Among the different technologies developed along the last decades, batteries and electrochemical capacitors succeed to stablish in the market as competitive solutions. Lithium-ion batteries (LIBs) are the technology of choice when high energy density is required, while electrochemical double layer capacitors (EDLCs) are the alternative for high power applications. However, a considerable energy to power gap yet exists between these two technologies, that needs to be covered in order to give response to new high-energy and high-power demanding applications.
Alternative technologies are on the race to reach the energy to power threshold while providing a competitive cycle life to capture this emerging market share. Lithium-ion capacitor (LIC) technology is a serious candidate to do so. Being a hybrid technology born from the union of LIBs and EDLCs, it is able to store mid energy values at high power densities while maintaining a cycle life approaching that of EDLCs.
These types of capacitors have the potential to act as a bridge between batteries and supercapacitors; a key factor in the process of decarbonizing the economy towards electric mobility and sustainable generation based on renewable energies, as well as the Internet of Things. These kinds of challenges demand storage systems that have both high energy and power density, as well as long lasting cyclability, security and a competitive cost.
Since LICs were first reported in the beginning of the 2000s, academic research has been mainly focused on the increase of the energy density. Sometimes, even sacrificing power and cyclability for the sake of it, reporting only faulty batteries. However, especially in the last years, several LICs based on novel materials and configurations, presenting well balanced energy, power and cyclability features have been reported.
Figure 1. Qualitative Ragone plot reported in normalized terms of mass of the whole device. The shaded areas are reported in gravimetric terms. Energies 2021, 14(11), 3010; https://doi.org/10.3390/en14113010.
LICs can play an important role in applications that need high power capabilities and a considerable amount of energy content. Bus, metros, trams, load levelling, back-up power and cranes are some examples of perfect candidates. The commercial application of current LICs is still impeded by their inferior energy density, which is mainly due to the low capacity of the cathode. Therefore, tremendous efforts have been made in developing novel cathode materials with high capacity and excellent rate capability.
Graphene-based nanomaterials have been recognized as one of the most promising cathodes for LICs due to their unique properties, and exciting progress has been achieved. It should be noted that graphene has comparable or even superior properties to other nanocarbon-based materials, making it an excellent candidate either as a high-performance active material or as an attractive flexible support to load other materials for applications in LIBs, SCs and hybrid devices.