Scientists at the Indian Institute of Technology (IIT Bombay) have claimed a breakthrough in developing sodium-ion (Na-ion) batteries by addressing the challenges of air/water instability and structural-cum-electrochemical instability in cathode materials.

The breakthrough has led to the creation of new air/water-stable cathode materials that exhibit exceptional electrochemical cyclic stability and resilience when exposed to air and water.

These advancements are expected to drive the development of cost-effective and sustainable energy storage systems for various applications, including consumer electronics, grid energy storage, renewable energy storage, and electric vehicles.

As the global demand for battery-driven electric vehicles continues to rise, there is a pressing need to develop a cost-effective, resource-friendly, safe, and sustainable battery system beyond the conventional lithium-ion (Li-ion) technology.

Given the abundance of sodium resources, the upcoming Na-ion battery system holds significant importance in India.

Stability Challenges

Like other alkali metal-ion battery cells, a Na-ion cell consists of cathode and anode active materials supported on metallic current collector foils.

These materials enable the reversible insertion and removal of the charge carrier, Na-ion, during the cell’s charge and discharge processes.

The performance of Na-ion batteries depends on the structural/electrochemical stability of the electrodes, Na-transport kinetics, and various dynamic resistances.

Despite the many advantages of Na-ion batteries, the electrochemical behavior and stability of the ‘layered’ Na-transition-metal-oxide-based cathode materials, especially in moist environments, require significant improvements for widespread adoption.

The lack of stability poses challenges in handling and storing these materials and adversely affects their electrochemical performance.

Furthermore, the water instability necessitates the use of toxic, hazardous, and expensive chemicals like N-Methyl-2-pyrrolidone for electrode preparation instead of more environmentally friendly water-based slurries.

Universal Design Criterion

To address these challenges, Amartya Mukhopadhyay’s research group at the institute leveraged materials science and electrochemical principles.

They identified dominant factors and controlling parameters that contribute to the development of high-performance Na-ion batteries.

The researchers have proposed a universal design criterion that offers a path toward the successful design and widespread adoption of environmentally stable and high-performance cathodes for Na-ion batteries and beyond.

Their findings have been published in the journal Chemical Communications.

The conventional ‘layered’ Na-transition-metal-oxide structure comprises alternating slabs of NaO2 and transition metal oxide.

Within this structure, the oxygen ions, carrying a net negative charge, are shared by the transition metal ions (TM-ions) and the sodium ions (Na-ions) in their respective layers.

While the TM-O bond is iono-covalent in nature, the Na-O bond is predominantly ionic.

The team discovered that adjusting the degree of covalency (the number of electrons that an atom gains, loses, or even shares during a chemical reaction) in the TM-O bond of the layered Na-transition-metal-oxide structure can have a significant impact on the performance and stability of Na-ion batteries.

Increasing the covalency strengthens the Na-O bond, leading to improved air/water stability and suppressing structural/phase transitions.

This enables the development of high-performance, water-stable cathodes using eco-friendly processing methods. On the other hand, reducing the covalency enhances Na-transport kinetics, increasing the power density of Na-ion batteries.

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