A method for preparing a LATP-based solid electrolyte interface layer and a LATP-based solid-state lithium battery

Abstract

The present invention discloses a method for preparing a LATP-based solid electrolyte interface layer and a LATP-based solid-state lithium battery, comprising: drying a commercial boron nitride release agent slurry into powder, placing it around a LATP cold-pressed sheet, and sintering it at a high temperature so that the powder is coated on the surface of the LATP cold-pressed sheet to form the interface layer. The solid-state lithium battery of the present invention solves the problem that Ti4+ in the LATP solid-state electrolyte is easily reduced by metallic lithium in the LATP-based solid-state lithium battery, causing interface problems, resulting in battery failure, and preventing the LATP solid-state electrolyte from being widely used.

Description

Technical Field

[0001] The present invention relates to the field of solid-state batteries, and in particular to a method for preparing a LATP-based solid-state electrolyte interface layer and a solid-state battery. Background Art

[0002] In the past 30 years, the rapid development of lithium-ion batteries has made it dominate the field of energy storage, and gradually expanded its use from portable devices to electronic vehicles, energy storage power stations, military equipment, etc. However, the market has put forward more challenges and requirements for high energy density and safety and reliability. Metallic lithium is considered to be an ideal material to replace commercial graphite for high energy density batteries due to its extremely high theoretical capacity and low redox potential. However, the unstable solid electrolyte interface in liquid organic electrolytes and the growth of lithium dendrites bring low Coulomb efficiency and safety issues. Especially recently, the frequent safety problems of lithium-ion batteries have attracted more and more attention. Solid-state batteries are often regarded as the ultimate solution, using solid electrolytes to replace flammable small molecule liquid electrolytes. Due to high Young's modulus and excellent mechanical properties, solid electrolytes can prevent lithium dendrites from piercing. However, with the deepening of research, more and more studies have reported side reactions at the interface between solid-state battery components, especially electrolytes in contact with lithium metal may bring safety problems and even thermal runaway.

[0003] NASICON structure solid electrolytes are considered to be a promising structure. Compared with polymers, sulfides and other oxides, they have excellent air stability and are easy to prepare and low cost. Specifically, among NASICON structure solid electrolytes, LATP has a 10 -4 ~10 -3 S cm -1 Despite its many advantages, LATP still faces a fatal shortcoming, namely, Ti 4+ In LATP, it is reduced to Ti by lithium 3+ . Lithium instability limits the electrochemical stability window, increases grain boundary resistance, forms cracks between adjacent ion channels, and ultimately leads to battery failure. In order to achieve practical applications of LATP, researchers usually improve lithium stability from three aspects: (1) element doping. Ge 4+ Usually used to partially or completely replace Ti 4+ , has higher ionic conductivity and excellent mechanical strength to prevent the penetration of lithium dendrites. However, due to Ge 4+The same reduction reaction occurs between LATP and Li, as well as the high cost of germanium metal and germanium compounds, which is not an ideal strategy; (2) Inorganic surface modification. Cover LATP with inorganic layers such as Al2O3, ZnO and BN to avoid direct contact with lithium. (LIU Yulong, SUN Qian, ZHAO Yang, et al. Stabilizing the interface of NASICON solid electrolyte against Li metal with atomic layer deposition [J]. ACS Applied Materials & Interfaces, 2018, 10 (37): 31240-31248.) Although effective, there are still some challenges that limit practical production, such as cost and scalability, because most modifications are deposited by complex preparation methods such as atomic layer deposition, chemical vapor deposition and magnetron sputtering. In addition, rigid coatings will also bring more serious interface problems; (3) Polymer surface modification. Thanks to the electronic insulation and flexibility of polymers, this method can not only separate LATP from Li, but also reduce the interfacial contact resistance. However, the weight and thickness of the polymer layer will affect the battery energy density. (YU Shicheng, SCHMOHL S, LIU Zigeng, et al. Insights into a layered hybrid solid electrolyte and its application in long life span high voltage all-solid-state lithium batteries [J]. Journal of Materials Chemistry A, 2019, 7 (8): 3882-3894.) Therefore, it is necessary to seek a simple and low-cost LATP modification method while seeking a balance between effective protection and low interfacial resistance. Summary of the invention

[0004] The purpose of the present invention is to solve the problem that in LATP-based solid-state lithium batteries, Ti4+ in the LATP solid electrolyte is easily reduced by metallic lithium, which brings about interface problems, leads to battery failure, and prevents the LATP solid electrolyte from being widely used.

[0005] In order to achieve the above-mentioned object, the present invention provides a method for preparing a LATP-based solid electrolyte interface layer, characterized in that it comprises: drying a commercial boron nitride release agent slurry into powder, placing it around a LATP cold-pressed sheet, and sintering it at a high temperature so that the powder is coated on the surface of the LATP cold-pressed sheet to form the interface layer.

[0006] Optionally, the commercial boron nitride release agent comprises at least boron nitride, a binder and an emulsifier dispersed in an acetone solvent.

[0007] Optionally, the boron nitride is a sheet-like structure, and the binder is a tubular structure.

[0008] Optionally, the interface layer is formed in-situ on the surface of the LATP cold-pressed sheet during high-temperature sintering.

[0009] Optionally, the interface layer is a dense three-dimensional multilayer structure.

[0010] Optionally, the thickness of the interface layer is 2 μm-4 μm.

[0011] The present invention also provides a method for preparing a LATP-based solid-state lithium battery, comprising: processing an interface layer on a LATP solid electrolyte by the above-mentioned preparation method, processing a metal lithium negative electrode on a side of the solid electrolyte containing the interface layer, and then assembling it with a positive electrode into a solid-state lithium battery.

[0012] Optionally, the method of processing metallic lithium includes: evaporation.

[0013] Optionally, during the process of evaporating the metallic lithium, the metallic lithium reacts with the boron nitride to generate lithium nitride.

[0014] Optionally, the thickness of the metal lithium negative electrode is 2 μm-5 μm.

[0015] The beneficial effects of the present invention are:

[0016] (1) A commercial boron nitride release agent containing inorganic boron nitride, an organic binder and an emulsifier is applied to the interface layer of a solid-state battery, and an organic-inorganic composite interface layer is instantly formed on the surface of the solid electrolyte sheet in one step. The interface layer is used to reduce the interface impedance of the solid-state battery, and the inorganic rigid filler is used to improve the mechanical strength of the interface layer. The raw materials are easily available, the cost is low, and the method is simple to operate.

[0017] (2) The formation of a dense interface layer with a three-dimensional multilayer structure can not only isolate the solid electrolyte LATP from the metal lithium negative electrode to avoid reduction reaction, but also effectively reduce the local current density and interface resistance during the subsequent battery operation, resulting in uniform Li+ deposition / stripping, effectively avoiding the lithium dendrite problem.

[0018] (3) The metallic lithium negative electrode in the solid-state lithium battery provided by the present invention is formed by evaporating lithium. During the lithium evaporation process, the lithium vapor reacts with the boron nitride in the interface layer to generate lithium nitride with high ionic conductivity and strong compatibility with lithium at the interface, which helps to obtain a solid-state lithium battery with excellent capacity, cycle performance, and rate performance.

[0019] (4) The good thermal conductivity of boron nitride in the interface layer can allow heat to diffuse rapidly, improving the safety issues of LATP-based solid metal lithium and avoiding thermal runaway. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is an electron microscope image of the interface layer prepared in Example 1 of the present invention.

[0021] Figure 2 Optical photographs of a solid electrolyte with an interface layer and a solid electrolyte without an interface layer prepared in Example 1 of the present invention after evaporation of a lithium negative electrode.

[0022] Figure 3 This is a graph showing the XPS test results of the interface layer prepared in Example 2 of the present invention.

[0023] Figure 4 This is the cycle performance curve of the solid-state lithium battery prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0024] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] The present invention provides an internal structure of a solid-state lithium battery, wherein the battery body consists of a positive electrode, a solid electrolyte, an interface layer and a lithium negative electrode.

[0026] The positive electrode is at least one of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material and lithium-rich manganese-based material. The solid electrolyte is a LATP-based oxide ceramic sheet with a chemical formula of Li 1+x Al x Ti 2–x (PO4)3(x=0~0.4)。

[0027] The interface layer is formed by spraying a commercial boron nitride release agent on the surface of the solid electrolyte. The commercial boron nitride release agent is mainly composed of boron nitride, a binder and an emulsifier dispersed in an acetone solvent. Optionally, the binder includes any one or a combination of sodium carboxymethyl cellulose and styrene-butadiene rubber; the emulsifier is any one or more of alkylbenzene sulfonate (sodium dodecylbenzene sulfonate), quaternary ammonium salt, and alkyl sulfate (sodium dodecyl sulfate). The interface layer helps to improve the stability to lithium, and Li3N is formed in situ at the interface, providing high ionic conductivity and low resistance for LATP lithium batteries.

[0028] The commercial boron nitride release agent used in the present invention was purchased from Dongguan Jiadan Chemical Co., Ltd., model JD-3028, named "Jiadan Boron Nitride Release Spray", batch 2021 / 09 / 26.

[0029] The preparation method of the present invention is as follows:

[0030] (1) LATP-based solid electrolyte interface layer

[0031] Step 1: Prepare LATP cold-pressed tablets;

[0032] Step 2: Dry the commercial boron nitride release agent slurry into powder, place it around the LATP cold-pressed sheet, and sinter it at high temperature so that the powder is coated on the surface of the LATP cold-pressed sheet to form the interface layer.

[0033] Since there are pores in the LATP cold-pressed sheet, and the pores disappear during high-temperature sintering, the disc will shrink and deform. Therefore, during the sintering process of the solid electrolyte ceramic sheet, the powder embedding step is used before high-temperature sintering to supplement this part of the defect. The prior art usually uses the solid electrolyte powder itself as the powder when embedding the powder. The present invention uses a commercial boron nitride release agent dried into a powder as the powder for embedding the powder. The commercial boron nitride release agent at least includes boron nitride with a sheet structure dispersed in an acetone solvent, a binder with a tubular structure, and an emulsifier. The boron nitride in the release agent can withstand a high temperature of 1200°C, will not volatilize during the sintering process, and can make the coating of the release agent more firm, forming an interface layer with a dense three-dimensional multilayer structure on the surface of the LATP cold-pressed sheet in situ, which has a better coating effect. The interface layer is directly formed on the LATP cold-pressed sheet during sintering, so that the interface layer can be in good contact with the LATP, completely fit, and there is no gap. The interface layer is stacked by two-dimensional sheet-like boron nitride, and the interface layer is firmly coated with a binder. Preferably, the thickness of the interface layer is 2 μm-4 μm.

[0034] The interface layer with a dense three-dimensional multilayer structure is a lithium-philic structure. Lithium can penetrate into the interface layer containing boron nitride to generate lithium nitride. Therefore, the lithium nitride in the solid-state lithium battery is not only on the surface of the interface layer, but can still conduct lithium even if the interface layer has a certain thickness.

[0035] (2) Solid-state batteries

[0036] Step 1: Prepare the positive electrode sheet;

[0037] Step 2: obtaining a metallic lithium negative electrode by evaporating lithium on one side of the LATP solid electrolyte containing the interface layer;

[0038] Step 3: Assemble the positive electrode and solid electrolyte / interface layer / negative electrode into a solid-state lithium battery.

[0039] Preferably, the thickness of the metal lithium negative electrode is 2 μm-5 μm.

[0040] Example 1

[0041] (1) LATP-based solid electrolyte interface layer

[0042] Step 1: Put the solid electrolyte powder into an agate mortar, grind it manually for half an hour, add an appropriate amount of ethanol solution with a mass fraction of 3% PVB, grind it again until the solvent is completely evaporated, weigh about 0.8g of powder, put it into a mold, keep it under a pressure of 10MPa for 20min, and press it into a solid disc with a diameter of 10mm to obtain LATP cold-pressed tablets.

[0043] Step 2: Place the LATP cold-pressed sheet flat in a crucible, spread the commercial boron nitride release agent dried into powder around the LATP cold-pressed sheet, place it in a tubular furnace, heat it to 960°C at 5°C / min, and keep it warm for 12 hours to form an interface layer on the LATP-based solid electrolyte.

[0044] (2) Solid-state batteries

[0045] Step 1: 77wt.% lithium nickel cobalt manganese oxide positive electrode material, 10wt.% carbon black, 10wt.% polyvinylidene fluoride (PVDF) and 3wt.% LiClO4 are dispersed in N-methyl-2-pyrrolidone (NMP), mixed evenly and then coated on aluminum foil, dried in a blast oven at 80°C for 6 hours, and then transferred to a vacuum oven at 110°C for drying for 24 hours to obtain a positive electrode sheet;

[0046] Step 2: A metallic lithium negative electrode is obtained by evaporating lithium on one side of the LATP solid electrolyte containing the interface layer, and the thickness of the metallic lithium negative electrode layer is 2 μm;

[0047] Step 3: The positive electrode, solid electrolyte / interface layer / lithium layer are stacked into the stainless steel shell of the button battery, assembled into a button battery in a glove box and tested. In order to eliminate the contact impedance between the positive electrode and the solid electrolyte, a small amount of liquid electrolyte is dripped at the interface between the positive electrode and the solid electrolyte.

[0048] The scanning electron microscope image of the interface layer prepared in Example 1 is shown in Figure 1. It can be seen from the figure that the thickness of the interface layer is less than 3 μm, which is much smaller than the thickness of a general polymer coating and will not have a significant impact on the energy density of the battery. The interface layer is a dense three-dimensional multilayer structure that can cover all surfaces of the LATP sheet to completely separate LATP and Li. At the same time, this three-dimensional structure can effectively reduce local current density and interface resistance.

[0049] The optical photograph of the solid electrolyte / lithium prepared in Example 1 is as follows Figure 2As shown in the figure, in order to verify the performance of the interface layer, the LATP ceramic sheet was divided into two parts, one part was the exposed LATP, and the other part was the LATP containing the interface layer. After lithium was deposited by evaporation, the unprotected part of the LATP turned completely black, indicating that a reduction reaction had occurred, while the other half of the protected LATP was covered by bright Li metal, once again proving the protective effect of the interface layer on the LATP.

[0050] Example 2

[0051] (1) LATP-based solid electrolyte interface layer

[0052] Step 1: Put the solid electrolyte powder into an agate mortar, grind it manually for half an hour, add an appropriate amount of ethanol solution with a mass fraction of 3% PVB, grind it again until the solvent is completely evaporated, weigh about 0.8g of powder, put it into a mold, keep it under a pressure of 10MPa for 20min, and press it into a solid disc with a diameter of 10mm to obtain LATP cold-pressed tablets.

[0053] Step 2: Place the LATP cold-pressed sheet flat in a crucible, spread the commercial boron nitride release agent dried into powder around the LATP cold-pressed sheet, place it in a tubular furnace, heat it to 960°C at 5°C / min, and keep it warm for 12 hours to form an interface layer on the LATP-based solid electrolyte.

[0054] (2) Preparation of solid-state lithium batteries

[0055] Step 1: 77wt.% lithium iron phosphate positive electrode material, 10wt.% carbon black, 10wt.% polyvinylidene fluoride (PVDF) and 3wt.% LiClO4 are dispersed in N-methyl-2-pyrrolidone (NMP), mixed evenly and then coated on aluminum foil, dried in a blast oven at 80°C for 6 hours, and then transferred to a vacuum oven at 110°C for 24 hours to obtain a positive electrode sheet;

[0056] Step 2: A metallic lithium negative electrode is obtained by evaporating lithium on one side of the LATP solid electrolyte sheet containing the interface layer, and the thickness of the metallic lithium negative electrode layer is 5 μm;

[0057] Step 3: The positive electrode, solid electrolyte / interface layer / lithium layer are stacked into the stainless steel shell of the button battery, assembled into a button battery in a glove box and tested. In order to eliminate the contact impedance between the positive electrode and the solid electrolyte, a small amount of liquid electrolyte is dripped at the interface between the positive electrode and the solid electrolyte.

[0058] The interface layer prepared by this embodiment 2 was tested by XPS, and the results are as follows: Figure 3 As shown, BN and Li3N were observed simultaneously at the interface after lithium evaporation, proving the formation of lithium nitride.

[0059] The cycle curve of the solid-state lithium battery prepared in Example 2 is as follows: Figure 4 As shown, the battery is charged and discharged in the voltage range of 2.5-4.2V at room temperature with a current density of 15mA / g. The initial discharge specific capacity is 152.3mAh / g, and the capacity remains unchanged after 50 cycles. The capacity performance and cycle stability are both excellent, and it is expected to be used in energy storage power supplies and power batteries.

[0060] In summary, in the present invention, a commercial boron nitride release agent is introduced into the interface between the LATP solid electrolyte and metallic lithium, and the main components contained in the release agent are used to form an organic-inorganic composite three-dimensional dense interface protection layer, which can reduce the interface impedance while avoiding the formation of lithium dendrites, and form Li3N with high ionic conductivity at the interface, achieving multiple goals at one stroke, so that the LATP-based solid-state lithium battery exhibits excellent performance.

[0061] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A method for preparing a LATP-based solid electrolyte interface layer, characterized in that: include: The commercial boron nitride release agent slurry is dried into powder, placed around the LATP cold pressed sheet, and sintered at high temperature so that the powder is coated on the surface of the LATP cold pressed sheet to form the interface layer.

2. The preparation method according to claim 1, characterized in that The commercial boron nitride release agent at least comprises boron nitride, a binder and an emulsifier dispersed in an acetone solvent.

3. The preparation method according to claim 2, characterized in that: The boron nitride is a sheet-like structure, and the binder is a tubular structure.

4. The preparation method according to claim 1, characterized in that: The interface layer is formed in-situ on the surface of the LATP cold-pressed sheet during the high-temperature sintering process.

5. The preparation method according to claim 1, characterized in that: The interface layer is a dense three-dimensional multi-layer structure.

6. The preparation method according to claim 1, characterized in that: The thickness of the interface layer is 2 μm-4 μm.

7. A method for preparing a LATP-based solid-state lithium battery, characterized in that: include: On the LATP solid electrolyte, an interface layer is processed by the preparation method according to any one of claims 1 to 6, and after a metal lithium negative electrode is processed on the side of the solid electrolyte containing the interface layer, it is assembled with the positive electrode into a solid-state lithium battery.

8. The preparation method according to claim 7, characterized in that: Methods for processing metallic lithium include: evaporation.

9. The preparation method according to claim 8, characterized in that: During the process of evaporating the metallic lithium, the metallic lithium reacts with the boron nitride to generate lithium nitride.

10. The preparation method according to claim 7, characterized in that: The thickness of the metal lithium negative electrode is 2 μm-5 μm.

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