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 a cathode active material-, NCM (e.g. Li1Ni0.33Co0.33Mn0.33O2) is widely used and provides high specific capacity, However, despite its great potentials, there are still limitations to surmount, especially in manufacturing dense and thermodynamically stable composite electrodes. Because 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 (UP) 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.
To improve the densification at 950°C, cold isostatic pressing (CIP) was applied before sintering in air. With both, uniaxial and cold isostatic pressing at 435 MPa a densification of > 80 % was achievied, while CIPed pellets were more stable and thus easier to handle.
In future experiments, CIPed samples will be sintered in Ar-atmosphere at 950 °C to prevent decomposition. Furthermore the usage of sintering additives and the effect on densification will be investigated.
1. Zhang, Zhizhen, et al., Energy & Environmental Science 11.8 (2018): 1945-1976.
2. Ren, Yaoyu, et al., Journal of Materiomics 2.3 (2016): 256-264.
3. Kim, Younggyu, et al., Chemistry of Materials 32.22 (2020): 9531-9541.
4. Ihrig, Martin, et al., Journal of Power Sources 482 (2021): 228905.
5. Shin, Ran-Hee, and Sung-Soo Ryu., Journal of Nanoscience and Nanotechnology 19.3 (2019): 1809-1813.
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