Boroxide-molten salt-based solid electrolyte and method for producing the same

Abstract

This invention relates to a solid electrolyte, which is a core material for an all-solid-state secondary battery, and Li 4+x B7O 12+x / 2 The present invention provides a composite solid electrolyte comprising lithium chloroborasite having a composition of Cl(x=0~1) and a LiCl-LiBr-LiF molten salt. This borite-molten salt-based solid electrolyte can solve the safety problems of conventional sulfide solid electrolytes such as P2S5 and Li2S, which are used as raw materials.

Description

[Technical Field]

[0001] This invention relates to an all-solid-state electrolyte for lithium-ion batteries with improved safety and ionic conductivity. [Background technology]

[0002] A secondary battery is a battery composed of one or more electrochemical cells that can be charged and discharged. Currently, lithium-ion batteries are leading the way, being used in essential small electronic devices (such as smartphones). Lithium-ion batteries have advantages over conventional lead-acid batteries in terms of environmental toxicity and the memory effect (a phenomenon in which the discharge voltage decreases and the discharge capacity becomes smaller than the nominal capacity when the battery is repeatedly charged during use), and they also have superior energy density per unit mass / volume.

[0003] Furthermore, while rechargeable batteries generally have a higher initial cost than disposable batteries, their advantage lies in their ability to be recharged multiple times before replacement, resulting in significantly lower overall costs and environmental impact. Some rechargeable battery types can be used with the same size and voltage as disposable batteries and can be replaced.

[0004] However, the electrolyte (liquid or gel) inside lithium-ion batteries poses a risk of ignition and explosion due to thermal propagation. In fact, numerous explosion incidents have led to increasing concerns about their stability.

[0005] Therefore, replacing liquid electrolytes with solid electrolytes to compensate for these shortcomings offers the following advantages:

[0006] The risk of fire and explosion due to temperature changes and external shocks is significantly reduced, and since safety devices and separation membranes to protect against temperature changes and external shocks are unnecessary, it is possible to reduce costs and achieve higher capacity while maintaining the same size.

[0007] Because there is no risk of fire, the space freed up by the removal of the cooling system, which occupies more than 30% of the battery pack space, can be filled with additional battery cells, thereby increasing the energy density.

[0008] In liquid electrolytes, the separation membrane that physically isolates the positive and negative electrodes to prevent electrical short circuits is unnecessary, thus enabling volume reduction and cost savings.

[0009] Lithium metal, which has excellent performance such as having a capacity 10 times greater than graphite but could not be used due to the risk of fire and explosion between electrodes, can now be used as a negative electrode active material, thus enabling high capacity in the same size.

[0010] On the other hand, sulfide-based solid electrolytes using Li2S-P2S5-LiCl ternary raw materials with an argyrodite crystal structure have been developed as such all-solid-state electrolytes. However, these conventional sulfide-based solid electrolytes pose a risk because they may generate harmful compounds such as hydrogen sulfide (H2S). In response to this, the need has emerged for the development of a safe and mass-producible solid electrolyte that can essentially eliminate the risks of conventional sulfide-based solid electrolytes. [Overview of the project] [Problems that the invention aims to solve]

[0011] The present invention aims to manufacture a borate-based solid electrolyte material with high purity and ionic conductivity and a particle shape, and to apply it to the lithium-ion conductor of an all-solid-state battery.

[0012] Furthermore, by supplying borate-based solid electrolyte materials to industry, we aim not only to gain an economic advantage but also to ensure superior safety in raw material handling. [Means for solving the problem]

[0013] As one method for improving the ionic conductivity of oxide-based solid electrolytes, we studied various approaches that increase the density of molded bodies such as pellets, thereby increasing the solid electrolyte content in the same space.

[0014] The solid electrolyte of the present invention is Li 4+x B7O 12+x / 2 This is a composite of lithium chloroborasite having a composition of Cl(x=0~1) and a LiCl-LiBr-LiF molten salt.

[0015] The composite mixture of lithium chloroborasite and molten salt exhibits a density increase of 20% or more in molded bodies such as pellets compared to the mixture before mixing, and a Li ion conductivity of 1.0 × 10⁻⁶ at room temperature (25°C). -7 Characterized by being S / cm or higher.

[0016] Furthermore, the composite of the present invention is characterized by being nanoparticles with a diameter of 10 nm to 300 μm. Preferably, the particle size is 0.8 to 80 μm in diameter, more preferably 1 to 20 μm. As the particle size decreases, the density can be increased, which is advantageous for ionic conductivity. However, there are difficulties in the manufacturing process, and there are limitations to improving the density of the molded article by controlling the particle size. For the reasons mentioned above, the density of the composite was improved by adding a molten salt.

[0017] The composite of the present invention is Li 4+x B7O 12+x / 2 The process is characterized by comprising the steps of: preparing lithium chloroborasite having a composition of Cl(x=0~1); preparing a LiCl-LiBr-LiF molten salt; mixing the LiCl-LiBr-LiF molten salt with the lithium chloroborasite; pulverizing the raw material mixture; and calcining the raw material mixture at 500°C or higher.

[0018] The composite solid electrolyte of the present invention can solve the problem of hydrogen sulfide generation in the atmosphere containing moisture caused by P2S5, Li2S, etc., which are used as raw materials for conventional sulfide solid electrolytes. By manufacturing a secondary battery containing this, it is possible to replace the secondary batteries using conventional liquid electrolytes and sulfide-based solid electrolytes in the market.

Advantages of the Invention

[0019] Synthesize the borate-molten salt-based solid electrolyte of the present invention to provide a solid electrolyte having a particle shape, high purity, and high ionic conductivity.

[0020] In addition, the borate-molten salt-based solid electrolyte of the present invention is free from safety problems such as hydrogen sulfide gas generation due to moisture in conventional sulfide solid electrolytes.

[0021] Furthermore, by manufacturing a secondary battery containing the borate-molten salt-based solid electrolyte of the present invention, it is possible to replace the secondary batteries using conventional liquid electrolytes and sulfide-based solid electrolytes in the market.

Brief Description of the Drawings

[0022] [Figure 1] XRD data of lithium chloroboracite. [Figure 2] SEM image showing the particle size of lithium chloroboracite of 2 to 40 μm. [Figure 3] Image of lithium chloroboracite after gold plating. [Figure 4] SEM-BSE (Back Scattered Electron), a photograph for confirming the coating thickness of lithium chloroboracite after gold plating in FIG. 3. [Figure 5] Data of Li ion conductivity measured at room temperature after gold plating.

Best Mode for Carrying Out the Invention

[0023] A preferred embodiment of the present invention will be described in detail. The following description of the embodiments is provided by way of example and is not limited thereto. 1. Structure of the solid electrolyte

[0024] The present invention uses LiOH or Li2CO3, H3BO3 or B2O3, and LiCl raw materials to prepare lithium chloroboracite having a composition of Li 4+x B7O 12+x / 2 Cl(x = 0 to 1).

[0025] And a LiCl-LiBr-LiF molten salt is prepared.

[0026] The Li 4+x B7O 12+x / 2 Cl(x = 0 to 1) composition lithium chloroboracite is mixed with the LiCl-LiBr-LiF molten salt, and the mixture is sintered to produce a composite of lithium chloroboracite and the LiCl-LiBr-LiF molten salt. 2. Confirmation of the density and ionic conductivity of the molded body

[0027] The solid electrolyte according to an embodiment of the present invention has a density increased by up to 26% based on the sintered n molded body compared to before mixing, and when measured at a room temperature of 25 ° C, the Li ion conductivity is 1.0 × 10 -7 S / cm or more.

[0028] - Electrical conductivity model / measurement conditions - EIS SP-300, Scan fi = 7.0MHz, ff = 1.0 Hz, Nd = 10 points per decade sinus amplitude Va = 20.0 mV, pw = 0.10, Na = 2, E Range = -10V~10V 3. Manufacture of the all-solid-state secondary battery

[0029] The all-solid-state secondary battery is manufactured by fabricating a positive electrode layer, a negative electrode layer, and a solid electrolyte layer in the manner described below and then laminating them.

[0030] The solid electrolyte may be a solid electrolyte having a composition according to one embodiment of the present invention. In one embodiment, the boronite-based solid electrolyte is produced by processing the starting material of the boronite-based solid electrolyte by a melt-and-quench method or a mechanical milling method. Furthermore, calcination can be performed after such processing. If additional calcination is performed, the crystals of the solid electrolyte may become stronger.

[0031] A solid electrolyte according to one embodiment of the present invention is manufactured by the following manufacturing method. First, the Li 4+x B7O 12+x / 2 A lithium chloroborasite having a composition of Cl(x=0~1) is mixed with a LiCl-LiBr-LiF molten salt, and the mixture is sintered to produce a composite of lithium chloroborasite and the LiCl-LiBr-LiF molten salt.

[0032] The LiCl-LiBr-LiF molten salt composite used above is also produced by processing the starting materials using methods such as melting and quenching or mechanical milling.

[0033] A positive electrode layer, a negative electrode layer, and a solid electrolyte layer can be laminated such that the solid electrolyte layer is sandwiched between the positive electrode layer and the negative electrode layer, and then rolled to produce an all-solid-state secondary battery according to one embodiment of the present invention. Comparative Examples and Examples

[0034] [Table 1] Comparative Example 1: Lithium Chloroborazite Comparative Example 2: Degree of density improvement due to grinding Comparative Example 1 vs. Examples 1, 2, 3, 4, 5: Controls based on the composite salt mixing ratio Comparative Example 2 vs. Example 6: Control by composite salt mixing ratio in fine particles Comparative Example 1 vs. Example 6: Control by grinding and flame mixing Examples 1-5. Production of composite solid electrolytes

[0035] This invention uses LiOH or Li2CO3, H3BO3 or B2O3, and LiCl as raw materials. 4+x B7O 12+x / 2 A lithium chloroborasite having the composition Cl(x=0~1) is prepared. Then, a LiCl-LiBr-LiF molten salt is prepared.

[0036] The Li 4+x B7O 12+x / 2 Lithium chloroborasite having a composition of Cl(x=0~1) is mixed with 1~5% of LiCl-LiBr-LiF molten salt, the mixture is pelletized, and then sintered to produce a composite of lithium chloroborasite and LiCl-LiBr-LiF molten salt. Comparative Examples 1 and 2: Evaluation of density and ionic conductivity of lithium chloroborasite solid electrolytes

[0037] The density and ionic conductivity were measured using only lithium chloroborasite that had been calcined after synthesis, by the following method. Comparative Example 1 used solid electrolyte powder with a particle size of 50-100 μm, and Comparative Example 2 used powder with a particle size of 1-20 μm, pressed (pressure 100 MPa / cm²). 2 Pellets were prepared by ) . The density of the prepared pellets was 1.91 g / cm³ depending on the particle size. 3 From 1.95 g / cm³ 3 The density showed an increasing trend, and after applying an Au coating and checking the Li ion conductivity using an AC impedance measuring device at room temperature, it was found that as the density increased, it increased to 2.0 × 10⁻⁶. -8 S / cm to 3.0 x 10 -8 We confirmed a slight increase in S / cm. Comparative Example 1 vs. Example 6: Density and ionic conductivity obtained by grinding and mixing of composite salts

[0038] When lithium chloroborasite powder, which was synthesized and calcined, was ground to a particle size of 1-20 μm and mixed with 5% of LiCl-LiBr-LiF molten salt, the density increased by up to 26%, reaching 2.41 g / cm³. 3 After confirming the results, the Li ion conductivity was also 1.0 × 10⁻⁶. -7 We confirmed that the S / cm level was above this.

[0039] The booxide-molten salt-based solid electrolyte of the present invention has the following advantages.

[0040] The solid electrolyte of the present invention has the following advantages.

[0041] 1) Solid electrolytes offer superior safety because they are free from the risk of evaporation due to temperature changes or leakage due to external impacts, do not undergo volume swelling, and do not exhibit explosive or ignition properties even under extreme external conditions such as heat and pressure.

[0042] 2) Because the risk of ignition and explosion is eliminated, related components can be omitted, and the remaining space can be filled with more active material. In particular, the separation membrane and PKG (current collector, cell casing material) are reduced, resulting in a smaller volume, and the battery cooling-related BMS (battery management system) is minimized, thus enabling a high energy density per unit volume.

[0043] 3) Unlike liquid electrolytes, a desolvation reaction is unnecessary to separate lithium ions from the solvent. High power output is possible because the charge-discharge reaction is immediately reflected as a diffusion reaction of lithium ions within the solid.

[0044] 4) Compared to conventional organic electrolytes, it can ensure stable performance over a wide temperature range. In particular, high ionic conductivity is expected at low temperatures. The biggest concern for electric vehicle users is that battery performance deteriorates in winter, reducing driving range. If a Tesla Model X is left overnight with a 50% charge on a cold day, the charge level will drop to 30% the next morning. The advent of solid-state batteries will eliminate concerns about low-temperature environments.

[0045] 5) The battery structure is simple and does not require a separation membrane. In the manufacturing process, a slurry-like solid electrolyte is coated onto the positive electrode active material. Various forms of multilayer cells can be realized through a continuous process without a liquid electrolyte injection step.

Claims

1. Li ４＋ｘ B ７ O １２＋ｘ／２ A composite solid electrolyte comprising lithium chloroborasite having a composition of Cl (x = 0 to 1) and a LiCl-LiBr-LiF molten salt.

2. The Li ion conductivity is 1.0 × 10⁻⁶ at room temperature (25°C). －７ The solid electrolyte according to claim 1, characterized in that it is S / cm or higher.

3. The solid electrolyte according to claim 1, characterized in that the composite is a nanoparticle with a diameter of 10 nm to 300 μm.

4. A secondary battery comprising a solid electrolyte according to any one of claims 1 to 3.

5. Li ４＋ｘ B ７ O １２＋ｘ／２ A step of preparing lithium chloroborasite having a composition of Cl (x = 0 to 1); Steps for preparing a LiCl-LiBr-LiF molten salt; A step of mixing the lithium chloroborate with a LiCl-LiBr-LiF molten salt; A step of grinding the raw material mixture; and A step of firing the raw material mixture at 500°C or higher; A method for producing a composite solid electrolyte of lithium chloroborasite containing a LiCl-LiBr-LiF molten salt.

6. The above Li ４＋ｘ B ７ O １２＋ｘ／２ The method for producing a solid electrolyte according to claim 5, characterized in that the mixing ratio of lithium chloroboracite having a composition of Cl (x = 0 to 1) and a LiCl-LiBr-LiF molten salt is 1 to 10 parts by weight, respectively.

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