A cathode composition and a rechargeable electrochemical cell comprising same are disclosed. The cathode composition is described as comprising particles of one or more transition metal, alkali halometallate having a melting point of less than about 300 degrees Celsius, and at least one phosphorus composition additive selected from P—O compositions, P-halogen compositions, P—O-halogen compositions, and their reaction products and combinations. Also described is a rechargeable electrochemical cell comprising the composition. The phosphorus composition additive in the cathode composition of a cell is effective to lower the capacity degradation rate of the cell during operation relative to absence of the additive, and effective to lower the internal resistance of the cell when under operating conditions relative to absence of the additive.