Sodium-ion batteries are attracting much interest due to their potential as viable future alternatives for lithium-ion batteries, in view of the much higher earth abundance of sodium over that of lithium. Although both battery systems have basically similar chemistries, the key celebrated negative electrode in lithium battery, namely, graphite, is unavailable for the sodium-ion battery due to the larger size of the sodium ion. This need is satisfied by "hard carbon", which can internalize the larger sodium ion and has desirable electrochemical properties. Unlike graphite, with its specific layered structure, however, hard carbon occurs in diverse microstructural states. Herein, the relationships between precursor choices, synthetic protocols, microstructural states, and performance features of hard carbon forms in the context of sodium-ion battery applications are elucidated. Derived from the pertinent literature employing classical and modern structural characterization techniques, various issues related to microstructure, morphology, defects, and heteroatom doping are discussed. Finally, an outlook is presented to suggest emerging research directions.

A unique morphology-controlling protocol involving hydrothermal preprocessing before pyrolysis is presented to convert cellulosic waste in the form of sugarcane bagasse into three-dimensional (3D) interconnected, conducting, and high surface area carbon nanochannels. The large buffer spaces in such a porous carbon sample yield impressive electrochemical capacitance (C sp ) of 280 F/g at a current density of 1 A/g (and 275 F/g at 5 mV/s), with 72% retention even at a very high current density of 20 A/g. In contrast, the non-hydrothermally treated sample exhibits a C sp value of 180 F/g at 1 A/g and only 52% retention at 20 A/g. The much better performance of the hydrothermally preprocessed bagasse-derived carbon (BHAC) can be ascribed to the solvent retention inside the buffer spaces created, negating the diffusional limitations of pore inaccessibility at higher scan rates. The BHAC has a high surface area of 1260 m 2 /g with a fairly good (11 wt %) concentration of oxygen functionality. The material renders a good energy density of 5 Wh/kg at a power density of 3.5 kW/kg and shows good cyclability of 90% after 1000 charge/discharge cycles.

The transformation of the globular structures of yogurt into a porous high-capacitive carbon material, displaying the perfect double layer capacitive behavior.

In this study, the potential of conversion-type anode materials for alkali-ion batteries has been examined and analyzed in terms of the parameters of prime importance for practical alkali-ion systems. Issues like voltage hysteresis, discharge profile, rate stabilities, cyclic stabilities, irreversible capacity loss, and Columbic efficiencies have been specifically addressed and analyzed as the key subjects. Relevant studies on achieving a better performance by addressing one or more of the issues have been carefully selected and outlook has been presented on the basis of this literature. Mechanistic insights into the subject of conversion reactions are discussed in light of the use of recent and advanced techniques like in situ transmission electron microscopy, in operando X-ray diffraction, and X-ray absorption spectroscopy. Three-dimensional plots depicting the performance of different materials, morphologies, and compositions with respect to these parameters are also presented to highlight the systematic of multiparameter dependencies. Inferences are drawn from these plots in the form of a short section at the end, which should be helpful to the readers, especially young researchers. We believe that this study differs from others on the subject in being focused toward addressing the practical limitations and providing possible research directions to achieve the best possible results from conversion-type anode materials.