This study illustrates the high complexity of phosphorus-based decomposition products in thermally treated state-of-the-art lithium ion battery (LIB) electrolytes. Liquid chromatographic techniques hyphenated to ion trap time-of-flight mass spectrometry reveal 122 different organophosphate (OP) and organofluorophosphate (OFP) species, the majority of which are not reported in the literature so far. The application of hydrophilic interaction liquid chromatography and reversed-phase chromatography enables the investigation of the acidic as well as nonacidic spectrum of aging products. Furthermore, the generation of high structure certainty by consideration of (i) mass accuracy of the precursor ions and subsequent MS2/3 fragments, (ii) fragment intensity distribution in the mass spectra, and (iii) retention times in hydrophilic interaction liquid chromatography (HILIC) and reversed-phase (RP) separation allows a target analysis of further work in the LIB electrolyte context. In an ethyl methyl carbonate-based battery electrolyte, 82 OP compounds, 27 OFPs, and 13 cyclic O(F)Ps are identified. Additionally, the formation of 8-membered organo(fluoro)phosphate rings in lithium ion battery electrolytes is reported for the first time. Since the high toxic potential of organo(fluoro)phosphates has emerged interest in safety assessments of electrolytes, the knowledge of possibly formed substances supports further quantification approaches and toxicological assessments compared to nontarget investigations.