Magnesium-Air Battery: A Breakthrough

An Interview with Mr. Susumu Suzuki & Dr. Hideki Kurihara

Many researchers have approached the challenge of the development of magnesium battery for practical use, but so far no one has succeeded. An entirely new concept developed by Mr. Susumu Suzuki and Dr. Hideki Kurihara seems to be a breakthrough that provides the way to a commercially available practical magnesium battery.

Magnesium is an Ideal Material for Batteries

Yamaji: I learned that you are developing magnesium batteries. Could you tell us what kind of battery you are developing?

Dr. Kurihara: We have been developing two kinds of primary battery (not rechargeable battery) and a secondary battery (a rechargeable battery).

Yamaji: Why did you select magnesium?

Dr. Kurihara: The oxidation-reduction reaction determines the performance capabilities of a battery. The ionization tendency of metals is the key to find a suitable battery material. The following is a list of metal elements in order of higher ionization tendency: Lithium (Li), Kalium (K), Calcium (Ca), Sodium (Na), Magnesium (Mg), Aluminum (Al), Zinc (Zn), Iron (Fe), Nickel (Ni) and so on. It is because of the ionization tendency that lithium is inviting attention nowadays. Many people believe if they use lithium, the best available batteries could be made. But the known amount of lithium reserve is not large and said to be 11million tons, and it is unevenly distributed. As lithium reacts violently with water and catches fire, water cannot be used as electrolyte for lithium battery. Among those metal elements with which we can use water as electrolyte, magnesium has the largest ionization tendency. Magnesium offers the best possibility to make a high performance battery. Further, magnesium is found in large deposit all over the Earth, and even it exists almost inexhaustibly dissolved in the sea water.

Structure of Ordinary Batteries and a Metal-Air Battery

Yamaji: Once I read an article for junior high students explaining how to make a magnesium-air battery in their physics class. The material can be abundantly obtained and it seems making a battery using magnesium is not complicated. Why is it that magnesium batteries have not been made commercially available so far?

Dr. Kurihara: The chemical reaction used in experimental magnesium-air batteries can be found in any electrochemistry textbook, and known very well. In fact, one of the primary batteries uses this reaction.

Let me explain first the structure of the most commonly available batteries. The negative electrode has a negative terminal active substance which can be easily ionized, and the positive electrode has an easily reducible positive terminal active substance. The negative terminal active substance is oxidized by releasing electrons, while the positive terminal active substance is reduced receiving the electrons given off by the negative terminal. At the same time ions migrate through the electrolyte electrically balancing the system to create continuous reactions that make it possible for users to tap electricity from the battery.

In a magnesium-air battery, metal magnesium is the negative electrode active substance, and positive electrode active substance is oxygen in the air. Magnesium in the negative electrode discharges electrons and becomes magnesium ions which are dissolved into the electrolyte. Meanwhile, oxygen and water on the positive electrode side receives electrons and form hydroxide ions. In the entire reaction, magnesium, oxygen and water form magnesium hydroxide (Mg (OH) 2). And when water is consumed, magnesium hydroxide finally turns into magnesium oxide (MgO).

Magnesium-air Batter reaction Formula

Yamaji: What was really the problem?

Dr. Kurihara: Generally speaking, alkaline electrolyte is chosen to prevent a battery from self-discharging. Self-discharging is a phenomena that occurs when at the same time as a metal used as the negative electrode dissolves, discharged electrons reacts with hydrogen ions to form hydrogen, preventing electrons from migrating to the positive electrode stopping the flow of electricity. The higher the hydrogen ion density in the acid solution, the most notable and severer the self-discharging. But, in high alkaline electrolyte, a film impervious to electricity or ions is formed on the surface of magnesium electrode, preventing the flow of electricity. This reaction accompanies the generation of heat. Many researchers have challenged to resolve the problem of film formation and heat generation in vain. This led many people to believe that the use of magnesium in practical batteries is not possible.

Succeeded in Completing A Practical Magnesium-Air Battery Believed to be Impossible!

Yamaji: Could you tell me how you have done it?

Dr. Kurihara: The reason why the film is formed on the electrode surface is that generated magnesium hydroxide is hard to dissolve into water. If there is some way to prevent the magnesium hydroxide generation and it becomes possible for magnesium ions to stay in water in stabilized state? Then the negative electrode magnesium will continue to dissolve itself into the electrolyte and the reaction will not cease. A substance developed by Mr. Suzuki, President made it possible to make this happen. The composition of the substance is proprietary and cannot be disclosed. Let us call it the Catalyst [X] here. The magnesium-air battery uses a piece of nonwoven fabric coated with the Catalyst [X], and when water is poured on the nonwoven fabric, the Catalyst [X] is dissolved into the electrolyte. In the case of magnesium-air battery, no heat generation occurs in the reaction, contrary to the conventional magnesium-air batteries which has heat generation in their reaction process.

Yamaji: You said the negative electrode magnesium continues to dissolve itself into the electrolyte. Do you mean the negative electrode magnesium is finally consumed?

Dr. Kurihara: Yes, the metal magnesium has gone. If you analyze dried electrolyte, you can find magnesium oxide and magnesium hydroxide. At laboratory, we have also made a magnesium-air battery and tried various types of electrolyte, but the electrochemical reaction in our battery also stopped in a short time due to the formation of black film on the surface of the magnesium electrode. The only electrolyte with the Catalyst [X] made it possible for the negative electrode magnesium to dissolve itself into the electrolyte. We believe we could utilize 90% of the potential that magnesium has with the Catalyst [X]. In any battery, utilizing 90% of the potential of active material is almost incredible phenomenon.

YAMAJI: What is the output voltage and amperage capacity?

Dr. Kurihara: In case of prototypes, the output voltage is in the range of 1.5V~1.6V per cell or one layer. The known oxidation-reduction potential of a magnesium-air battery is 2.76V, but as their reaction resistance and so on, it is not possible to produce that high voltage. Further, in case of our prototypes, we depend on manual process for coating the unwoven fabric with Catalyst [X], and commonly available filter paper is used for a membrane which we call a separator placed between the positive and negative electrodes to prevent short circuit. These are causes of higher inner electrical resistance. When measurement is made on a triple pole cell in which such inner electrical resistance can be neglected, the cell potential is 2.0V. This means, if we mechanize the production process and use an optimum separator material and with improvements of other parts of the cell, it will be possible to attain the output voltage that is close to 2.0V.

However, I would say the potential of the magnesium-air battery will not exceed the potential of the output voltage of lithium primary battery which is 3.0V. On the other hand, the capacity of the magnesium-air battery far exceeds that of lithium primary battery. Theoretical capacity of magnesium is 2290mAh/g, and the negative electrode capacity of the magnesium-air battery presently attained is 2000mAh/g. This is the reason why I said magnesium-air battery could bring out 90% of the potential that magnesium has.

The positive electrode is air, and capacity is in a way limitless; however, same as the other types of air batteries, cell reaction stops because of the formation of a film, which obstructs the supply of oxygen, or impedes reaction with oxygen. One thing which is promising is that we found the proprietary Catalyst [X] can also curb the formation of the film on the positive electrode. Therefore, in case of magnesium-air battery, the battery is considered exhausted when the allowable limit of magnesium hydrate dissolution of the Catalyst [X] is exceeded, or when the magnesium negative electrode is totally consumed.

On the contrary, a lithium primary battery has lithium negative electrode, whose capacity far exceed that of magnesium, but the positive electrode presents serious drawbacks. For example, the manganese dioxide used has a theoretical capacity of 308mAh/g, but the actual capacity produced is in the range of 100mAh/g. Some might ask why you do not use the positive electrode of air which has limitless capacity. But as the atmospheric air has high amount of water vapor or humidity, when oxygen is taken in from the atmosphere, lithium electrode reacts with water in the air, and catches fire. It is not easy to construct an air battery using lithium electrode in the water based electrolyte.

Magnesium that allows the use of water based electrolyte poses no such problem, and constructing an air battery using magnesium is relatively not difficult. It is not a mere dream that someone will produce a magnesium battery that has a capacity 10 times as large as the Lithium primary batteries now commercially available in the market.

Mr. Suzuki: I would say commercialization of such magnesium battery is already possible at this stage, as we have attained the capacity value very close to the theoretical capacity of magnesium.

Magnesium air battery, the burden on the environment

Yamaji: What is your commercialization plan?

Mr. Suzuki: We plan to start sales of the test products toward May of this year (2010). The first product is a charger for cellular phones, for one-time use. The sixe is 2.5 x 4cm. The battery has a water tank inside, and by pushing a button, the Catalyst [X] will dissolve itself into the electrolyte to start electrical discharge. We should like to make batteries that will be able to charge cellular phones three to four times after the market introduction of the test products.

Dr. Kurihara: It will be possible to start marketing of the cellular phone charger in Thailand, Cambodia and China. In Cambodia, people widely use cellular phones, but power supply is not available in many places. The time that people can use their phone is limited not by telephone charge, but by phone battery charging time. The Catalyst [X] is not made from exotic materials, a local factory in Cambodia will be able to economically produce the battery charger and sell them at a reasonable price.

Mr. S. Suzuki: What we are really interested is the emergency use battery market. At present, ministries and agencies of the Japanese Government store batteries for use in emergencies. Because of the life of conventionally available batteries, half the amount of stored batteries is disposed in every two years. This is impermissible waste. The advantage of magnesium-air batteries is that they can be stored for years, if water is not injected.

The magnesium-air batteries can be used as power source for marine life jackets and emergency lamps for cars. Many roads in the USA have no roadside lights and in case of accidents along such roadways, emergency lamps with long care free battery life will go a long way. Some people said they would use magnesium-air batteries for outdoor advertisement boards. Conventional batteries cease to function when soaked in the rain, but magnesium-air batteries have no problem in such environment.

Yamaji: As far as I see your explanation, magnesium-air battery does not use any toxic material, am I right?

Dr. Kurihara: It is also a merit of magnesium-air batteries. In fact, disposal cost of power cells in the market is more than their material costs. For example, the electrolyte of alkali dry cells is strong alkaline solution and if it gets in to the eyes, eye sight will be lost.

Mr. Suzuki: Advanced countries have disposal facilities, but in developing countries, abandoned used batteries could cause environmental contamination. Batteries inexpensively made and without toxic or dangerous materials are most suitable for developing country environment.

Yamaji: I can see that the Catalyst [X] is the key to magnesium-air battery; could you tell me how you come to discover the proprietary Catalyst [X]?

Mr. Suzuki: My business is originally the development of construction materials. Among materials I developed, there is “RealSand”, form inorganic calcium carbonate, which is light but as hard as a rock and “RealGuard”, which prevents deterioration of concrete structure.

In April 2000 I was going through trial and error process to complete a kind of concrete that allows weak flow of electric current. I wanted to have a wall made of concrete that warms up slightly. Graphite could be an easy answer, but materials for construction should be inexpensive first of all and was trying various materials that fit this requirement from coal dust, iron sand, alumina, and　what not.

At one time, I inadvertently spilled oolong tea over a certain material, and saw a current meter connected to the material swung once noticeably. I did not pay any attention to the incident, and continued my work. In one midnight when I woke up and thought, “Why the hell did the current meter react at that time?” I thought water or oolong tea had something to do with it, and poured oolong tea or water on various materials with no avail. But I later found out that what really mattered was the “curing,” which meant let the mixed material stand for some time. And cured materials sometimes show utterly different characteristics from the original materials.

I made a simple battery with these materials and by dropping water on it lead to a long electrical discharge. I first called this phenomenon, “Water Power Generation”, but I could not figure out the principle underlying it, and decided to do a laboratory analysis for proving it.

Dr. Kurihara: At the beginning, we thought it was something fishy. (Laughter), but when we asked the details, we found out magnesium was used as battery material. As mentioned in the beginning, magnesium is an ideal battery material if film formation and heat generation problems are solved, and we thought this could be a kind of breakthrough.

Our analysis showed that it was not “water power generation”, but it was a magnesium-air battery. We finally found that the battery had a kind of unique electrolyte and brushed up its construction to produce electricity efficiently.

Yamaji: I am very curious about what is the material of the Catalyst [X].

Mr. S. Suzuki: It has commonly found materials, but in the eyes of battery experts, the composition of materials looks crazy. Sometime ago, I sought an advise of a battery expert, he said, “If you put that kind of a thing in a battery, there will be no electric discharge.” As I am not a battery specialist and I always think in terms of ion exchange. That would have been the reason why I did not become slaves to the commonsense of battery experts. If I knew battery well, I did not bother to try from the beginning using magnesium as battery material. Everybody in batteries knew you could not make practical battery using magnesium in electrodes.

Yamaji: Even so, magnesium-air battery has a simple structure.

Mr. Suzuki: The basic structure is made up of a piece of metal magnesium plate and separator, unwoven fabric coated with the Catalyst [X], and a piece of copper plate as a collector. It is too simple, and when specialists see it, their remarks are, “it must be some sort of conjuring trick.” And I also got worried myself.

Dr. Kurihara: I think any right stuff is simple. The real point is the clever use of commonly available materials, and there is no exotic material involved. The battery makes use of phenomena daily occurring around us in the realm of nature.

The above shows knocked down parts of the magnesium-air battery. From the left, a piece of metal magnesium plate, a separator, unwoven fabric coated with the Catalyst [X], and copper plate collector.

Another Magnesium Primary Battery a Large Amperage

Yamaji: Can you only use magnesium-air battery for smaller gadget, such as the charger you mentioned for cellular phones?

Dr. Kurihara: The speed of chemical reaction determines the amount of electric current produced in the reaction. In the present magnesium-air battery, as magnesium has a fast ionization speed, the reaction rate controlling factor mainly depends on the speed of oxygen taken in for reaction. For this reason, in order to produce a large electric current, it is necessary to have a structure that improves the speed of oxygen taken in, while keeping the optimum level of electrolyte. In my view, it would be not easy to scale up using the present structure.

Therefore, we are developing another magnesium primary battery based on a different system. This battery use magnesium and water. The catalyst being used is not exactly Catalyst [X], but a manganese-based compound. However, as the manganese compound commonly used in alkaline battery has insufficient catalytic effect against water, voltage cannot be more than 0.5V, and not suitable for practical product. If, however, we use the Catalyst [X] in addition to the manganese compound, we can produce 1.2~1.4V. Water has a large theoretical amperage capacity of 1488mAh/g. When actually standardized by the weight of manganese dioxide catalyst, the battery capacity is 1750mAh/g, which is a substantial value. (The known reaction of manganese dioxide can never exceed the theoretical capacity of 308mAh/g.)

The voltage of this magnesium-water battery is lower than 1.6V~2.0V of the magnesium-air battery earlier explained, however, this magnesium-water battery can be scaled up as it can be constructed in the same way as the lead-acid battery. We have constructed prototypes with the capacity of 150V80A, and 300V40A, which can easily run a home-use refrigerator.

Yamaji: In a primary battery that uses reaction with water, I think there is hydrogen generation.

Dr. Kurihara: You are right. Hydrogen is generated in the reaction, which is now dispersed　in the atmosphere. There will be no danger unless it is stored in a closed space.

Yamaji: Do you think you can use the magnesium-water battery as battery for electric cars?

Dr. Kurihara: Yes, that is one possibility. But as this magnesium-water battery is a primary battery which cannot be recharged, when the metal magnesium negative electrode is consumed, it must be replaced with a new magnesium electrode.

Yamaji: If magnesium, abundantly found and safe, is widely used in batteries, there will be possibilities of changing the present energy supply of the world, I presume.

Mr. Suzuki: Personally I thank magnesium can replace petroleum in the future. We do not have lithium reserve in Japan. Countries that have rich lithium deposit are quite limited. As the utilization of lithium batteries become higher, the import price of lithium will also soar. We need to have something that will prove to be an effective alternative to lithium.

(Note: The interview included Dr. Kurihara’s explanations about a magnesium-ion secondary battery that he is now developing, which is quite informative, but it is excluded in this translation, as secondary battery is not the relevant topic.)