Identifying the plateau sodium storage behaviour of hard carbon through the spin state

Hard carbon has emerged as a promising anode material for rechargeable sodium-ion batteries, owing to its abundant precursor sources and remarkable reversible charge storage capacity. The key to achieving an extended sodium storage capacity lies in the plateau capacity largely associated with the closed-pore structure, yet a comprehensive understanding in this area remains incomplete. In our study, we utilized electron paramagnetic resonance (EPR) spectra to discern the spin state of unpaired electrons within hard carbon materials, enabling the identification of sodium storage plateau regions. Hard carbon anodes with plateau capacity predominantly exhibit delocalized spins, evident in the EPR spectrum by limited spin density and a linewidth exceeding 100 G. These spins originate from the extended aromatic structure within small-sized but curved nano-graphite and the closed-pore walls, which are crucial for plateau capacity. Conversely, hard carbon anodes exclusively possessing slope capacity are characterized by localized spins, presenting a typical narrow and intense Lorentzian EPR signal. These localized spins arise from the edge defects of the nano-graphite structure, serving as favourable sites for sodium adsorption. Our development of a methodology to discern hard carbon plateau capacity behaviour based on EPR spectra presents a facile and rapid approach for advancing high-performance hard carbon materials.