Researchers decrypted the formation of passivation layers for the promising bis-(fluorosulfonyl)-imide (FSI-) based ionic liquid electrolyte on carbon electrodes at high cell voltages. This dual chemical and electrochemical route to forming stable interphase layers could revolutionize design of cell chemistry to promote high performance batteries.
The presence of a high quality, ionically conducting passivation layer on electrodes surfaces is a key component of stable, high-performance batteries. There has been recent interest in anions which decompose to form fluoride salts at the surface in Li-ion and beyond Li-ion battery systems. The FSI- anion in particular has shown great promise in electrochemical data, but there has been a knowledge gap in the understanding of how the FSI- anion passivates the electrode surface relative to similar anions such as bis(trifluoromethanesulfonyl)imide (TFSI-). To address this knowledge gap, an operando neutron scattering cell was used to study the mechanism of passivation layer formation on carbon electrodes in a discharging battery via small angle neutron scattering at the Spallation Neutron Source. Along with ex-situ X-ray photoelectron spectroscopy (XPS), these data reveal lithium-rich salt reduction products (e.g., LiF) formed at relatively high cell potentials (≈2 V) in the 0.5 M LiFSI/EMI.FSI electrolyte, resulting in a more beneficial and stable SEI. Unexpectedly, we find the FSI- anion can react chemically at the open cell voltage (OCV) to produce a salt passivation layer. Imidazole carbenes formed at the lithium counter electrode diffuse to the carbon electrode and cause chemical reactions to form LiF, and other lithium salts such as Li2O and LiOH, at OCV. This study improves understanding of how the facile reactivity of the FSI- anion leads to improved surface passivation, both chemically and electrochemically. Understanding the driving forces behind both chemical and electrochemical passivation will help guide the design of next-generation electrochemical energy storage systems.
