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Abstract To achieve cohesive interfaces of solid components in next‐generation Li‐ion batteries (LIBs) composed of oxide cathodes and solid electrolytes (SEs) high‐temperature sintering is necessary which leads to material degradation and phase decompositions. Thermochemically compatible components can address these issues. In this work, advanced electron microscopy techniques are employed to investigate the structural and chemical evolution taking place at the interfaces between a LiFePO4 (LFP) cathode and an in‐house optimized Li1.3Al0.3Ti1.7(PO4)3 (LATP) SE co‐sintered between 650 °C – 850 °C. While the LFP particles exhibit excellent structural stability, the LATP particles phase transforms at elevated temperatures. At 650 °C, FeLi anti‐site defect is observed at the LFP grain boundaries. Between 750 and 850 °C, the LATP (R‐3c) transforms into two orthorhombic phases (Pbca and Pbcn) of the type Li1.3+xAl0.3FexTi1.7‐x(PO4)3 depending on Fe substitution while forming the cohesive interfaces. The discharge capacity of the composite decreases from 148 mAh g−1 at 650 °C to 130 mAh g‐1 at 850 °C confirming that the new phases are still electrochemically active. This study provides a spatially resolved insight into the formation of cohesive interfaces highlighting the advantage of employing components with similar anionic structures for further improvements in LIBs.

Solid-state batteries (SSBs) with Li-ion conductive electrolytes made from polymers, thiophosphates (sulfides) or oxides instead of liquid electrolytes have different challenges in material development and manufacturing. For oxide-based SSBs, the co-sintering of a composite cathode is one of the main challenges. High process temperatures cause undesired decomposition reactions of the active material and the solid electrolyte. The formed phases inhibit the high energy and power density of ceramic SSBs. Therefore, the selection of suitable material combinations as well as the reduction of the sintering temperatures are crucial milestones in the development of ceramic SSBs. In this work, the co-sintering behavior of Li1.3Al0.3Ti1.7(PO4)3 (LATP) as a solid electrolyte with Li-ion conductivity of ≥0.38 mS/cm and LiFePO4 with a C-coating (LFP) as a Li-ion storage material (active material) is investigated. The shrinkage behavior, crystallographic analysis and microstructural changes during co-sintering at temperatures between 650 and 850 °C are characterized in a simplified model system by mixing, pressing and sintering the LATP and LFP and compared with tape-casted composite cathodes (d = 55 µm). The tape-casted and sintered composite cathodes were infiltrated by liquid electrolyte as well as polyethylene oxide (PEO) electrolyte and electrochemically characterized as half cells against a Li metal anode. The results indicate the formation of reaction layers between LATP and LFP during co-sintering. At Ts > 750 °C, the rhombohedral LATP phase is transformed into an orthorhombic Li1.3+xAl0.3−yFex+yTi1.7−x(PO4)3 (LAFTP) phase. During co-sintering, Fe3+ diffuses into the LATP phase and partially occupies the Al3+ and Ti4+ sites of the NASICON structure. The formation of this LAFTP leads to significant changes in the electrochemical properties of the infiltrated composite tapes. Nevertheless, a high specific capacity of 134 mAh g−1 is measured by infiltrating the sintered composite tapes with liquid electrolytes. Additionally, infiltration with a PEO electrolyte leads to a capacity of 125 mAh g−1. Therefore, the material combination of LATP and LFP is a promising approach to realize sintered ceramic SSBs.

The density, microstructure, and ionic conductivity of solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP) ceramics prepared by cold sintering using liquid and solid sintering additives are studied. The effects of both liquid (water and water solutions of acetic acid and lithium hydroxide) and solid (lithium acetate) additives on densification are investigated. The properties of cold-sintered LATP are compared to those of conventionally sintered LATP. The materials cold-sintered at temperatures 140–280 °C and pressures 510–600 MPa show relative density in the range of 90–98% of LATP’s theoretical value, comparable or higher than the density of conventionally sintered ceramics. With the relative density of 94%, a total ionic conductivity of 1.26 × 10−5 S/cm (room temperature) is achieved by cold sintering at the temperature of 200 °C and uniaxial pressure of 510 MPa using water as additive. The lower ionic conductivities of the cold-sintered ceramics compared to those prepared by conventional sintering are attributed to the formation of amorphous secondary phases in the intergranular regions depending on the type of additives used and on the processing conditions selected.

A zirconia-based potentiometric solid electrolyte gas sensor with internal solid state reference was used to study the response behavior of platinum cermet and indium tin oxide sensing electrodes. Target gases included both oxygen and carbon monoxide in nitrogen-based sample gas mixtures. It was found that with the indium tin oxide sensing electrode, the low-temperature behavior is mainly a result of incomplete equilibration due to contaminations of the electrode surface. On the other hand, some of these contaminant species have been identified as being pivotal for the higher carbon monoxide sensitivity of the indium tin oxide sensing electrode as compared to platinum cermet electrodes.

By combining results of adsorption/desorption measurements on powders and electrical conductivity studies on thick and thin films, the interaction of indium tin oxide with various ambient gas species and carbon monoxide as potential target gas was studied between room temperature and 700 °C. The results show that the indium tin oxide surfaces exhibit a significant coverage of water-related and carbonaceous adsorbates even at temperatures as high as 600 °C. Specifically carbonaceous species, which are also produced under carbon monoxide exposure, show a detrimental effect on oxygen adsorption and may impair the film’s sensitivity to a variety of target gases if the material is used in gas sensing applications. Consequently, the operating temperature of an ITO based chemoresistive carbon monoxide sensor should be selected within a range where the decomposition and desorption of these species proceeds rapidly, while the surface oxygen coverage is still high enough to provide ample species for target gas interaction.

All-solid-state batteries (ASSB) are considered promising candidates for future energy storage and advanced electric mobility. When compared to conventional Li-ion batteries, the substitution of Li-ion conductive, flammable liquids by a solid electrolyte and the application of Li-metal anodes substantially increase safety and energy density. The solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 (LATP) provides high Li-ion conductivity of about 10−3 S/cm and is considered a highly promising candidate for both the solid electrolyte-separator and the ionically conductive part of the all-solid state composite cathode, consisting of the cathode material, the solid electrolyte, and an electron conductor. Co-sintering of the composite cathode is a sophisticated challenge, because temperatures above 1000 °C are typically required to achieve the maximum ionic conductivity of LATP but provoke reactions with the cathode material, inhibiting proper electrochemical functioning in the ASSB. In the present study, the application of sintering aids with different melting points and their impact on the sinterability and the conductivity of LATP were investigated by means of optical dilatometry and impedance spectroscopy. The microstructure of the samples was analyzed by SEM. The results indicate that the sintering temperature can be reduced below 800 °C while maintaining high ionic conductivity of up to 3.6 × 10−4 S/cm. These insights can be considered a crucial step forward towards enable LATP-based composite cathodes for future ASSB.

The electrode processes governing response behavior of a solid electrolyte CO2 sensor have been studied using electrochemical impedance spectroscopy on symmetric cells exposed to various temperatures and gas compositions. It was shown that the electrochemical processes leading to potential formation at the Au/Na2CO3 electrode can be described by adapting models commonly used in the field of molten carbonate fuel cells.

The influence of various gas compositions on surface adsorbate species and electrical conductivity of indium tin oxide (ITO) powders and thick films was studied. By combining results of temperature dependent desorption (TPD) with electrical conductivity measurements it was shown that, after exposure to ambient air, the surfaces of both powders and films are covered with significant amounts of oxygen, water and carbon related species. While the influence of oxygen adsorbates has already been described for temperatures below 500 °C, desorption of some of these species could be detected at temperatures as high as 675 °C, with a significant influence on electrical film conductivity.