Sodium-ion batteries (SIBs) have emerged as a compelling alternative to lithium-ion batteries (LIBs), exhibiting comparable electrochemical performance while capitalizing on the abundant availability of sodium resources. In SIBs, P2/O3 biphasic cathodes, despite their high energy, require furthur improvements in stability to meet current energy demands. This study introduces a systematic methodology that leverages the meta-heuristically assisted NSGA-II algorithm to optimize multi-element doping in electrode materials, aiming to transcend conventional trial-and-error methods and enhance cathode capacity by the synergistic integration of P2 and O3 phases. A comprehensive phase analysis of the meta-heuristically designed cathode material Na0.76Ni0.20Mn0.42Fe0.30Mg0.04Ti0.015Zr0.025O2 (D-NFMO) is presented, showcasing its remarkable initial reversible capacity of 175.5 mAh g-1 and exceptional long-term cyclic stability in sodium cells. The investigation of structural composition and the stabilizing mechanisms is performed through the integration of multiple characterization techniques. Remarkably, the irreversible phase transition of P2→OP4 in D-NFMO is observed to be dramatically suppressed, leading to a substantial enhancement in cycling stability. The comparison with the pristine cathode (P-NFMO) offers profound insights into the long-term electrochemical stability of D-NFMO, highlighting its potential as a high-voltage cathode material utilizing abundant earth elements in SIBs. This study opens up new possibilities for future advancements in sodium-ion battery technology.

Z phase is one of the three basic units by which the Frank-Kasper (F-K) phases are generally assembled. Compared to the other two basic units, that is, A15 and C15 structures, the Z structure is rarely experimentally observed because of a relatively large volume ratio among the constituents to inhibit its formation. Moreover, the discovered Z structures are generally the three-dimensional ordered Gibbs bulk phases to conform to their thermodynamic stability. Here, we confirmed the existence of a metastable two-dimensional F-K Z phase that has only one unit-cell height in the crystallography in a model Mg-Sm-Zn system, using atomic-scale scanning transmission electron microscopy combined with the first-principles calculations. Self-adapted atomic shuffling can convert the simple hexagonal close-packed structure to the topologically close-packed F-K Z phase. This finding provides new insight into understanding the formation mechanism and clustering behavior of the F-K phases and even quasicrystals in general condensed matters.

In this study, 22Cr25NiWCuCo(Nb) heat-resistant steel specimens with high Cr and Ni contents were adopted to investigate the effect of Nb content on thermal and precipitation behavior. Differential scanning calorimetry profiles revealed that the melting point of the 22Cr25NiWCuCo(Nb) steel specimens decreased slightly with the Nb content. After heat treatment at 1200 °C for 2 h, the precipitates dissolved in a Nb-free steel matrix. In addition, the Z phase (CrNb(C, N)) and MX (Nb(C, N), (Cr, Fe)(C, N), and NbC) could be observed in the Nb-containing steel specimens. The amount and volume fraction of the precipitates increased with the Nb content, and the precipitates were distributed heterogeneously along the grain boundary and inside the grain. Even when the heat treatment duration was extended to 6 h, the austenitic grain size and precipitates became coarser; the volume fraction of the precipitates also increased at 1200 °C. The Z phase, rather than the MX phase, became the dominant precipitates at this temperature.