Composites of Alkali Carbonphosphonitride Thermosets with LLZO and other Ion Conducting Ceramics

Lithium dicyanamide (LiN(CN)₂) reacts with phosphorus cyanides (P(CN)₃ or RP(CN)₂) in a 2:1 or greater mole ratio in an anhydrous aprotic mutual solvent to form a resin, which then cures at temperatures of 200-300 °C to a non-flammable ion conductor. When fully cured, those films have a low ionic conductivity, probably due to rigid crosslinking of the thermoset. We found previously that adding compounds to the resin, such as LiCN that contain a higher concentration of Li than LiN(CN)₂, and some non-volatile organic plasticizers, increase the room temperature ionic conductivity from 10^-12 S/cm to around 10^-8 S/cm, and the Na versions can also be more conductive than those with Li. Post-cure treatment with diglyme also causes a dramatic conductivity increase, presumably by complexing the mobile Li ions and plasticizing the material. While it seems unlikely that practical conductivities can be obtained in the pure resins, they might be attractive as binders for more highly conducting ceramic powders such as lithium lanthanum zirconium oxide (LLZO), in order to produce a composite material that can be cured to a solid electrolyte at temperatures compatible with other components. We will report our systematic efforts to use these dicyanamides and A-P-C-N (A = Li, Na) thermosets as binders for commercially available ion conducting ceramics. The ratios of resin to ceramic and the conditions of preparation are varied to produce solid composite films for measurement, and the ionic conductivity and chain motions of these films are characterized by impedance and dielectric relaxation spectroscopy. Test cells with Li foil are constructed and measured to evaluate electrochemical stability and lithium transference numbers. Additionally, solid state NMR is used to examine the compositional stability of the composite and the ion mobility between different ionic conductors.