17Jan 2019

Ted Miller, senior manager of energy storage at Ford Motor Co., recently stated:  We don’t see another way to get there without solid-state technology.” The statement is in regard to more powerful batteries for electric vehicles. Mr. Miller goes on clarifying: “What I can’t predict right now is who is going to commercialize it.”

So what is a solid state battery and why is it so difficult to commercialize? 

First, let’s clarify some misconceptions. 

A polymer battery, known as a LiPo, is a lithium-ion battery. 

A cylindrical battery, like an 18650 cell (used in the early Tesla models) is also a lithium-ion battery.

A prismatic battery is too a lithium-ion battery with a hard shell.

And so is a solid-state battery. It involves newer manufacturing processes, but it is a lithium-ion battery. 

All of these variances of lithium-ion batteries have one physical principle in common: the lithium ions contribute to storing the electrical energy.

Simplistically, a lithium-ion battery operates with lithium ions shuffling back and forth between two electrical layers: an anode and a cathode. When the ions are at the cathode, the battery is discharged. When they move to the anode, then the battery is charged. The cathode and anode are called electrodes.

The motion of the ions between these two electrodes is facilitated by an intermediate medium called electrolyte. It is a solution that is electrically conductive: it permits ions to travel through it with little impediment. One key property is called conductivity: it is a scientific measure of the ease at which ions can travel through the electrolyte. High conductivity means the ions can travel easily and quickly. Low means the opposite.

In a lithium-ion battery, the two electrodes are immersed in an electrolyte solution. Today’s batteries use a liquid or gel-like electrolyte. Battery manufacturers go to great lengths to formulate unique electrolytes for their batteries. The formulations do have an impact on many of the battery’s specifications, in particular cycle life (the number of times a battery can be charged and discharged). 

In a solid-state battery, the liquid or gel electrolyte disappears. It is instead replaced by a “solid-state” layer sandwiched between the two electrodes. “Solid-state” means this layer is not a liquid, but a physical solid. The material can consist of a ceramic, glass, or even a plastic-like polymer, or some type of mixture of all three.

So why use a solid electrolyte? There are two major reasons. First, a battery with solid electrolyte occupies a lot less space than one with liquid electrolyte. That means one can pack more energy in the same volume. Consequently, energy density — an important metric of batteries — goes up.

The second reason is safety. Liquid or gel electrolytes are more prone to catching fire than a solid electrolyte.

Traditionally, the primary challenge with solid electrolytes is poor conductivity especially at room temperature (25 °C or 77 °F). A liquid or gel electrolyte has a conductivity that is about 1,000 times better than that of solid electrolyte. In other words, solid electrolytes exhibit a far higher resistance to the flow of lithium ions. This results in several performance challenges, starting with poorer cycle life and inability to charge at fast rates. 

Some companies proposed operating their solid-state batteries at elevated temperatures (> 80 °C) to improve conductivity. But this is not practical under most use scenarios.

Therefore the quest for solid electrolyte materials continues to be a much active field of exploration and discovery. There is confidence in the industry better materials will be discovered, yet, we really can’t predict when a breakthrough will be widely adopted. 

Another challenging aspect is the surface stability and manufacturability of solid electrolytes.  Unlike liquid solutions, glass and ceramic electrolytes are not deformable. They must be assembled with the two electrodes using high external pressure, equivalent to about 1,000 atmospheres. It becomes questionable whether existing battery manufacturing factories can be retooled for this purpose. If not, the economics of solid-state batteries will undoubtedly suffer as is the present case.

In a nutshell, there is much promise in breakthrough material innovations to make solid-state batteries a reality. Yet, many challenges remain ahead. I personally do not expect to see solid-state batteries in commercial scale for several years to come. We will continue to see evolutionary progress with traditional lithium-ion batteries especially as prices continue to decline.

But in all cases, solid-state batteries are subject to the same physical principles that govern traditional lithium-ion batteries. Consequently, many of the battery management solutions developed for traditional lithium-ion batteries will evolve and continue to apply. And that is good news.