Elucidating Lithium- and Sodium-Ion Storage Mechanisms in Layered Na ₂ Ti ₃ O ₇

Layered titanates are widely recognized for their energy storage applications; however, their practical performance is often limited by low energy density arising from limited vacant sites. In this study, Na 2 Ti 3 O 7 (NTO) is investigated as an anode material for both LIB and SIB to systematically understand the effect of ionic size on electrochemical behavior and structural stability. The NTO||Li cell exhibits capacity retention values of 70 and 17% at 0.2 C and 2 C, respectively, whereas the NTO||Na system retains only 50 and 10% capacity at the corresponding C-rates. In situ Raman spectroscopy further reveals the structural evolution of Na 2 Ti 3 O 7 during electrochemical cycling through changes in Ti-O vibrations and lattice distortion, with a larger decrease in the I D / I G ratio for NTO||Li (25.31%) than for NTO||Na (21.73%), indicating stronger structural and electronic perturbation in the lithium system. In situ EIS analysis reveals significantly higher R SEI and R ct in the NTO||Na compared to NTO||Li, with R ct in the Na system rising sharply (up to 371 Ω) at low voltages during discharge. The NTO||Li cell exhibits lower interfacial resistance and improved kinetics from the 1 st to 2 nd charge cycle, consistent with its lower activation energy and more favorable charge-transfer behavior. ToF-SIMS analysis, both at low and high C-rates, along with ex situ cross-sectional FESEM imaging, reveals significant surface cracking in the NTO||Na electrode after GCD cycling, accompanied by the presence of COOH −, PO −, and C 2 HO − species on the surface. These observations are further corroborated by ex situ EXAFS studies, highlighting the structural distortion induced by the insertion of larger sodium ions. Overall, this work emphasizes the necessity of detailed investigations into Li + and Na + intercalation mechanisms to optimize the electrochemical performance and long-term stability of NTO-based anodes for advanced energy storage applications.