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+2𝑶𝑯− →ZnO+H2O+2𝒆−
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.
𝟏
O2+ H2O+2𝒆− →2𝑶𝑯−
𝟐
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+3𝑶𝑯− →Al(OH)3+3𝒆−
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+4𝒆− →4𝑶𝑯−
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