Nonaqueous Polymer Combustion Synthesis of Cubic Li7La3Zr2O12 Nanopowders

Accelerating the Development of LLZO in Solid-State Batteries Toward Commercialization: A Comprehensive Review

Solid-state batteries (SSBs) are under development as high-priority technologies for safe and energy-dense next-generation electrochemical energy storage systems operating over a wide temperature range. Solid-state electrolytes (SSEs) exhibit high thermal stability and, in some cases, the ability to prevent dendrite growth through a physical barrier, and compatibility with the "holy grail" metallic lithium. These unique advantages of SSEs have spurred significant research interests during the last decade. Garnet-type SSEs, that is, Li7La3Zr2O12 (LLZO), are intensively investigated due to their high Li-ion conductivity and exceptional chemical and electrochemical stability against lithium metal anodes. However, poor interfacial contact with cathode materials, undesirable lithium plating along grain boundaries, and moisture-induced chemical degradation greatly hinder the practical implementation of LLZO-based SSEs for SSBs. In this review, the recent advances in synthesis methods, modification strategies, corresponding mechanisms, and applications of garnet-based SSEs in SSBs are critically summarized. Furthermore, a comprehensive evaluation of the challenges and development trends of LLZO-based electrolytes in practical applications is presented to accelerate their development for high-performance SSBs.

Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering

Ta-doped Li7La3Zr2O12 (LLZTO) garnet is a promising Li-ion-conducting ceramic electrolyte for solid-state batteries. However, it is still challenging to use LLZTO in Li metal batteries operating at high current densities because of the tendency for Li metal to nucleate and propagate along the grain boundaries. In this study, we carry out a detailed investigation to elucidate the effect of microstructure and grain size on the electrochemical properties and short circuit behavior in LLZTO. Pellets were prepared using reactive sintering from pyrochlore precursors (a method called pyrochlore-to-garnet, P2G) and compared with LLZTO synthesized using solid-state reaction (SSR) followed by conventional pressureless sintering. Both preparation methods were controlled to keep the phase and elemental composition, ionic and electronic conductivity, relative density, and area-specific resistance of the pellets constant. Reflection electron energy loss spectroscopy and X-ray photoelectron spectroscopy confirm that both types of LLZTO have similar band gaps and chemical states. Microstructure analysis shows that the P2G method results in LLZTO with an average grain size of around 3 μm, which is much smaller than the grain sizes (as large as 20 μm) seen in SSR LLZTO. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm2 in bidirectional cycling and >1 mA/cm2 for unidirectional) than those seen in SSR LLZTO. Post-mortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. The improved cycling behavior in P2G LLZTO despite the higher grain boundary area could be from more homogeneous current density at the interfaces and different grain boundary properties arising from the liquid-phase, reactive sintering method. These results suggest that the effect of grain size on Li dendrite propagation in LLZO may be highly dependent on the synthesis and sintering method employed.

Processing and Properties of Garnet-Type Li7 La3 Zr2 O12 Ceramic Electrolytes

Garnet-type solid electrolyte Li₇ La₃ Zr₂ O₁₂ (LLZO) is widely considered as one of the most promising candidates for solid state batteries (SSBs) owing to its high ionic conductivity and good electrochemical stability. Since its discovery in 2007, great progress has been made in terms of crystal chemistry, chemical and electrochemical properties, and battery application. Nonetheless, reliable and controllable preparation of LLZO ceramics with desirable properties still remains as big challenges. Herein, this review summarizes various synthetic routes of LLZO ceramics and examines the influence of various key processing parameters on the chemical and electrochemical properties. Focusing on correlation of processing parameters and properties, this review aims to provide new insights on a reliable and controllable production of high-quality LLZO ceramic electrolytes for SSB application.

La2Zr2O7 Nanoparticle-Mediated Synthesis of Porous Al-Doped Li7La3Zr2O12 Garnet

In this work, we modified the reaction pathway to quickly (minutes) incorporate lithium and stabilize the ionic conducting garnet phase by decoupling the formation of a La-Zr-O network from the addition of lithium. To do this, we synthesized La₂Zr₂O₇ (LZO) nanoparticles to which LiNO₃ was added. This method is a departure from typical solid-state synthesis methods that require high-energy milling to promote mixing and intimate particle-particle contact and from sol-gel syntheses as a unique porous microstructure is obtained. We show that the reaction time is limited by the rate of nitrate decomposition and that this method produces a porous high-Li-ion-conducting cubic phase, within an hour, that may be used as a starting structure for a composite electrolyte.

Highly Conductive Garnet-Type Electrolytes: Access to Li6.5La3Zr1.5Ta0.5O12 Prepared by Molten Salt and Solid-State Methods

Tantalum-doped garnet (Li_6.5La₃Zr_1.5Ta_0.5O₁₂, LLZTO) is a promising candidate to act as a solid electrolyte in all-solid-state batteries owing to both its high Li⁺ conductivity and its relatively high robustness against the Li metal. Synthesizing LLZTO using conventional solid-state reaction (SSR) requires, however, high calcination temperature (>1000 °C) and long milling steps, thereby increasing the processing time. Here, we report on a facile synthesis route to prepare LLZTO using a molten salt method (MSS) at lower reaction temperatures and shorter durations (900 °C, 5 h). Additionally, a thorough analysis on the properties, i.e., morphology, phase purity, and particle size distribution of the LLZTO powders, is presented. LLZTO pellets, either prepared by the MSS or the SSR method, that were sintered in a Pt crucible showed Li⁺ ion conductivities of up to 0.6 and 0.5 mS cm^-1, respectively. The corresponding activation energy values are 0.37 and 0.38 eV, respectively. The relative densities of the samples reached values of approximately 96%. For comparison, LLZTO pellets sintered in alumina crucibles or with γ-Al₂O₃ as sintering aid revealed lower ionic conductivities and relative densities with abnormal grain growth. We attribute these observations to the formation of Al-rich phases near the grain boundary regions and to a lower Li content in the final garnet phase. The MSS method seems to be a highly attractive and an alternative synthetic approach to SSR route for the preparation of highly conducting LLZTO-type ceramics.

Sulfide and Oxide Inorganic Solid Electrolytes for All-Solid-State Li Batteries: A Review

Energy storage materials are finding increasing applications in our daily lives, for devices such as mobile phones and electric vehicles. Current commercial batteries use flammable liquid electrolytes, which are unsafe, toxic, and environmentally unfriendly with low chemical stability. Recently, solid electrolytes have been extensively studied as alternative electrolytes to address these shortcomings. Herein, we report the early history, synthesis and characterization, mechanical properties, and Li+ ion transport mechanisms of inorganic sulfide and oxide electrolytes. Furthermore, we highlight the importance of the fabrication technology and experimental conditions, such as the effects of pressure and operating parameters, on the electrochemical performance of all-solid-state Li batteries. In particular, we emphasize promising electrolyte systems based on sulfides and argyrodites, such as LiPS5Cl and β-Li3PS4, oxide electrolytes, bare and doped Li7La3Zr2O12 garnet, NASICON-type structures, and perovskite electrolyte materials. Moreover, we discuss the present and future challenges that all-solid-state batteries face for large-scale industrial applications.