NASICON-type Ta5+ substituted LiZr2(PO4)3 with improved ionic conductivity as a prospective solid electrolyte

The need to develop safe solid-state lithium batteries has stimulated intense research efforts for Li + solid electrolytes. However, the low conductivity limits the development of NASICON LiZr 2 (PO 4 ) 3 (LZP) electrolyte. Here, the doping effects of Ta on the structure, surface morphologies and electrochemical properties of Li 1- x Zr 2- x Ta x (PO 4 ) 3 (LZTP, x  = 0, 0.01, 0.02, 0.04, 0.06 and 0.08) solid electrolyte were analyzed. LZTP was prepared using a simple solid-state reaction route, followed by sintering at 1200 °C for 12 h. A proper content of Ta 5+ substitution for Zr 4+ is beneficial to stabilize the high conductive rhombohedral (α) phase of LZP at room temperature. Doping Ta 5+ is conducive to unblocking of Li + at the M1 site and facilitates the occupation of Li + at the M2 site, thereby expanding the pathway for Li + conduction. Rietveld refinement data demonstrated that the Zr–O and P–O bond lengths ( d Zr-O and d P-O ) increased with a decrease in Zr–O–P bond angles ( θ Zr-O-P ) as x rose. The distortions in the ZrO 6 octahedron may weaken the coulomb attraction in Li + -O 2- , resulting in a lower activation energy ( E a ) and a higher Li + conductivity. The highest room-temperature conductivity (6.06 × 10 −5  S cm −1 ) was obtained at x  = 0.06, which reached 1.5 × 10 −4  S cm −1 at 50 °C. The E a was found to decrease from 0.388 eV ( x  = 0) to 0.306 eV ( x  = 0.06). In addition, Ta doping resulted in improved connectivity and reduced pore formation, which also contributed to the decrease in resistance. The Raman spectrum demonstrated that some phonon modes of bending vibration in PO 4 were degenerate and external modes became too weak to be observed or even disappeared as Ta content increased. This change in the modes also had an impact on the Li + conductivity. Overall, the LZTP-0.06 appears to be a promising candidate for the solid electrolyte.