Ultrafast Decay of the Solvated Electron in a Neat Polar Solvent: The Unusual Case of Propylene Carbonate
Résumé
The behavior of carbonates is critical for a detailed understanding of aging phenomena in Li-ion batteries. Here we study the first reaction stages of propylene carbonate (PC), a cyclical carbonate, by picosecond pulse radiolysis. An absorption band with a maximum around 1360 nm is observed at 20 ps after the electron pulse and is shifted to 1310 nm after 50 ps. This band presents the features of a solvated electron absorption band, the solvation lasting up to 50 ps. Surprisingly, in this polar solvent, the solvated electron follows an ultrafast decay and disappears with a half time of 360 ps. This is attributed to the formation of a radical anion PC − •. The yield of the solvated electron is low, suggesting that the radical anions are mainly directly produced from presolvated electrons. These results demonstrate that the initial electron transfers mechanisms are strongly different in linear compared with cyclical carbonates. T he commonly used solvents in lithium-ion batteries (LIBs) are a mixture of linear and cyclical carbonates. 1,2 The mixture of solvents is required to get the desired properties of the electrolyte, i.e., a high electrolytic conductivity, a fair chemical and electrochemical stability, etc. The excess electron in the solvent could then play different roles according to the nature of the carbonates. For instance, the anion radical arising from cyclical carbonates is often postulated in electrochemical reactions. 1,2 In this context, the radiolysis approach is a very suitable method to focus on the electron fate and reactivity. We have recently studied the aging mechanisms in a linear carbonate, diethyl carbonate, by pulse and steady-state radiolysis. 3,4 The purpose of the present work is to investigate the short-time behavior of excess electrons in a prototype cyclical carbonate, i.e., propylene carbonate (PC, see the chemical structure in the inset of Figure 1) which is polar with a static dielectric constant around ε s = 66, and a dipolar moment of 4.9 D. The transient optical absorption spectra detected in neat PC after the electron pulse are given in Figure 1. At the shortest delay time after the end of the pulse, a broad absorption band centered at 1360 nm in the near-infrared (NIR) is observed. This band is shifted slightly to the blue and reaches a maximum at 1310 nm at 50 ps, and then disappears rapidly with a half time of 360 ps (Figure 1). In the presence of H + , by means of HClO 4 addition, the decay of the band is still faster (Figure 2); in the presence of perchloric acid at one molar concentration, the band has fully vanished within the electron pulse (Figure 1, dotted line). Note that, even if HClO 4 contains a small amount of water, the reaction of PC with water is extremely slow (k = 7.1 × 10