The Li-ion battery (LIB) market increases rapidly due to the development of portable electronic devices and energy storage systems (ESSs). By increasing the demand for commercialization of scale-up LIB, large amounts of accumulated lithium resources will be consumed in the near future. Recently, Na-ion battery (NIB) system is considered as an alternative to LIB system because sodium resources are abundant and distributed around the world as well as sodium storage mechanism is similar to that of lithium. In contrast to LIB, graphite is not suitable for NIB anode material due to the small interlayer for large ionic radius of Na⁺ ion. Therefore, previous studies have investigated that non-graphitic carbon materials can deliver reversible Na⁺ ion insertion and extraction with initial capacity up to 300mAh g^-1, however, the cycle stability of non-graphitic carbon materials is very poor.

Ordered mesoporous carbon (OMC) has the advantage of large surface area due to the development of nano-sized pores inside a micron-sized particle, which results in large electrode/electrolyte interface, easy penetration of electrolyte, and fast diffusion of Li⁺/Na⁺ ion. The ordered hexagonal structure of the OMC was formed as an inverse replica of a mesoporous silica SBA-15 structure and started from phenanthrene as a carbon source. In carbonate-based electrolyte system, the first and second discharge capacities of the OMC were 800 mAh g^-1 and 350 mAh g^-1 at a current density of 100 mA g^-1, respectively, which means a large irreversible capacity loss, whereas in ether-based electrolyte system, the first irreversible capacity decreased considerably, implying an enhanced reaction reversibility. Even though the first discharge capacity in the ether-based electrolyte was about 200 mAh g^-1, the capacity in the ether-based electrolyte increased during repeated cycling and achieved 350 mAh g^-1 after 100 cycles. In contrast, the cycle retention in carbonate-based electrolyte degraded down to 200mAh g^-1 consistently during 100 cycles.

To elucidate the electrolyte system-dependent Na⁺ storage of the OMC, various electrochemical analyses and synchrotron X-ray based techniques were performed using two-type electrolytes of 1 M NaPF₆ in EC/DMC=1/1 and 1 M NaPF₆ in DEG/DME.