The pursuit of next-generation high-energy-density batteries has intensified the focus on lithium-metal batteries (LMBs) employing solid electrolytes. Solid-state electrolytes offer significant advantages over liquid electrolytes, primarily enhanced safety and the potential for substantially higher energy densities. However, a critical challenge hindering the widespread adoption of LMBs is the interfacial instability between the lithium metal anode and the solid electrolyte. This instability manifests as the formation of dendrites, leading to short circuits and compromised battery lifespan. The formation of a stable and efficient solid electrolyte interphase (SEI) layer is paramount to overcome this challenge. This research directly tackles this issue by focusing on garnet-type Li7-xLa3Zr2-yO12 (LLZO) solid electrolytes, known for their high ionic conductivity but plagued by interfacial problems with lithium metal. The study's purpose is to demonstrate the fabrication of high-energy and durable LMBs using strategically tailored LLZO electrolytes, addressing the key limitation of interfacial instability. The importance of this work lies in its potential to accelerate the transition towards commercially viable solid-state batteries, which are crucial for various applications including electric vehicles and grid-scale energy storage, demanding high energy density, long cycle life, and improved safety.

Extensive research has been dedicated to improving the stability and performance of solid-state batteries, particularly focusing on the lithium metal anode. Several studies have highlighted the critical role of the interface between the lithium metal and the solid electrolyte in determining the overall battery performance and lifespan. The garnet-type LLZO has emerged as a promising solid electrolyte due to its high ionic conductivity. However, the inherent challenges associated with its chemical and electrochemical compatibility with lithium metal have been extensively documented. Previous work has explored various strategies to improve the interfacial stability, including surface modifications and the use of interlayers. Density functional theory (DFT) calculations have been instrumental in understanding the thermodynamic stability of different electrolyte compositions and their interactions with lithium metal. These prior studies often focused on single modifications either to the bulk or the surface of the electrolyte, and often resulted in only incremental improvements. The current research builds upon this body of knowledge by employing a combined approach involving both bulk and interface engineering to achieve a synergistic improvement in battery performance.

The researchers synthesized Ta-, Al-, Nb-, and Ga,W-doped LLZO solid electrolytes using a solid-state synthesis technique. Starting precursors included Li2CO3, La2O3, and ZrO2, along with the respective dopants (Ta2O5, Al2O3, Nb2O5, Ga2O3, and WO3). The powders were mixed, calcined, and ball-milled to ensure homogeneity. Hot-pressing was employed to create high-density pellets. These pellets were cut, polished, and subjected to various characterization techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM). The chemical stability of the doped LLZO with lithium metal was evaluated using color change tests at high temperature, assessing the degree of Li penetration and electrolyte degradation. Electrochemical impedance spectroscopy (EIS) was used to investigate the interfacial impedance between the Li metal and the solid electrolyte. Galvanostatic cycling tests were conducted on symmetric Li/LLZO/Li cells to determine the critical current density and long-term cycling stability. Density functional theory (DFT) calculations were performed to predict the thermodynamic stability of the doped LLZOs against reduction by lithium metal. Crucially, a solution-based acid treatment (using 1 M HCl) was applied to selectively protonate the grain boundaries and interfaces of the LLZO pellets, aiming to improve compatibility with lithium metal. This protonation was verified using thermal gravimetric analysis (TGA). Finally, full cell tests were performed using NCM111 and NCM811 cathodes with different loading capacities, both with and without the addition of an ionic liquid to enhance cathode-electrolyte contact. The full cells were cycled at various current densities and temperatures to evaluate their performance and lifespan. The researchers analyzed the data using Rietveld refinement for XRD patterns, Williamson-Hall analysis for lattice strain calculation, and other statistical methods to quantify the improvements achieved.

The study revealed a strong correlation between the chemical stability of the doped LLZO and its electrochemical performance in Li-metal batteries. Through color change tests, XRD, and SIMS analysis, the researchers found that Nb- and Ga,W-doped LLZOs exhibited significant chemical instability with lithium metal, showing extensive micro-crack formation and Li penetration. In contrast, Ta- and Al-doped LLZOs demonstrated improved chemical stability. DFT calculations confirmed this trend, predicting that LLZO doped with certain elements was highly susceptible to reduction by lithium metal, generating electronically conductive byproducts. Importantly, the researchers demonstrated that protonation of the grain boundaries and interfaces of the LLZO significantly enhanced its stability towards lithium metal. This was achieved through a selective acid treatment which was confirmed by XPS, showing that Li2CO3 was effectively removed from the surface. The protonation led to the formation of electronically insulating but ionically conducting byproducts (like hydroxides), effectively passivating further decomposition reactions. The protonation also improved the mechanical strength of the LLZO pellets. The electrochemical performance of Li/LLZO/Li symmetric cells with protonated Ta- and Al-doped LLZOs showed remarkably improved critical current densities (2.6 and 2.0 mA cm-2 at 60 °C respectively) compared to the pristine counterparts. The hybrid solid-state full cells with protonated Ta-LLZO and a 3.2 mAh cm-2 NCM111 cathode exhibited excellent cycling performance at a high current density of 3 mA cm-2 at 60 °C, delivering a cumulative capacity exceeding 3200 mAh cm-2 over 1000 cycles with high Coulombic efficiency. Moreover, an all-solid-state battery with a 5 mAh cm-2 composite cathode (NCM811 and Li2PSCl electrolyte) and protonated Ta-LLZO achieved a cumulative capacity of over 4000 mAh cm-2 at 3 mA cm-2 over 1000 cycles. This result represents the highest long-term cycling parameters reported to date for Li-metal batteries using garnet-oxide electrolytes.

The findings of this study directly address the critical challenge of interfacial instability in LMBs using solid electrolytes. The combined strategy of bulk dopant selection and dopant-specific interfacial treatment via protonation/etching proved highly effective in achieving a synergistic improvement in battery performance. The enhanced stability of the tailored LLZO against lithium metal is attributed to both the suppression of electronically conductive byproducts and the modification of the interfacial morphology which reduces the interfacial resistance. The results highlight the importance of considering both bulk and interface properties when designing high-performance solid electrolytes. The demonstration of high-energy-density full cells with exceptional cycle life and Coulombic efficiency showcases the immense potential of the tailored LLZO electrolytes for practical applications. The ability to achieve such high cumulative capacities at high current densities significantly advances the feasibility of solid-state batteries for demanding applications such as electric vehicles and grid-scale energy storage.

This research successfully demonstrates a strategy for significantly improving the long-term stability and performance of lithium-metal batteries using garnet-type solid electrolytes. The combined approach of selecting appropriate bulk dopants (e.g., Ta) and tailoring the interface through protonation enabled the fabrication of high-energy-density all-solid-state batteries capable of exceeding 1000 cycles at high current densities. These findings highlight the importance of coupled bulk and interface engineering for advancing the development of commercially viable solid-state batteries. Future research could focus on exploring other dopants and surface treatments to further optimize the performance of LLZO and other solid electrolytes, as well as investigating different cathode materials and cell architectures for even higher energy densities and improved cycle life.

While this study demonstrates exceptional improvements, several limitations should be noted. The acid treatment process may introduce some degree of proton incorporation into the bulk LLZO, potentially affecting its overall ionic conductivity. This effect requires further investigation to fully quantify the trade-off between stability and conductivity. The study primarily focused on NCM111 and NCM811 cathodes. The applicability of this strategy to other cathode chemistries remains to be explored. The long-term cycling tests were conducted under specific conditions (temperature, current density). Further investigation is needed to assess the performance of these batteries under a wider range of operating conditions and explore the impact of potential degradation mechanisms over extended periods.