Sodium-ion batteries are the most promising solution for low-cost, short- and medium-term, large-scale energy storage needs, but their practical applications are restricted by the high manufacturing cost, low capacity, low initial coulombic efficiency, and slow kinetics of hard carbon anodes. The research and development of hard carbons focus on overcoming the following issues: The active structures for Na-ion storage are unclear, which restricts the structural designs of hard carbons. On the other hand, there lack of hard carbon materials of low costs, high-sodium storage capacities, and scalable preparation.

In view of this, the staff of the Environmental Science and Technology Laboratory of Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, developed a synthesis strategy for heteroatom pyrolysis to construct closed pores for sodium storage (Fig. 1). The optimized hard carbon material has abundant pores (0.5∼0.9 nm) and surface carbonyl functional groups. The special pore structure significantly increases the plateau capacity, while the carbonyl functional groups induce the inorganic-rich solid electrolyte interface (SEI) to improve the initial coulombic efficiency (ICEs). The prepared hard carbon delivers a high reversible capacity of 352.9 mAh/g and an ICE of 88.0%. The related research was published in Energy Storage Materials with the title of "Molecular engineering of pore structure/interfacial functional groups toward hard carbon anode in sodium-ion batteries". (Energy Storage Materials,2025, 75, 104008)

Figure 1. Heteroatom pyrolysis strategy for the closed pores of sodium-ion storage.

Plateau capacities of hard carbons are critical to achieving high energy densities of sodium-ion batteries. However, the loss of plateau capacities due to high polarizations at high current densities severely limit the further development of sodium-ion battery technology. Ultra-micropores could provide high plateau capacities at high current densities, but the construction of ultra-micropores is always accompanied by a large number of micropores and mesopores, which seriously reduces the plateau capacities and the ICEs. The research team prepared a hard carbon with ultra-micropores of concentrated pore sizes of 0.4–0.8 nm through a protonic-mediated strategy (Fig. 2). Results show that P heteroatoms weaken the interaction between N and C atoms, which leads to the N pyrolysis in terms of NH3 small molecules, resulting in the formation of ultra-micropores with specific pore sizes. The developed hard carbon has a reversible capacity of 386 mAh/g, a high plateau capacity of 173 mAh/g at a current density of 20 mA/g, and a good rate performance of 106 mAh/g at a current density of 2 A/g. The result was published in Small, entitled "Uncovering the Salt‐Controlled Porosity Regulation in Coal-Derived Hard Carbons for Sodium Energy Storage". (Small,2025, 21, 2409116)

Figure 2. Strategies for the ultra-micropore construction.

Based on the above research results, the research team used typical Zhundong coal and biomass of Xinjiang as raw materials to carry out core research and development of sodium storage hard carbon products. In order to solve the problem that the dense structure of coal aromatic ring π-π accumulation is difficult to control and construct the microcrystalline and pore structure of sodium storage, different coal structures were systematically analyzed, and the hard carbon with a high reversible capacity (340 mAh/g) and a high ICE (88%) was successfully developed by using Zhundong coal as the precursor, by taking advantage of the in-situ template activation effect of salt and the volatilization and calolysis of aliphatic chain hydrocarbons in the coal. On the other hand, the team utilized Xinjiang typical cotton straw and fruit wood as feedstock, engineered with molecular level pyrolysis strategy for pore creation, and developed a high-rate biomass-based hard carbon material. The current research progress not only provides new ideas for the basic research on the structure design of hard carbons, but also provide new solutions for achieving the sodium-ion batteries of high-energy densities and high-power densities. The result was published on Journal of Materials Chemistry A, with a title of “Ultra-micropores of hard carbons for ultrafast Na-ion storage”. (Journal of Materials Chemistry A,2025)