Electrolytes toward High‐Voltage Na3V2(PO4)2F3 Positive Electrode Durable against Temperature Variation

• Published in: Advanced energy materials Vol. 10; no. 34
• Main Authors: Hwang, Jinkwang, Matsumoto, Kazuhiko, Hagiwara, Rika
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 08.09.2020
• Subjects: Additives; Cathodic protection; Electrochemical analysis; Electrode materials; Electrodes; Electrolytes; Fading; Flux density; High temperature; High voltages; Ionic liquids; Ions; Na 3V 2(PO 4) 2F 3; Nonaqueous electrolytes; Photoelectrons; sodium secondary batteries; Storage batteries; Temperature gradients; wide operating temperatures
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.202001880

High power and energy density, long cyclability, and tolerance for wide temperature (seasonal and daily operational temperature differences) must be considered to construct large‐scale sodium secondary batteries. In this regard, Na3V2(PO4)2F3 (NVPF) has become a subject of interest as a high‐performance positive electrode material owing to its high energy density. However, the high operating voltage of NVPF causes continuous decomposition of electrolytes during cycles, resulting in significant capacity fading and low Coulombic efficiency. In this study, the electrochemical performance of the NVPF electrode in organic solvent electrolytes with and without additives and an ionic liquid is investigated at high voltage regimes over a wide temperature range (−20 °C to 90 °C). The results reveal that the performance of organic electrolytes is still insufficient even with additives, and the ionic liquid electrolyte demonstrates high electrochemical stability and cyclability with NVPF electrodes over a temperature range from −20 °C to 90 °C, achieving stable cycling over 500 cycles. The detailed electrochemical analysis combined with X‐ray photoelectron and energy dispersive X‐ray spectroscopy indicates that a sturdy cathode electrolyte interphase layer around the electrode protects it from capacity fading at high voltage and elevated temperature, resulting in high Coulombic efficiency. The scarcity of electrolytes suitable for high‐voltage electrode materials, such as Na3V2(PO4)2F3, greatly undermines the implementation of sodium secondary batteries. Herein, Na3V2(PO4)2F3 performance is analyzed through the screening of several organic and ionic liquid electrolytes. Results show ionic liquids are the most stable across a wide temperature range due to the formation of an appropriate cathode electrolyte interface layer around the electrode.