In an effort to achieve higher energy density batteries, many higher voltage cathodes are being investigated.  However, at high voltage (V>4.5 vs. Li/Li⁺), conventional carbonate electrolytes are unstable, leading to electrolyte oxidation and subsequent drop in battery performance.  Two possible methods for mitigating electrolyte oxidation are through the use of intrinsically stable electrolytes at the elevated cathode voltage, or through the use of electrolyte additives that sacrificially decompose on the charged cathode surface, impeding further oxidation of the electrolyte.  Evaluation of electrolyte or additive performance is traditionally accomplished by cycling tests or through voltammetry with platinum or carbon working electrodes, however each of these methods has intrinsic gaps.  Capacity retention can be affected by electrode ‘slippage’ as well as electrode-electrode effects, and voltammetry on inert working electrodes is not evaluating reaction rates on a representative surface.  Determining oxidation rates at the cathode surface is of great value in the design of passivating additives.

In this work, stability of electrolytes was assessed using high voltage potentiostatic holds (4.6V vs. Li/Li⁺, for at least 60 hours) at elevated temperature with a NCM (LiNi_0.5Co_0.2Mn_0.3O₂) electrode as the working electrode and an LTO (Li₄Ti₅O₁₂) electrode as a counter and reference.  A 60 hour potentiostatic hold allows for the relaxation of electrode polarization such that the dominant contributor to external current is oxidation reactions, allowing a quantitative measure of electrolyte decomposition rate.  Within this protocol, the passivating effect of various additives can be evaluated both during and after the potentiostatic hold by comparing oxidation current for an additive-containing electrolyte to the baseline.  Upon completion of the potentiostatic hold, the capacity and impedance are measured, and cells were disassembled with components characterized by XPS, LC-MS, and IR. 

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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