of temperature 30℃, charge and discharge current 0.1C, and pressure 50MPa, and the test results are shown in Table 2 and FIG. 2. Figure 2 .

[0033] The pressure evolution of the all-solid-state battery was detected under the conditions of temperature 30℃, charge and discharge current 0.1C, and pressure 50MPa, and the test results are shown in Table 1 and FIG. 1. Figures 3-6 .

[0034] Table 1 Pressure evolution results of all-solid-state batteries with different negative electrode materials

[0035] The degree of lithiation in the table is calculated according to the maximum lithiation phase of silicon Li 4.4 Si, which is Si-100%Li here, and the percentage here represents the degree of lithiation of silicon, which is 0%, that is, pure silicon; 20% means that the silicon is lithiated by 20%, that is, Li （4.4×20%） Si; and so on.

[0036] From Table 1, Figures 2-6 It can be seen that: (1) The test results of Example 1 to Example 3 and Comparative Example 1 show that, compared with pure silicon negative electrode material, the prelithiation method of the application can effectively alleviate the volume expansion and improve the cycle stability of the full battery. The reason is that: the Young's modulus of the lithium-silicon alloy prepared by the method of the application is much lower than that of silicon. The Young's modulus of the original silicon is as high as 10.0-11.6 GPa, while after complete lithiation, the Young's modulus can be reduced to 1.3-1.5 GPa. The lithium-silicon alloy with lower hardness is more suitable for the cold pressing process of the full solid-state battery, which is conducive to forming good solid-solid contact, reducing the interface impedance in the full solid-state battery, and the structure can better adapt to the volume change in the cycle process, thereby maintaining the structural stability and cycle performance of the full solid-state battery.

[0037] (2) Nano-silicon exhibits typical semiconductor properties of silicon, with low electronic conductivity and almost no lithium ion transport capacity. It is usually necessary to mix conductive carbon to prepare a composite electrode. The application of lithium treatment directly uses lithium-silicon alloy as negative electrode material, which helps to build a conductive network in the negative electrode and promotes the transport of lithium ions, thereby avoiding the formation of dendrites. On the other hand, the pre-embedded lithium ions play a role in lithium supplementation during the cycle process of the full solid-state battery, thereby improving the cycle performance of the full solid-state battery.

[0038] The above-described embodiments are only preferred embodiments of the application, and the protection scope of the application is not limited thereto. Any equivalent replacement or transformation made by those skilled in the art based on the application is within the protection scope of the application.

Claims

1. A method for producing a modified silicon negative electrode material, characterized by, The method comprises the following steps: S1, pressing silicon powder under a certain pressure; S2, coating the pressed silicon with lithium foil according to a certain proportion, grinding and mixing to obtain a precursor I; S3, primary heating the precursor I under inert gas protection to obtain a powder precursor II; S4, grinding and pressing the precursor II, heating to a predetermined temperature in vacuum or inert atmosphere, and grinding to obtain a lithium-silicon alloy material, and the volume change of silicon is controlled by changing the pre-lithiation degree of silicon.

2. The method for preparing the modified silicon anode material according to claim 1, characterized in that, The particle size of the silicon powder is 10-250 nm.

3. The method for producing the modified silicon negative electrode material according to claim 1 or 2, characterized by, The pressing pressure of the silicon powder is 300-500 MPa.

4. The method for preparing the modified silicon anode material according to claim 1, characterized in that, The molar ratio of lithium to silicon is 0.88-4.

4.

5. The method for preparing the modified silicon anode material according to claim 1, characterized in that, The temperature of the primary heating is 200-800℃, and the time is 15-60 min.

6. The method for preparing the modified silicon anode material according to claim 1 or 5, characterized in that, The temperature of the secondary heating is 200-800℃, and the time is 1-10 h.

7. The method for preparing the modified silicon anode material according to claim 6, characterized in that, The pressing pressure of the precursor II is 300-800 MPa.

8. The method of claim 1, wherein the modified silicon anode material is prepared by a process comprising: providing a silicon material; and coating the silicon material with a coating material. The inert gas is N2, He or Ar.

9. The modified silicon anode material produced by the method of any one of claims 1-8, characterized in that, The negative electrode material is a lithium-silicon alloy material formed by uniformly embedding lithium ions into a silicon negative electrode.

10. Use of the modified silicon negative electrode material according to claim 9, characterized in that The modified silicon negative electrode material is used as a negative electrode of a full solid-state battery.

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