What are Flexible vs Thin Film Batteries?

Wearable electronics and the internet of things have seen increasing interest in the past few years. Many of these connected devices are often thin, flexible, or both, and need power sources to match. This has led to an increasing interest in thin film and flexible batteries. That said there is some confusion as to what these terms mean exactly, given the partial overlap between thin film and flexible batteries.

Thin Film Battery

For starters, the term thin film battery for those coming from an electronics background, refers to a battery made using conventional thin film processes, such as CVD or sputtering. There are, in fact, commercially available batteries made using these methods. A number of companies sell batteries with a thin film LiPON electrolyte, a conventional lithium-ion cathode such as LiCoO2, and an electrodeposited lithium anode. LiPON or lithium phosphorus oxynitride, is a glassy material used as an electrolyte material in thin film batteries.

Commonly, however, the term thin film battery refers to batteries which are not made using conventional thin film techniques, but are substantially thinner than conventional batteries. These batteries use carbon-zinc, primary lithium, and lithium-polymer chemistries, along with some others, and are made using techniques like printing and coating. These so-called thin film batteries are generally on the order of 100’s of microns (tenths of a millimeter) thick; for the sake of comparison, conventional batteries are on the order of millimeters thick, or thicker.

The term thin film battery refers more to the thickness of a battery than to the method used to construct it. Furthermore, thin film batteries are not necessarily flexible. For instance, some batteries made using thin film techniques described above are encapsulated using conventional electronics packaging techniques, or use rigid substrates for deposition. These batteries are not very flexible, though they are thin. Other thin film batteries can be rigid if the casing or another component is rigid—some applications don’t need a flexible battery, and making the battery flexible can increase costs by requiring flexible current collectors, separator, packaging, and so on.  For example, current collectors for carbon-zinc batteries are substantially more expensive when they are required to be flexible, given the nature of the electrolyte.

These various batteries target a wide range of applications and are summarized in the table below.

Technology

Thickness

Rechargeable?

Cost

Maturity

Flexible

Applications

Carbon-zinc

100’s of microns

No

Low

High

Possibly

Transdermal patch, single use thermometers

Thin film (vapor process) lithium/lithium-ion

10’s to low 100’s of microns

Yes

High

High

Possibly, if made very thin

Memory backup, medical devices, energy harvesting storage

Thick film lithium

100’s of microns

No

Moderate

High

Possibly

Smart cards, RFID, medical devices

Thick film lithium-ion

100’s of microns

Yes

Moderate

Moderate-High

Possibly

Wearables, medical devices, IOT

Supercapacitors

100’s of microns

Yes

Moderate-High

Moderate

Possibly

Power boost

Zinc ionic liquid

100’s of microns

Yes (claimed)

Moderate-High

Low

Possibly

Wearables, IOT

Most thin film batteries are either carbon-zinc batteries, or lithium/lithium-ion batteries of some sort. These are mature chemistries, which are adaptable with a few “tricks” to the thin film form factor.

Within the lithium/lithium-ion space, there are batteries based upon thin film technology and batteries based on thick film technology, both of which are somewhat confusingly called thin film batteries. Lithium and lithium-ion batteries based on thin film technology are a relatively mature technology, which offer low self-discharge, good safety, and long cycle life. They are, however, relatively high cost, and are generally very thin (low 100’s of microns maximum thickness.) They target a number of applications, including memory backup, energy storage to backup energy harvesting, and medical devices.  Thick film lithium metal and lithium ion batteries are also available from a number of suppliers. The lithium metal batteries are primary batteries, while lithium ion batteries are rechargeable; they often use solid electrolytes in an effort to improve device safety. Thick and thin film lithium batteries target a number of applications, including smartcards, RFID’s, and medical devices.

Carbon-zinc batteries are a thick-film battery technology which tends to target the low end of the market; they require less expensive processing than lithium and lithium-ion batteries, and so are lower cost. Their chemistry is limited to primary batteries, and they offer a reasonably good safety profile. Carbon-zinc thin film batteries’ low cost-single use profile has found some use in applications such as single-use thermometers and drug delivery patches.

Flexible Battery

A flexible battery as the name implies is flexible. It is possible to make batteries somewhat thicker than ‘thin film batteries’ which are flexible. Another point of confusion is just how flexible so-called flexible batteries are.  How much force does it take to flex? How many times is it going to need to withstand bending? What sort of bend radius does the application require? These are just some of the questions you need to ask yourself if you’re thinking about an application requiring a flexible battery.

There is no accepted standard on what exactly constitutes a flexible battery, and ultimately your application dictates the level of flexibility required. Some applications may need enough flexibility for the battery to conform to the body, while others may only need enough flexibility to enable application to a rigid substrate.

Conclusion

As you can see, the terms thin film battery and flexible battery can be a bit confusing. Thin film batteries are generally on the order of hundreds of microns thick or less, and do not necessarily use so-called thin film methods. Thin film batteries are also not necessarily flexible. And those that are flexible may not be suitable for a given application.

As always, talk to us if you have questions on flexible batteries or application specific battery development. You can also visit our battery resources page or battery fundamentals page for more basic information on batteries.

About the Author

John

Hi, I'm John, editor-in-chief of an Flexel Battery online magazine!

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