Effect of Conducting Salts in Ionic Liquid Electrolytes for Enhanced Cyclability of Sodium-Ion Batteries

Minh Phuong Do; Nicolas Bucher; Arun Nagasubramanian; Iulius Markovits; Tian Bingbing; Pauline J Fischer; Kian Ping Loh; Fritz E Kühn; Madhavi Srinivasan

doi:10.1021/acsami.9b03279

Effect of Conducting Salts in Ionic Liquid Electrolytes for Enhanced Cyclability of Sodium-Ion Batteries

ACS Appl Mater Interfaces. 2019 Jul 10;11(27):23972-23981. doi: 10.1021/acsami.9b03279. Epub 2019 Jun 28.

Authors

Minh Phuong Do¹, Nicolas Bucher², Arun Nagasubramanian², Iulius Markovits³, Tian Bingbing⁴, Pauline J Fischer⁵, Kian Ping Loh⁴, Fritz E Kühn⁵, Madhavi Srinivasan¹

Affiliations

¹ School of Materials Science and Engineering , Nanyang Technological University , Singapore 639798 , Singapore.
² TUMCREATE , Singapore 138602 , Singapore.
³ TUM Asia , Singapore 139660 , Singapore.
⁴ Department of Chemistry , National University of Singapore , Singapore 117543 , Singapore.
⁵ Molecular Catalysis, Department of Chemistry and Catalysis Research Center , Technical University of Munich , Garching 85748 , Germany.

PMID: 31251014
DOI: 10.1021/acsami.9b03279

Abstract

The electrochemical performance of ionic liquid electrolytes containing different sodium salts dissolved in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) evaluated in a half-cell configuration using spherical P2-Na_0.6Co_0.1Mn_0.9O_2+z (NCO) cathodes are reported. Among the various electrolytes investigated, sodium bis(fluorosulfonyl)imide (NaFSI) (0.5 M) in BMPTFSI shows the best electrochemical performance with a significant improvement in cycling stability (90% capacity retention after 500 cycles at 50 mA g^-1 in a half cell versus Na metal anode) compared with conventional NaClO₄ (1 M) in ethylene carbonate/propylene carbonate electrolytes (39% retention after 500 cycles). Cyclic voltammetry (CV) studies reveal that ionic liquid electrolytes are stable up to 4.8 V versus Na/Na⁺. When NaFSI and NaTFSI are used as conducting salts, X-ray photoelectron spectroscopy results prove that the cathode electrolyte interface (CEI) is composed of components resulting from the decomposition of the TFSI anion and the deposition of the BMP cation. On the other hand, the CEI layer of the electrode cycled in an electrolyte containing NaClO₄ in BMPTFSI follows a different pathway of TFSI decomposition and consists mainly of sodium fluoride. Similarly, plating studies were used to understand the stability of different ionic liquids in contact with metallic sodium. It was found that the excellent capacity retention for the electrolyte consisting of NaFSI salt is related to the formation of a stable CEI and solid electrolyte interphase layers.

Keywords: X-ray photoelectron spectroscopy; cathode electrolyte interfaces; ionic liquid; sodium ion battery; sodium manganese oxide; sodium stripping plating; solid electrolyte interfaces.