With the proliferation of microelectronic devices, the need for portable power supplies is evidently increasing. Possible candidates for micro energy storage devices are Li-ion batteries and supercapacitors. Among them, the flexible solid-state supercapacitors combined with exceptionally long cycle life, high power density, environmental friendline. ••A better practice by calculating released energy to evaluate material and device performance is proposed.••The review discussed electrode materials of solid-state supercapacitors.••The review emphasized novel configurations for high performance flexible solid-state supercapacitors.Energy storageFlexibleSolid-stateSupercapacitorDevices for the future electronics will be twistable and deformable, thereby enabling applications that would be impossible to achieve by using the hard, rigid electronics of today. Energy storage devices possessing flexibility, light weight, and even safety may meet the large proliferation of consumer electronics. Sustainable energy research is attracting increasing attention as our planet facing enormous challenges related to environment and energy. Electrochemical capacitors, or named as supercapacitors (SCs), combined with exceptionally long cycle life, very high power density and enhanced energy density, afford a smart maneuver,. Figure 1a shows the Ragone plot for the most important energy storage systems. SCs are used to provide high power in a short time, whereas batteries (commonly Li-ion battery, LIB) are preferred for long-time operation of a device. Compared with liquid electrolyte based SCs, solid-state SCs have many advantages, such as portability, environmental friendliness, flexibility and stability, which can broaden the application area of SCs,. Therefore the need for solid-state SCs arises, which expedites the development of stand-alone microelectronic devices and enhance the applicability in the in vivo systems. To enhance the power and energy density of these solid-state SCs new advanced concepts have been proposed, which are based on the exploration of new materials and architectures.EDLCIn EDLC, electrostatic energy storage is achieved by separation of charges in a Helmholtz double layer at the interface between the surface of a conductive electrode and an electrolyte. The distance of the static separation of charges in a double-layer is on the order of a few Angstroms which is extremely small. The key to reaching high capacitance by charging the double layer is using high specific surface area (SSA) and electronically conducting electrodes. During the discharge process, the voltage of an ideal EDLC decreases linearly, but an ideal LIB remains constant (Figure 1b). As a result, the energy stored in ideal EDLC is proportional to the voltage squared, whereas the energy stored in ideal LIB is just proportional to the voltage. In addition, the voltage provides a convenient measure of the state of charge in EDLC, but not in LIB. In the condition of EDLC, carbon materials and their derivatives have been investigated and used in EDLC electrodes. The EDLCs generally store charges through surface absorption (up to 0.17–0.20 electrons per atom at accessible surface), so they reveal high power density, long cycle life but low energy density.Pseudocapacitor.
