Unraveling the role of LiODFB salt as a SEI-forming additive for sodium-ion battery

The sodium-ion battery is a strong candidate for the large-scale energy storage device due to its low cost and abundant resources. However, the severe self-discharge issue, which is rooted in the dissolution of the solid-electrolyte interphase (SEI) film in the sodium-ion battery, impedes its practical application. Thus, the central question is how to build a stable SEI film onto the electrode surface. Here, we propose and experimentally demonstrate a LiF-rich SEI film at the surface of hard carbon (HC) anode in sodium-ion battery, which is generated by adding lithium difluoro(oxalate)borate (LiODFB) additive into the electrolyte of 1 M NaPF 6 in EC:DMC (1:1 in volume ratio). The X-ray photoelectron spectroscopy (XPS) and electron microscopy (SEM and TEM) results confirm that we obtain the LiF-rich SEI film at the HC electrode surface, which grows up with the increasing of the concentration of added LiODFB additive. But it blocks the transmission of Na ions into HC as evidenced by the initial galvanostatic charge/discharge, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) results. Although this work shows a negative result, it denies the possibility of using the lithium compounds with lower solubility as SEI components for Na-ion battery since it allows to transfer the lithium ions rather than the Na ions.