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

Abstract

Sufficient interphase formation during the first cycles is crucial for the long‐term performance of lithium ion batteries. During the first cycles, electrolyte salt and solvent molecule decomposition caused by electrochemical instabilities leads to a wide range of decomposition species contributing to the formation of performance‐beneficial interphases at the electrodes as well as performance‐impairing side reactions. Due to structural similarities of carbonate educts, elucidation of underlying reaction pathways is complex. In this work, isotope‐labeled ethylene carbonate (13C3−EC) is applied to differentiate contributions of cyclic and linear carbonates to occurring interphase and decomposition reactions. Thereby, carbon atom tracing of electrolyte soluble species is performed by means of gas chromatography‐mass spectrometry (GC−MS). Reaction pathways are postulated based on identified molecular origins. Among others, a new DMC‐EC cross reactivity via lithium ethylene monocarbonate is considered. Moreover, radical reactions resulting in C−C bond formation are investigated and radical polymerizations of stoichiometric equivalents of ethene formed in situ during EC reduction are proposed. Finally, alkyl carbonates with C>4 alkyl chains are identified considering underlying reactivities. Discussed electrolyte decomposition reactions extend the understanding of EC‐based solid electrolyte interphase (SEI) formation emphasizing possible radical reactivities and solvent molecule contributions.

The Origin of Gaseous Decomposition Products Formed During SEI Formation Analyzed by Isotope Labeling in Lithium‐Ion Battery Electrolytes

“…The cells were filled with 700 μL of the respective electrolyte formulation (Table 1) via GSP. Following the cell was cut open to enable vacuum-sealing at 165 °C and a relative pressure of À 90 kPa for 5 s. For detailed description of the cell preparation and the GSP the reader is kindly referred to Schmiegel et al [29] After wetting for 20 h, two constant current/constant voltage (CC/ CV) charge (0.2 C; < 0.05 C) and CC discharge (0.2 C) cycles were performed in a voltage window between 3.0 and 4.2 V. [41] After formation, the emerged gas was directly extracted from the cell via GSP and injected to the GC system. The results of the DEC containing electrolyte are discussed in the main part of this paper, mass spectra (MS) of DMC and EMC containing electrolytes can be found in the supporting information.…”

“…[25][26][27][28][29][30][31][32][33][34][35][36][37][38] Moreover, application of isotope labeled electrolytes offers the possibility to clarify decomposition pathways and assign reaction products to certain electrolyte components. [26,28,[39][40][41] Onuki et al used 13 C 3 -EC and 13 C 5 -DEC as part of the electrolyte to confirm inter alia the reaction of EC forming C 2 H 4 in LIBs. [28] This publication is part of a series of studies on isotopically labeled electrolytes, which in this case focuses not on the liquid, but particularly on the gaseous products formed during formation.…”

“…[28] This publication is part of a series of studies on isotopically labeled electrolytes, which in this case focuses not on the liquid, but particularly on the gaseous products formed during formation. [40,41] Except for a few gaseous products like C 2 H 4 , where some studies indicate their origin, the data situation for several other gaseous decomposition products are insufficient and many proposed decomposition pathways in LIB are based on mechanistic conclusions rather than true analytical evidence. [4,42] This study aims to contribute more experimental data to the discussion.…”

The Impact of the C‐Rate on Gassing During Formation of NMC622 II Graphite Lithium‐Ion Battery Cells

“…C 2 H 4 is a well investigated electrolyte decomposition product, which is formed during formation of LIB cells. C 2 H 4 can clearly be traced back to decomposition of the cyclic carbonate EC in a one or two electron reduction process [23] . Resulting solid products like Li 2 CO 3 and (CH 2 OCO 2 Li) 2 are essential for the SEI formation [42,54,55] .…”

“…Hence, the SEI is composed of degradation products, which are generated via electrochemical reduction and precipitation on the surface [19,20] . Examples for solid inorganic and organic precipitates are Li 2 O, Li 2 CO 3 , LiF, ROCO 2 Li and a variety of polymers [20–23] . Due to an electronically insulating effect, the SEI protects the electrolyte from further degradation and the graphite from co‐intercalation of electrolyte and solvent molecules which would lead to exfoliation of graphite.…”