Quantitative In Operando Spectroscopic Measurements of Species Concentrations in a Sodium-Halide Cell

Introduction

                In order to meet the increasing demand for energy storage, new battery technologies are needed to fulfill the needs of a world where electricity is of increasing importance (1, 2).  One of the difficulties of developing new battery technologies is characterizing the electrochemical reactions that occur within the battery electrolyte. In this work, the authors present a novel approach for quantitative in-operando optical measurements of electrochemistry in battery electrolytes.

                An optically accessible cell was fabricated using a ceramic ion exchange membrane made from NASICON (Na Ion Super Conductor), a promising alternative to β-alumina for molten sodium batteries for stationary storage applications (3, 4, 5)­­.  Optical access was achieved by sandwiching the battery between two UV-transparent windows (Figure 1).  The battery chemistry utilized a copper cathode contained in an aqueous sodium iodide catholyte solution.  During charging, copper is stripped from the cathode to form aqueous CuI₂^-.  The full cell reaction is

Na⁺ + 2I^- +Cu_(s) ↔ Na + CuI₂^-

The aim of this work was to monitor the species present in the catholyte solution spectroscopically.  Species concentrations were determined from UV absorption spectra using absorption cross-sections measured through fundamental experiments and calculated through ab initio simulations.

Experimental

Figure 1: Conceptual drawing of the optically accessible sodium halide cell.  Optical access allows for in-operando measurements of electrochemical species concentrations via spectroscopic methods. The channels for the anode and cathode are each 2 mm wide.  The cathode is spaced 1 mm from the NASICON separator.  The separator is 1 mm thick.

                A molten sodium battery was constructed inside the optical cell by machining a NASICON wafer to accommodate a molten sodium anode and an aqueous catholyte.  The front window of the cell was machined to allow electrodes and plumbing to be passed into the cell.  Viton gaskets were used to seal around the NASICON, separating the aqueous solution from the sodium.

UV absorption spectroscopy was used to measure changes in CuI₂^-concentration as the cell was operated. Fundamental spectroscopic experiments were carried out to isolate the spectra of relevant electrochemical species.  Measurements of the absorption cross-sections of the electrochemical species were then used to make quantitative measurements of species concentration in operando. 

                Ab initio calculations of the UV-visible absorption spectra of relevant electrochemical species and possible products of side reactions (Na⁺, I^-, I₃^-, I₂, and CuI₂^-) were performed at the TD-B3LYP//aug-ccpv5z-pp, TD-M062X//aug-ccpv5z-pp, and EOM-CCSD//aug-ccpv5z-pp levels of theory.  Calculations were carried out in Gaussian 09. Results of the calculations were compared to experimental results to verify assignment of absorption features to species present in the electrolyte. 

Conclusions

                A fully functional, optically accessible cell based on molten sodium-halide chemistry with a NASICON ion exchange membrane has been demonstrated.  Furthermore, quantitative measurements of relevant electrochemical species concentrations have been demonstrated.  UV absorption cross-sections of the electrochemical species and possible side reaction products have also been measured experimentally and calculated through ab initio methods.

Acknowledgements

                The authors would like to thank Dr. Imre Gyuk at the Department of Energy Office of Electricity for providing the funding for this work.  The authors also thank Dr. David Ingersoll from Sandia National Laboratories for overseeing the project, Dr. Sai Bhavaraju and Matt Robins from Ceramatec for their battery expertise and advice, Dr. Huayang Zhu and Dr. Robert Kee at CSM for support in computational battery modeling, and Dr. C. Mark Maupin for supporting the ab initio calculations.

References

 

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