One of the key factors, which enables stable performance, cycle life and safety of lithium-ion batteries is the formation of the solid electrolyte interphase (SEI)¹. SEI is formed on the surface of the graphite during the first and few initial cycles due to the reduction of the electrolyte components. The formation of this passive layer consumes Li ions, thus leading to the irreversible capacity loss. The control of the SEI formation and growth is complicated as it depends on a number of factors, such as type of graphite, electrolyte composition, cycling conditions and temperature².

Organic electrolyte composition greatly influences the formation of the passive layer. State-of-the-art electrolytes are usually composed of a combination of cyclic (EC) and linear (DMC, EMC or DEC) carbonates and lithium salt (LiPF₆). To provide an SEI with well-balanced physical-chemical characteristics additives are included in the electrolyte formulation. These additives usually decompose prior to the electrolyte main components during the first charge leading to the formation of more compact, stable and ionically conductive passive layer³.

The cell formation protocol also influences the properties of the SEI layer. It has been shown that high current density leads to the formation of porous SEI with high ionic and electronic conductivities, whereas low current density contributes to the formation of denser SEI with decreased electronic and increased ionic conductivities⁴.

Herein we investigate the influence of the imide-based salts (e.g. LiTFSI, LiFSI and LiFTFSI) as additives on the cyclability of Li-ion cells, comprising graphite and LiFePO₄ electrodes with practical loading of ~2.5 mAh cm^-2. The electrolytes are investigated focusing on their ionic conductivity and thermal stability. Furthermore, using half- and full-cells, the impact of different formation protocols on the capacity retention and cycling efficiency is evaluated. The SEI formed in different conditions is characterized by electrochemical impedance spectroscopy (EIS) and ex-situ x-ray photoelectron spectroscopy (XPS).

Acknowledgement

The authors kindly acknowledge the financial support from the EU under the H2020 project grant agreement no. 653373 (SPICY – Silicon and polyanionic chemistries and architectures of Li-ion cell for high energy battery).

 

References

1. Peled, E., J. Electrochem. Soc.  126 (12) 1979

2. Verma, P., et al., Electrochim. Acta  55 (22) 2010

3. Kalhoff, J., et al., ChemSusChem  8 (13) 2015

4. An, S. J., et al., Carbon  105 2016