Effects of Air/Water-Stability, “Aqueous” Processing and Binder-Type on the Chemical-Mechanical-Electrochemical Stability of Na-Titanate Based Anodes for Na-Ion Batteries

Abstract

In the contexts of easing the handling/storage and facilitating “aqueous” processing of Na-titanate based electrodes (as potential anodes for Na-ion batteries), the present work compares the compositional/phase/structural stability of Na2Ti3O7, Na2Ti6O13 and “Bi-phase NTO” (having ∼72 wt%Na2Ti3O7 + ∼28 wt%Na2Ti6O13) upon being exposed to air and water. This reveals the long-term air/water-stability of Na2Ti6O13 and “Bi-phase NTO”, having the Na2Ti6O13 component at/closer to particle surface and the Na2Ti3O7 component (primarly, at/closer to core) with shrunk lattice parameters; thus, rendering “aqueous” processing of electrodes feasible even for “Bi-phase NTO”, unlike for Na2Ti3O7. Furthermore, the usage of Na-alginate binder, as rendered possible by “aqueous” processing, bestows the “Bi-phase NTO” electrode with vastly superior chemical, mechanical and electrochemical stability, as compared to the usage of PVDF for “non-aqueous” processing. As confirmed by top-view/cross-section electron microscopy, depth-profiling XPS and in-situ/operando stress measurements during rest (post cell-assembly), as well as electrochemical cycling, this is primarily due to the deterioration/decomposition and swelling of PVDF in contact with cyclic carbonate-based electrolyte. Overall, this work reveals that the superior air/water-stability of “Bi-phase NTO” electrodes not only aids in storage/handling, but also facilitates cost-effective and environment/health-friendly “aqueous processing” of Na-titanate based anodes for Na-ion batteries, where the usage of Na-alginate as binder bestows further stability.