Solid-State Lithium Batteries: Where Is the New Battleground for Electric Vehicles?

The year 2024 is pivotal for solid-state lithium batteries.

First, let's understand what a solid-state lithium battery is.

We need to break it down into two parts: "solid-state" and "lithium battery." You might think you're familiar with lithium batteries since they're in smartphones.

But to be precise, the batteries in smartphones are "lithium-ion batteries." True lithium batteries use metallic lithium as the anode. You might not be familiar with battery anode and cathode materials, so here's the biggest difference in use: lithium-ion batteries can be recharged and used repeatedly, while true lithium batteries are not rechargeable and are single-use.

Comparatively, lithium-ion batteries are the best rechargeable batteries available – they have a large capacity and recharge quickly. True lithium batteries, however, have a short lifespan, run out quickly, and can't be recharged. Hence, they have no real advantage and haven't been widely adopted. Over time, many forgot such batteries existed, and "lithium battery" became shorthand for lithium-ion batteries.

For this discussion, we must distinguish between the two to understand what a solid-state lithium battery really is.

What's the fundamental difference between lithium-ion and true lithium batteries? The form of current is key.

Batteries discharge by creating an electric current. Lithium-ion batteries do this through the movement of lithium ions, while in lithium batteries, it's the flow of electrons.

What's the difference? I was quite mischievous as a kid and dismantled dry-cell batteries. True lithium batteries are a type of dry-cell. If

you've ever taken one apart, you might have noticed a carbon rod inside; that's the conductor that allows electrons to flow through.

Electrons can pass through the carbon rod because they're very small and move easily. Lithium ions are much larger, almost a million times the diameter of electrons. If electrons were the size of dust particles, lithium ions would be the size of elephants. They're too big to pass through the carbon rod.

So, lithium-ion batteries face a practical issue: how do lithium ions move inside the battery? We need to think differently in terms of design.

In everyday life, we understand that since roads have limited width and load capacity, oversized and overweight cargo can't be transported by road but by water instead.

Likewise, lithium ions in batteries have abandoned the "land route" of the carbon rod in favor of the "water route" of the electrolyte. The carbon rod is like a fixed railway for electrons only, while the electrolyte is a liquid, like a canal, allowing lithium ions to pass through easily.

Now, let's discuss "solid-state lithium batteries."

We're not digging canals anymore; we're designing special railways for lithium ions to travel by land. Solid-state lithium batteries replace the liquid electrolyte with a solid one, allowing lithium ions to flow through a solid electrolyte.

You may have noticed that true lithium batteries involve electron flow, while lithium-ion batteries involve lithium ion flow. So, the accurate full name of what we're discussing should be "solid-state lithium-ion batteries."

After hearing the above, you might think the concept of solid-state lithium batteries is simple – just replacing a liquid electrolyte with a solid one. So why are they considered the new battleground for electric vehicles?

Let's look at the issues with traditional lithium-ion batteries.

Traditional lithium-ion batteries require a significant amount of electrolyte, which leads to two major, difficult-to-overcome problems.

The first is safety. The electrolyte contains a component, carbonate esters, that's highly flammable. If there's a fault, like a short circuit, causing a temperature rise or a spark, the electrolyte can combust violently.

Thus, the constant companion

to the widespread use of lithium-ion batteries has been the risk of combustion and explosion. Phones charging could explode; cars driving could spontaneously combust... These issues stem from inherent design flaws.

The second issue, though not fatal, might be more significant: the energy density of batteries using an electrolyte can't be increased. What does this mean?

Using the earlier analogy, to transport the special cargo of lithium ions between two cities, we had to dig a canal.

But transport via canal is slower compared to railways or highways.

Similarly, the electrolyte has a much higher resistance to lithium ions than carbon rods do to electrons. Therefore, a significant portion of the battery's energy is consumed fighting this resistance.

The lithium-ion batteries with electrolytes have reached close to their theoretical limit in terms of energy density, necessitating innovation. The two issues we've discussed are caused by the liquid state of the electrolyte, which solid-state lithium batteries overcome. These batteries offer better safety, more efficient lithium-ion transport, and a longer lifespan.

However, these breakthroughs have already been achieved. The year 2024 is critical for solid-state lithium batteries because they have moved from being unproducible in quantity to being mass-produced.

Typically, the mass production of materials occurs when research is fully mature. However, mass production is part of the research for solid-state lithium batteries. Researchers continue to identify and solve problems during production, advancing the technology of solid-state lithium batteries – this is where the material is truly cutting-edge.

Currently, solid-state lithium batteries haven't achieved full solidification. The electrolytes used, usually polymer-based, present even greater resistance to lithium ions than liquid electrolytes, negating the advantages of a solid state. We haven't found the ideal solid electrolyte yet.

Moreover, solid electrolytes lack the wetting advantage of liquid electrolytes. Why is wetting important? Getting two solids to stick together tightly, without gaps, is challenging. But covering a solid with a liquid is much easier.

Lithium ions need to transfer

smoothly between the electrode and electrolyte to form an effective current path. This is not difficult with a liquid electrolyte. But with a solid electrolyte and solid electrodes, it's not as easy for lithium ions to make this transition.

Therefore, in actual battery designs, retaining a small amount of liquid electrolyte is common. However, this compromises the significant advantages that solid-state batteries might have over traditional lithium batteries. From this perspective, the formulation of solid-state lithium batteries still requires significant optimization.

Another point is that the performance of solid-state lithium batteries is not as ideal as imagined.

For instance, lithium-ion batteries have a common flaw where lithium ions reduce to metallic lithium during charging, forming "lithium dendrites," which are branch-like lithium crystals. These dendrites can pierce through the battery causing short circuits, a major issue for the stability and safety of lithium-ion batteries.

In theory, solid electrolytes could effectively prevent this problem. But in practice, lithium ions can still permeate solid electrolytes under certain conditions and form crystals, which does not align with researchers' expectations.

Additionally, researchers are still uncertain about the mechanisms behind this issue and lack effective solutions, requiring further exploration through practical application.

Although the development of solid-state lithium batteries has many immature aspects, a newborn baby surely can't compete with an adult. But who can say what this baby will look like once it reaches adulthood?

Toyota has already achieved a 1,000 km range with solid-state lithium batteries in cars, and entering the battleground is a certainty.

Traditional lithium battery manufacturers like BYD have begun to invest in solid-state lithium battery development.

Even car manufacturers that don't produce batteries, such as Volkswagen, have invested this year in solid-state lithium battery development companies like QuantumScape.

The 2019 Nobel Prize in Chemistry was awarded to three scientists for the development of lithium-ion batteries. As a Nobel Prize-level new material, lithium-ion batteries have already changed the world; and we are about to see this material change the world again in even richer forms.