image: Nanopore engineering strategy is designed for tuning the pore architecture of hard carbons. These hard carbons with different pore architectures possess remarkably different sodium-ion storage kinetics and cycling stability.
Credit: ©Science China Press
With the widespread adoption of electric vehicles and large-scale energy storage facilities, the demand for batteries is increasing significantly. Although lithium-ion batteries have outstanding performance, lithium resources are limited. Sodium-ion batteries are seen as the rising star of large-scale energy storage because of their abundance and low cost. Hard carbon is the most promising commercial anode material, but its densely packed internal pore structure has always been like a "black box". This closed-pore “black box” is difficult to regulate and characterize. In the past, scientists did not fully understand how the size and architecture of the pores affect the rate of charge and discharge and the cycling stability of hard carbon anode. The research team from Guangdong University of Technology used lignin precursor to ingeniously modify the internal micropore structure of hard carbon, uncovering the secrets between the pore structure and battery performance, and providing new ideas for the design of high-performance sodium-ion batteries.
The size of the nanopores determines the sodium-ion storage performance of the battery.
To solve this mystery, the research team selected four different lignin-based precursors: alkaline lignin, phenolic resin, lignin-phenolic resin and sodium lignosulfonate. By regulating their molecular aggregation structures and chemical compositions, they precisely designed hard carbon materials with different pore structures like building blocks. The intermolecular distance of alkaline lignin is large, and the resulting hard carbon nanochannels after carbonization are also large (about 1.7–2 nm). When alkaline lignin is polymerized with phenol to form a more compact lignin-phenolic resin, the carbonized hard carbon pore diameter is reduced to approximately 1.5 nm. By taking advantage of the self-template effect of lignosulfonate, hard carbon with the largest pore diameter (about 2 nm) can be fabricated.
It is found that the HC with smaller closed pores and more abundant pores is easier to form sodium clusters and conduct charge faster, which can provide ion-diffusion channels. It has been confirmed that if the graphene layer of HC has many defects, the proportion of sp3 hybrid carbon is high, and the size of nanopores is large, the internal nanostructure is easily destroyed when the charging and discharging cycle is repeated. The graphene layer becomes disordered, the pore volume decreases, and the reversible sodium-ion storage capacity will rapidly decline. What is even more alarming is that the disordered graphene layer will impede the passage of sodium ions and increase the risk of metal sodium dendrite formation, laying a safety hazard of battery short circuit.
The nano-pore structure determines the "running speed" of sodium ions.
The speed of battery charging and discharging essentially depends on how smoothly sodium ions move within the anode material, which is related to the reaction kinetics. Sodium ions adsorb and intercalate into the graphene layer in the slope-potential region, and the reaction rate is very fast. However, in the plateau-potential area, sodium ions form metallic sodium clusters in the closed pores. This process presents greater resistance and is also a weakness that limits the overall charging and discharging performance of HC anode. The team, with the help of advanced characterization methods such as small Angle X-ray scattering (SAXS), directs the whole process of sodium-ion storage. Combined with theoretical calculations, it is found that the geometric structure of nanopores will directly change the difficulty of forming sodium clusters. In closed pores with smaller pore diameters, the formation energy of sodium clusters is lower, the formation potential is higher, and the resistance to charge transfer is significantly reduced. The densely interwoven pore network can also create more diffusion pathways for sodium ions, making the ion migration smoother. In contrast, larger pores create larger sodium clusters, which not only form with higher energy, but also lead to greater electrochemical polarization. At large current, the intensified polarization will depress the formation potential of sodium clusters, even approaching 0 V, which can easily induce the precipitation of sodium metal, which leads to safety hazard. The test of sodium-ion diffusion coefficient also proved that the sodium-ion diffusion capacity of HC anode with small-sized pores and high-density pore structure was significantly better than that of large-pore HC anodes.
This study elucidated the relationship of structural and activity between the pore structure of HC and sodium-ion storage performance, providing theoretical guidance for the design of high-performance HC anodes. By precisely regulating the molecular structure of the precursor, constructing closed pores with small-pore diameters, high density and stable structure is the key to achieving long cycling life of sodium-ion batteries.