First, let's talk about why we need to develop sodium-ion batteries.
my country's batteries are mainly used in three major industries: electric vehicles, energy storage, and consumer electronics. Electric vehicles and energy storage have seen rapid development in recent years, primarily driven by lithium-ion batteries. Power battery production grew by about 80% in 2015, and exceeded 30 GWh in 2016. This has led to prominent issues regarding lithium battery disposal and recycling, and the limited availability of lithium resources. Meanwhile, energy storage is also a rapidly developing industry, especially in microgrids, where there is a significant demand for energy storage. It is projected that by 2020, energy storage will grow threefold compared to 2015. Such a large demand for batteries presents two problems if all batteries are lithium-ion batteries: lithium resource scarcity and lithium recycling. Therefore, we need new options beyond lithium batteries. This involves choosing the right battery system and materials. Based on this consideration, could we find a system with abundant reserves and cheaper materials? Ultimately, we chose sodium-ion batteries.
Sodium-ion batteries and lithium-ion batteries have similar reaction mechanisms. Besides phosphates or fluorinated phosphates, nickel-manganese layered transition metal oxides can also be used as cathode materials. For anode materials, carbon-based materials, alloys, and compounds can be selected. Among these three main categories, we chose the cheapest carbon materials. We further studied carbon anode materials by classifying them into three categories: soft carbon, hard carbon, and graphene.
One of our recent research findings involves using a layered structure Na0.67Ni0.33-xMxMn0.67O2 as the cathode material. Through experimental research and comparison, we believe that acetate or oxalate are better choices for cathode raw materials. According to literature reports, cathode materials using only nickel-manganese oxide exhibit poor cycle performance and stability at high charging potentials. Therefore, some literature reports the use of magnesium doping to replace nickel sites, which is expected to increase capacity. This method is very helpful for obtaining high-energy-density sodium-ion batteries. Besides magnesium, can other elements be used for doping? We selected elements with similar ionic radii to the replacement elements for doping. For example, to replace nickel sites, we chose zirconium (Zr) and copper (Cu) ions. The electrochemical and cycle performance of the material improved after doping compared to before doping. Compared to Zr doping, Cu doping showed better cycle stability.
Regarding the anode, since there are many methods for processing soft carbon materials, we experimented with phosphorus-doped soft carbon. Phosphorus doping increased the discharge capacity by over 30% and improved cycle characteristics. Why does phosphorus doping improve material performance? This is because phosphorus doping increases the number of active sites for sodium adsorption. In addition to the traditional intercalation reaction, it adds more active sites for sodium ion adsorption. Furthermore, for hard carbon, we selected biomass materials such as coconut shells and apricot shells, and through processing, obtained hard carbon materials. Raman analysis revealed that these materials have a short-layer ordered and long-layer disordered structure with a large interlayer spacing, suitable for sodium ion intercalation. Cycling experiments showed that after 200 cycles, the capacity showed almost no decay, demonstrating excellent cycle stability. Therefore, these biomass materials are excellent and inexpensive anode materials for sodium-ion batteries. We also conducted research on graphene anodes. The biggest problem with graphene is its relatively low density; whether it can be used to create batteries with high volumetric energy density remains a question. Therefore, we can consider combining graphene with other anode materials such as hard carbon, soft carbon, and compound or alloy materials.
We fabricated two types of pouch cells, 1.5Ah and 0.5Ah, using the aforementioned nickel-manganese oxide as the positive electrode and biomass-based hard carbon material as the negative electrode. After 300 cycles, the capacity decayed to 15%. This demonstrates that sodium-ion batteries can be fabricated using inexpensive materials and exhibit good electrical performance.
In summary, for sodium-ion battery cathode materials, we doped nickel-manganese oxide to improve its electrical performance. For anode materials, we investigated three materials: hard carbon, soft carbon, and graphene. Doping soft carbon with phosphorus can improve capacity; hard carbon materials show good cycle stability; graphene has high capacity but low initial efficiency. Finally, we expect that sodium-ion batteries based on inexpensive materials will achieve energy densities approaching or exceeding those of lithium iron phosphate batteries, finding applications in electric vehicles and energy storage.
