Murata experts take you to understand the working principle and characteristics of lithium-ion batteries

[Introduction]Lithium-ion batteries are not only used as rechargeable batteries for computers and smartphones, but are also widely used in various fields such as electrical products, electric vehicles, industrial machinery, and cutting-edge technology in our lives. Why is it used by so many products? What changes has it brought to our lives? As well as future development trends, let us take a look at it based on the insights of experts~

In this series, we will give a comprehensive introduction in five lectures on the characteristics, usage scenarios, history and future possibilities of lithium-ion batteries, under the supervision of Mr. Ryoji Kanno, a special professor of Tokyo Institute of Technology.

Supervisor: Ryoji Kanno

Distinguished Professor (Honorary Professor), Institute of Science and Technology Creation, Tokyo Institute of Technology

Mr. Ryoji Kanno graduated from the Inorganic and Physical Chemistry major of the Graduate School of Science, Osaka University in 1980. In 1985, he became a doctor of science. After serving as an associate professor in the Faculty of Science, Kobe University, in 2001, he was appointed as a professor at the Graduate School of Integrated Science and Technology, Tokyo Institute of Technology. Team leader, appointed as a Distinguished Professor and Director of the All-Solid-State Battery Research Center of the Institute of Science and Technology Creation in 2021.

1. What is a lithium-ion battery?

Lithium-ion batteries are rechargeable batteries that are installed in devices such as smartphones and laptops that we use every day. The prototype of the battery was invented at the end of the 18th century, and it has been developed for more than 200 years since then. Lithium-ion batteries are one of the newest types of batteries born in the battery development process.

Cylindrical lithium-ion battery

Laminated lithium-ion battery

Features of Lithium Ion Batteries:

Batteries are divided into “primary batteries” that can only be used once and “secondary batteries” that can be used multiple times. Lithium-ion batteries are rechargeable secondary batteries. Compared with other types of batteries, they are not only small and lightweight, but also can store high electrical energy.

2. The working principle of lithium-ion battery to generate electricity

In addition to lithium-ion batteries, there are various other types of batteries, and in fact the basic working principle of the battery to generate electricity is the same.

A battery has a positive electrode (positive electrode) and a negative electrode (negative electrode) using metal materials, and a substance (electrolyte) that conducts electricity by ions is filled between the positive and negative electrodes. The metal electrode is melted by the electrolyte and divided into ions and electrons. The electrons move from the negative electrode to the positive electrode to generate current, and then electricity is generated. The secondary battery is charged before starting to use the battery, and electrons are stored in the negative electrode in advance, and the stored electrons move to the positive electrode when the battery is used to generate electricity.

In a lithium-ion battery, a lithium-containing metal compound is previously used for the positive electrode, and carbon (graphite) capable of absorbing and storing lithium is used for the negative electrode. Through such a structure, electricity can be generated without the need to melt electrodes with electrolytes as in traditional batteries, thus slowing down the aging of the battery itself, not only saving more electricity, but also increasing the number of charging and discharging times. In addition, lithium is a very small and light substance, which enables the battery to have various advantages such as small size and light weight.

How Lithium-Ion Batteries Generate Electric Current

3. Are there different types of Li-ion batteries?

Lithium-ion batteries are classified into several types according to the metal materials used in the positive electrode. The metal material originally used for the positive electrode of lithium-ion batteries was cobalt. However, cobalt is produced in almost as little as lithium, and is also a rare metal with high manufacturing costs. Therefore, materials such as manganese, nickel, iron, etc., which are inexpensive and low in environmental impact, have been used. Lithium-ion batteries are classified by the materials they use.

Types and characteristics of lithium-ion batteries

Let’s take a look at the characteristics of each category.

Cobalt Lithium Ion Batteries

The positive electrode uses lithium cobalt oxide. Lithium cobalt oxide is relatively easy to synthesize and easy to use, so the earliest mass production of lithium ion batteries is lithium cobalt oxide lithium ion batteries. But because cobalt is a rare metal and expensive, it is hardly used in auto parts.

Manganese lithium ion battery

The positive electrode uses lithium manganate. The advantage is that the voltage can be similar to that of cobalt-based lithium-ion batteries, and the manufacturing cost is cheap. The disadvantage is that manganese may be melted into the electrolyte during charging and discharging, shortening the life of the battery.

Iron Phosphate Lithium Ion Batteries

The positive electrode uses lithium iron phosphate. The advantages of iron phosphate-based lithium-ion batteries are that even if the internal heat generating structure is difficult to damage, the safety is high, and the production cost is lower than that of manganese-based batteries by using iron as a raw material. But the voltage is lower than other lithium-ion batteries.

Ternary lithium-ion battery

The ternary lithium-ion battery is a battery made of cobalt, nickel and manganese in order to reduce the amount of cobalt. At present, most of the ternary lithium-ion batteries have a higher proportion of nickel. Although the voltage is slightly lower than that of cobalt-based and manganese-based systems, manufacturing costs can be reduced. Even so, the synthesis and preparation of each material is difficult and the stability is low, and there are still problems to be solved as practical materials.

4. What is the difference between lead-acid batteries and lithium-ion batteries?

In addition to lithium-ion batteries, there are several types of rechargeable batteries. Among them, lead-acid batteries are batteries with a long history that have been used more than 100 years ago, and they are still used as batteries for automobiles even after the development of new types of batteries such as lithium-ion batteries.

The difference between lead battery and lithium ion battery

Both the positive and negative electrodes of lead-acid batteries use lead, so the manufacturing cost is very cheap compared to lithium-ion batteries. But since lead is heavier than other metals, the battery itself is also heavier. In addition, the maximum voltage can only reach 2V, and the large self-discharge is also a disadvantage of lead-acid batteries.

The reason why lead-acid batteries are still used

Although the lead-acid battery has these shortcomings, the reason why the battery of the automobile is not replaced with a high-performance secondary battery such as a lithium-ion battery is that it is cheap, the technology is basically mature, and the reliability is high. Automobiles take full advantage of the characteristics of lead-acid batteries and establish a recirculation system. If it is replaced with a new type of battery, the circuit design needs to be modified. Based on the current situation that the lead-acid battery is still enough to function, the manufacturer does not want to pay extra.

Comparison of lead-acid batteries and lithium-ion batteries

Having said that, lithium-ion batteries are already used as secondary batteries for driving engines of electric vehicles, electric hybrid vehicles, etc., and there is a possibility that lead-acid batteries will not be used in cars in the future.

5. Where are lithium-ion batteries used?

In the first half of the 1990s, the first commercialized lithium-ion batteries for home appliances were used in video cameras to meet their needs for small size and weight. Since then, it has been continuously used in mobile phones that were rapidly popularized at that time, and the market demand has skyrocketed instantly.

Today, lithium-ion batteries have been widely used in various scenarios in our lives, such as smartphones, laptops, and electric vehicles, electric bicycles and other fields.

6. How safe are lithium-ion batteries?

In fact, batteries can be said to be energy cans. Lithium-ion batteries, which can store energy at a high density per unit volume, require more safety considerations than other types of batteries. And from the point of view of using flammable organic solvents, it is also necessary to be more careful in operation and use than other batteries that use aqueous solutions.

The most important thing to avoid is the internal short circuit problem. Internal short circuit refers to the state in which the battery is deformed by external force, and the positive electrode and the negative electrode are in direct contact. The current is concentrated here, causing serious accidents such as temperature rise, battery fire, etc. Even very small impurities, once mixed into the battery, may cause an internal short circuit, which requires designing a protection circuit so that there is no excess current flowing in the battery, so that the battery has an accident-proof function.

Others include the use of cooling devices and other means to keep the working environment of the battery below 60°C, and it is also important to control the temperature of the battery. A diaphragm is set between the positive and negative electrodes. If the temperature exceeds a certain temperature, the diaphragm acts as a complete barrier between the positive and negative electrodes, etc., using various methods to improve safety.

As described in this lecture, it seems that small, lightweight and high-performance lithium-ion batteries will continue to be used in various scenarios in our lives in the future compared to previous secondary batteries.