Insight into the ion-dependent capacity mismatch in alkali metal ion batteries by in situ magnetometry

Sodium-ion batteries (SIBs) share similar working principles and cell configurations with lithium-ion batteries (LIBs), thus the advances of LIBs also spur the progress of SIBs. However, the obvious electrochemical performance discrepancy has been widely observed, and a comprehensive understanding of the underlying mechanism remains elusive. Herein, we develop and demonstrate a methodology to elucidate the physical and chemical origin of such a cell performance mismatch. By selecting FeTiO3/Na (Li) systems as representation, coupling in situ magnetometry with first-principles calculations, it is revealed that the inferior depth of conversion chemistry and limited redox reversibility of discharged products underlie the insurmountable plights of the insufficient specific capacity of SIBs. Furthermore, in situ magnetic response together with thermodynamic considerations strongly corroborates the spin-polarized capacitance in both systems that further heighten their differences in electrochemical properties. Our work unambiguously elucidates the distinguished reaction mechanisms of different charge carriers and paves the avenue to promote the development of high-performance electrodes for SIBs.