Nowadays, with the rapid development of human society, the exploration of green, economic and sustainable energy storage devices has become an indispensable demand. At present, the most studied energy storage devices mainly include various batteries and capacitors, such as lithium-ion batteries, sodium-ion batteries, zinc-ion batteries, electric double-layer capacitors, and pseudocapacitors [
1–
9]. For energy storage devices, one of the most important components is their various energy storage materials. Such as for lithium-ion batteries, the energy storage materials include the widely studied LiCoO
2- or LiFePO
4-based cathode materials and the graphite or silicon-based anode materials [
10–
12]. For supercapacitors, the electrode materials mainly include carbon material, metal oxide, and conductive polymer [
13–
15]. To pursue energy storage materials with greater capacity, high power, better safety and non-pollution, it is also necessary to develop various testing techniques to systematically evaluate the energy storage materials/devices. For example, the cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) testing techniques can be used to characterize the charge-discharge voltage, capacity, rate, cycle and other properties of energy storage materials. The variations of morphology and volume of energy storage materials before, during and after charging and discharging process can be intuitively described by scanning electron microscope (SEM) and atomic force microscope (AFM). Park
et al. clearly characterized the volume increase and shrinkage, and the damage of the silicon-based lithium-ion battery anode particles during the charging and discharging process by SEM technology [
16]. The change of aliovalent cation valence state and the oxygen vacancy concentration in energy storage materials can be (semi-)quantitatively evaluated by XPS, EPR and chemical titration methods [
17,
18]. For different charge carriers charging/discharging in energy storage materials, the possible insertion site, transport route and activation energy can be calculated by theoretical computation. For example, the voltage profile for the discharge/charge curves can be described by the density-of-state diagram, which is mainly based on the corresponding positions of the bottom/top band gap and the aliovalent cation redox couples relative to the Fermi energy of lithium [
19]. The possible changes in the crystal structure, bond length/angle, phase transition,
etc. of energy storage materials during the charge and discharge process also can be characterized by XRD, TEM and Raman testing techniques [
20–
22]. Despite much progress, as mentioned above, has been made in the past decades to get a more accurate assessment for energy storage materials, however, a clear understanding of the relationship between the charge-discharge process of energy storage materials and the corresponding changes of energy band structure is still lacking.