Analysis of the Compressive and Conductive Properties of Silicon-Carbon and Silicon-Oxygen Materials

Lithium-ion batteries have gradually been widely used in portable electronic products and electric vehicles due to their advantages such as high energy density, long cycle life, and environmental protection. At present, the capacity of lithium-ion batteries with graphite materials as the negative electrode has gradually failed to meet the requirements of long battery life for electric vehicles，silicon-based materials are the most promising next-generation lithium battery anode materials due to their advantages such as large specific capacity, low discharge platform, and abundant energy storage. However, silicon-based materials have severely limited their commercial applications due to their own factors., First, the large volume change in the lithium-deintercalation process can easily lead to particle pulverization, active material detachment from the current collector, and continuous production of the SEI film, which eventually leads to electrochemical performance degradation. Figure 1 is a schematic diagram of the failure mechanism of silicon.

In addition, the electrical conductivity of silicon-based materials is relatively low, and the diffusion rate of lithium in silicon is also relatively low, which is not conducive to the transmission of lithium-ion and electrons; for the problem of poor cycle stability caused by the volume expansion of elemental silicon. At present, the main solutions are nanometerization and compounding, and the practical application is mainly to improve its conductivity and lithium-ion transport by doping with carbon materials or modifying the structural end of silicon materials. This article mainly combines silicon-carbon materials with different doping ratios and silicon oxide-based materials with different sintering conditions, combined with scanning electron microscope, powder conductivity, compaction density and other testing equipment. The material is systematically tested and analyzed in terms of morphology, electronic conductivity, compacted density and compressive properties.

Figure 1. Failure mechanism of Si electrode: (a) material crushing; (b) shape and volume change of the whole Si electrode; (c)SEI continues to grow[1].

1.Test Method

1.1 SEM Morphology Test of SiO Material and Si/C Material.

1.2Use PRCD3100 (IEST) to test the electrical conductivity, compacted density and compressive properties of the material respectively. The test equipment is shown in Figure 2. Test parameters: pressure range 10~200MPa, interval 10MPa, hold pressure 10s.

Figure 2. (a) Appearance of PRCD3100; (b) Structure of PRCD3100

2.Test Results

2.1Silicon Carbon Anode Material

Among the new negative electrode materials, the silicon negative electrode has attracted extensive attention from researchers with its ultra-high theoretical specific capacity of 4200mAh/g. For the silicon negative electrode, the huge volume swelling accompanied by the charging and discharging process will generate a large mechanical stress, which will cause the active material to pulverize and lose contact with the current collector, resulting in a rapid decay of the reversible capacity of the electrode.

In this experiment, three silicon-carbon hybrid materials with a silicon content of 3% (SiC-1), 6% (SiC-2), and 10% (SiC-3) were selected to test the differences in electronic conductivity, compaction density, and compression properties. Combined with the scanning electron microscope, the difference of the morphology test of the three materials was compared. Because the silicon content of the three materials is not high, and it involves the difference of sample preparation, no obvious difference can be seen under the electron microscope. Figure 3 shows the SEM topography images under different magnifications with a silicon content of 6%, in which the silicon material is mostly spherical, with a particle size of 5-10 μm.

The swelling and cracking of silicon particles is often related to the size of the particles. Generally speaking, the cracking of larger-sized μm-sized silicon particles is more severe, while the nano-sized particles with a size smaller than a certain critical value will have fewer cracks. The best way to utilize μm-sized Si particles is to compound them with graphite.

Figure 3.SEM topography images of the same silicon-carbon hybrid material under different magnifications

In order to further evaluate the difference of mixed materials with different silicon contents, this part uses the PRCD series powder resistivity &compaction density dual-function equipment to evaluate the conductivity, compaction density and compression performance. Figure 4 and Table 1 show the stress-strain curves and deformation comparisons of the three materials respectively. From the perspective of deformation ratio, the elastic and plastic deformations of the three materials are not much different. This shows that the addition of a small amount of silicon spheres has little effect on the overall deformation of the carbon material.

Table 1.Summary of Deformation Data of Three Silicon-carbon Hybrid Materials

Figure 4. Stress-strain curves of three silicon-carbon hybrid materials

Figure 5 shows the measurement results of the resistivity and compaction density of the three materials as a function of pressure. It can be seen from Figure (A) that as the proportion of silicon increases, the conductivity of the mixed material gradually deteriorates, this is mainly due to the poor conductivity of the silicon material, which leads to poor overall performance of the hybrid material as its proportion increases.

As for the measurement results of the compacted density of the three materials (B), it can be seen that as the proportion of silicon material increases, the compacted density tends to decrease significantly, this is mainly because the compaction density of silicon materials is relatively small compared to carbon materials, and in mixed materials, there will be obvious changes with the proportion difference between materials.

Therefore, the design and preparation of the pole piece of the silicon-carbon composite negative electrode needs to optimize the electrode parameters such as the conductive agent formulation and compaction density of the pole piece. Studies have shown that compared with graphite anodes, appropriately reducing the compaction density and increasing porosity of silicon-carbon anodes is conducive to buffering the volume swelling of silicon particles and inhibiting crack generation. On the one hand, the conductive agent uses a zero-dimensional conductive agent to coat the active particles to form a tight short-range electronic conduction network, and uses a one-dimensional conductive agent such as CNT to form a long-range electronic conduction network from the current collector to the entire electrode thickness direction.

Figure 5. (A) and (B) are the resistivity and compaction density of three silicon and carbon hybrid materials as a function of pressure

2.2 SiO-based Anode Materials

Compared with elemental silicon, silicon oxide-based composites react during the first lithium intercalation process to generate Li2O, Li4SiO4 and Si in situ, among which Li2O and Li4SiO4 are electrochemically inert components, it does not participate in the subsequent electrochemical reaction, and is uniformly dispersed with the generated elemental Si, which to a large extent buffers the volume swelling of the elemental Si during charge and discharge, and improves the cycle stability of the overall electrode material. However, silicon oxide-based materials still have an swelling effect in the process of lithium intercalation and deintercalation, resulting in capacity fading and poor conductivity. Its application is mainly modified by means of carbon coating, nanometerization, porous structure design, and compounding with high-conductivity phases.

This part selects four silicon oxide-based materials SiO-1, SiO-2, SiO-3, and SiO-4 with 0.1% carbon on the surface at different sintering temperatures (material sintering temperature: SiO-1<SiO-2 <SiO-3<SiO-4), which were measured and analyzed from the perspectives of SEM morphology, electrical conductivity, compacted density, and compressive properties. As shown in Figure 6, the comparison of the morphology test differences of the four materials shows that there is no significant difference in the morphology results of the four materials. Compared with the elemental silicon material, the silicon oxide-based material presents irregular surface loose morphology.

Figure 6. SEM topography of four kinds of silicon oxide-based materials

Similarly, for silicon oxide-based materials, comparative tests and evaluations were carried out from the aspect of compressive properties. Figure 7 and Table 2 show the stress-strain curves and deformation comparisons of the four materials. From the perspective of deformation ratio, for the four materials with different sintering temperatures, the overall compressibility of SiO-2 and SiO-3 is not much different in terms of compressibility，however SiO-1 with the lowest sintering temperature and SiO-4 with the highest sintering temperature have significant differences in maximum deformation, preliminary judgment may be that as the sintering temperature increases, the overall compactness of the material is better, and the material's ability to resist compression becomes larger.

According to the data representing the plastic deformation parameters of the material, i.e. the irreversible deformation, the plastic deformation of the SiO4 material with a higher sintering temperature is the smallest, and for the elastic deformation and reversible deformation under the action of material stress, the overall difference is not big from the data. However, in the actual powder particle compression process, multiple forces act together, and the stress is also a comprehensive change process, which can be combined with other testing methods for further analysis.

Table 2.Summary of Deformation Data of Four Kinds of Silicon Oxide-based Materials

Figure 7.Stress-strain curves of four silicon oxide-based materials

Figure 8 shows the measurement results of resistivity and compaction density of four kinds of silicon oxide-based materials as a function of pressure，it can be seen from Figure (A) that the resistivity of the four materials is SiO-1<SiO-2<SiO-3<SiO-4, that is, as the sintering temperature increases, the conductivity of the material is getting better and better，this may be because as the sintering temperature increases, the overall coating of the material becomes better, which in turn improves its conductivity. Figure (B) shows the variation curves of the compacted density of the four materials with the pressure. It can be clearly seen from the figure that the overall difference in the compacted density is not large when the pressure is small，with the increase of pressure, the difference of compaction density is gradually distinguished, but the overall difference is less than 0.05g/cm3.

In conclusion, the surface-coated carbon materials enhance the electrochemical performance due to the following reasons:  (1)  The carbon layer provides an elastic shell and reduces the volume change during alloying/dealloying. (2) Reduces the side reactions between active materials and electrolytes. (3) The carbon layer provides a large number of lithium ion and electron transport channels, thus improving the applicability of silicon-oxygen materials.

Figure 8. (A) and (B) are the resistivity and compaction density of four silicon oxide-based materials as a function of pressure.

Summarize

In this paper, PRCD3100 is used to test the conductivity, compaction density and compression performance of silicon-based materials, and to evaluate the difference analysis of materials under different mixing ratios and different modification process conditions. it provides a new idea and direction for material modification and differential analysis and evaluation.

Reference Literature

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[3] Lin Ning. Preparation and electrochemical performance of silicon-based anode materials for lithium-ion batteries [D]. University of Science and Technology of China, 2016.