(Battery Division Research Award) Electrolyte Oxidation Mechanisms in Lithium-Ion Batteries and Related Follow-Up Reactions
Authors: Beyer Hans , Freiberg Anna Teresa Sophie , Gasteiger Hubert A. , Jung Roland , Metzger Michael , Ogrodnik Alexander , Sicklinger Johannes , Solchenbach Sophie , Strehle Benjamin , Wandt Johannes 
Published: December 2020

In this contribution we will discuss our current understanding of the electrolyte oxidation mechanisms in lithium-ion batteries as well as of the various follow-up reactions that are triggered by electrolyte oxidation. Using online electrochemical mass spectrometry (OEMS) in combination with a two-compartment cell, in which a solid lithium ion conductor serves as a diffusion barrier between the anolyte and the catholyte compartments, it could be shown that the electrochemical oxidation of electrolyte solvents at high potentials leads to the formation of protic species [1]. In one-compartment OEMS cells it was thus observed that this electrochemical electrolyte oxidation is accompanied by the evolution of H2 that is formed by the reduction of protic species at the anode, while no H2 evolution was observed when using a two-compartment cell, which prevents the diffusion of protic species to the anode compartment. In addition to H2 evolution, the protic species formed upon electrochemical electrolyte oxidation were also shown to lead to the formation of HF and PF5 gas in an LiPF6 based electrolyte (PF6
- + H+ ® HF + PF5) [2] and to the decomposition of Li2CO3 (2H+ + 2PF6
- + Li2CO3 ® 2LiF + H2O + CO2 + PF5) that is present as surface impurity of cathode active materials [3].
To investigate why all of the above described OEMS experiments also indicated the formation of minor amounts of protic species at potentials as low as »4.0 V vs. Li+/Li, even on carbon-only model electrodes [2], the effect of alcoholic contaminants of lithium-ion battery electroytes (e.g., ethylene glycol, formed by the slow decomposition of ethylene carbonate with water impurities [4]) was examined. This revealed that the electrochemical oxidation of alcoholic contaminants that commences at »4.0 V vs. Li+/Li leads to the formation of protons (R-CH2OH ® RCHO + 2H+ + 2e-) and thus triggers the above described reactions [3].
In contrast to the electrochemical electrolyte oxidation, another mechanisms which leads to the decomposition of lithium-ion battery electrolytes is the chemical oxidation of the electrolyte that can be observed at high degrees of delithiation of layered transition metal oxide based cathode active materials. It was found, that lattice oxygen that is released at »80% delithiation of NCMs leads to the chemical oxidation of the electrolyte. As the potential at which »80% delithiation are obtained decreases with increasing nickel content, the chemical electrolyte oxidation initiates at different potentials (»4.2-4.6 V vs. Li+/Li) for different NCM compositions [5]. For cathode active materials which do not release oxygen upon complete delithiation (e.g., LNMO) or for carbon-only model electrodes, this chemical electrolyte oxidation mechanism is not observed [6]. For NCMs and Li- and Mn-rich NCMs it could be shown that at least part of the released lattice oxygen is formed as singlet oxygen, which we believe is responsible for initiating chemical electrolyte oxidation [7]. This is also based on the finding, that singlet oxygen reacts readily with ethylene carbonate (EC), forming vinylene carbonate and H2O2 whose oxidation at ³4.0 V vs. Li+/Li produces protons [8]. While we therefore believe that this is a relevant decomposition mechanisms, others suggested that the chemical electrolyte oxidation pathway proceeds through the reaction of EC on the surface of highly delithiated NCM surfaces [9].
References:
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