There are a fair number of flexible battery companies and research groups out there, each with their own flexible battery designs. Most of these designs, however, use the same few battery chemistries. That isn’t to say that all of the flexible batteries using a given chemistry are alike—indeed the cell design, rather than the chemistry, can be novel. And very often the cell design is the make-or-break factor for issues like manufacturability, shelf life, and safety.
Nonetheless, the battery’s underlying chemistry will strongly influence its “personality”, and will set limits for what is and isn’t possible. So, if you’re considering a custom battery it can be very instructive to look at what flexible battery chemistries are most common; a few battery chemistries which are used in most flexible batteries are discussed below. These include lithium-ion batteries, lithium primary batteries, and carbon-zinc batteries. While this discussion does not cover some of the chemistries at the R&D stage or targeting niche applications, it does cover most of the flexible batteries currently on the market.
Lithium Primary Chemistry
Lithium metal batteries are widely used as high performance primary batteries. They offer high energy and power density, long shelf life, and a wide temperature range. Historically, lithium batteries have attracted significant interest as secondary (rechargeable) batteries. However, with common electrolytes they suffer from dendrite formation during charging, resulting in sometimes spectacular cell failures and creating an unacceptable safety risk. Therefore, lithium metal batteries are generally sold as primary batteries.
One thing to keep in mind is that there are multiple primary lithium battery chemistries. All of these battery chemistries use a lithium metal anode (negative electrode), but they use different cathode materials and electrolytes. Changing the cathode and electrolyte will change the performance of the battery.
Over the years, a wide range of cathode materials have been investigated for use in lithium batteries. Because of various issues, including cost, stability, performance, and interaction with the electrolyte, only a relative few cathode materials have been commercialized in primary lithium batteries. Currently, the majority of small form factor lithium cells, both flexible and rigid, use manganese dioxide as the cathode, due to its low cost, high energy density, good stability, voltage compatibility with common electronics, and relatively low toxicity.
There are also a number of different electrolytes, since there are a wide range of materials which conduct lithium ions. These include organic solvents with lithium salts, ionic liquids, dry polymer-salt blends, ceramic materials, and gelled polymer electrolytes containing both liquids and solids (Check out our post on electrolytes for more details). Most primary lithium batteries use either organic solvents with lithium salts or gelled polymer electrolytes. Because of the improved mechanical integrity of a gelled polymer electrolyte compared to a wet separator with a liquid electrolyte, to date most flexible lithium primary batteries have used gelled polymer electrolytes.
Regardless of the electrolyte chosen, flexible lithium batteries all require hermetic packaging to protect the cell from oxygen and moisture in the air. They are also subject to various transportation restrictions, depending on jurisdiction. And they require a controlled environment during production.
Flexible primary lithium batteries have emerging and established applications such as smart cards, RFID, and various internet of things (IOT) applications. Currently, primary lithium-MnO2 batteries have energy densities of between roughly 100 and 700 W-h/L, depending on the size of the battery (smaller batteries will have lower capacity due to the higher fraction of the battery taken up by inactive components such as the packaging). They also offer shelf life of up to five years, and can discharge up to C/2 (i.e. continuously discharge the cell over two hours.)
Lithium-ion batteries are currently the most commonly used rechargeable battery, with a very wide range of applications. These applications include miniaturized medical devices, grid storage, and virtually everything in between. As the name implies, these batteries operate by transporting lithium ions—lithium ions go from the cathode to the anode during charge and from the anode to the cathode during discharge. To balance the ions’ transport, electrochemical reactions take place at the two electrodes, allowing the cell to store energy.
Just like with lithium batteries, there are many different materials which have been tested for the anode, cathode, and electrolyte in lithium-ion batteries, but only a few select materials have proven sufficiently viable for commercial devices. As with primary lithium metal batteries, lithium-ion batteries can use a number of electrolytes such as organic solvents with lithium salts, ionic liquids, dry polymer-salt blends, ceramic materials, and gelled polymer electrolytes containing both liquids and solids. And, just like flexible lithium batteries, most flexible lithium-ion batteries use gelled polymer electrolytes because of their combination of flexibility, conductivity, and mechanical robustness.
Graphite is still the most common lithium-ion battery anode; other materials such as lithium titanate are also used in some commercial batteries. On the cathode side, oxides such as lithium manganese oxide, lithium cobalt oxide, lithium iron phosphate, and various mixed metal oxides are generally used. Each anode and cathode material has its own advantages and disadvantages, and they are in various states of development.
On a device level, lithium-ion batteries have a number of things in common. All else equal, they cost more and have lower capacity than a comparable lithium primary battery, owing to the required pre-charging and the use of an alternate anode to lithium. They also require hermetic packaging, due to the presence of oxygen and moisture in the air, just like lithium primary batteries; they are also subject to the same sorts of transportation restrictions as primary lithium batteries. And, they also require protective circuitry to avoid issues with electrically abusing the cells, e.g. over-discharging or over-charging. This protective circuitry is in excess of the current limiters added to on some lithium batteries, since many of the deleterious effects of abuse only occur when recharging the battery.
Flexible lithium-ion batteries range from 100 W-h/L or so to claims of 800 W-h/L. Depending on cell construction, discharge at up to 5C (full continuous discharge in 12 minutes) can be possible. They offer improved performance in terms of power over primary lithium batteries along with the advantage of rechargeability, but this comes at a higher cost than primary lithium batteries because of processing and protective circuitry. Flexible lithium-ion batteries can have longer service life than primary lithium batteries in applications like RFID, internet of things, and smart cards, owing to their rechargeability.
Carbon-zinc batteries are one of the more widely used chemistries for flexible batteries. The carbon-zinc battery chemistry uses a zinc anode, a manganese dioxide cathode, and a zinc chloride electrolyte. The “carbon” in “carbon-zinc battery” comes from the relatively large amount of conductive carbon historically used in the cathode in these batteries. Carbon-zinc batteries (sometimes referred to as “heavy duty batteries”) are known for being the lowest performing and cheapest conventional (rigid) batteries, and this carries over to the thin film battery world.
Carbon-zinc batteries are generally single use only batteries, i.e. not rechargeable. They have lower power and energy density than flexible lithium batteries, and have a lower nominal voltage (1.5 vs 3 V). On the other hand, carbon-zinc batteries are cheaper to produce than lithium or lithium-ion batteries. They do not need as stringent process control during manufacturing, do not need as hermetic packaging, and are not subject to the same disposal and transportation restrictions as lithium and lithium ion batteries. Flexible carbon-zinc batteries offer between roughly 10 and 110 W-h/L capacity depending on size, as compared to 100 to 700 W-h/L for primary lithium-MnO2 flexible batteries. Because of their low price and excellent safety profile, carbon-zinc flexible batteries are useful for a number of applications such as drug delivery patches, electronic thermometers, short-life RFID, and so on.
Incidentally, you may be wondering why flexible carbon-zinc batteries are significantly more commonly mentioned than flexible alkaline zinc-MnO2 batteries or nickel-metal hydride batteries. One of the major reasons is that alkaline and nickel metal hydride batteries use caustic (alkaline) electrolytes, which are much more hazardous than zinc salt electrolytes. Another is that caustic electrolytes are harder to package; they are known for “creeping” along the seals.
There are many flexible batteries out there, but most of them use the same few battery chemistries. These include carbon-zinc, lithium primary, and lithium-ion chemistries—all of which have been proven out over many years in conventional batteries. If you’re looking for a rechargeable flexible cell, you probably want lithium-ion, while if you’re looking for very low cost and easy transport/disposal you are going to want to use carbon-zinc, and for very long shelf life or high power/energy single-use applications you’re going to want to go with primary lithium metal cells. 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.