What is an all-solid-state battery? How likely is it to be practical?

“Another highly anticipated use for all-solid-state batteries is in electric vehicles. Lithium-ion batteries are currently used in electric vehicles, and all-solid-state batteries are expected to reduce risks such as fires caused by accidents because they do not contain flammable organic solvents. In addition, today’s electric vehicles take longer to charge than gasoline, and can be charged more quickly if they use an all-solid-state battery.

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Lithium-ion batteries that change our lives Lecture 4: What is an all-solid-state battery? How likely is it to be practical?

In this series of articles, under the supervision of Mr. Ryoji Kanno, a special professor of Tokyo Institute of Technology who has been engaged in research on improving battery performance for more than 30 years, we have studied from what is a lithium-ion battery to an all-solid-state battery called the next-generation lithium-ion battery. situation, make a comprehensive introduction to the topic.

The fourth lecture focuses on “all-solid-state batteries” called “next-generation batteries”, which have similar characteristics to lithium-ion batteries, and we will talk about their differences from current lithium-ion batteries, their envisaged uses, and their move toward practicality ization topics, etc. .

1. What is an all-solid-state battery?

As the name suggests, an all-solid-state battery is one in which all the components that make up the battery are “solid-state.” Secondary batteries (batteries that can be charged and reused) such as lithium-ion batteries are basically composed of two electrodes (positive and negative electrodes) made of metal and an electrolyte filled in between. The electrolyte of conventional secondary batteries uses liquids, while the electrolytes of all-solid-state batteries use solids.

After the electrolyte becomes a solid, it is expected to introduce a battery with a larger capacity and higher power than a lithium-ion battery. In addition, by making the electrolyte into a solid state, it has the advantage of being safer than a lithium-ion battery, and the possibility of being mounted on an electric vehicle or the like is also attracting attention.

If all-solid-state batteries can be practical, they will have various advantages. Now, companies are competing fiercely with each other for product development and mass production for a large supply.

2. The working principle of all-solid-state battery

All-solid-state batteries are pretty much the same as lithium-ion batteries when it comes to how they work to extract power from the battery. Metals are used as electrode materials, and ions move between the positive and negative electrodes through the electrolyte, creating the flow of electricity.

The main difference between an all-solid-state battery and a lithium-ion battery is whether the electrolyte is solid or not. If the electrolyte is a liquid, there is a separator separating the positive and negative electrodes to prevent the liquid on the positive side and the liquid on the negative side from mixing violently, while in the case of a solid electrolyte, a separator is not required.

The key to all-solid-state battery research is to find and develop solid materials. Previously, no solid material has been found in which ions can move inside to allow enough electricity to flow to the electrodes, and the development of all-solid-state batteries has become increasingly active after the discovery. By changing the electrolyte from a liquid to a solid, ions move frequently within the battery, enabling batteries with higher capacity and higher power than lithium-ion batteries.

3. Types of all-solid-state batteries

All-solid-state batteries are classified into two categories: “stacked” and “thin-film” according to how they are made, with different amounts of energy that can be stored.

Features of stacked all-solid-state batteries:

Powders (aggregated substances such as powders and particles) are used as materials for electrodes and electrolytes. It is possible to make high-capacity batteries that can store more energy. Envisioned mainly for large objects such as electric vehicles;

Features of thin-film all-solid-state batteries:

A battery manufactured by depositing a thin-film electrolyte on an electrode in a vacuum state. The energy that can be stored is small, and it cannot output large capacity. However, it has the advantages of long cycle life and easy manufacture. Because it is small, it is suitable for small devices such as sensors.

4. The difference from lithium-ion batteries, the advantages of all-solid-state batteries

All-solid-state batteries expected as next-generation secondary batteries have various advantages as shown below:

Low temperature to high temperature resistance:

Since a flammable organic solvent (a liquid that dissolves a substance insoluble in water) is used for the electrolyte of a lithium-ion battery, there is concern about use in a high temperature environment. The electrolyte of all-solid-state batteries does not use flammable materials, so it can also be used at higher temperatures.

Also, at low temperatures, the movement of ions in liquid electrolytes sometimes becomes sluggish, resulting in reduced battery performance and reduced voltage. At low temperatures, the solid electrolyte does not freeze like a liquid, so the internal resistance does not increase much, and the battery performance does not decrease much.

Can fast charge:

The advantage of high temperature resistance is also beneficial when charging quickly. The faster the charge, the higher the temperature of the battery, and the high-temperature all-solid-state battery can charge faster than today’s lithium-ion batteries.

long life:

The life of a battery varies depending on the nature of the electrolyte. Because the lithium-ion battery does not use the battery reaction like other secondary batteries, the electrode aging is less and the life is long, but the aging of the electrolyte is still visible during long-term use. In this regard, life can be further extended because the electrolyte of an all-solid-state battery ages less than a liquid.

High degree of freedom of shape:

In order to prevent liquid leakage, liquid electrolytes have structural limitations, and all-solid-state batteries have no such limitations, so they are easy to be miniaturized and thinned, and they can also be stacked and bent, and can be used in various shapes.

5. The use of all solid-state batteries

Envisioned uses for all-solid-state batteries

Another highly anticipated use for all-solid-state batteries is in electric vehicles. Lithium-ion batteries are currently used in electric vehicles, and all-solid-state batteries are expected to reduce risks such as fires caused by accidents because they do not contain flammable organic solvents. In addition, today’s electric vehicles take longer to charge than gasoline, and can be charged more quickly if they use an all-solid-state battery.

In addition, one of the reasons for actively promoting the practical application of all-solid-state batteries is to make up for the weakness of lithium-ion batteries that are not resistant to high temperatures. If the heat-resistant feature is used, it can be directly soldered to the Electronic substrate, so it is envisaged that it can be used as a backup power supply for electronic equipment, IoT sensors, etc. If it is used for computers, smartphones, etc., it can work longer and more powerfully.

Moreover, compared with lithium-ion batteries, since they can achieve larger capacity and higher power, they are expected to be used in aircrafts and ships, etc., and because they can adapt to temperature changes from high to low temperatures, they are also expected to be used in space. equipment used in the space, etc.

6. Safety of all solid-state batteries

Since lithium-ion batteries use easily vaporized organic solvents as electrolytes, there is concern about use in high-temperature environments. In addition, when using a liquid electrolyte, a separator or the like that separates the positive electrode and the negative electrode is required so that the positive electrode and the negative electrode do not come into direct contact (short circuit) due to impact.

Because the electrodes of an all-solid-state battery are separated by solids, short circuits are less prone to occur, and because a highly heat-resistant electrolyte is used, it can be used at higher temperatures. That said, all-solid-state batteries are also risky because they are “energy cans.” The electrodes may be shorted for some reason, so be careful when handling them.

7. Towards the practical application of all-solid-state batteries

Now, with the goal of realizing the practical application of all-solid-state batteries in the first half of the 2020s, higher-performance solid-state electrolyte materials are being developed. In order to be practical, it is necessary to solve the following problems:

The subject of solid electrolytes:

For a battery to perform at high performance, the electrodes and electrolyte need to be kept close together at all times. Because the shape of the liquid electrolyte is always variable, the electrodes stay close together even if they change slightly. And it is difficult for solids to stick together all the time, which is a problem.

The subject of electrode material:

Compared with existing lithium-ion batteries, to greatly improve the energy density of all-solid-state batteries, it is necessary to develop electrodes that can store more electricity with the same weight and size.

The subject of the manufacturing process:

Because the electrolyte changes from liquid to solid, it requires a different manufacturing process than lithium-ion batteries.

For example, all-solid-state batteries include oxide-based, sulfide-based, nitride-based, etc., depending on the material. The solid electrolyte used in sulfide-based all-solid-state batteries, one of the mainstream, is not water-resistant even if it encounters moisture in the air. nature. Therefore, the production of all-solid-state batteries that require strict moisture management requires specialized equipment such as drying chambers.

As described above, the all-solid-state battery, which is expected as a battery capable of further improving the performance of a lithium-ion battery, is currently being put into practical use by various companies. On the other hand, lithium-ion batteries are also making their mark in a wide range of fields.

In the next lecture, we will talk about what role lithium-ion batteries can play in realizing a sustainable society. Stay tuned!