Understanding the role of pressurized CO2 in the direct recycling process of Li-ion battery positive electrode

Abstract

The ternary mixture of carbon dioxide (CO₂), triethyl phosphate (TEP), and acetone enabled an efficient delamination of Li-ion battery (LIB) positive electrode inciting an interest to study the ternary mixture behavior. Pressurized fluids, such as CO₂, are known to be beneficial in various chemical processes. However, the behavior of CO₂, when mixed with TEP and acetone is not well understood, particularly under pressure and temperature conditions. This study investigates the behavior of CO₂ or nitrogen (N₂) in mixtures with TEP and acetone at various compositions, using experimental investigations of the ternary system. Experimental data covers four temperatures at 35°C, 70°C, 100°C and 120°C at a constant pressure of 100 bar. The phase behaviors of the binary and ternary mixtures were observed using a transparent reactor, while the compositions were analyzed in situ with Raman spectroscopy. Under isobaric conditions, a single phase was observed with CO₂ at 35°C, both in the binary systems with either TEP or acetone, as well as in the ternary mixture. In contrast, a biphasic system was observed at higher temperatures (70°C, 100°C, and 120°C) in all mixtures containing CO₂. Specifically, the biphasic condition at 55°C at 100 bar, of the mixtures were semi-quantitatively investigated using Raman spectroscopy to probe the compositions in the vapor and liquid phases. These observations elucidate the crucial role of CO₂ in the delamination of the positive electrode in LIB using the TEP-acetone-CO₂ system enabling high efficiency, low solvent consumption, and a faster processing time.