The pressing need for sustainable energy storage solutions has been accelerated by global efforts to transition to renewable energy sources and mitigate climate change. Conventional energy storage technologies predominantly rely on inorganic materials such as lithium, cobalt and nickel, which present significant challenges in terms of resource scarcity, environmental impact and supply chain ethics. Organic batteries, composed of carbon-based molecules, offer an alternative that addresses these concerns.

Unlike inorganic batteries, organic batteries utilize materials that are abundant, low-cost and environmentally benign. Furthermore, their molecular structure can be engineered at the synthetic level, providing unique opportunities for optimization in terms of energy density.

Used batteries for disposal. Source: Roberto Sorin/Unsplash

Limitations of conventional inorganic materials

Traditional lithium-ion batteries (LIBs) and other inorganic-based rechargeable batteries face multiple challenges, including limited resource availability, high costs and environmental concerns. Layered oxides, spinel oxides, polyphosphates and Prussian blue compounds, while efficient, are not sustainable in the long term.

Organic electrode materials (OEMs) offer a compelling alternative due to their environmental friendliness, abundant resources and structural designability. These materials are not only versatile but can also be synthesized from easily accessible raw materials via common organic synthesis reactions.

How lithium-ion batteries work diagram. Source: U.S. Argonne National Laboratory/Flickr

Historical context

The concept of using OEMs in rechargeable batteries dates back to 1969, paralleling the development of LIBs. Early experiments focused on quinone electrodes and demonstrated reversible redox reactions.

In the 1970s, the discovery of conductive polymers like polyaniline paved the way for their use in rechargeable batteries. While they displayed good cycling stability, their capacity was low, attributed to low doping levels.

The development of synthesis technologies has led to the creation of conjugated polymers with multiple redox-active groups. These are emerging as strong candidates for high-performance rechargeable batteries, gaining increased attention over the past decade.

The evolution of OEMs in energy storage systems

Since the initial study on quinone electrodes back in 1969, OEMs have undergone significant development, diversifying into various types of redox-active materials like small molecules and conductive polymers. The versatility of these materials lies in their facile synthesis and tunable structures, allowing for a wide range of applications from traditional lithium-ion batteries to innovative metal-ion batteries. Yet, despite their promise, OEMs have been hampered by poor electronic conductivity and stability.

Battery design based on OEMs

OEM-based full batteries have gradually emerged as a promising alternative to traditional LIBs. These full batteries typically employ a p-type organic electrode in combination with a common n-type organic electrode. The mass-energy density of full organic batteries is significantly influenced by factors such as electrode materials, the ratio of anode to cathode materials, and the electrolyte type and quantity.

All-organic full batteries

In the domain of lab-level research, all-organic full batteries have made significant strides. For instance, some researchers utilized a series of oxocarbon salts to prepare fast rechargeable batteries that demonstrated a discharge capacity of 212 mAh/g due to rapid K+ diffusion. Similarly, researchers have reported impressive energy densities and cycle lives in their respective all-organic symmetric batteries. Recent advancements also include research on all-organic proton batteries, showing high reversible capacity and good cycling stability.

OEM-based redox flow cells are also gaining traction for large-scale energy storage. These cells offer promising long-term cycling performance and capacity.

OEM-based hybrid full batteries

OEMs are not restricted to all-organic setups; they can be combined with inorganic cathodes like PbO₂, LiMn₂O₄, LiFePO4 and NaVPO₄F to extend the operational voltage window and elevate mass-energy density. Researchers have replaced the inactive carbon with electronically conductive inorganic cathode materials, achieving a high energy density. Recently, aqueous full batteries with impressive energy densities and cycle lives have also been reported.

Quasi-practical batteries

One of the hurdles for the commercial adaptation of OEMs is the low areal capacity in lab-level cells. While some efforts, such as a PTCLi4 electrode designed with high mass loading, show promise, more emphasis on electrode design and the judicious use of electrolytes is needed to meet the energy density goals of next-generation LIBs.

Biodegradable batteries

The growth of rechargeable batteries has brought about concerns related to the eco-friendly disposal of electrode materials. OEMs, owing to their biodegradable nature, offer a sustainable solution to this problem. Fully polypeptide-based biodegradable cells with good capacity retention have been developed, and a strategy for the electrochemical degradation of organic flow cells has also been researched. These suggest a path toward recyclable and sustainable OEM-based batteries.

Current limitations for organic batteries

However, it is imperative to recognize that current redox-active OEMs are plagued by several challenges. These include inferior electronic conductivity, solubility issues, lower redox potentials and complex redox reaction mechanisms. Unlike their inorganic counterparts, the structural transformations of OEMs, which are free of heavy metals and are structurally flexible, are challenging to monitor using conventional tools.

That said, advancements in in situ spectroscopy measurements like FTIR, UV, Raman and EPR, coupled with emerging theoretical calculation methods, have proven invaluable in monitoring structural changes in OEMs.

Strategies for overcoming barriers

Myriad efforts are underway to enhance the practical utility of rechargeable organic batteries. It is speculated that the key to constructing high-performance organic batteries lies in function-oriented structural engineering for novel OEMs. By introducing fewer inactive ingredients and optimizing redox potentials, it is possible to improve the electronic conductivity of redox-active organics. Other practical parameters like synthesis routes, cost of raw materials, active material-loading and gravimetric density also warrant attention for large-scale production and commercial applications.

Cross-application potential

OEMs also show promise beyond traditional organic batteries. Their applicability extends to LIBs, lithium-sulfur (Li-S) batteries and lithium-oxygen batteries as sulfur-loading substrates, electrochemically active binders, or redox reaction mediums. Their fast ion transfer rates and structural flexibility also make them ideal candidates for electrolyte optimization and negative electrode production.

The future landscape

Even as researchers achieve a molecular-level understanding of the redox reaction mechanisms in OEMs, details of electron or ion transfer routes and interface structures between organic electrodes and electrolytes remain elusive. Advanced characterization techniques and theoretical calculations, perhaps augmented by machine learning methodologies, can offer deeper insights into these areas.

In conclusion, there is a need to evaluate OEM-based batteries under practically relevant conditions to meet industrial requirements. Efficient recycling methods, green synthesis routes, and low-cost macrofabrication technologies for OEMs will also be of significant interest in the years to come. With the ongoing development in characterization and computational techniques, OEMs are poised for broader practical applications in the foreseeable future.

Author Byline

Jody Dascalu is a freelance writer in the technology and engineering niche. She studied in Canada and earned a Bachelor of Engineering. As an avid reader, she enjoys researching upcoming technologies and is an expert on a variety of topics.