The sodium cobalt bronze, , has been prepared in four different phases: O3, O′3, P3, and P2. The phase evolution and capacity of these bronzes has been investigated during sodium ion insertion and extraction in a nonaqueous electrolyte. The three phases, O3, O′3, and P3, exhibit equivalent behavior in that, regardless of initial phase, an evolution, O3 ↔ O′3 ↔ P3, occurs within the potential range, 2.3–3.5V. Measured capacity and discharge voltage are equivalent to 300 Wh/kg, and cycle life to 50% capacity is projected to be 340 cycles at 1 mA/cm2. The bronze prepared in the P2 phase exhibits thermodynamically reversible behavior in the potential range, 2.2–4.0V. The P2 phase displays a semi‐empirical energy density, equivalent to 440 Wh/kg as well as superior cycle life (1000 cycles to 50% capacity when cycled from 1.5 to 3.3V at 1 mA/cm2).