Using a dual-site substitution strategy to inhibit staircase-like electrochemical profiles for high-performance Na-ion battery cathodes

Abstract

Layered oxide materials with P2-type structure have attracted intensive interest because of their low cost and high security and the abundance of sodium sources. However, the application of P2-type materials is severely inhibited by complex Na⁺/vacancy ordering and unfavorable phase transitions in electrochemical processes, which lead to staircase-like voltage profiles accompanied by fast capacity decay and poor rate capability. In this work, we utilize the dual-site substitution strategy to design a P′2-type (Cmcm symmetry) material, K_0.05Na_0.62[Mn_0.9Ti_0.1]O₂ (KNMTO). Structural characterization indicates that Mn^3+/4+ and Ti⁴⁺ ions occupy the transition metal layers and K⁺ and Na⁺ ions occupy the alkali-metal layers. Electrochemical tests indicate that KNMTO exhibits excellent electrochemical behavior with comparatively smooth voltage profiles and staircase-like voltage plateaus are effectively inhibited. KNMTO delivers a reversible capacity of 210 mA h g⁻¹ in the voltage range of 1.5–4.3 V and has a capacity retention of 80% after 100 cycles at a current density of 150 mA g⁻¹ in full cells. In the low-voltage region, the material undergoes a solid-solution reaction without unfavorable phase transitions due to the synergetic effect of dual-site substitution on interrupting the ordered Na/vacancy arrangement/re-arrangement in the lattice. In the high-voltage region, the material shows P′2–OP4 phase transition with the occurrence of an intermediate, which can be accurately described as an intergrowth of P-type stacking and O-type stacking between P′2 and OP4 phases (if we consider the OP4 phase as 50% O-type stacking within the P2 structure). The intergrowth structure somewhat defers the instant structural collapse and smooths the steep electrochemical plateau caused by the biphasic reaction. This work reports the dual-site substitution strategy in improving the electrochemical behavior of layered oxide cathode materials and provides a synergistic modification method to achieve high-performance layered cathode materials for sodium-ion batteries.