O3- vs P2-type Nax(Ni,Zn,Mn,Ti)O2 layered oxides: Comparative study on electrode-electrolyte reactivity and structural stability for cycling performance

Abstract

Sodium layered oxides, having either O3, P2 or P3 stacking, are extensively studied as low-cost cathode materials for high energy Na-ion batteries (NIBs). Previous efforts focused on the optimization of layered oxide compositions resulted in the O3-Na_0.85Ni_0.38Zn_0.04Mn_0.48Ti_0.1O₂ and P2-Na_0.67Ni_0.3Zn_0.03Mn_0.52Ti_0.15O₂ phases as potential candidates to establish prototype cylindrical 18650 cells with 120-150 Wh/kg specific cell energy. In this study, we focus particularly on the electrode-electrolyte reactivity of these phases, especially at high state of charge (∼70 % or more) and at high temperatures. Our results indicate that the end-of-charge phase, O1 and O2 formed during complete de-sodiation of O3 and P2, respectively, plays a major role in determining their reactivity. The O1 phase is particularly prone to transition metal migration and oxygen oxidation, having increased reactivity with electrolyte. On the other hand, the P2 layered oxide, while having lower capacity than O3, offers better cycling stability (90 % retention after 1000 cycles at 25 °C) due to the greater stability of the O2 end-of-charge structure. These results once again underline the fact that specific capacity should not be the sole metric for determining the most suitable electrode materials for Na-ion or other battery chemistries.