Showing papers by "Martin Finsterbusch published in 2021"

Li7La3Zr2O12 solid electrolyte sintered by the ultrafast high-temperature method

Abstract: All-solid-state Li batteries (ASSLBs) are regarded as the systems of choice for future electrochemical energy storage. Particularly, the garnet Li7La3Zr2O12 (LLZO) is one of the most promising solid electrolytes due to its stability against Li metal. However, its integration into ASSLBs is challenging due to high temperature and long dwell time required for sintering. Advanced sintering techniques, such as Ultrafast High-temperature Sintering, have shown to significantly increase the sintering rate. Direct contact to graphite heaters allows sintering of LLZO within 10 s due to extremely high heating rates (up to 104 K min−1) and temperatures up to 1500 °C to a density around 80 %. The LLZO sintered in vacuum and Ar atmosphere has good mechanical stability and high phase purity, but kinetic de-mixing at the grain boundaries was observed. Nevertheless, the Li-ion conductivity of 1 mS cm−1 at 80 °C was comparable to conventional sintering, but lower than for Field-Assisted Sintering Technique/Spark Plasma Sintering.

Controlling the lithium proton exchange of LLZO to enable reproducible processing and performance optimization

Abstract: Ceramic solid state-electrolytes attract significant attention due to their intrinsic safety and, in the case of the garnet type Li6.45Al0.05La3Zr1.6Ta0.4O12 (LLZO), the possibility to use Li-metal anodes to provide high energy densities on a cell and battery level. However, one of the major obstacles hindering their wide-spread application is the translation and optimization of production processes from laboratory to industrial scale. Even though the plausibility of manufacturing components and cells via wet processing routes like tape casting and screen printing has been shown, the impact of the sensitivity of LLZO to air and protic solvents due to Li+/H+-exchange is not fully understood yet. An uncontrolled alteration of the powder surface results in poorly reproducible processing characteristics and electrochemical performance of the final battery components and full cells. This knowledge gap is the cause of the large performance variations reported across different research labs worldwide and is unacceptable for up-scaling to industrial level. To close this gap, the influence of the Li+/H+-exchange taking place at various steps in the manufacturing process was systematically investigated in this study. For the first time, this allowed a mechanistic understanding of its impact on the processability itself and on the resulting electrochemical performance of a free-standing LLZO separator. The importance of a close control of the pre-treatment and storage conditions of LLZO, as well as contact time with the solvent could be extracted for each step of the manufacturing process. As a result, we were able to optimize the processing of thin, dense, free standing LLZO separators and significantly improve the total Li-ion conductivity to 3.90 × 10−4 S cm−1 and the critical current density to over 300 μA cm−2 without making structural changes to separator or the starting material. These findings do not only enable a deeper understanding and control over the manufacturing process, but also show potential for further improvement of cell concepts already existing in literature.

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

Abstract: 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.

Evaluation of Scalable Synthesis Methods for Aluminum-Substituted Li7La3Zr2O12 Solid Electrolytes.

Abstract: Solid electrolyte is the key component in all-solid-state batteries (ASBs). It is required in electrodes to enhance Li-conductivity and can be directly used as a separator. With its high Li-conductivity and chemical stability towards metallic lithium, lithium-stuffed garnet material Li7La3Zr2O12 (LLZO) is considered one of the most promising solid electrolyte materials for high-energy ceramic ASBs. However, in order to obtain high conductivities, rare-earth elements such as tantalum or niobium are used to stabilize the highly conductive cubic phase. This stabilization can also be obtained via high levels of aluminum, reducing the cost of LLZO but also reducing processability and the Li-conductivity. To find the sweet spot for a potential market introduction of garnet-based solid-state batteries, scalable and industrially usable syntheses of LLZO with high processability and good conductivity are indispensable. In this study, four different synthesis methods (solid-state reaction (SSR), solution-assisted solid-state reaction (SASSR), co-precipitation (CP), and spray-drying (SD)) were used and compared for the synthesis of aluminum-substituted LLZO (Al:LLZO, Li6.4Al0.2La3Zr2O12), focusing on electrochemical performance on the one hand and scalability and environmental footprint on the other hand. The synthesis was successful via all four methods, resulting in a Li-ion conductivity of 2.0–3.3 × 10−4 S/cm. By using wet-chemical synthesis methods, the calcination time could be reduced from two calcination steps for 20 h at 850 °C and 1000 °C to only 1 h at 1000 °C for the spray-drying method. We were able to scale the synthesis up to a kg-scale and show the potential of the different synthesis methods for mass production.

Free standing dual phase cathode tapes – scalable fabrication and microstructure optimization of garnet-based ceramic cathodes

Abstract: To make ceramic based all-solid-state batteries competitive on the battery market, a shift from the separator supported cell-design for lab cells to a scalable, cathode-supported one is necessary to improve the energy density. Using tape casting, we were able to demonstrate for the first time all-ceramic free-standing LiCoO2 (LCO)/ Li6.45Al0.05La3Zr1.6Ta0.4O12 (LLZO) mixed cathodes with high capacities and active material utilization. Further morphology engineering by introduction of a sequential layer casting enabled us to tailor the microstructure of the mixed cathodes resulting in opposite concentration gradients for the active material and the electrolyte over the thickness of the cathode. With this optimized microstructure, we were able to increase the discharge capacity of the free-standing mixed cathodes to 2.8 mAh cm-2 utilizing 99% of the theoretical capacity. For oxide garnet-based system, both the scalable fabrication method and the achieved electrochemical performance demonstrates industrial relevance for the first time. Additionally, the obtained free-standing cathodes have sufficient mechanical stability to allow the application of hybrid and ultra-thin separators to further increase the energy density on the full cell level.

The influence of hafnium impurities on the electrochemical performance of tantalum substituted Li7La3Zr2O12 solid electrolytes

Abstract: Garnet-based Li7La3Zr2O12 (LLZO) is considered one of the most promising oxide-ceramic solid electrolyte materials for inorganic all-solid-state batteries. Dopants and substituents like Al, Ta, Nb, Ga, and W were shown to have a high impact on the total ionic conductivity, increasing it from 10−6 S/cm up to 10−3 S/cm. However, natural zirconium sources always contain a small amount of hafnium which could also act as dopant, but the separation of these two elements is complicated and expensive. In this work, we investigate the influence of various Hf-impurity concentrations on the performance of tantalum-doped LLZO. We synthesised Li6.45Al0.05La3Zr1.6−xHfxTa0.4O12 (LLZHO with x = 0 – 1.6) via conventional solid-state synthesis and have demonstrated that up to x = 0.1, hafnium impurities do not have a significant impact on the performance of the material. Above this concentration, the Li-ion conductivity is steadily reduced to around 70% when zirconium is fully substituted by hafnium resulting in Li6.45Al0.05La3Hf1.6Ta0.4O12. As the purity of Zr precursors has a great impact on their price, these findings can help to reduce the price of LLZO in general, as lower grade zirconium can be used in industrial scale applications.

Absolute Local Quantification of Li as Function of State-of-Charge in All-Solid-State Li Batteries via 2D MeV Ion-Beam Analysis

Abstract: Direct observation of the lithiation and de-lithiation in lithium batteries on the component and microstructural scale is still difficult. This work presents recent advances in MeV ion-beam analysis, enabling quantitative contact-free analysis of the spatially-resolved lithium content and state-of-charge (SoC) in all-solid-state lithium batteries via 3 MeV proton-based characteristic x-ray and gamma-ray emission analysis. The analysis is demonstrated on cross-sections of ceramic and polymer all-solid-state cells with LLZO and MEEP/LIBOB solid electrolytes. Different SoC are measured ex-situ and one polymer-based operando cell is charged at 333 K during analysis. The data unambiguously show the migration of lithium upon charging. Quantitative lithium concentrations are obtained by taking the physical and material aspects of the mixed cathodes into account. This quantitative lithium determination as a function of SoC gives insight into irreversible degradation phenomena of all-solid-state batteries during the first cycles and locations of immobile lithium. The determined SoC matches the electrochemical characterization within uncertainties. The presented analysis method thus opens up a completely new access to the state-of-charge of battery cells not depending on electrochemical measurements. Automated beam scanning and data-analysis algorithms enable a 2D quantitative Li and SoC mapping on the µm-scale, not accessible with other methods.

Energetic Stability and Its Role in the Mechanism of Ionic Transport in NASICON-Type Solid-State Electrolyte Li1+xAlxTi2-x(PO4)3.

Abstract: We apply high-temperature oxide melt solution calorimetry to assess the thermodynamic properties of the material Li1+xAlxTi2-x(PO4)3, which has been broadly recognized as one of the best Li-ion-conducting solid electrolytes of the NASICON family. The experimental results reveal large exothermic enthalpies of formation from binary oxides (ΔHf,ox°) and elements (ΔHf,el°) for all compositions in the range 0 ≤ x ≤ 0.5. This indicates substantial stability of Li1+xAlxTi2-x(PO4)3, driven by thermodynamics and not just kinetics, during long-term battery operation. The stability increases with increasing Al3+ content. Furthermore, the dependence of the formation enthalpy on the Al3+ content shows a change in behavior at x = 0.3, a composition near which the Li+ conductivity reaches the highest values. The strong correlation among thermodynamic stability, ionic transport, and clustering is a general phenomenon in ionic conductors that is independent of the crystal structure as well as the type of charge carrier. Therefore, the thermodynamic results can serve as guidelines for the selection of compositions with potentially the highest Li+ conductivity among different NASICON-type series with variable dopant contents.

Co-Sintering Study of Na0.67[Ni0.1Fe0.1Mn0.8]O2 and NaSICON Electrolyte–Paving the way to High Energy Density All-Solid-State Batteries

Abstract: Sodium is a promising candidate for stationary storage applications, especially when the demand for lithium-ion batteries increases due to electromobility applications. Even though its energy density is lower, Na-ion technology is estimated to lead to a cost reduction of 30 % compared to Li-ion technology. To improve safety as well as energy density, Na-based all-solid-state-batteries featuring solid electrolytes such as beta-alumina and sodium superionic conductors and cathode materials such as Na3V2(PO4)3 and NaxCoO2 have been developed over the past years. However, the biggest challenge are mixed cathodes with highly conductive interfaces, especially when co-sintering the materials. For example, a promising sodium superionic conductor type Na3Zr2Si2PO12 electrolyte sinters at 1250 °C, whereas the corresponding Na3V2PO12 cathode decomposes at temperatures higher than 900 °C, posing a bottleneck. Thus in this paper, we synthesized Na0.62[Ni0.10Fe0.10Mn0.80]O2 as cathode material for all-solid-state sodium-ion batteries via a relatively cheap and easy solution-assisted solid state reaction processing route. The thermal investigations of the pure cathode material found no degradation up to 1260 °C, making it a perfect match for Na3.4Zr2Si2.4P0.6O12 electrolyte. In our aim to produce a co-sintered mixed cathode, electron microscopy investigation showed a highly dense microstructure and the elemental mapping performed via energy dispersive X-ray spectroscopy and secondary ion mass spectrometry confirm that Na3.4Zr2Si2.4P0.6O12 and Na0.62[Ni0.10Fe0.10Mn0.80]O2 do not react during sintering. However, the active cathode material forms a sodium rich and a sodium deficient phase which needs further investigation to understand the origin and its impact on the electrochemical performance.