On the Reliability of Half-Cell Tests for Monovalent (Li+, Na+) and Divalent (Mg2+, Ca2+) Cation Based Batteries

D S Tchitchekova,P Johansson,F Bardé,A Ponrouch,M R Palacín,D Monti,A Randon-Vitanova

doi:10.1149/2.0411707jes

Abstract

A comprehensive study is reported entailing a comparison of Li, Na, K, Mg, and Ca based electrolytes and an investigation of the reliability of electrochemical tests using half-cells. Ionic conductivity, viscosity, and Raman spectroscopy results point to the cation-solvent interaction to follow the polarizing power of the cations, i.e. Mg2+ > Ca2+> Li+ > Na+ > K+ and to divalent cation based electrolytes having stronger tendency to form ion pairs – lowering the cation accessibility and mobility. Both increased temperature and the use of anions with delocalized negative charge, such as TFSI, are effective in mitigating this issue. Another factor impeding the divalent cations mobility is the larger solvation shells, as compared to those of monovalent cations, that in conjunction with stronger solvent - cation interactions contribute to slower charge transfer and ultimately a large impedance of Mg and Ca electrodes. An important consequence is the non-reliability of the pseudo-reference electrodes as these present both significant potential shifts as well as unstable behaviors. Finally, experimental protocols in order to achieve consistent results when using half-cell set-ups are proposed.

Highlights

Sodium metal anodes are already used in the liquid state (m.p. ∼97◦C) in the Na/S technology[2] and room-temperature Na-ion technology is currently intensively investigated with hundreds of papers appearing per year, with progress being summarized in several review papers amongst which[3,4,5] are the most recent
For battery purposes, rather low concentrations almost total salt dissociation is expected, and the difference in conductivities between monovalent and divalent cation based electrolytes might be explained by the differences in the moles of charge carriers created combined with the total unit charges
A comparative study on Li, Na, Ca, and Mg based electrolytes shows that results of ionic conductivity and viscosity measurements can be correlated to Raman spectra probing solvation shell and allow to assess that divalent cation (Mg or Ca) containing electrolytes suffer from significantly stronger cation-solvent and ion-ion interactions than monovalent cation, Li or Na, based electrolytes

Summary

Introduction

Sodium metal anodes are already used in the liquid state (m.p. ∼97◦C) in the Na/S technology[2] and room-temperature Na-ion technology is currently intensively investigated with hundreds of papers appearing per year, with progress being summarized in several review papers amongst which[3,4,5] are the most recent.

Results

Conclusion