Behind the Candelabra: A Facile Flame Vapor Deposition Method for Interfacial Engineering of Garnet Electrolyte To Enable Ultralong Cycling Solid-State Li-FeF3 Conversion Batteries

In-Situ Formed Electron Inhibitor Enabling Dendrite-free Garnet Electrolytes: A Case Study of Fumed Silica

Ta-doped Li7La3Zr2O12 (LLZTO) solid-state electrolytes (SEs) show great promise for solid-state batteries due to its high conductivity and safety. However, one of the challenges it faces is lithium dendrite propagation upon long-term cycling. To address this issue, we propose the incorporation of fumed silica (FS) at the grain boundaries of LLZTO to modify the properties of the garnet pellet, which effectively inhibits the dendrite growth. The introduction of FS has demonstrated several beneficial effects. Firstly, it reduces the migration barrier of lithium ions, which helps prevent dendrite formation and propagation. Additionally, FS reduces the electronic conductivity of the SEs pellet, suppressing the dendrite formation. Moreover, the formed lithium silicates from FS might also be acted as electron inhibitor, thus inhibiting the lithium dendrite growth upon cycling. By investigating the use of FS as a modifier in LLZTO-based electrolytes, our study contributes to advancing dendrite-free solid-state electrolytes and thus the development of high-performance all-solid-state batteries.

Boosting the Cycle Performance of Iron Trifluoride Based Solid State Batteries at Elevated Temperatures by Engineering the Cathode Solid Electrolyte Interface

Iron trifluoride (FeF3) is attracting tremendous interest due to its lower cost and the possibility to enable higher energy density in lithium-ion batteries. However, its cycle performance deteriorates rapidly in less than 50 cycles at elevated temperatures due to cracking of the unstable cathode solid electrolyte interface (CEI) followed by active materials dissolution in liquid electrolyte. Herein, by engineering the salt composition, the Fe3O4-type CEI with the doping of boron (B) atoms in a polymer electrolyte at 60 °C is successfully stabilized. The cycle life of the well-designed FeF3-based composite cathode exceeds an unprecedented 1000 cycles and utilizes up to 70% of its theoretical capacities. Advanced electron microscopy combined with density functional theory (DFT) calculations reveal that the B in lithium salt migrates into the cathode and promotes the formation of an elastic and mechanic robust boron-contained CEI (BOR-CEI) during cycling, by which the durability of the CEI to frequent cyclic large volume changes is significantly enhanced. To this end, the notorious active materials dissolution is largely prohibited, resulting in a superior cycle life. The results suggest that engineering the CEI such as tuning its composition is a viable approach to achieving FeF3 cathode-based batteries with enhanced performance.

Liquid Metal Mediated Heterostructure Fluoride Solid Electrolytes of High Conductivity and Air Stability for Sustainable Na Metal Batteries

Fluoride-based solid electrolytes (SEs) have emerged as a promising component for high-energy-density rechargeable solid-state batteries (SSBs) in view of their wide electrochemical window, high air stability, and interface compatibility, but they still face the challenge of low ion conductivity and the lack of a desired structure for sodium metal SSBs. Here, we report a sodium-rich heterostructure fluoride SE, Na3GaF6-Ga2O3-NaCl (NGFOC-G), synthesized via in situ oxidation of liquid metal gallium and in situ chlorination using low-melting GaCl3. The distinctive features of NGFOC-G include single-crystal Na3GaF6 domains within an open-framework structure, composite interface decoration of Ga2O3 and NaCl with a concentration gradient, exceptional air stability, and high electrochemical oxidation stability. By leveraging the penetration of gallium at NaF grain boundaries and the in situ self-oxidation to form Ga2O3 nanodomains, the solid-phase reaction kinetics of NaF and GaF3 is activated for facilitating the synthesis of main component Na3GaF6. The introduction of a small amount of a chlorine source during synthesis further softens and modifies the boundaries of Na3GaF6 along with Ga2O3. Benefiting from the enhanced interface ion transport, the optimized NGFOC-G exhibits an ionic conductivity up to 10-4 S/cm at 40 °C, which is the highest level reported among fluoride-based sodium-ion SEs. This SE demonstrates a "self-protection" mechanism, where the formation of a high Young's modulus transition layer rich in NaF and Na2O under electrochemical driving prevents the dendrite growth of sodium metal. The corresponding Na/Na symmetric cells show minimal voltage hysteresis and stable cycling performance for at least 1000 h. The Na/NGFOC-G/Na3V2(PO4)3 cell demonstrates stable capacity release around 100 mAh/g at room temperature. The Na/NGFOC-G/FeF3 cell delivers a high capacity of 461 mAh/g with an excellent stability of conversion reaction cycling.

Titanium-Oxygen Clusters Brazing Li with Li6.5La3Zr1.5Ta0.5O12 for High-Performance All-Solid-State Li Batteries

Garnet-based solid-state electrolytes are considered crucial candidates for solid-state Li batteries due to their high Li+ conductivity and nonflammability; however, poor interfacial contact with the Li anode and growth of Li dendrites limit their application. Herein, a high-activity titanium-oxygen cluster is used as a brazing filler to braze the Li6.5La3Zr1.5Ta0.5O12 (LLZTO) with an Li anode into the whole unit. The brazing layer leads to a significantly lower interfacial impedance of 8.32 Ω cm2. Furthermore, the brazing layer is an isotropic amorphous ion-electron hybrid conductive layer, which significantly promotes Li+ transport and regulates the distribution of the electric field, therefore inhibiting the growth of Li dendrites. The cell exhibits an ultrahigh critical current density of 2.3 mA cm-2 and stable cycling of over 4000 h at 0.5 mA cm-2 (25 °C).

H3PO4-Induced Nano-Li3PO4 Pre-reduction Layer to Address Instability between the Nb-Doped Li7La3Zr2O12 Electrolyte and Metallic Li Anode

Solid-state batteries based on a metallic Li anode and nonflammable solid electrolytes (SEs) are anticipated to achieve high energy and power densities with absolute safety. In particular, cubic garnet-type Nb-doped Li₇La₃Zr₂O₁₂ (Nb-LLZO) SEs possess superior ionic conductivity, are feasible to prepare under ambient conditions, have strong thermal stability, and are of low cost. However, the interfacial compatibility with Li metal and Li dendrite hazards still hinder the applications of Nb-LLZO. Herein, a quick and efficient solution was applied to address this issue, generating a nano-Li₃PO₄ pre-reduction layer from the reaction of H₃PO₄ with the ion-exchanged passivation layer (Li₂CO₃/LiOH) on the surface of Nb-LLZO. A lithiophilic, electrically insulating interlayer is in situ created when the Li₃PO₄ modified layer interacts with molten Li, successfully preventing the reduction of Nb⁵⁺. The interlayer, which mostly consists of Li₃P and Li₃PO₄, also has a high shear modulus and relatively high Li⁺ conductivity, which effectively inhibit the growth of Li dendrites. The Li|Li₃PO₄|Nb-LLZO|Li₃PO₄|Li symmetric cells stably cycled for over 5000 h at 0.05 mA cm^-2 and over 1000 h at a high rate of 0.15 mA cm^-2 without any short circuits. The LiFePO₄ and S/C hybrid solid-state batteries using the modified Nb-LLZO electrolyte also demonstrated good electrochemical performances, confirming the practical application of this interfacial engineering in various solid-state battery systems. This work offers an efficient solution to the instability issue between the Nb-LLZO SE and metallic Li anode.

Metal Fluoride Cathode Materials for Lithium Rechargeable Batteries: Focus on Iron Fluorides

Exploring prospective rechargeable batteries with high energy densities is urgently needed on a worldwide scale to address the needs of the large-scale electric vehicle market. Conversion-type metal fluorides (MFs) are attractive cathodes for next-generation rechargeable batteries because of their high theoretical potential and capacities and provide new perspectives for developing novel battery systems that satisfy energy density requirements. However, some critical issues, such as high voltage hysteresis and poor cycling stability must be solved to further enhance MF cathode materials. In this review, the recent advances in mechanisms focused on FeF₃ cathodes under lithiation/delithiation processes are discussed in detail. Then, the classifications and advantages of various synthesis methods to prepare MF-based materials are first minutely discussed. Moreover, the performance attenuation mechanisms of MFs and the effort in the development of mitigation strategies are comprehensively reviewed. Finally, prospects for the current obstacles and possible research directions, with the aim to provide some inspiration for the development of MF cathode-based batteries are presented.

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.