- Magnesium Battery Definition: A magnesium battery is a primary battery that uses magnesium as the anode material due to its high potential and cost-effectiveness.
- Chemistry Components: The battery uses a magnesium alloy anode, manganese dioxide cathode mixed with acetylene black for conductivity, and magnesium perchlorate electrolyte with additives for corrosion prevention.
- Construction: Magnesium batteries have a similar construction to zinc-carbon batteries but use magnesium alloy containers and need careful sealing to manage moisture and hydrogen gas.
- Advantages: These batteries offer good shelf life, high capacity, and higher voltage compared to zinc-carbon batteries.
- Disadvantages: They face issues like voltage delay, hydrogen evolution during discharge, heat generation, and poor storage after partial discharge.
Magnesium is used as an anode material in primary battery due to its high standard potential. It is a light and low-cost metal. The magnesium/manganese dioxide (Mg/MnO2) battery has double the capacity of the zinc/manganese dioxide (Zn/MnO2) battery of the same size. It can retain its capacity even during storage at high temperatures. Magnesium batteries are durable and storable because they have a natural protective cover on the surface of the magnesium anode.
Magnesium batteries lose their storability after partial discharge, making them unsuitable for long-term intermittent use. This is why they are losing popularity, with lithium batteries taking over their market share.
Chemistry of Magnesium Battery
In primary magnesium batteries, a magnesium alloy is used as the anode and manganese dioxide as the cathode material. Manganese dioxide lacks the necessary conductivity, so acetylene black is added to improve it. Magnesium perchlorate serves as the electrolyte, with barium and lithium chromate added to prevent corrosion. Magnesium hydroxide is also included as a buffering agent to improve storability.
The oxidation reaction which occurs in the anode is,
The reduction reaction which occurs in the cathode is,
Overall reaction,
The open circuit voltage, this cell gives around 2 volt but the theoretical value of the cell potential is 2.8 volt.
The chance of corrosion of magnesium is very less even under extreme environmental conditions. Raw magnesium reacts with moisture and forms a coating of thin film of Mg(OH)2 on its surface.
This thin film of magnesium peroxide serves as a corrosion protective layer over the magnesium. In addition to that chromate treatment on magnesium improves this protection to a very large extent. But when this protective film of magnesium peroxide is punchered or removed due to the discharge of the battery, corrosion takes place with the formation of hydrogen gas.
This is the basic chemistry of magnesium battery.
Construction of Magnesium Battery
Construction wise a cylindrical magnesium battery cell is similar to a cylindrical zinc-carbon battery cell. Here an alloy of magnesium is used as the main container of the battery. This alloy is formed by magnesium and a small quantity of aluminum and zinc. Here, manganese dioxide is used as cathode material. As the manganese dioxide has poor conductivity, acetylene black is mixed with this to improve its conductivity. This also helps to retain water inside cathode. In this cathode mixture barium chromate is added as an inhibitor, and also magnesium hydroxide is added as a pH buffer. Magnesium perchlorate with lithium chromate mixed with water is used as the electrolyte. Carbon is inserted into the cathode mix as a current collector. Kraft papers, absorbed with electrolyte solution are placed in between cathode and anode materials as separators. Special attention is to be given during designing of sealing arrangement in magnesium battery. The sealing of the battery should not be so porous that the moisture inside the battery will be evaporated during storing of the battery and it should not be so nonporous that the hydrogen gas formed during discharge cannot escape from the battery. So the seal of the battery should retain the moisture inside it, and at the same time, it gives sufficient vent to the hydrogen gas formed during discharge. This can be done by providing a small hole on the top of the plastic seal washed under the Retainer ring. When excess gas comes out from the hole, this retainer ring is deformed due to pressure and resulting escaping from the gas.
Magnesium anode forms the outer cover of the battery, but another construction of magnesium battery is also available where carbon forms the outer container of the battery. Here a typically shaped container is formed from highly conductive carbon. This container is formed in a cylindrical cup shape, and one rod-like shape is projected from its center as shown in the picture. Anode of the battery is formed by a cylinder or drum of magnesium. The diameter of the cylindrical anode is about half of the carbon cup. The cathode mix is placed inside this anode cylinder and separated from the inner wall of the cylinder by a paper separator. The space between the inner surface of the carbon cup and outer surface of the anode cylinder also filled with cathode mix and here also the outer surface of the anode cylinder is separated from cathode mix by a paper separator. The cathode mix is produced by mixing manganese dioxide, carbon black, and small quantity of aqueous magnesium bromide or perchlorate as the electrolyte. The positive terminal is connected to the end of the carbon cup. The negative terminal is connected to the end of anode drum. The entire system is encapsulated in a crimped tin-plated steel jacket.
Advantage of Magnesium Battery
- It has a very good self life; it can be stored for a long time even under high-temperature. These battery can be stored up to 5 years at the temperature 20oC.
- It has twice capacity compared to equivalent size Leclanche battery.
- Higher battery voltage than zinc-carbon battery.
- Cost is also moderate.
Disadvantages of Magnesium Battery
- Delayed action.(voltage delay)
- Evolution of hydrogen during discharge.
- Heat generated during use.
- Poor storage after partial discharge.
The battery are no longer manufactured commercially.
Sizes And Types Of Mg/Mno2 Batteries
| Battery type | Diameter in mm | Height in mm | Weight in gm | Capacity in Ah |
| N | 11 | 31 | 5 | 0.5 |
| B | 19.2 | 53 | 26.5 | 2 |
| C | 25.4 | 49.7 | 45 | 3 |
| 1LM | 22.8 | 84.2 | 59 | 4.5 |
| D | 33.6 | 60.5 | 105 | 7 |
| FD | 41.7 | 49.1 | 125 | 8 |
| No. 6 | 63.5 | 159 | 1000 | 65 |
