Wet-Environment-Induced Structural Alterations in Single- and Polycrystalline LLZTO Solid Electrolytes Studied by Diffraction Techniques

Ion-Exchange-Induced Phase Transition Enables an Intrinsically Air Stable Hydrogarnet Electrolyte for Solid-State Lithium Batteries

Inferior air stability is a primary barrier for large-scale applications of garnet electrolytes in energy storage systems. Herein, a deeply hydrated hydrogarnet electrolyte generated by a simple ion-exchange-induced phase transition from conventional garnet, realizing a record-long air stability of more than two years when exposed to ambient air is proposed. Benefited from the elimination of air-sensitive lithium ions at 96 h/48e sites and unobstructed lithium conduction path along tetragonal sites (12a) and vacancies (12b), the hydrogarnet electrolyte exhibits intrinsic air stability and comparable ion conductivity to that of traditional garnet. Moreover, the unique properties of hydrogarnet pave the way for a brand-new aqueous route to prepare lithium metal stable composite electrolyte on a large-scale, with high ionic conductivity (8.04 × 10-4 S cm-1), wide electrochemical windows (4.95 V), and a high lithium transference number (0.43). When applied in solid-state lithium batteries (SSLBs), the batteries present impressive capacity and cycle life (164 mAh g-1 with capacity retention of 89.6% after 180 cycles at 1.0C under 50 °C). This work not only designs a new sort of hydrogarnet electrolyte, which is stable to both air and lithium metal but also provides an eco-friendly and large-scale fabrication route for SSLBs.

Understanding the Surface Regeneration and Reactivity of Garnet Solid-State Electrolytes

Garnet solid-electrolyte-based Li-metal batteries can be used in energy storage devices with high energy densities and thermal stability. However, the tendency of garnets to form lithium hydroxide and carbonate on the surface in an ambient atmosphere poses significant processing challenges. In this work, the decomposition of surface layers under various gas environments is studied by using two surface-sensitive techniques, near-ambient-pressure X-ray photoelectron spectroscopy and grazing incidence X-ray diffraction. It is found that heating to 500 °C under an oxygen atmosphere (of 1 mbar and above) leads to a clean garnet surface, whereas low oxygen partial pressures (i.e., in argon or vacuum) lead to additional graphitic carbon deposits. The clean surface of garnets reacts directly with moisture and carbon dioxide below 400 and 500 °C, respectively. This suggests that additional CO2 concentration controls are needed for the handling of garnets. By heating under O2 along with avoiding H2O and CO2, symmetric cells with less than 10 Ωcm2 interface resistance are prepared without the use of any interlayers; plating currents of >1 mA cm-2 without dendrite initiation are demonstrated.

Li+/H+ exchange of Li7La3Zr2O12 single and polycrystals investigated by quantitative LIBS depth profiling

Li₇La₃Zr₂O₁₂ (LLZO) garnets are highly attractive to be used as solid electrolyte in solid-state Li batteries. However, LLZO suffers from chemical interaction with air and humidity, causing Li⁺/H⁺ exchange with detrimental implication on its performance, processing and scalability. To better understand the kinetics of the detrimental Li⁺/H⁺ exchange and its dependence on microstructural features, accelerated Li⁺/H⁺ exchange experiments were performed on single crystalline and polycrystalline LLZO, exposed for 80 minutes to 80 °C hot water. The resulting chemical changes were quantified by analytical methods, i.e. inductively coupled plasma optical emission spectroscopy (ICP-OES) and laser induced breakdown spectroscopy (LIBS). From the time dependence of the Li⁺ enrichment in the water, measured by ICP-OES, a bulk interdiffusion coefficient of Li⁺/H⁺ could be determined (7 × 10^-17 m² s^-1 at 80 °C). Depth dependent concentrations were obtained from the LIBS data for both ions after establishing a calibration method enabling not only Li⁺ but also H⁺ quantification in the solid electrolyte. Short interdiffusion lengths in the 1 μm range are found for the single crystalline Ga:LLZO, in accordance with the measured bulk diffusion coefficient. In polycrystalline Ta:LLZO, however, very long diffusion tails in the 20 μm range and ion exchange fractions up to about 70% are observed. Those are attributed to fast ion interdiffusion along grain boundaries. The severe compositional changes also strongly affect the electrical properties measured by impedance spectroscopy. This study highlights that microstructural effects may be decisive for the Li⁺/H⁺ ion exchange kinetics of LLZO.

Garnet Li7La3Zr2O12-Based Solid-State Lithium Batteries Achieved by In Situ Thermally Polymerized Gel Polymer Electrolyte

Garnet Li₇La₃Zr₂O₁₂ (LLZO) is a potential solid electrolyte for solid-state batteries (SSBs) because of its high ionic conductivity, electrochemical stability, and mechanical strength. However, large interface resistances arising from deserted cathodes and rigid garnet/electrode interfaces block its application. In order to deal with this issue, a gel polymer electrolyte (GPE) was introduced into the cathode and both sides of LLZO to achieve a solid-state battery. Especially, the provided GPE could be thermally polymerized and solidified in situ, which would integrate LLZO with both anode and cathode and dramatically simplify the battery manufacturing process. Since the interface from rigid LLZO is improved by the flexible GPE buffer, the inability of flexible GPE to inhibit lithium dendrites is compensated by the rigid LLZO in return. As a result, the interface resistances are reduced from 6880 to 473 Ω, the Li symmetric cell exhibits a flat galvanostatic charge/discharge for 400 h without lithium dendrites, and the solid-state Li|GPE@LLZO|LiCoO₂ battery exerts a capacity retention of 82.6% after 100 cycles at 0.5 C at room temperature. Such an interfacial engineering approach represents a promising strategy to address solid-solid interface issues and provides a new design for SSBs with high performance.

Deep hydration of an Li7-3xLa3Zr2MIIIxO12 solid-state electrolyte material: a case study on Al- and Ga-stabilized LLZO

Single crystals of an Li-stuffed, Al- and Ga-stabilized garnet-type solid-state electrolyte material, Li7La3Zr2O12 (LLZO), have been analysed using single-crystal X-ray diffraction to determine the pristine structural state immediately after synthesis via ceramic sintering techniques. Hydrothermal treatment at 150 °C for 28 d induces a phase transition in the Al-stabilized compound from the commonly observed cubic Ia-3d structure to the acentric I-43d subtype. LiI ions at the interstitial octahedrally (4 + 2-fold) coordinated 48e site are most easily extracted and AlIII ions order onto the tetrahedral 12a site. Deep hydration induces a distinct depletion of LiI at this site, while the second tetrahedral site, 12b, suffers only minor LiI loss. Charge balance is maintained by the incorporation of HI, which is bonded to an O atom. Hydration of Ga-stabilized LLZO induces similar effects, with complete depletion of LiI at the 48e site. The LiI/HI exchange not only leads to a distinct increase in the unit-cell size, but also alters some bonding topology, which is discussed here.

A Review on Li+ /H+ Exchange in Garnet Solid Electrolytes: From Instability against Humidity to Sustainable Processing in Water

Garnet-based Li-ion conductors are one of the most promising oxide-ceramic solid electrolytes for next-generation Li batteries. However, they undergo a Li+ /H+ exchange (LHX) reaction with most protic solvents used in component manufacturing routes and even with moisture in ambient air. These protonated garnets show a lower Li-ionic conductivity, and even if only the surface is protonated, this degraded layer hinders the Li-ion exchange with, for example, a metallic Li anode. Furthermore, the resulting unstable surface properties during the processing in air lead to challenges with respect to reproducibility of the final component performance, limiting their commercial applicability. However, in recent years, the knowledge about the underlying chemical mechanisms has led to the development of mitigation strategies and enabled a push of this promising material class towards sustainable and scalable fabrication routes. This Minireview covers the following four aspects, which are relevant for a comprehensive understanding of these developments: (1) reports of LHX phenomenon in garnets exposed to air and solvents; (2) recent understandings of the fundamentals and properties of LHX; (3) strategies to prevent LHX and to recover garnets; and (4) sustainable application of LHX for material processing and energy-related devices.

Investigating the electrochemical stability of Li7La3Zr2O12 solid electrolytes using field stress experiments

Cubic Li₇La₃Zr₂O₁₂ (LLZO) garnets are among the most promising solid electrolytes for solid-state batteries with the potential to exceed conventional battery concepts in terms of energy density and safety. The electrochemical stability of LLZO is crucial for its application, however, controversial reports in the literature show that it is still an unsettled matter. Here, we investigate the electrochemical stability of LLZO single crystals by applying electric field stress via macro- and microscopic ionically blocking Au electrodes in ambient air. Induced material changes are subsequently probed using various locally resolved analysis techniques, including microelectrode electrochemical impedance spectroscopy (EIS), laser induced breakdown spectroscopy (LIBS), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and microfocus X-ray diffraction (XRD). Our experiments indicate that LLZO decomposes at 4.1-4.3 V vs. Li⁺/Li, leading to the formation of Li-poor phases like La₂Zr₂O₇ beneath the positively polarized electrode. The reaction is still on-going even after several days of polarization, indicating that no blocking interfacial layer is formed. The decomposition can be observed at elevated as well as room temperature and suggests that LLZO is truly not compatible with high voltage cathode materials.

Aging Behavior of Al- and Ga- Stabilized Li7La3Zr2O12 Garnet-Type, Solid-State Electrolyte Based on Powder and Single Crystal X-ray Diffraction

Li7La3Zr2O12 garnet (LLZO) belongs to the most promising solid electrolytes for the development of solid-state Li batteries. The stability of LLZO upon exposure to air is still a matter of discussion. Therefore, we performed a comprehensive study on the aging behavior of Al-stabilized LLZO (space group (SG) Ia3¯d) and Ga-stabilized LLZO (SG I4¯3d) involving 98 powder and 51 single-crystal X-ray diffraction measurements. A Li+/H+ exchange starts immediately on exposure to air, whereby the exchange is more pronounced in samples with smaller particle/single-crystal diameter. A slight displacement of Li from interstitial Li2 (96h) toward the regular tetrahedral Li1 (24d) sites occurs in Al-stabilized LLZO. In addition, site occupancy at the 96h site decreases as Li+ is exchanged by H+. More extensive hydration during a mild hydrothermal treatment of samples at 90 °C induces a structural phase transition in Al-LLZO to SG I4¯3d with a splitting of the 24d site into two independent tetrahedral sites (i.e., 12a and 12b), whereby Al3+ solely occupies the 12a site. Li+ is preferably removed from the interstitial 48e site (equivalent to 96h). Analogous effects are observed in Ga-stabilized LLZO, which has SG I4¯3d in the pristine state.