A brief article on how to choose between Lithium ION and Lithium Polymer Batteries
A Look Back
Lithium ion batteries have been commercially available since the early 1990s, possessing the highest energy density of all rechargeable battery chemistries and finding their way into almost every portable electronic device to date. In response to potential safety issues related to potential abuse events or conditions, battery pack designers and engineers have added several lines of defense with regard to battery safety. These include active circuits to maintain the battery voltage within a specific voltage window as well as short circuit protection, external PTCs to control the thermal environment of the battery pack and mechanical devices internal to the cell that disconnect the cell and arrest operation when abuse conditions occur.
In the early to mid-1990s, in response to safety considerations, as well as market pressures to make thinner and thinner cells that might not need active protection circuits, battery scientists looked for design techniques to mitigate the effects of overcharging or short circuits which could potentially harm consumers and users with shards of metal from a breeched cell can in the event of an explosion from abuse. Even though the many lines of defense were in place, continued attention to safety problems were at the forefront of engineer's minds. Lithium polymer technology provided an answer, whereby a gelled separator / electrolyte and a laminated aluminum bag substitute for a flooded electrolyte system and a sealed metal can. Less volume of volatile electrolyte was required for acceptable power rates and the bag would rupture a seal rather than violently release metal shards if over voltage or thermal runaway conditions occurred.
Polymer on the Rise
The adoption rate of lithium polymer technology in the marketplace since the early 1990s has been delayed by the instability of the supply chain as researchers struggled with manufacturing processes that combined plastics process technology with electrochemical engineering concepts, the need to manufacture the technology within reasonable economic limits and business models, and the requirement to keep pace with the rapidly growing performance of standard lithium ion chemistry. Early polymer cell and battery designs were very similar to that of conventional lithium ion batteries, offering no significant points of differentiation or enhanced product features and application engineers could not justify the tradeoffs with regard to increased design costs with the promised safety enhancements afforded by lithium polymer designs.
In the late 1990s, the combination of improving manufacturing yields, lower production costs, new and higher quality materials and a better understanding of cell design capabilities led the way to increasing the adoption rate of lithium polymer technology. Applications emerged, both in mass and niche markets, where new designs that utilized the thin, flat, form factor of lithium polymer justified design costs, market risk and consideration. Cell designs were better understood and could compete and surpass those afforded by conventional lithium ion batteries.
What's the Difference?
Lithium ion batteries consist of a cathode of lithiated metal oxide or phosphate, and a graphite anode separated by a polyethylene or polypropylene separator material in a non-aqueous electrolyte. When the battery is charged, lithium ions leave the cathode and are intercalated (inserted) into the layered structure of the graphite. As the battery discharges, lithium ions leave the anode and return to the cathode, while electrons travel in the opposite direction, completing the circuit.
Lithium polymer batteries have the same common electrochemistry as conventional lithium ion batteries - they have a lithiated oxide cathode and a graphite anode held together in a binder matrix of polyvinlydiene fluoride and coated and/or laminated to a current collector grid. However, they contain a highly porous separator, which converts to a gel when a minimum amount of electrolyte is added to operate the cell. Lithium polymer technology uses stacked plate cell architecture, as opposed to a wound "jelly roll" found in conventional lithium ion cells, which means that they consist of die-cut anode and cathode plates that are about the size of the battery and are stacked on top of each other like a deck of cards, with the plates connected in parallel.
Lithium polymer batteries have comparable energy density to lithium ion batteries and are achieving cost parity. The cells are sealed in a plastic aluminum laminate, so if the battery sees an overcharge condition, the seal ruptures and vents any gases due to electrolyte oxidation and the cell shuts down rather than having steel or aluminum casings fragment, as in the case of lithium ion batteries that may result in metal shards.
How to Choose
When our Applications Engineers receive inquiries from customers looking for small rechargeable batteries, they guide them through a number of questions in order make a sound recommendation.
First, they need to understand the application. Is it commercial, industrial, military or consumer? This invariably leads to sizing questions, mechanical constraints, and packaging parameters. From here, electrical needs are addressed including voltage, capacity and drain. And finally, yearly quantities and cost structures are discussed at very introductory levels.
From this round of discussions and guidance from the customer, a thoughtful recommendation can be made.
Generally, battery cavity restrictions and cost structures built into the product or application determine whether or not Lithium ion or Lithium polymer batteries are recommended. If an application needs a very thin battery, typically less than 4 or 5mm, lithium polymer is recommended as it can be manufactured to such degrees of thinness that can't be matched by Lithium ion. If the application's battery cavity is a bit more forgiving, then Lithium ion is typically recommended largely due it being less expensive to manufacture.
Other factors, of course, will play into whether or not Lithium ion or Lithium polymer is recommended such as weight and customization needs. But on a very general level, thickness and cost structures play a very large role in determining which battery type is right for you.