The synthesis of cyclic carbonates from epoxides and carbon dioxide using metal-free catalyst systems is critically reviewed.

superior performance and have been used for practical applications. [13] Recently, hard carbons including highly graphitized hard carbons and porous hard carbons have been widely investigated, and a specific capacity of 200-500 mA h g −1 , initial Coulombic efficiency (ICE) in the 30-80% range, and an acceptable rate capability were achieved. [14][15][16][17][18] However, these three key indicators of electrochemical performance are often contradictory. For example, the high rate capability of porous hard carbons is always accompanied by a low ICE, whereas the opposite is obtained for highly graphitized hard carbons. This phenomenon originates from the complex sodium-storage behaviors arising from the diversity of the active sites in hard carbons. The control of homogeneous active sites of hard carbons remains a great challenge and requires ongoing research effort.Sodium storage behaviors are mainly divided into three categories: adsorption on the edges, defects, and functional groups, insertion in the graphitic interlayers, and pore filling. In the charge curve of anode materials, highly graphitized hard carbons normally exhibit two regions: a low-potential plateau (0.01-0.10 V) and a high-potential slope (0.10-1.00 V). The reversible capacity of the materials is mainly due to the plateau region. However, this plateau capacity is easily affected by polarization at a high current density, resulting in a decreased sodium storage capacity. Although the correspondence of sodium storage behaviors to different potential regions has been controversial, it is known that the diffusion kinetics of Na + can be improved by the introduction of porosity as an effective strategy for reducing the influence of polarization. [13,[19][20][21] Previous studies have shown that porous hard carbons normally have high reversible capacity and high rate capability, yet low ICE due to a large irreversible capacity loss arising from numerous defects and a large specific surface area. [13,[22][23][24] Their sodium storage behaviors generally exhibit a slope potential region (0.01-3.00 V), but the charge specific capacity below 1.00 V is usually lower than 50%. [18,25,26] In practical use, the charge capacity below 1.00 V of a halfcell is considered to determine its realistic capacity, and a Hard carbon attracts considerable attention as an anode material for sodiumion batteries; however, their poor rate capability and low realistic capacity have motivated intense research effort toward exploiting nanostructured carbons in order to boost their comprehensive performance. Ultramicropores are considered essential for attaining high-rate capacity as well as initial Coulombic efficiency by allowing the rapid diffusion of Na + and inhibiting the contact of the electrolyte with the inner carbon surfaces. Herein, hard carbon nanosheets with centralized ultramicropores (≈0.5 nm) and easily accessible carbonyl groups (CO)/hydroxy groups (OH) are synthesized via interfacial assembly and carbonization strategies, delivering a large capacity (318 mA h ...