A silicon negative electrode material, a preparation method thereof, and a solid-state battery

Abstract

The present invention belongs to the technical field of battery materials, and discloses a silicon negative electrode material, a preparation method thereof, and a solid-state battery. The silicon negative electrode material includes silicon and a lithium iodide layer coated on the surface of the silicon, and the particle size D of the silicon 50 is less than 100 μm. For the silicon negative electrode material of the present invention, lithium iodide is used as a coating material to coat the surface of silicon materials with a specific particle size, which can significantly improve the conductivity of the silicon negative electrode, reduce polarization, improve the rate performance, and the lithium iodide shell can also inhibit the volume expansion of silicon to a certain extent, reduce the damage to the contact between the silicon material and the sulfide solid electrolyte during the lithiation and delithiation processes of the silicon material, and improve the cycle stability and rate performance of the material.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to a silicon negative electrode material, a preparation method thereof, and a solid-state battery. Background Art

[0002] In recent years, with the development of new energy vehicles, electronic communication devices, and large-scale energy storage power grids, consumers' demand for lithium-ion batteries with high capacity, long life, high stability, and fast charge and discharge performance has been increasing. However, current lithium-ion batteries based on liquid electrolytes may not be able to meet the growing energy and power density requirements, as well as safety needs. Therefore, research on high-capacity ion batteries is being carried out in depth. Sulfide solid-state batteries are considered potential next-generation high-capacity batteries. In addition, by removing flammable electrolyte liquids, sulfide solid-state batteries are expected to provide higher safety, and the negative electrode material plays a crucial role in the energy density of sulfide solid-state batteries.

[0003] Currently, various types of negative electrodes have been explored, such as insertion-type negative electrodes (e.g., graphite lithium titanate), alloy-type negative electrodes, conversion-type negative electrodes, and lithium metal negative electrodes. Among them, pure lithium metal negative electrodes have the highest specific capacity (3860 mAh / g -1 ), and the lowest potential (-3.04 V vs standard hydrogen electrode). However, the practical application of lithium negative electrodes in sulfide solid-state batteries is still limited by factors such as interface instability, solid electrolyte interface phase (SEI) formation, low critical current density, and lithium dendrite penetration. Compared with lithium, silicon exhibits the same high theoretical capacity (3590 mAh / g based on Li 3.75 Si at room temperature -1 ), which is almost 10 times that of graphite. The lithiation potential of silicon is 0.4 V (vs Li + / Li), which can not only prevent the risk of lithium plating and lithium dendrite growth but also obtain a higher energy density than using other alloy negative electrodes. In addition, abundant natural resources, low cost, and environmental friendliness make silicon negative electrodes a very promising solid-state silicon negative electrode material.

[0004] Silicon has been explored as a negative electrode material or a negative electrode component of batteries, but its rate performance and cycling performance in sulfide solid-state batteries are poor. Its poor rate performance is due to the low ionic conductivity of the silicon negative electrode, low lithium-ion transport rate, and large polarization of the silicon negative electrode. As the rate increases continuously, the capacity decreases significantly; while the poor cycling performance is due to about 300% volume expansion during the lithium insertion and extraction process of silicon. The excessive volume expansion leads to the continuous expansion of the gap between silicon and the sulfide solid electrolyte, and the continuous deterioration of the contact between the two, resulting in the continuous deterioration of electron and ion transport between the two, leading to the rapid attenuation of cycling performance.

[0005] In the prior art, the silicon negative electrode is modified by a coating method, so that the volume change of silicon during charge and discharge is small. However, the volume expansion of the silicon negative electrode obtained by the existing coating method is not improved well, and the cycle stability of the battery also needs to be further improved. The conductivity and rate performance of the modified silicon negative electrode also need to be improved.

[0006] Therefore, there is an urgent need for a new preparation method of silicon negative electrode to solve the problems of volume expansion and cycle stability, and further solve the problem of large polarization. Summary of the Invention

[0007] The present invention aims to solve at least one of the technical problems existing in the above prior art. For this purpose, the present invention provides a silicon negative electrode material, a preparation method thereof, and a solid-state battery. The silicon negative electrode material of the present invention has small polarization, good rate performance, and can well solve the problem of volume expansion, and has good cycle stability.

[0008] Lithium iodide, as a material with excellent ionic conductivity, is used as a coating material to coat the surface of silicon material, which can significantly improve the conductivity of the silicon negative electrode, reduce polarization, improve rate performance, and the lithium iodide shell can also inhibit the volume expansion of silicon to a certain extent, reduce the damage to the contact between the silicon material and the sulfide solid electrolyte during the lithiation and delithiation volume change of the silicon material, and improve the cycle stability and rate performance of the material.

[0009] The first aspect of the present invention provides a silicon negative electrode material.

[0010] Specifically, a silicon negative electrode material (denoted as silicon@lithium iodide) includes silicon and a lithium iodide layer coated on the surface of the silicon, and the particle size D of the silicon 50 is less than 100 μm.

[0011] Preferably, the particle size D of the silicon 50 is less than or equal to 50 μm. Further preferably, the particle size D of the silicon 50 is 20 nm to 1 μm. For example, it is 30 nm, 50 nm, 100 nm, 1 nm, 50 μm. Silicon with a suitable particle size helps to obtain a uniformly coated lithium iodide layer on the surface. A suitable particle size can achieve smaller porosity control and obtain a silicon negative electrode material with a smaller internal resistance.

[0012] Preferably, the thickness of the lithium iodide layer is 5 - 100 nm, and further preferably 10 - 60 nm. For example, it is 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm.

[0013] Preferably, in the silicon negative electrode material, the mass ratio of silicon is 80 - 95%, and further preferably 88 - 90%.

[0014] Preferably, in the silicon negative electrode material, the mass ratio of the lithium iodide layer is 5-20%, more preferably 10-12%. An appropriate mass of lithium iodide helps to form a uniform lithium iodide layer, thereby reducing the internal resistance between the silicon negative electrode material and the sulfide electrolyte.

[0015] The second aspect of the present invention provides a method for preparing a silicon negative electrode material.

[0016] Specifically, a method for preparing a silicon negative electrode material includes the following steps:

[0017] Dissolve lithium iodide in an organic solvent, then add silicon, stir and mix to obtain a mixture, and then remove the organic solvent and calcine to obtain the silicon negative electrode material.

[0018] Preferably, the organic solution is at least one of methanol, ethanol, propanol, acetone, acetonitrile, and ester organic solvents, more preferably ethanol.

[0019] Preferably, the concentration of lithium iodide in the organic solvent is 1-5 g / L, more preferably 2-2.5 g / L.

[0020] Preferably, the mass ratio of lithium iodide to silicon is 1:100 to 10:100, more preferably (5-10):100.

[0021] Preferably, the stirring speed of the stirring and mixing is 500-1000 rpm, more preferably 700-800 rpm.

[0022] Preferably, the duration of the stirring and mixing is 1-4 h, more preferably 2-3 h.

[0023] Preferably, the method for removing the organic solvent is one or more of rotary evaporation, freeze drying, vacuum drying, and filtration, more preferably rotary evaporation.

[0024] Preferably, the temperature of the rotary evaporation is 25-50 °C, more preferably 30-40 °C, the rotation speed of the rotary evaporation is 30-100 rpm, more preferably 60-80 rpm, and the duration of the rotary evaporation is 1-5 h, more preferably 2-3 h.

[0025] Preferably, the temperature of the vacuum drying is 60-100 °C, the vacuum pressure is -0.04 to -0.1 MPa, and the duration of the vacuum drying is 12-48 h.

[0026] More preferably, the temperature of the vacuum drying is 60-70 °C, the vacuum pressure is -0.05 to -0.1 MPa, and the duration of the vacuum heating is 24-36 h.

[0027] Preferably, the calcination temperature is 400 - 800 °C, and the calcination time is 1 - 10 h.

[0028] Preferably, the calcination temperature is 600 - 700 °C, and the calcination time is 4 - 6 h.

[0029] Preferably, the calcination is carried out under a protective atmosphere.

[0030] Preferably, the protective atmosphere is one or more of Ar, He, and N2.

[0031] Preferably, in the process of calcination under a protective atmosphere, it is first kept at 250 - 300 °C for 0.5 - 1.5 h, and then heated to 400 - 800 °C at a heating rate of 5 - 10 °C / min and kept for 0.5 - 9.5 h.

[0032] The third aspect of the present invention provides an application of a silicon negative electrode material.

[0033] Specifically, a solid-state battery includes a negative electrode, a sulfide solid electrolyte, and a positive electrode, and the negative electrode includes the above-mentioned silicon negative electrode material.

[0034] Preferably, the negative electrode further includes at least one of a sulfide solid electrolyte, a binder, and a conductive agent.

[0035] Preferably, the sulfide solid electrolyte includes LiP6S5Cl.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] (1) For the silicon negative electrode material of the present invention, lithium iodide is used as a coating material to coat the surface of silicon materials with a specific particle size, which can significantly improve the conductivity of the silicon negative electrode, reduce polarization, improve the rate performance, and the lithium iodide shell can also inhibit the volume expansion of silicon to a certain extent, reduce the damage to the contact between the silicon material and the sulfide solid electrolyte during the lithiation and delithiation processes of the silicon material, and improve the cycle stability and rate performance of the material.

[0038] (2) For the silicon negative electrode material of the present invention, the uniformly distributed lithium iodide on its surface can form a three-dimensional ion conduction network, improve the ion transport rate of the material, realize the rapid transport of lithium ions, reduce the polarization of the silicon negative electrode, and improve the rate performance of the sulfide solid-state battery. The lithium iodide material shell can reduce the influence of the volume expansion of silicon on the electrode structure, inhibit the increase in the porosity of the silicon negative electrode material, reduce the contact failure between the silicon negative electrode material and the sulfide solid electrolyte, and improve the problem of accelerated capacity decay of the silicon negative electrode material during long-term cycling.

[0039] (3) In the preparation method of the present invention, lithium iodide is dissolved in an organic solvent, and silicon materials are dispersed in the organic solvent, so that lithium iodide is uniformly dispersed on the surface of the silicon materials. Then, the organic solvent is removed, and silicon@lithium iodide is obtained through high-temperature calcination. The uniformly coated lithium iodide can avoid the contact between silicon and the sulfide solid electrolyte. Its high lithiation potential can effectively inhibit the decomposition of the sulfide electrolyte, reduce the generation of inert products, and lower the interfacial resistance, thereby improving the rate performance of the silicon negative electrode material in the sulfide solid-state battery. Secondly, the hard shell formed by lithium iodide can relieve the volume expansion and contraction during the lithiation and delithiation processes of silicon, inhibit the contact failure between silicon and the sulfide solid electrolyte, and effectively improve the cycle stability of the silicon negative electrode material in the sulfide solid-state battery. Using lithium iodide as the lithium source realizes the uniform coating of lithium iodide on the silicon surface, improves many disadvantages of the silicon negative electrode material in the sulfide solid-state battery, and is of great significance for the commercialization of sulfide solid-state batteries. Description of the Drawings

[0040] Figure 1 SEM image and energy dispersive spectrometer (EDS) scan result corresponding to Sample 1 prepared in Example 1;

[0041] Figure 2 Energy dispersive spectrometer (EDS) scan result of Comparative Sample 3 prepared in Comparative Example 3;

[0042] Figure 3 Initial Coulombic efficiency test result of the half-cell composed of the composite negative electrode corresponding to Sample 1 prepared in Example 1;

[0043] Figure 4 Rate performance test result of the half-cell composed of the composite negative electrode corresponding to Sample 1 prepared in Example 1. Detailed Description of the Embodiments

[0044] In order to make the technical solution of the present invention more clearly understood by those skilled in the art, the following examples are listed for illustration. It should be noted that the following examples do not limit the scope of protection required by the present invention.

[0045] Unless otherwise specified, the raw materials, reagents or devices used in the following examples can be obtained from conventional commercial channels or can be obtained by existing known methods.

[0046] Example 1: Preparation of Silicon Negative Electrode Material

[0047] A silicon negative electrode material (silicon@lithium iodide) includes silicon and a lithium iodide layer coated on the surface of the silicon, and the particle size D of the silicon 50 is 50 nm.

[0048] A preparation method of a silicon negative electrode material includes the following steps:

[0049] Dissolve 0.05 g of lithium iodide in 20 mL of absolute ethanol to obtain a pale yellow solution; then take 1 g of Si with D 50 = 50 nm and add it to the pale yellow solution, and stir at 800 rpm for 2 h;

[0050] Put the above-mentioned uniformly mixed solution into a rotary evaporation flask, rotate at 60 rpm, heat and vacuum rotate to evaporate ethanol at 40 °C, and then dry it in a vacuum oven at 60 °C under -0.1 MPa for 24 h to obtain a powder;

[0051] Finally, the obtained powder is calcined at high temperature to obtain a silicon negative electrode material. The specific calcination procedure is as follows: maintain at 300 °C for 1 h in a N2 atmosphere, and then heat to 600 °C at a heating rate of 5 °C / min and maintain for 4 h. The silicon negative electrode material sample obtained in this example is named sample 1.

[0052] Example 2: Preparation of silicon negative electrode material

[0053] Change the dosage of lithium iodide in Example 1 to 0.1 g, and keep other steps unchanged. The obtained sample is named sample 2.

[0054] Example 3: Preparation of silicon negative electrode material

[0055] Change the dosage of lithium iodide in Example 1 to 0.02 g, and keep other steps unchanged. The obtained sample is named sample 3.

[0056] Example 4: Preparation of silicon negative electrode material

[0057] Change the dosage of lithium iodide in Example 1 to 1 g, and keep other steps unchanged. The obtained sample is named sample 4.

[0058] Comparative Example 1

[0059] Change the particle size of silicon in Example 1 to D 50 = 100 μm, and keep other steps unchanged. The obtained sample is named comparative sample 1;

[0060] Comparative Example 2

[0061] Change lithium iodide in Example 1 to lithium chloride, and keep other steps unchanged. The obtained sample is named comparative sample 2.

[0062] Comparative Example 3

[0063] Change lithium iodide in Example 1 to sodium iodide, and keep other steps unchanged. The obtained sample is named comparative sample 3.

[0064] Product effect test

[0065] 1. Scanning electron microscope

[0066] Through SEM testing, it can be seen that in Figure 1 (a) of Figure 1 , Sample 1 is composed of a core of silicon and a shell of lithium iodide. The small particles of lithium iodide are evenly distributed on the surface of the silicon, firmly coating the silicon, which can effectively reduce the contact between silicon and the sulfide solid electrolyte and improve the lithium conduction performance of the material. From Figure 1 (b), (c), and (d) of Figure 1 , it can also be seen that Sample 1 contains Si, O, and I elements.

[0067] For the comparative sample 3, when energy dispersive spectrometer (EDS) testing is carried out ( Figure 2 ), no iodine element can be seen at all, and there is an obvious phenomenon that sodium iodide is not coated. This is attributed to the fact that it is difficult for sodium iodide to adhere evenly to the surface of silicon and form a uniform sodium iodide coating layer.

[0068] 2. Electrochemical performance testing

[0069] Samples prepared from Samples 1 - 4 and Comparative Samples 1 - 3 were used as active materials respectively. The active materials were mixed with LiP6S5Cl and PTFE (polytetrafluoroethylene) in a mass ratio of 60:40:1 to form a composite negative electrode, and then the negative electrode sheet was obtained through the processes of rolling and slicing. The negative electrode sheet was placed into a polyether ether ketone tube with a diameter of 10 mm. Then, 100 mg of LiP6S5Cl was weighed and added into the polyether ether ketone tube, and it was pressed into shape with a pressure of 12 MPa and closely contacted with the negative electrode sheet. Finally, a Li - In alloy sheet was added on the other side of LiP6S5Cl and pressed into shape with a pressure of 360 MPa to prepare a half - cell with a three - layer structure (composite negative electrode / solid electrolyte LiP6S5Cl / Li - In alloy).

[0070] Test conditions: Charge - discharge tests were carried out at 60 °C with currents of 0.1C, 0.3C, 0.5C, 0.8C, and 1C and a cut - off voltage of - 0.61 to 0.2V. The battery was tested under a pressure of 15 MPa.

[0071] 2.1. Electrochemical impedance testing

[0072] The internal resistance of the half - cell was tested using electrochemical impedance spectroscopy (EIS) to further judge the contact and lithium conduction performance between the obtained negative electrode material and the sulfide solid electrolyte LiP6S5Cl. The internal resistances of the half - cells composed of Samples 1 - 4 and Comparative Samples 1 - 3 are shown in Table 1.

[0073] Table 1: Internal resistances of the half - cells composed of Samples 1 - 4 and Comparative Samples 1 - 3

[0074] Sample Sample 1 Sample 2 Sample 3 Sample 4 Control Sample 1 Control Sample 2 Control Sample 3 Internal Resistance (Ω) 94.6 96.7 95.9 126.3 185.6 133.8 140.2

[0075] By comparing the internal resistances of the half-cells composed of Samples 1-4 and Comparative Samples 1-3, it can be found that the contact between Sample 1 and the sulfide solid electrolyte is the best, and the combination of the two achieves the lowest internal resistance, which is beneficial to the conduction of lithium ions and electrons in the composite anode, facilitating better rate performance. Sample 1 has achieved good coating of lithium iodide on graphite, effectively inhibiting the contact between silicon and the sulfide solid electrolyte. At the same time, lithium iodide can rapidly transfer lithium ions and electrons as an intermediate layer. From Comparative Samples 1-3, it can be seen that if the particle size of silicon is too large, or lithium chloride or sodium iodide is used instead of lithium iodide, the internal resistance of the prepared silicon anode materials is very large. Thus, it can be seen that in the technical solution of the present invention, the particle size of silicon and the substance of the coating layer are selective.

[0076] 2.2. First Coulombic Efficiency Test

[0077] The first Coulombic efficiency (initial efficiency, ICE) of the half-cells prepared according to the above method was tested, and the results are shown in Table 2. The efficiency of the half-cell corresponding to the composite anode of Sample 1 was 89.4% ( Figure 3 ).

[0078] Table 2: First Coulombic Efficiency of Half-Cells Composed of Samples 1-4 and Comparative Samples 1-3

[0079] Sample Sample 1 Sample 2 Sample 3 Sample 4 Control Sample 1 Control Sample 2 Control Sample 3 First Efficiency (%) 89.4 88.6 87.9 81.4 75.7 82.8 82.3

[0080] By comparing the initial efficiencies of the half-cells composed of Samples 1-4 and Comparative Samples 1-3, it can be seen that the initial efficiency of Sample 1 is the highest, indicating that the lithium iodide coating on its surface is the most uniform. The uniformly coated lithium iodide effectively reduces the contact between silicon and the sulfide solid electrolyte. Therefore, the side reaction between silicon and the sulfide solid electrolyte is correspondingly reduced, and the initial efficiency is improved. It can also be seen from Table 1 that the particle size of silicon or the type of substance in the coating layer has a great influence on the initial efficiency of the half-cell.

[0081] 2.3. Rate Performance Test

[0082] The rate performance of the half-cell composed of Sample 1 was tested, and the results are as Figure 4 shown. Sample 1 exhibited excellent rate performance, and it could even maintain a specific capacity of 2808 mAh / g at a rate of 1C. This fully demonstrates that the silicon@lithium iodide silicon anode material prepared by the present invention has excellent performance and commercial potential.

Claims

1. A sulfide solid-state battery, characterized in that: The invention comprises a negative electrode, a sulfide solid electrolyte and a positive electrode, wherein the negative electrode comprises a silicon negative electrode material, wherein the silicon negative electrode material is composed of silicon and a lithium iodide layer coated on the surface of the silicon, and the particle size D of the silicon is 50 Less than 100μm.

2. The sulfide solid-state battery according to claim 1, characterized in that: The particle size D of the silicon 50 Less than or equal to 50μm.

3. The sulfide solid-state battery according to claim 1, characterized in that: The thickness of the lithium iodide layer is 5-100 nm.

4. The sulfide solid-state battery according to claim 1, characterized in that: In the silicon negative electrode material, silicon accounts for 80-95% by mass.

5. The sulfide solid-state battery according to claim 1, characterized in that: The method for preparing the silicon negative electrode material comprises the following steps: Lithium iodide is dissolved in an organic solvent, and then silicon is added and stirred to obtain a mixture. The organic solvent is then removed and the mixture is calcined to obtain the silicon negative electrode material.

6. The sulfide solid-state battery according to claim 5, characterized in that: The concentration of the lithium iodide in the organic solvent is 1-5 g / L.

7. The sulfide solid-state battery according to claim 5, characterized in that: The method of removing the organic solvent is one or more of rotary evaporation, freeze drying, vacuum drying and filtration.

8. The sulfide solid-state battery according to claim 7, characterized in that: The temperature of the rotary evaporation is 25-50° C., the speed of the rotary evaporation is 30-100 rpm, and the duration of the rotary evaporation is 1-5 h.

9. The sulfide solid-state battery according to claim 7, characterized in that: The vacuum drying temperature is 60-100° C., the vacuum pressure is -0.04 to -0.1 MPa, and the vacuum drying time is 12-48 h; and / or the calcination temperature is 400-800° C., and the calcination time is 1-10 h.

Images