Ethylene carbonate (EC) is the archetype solvent in Li-ion batteries. Still, questions remain regarding the numerous possible reaction pathways of EC. Although the reaction pathway involving direct EC reduction and solid electrolyte interface (SEI) formation is most commonly discussed, EC ring-opening is often observed, but seldom addressed, especially with respect to SEI formation. By applying online electrochemical mass spectrometry, the EC ring-opening reaction on carbon is found to start already at ~2.5 V vs. Li+/Li as initiated by oxygenic carbon surface groups. Later, OH- generated from H2O reduction reaction at ~1.6 V further propagates EC to ring-open. The EC reduction reaction occurs <0.9 V, but is suppressed depending on the extent of EC ring-opening at higher potentials. Electrode/electrolyte impurities and handling conditions are found to have a significant influence on both processes. In conclusion, SEI formation is shown to be governed by several kinetically competing reaction pathways whereby EC ring-opening can play a significant role.
Data-driven experimentation can accelerate battery research dramatically by closing the experimentation–analysis loop. Experimentation in traditional battery research is acknowledged to be heavily time-consuming and often suffers from large cell-to-cell variations. For closed-loop approaches, however, reliable and rapid performance evaluation is vital. Automation promises to enhance both the rate of testing and reproducibility. Herein, we present ODACell, an automated electrolyte formulation and battery assembly system, capable of preparing large batches of coin cells. We demonstrate the feasibility of Li-ion cell assembly in ambient atmosphere by preparing LiFePO4 || Li4Ti5O12 –based full cells with dimethyl sulfoxide–based model electrolyte. Furthermore, the influence of water is investigated to account for the hygroscopic nature of the non-aqueous electrolyte when exposed to ambient air. Reproducibility tests demonstrate a conservative fail rate of 5%, while the relative standard deviation of the discharge capacity after 10 cycles was 2% for the studied system. Electrolytes with 2 vol% and 4 vol% of water showed overlapping performance trends, highlighting the nontrivial relationship between water contaminants in electrolytes and cycling performance. Thus, reproducible data are essential to ascertain whether or not there are minor differences in performance for high-throughput electrolyte screenings. ODACell is broadly applicable to coin cell assembly with liquid electrolytes and therefore presents an essential step towards accelerating research and development of such systems.
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