In recent years, new energy is undoubtedly one of the most concerned industries. Comparing lithium vs sodium battery, lithium-ion batteries have received more attention due to their higher energy density. However, sodium-ion batteries, which have the advantages of raw material resources, also have the potential to become large-scale electrochemical energy storage devices, so they have been extensively studied and the industrialization process has been accelerating.
At present, the commercial application of sodium-ion batteries mainly depends on the development of high-performance cathode and anode materials. However, the intrinsic differences between Li and Na in terms of ionic radii and electrochemical potentials lead to some differences between sodium-ion battery and lithium-ion battery technologies, especially the anode as host material providing Na+ storage.
Studies have shown that graphite, an ideal anode material for lithium-ion batteries, is not suitable for sodium-ion batteries. In contrast, as one of the carbon-based materials, hard carbon with extended interlayer spacing is considered to be the most mature and most likely commercial anode for sodium-ion batteries.
1. Comparison of hard carbon with other anode materials
At present, research on carbon materials is focused on graphite, amorphous carbon, heteroatom-doped carbon, and biomass-synthesized carbon. Among them, both soft carbon and hard carbon belong to amorphous carbon materials, and the graphitization difficulty of the two is different.
Hard carbon vs graphite: higher specific capacity potential
The general specific capacity of hard carbon can reach 300-350mAh/g, and after optimization and modification, it can reach 400mAh/g, which will exceed the theoretical specific capacity of lithium battery graphite (372mAh/g). Hard carbon has a more disordered structure, a higher defect concentration, a higher heteroatom content and a larger distance between graphitic layers, and a more closed pore structure.
This is beneficial to provide more storage points and diffusion pathways for Na+ ions. Including: storing sodium through intercalation reaction, forming atomic clusters in closed pores, storing sodium through capacitive adsorption on the surface contacting the electrolyte, and storing sodium through pseudocapacitance at the sites related to defects on the internal surface.
Hard carbon vs soft carbon: better performance, greater room for cost reduction
Hard carbon usually refers to carbon that is difficult to be completely graphitized after high temperature treatment, and its disordered structure is difficult to eliminate at high temperature, also known as non-graphitizable carbon.
Soft carbon, in contrast, exhibits graphitization characteristics at high temperatures. Compared with soft carbon, hard carbon has higher gram capacity, first effect and potential stability, but the economy of hard carbon is slightly worse than that of soft carbon, and it has more room for cost reduction.
2. Problems and solutions of hard carbon
Hard carbon is easy to achieve commercial application due to its wide range of sources and excellent performance, but it still faces problems such as low initial coulombic efficiency (ICE), poor rate performance and cycle performance.
Low initial coulombic efficiency (ICE)
Hard carbon has a large specific surface area and a large number of defects, resulting in low first coulombic efficiency. The low initial coulombic efficiency reflects that a large number of irreversible reactions occurred during the first charge and discharge process of the battery, including the consumption of some sodium ions by the decomposition of the electrolyte to form the electrolyte interfacial film (SEI) during the recirculation process.
At present, reducing the specific surface area of hard carbon anode materials, reducing defects and closing some pores is the key to improving the initial coulombic efficiency (ICE).
Poor rate performance and cycling stability
Poor rate performance and cycle stability are another major factor affecting the commercialization of hard carbon anodes. For hard carbon anodes in sodium-ion batteries, most of the capacity is provided by the low-voltage plateau region.
The short-range ordered structure of hard carbon makes it have low conductivity, resulting in a large voltage hysteresis during high-rate charge/discharge, which may lead to a partial plateau capacity in a full battery at a voltage lower than that of Na metal deposition. This means that higher current densities increase the likelihood of metal deposition and present safety concerns.
The currently reported hard carbon anodes with high specific capacity are often accompanied by rapid capacity fading when the current density increases sharply. Second, the SEI film formed on the hard carbon anode is generally unstable, and its thickness and composition will change during cycling.
A thicker SEI film increases ohmic resistance and hinders the diffusion of ions, resulting in high voltage hysteresis and capacity fading. In addition, the co-intercalation of Na+ with solvent molecules and the exfoliation of graphite microchip layers may also lead to capacity fading.
3. Industrialization process of hard carbon
At present, hard carbon has good application scenarios in top 10 sodium ion battery companies related fields such as sodium-ion battery electrodes, sodium-ion capacitor electrodes, and sodium-based dual-ion battery electrodes. The manufacturers of hard carbon material products that are more mature in the market are mainly Japanese manufacturers.
Due to the small number of mass production companies in the world and the high import price of hard carbon, it is obviously not good for the development of China's sodium ion battery industry. Therefore, in recent years, the number of companies entering the hard carbon/soft carbon anode material market in my country has gradually increased.
For example: Shanshan has successfully developed a high-capacity, high-first-efficiency hard carbon anode material and is the first to realize industrialization; BTR has the ability to industrialize hard carbon anode materials and is building a mass production line.
Due to its unique structure, low cost, high capacity, and abundant sources, hard carbon has become the most commercially potential anode material for sodium-ion batteries. In addition, the practical application of hard carbon faces problems such as low first-week coulombic efficiency, insufficient cycle stability, and poor rate performance.
In recent years, in order to prepare high-level hard carbon materials, researchers have used modification methods such as morphology and structure design, electrolyte regulation, and pre-sodiumization to optimize the performance of hard carbon negative electrodes.
In short, with the characteristics of low cost and high safety performance, sodium-ion batteries are emerging in the context of dual carbon, and they are becoming more and more courageous in the wave of new energy transformation promoted by various countries. Although sodium-ion batteries are still far from being widely used, it is believed that through further ingenious design of materials such as hard carbon anodes, sodium-ion batteries are still promising in the future.