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

In this work, graphite anodes and lithium iron phosphate (LFP) cathodes are used to examine the effects of sodium hexafluorophosphate (NaPF6) and potassium hexafluorophosphate (KPF6) electrolyte additives on the formation of the solid electrolyte interphase and the performance of lithium-ion batteries in both half-cell and full-cell designs. The objective is to assess whether these additives may increase cycle performance, decrease irreversible capacity loss, and improve interfacial stability. Compared to the control electrolyte (1.22 M Lithium hexafluorophosphate (LiPF6)), cells with NaPF6 and KPF6 additives produced less SEI products, which decreased irreversible capacity loss and enhanced initial coulombic efficiency. Following the formation of the solid electrolyte interphase, the specific capacity of the control cell was 607 mA·h/g, with 177 mA·h/g irreversible capacity loss. In contrast, irreversible capacity loss was reduced by 38.98% and 37.85% in cells containing KPF6 and NaPF6 additives, respectively. In full cell cycling, a considerable improvement in capacity retention was achieved by adding NaPF6 and KPF6. The electrolyte, including NaPF6, maintained 67.39% greater capacity than the LiPF6 baseline after 20 cycles, whereas the electrolyte with KPF6 demonstrated a 30.43% improvement, indicating the positive impacts of these additions. X-ray photoelectron spectroscopy verified that sodium (Na+) and potassium (K+) ions were present in the SEI of samples containing NaPF6 and KPF6. While K+ did not intercalate in LFP, cyclic voltammetry confirmed that Na+ intercalated into LFP with negligible impact on the energy storage of full cells. These findings demonstrate that NaPF6 and KPF6 are suitable additions for enhancing lithium-ion battery performance in the popular artificial graphite/LFP system.