Structural evolution and electrochemistry of the Mn-Rich P2– Na2/3Mn0.9Ti0.05Fe0.05O2 positive electrode material

Positive electrodes still limit the maximum attainable energy density of Na-ion batteries. Increasing the amount of electrochemically active transition metal is one way of improving energy density. However, this is complicated by the balance between initial capacity and structural stability/capacity retention. Here, the Mn-rich P2– Na2/3Mn0.9Fe0.05Ti0.05O2 is synthesised via the solid-state method and its structural evolution during operation, between 1.9 and 4.2 V, investigated. Through operando X-ray powder diffraction data, no evidence is found for the formation of Z, OP4 or O2 phases and the material primarily displays regions of two-phase coexistence. P2– Na2/3Mn0.9Fe0.05Ti0.05O2 delivers a second charge/discharge capacity of 152/164 mAh.g−1 at C/10, within the voltage range 4.0–2.0 V and retains 75% of the dis-charge capacity at the 50th cycle. A detailed comparison, in terms of both the electrochemical performance and structural evolution, to related Mn-rich phases is provided. The results further demonstrate that structural stability and battery performance can be improved through subtle co-substitutions.