NOVELTY - Battery electrolyte solution (I), comprises: at least one lithium salt and a non-aqueous solvent in which the lithium salt is soluble, where 0.01-80 wt.% of (I) is at least one aromatic phosphorus compound (II).    USE - (I) is useful in an electrical battery, which is a secondary battery, preferably a lithium ion, lithium sulfur, lithium metal and lithium polymer battery (all claimed), which is useful: in electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, aerospace and e-bikes; for operating a large number of electrical and electronic devices (computers, cameras, video cameras, cell phones, personal digital assistants, MPEG-1 audio layer 3 (MP3) and other music players, televisions, toys, video game players, household appliances, power tools and medical devices (pacemakers and defibrillators)).    ADVANTAGE - The aromatic phosphorus compound: provides increased thermal stability for the electrolyte, and reduces thermal degradation, thermal runaway reactions and the possibility of burning; has little adverse impact on the electrical properties of the battery, and improves the battery performance; increases battery capacity utilization, discharge rate capability and/or cycling stability of the battery; and is effective at low levels of addition.    DETAILED DESCRIPTION - Battery electrolyte solution (I), comprises: at least one lithium salt and a non-aqueous solvent in which the lithium salt is soluble, where 0.01-80 wt.% of (I) is at least one aromatic phosphorus compound of formula (A1-(R-P(=O)-(R1)2)x) (II).    A1 = radical that contains aromatic rings;    R = alkylene diradical containing 1-3C and is bonded directly to C of an aromatic ring of the A1, preferably methylene, ethylene or isopropylene; either    R1 = H, halo, OH, hydrocarbyl having up to 12C or alkoxy having up to 12C; or    R1R1 = ring structure that includes P; and    x = 2.    An INDEPENDENT CLAIM is also included for an electrical battery comprising: an anode; cathode; separator disposed between the anode and cathode; and an electrolyte solution in contact with the anode and cathode, where the electrolyte solution (I).