Investigation of Gas Evolution by Operando GC/MS from SEI Forming Additives in Lithium-Ion-Batteries

Abstract

Due to the chemical reactivity of the electrolyte, electrolyte decomposition often occurs at the electrode-electrolyte interface during cycling, which can lead to a loss in LIB performance. On the anode side, however, the formation of a stable Solid Electrolyte Interphase (SEI) is essential for the safe operation of Lithium-ion batteries (LIBs). The SEI layer, which is permeable for Li+ ions, protects the electroactive materials from direct contact with the electrolyte, thereby inhibiting further electrolyte decomposition. In the formation cycles of fresh cells, electrolyte degradation is primarily attributed to the SEI formation processes at the anode. Several literature reports have confirmed that the newly formed interphase consists of organic and inorganic decomposition products of the electrolyte.1 To increase battery performance, cycle life and safety, it is essential to form a stable SEI.2 A common strategy to stabilize the SEI is to use additives in the electrolyte composition. The additives should exhibit higher reduction potentials than other electrolyte components to enable preferential reduction and incorporation into the SEI. The most common SEI forming additives are fluoroethylene carbonate (FEC) and vinylene carbonate (VC). They both decompose during the formation cycle to form a polymeric layer at the electrode surface.The electrolyte decomposition reactions which lead to SEI formation are usually accompanied by gaseous side products. The evolving gas species form a complex gas mixture which can be separated by gas chromatography (GC) into their individual components and subsequently identified by mass spectrometry (MS). While operando GC/MS gives information on the volatile side products of the electrolyte decomposition reactions, X-ray photoelectron spectroscopy (XPS) can be used to identify the degradation products that form on the surface of the electrode and are incorporated into the SEI. The combination of the two techniques allows a more comprehensive picture of SEI formation processes to be drawn.In this work, operando GC/MS of Lithium-ion full cells with NMC 811 cathodes and graphite anodes was used to compare the gas species evolving during the formation reactions in an electrolyte with the composition of 1M LiPF6 in EC/DEC 1:1 with FEC and VC electrolyte additives in a ratio of 1w%, respectively. In a comparative study we present the decomposition pathways of the electrolytes with and without SEI forming additives by analysing the composition of the gas phase as a function of cell potential. We observe that the complex gas mixture consists of carbonate components originating from transesterification reactions and evaporation of the volatile electrolyte. In addition, inorganic components such as carbon monoxide and carbon dioxide are produced, which mainly stem from the degradation of cyclic carbonates. Furthermore, the gas phase consists of saturated and unsaturated hydrocarbons which can be ascribed to the linear carbonate components as well as ether and carbonyl components. We have also observed that it is possible to distinguish between volatile compounds which form during the charge and discharge processes, respectively. The increased gas evolution at the beginning of the formation cycle confirms that a substantial part of the passivation layer is formed during charge. Finally, operando GC/MS was also used to monitor the evolution of the decomposition products during battery operation at overcharge conditions.Acknowledgement:The author gratefully acknowledges the FFG (Austrian Research Promotion Agency) for funding this research within project No. 879613