Asymmetrically coated LAGP/PP/PVDF–HFP composite separator film and its effect on the improvement of NCM battery performance

Abstract

Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) is an inorganic solid electrolyte with a Na superionic conductor (NASICON) structure that provides a channel for lithium ion transport. We coated LAGP particles on one side of a polypropylene (PP) separator film to improve the ionic conductivity of the separator, and water-dispersed polyvinylidene fluoride–hexafluoropropylene (PVDF–HFP) on the other side to reduce the interfacial resistance between the separator and the lithium metal anode. The results show that the LAGP/PP/PVDF–HFP separator has a high ionic conductivity (1.06 mS cm⁻¹) at room temperature (PP separator: 0.70 mS cm⁻¹), and an electrochemical window of 5.2 V (vs. Li⁺/Li). The capacity retention of a NCM|LAGP/PP/PVDF–HFP|graphite full cell is 81.0% after 300 charge–discharge cycles at 0.2C. When used in a NCM|LAGP/PP/PVDF–HFP|Li half-cell system, the initial discharge capacity is 172.5 mA h g⁻¹ at 0.2C, and the capacity retention is 83.2% after 300 cycles. More significantly, the surface of the Li anode is smooth and flat after 200 cycles. The interface resistance increased from 7 to 109 Ω after 100 cycles at 0.2C. This indicates that the synergistic effect of the asymmetric coated LAGP and PVDF–HFP is beneficial to inhibiting the growth of lithium dendrites in the battery and reduces the interface resistance.