JPS63128572A - Solid electrolyte for use in secondary battery - Google Patents

Abstract

PURPOSE:To decrease voids of a solid electrolyte with objective to lengthen the life of a battery by constituting ceramics with unit particles having an average particle size in the short axis of 1 mum or more and a specific particle size distribution. CONSTITUTION:The battery comprises ceramics with unit particles having a particle size in the short axis of 1 mum or more and a particle size distribution with standard deviation of 1.5 mum or more. Practically, in order to decrease voids of a solid electrolyte and strengthen crystal grain boundary thereof, the size of raw material powder is adjusted and small particles are mixed among large particles. By molding and sintering, large particles grow coarse crystal grains and small particles grow small crystal grains, and the latter embed spaces formed among the former. By the arrangement, a completely sintered, dense and strong solid electrolyte is obtained and internal strain therein is reduced and the life of sodium-sulfur battery is lengthened.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a solid electrolyte for secondary batteries, and particularly to a solid electrolyte for sodium-sulfur batteries.

[Conventional technology]

Conventionally, solid electrolytes are produced using beta alumina or beta double prime alumina.

Alternatively, a method is used in which a powder of a mixture of these is molded and sintered.

As described in JP-A-51-13814, the solid electrolyte used in sodium-sulfur batteries is prepared by pulverizing raw material powder to 1 μm or less, molding it, and sintering it.

Efforts have been made to obtain dense and strong solid electrolytes.

However, no consideration was given to the crystal grain size of the solid electrolyte after sintering.

[Problem that the invention seeks to solve]

In the above conventional technology, since beta double prime alumina is a layered compound, the direction of the crystal grains during sintering is not constant.
As a result, the gaps between the corners of the crystal grains are not filled, and no consideration is given to the formation of pores. During battery operation, sodium ions precipitate in the pores, and eventually push the inside of the pores and expand the solid electrolyte. There was a problem that cracks occurred.

An object of the present invention is to reduce pores in a solid electrolyte and thereby extend the life of a battery.

[Means for solving problems]

The above purpose is to ensure that the average particle diameter in the minor axis direction of the constituent unit particles is 1.
This is achieved by a solid electrolyte for secondary batteries characterized by being made of ceramics having a particle size distribution of 1.5 μm or more and a field standard deviation of 1.5 μm or more.

[Effect]

The size of the raw material powder is adjusted to reduce the number of pores in the solid electrolyte and strengthen the grain boundaries.

Molding by mixing small particles between large particles.

When sintered, the large particles become coarse crystal grains and the small particles become small crystal grains to fill the gaps between the coarse crystal grains, resulting in a completely sintered, Jl-dense, solid electrolyte.

〔Example〕

Next, a sample of the solid electrolyte for secondary batteries according to the present invention will be explained.

Beta double prime alumina powder, which is a raw material for the solid electrolyte, is prepared according to the method described in Japanese Patent Publication No. 57-15063 (Applicant: Ford Motor Co., Ltd.).

Lithium required for beta double prime alumina
Provided by calcining zo-5AI2zO+s. The manufacturing method of beta double prime alumina was published in 1977-156.
According to the method described in Publication No. 3 (Applicant: Ford: Motor 0 Name: Manufacturing method of cation-conductive multi-crystalline sintered body).

The composition of this powder is AQzOa89.4 in weight percent.
%, NazO 9.8%, and LizO 0.8%, and it was confirmed by X-ray diffraction that it shows the composition of beta double prime alumina. Three types of samples were prepared as shown in Table 1.

Table 1 Percentage ratio (%) of alumina powder by particle size First example The three types of samples shown in Table 1 were filled in a predetermined amount into a pellet mold with an inner diameter of 20I, and molded with a load of 1 ton per square centimeter. , furthermore, the above-mentioned Special Publication No. 50-1563
Based on the manufacturing method described in the publication.

1. After sintering at 560℃ for 6 minutes, annealing at 1,400℃
℃ and produced sintered pellets with a diameter of about 16 aw.

Using these sintered pellets, a sodium/sodium cell was assembled, and the resistance of the solid electrolyte pellet was measured using an LCR meter at an AC frequency of 20 KHz, and a charge/discharge cycle test and critical current density test were conducted using a charge/discharge device. Ta.

In the charge/discharge cycle test, one cycle was defined as charging by sending a current of 2 amperes from positive to negative for one hour, and discharging from the opposite direction.

The limiting current density test is a method for evaluating the strength against the force that tends to destroy the solid electrolyte from within, which is generated in the solid electrolyte during charging and discharging of a battery.

The limiting current density is when both sides of the sample are filled with sodium,
If you apply a current to move sodium ions in one direction and gradually increase the current value, the amount of ion movement will increase and the voltage will rise, but at a certain current value the sample will be destroyed and the sodium will short-circuit. The voltage suddenly drops to zero, and the current value at that time is expressed as the surface area of the part of the sample in contact with sodium (
(effective surface area).

Table 2 shows the resistance values and charge/discharge cycle test results of sintered pellets made by squaring the three types of alumina powder samples shown in Table 1.

Although the initial resistance value of Sample No. 1 in Table 2 is low, the resistance value gradually increases as the charge/discharge cycles increase.

The resistance value of sample number 2 was almost the same as the initial value even after 200 cycles. The charge/discharge cycle test results also showed good values.

FIG. 1 shows the results of the limiting current density test, and the value under the conditions of sample number 2 was more than double that of sample number 1.

By combining this value with the above resistance value and the results of the charge/discharge cycle test, it was confirmed that a solid electrolyte with one grain boundary strengthened was obtained. In addition, in the case of sample number 3, the resistance value was high.

The results of the charge/discharge cycle test also showed no improvement, and the limiting current density was low.

Second Example Beta double prime alumina powder consisting of three types of alumina powder shown in Table 1 of the first example was filled into a rubber press mold for each sample, and the mold was placed in a pressure vessel. Then, water pressure is applied to compress the powder into a molded body. This molded body was sintered and annealed in the same manner as in the first example to produce a bag tube.

The size of these bag tubes is approximately 18 nm in outer diameter and approximately 44 m in length.
The thickness is approximately 1.5m. A sodium/sodium cell was made using this bag tube, and the resistance was measured and the charge/discharge cycle test was performed in the same manner as in the first example.

In addition, radial crushing strength was measured as mechanical strength.

The evaluation test results for the bag tube are shown in Table 3 below. In this bag tube as well, the radial crushing strength was high when the powder of sample number 2 was used, and both the resistance value and the results of the charge/discharge cycle test were excellent.

Table 3 Next, the polished surface of the pellet of the first example will be explained. Obtained in the first example. The surface of a sintered pellet of beta double prime alumina was polished, immersed in 150"C phosphoric acid, the polished surface was etched, and the degree of grain boundary erosion was observed using a scanning electron microscope. Photographs are shown in Figures 2A-2C.

In FIG. 2A, it was observed that on the surface of the sintered pellet produced using the powder of sample number 1, small pores were relatively uniformly distributed between fine crystal grains. In FIG. 2B, it was observed w4 that on the surface of the sintered pellet produced using the powder of sample number 2, fine particles were intercalated between the coarse crystal grains and were extremely tightly interlocked.

In FIG. 2C, it was observed that the surface of the sintered pellet produced using the powder of sample number 3 was rough due to pores where one particle had fallen out and pores between crystal particles. The immersion time in phosphoric acid was 4 minutes, and the magnification of the electron micrograph was s, ooo times.

The results of etching with phosphoric acid also show that grain boundaries in beta double prime alumina were strengthened by adjusting the particle size of alumina powder, which is one of the raw materials for beta double prime alumina. In addition, the particle diameter in the minor axis direction of the particles on the surface of the sintered pellets produced using the powder of sample number 2 in Table 1 was found to be 1 μm or less in 44% of the particles when electron micrographs were analyzed using an image processing device. 1 to 5 μm is 54
%.

5i10μm was 2%.

〔Effect of the invention〕

According to the present invention, the grain boundaries of the solid electrolyte are strengthened. In addition, the pores on the surface and inside of the solid electrolyte are reduced, and when used as a solid electrolyte in a sodium-sulfur battery, there is less room for sodium ions to precipitate within the solid electrolyte.
The internal stress generated in the solid electrolyte was also reduced. As a result, the lifespan of sodium-sulfur batteries was increased by about 30%.

[Brief explanation of the drawing]

Claims (1)

[Scope of Claims] 1. A secondary battery characterized by being made of a ceramic having a particle size distribution in which the average particle size in the short axis direction of the constituent unit particles is 1 μm or more and the standard deviation is 1.5 μm or more. Solid electrolyte for use. 2. The grain size in the minor axis direction of the constituent unit particles of ceramics is
40-60% is 1 μm or less, 50-60% is 1-5 μm
, 5 to 10 μm is 1 to 5% of the solid electrolyte for a secondary battery according to claim 1. 3. The solid electrolyte for a secondary battery according to claim 1, wherein the constituent unit particles of the ceramic are beta alumina or beta double prime alumina.