Fluorine-Free Li-Ion Battery Features Comparable Cycling Performance to a Highly-Fluorinated Equivalent†

Abstract

Fluorinated compounds, including polyvinylidene fluoride (PVdF) binder and lithium hexafluorophosphate salt, are considered essential components in lithium-ion batteries due to their ability to provide good performance and cycle life. However, these compounds raise potential environmental concerns, as they can lead to the formation of toxic, corrosive and persistent compounds, such as hydrofluoric acid, phosphorus pentafluoride (PF₅) and per- and polyfluoroalkyl substances. In this study, the effects of fluorine-free electrolyte consisting of lithium bis(oxalato)borate (LiBOB) salt and vinylene carbonate (VC) addtive and an aqueous-based binder based on carboxymethyl cellulose (CMC) and latex are investigated for full cells combining a silicon–graphite composite anode with LiNi_0.6Mn_0.2Co_0.2O₂ cathodes. Higher capacity retention is obtained at C/2 after 500 cycles with the fluorine-free binder in the cathode (61 and 65% for the fluorine-free and fluorinated electrolytes, respectively) compared to the PVdF-based binder (58 and 56% respectively). X-ray photoelectron spectroscopy analysis of the passivating layers on the cathode and anode across the four systems revealed a F-rich interfacial composition in the presence of the fluorinated electrolyte, while the fluorine-free electrolyte led to the formation of oxygen-rich layers at the interphases. Despite the chemical differences, both layers protect the cathode and anode during cycling, resulting in similar electrochemical performance. Moreover, the solid electrolyte interphase composition on the anode is dependent on the cathode’s formulation. The aqueous-processed cathode resulted in higher coverage of the active material, which mitigated salt decomposition and facilitated the formation of a more stable passivating layer. This contributed to higher capacity retention despite a lower initial discharge capacity compared to the PVdF-based cathode. This study demonstrates the potential of fluorine-free components (electrolyte and binder with aqueous-processed cathode) to achieve high-energy-density full cells with comparable performance to conventional highly fluorinated lithium-ion batteries.