All-solid rechargeable battery, method for manufacturing same, and electronic apparatus

The present invention relates to an all-solid rechargeable battery, a method for manufacturing the same, and an electronic apparatus, makes the annealing step for crystallization unnecessary in the all-solid rechargeable battery. The present invention includes a substrate, a negative electrode, a solid electrolyte, and a positive electrode, wherein LiFePO4 in an amorphous state is used as the positive electrode.

FIELD

The present invention relates to an all-solid rechargeable battery, a method for manufacturing the same, and an electronic apparatus, and in particular, an all-solid rechargeable battery using a positive electrode material for which no crystallization step is necessary, a method for manufacturing the same, and an electronic apparatus.

BACKGROUND

In recent years, lithium ion rechargeable batteries have attracted attention for the application to hybrid vehicles, electric vehicles and the like. In addition, rechargeable batteries are charged with electrical energy that has been generated and supply the charged electrical energy, and thus, there is a large potential for various applications which also has attracted attention.

A typical example of an all-solid rechargeable battery, particularly an all-solid lithium rechargeable battery, where no liquid is used for the electrolyte is a thin film rechargeable battery. Such a thin film rechargeable battery is fabricated by forming films on a substrate such as of silicon in accordance with a vapor deposition method, a PLD (pulse laser ablation) method, a sputtering method, a CVD method and the like, and is characterized in that the film thickness is as thin as several μm.

Such a thin film rechargeable battery is characterized in that the level of safety is high due to the use of a solid electrolyte, and it is possible to integrate it into devices of various sizes. A metal oxide or a metal oxoate(oxyacid salts) is used as the positive electrode of a thin film rechargeable battery. In addition, a metal oxide, a metal oxoate or any of these of which the oxygen that forms these is partially replaced with nitrogen is used as the solid electrolyte. An alkali metal such as lithium or sodium is used as the negative electrode (see Patent Literature 1 or Patent Literature 2).

FIG. 12is a diagram illustrating the structure of a conventional all-solid Li ion battery where films of a current collector52, a positive electrode53and a solid electrolyte54are formed on a substrate51. Next, an annealing treatment is carried out in order to crystalize the positive electrode53. Then, films of a negative electrode55and a current collector56are formed. In this case generally, Pt or SUS is used as the current collectors52and56, LiCoO2is used as the positive electrode53, LiPON is used as the solid electrolyte54, and metal Li is used as the negative electrode55.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Unexamined Patent Publication 2012-248414

Patent Literature 2: Japanese Unexamined Patent Publication 2012-059497

As for the order of film formation in the conventional thin film rechargeable batteries, only the order of a current collector, a positive electrode active material, a solid electrolyte, a negative electrode active material and a current collector can be applied, which is a large restriction for the circuit design in the case where a thin film rechargeable battery is integrated.

The reason why the opposite order of film formation cannot be applied is that the negative electrode material cannot bear the temperature during the above-described annealing step for crystallizing the positive electrode material. Conventional films of a positive electrode active material and a solid electrolyte are formed in accordance with a PLD method, a sputtering method or a CVD method, and the formed films are in an amorphous state. Here, the amorphous state indicates a state where no diffraction peak can be detected when X-ray diffractometry is carried out. It is considered that these films need to have a crystal structure in order to make it possible for lithium ions to move at the time of charging and discharging. Therefore, heat treatment at 400° C. or higher is necessary for crystallization.

In the case where lithium (of which the melting point is 180° C.) is used as the negative electrode, however, it is possible for the negative electrode to be melted due to its low melting point when heat treatment is carried out on the positive electrode or the solid electrolyte in a state where the film of the negative electrode has already been formed, and thus, there is a risk of internal short-circuiting or other problems.

Therefore, there is such a problem that the annealing step for crystallization bottlenecks the implementation of a tandem structure, even when a tandem structure is desired to increase the output voltage. In the case where an electronic device and an all-solid rechargeable battery are integrated, there is a problem that the circuit design is restricted due to the problematic locations of connections of the positive and negative electrodes.

SUMMARY

One aspect of the disclosed invention provides an all-solid rechargeable battery including a substrate; a negative electrode; a solid electrolyte; and a positive electrode, and the all-solid rechargeable battery is characterized in that LiFePO4in an amorphous state is used as the positive electrode.

Another aspect of the disclosed invention provides a method for manufacturing an all-solid rechargeable battery that is characterized in that films in a multilayer structure made of a current collector, an LiFePO4film, a solid electrolyte, a negative electrode and a current collector or made of a current collector, a negative electrode, a solid electrolyte, an LiFePO4film and a current collector are formed on a substrate at room temperature without carrying out heat treatment during a manufacturing process.

Still another aspect of the disclosed invention provides an electronic apparatus that is characterized by having an all-solid rechargeable battery including a substrate on which an electronic device is mounted; and an all-solid rechargeable battery formed on a surface of the substrate, wherein a positive electrode is made of an LiFePO4film in an amorphous state.

DESCRIPTION OF EMBODIMENTS

Here, the all-solid rechargeable battery according to an embodiment of the present invention is described in reference toFIGS. 1 through 5.FIG. 1is a schematic diagram illustrating the all-solid rechargeable battery according to an embodiment of the present invention. Though the description relates to an example where the negative electrode is provided on the substrate side, it may be applied to an example where the positive electrode is provided on the substrate side.

Films of a current collector2, a negative electrode3, a solid electrolyte4, a positive electrode5and a current collector6are sequentially formed on a substrate1at room temperature without carrying out heat treatment during a manufacturing process of the films. Here, an LiFePO4film is used as the positive electrode without carrying out heat treatment, and therefore, this film stays as an amorphous LiFePO4film. In order to form films at room temperature in this manner, as illustrated inFIG. 2, an RF sputtering method with a crystal LiFePO4target is used.

A silicon substrate, a crystal substrate, a glass substrate or an organic substrate such as a polyimide film is used as the substrate1. Though Pt is typically used as the current collectors2and6, Cu may be used or a Pt film may be provided with a Ti film in between in order to increase the adhesiveness with the substrate1. LiPON or LAPP (LiAl(PO4)(P2O7)) is used as the solid electrolyte4.

FIG. 2is a diagram illustrating the method for forming an LiFePO4film according to an embodiment of the present invention. Here, an example where the positive electrode is provided on the substrate side is described for the purpose of making the illustration on the substrate side easier to understand. An RF power supply8is connected between the substrate1on which a current collector2is formed and a crystal LiFePO4target7, and Ar ions are generated by means of the RF power so that sputtering is carried out. When the distance between the substrate1and the crystal LiFePO4target is set to 150 mm, the Ar gas pressure is set to 0.1 Pa, and an RF power of 140 W is applied, for example, an amorphous LiFePO4film5is deposited on the current collector2at a film formation rate of 0.12 μm/hr.

FIG. 3is a graph illustrating the charging and discharging characteristics of the all-solid rechargeable battery according to an embodiment of the present invention, where the charging and discharging operations are carried out ten times. As illustrated inFIG. 3, it was confirmed that the structure having the positive electrode in the upper portion allowed the rechargeable battery to perform properly. Here, the capacity was approximately ½ and the voltage was 70% as compared to the rechargeable battery with the structure having the negative electrode in the upper portion where a crystal LiFePO4film on which heat treatment was carried out was used as the positive electrode.

FIG. 4is a graph illustrating the X-ray diffraction pattern of the amorphous LiFePO4film used in the all-solid rechargeable battery according to an embodiment of the present invention, which also illustrates examples where heat treatment was carried out for one hour at various temperatures in an Ar gas atmosphere. As illustrated inFIG. 4, a diffraction peak of the (200) plane exhibiting crystallinity appeared as a result of heat treatment at 500° C. or higher. In the case of heat treatment at a temperature lower than that, no diffraction peak of the (200) plane appeared, and thus, it was confirmed that the film was in an amorphous state.

FIG. 5is a graph illustrating the infrared ray wavelength absorbing characteristics of the amorphous LiFePO4film used in the all-solid rechargeable battery according to an embodiment of the present invention, which also illustrates the wavelength absorbed by crystal LiFePO4in the lower portion. Here, the absorption strength is in an arbitrary unit. As illustrated in the graph, acute absorption peaks were confirmed in the case of crystal LiFePO4, while expansion and contraction as well as bending vibrations were observed on the basis of the PO4structure in the case of amorphous LiFePO4.

According to the prior art, it is considered that the positive electrode needs to have a basket structure in order to make it possible for Li to move at the time of charging and discharging, and therefore, heat treatment for crystallization is indispensable. As a result of diligent research by the present inventor, however, it has been confirmed that a positive electrode made of LiFePO4has a PO4structure that is necessary for Li to move even when the used LiFePO4is in an amorphous state.

As a consequence, the rechargeable battery performs sufficiently even without heat treatment, which makes it unnecessary to take the melting of Li in the negative electrode into consideration. Thus, films can be formed in any order, and the fabrication of a tandem structure becomes possible. In addition, the films can be formed basically at room temperature, and therefore, a wider range of material can be chosen for the substrate. Accordingly, a hybrid rechargeable battery with various types of electronic apparatuses becomes possible.

For example, it is possible to provide an all-solid rechargeable battery through a direct formation of films on a substrate on which an electronic device has been mounted. Here, any type of battery, including a type where the negative electrode is in the top portion, a type where the positive electrode is in the top portion, and a tandem type, can be fabricated, which can reduce the restrictions in designing a circuit.

It is also possible to form films on a substrate such as of a polyimide film so as to provide an all-solid rechargeable battery as a chargeable power supply unit having a structure with a number of p-n junction diodes using an organic semiconductor that can generate a photoelectromotive force and a thermoelectromotive force. In this case, a switching means such as a transistor circuit may be provided so that the p-n junction diodes can be connected in parallel when the structure is used as a photoelectromotive force element, and the p-n junction diodes can be connected in series when the structure is used as a thermoelectromotive force element.

Thus, there is no risk of the negative electrode metal being melted during the manufacture of the all-solid rechargeable battery in accordance with the embodiment of the present invention, and therefore, the order in which films are formed is less restricted, which gives more freedom to the designing of a circuit in which the all-solid rechargeable battery is incorporated. Here, “LiFePO4in an amorphous state” means there are no (200) diffraction peaks of X-rays, and the full width at half maximum of the isolated peaks observed in a range from 640 cm−1to 660 cm−1in the infrared absorption spectrum is no smaller than 50 cm−1.

Next, the all-solid Li ion battery according to Example 1 of the present invention is described in reference toFIGS. 6A through 7B.FIGS. 6A and 6Bare diagrams illustrating the structure of the all-solid Li ion battery according to Example 1 of the present invention.FIG. 6Ais a plan diagram, andFIG. 6Bis a cross-sectional diagram along the single-dotted chain line A-A′ inFIG. 6A.

A Ti film12having a thickness of 170 nm and a Pt film13having a thickness of 30 nm, which becomes the negative current collector, are formed on a silicon substrate11at room temperature. Next, an Li film14having a thickness of 100 nm is formed, which becomes the negative electrode of which the area is smaller than that of the negative current collector. Next, an LiPON film15having a thickness of 1 μm, which becomes a solid electrolyte, is formed so as to cover the entirety of the Li film14.

Subsequently, an LiFePO4film16of which the area is smaller than that of the LiPON film15and having a thickness of 30 nm is formed, and on top of this a Pt film17having a thickness of 30 nm is provided, which becomes the positive current collector. Next, a lead-out electrode18is provide on the Pt film13and a lead-out electrode19is provided on the Pt film17in order to complete the basic structure of the all-solid Li ion battery. Here, the Li film14is formed through vapor deposition, the Ti film12and the Pt films13and17are formed through DC sputtering, and the LiPON film15and the LiFePO4film16are formed through RF sputtering.

FIGS. 7A and 7Bare graphs illustrating the charging and discharging properties of the all-solid Li ion battery according to Example 1 of the present invention.FIG. 7Ais a graph illustrating the charging and discharging properties of the all-solid Li ion battery according to Example 1 of the present invention, andFIG. 7Bis a graph illustrating the charging and discharging properties of the all-solid Li ion battery on which heat treatment for crystallization has been carried out. As illustrated inFIG. 7A, an LiFePO4film is used as the positive electrode, and therefore, it can be seen that the rechargeable battery operates even when the LiFePO4film is in an amorphous state on which no heat treatment has been carried out. The voltage is low as compared to that inFIG. 7B, and the film thickness of LiFePO4in the sample inFIG. 7Ais three times greater than the film thickness of LiFeO4in the sample inFIG. 7B, and therefore, the capacity is approximately ½, which exhibits sufficient properties for the rechargeable battery to perform.

Thus, LiFePO4is used as the positive electrode active material in Example 1 of the present invention, and therefore, the rechargeable battery has sufficient performance even when the films are formed at room temperature. As a result, a type of all-solid Li ion battery of which the negative electrode is in the bottom portion can be implemented by placing the negative electrode on the bottom side.

Next, the all-solid multilayer Li ion battery according to Example 2 of the present invention is described in reference toFIGS. 8 through 9B.FIG. 8is a diagram illustrating the structure of the all-solid multilayer Li ion battery according to Example 2 of the present invention. A Pt film22having a thickness of 170 nm is formed on a silicon substrate21at room temperature, and on top of that an LiFePO4film23having a thickness of 100 nm is formed. Next, an LAPP film24having a thickness of 1 μm, an Li film25having a thickness of 100 nm, and a Cu film26having a thickness of 0.5 μm, which becomes a current collector, are formed in sequence.

Subsequently, films are formed in the same manner as in the above, starting from the Pt film22through the Cu film26so as to provide a unit cell structure, and after that, a Pt film22having a thickness of 30 nm is formed. Finally, a lead-out electrode is formed on the Pt film22at the bottom, and at the same time, a lead-out electrode is formed on the Pt film22at the top in order to complete the basic structure of the all-solid multilayer Li ion battery having a tandem structure.

FIGS. 9A and 9Bare graphs illustrating the charging and discharging properties of the all-solid multilayer Li ion battery according to Example 2 of the present invention.FIG. 9Ais a graph illustrating the charging and discharging properties of the all-solid multilayer Li ion battery according to Example 2 of the present invention, andFIG. 9Bis a graph illustrating the charging and discharging properties of a unit rechargeable battery. Here, the all-solid multilayer Li ion battery has carried out the charging and discharging operations twice, and the unit rechargeable battery has carried out the charging and discharging operations 15 times.

As is clear fromFIG. 9A, the properties of the rechargeable battery can be confirmed even when a tandem structure is fabricated thanks to a heat treatment process being unnecessary. As is clear from the comparison ofFIGS. 9A and 9B, the operational voltage is approximately two times higher than that of the unit rechargeable battery.

Thus, it is possible to implement an all-solid Li rechargeable battery having a tandem structure by using an LiFePO4film in an amorphous state as a positive electrode in Example 2 of the present invention as well. Though the positive electrode is on the bottom side inFIG. 8, the same properties can be gained in the tandem structure where the negative electrode is on the bottom side.

Next, the power supply unit according to Example 3 of the present invention is described in reference toFIGS. 10A and 10B.FIGS. 10A and 10Bare diagrams illustrating the power supply unit according to Example 3 of the present invention.FIG. 10Ais a plan diagram, andFIG. 10Bis a cross-sectional diagram along the single-dotted chain line A-A′ inFIG. 10A. As illustrated in the figures, a p type semiconductor32made of P3HT (poly-3-hexylthiophene) is formed on a polyimide film31in accordance with a screen printing method. Next, on top of this, an n type semiconductor33made of PCBM (phenyl-C61-butyric acid methyl ester) is formed so as to provide a p-n junction diode.

Next, the p type semiconductor32, the n type semiconductor33and the switching circuit35are connected through wires34in order to make switching between the series connection and the parallel connection possible. Though the wires34are illustrated as simple lines, they actually have more complicated connection structures. Meanwhile, an all-solid multilayer Li ion battery36having a tandem structure according to Example 2 is formed on the polyimide film31through film formation at room temperature and connected to the output terminal of the switching circuit35.

In order to operate this structure with the p-n junction diodes as a photoelectromotive element, the p-n junction diodes are connected in parallel by means of the switching circuit35. In order to operate the structure as a thermoelectromotive element, the p-n junction diodes are connected in series by means of the switching circuit35. At this time, one end of each p-n junction diode, for example, the upper terminals inFIG. 10A, is made to make contact with a heat source, and one end on the opposite side, for example, the lower terminals inFIG. 10A, is located on the heat releasing side in order to gain a thermoelectromotive force. The generated photoelectromotive force or thermoelectromotive force charges the all-solid multilayer Li ion battery36via the switching circuit35.

Thus, in Example 3 of the present invention, a power supply unit that can contribute to energy harvesting can be implemented through the integration with a photoelectromotive element or a thermoelectromotive element. This power supply unit is formed on a flexible substrate, and therefore is very effective in realizing a wearable healthcare device.

Next, the electronic apparatus according to Example 4 of the present invention is described in reference toFIG. 11.FIG. 11is a diagram illustrating the structure of the electronic apparatus according to Example 4 of the present invention. As illustrated in the figure, an all-solid Li ion battery is provided through the film formation at room temperature on a silicon substrate on which an electronic device47has been mounted. Here, a rechargeable battery of a type where the negative electrode is in the lower portion made of a negative current collector42, a negative electrode43, a solid electrolyte44, an LiFePO4film45and a positive current collector46, and a rechargeable battery of a type where the positive electrode is in the lower portion made of a positive current collector46, an LiFePO4film45, a solid electrolyte44, a negative electrode43and a negative current collector42are formed at the same time.

Here, the rechargeable battery of a type where the negative electrode is in the lower portion and the rechargeable battery of a type where the positive electrode is in the lower portion are connected in series through a Cu connection electrode49, and at the same time, the positive current collector46in the rechargeable battery of a type where the negative electrode is in the lower portion and the electronic device47are connected through a Cu wire48. Here, a vibration sensor is used as the electronic device47, which may be combined with the photoelectromotive element or the thermoelectromotive element in Example 3, so that a wireless sensor terminal without needing a power supply can be realized.

Thus, LiFePO4in an amorphous state is used as the positive electrode in Example 4 of the present invention, and therefore, an all-solid rechargeable battery can be formed in any location without depending on the order of the film formation or the substrate material. As a result, the all-solid rechargeable battery is not subject to a restriction in designing a circuit.

The disclosed all-solid rechargeable battery, the method for manufacturing the same and the electronic apparatus make the annealing step for crystallization unnecessary in the all-solid rechargeable battery.