Positive electrode of all-solid-state battery and preparation method thereof

Abstract

The present invention discloses a positive electrode of an all-solid-state battery and a preparation method thereof. The preparation method includes the following steps: dispersing a positive electrode active material in an organic solvent, adding an ionic plastic crystal and a lithium salt and mixing them, then removing the organic solvent and drying to obtain a coating material; preparing an electrode sheet from the coating material, a conductive agent, an electrolyte and a binder through a dry electrode process; and then soaking the electrode sheet in an ionic melt, heating it under negative pressure to obtain the positive electrode. Among them, the ionic melt is a mixed melt of an ionic plastic crystal and a lithium salt. By means of wet coating, the present invention coats the lithium-ion conductive plastic crystal on the surface of the particles of the positive electrode active material, and uses the melted lithium-ion conductive plastic crystal to infiltrate the gaps inside the electrode sheet, so as to increase the contact between various substances inside the positive electrode of the all-solid-state battery and the contact between the external electrode sheet and the electrolyte layer, thereby increasing the lithium conduction path, reducing the interfacial impedance of the battery, and improving the electrochemical performance of the all-solid-state battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and particularly relates to a positive electrode of an all-solid-state battery and a preparation method thereof. Background Art

[0002] Lithium-ion batteries have been widely used in the fields of portable electronic products, electric vehicles, aerospace, and energy storage systems due to their advantages such as high energy density, good cycle stability, environmental friendliness, and wide operating temperature range. However, most of the current lithium-ion batteries use organic liquid electrolytes, which are prone to defects such as gas swelling and liquid leakage, and have relatively large safety hazards. Using solid electrolytes can fundamentally make up for the deficiencies of organic liquid electrolytes. Therefore, the development of all-solid-state batteries with high safety, high energy density, and long cycle life has become a research focus.

[0003] An all-solid-state battery is a battery that uses solid electrodes and solid electrolytes. Solid electrolytes are the core materials of all-solid-state batteries and are crucial for the performance of all-solid-state batteries. The currently developed solid electrolytes include polymers, oxides, sulfides, and halides. Among them, sulfides have the highest ionic conductivity, so sulfide all-solid-state batteries have important development prospects.

[0004] Currently, due to the instability between the electrode and the solid electrolyte, sulfide all-solid-state batteries face a series of interface problems, including space charge layers, interfacial side reactions, and mechanical instabilities, among which: the interface problems will lead to the formation of a Li + depletion layer at the interface between the active material and the electrolyte, hindering charge transfer. In the positive electrode of a sulfide all-solid-state battery, since all internal interfaces are "solid-solid" contacts, the interfacial impedance is much greater than that of a liquid battery infiltrated with electrolyte. To increase the contact between the components inside the positive electrode of a sulfide solid-state battery, a relatively large pressure needs to be applied to the battery. However, there are inevitably voids between the rigid positive electrode active materials, and the existence of the voids will hinder the transport of lithium ions during charge and discharge, affecting the battery performance.

[0005] Therefore, there is an urgent need to develop a material that can reduce the voids inside the positive electrode of an all-solid-state battery and reduce the high impedance of "solid-solid" contact inside the positive electrode plate. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems existing in the above-mentioned prior art. For this purpose, the present invention provides a positive electrode of an all-solid-state battery and a preparation method thereof. The present invention reduces the interfacial impedance between the materials inside the positive electrode of an all-solid-state battery by coating the positive electrode active material with ionic plastic crystals and infiltrating the gaps inside the electrode plate, thereby improving the comprehensive performance of the battery.

[0007] The inventive concept of the present invention is as follows: The present invention wet-coats ionoplastics with certain plasticity and lithium conductivity on the surface of the cathode active material particles, and immerses the obtained electrode sheet in a melt mixture of the ionoplastics and lithium salts, so that the lithium-ion conductive ionoplastic melt infiltrates into the gaps inside the electrode sheet, thereby increasing the contact between various substances inside the cathode of the all-solid-state battery and the contact between the external electrode sheet and the electrolyte layer, thereby increasing the lithium conduction path, reducing the interfacial impedance of the battery, and further improving the electrochemical performance of the all-solid-state battery.

[0008] To solve the above technical problems, a first aspect of the present invention provides a method for preparing a cathode, including the following steps:

[0009] (1) Disperse the cathode active material in an organic solvent, add ionoplastics and lithium salts and mix them, then remove the organic solvent and dry to obtain a coating material (i.e., the cathode active material coated with lithium-ion conductive ionoplastics);

[0010] (2) Prepare an electrode sheet from the coating material, a conductive agent, an electrolyte and a binder through a dry electrode process; then immerse the electrode sheet in an ionic melt, heat it, and apply negative pressure to obtain the cathode;

[0011] The ionic melt is a melt mixture of the ionoplastics and the lithium salts.

[0012] Preferably, in step (1), the ionoplastics are composed of anions and cations, the anion is bis(fluorosulfonyl)imide ion or bis(trifluoromethylsulfonyl)imide ion, and the cation is any one of N,N-diethylpyrrolidinium, N-ethyl-N-methylpyrrolidinium, N,N-dimethylpyrrolidinium, N-methylpyridinium, N-ethyl-N-methylpiperidinium, 3,3-dimethyloxazolidinium, 4-ethyl-4-methylmorpholinium, 4-isobutyl-4-methylmorpholinium, tetramethylammonium, tetraethylammonium, tetrabutylammonium, tetramethylphosphonium, tetraethylphosphonium and tetrabutylphosphonium.

[0013] More preferably, the ionoplastics are N,N-dimethylpyrrolidonium bis(fluorosulfonyl)imide (P 11 FSI), tetramethylammonium bis(fluorosulfonyl)imide (N 1111 FSI), tetraethylammonium bis(fluorosulfonyl)imide (N 2222 FSI), N,N-diethylpyrrolidonium bis(fluorosulfonyl)imide (P 22 FSI).

[0014] Preferably, the anion of the lithium salt is the same as the anion of the ionoplastics, that is, the lithium salt is lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide salt.

[0015] Preferably, in step (1), the mass ratio of the ionoplastics to the lithium salt is (3-5):1.

[0016] Preferably, the positive electrode active material is selected from at least one of nickel-cobalt-manganese ternary materials, nickel-cobalt-lithium ternary materials, lithium cobalt oxide, lithium iron phosphate, and lithium-rich manganese-based materials.

[0017] More preferably, the positive electrode active material is selected from Li 0.6 Co 0.2 Mn 0.2 O 2 (NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523), LiNi 0.33 Co 0.33 Mn 0.33 O 2 (NCM111).

[0018] Preferably, the particle size of the positive electrode active material is 2 - 15 μm.

[0019] Preferably, the mass ratio of the positive electrode active material to the lithium salt is (92 - 98):1.

[0020] Preferably, the organic solvent is selected from at least one of toluene, dichloromethane, tetrahydrofuran, acetonitrile, ethanol, acetone, ethyl acetate, butyl butyrate, anisole, isobutyl isobutyrate, N,N-dimethylformamide, and N-methylpyrrolidone.

[0021] Preferably, in step (1), the method for removing the organic solvent is rotary evaporation.

[0022] Preferably, in step (1), the drying is carried out at a temperature of 55 - 65 °C for 20 - 28 hours.

[0023] Preferably, the electrolyte is a sulfide electrolyte, an oxide electrolyte, or a halide electrolyte.

[0024] More preferably, the electrolyte is a sulfide electrolyte, and the sulfide electrolyte is selected from Li 3 PS 4 、Li 4 P 2 S 6 、Li 7-a PS 6-a X a at least one of them, where: X is any one of Cl, Br, I, and the value of a is 0.5 - 2.

[0025] Preferably, the conductive agent includes vapor-grown carbon fiber (VGCF).

[0026] Preferably, the binder includes polytetrafluoroethylene (PTFE).

[0027] Preferably, in the ionic melt, the mass ratio of the ionic plastic crystal to the lithium salt is (3 - 5):1.

[0028] Preferably, in step (2), the heating is carried out under vacuum conditions at a temperature of 55 - 65 °C for 8 - 15 hours.

[0029] Preferably, in step (2), the pressure of the negative pressure is 50 - 150 mbar.

[0030] The second aspect of the present invention provides a positive electrode, which is prepared by the preparation method of the above positive electrode. The surface of the positive electrode active material in the positive electrode is coated with a lithium-ion conducting plastic crystal, and the internal gaps of the positive electrode are filled with a lithium-ion conducting plastic crystal.

[0031] The third aspect of the present invention provides a all-solid-state battery, which includes the above positive electrode.

[0032] The above technical solutions of the present invention, compared with the prior art, have at least the following technical effects or advantages:

[0033] The present invention coats the lithium-ion conducting plastic crystal with certain plasticity on the surface of the particles of the positive electrode active material by means of wet coating, and uses the melted lithium-ion conducting plastic crystal to infiltrate the internal gaps of the electrode sheet, so as to increase the contact between various substances inside the positive electrode of the all-solid-state battery and the contact between the external electrode sheet and the electrolyte layer, thereby increasing the lithium conduction path, reducing the interfacial impedance of the battery, and improving the electrochemical performance of the all-solid-state battery. The interfacial impedance of the all-solid-state battery is 88 - 120 Ω, the initial Coulomb efficiency is 77.4 - 79.0%, and the 0.1C cycle life is 90 - 106 cycles. Description of the Drawings

[0034] Figure 1 SEM image of the positive electrode prepared in Example 1;

[0035] Figure 2 SEM image of the positive electrode prepared in Comparative Example 1. Detailed Description of the Invention

[0036] The present invention will be specifically described below in conjunction with embodiments to facilitate the understanding of those skilled in the art. It is necessary to specifically point out here that the embodiments are only used to further illustrate the present invention and should not be construed as limiting the protection scope of the present invention. Those skilled in the art, based on the above-mentioned invention content, make non-essential improvements and adjustments to the present invention, which should still fall within the protection scope of the present invention. At the same time, for the raw materials not specifically described below, they are all commercially available products; for the process steps or preparation methods not specifically mentioned, they are all process steps or preparation methods known to those skilled in the art.

[0037] Example 1

[0038] A method for preparing a positive electrode includes the following steps:

[0039] (1) Disperse the positive electrode active material NCM622 in the organic solvent butyl butyrate, and perform ultrasonic dispersion for 30 min; then add the ionic plastic crystal N 1111 FSI and the lithium salt LiFSI in a mass ratio of 4:1 and mix evenly (the mass ratio of NCM622 to the mixture of N 1111 FSI and LiFSI is 95:5), stir at room temperature for 24 hours; then perform rotary evaporation to remove the excess butyl butyrate, and dry at 60 °C for 24 hours to obtain the positive electrode active material coated with a lithium-ion conducting plastic crystal;

[0040] (2) Mix the positive electrode active material coated with a lithium-ion conducting plastic crystal prepared in step (1) with the sulfide electrolyte Li 3 PS 4 , the conductive agent VGCF and the binder PTFE in a mass ratio of 70:30:3:3 and mix evenly, then roll and mix on a heating table at 50 °C, slice to obtain an electrode sheet; then soak the electrode sheet in an ionic melt (formed by melting the ionic plastic crystal N 1111 FSI and the lithium salt LiFSI in a mass ratio of 4:1); finally, place it in a glove box and store it at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheet, thus obtaining the positive electrode of this example.

[0041] Example 2

[0042] A method for preparing a positive electrode includes the following steps:

[0043] (1) Disperse the positive electrode active material NCM622 in the organic solvent butyl butyrate, and perform ultrasonic dispersion for 30 min; then add the ionic plastic crystal N 1111 FSI and the lithium salt LiFSI in a mass ratio of 4:1 and mix evenly (the mass ratio of NCM622 to the mixture of N 1111(The mass ratio of the FSI and LiFSI mixture is 95:5), and it is stirred at room temperature for 24 hours; then rotary evaporation is carried out to remove the excess butyl butyrate, and it is dried at 60 °C for 24 hours to obtain the positive electrode active material coated with lithium-ion conducting plastic crystal;

[0044] (2) The positive electrode active material coated with lithium-ion conducting plastic crystal prepared in step (1) is mixed with the sulfide electrolyte Li 4 P 2 S 6 , the conductive agent VGCF and the binder PTFE are fully mixed evenly according to the mass ratio of 70:30:3:3, then roll-mixed on a heating table at 50 °C, sliced to obtain an electrode sheet; then the electrode sheet is soaked in an ionic melt (formed by melting the ionic plastic crystal N 1111 FSI and the lithium salt LiFSI); finally, it is placed in a glove box and stored at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheet, thus obtaining the positive electrode of this example.

[0045] Example 3

[0046] A method for preparing a positive electrode, comprising the following steps:

[0047] (1) Disperse the positive electrode active material NCM622 in the organic solvent butyl butyrate, and perform ultrasonic dispersion for 30 min; then add the ionic plastic crystal N 1111 FSI and the lithium salt LiFSI and mix evenly (the mass ratio of NCM622 to the mixture of N 1111 FSI and LiFSI is 95:5), and it is stirred at room temperature for 24 hours; then rotary evaporation is carried out to remove the excess butyl butyrate, and it is dried at 60 °C for 24 hours to obtain the positive electrode active material coated with lithium-ion conducting plastic crystal;

[0048] (2) The positive electrode active material coated with lithium-ion conducting plastic crystal prepared in step (1) is mixed with the sulfide electrolyte Li 6 PS 5 Cl, the conductive agent VGCF and the binder PTFE are fully mixed evenly according to the mass ratio of 70:30:3:3, then roll-mixed on a heating table at 50 °C, sliced to obtain an electrode sheet; then the electrode sheet is soaked in an ionic melt (formed by melting the ionic plastic crystal N 1111 FSI and the lithium salt LiFSI); finally, it is placed in a glove box and stored at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheet, thus obtaining the positive electrode of this example.

[0049] Example 4

[0050] A method for preparing a positive electrode, comprising the following steps:

[0051] (1) Disperse the positive electrode active material NCM622 in the organic solvent butyl butyrate and perform ultrasonic dispersion for 30 min; then add the ionic plastic crystal N 2222 FSI and the lithium salt LiFSI and mix them evenly (the mass ratio of NCM622 to the mixture of N 2222 FSI and LiFSI is 95:5), stir at room temperature for 24 hours; then perform rotary evaporation to remove the excess butyl butyrate, and dry at 60 °C for 24 hours to obtain the positive electrode active material coated with lithium-ion conductive plastic crystal;

[0052] (2) Mix the positive electrode active material coated with lithium-ion conductive plastic crystal prepared in step (1) with the sulfide electrolyte Li 3 PS 4 、the conductive agent VGCF and the binder PTFE evenly at a mass ratio of 70:30:3:3, then roll and mix them evenly on a heating table at 50 °C, slice to obtain an electrode sheet; then soak the electrode sheet in an ionic melt (formed by melting the ionic plastic crystal N 2222 FSI and the lithium salt LiFSI in a mass ratio of 4:1); finally, place it in a glove box and store it at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheet, and prepare the positive electrode of this example.

[0053] Example 5

[0054] A method for preparing a positive electrode, comprising the following steps:

[0055] (1) Disperse the positive electrode active material NCM622 in the organic solvent butyl butyrate and perform ultrasonic dispersion for 30 min; then add the ionic plastic crystal P 11 FSI and the lithium salt LiFSI and mix them evenly (the mass ratio of NCM622 to the mixture of P 11 FSI and LiFSI is 95:5), stir at room temperature for 24 hours; then perform rotary evaporation to remove the excess butyl butyrate, and dry at 60 °C for 24 hours to obtain the positive electrode active material coated with lithium-ion conductive plastic crystal;

[0056] (2) Mix the positive electrode active material coated with lithium-ion conductive plastic crystal prepared in step (1) with the sulfide electrolyte Li 3 PS 4 、the conductive agent VGCF and the binder PTFE evenly at a mass ratio of 70:30:3:3, then roll and mix them evenly on a heating table at 50 °C, slice to obtain an electrode sheet; then soak the electrode sheet in an ionic melt (formed by melting the ionic plastic crystal P 11(It is) melted in FSI and lithium salt LiFSI; finally, it is placed in a glove box and stored at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheet, thus obtaining the positive electrode of this example.

[0057] Example 6

[0058] A method for preparing a positive electrode, comprising the following steps:

[0059] (1) Disperse the positive electrode active material NCM622 in the organic solvent butyl butyrate and perform ultrasonic dispersion for 30 min; then add ionic plastic crystal P in a mass ratio of 4:1 22 Mix FSI and lithium salt LiFSI evenly (the mass ratio of NCM622 to the mixture of P 22 FSI and LiFSI is 95:5), stir at room temperature for 24 hours; then perform rotary evaporation to remove the excess butyl butyrate, and dry at 60 °C for 24 hours to obtain the positive electrode active material coated with lithium-ion conducting plastic crystal;

[0060] (2) Mix the positive electrode active material coated with lithium-ion conducting plastic crystal prepared in step (1) with the sulfide electrolyte Li 3 PS 4 , conductive agent VGCF and binder PTFE evenly at a mass ratio of 70:30:3:3, then roll and mix them evenly on a heating table at 50 °C, slice to obtain an electrode sheet; then soak the electrode sheet in an ionic melt (composed of ionic plastic crystal P 22 FSI and lithium salt LiFSI melted); finally, place it in a glove box and store it at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheet, thus obtaining the positive electrode of this example.

[0061] Example 7

[0062] A method for preparing a positive electrode, comprising the following steps:

[0063] (1) Disperse the positive electrode active material NCM523 in the organic solvent butyl butyrate and perform ultrasonic dispersion for 30 min; then add ionic plastic crystal N in a mass ratio of 4:1 1111 Mix FSI and lithium salt LiFSI evenly (the mass ratio of NCM523 to the mixture of N 1111 FSI and LiFSI is 95:5), stir at room temperature for 24 hours; then perform rotary evaporation to remove the excess butyl butyrate, and dry at 60 °C for 24 hours to obtain the positive electrode active material coated with lithium-ion conducting plastic crystal;

[0064] (2) Mix the positive electrode active material coated with lithium-ion conducting plastic crystal prepared in step (1) with the sulfide electrolyte Li 3 PS4 、VGCF as the conductive agent and PTFE as the binder are fully mixed evenly at a mass ratio of 70:30:3:3, then roll-pressed and mixed evenly on a heating table at 50 °C, sliced to obtain electrode sheets; then the electrode sheets are soaked in an ionic melt (formed by melting ionic plastic crystal N 1111 FSI and lithium salt LiFSI in a mass ratio of 4:1); finally, it is placed in a glove box and stored at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheets, thus obtaining the positive electrode of this example.

[0065] Example 8

[0066] A method for preparing a positive electrode includes the following steps:

[0067] (1) Disperse the positive electrode active material NCM111 in the organic solvent butyl butyrate and perform ultrasonic dispersion for 30 min; then add ionic plastic crystal N 1111 FSI and lithium salt LiFSI and mix evenly (the mass ratio of NCM111 to the mixture of N 1111 FSI and LiFSI is 95:5), stir at room temperature for 24 hours; then perform rotary evaporation to remove the excess butyl butyrate and dry at 60 °C for 24 hours to obtain the positive electrode active material coated with lithium-ion conductive plastic crystal;

[0068] (2) Mix the positive electrode active material coated with lithium-ion conductive plastic crystal prepared in step (1) with the sulfide electrolyte Li 3 PS 4 、VGCF as the conductive agent and PTFE as the binder are fully mixed evenly at a mass ratio of 70:30:3:3, then roll-pressed and mixed evenly on a heating table at 50 °C, sliced to obtain electrode sheets; then the electrode sheets are soaked in an ionic melt (formed by melting ionic plastic crystal N 1111 FSI and lithium salt LiFSI in a mass ratio of 4:1); finally, it is placed in a glove box and stored at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheets, thus obtaining the positive electrode of this example.

[0069] Comparative Example 1

[0070] A method for preparing a positive electrode includes the following steps:

[0071] Mix the positive electrode active material NCM622 with the sulfide electrolyte Li 3 PS 4 、VGCF as the conductive agent and PTFE as the binder are fully mixed evenly at a mass ratio of 70:30:3:3, then roll-pressed and mixed evenly on a heating table at 50 °C, sliced to obtain the electrode of this comparative example.

[0072] Comparative Example 2

[0073] A preparation method of a positive electrode, comprising the following steps:

[0074] (1) Disperse the positive electrode active material NCM622 in the organic solvent butyl butyrate, and perform ultrasonic dispersion for 30 min; then add molecular plastic crystal succinonitrile and lithium salt LiFSI in a mass ratio of 4:1 and mix evenly (the mass ratio of NCM622 to the mixture of succinonitrile and LiFSI is 95:5), and stir at room temperature for 24 hours; then perform rotary evaporation to remove the excess butyl butyrate, and dry at 60 °C for 24 hours to obtain the positive electrode active material coated with lithium-ion conductive plastic crystal;

[0075] (2) Mix the positive electrode active material coated with lithium-ion conductive plastic crystal prepared in step (1) with the sulfide electrolyte Li 3 PS 4 , conductive agent VGCF and binder PTFE in a mass ratio of 70:30:3:3 and mix evenly, then roll and mix on a heating table at 50 °C, slice to obtain an electrode sheet; then soak the electrode sheet in an ionic melt (formed by melting molecular plastic crystal succinonitrile and lithium salt LiFSI in a mass ratio of 4:1); finally, place it in a glove box and store it at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheet, thus obtaining the positive electrode of this comparative example.

[0076] Comparative Example 3

[0077] A preparation method of a positive electrode, comprising the following steps:

[0078] (1) Weigh ionic plastic crystal N 1111 FSI and lithium salt LiFSI according to a mass ratio, dissolve them in the organic solvent butyl butyrate to obtain a mixed solution; then scrape the mixed solution onto the surface of the positive electrode active material NCM622; dry at 60 °C for 24 hours to remove the excess butyl butyrate, and obtain a coated positive electrode;

[0079] (2) Mix the coating material prepared in step (1) with the sulfide electrolyte Li 3 PS 4 , conductive agent VGCF and binder PTFE in a mass ratio of 70:30:3:3 and mix evenly, then roll and mix on a heating table at 50 °C, slice to obtain an electrode sheet; then soak the electrode sheet in an ionic melt (formed by melting ionic plastic crystal N 1111 FSI and lithium salt LiFSI in a mass ratio of 4:1); finally, place it in a glove box and store it at 60 °C under vacuum conditions (pressure is 50 mbar) for 8 hours to allow the ionic melt to infiltrate into the internal gaps of the electrode sheet, thus obtaining the positive electrode of this comparative example.

[0080] Comparative Example 4

[0081] A method for preparing a positive electrode, comprising the following steps:

[0082] (1) Disperse the positive electrode active material NCM622 in the organic solvent butyl butyrate, and perform ultrasonic dispersion for 30 min; then add the ionic plastic crystal N 1111 FSI and the lithium salt LiFSI are mixed evenly (the mass ratio of NCM622 to the mixture of N 1111 FSI and LiFSI is 95:5), stir at room temperature for 24 hours; then perform rotary evaporation to remove the excess butyl butyrate, and dry at 60 °C for 24 hours to obtain the positive electrode active material coated with a lithium-ion conducting plastic crystal;

[0083] (2) Mix the positive electrode active material coated with a lithium-ion conducting plastic crystal prepared in step (1) with the sulfide electrolyte Li 3 PS 4 , the conductive agent VGCF and the binder PTFE are fully mixed evenly according to the mass ratio of 70:30:3:3, then roll and mix on a heating table at 50 °C, and slice to obtain the positive electrode of this comparative example.

[0084] Performance test

[0085] Respectively place the positive electrodes prepared in Examples 1-8 and Comparative Examples 1-4 in a solid-state battery mold; then weigh a certain amount of the sulfide electrolyte Li 3 PS 4 into the solid-state battery mold equipped with the positive electrode; then use a lithium metal electrode as the negative electrode, perform die cutting on a 50-μm-thick lithium metal strip to obtain a negative electrode sheet, and place it in the solid-state battery mold equipped with the positive electrode and the sulfide electrolyte; finally, stack the positive electrode, the solid-state electrolyte membrane, and the negative electrode, and apply a pressure of 4 t to the battery mold to pressurize and assemble a sulfide all-solid-state battery.

[0086] Test the interface impedance, initial Coulomb efficiency, and cycle performance of the assembled sulfide all-solid-state battery. The test methods are as follows:

[0087] Interface impedance: Test the EIS of the battery on an electrochemical workstation, with an amplitude of 10 mV and a frequency of 1-106 Hz.

[0088] Initial Coulomb efficiency: Charge and discharge the battery at a rate of 0.1C, and the initial Coulomb efficiency = initial discharge capacity / initial charge capacity × 100%.

[0089] Cycle performance: At room temperature, test the cycle performance of the battery under the condition of 0.1C. When the discharge capacity is lower than 80% of the initial discharge capacity, the life is terminated.

[0090] The test results are shown in Table 1.

[0091] Table 1:

[0092] Electrochemical performance Interface impedance (Ω) Initial Coulombic efficiency (%) Cycle life (cycles) Example 1 102 78.9 93 Example 2 98 77.8 99 Example 3 88 77.6 106 Example 4 90 79.0 102 Example 5 95 78.3 98 Example 6 98 77.4 93 Example 7 96 78.2 96 Example 8 103 78.9 90 Comparative Example 1 130 75.0 85 Comparative Example 2 120 74.0 70 Comparative Example 3 130 74.2 75 Comparative Example 4 118 75.8 85

[0093] As can be seen from Table 1, the all-solid-state batteries assembled with the positive electrodes prepared in Examples 1-8 have significantly better electrochemical performance than Comparative Example 1 without added ionic plastic crystals and lithium salts. This is because the combination of ionic plastic crystals and lithium salts has certain lithium conductivity and can also fill the gaps inside the electrode sheets, thereby reducing the interfacial impedance of the electrode sheets and improving the electrochemical performance.

[0094] Comparative Example 2 uses molecular plastic crystal succinonitrile to replace ionic plastic crystal N 1111 FSI. Due to the strong polarity of the -CN group in succinonitrile, even in the case of a low concentration of lithium salt, the combination of succinonitrile and lithium salt will be liquefied. When it is liquefied, the fluidity increases and the contact area with the sulfide electrolyte increases, and the damage to the sulfide electrolyte will also increase, resulting in inferior electrochemical performance compared to Example 1.

[0095] In Comparative Example 3 compared with Example 1, the ionic plastic crystal and lithium salt are directly coated on the surface of the positive electrode layer. Since the conductivity of the ionic plastic crystal and lithium salt is much smaller than that of the sulfide electrolyte, it affects the ionic conductivity of the intermediate electrolyte layer in the battery; in Comparative Example 4 compared with Example 1, the electrode sheet is not immersed in the melt mixture of the ionic plastic crystal and lithium salt, and the extremely tiny gaps remaining inside the electrode sheet cannot be well filled. Therefore, the all-solid-state batteries assembled with the positive electrodes prepared in Comparative Examples 3-4 have electrochemical performance inferior to that of Example 1.

[0096] Figure 1 SEM image of the positive electrode prepared for Example 1, from Figure 1 it can be seen that due to the plasticity of the ionic plastic crystal, the gaps covering the positive electrode active material are significantly filled. When assembled into a sulfide solid-state battery, the ionic plastic crystal penetrates into each gap inside the electrode under the action of pressure, thereby reducing the positive electrode interfacial impedance.

[0097] Figure 2 SEM image of the positive electrode prepared for Comparative Example 1, from Figure 2 it can be seen that in the positive electrode without ionic plastic crystals, obvious gaps exist between the particles, and these gaps will cause an incomplete lithium conduction channel to be formed inside the positive electrode, thereby increasing the battery impedance and affecting the battery performance.

[0098] For those of ordinary skill in the art to which the present invention pertains, several simple deductions or substitutions can be made without departing from the concept of the present invention, without the need for creative labor. Therefore, the simple improvements made by those skilled in the art based on the disclosure of the present invention should all fall within the protection scope of the present invention. The above embodiments are the preferred embodiments of the present invention, and all processes similar to the present invention and equivalent changes made thereto should fall within the protection scope of the present invention.

Claims

1. A method for preparing a positive electrode, characterized in that: The following steps are involved: (1) dispersing the positive electrode active material in an organic solvent, adding ion plasticizer and lithium salt to mix, removing the organic solvent, and drying to obtain a coating material; (2) preparing an electrode sheet by a dry electrode process using the coating material, a conductive agent, an electrolyte and a binder; then immersing the electrode sheet in an ion melt, heating it and applying negative pressure to obtain the positive electrode; the ion melt is a mixed melt of the ion plastic crystal and the lithium salt; In step (1), the mass ratio of the ion plasticizer to the lithium salt is (3-5): 1; In the ion melt, the mass ratio of the ion plastic and the lithium salt is (3-5):

1.

2. The method for preparing a positive electrode according to claim 1, characterized in that: In step (1), the ionic crystal is composed of anions and cations, the anions are bis(fluorosulfonyl)imide ions or bis(trifluoromethylsulfonyl)imide ions, and the cations are any one of N,N-diethylpyrrolidinium, N-ethyl-N-methylpyrrolidinium, N,N-dimethylpyrrolidinium, N-methylpyridinium, N-ethyl-N-methylpiperidinium, 3,3-dimethyloxazolidinium, 4-ethyl-4-methylmorpholinium, 4-isobutyl-4-methylmorpholinium, tetramethylammonium, tetraethylammonium, tetrabutylammonium, tetramethylphosphine, tetraethylphosphine and tetrabutylphosphine; And / or, the lithium salt is lithium bis(trifluoromethanesulfonyl imide) and / or lithium bis(fluorosulfonyl imide) salt.

3. The method for preparing a positive electrode according to claim 1, characterized in that: The positive electrode active material is selected from at least one of nickel-cobalt-manganese ternary materials, nickel-cobalt-lithium oxide ternary materials, lithium cobalt oxide, lithium iron phosphate, and lithium-rich manganese-based materials; and / or the particle size of the positive electrode active material is 2-15 μm.

4. The method for preparing a positive electrode according to claim 1 or 3, characterized in that: In step (1), the mass ratio of the positive electrode active material to the lithium salt is (92-98):

1.

5. The method for preparing a positive electrode according to claim 1, characterized in that: The electrolyte is a sulfide electrolyte, an oxide electrolyte or a halide electrolyte.

6. A positive electrode, characterized in that The positive electrode is prepared by the positive electrode preparation method according to any one of claims 1 to 5.

7. An all-solid-state battery, characterized in that: The all-solid-state battery comprises the positive electrode according to claim 6.

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