LLZTO (Li6.6La3Zr1.6Ta0.4O12) is a promising oxide solid electrolyte for solid state batteries, exhibiting a high chemical stability against Li metal. At the same time, it offers a wide electrochemical window and high ionic conductivity reaching close to 10-3 S cm-1 at room temperature. As cathode active materials, NCM (e.g. Li1Ni0.33Co0.33Mn0.33O2) is widely used providing high specific capacity. Despite their great potentials, there are still limitations to surmount, especially in manufacturing dense and thermodynamically stable composite electrodes. This is, since LLZTO and NCM are not stable at elevated temperatures in ambient atmosphere presumably because of the presence of CO2. This emphasizes the need for advanced sintering methods and/or the usage of sintering additives[4,5] as well as further investigations of the sintering behavior in inert atmosphere.
In this study, the sintering behavior of LLZTO/NCM composite cathodes was investigated as a function of sintering temperature by X-ray-diffraction (XRD) and scanning electron microscopy (SEM). LLZTO powder was synthesized by solid-state reaction and the obtained LLZTO was mixed with NCM using Thinky mixer for 5 min at 300 rpm to prevent particle cracking. These composite powders were uniaxially pressed and sintered in air and Ar-atmosphere at various temperatures.
XRD results of samples sintered in air show that for the tested temperatures decomposition of the NCM starts at around 700°C and gets more pronounced with increasing temperature until most of the NCM is decomposed at 1100°C (Fig. 1a & 1b). At higher temperatures, decomposition products such as La2Zr2O7, La(Mn,Ni)O3 and Li-Co-oxides are formed through the reaction with the solid electrolyte (Fig. 1a & 1b). At the same time, we observed hints of grain growth of NCM-primary particles above 900°C (Fig. 2). Increasing the temperature up to 1100 °C in order to get higher densification results in decomposition of the active material (Fig 1b & 2).
In parallel, we investigated sintering behavior under Ar-atmosphere. At 750°C, we did not find any significant differences in SEM images compared to air-sintering (Fig. 2). However, XRD results demonstrate that sintering in inert Ar-atmosphere inhibits the decomposition of the NCM up to a temperature of 950°C (Fig. 1c & 1d). Similar to air-sintering, hints of grain growth of NCM-primary particles are observed above 900°C, too (Fig. 2). Nevertheless, NCM is not stable in inert atmosphere at 1100°C showing a different decomposition mechanism compared to oxidic reactions in air. At this temperature we observed that NCM gets reduced in Ar-atmosphere forming intermetallic phases. For the purpose of achieving improved densification at 950°C, cold isostatic pressing will be applied before thermal treatment and sintering additives will be used in further studies.
Figure 1. (a) XRD patterns of LLZTO/NCM composite cathodes sintered at different temperatures in ambient atmosphere. (b) Enlarged view of the pattern (31-38°) for the sample sintered at 1100°C in ambient atmosphere. (c) Comparison of XRD patterns of composites cathodes sintered in ambient and inert atmosphere. (d) Enlarged view of the XRD patterns (31.50-33.50°) of composite cathodes sintered in air and inert atmosphere at 900°C and 950°C.
Figure 2. SEM BSE images of LLZTO/NCM composite cathodes sintered at different temperatures in Air and Ar-atmosphere.
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