Understanding the Beneficial Role of Transition-Metal Layer Na+ Substitution on the Structure and Electrochemical Properties of the P2-Layered Cathode Na2+xNi2–x/2TeO6

Abstract

Layered Na_xMO₂ sodium oxide positive electrode materials have experienced renewed interest owing to the current commercial attention on sodium-ion batteries. Although there are many attractive qualities of these materials, they suffer from serious shortcomings owing to Na⁺ ordering and transition-metal layer gliding that cause a plethora of voltage plateaus during cycling. The P2-layered Na_2+xNi_2–x/2TeO₆ (0 ≤ x ≤ 0.5) system provides a framework for investigating the effect of dual Na⁺ substitution into the sodium layer and the transition-metal layer of the structure and its effects on the electrochemical properties of the materials. A careful investigation into the synthesis and properties of these materials reveals that the sodium content used during material preparation has a drastic effect on the composition and electrochemical profile of these materials. The sodium substitution disrupts ordering within the transition-metal layer, thereby disrupting Na⁺ ordering in the adjacent sodium layers. Beyond a critical sodium concentration, the layer stacking shifts, and all voltage plateaus of the P2-Na₂Ni₂TeO₆ material are no longer observed at 4.4 V versus Na⁺/Na. These results also question the common belief that additional sodium precursor is required when preparing layered sodium oxide cathodes, providing new guidelines for material synthesis and characterization.