Crystallization of Nanoscale Garnet-Type Li-Ion Conductors for All-Solid-State Batteries (ASSBs)

Abstract.

All-solid-state lithium-ion batteries (ASSLIBs) are considered to have substantial benefits over conventional Li-ion batteries based on flammable organic solvent electrolytes, such as high energy density, superior safety, and excellent mechanical strength [1–2]. The two types of battery architecture are compared in Fig. 1. As the core component of ASSLIBs, a lot of solid-state Li-ion conductors (electrolytes) have been developed and extensively explored in recent years, including crystalline Li_1 + xAl_xTi_2 − x(PO₄)₃ (NASICON), Li₇La₃Zr₂O₁₂ (Garnet), Li_3xLa_2/3 − xTiO₃ (Perovskite), Li₁₀GeP₂S₁₂ (thio-LISICON), amorphous LiPON, etc [3–4]. Among them, Al-doped cubic-structured garnet-type Li₇La₃Zr₂O₁₂ (Al-c-LLZO) solid-state Li-ion conductor has attracted the most attention due to its relatively high lithium-ion conductivity (10⁻³ ~ 10⁻⁴ S cm⁻¹ at ambient temperature), wide electrochemical window up to 6 V (vs. Li / Li⁺), and excellent chemical stability against lithium metal used as anode [5]. Despite of its good electrochemical properties, the commercial application potential of Al-c-LLZO is unfortunately hindered by its complex synthesis procedure and fabrication, which are time and energy consuming but also its interfacing with the electrodes. Up to now, the synthesis and processing of cubic-structured garnet conductors are based on sol-gel and solid-state synthesis and sintering usually done at temperatures above 1100 °C, as well as multiple cycles of sintering and milling [5]. This process results in micro-sized particles and potential Li-loss, which are unfavorable for further processing and electrode–electrolyte assembly with high Li-ion conductivity [6]. Here, we tackle this problem and firstly report a novel low-temperature synthesis-processing route to stabilize the cubic phase of Li_6.1Al_0.3La₃Zr₂O₁₂ (Al-c-LLZO), while limiting particle growth to ~100–200 nm. This synthesis route involves three steps: (1) co-precipitation, (2) hydrothermal aging, and (3) calcination. The cubic Al-c-LLZO phase is obtained at temperatures as low as 600 °C by reactive transformation/calcination of aqueous co-precipitated metal hydroxide intermediate of controlled crystal orientation. Through extensive structural characterization of the samples collected after different synthesis steps, the crystalline phase transition from M(OH)_{x (amorphous)} to M(OH)_{x (crystalline)} to La_zZr_yO_w and finally to Li_7-vAl_v/3La₃Zr₂O₁₂ (Al-c-LLZO) was observed and presented in Fig. 1. Based on the XRD patterns, the crystallization of Al-c-LLZO phase is fully developed when temperature reached 600 °C, and no further phase transition occurred when the temperature was raised to 700 °C, which suggests that the pure Al-c-LLZO phase can be obtained at 600 °C, the lowest calcination temperature compared to the published results so far. Moreover, according to the XRD results, the size of the primary nanocrystals of the low-T synthesized Al-c-LLZO was calculated through Scherrer equation to be ~29 nm. The morphology and particle size distribution of the low-T synthesized Al-c-LLZO particles are presented in Fig. 2. The mean size of the particles is 180 nm. As for the Li-ion conductivity, the low-T synthesized Al-c-LLZO particles after one-step pellet compaction and sintering were characterized by Electrochemical Impedance Spectroscopy (EIS) at room temperature (21 °C) as well as from 30 to 80 °C with 10 °C temperature intervals. The Nyquist plots are shown in Fig. 3. The Li-ion conductivity at RT is approaching 10-3 S ∙ cm⁻¹ and the calculated activation energy of Li-ion migration is 0.174 eV. Through this novel synthesis route, nanoscale Al-c-LLZO particles with high ionic conductivity are particularly attractive in promoting compatibility between cathode materials and solid electrolytes for ASSLIBs. The mechanism of the low-T synthesis route, as well as the results in detail, will be provided during the oral presentation (Fig. 4).

Senhao Wang is the presenting author of this chapter.