The development and progress of portable electric devices require lithium-ion batteries with higher energy density. In addition, the safety of batteries also becomes more important, particularly for electric vehicles. As one of solutions to fulfill such requirements, all-solid-state lithium-ion batteries have attracted much attention. One of prospective solid electrolyte for all-solid-state lithium-ion batteries is Li₇La₃Zr₂O₁₂ (LLZ), which has a relatively high Li⁺ ion conductivity of ~10^–4 S cm^–1 and the chemical stability against Li metal. However, LLZ requires a very high sintering temperature at around 1100 °C, and basically the preparation of dense LLZ pellets is difficult. To realize the lower sintering temperature and densification of LLZ pellets, sintering additive such as Al and Nb have been investigated. However, there are few studies about the influence of starting materials for LLZ synthesis on the quality of resulting LLZ pellets. In this study, we synthesized Al-doped garnet-type Li_6.25La₃Zr₂Al_0.25O₁₂by changing the Li sources to investigated the influence of Li sources on sintering process of Al-doped LLZ.

Experimental

Al-doped LLZ was synthesized from each Li sources, LiOH·H₂O, Li₂CO₃ and CH₃COOLi, by solid phase method. Stoichiometric amounts of Li sources (10% excess) La(OH)₃ and ZrO₂ were mixed by planetary ball-milling with zirconia balls and hexane, and then calcined at 900 °C for 5 h in alumina crucible. The calcined powders and stoichiometric amounts of γ-Al₂O₃were mixed by planetary ball-milling with zirconia balls and hexane. The mixed powders were pelletized, and then the pellets were sintered at 900 °C for 3 h and 1200 °C for 12 h with mother powder.

Results and discussion

X-ray diffraction (XRD) patterns of the samples prepared using LiOH·H₂O, Li₂CO₃ and CH₃COOLi as Li sources shows that all the samples are single phase of cubic garnet without impurity. Observation of cross sectional of each pellet was performed by scanning electron microscope (SEM). Al-doped LLZ pellet derived from LiOH·H₂O was uniformly dense (Fig. 1(a)). However, many grain boundaries are found when Li₂CO₃ or CH₃COOLi were used as Li sources (Fig. 1 (b) and (c)). These results indicated that Li sources have an influence on sintering behavior of Al-doped LLZ. To investigate difference in sintering behavior, XRD patterns of the calcined powders were measured. The calcined powders derived from Li₂CO₃ or CH₃COOLi included La₂Zr₂O₇ as main component and cubic LLZ, whereas XRD patterns of the calcined powder derived from LiOH·H₂O were assigned to single phase of cubic LLZ. Difference between LiOH·H₂O and other two Li sources is presence or absence of C element. The C element changed to CO₂ and then this CO₂ generated Li₂CO₃ by reacting with raw or calcined materials during calcination. It is known that Li₂CO₃ inhibits synthesis and sintering of LLZ. Therefore, sintering density of Al-doped LLZ derived from Li₂CO₃ and CH₃COOLi was low. There results indicated that it is important to prepare calcined powders with single phase of cubic LLZ for densification of Al-doped LLZ pellets.

Acknowledgement

This work was partly supported by Advanced Low Carbon Technology Research and Development Program (ALCA-SPRING) of Japan Science and Technology Agency (JST).

Figure captions

Fig. 1  Cross sectional SEM images of LLZ pellets derived from (a) LiOH·H₂O, (b) Li₂CO₃ and (c) CH₃COOLi.