Effect of rare earth ytterbium (RE-Yb 3+ ) ions on structure and electrical properties of the Li 1+x Ti 2−x Yb x (PO 4 ) 3 (0 ≤ x ≤ 0.3) NASICON series

Li 1+x Ti 2 −x Yb x (PO 4 ) 3 (0 ≤ x ≤ 0.3) materials were prepared via a conventional solid-state reaction and characterised via powder X-ray diffraction (XRD), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) and electrochemical impedance spectroscopy (EIS). The substitution of Ti 4+ by Yb 3+ in LiTi 2 (PO 4 ) 3 increases unit cell parameters and the amount of lithium in NASICON phase (S.G. R 3 c), but its incorporation is limited to 0.24/formula. The partial substitution of titanium by ytterbium enlarges diffusion channels and increases Li + ion mobility in Li 1+x Ti 2−x Yb x (PO 4 ) 3 (0 ≤ x ≤ 0.3) series. Li–Li repulsion favours the allocation of lithium at M3 (0.07, 0.34, 0.07) at the expense of M1 (0,0,0) sites in the NASICON structure, increasing Li + ion conductivity. The presence of paramagnetic Yb 3+ ions causes severe broadening of NMR signals which precludes structural analyses. Among the investigated samples, Li 1.2 Ti 1.8 Yb 0.2 (PO 4 ) 3 showed the best “bulk” conductivity (1.98 × 10 −3 S.cm −1 at room temperature) and a lower activation energy ( E a = 0.29 eV). The increment of ceramic pellet density decreased the grain-boundary resistance, improving transport properties. In particular, the “overall” conductivity at room temperature increased from 1.18 × 10 −7 ( E a = 0.45 eV) in LiTi 2 (PO 4 ) 3 to 1.01 × 10 −5 S.cm −1 ( E a = 0.33 eV) in Li 1.2 Ti 1.8 Yb 0.2 (PO 4 ) 3 . The stability of NASICON phases and the small amount of secondary phases make x = 0.2 sample a good candidate for solid electrolytes in all solid-state batteries (ASSBs). In these samples, “overall” conductivity is conditioned by “core–shell” structures, where external shells are enriched in Yb and Li ions. Above x = 0.2, the formation of secondary YbPO 4 and LiTiPO 5 phases at the NASICON particle surface decreased “overall” conductivity.