Degradation Mechanism of Phosphate-Based Li-Nasicon Conductors in Alkaline Environment

Li-NASICON solid-state superionic conductors have traditionally been regarded as promising candidates for Li-air applications because of their stability in ambient air and water [1], [2] . However, the presence of water in the cathode can change the discharge product from Li 2 O 2 to LiOH [3], [4] , inducing a highly alkaline environment which may degrade cathode and separator materials. Here, we investigate the impact of octahedral substitution on the alkaline stability of common Li-NASICON chemistries through a systematic experimental case study of LiTi x Ge 2-x (PO 4 ) 3 (LTGP) with varying x = 0 - 2.0. Density functional theory calculations are combined to gain mechanistic understandings of the alkaline instability. We demonstrated that the alkaline instability of LTGP is mainly driven by the dissolution of PO 4 polyanion groups, which subsequently precipitate as Li 3 PO 4 . The introduction of Ti facilitates the formation of a Ti-rich compound on the surface that eventually passivates the material, but only after significant bulk degradation. Consequently, phosphate-based Li-NASICON materials exhibit limited alkaline stability, raising concerns of their viability in humid Li-air batteries. [1] N. Imanishi, S. Hasegawa, T. Zhang, A. Hirano, Y. Takeda, and O. Yamamoto, “Lithium anode for lithium-air secondary batteries,” Journal of Power Sources , vol. 185, no. 2, pp. 1392–1397, Dec. 2008, doi: 10.1016/j.jpowsour.2008.07.080. [2] R. Chen, Q. Li, X. Yu, L. Chen, and H. Li, “Approaching Practically Accessible Solid-State Batteries: Stability Issues Related to Solid Electrolytes and Interfaces,” Chem. Rev. , vol. 120, no. 14, pp. 6820–6877, Jul. 2020, doi: 10.1021/acs.chemrev.9b00268. [3] S. Ma, J. Wang, J. Huang, Z. Zhou, and Z. Peng, “Unveiling the Complex Effects of H2O on Discharge–Recharge Behaviors of Aprotic Lithium–O2 Batteries,” J. Phys. Chem. Lett. , vol. 9, no. 12, pp. 3333–3339, Jun. 2018, doi: 10.1021/acs.jpclett.8b01333. [4] T. Liu et al. , “Cycling Li-O2 batteries via LiOH formation and decomposition,” Science , vol. 350, no. 6260, pp. 530–533, Oct. 2015, doi: 10.1126/science.aac7730.