Conductivity and Compaction Density Test of Sodium Cathode and Anode Materials

Sodium and lithium belong to the same group of elements, and their chemical properties are similar, but compared with lithium, sodium has obvious advantages in resource reserves and cost. At the same time, sodium-ion batteries have the characteristics of fast charging and discharging, excellent low-temperature performance, good safety performance, and the same production process as lithium batteries, making them a potential substitute for lithium-ion batteries and are expected to become the next generation of commercial energy storage devices. With the gradual advancement of research on sodium-ion batteries, breakthroughs have been made in positive and negative energy storage materials for sodium-ion batteries. The positive electrode materials for sodium-ion batteries mainly include oxides, polyanions, Prussian blues, and organics; Anode materials mainly include carbon-based, titanium-based, organic, alloy and other anode materials.

Prussian blue (PB), as a representative material of metal-organic frameworks (MOFs) in the research of sodium ion cathode materials, has attracted attention due to its low cost, facile preparation process and hollow framework structure.It has been shown that PB-derived nanomaterials can inherit some of their characteristics, exhibiting large surface area, interconnected pores, and graded pore size, which can facilitate charge transfer when used in energy storage and conversion systems. By adjusting the synthesis conditions (such as temperature and atmosphere), nanomaterials with ideal structures and properties can be obtained, which can be widely used in the field of energy storage [1].Figure 1 is a schematic diagram of the crystal structure of Prussian blue and its derivatives, and Figure 2 is the SEM image of Prussian blue and its derivatives.

Figure 1. Schematic diagram of the crystal structure of Prussian blue and its derivatives

Figure 2.SEM images of Prussian blue and its derivatives: (a)Na_0.67Ni_0.33Mn_0.67O₂ (b)Na_0.67Ni_0.33Mn_0.66Sn_0.01O₂ (c) Na_0.67Ni_0.33Mn_0.64Sn_0.03O₂ 和 (d) Na_0.67Ni_0.33Mn_0.62Sn_0.05O₂^[2]

Among the anode materials, carbon-based anode not only has a lower sodium intercalation platform, higher capacity, and good cycle stability, but also has the advantages of abundant resources and simple preparation. It is currently the most promising anode material for sodium storage. Among them, hard carbon materials have become ideal materials for commercialization due to their own advantages such as large interlayer spacing, low cost, simple synthesis method, and the possibility of using renewable resources as precursors. Figure 3 is a schematic diagram of hard carbon synthesis and a microscopic morphology and structure characterization diagram.

Figure 3. Schematic diagram of hard carbon synthesis and microstructure characterization diagram.

In this paper, four Prussian blue (PB) and hard carbon (HC) materials were selected, and the differences between the materials were evaluated by testing the electrical conductivity and compaction density under different pressure conditions.

1.Test Method

1.1Use PRCD3100 (IEST) to test four types of Prussian blue (PB-1/PB-2/PB-3/PB-4) materials and four types of hard carbon (HC-1/HC-2/HC-3/ HC-4) Materials are tested for electrical conductivity and compaction density. Among them, Prussian blue materials are measured in two-probe mode, and hard carbon materials are measured in four-probe mode. The test equipment is shown in Figure 2.

Test Parameters: applied pressure range 10-200MPa, interval 20MPa, hold pressure 10s.

Figure 4. (a) Appearance of PRCD3100; (b) Structure of PRCD3100

2.Test Results and Analysis

Prussian blue (PB) and its analogues have channels composed of a three-dimensional framework structure, which can facilitate the intercalation and deintercalation of sodium ions and are ideal cathode materials for sodium-ion batteries. The material can provide a theoretical specific capacity of 170mAh/g, and has good cycle stability. However, their poor cycle stability and rate performance in electrochemical tests limit their practical applications in Na-ion batteries. The main reason for affecting its electrochemical performance is that there are many vacancies in the crystal structure of the material and the coordination water occupies many electrochemical reaction sites, which reduces the specific capacity of the material.At the same time, the existence of vacancies will cause the structure to collapse due to the migration of sodium ions, while the coordination water in the structure reduces the electrical conductivity of the material. In practical applications, researchers optimize its physical and electrochemical properties by modifying it. , and the evaluation of electronic conductivity at the material end can be used as an effective evaluation method. Figure 5 shows the test results of resistivity and conductivity of four Prussian blue materials. Among them, PB-2 is a modification based on PB-1, and PB-4 is a modification based on PB-3. From the results of the resistivity test, PB-1, PB-3>PB-2>PB- 4. The two modified materials have better electrical conductivity. 

During the manufacturing process of lithium-ion power batteries, the compaction density has a great influence on the battery performance. Compacted density is closely related to specific capacity, efficiency, internal resistance, and battery cycle performance. Figure 6 shows the compaction density test results of four Prussian materials, PB-1>PB-3>PB-4>PB-2, the compaction density of the two modified materials under the current test conditions did not show the same It can be seen that in the actual research and development work, it is necessary to combine multiple means to comprehensively evaluate the overall performance of the material, so as to finally obtain a material with better overall performance.

Figure 5 . (A) Resistivity test results of four Prussian blue materials. (B) Conductivity test results of four Prussian blue materials.

Figure 6. Compaction density test results for four Prussian blue materials.

Hard carbon materials are considered to be the most potential anode materials for the development of sodium-ion batteries. Researchers control the morphology of hard carbon materials, introduce pore structures into hard carbon materials or build three-dimensional inline structures to improve their rate performance. By controlling the different carbonization processes and adjusting the microstructure of hard carbon materials, especially the graphite-like microcrystalline structure, the thermodynamic process of sodium ion insertion can be improved, and the sodium storage capacity of the material can be improved [4]. The resistivity and conductivity test results of the four hard carbon materials selected in this paper are shown in Figure 7 A and B. From the results of the conductivity test, HC-1>HC-4>HC-2>HC-3, that is, HC -1 shows better electrical conductivity; the compaction density test results of the four materials are shown in Figure 8. From the compaction density test results, HC-4>HC-1>HC-2>HC-3, four There is a clear distinction between the two materials; the difference in conductivity and compaction density between materials is related to their process, crystal morphology, internal structure and surface state.

Figure 7. (A) Resistivity test results of four hard carbon materials. (B)Conductivity test results of four hard carbon materials.

Figure 8. Compaction density test results of four hard carbon materials

3 Summary

In this paper, the powder resistance & compaction density (PRCD3100) equipment is used to detect the difference in conductivity and compaction density of the positive electrode material Prussian blue and the negative electrode material hard carbon of sodium ion batteries. The test results show that the differences between different materials can be clearly distinguished, which can be As an effective means of testing the physical properties of materials, it helps researchers to quickly evaluate the differences in electrical conductivity and compaction density at the material level.

References

[1] Chen J, Wei L, Mahmood A, et al. Prussian blue, its analogues and their derived materials for electrochemical energy storage and conversion - ScienceDirect[J]. Energy Storage Materials, 2020, 25:585-612.

[2] Li J, Risthaus T, Wang J, et al. The effect of Sn substitution on the structure and oxygen activity of Na0.67Ni0.33Mn0.67O2 cathode materials for sodium ion batteries[J]. Journal of Power Sources, 2019, 449:227554.

[3] Yin X, Lu Z, Wang J, et al. Enabling Fast Na+ Transfer Kinetics in the Whole-Voltage-Region of Hard-Carbon Anodes for Ultrahigh-Rate Sodium Storage[J]. Advanced Materials, 2022.

[4] Wu Junda, Zhao Yabin, Zhang Fuming. Research progress on hard carbon materials as anode materials for room temperature sodium-ion batteries [J]. Shandong Chemical Industry, 2019, 488.