Improved Li-Ion Conduction and (Electro)Chemical Stability at Garnet-Polymer Interface through Metal-Nitrogen Bonding

Organic-inorganic composite solid electrolytes consisting of garnet fillers dispersed in polyvinylidene difluoride (PVDF) frameworks have shown promise to enable high-energy solid-state Li-metal batteries. However, the air-sensitive garnets easily form poorly-conductive residues, which hinders fast Li-ion exchange at the garnet-polymer interface and results in low ionic conductivity. The highly alkaline residues trigger instant dehydrofluorination of PVDF to form unsaturated C-C bonds, which are unstable against high-voltage cathode materials. Here it is shown that, by applying a 10-nm polydopamine coating on the residue-removed garnet surface, the modified garnet filler becomes air-stable and does not generate alkaline residues, so PVDF remains an intact structure. Surface characterizations reveal substantial metal-nitrogen bonding between the La atoms of garnet and the amino groups of polydopamine, which can invite stronger adsorption of Li ions at the heterointerface. A new interparticle Li-ion conduction mechanism is disclosed for the composite electrolyte, in which Li ions preferably migrate through the garnet-polydopamine interface, forming an efficient ion-percolation network. As a result, the composite electrolyte demonstrates an effective room-temperature Li⁺ conductivity of 1.52 × 10^–4 S cm^–1 and a high cutoff voltage of up to 4.7 V versus Li⁺/Li to support stable operation of all-solid-state Li-LiCoO₂ batteries.