High performance red phosphorus electrode in ionic liquid-based electrolyte for Na-ion batteries

• Published in: Journal of power sources Vol. 363; pp. 404 - 412
• Main Authors: Dahbi, Mouad, Fukunishi, Mika, Horiba, Tatsuo, Yabuuchi, Naoaki, Yasuno, Satoshi, Komaba, Shinichi
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.09.2017
• Subjects: Ionic liquid electrolyte; Phosphorus; Sodium-ion batteries; Solid electrolyte interphase; Phosphorus; Solid electrolyte interphase; Ionic liquid electrolyte; Sodium-ion batteries
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2017.07.089

Electrochemical performance of the red phosphorus electrode was examined in ionic-liquid electrolyte, 0.25 mol dm−3 sodium bisfluorosulfonylamide (NaFSA) dissolved N-methyl-N-propylpyridinium-bisfluorosulfonylamide (MPPFSA), at room temperature. We compared its electrochemical performance to conventional EC/PC/DEC, EC/DEC, and PC solutions containing 1 mol dm−3 NaPF6. The electrode in NaFSA/MPPFSA demonstrated a reversible capacity of 1480 mAh g−1 and excellent capacity retention of 93% over 80 cycles, which is much better than those in the conventional electrolytes. The difference in capacity retention for the electrolytes correlates to the different solid electrolyte interphase (SEI) layer formed on the phosphorus electrode. To understand the SEI formation in NaFSA/MPPFSA and its evolution during cycling, we investigate the surface layer of the red phosphorus electrodes with hard X-ray photoelectron spectroscopy (HAXPES) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). A detailed analysis of HAXPES spectra demonstrates that SEI layer consists of major inorganic and minor organic species, originating from decomposition of MPP+ and FSA−. Homogenous surface layer is formed during the first cycle in NaFSA/MPPFSA while in alkyl carbonate ester electrolytes, continuous growth of surface film up to the 20th cycle is observed. Possibility of red phosphorous electrode for battery applications with pure ionic liquid is discussed. •Na storage performance of red P in MPPFSA ionic liquid electrolyte at 25 °C.•High-reversible capacity (1480 mAh g−1) and excellent-capacity retention of red P in NaFSA/MPPFSA electrolyte.•NaFSA and MPPFSA ionic liquid for improving the cyclability of red P electrodes.•SEI layer studied by photoelectron and secondary-ion mass spectroscopies.