Beyond Lithium Paradigms: Distinct Electrochemo-Mechanical Behaviors of Sodium-Ion Batteries

Abstract

Understanding electrochemo-mechanical failure in sodium-ion batteries (SIBs) under mechanical abuse is essential for their safe deployment. Owing to similarities in electrochemical dynamics between lithium-and sodium-ion systems, electrochemomechanical coupling in SIBs is often assumed to follow lithium-ion paradigms. Here, we demonstrate that, in contrast to the pronounced state-of-charge (SOC)-induced hardening observed in lithium-ion batteries, SIBs exhibit a largely SOC-independent macroscopic mechanical response. Component-level characterization reveals an intrinsic mechanical compensation mechanism, in which mild hardening of the hard carbon anode is counterbalanced by concurrent softening of the cathode. Despite this mechanical invariance, the electrochemical response exhibits strong SOC dependence, manifested as a voltage bifurcation, with anomalous increases at low SOCs and monotonic decay at high SOCs.Decoupled half-cell analysis attributes this bifurcation to the competition between stressdependent thermodynamic potentials of the individual electrodes. Moreover, a universal failure sequence is identified across all SOCs, wherein a distinct voltage mutation, triggered by tensile fracture of electrode coatings, consistently precedes catastrophic structural collapse. These findings challenge the direct transfer of lithium-ion safety paradigms to sodium-ion systems and establish voltage evolution as a sensitive, zero-lag precursor for mechanically induced failure, providing fundamental insight into stress-potential coupling in SIBs.