Structure-Performance Relationship of Zn2+ Substitution in P2–Na0.66Ni0.33Mn0.67O2 with Different Ni/Mn Ratios for High-Energy Sodium-Ion Batteries

The P2-type Na0.66Ni0.33Mn0.67O2 electrode is one of the most promising layered cathodes, which exhibits high average potential of 3.5 V versus Na+/Na. When working at the high voltage range beyond 4.1 V versus Na+/Na, the Na0.66Ni0.33Mn0.67O2 electrode suffers from fast capacity decay. Substitution with electrochemically inactive metallic cations, such as Zn, as exhibited in our previous works, can dramatically relieve this phenomenon and improve the cyclability of Na0.66Ni0.33Mn0.67O2. Here, we designed a new Na0.66Ni0.33Zn0.07Mn0.60O1.93 compound by substituting Mn4+ ions in Na0.66Ni0.33Mn0.67O2 with Zn2+ and use it as cathode for sodium ion batteries. The successful preparation of targeted material was demonstrated by X-ray powder diffraction, 23Na MAS NMR, X-ray absorption spectroscopy, X-ray photoemission spectroscopy, and IEC-AES measurements. The electrochemical results demonstrate that Na0.66Ni0.33Zn0.07Mn0.60O1.93 sample exhibits improved cycling stability as compared to Na0.66Ni0.33Mn0.67O2 within both 2.0–4.4 and 2.0–4.1 V. However, the capacity retention after 100 cycles within 2.0–4.4 V of Na0.66Ni0.33Zn0.07Mn0.60O1.93 is lower than that of Na0.66Ni0.26Zn0.07Mn0.67O2. The smooth voltage profiles and increased Na+ ion diffusion coefficient beyond 4.0 V of Na0.66Ni0.26Zn0.07Mn0.67O2 indicate that the improved performances are originated from preventing Na+/vacancy ordering by substituting Ni2+ with Zn2+. Our results not only suggest that Zn2+ ions in the transition metal layers may serve as "pillars" to alleviate the structure destruction during repeated electrochemical cycling but also indicate that properly select elemental substitution sites are important for optimizing the chemical design strategy of high-energy Na ion battery cathodes.