NOVELTY - Battery comprises an anode, a cathode, a separator between the anode and the cathode, and an electrolyte composition in contact with the anode, the cathode, and the separator, where the electrolyte composition comprises one or more phosphonium ionic liquids, or one or more phosphonium salts dissolved in a solvent, and the phosphonium ionic liquids or phosphonium salts comprises one or more phosphonium based cations (I) and one or more anions.    USE - The battery is useful in electronic devices such as memory devices including static, permanent and dynamic random access memory, energy storage devices such as batteries, electrochemical double layer capacitors or super capacitors or ultracapacitors, electrolytic capacitors, as electrolytes in dye-sensitized solar cells, fuel cells, a heat transfer medium, high temperature reactions and extraction media.    ADVANTAGE - The battery exhibits thermodynamic stability, low volatility, wide liquidus range and ionic conductivity.    DETAILED DESCRIPTION - Battery comprises an anode, a cathode, a separator between the anode and the cathode, and an electrolyte composition in contact with the anode, the cathode and the separator, where the electrolyte composition comprises one or more phosphonium ionic liquids or one or more phosphonium salts dissolved in a solvent, and the phosphonium ionic liquids or phosphonium salts comprising one or more phosphonium based cations of formula (R1R2R3R4P) (I) and one or more anions.    R1-R4 = alkyl group.    INDEPENDENT CLAIMS are included for:    (1) a hybrid energy storage system comprising an array of batteries and an array of electrochemical double layer capacitors (EDLCs); and    (2) a lithium battery having an anode, a cathode and a separator between the anode and the cathode, and an electrolyte composition in contact with the anode, the cathode, and the separator, characterized in that: the electrolyte composition comprises one or more lithium salts and or one or more phosphonium salts dissolved in a solvent.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of bipolar electrode and multi-cell stack structures of a battery.    Multi-cell battery (30)    Uint cell. (32, 34, 36, 38)