Tailoring the microstructure of high mass loading cathodes for sodium-ion batteries: The effect of densification on electronic and electrochemical properties

Sodium-ion batteries (SIBs) are a more sustainable alternative to lithium-ion batteries (LIBs), primarily due to sodium's greater abundance and accessibility. While SIBs principle of operation is similar to LIBs, and comparable intercalation cathode active materials (CAM) such as NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFMO) are used, their energy density is lower due to their lower electrochemical potential and specific capacity. Improving the properties of CAM and optimizing electrode design are both key strategies for advancing SIB performance. For cathodes with a high active mass loading, which are needed for competitive sodium-ion batteries, the electrode microstructure is key, as it significantly affects electrode kinetics. Microstructure-performance correlations have been intensively studied for lithium-ion batteries. However, for SIBs with their new cathode active materials, there are only a few datasets available. In our study, we investigate how the densification during calendering affects NFMO-based cathodes with a high mass loading of 23.8 mg cm −2 (2.86 mAh cm −2 ), focusing on changes in electrode microstructure, electronic and ionic conductivity, and overall electrochemical performance. By increasing the electrode density from 1.87 to 2.44 g cm −3 , we examine the impact of densification on microstructural evolution using scanning electron microscopy (SEM) and Raman spectroscopy. Moreover, we assessed the influence of microstructure on the electrode performance characteristics using complementary electronic and electrochemical methods, including rate-capability tests, electrochemical impedance spectroscopy (EIS), and through-plane electronic conductivity measurements to develop a comprehensive understanding of the governing microstructure-property correlations and to derive electrode design guidelines. • Microstructural evolution of high-mass loading NFMO cathodes due to compaction to densities of 1.87, 2.22 and 2.44 g cm −3 . • Electronic conductivity measurements and impedance spectroscopy for electronic and ionic conductivity contributions. • Performance evaluation up to a current rate of 5C. • Correlation of electrode densification with microstructural evolution and electrochemical performance.