PerspectiveMaterials Science

Toward Flexible Batteries

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Science  08 Feb 2008:
Vol. 319, Issue 5864, pp. 737-738
DOI: 10.1126/science.1151831

The design of soft portable electronic equipment, such as rollup displays and wearable devices, requires the development of batteries that are flexible. Active radio-frequency identification tags and integrated circuit smart cards also require bendable or flexible batteries for durability in daily use. Several routes toward the development of flexible batteries are being explored. Some involve batteries made mostly or entirely from plastic, with the added advantage of avoiding ignitable and toxic substances such as lithium and lead. An inorganic primary battery that can be bent like a piece of paper has been developed for disposable-card applications (1, 2). However, primary batteries produce current by a one-way chemical reaction and are not rechargable; their usefulness of in portable electronic equipment is therefore limited. Rechargeable secondary batteries are generally used to power portable equipment. There have been recent efforts to make secondary lithium-ion batteries into thin films while maintaining their high energy capacity (3).

Making a bendable lithium-ion battery requires the development of soft electrode- active materials, such as metal oxide nanoparticles or nanocoatings for cathodes and lithium foil or nanocarbon materials for anodes (4, 5). Virus-templated Co3O4 nanowires have been shown to improve the capacity of thin, bendable lithium-ion batteries (6). The charging/discharging process of batteries is generally dominated by the electron and counterion transport at the surface of the electrodes. By using nanostructured inorganic materials, the rates of the electron and counterion transport are increased. The nanostructured inorganic materials are also soft and bendable, helping in the development of bendable lithium-ion batteries. However, transport rates at inorganic electrodes are often limited by the slow kinetics of ion intercalation and migration in these materials (7). The transport properties have been improved somewhat by using nanoarchitectured electrodes with large surface areas (8).

Plastic batteries using organic electrodes have inherent advantages over lithium-ion batteries, because the organic materials are flexible and their properties can be tuned through chemical synthesis. Several avenues toward such batteries have been explored, and test batteries have been demonstrated.

Research on plastic batteries has a long history, which began with the discovery of the electric conductivity of doped polyacetylene in the late 1970s. However, early attempts to develop organic polymer batteries based on polyacetylene (9) did not lead to commercialization because of the chemical instability of the doped and virgin polyacetylene. Electrically conducting polymers—such as polyaniline, polypyrrole, and their derivatives—have also been examined as electrode-active materials, on the basis of their reversible electrochemical doping behaviors. However, no successful battery will emerge from this work, because the doping levels are insufficient, the resulting redox capacities are low, and the doped states are not chemically stable, leading to self-discharge and degradation of the rechargeable properties of the resulting batteries.

Another effort to making plastic batteries uses an electrolyte layer sandwiched between thin layers of polymers that have low conductivity but incorporate redox-active groups, with a view to increase the overall redox capacity of the battery (see the figure). In this case, the polymer backbones provide a matrix, rather than a conducting path, to interconnect innumerable redox sites for the hopping of electrons by a self-exchange mechanism, resulting in the storage and transport of charge in a homogeneous solid. However, concurrent chemical reactions such as chemical bond cleavage and formation, accompanied by the redox reaction of closed-shell molecules, generally result in an electrochemically irreversible reaction characterized by slow kinetics (10, 11). Organic electrode reactions have to be reversible in order for organic batteries to be developed that can be rapidly charged and discharged with a large current.

Example of a flexible plastic battery.

The R groups in the cathode and in the anode have different redox potentials. During the charging process, charge is stored by oxidizing R groups at the cathode and reducing R groups at the anode. The output voltage of the battery corresponds to the gap between the redox potentials. The curves connecting the R groups are polymer chains, which give flexibility. Many R groups are attached to the polymer chain, so that electrons can hop between neighboring R groups to produce the output current.

Another approach to the development of organic electrode-active materials is based on the large redox capacity of aliphatic redox polymers, which are densely populated with pendant redox-isolated sites (12, 13). Purely organic polymers based on open-shell molecules called radicals have also been studied. These radicals, such as nitroxides and galvinoxyls, allow fully reversible one-electron redox reactions featuring fast electrode kinetics, reactant recyclability, and high redox capacity. The radical polymers act as both cathode-and anode-active materials, because their redox potentials can be tuned by varying the organic substituents (14). These “radical polymer batteries” can be charged in less than 30 s, can generate burst power at rated voltages, and can be transformed into completely flexible, foldable, and semi-transparent batteries (15).

The technologies of lithiumion batteries are regarded as mature, but they have limitations because of safety concerns (prompted by accidents involving ignition and explosion) and because it is very difficult to make highly flexible lithium-ion batteries. Compared with their inorganic counterparts, plastic batteries are safer, adaptable to both roll-to-roll and inkjet printing processes, and comparatively easy to dispose (they can be burned away without toxic gas and ash formation); furthermore, they can be fabricated from less-limited resources. Plastic batteries are intrinsically more bulky than lithium-ion batteries. However, if the batteries are sufficiently light, flexible, and environmentally benign, then bulkiness will not be a significant problem. We may then see the commercialization of flexible plastic batteries for use in electronic equipment.

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