In this blog post, let’s explore the basic operating principles and components of lithium-ion batteries—which are widely used in various devices such as smartphones and electric vehicles—as well as their charging and discharging processes and their characteristics compared to other rechargeable batteries.
One of the things many people pay attention to when using smartphones frequently is battery capacity. Anyone who has ever used a cell phone has likely heard at least once that its battery is a lithium-ion battery. Lithium-ion batteries have relatively high energy density and excellent charge and discharge characteristics, making them widely used in various portable electronic devices, including smartphones. So, let’s explore how lithium-ion batteries—which are lightweight and capable of storing large amounts of energy—actually work.
A lithium-ion battery is a type of chemical cell that utilizes oxidation-reduction reactions. Oxidation-reduction reactions can be explained by the movement of electrons. When electrons are gained, a reduction occurs; when electrons are lost, an oxidation occurs. Oxidation and reduction do not occur separately but are interconnected; therefore, when an oxidation reaction occurs on one side, a reduction reaction occurs simultaneously on the other side. A common example of an oxidation-reduction reaction we see in everyday life is the corrosion of iron, or rusting.
Before examining the exact principles of lithium-ion batteries, let’s first explain chemical batteries: a chemical battery is a device that converts chemical energy into electrical energy using oxidation-reduction reactions. The dry cell batteries commonly found around us are also a type of chemical cell. When two electrodes with different reactivity are connected to an ion-conductive substance called an electrolyte, an oxidation reaction occurs at the more reactive electrode, causing electrons to move through an external circuit to the other electrode and generating an electric current. The electrode where the oxidation reaction occurs—releasing electrons—is called the cathode, while the electrode that accepts electrons and undergoes a reduction reaction is called the anode. However, since the direction of the reactions occurring at the electrodes can vary depending on whether the battery is charged or not, it is important to understand the electrodes based on oxidation and reduction reactions. For example, in a battery using zinc and copper, zinc releases electrons and is oxidized to zinc ions (Zn²⁺), so it can be said to be a substance that oxidizes more easily than copper. The electrons released by zinc travel along the wire to the copper electrode.
Lithium-ion batteries also fundamentally utilize these electrochemical principles, but they differ from typical voltaic cells in both their operating mechanism and structure. A lithium-ion battery consists of a cathode, an anode, an electrolyte, a separator, and other components. In various lithium-ion batteries, both past and present, the cathode uses a variety of materials, including lithium cobalt oxide (LiCoO₂), nickel-manganese-cobalt-based oxides, and lithium iron phosphate (LiFePO₄), while graphite is the most commonly used material for the anode. The electrolyte allows lithium ions to move between the cathode and anode, while the separator prevents the cathode and anode from coming into direct contact, thereby blocking the flow of electrons within the cell. The separator is designed with a porous structure featuring microscopic pores that allow lithium ions to pass through.
When a lithium-ion battery is charged, electrical energy is supplied from an external source, causing lithium ions to be released from the cathode. These ions travel through the electrolyte and separator to the anode, where they are stored between the layers of the anode material, such as graphite. At this time, electrons flow to the anode through the external circuit. Conversely, when a lithium-ion battery is discharged, the lithium ions stored at the anode move toward the cathode, and electrons flow from the anode to the cathode through the external circuit, generating an electric current. In other words, the direction of lithium-ion movement is opposite during charging and discharging. The storage and release of electrical energy occur as lithium ions and electrons move in opposite directions through the electrolyte and the external circuit, respectively.
This ability to be reused repeatedly through charging and discharging cycles is a key feature of lithium-ion batteries. However, the battery’s performance does not remain unchanged indefinitely simply because it undergoes repeated charging and discharging cycles. During repeated charging and discharging cycles, various chemical changes occur at the interface between the electrode and the electrolyte, and the electrode structure may also change. Consequently, over time, the amount of energy that can be stored in a single charge decreases, and the battery’s lifespan shortens. Therefore, it is a natural phenomenon for the battery’s performance to gradually degrade with each repeated charge and discharge cycle.
Although lithium-ion batteries were developed based on the same electrochemical principles as conventional chemical batteries, they differ in that they store and release energy by having lithium ions move between the structures of the electrode materials, rather than through the consumption of the electrode materials themselves as they react. A typical example of the chemical batteries described above is the Voltaic cell. In a Voltaic cell, the material in the zinc plate is consumed during use, so it generally cannot be reused after a single use. In contrast, lithium-ion batteries are rechargeable secondary batteries because lithium ions can return to their original electrode during the charging process. Of course, lithium-ion batteries are not the only type of rechargeable secondary battery. There are also nickel-cadmium batteries and nickel-metal hydride batteries, among others, and each type of battery has its own distinct characteristics and applications. Among these, lithium-ion batteries are widely used in various fields—such as portable electronic devices, electric vehicles, and energy storage systems—due to their advantages, including high energy density and relatively light weight.
As such, lithium-ion batteries have been widely used in various portable devices, such as smartphones, based on their advantages of high energy density and charging/discharging characteristics, and their range of applications has now expanded further to include electric vehicles and large-scale energy storage systems. In particular, lithium-ion batteries offer the advantage of being able to store a relatively large amount of energy by leveraging the properties of lithium and the structure of electrode materials, and they have established themselves as a key technology in batteries for electric vehicles. However, to achieve higher energy density, longer lifespan, faster charging, and improved safety, continuous research and improvement are needed not only for the cathode and anode but also for various other components of the battery, such as the electrolyte and separator. In fact, various materials—such as nickel-manganese-cobalt-based cathode materials, lithium iron phosphate cathode materials, and silicon-based anode materials—are currently being developed, and research continues to create next-generation energy storage technologies that are safer and have a longer service life. Therefore, although lithium-ion batteries are already a widely commercialized technology, there remains room for further improvement in performance and safety, and they are expected to continue playing a vital role across various industries and technological fields in the future.
