Single-Crystal Growth of P2-Type Layered Oxides with Increased Exposure of {010} Planes for High-Performance Sodium-Ion Batteries

An increase in the size of single-crystal particles can effectively reduce the interfacial side reactions of layered oxides for sodium-ion batteries at high voltages but may result in sluggish Na⁺ transport. Herein, single-crystal Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂ with increased proportions of {010} planes is synthesized by adding low-cost NaCl as the molten salt. With the assistance of a NaCl molten salt, the median diameter (D50) of single-crystal Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂ increases to 10.46 μm relative to that of the comparison sample without NaCl (6.57 μm). Electrolyte decomposition on the surface of single-crystal Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂ is considerably suppressed, owing to a decrease in the specific surface area. Moreover, the increased exposure of {010} planes is favorable for improving the Na⁺ transport kinetics of single-crystal particles. Therefore, at 100 mA g^-1, single-crystal Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂ exhibits a high-capacity retention of 96.6% after 100 cycles, which is considerably greater than that of the comparison sample (86.8%). Moreover, the rate performance of single-crystal Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂ (average discharge capacity of 81.2 mAh g^-1) is superior to that of the comparison sample (average discharge capacity of 61.2 mAh g^-1) at 2000 mA g^-1. This work provides a new approach for promoting the single-crystal growth of layered oxides for highly stable interfaces at high voltages without compromising Na⁺ transport kinetics.