Nickel-rich layered oxide cathode materials have seen widespread deployment due to their high gravimetric capacity and high average discharge voltage. However, achieving this high capacity requires charging voltages above the potential at which the electrolytes decompose at the cathode surface (>4.5 V vs. Li/Li⁺). This interfacial instability leads to impedance rise and capacity loss. One strategy to stabilize this interface is by the addition of additives to the electrolyte, in which small amounts of a new component change the interactions of the interface. Potentiostatic holds with LiNi_0.5Mn_0.3 Co_0.2O₂ (NMC) as the working electrode and Li₄Ti₅O₁₂ (LTO) as the counter/reference allow a quantitative measurement of the oxidation reactions occurring at cathode. A 60 hour hold allows for concentration polarizations to relax and oxidative side reactions to dominate the current. A more stable electrolyte at the cathode/electrolyte interface would lead to a lower current during the potentiostatic hold.

In this work, the performance of five phosphorus-based additive electrolyte formulations (1% wt. ratio) is evaluated against a baseline organic carbonate electrolyte in NMC (LiNi_0.5Mn_0.3 Co_0.2O₂) / LTO (Li₄Ti₅O₁₂) full cells. These additives include triethylphosphite (TEPi), tris(2,2,2-trifluoroethyl)phosphite (TTFPi), tris(trimethylsilyl) phosphite (TMSPi), and tris(trimethyl silyl) phosphate (TMSPa). The electrochemical protocol used to evaluate these additives comprises multiple cycles up to 4.4 V vs. Li⁺/Li, a potentiostatic hold at 4.6 V vs. Li⁺/Li, followed by AC impedance spectroscopy and extended cycling. Of the formulations surveyed, only electrolytes containing TMSPa and TMSPi (aged for one week) increased the coulombic efficiency during the initial cycling. For the potentiostatic hold, only the aged TMSPi electrolyte lowered the terminal oxidation current compared to the baseline electrolyte, indicating increased stability of the cathode/electrolyte interface. AC impedance spectra indicated the aged TMSPi electrolyte had a slightly elevated high-frequency semicircle (traditionally associated with phenomena at the solid-electrolyte interphase) and a vastly decreased mid-frequency semicircle (traditionally associated with charge-transfer phenomena), compared with all other electrolytes tested. Characterization of cycled electrodes was performed with XPS and SEM will be presented.

Acknowledgements

The work at Argonne National Laboratory was performed under the auspices of the U.S. Department of Energy (DOE), Office of Vehicle Technologies, under Contract No.
DE-AC02-06CH11357.

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