Entropic Contributions to Sodium Solvation and Solvent Stabilization upon Electrochemical Sodium Deposition from Diglyme and Propylene Carbonate Electrolytes

Abstract

The formation of an appropriate solid electrolyte interphase (SEI) at the anode of a sodium battery is crucially dependent on the electrochemical stability of solvent and electrolyte at the redox potential of Na/Na + in the respective system. In order to determine entropic contributions to the relative stability of the electrolyte solution, we measure the reaction entropy of Na metal deposition for diglyme (DG) and propylene carbonate (PC) based electrolyte solutions by electrochemical microcalorimetry at single electrodes. We found a large positive reaction entropy for Na + deposition in DG of), which signals substantial entropic destabilization of Na + in DG by about 0.73 eV, thus increasing the stability of solvent and electrolyte relative to Na + reduction. We attribute this strong entropic destabilization to a highly negative solvation entropy of Na + , due to the low dielectric constant and high freezing entropy of DG.

Detection of Charge‐Neutral Near‐Equilibrium Processes at Na‐Metal Electrodes by Electrochemical Microcalorimetry

“…For all experiments presented here, we chose 1 m NaPF 6 in diglyme as electrolyte, since a stable SEI can be formed in this system (see Refs. [2,3,5,[7][8][9] and references therein). The two upper panels of Figure 1a,b show potential and current transients upon 10 ms potential steps of ± 100 mV amplitude, causing Na deposition and dissolution on the Na layer, which in this case was completely dissolved and redeposited five times prior to the microcalorimetry experiment (in the following, this is called "aged" Na surface).…”

“…The depicted temperature transient was recorded in the same setup, directly following the Na experiments and was also used for calibration of the heat scale (see Ref. [5]). It can be clearly seen that the temperature relaxation following the electrochemical Fe reduction is considerably faster than that observed for Na deposition or dissolution.…”

“…These are in detail discussed in Ref. [5]. Since experimental values for the heat of transport of Na + and PF 6 − in diglyme are not available, we employed the approximation suggested by Agar, [13] which yielded −5.3 kJ mol −1 for the transport contribution to the heat from ion transport by migration, which is much smaller than the observed reversibly exchanged heat (up to ca.…”

“…The origin of the entropy changes during Na metal deposition in NaPF 6 /diglyme was discussed in detail in Ref. [5]. There it was found that the overall entropy change of 234 J mol −1 K −1 [16] is dominated by desolvation of Na + , that is, by liberation of solvent molecules in the first solvation shell (191 J mol −1 K −1 ) and release of Born entropy from higher solvation shells (81 J mol −1 K −1 ), which strongly overcompensate the entropy loss by fixation of Na at the electrode (−37,5 J mol −1 K −1 ).…”

“…[4] Recently, we presented a study on sodium bulk metal deposition and dissolution from diglyme and propylene carbonate-based electrolytes. [5] We determined the heat, which was liberated or absorbed during the electrochemical reactions at a Na metal electrode with a specially designed microcalorimeter, where the temperature of a thin working electrode is measured at its backside with a pyroelectric sensor (see Refs. [4,6] and Supporting Information for the principle of the measurement).…”