Dual regulation of Li⁺ migration of Li_6.4La₃Zr_1.4M_0.6O₁₂ (M = Sb, Ta, Nb) by bottleneck size and bond length of M−O

Abstract

Bottleneck size is the minimum Li+ migration channel of Li7La3Zr2O12 (LLZO) and it greatly influences the Li+ conductivity. Doping different elements on the Zr site of LLZO can adjust the bottleneck size and improve the Li+ conductivity. However, the regulation mechanism is not clear. In this work, Li6.4La3Zr1.4M0.6O12 (M = Sb, Ta, Nb) has been prepared and the bottleneck size has been adjusted by doping different pentavalent ions. The results manifest that the cell parameter and bottleneck size decrease with the rise of the radius of doped pentavalent ions. This is because larger pentavalent ion leads to larger bond length of M–O, and weaker covalent component between M5+ and O2‐, corresponding, the formal charge on the M5+ become larger, and the bond length of La–O slightly decreases due to the coulomb repulsion between La3+ and M5+ increase. While, the activation energy drop firstly and then rise with the rise of bottleneck size because of the migration of Li+ not only relate to the size of the migration channel but also to the strength of M–O covalent bonding. The bottleneck size and bond length of M–O synergistically affect the migration of Li+.

Study of Codoping Effects of Ta5+ and Ga3+ on Garnet Li7La3Zr2O12

“…This result is consistent with an increase in the peak intensity of the (422) peak with Ta 5+ doping in the garnet structure. 34 The intensity ratio of (420)/(422) was observed to be decreased for all of the sintered samples. A linear fitting showed steeper slopes for 12 h sintering than for 6 h ( Figure 2 e,f).…”

“…Generally, Ta-doping requires a relatively high temperature and long sintering duration compared with Ga-doping in LLZO. 29 − 34 Long-time sintering at higher temperatures could induce undesirable Li loss. Therefore, it is very significant to secure a sintering process at a lower temperature and short sintering time, providing cubic-phase LLZO with suppression of Li loss.…”

Tri-Doping of Sol–Gel Synthesized Garnet-Type Oxide Solid-State Electrolyte

“…The ionic conductivity is improved by Zr 4+ site doping owing to expansion of the Li + transfer bottleneck. Previous research [ 43 , 44 ] has described this mechanism as widening of the migration channel through which Li + can be conducted, thus making diffusion easier. Furthermore, this widening is greater in the case of Nb doping than Ta doping.…”

The influence of hafnium impurities on the electrochemical performance of tantalum substituted Li7La3Zr2O12 solid electrolytes

“…This behaviour can be explained when considering the conductivity mechanism in LLZO. The conductivity of a Li-ion conductor depends mainly on two different parameters: the concentration of mobile Li-ions and their mobility [22]. The optimal concentration of mobile Li + -ions was subject of many investigations and was found to have an optimal value between 6.4 and 6.5 mol Li per unit cell [28,29,36].…”

“…for tantalum and niobium. The chemical properties of both elements are very similar [22], and they are often found together in ores. However, their separation is complicated and expensive.…”