Mitigating Mechanical Stress by the Hierarchical Crystalline Domain for High-Energy P2/O3 Biphasic Cathode Materials

Sodium-ion batteries (SIBs) have captured widespread attention for grid-scale energy storage owing to the wide distribution and low cost of sodium resources. Delivery of high energy density with stable retention remains a challenge in developing cathode candidates for rechargeable SIBs. Inspired by the concept of "cationic potential", here, we present a hierarchical crystalline domain in hexagonal particles with target chemical composition (Na_0.8Li_0.03Mg_0.05Ni_0.28Fe_0.05Mn_0.54Ti_0.05O₂) from the inner bulk O3 phase (71.1 wt %) to the outer P2-type shell (28.9 wt %) of the structure. Benefiting from the mitigated mechanical stress of the predominant bulk O3 phase under the protection of the surficial P2 crystalline domain at the microscale during Na⁺ (de)intercalation, the brittle fracture, plastic yielding, and structural damage of the bulk O3 phase are effectively prohibited during battery cycling, thereby achieving good structural integrity. As a consequence, the biphasic P2/O3-Na_0.8Li_0.03Mg_0.05Ni_0.28Fe_0.05Mn_0.54Ti_0.05O₂ material exhibits satisfactory electrochemical properties, with a high energy density of 506 Wh kg^-1 and good capacity retention of 85.5% over 200 cycles. This work highlights the importance of tailoring the crystalline domain to mitigate the reaction-induced stress and particle fracture of layered biphasic cathode materials for high-energy SIBs.