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In this work, the effects of dopant size and oxidation state on the structure and electrochemical performance of LiNi0.8Co0.1Mn0.1O2 (NCM811) are investigated. It is shown that doping with boron (B) which has a small ionic radius and an oxidation state of 3+, leads to the formation of a boron oxide-containing surface coating (probably Li3BO3), mainly on the outer surface of the secondary particles. Due to this effect, boron only slightly affects the size of the primary particle and the initial capacity, but significantly improves the capacity retention. On the other hand, the dopant ruthenium (Ru) with a larger ionic radius and a higher oxidation state of 5+ can be stabilized within the secondary particles and does not experience a segregation to the outer agglomerate surface. However, the Ru dopant preferentially occupies incoherent grain boundary sites, resulting in smaller primary particle size and initial capacity than for the B-doped and pristine NCM811. This work demonstrates that a small percentage of dopant (2 mol%) cannot significantly affect bulk properties, but it can strongly influence the surface and/or grain boundary properties of microstructure and thus the overall performance of cathode materials.
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Ion Beam Analysis (IBA) utilizing MeV ion beams provides valuable insights into surface elemental composition across the entire periodic table. While ion beam measurements have advanced towards high throughput for mapping applications, data analysis has lagged behind due to the challenges posed by large volumes of data and multiple detectors providing diverse analytical information. Traditional physics-based fitting algorithms for these spectra can be time-consuming and prone to local minima traps, often taking days or weeks to complete. This study presents an approach employing a Mixture Density Network (MDN) to model the posterior distribution of Elemental Depth Profiles (EDP) from input spectra. Our MDN architecture includes an encoder module (EM), leveraging a Convolutional Neural Network-Gated Recurrent Unit (CNN-GRU), and a Mixture Density Head (MDH) employing a Multi-Layer Perceptron (MLP). Validation across three datasets with varying complexities demonstrates that for simple and intermediate cases, the MDN performs comparably to the conventional automatic fitting method (Autofit). However, for more complex datasets, Autofit still outperforms the MDN. Additionally, our integrated approach, combining MDN with the automatic fit method, significantly enhances accuracy while still reducing computational time, offering a promising avenue for improved analysis in IBA.
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Solid-state batteries based on Li7La3Zr2O12 (LLZO) garnet electrolyte are a robust and safe alternative to conventional lithium-ion batteries. However, the large-scale implementation of ceramic composite cathodes is still challenging due to a complex multistep manufacturing process. A new one-step route for the direct synthesis of LLZO during the manufacturing of LLZO/LiCoO2 (LCO) composite cathodes based on cheap precursors and utilizing the industrially established tape casting process is presented. It is shown that Al, Ta:LLZO can be formed directly in the presence of LCO from metal oxide precursors (LiOH, La2O3, ZrO2, Al2O3, and Ta2O5) by heating to 1050 °C, eliminating the time- and energy-consuming synthesis of preformed LLZO powders. In addition, performance-optimized gradient microstructures can be produced by sequential casting of slurries with different compositions, resulting in dense and flat phase-pure cathodes without unwanted ion interdiffusion or secondary phase formation. Freestanding cathodes with a thickness of 85 µm, a relative density of 95%, and an industrial relevant LCO loading of 15 mg show an initial capacity of 82 mAh g-1 (63% of the theoretical capacity of LCO) in a solid-state cell with Li metal anodes, which is comparable to conventional LCO/LLZO cathodes and can be further improved in the future.
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The phosphate lithium-ion conductor Li1.5Al0.5Ti1.5(PO4)3 (LATP) is an economically attractive solid electrolyte for the fabrication of safe and robust solid-state batteries, but high sintering temperatures pose a material engineering challenge for the fabrication of cell components. In particular, the high surface roughness of composite cathodes resulting from enhanced crystal growth is detrimental to their integration into cells with practical energy density. In this work, we demonstrate that efficient free-standing ceramic cathodes of LATP and LiFePO4 (LFP) can be produced by using a scalable tape casting process. This is achieved by adding 5 wt % of Li2WO4 (LWO) to the casting slurry and optimizing the fabrication process. LWO lowers the sintering temperature without affecting the phase composition of the materials, resulting in mechanically stable, electronically conductive, and free-standing cathodes with a smooth, homogeneous surface. The optimized cathode microstructure enables the deposition of a thin polymer separator attached to the Li metal anode to produce a cell with good volumetric and gravimetric energy densities of 289 Wh dm-3 and 180 Wh kg-1, respectively, on the cell level and Coulombic efficiency above 99% after 30 cycles at 30 °C.
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All-solid-state lithium batteries are promising candidates for next-generation energy storage systems. Their performance critically depends on the capacity and cycling stability of the cathodic layer. Cells with a garnet Li7La3Zr2O12 (LLZO) electrolyte can show high areal storage capacity. However, they commonly suffer from performance degradation during cycling. For fully inorganic cells based on LiCoO2 (LCO) as cathode active material and LLZO, the electrochemically induced interface amorphization has been identified as an origin of the performance degradation. This study shows that the amorphized interface can be recrystallized by thermal recovery (annealing) with nearly full restoration of the cell performance. The structural and chemical changes at the LCO/LLZO heterointerface associated with degradation and recovery were analyzed in detail and justified by thermodynamic modeling. Based on this comprehensive understanding, this work demonstrates a facile way to recover more than 80% of the initial storage capacity through a thermal recovery (annealing) step. The thermal recovery can be potentially used for cost-efficient recycling of ceramic all-solid-state batteries.
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The garnet-type Li7La3Zr2O12 (LLZO) ceramic solid electrolyte combines high Li-ion conductivity at room temperature with high chemical stability. Several all-solid-state Li batteries featuring the LLZO electrolyte and the LiCoO2 (LCO) or LiCoO2-LLZO composite cathode were demonstrated. However, all batteries exhibit rapid capacity fading during cycling, which is often attributed to the formation of cracks due to volume expansion and the contraction of LCO. Excluding the possibility of mechanical failure due to crack formation between the LiCoO2/LLZO interface, a detailed investigation of the LiCoO2/LLZO interface before and after cycling clearly demonstrated cation diffusion between LiCoO2 and the LLZO. This electrochemically driven cation diffusion during cycling causes the formation of an amorphous secondary phase interlayer with high impedance, leading to the observed capacity fading. Furthermore, thermodynamic analysis using density functional theory confirms the possibility of low- or non-conducting secondary phases forming during cycling and offers an additional explanation for the observed capacity fading. Understanding the presented degradation paves the way to increase the cycling stability of garnet-based all-solid-state Li batteries.
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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.
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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.
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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.
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The development of high-capacity, high-performance all-solid-state batteries requires the specific design and optimization of its components, especially on the positive electrode side. For the first time, we were able to produce a completely inorganic mixed positive electrode consisting only of LiCoO2 and Ta-substituted Li7La3Zr2O12 (LLZ:Ta) without the use of additional sintering aids or conducting additives, which has a high theoretical capacity density of 1 mAh/cm2. A true all-solid-state cell composed of a Li metal negative electrode, a LLZ:Ta garnet electrolyte, and a 25 µm thick LLZ:Ta + LiCoO2 mixed positive electrode was manufactured and characterized. The cell shows 81% utilization of theoretical capacity upon discharging at elevated temperatures and rather high discharge rates of 0.1 mA (0.1 C). However, even though the room temperature performance is also among the highest reported so far for similar cells, it still falls far short of the theoretical values. Therefore, a 3D reconstruction of the manufactured mixed positive electrode was used for the first time as input for microstructure-resolved continuum simulations. The simulations are able to reproduce the electrochemical behavior at elevated temperature favorably, however fail completely to predict the performance loss at room temperature. Extensive parameter studies were performed to identify the limiting processes, and as a result, interface phenomena occurring at the cathode active material/solid-electrolyte interface were found to be the most probable cause for the low performance at room temperature. Furthermore, the simulations are used for a sound estimation of the optimization potential that can be realized with this type of cell, which provides important guidelines for future oxide based all-solid-state battery research and fabrication.