Electrochemical properties and structural stability of Ga- and Y- co-doping in Li7La3Zr2O12 ceramic electrolytes for lithium-ion batteries

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.

Recent Strategies for Lithium-Ion Conductivity Improvement in Li7La3Zr2O12 Solid Electrolytes

The development of solid electrolytes with high conductivity is one of the key factors in the creation of new power-generation sources. Lithium-ion solid electrolytes based on Li7La3Zr2O12 (LLZ) with a garnet structure are in great demand for all-solid-state battery production. Li7La3Zr2O12 has two structural modifications: tetragonal (I41/acd) and cubic (Ia3d). A doping strategy is proposed for the stabilization of highly conductive cubic Li7La3Zr2O12. The structure features, density, and microstructure of the ceramic membrane are caused by the doping strategy and synthesis method of the solid electrolyte. The influence of different dopants on the stabilization of the cubic phase and conductivity improvement of solid electrolytes based on Li7La3Zr2O12 is discussed in the presented review. For mono-doping, the highest values of lithium-ion conductivity (~10-3 S/cm at room temperature) are achieved for solid electrolytes with the partial substitution of Li+ by Ga3+, and Zr4+ by Te6+. Moreover, the positive effect of double elements doping on the Zr site in Li7La3Zr2O12 is established. There is an increase in the popularity of dual- and multi-doping on several Li7La3Zr2O12 sublattices. Such a strategy leads not only to lithium-ion conductivity improvement but also to the reduction of annealing temperature and the amount of some high-cost dopant. Al and Ga proved to be effective co-doping elements for the simultaneous substitution in Li/Zr and Li/La sublattices of Li7La3Zr2O12 for improving the lithium-ion conductivity of solid electrolytes.

Li-stuffed garnet electrolytes: structure, ionic conductivity, chemical stability, interface, and applications

Current lithium-ion batteries have been widely used in portable electronic devices, electric vehicles, and peak power demand. However, the organic liquid electrolytes used in the lithium-ion battery are flammable and not stable in contact with elemental lithium and at a higher voltage. To eliminate the safety and instability issues, solid-state (ceramic) electrolytes have attracted enormous interest worldwide, owing to their thermal and high voltage stability. Among all the solid-state electrolytes known today, the Li-stuffed garnet is one of the most promising electrolytes due to its physical and chemical properties such as high total Li-ion conductivity at room temperature, chemical stability with elemental lithium and high voltage lithium cathodes, and high electrochemical stability window (6 V vs. Li+/Li). In this short review, we provide an overview of Li-stuffed garnet electrolytes with a focus on their structure, ionic conductivity, transport mechanism, chemical stability, and battery applications.

Efficient Anion Fluoride-Doping Strategy to Enhance the Performance in Garnet-Type Solid Electrolyte Li7La3Zr2O12

Garnet-type solid-state electrolyte Li₇La₃Zr₂O₁₂ (LLZO) is expected to realize the next generation of high-energy-density lithium-ion batteries. However, the severe dendrite penetration at the pores and grain boundaries inside the solid electrolyte hinders the practical application of LLZO. Here, it is reported that the desirable quality and dense garnet Li_6.8Al_0.2La₃Zr₂O_11.80F_0.20 can be obtained by fluoride anion doping, which can effectively facilitate grain nucleation and refine the grain; thereby, the ionic conductivity increased to 7.45 × 10^-4 at 30 °C and the relative density reached to 95.4%. At the same time, we introduced a transition layer to build the Li_6.8Al_0.2La₃Zr₂O_11.80F_0.20-t electrolyte in order to supply a stable contact; as a result, the interface resistance of Li|Li_6.8Al_0.2La₃Zr₂O_11.80F_0.20-t decreases to 12.8 Ω cm². The Li|Li_6.8Al_0.2La₃Zr₂O_11.80F_0.20-t|Li symmetric cell achieved a critical current density of 1.0 mA cm^-2 at 25 °C, which could run stably for 1000 h without a short circuit at 0.3 mA cm^-2 and 25 °C. Moreover, the Li|LiFePO₄ battery exhibited a high Coulombic efficiency (>99.5%), an excellent rate capability, and a great capacity retention (123.7 mA h g^-1, ≈80%) over 500 cycles at 0.3C and 25 °C. The Li|LiNi_0.8Co_0.1Mn_0.1O₂ cell operated well at 0.2C and 25 °C and delivered a high initial discharge capacity of 151.4 mA h g^-1 with a good capacity retention (70%) after 195 cycles. This work demonstrates that the anion doping in LLZO is an effective method to prepare a dense garnet ceramic for the high-performance lithium batteries.