Tim Melin

Despite the lack of fundamental understanding of their operating mechanisms, layer‐forming electrolyte additives are key to stabilizing modern Li‐ion cells. Herein, the reduction mechanism of sulfur‐containing additive prop‐1‐ene‐1,3‐sultone (PES) is investigated using a model inert electrode/electrolyte system to isolate additive‐driven reactions. Attenuated total reflection fourier‐transform infrared spectroscopy, online electrochemical mass spectrometry, electrochemical quartz crystal microbalance with dissipation monitoring, and electrochemical impedance spectroscopy (EIS) are applied operando along with ex situ X‐ray photoelectron spectroscopy. PES satisfies the main criteria for layer‐forming additives, to reduce at higher potentials than typical carbonate ester‐based electrolytes and to suppress further electrolyte decomposition. Inclusion of PES in the electrolyte significantly reduced gas evolution, forming a thin, dense, and Li ⁺ ‐conductive SEI. Despite significant charge passed already at 1.4 V vs. Li ⁺ /Li, no major interfacial change is observed until 1.0 V, where a high‐resistance, low‐capacitance interphase forms containing sulfur–oxygen species, such as ROSO ₂ Li, RSO ₃ Li, and Li ₂ SO ₃ . Such species primarily form in chemical steps after PES reduction, which in turn continue the layer growth long after the electrochemical PES reduction has ended. The combined operando methodology applied herein provides unique insights into electrode layer formation in batteries and thereby guides understanding and development of future electrolyte additives.

Prussian blue analogues (PBAs) for sodium ion battery (SIB) cathodes are growing in popularity as next generation energy storage devices. Prussian White (PW) with formula NaxFe[Fe(CN)6]y•nH2O is leading the trend, having already been commercialized. However, capacity fade (PW/electrolyte degradation) and safety concerns (cyanide/cyanogen release) still raise concerns. Online electrochemical mass spectrometry (OEMS), supported by both operando Fourier transform infrared spectroscopy (FTIR) and Mössbauer spectroscopy (MöS), is herein used to analyse degradation processes in PW based Na-ion full cells. Apart from the typical cell formation reactions, hydrogen is observed to evolve during cell discharge and evidenced to stem from oxidation of NaH, accumulated upon charge. Over-oxidation of PW after full desodiation releases CN, which not only forms (CN)2 but also degrades the electrolyte. Loss of CN likely results in a nanometric (~4 nm) surface-reconstructed passivation layer on PW thus inhibiting further degradation. Fundamental understanding of degradation reactions in PW full-cells, as gathered herein, shows that the aforementioned capacity fade and safety concerns are wholly addressable and hence guides the further development of Na-ion batteries for wider ranges of applications.

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.

The solid electrolyte interphase (SEI) is arguably one of the most critical components of the Li‐ion cell. Despite decades of studies of the SEI, its intrinsic complexity and the lack of suitable characterization tools still prevent a real consensus on the governing mechanisms to be reached. Herein, operando Raman spectroscopy supported by complimentary online electrochemical mass spectrometry is employed to study the SEI formation on Au in a model electrolyte based on LiClO₄ in ethylene carbonate (EC). Both the electrolyte itself and cell contaminants, such as O₂, CO₂, and H₂O, contribute in stepwise electro‐/chemical processes to the build‐up of the SEI. Effects associated with electrode/electrolyte double‐layer charging, electrode adsorbate polarization (stark effect), and SEI dissolution are discerned. Lithium carbonate and lithium oxide are identified as major products formed already ≈2 V versus Li⁺/Li. Although Raman spectroscopy provides deeper insights into the underlying mechanisms, complementary techniques are necessary to support spectral interpretations. Classical challenges in the field of surface science, such as contaminations, have to be systematically addressed if the puzzle of the SEI ever will be completed.

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.