How does an electric battery work and what are the different types? (GS Paper 3, Science and Technology)
Why in news?
- Electric batteries hold and release electrical energy that they have acquired by converting other forms of energy.
- As portable sources of electric power, batteries are at the foundation of what convenience means today in industrialised societies.
- Innovations to improve the efficiency will determine what such convenience as well as sustainability mean tomorrow.
What led to the invention of the electric battery?
- All chemical reactions are fundamentally about how the electrons in the bonds between atoms are rearranged. The bridge between this fact and the electrochemical cells that were the precursors of modern batteries is most apparent in an experiment that Luigi Galvani conducted in 1780.
- He discovered that the muscles of dead frogs' legs twitched when struck by an electrical spark.
- Alessandro Volta invented the first electric pile, the forerunner of the modern battery in 1800. This cell consisted of copper and zinc plates arranged in alternating fashion, separated by sheets of paper soaked in salty water. He found that this set-up could produce a steady current for some time but, like Galvani, couldn’t explain why.
- The British chemist John Daniel improved on Volta’s design: he filled a copper pot with copper sulphate, and inside that placed an earthen pot containing a zinc electrode sitting inside sulphuric acid. This more sophisticated set-up could produce an electric current for an even longer duration.
- Then, in the early 19th century, Michael Faraday worked out why these cells worked the way they did, and named its various components (anode, cathode, electrolyte, etc.).
What is an electric battery?
- A voltaic, or galvanic, cell uses redox reactions to produce an electric current.
- The cell consists of two half-cells. Each half-cell is made of a metal electrode immersed in an electrolyte of that metal, say, a zinc electrode in zinc sulphate and a copper electrode in copper sulphate.
- The two metal electrodes are connected by a wire and the two tubs of electrolyte are connected by a salt bridge (a material that conducts ions while remaining electrically neutral).
- In the zinc half-cell, zinc ions (Zn2+) from the electrode dissolve in the zinc sulphate, releasing two electrons into the electrode. In the copper half-cell, the reverse happens: copper ions (Cu2+) from the copper sulphate deposit onto the electrode, which now requires two electrons. So the wire connecting the electrodes transports two electrons from the zinc to the copper electrode.
- Similarly, the salt bridge connecting the two electrolytes allows the Zn2+ and the sulphate (SO42-) ions to meet and exchange electrons.
- Since the wire connecting the zinc to the copper electrodes carries electrons, an external circuit connected to it can draw the electron flow for various applications.
What are the concepts of a battery?
- The cathode is the positively charged electrode, the one to which electrons arrive (copper in the example above). The anode is the negatively charged electrode, which ‘supplies’ electrons.
- In an oxidation reaction, electrons are released, and in a reduction reaction they are consumed. So zinc oxidises at the anode and copper reduces at the cathode. This is the redox reaction at the heart of every electrochemical cell.
- The energy imparted to the electrons by the half-cells is called the source voltage (previously called the electromotive force). The terminal voltage is like a driving force that pushes the electrons from the anode to the cathode.
- In ideal conditions, the source voltage is equal to the terminal voltage. The higher the source voltage, the greater the cell’s electrochemical potential. For example, nickel-cadmium batteries and zinc-copper cells have voltages of 1.2 V and 1.5 V respectively, whereas lithium-ion cells have more than 3 V.
- A well-known problem that degrades the performance of electrochemical cells is corrosion. For example, in humid conditions, water droplets can condense on the electrodes.
- If atmospheric carbon dioxide levels are high, the water can combine with the gas to produce carbonic acid, which can corrode the electrode.
Galvanic corrosion:
- Another source is galvanic corrosion, whereby one of the electrodes in a cell dissolves faster into the electrolyte over time because it is more reactive, before the less reactive electrode starts to erode. For example, in a (non-rechargeable) carbon-zinc battery, zinc erodes preferentially as the battery is used.
- In the late 18th century, the British Navy found that in wooden ships with copper plating nailed to the hull (for protection), the plating was intact but had become detached from the hull because the iron nails had turned to paste.
- This is because the nails had preferentially eroded due to galvanic corrosion, with sea water as electrolyte.
What are the types of batteries?
- Two batteries that are often on the news these days are the lithium-ion (Li-ion) battery and the batteries used in Electric Vehicles (EVs).
- The Li-ion battery won the developers of its foundational principles the Nobel Prize for chemistry in 2019. This battery is a voltaic as well as an electrolytic cell.
- A voltaic cell converts chemical energy to electrical energy. An electrolytic cell converts electrical energy to chemical energy. A battery that can do both is thus rechargeable.
- In a Li-ion polymer cell, used in smartphones, a lithium metal oxide is the cathode and graphite is the anode. The electrolyte is a semisolid polymer gel. Microporous polyethylene is used to separate the two half-cells.
Voltaic phase:
- In the voltaic phase, lithium oxidises to Li+ in the anode and releases an electron. The electron moves via the external circuit to the cathode whereas the Li+ moves via the electrolyte to the cathode.
- There, the ion slips between the layers of carbon sheets that graphite is made of in a process called intercalation.
Electrolytic phase:
- In the electrolytic phase, an over-voltage is applied to the cell so that it charges and the Li+ moves from the graphite to intercalate in the metal oxide, getting ready for the next discharge.
EVs:
- Li-ion batteries are an important research focus worldwide, with a large variety of batteries with different configurations and different pros and cons. These batteries can also be used to power EVs.
- For example, the P85 battery used in Tesla’s Model S cars consists of 18,650 Li-ion cells, weighing half a tonne in all with an energy output of 80-90 kWh.
Hydrogen fuel cells:
- Hydrogen fuel cells are also of great interest today. At the anode, a catalyst separates hydrogen into protons and electrons. The electrons flow through an external circuit and the protons through the electrolyte – both to the cathode.
- At the cathode, the particles react with oxygen from the air to create heat and water. A cell like this will work as long as hydrogen is supplied, and is expected to be a key component of the hydrogen economy.
National Green Hydrogen Mission:
- In January 2023, the Indian government approved the ₹19,744 crore National Green Hydrogen Mission to make India a “global hub” to utilise and export green hydrogen.
