An inorganic / polymer composite solid electrolyte and its application

Abstract

The present invention provides an inorganic / polymer composite solid electrolyte and its application, which includes a framework material that plays a role in fast ion conduction and a toughening material filled in the framework material. The framework material is selected from garnet-type solid electrolyte lithium lanthanum zirconium oxide and perovskite-type solid electrolyte lithium lanthanum titanate. Due to the combination of the high modulus of the inorganic framework and the high toughness of the polymer, this composite solid electrolyte has good mechanical properties, and successfully solves the problem that lithium dendrites pierce the electrolyte and cause battery short circuit. The introduction of the polymer greatly improves the brittleness of the inorganic framework, and at the same time solves the problem of high interfacial impedance caused by poor contact between the inorganic ceramic fibers and the positive and negative electrodes. Finally, the use of this electrolyte improves the safety performance and service performance of lithium metal batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of solid-state lithium metal batteries, and particularly to an inorganic / polymer composite solid electrolyte. Background Art

[0002] With the progress of society, people's requirements for energy storage devices are constantly increasing, and the disadvantages of low capacity of traditional liquid lithium-ion batteries are continuously magnified. At the same time, organic electrolytes have problems such as easy volatilization, low flash point, and easy leakage and gas swelling. The most fatal thing is that the uneven deposition of lithium metal during the charge and discharge process of the battery will lead to the growth of lithium dendrites. Traditional liquid lithium-ion batteries cannot inhibit the growth of lithium dendrites, which will eventually cause the dendrites to pierce the diaphragm, resulting in battery short circuit failure and even battery explosion. Therefore, people urgently need higher-capacity and safer batteries as substitutes. Lithium metal batteries have gradually become a popular research direction due to their high energy density (3860 mAh g -1 ), and the lowest redox potential (-3.04 V vs standard hydrogen electrode). At the same time, solid electrolytes with high mechanical strength replacing liquid electrolytes are expected to solve the problem of battery short circuit caused by lithium dendrites piercing the diaphragm, thereby improving the safety performance of the battery.

[0003] Solid electrolytes are divided into inorganic solid electrolytes, polymer solid electrolytes, and inorganic / polymer composite solid electrolytes. Common inorganic solid electrolytes include garnet-type solid electrolytes, NASICON-type solid electrolytes, LISICON-type solid electrolytes, and perovskite-type solid electrolytes. Due to their rough surfaces, poor contact between the electrolyte and the lithium negative electrode and the positive electrode in the battery is caused, which will ultimately lead to high interfacial impedance and affect the cycling performance of the battery. Polymer solid electrolytes transport lithium ions mainly by the movement of polymer chain segments, and this lithium ion transport method is generally one to two orders of magnitude slower than that of inorganic fast ion conductors.

[0004] Therefore, inorganic / polymer composite solid electrolytes have become a relatively suitable choice. However, at present, most composite solid electrolytes simply physically blend inorganic substances with polymers. Since the inorganic substances in such electrolytes usually exist in the form of particles and cannot actually form a continuous lithium ion transport channel, the lithium ion transport efficiency of such solid electrolytes is not high.

[0005] In view of the above problems, developing a solid electrolyte with both high ionic conductivity and good interfacial compatibility has become a great challenge. Summary of the Invention

[0006] To solve the problem that lithium dendrite growth in traditional liquid lithium-ion batteries pierces the separator and causes battery failure, the present invention provides a preparation method of an asymmetric double-layer ceramic fiber / polymer composite solid electrolyte and its application field in solid-state lithium metal batteries. By using a double-layer inorganic fiber as the skeleton and the high molecular polymer polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) as the filler, an inorganic fiber / polymer composite solid electrolyte with an asymmetric double-layer structure is prepared. Due to the addition of the inorganic fiber, the lithium-ion transport efficiency of the electrolyte is increased, making the solid electrolyte have a high ionic conductivity at room temperature. The addition of PVDF-HFP can well improve the mechanical properties of the solid electrolyte and improve the interfacial contact between the electrolyte and the positive and negative electrodes of the battery, thereby improving the safety performance and service performance of the battery.

[0007] To achieve the above object, in the first aspect, the present invention provides a composite solid electrolyte, which includes a double-layer inorganic fiber as the skeleton material and a toughening material as the filler for filling the skeleton. The skeleton material is a garnet-type solid electrolyte or a perovskite-type solid electrolyte, and the filler material is selected from the high molecular polymer polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP).

[0008] The composite solid electrolyte provided by the present invention uses a double-layer inorganic fiber as the skeleton material. The inorganic skeleton material is lithium lanthanum titanate (LLTO) and lithium lanthanum zirconium oxide (LLZO), where the LLZO layer serves as a stable protective layer, that is, it protects the LLTO inorganic fiber with high ionic conductivity from reacting with lithium metal. The double-layer inorganic fiber can not only be used as a fast ion conductor but also as the skeleton of the solid electrolyte, and this skeleton increases the modulus of the solid electrolyte. During the growth of lithium dendrites, the protruding lithium dendrites cannot pierce the electrolyte when encountering the solid electrolyte, and then tend to stretch around, thus solving the safety problem in battery cycling. At the same time, using PVDF-HFP as the filling material can improve the brittleness of the inorganic fiber and enhance the comprehensive mechanical properties of the solid electrolyte. And this polymer has a smooth surface, so using it as the filling material can solve the problem of poor interfacial compatibility of the inorganic fiber, greatly reducing the interfacial impedance during battery cycling and improving the cycling performance of the battery.

[0009] In existing lithium metal batteries in the prior art, a commercial separator and an organic electrolyte are combined as the electrolyte of the lithium metal battery. When the battery is cycling, due to the unequal local current density between the electrolyte and the lithium negative electrode, lithium metal tends to deposit in the place with a high current density, so the grown lithium dendrites will pierce the commercial separator and cause the battery to short-circuit and fail.

[0010] Principle of the present invention: The skeleton structure layer in the composite solid electrolyte provided by the present invention is composed of a bilayer inorganic fiber with a continuous lithium-ion conduction path, namely, a bilayer inorganic fiber of LLTO and LLZO. This bilayer inorganic fiber can effectively increase the ionic conductivity of the electrolyte as a skeleton. The high mechanical strength of this solid electrolyte can also suppress the formation of lithium dendrites, that is, the lithium dendrites growing into the solid electrolyte will not continue to grow, thus effectively avoiding the risk of short circuit between the positive and negative electrodes caused by the growth of lithium dendrites. And because the present invention uses bilayer fibers, in practical applications, the LLZO layer fibers, as a protective layer, cleverly solve the problem of the reaction between high-valent titanium and lithium metal. In addition, PVDF-HFP as a filling material can effectively solve the problem of poor interfacial compatibility between inorganic fibers and electrodes. Through this reasonable electrolyte structure design, both the problem of lithium dendrite growth commonly found in lithium metal batteries and the interfacial compatibility between the electrolyte and electrodes can be increased while introducing inorganic fibers are solved. This reasonable invention design enables the battery to have higher safety performance and better cycling performance during cycling.

[0011] Compared with the prior art, the effects of the present invention are as follows:

[0012] 1. The asymmetric bilayer inorganic ceramic fiber as a skeleton not only enhances the modulus of the solid electrolyte but also provides a continuous high-speed channel for the transmission of lithium ions. More importantly, due to the design of the bilayer fiber, the LLZO layer can serve as a protective layer to prevent the reaction between high-valent titanium in the LLTO layer and lithium metal.

[0013] 2. The introduction of PVDF-HFP as a filling material not only increases the toughness of the solid electrolyte but also solves the problem of poor interfacial compatibility between inorganic ceramic fibers and electrodes, enabling the battery to operate stably during long cycling. Description of the Drawings

[0014] Figure 1 It is a scanning electron microscope image of the composite solid electrolyte provided in Example 1 of the present invention.

[0015] Figure 2 It is a comparison chart of the mechanical strengths of the composite solid electrolytes obtained in Examples 1, 2, and 3 of the present invention.

[0016] Figure 3 It is a test chart of the composite solid electrolytes obtained in Examples 1, 2, and 3 of the present invention; among them Figure 3 (a) is the ionic conductivity chart of the obtained composite solid electrolyte; Figure 3 (b) is the long cycling test chart of the obtained composite solid electrolyte; Figure 3 (c) is the rate cycling test chart of the obtained composite solid electrolyte. Detailed Embodiments

[0017] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0018] The present invention provides an asymmetric double-layer ceramic fiber / polymer composite solid electrolyte. The solid electrolyte includes a skeleton material and a toughening material filled in the skeleton. The skeleton material is selected from garnet-type solid electrolytes and perovskite-type electrolytes. The toughening material is selected from the high molecular polymer polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP).

[0019] For the lithium metal batteries in the prior art, only commercial diaphragms and organic electrolytes are used as electrolytes. When lithium dendrites grow, due to the low mechanical strength of commercial diaphragms, the growth of lithium dendrites cannot be inhibited, and the lithium dendrites will pierce through the commercial diaphragms and grow to the positive electrode, which will cause the battery to short-circuit and fail, and even pose a safety hazard of battery explosion. The composite solid electrolyte in this invention uses inorganic ceramic fibers as the skeleton, which can effectively increase the modulus of the solid electrolyte and achieve the effect of inhibiting the growth of lithium dendrites in actual battery applications. And this composite solid electrolyte with its unique structural design (using LLZO as the isolation layer between LLTO and lithium metal) perfectly avoids the disadvantage of the reaction between Ti in the inorganic ceramic fiber LLTO and lithium metal. 4+ And the disadvantage of reacting with lithium metal.

[0020] Moreover, although common inorganic ceramic fibers have the advantages of being able to inhibit the growth of lithium dendrites and having high ionic conductivity when used as electrolytes, the obvious defects are that the inorganic fibers are brittle and have poor interfacial contact with the positive and negative electrodes. To solve this thorny problem, we introduce the high molecular polymer polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) as a filling material in the present invention. As a flexible polymer, PVDF-HFP can solve the problem of rough interface when filled into inorganic fibers, providing a new idea for the interface problem between the solid electrolyte and the positive and negative electrode materials.

[0021] Among them, the garnet-type solid electrolyte is specifically Li 6.4 La3Zr2Al 0.2 O 12 , and the perovskite-type solid electrolyte is Li 0.33 La 0.56 TiO3.

[0022] The filling material of the present invention is selected from the polymer polyvinylidene fluoride - hexafluoropropylene (PVDF-HFP). The proportion of the filling material in the entire solid electrolyte will affect the ionic conductivity of the entire solid electrolyte. It is the inorganic fibers that play the role of enhancing the ionic conductivity in the electrolyte. Therefore, when the proportion of the polymer filling material is too high, the ionic conductivity of the solid electrolyte will be too low.

[0023] Therefore, the polymer content of this solid electrolyte accounts for 45% of the entire solid electrolyte material.

[0024] Furthermore, the thickness of this solid electrolyte is about 150 μm.

[0025] Preferably, the diameter of LLZO fibers is about 80 - 100 nm, and the diameter of LLTO fibers is about 300 - 500 nm.

[0026] Because the ionic conductivity of LLTO fibers is higher than that of LLZO at room temperature, in this solid electrolyte, the thickness of the LLTO layer is about 100 μm, while the LLZO layer as the protective layer is about 50 μm.

[0027] PVDF-HFP used as the filler is purchased from ALDRICH, where M w ~400000, M n ~130000.

[0028] Furthermore, the two inorganic fibers LLZO and LLTO used as the framework materials are prepared by the traditional electrospinning method. The LLZO and LLTO electrospinning solutions are prepared according to the stoichiometric ratio respectively. The solvent is DMF and acetic acid, and PVP is used as the spinning aid.

[0029] Example 1

[0030] (1) Take two 20 mL glass bottles and divide them into Bottle 1 and Bottle 2. Prepare the LLTO electrospinning solution in Bottle 1 and the LLZO electrospinning solution in Bottle 2. First, add 8 mL of DMF and 2 mL of acetic acid solution to Bottle 1, and add 4.8 mL of DMF and 0.75 mL of acetic acid solution to Bottle 2. Then, add lithium nitrate, lanthanum nitrate nonahydrate, zirconium propoxide, and tetrabutyl titanate to Bottle 1 and Bottle 2 respectively according to a fixed molar ratio, and stir the prepared solution for two hours. After the salts are completely dissolved, add the previously prepared PVP powder. Finally, stir the mixed solution overnight to complete the preparation of the electrospinning solution.

[0031] Load the prepared spinning solution into a 10 mL syringe and place it in an electrospinning machine to prepare for electrospinning. Before starting the electrospinning, set the temperature to room temperature of 25 °C and the humidity to 37%. Turn on the electrospinning machine for electrospinning operation, set the positive high voltage of the electrospinning machine to 15 kV and the negative high voltage to 0.5 kV. Then set the injection, receiving and translation speeds to complete the electrospinning operation.

[0032] Put the successfully prepared fibers into a tube furnace and sinter them at high temperature using a specific program to obtain the inorganic ceramic fibers required by the present invention.

[0033] (2) Move the LLZO and LLTO inorganic ceramic fiber membranes obtained in step (1) to a customized polytetrafluoroethylene mold, which consists of a cylinder with a radius of 18 mm. Move the mold into the glove box to prepare for the preparation of the electrolyte.

[0034] (3) Add PVDF-HFP to the DMF solution in a certain proportion. After PVDF-HFP is completely dissolved, transfer the solution into the glove box to prepare for the preparation of the electrolyte.

[0035] (4) Add a certain amount of lithium salt to the PVDF-HFP solution in the glove box and stir for two hours until the lithium salt is dissolved. After the lithium salt is dissolved, drop the PVDF-HFP solution onto the previously prepared inorganic ceramic fibers in a certain proportion. Then wait for the DMF to naturally volatilize in the glove box to obtain the solid electrolyte we need.

[0036] (5) Assembly of the lithium metal solid electrolyte battery: According to the assembly method of button batteries, take out the 2025 type battery case, place the negative electrode case at the bottom, and then in turn are the shrapnel, gasket, and lithium negative electrode. Then drop an appropriate electrolyte between the electrolyte and the lithium negative electrode to increase the interfacial compatibility. Finally, add the positive electrode sheet and the positive electrode case to complete the preparation of the button battery.

[0037] It can be seen from Figure 1 that the thickness of the prepared PVDF-HFP / LLZO / LLTO composite solid electrolyte is about 150 μm, of which the LLZO layer is about 50 μm and the LLTO layer is 100 μm.

[0038] It can be seen from Figure 2 that the mechanical strength of the prepared PVDF-HFP / LLZO / LLTO composite solid electrolyte is higher than that of single-layer solid electrolytes and polymer solid electrolytes.

[0039] As Figure 3 shown, where Figure 3 (a) is the ionic conductivity diagram of the obtained composite solid electrolyte; Figure 3 (b) is the long-cycle test diagram of the obtained composite solid electrolyte;Figure 3 (c) is the rate cycling test chart of the obtained composite solid electrolyte. It can be seen that the prepared PVDF-HFP / LLZO / LLTO composite solid electrolyte has a high ionic conductivity. Although the ionic conductivity of the PVDF-HFP / LLZO / LLTO solid electrolyte is slightly lower than that of PVDF-HFP / LLTO, due to the existence of the LLZO fiber protective layer, the former can prevent Ti 4+ from reacting with lithium metal, ensuring the cycling performance in the actual battery. From the cycling performance and rate performance of the full battery, the advantages of the double-layer inorganic ceramic fiber skeleton and polymer filling material in the actual battery cycling can be seen.

[0040] Example 2

[0041] (1) Take a 20 mL glass bottle as Bottle 1, and prepare the LLTO electrospinning solution in Bottle 1. First, add 8 mL of DMF and 2 mL of acetic acid solution to Bottle 1. Then, according to a fixed molar ratio, add lithium nitrate, lanthanum nitrate nonahydrate, and tetrabutyl titanate to Bottle 1 respectively, and stir the prepared solution for two hours. After the salts are completely dissolved, add the pre-prepared PVP powder. Finally, stir the mixed solution overnight to complete the preparation of the electrospinning solution.

[0042] Load the prepared spinning solution into a 10 mL syringe and place it in the electrospinning machine for spinning preparation. Before starting the spinning, set the temperature to room temperature 25 °C and the humidity to 37%. Turn on the electrospinning machine for electrospinning operation, set the positive high voltage of the spinning machine to 15 kV and the negative high voltage to 0.5 kV. Then set the injection, receiving, and translation speeds to complete the spinning operation.

[0043] Put the successfully prepared fibers into a tube furnace and sinter them at high temperature with a specific program to obtain the inorganic ceramic fibers we need.

[0044] (2) Move the LLTO inorganic ceramic fiber membrane obtained in step (1) to a customized polytetrafluoroethylene mold, which consists of a cylinder with a radius of 18 mm. Move the mold into the glove box to prepare the electrolyte.

[0045] (3) Add PVDF-HFP to the DMF solution in a certain proportion. After PVDF-HFP is completely dissolved, transfer the solution into the glove box to prepare the electrolyte.

[0046] (4) Add a certain amount of lithium salt to the PVDF-HFP solution in the glove box and stir for two hours until the lithium salt is dissolved. After the lithium salt is dissolved, drop the PVDF-HFP solution onto the pre-prepared inorganic ceramic fibers in a certain proportion. Then wait for the DMF to naturally volatilize in the glove box to obtain the solid electrolyte we need.

[0047] (5) Assembly of lithium metal solid electrolyte battery: According to the assembly method of button batteries, take out the 2025 type battery case, place the negative electrode case at the bottom, and then successively place the shrapnel, gasket, and lithium negative electrode. Then, add an appropriate electrolyte solution between the electrolyte and the lithium negative electrode to increase the interfacial compatibility. Finally, add the positive electrode sheet and the positive electrode case to complete the preparation of the button battery.

[0048] Example 3

[0049] (1) Take a 20 mL glass bottle as Bottle 1, and prepare a PVDF-HFP / LiTFSI solution in Bottle 1.

[0050] (2) Dissolve PVDF-HFP in the DMF solution. After complete dissolution, transfer the prepared solution into the glove box. Add LiTFSI to the PVDF-HFP solution in the glove box at a certain ratio and stir for two hours until the lithium salt is completely dissolved.

[0051] (3) After the lithium salt is dissolved, drop the PVDF-HFP solution into a pre-prepared polytetrafluoroethylene mold at a certain ratio. Then, wait for the DMF to naturally volatilize in the glove box to obtain the polymer solid electrolyte we need.

[0052] (5) Assembly of lithium metal solid electrolyte battery: According to the assembly method of button batteries, take out the 2025 type battery case, place the negative electrode case at the bottom, and then successively place the shrapnel, gasket, and lithium negative electrode. Then, add an appropriate electrolyte solution between the electrolyte and the lithium negative electrode to increase the interfacial compatibility. Finally, add the positive electrode sheet and the positive electrode case to complete the preparation of the button battery.

[0053]

[0054] Finally, it should be noted that the above are only the preferred embodiments of the present invention and do not impose any formal limitations on the present invention. Obviously, those skilled in the art can make various modifications and variations to the present invention without departing from the spirit and scope of the present invention, and the present invention also intends to include these modifications and variations.

Claims

1. An inorganic / polymer composite solid electrolyte, characterized in that, The composite solid electrolyte is composed of a double-layer inorganic fiber as the skeleton and a polymer material filled in the skeleton, and the skeleton is an asymmetric double-layer inorganic fiber structure; The skeleton is lithium lanthanum titanate and lithium lanthanum zirconium oxide; the polymer material is the high molecular polymer polyvinylidene fluoride-hexafluoropropylene; the thickness of the lithium lanthanum titanate layer is 100 μm, and the thickness of the lithium lanthanum zirconium oxide layer is 50 μm; the fiber diameter of the lithium lanthanum titanate is 80-100 nm, and the fiber diameter of the lithium lanthanum zirconium oxide layer is 300-500 nm; the polymer content of the composite solid electrolyte accounts for 45% of the entire solid electrolyte.

2. The inorganic / polymer composite solid electrolyte according to claim 1, wherein The skeleton is prepared by electrospinning.

3. A solid-state lithium metal battery, characterized in that, It includes a positive electrode, a negative electrode, and the composite solid electrolyte according to any one of claims 1-2.

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