Understanding and controlling the reactions between the electrolyte and positive electrodes for Li-ion batteries

Abstract

Improving electrochemical energy storage devices is critical for the development of renewable energy storage devices for transportation, grid and residential applications. Lithium-ion batteries are the leading technology in the markets due to its good cycle life and high-energy density. However, current high production cost and safety concerns constrain their usage for various applications. Understanding the reactivity between the positive electrode and electrolyte is crucial for developing next-generation safer and cheaper lithium-ion batteries. The focus of this thesis is on the effect of the positive electrode/electrolyte interface (EEI) on the electrochemical performance of lithium-ion cells. To avoid any ambiguities from the presence of carbon or binder, oxide-only electrodes were used for these studies.

First, the EEI layer on LiCoO₂ electrodes was studied using X-ray Photoelectron Spectroscopy (XPS) and a correlation between interface composition and the ethylene carbonate (EC) dissociation on positive electrode surfaces was observed. This concept was extended to lithium nickel manganese cobalt oxides with different nickel contents (NMCl 11, NMC622, and NMC811 electrodes). Using these electrodes, we showed experimental evidence for EC dehydrogenation on charged NMC electrodes, which became more pronounced as the nickel content increased. Greater salt decomposition was coupled with the earlier onset of EC dehydrogenation with increasing nickel content or delithiation amount. Building upon these studies, we investigated different ways to improve cycling performance and to reduce or eliminate the effect of EC dehydrogenation on NMC surfaces. In this thesis, we explore various methods to stabilize high-energy positive electrodes such as coatings and electrolyte additives.

Through studying different coatings, we propose that high band-gap insulators such as A1₂O₃ are the best coating materials for positive electrodes due to reduced reactivity with electrolyte solvent (EC dehydrogenation) and salts (formation of lithium nickel fluoride/oxyfluoride species). We also show that adding chemically stable but electrochemically unstable electrolyte additives can reduce the effect of EC dehydrogenation even for NMC811 electrodes. We believe that by connecting surface reactivity on oxides with cycling performance, we can pinpoint key parameters to better lithium-ion cells.