Halogenation of Li₇La₃Zr₂O₁₂ solid electrolytes: a combined solid-state NMR, computational and electrochemical study

Garnet-based solid electrolytes have been proposed as promising candidates for next generation all-solid-state batteries. Whilst multiple cation substitution studies of these garnets have been undertaken to try and improve their overall performance, anion doping of garnets has rarely been attempted, owing to the synthetic challenges associated with this particular doping strategy. In this work, we present the halogenation (F, Cl) of the solid state electrolyte Li₇La₃Zr₂O₁₂ (LLZO) via a low temperature solid state synthetic route using PTFE and PVC polymers. A reduction in tetragonal distortion (F incorporation) and a tetragonal to cubic phase transition (Cl incorporation) is observed in halogenated LLZO, suggesting the replacement of O²⁻ with F⁻ or Cl⁻ is associated with the creation of lithium vacancies. Combined solid-state NMR and computational studies support the presence of F⁻ or Cl⁻ in halogenated LLZO. The effects of surface fluorination were also investigated for Al-doped LLZO (Li_6.4Al_0.2La₃Zr₂O₁₂, LLAZO) with the results suggesting that this strategy has the ability to prevent full dendrite penetration at high current densities (up to 10 mA cm⁻²).