Solid electrolyte, preparation method thereof and solid-state battery

Abstract

The invention discloses a solid electrolyte, a preparation method thereof and a solid battery, the solid electrolyte is a single crystal, the phase is a C2 / m space group of a monoclinic structure, the chemical formula is Li3MX6, and M comprises one or more of Y, In and Sc; x comprises one or more of halogens Cl, Br, I and F. According to the invention, the solid electrolyte of which the phase is a C2 / m space group of a monoclinic structure is subjected to single crystallization treatment, so that the ionic conductivity and the environmental stability of the solid electrolyte are greatly improved; and when the material is applied to an all-solid-state battery, the growth of lithium dendrites can be effectively inhibited, so that the battery has higher cycling stability.

Description

Technical Field

[0001] The present invention relates to the technical field of solid-state battery preparation, and in particular to a solid-state electrolyte and a preparation method thereof, and a solid-state battery. Background Art

[0002] All-solid-state batteries have excellent characteristics such as high safety and high specific energy, and have the potential to become the next generation of power batteries. Solid electrolyte materials with excellent performance are the key to all-solid-state batteries and directly determine the performance of all-solid-state batteries. Solid electrolytes based on oxides, polymers, sulfides and halides are currently the main development direction of solid electrolyte materials.

[0003] In 2018, Tetsuya Asano and others from Panasonic of Japan reported new halide solid electrolytes Li3YCl6 and Li3YBr6. This type of electrolyte has excellent ionic conductivity, high oxidation stability potential and good matching of positive electrode materials, and quickly became a research hotspot. Subsequently, a large number of Li3MX6-type halide electrolytes were reported. At present, this type of electrolyte mainly has three phase structures: monoclinic (C2 / m space group), orthorhombic (Pnma space group) and trigonal (P-3m1 space group). Among them, electrolytes with C2 / m structure, such as Li3InCl6, Li3YBr6, and Li3ScCl6, have higher ionic conductivity than other electrolytes with Pnma or P-3m1 structure. The literature reports that their room temperature ionic conductivity reaches 1.5mS / cm, 1.7mS / cm and 3.0mS / cm respectively.

[0004] However, there are still many outstanding problems for Li3MX6 solid electrolytes to be used in practice. On the one hand, the room temperature ionic conductivity of the material is still far behind the level of 10mS / cm of liquid electrolytes and sulfide electrolytes. On the other hand, the material is extremely sensitive to moisture in the environment and easily absorbs moisture, resulting in a decrease in ionic conductivity. For example, the ionic conductivity retention rate of the powder material after being exposed to a dew point environment of -40°C for 24 hours is less than 50% of the original performance, causing the electrolyte to be too demanding on the environment during use and difficult to meet the needs of practical applications. In addition, it also has common problems similar to other solid electrolytes, that is, it cannot really effectively prevent the formation of lithium dendrites, and the cycle stability of the prepared solid-state battery is poor. Summary of the invention

[0005] The present invention significantly improves the ionic conductivity of the electrolyte by crystallizing the Li3MX6 solid electrolyte, eliminating the internal interface and defects of the electrolyte; at the same time, significantly reduces the degree of reaction between the electrolyte and moisture in the environment, thereby improving the environmental stability of the material; and then applies it to all-solid-state batteries, effectively inhibiting the generation of lithium dendrites and improving the cycle stability of the battery.

[0006] A first aspect of an embodiment of the present invention provides a solid electrolyte, characterized in that the solid electrolyte is a single crystal, the physical phase is a monoclinic structure of C2 / m space group, the chemical formula is Li3MX6, M includes: one or more of Y, In, Sc; X includes: one or more of halogens Cl, Br, I and F.

[0007] Furthermore, the room temperature ionic conductivity of the solid electrolyte is ≥3×10 -3 S / cm.

[0008] Preferably, M is Y, and X is Br, or a mixture of Cl and Br.

[0009] Preferably, M is In or Sc, and X is Cl.

[0010] Furthermore, the room temperature ionic conductivity of the solid electrolyte is ≥5×10 -3 S / cm.

[0011] Accordingly, a second aspect of an embodiment of the present invention provides a method for preparing a solid electrolyte, which is used to prepare any of the above-mentioned solid electrolytes, comprising the following steps:

[0012] Mixing lithium halide and rare earth halide in a preset ratio to obtain a mixed material;

[0013] The mixed material is placed in a quartz crucible, vacuum-sealed, and placed in a Bridgman crystal furnace for single crystal growth, wherein the temperature of the crystal furnace heating zone is a first preset value, the temperature gradient of the cooling zone is a second preset value, and the crucible descending rate is a third preset value. After the growth is completed, a halide single crystal is obtained at a cooling rate of a fourth preset value;

[0014] The halide single crystal is cut to obtain a single crystal sheet of a preset thickness, namely the solid electrolyte layer.

[0015] Furthermore, during the crystal growth, the preset temperature in the high temperature zone has a numerical range of 600° C. to 900° C.;

[0016] The temperature gradient range during the crystal growth is 5°C / cm to 30°C / cm.

[0017] The crystal growth rate is: 0.1mm / h~1mm / h.

[0018] The cooling rate during the crystal growth is 5°C / h to 20°C / h.

[0019] Accordingly, a third aspect of an embodiment of the present invention provides a solid-state battery, comprising: a positive electrode sheet, a negative electrode sheet, and a solid electrolyte layer located therebetween.

[0020] In detail, the positive electrode active material in the positive electrode sheet includes any one of lithium iron phosphate, NCM ternary material, NCA ternary material and lithium-rich manganese-based material.

[0021] In detail, the negative electrode active material in the negative electrode sheet includes any one of carbon, silicon carbon, lithium metal and lithium alloy.

[0022] The above technical solution of the embodiment of the present invention has the following beneficial technical effects:

[0023] 1. The monoclinic structure (C2 / m) halide electrolyte has a three-dimensional isotropic Li+ transport channel, and the Li+ is solidified after single crystallization. + transmission channel, improving the Li + Conductivity.

[0024] 2. After the electrolyte is crystallized, the material becomes more compact, reducing the degree of reaction with water in the environment and improving the environmental stability of the material (under a dew point environment of -40°C, the 24h electrolyte ion conductivity retention rate is ≥80%).

[0025] 3. After the electrolyte is crystallized, the internal interface and defects of the crystal are reduced, and it is not easy to produce Li during the use of the assembled solid-state battery. + Dendrites can increase the cycle life of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flow chart of a method for preparing a solid electrolyte and a solid-state battery provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0028] A first aspect of an embodiment of the present invention provides a solid electrolyte, which is a single crystal, has a monoclinic structure, a C2 / m space group, a chemical formula of Li3MX6, M is one or more of Y, In, Sc, and X is one or more of Cl, Br, I, and F.

[0029] Furthermore, the room temperature ionic conductivity of the solid electrolyte is ≥3×10 -3 S / cm.

[0030] In a preferred embodiment of the present invention, M is Y, and X is Br, or a mixture of Cl and Br.

[0031] In another preferred embodiment of the present invention, M is In or Sc, and X is Cl.

[0032] In the above two preferred embodiments, the room temperature ionic conductivity of the solid electrolyte is ≥5×10 -3 S / cm.

[0033] The results show that the halide electrolyte with monoclinic structure and C2 / m space group has three-dimensional isotropic Li + conduction mode, thus increasing the crystallinity of the material has a stable Li + The transmission channel reduces impedance and increases the ionic conductivity of the material. In addition, the halide electrolyte of this structure is flexible and the single crystal is not easy to break, which reduces the difficulty of assembling solid-state batteries.

[0034] Correspondingly, a second aspect of an embodiment of the present invention provides a solid-state battery, comprising: a positive electrode sheet, a negative electrode sheet and a solid electrolyte layer located therebetween, wherein the solid electrolyte layer comprises the solid electrolyte as described above.

[0035] Furthermore, the positive electrode active material in the positive electrode sheet includes any one of lithium iron phosphate, NCM ternary material, NCA ternary material and lithium-rich manganese-based material.

[0036] Furthermore, the negative electrode active material in the negative electrode sheet includes any one of carbon, silicon carbon, lithium metal and lithium alloy.

[0037] Accordingly, a third aspect of an embodiment of the present invention provides a method for preparing a solid electrolyte, which is used to prepare any of the above-mentioned solid electrolytes, comprising the following steps:

[0038] S100, mixing lithium halide and rare earth halide according to a preset ratio to obtain a mixed material.

[0039] S200, placing the mixed material in a quartz crucible, vacuum-sealing it, and placing it in a Bridgman crystal furnace for single crystal growth, wherein the temperature of the crystal furnace heating zone is a first preset value, the temperature gradient of the cooling zone is a second preset value, and the crucible descent rate is a third preset value. After the growth is completed, a halide single crystal is obtained at a cooling rate of a fourth preset value.

[0040] S300, cutting the halide single crystal to obtain a single crystal sheet of a preset thickness, namely, the solid electrolyte layer.

[0041] Optionally, during the crystal growth, the preset temperature in the high temperature zone of the furnace body has a numerical range of 600° C. to 900° C.;

[0042] During the crystal growth, the temperature gradient ranges from 5°C / cm to 30°C / cm, preferably from 10°C / cm to 20°C / cm; when the temperature gradient is lower than 5°C / cm, the latent heat of crystallization is not easily released, and crystal growth is difficult; when the temperature gradient is greater than 30°C / cm, the furnace body needs to forcibly dissipate heat, and the crystal furnace structure is complex and difficult to achieve; when the temperature gradient is 10°C / cm to 20°C / cm, the crystal growth yield is high and the furnace body structure is simple.

[0043] Optionally, the crystal growth rate is: 0.1mm / h~1mm / h.

[0044] Optionally, the cooling rate during the crystal growth is 5°C / h to 20°C / h.

[0045] The numerical range of the preset thickness is 0.01 mm to 3.0 mm, preferably 0.1 mm to 1 mm. If the thickness of the single crystal electrolyte sheet is too small, the processing is difficult, and if the thickness is too large, waste is caused and the material utilization rate is low.

[0046] The material selection and preparation process of the solid electrolyte and the solid battery containing the solid electrolyte are further described below with several embodiments and comparative examples:

[0047] Example 1

[0048] Preparation of Li3YBr6 electrolyte single crystal: YBr3 and LiBr are mixed in a molar ratio of the chemical formula Li3YBr6, and the mixed materials are sealed in a quartz crucible. The single crystal is grown by the Bridgman method, with a high temperature zone temperature of 650°C, a temperature gradient of 30°C / cm, and a growth rate of 0.86mm / h. The well-grown crystal is taken out of the crucible in a glove box and cut into 0.05mm for use.

[0049] Comparative Example 1

[0050] Preparation of Li3YBr6 electrolyte powder: YBr3 and LiBr are mixed in a molar ratio according to the chemical formula Li3YBr6. The mixed materials are sealed in a quartz crucible and placed in a muffle furnace to react at 650°C for 4 hours. After the reaction is completed, the electrolyte is taken out from the glove box and ground into powder.

[0051] Example 2

[0052] Preparation of Li3YBr3Cl3 electrolyte single crystal: YCl3 and LiBr are mixed in a molar ratio of the chemical formula Li3YBr3Cl3, and the mixed materials are sealed in a quartz crucible. The single crystal is grown by the Bridgman method, with a high temperature zone temperature of 900°C, a temperature gradient of 10°C / cm, and a crystal growth rate of 0.1mm / h. The grown single crystal is cut into a single crystal with a thickness of 0.01mm.

[0053] Comparative Example 2

[0054] Preparation of Li3YBr3Cl3 electrolyte powder: YCl3 and LiBr are mixed in a molar ratio according to the chemical formula Li3YBr3Cl3. The mixed materials are sealed in a quartz crucible and placed in a muffle furnace to react at 650°C for 4 hours. After the reaction is completed, the electrolyte is taken out from the glove box and ground into powder.

[0055] Example 3

[0056] Y3 5.7 F 0.3 Preparation of electrolyte single crystal: YBr3, LiBr, and YF3 are prepared according to the chemical formula Li3YBr 5.7 F 0.3 The mixed materials are sealed in a quartz crucible and the single crystal is grown by the Bridgman method, with a high temperature zone temperature of 600°C, a temperature gradient of 5°C / cm, and a pull-down speed of 1mm / h. The grown single crystal is cut into single crystals with a thickness of 0.1mm.

[0057] Comparative Example 3

[0058] Y3 5.7 F 0.3 Preparation of electrolyte: YBr3, LiBr, YF3 according to the chemical formula Li3YBr 5.7 F 0.3 The remaining operations were the same as those in Comparative Example 1.

[0059] Example 4

[0060] Preparation of Li3InCl6 electrolyte single crystal: InCl3 and LiCl are mixed according to the molar ratio of the chemical formula Li3InCl6, and the mixed materials are sealed in a quartz crucible. The Bridgman method is used to grow the single crystal, with a high temperature zone temperature of 600℃, a temperature gradient of 15℃ / cm, and a pull-down speed of 0.5mm / h. The grown single crystal is cut into a single crystal with a thickness of 0.2mm.

[0061] Comparative Example 4

[0062] Preparation of Li3InCl6 electrolyte powder: InCl3 and LiCl are mixed in a molar ratio according to the chemical formula Li3InCl6, and the remaining operations are the same as those in Comparative Example 1.

[0063] Preparation of solid electrolyte layer: ScCl3 and LiCl are mixed in a molar ratio of the chemical formula Li3ScCl6, and the mixed materials are sealed in a quartz crucible. The single crystal is grown by the Bridgman method, with a high temperature zone temperature of 650°C, a temperature gradient of 30°C / cm, and a pull-down speed of 0.86mm / h. The grown single crystal is cut into a single crystal with a thickness of 3mm.

[0064] The electrolyte crystal growth process of Examples 5, 6 and Comparative Examples 7, 9 and 11 is the same as that of Example 1, except that the raw materials used are proportioned according to their chemical formulas.

[0065] The preparation process of the electrolyte powders of Comparative Examples 5, 6, 8, 10 and 12 is the same as that of Comparative Example 1, except that the raw materials used are proportioned according to their chemical formulas.

[0066] Ionic conductivity test method:

[0067] 1) Ion conductivity test of electrolyte single crystal: In a glove box, gold is sprayed on both sides of a cut electrolyte with a certain thickness, and the mold battery is pressurized at a pressure of 2MPa. Then, a symmetrical blocked electrode battery of carbon / electrolyte carbon is formed in the mold battery. The AC impedance of the battery under open circuit conditions is measured, and the obtained impedance value is recorded as R, which is calculated using the formula σ=L / (R·A), where σ is the ionic conductivity, L is the thickness of the electrolyte layer, R is the impedance value, and A is the electrode area of ​​the electrolyte sheet.

[0068] 2) Ion conductivity test of electrolyte powder: 150 mg of electrolyte material was weighed in the glove box, pressurized at 300 MPa in the mold battery, the thickness of the electrolyte layer was measured and recorded as L, and then a symmetrical blocked electrode battery of carbon / electrolyte carbon was formed in the mold battery, and the AC impedance of the battery under open circuit conditions was measured. The obtained impedance value was recorded as R and calculated using the formula σ=L / (R·A), where σ is the ionic conductivity, L is the thickness of the electrolyte layer, R is the impedance value, and A is the electrode area of ​​the electrolyte sheet.

[0069] Environmental stability test method:

[0070] The electrolyte sample with known ion conductivity was exposed in a drying oven at a dew point of -40°C for 24 hours, and then its ion conductivity was tested and compared with the initial value.

[0071] Assembly and performance testing of solid-state batteries:

[0072] Preparation of positive electrode sheet: Weigh the positive electrode active material, any type of electrolyte and conductive agent in a mass ratio of 70:30:1, grind them thoroughly and mix them evenly. For example, the positive electrode active material is NC ternary material, the solid electrolyte material is Li3YCl6 powder, and the conductive agent is graphite. Weigh 0.3g of the mixed material and press it into a positive electrode sheet under a pressure of 120MPa.

[0073] Preparation of negative electrode sheet: The negative electrode material is prepared under a pressure of 120 MPa, and the thickness is about 0.2 mm. For example, 0.2 g of metallic lithium is pressed under a pressure of 120 MPa, and the pressed thickness is 0.2 mm.

[0074] Battery assembly: stack the positive electrode sheet, solid electrolyte, and negative electrode sheet in sequence and press them. For example: 1) Place the positive electrode sheet, the cut and processed Li3YBr6 single crystal electrolyte layer, and the negative electrode sheet into the battery mold at the same time and press them at a pressure of 5MPa for 5min. 2) Place the positive electrode sheet, 0.1g Li3YBr6 electrolyte powder, and the negative electrode sheet into the battery mold at the same time and press them at a pressure of 100MPa for 5min.

[0075] Solid-state battery cycle stability test:

[0076] The assembled all-solid-state lithium secondary battery was subjected to a cyclic voltammetry test, and the test conditions were: a current density of 0.1C and a voltage range of 2.7 to 4.1V (Li+ / Li).

[0077] Table 1 Test of ionic conductivity and environmental stability

[0078]

[0079]

[0080] It can be seen from the embodiments and comparative examples that the ion conductivity of the halide electrolyte with C2 / m structure after single crystallization is greatly improved compared with its powder ion conductivity, while the ion conductivity improvement effect of the halide electrolyte with Pnma or P-3m1 structure after single crystallization is not obvious, among which comparative examples 9 and 11 have shown significant degradation.

[0081] In the environmental stability test, after being stored in a -40°C dew point environment for 24 hours, the ion conductivity retention rate of all embodiments is much higher than that of the comparative example. The experimental results in Table 1 prove that the ion conductivity of the halogenated electrolyte with a C2 / m structure is greatly improved after single crystallization, and the electrolyte has better air stability after single crystallization.

[0082] The ionic conductivity and the discharge capacity retention rate after 200 cycles of Examples 10-14 and the comparative example are shown in Table 2. After 200 cycles, the battery was disassembled and the growth of lithium dendrites on the negative electrode side of the electrolyte layer was observed. The results are shown in Table 2.

[0083] Table 2 Cyclic stability of batteries and lithium dendrite growth

[0084]

[0085]

[0086] From the comparison of the examples and comparative examples in Table 2, it can be seen that under the same chemical composition, the discharge capacity retention rate of the battery prepared based on the solid electrolyte single crystal layer prepared by the present invention after 200 cycles is more than 90%, while the discharge capacity retention rate of the ceramic battery prepared based on the powder electrolyte lamination after 200 cycles is less than 60%. The data show that the use of single crystal electrolytes has obvious advantages in improving the performance of batteries and increasing the service life of batteries. From the growth of lithium dendrites after 200 cycles, in all embodiments using single crystal electrolytes, no lithium dendrites were observed on the negative electrode side, while in the embodiments using powder electrolytes, lithium dendrites were observed, and the precipitation of lithium dendrites in some embodiments was already serious. The above results show that single crystal electrolytes can effectively prevent the growth of lithium dendrites.

[0087] It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principles of the present invention, and do not constitute a limitation of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all changes and modifications that fall within the scope and boundaries of the appended claims, or the equivalent forms of such scope and boundaries.

Claims

1. A solid electrolyte, characterized in that The solid electrolyte is a single crystal, the physical phase is a monoclinic structure of C2 / m space group, the chemical formula is Li3MX6, M is one or more of Y, In, Sc, and X is one or more of Cl, Br, I, F.

2. The solid electrolyte according to claim 1, characterized in that M is Y, and X is Br, or a mixture of Cl and Br.

3. The solid electrolyte according to claim 1, characterized in that M is In or Sc, and X is Cl.

4. The solid electrolyte according to claim 1, characterized in that The room temperature ionic conductivity of the solid electrolyte is ≥3×10 -3 S / cm.

5. The solid electrolyte according to claim 2 or 3, characterized in that: The room temperature ionic conductivity of the solid electrolyte is ≥5×10 -3 S / cm.

6. A method for preparing a solid electrolyte layer, characterized in that: The steps include: Mixing lithium halide and metal halide in a preset ratio to obtain a mixed material; The mixed material is placed in a quartz crucible, vacuum-sealed, and placed in a Bridgman crystal furnace for single crystal growth, wherein the temperature of the crystal furnace heating zone is a first preset value, the temperature gradient of the cooling zone is a second preset value, and the crucible descending rate is a third preset value. After the growth is completed, a halide single crystal is obtained at a cooling rate of a fourth preset value; The halide single crystal is cut to obtain a single crystal sheet of a preset thickness, namely the solid electrolyte layer.

7. The method for preparing a solid electrolyte layer according to claim 6, characterized in that: The first preset value range is: 600°C to 900°C; The second preset value range is: 5°C / cm to 30°C / cm; The third preset value range is: 0.1mm / h~1mm / h; The fourth preset value range is: 5°C / h to 20°C / h.

8. A solid-state battery, characterized in that: include: A positive electrode sheet, a negative electrode sheet and a solid electrolyte layer located therebetween, wherein the solid electrolyte layer comprises the solid electrolyte as described in any one of claims 1 to 5, or is prepared by the method as described in any one of claims 6 to 7.

9. The solid-state battery according to claim 8, characterized in that: The positive electrode active material in the positive electrode sheet includes: any one of lithium iron phosphate, NCM ternary material, NCA ternary material and lithium-rich manganese-based material; The negative electrode active material in the negative electrode sheet includes any one of carbon, silicon carbon, lithium metal and lithium alloy.

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