Revisiting the Ethylene Carbonate–Propylene Carbonate Mystery with Operando Characterization

Abstract

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.

Anion Intercalation into Graphite Drives Surface Wetting

“…The latter is ascribed to the cathodic decomposition of the solvent with a possible contribution from the hydrogen evolution reaction (HER) triggered by water impurities (since the experiments were conducted at ambient temperature). 47,67 In more detail, the overall cathodic process involves the evolution of CO 2 due to the water catalyzed ring-opening of PC as well as the formation of other byproducts such as carbonates. 67 Returning to the θ vs E plots in Figure 2a, it is evident that the increase in the rate of the above mentioned processes with E inhibits the electrowetting response, a phenomenon depicted in the low CA changes recorded for E < E pzc and is further highlighted in the contact angle saturation observed for E < −1.2 V. Therefore, we can conclude that solvent decomposition occurring at E < −0.8 V is detrimental to the electrowetting phenomenon due to the electrochemical modification of the surface of HOPG as a result of the byproducts (often insoluble) formed upon solvent decomposition 47,67 that increase the solid−electrolyte interfacial surface tension.…”

Elucidating the Step‐Wise Solid Electrolyte Interphase Formation in Lithium‐Ion Batteries with Operando Raman Spectroscopy

“…Even if the surface‐enhancement of the Au SERS substrate is very minor or next to nothing, the Au surface is highly reflective (more photons are scattered into the spectrometer) and its nano‐structured roughness provides a higher electrode area (lower flooding factor) compared to planar Au. [ 44 ]…”

“…Even if the surfaceenhancement of the Au SERS substrate is very minor or next to nothing, the Au surface is highly reflective (more photons are scattered into the spectrometer) and its nano-structured roughness provides a higher electrode area (lower flooding factor) compared to planar Au. [44] Finally, multiple replicates of identical cells with the same electrolyte were performed and a spread in results between cells is observed (Figure S2, Supporting Information). The most common difference between the cells is often observed at >2.0 V in the region 1200-1600 cm -1 associated with electrolyte contaminants, which is also a conceivable result from the human factor.…”

Unlocking the Low-Temperature Potential of Propylene Carbonate to −30 °C via N-Methylpyrrolidone

“…Although PC has only modified EC’s structure with an additional methyl group, it has cost its ability to form a stable interphase with graphitic anodes. Graphitic anodes are usually exfoliated structurally in PC-based electrolytes, which has been ascribed to the cointercalation of PC with Li + and its immediate gas-evolving reduction upon entering the host’s lattice. − Although the reason the minor structural difference has created the chasm in performance remains elusive, many strategies have been demonstrated to improve the compatibility between PC and graphite, such as surface modification of raw materials, new binders, and so on. − Among them, electrolyte engineering via the adoption of new salts, additives, and cosolvents is the most appealing route, in terms of its readiness to be integrated with the existing battery manufacturing technology. − The common emphasis of these approaches is the controlled growth of a robust SEI layer before the onset of catastrophic side reactions. The mechanism critically relies on the solvation structure of Li + , since it is already determined that only the ligandsanions, solvent molecules, and additivespresent in the solvation sheath of Li + are relevant for the interfacial reactions generating the SEI layer .…”