JPH0477422B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Field of Application The present invention provides a secondary battery that can be charged with light rather than electricity;
That is, the present invention relates to an all-solid-state photonic secondary battery that functions as a combination of a solar cell and a secondary battery that can store light energy as electric power.

Conventional technology Attempts have been made to develop secondary batteries that can be charged with light, as shown in the review by Masao Kaneko, Electronics, pages 97-104 (S59, 10), but only solar cells have been put into practical use. This is a method for charging ordinary secondary batteries.

In addition to the two-stage type, which stores electricity generated by solar cells in a secondary battery, there are two-stage electrodes, one made of a semiconductor such as n- _TiO2 , one made of a noble metal such as platinum, or a semiconductor such as p-GaP. An attempt is made to immerse the other electrode in an electrolytic solution, irradiate the semiconductor electrode with light to separate the charges, oxidize and reduce the electrolyte with the minority carriers generated by photoexcitation, store it as an active material, and use it during discharge. However, it has not yet been put into practical use for the reasons described below. ()
For the photoexcited charges to carry out the subsequent oxidation/reduction reactions, the oxidation/reduction potential of the electrolyte must be within the valence and conduction bands of the semiconductor electrode (the oxidation potential is above the upper edge of the valence band, It is necessary that the reduction potential be below the lower edge of the conductive band, and that the bandgap of the semiconductor electrode be as small as possible in order to cause as much charge separation as possible by light. On the other hand, the band gap of a semiconductor electrode is preferably about 1 to 2.5 eV in order to satisfy the conditions () and () and to proceed with the reaction efficiently using light from a solar cell or a fluorescent lamp. semiconductor electrode (n-Si:
1.1 eV, n-GaAs: 1.35 eV, CdS: 2.4 eV) had the disadvantage that the electrode itself was involved in the corrosion reaction when using an aqueous electrolyte.

Most recently, H.Tributeh has been working on Structure and Bonding (H.Tributeh:
“Structure and Bondig”) 49, 162–166 (1982)
In this paper, he summarizes previous research and discusses the possibility of light-chargeable batteries using intercalation compounds as positive electrode materials. Considering that sunlight is used as the light source, it is difficult to achieve the conditions () and () above with a battery using Li as the negative electrode, so Cu or Ag, which has a noble potential, is used as the negative electrode. It also states that ZrS ₂ and HfS ₂ with appropriate band gaps are promising as semiconductor electrodes.

However, H. Trivitucci's review only describes the possibilities and prospects, and does not discuss the photo-corrosion problems that have been a problem with this type of photo-semiconductor electrodes in the past, methods for smooth photo-charging and discharging, and extending the cycle life. There is no mention of any measures to do so. In fact, when an aqueous electrolyte is used, the problem of photocorrosion occurs as usual.

The inventors previously discovered that this corrosion did not occur when a Cu ⁺ ion conductive solid electrolyte was used instead of a solution electrolyte, and found that ZrS ₂ or HfS ₂ ｜KxRb _1-X
We proposed an all-solid-state photovoltaic secondary battery with a composition of Cu ₄ I _1.5 Cl _3.5 ｜Cu + Cu ₂ S, and subsequently discovered that the addition of NbS ₂ and Pd was effective in improving charge-discharge characteristics, and that it was effective at high temperatures. We have discovered that the presence of S vacancies is more effective in improving lifespan than using stoichiometric ZrS ₂ or HfS ₂ , and have continued to make improvements one after another.

Problems to be Solved by the Invention The present invention aims to further increase the charging efficiency by light and further improve the discharge characteristics.

Means for Solving the Problems The present invention provides a photoelectrode mainly composed of ZrS _2- 〓 (0<δ<0.2), a counter electrode made of a mixture of Cu and Cu ₂ S,
This is a secondary battery that is composed of a Cu ⁺ ion conductive solid electrolyte and that is charged by light with the light incident surface of the photoelectrode oriented in a direction perpendicular to the C axis of the sulfide of the photoelectrode.

Function The photovoltaic cell of the present invention is reversibly charged and discharged by light. Here, the intercalation and deintercalation of Cu into ZrS _2- 〓 is performed perpendicular to the C axis, so the C axis of ZrS _2- 〓 is arranged parallel to the interface between the electrolyte and ZrS _2- 〓. It becomes easier for the reaction to occur. Further, electronic excitation by light is performed on the a-plane of ZrS _2- . Therefore, aligning the C-axis with the direction of light incidence increases the number of excited electrons. This particularly improves the photocharging efficiency.

Example In order to increase the charging efficiency by light, it is desirable that the incident light be incident from a direction close to the C-axis direction of the photoelectrode material, and in order to make the discharge reaction occur smoothly (to reduce the discharge polarization) It is effective to make the C-axis direction of the substance perpendicular to the direction of incidence of light, and it is appropriate to use surface flow. ZrS ₂ , HfS ₂ and
Materials like NbS ₂ have a layered structure, just like graphite crystals. These powders have a flat structure with a short C-axis direction. When such powder is made to flow in one direction, the C-axes of the powder particles align perpendicularly to the flow direction because the smaller the cross-sectional area, the lower the resistance. On the other hand, in the part in contact with the electrolyte layer, there is almost no flow, and the particles move toward the gap between the electrolyte particles, so in the part in contact with the electrolyte, the direction of the C axis is parallel to the interface. Particle orientation can be performed by utilizing the fact that the particles come together. Methods for producing surface flow include a doctor blade method and a roller press method, but the screen printing method has a slow photocharging reaction rate.

When a charged battery is discharged, the counter electrode discharges as shown in Figure 1b, releasing Cu ⁺ ions into the electrolyte as shown in xCu→xCu ⁺ +xe (1), and the generated electrons are transferred to the external load. It flows into the conductive band of the photoelectrode through the electrolyte, reaches the interface with the electrolyte, and reduces the Cu ⁺ ions in the electrolyte, xCu ⁺ +xe ^- +ZrS _2- 〓→Cu _x ZrS _2- 〓 ……(2) An intercalation compound Cu _x ZrS _2- is formed. The terminal voltage decreases linearly as the interlayer compound forms. In other words, the energy level increases linearly as it approaches the potential of Cu/Cu ⁺ .

This cell reaches the limit where Cu can intercalate at the photoelectrode, and just before a new compound is formed, the counter electrode is completely discharged, and the potential changes from Cu/Cu ^+.
It shifts to Cu ₂ S/Cu ⁺ and the terminal potential becomes zero (it is no longer possible to discharge any further on its own). At that time, the energy levels at both poles are as shown in Figure 1c.

On the other hand, when light is applied to the discharged photoelectrode, approximately
1.68EV or more (band gap V _G of ZrS _2-x photoelectrode
By absorbing the light (above), electrons in the valence band are excited to the conductive band, forming holes in the valence band. This hole and ^the intercalation compound Cu _x ZrS _{2- 〓} react with ^each _other to release Cu ⁺ _ions . The terminal voltage increases linearly. Furthermore, the excited electrons move to the counter electrode and precipitate Cu as follows: Cu ⁺ +e ^- →Cu (4).

The electrode discharged in this manner reversibly returns to its original state by light.

Here, the intercalation of ZrS _2- 〓 of Cu,
Since deintercalation is performed perpendicular to the C axis, C of ZrS _2- 〓 is present at the interface between the electrolyte and ZrS _2-
Arranging the axes in parallel makes it easier for these reactions to occur. Further, electronic excitation by light is performed on the a-plane of ZrS _2- . Therefore, aligning the C-axis with the direction of light incidence is useful for increasing the number of excited electrons.

Example 1 When the thermal decomposition of ZrS ₂ was investigated using a thermobalance, 2
There were two stages, the first stage was at 34°C with a weight loss of about 0.1%, and the second stage was at 131°C. It was found that the first stage decomposition resulted in a ZrS composition of _1.93-1.94 . The compound with this composition was confirmed to be n-type by thermoelectromotive force measurement, so
The following ZrS ₂ raw material was heated at 90°C for one day and night.

The ratio of Cu:Cu ₂ :RbCu ₄ I _1.5 Cl _3.5 is 60:20:20
3.25 g of counter electrode powder mixed in the ratio of 50 x 50 mm ²
After putting it in a mold and pressing it with a pressure of 100Kg/cm ² ,
RbCu ₄ I _1.5 Cl _3.5 Add 6.5g of electrolyte powder to 4t/cm ²
A photoelectrode was applied onto the electrolyte using the following three methods. The photoelectrode slurry at that time was Cu _0.1 ZrS _2- 〓
1:1 mixture of 75% and electrolyte, 20% methanol
%, polyvinyl butyral 3%, and butyl diphthalate 2% were placed in a pot mill containing about 1/3 of agate cobblestone, and the mixture was mixed by rotation at 150 to 200 rpm for 24 hours. The paste was applied to a thickness of approximately 120μ using a doctor blade, rolled to a thickness of approximately 120μ using a Teflon roll, filled to approximately 120μ by screen printing, and then rolled to a thickness of approximately 120μ using a Teflon roll.
After drying at ℃ for 2 hours, it was covered with a transparent electrode to construct a battery with an electrode area of 10 x 10 mm ² as shown in Fig. 3.

After fully charging this battery with a constant voltage of 0.6V,
The discharge characteristics were determined by discharging to 0.3 V at 10 μA/cm ² and are shown as dotted lines in FIG. After connecting three cells in series and connecting diodes in parallel to all cells so that current flows in the charging direction, each cell is connected in series.
It was photo-charged by irradiating it with a 500WXe lamp for 8 hours from a distance of 50cm. These cells were again discharged at a current density of 10 μA/cm ² and the discharge curve is shown by the solid line in FIG.

Example 2 ZrS _{2- 〓+ instead of ZrS 2- 〓} as raw material for photoelectrode
A battery was made in exactly the same manner as in Example 1 except that NbS ₂ (1:1) was used, and the discharge characteristics are shown in FIG.

Example 3 One part of Pd black powder is added to ZrS _2- 〓 + NbS ₂ , which is the raw material for the photoelectrode.
% was used, and the discharge current density was
Batteries were made in the same manner as in Examples 1 and 2, except that the voltage was 100 μA/cm ² , and the discharge curves are shown in FIG.

In addition, in Figures 4 to 6, the dotted lines represent the characteristics after charging by energization, the solid lines represent the discharge characteristics after photocharging,
A shows filling with a doctor blade, B shows filling with a roller, and C shows filling with screen printing.

As shown in FIGS. 4 and 5, the discharge capacity and voltage after photocharging were higher when filling and coating was performed by a method other than screen printing. This is because in the case of screen printing, there is almost no flow in the direction parallel to the electrode surface, and the filling flows in the perpendicular direction, whereas the other two methods of filling application are applied to the surface of the photoelectrode. This is because the near portion flows in a direction parallel to the electrode surface.

In addition, in order to observe the difference in the photoelectrode in the battery of the above-mentioned example, the negative electrode material was constituted in excess. In reality, unless the discharge is stopped within the capacity at which Cu can be intercalated in the intercalation compound, the life of the photoelectrode will be significantly shortened, and for this reason it is desirable to regulate the discharge capacity with the negative electrode. remains unchanged.

Effects of the Invention As described above, the present invention makes the crystal orientation of the intercalation compound powder used as a raw material for the photoelectrode so that the C-axis is parallel to the interface in the part near the electrolyte, and perpendicular to the surface in the part near the light incidence plane. This improves the efficiency of charging with light.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the energy levels of both electrodes during discharging to explain the action of a photo secondary cell according to an embodiment of the present invention, and FIG. 2 is a diagram showing the energy levels of both electrodes during charging of the same battery. , FIG. 3 is a block diagram of a secondary photovoltaic battery according to one embodiment of the present invention, and FIGS. 4, 5, and 6 are discharge characteristic diagrams of secondary photovoltaic batteries according to different embodiments of the present invention. 1...Transparent body that serves as a battery lid and light entrance window, 2...Transparent electrode, 3...Photoelectrode, 4...Cu ⁺
Ionic conductive solid electrolyte, 5... reversible copper electrode,
6...Metal body serving as negative electrode current collector and container, 7...
Insulation packing.

Claims (1)

[Claims] 1. A photoelectrode mainly composed of ZrS _2- (0<δ<0.2), a Cu ⁺ ion conductive solid electrolyte, and Cu and Cu ₂ S
1. A photosecondary cell comprising a counter electrode made of a mixture of the above, and a surface of the photoelectrode on which light is incident is oriented in a direction perpendicular to the C-axis of the sulfide of the photoelectrode. 2. The photosecondary cell according to claim 1, wherein the photoelectrode contains at least one of NbS ₂ and Pd. 3. The photo secondary cell according to claim 1 or 2, wherein the surface is oriented by surface flow.