Harnessing Cold Sintering to Fabricate Composite Polymer Electrolytes - A Paradigm Shift in Organic-Inorganic Material Assembly

The development of composite electrolytes for all-solid-state batteries is an emerging field, but the creation of predominantly inorganic electrolytes remains challenging. In this study, Li6.25Al0.25La3Zr2O12 (Al-LLZO), a ceramic material selected for its high ionic conductivity (1x104 S.cm-1 at ambient temperature) was shaped by the cold-sintering process (CSP). The organic phase was synthesized by free-radical polymerization of two poly(ethylene oxide) methacrylate derivatives in the presence of lithium bis(trifluoromethanesulfonyl)imide salts (LiTFSI). The polymethacrylate network with dangling poly(ethylene oxide) (PEO) chains was thus obtained. This in-situ polymerization allows the one-pot synthesis of the composite electrolyte during CSP. Remarkably, the ionic conductivity of the CSP pellet varied with the nature of the organic phase, ranging from 1x10−4 to 1x10−5 S.cm-1 for non-grafted and grafted TFSI anion on the PEO-based network, respectively. Additionally, the transport of Li+ remained unaffected by the inorganic material's nature as long as it contained Li species. Furthermore, a significant enhancement of the ionic conductivity was observed in the composite pellet compared to the TFSI grafted network (10−5 to 10−7 S.cm-1, respectively). Electrochemical impedance spectroscopy measurements revealed changes in the Al-LLZOPEO-based polymer interface during CSP with the formation of an interphase, confirmed by a low activation energy value (0.1 eV).