Operando Spectroscopy of Alkyl Carbonate-Based Electrolyte Decomposition in LiCoO2 Batteries
Authors: Porter Jason Morgan , Saqib Najmus 
Published: February 2020

Electrolytes composed of alkyl carbonate solvents with lithium hexafluorophosphate (LiPF6) salt have enjoyed near universal adoption in lithium ion batteries. Despite the wide adoption of these electrolytes, significant safety issues can occur under mechanical, electrochemical, or thermal abuse. Safety concerns will only increase with an expanded number of battery applications (including emerging markets for used batteries), higher energy density, and a need for extreme fast charging in electric vehicle applications.

The stability of ethylene carbonate (EC) against oxidation at the cathode while forming a stable solid electrolyte interface (SEI) upon reduction at the anode has made this solvent a key ingredient in almost all lithium ion batteries. Linear carbonates (e.g. diethyl carbonate (DEC) or dimethyl carbonate (DMC)) are mixed with EC to reduce viscosity and increase ion mobility, while maintaining a wide electrochemical stability window. While stable against reduction at the anode, the higher volatility and lower boiling points of linear alkyl carbonates increase risk of vapor buildup within the battery. At elevated temperature, these carbonate-based electrolytes become unstable as SEI decomposition, solvent-salt reactions, anode reduction, cathode oxidation, and thermal decomposition lead to thermal runaway, battery venting, fire, or explosion.

In an effort to improve safety and explore alternative electrolytes, numerous studies have sought to identify the underlying mechanisms of electrolyte decomposition. Differential scanning calorimetry and accelerated rate calorimetry experiments are commonly used to identify onset temperatures of key reactions, global kinetic parameters, and heat release rates. Ex-situ electrolyte and gas analyses are also used to identify decomposition mechanisms and reaction products. However, much remains unknown about the detailed decomposition chemistry in lithium ion batteries. Furthermore, decomposition chemistry is highly dependent on the composition of the electrolyte and electrode, making it difficult to predict thermal stability in new or modified battery chemistries.

Here we present operando Fourier transform infrared (FTIR) spectroscopy measurements of electrolyte decomposition in lithium ion batteries. Using attenuated total reflection (ATR) spectroscopy, we simultaneously monitored electrolyte composition and electrochemical performance of heated LiCoO2 coin cells with EC/DEC/LiPF6 electrolyte and Celgard separators. Using the ATR method, an infrared evanescent wave propagates several microns into the electrolyte-saturated pores of the anode and cathode layers, respectively. The resulting absorption spectra are analyzed to identify electrolyte species and their molecular bonds. The coin cell geometry allows for careful control and monitoring of electrolyte temperature and composition at charge/discharge rates > 1C.

Electrolyte decomposition was monitored during battery cycling as a function of electrolyte composition, battery temperature, C-rate, and state of charge. The role of LiPF6 in solvent decomposition reactions was also explored. These measurements captured the dynamics of SEI formation on the graphite anode, including identification of key molecular bonds involved in EC decomposition during formative cycles from density functional theory modeling. At temperatures above 70°C, SEI decomposition was observed, followed by an increase in EC decomposition. Overcharging resulted in irreversible damage of the LiCoO2 cathode without any observable change in electrolyte composition. Spectral analysis further revealed distinct reaction mechanisms for temperature-induced EC decomposition and EC decomposition associated with SEI formation.