Facile SEI Improvement in the Artificial Graphite/LFP Li-ion System: via NaPF6 and KPF6 Electrolyte Additives

Abstract

Lithium-ion batteries (LIBs) are the most widely used energy storage technologies for grid-scale applications, electric cars, and portable devices because of their consistent voltage profiles, high energy density, and extended cycle life. However, interfacial degradation events, especially at the anode, frequently restrict their long-term performance. The creation of the solid electrolyte interphase (SEI) layer, a passivation film that occurs on the graphite anode surface during the initial cycles, is one of the most important issues in this context. Increased impedance, irreversible capacity loss, and decreased coulombic efficiency are caused by unstable or excessive SEI production, which is necessary for stabilizing the electrode–electrolyte interface. These issues are made worse by fast charging or prolonged cycling. This thesis uses artificial graphite anodes and lithium iron phosphate (LFP) cathodes to examine the effects of sodium hexafluorophosphate (NaPF₆) and potassium hexafluorophosphate (KPF₆) additives on SEI properties and electrochemical performance in LIBs. These alkali salt additives, which are rarely investigated in lithium-ion systems, are being investigated as scalable and affordable substitutes for traditional SEI-modifying additives. The effects of the additives were evaluated by electrochemical testing, which included galvanostatic cycling, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), as well as surface and structural characterization (SEM and XPS). According to our results, using KPF₆ and NaPF₆ considerably lowers irreversible capacity loss (by 38.98% and 37.85%, respectively) as compared to a baseline LiPF₆ electrolyte. Additionally, full cell tests show enhanced capacity retention without adversely altering ionic conductivity, with 67.39% for NaPF₆ and 30.43% for KPF₆ across 20 cycles. High-conductivity electrodes used for SEI generation at 1C further validated the additives' capacity to continue functioning during faster formation. These results provide new perspectives on mixed-ion SEI engineering and show how promising NaPF₆ and KPF₆ are for improving LIB performance in demanding applications like electric vehicles.