Polymer-based battery


A polymer-based battery uses organic materials instead of bulk metals to form a battery. Currently accepted metal-based batteries pose many challenges due to limited resources, negative environmental impact, and the approaching limit of progress. Redox active polymers are attractive options for electrodes in batteries due to their synthetic availability, high-capacity, flexibility, light weight, low cost, and low toxicity. Recent studies have explored how to increase efficiency and reduce challenges to push polymeric active materials further towards practicality in batteries. Many types of polymers are being explored, including conductive, non-conductive, and radical polymers. Batteries with a combination of electrodes are easier to test and compare to current metal-based batteries, however batteries with both a polymer cathode and anode are also a current research focus. Polymer-based batteries, including metal/polymer electrode combinations, should be distinguished from metal-polymer batteries, such as a lithium polymer battery, which most often involve a polymeric electrolyte, as opposed to polymeric active materials.
Organic polymers can be processed at relatively low temperatures, lowering costs. They also produce less carbon dioxide.

History

Organic batteries are an alternative to the metal reaction battery technologies, and much research is taking place in this area.
An article titled "Plastic-Metal Batteries: New promise for the electric car" wrote in 1982: "Two different organic polymers are being investigated for possible use in batteries" and indicated that the demo he gave was based on work begun in 1976.
Waseda University was approached by NEC in 2001, and began to focus on the organic batteries. In 2002, NEC researcher presented a paper on Piperidinoxyl Polymer technology, and by 2005 they presented an organic radical battery based on a modified PTMA, poly.
In 2006, Brown University announced a technology based on polypyrrole. In 2007, Waseda announced a new ORB technology based on "soluble polymer, polynorborene with pendant nitroxide radical groups."
In 2015 researchers developed an efficient, conductive, electron-transporting polymer. The discovery employed a "conjugated redox polymer" design with a naphthalene-bithiophene polymer that has been used for transistors and solar cells. Doped with lithium ions it offered significant electronic conductivity and remained stable through 3,000 charge/discharge cycles. Polymers that conduct holes have been available for some time. The polymer exhibits the greatest power density for an organic material under practical measurement conditions. A battery could be 80% charged within 6 seconds. Energy density remained lower than inorganic batteries.

Electrochemistry

Like metal-based batteries, the reaction in a polymer-based battery is between a positive and a negative electrode with different redox potentials. An electrolyte transports charges between these electrodes. For a substance to be a suitable battery active material, it must be able to participate in a chemically and thermodynamically reversible redox reaction. Unlike metal-based batteries, whose redox process is based on the valence charge of the metals, the redox process of polymer-based batteries is based on a change of state of charge in the organic material. For a high energy density, the electrodes should have similar specific energies.

Classification of active materials

The active organic material could be a p-type, n-type, or b-type. During charging, p-type materials are oxidized and produce cations, while n-types are reduced and produce anions. B-type organics could be either oxidized or reduced during charging or discharging.

Charge and discharge

In a commercially available Li-ion battery, the Li+ ions are diffused slowly due to the required intercalation and can generate heat during charge or discharge. Polymer-based batteries, however, have a more efficient charge/discharge process, resulting in improved theoretical rate performance and increased cyclability.

Charge

To charge a polymer-based battery, a current is applied to oxidize the positive electrode and reduce the negative electrode. The electrolyte salt compensates the charges formed. The limiting factors upon charging a polymer-based battery differ from metal-based batteries and include the full oxidation of the cathode organic, full reduction of the anode organic, or consumption of the electrolyte.

Discharge

Upon discharge, the electrons go from the anode to cathode externally, while the electrolyte carries the released ions from the polymer. This process, and therefore the rate performance, is limited by the electrolyte ion travel and the electron-transfer rate constant, k0, of the reaction.
This electron transfer rate constant provides a benefit of polymer-based batteries, which typically have high values on the order of 10−1 cm s−1. The organic polymer electrodes are amorphous and swollen, which allows for a higher rate of ionic diffusion and further contributes to a better rate performance. Different polymer reactions, however, have different reaction rates. While a nitroxyl radical has a high reaction rate, organodisulfades have significantly lower rates because bonds are broken and new bonds are formed.
Batteries are commonly evaluated by their theoretical capacity. This value can be calculated as follows:
where m is the total mass of active material, n is the number of transferred electrons per molar mass of active material, M is the molar mass of active material, and F is Faraday's constant.

Charge and discharge testing

Most polymer electrodes are tested in a metal-organic battery for ease of comparison to metal-based batteries. In this testing setup, the metal acts as the anode and either n- or p-type polymer electrodes can be used as the cathode. When testing the n-type organic, this metal-polymer battery is charged upon assembly and the n-type material is reduced during discharge, while the metal is oxidized. For p-type organics in a metal-polymer test, the battery is already discharged upon assembly. During initial charging, electrolyte salt cations are reduced and mobilized to the polymeric anode while the organic is oxidized. During discharging, the polymer is reduced while the metal is oxidized to its cation.

Types of active materials

Conductive polymers

can be n-doped or p-doped to form an electrochemically active material with conductivity due to dopant ions on a conjugated polymer backbone. Conductive polymers are embedded with the redox active group, as opposed to having pendant groups, with the exception of sulfur conductive polymers. They are ideal electrode materials due to their conductivity and redox activity, therefore not requiring large quantities of inactive conductive fillers. However they also tend to have low coulombic efficiency and exhibit poor cyclability and self-discharge. Due to the poor electronic separation of the polymer's charged centers, the redox potentials of conjugated polymers change upon charge and discharge due to a dependence on the dopant levels. As a result of this complication, the discharge profile of conductive polymer batteries has a sloped curve.
Conductive polymers struggle with stability due to high levels of charge, failing to reach the ideal of one charge per monomer unit of polymer. Stabilizing additives can be incorporated, but these decrease the specific capacity.

Non-conjugated polymers with pendant groups

Despite the conductivity advantage of conjugated polymers, their many drawbacks as active materials have furthered the exploration of polymers with redox active pendant groups. Groups frequently explored include carbonyls, carbazoles, organosulfur compounds, viologen, and other redox-active molecules with high reactivity and stable voltage upon charge and discharge. These polymers present an advantage over conjugated polymers due to their localized redox sites and more constant redox potential over charge/discharge.

Carbonyl pendant groups

Carbonyl compounds have been heavily studied, and thus present an advantage, as new active materials with carbonyl pendant groups can be achieved by many different synthetic properties. Polymers with carbonyl groups can form multivalent anions. Stabilization depends on the substituents; vicinal carbonyls are stabilized by enolate formation, aromatic carbonyls are stabilized by delocalization of charge, and quinoidal carbonyls are stabilized by aromaticity.

Organosulfur groups

Sulfur is one of earth's most abundant elements and thus are advantageous for active electrode materials. Small molecule organosulfur active materials exhibit poor stability, which is partially resolved via incorporation into a polymer. In disulfide polymers, electrochemical charge is stored in a thiolate anion, formed by a reversible two-electron oxidation of the disulfide bond. Electrochemical storage in thioethers is achieved by the two-electron oxidation of a neutral thioether to a thioether with a +2 charge. As active materials, however, organosulfur compounds, however, exhibit weak cyclability.

Radical groups

Polymeric electrodes in organic radical batteries are electrochemically active with stable organic radical pendant groups that have an unpaired electron in the uncharged state. Nitroxide radicals are the most commonly applied, though phenoxyl and hydrazyl groups are also often used. A nitroxide radical could be reversibly oxidized and the polymer p-doped, or reduced, causing n-doping. Upon charging, the radical is oxidized to an oxoammonium cation, and at the cathode, the radical is reduced to an aminoxyl anion. These processes are reversed upon discharge, and the radicals are regenerated. For stable charge and discharge, both the radical and doped form of the radical must be chemically stable. These batteries exhibit excellent cyclability and power density, attributed to the stability of the radical and the simple one-electron transfer reaction. Slight decrease in capacity after repeated cycling is likely due to a build up of swollen polymer particles which increase the resistance of the electrode. Because the radical polymers are considerably insulating, conductive additives are often added that which lower the theoretical specific capacity. Nearly all organic radical batteries feature a nearly constant voltage during discharge, which is an advantage over conductive polymer batteries. The polymer backbone and cross-linking techniques can be tuned to minimize the solubility of the polymer in the electrolyte, thereby minimizing self-discharge.