[Complete Battery Mastery]⑧ Oxidation and Reduction Explained for Liberal Arts Students

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How do batteries store and release electricity? The basic principle of a battery is to convert chemical energy into electrical energy. It is easy to think that energy or electricity is stored, but in reality, electrical energy is generated through chemical reactions occurring inside the cell. From the so-called 'Voltaic pile' made by Italian scientist Alessandro Volta in 1800 to the lithium-ion batteries widely used in electric vehicles today, all operate on the same principle. The difference lies in which materials are combined and how they are used in the cell, affecting performance. Even now, countless scientists tirelessly research day and night to find the optimal combination that produces high-performance and safe batteries.

Oxidation and Reduction: Losing and Gaining Electrons

To understand the principle of a battery, one must first grasp the concepts of oxidation and reduction. When a substance is broken down repeatedly, atoms are formed. Atoms consist of a nucleus and electrons orbiting around it. Under certain conditions, these electrons can leave the atom and move freely; these are called free electrons. The flow of electricity (current) in a battery is the phenomenon of electrons moving freely between the materials that make up the cathode and anode. The processes of oxidation and reduction explain this movement of electrons.

The classical meaning of oxidation and reduction is a substance gaining oxygen (oxidation) or losing oxygen (reduction). This concept evolved to include losing hydrogen (oxidation) and gaining hydrogen (reduction). More broadly, oxidation refers to losing electrons, and reduction refers to gaining electrons. The concept of 'oxidation number' numerically expresses the loss or gain of electrons to explain oxidation and reduction. An increase in oxidation number indicates oxidation, while a decrease indicates reduction.

Oxidation and reduction occur simultaneously in chemical bonds. Together, they are called redox reactions. When silver nitrate solution is immersed with copper metal, the solution turns blue, and silver metal precipitates (solid forms in liquid). Copper gives two electrons to the silver ions in silver nitrate and becomes copper ions that dissolve into the solution (oxidation). The silver ions in silver nitrate accept two electrons and become silver metal (reduction). This can be represented by the following chemical equation.

When copper reacts with oxygen, copper oxide (2CuO) forms; copper is oxidized by giving two electrons, and oxygen is reduced by accepting two electrons from copper. A substance that is reduced and causes another substance to be oxidized is called a reducing agent, while a substance that is oxidized and causes another substance to be reduced is called an oxidizing agent. In copper oxide, oxygen is the oxidizing agent, and copper is the reducing agent.

Batteries operate through these oxidation and reduction processes. A battery is mainly composed of a cathode and an anode. Together, the cathode and anode are called electrodes.

In a battery, the site where the reduction reaction occurs is defined as the cathode, and the site where the oxidation reaction occurs is defined as the anode. Electrons leave the anode (where oxidation occurs and electrons are lost), travel through a wire, and move to the cathode (where reduction occurs and electrons are gained).

Current is defined as moving in the opposite direction, from cathode to anode. The reason the direction of electron flow and current flow are opposite is that the electron flow was discovered later. Scientists initially defined current as flowing from cathode to anode after discovering electrical phenomena, but later it was found that electrons actually move from anode to cathode.

The first chemical battery, the Voltaic pile, can be explained by oxidation and reduction. The Voltaic pile uses a dilute sulfuric acid solution (H2SO4) with zinc (Zn) and copper (Cu) metal plates connected by a wire.

At this time, zinc metal undergoes oxidation by giving electrons, and copper undergoes reduction by receiving electrons. Electrons from zinc move through the wire toward the copper plate. Current flows in the opposite direction, from copper to zinc. This chemical reaction continues in the Voltaic pile, allowing current to flow.

Zinc that loses electrons becomes zinc ions (Zn2+), leaving the metal plate and dissolving into the sulfuric acid solution. Electrons that reach the cathode combine with hydrogen ions (H+) dissolved in the solution and are reduced to hydrogen gas (H2).

This can be expressed by the following chemical equation.

The substance that provides electrons in a battery is called the anode active material, and the substance that receives electrons is called the cathode active material. In the Voltaic pile, zinc is the anode active material, and hydrogen ions are the cathode active material. Also, hydrogen ions act as the oxidizing agent, and zinc acts as the reducing agent.

So, why is some material reduced and some oxidized? The determining factor is the ionization tendency. Depending on the ionization tendency, a material can serve as either an anode or a cathode in a battery. A material with a strong tendency to oxidize becomes the anode, and one with a strong tendency to reduce becomes the cathode material.

Ionization tendency refers to the property of metal substances to lose electrons and become cations (oxidation tendency). Scientists have calculated ionization tendencies based on the reactivity of substances and compiled them into a table. The mnemonic learned in middle and high school chemistry classes, 'KalKalNaMaAlAChulNiJuNapSuGuSuEukBaekGeum,' corresponds to this. It means the order of oxidation tendency is Potassium (K) > Calcium (Ca) > Sodium (Na) > Magnesium (Mg) > Aluminum (Al) > Zinc (Zn) > Iron (Fe) > Nickel (Ni) > Tin (Sn) > Lead (Pb) > Hydrogen (H) > Copper (Cu) > Mercury (Hg) > Silver (Ag) > Platinum (Pt) > Gold (Au). Moving left from hydrogen indicates stronger oxidation tendency, and moving right indicates stronger reduction tendency.

For example, when zinc (Zn) is placed in dilute sulfuric acid (2HCl), zinc, which has a higher ionization tendency than hydrogen, gives two electrons (oxidation). These electrons meet hydrogen ions (2H+) in sulfuric acid and produce hydrogen gas (H2) (reduction).

An ion is an atom or molecule that has gained or lost electrons and thus carries an electric charge. Losing electrons results in a cation, and gaining electrons results in an anion. The reason we distinguish lithium-ion batteries from lithium metal batteries is that lithium exists in the battery as lithium ions, not lithium metal.

How Does Voltage Arise? ... Standard Reduction Potential

When electrons move at the cathode or anode, electric potential energy is generated, called electric potential. Just as water at a higher place has more force to fall, a higher electric potential means electrons have a stronger force to move in the opposite direction. Electric potential varies depending on the metal's reactivity, i.e., ionization tendency.

Scientists set the electric potential between hydrogen and hydrogen ions under standard conditions as a reference and calculated the relative potentials of other metals. This is called the Standard Reduction Potential. Standard conditions mean a temperature of 25°C, pressure of 1 atmosphere (1 atm), and ion concentration of 1 mole (M). The unit of electric potential is volts (V). For example, zinc's standard reduction potential is -0.763 V, meaning it is 0.763 V lower than the potential between hydrogen and hydrogen ions (0 V).

If the standard reduction potential is positive (+), the substance reduces better than hydrogen ions under standard conditions; if negative (-), it reduces less well. Since standard reduction potential is expressed based on reduction properties, oxidation properties are understood oppositely. A larger negative (-) value means the substance oxidizes more easily (loses electrons readily).

Lithium's standard reduction potential is -3.045. Lithium is a metal that oxidizes very easily. Because of this property, early battery research actively pursued using lithium metal as the anode. However, later, Dr. Akira Yoshino from Japan used a material called lithium cobalt oxide (LCO), developed by Professor John Goodenough, as the cathode material, leading to the birth of the lithium-ion battery. This achievement was the basis for their joint Nobel Prize in Chemistry in 2019.

Standard reduction potential measures only half of the electrode and is called half-cell potential. When two materials with different potentials meet, a potential difference occurs, called electromotive force (EMF). The Chinese characters for electromotive force mean 'the force that generates electricity.' It works like water flowing from a higher place to a lower place. The unit of EMF is also volts (V). In batteries, EMF (voltage) arises from the potential difference between the cathode and anode.

The electromotive force (E) of a battery is calculated as 'standard reduction potential of cathode - standard reduction potential of anode.' This is similar to calculating the total number of floors from the top floor to the basement in a building.

In the Voltaic pile, hydrogen ions (0 V) effectively act as the cathode active material, so the EMF is 0 - (-0.763) = 0.763 V. Theoretically, using metals with a large potential difference as cathode and anode materials can yield high voltage.

Voltaic pile EMF (E) = 0 - (-0.763) = 0.763 V

Voltaic Pile and Daniell Cell

In 1794, Italian physicist Alessandro Volta discovered that stacking silver and zinc plates with saltwater-soaked paper in between and connecting the two metals with a copper wire caused current to flow. This was called Volta's pile. In 1800, Volta improved the pile by using copper and zinc plates as electrodes and dilute sulfuric acid instead of saltwater, inventing the Voltaic pile. It is known as the world's first chemical battery. Volta was honored by Napoleon with a noble title for his contributions to electrical research, and the unit of voltage, the volt (V), was named after him.

Despite its scientific achievement, the Voltaic pile had practical difficulties due to the 'polarization phenomenon,' where hydrogen gas adhered to the copper plate at the cathode. The first person to improve the Voltaic pile and create the world's first practical chemical battery was British chemist John Frederic Daniell. In 1836, Daniell made a battery by placing a zinc electrode in a porous earthenware container filled with sulfuric acid solution, which was then floated in a copper pot filled with copper sulfate solution. The earthenware container had tiny holes allowing ions generated in each solution to move between containers. The Daniell cell was more stable and longer-lasting than the Voltaic pile and became the basis for modern primary batteries. The EMF of the Daniell cell is 1.1 V.

The actual appearance of a voltaic cell. Photo by the Ministry of Education official blog

The early form of the battery developed in 1836 by British chemist John Frederic Daniell. It is exhibited at the National Museum of American History in Washington DC. Photo by Wikimedia