Patent Application
20110300422
This invention relates to a battery using a solid electrolyte plate and liquid Na.
In recent years, attention is attracted to a sodium battery represented by a sodium-sulfur battery capable of storing a high-capacity power. Further, development has been made of the sodium-sulfur battery used as a power source for supplying a stabilized output in combination with wind power generation and photovoltaic generation having large output fluctuation.
The battery of the type is, as disclosed in Patent Documents 1 and 2 and the like, a kind of a secondary battery using sodium on a negative electrode side, sulfur for a positive electrode, and a ceramic alumina material (β-alumina) for an electrolyte. The battery has excellent characteristics, such as an energy density per volume as high as about three times of that of a common lead storage battery, an excellent charge-discharge cycle characteristic, and a low self-discharge.
Herein, a principle of the sodium-sulfur battery will briefly be described.
Sodium (Na) on the negative electrode side has a work function as low as 2.8 eV and, therefore, easily donates an electron to an Al alloy to become a Na+ ion. The Na+ ion passes through β-alumina having a conductivity with respect to the Na+ ion. The Na+ ion which has passed through the β-alumina forms a compound of sodium polysulfide (NaSx) to gain an electron from sulfur to be neutralized. Herein, sodium (Na), sulfur (S), and sodium polysulfide (NaSx) have melting points of 98° C., 120° C., and 285° C., respectively. Therefore, in order to keep all these substances in a liquid state, it is indispensable for the sodium-sulfur battery to be operated at a temperature of about 300° C. or above.
Further, for the purpose of dissociating Na from sodium polysulfide to return Na to its original position, cyclic battery charge is required. In a charging process, NaSx is dissociated to produce a Na+ ion which returns to liquid Na as the original position. In a conventional sodium-sulfur battery, there is a following problem. When sodium is directly brought into contact with sulfur due to a breakage or the like, a sodium-sulfur reaction is caused to occur with reaction heat. An electronic container or the like is melted down due to the reaction heat to cause leakage of active materials, such as sodium and sulfur, to the outside of the battery.
As described above, in the conventional sodium-sulfur battery, a temperature of about 300° C. or above is required to operate the battery and an energy consumption for heating is indispensable. Further, the charging process for returning NaSx to Na and a structural design for preventing the sodium-sulfur reaction are required. Thus, an increase in cost is caused.
It is therefore an object of the present invention to provide a battery using liquid sodium and operable at a temperature lower than 300° C.
It is another object of the present invention to provide a battery using liquid sodium and operable without using sulfur.
It is still another object of the present invention to provide a battery using liquid sodium and requiring no charging process.
A liquid Na battery according to the present invention may have structures as follows.
(Structure 1)
According to the present invention, it is possible to provide a liquid Na battery comprising a first electrode, liquid Na arranged so as to be brought into contact with the first electrode, a partition wall arranged so as to be brought into contact with the liquid Na and formed of a Na-ion conducting solid substance, and a second electrode arranged on a side opposite to the first electrode with respect to the partition wall through the liquid Na, characterized in that:
the first electrode has a portion brought into contact with the liquid Na, at least a part of the portion being formed of a first conductive member having a work function whose absolute value is smaller than 2.8 eV, and
the second electrode has a portion brought into contact with the liquid Na, at least a part of the portion being formed of a second conductive member having a work function whose absolute value is greater than 2.8 eV.
(Structure 2)
According to the present invention, it is possible to provide the liquid Na battery as mentioned in the above-mentioned Structure 1, characterized by further comprising:
a first space defined between the partition wall and the first electrode and filled with the liquid Na;
a second space defined between the partition wall and the second electrode and filled with the liquid Na;
a third space filled with a pressurized inactive gas so as to be brought into contact with the liquid Na communicating with the liquid Na filling the second space;
a liquid Na flow path arranged so as to return the liquid Na from the first space to the second space and constructed so as to prevent electrical short-circuiting between the liquid Na filling the first space and the liquid Na filling the second space; and
a first output terminal arranged so as to be electrically connected to the first electrode and a second output terminal arranged so as to be electrically connected to the second electrode.
(Structure 3)
According to the present invention, it is possible to provide a liquid Na battery including a container for containing liquid Na, characterized in that:
a first portion of an inner wall of the container is formed of a first conductive member having a work function whose absolute value is smaller than 2.8 eV; and
a second portion of the inner wall of the container is formed of a second conductive member having a work function whose absolute value is greater than 2.8 eV;
the liquid sodium battery further comprising:
a partition wall formed of a Na-ion conducting solid substance and located between the first portion and the second portion in a space inside the container;
a first space defined between the partition wall and the first portion and filled with the liquid Na;
a second space defined between the partition wall and the second portion and filled with the liquid Na;
a third space filled with a pressurized inactive gas so as to be brought into contact with the liquid Na communicating with the liquid Na filling the second space;
a liquid Na flow path arranged so as to return the liquid Na from the first space to the second space and constructed to prevent electrical short-circuiting between the liquid Na filling the first space and the liquid Na filling the second space; and
a first output terminal arranged so as to be electrically connected to the first member and a second output terminal arranged so as to be electrically connected to the second member.
(Structure 4)
According to the present invention, it is possible to provide the liquid Na battery as mentioned in the above-mentioned Structure 2 or 3, characterized in that the liquid Na flowing path has a structure allowing the liquid Na to be discharged in droplets into the third space.
(Structure 5)
According to the present invention, it is possible to provide the liquid Na battery as mentioned in any one of the above-mentioned Structures 2-4, characterized in that the liquid Na filling the second space donates an electron to the second member to become a positive Na ion, the Na ion thus formed passing through the partition wall and receiving an electron from the first member to become electrically neutral, and that the liquid Na in the first space is increased by the flow of the liquid Na and an increased amount of the liquid Na returns to the second space through the liquid Na flow path,
thereby the first output terminal becoming a positive electrode, the second output terminal becoming a negative electrode.
(Structure 6)
According to the present invention, it is possible to provide the liquid Na battery as mentioned in any one of the above-mentioned Structures 1-5, characterized in that the first member is a material selected from LaB6, NbC, ZrN, and Cs.
(Structure 7)
According to the present invention, it is possible to provide the liquid Na battery as mentioned in any one of the above-mentioned Structures 1-6, characterized in that the second member is a metal selected from Au, Ni, Pt, and Pd.
(Structure 8)
According to the present invention, it is possible to provide the liquid Na battery as mentioned in any one of the above-mentioned Structures 2-7, characterized in that, among the liquid Na's, at least the liquid Na filling the first space is added with NaF.
(Structure 9)
According to the present invention, it is possible to provide the liquid Na battery as mentioned in any one of the above-mentioned Structures 1-8, characterized in that the first member is deposited using rotary magnet sputtering.
According to the present invention, it is possible to obtain a liquid Na battery operable at a temperature lower than 300° C. Further, according to the present invention, it is possible to obtain a battery using liquid sodium and operable without using sulfur.
According to the present invention, it is possible to obtain a low-cost battery using liquid sodium and requiring no charging process.
Next, embodiments of the present invention will be described with reference to the drawings.
A space (first space) inside the container 1-1, which includes the partition wall (hereinbelow called a β-alumina partition wall) 1-4 formed of the β-alumina and the LaB6 electrode portion 1-2, is filled with liquid sodium (Na) indicated by 1-5. At the same time, a space (second space) inside the container 1-1, which includes the β-alumina partition wall 1-4 and the Pt electrode portion 1-3, is also filled with liquid sodium (Na) indicated by 1-6. Thus, it is noted here that, in the example shown in the figure, sulfur or the like other than sodium (Na) is not used.
Further, a space (third space) 1-7 filled with a pressurized Ar gas (inactive gas) so as to pressurize liquid sodium (Na) communicating with the liquid sodium (Na) 1-6 is formed inside the space filled with the liquid sodium (Na) 1-6. The Ar gas filling the space 1-7 has a pressure of 1.1 atmosphere. In order to return the sodium (Na) 1-5 on the side of the LaB6 electrode portion 1-2 to the space 1-6 on the side of the Pt electrode portion 1-3, a liquid sodium (Na) flow path 1-8 formed by an alumina tube is arranged so as to connect upper portions of the space 1-5 and the space 1-7. The liquid sodium flow path 1-8 of the alumina tube has an outlet located in the space on the side of the Pt electrode portion 1-3 and has a structure such that the liquid sodium (Na) is discharged in droplets as indicated by 1-12.
The container 1-1 has a bottom portion 1-9 formed of alumina as an insulator and a container wall portion 1-21 which is a portion other than the bottom surface and which is formed of an electroconductive material (for example, an aluminum alloy). In the first embodiment, the LaB6 electrode portion 1-2 is formed by sputtering deposition on the inner wall of the container wall portion 1-21 formed of the aluminum alloy. The LaB6 electrode portion 1-2 has a film thickness of 200 nm. 1-10 represents a first output terminal which is fixed to an exterior of the container wall portion 1-21 and which is electrically connected to the LaB6 electrode portion 1-2. The Pt electrode portion 1-3 is also formed by sputtering deposition on an outer wall surface of an aluminum-alloy tubular member 1-22 which has a tubular shape with a sealed upper portion and a bottom portion having a hole. The Pt electrode portion 1-3 has a film thickness of 200 nm.
The tubular member 1-22 encloses the pressurized space 1-7 described above with its inside bottom portion filled with the liquid sodium (Na) communicating with the liquid sodium (Na) 1-6 filling the space containing the β-alumina partition wall 1-4 and the Pt electrode portion 1-3. The tubular member 1-22 has an exterior connected to a second output terminal 1-11 which is electrically connected to the Pt electrode portion 1-3. In the first embodiment, the container of an aluminum alloy is used as a base material of the container wall portion 1-21 and the tubular member 1-22 on which the electrodes are to be deposited. However, the container material is not limited to the aluminum alloy as long as it is electrically conductive.
Each of the β-alumina partition wall 1-4, and the LaB6 electrode portion 1-2 and the Pt electrode portion 1-3 opposite to each other has a flat plate shape.
Next, an operation of the battery will be described in detail. Pt forming the Pt electrode portion 1-3 has, as an energy level corresponding to a work function (hereinbelow called an energy level), a very low energy level of −5.7 eV and, therefore, can easily take an electron from Na having an energy level of −2.8 eV. That is, a metal (second member) (for example, Pt) having a work function whose absolute value is greater than that of the work function of Na can take an electron from Na.
The liquid sodium (Na) 1-6 in the space on the side of the Pt electrode portion 1-3 donates an electron to Pt to become a positive Na+ ion. The Na+ ion thus formed passes through the β-alumina partition wall 1-4 having a conductivity with respect to the Na+ ion only. The LaB6 thin film has an energy level of −2.5 eV. Therefore, the LaB6 electrode portion 1-2 can give an electron to the Na+ ion which has reached there. The Na+ ion receives the electron from the LaB6 electrode portion 1-2 to become electrically neutral. That is, by using a metal (first member) (for example, LaB6) having a work function whose absolute value is smaller than that of the work function of Na, it is possible to give an electron to the Na+ ion to neutralize the Na+ ion.
By a flow of liquid sodium (Na) thus generated, the liquid sodium (Na) 1-5 in the space on the side of the LaB6 electrode portion 1-2 is increased in amount. The increased amount of liquid sodium passes through the liquid sodium (Na) flow path 1-8 and, as droplets 1-12, discontinuously falls down in the pressurized space 1-7 to return to the liquid sodium (Na) communicating with the liquid sodium (Na) 1-6 in the space on the side of the Pt electrode portion 1-3. As a result, the liquid sodium (Na) battery is operated with the first output terminal 1-10 as a positive electrode and the second output terminal 1-11 as a negative electrode. When the liquid sodium (Na) is returned to the liquid sodium (Na) 1-6 on the side of the Pt electrode portion 1-3, if it is connected with the liquid sodium (Na) 1-5 on the side of the LaB6 electrode portion 1-2, the liquid sodiums (Na) in the both spaces are electrically short-circuited. As a consequence, an electromotive force is not generated.
In order to avoid electrical short circuiting between the space on the side of the LaB6 electrode portion 1-2 and the pressurized space 1-7, it is essential to discontinuously return the liquid sodium (Na) in droplets as indicated by 1-12 in
Liquid sodium (Na) has a melting point of 98° C. and has a density at each of 100° C. and 200° C. as follows.
100° C.: density 0.926 g/cm3, 2.424×1022 atom/cm3
200° C.: density 0.902 g/cm3, 2.362×1022 atom/cm3
In order to perform a battery operation, it is required to make a battery have a temperature of at least 98° C. or above so as to liquefy sodium (Na). It is noted here that, with an increase of an operation temperature, an energy consumption required for heating is increased. It is therefore desirable to use the battery in a temperature range slightly higher than the melting point. In the first embodiment, liquid sodium (Na) is heated to 110° C. and used. Meanwhile, in order to increase efficiency of the battery, it is important to enhance heat insulation property of the exterior of the container. In the first embodiment, the exterior of the container is surrounded by a vacuum insulating material having a thermal conductivity as extremely low as 0.0012 W/mK so as to minimize heat loss.
In order to achieve the above-mentioned battery operation, as an electrode on a positive electrode side, a thin film is required which has a work function whose absolute value is smaller than 2.8 eV of Na and which is chemically stable. Typically, electrons are easily removed from a low-work-function material. Therefore, the material is easily oxidized and a chemically stable thin film is difficult to be formed therefrom. In the first embodiment, by a rotary magnet sputtering technique (described in Patent Document 3 and so on) capable of performing low-damage film formation, film formation is performed with an ion irradiation amount and an ion irradiation energy controlled. As a consequence, it is possible to achieve a LaB6 thin film which exhibits a strong orientation in a (100) direction as a crystallinity, which is chemically stable and resistant to liquid Na, and which has a work function of 2.5 eV. It is noted here that a material for the electrode on the positive electrode side (first member or first portion) is not limited to LaB6, if the material has a work function smaller than that of sodium (Na) and a resistance to the liquid sodium (Na). For reference, examples of a low-work-function material for use as the electrode on the positive electrode side and work function values thereof are shown below.
The work functions have a range in value because they are dependent on a crystalline structure and a manufacturing method. When the LaB6 thin film is formed by rotary magnet sputtering, a flat-plate electrode is suitable as compared to a cylindrical electrode or the like. This is because the film is formed with ion irradiation uniformly performed at all positions thereof.
If a material for an electrode on a negative electrode side (second member or second portion) has a work function whose absolute value is greater than 2.8 eV of the liquid sodium (Na) and a difference therebetween is greater, electrons are easily received to enhance ionization of the liquid sodium (Na) so that a power generation efficiency is improved. In case of a high-work-function material, it is difficult to remove electrons so that the material is hardly oxidized. Therefore, a chemically stable thin film can relatively easily be formed. For reference, work function values of electrode materials on the negative electrode side are shown below.
The work functions have a range in value because they are dependent on a crystalline structure and a manufacturing method.
The present inventors have found that, when the battery is operated at 110° C., a current density of about 77 mA/cm2 is obtained at each of the Pt electrode portion and the LaB6 electrode portion. The above-mentioned current density corresponds to a Na+ ion stream of 4.8×1017 Na+ ion/cm2·sec. Per 1 m2 of an electrode area:
Physical property values of sodium (Na) are shown in the following table 1.
Considering the case at around 100° C., an amount of Na flowing through the β-alumina partition wall 1-4 of 1 m2 is as follows:
Thus, it is sufficient to continuously flow this level of liquid sodium (Na) from the side of the LaB6 electrode portion to the side of the Pt electrode portion (when the area of each of the opposite electrodes is about 1 m2).
In
Using
By the above-described iterative operation, it becomes possible to return the liquid sodium (Na) from the side of the LaB6 electrode portion to the side of the Pt electrode portion without causing electrical conduction. Herein, one example of the structure of the liquid sodium (Na) flow path is shown. In this regard, it is important to have a structure in which electrical short-circuiting is not caused (preferably, a structure allowing discharge in droplets). As long as the above-mentioned structure is achieved, the flow path is not limited to the structure being illustrated.
A second embodiment of the present invention will be described using
The LaB6 electrode portion 1-2 gives an electron to a Na+ ion and the Pt electrode portion 1-3 takes an electron from a Na atom to convert the Na atom into a Na+ ion. Accordingly, an electric current I flows at the outside from the LaB6 electrode portion 1-2 to the Pt electrode portion 1-3. When a load is connected, the LaB6 electrode portion 1-2 has a positive voltage and the Pt electrode portion 1-3 has a negative voltage.
In this case, the positive voltage generated at the LaB6 electrode portion 1-2 tries to inhibit the Na+ ion from passing to the side of the LaB6 electrode portion 1-2. However, by adding NaF, it is possible to reduce the above-mentioned effect. Specifically, a F− ion generated by dissociation of NaF approaches the LaB6 electrode portion 1-2 to inhibit the effect of the generated positive voltage from reaching the inside the liquid sodium (Na). A F atom has an energy level of −9.42 eV. Therefore, even if the LaB6 electrode having a work function whose absolute value is 2.5 eV (energy level being −2.5 eV) has a positive voltage of 2.0V to 2.5V, it is not possible to take an electron from the F− ion and the F− ion approaches the LaB6 electrode to continuously inhibit the effect of the positive voltage. The F− ions indicated by 3-1 are constantly present on the entire surface of the LaB6 electrode portion 1-2 and cancel the effect that the positive voltage generated at the LaB6 electrode portion 1-2 repels the Na+ ion. Even when the electric power is extracted to the outside and the positive voltage is generated at the LaB6 electrode portion 1-2, the Na+ ion efficiently passes through the β-alumina partition wall 1-4. Thus, NaF functions as a power generation efficiency improver.
The liquid sodium batteries according to the embodiments of the present invention need not use sulfur and, therefore, can generate an electric power without heating to a high temperature.
The present invention may be achieved by the following modes (1) and (2).
(1) A liquid sodium battery characterized by comprising a partition wall formed of a Na-ion conducting solid substance, a first member which is formed of a metal having a work function whose absolute value is smaller than that of a work function of sodium and which is arranged on the side of one surface of the partition wall with a space kept therefrom, and a second member which is formed of a metal having a work function whose absolute value is greater than that of the work function of sodium and which is arranged on the side of the other surface of the partition wall with a space kept therefrom.
(2) A liquid sodium battery using liquid sodium, characterized in that electrodes are constructed by two metals selected in relation to a work function of sodium and having work functions different from each other, thereby generating an electric power with sodium only.
The liquid sodium battery according to the present invention is applicable to an emergency power source during power outage, an output stabilizing power source for wind power generation or the like, an automotive power source, and the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-039700 | Feb 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/052150 | 2/15/2010 | WO | 00 | 8/18/2011 |
