Solvation Structure around Li⁺ Ions in Organic Carbonate Electrolytes: Spacer-Free Thin Cell IR Spectroscopy

Abstract

Organic carbonate electrolytes are widely used materials for lithium-ion batteries. However, detailed solvation structures and solvent coordination numbers (CNs) of lithium cations in such solutions have not been accurately described nor determined yet. Because transmission-type IR spectroscopy is not of use for measuring the carbonyl stretch modes of electrolytes due to their absorption saturation problem, we here show that simple spacer-free thin cell IR spectroscopy can provide quantitative information on the number of solvating carbonate molecules around each lithium ion. We could estimate the solvent (carbonate) CNs of lithium ions in dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, and butylene carbonate over a wide range of lithium salt concentrations accurately, and they are compared with the previous results obtained with attenuated total reflection IR spectroscopy technique. We anticipate that our spacer-free thin cell approach will potentially be used to investigate the solvation dynamics, chemical exchange process, and vibrational energy transfers between solvating carbonate molecules in lithium salt electrolytes when combined with time-resolved IR spectroscopy.

Lithium-Ion Solvation Structure in Organic Carbonate Electrolytes at Low Temperatures

“…The red-shift of free carbonate absorption causes the dispersive difference spectra due to the temperature decrease, which is also observed in pure carbonates (Figure S3). The Li-bound peaks in the O− C−O asymmetric stretching modes also increased as the temperature decreased, which share the same origin as the Libound C=O peaks 19,25 (Figure S4). This also implies that carbonates interacting with Li + ions increase as the temperature decreases.…”

“…At a 1 M LiPF 6 concentration, dimethyl carbonate (DMC) has a notably larger amount of CIPs than propylene carbonate (PC); hence, DMC should have a smaller carbonate coordination number (CN) than PC based on the tetrahedral solvation structure. However, the carbonate CN of DMC is similar to or slightly larger than that of PC, , which is inconsistent with the assumption of the tetrahedral solvation structure. Another contradiction more clearly appears when temperatures are lowered.…”

“…Although organic carbonates are commercially used as LIB electrolytes, the quantitative Li + ion solvation structure and Recently, we demonstrated a spacer-free thin-cell method to observe carbonyl stretching modes and determined the carbonate CN by transmission-type FTIR spectroscopy. 19 Here, we combined a cryostat and transmission-type FTIR spectrometer with a spacer-free thin-cell method to investigate the solvation structure change of Li + ions depending on the temperature in DMC, diethyl carbonate (DEC), PC, and DMC:PC mixed solutions with lithium hexafluorophosphate (LiPF 6 ). Since vibrational spectroscopy resolves noncoordinating and coordinating carbonates and anions toward Li + ions, we successfully studied the solvation structures of fully solvated and CIP forms by analyzing the absorption spectrum of carbonates and anions, respectively.…”

“…We recently reported that the molar absorption coefficient ratios (ϵ b carb /ϵ f carb ) between Li-bound and free carbonates are 1.39, 1.51, and 1.24 for DMC, DEC, and PC, respectively. 19 Here, we assumed that the change in molar absorption coefficient ratios with temperature is negligible because the molar absorption coefficient changes of absorbance area have little temperature dependence. 26 Moreover, these small changes would be compensated when they are ratioed, since they are from the same C=O stretching modes of similar systems.…”

“…The quantitative measurement of the solvation structure in organic carbonate by transmission-type IR spectroscopy has been limited due to the absorption saturation problem of carbonyl stretching modes of electrolytes. Recently, we demonstrated a spacer-free thin-cell method to observe carbonyl stretching modes and determined the carbonate CN by transmission-type FTIR spectroscopy . Here, we combined a cryostat and transmission-type FTIR spectrometer with a spacer-free thin-cell method to investigate the solvation structure change of Li + ions depending on the temperature in DMC, diethyl carbonate (DEC), PC, and DMC:PC mixed solutions with lithium hexafluorophosphate (LiPF 6 ).…”