Li-Ion Conductivity of Single-Step Synthesized Glassy-Ceramic Li10GeP2S12 and Post-heated Highly Crystalline Li10GeP2S12

Li₁₀GeP₂S₁₂ is a phosphosulfide solid electrolyte that exhibits exceptionally high Li-ion conductivity, reaching a conductivity above 10^-3 S cm^-1 at room temperature, rivaling that of liquid electrolytes. Herein, a method to produce glassy-ceramic Li₁₀GeP₂S₁₂ via a single-step utilizing high-energy ball milling was developed and systematically studied. During the high energy milling process, the precursors experience three different stages, namely, the 'Vitrification zone' where the precursors undergo homogenization and amorphization, 'Intermediary zone' where Li₃PS₄ and Li₄GeS₄ are formed, and the 'Product stage' where the desired glassy-ceramic Li₁₀GeP₂S₁₂ is formed after 520 min of milling. At room temperature, the as-milled sample achieved a high ionic conductivity of 1.07 × 10^-3 S cm^-1. It was determined via quantitative phase analyses (QPA) of transmission X-ray diffraction results that the as-milled Li₁₀GeP₂S₁₂ possessed a high degree of amorphization (44.4 wt %). To further improve the crystallinity and ionic conductivity of the Li₁₀GeP₂S₁₂, heat treatment of the as-milled sample was carried out. The optimal heat-treated Li₁₀GeP₂S₁₂ is almost fully crystalline and possesses a room temperature ionic conductivity of 3.27 × 10^-3 S cm^-1, an over 200% increase compared to the glassy-ceramic Li₁₀GeP₂S₁₂. These findings help provide previously lacking insights into the controllable preparation of Li₁₀GeP₂S₁₂ material.

Disentangling Phase and Morphological Evolution During the Formation of the Lithium Superionic Conductor Li10 GeP2 S12

The structural and morphological changes of the Lithium superionic conductor Li₁₀ GeP₂ S₁₂ , prepared via a widely used ball milling-heating method over a comprehensive heat treatment range (50 - 700 °C), are investigated. Based on the phase composition, the formation process can be distinctly separated into four zones: Educt, Intermediary, Formation, and Decomposition zone. It is found that instead of Li₄ GeS₄ -Li₃ PS₄ binary crystallization process, diversified intermediate phases, including GeS₂ in different space groups, multiphasic lithium phosphosulfides (Li_x P_y S_z ), and cubic Li₇ Ge₃ PS₁₂ phase, are involved additionally during the formation and decomposition of Li₁₀ GeP₂ S₁₂ . Furthermore, the phase composition at temperatures around the transition temperatures of different formation zones shows a significant deviation. At 600 °C, Li₁₀ GeP₂ S₁₂ is fully crystalline, while the sample decomposed to complex phases at 650 °C with 30 wt.% impurities, including 20 wt.% amorphous phases. These findings over such a wide temperature range are first reported and may help provide previously lacking insights into the formation and crystallinity control of Li₁₀ GeP₂ S₁₂ .

Active Interphase Enables Stable Performance for an All-Phosphate-Based Composite Cathode in an All-Solid-State Battery

High interfacial resistance and unstable interphase between cathode active materials (CAMs) and solid-state electrolytes (SSEs) in the composite cathode are two of the main challenges in current all-solid-state batteries (ASSBs). In this work, the all-phosphate-based LiFePO₄ (LFP) and Li_1.3 Al_0.3 Ti_1.7 (PO₄ )₃ (LATP) composite cathode is obtained by a co-firing technique. Benefiting from the densified structure and the formed redox-active Li_{3- x} Fe_{2- x - y} Ti_x Al_y (PO₄ )₃ (LFTAP) interphase, the mixed ion- and electron-conductive LFP/LATP composite cathode facilitates the stable operation of bulk-type ASSBs in different voltage ranges with almost no capacity degradation upon cycling. Particularly, both the LFTAP interphase and LATP electrolyte can be activated. The cell cycled between 4.1 and 2.2 V achieves a high reversible capacity of 2.8 mAh cm^-2 (36 µA cm^-2 , 60 °C). Furthermore, it is demonstrated that the asymmetric charge/discharge behaviors of the cells are attributed to the existence of the electrochemically active LFTAP interphase, which results in more sluggish Li⁺ kinetics and more expansive LFTAP plateaus during discharge compared with that of charge. This work demonstrates a simple but effective strategy to stabilize the CAM/SSE interface in high mass loading ASSBs.

Single-Ion-Conducting "Polymer-in-Ceramic" Hybrid Electrolyte with an Intertwined NASICON-Type Nanofiber Skeleton

The fast Li⁺ transportation of "polymer-in-ceramic" electrolytes is highly dependent on the long-range Li⁺ migration pathways, which are determined by the structure and chemistry of the electrolytes. Besides, Li dendrite growth may be promoted in the soft polymer region due to the inhomogeneous electric field caused by the commonly low Li⁺ transference number of the polymer. Herein, a single-ion-conducting polymer electrolyte is infiltrated into intertwined Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) nanofibers to construct free-standing electrolyte membranes. The composite electrolyte possesses a large electrochemical window exceeding 5 V, a high ionic conductivity of 0.31 mS cm^-1 at ambient temperature, and an extraordinary Li⁺ transference number of 0.94. The hybrid electrolyte in the lithium symmetric cell shows stable Li plating/stripping up to 2000 h under 0.1 mA cm^-2 without dendrite formation. The Li|hybrid electrolyte|LiFePO₄ battery exhibits enhanced rate capability up to 1 C and a stable cycling performance with an initial discharge capacity of 131.8 mA h g^-1 and a retention capacity of 122.7 mA h g^-1 after 500 cycles at 0.5 C at ambient temperature. The improved electrochemical performance is attributed to the synergistic effects of the LATP nanofibers and the single-ion-conducting polymer. The fibrous fast ion conductors provide continuous ion transport channels, and the polymer improves the interfacial contact with the electrodes and helps to suppress the Li dendrites.

Bulk and grain boundary Li-diffusion in dense LiMn2O4 pellets by means of isotope exchange and ToF-SIMS analysis

Lithium diffusion in LiMn₂O₄ pellets is studied by means of isotope exchange and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). A ⁶Li-enriched film deposited by Pulsed Laser Deposition (PLD) on a dense LiMn₂O₄ pellet with natural abundance of lithium isotopes is used to study the tracer diffusion of lithium. The measured profiles are analyzed by numerical models describing the ⁶Li tracer diffusion from the film into the pellet. Experiments in the Harrison type B regime of diffusion kinetics allow for the distinction and simultaneous determination of bulk and grain boundary diffusion coefficients. Changing the experimental conditions to reach Harrison type A behavior yields effective diffusion coefficients for lithium tracer diffusion in LiMn₂O₄. Activation energies for bulk and grain boundary diffusion were obtained from experiments at different temperatures. Our values are critically compared to previous studies.

About the Compatibility between High Voltage Spinel Cathode Materials and Solid Oxide Electrolytes as a Function of Temperature

The reactivity of mixtures of high voltage spinel cathode materials Li₂NiMn₃O₈, Li₂FeMn₃O₈, and LiCoMnO₄ cosintered with Li_1.5Al_0.5Ti_1.5(PO₄)₃ and Li_6.6La₃Zr_1.6Ta_0.4O₁₂ electrolytes is studied by thermal analysis using X-ray-diffraction and differential thermoanalysis and thermogravimetry coupled with mass spectrometry. The results are compared with predicted decomposition reactions from first-principles calculations. Decomposition of the mixtures begins at 600 °C, significantly lower than the decomposition temperature of any component, especially the electrolytes. For the cathode + Li_6.6La₃Zr_1.6Ta_0.4O₁₂ mixtures, lithium and oxygen from the electrolyte react with the cathodes to form highly stable Li₂MnO₃ and then decompose to form stable and often insulating phases such as La₂Zr₂O₇, La₂O₃, La₃TaO₇, TiO₂, and LaMnO₃ which are likely to increase the interfacial impedance of a cathode composite. The decomposition reactions are identified with high fidelity by first-principles calculations. For the cathode + Li_1.5Al_0.5Ti_1.5(PO₄)₃ mixtures, the Mn tends to oxidize to MnO₂ or Mn₂O₃, supplying lithium to the electrolyte for the formation of Li₃PO₄ and metal phosphates such as AlPO₄ and LiMPO₄ (M = Mn, Ni). The results indicate that high temperature cosintering to form dense cathode composites between spinel cathodes and oxide electrolytes will produce high impedance interfacial products, complicating solid state battery manufacturing.