Lithium iron phosphate cathode supported solid lithium batteries with dual composite solid electrolytes enabling high energy density and stable cyclability

In this research, we present a report on the fabrication of a Lithium iron phosphate (LFP) cathode using hierarchically structured composite electrolytes. The fabrication steps are rationally designed to involve different coating sequences, considering the requirements for the electrode/electrolyte interfaces. Two layers of composite solid electrolyte, consisting of poly(propylene carbonate) (PPC) + lithium bis (trifluoromethanesulfonyl)imide (LiTFSI) salt and poly(ethylene oxide) (PEO) + LiTFSI + Li7La3Zr2O12 (LLZTO), are employed as hierarchical structured composite electrolytes on the LFP cathode. Experimental results indicate that the optimal combination consists of a thinner PPC + LITFSI layer on the LFP cathode and a thicker PEO + LiTFSI + LLZTO facing Li metal. The Li-metal batteries, equipped with LFP supported hierarchical electrolytes exhibited an ultra-high specific capacity (~155 mAh g−1). Additionally, they demonstrated electrode polarization in the reduced form of −0.15 V, and an excellent cyclic stability with the retention capacity of 87.6 % even in 200 cycles. The ionic conductivity of the dual-layer electrolytes is achieved as high as 1.63–2.60 × 10−4 S cm−1 at ambient temperature. The impressive cycling performance demonstrated in this study is primarily attributed to the significance of the coating sequence in the design of dual composite solid electrolytes. The PPC + LiTFSI composite layer appears to function as a flexible filler on the LFP side. On the other hand, the PEO + LiTFSI + LLZTO composite layer, which faces the Li metal anode, not only provides excellent ionic conductivity but also serves as a barrier against the mechanical stress by the formation of Li-dendrite. The dual-layer electrolyte configuration, as demonstrated in this work, can be engineered to enable high energy density and stable cyclability of Li-metal batteries.