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The synthesis and processing of garnet-type conductors is usually done at temperatures above 1100 °C to reach the high Li-ion conducting cubic phase resulting in micron-sized particles and potential Li-loss, which are unfavorable for further processing and electrode-electrolyte assembly. Here, we tackle this problem and report a novel low-temperature aqueous synthesis route to stabilize the cubic phase of Li 6.1 Al 0.3 La 3 Zr 2 O 12 (c-Al-LLZO) at a temperature as low as 600 °C , while obtaining nano-crystallites at around 100 - 200 nm. Moreover, a dual-reaction mechanism was proposed to describe the process of c-Al-LLZO crystallization. The as-synthesized nanoscale c-Al-LLZO particles facilitate the densification of the solid-state electrolyte (SSE) pellets with less endowed grain boundaries (exhibiting a high relative density of 97.8 %). The ionic conductivity reaches 0.42 mS cm−1 at 21 °C and the low activation energy (half of the published values) is 0.17 eV. The results presented here show that symmetric cells with Li metal exhibit excellent stability at the current density of 0.2 mA/cm2 and 0.5 mA/cm2, and an improved critical current density (CCD) of 2.16 mA/cm2. This work provides a feasible method of low-temperature crystallization of nanoscale cubic garnet SSEs with good performance for the fabrication of stable SSLBs. [Display omitted] • Direct crystallization of cubic garnet (LLZO) solid-state electrolytes at 600 °C. • A new variety of near nanoscale cubic garnet solid electrolyte. • Excellent densification and high lithium-ion conductivity. • Symmetric cells with high CCD and Li metal plating-stripping stability. [ABSTRACT FROM AUTHOR]

Lithium iron orthosilicate (Li 2 FeSiO 4 ) is an important alternative cathode for next generation Li-ion batteries due to its high theoretical capacity (330 mA h/g). However, its development has faced great challenges arising from significant structural complexity, including the disordered arrangement/orientation of Fe/Si tetrahedra, polytypes and its poorly understood Li storage and transport properties. In this context, ab-initio calculations are employed to investigate the phase stability and Li diffusion profiles of both monoclinic ( P 2 1 ) and orthorhombic ( Pmn 2 1 ) Li 2 FeSiO 4 orthosilicates. The calculations demonstrate that formation of Li Fe antisites can induce a metastability competition between both phases, with neither dominating across nearly the entire discharging profile from Li 2 FeSiO 4 through to LiFeSiO 4 . Furthermore, structural instability is shown to be a serious concern at discharge concentrations below LiFeSiO 4 (1 Li extraction) due to the shared occupation of Li donated electrons with oxygen 2p orbitals – rather than the hypothesized transition to a tetravalent Fe 4+ state. This finding is further supported by diffusion calculations that have determined a high activation energy barrier towards fast charging and rapid phase transitions. In summary, these theoretical results provide critical and timely insight into the structural dynamics of lithium iron orthosilicate, in pursuit of high energy density cathodes. [ABSTRACT FROM AUTHOR]