Clarification of Decomposition Pathways in a State‐of‐the‐Art Lithium Ion Battery Electrolyte through ¹³C‐Labeling of Electrolyte Components

Abstract

The decomposition of state‐of‐the‐art lithium ion battery (LIB) electrolytes leads to a highly complex mixture during battery cell operation. Furthermore, thermal strain by e.g., fast charging can initiate the degradation and generate various compounds. The correlation of electrolyte decomposition products and LIB performance fading over life‐time is mainly unknown. The thermal and electrochemical degradation in electrolytes comprising 1 m LiPF6 dissolved in 13C3‐labeled ethylene carbonate (EC) and unlabeled diethyl carbonate is investigated and the corresponding reaction pathways are postulated. Furthermore, a fragmentation mechanism assumption for oligomeric compounds is depicted. Soluble decomposition products classes are examined and evaluated with liquid chromatography‐high resolution mass spectrometry. This study proposes a formation scheme for oligo phosphates as well as contradictory findings regarding phosphate‐carbonates, disproving monoglycolate methyl/ethyl carbonate as the central reactive species.

Analysis of Carbonate Decomposition During Solid Electrolyte Interphase Formation in Isotope‐Labeled Lithium Ion Battery Electrolytes: Extending the Knowledge about Electrolyte Soluble Species

“…The distinct assignment of labeled atoms from EC and unlabeled atoms from the linear solvent has to be ensured. This was proven for EL DEC, lab in a previous study [35] . The same behavior was observed for EL EMC, lab .…”

“…This would lead to EC‐based butyl carbonate radicals which could further react with any C−H bond via H‐abstraction and subsequent alkoxy termination to form the detected butyl carbonates (3) . Further, decarboxylation reactions as described in previous labeling study can explain the variation of the carbonyl carbon origin (4) [35] . The formation of ethene in EL DMC, lab and EL DEC, lab after cell formation was measured with a gas sampling port and could be clearly assigned to EC (Figure S6) [54,55] .…”

“…Due to the solvent molecule cross reactivity in EL DMC, lab , further conclusions regarding the carbonyl carbon origins in decomposition products cannot be ensured for this electrolyte formulation. A previous labeling study showed that carbonyl carbons of decomposition species in EL DEC, lab can be EC‐ and DEC‐based, probably due to decarboxylation reactions [35] . Therefore, further discussions of decomposition pathways will focus on alkyl chain atoms and carbonyl carbon atom origins are not addressed in this study.…”

“…revealed (di)lithium ethylene monocarbonate ((D)LEMC) as the main SEI component present [11] . Based on LEMC or DLEMC being the major SEI component and therefore, likely involved in most carbonate decomposition pathways, electrochemical carbonate decomposition was defined more precisely with an analytical study of a 13 C 3 ‐labeled EC/DEC electrolyte [35] . Analogously to nuclear magnetic resonance spectroscopy (NMR) studies performed by Grey and co‐workers, the origins of carbon atoms of different carbonate, carbonate‐phosphate and organo phosphate species were clarified and literature‐proposed reactivities reconsidered [36,37] .…”

Optimisation of Formation and Conditioning Protocols for Lithium‐Ion Electric Vehicle Batteries

“…LEMC can react further to produce various oligo-carbonates, and related compounds. [18] Several of these SEI components can be decomposed by HF. They may also dissolve and re-precipitate during conditioning.…”