Analysis of the Compression and Conductive Properties of Silicon-Carbon and Silicon-Oxide Materials

1. Preface

This paper mainly combines different doping ratios of silicon-carbon materials and different sintering processes of silica-based materials, combined with scanning electron microscopy, powder conductivity, compaction density and other testing equipment, from the morphology, electronic conductivity, compaction density and compression properties of the material on the systematic testing and analysis.

Lithium-ion batteries have gradually been widely used in portable electronic products and electric vehicles due to their advantages of high energy density, long cycle life and environmental protection. Currently, the capacity of lithium-ion batteries with graphite-based materials as the anode has gradually failed to meet the requirements of electric vehicles with long range, and silica-based materials are the most promising anode materials for the next-generation lithium batteries due to their advantages of large specific capacity, low discharge platform, and rich energy storage. However, silicon-based materials have severely limited its commercial application due to its own factors. First, the large volume change in the process of de-embedded lithium, which is easy to lead to particle pulverization, active materials out of the collector, and the continuous production of SEI film, which ultimately leads to the decline of electrochemical performance, as shown in Figure 1, which is a schematic representation of the failure mechanism of silicon.

In addition, silicon-based materials have a relatively low electrical conductivity, and the rate of lithium diffusion in the silicon is relatively low, which is not conducive to the Lithium ion and electron transport; for the monolithic silicon exists in the volume expansion caused by the poor cycling stability of the problem, the current main measures to solve the nano- and composite, the practical application is also mainly through the doping with carbon materials or silicon material structure end of the design of the modification to enhance its electrical conductivity and lithium ion transport 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.

2. Test Methods

2.1 SEM morphology test of SiO material and Si/C material;

2.2 Conductivity, compaction density and compression properties of the materials were tested by using PRCD3100 (IEST), respectively, and the testing equipment is shown in Figure 2.

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

2.3 Test parameters

Pressure range 10~200MPa, interval 10MPa, holding pressure 10s.

3. Test results

3.1 Silicon-carbon anode materials

Among the new negative electrode materials, silicon negative electrode has gained wide attention from researchers for its ultra-high theoretical specific capacity of 4200mAh/g. For silicon negative electrode, the huge volume expansion that accompanies the charging and discharging process generates a large mechanical stress, which pulverizes the active material and loses contact with the collector, thus leading to the rapid decay of the reversible capacity of the electrode. 3% (SiC-1) of silicon content is selected for the present experiments, 6% (SiC-2) and 10% (SiC- 3) SiC hybrid materials were selected to test the differences in electronic conductivity, compaction density and compression properties.

Combined with the scanning electron microscope, the three materials were compared with each other in terms of the differences in morphology, which could not be seen under the electron microscope due to the low silica content and the differences in sample preparation. As shown in Figure 3, the SEM morphology at different magnifications with 6% Si content, where the Si material morphology 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 morphology of the same SiC hybrid material at 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 electrode of the silicon-carbon composite negative electrode needs to optimize the electrode parameters such as the conductive agent formulation and compaction density of the electrode. 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)Variation of resistivity with pressure for three silicon-carbon hybrid materials

Figure 6. (B)Variation of compacted density with compression for three silicon-carbon hybrids

3.2 Silicon oxide-based anode materials

Compared with monomaterial Si, the silicon oxide-based composite material reacts during the first lithium embedding process to generate Li₂O, Li₄SiO₄, and Si in situ, of which Li₂O and Li₄SiO₄ are electrochemically inert components that do not take part in subsequent electrochemical reactions, and are evenly dispersed with each other with the generated monomaterial Si. chemical reaction, and the generated monolithic Si is uniformly dispersed with each other, which largely buffers the volume expansion of monolithic Si in the charge/discharge process and improves the cycling stability of the overall electrode material. However, SiO2-based materials still have the expansion effect in the process of de-embedded lithium, which leads to capacity degradation and poor electrical conductivity, and their applications are mainly modified by means of carbon coating, nanosizing, porous structure design, and composite with highly conductive phases, etc. The surface-coated 0.5 mm Si oxide-based materials were selected in this part. In this part, four SiO-based materials SiO-1, SiO-2, SiO-3 and SiO-4 (sintering temperature: SiO-1 < SiO-2 < SiO-3 < SiO-4) with 0.1% carbon coating on the surface and different sintering temperatures were analyzed from the angles of SEM morphology, electrical conductivity, compacted density, and compression properties, respectively. As Figure 6 shows the comparison of the differences in the morphology test of the four materials, there is no obvious difference between the four materials from the morphology results. Compared with the monolithic silica material, the silica oxide-based material presents an irregular surface loose morphology.

Figure 7. SEM morphology of four 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 8. 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.

4. Summary

In this paper, the electrical conductivity, compaction density and compression properties of silicon-based materials were tested by using PRCD3100 to evaluate the difference analysis of the materials under different mixing ratios and modification process conditions, which provides a new way of thinking and direction for the evaluation of material modification and difference analysis.

5. References 

[1] Wu H, Cui, Y. Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today, 7, 414-429, (2012).

[2] Guerfi A , Hovington P , Charest P , et al. Nanostructured Carbon Coated Si and SiOx Anodes for High Energy Lithium-ion Batteries. 2011 ECS – The Electrochemical Society

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