JPH0547943B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a lithium battery, and particularly to an all-solid-state thin film lithium battery suitable for ultra-thin design.

[Background of the invention]

BACKGROUND ART In recent years, electronic devices have become significantly smaller and have lower power consumption. Under these circumstances, there is an increasing need for small, stable, and highly reliable batteries. Among these, lithium batteries in particular have high energy density and high electromotive force. It is attracting attention as a new battery. A lithium battery consists of a metallic lithium negative electrode, a lithium ion conductive electrolyte, and a positive active material.
Lithium batteries have already been put into practical use as primary batteries that use manganese dioxide, graphite fluoride, or the like as positive electrode active materials. Currently, the next research topic is the conversion of lithium batteries into secondary batteries. If a lithium battery is successfully converted into a secondary battery, it will be possible to use the battery semi-permanently. Furthermore, since they can be charged more easily than primary batteries, they do not require a very large discharge capacity, making it possible to miniaturize the battery.

The characteristics of a lithium secondary battery are expressed by () voltage and current characteristics of the battery, () chemical diffusion coefficient of lithium ions in the positive electrode, () discharge capacity of the battery, () repeatability of charging and discharging, etc. These characteristics largely depend on the characteristics of the positive electrode.
The choice is very important. To date, there have been many reports on positive electrode materials for lithium secondary batteries, but they have advantages and disadvantages in their characteristics as batteries, and the search for positive electrode materials is still continuing. Among the materials reported to date, titanium disulfide (TiS ₂ ) has a large diffusion coefficient and excellent charge/discharge reversibility. However, this titanium disulfide has a two-dimensional layered structure and has anisotropy with respect to the diffusion of lithium ions. Therefore, when forming such a compound into a thin film, it is necessary to orient the crystals in the direction in which lithium ions diffuse more. However, it is generally technically difficult to create an alignment film, and it is expected that this will become even more difficult when increasing the thickness of the positive electrode film in order to increase the discharge capacity of the battery. Therefore, finding a material that has no anisotropy with respect to lithium ion diffusion, has a large diffusion coefficient, and has excellent charge/discharge repeatability is an important step toward the practical application of lithium secondary batteries. This is a very important issue. In the present invention,
In order to solve the above-mentioned problems, we focused on positive electrode materials with a three-dimensional network structure and selected tungsten oxide.

To date, among tungsten oxides, tungsten trioxide has been used as a positive electrode material for lithium batteries or as a material for electrochromic display elements.
It has been considered many times. Among these, tungsten trioxide as a positive electrode material for lithium secondary batteries is known to be a promising material that generally has a large discharge capacity and excellent repeatability of charging and discharging. Furthermore, recently, tungsten trioxide has been made into a thin film and its electrochemical properties are also being studied. There are the following problems when making tungsten trioxide into a thin film. First of all,
When making lithium batteries ultra-thin, there are limits to how battery constituent materials can be made using powder compaction or sintering methods, and it is necessary to consider other methods to make the battery thinner. The thinning method currently used is a vacuum evaporation method or a sputtering method. However, with the vacuum evaporation method, it is difficult to change the composition ratio of tungsten and oxygen precisely and with good reproducibility. In addition, the creation of tungsten trioxide thin films by the sputtering method mainly uses metallic tungsten as the target material.
Although this method is performed in an oxygen-containing atmosphere, it is difficult to precisely control the composition ratio of tungsten and oxygen. Second, tungsten trioxide itself has low electronic conductivity, and to be used as a positive electrode material, a conductive material such as graphite must be added. However, the addition of a conductive material has the disadvantage of reducing the discharge capacity of the battery, and the addition of such a material is extremely disadvantageous, especially when used as a positive electrode thin film material. Thirdly, important properties required of a positive electrode material are a large chemical diffusion coefficient for lithium ions and excellent repeatability of charging and discharging. The largest value of the diffusion coefficient of lithium ions in a tungsten trioxide thin film reported to date is 4×10 ^-7 m ² /s.
（MINO GREEN and KSKANG，Thin Solid
Film 50 , 145 (1978). However, in order to use tungsten trioxide as a positive electrode material for thin-film lithium batteries, it is necessary to further improve the value of the diffusion coefficient by about one to two orders of magnitude. Furthermore, in order to use this material as a positive electrode material for secondary batteries, it is required to improve the reversibility of lithium ion inflow and outflow during charging and discharging.

As described above, the tungsten trioxide thin film produced by the conventional thin film manufacturing method has drawbacks, and there are also problems that must be solved in order to use it as a positive electrode thin film material for lithium batteries.

[Purpose of the invention]

An object of the present invention is to provide a thin film lithium battery, particularly a thin film lithium secondary battery, which has no anisotropy in the diffusion of lithium ions, has a large diffusion coefficient, and has a reversible flow of lithium ions in and out during charging and discharging. An object of the present invention is to provide a positive electrode for a lithium battery that has excellent properties and a large discharge capacity.

[Summary of the invention]

In the present invention, the positive electrode is made of tungsten oxide having a composition of WO _3- 〓 (0 < δ1) or WO _3- 〓 (0 < δ1) containing a transition metal oxide (for example, molybdenum trioxide, vanadium pentoxide, etc.). The key point is to form a tungsten oxide thin film made of a substance whose composition is primarily tungsten oxide.

Among currently reported positive electrode materials, materials having a one-dimensional chain or two-dimensional layered structure have anisotropy with respect to lithium ion diffusion. When these materials are used as cathode materials for lithium batteries, their anisotropy makes it necessary to take into account the tendency of crystals when forming thin films, which makes thin film formation extremely difficult. Therefore, we focused on compounds with a three-dimensional network structure that exhibits no anisotropy with respect to lithium ion diffusion as positive electrode materials, and selected tungsten trioxide among them. This material has a large discharge capacity among compounds with a three-dimensional structure,
It also has good reversibility with repeated charging and discharging, and the chemical diffusion coefficient of lithium ions is 4×10 ^-17 m ² /
s, which is relatively large, and is considered to be promising as a positive electrode material for lithium batteries. However, in order to use this material as a positive electrode material, the following problem must be solved. That is, ()
Increasing discharge capacity, () improving the diffusion coefficient of lithium ions, () improving the reversibility of lithium ions in and out during charging and discharging, () increasing electronic conductivity without adding conductive additives, etc. be. The measures adopted in the present invention to solve these problems will be described in detail below.

First, in order to improve the diffusion coefficient of lithium ions, we attempted to increase the size and number of channels through which lithium ions can move by making the thin film amorphous when creating the positive electrode. They also planned to generate oxygen defects in the positive electrode thin film in order to increase electronic conductivity. The resulting oxygen-deficient compound can be expected to increase discharge capacity and improve the reversibility of lithium ions in and out during charging and discharging. Therefore, the thin film was formed by sputtering in a hydrogen-containing reducing atmosphere using tungsten trioxide WO ₃ (the case containing a transition metal oxide will be described later) as a target. Here, the thin film was formed in a reducing atmosphere because it is easy to create oxygen defects in the thin film. In other words, normally oxygen-deficient compounds
To synthesize the non-stoichiometric compound represented by WO _3- , a mixture of metallic tungsten W, ammonium tungstate NH ₄ WO ₄ , tungstate H ₂ WO ₄ , and tungsten trioxide WO ₃ is vacuum sealed in a quartz tube. It is made by reacting the tungsten and oxygen at 900℃ for 24 to 48 hours, but even if the raw material synthesized in this way is used as an evaporation source and made into a thin film by vacuum evaporation or sputtering, the original composition ratio of tungsten and oxygen cannot be maintained. It is difficult, and furthermore, it requires a great deal of time and effort from synthesis of raw materials to evaluation using batteries. Therefore, as a method to easily control the amount of oxygen vacancies in the thin film with good reproducibility, a method of sputtering in a reducing atmosphere using WO ₃ as a target was used. In addition to the sputtering method, ion plating method, vacuum evaporation method, or CVD method can be used to create the positive electrode.
It may be created by law.

In the production of the above thin film, the amount of oxygen vacancies is controlled by the high frequency output applied to the target.
and by changing the amount of hydrogen contained in the discharge gas. In addition, evaluation of the produced positive electrode was performed by producing a lithium battery. That is, first, a positive electrode thin film was formed on a glass substrate by a sputtering method. Next, the produced positive electrode thin film was evaluated for crystallinity by X-ray diffraction and measured for electronic conductivity. As a result, when the amount of hydrogen contained in the discharge gas was constant, crystallization progressed as the high frequency output increased, and the electronic conductivity also increased. Further, when the high frequency output was constant, as the amount of hydrogen gas was increased, the electronic conductivity increased, but no change in crystallinity was observed by X-rays, and all were amorphous. After this thin film evaluation, a solid electrolyte thin film was formed on the positive electrode thin film by a sputtering method, and finally a lithium negative electrode was formed by a vacuum evaporation method to create a lithium battery. The battery was first evaluated on () the voltage and current characteristics of the battery, and () the chemical diffusion coefficient of lithium ions in the positive electrode. As a result, the positive electrode thin film created according to the present invention has voltage/current characteristics and diffusion coefficient (10 ^-16 to ^{10 -15} m ² /s).
Additionally, as a tendency, the amorphous thin film was slightly larger than the crystalline thin film. In addition, tungsten oxide thin films created in a stronger reducing atmosphere,
When δ>1 in WO _3- , both the electromotive force and discharge capacity of the battery were small, and it was unsuitable for use as a battery. Next, to evaluate the battery, we examined the reversibility of lithium ion inflow and outflow through repeated charging and discharging. That is, the battery was repeatedly charged and discharged between certain potentials with a constant current, and the time required for charging or discharging at that time was measured. As a result, it was found that a thin-film lithium secondary battery with a discharge capacity of 0.15 mA/cm ² and less deterioration due to repeated charging and discharging could be obtained.

From the above studies, we found that by using tungsten trioxide as a target and a thin film obtained by sputtering in a reducing atmosphere as a positive electrode active material, we can improve the voltage and current characteristics of the battery, and improve the diffusion coefficient of lithium ions in the positive electrode. It was confirmed that the discharge capacity and the reversibility of lithium ion inflow and outflow accompanying repeated charging and discharging can be greatly improved. The above positive electrode thin film has a composition of WO _3- 〓, and the value of δ is:
From the results of evaluation of crystallinity by X-ray diffraction (in the case of amorphous, estimated values obtained from values for crystalline thin films prepared under similar conditions), it was in the range of 0<δ1.

The raw material is tungsten oxide with a transition metal oxide added to it, sealed in a quartz tube under reduced pressure, and heat treated to change its crystal structure.
It is known that it is possible to create channels in which lithium ions can easily diffuse, thereby improving the diffusion coefficient. Also in the present invention, when creating the positive electrode,
Batteries using positive electrodes made by sputtering using targets containing transition metal oxides such as molybdenum trioxide, vanadium pentoxide, and niobium pentoxide in tungsten trioxide also have voltage and current characteristics, and lithium ions in the positive electrode. Almost the same properties as tungsten trioxide were obtained, including the diffusion coefficient of , discharge capacity, and reversibility of lithium ion inflow and outflow with repeated charging and discharging.

[Embodiments of the invention]

Example 1 of a thin film lithium battery according to the present invention is a thin film positive electrode composed of only tungsten oxide.
~14, Examples 15 to 20 containing transition metal oxides
This will be explained in detail below.

Examples 1 to 8: FIG. 1 shows the overall structure of a thin film lithium battery.
That is, it has a structure in which a solid positive electrode thin film 2, a lithium ion conductive solid electrolyte thin film 3, and a lithium negative electrode thin film 4 are laminated in this order on a glass substrate 1, and a lead wire 5 is provided. It is.

In this example, the battery was created as follows. That is, first, a positive electrode thin film 2 was formed by sputtering on a quartz substrate 1 whose both sides were mirror-polished. The sputtering conditions were as follows: tungsten trioxide with a purity of 99.9% was used as the target material, Ar and H ₂ mixed in the ratio described later were used as the discharge gas, and the discharge gas pressure was 3×10 ⁻² Torr. Sputtering was performed under these conditions using a commercially available RF sputtering device. During sputtering, the substrate was cooled with water. here,
The ratio of tungsten to oxygen was controlled by changing the hydrogen content in the discharge gas or by changing the high frequency output. First, using 90% Ar-10% H ₂ as a discharge gas, sputtering was performed at high frequency outputs of 2.5, 3.8, 5.0, 6.4, and 7.6 W/cm ² to create positive electrode thin films (Examples 1 to 5). Next, sputtering was performed while keeping the high frequency output constant at 2.5 W/cm ² and varying the hydrogen content in the discharge gas.
That is, the discharge gas contains 95% Ar-5% H ₂ and 90% Ar.
-10% _H2 , 70%Ar-30% _H2 (Example 6 and 5% _H2 and 30% _H2 , respectively) were used.
7. H ₂ 10% is the same as in Example 1). The crystallinity of the produced positive electrode thin film was examined using an X-ray diffraction method. After that, 0.6Li ₄ was applied on the positive electrode thin film.
SiO ₄ −0.4Li ₃ PO ₄ solid electrolyte thin film 3 (thickness 3.5μm)
was formed by a sputtering method, and finally, a metal lithium negative electrode thin film 4 (thickness: 4 μm) was formed by a vacuum evaporation method to create a thin film lithium battery.

The produced thin film battery was evaluated as follows. First, as initial characteristics, the voltage and current characteristics of the battery and the chemical diffusion coefficient of lithium ions in the positive electrode were measured. The voltage/current characteristics were expressed by short-circuit current (current that flows when the battery is short-circuited). Finally, the repeated charging and discharging characteristics were examined as follows. That is, evaluation was made by charging and discharging the battery with a constant current and measuring the time required for charging or discharging at that time.

The results of the evaluation of the battery and the results of the study using the X-ray diffraction method are summarized and shown as Examples 1 to 7 in the chart of FIG. 2, and will be described in detail below. First, as a result of the crystallinity evaluation of the positive electrode thin film by X-ray diffraction method, Example 1
The thin films made with low sputtering powers of -3 were amorphous, whereas those made with high sputtering powers (Examples 4 and 5) were oriented crystalline thin films. That is, the material of Example 4 was W ₂₀ O ₅₈ oriented in (110), and the material in Example 5 was W ₁₈ O ₄₉ oriented in (010). From the above results, it was found that as the high frequency output was increased, the amount of oxygen vacancies in the thin film increased. Moreover, the results of sputtering with varying hydrogen content in the discharge gas were found to be amorphous by X-ray examination.

Next, batteries were created using these positive electrode thin films and their characteristics were investigated, which will be explained below. First, we investigated the relationship between sputter output and battery characteristics. That is, using tungsten trioxide as a starting material, keeping the hydrogen content in the discharge gas constant, increasing the high frequency output and sputtering to create a positive electrode thin film (Examples 1 to 4), making it into a battery using the method described above, and Characteristics were determined. As a result, no major changes were observed in either the voltage/current characteristics of the battery or the chemical diffusion coefficient of lithium ions in the positive electrode. The chemical diffusion coefficient of lithium ion is
10 ^-15 to 10 ^-16 m ² /s, and titanium disulfide (diffusion coefficient 5 × 10 ^-15
m ² /s). Next, the high frequency output is 2.5W/cm ² , which is the value when the largest short circuit current and diffusion coefficient are obtained from the previous study.
were selected, and positive electrode thin films were created by changing the hydrogen content in the discharge gas (5%, 10%, and 30%, respectively) (Examples 6, 1, and 7). In other words, the relationship between the hydrogen content in the discharge gas and battery characteristics was investigated. As a result, as in the case of changing the high-frequency output, there were no significant changes in the voltage/current characteristics of the battery or the chemical diffusion coefficient of lithium ions in the positive electrode, and these values were similar to those of the titanium disulfide mentioned above. The value was almost the same as that of . Next, we investigated the effect of thin film crystallinity on battery characteristics. First, the high frequency output is 3.8W/cm ² with the substrate heated (300℃).
A comparison was made between a sample crystallized by sputtering (Example 8) and an amorphous sample (Example 2) created by sputtering the substrate with the same high frequency output while cooling the substrate with water. There were no major differences in the voltage/current characteristics of the batteries or the diffusion coefficients of lithium ions in the positive electrode between the two, and both values were approximately the same as those of titanium disulfide. Furthermore, by changing the high-frequency output, we created crystalline thin films by changing the reduction state of tungsten trioxide, that is, the amount of oxygen vacancies in the thin film (Examples 1, 4, 5, and 8), and investigated the effect on battery characteristics. investigated. As a result, even in the case of crystalline thin films, there were almost no differences in battery characteristics due to differences in reduction state. Here, the difference in battery characteristics between a crystalline thin film and an amorphous thin film is that the crystalline thin film has a smaller short circuit current and a smaller diffusion coefficient, but it is far better than other materials. From the above considerations, tungsten trioxide
Using WO ₃ as a starting material, tungsten oxidation is performed by varying the high frequency output in a reducing atmosphere with a constant hydrogen content in the discharge gas, or by sputtering by increasing the hydrogen content in the discharge gas with a constant high frequency output. We were able to reduce the value of δ in the product WO _3- by gradually increasing it from zero. and,
As a result of examining its thin film properties, it was found that in this composition range, the short-circuit current ^and the chemical diffusion coefficient of lithium ions in the positive electrode tended to decrease slightly as ^reduction progressed ^; It showed a large value of s.

Next, we investigated the reversibility of lithium ion inflow and outflow during charging and discharging of thin film lithium batteries using these positive electrode thin films. As a result, the deterioration caused by repeated charging and discharging was small in all of the batteries of Examples, and was 10% after approximately 10 repetitions. After that, the discharge capacity gradually decreased due to repeated charging and discharging, but no significant decrease in capacity was observed after approximately 100 cycles, confirming that good charging and discharging could be performed. Furthermore, among these, the batteries of Examples 1 and 2 have a large discharge capacity and a thin film thickness.
2.5μm, diameter 2mm, repeating 2.5V1.5V,
It was about 0.15mAh/ ^cm2 .

From the above studies, it was found that the positive electrode thin film of the present invention, which was created by the sputtering method using tungsten trioxide as a target in a hydrogen-containing reducing atmosphere, has voltage and current characteristics without adding conductive additives such as graphite. It was confirmed that the chemical diffusion coefficient of lithium ions was significantly improved. Furthermore, as a result of examining the reversibility of lithium ion inflow and outflow during repeated charging and discharging, we found that these thin-film lithium secondary batteries have good reversibility and a large discharge capacity of 0.15mAh/ ^cm2 . was completed.

In each of the above examples, when the positive electrode thin film is crystalline, the value of δ was determined from the results of crystallinity evaluation by X-ray diffraction. In addition, in the case of an amorphous film, an estimated value is shown based on the value of δ of a crystalline thin film prepared under similar conditions. All values are in the range of 0<δ1.

Examples 9 to 14: Using 50% Ar-50% _H2 with a higher hydrogen content than in the previous example as the discharge gas, changing the high frequency output to 5.0, 6.4, 7.6 W/ ^cm2 , For each high frequency output, the substrate is water cooled and the substrate is heated to 300℃.
A positive electrode thin film was created under the same sputtering conditions. The results are shown as Examples 9 to 14 in FIG. As a result, the tendency was similar to that of Examples 1 to 8 described above, and both the diffusion coefficient and short circuit current were the same as those of the previous examples.

Examples 15 to 20: Examples using a tungsten trioxide-based positive electrode thin film containing a transition metal oxide will be described. That is,
Tungsten trioxide with molybdenum trioxide, panadium pentoxide, and niobium pentoxide added as pellets was used as the target material, high frequency output 2.5 W/cm ² , discharge gas pressure 3 × 10 ^-2 Torr, discharge gas 90%. A positive electrode thin film was created by sputtering with Ar-10% H ₂ for 2 hours. The method for manufacturing the battery and the evaluation of the battery were performed using the same method as in the case of tungsten trioxide described above. The results are summarized in the chart in Figure 3. As a result, no major differences in voltage/current characteristics or lithium ion diffusion coefficients were observed due to the addition of transition metal oxides. on the other hand,
Although the discharge capacity slightly decreased to 0.08 mAh/cm ² due to the addition, the reversibility of lithium ion inflow and outflow during repeated charging and discharging was slightly improved.

As explained in the above examples, according to the present invention, tungsten trioxide, which has a three-dimensional network structure and has no anisotropy with respect to lithium ion diffusion, is used as a positive electrode material for thin-film lithium batteries. A positive electrode thin film was created using the sputtering method in a hydrogen-containing reducing atmosphere. By using this thin film, it was possible to significantly improve the voltage/current characteristics and lithium ion diffusion coefficient of the battery without adding an electronically conductive material to the positive electrode. Furthermore, a lithium secondary battery was obtained in which the repeatability of charging and discharging could be improved and the lithium secondary battery had excellent reversibility. Further, the discharge capacity was as large as 0.15 mAh/cm ² at a 2 mm diameter positive electrode film thickness of 2.5 μm and a discharge depth of 70%. It has been found that the effective composition range of the tungsten acid surface of the positive electrode thin film in which these effects are noticeable is a range of 0<δ1 as the value of δ in the tungsten oxide WO ₃₋ .

In addition, the positive electrode thin film using tungsten oxide according to the present invention has no irregularity with respect to the diffusion of lithium ions, and crystals are directed in the direction of large diffusion of lithium ions as seen in the case of two-dimensional layered compounds. The technical problem of orienting can be solved.

〔Effect of the invention〕

As explained above, according to the present invention, there is no anisotropy in the diffusion of lithium ions, the diffusion coefficient is large, the reversibility of lithium ions in and out during charging and discharging is excellent, and the discharge capacity is large. Since a positive electrode for thin film lithium secondary batteries can be obtained, the effect in the field of thin film lithium secondary batteries is significant.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing the overall structure of a thin-film lithium battery, Fig. 2 is a chart summarizing data of an example of the present invention using tungsten oxide as the positive electrode, and Fig. 3 contains a transition metal oxide in the positive electrode. 1 is a chart summarizing data of an example of the present invention using a substance mainly composed of tungsten oxide. Explanation of symbols, 1...Substrate, 2...Positive electrode thin film, 3
...Solid electrolyte thin film, 4...Lithium negative electrode thin film,
5... Lead wire.

Claims (1)

[Claims] 1. A negative electrode thin film containing solid lithium as an active material;
In a thin film lithium battery consisting of a lithium ion conductive solid electrolyte thin film and a positive electrode thin film using solid tungsten oxide as an active material, the positive electrode thin film is made of tungsten oxide having a composition of WO _3- 〓 (0 < δ1) or transition WO _3- 〓(0
An all-solid-state thin film lithium battery characterized by being made of a substance mainly composed of tungsten oxide with a composition <δ1). 2. The all-solid-state thin film lithium battery according to claim 1, wherein the positive electrode thin film is amorphous. 3. The all-solid-state thin film lithium battery according to claim 1, wherein the transition metal oxide is at least one compound selected from the group consisting of molybdenum trioxide, vanadium pentoxide, and niobium pentoxide.