The effect of operating temperature on performance and aging of Zebra cells was investigated. The cells were cycled at 260°C and 350°C using a modified charging regime to accelerate damage. The cathode and solid electrolyte of degraded cells at the end of cycling were analyzed by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. It is hypothesized that the formation of AlF3 on the surface of the β″ alumina electrolyte contributes to the failure of cells at low temperatures. A mechanism based on the overcharge reaction is proposed for the formation of this phase. At 350°C, a significant capacity loss was observed with cycle life, which correlates to nickel particle growth as the cause of this phenomenon. The factors responsible for Ni grain growth are discussed.