New insights into the effect of hard carbons microstructure on the diffusion of sodium ions into closed pores

Closed pores formed in hard carbons play an essential role in sodium storage at plateau region.However,the effect of different structural features on the diffusion of sodium ions into closed pores remains unclear.Herein,a precursor reconstruction strategy is conducted to regulate carbon microstructures including interlayer spacing,defect concentration,and closed pore volume by changing the ratio of aromatic and polysaccharide components.Aromatic structure parts tend to develop disordered carbons with fewer defects,larger interlayer spacing,and smaller closed pore volume,while polysaccharide components prefer to form disordered carbons with more defects,smaller interlayer spacing,and larger closed pore volume.Through the correlation analysis of microstructure features and the sodium storage capacity below 0.1 V.It finds that the intercalation capacity is proportional to the ratio of pseudo-graphitic domains,whereas the pore filling capacity appeared at lower potential gradually decreases with the increasing defect concentration due to homo-ionic repulsion effect,without linear correlation with shortrange microcrystalline and closed pore volume.The optimized sample with suitable interlayer spacing and defect concentration exhibits a high plateau capacity of 241.7 m Ah/g.This work provides insights into the exploitation of closed pore sodium storage performance.

Free radicals trigger the closure of open pores in lignin-derived hard carbons toward improved sodium-storage capacity

The chemical activation of various precursors is effective for creating additional closed pores in hard carbons for sodium storage.However,the formation mechanism of closed pores under the influence of pore-forming agents is not well understood.Herein,an effective chemical activation followed by a high-temperature self-healing strategy is employed to generate interconnected closed pores in lignin-derived hard carbon(HCs).By systematic experimental design combined with electron paramagnetic res-onance spectroscopy,it can be found that the content of free radicals in the carbon matrix influences the closure of open pores at high temperatures.Excessively high activation temperature(>700 C)leads to a low free radical concentration,making it difficult to achieve self-healing of open pores at high tempera-tures.By activation at 700°C,a balance between pore making and self-healing is achieved in the final hard carbon.A large number of free radicals triggers rapid growth and aggregation of carbon microcrys-tals,blocking pre-formed open micropores and creating additional interconnected closed pores in as-obtained hard carbons.As a result,the optimized carbon anode(LK-700-1300)delivers a high reversible capacity of 330.8 mA h g^(-1) at 0.03 A g^(-1),which is an increase of 86 mA h g^(-1) compared to the pristine lignin-derived carbon anode(L-700-1300),and exhibits a good rate performance(202.1 mA h g^(-1) at 1 A g^(-1)).This work provides a universal and effective guidance for tuning closed pores of hard carbons from otherprecursors.

Fabrication and Properties of Closed-pore Al_2O_3-based Refractory Aggregates

A novel Al2O3 based refractory aggregate with closed-pore structure was fabricated utilizing superplastic- ity with submicro-sized Al2O3 and MgO as raw materi- als, and SiC as a high temperature pore-forming agent. The effect of MgO on porosity, phase composition and microstructure of the refractory aggregate has been inves- tigated. For comparison, the common Al2O3-based refractory aggregates and porous ones with open-pore structure were also prepared. The results indicate that the closed porosity of Al2O3-based refractory aggregate in- creases as the content of MgO increases. When the con- tent of MgO is 15 mass%, the closed and apparent poros- ities are 14.5% and 1.1%, respectively. The main phase compositions are Al2O3 and MgAl204. The formation mechanism of closed pores is that the fine-crystalline- grain Al2O3 ceramic possesses superplastic deformation ability after adding MgO at high temperatures. When SiC powder is added to the Al2O3 ceramic, the generated gases by the reaction of SiC at the sintering temperature can provide a pressure to make grain boundaries slide. Then, the gases are enclosed by crystalline grains to form the closed pores. The slag corrosion resistance of the fabrica- ted closed-pore Al2O3-based refractory aggregate is better than the common refractory aggregate and porous ones.

Regulating closed pore structure enables significantly improved sodium storage for hard carbon pyrolyzing at relatively low temperature

The closed pore has been considered as the key structure for Na ion storage in hard carbon.However,the traditional view is that closed pores can only be formed by the curling of graphite-like crystallites in the case of high temperature carbonization.Ingenious designing of closed pore structures at lower temperature is still blank.Herein,for the first time,engineering the wall thickness and number of closed pores in waste rosewood-derived hard carbon was successfully achieved at a low temperature of 1100℃ by removing the lignin and hemicellulose components in wood precursor.When applied as an anode material,the optimum sample exhibits a high capacity of 326 mAh/g at 20 mA/g and a remarkable rate capability of 230mAh/g at 5000 mA/g,significantly higher than those of the untreated sample(only 33 mAh/g at 5000 mA/g).The significantly improved Na storage performance should be attributed to abundant closed pores that provide sufficient spaces forNa storage and thin porewall structure that is beneficial to the diffusion of Na^(+)in the bulk phase.This work provides a new idea for the future application of biomass-based hard carbon for advanced Na ion batteries.

Hybrid hard carbon framework derived from polystyrene bearing distinct molecular crosslinking for enhanced sodium storage

Exploiting high-performance yet low-cost hard carbon anodes is crucial to advancing the state-of-the-art sodium-ion batteries.However,the achievement of superior initial Coulombic efficiency(ICE)and high Na-storage capacity via low-temperature carbonization remains challenging due to the presence of tremendous defects with few closed pores.Here,a facile hybrid carbon framework design is proposed from the polystyrene precursor bearing distinct molecular bridges at a low pyrolysis temperature of 800℃ via in situ fusion and embedding strategy.This is realized by integrating triazine-and carbonylcrosslinked polystyrene nanospheres during carbonization.The triazine crosslinking allows in situ fusion of spheres into layered carbon with low defects and abundant closed pores,which serves as a matrix for embedding the well-retained carbon spheres with nanopores/defects derived from carbonyl crosslinking.Therefore,the hybrid hard carbon with intimate interface showcases synergistic Na ions storage behavior,showing an ICE of 70.2%,a high capacity of 279.3 mAh g^(-1),and long-term 500 cycles,superior to carbons from the respective precursor and other reported carbons fabricated under the low carbonization temperature.The present protocol opens new avenues toward low-cost hard carbon anode materials for high-performance sodiumion batteries.

Fabrication and properties of lightweight zirconia with fine closed porosity

Lightweight refractory materials with thermal insulation properties and erosion resistance are advantageous for hightemperature applications.Lightweight zirconia refractories were prepared using starch as a pore-forming agent,basic magnesium carbonate as a stabilizer,and nano-zirconia as an additive.The effects of the nano-zirconia content on the pore and thermal insulation properties of the lightweight zirconia refractories were investigated based on the porosity,phase composition,microstructure,and thermal conductivity.Nano-zirconia was shown to have a high surface energy,and its addition effectively increased the driving force for sintering,reduced the sintering temperature,and promoted the sintering reaction,thereby reducing the apparent porosity and improving the density of the prepared material.Owing to the superplasticity of nano-zirconia,the surface stress caused plastic deformation between particles,which increased the migration rate of grain boundaries and trapped more gas inside the material before it diffused to the surface,thereby enhancing the closed porosity of the material.The presence of closed pores could extend the thermal conduction path,decrease the conduction rate,and hinder the conduction effect to effectively reduce the thermal conductivity of the material.At a nano-zirconia content of 0.75 wt.%,the prepared lightweight zirconia had the highest closed porosity and the lowest thermal conductivity.The apparent porosity,closed porosity,and total porosity of the material were 2.8%,7.0%,and 9.8%,respectively,and the thermal conductivity at 800℃was 1.37 W m^(-1)k^(-1).