Effective Stabilization of NCM622 Cathodes in Aqueous/Non-Aqueous Hybrid Electrolytes by Adding a Phosphazene Derivate

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Summary
In recent years, the occurrence of battery fire incidents has aroused public concerns, in which the used flammable electrolytes in most batteries plays a key role.[1, 2] Regarding this, aqueous electrolytes have become a promising alternative to enhance the battery safety by their intrinsic non-flammability. Especially, after reporting so-called “water-in-salt” electrolytes,[3] aqueous electrolytes are being re-visited by researchers due to their safety superiority, while the “high salt concentration” approach improves the electrochemical stability of electrolytes. Furthermore, employing the co-solvent system (e.g., H2O/ dimethyl carbonate (DMC) or H2O/ acetonitrile (ACN)) not only decreases electrolyte viscosity because of high salt concentration, but also facilitates to expand the electrochemical stability window (ESW) of the electrolyte, especially by lowering the cathodic (reductive) stability limit.

In order to further improve the energy density of aqueous Lithium ion batteries (ALIBs), layered NCM-type cathodes are a promising material class because of their high capacity. In this work, considering the excellent abilities of CEI formation and flame retardance, ethoxy-(pentafluoro)-cyclotriphosphazene (PFN)[4] was selected as new co-solvent for hybrid aqueous electrolytes. A 13M aqueous/non-aqueous hybrid electrolyte (13M lithium bis (trifluoromethane sulfonyl) imide (LiTFSI) in 1 kg H2O/DMC mixture (molar ratio: 1:1)) was prepared as reference electrolyte, while 10 wt.% and 20 wt.% of PFN were added as co-solvent. The contribution of the PFN component on the electrochemical performance and CEI formation was systemically investigated in NCM622 || TiO2@LiTi2(PO4)3 full-cells. The tailor-made CEI film, consisting of LiF, Li2CO3, Li3PO4 and organic species such as monoester phosphate, can effectively facilitate the (de)lithiation of NCM622 in hybrid electrolyte without heavy oxidative side reactions.

The effective CEI film on the cycled NCM622 particle surface, stabilizing NCM622 in a water-containing environment and suppresses electrolyte decomposition. As a result, a high initial specific energy of 123 Wh kg-1 (on active materials level) is achieved from NCM622 || TiO2@LiTi2(PO4)3 cell, the capacity retention reaches 79.3% after 200 cycles. At the same time, the concentration of LiTFSI is reduced to 13M per kilogram solvent, easing (but not eliminating) cost worries imposed by the use of LiTFSI. This work provides insight on the utilization of layered NCM cathode in aqueous electrolytes aiming to further improve the energy density of ALIBs.

References
[1] Luo, J.-Y., et al., Raising the cycling stability of aqueous lithium-ion batteries by eliminating oxygen in the electrolyte. Nature chemistry, 2010. 2(9): p. 760-765.
[2] Parker, J.F., et al., Rechargeable nickel–3D zinc batteries: An energy-dense, safer alternative to lithium-ion. Science, 2017. 356(6336): p. 415-418.
[3] Suo, L., et al., “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Science, 2015. 350(6263): p. 938-943.
[4] Liu, J., et al., Fluorinated phosphazene derivative – A promising electrolyte additive for high voltage lithium ion batteries: From electrochemical performance to corrosion mechanism. Nano Energy, 2018. 46: p. 404-414.

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