Electron Paramagnetic Resonance as a Probe for Sodium-Ion Battery Materials: Recent Advances and Future Perspectives

Sodium-ion batteries (SIBs) are emerging as cost-effective alternatives to lithium-ion batteries (LIBs), particularly for large-scale stationary energy storage systems. The design of novel electrode materials depends on a deeper understanding of their kinetics and structural properties to optimize battery performance and commercialize SIBs. The structural evolution of electrodes, sodium-ion diffusion, and the reconstruction of electrode-electrolyte interfaces strongly influence the battery metrics of SIBs. Among all the material characterization techniques for SIBs, including diffraction, microscopy, and spectroscopy, electron paramagnetic resonance (EPR) spectroscopy has emerged as a vital tool for probing the electronic states of materials. EPR, capable of analyzing free radicals, paramagnetic species, and unpaired electron spins, offers unique advantages for understanding charge storage mechanisms involving redox reactions in SIBs. Although challenging, recent advancements in operando and in situ experiments enable precise studies of the mechanisms of sodium-ion insertion and deinsertion during charge-discharge cycles. The review examines the impact of EPR on advancing SIB research, highlighting pivotal contributions in experimental methodologies and their applications across diverse materials. By showcasing recent advances in navigating the complexities of EPR integration, we provide a perspective on realizing the full potential of EPR in the design of SIB electrode materials.