Decomposition Behavior of LLZTO/NCM Composite Cathodes in Air vs. Argon Atmosphere During Sintering

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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[1], However, despite its great potentials, there are still limitations to surmount, especially in manufacturing dense and thermodynamically stable composite electrodes[2]. Because LLZTO and NCM are not stable at elevated temperatures in ambient atmosphere presumably because of the presence of CO2[3]. 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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