TECHNOLOGY FOCUS - METALLURGY - Preferred Component: The electrolyte salt comprises at least one cation and at least one weakly coordinating anion. The cation is hydrogen, alkali metal, alkaline earth metal, Group 11B metal, Group IIIB metal, transition metal, rare earth metal, or ammonium cations. The electrolyte salt is a lithium salt. The positive electrode is a composite oxide comprising lithium and transition metal element(s). The negative electrode is carbon materials, lithium (preferably), lithium containing alloys, or compounds that alloy with lithium. The lithium (Li) species are oxidized or reduced at a potential of greater than or equal to3V (4.2-8.5V) versus Li/Li+. The cathode is LiaNibCocMndO2. The anode is carbon materials, lithium, or lithium alloys. The carbon materials are graphitized carbon, partially graphitized carbon, or non-graphitized carbon. The battery has a graphitized carbon anode.a=greater than 0.8 but less than 1.2;b=greater than 0 but less than 1;c=greater than 0 but less than 1; andd=greater than 0 but less than 1.Preferred Property: The composition has an oxidative stability of up to 8V (up to 5V) versus Li/Li+. The battery has a discharge capacity of greater than 80% (greater than 40%) at a discharge current of up to 12 CmA, and retains over 90% of its discharge capacity at 25degreesC when exposed to 0degreesC to -20degreesC. It can retain a discharge capacity of greater than 150 mAh per gram of cathode over up to 30 charging cycles at up to 4.5V.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Component: The oxymethylene group is substituted. At least one carbon-bonded hydrogen atom of the oxymethylene group is replaced with an alkyl or fluoroalkyl group that optionally contains catenated heteroatom(s). The hydrogen atom of the fluoroalkyl groups is part of a monofluoromethylene moiety. The compound is of formula Rf'-CF(CFH-Rf')CR1R2-O-CF(Rf')-CFH-Rf'" (I) or H(CF2)p-CR1R2-O-CF(Rf')-CFH-Rf' (II). The solvent composition comprises at least one organic or fluorine-containing electrolyte solvent that is different from the hydrofluoroether compound. The organic or fluorine-containing electrolyte solvent is propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethoxyethane, gamma-butyrolactone, diglyme, tetraglyme, tetrahydrofuran, alkyl-substituted tetrahydrofuran, 1,3-dioxolane, alkyl-substituted 1,3-dioxolane, monofluoroethylene carbonate, tetrahydropyran, and/or alkyl-substituted tetrahydropyran. The electrolyte salt comprises fluorine-containing inorganic anions; chlorate, sulfuric acid, phosphoric acid, alkane, aryl, and alkaryl sulfonates; fluorine-containing and unfluorinated tetraarylborates; carborane anions; halogen-, alkyl-, or haloalkyl-substituted carborane anions; or fluorine-containing organic anions. The anion is fluorine-containing inorganic anions or fluorine-containing organic anions.Rf'=F or a perfluoroalkyl group that is linear, branched, and/or cyclic and that optionally contains catenated heteroatom(s), preferably F or C3F7-;Rf"'=F or a perfluoroalkyl group that is linear, branched, and/or cyclic and that optionally contains catenated heteroatom(s), preferably F, C3F7O-, C4F9O-, C3F7OC3F6O-, CF3OC3F6O-, or CF3;Rf"=perfluoroalkyl group that is linear, branched, and/or cyclic and that optionally contains catenated heteroatom(s), preferably C3F7O-, C4F9O-, C3F7OC3F6O-, CF3OC3F6O-, or CF3-;R1=H, alkyl group that is linear, branched, and/or cyclic and that optionally contains catenated heteroatom(s), or a fluoroalkyl group that is linear, branched, and/or cyclic and that optionally contains catenated heteroatom(s), preferably H or Me;R2=H or alkyl group that is linear, branched, and/or cyclic (preferably), or -(CR1R3)-O-CF(Rf')-CFH-Rf", preferably Me;R3=H or alkyl group that is linear, branched, and/or cyclic; or -CF(Rf')-CFH-Rf";n=1-8; andp=2-10.Preferred Composition: The solvent composition comprises more than zero and up to 80 (10-80) vol.% organic or fluorine containing electrolyte solvent. The composition comprises 0.1-2 mol/l inorganic electrolyte salt. Preferred Property: The hydrofluoroether compound in the composition has an oxidative stability of up to 8V (up to 5V) versus Li/Li+. SPECIFIC COMPOUNDS - There are 73 specific compounds, e.g. CF3CFHCF2CH2OCF2CFHOC3F7.    EXAMPLE - A sample was prepared by dissolving 1 mole of lithium hexafluorophosphate in 1 L solvent mixture, and brought to equilibrium at a temperature. The solvent mixture contained varying proportions of ethylmethylcarbonate, and a hydrofluoroether. The electrolyte compositions were clear, uniform, single phase at a given temperature, and stable at that temperature.