CARiM’s research team had presented results and achievements on the project at a national conference FIFTH WORKING MEETING of the ESHER GENERAL ASSEMBLY, June 19-20, 2023, Sofia, Bulgaria.

The presenting author was Prof. Dr. Violeta Koleva.

Electrochemical performance of tunnel-type sodium iron phosphates as positive electrodes in sodium ion batteries using pyrrolidinium-based ionic liquid electrolyte

Violeta Koleva, Trajche Tushev, Sonya Harizanova, Radostina Stoyanova

Abstract: Besides the positive and negative electrodes, the electrolyte is a key component in each electrochemical cell as it provides the ion transport between the positive and negative electrodes. Recently, ionic liquids (ILs) based on imidazolium and pyrrolidinium cations have been proposed as promising alternative to the conventional organic electrolytes (often NaPF6 or NaClO4 salts dissolved in a mixture of organic carbonates such as propylene carbonate, ethylene carbonate, diethyl carbonate, etc.). ILs are nonflammable, non-volatile, with excellent thermal and electrochemical stability, enabling battery operation at higher temperatures (40-90C) and within wide electrochemical windows (up to 6V). These properties make ILs safety and green alternative to the organic electrolytes, but the high manufacturing cost currently limits their large-scale application.
To date, only limited phosphate-based positive electrode materials such as NaFePO4, Na2FeP2O7 and
Na3V2(PO4)3 have been investigated using ILs as electrolytes and very promising electrochemical characteristics in terms of energy density and capacity retention that outperform the organic electrolyte-based devices have been reported.
Here, we first report electrochemical properties of tunnel–type sodium iron phosphates, Na4Fe3(PO4)2P2O7 and Na2Fe3(PO4)3, in sodium half-cells using NaFSI:[Pyr13]+[FSI]- electrolyte (Pyr13FSI is N-methyl-N-propylpyrrolidinium bis-(fluorosulfonyl)imide) (1:9 mole ratio).
The phosphate materials are prepared by thermal decomposition at 500-600 C of phosphate-formate precursors, the latter being obtained by freeze-drying of solutions containing NaH2PO4, Fe(HCOO)2.2H2O and (NH4)2HPO4 in needed proportions. The electrochemical performance is studied in galvanostatic and potentiostatic regimes at 20 and 40 C. The observation of clear redox peaks in the CV curves owing to Fe2+/Fe3+ redox pairs, that remain unchanged during the cycling, confirms the electrochemical activity and stability of the phosphate electrodes in the NaFSI:Pyr13FSI electrolyte. An improved chargedischarge performance is established at elevated temperature: the achieved discharged capacity is 94 mAh/g (73 % of the theoretical value) at C/2 rate after 100 cycles and excellent cycling stability with 96 % capacity retention. The results obtained demonstrate the potential of tunel-type Na4Fe3(PO4)2P2O7 and Na2Fe3(PO4)3 as cathodes in sodium ion batteries using NaFSI:Pyr13FSI electrolyte at intermediate temperature.

Acknowledgements: The authors are kindly acknowledged for financial support to project National Roadmap for Research Infrastructure 2017-2023 “Energy Storage and Hydrogen Energy” approved by DCM No 354/29.08.2017 under Grant Agreement (NSI-ESHER, № DO1-161/28.07.2022) for incubators used and to project CARiM (NSP Vihren, КП-06-ДB-6/16.12.2019) for the freeze-drying apparatus.

The authors from CARiM’s Research Team are bolded.