Basic principles and composition of sodium-ion batteries

Basic principles and composition of sodium-ion batteries

1. Overview of sodium-ion batteries

In various energy storage systems, lithium-ion batteries are widely used due to their advantages such as high energy and power density, long life, environmental friendliness and lack of memory effect. Since the successful commercialization of lithium-ion batteries in 1991, they have played an important and irreplaceable leading role in many important fields, such as consumer electronics industry, new energy vehicles, large-scale energy storage, etc. At present, although the relevant technologies and processes of lithium-ion batteries have matured, and lithium-ion batteries have unique advantages in various fields, the low safety, low cycle life, low temperature resistance and high cost of lithium-ion batteries cannot be ignored. Therefore, it is urgent to develop low-cost batteries with high safety, high reliability, cold and heat resistance as alternatives to lithium-ion batteries. In contrast, sodium resources are the sixth most abundant element in the earth's crust (about 150 million tons, accounting for 2.74% of the total elements in the earth's crust), and sodium, as the main component of sea salt, is widely distributed in the ocean, with the advantages of wide and uniform distribution, easy acquisition and purification. In addition, sodium is an element of the first main group like lithium, and its physical and chemical properties such as ionic radius and atomic mass are similar to those of lithium (Table 1-1). Metallic sodium has a relatively high theoretical specific capacity (1166 mAhgl) and an electrochemical potential of -2.71 V (relative to a standard hydrogen electrode). In summary, sodium-ion batteries are expected to become a substitute for current lithium-ion batteries, and the development and research of efficient sodium-ion batteries has important strategic significance and commercial application value.

2. Basic principles and composition of sodium-ion batteries

1) Working mode
When the battery is charged, sodium ions are released from the positive electrode material into the electrolyte, and the free sodium ions in the electrolyte are embedded in the negative electrode material; in the external circuit, electrons migrate from the positive electrode to the negative electrode. When the battery is discharged, sodium ions are released from the negative electrode and re-embedded into the positive electrode material; the external circuit electrons flow from the negative electrode to the positive electrode under the potential field.

2) Composition
Positive electrode
As an important component of sodium-ion batteries, the positive electrode material provides sodium ions during the first cycle of charging and discharging. In addition, the structural stability of the positive electrode material is largely related to the cycle stability of the sodium-ion battery. In an ideal positive electrode material, the volume shrinkage and expansion caused by the extraction and insertion of sodium ions can cause negligible distortion and damage to the crystal structure, and can effectively improve the electrochemical performance. Generally speaking, organic polymer materials with octahedral structures and layered oxide materials with two-dimensional structures can effectively bind sodium ions in octahedrons and are ideal positive electrode materials for storing sodium ions.
The binding energy of lithium and sodium is different. In the same structure, the embedding voltage of sodium ions is significantly lower than that of lithium ions (0.18-0.57V). Compared with lithium ions, sodium ions have a larger mass and size, which indicates that their diffusion rate is also significantly lower. In order to increase the diffusion rate of sodium ions in electrode materials, nanosizing the electrode material size is an effective way.

Negative electrode
In a full battery, the negative electrode material is equally important to the battery's capacity, rate, cycle stability and other performance. The theoretical specific capacity of the sodium metal cathode (1166mAhg-1) is lower than that of the lithium metal cathode, and it has a higher reduction potential. Metallic sodium is more likely to react and decompose in organic electrolytes, leading to the formation of sodium dendrites. Not only that, due to the low melting point of sodium metal (98°C), sodium metal is easier to melt and diffuse during the charge and discharge process, which endangers the health of the battery. Therefore, the application prospects of sodium metal batteries are slim. However, by using sodium ions as the ion source for embedding and de-embedding, the positive and negative electrode materials can be embedded and de-embedded in a "rocking chair" manner to realize the charging and discharging of the battery, and the recycling of sodium ions can be achieved. Such a design avoids the dangers associated with low sodium ion activity. Unfortunately, it is difficult to seamlessly combine with other components of the battery material to form a complete battery. Therefore, most studies have only studied the electrochemical properties of new electrode materials and metal sodium half-cells.