Term paper Metal air batteries (Zn-Air and Al –Air battery) by RAM KESHAV M (SE21MEEE005) Introduction: Modern day society is currently in a transition phase from a fossil fuel based economy to the clean energy alternatives required to minimize environmental pollution.To minimize environmental pollution, a battery-powered vehicle enters the picture. An electric car's main heart is a battery, which will provide energy to power the vehicle. Batteries are made up of two electrodes that transform chemical energy into electrical energy. Batteries can be used for both storage and as a power source. So far, we've encountered lead-acid, lithium-ion, sodium-based batteries, and nickel-based batteries. Each of these batteries has its own set of benefits and drawbacks. Li-ion batteries have high specific energy and power when compared to other types of batteries. With the help of the Li-ion battery, we can stay on the road for a long time and accelerate quickly. Metal air batteries are a novel form of battery that has been produced through research. As a result of research, metal-air batteries that provide a relatively higher specific energy than Li-ion batteries have been developed. Metal air batteries serve as a link between primary and secondary cells. These batteries are not rechargeable, but they can be mechanically recharged by replacing the metal electrode. Metal air batteries are made up of a metal anode, an air-breathing cathode, and a suitable electrolyte for conduction. They are divided into lithium-air, zinc-air, al-air, and magnesium-air batteries based on the metal employed in the anode. Before diving into the intricacies of construction, functioning advantages, and downsides of these batteries. Since lithium based batteries are dominating the global EV market, let's take a look at Li-air batteries . These batteries have high energy density and open-circuit voltage are thought to be superior to other forms of batteries. Nonetheless, it has significant drawbacks due to porous carbon blocking cathodes by discharge products, lithium instability in humid conditions, and side products (lithium alkyl carbonates). These drawbacks will have an impact on the charging and discharging cycles of lithium-air batteries. This paper will go over zinc-air and aluminumair batteries in detail. Zinc-Air batteries: Of all the metal-air batteries, zinc-air batteries are the most commonly used because of their safety and cost. Zinc-air batteries can be recycled. Zinc availability is not a major problem since zinc reserves are high around the world, with the current largest producers being China, Australia, Canada, and the USA. Zinc-air batteries consist of two electrodes. The anode is made of zinc metal, whereas the cathode is made of porous and carbonaceous materials to allow atmospheric air. The electrolyte is potassium hydroxide (KOH). The electrolyte is pumped from an external electrolyte tank at a controlled flow rate to the battery to suppress anode passivation. Figure1: Construction of Zn-air cell Working: At the anode: When the battery is discharging the anode undergoes oxidation, it releases two electrons. The total reaction takes placed in the anode are: Zn+2πΆπ―− →ZnO+H2O+2π− At the cathode: In the cathode, oxygen from the air reacts with the electrons released in the anode through an external circuit and is reduced to hydroxide ions, During discharging cathode sees a reduction or gain electrons from the anode. π O2+ H2O+2π− →2πΆπ―− π The resultant product obtained at the cathode is hydroxyl ions. These hydroxyl ions again combine with the anode material and this process continues until discharging process happens. These are the construction and working of Zinc air batteries. Let’s look into the performance metrics of these batteries. Zinc-Air battery parameters: Specific energy 1370 WH/kg (theoretical), 470 WH/kg(practical) Specific power 100 w/kg Nominal cell voltage 1.65V theoratical,1.35-1.42 practical Table1: Important parameters of Zinc-Air battery Packaging: Zinc–air batteries should not be used in a sealed battery holder. Zinc air cells have longer shelf life .The sealed miniature cells can be stored for three years at room temperature when it is not in use. Miniature cells have high self –discharge. Operating life: The operating life of zinc air battery is based on the environmental conditions.If the battery is exposed to high temperature,the electrolyte loses water immediately. Since the electrolyte used in Zinc air battery is deliquescent in nature,in very humid conditions these electrloytes will have more water concentration which destroys the active properties of the cell. Potassium hydroxide reacts with atmospheric carbon-dioxide forms a carbonate formation which reduces conductivity. Discharge profile: During discharge, terminal voltage is quite constant upto the 90 percent of capacity removed. The terminal voltage is constant because cathode does not change its properties. These are the performance metrics of Zn-Air battery. Let’s see what are its advantages ,disadvantages and its applications. Advantages: 1. The combination of both primary and secondary batteries 2. High energy density 3. Inexpensive materials 4. Available in a range of button and coin cell size. 5. No electricity is required for charging these cells Disadvantages: 1. Low specific power 2. They corrode easily 3. Evolution of hydrogen gas Applications: 1. Vehicle propulsion 2. Grid storage 3. Hearing aids Aluminum-air batteries: It is a battery which is used as aluminum as anode and oxygen in the air is used as a cathode. Energy produced in this battery is comparatively high compared to other batteries. Electrolyte is Potassium hydroxide(KOH) Fig2: Construction of Aluminium –Air battery Working: At the anode: When the battery is discharging the anode undergoes oxidation, it releases two electrons. The total reaction takes placed in the anode are: Al+3πΆπ―− →Al(OH)3+3π− At the cathode: In the cathode, oxygen from the air reacts with the electrons released in the anode through an external circuit and is reduced to hydroxide ions, During discharging cathode sees a reduction or gain electrons from the anode. O2+2H2O+4π− →4πΆπ―− The resultant product obtained at the cathode is hydroxyl ions. These hydroxyl ions again combine with the anode material and this process continues until discharging process happens. These are the construction and working of Aluminium- Air batteries air batteries. Let’s look into the performance metrics of this batteries. Aluminum-Air battery parameters: Specific energy 1300 WH/kg(practical), 6000-8000WH/kg(theoretical) Specific power Nominal cell voltage 200w/kg 1.2V Table 2: Parameters of Aluminium-Air battery Discharge profile of Aluminum–Air battery: Fig-3: Discharge profile of Aluminium-Air battery The above figure shows the discharge performance of the Aluminium-air battery with 1 M of KOH. It indicated that the battery discharge with 10 mA took a longer time to become completely exhausted. It could last for about 1 h and 36 min before the battery dried out. The discharge duration is inversely proportional to the discharge current. At a discharge current of 50 mA, the battery could last for about 20 min only. During the discharge process, the aluminium and hydroxyl ions will be consumed and lead to a reduction of battery voltage. Operating life: Traditional Al–air batteries had a limited shelf life because the aluminium reacted with the electrolyte and produced hydrogen when the battery was not in use. The problem can be avoided by storing the electrolyte in a tank outside the battery and transferring it to the battery when it is required for use. Experimental setup of Aluminium–Air battery: I have made an Al-air battery in which I have stacked an aluminium sheets of 10 micron thickness which acts as an anode ,cathode I have used a Graphite powder which is a carbon based material to allow air to pass through to react with aluminium. Electrolyte which I have used in this experiment is saltwater for conduction. The open circuit voltage which comes to be 0.736V. Fig 4: Experimental setup of Aluminium-air battery Output Voltages at Different time stamps: Time(mins) 0 5 10 15 20 25 30 Output voltage 0.736 0.723 0.712 0.705 0.704 0.702 0.698 DISCHARGE PROFILE: The above graph is plotted against Open circuit voltage (volts) (vs) Time (mins). From this graph, my observation is that terminal voltage is constant for a period of time. SOC measurement is difficult for these batteries. Advantages: 1. These batteries have high energy density 2. These batteries are light in weight 3. Good recyclability 4. Safety, 5. Abundance of raw materials. 6. Long range. Disadvantages: 1. Corrosion of Aluminum metals. 2. They cannot be recharged like Li-ion batteries. 3. Battery swapping stations are needed. Applications: 1. Reserve batteries in telephone exchanges and as backup power sources. 2. These batteries are used in Marine applications. 3. These batteries are used in Military applications Present Challenges There are several challenges which make the practicality of metal-air batteries very difficult. Three of these main challenges has been mentioned here. First the main challenge is the metal anodes react with the electrolyte to form a passivation layer called solid electrolyte interphase (SEI) film. Second, the reason behind internal short circuit in batteries which lead to explosion is the dendrite growth on anodes. Third, it is difficult to find an electrolyte with all desired properties which include high stability, low volatility, non-toxicity and high oxygen solubility and wide electrochemical window Conclusion: The high energy density of metal-air batteries makes them desirable for electrification of vehicles and storing energy from renewable sources, which are only available intermittently. However, there are several limitations which have to be resolved before replacing the currently used Li-ion batteries, which has about ten times less theoretical energy density. These limitations include corrosion of metals, evolving of hydrogen gas, carbonate formation etc. Currently, there is a great deal research being undertaken by scientists around the world to better understand these limitations and devise solutions. References: 1. https://link.springer.com/article/10.1007/s40243-014-0028-3 2. https://www.sciencedirect.com/science/article/pii/S246802571730081X 3. https://en.wikipedia.org/wiki/Zinc%E2%80%93air_battery 4. https://en.wikipedia.org/wiki/Aluminium%E2%80%93air_battery