NASICON LiM₂(PO₄)₃ electrolyte (M = Zr) and electrode (M = Ti) materials for all solid-state Li-ion batteries with high total conductivity and low interfacial resistance

All solid-state batteries based on NASICON-Type LiM₂(PO₄)₃ electrolyte phases are highly promising owing to their high ionic conductivities and chemical stabilities. Unlike Ti-based phases, extensively studied as Li⁺ solid electrolyte membranes, LiZr₂(PO₄)₃ (LZP) is expected to form a stable interface with a metallic lithium anode, a challenge which has posed a serious roadblock to realising safe all solid-state batteries. However, prohibitively large grain boundary resistances are often observed in this material and this issue, combined with processing difficulties in fabricating LZP in dense forms, has impinged on the application of LZP as a solid electrolyte for all solid-state batteries. To overcome these shortcomings and demonstrate the excellent potential of LZP as a solid electrolyte, we have developed a simple approach, based on sol-gel chemistry, to effectively improve the densification of the material leading to higher total conductivity than previously reported (1.0 × 10^-4 S cm^-1 at 80 °C) and enabling the investigation of the material as a Li⁺ solid electrolyte without the need for elaborate post-processing steps. The interfacial resistance decreases dramatically on using thin layers of Au buffer to improve the contact between Li and the LZP surface. The Li/LZP interface shows constant resistance upon Li⁺ cycling (at 40 μA cm^-2), despite the formation of a passivation layer of Li₃P/Li₈ZrO₆ on the LZP surface. This is consistent with the prediction that this surface layer serves as a Li⁺ conductive, solid electrolyte interface between Li and LZP. Finally, an analogue material, LiTi₂(PO₄)₃, is also introduced and demonstrated as an electrode material for proposed LZP-based all-solid-state batteries.