11Mar 2015

Did you know that the European semiconductor giant ST Micro makes rechargeable lithium-ion batteries? Probably not. You don’t believe me; go ahead and google “ST Micro thin-film batteries.” It’s really tiny. It  is a square that measures one inch (25.7 mm) on a side, and is a mere 0.2 mm thick (that’s about the thickness of a human hair). But before you jump out of your seat to order one, let me tell you that it has a capacity of 0.7 mAh. That’s right, that’s 0.7 milli Ah. It will take about 2,600 of these little cells to give a capacity equivalent to the iPhone 6 battery.  But they are useful for applications that require very little power and energy, such as RFID tags or smart cards.

The cells from ST Micro and other suppliers such as Cymbet and Front Edge Technology represent a new category of rechargeable lithium-ion batteries that are called solid-state thin-film batteries. The name says it all. They are solid-state, in other words, no gels or liquids inside the structure. They are thin-film, in other words, made of very thin layers (films) of materials. Naturally, this implies that they can be manufactured in similar ways to semiconductor chips. This is a powerful argument for manufacturing with high precision yet delivering extremely low cost. So if that is the case, why don’t we see them more commonly in mobile devices. Before we tackle this question, let’s dive a little into the internal structure of solid-state batteries.

Basic structure of a lithium-ion battery includes two electrodes and an electrolyte in the middle. Courtesy: Wikipedia.

A lithium-ion battery consists fundamentally of two electrodes, an anode and a cathode, sandwiching an electrolyte medium that allows the lithium ions to shuttle back and forth between the electrodes;  in battery parlance, it has to be electrically conductive to the ions. The anode and the cathode are commonly made of carbon and lithium-cobalt-oxide (LCO), both of which are solid materials that can be layered down using semiconductor-like techniques. But the electrolyte is usually a liquid or a gel…hence, it defies our stated objective of thin layer deposition. The hunt has been for decades now to find a electrolyte that is suitable for the transport of ions through it, yet be made of a solid material. Unfortunately, very few candidates present themselves so far as commercially viable — but that has not deterred small and large companies from continuing the search and exploration. Examples include startups such as SEEO in California and Sakti3 in Michigan.

Lithium Phosphorous OxyNitride, or in short LiPON, is a glass initially developed at Oak Ridge National Laboratory in Tennessee and has evolved into the material of choice today for commercially available thin-film solid-state batteries. But it is far from ideal. It exhibits a 1,000X higher resistance to ion flow than do liquid or gel electrolytes.  Not good! That means few ions can shuttle back and forth consequently limiting the capacity of the battery to a very small figure, usually on the order of mAh or less. Other exotic candidates include zirconia-based ceramics but I am not aware of any commercial deployment. The result is that the energy density of these cells is low: for the ST Micro cell, it is a meager 20 Wh/l, or 30X worse than state-of-the-art lithium-ion polymer batteries.

The other challenge is cost, presumably driven by the lack of manufacturing scale, potentially low manufacturing yields, and the high cost of the exotic materials. Presently, a small solid-state cell can retail for $10 – $30 each. That works out to more than $5,000 per Wh vs. $0.20 per Wh for commercial polymer batteries.  But some of the new startups are trying to change this and reduce the cost by several orders of magnitude.

But on the upside, solid-state cells typically exhibit long cycle life and an excellent safety performance — they are not prone to fire the same way liquid or gel electrolytes are.

In summary, solid-state thin-film batteries present a very attractive story but much research and exploration in materials need to be completed first. I will continue to applaud for more breakthroughs in this area but I don’t see one yet on the horizon.