Layered Organic Cathodes for High-Energy, High-Power Sodium-Ion Batteries: Overcoming Conductivity and Stability Challenges

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs) due to the natural abundance and cost-effectiveness of sodium. They are particularly suitable for large-scale energy storage applications, such as microgrids and renewable energy systems. However, the widespread adoption of SIBs is hindered by the limited energy density and power performance of conventional cathode materials. Traditional inorganic cathodes, including layered transition metal oxides, Prussian blue analogues, and polyanionic compounds, fail to simultaneously achieve high energy and power densities at the electrode level. Redox-active organic electrode materials (OEMs) have been proposed as an alternative due to their high theoretical specific capacities and moderate redox potentials. However, they suffer from critical drawbacks such as poor electrical conductivity, slow Na-ion diffusion, and excessive dissolution in electrolytes, leading to poor cycling stability. These limitations necessitate the use of significant amounts of conductive additives, which in turn reduce the electrode-level energy density. Moreover, the absence of a clear structure-property relationship in organic materials further complicates the optimization of Na-ion diffusion kinetics.

To address these challenges, new research paper published in Journal of the American Chemical Society  and led by Professor Mircea Dincă  from the Massachusetts Institute of Technology  and conducted by Tianyang Chen, Jiande Wang, Bowen Tan, Kimberly Zhang, Harish Banda, Yugang Zhang and Dr. Dong-Ha Kim, the researchers developed a novel layered organic cathode material, bis-tetraaminobenzoquinone (TAQ). TAQ possesses a narrow electronic bandgap, high electrical conductivity, and a two-dimensional (2D) layered structure with strong hydrogen bonding, facilitating fast Na-ion diffusion while maintaining stability. Unlike traditional organic cathodes, TAQ is intrinsically insoluble, eliminating the need for additional stabilization strategies. By leveraging these structural advantages, the study aims to enhance both the energy density and power performance of SIBs while ensuring long-term cycling stability.

The study investigated the electrochemical performance of TAQ as a cathode material in SIBs. The researchers synthesized TAQ as microcrystals, where strong hydrogen bonding between carbonyl and amino groups forms closely stacked 2D molecular layers. This structural arrangement allows for efficient electron transport and facile Na+ intercalation. UV−vis−NIR spectroscopy and density functional theory (DFT) calculations confirmed a narrow bandgap of 0.75 eV, significantly lower than that of conventional organic cathodes, indicating enhanced electrical conductivity. Electrochemical testing of TAQ-based cathodes in sodium-ion half-cells revealed a high theoretical capacity of 355 mAh g⁻¹ per formula unit, facilitated by a four-electron redox process. The initial galvanostatic charge-discharge cycles exhibited distinct voltage plateaus, corresponding to sequential reductions of carbonyl functional groups. Operando powder X-ray diffraction (PXRD) analysis demonstrated minimal structural distortion during cycling, indicating excellent structural integrity. The authors also found that Na-ion diffusion within TAQ followed a quasi-2D pathway, resulting in a high intrinsic rate capability. To further enhance charge transport, the researchers incorporated carboxyl-functionalized single-walled carbon nanotubes (cSWCNTs) during TAQ crystallization. This approach enabled the formation of a TAQ-CNT composite, where intimate contact between TAQ crystallites and CNTs significantly improved electrical conductivity (8.1 mS cm⁻¹, a 400-fold increase over pristine TAQ). Electrochemical impedance spectroscopy (EIS) measurements confirmed a substantial reduction in charge transfer resistance and interfacial resistances, further enhancing rate performance. The optimized TAQ-CNT cathodes exhibited exceptional electrochemical performance. They delivered an electrode-level energy density of 606 Wh kg⁻¹ at 25 mA g⁻¹ and maintained 472 Wh kg⁻¹ at 10 A g⁻¹, demonstrating superior rate capability. The cells also displayed remarkable cycling stability, with a capacity retention of 94% after 5000 cycles at 2 A g⁻¹. Even at an ultrahigh current density of 20 A g⁻¹, the cathodes maintained an energy density of 241 Wh kg⁻¹ and a power density of 31.6 kW kg⁻¹.

In conclusion, Professor Mircea Dincă  and colleagues developed novel TAQ-based cathodes overcome the long-standing limitations of traditional organic electrodes by combining high energy density, excellent rate capability, and long-term cycling stability. The intrinsic insolubility of TAQ eliminates the need for excessive conductive additives, maximizing active material content and improving overall battery performance. Moreover, the two-dimensional diffusion pathways and enhanced electrical conductivity enable rapid Na-ion intercalation, making these cathodes suitable for high-power applications. The real-world applications of this research extend beyond SIBs. The development of high-performance, sustainable organic cathodes aligns with the growing demand for environmentally friendly energy storage solutions. Potential applications include grid-level storage for renewable energy integration, backup power systems for data centers, and electric vehicle batteries. Furthermore, the insights gained from this study can inform the design of next-generation organic electrode materials for other energy storage technologies.