Enabling double layer polymer electrolyte batteries: Overcoming the Li-salt interdiffusion

Solid state lithium metal batteries based on polymer electrolytes hold the most promising prospect to face energy density and safety issues encountered by conventional Li ion batteries. The use of two different polymers, one for the cathode and another one as electrolyte, brings a sufficient energy gap and chemical stability allowing compatibility with the positive electrode and lithium metal anode; thus, achieving remarkable benefits towards high-performance cells. The present work unveils the Li salt interdiffusion occurring between two different dual-ion conducting polymer electrolytes consisting of lithium bis(trifluoromethanesulfonyl)imide] (LiTFSI) dissolved in poly(ethylene oxide) (PEO) and poly(propylene carbonate) (PPC). Combining these polymers within the same device results in cell failure due to the migration of LiTFSI to the more solvating PEO. The replacement of LiTFSI by lithium poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide] (LiPSTFSI), in which the anion is immobilized at a polymeric backbone, proves to overcome salt migration between the polymer phases. LiFePO4 -based solid state lithium metal batteries using LiTFSI fail within the initial cycles, while cells with LiPSTFSI display an outstanding cycling performance, with above 80% capacity retention at C/10, over 120 cycles, and excellent coulombic efficiency of ca. 100%. The rational design and in-depth knowledge provided in this work are highlighted as key elements for the development of high-performance solid state lithium metal batteries.