Unravelling the structure-stability interplay of O3-type layered sodium cathode materials via precision spacing engineering

The O3-type layered oxide represents a highly promising candidate for sodium-ion batteries (SIBs). However, the intrinsic stability law of these cathodes remains elusive due to the complex phase transition mechanism and migration of transition metal (TM) ions. Here, we underscore how the ratio between the spacings of alkali metal layer and TM layer (R = d_O-Na-O/d_O-TM-O) plays a critical role in determining the structural stability and the corresponding electrochemical performance. We design a peculiar family of Na_xMn_0.4Ni_0.3Fe_0.15Li_0.1Ti_0.05O₂ (0.55 ≤ x ≤ 1) composition that is thermodynamically stable as an O3-type structure even when R is as high as 1.969, far exceeding 1.62 that normal O3-type structures can reach at most. The high R-value puts the O3 cathode in the preparatory stage for the O3-P3 phase transition, resulting in a rapid yet smooth phase transition process. It also induces a significantly stretched interstitial tetrahedral structure to the Na layer, thus effectively impeding TM migration. Leveraging this mechanism, we reexamine the underlying cause for enhanced stability in P2/O3 hybrid structure. Besides the conventional wisdom of an interlocking effect, the high R-value nature of its O3 sub-phase also plays a pivotal role.