High Performance Sodium Secondary Batteries Using Ionic Liquid Electrolytes
Authors: Matsumoto Kazuhiko , Hwang Jinkwang , Kaushik Shubham , Hagiwara Rika 
Published: December 2020

Sodium secondary batteries are attracting more and more attention as large-scale energy storage devices owing to high abundance and low cost of sodium resources.[1-3] Ionic liquid electrolytes have unique properties such as low volatility, low flammability, and thermal stability and enable construction of safe electrochemical devices, including sodium secondary batteries.[4,5] In particular, intermediate temperature operation of sodium secondary batteries using ionic liquid electrolytes is attractive by considering the improvement of battery performance. Such environments are ubiquitous in our daily life or industries and can be achieved with minimal extra energy. Self-heating of batteries can also contribute to operation at elevated temperatures.
Highly improved performance is observed for some electrode materials by intermediate-temperature operation of sodium secondary batteries with bis(fluorosulfonyl)amide-based ionic liquid electrolytes. As for the sodium metal negative electrode, morphology of sodium metal electrodeposit is significantly dependent on operation temperature; smooth sodium metal deposition with a high deposition/dissolution efficiency is observed at 90 deg C. High rate performance is observed for a number of positive electrode materials. The NASICON-type Na3V2(PO4)3 especially provides a long cycle life, exhibiting more than 5000 cycles at 90 deg C in a half cell configuration at a rate of 2C, which is also due to the high cycleability of sodium metal electrode.[6] Metal phosphide negative electrodes also show improved rate and cycle performance by intermediate-temperature operation. The copper phophide-carbon composite, CuP2/C, electrode prepared by high-energy ball-milling shows a stable cycling based on a conversion mechanism at 90 deg C over 300 cycles.[7]
Na metal is used as a counter electrode in a half-cell configuration to test electrode performance for Na secondary batteries, but its unstability and large interfacial resistance causes unreliable results, especially at high rates. The Na3V2(PO4)3 and NaV2(PO4)3 electrode prepared by mixing them in a target ratio exhibits a flat plateau at 3.4 V vs. Na+/Na and lower polarization than the Na metal. This new electrode enables trustable electrochemical evaluation of a target electrode even at a temperature above the melting point of Na metal.[8]
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