Research on Corrosion and Thermal Shock Resistance of Cordierite-Mullite Saggar Materials for Lithium Battery Material Sintering
Cordierite–Mullite Saggar Material
Cathode materials for lithium-ion batteries are a new type of green and environmentally friendly material in the high-tech field and have broad application prospects. With the rapid development of emerging industries, the application of lithium-ion battery cathode materials has become increasingly widespread. Consequently, the demand for saggar materials used in the sintering of lithium-ion battery cathode materials is increasing, and higher performance requirements are being imposed on these saggar materials. However, current domestically produced saggar materials for lithium-ion battery cathodes generally suffer from poor corrosion resistance and low thermal shock stability, while imported saggars are relatively expensive. These factors have, to some extent, restricted the further application of lithium-ion battery cathode materials. Therefore, the development of a new type of saggar material for lithium-ion battery cathode materials is of great significance.
To address the problems of poor thermal shock resistance, insufficient corrosion resistance, and short service life of existing saggar materials for lithium-ion batteries, this study adopts a high-temperature solid-phase synthesis method. Cordierite and mullite are used as the main raw materials, with fused magnesia, activated alumina, and other powders added as auxiliary materials. The volume expansion associated with mullite formation at high temperatures is utilized to offset the internal stresses caused by volume shrinkage resulting from liquid-phase formation during high-temperature sintering. In this way, a saggar material with excellent corrosion resistance and thermal shock resistance for lithium-ion batteries is prepared. In addition, the effects of raw material particle size distribution and sintering temperature on the phase composition, microstructure, thermal shock resistance, and corrosion resistance of the cordierite–mullite saggar material are investigated.
Part 1: Experiments
1. Sample Preparation
The raw materials used in the experiment were as follows: cordierite (particle sizes of 1–2.5 mm, 0.2–1 mm, and 0–0.2 mm, purity ≥ 97.0%), fused mullite (particle sizes of 1–1.6 mm and 0.2–1 mm, purity ≥ 97.0%), fused MgO (particle size of 74 μm, purity ≥ 97.0%), α-Al₂O₃ (particle size of 10 μm, purity ≥ 99.0%), silica fume (particle size of 0.1–0.3 μm, purity ≥ 97.0%), ZrO₂ (particle size of 10 μm, purity ≥ 99.0%), magnesia-alumina spinel (particle size of 10 μm, purity ≥ 97.0%), kaolin (particle size of 44 μm, purity ≥ 97.0%), and binder (concentration: 30.0%). The experimental formulations are shown in Table 1. The raw materials were mixed according to the formulation in Table 1, aged, and then pressed into shape. The samples were subsequently fired at temperatures of 1300 ℃, 1350 ℃, 1380 ℃, and 1400 ℃, with a holding time of 3 hours for each temperature.
The formulation of the corrosive medium used is shown in Table 2. Lithium carbonate (Li2CO3) was used as the primary component, with specified amounts of manganese dioxide (MnO2), cobalt trioxide (Co2O3), and nickel sesquioxide (Ni2O3) added to prepare the composite Li2(Ni0.8Co0.1Mn0.1)Ox as the corrosive medium. The corrosion resistance test was conducted using the static crucible method. The corrosive medium was loaded into crucible samples with outer dimensions of Φ50 mm×50 mm and an inner cavity dimension of H25mm×Φ20mm. The samples were then sintered five times in an air atmosphere at 800℃ (the typical synthesis temperature for lithium battery materials), with a holding time of 4 hours per sintering cycle.
2. Sample Testing
The instruments used are listed in Table 3. The phase composition of the samples was analyzed using an X-ray diffractometer (XRD). The microscopic morphology of the samples was observed with a field emission scanning electron microscope (FESEM). The elastic modulus of the samples was measured with a Young's modulus tester. The flexural and compressive strength of the samples were tested using a universal testing machine. Additionally, the bulk density, apparent porosity, and water absorption of the samples were measured using a porosity and bulk density tester.
The thermal shock stability of the samples was evaluated according to the metallurgical standard YB/T 376.1-1995. The samples were held at 1100 ℃ for 30 minutes and then quenched in water.
Part 2: Results and Discussion
1. Physical Properties
After firing the different samples at temperatures ranging from 1300 ℃ to 1400 ℃ and subjecting them to five thermal shock tests, the surfaces were inspected for cracks. It was observed that a small number of fine cracks appeared on the surfaces of some S2 and S3 samples, while no cracks were found on the S1 samples.
The Young's modulus of S1 samples was significantly higher than that of S2 and S3 samples. This can be attributed to the higher cordierite content in the S1 samples compared to S2 and S3. Given that cordierite has a lower coefficient of thermal expansion than mullite, the S1 samples exhibited superior thermal shock resistance and a higher residual Young's modulus than the latter two groups. Meanwhile, the Young's modulus of the samples showed considerable variation before and after the thermal shock tests, caused by the generation of fine internal cracks after thermal shock, which led to a decrease in Young's modulus.
Figure 2 shows the curve of the residual flexural strength of the samples as a function of firing temperature. As seen in Figure 2, the residual flexural strength of S1 samples was notably higher than that of S2 and S3 samples. This can be explained by the fact that the three groups of samples had similar mullite contents, whereas S2 and S3 samples contained a relatively lower amount of cordierite aggregate compared to S1. After five thermal shock tests, the residual strength mainly depended on the cordierite aggregate in the samples. Consequently, S2 and S3 samples showed poorer thermal stability than S1 samples, leading to the observed difference in residual flexural strength. The residual flexural strength of S1 samples ranged from 3.0 MPa to 4.5 MPa.
Figure 3 shows the curve of the residual compressive strength of the samples as a function of firing temperature. As shown in Figure 3, the residual compressive strength of all the samples decreased from 1300 ℃ to 1350 ℃, then began to rebound between 1350 ℃ and 1380 ℃. This phenomenon can be attributed to the formation of mullite crystals at 1350 ℃, which created small pores within the samples. At 1380 ℃, some of the cordierite raw material formed a liquid phase that filled the pores, thereby improving the strength of the samples. Among the samples, the residual compressive strength of S1 was higher than that of S2 and S3, with S1's residual compressive strength ranging from 17.5 MPa to 24.5 MPa.
Figures 4–6 show the curves of the samples' bulk density, apparent porosity, and water absorption as a function of firing temperature. The apparent porosity of the samples showed complex variations with temperature. This can be attributed to the partial melting of cordierite at around 1380 ℃, which filled some of the pores. As a result, the apparent porosity decreased significantly at 1380 ℃, improving the compactness of the samples and increasing the bulk density. Since water absorption is linearly correlated with apparent porosity, the trends in Figure 5 and Figure 6 are nearly identical.
Figure 7 shows the curve of the linear change rate of the samples as a function of firing temperature. After firing at different temperatures, the samples exhibited slight expansion or shrinkage, with the linear change rate ranging from -1.3% to 0.2%. The linear change rates of the different samples displayed similar trends with temperature: as the firing temperature increased, the linear change rate initially rose and then declined. For the S1 samples, this behavior can be attributed to the formation of new mullite phases, which caused sample expansion. When the temperature reached 1380 ℃, partial melting of cordierite generated a liquid phase, leading to sample shrinkage.
Figure 8: Apparent Morphologies of Samples After Five Corrosion Cycles (a) S1; (b) S2; (c) S3
Figures 8(a)–(c) show the apparent morphologies of the samples fired at different temperatures after five corrosion cycles. As seen in Figures 8(a)–(c), all the samples exhibited excellent corrosion resistance, with no signs of deep penetration or damage. This indicates that the S1, S2, and S3 samples all possess strong corrosion resistance.
2. Phase Analysis
Figure 9 shows the XRD patterns of different samples fired at 1380 ℃. As indicated in Figures 9(a)–(c), the phases are all cordierite phase, mullite phase and MgO phase, with no other phases formed.
(a)S1;(b)S2;(3)S3
3.3 Microstructure Analysis
Given the similar microstructures of the three sample groups after high-temperature firing, the middle group (Sample S2) was selected for microstructure analysis. Figure 10 shows the microstructures of Sample S2 fired at different temperatures. At 1300 ℃, both flaky and columnar mullite particles are clearly observed, with pores present between the crystal grains (Figure 10a). As the temperature increases, cordierite begins to melt at 1350 ℃, although the cordierite particles still maintain some crystalline structure (Figure 10b). At 1380 ℃, the melting became more pronounced; large pores were filled by the molten products, leaving only a few fine pores remaining (Figure 10c). At 1400 ℃, a significant amount of cordierite melted, and the molten products filled most of the pores, resulting in a denser structure and improved sample strength (Figure 10d).
(a)1300℃;(b)1350℃;(c)1380℃;(d)1400℃
(a)1300℃;(b)1350℃;(c)1380℃;(d)1400℃
Figure 11 shows the microstructures of S2 samples fired at different temperatures after five corrosion cycles. During sintering, the mismatch in thermal expansion coefficients between the different components generates thermal stress, leading to fine cracks in the samples. Moreover, the penetration of the corrosive medium exacerbates the propagation of these microcracks. When the firing temperature ranges from 1300 ℃ to 1350 ℃, the number of cracks increases with the temperature, indicating severe corrosion of the samples (Figures 11a and 11b). The corrosion resistance of the samples fired at 1380 ℃ and below 1400 ℃ is significantly improved. Combined with the results in Figures 10c and 10d, this improvement can be attributed to the molten cordierite products filling the pores and microcracks, which, to some extent, inhibit the corrosion and penetration of the corrosive medium (Figures 11c and 11d).
Part 3: Conclusions
(1)Cordierite-mullite composite materials were prepared using mullite and cordierite as the main raw materials, and fused magnesia, activated alumina, silica fume, and other micropowders as auxiliary materials, followed by firing at temperatures ranging from 1300 ℃ to 1400 ℃.
(2)After five thermal shock tests, the three groups of samples fired at 1300–1400 ℃ exhibited varying thermal shock resistance, with S1 samples showing superior performance. The residual flexural strength of the S1 samples ranged from 3.0 MPa to 4.5 MPa, and their residual compressive strength ranged from 17.5 MPa to 24.5 MPa.
(3)The prepared cordierite-mullite saggar materials demonstrated excellent corrosion resistance in the corrosion test conducted at 800 ℃.