In operando X-shaped cell online electrochemical mass spectrometry (OEMS): New online analysis enables insight into lab scale lithium ion batteries during operation

• New method for the in operando analysis of evolving gases during cyclic aging of lithium ion batteries
• Application of the new system to LiNi1/3Co1/3Mn1/3O2 (NCM)/Li half cells
• Identification of decomposition products with a new method in commercially electrolytes and self-assembled cells

A new method for the in operando analysis of evolving gases during cyclic aging of lithium ion batteries (LIBs) was developed to better assess safety concerning cell processes, especially those arising from the electrochemical degradation of the lithium hexafluorophosphate LiPF6/organic carbonate solvent based electrolyte. For electrochemical characterization at lab-scale, a cell in the shape of T-connector (“T-cell”) is usually used, offering connections to working, counter and reference electrode. To maintain comparability to this established system, an in operando X-shaped cell, i.e., a T-cell (“X-cell”), which varies only by an additional connector from the original setup, was designed. The new OEMS cell based on DEMS cell designs was linked to a modified GC–MS System and a potentiostat for in operando analysis of the evolving gases and the voltammetry experiments, respectively. This work comprises the evaluation of this new OEMS method in potentiostatic aging experiments of the conventional electrolyte 1M LiPF6 in EC:EMC (1:1, by wt.) in LiNi1/3Co1/3Mn1/3O2 (NCM)/Li half cells as a function of the applied cut-off potential. Mainly CO2 release at onset potentials > 4.6 V vs. Li/Li+ could be identified. At a potential of > 5.4 V vs. Li/Li+, the evolution of silicon tetrafluoride (SiF4) was observed mainly stemming from the HF induced degradation of the used glass fiber separator. Furthermore, triethyl phosphate (TEP) evolved from the LiPF6 decomposition at > 5.5 V vs. Li/Li+. Oxygen evolution either coming from the oxidative decomposition of the electrolyte or degradation of the NCM cathode material was not detected at even 5.5 V vs. Li/Li+ and at 20 °C.