With the advancement of technology and increasing demand for sustainable energy solutions, lithium-ion batteries are being widely used, including in electric vehicles and large-scale grid energy storage. In recent years, there has been rapid development in the quest for more efficient energy storage solutions. New battery technologies such as solid-state batteries and chemical batteries are being accelerated in research and development to provide better performance in mobile power applications.
Iron, being the fourth most abundant element on Earth after oxygen, silicon, and aluminum, is considered an ideal material for various applications due to its widespread distribution and abundance. The utilization of the iron oxidation process to generate current during discharge and the reverse process of reducing iron oxide with an external power source during charging are simple chemical reactions. Currently, this principle is being developed for large-scale electricity storage facilities(sources from medcom.com.pl).
Discharge process (conversion of chemical energy to electrical energy):
Anode (iron electrode) reaction: Fe → Fe3+ + 2e-
Cathode (air electrode) reaction: O2 + 2H2O + 4e- → 4(OH)-
Overall reaction: 4Fe + 3O2 + 6H2O → Fe(OH)3
During the discharge process, the iron anode is oxidized to form ferrous ions (Fe2+), releasing electrons. These electrons flow through the external circuit to the cathode, where they react with oxygen and water to generate hydroxide ions (OH−). Finally, ferrous ions combine with hydroxide ions to form ferric hydroxide (Fe(OH)3), commonly known as rust.
Charging process (conversion of electrical energy to chemical energy):
During charging, the reaction direction is reversed by forcing electrons to flow backward through an external power source. The reduction reactions are as follows:
Anode (iron electrode) reaction: Fe2+ + 2e- → Fe
Cathode (air electrode) reaction: 4OH- – 4e- → O2 + 2H2O
Overall reaction: 4Fe(OH)3 → 4Fe + 3O2 + 6H2O
In the charging process, ferric hydroxide (rust) is reduced back to iron, and oxygen is released back into the air.
As early as the 1960s, NASA began developing this principle to produce large-scale energy storage systems for space missions and deep space exploration. NASA’s space missions require long-term, stable, and efficient energy solutions. In the space environment, traditional energy storage systems (such as lithium batteries) may face weight, cost, and performance limitations. Iron-air batteries offer high energy density, allowing them to store more energy in a smaller volume and weight. This is crucial for saving valuable payload space and reducing the weight of spacecraft. Additionally, iron is one of the Earth’s most abundant metals, making it cost-effective and environmentally friendly. Iron-air batteries have advantages in terms of resource availability and environmental impact. Furthermore, compared to other battery technologies, iron-air batteries are theoretically safer, less prone to overheating or combustion. NASA’s research on iron-air batteries primarily focuses on improving their energy efficiency, cycle stability, and overall performance. The potential of these batteries lies in their theoretical high energy density and low cost, making them particularly suitable for applications requiring long-term, stable energy supply, such as space exploration.
In recent years, this technology has emerged from the “technology history museum” and has been pushed towards industrial applications by battery technology development companies and venture capital. This is mainly driven by the global demand for new energy and low-carbon technologies. The growth of solar and wind energy urgently requires large-scale, long-duration, and low-cost storage technologies. Iron-air batteries are particularly suitable for long-duration energy storage. It is reported that their unit cost is one-tenth of mainstream commercial batteries, while their discharge time is seventeen times that of ordinary batteries. This is crucial for balancing the intermittency of renewable energy, especially when solar and wind energy generation is insufficient due to weather and time factors. Iron-air batteries provide a reliable energy storage solution, helping overcome the limitations of renewable energy, ensuring a stable power supply even in windless or nighttime conditions. Additionally, iron, compared to other battery materials, is highly accessible with lower environmental impact, favoring large-scale production.
Iron-air batteries belong to the category of chemical batteries, specifically a type of metal-air battery. Their operation is based on chemical reactions, similar to lithium-ion batteries, rather than solid fuel cells. The key features include:
Basic Principle: Iron-air batteries are metal-air batteries that use the reaction between iron and oxygen to store and release energy. During discharge, the battery inhales oxygen from the air, converting iron metal into rust. During charging, applying current reverses the process, converting rust back to iron, and the battery releases oxygen. This operating principle is known as reversible rusting.
Energy Density: Iron-air batteries have an energy density exceeding 1200 Wh/kg, compared to around 600 Wh/kg for lithium-ion batteries. In terms of volumetric energy density, iron-air batteries outperform lithium-ion batteries, reaching 9700 watt-hours per liter (Wh/l), nearly five times that of current lithium-ion batteries (2000 Wh/l).
Cost and Applications: Iron-air batteries are considered low-cost, safe, durable, and suitable for long-term storage of renewable energy. A prototype developed by an energy company derived from MIT is used to obtain reliable power from renewable resources for large-scale energy storage.
Technical Comparison: Iron-air batteries can be seen as alternatives to iron-nickel batteries and alkaline batteries. One of their main advantages is that they do not produce iron dendrites during charging.
However, iron-based batteries have significant drawbacks, mainly their bulkiness. They are giant devices designed for large-scale charging and discharging, and scaling down may not be meaningful. The fundamental reason is that the atomic weight of iron is relatively large compared to lithium, making it less efficient in electron transfer and ion migration. Lithium-ion batteries, with their efficient electron transfer and ion migration, achieve easier charging and discharging due to the smaller atomic weight of lithium. Lithium batteries generally have longer cycle life, as the migration of lithium ions between electrode materials is relatively stable. In contrast, in iron-air batteries, the oxidation and reduction processes of iron may lead to rapid degradation of electrode materials, affecting the cycle stability of the battery. Research on the reduction and oxidation of iron and corrosion of metal materials needs further investigation to address these challenges. For iron-air batteries to be widely accepted in the market, some technical challenges need to be overcome(quotes from medcom.
Currently, four companies abroad are undergoing commercial deployment, with several actual projects scheduled for construction in 2023. Among them, California is building a 5 MW/500 MWh iron-air battery energy storage project, the largest long-term storage facility in the state, expected to be operational by the end of 2025. The main purpose is to provide a reliable and stable capacity to the grid, supporting grid reliability and local grid resilience, even during prolonged extreme weather or low renewable energy generation periods.
As the share of new energy increases in the overall energy mix, the large-scale deployment of iron-air batteries could open up a new realm of possibilities for this metal, which has been a cornerstone of civilization since ancient times.
