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

Abstract 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‐LiCoO 2 batteries.