With so many batteries, from cell phones to EVs, that are dependent on lithium, an alternative is always welcome. Solid-state chemistries could significantly reduce the risk of overheating and fire.

We’ve become dependent on lithium-ion (Li-ion) batteries to power cell phones, EVs, and everything in between. Today’s Li-ion batteries can produce low and high voltages. Plus, lithium is the third lightest element on the periodic table. That’s important because it minimizes weight, which maximizes efficiency, and it maximizes power density. Lithium, however, requires mining. Sodium-ion (Na-ion) has emerged as an alternative because it’s cheap and abundant. Oceans are full of it.

EE World spoke with Bor Jang, PhD, Board Chair and Chief Science Officer at Solidion Technology. We started with a brief discussion on how batteries work, and then Jang explained solid-state batteries and how they work. He also explained how sodium could be used in place of lithium. While lithium is currently the material of choice for battery anodes, it comes with safety risks. Researchers at Solidion have developed a technology that they claim can reduce the heating and fire risks associated with lithium.

EE World: Dr. Jang, thank you for coming on. Our readers at EE World may not know much about the internal workings of batteries, which are constantly changing. One of the things I’ve been hearing about is something called solid-state batteries. Please give us an overview. What is a solid-state battery?

Jang: In my perspective as a materials scientist, the so-called solid-state battery is a battery that makes use of a solid-state electrolyte that allows lithium ions, sodium ions, aluminum ions, or zinc ions to move back and forth between an anode and a cathode in a medium that is in a solid state. It’s like a fish swimming in a solid; it can be difficult. It is a battery that uses solid-state material as an electrolyte, which I call a solid-state battery. I noticed that when people talk about the so-called solid-state battery, they typically refer to a solid-state lithium-metal battery, with lithium metal as the anode (active material).

There is a debate in the narrow sense. However, it still makes sense that you have an anode material that can be metal — graphite, silicon, or any other type of anode material, and the cathode can be the traditional transition metal oxides or conversion-types of cathode active material, such as sulfur. Your electrolyte can be inorganic, organic, or a combination of organic and inorganic, which we call a solid-state composite electrolyte. That type of battery is typically referred to by the media or, in some cases, in the industry, as what we mean by a solid-state battery.

EE World: So, a battery that is not in a solid state would have a liquid electrolyte material?

Jang: Yes, a liquid electrolyte.

EE World: Where does the charge reside when a battery is charged? Is it entirely on the anode, and then the charge passes through to the cathode as the load uses it?

Jang: Yes. So typically, there are two types of solid-state batteries. One makes use of Lithium for the anode material. We call that a lithium metal battery. Another one is called the lithium-ion battery. In this case, typically, the lithium is initially stored in the cathode. In theory, you’re right; the lithium ions come out of the cathode material and travel through the electrolyte to reach the anode. You store the charges in the anode. When a battery is fully charged, then the charges are stored in the anode. During the discharge, when you use your device, the lithium ions come out of the anode material, travel through the electrolyte, and enter the cathode material. Concurrently, electrons move out of the battery cell, providing the electricity that a load or device needs.

EE World: What are some of the advantages of using a solid electrolyte as opposed to a liquid electrolyte?

Jang: Traditionally, the liquid electrolyte being used in a conventional lithium-ion battery that goes into your smartphone, for example, is typically organic liquid, which has the so-called high vapor pressure. It can easily vaporize and sometimes catch fire, particularly when the temperature is relatively high. This high temperature can come from abuse of the battery or some type of internal shorting that generates heat, which vaporizes the liquid electrolyte and will catch fire. You may also run into some explosion issues.

For solid-state electrolytes, this type of issue doesn’t exist because it will be more difficult to burn the solid. There will be no vapor that can catch fire. I think the most important feature of a solid-state battery versus a liquid electrolyte is safety against thermal-runaway failure.

There are other issues from a chemistry perspective. When you use certain types of solid electrolytes, you make using some higher-capacity materials slightly more suitable. So, in many cases, you can use lithium metal as the anode when you talk about solid-state lithium batteries. This metal can intrinsically store a lot of charges per unit weight and per unit volume. This is what we call the gravimetric energy density or volumetric energy density. You can get higher energy density with a solid-state electrolyte.

EE World: Many materials seem to be used for these anodes. We hear primarily about lithium, but we’ve heard about others. For example, silicon, sodium, and others. Are these other materials still in development? Are there applications for these materials other than lithium? Are there other reasons for using these materials, such as safety, sustainability, abundance, cost, and so on?

Jang: As I alluded to earlier, from a chemistry perspective, a battery is composed of an anode and a cathode, and in between, there is a separator that electronically isolates the anode from the cathode. Then, there is an electrolyte that can be liquid, solid, or somewhere in between, called a semi-solid or quasi-solid electrolyte.

For your smartphone, the anode material right now is natural or artificial graphite. Graphite can theoretically help you store a charge equivalent to 372 mAh per gram.

In contrast, silicon as an anode material for a lithium-ion battery can allow you to store up to about 3800 mAh or even 4000 mAh per gram. Silicon can store a lot more charge than graphite. That is the second type of anode material.

Then, you talk about lithium metal itself. You can deposit it to the anode side. For example, the anode side has a thin copper foil and just lithium, nothing else, deposited on the copper foil. The lithium itself can give a specific capacity of 3850 mAh per gram. That’s a very high capacity you can store because it’s 100% lithium.

Lithium has, however, an issue when you allow the lithium ions to come out of your cathode and move to the anode. They tend to deposit non-uniformly, not homogeneously. When hitting a certain spot, they (the lithium atoms) form a bump and then prefer to continue to grow that bump into some small needles that can grow in such a way that they can penetrate the separator. Lithium is a conductive material; you can imagine that if you had a nail penetrating through the anode and cathode, this nail would allow the anode and cathode to connect electrically. That is what we mean by internal shorting.

This shorting issue is very dangerous. Battery scientists are trying to solve what we call a lithium “needle” or, in scientific terms, what we call a “dendrite” type of issue. The dendrite issue may still occur but will occur to a much lesser extent if you use silicon or graphite as your anode material, but you must add the weight of graphite or silicon. If you do that, however, you sacrifice capacity for safety. With graphite and silicon, you can have a higher safety level than pure lithium. The anode material type differentiates a lithium-ion battery (if graphite or silicon is used) from a lithium metal battery (if lithium metal is the anode active material).

There are at least three types of anode material for lithium-ion batteries. The fourth type you mentioned is sodium, which is a totally different type of battery that we call sodium-ion or sodium metal battery.

EE World: If you put these materials on top of lithium for safety reasons, does that reduce the lithium’s energy density?

Jang: Yes, for anode materials such as silicon and graphite.

EE World: Going back to the solid state, does that offer an advantage in energy density so that batteries can either get smaller and lighter with the same capacity or have more capacity for a given size and weight?

Jang: Yes, definitely. Typically, if you use a solid-state battery or solid-state electrolyte, you will not have a severe danger of fire or explosion.

Some solid-state electrolytes, such as ceramic or metal-oxide types, will, in many cases, stop the penetration previously discussed. Because of that characteristic, the solid-state battery lets you use a thin lithium metal layer as the anode material, or initially no lithium metal (called “anode-less”). That will lead to a much higher energy density.

EE World: Is there an issue with lithium in terms of supply? How much of it is on Earth? These materials must come from somewhere. Do we need these other materials for sustainability? Is there enough lithium in the world to manufacture our batteries?

Jang: That’s a critical question. I just heard some statistics. If I recall correctly, I think more than 50%, up to about 58%, of the lithium resources reside in three South American countries: Bolivia, Argentina, and Chile. Those three countries combined control 58% of the mined lithium. Their lithium is in some form of lithium carbonate, a little bit of lithium hydroxide. They are dissolved in water; it’s just like salt water, but it’s in a lake. So they can mine it more easily, as opposed to China, where some of their lithium is buried in part of a mountain. They have to mine it, dig it out of the dirt, correct some chemical compounds, and then convert them into lithium. In the United States, I think we have about 9%, which is not too bad. Australia is 7%, China’s 6%. The Republic of the Congo is 3%, Canada 3%. And the rest of the war is about 14%-15% or so.

It takes a lot of effort [to obtain lithium], and the production of lithium can negatively impact the environment if you don’t do it carefully. But if you do it carefully, I think you will be okay. If you can use sodium, for example, which is everywhere. We have seven oceans and salt water everywhere that contains sodium chloride and, therefore, a lot of sodium. Indeed, the question you asked is very important in the sense that one of the reasons a lot of Chinese engineers and scientists decided to begin to develop the sodium-ion battery is the fear that China has only 6% of the world’s lithium resources. If the three South American countries decide to control the export of lithium, then China is in trouble. The Chinese government has made EVs a national priority. They certainly are concerned about the potential shortage of lithium. The Chinese recognize this and thus encourage the development and commercialization of sodium-ion batteries; there have been 15 to 20 companies developing sodium-ion batteries in the hope that, in the future, once these technologies are fully commercialized, they can stay ahead of the curve.

EE World: Dr. Jang, thank you very much for an insightful discussion on battery chemistries, construction, and material resources.

Click here for the Sodium-ion schematic source.