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Online Electrochemical Mass Spectrometry (OEMS) provides unparalleled access to the details of electrode/electrolyte interfacial reactions in electrochemical systems. Herein, the development and validation of an OEMS system along with detailed calibration protocols and limits of detection sensitivity are showcased. Combined partial and total pressure monitoring provides a clear advantage when detailing major and minor gas reactions as well as when determining unaccounted gases. A classical Li-ion LiCoO2/Graphite full cell is studied during overcharge to 4.9 V vs. Li+/Li at 50 degrees C at an unprecedented level of detail and the results are compared to LiCoO2/LiFePO4 and Graphite/LiFePO4 cells in order to differentiate between gases forming at the anode and cathode. The release of O-2 from LixCoO2 (x < 0.4) during both charge and discharge demonstrates that its degradation is dependent on state of charge 1-x rather than potential. The presented methodology establishes an improved experimental basis for deeper understanding of interfacial reactions in batteries and electrochemical systems alike.

The “ethylene carbonate (EC)–propylene carbonate (PC) mystery” has puzzled electrochemists for decades. Surprisingly, the minor structural difference between PC and EC, a methyl vis-à-vis a proton, prevents PC unlike EC to form a stable solid electrolyte interphase (SEI) on carbon (C), which along with the popularity of PC has impeded the development of Li-ion batteries with many years. Despite several hypotheses, the fundamental reason remains debated largely due to the lack of sufficient experimental evidence. Herein, SEI formed as a result of EC and PC reductions are analyzed by two state-of-the-art operando techniques, online electrochemical mass spectrometry and electrochemical quartz crystal microbalance with dissipation monitoring. Although both EC- and PC-based electrolytes appear to have virtually identical reaction pathways, PC is reduced much more extensively than EC and forms a much thicker SEI. However, while the SEI derived from EC remains on the electrode, PC reduction products redissolve in the electrolyte leaving the bare C electrode behind. The presented study illustrates the complex scheme of competing electro-/chemical reactions behind SEI formation and provides further scientific details needed to eventually form a consensus of the processes governing electrode/electrolyte interphases in Li-ion batteries.

Ethylene carbonate (EC) and vinylene carbonate (VC) are the archetypical electrolyte solvent and additive in Li-ion batteries (LIBs), respectively. However, our understanding of their reaction pathways remains incomplete. Herein, the reaction pathways of EC and VC are explored by using online electrochemical mass spectrometry complemented by nuclear magnetic resonance analysis. For EC, reduction occurs through two distinct pathways <0.8 V vs Li⁺/Li, one yielding C₂H₄ and the other yielding CO, depending on the electrode potential and the EC concentration. The CO-releasing pathway does not contribute to the solid electrolyte interphase formation. For VC, reduction commences at <1.9 V, but CO₂ gas evolution proceeds through a chemical step via a nucleophilic attack and VC ring opening. Additionally, VC scavenges H₂O and reduced protons via hydrolysis and via proton abstraction from the carbon electrode to form EC. Our study uncovers further reaction pathways and underscores the unique properties of EC and VC, both individually and in combination, and elucidates their roles in influencing the formation process in Li-ion batteries.

The solid electrolyte interphase (SEI) is the most critical yet least understood component to guarantee stable and safe operation of a Li-ion cell. Herein, the early stages of SEI formation in a typical LiPF6 and organic carbonate-based Li-ion electrolyte are explored by operando surface-enhanced Raman spectroscopy, on-line electrochemical mass spectrometry, and electrochemical quartz crystal microbalance. The electric double layer is directly observed to charge as Li+ solvated by ethylene carbonate (EC) progressively accumulates at the negatively charged electrode surface. Further negative polarization triggers SEI formation, as evidenced by H-2 evolution and electrode mass deposition. Electrolyte impurities, HF and H2O, are reduced early and contribute in a multistep (electro)chemical process to an inorganic SEI layer rich in LiF and Li2CO3. This study is a model example of how a combination of highly surface-sensitive operando characterization techniques offers a step forward to understand interfacial phenomena in Li-ion batteries.

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 SEI formation is most commonly discussed, EC ring-opening is often observed, but seldomly 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 similar to 2.5 V vs Li+/Li as initiated by oxygenic carbon surface groups. Later, OH- generated from H2O reduction reaction at similar to 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.

Electrolyte additives are indispensable to enhance the performance of Li-ion batteries. Lithium bis(oxalato)borate (LiBOB) has been explored for many years, as it improves both cathode and anode performance. No consensus regarding its reaction mechanisms has, however, been established. A model operando study combining attenuated total reflection infrared spectroscopy (ATR-FTIR), electrochemical quartz crystal microbalance (EQCM), and online electrochemical mass spectrometry (OEMS) is herein presented to elucidate LiBOB reduction and electrode/electrolyte interphases thus formed. Reduction of the BOB^– ion sets in at ∼1.8 V with solid lithium oxalate and soluble oxalatoborates as the main products. The reduced BOB^– ion also reacts with itself and its environment to evolve CO₂, which in turn impacts the interphase formed on the negative electrode. This study provides further insights into the reduction pathways of LiBOB and how they contribute to the interphase formation.