Flexible batteries, both primary and secondary ones are batteries that are designed to be conformal and flexible, while the traditional batteries are rigid with certain shapes. The increasing interest in portable and flexible electronics has led to the development of flexible batteries which can be implemented in products such as smart cards, wearable electronics, novelty packaging as well as flexible displays and transdermal drug delivery patches. Hence efforts are underway to make different flexible power sources including primary and rechargeable batteries.
In general, a battery is made of one or several galvanic cells, where each cell consists of cathode, anode, separator, and in many cases current collectors. In flexible batteries all these components need to be flexible. These batteries can be fabricated into different shapes and sizes and by different methods. One approach is to use polymer binders to fabricate composite electrodes where conductive additives are added to enhance their conductivity. The electrode materials can be printed or coated onto flexible substrates. The cells are assembled into flexible packaging materials to maintain bendability. Others approaches include the filtering of electrode suspension through filters to form free-standing films, or use flexible matrix to hold electrode materials. There are also other designs like cable batteries.
There have been much efforts in adapting conventional batteries such as zinc-carbon and lithium ion, and at the same time new materials such as those based on nanoparticle complexes are being developed for flexible battery and supercapacitor electrodes. For example, there are efforts at developing flexible lithium-ion batteries. Some studies have introduced nanocarbons into flexible lithium-ion batteries, and there are batteries with Li4Ti5O12 and LiFePO4 as anode and cathode, with graphene based current collector reported by Cheng. Carbon nanotube electrodes have been reported by Pushparaj. Yi Cui’s group coated electrode active materials onto carbon nanotubes to form Li4Ti5O12-CNT and LiCoO2-CNT composites and Ajayan et al. developed SnO2-CNT composites. Another flexible all solid lithium-ion battery with a maximum 4.2 V charging voltage and 106 μAh/cm capacity was reported by Lee et al. Another development is the paper-thin flexible self-rechargeable battery which combines a thin-film organic solar cell with an extremely thin and highly flexible lithium-polymer battery. This recharges itself when exposed to light.
Disposable, primary flexible primary batteries which are the equivalent of AA and AAA batteries are also of great interest with applicability in smart cards, medical patches, greeting cards, toys, and disposable devices. Advantages of primary batteries with aqueous electrolyte over lithium ion batteries include their eco friendliness and the ease of fabrication. A flexible zinc-carbon battery using single-walled carbon nanotubes as current was reported by Hiraral in 2010; later in 2013 one with conductive polymer and cheaper multiwalled carbon nanotubes was published by Mitra. Et. al. Binders were used to maintain the flexibility and integrity of electrodes, and the carbon nanotubes dramatically reduced electrode resistance. The primary alkaline battery is more durable than conventional zinc-carbon under heavy load. An alkaline battery uses MnO2 as the active material along with zinc anode and KOH is used as an electrolyte here. A flexible alkaline cell offers several challenges because compared to zinc-carbon cells using weak acidic or neutral electrolytes, KOH is more basic and corrosive. Gaikwad first proposed an alkaline battery using nylon mesh and more recently Wang and Mitra reported an alkaline cell using. A rechargeable flexible alkaline battery has also been reported by the same group.
Commercialization efforts for flexible lithium-ion and zinc-carbon systems are ongoing. LG is proposing to mass-produce a flexible cable battery. The global market for thin film batteries increased from $33.5 million in 2011 to $51.8 million in 2012, and is estimated to be valued at $87.3 million by the end of 2013. Manufacturers of zinc-based flexible disposable batteries include Blue Spark Technologies (Westlake, OH, US), FlexEl (College Park, MD, US), Printechnologics (Chemnitz, Germany), etc. While the suppliers of lithium-ion systems include GS NanoTech (Seoul, South Korea), Cymbet (Elk River, MN, USA), and Excellatron (Atlanta, GA,USA).