Low Interfacial Resistance and Superior Suppression to Li-Dendrite Penetration Facilitated by Air-Stable and Mechanically Robust Al/Mg-Co-Doped Li-La-Zirconate as Electrolyte for Li-Based Solid-State Cells

In the context of usage as a solid electrolyte (SE) for Li-based solid-state cells, the interfacial characteristics of Li-La-zirconate (LLZO) with the electrodes and the mechanical properties of LLZO influence the overall impedance and stability. In this regard, the newly developed air-stable Al/Mg-co-doped LLZO has been found to possess greater resistance to crack propagation (by ∼31%) and fracture stress (by ∼52%), along with elevated hardness and stiffness, as compared to simply Al-doped LLZO. Furthermore, as directly visualized via cross-section electron microscopy at the Li/LLZO interfaces, the air-stability, along with mechanical robustness of Al/Mg-co-doped LLZO, facilitates the complete absence of impurity phase and cracks at the Li/LLZO interface, unlike for the simply Al-doped LLZO. These result in a very low area specific resistance for the Li/"Al/Mg-co-doped LLZO" interface of ∼9 Ω cm2, which is ∼3 times lower than that at the Li/"Al-doped LLZO" interface and is also among the lowest reported to date for Li/LLZO interfaces, that too sans any surface/interfacial coating/engineering. Galvanostatic Li-plating/stripping cycles indicate that the critical current density toward initiating Li-dendrite nucleation/propagation is higher in the case of Al/Mg-co-doped LLZO SE, viz., ∼0.45 mA/cm2, than for the Al-doped counterpart (viz., ∼0.25 mA/cm2). Furthermore, Li-stripping/plating cycles @ 0.1 mA/cm2 have revealed outstanding stability of polarization voltage up to at least 100 cycles upon using Al/Mg-codoped LLZO as the SE, in contrast to the instability right from the 36th cycle onward with the Al-doped LLZO. This indicates superior suppression toward Li-dendrite nucleation/propagation by the Al/Mg-codoped LLZO, unlike by Al-doped LLZO, as also directly visualized via cross-section electron microscopy post-cycling. The air-stability induced a clean Li/LLZO interface (viz., good contact), which, together with the mechanical robustness of Al/Mg-codoped LLZO, resulted in the very low interfacial resistance and excellent suppression toward Li-dendrite nucleation/propagation, leading to high CCD and very stable long-term Li-stripping/plating. Overall, in addition to highlighting the notable advantages offered by the Al/Mg-co-doped LLZO solid electrolyte, the insights obtained as part of this work are expected to lead to the successful and facile development of high-performance solid-state Li-based cells.

Thermal Recovery of the Electrochemically Degraded LiCoO2/Li7La3Zr2O12:Al,Ta Interface in an All-Solid-State Lithium Battery

All-solid-state lithium batteries are promising candidates for next-generation energy storage systems. Their performance critically depends on the capacity and cycling stability of the cathodic layer. Cells with a garnet Li₇La₃Zr₂O₁₂ (LLZO) electrolyte can show high areal storage capacity. However, they commonly suffer from performance degradation during cycling. For fully inorganic cells based on LiCoO₂ (LCO) as cathode active material and LLZO, the electrochemically induced interface amorphization has been identified as an origin of the performance degradation. This study shows that the amorphized interface can be recrystallized by thermal recovery (annealing) with nearly full restoration of the cell performance. The structural and chemical changes at the LCO/LLZO heterointerface associated with degradation and recovery were analyzed in detail and justified by thermodynamic modeling. Based on this comprehensive understanding, this work demonstrates a facile way to recover more than 80% of the initial storage capacity through a thermal recovery (annealing) step. The thermal recovery can be potentially used for cost-efficient recycling of ceramic all-solid-state batteries.