
In particular, inorganic anode materials such as Sn, metallic selenides, and hybrid materials have gained recognition as promising candidates for SIBs. 6 Among the carbonaceous materials, hard carbons are considered one of the most promising solutions for anode materials in SIBs due, among others, to their turbostratic structure, providing a high volume of closed porosity. 7 The exploitation of hard carbons as anode materials in SIBs has shown promising electrochemical energy storage performance, reaching specific capacity values of more than 300 mA h g −1 with a long plateau close to sodium's reduction potential. [pdf]
For SIB anode materials, hard carbon is the most mature and currently the only material likely to be commercialized, but it is still far away from large-scale industrialization. Herein, we carry out a comprehensive overview of the current state of the art in terms of three main aspects.
Hard carbon (HC) is recognized as a promising anode material with outstanding electrochemical performance for alkali metal-ion batteries including lithium-ion batteries (LIBs), as well as their analogs sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs).
In recent years, coal-based hard carbon has received widespread attention as an anode material for sodium-ion batteries [19, 20]. To date, coal-based hard carbon is a promising anode material for sodium-ion batteries due to its high storage capacity, appropriately low operating potential and relatively stable source.
Hard carbon still suffers from unclear sodium storage mechanism, unsatisfactory performance, and low initial Coulombic efficiency (ICE). Herein, the current state-of-the-art advances in designing hard carbon anodes for high-performance SIBs is summarized.
Hard carbon (HC) is a promising anode candidate for Na-ion batteries (NIBs) because of its excellent Na-storage performance, abundance, and low cost. However, a precise understanding of its Na-storage behavior remains elusive.
This indicates the existence of three types of sodium ion storage sites in the hard carbon anode.

Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance; full-cycle lifetimes quoted for flywheels range from in excess of 10 , up to 10 , cycles of use), high (100–130 W·h/kg, or 360–500 kJ/kg), and large maximum power output. The (ratio of energy out per energy in) of flywheels, also known as round-trip efficiency, can be as high as 90%. Typical capacities range from 3 to 1. [pdf]
First-generation flywheel energy-storage systems use a large steel flywheel rotating on mechanical bearings. Newer systems use carbon-fiber composite rotors that have a higher tensile strength than steel and can store much more energy for the same mass. To reduce friction, magnetic bearings are sometimes used instead of mechanical bearings.
Flywheel energy storage systems are considered to be an attractive alternative to electrochemical batteries due to higher stored energy density, higher life term, deterministic state of charge and ecological operation. The mechanical performance of a flywheel can be attributed to three factors: material strength, geometry, and rotational speed.
Flywheels, one of the earliest forms of energy storage, could play a significant role in the transformation of the electrical power system into one that is fully sustainable yet low cost.
In 1999 , the University of Texas at Austin developed a 7-ring interference assembled composite material flywheel energy storage system and provided a stress distribution calculation method for the flywheel energy storage system.
High-strength steel flywheels have a high energy density (volume-based energy) due to their high mass density. Furthermore, they are superior to composite ones regarding thermal conductivity and design data availability, such as SN curves and fracture toughness.
Thanks to the unique advantages such as long life cycles, high power density, minimal environmental impact, and high power quality such as fast response and voltage stability, the flywheel/kinetic energy storage system (FESS) is gaining attention recently.

A typical system consists of a flywheel supported by connected to a . The flywheel and sometimes motor–generator may be enclosed in a to reduce friction and energy loss. First-generation flywheel energy-storage systems use a large flywheel rotating on mechanical bearings. Newer systems use composite Even if a carbon fiber flywheel is only 50% efficient it has the ability to store and provide more energy than Tesla's Li-ion battery with comparable mass. There would also be additional mass needed to house the flywheel and mechanisms, but these should be small compared to the maximum limit of energy storage. [pdf]
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