On Disrupting the Na⁺-Ion/Vacancy Ordering in P2-Type Sodium-Manganese-Nickel Oxide Cathodes for Na⁺-Ion Batteries

An investigation of the electrochemical and structural properties of layered P2-Na_0.62Mn_0.75Ni_0.25O₂ is presented. The effect of changing the Mn/Ni ratio (3:1) from what is found in Na_0.67Mn_0.67Ni_0.33O₂ (2:1) and consequently the introduction of a third metal center (Mn³⁺) was investigated. X-ray powder diffraction (in situ and ex situ) revealed the lack of Na⁺-ion/vacancy ordering at the relevant sodium contents (x = 0.33, 0.5, and 0.67). Mn³⁺ in Na_0.62Mn_0.75Ni_0.25O₂ introduces defects into the Ni-Mn interplane charge order that in turn disrupts the ordering within the Na-plane. The material underwent P2-O2 and P2-P2′ phase transitions at high (4.2 V) and low (∼1.85 V) voltages, respectively. The material was tested at several different voltage ranges to understand the effect of the phase transitions on the capacity retention. Interestingly, the inclusion of both phase transitions demonstrated comparable cycling performance to when both phase transitions were excluded. Last, excellent rate performance was demonstrated between 4.3 and 1.5 V with a specific capacity of 120 mA h/g delivered at 500 mA/g current density.