Single-Crystal P2-Na_0.67Mn_0.67Ni_0.33O₂ Cathode Material with Improved Cycling Stability for Sodium-Ion Batteries

Layered oxides constitute one of the most promising cathode materials classes for large-scale sodium-ion batteries because of their high specific capacity, scalable synthesis, and low cost. However, their practical use is limited by their low energy density, physicochemical instability, and poor cycling stability. Aiming to mitigate these shortcomings, in this work, we synthesized polycrystalline (PC) and single-crystal (SC) P2-type Na_0.67-δMn_0.67Ni_0.33O₂ (NMNO) cathode materials through a solid-state route and evaluated their physicochemical and electrochemical performance. The SC-NMNO cathode with a large mean primary particle size (D₅₀) of 12.7 μm was found to exhibit high cycling stability leading to 47% higher capacity retention than PC-NMNO after 175 cycles at 1C rate in the potential window 4.2-1.5 V. This could be attributed to the effective mitigation of parasitic side reactions at the electrode-electrolyte interface and suppressed intergranular cracking induced by anisotropic volume changes. This is confirmed by the lower volume variation of SC-NMNO (ΔV ∼ 1.0%) compared to PC-NMNO (ΔV ∼ 1.4%) upon charging to 4.2 V. Additionally, the SC-NMNO cathode displayed slightly higher thermal stability compared to PC-NMNO. Both cathodes exhibited good chemical stability against air and water exposure, thus enabling material storage/handling in the ambient atmosphere as well as making them suitable for aqueous processing. In this regard, PC-NMNO was investigated with two low-cost aqueous binders, carboxymethyl cellulose, and sodium trimetaphosphate, which exhibited higher binding strength and displayed excellent electrochemical performance compared to PVDF, which could potentially lead to significant cost reduction in electrode manufacturing.