Electrochemical energy storage (EES) is an essential technology in modern lifestyle. As two typical EES devices, batteries and supercapacitors have their own advantages and disadvantages. Pseudocapacitive materials have attracted recent attention. Such materials store charges via battery-like redox reactions, but have supercapacitor-like electrochemical behaviors. Therefore, pseudocapacitive materials have a considerable potential to simultaneously achieve both high energy and high power density as well as excellent stability, merging a unbridgeable gap between batteries and supercapacitors. However, the development of pseudocapacitive materials still encounter some serious challenges such as unclear energy storage mechanism, lack of alternative pseudocapacitive materials and vague future development directions. This review represented recent development on pseudocapacitive materials and briefly discussed their energy storage mechanism and the imperfections of the existing issues. Furthermore, we proposed a concept of bulk pseudocapacitance and realized a goal for the future development of pseudocapacitive materials. In addition, we also discussed some key factors and strategies for achieving such a bulk pseudocapacitance.The energy storage mechanisms of pseudocapacitive materials were discussed. The first pseudocapacitive material, i.e., RuO2 thin film, was reported by Trasatti et al. in 1971. The thin film electrode exhibited a supercapacitor-like cyclic voltammetry curve but differed from the electric double layer in faradic origin. Although the electrochemical performance of RuO2 electrodes were enhanced via fabricating various nanostructures and introducing structural water, the energy storage mechanism of this electrode was not fully understood, and limited to reversible redox reaction at the surface, the content of structural water in crystal, fast proton transport and crystallinity of electrode, etc.. MnO2 is another typical pseudocapacitive electrode, which was firstly reported by Lee and Goodenough in 1999. Simon and Gogotsi reported the pseudocapacitive response of this electrode to successive multiple surface redox reactions. Recently, Augustyn et al. indicated the pseudocapacitive response to the existence of nanoconfined interlayer structural water, mediating an interaction between charge carrier and electrode. Dunn et al. investigated the pseudocapacitive response for T-Nb2O5 electrode and proposed a model of pseudocapacitance, i.e., intercalation pseudocapacitance. Mxenes is one of pseudocapacitive materials with a unique structure that is responsible for its pseudocapacitive behaviors. Although it is still controversial whether it is a pseudocapacitive electrode, Co(OH)2 demonstrates a coupled performance of batteries and supercapacitors. The energy storage mechanism of this electrode was investigated by in-situ X-ray adsorption spectroscopy. It was found that the high specific capacitance originates from a battery-mimic bulk reaction, while the high stability performance roots in the crystal structure stability in charge-discharge process. Furthermore, we highlighted that the Co(OH)2 electrode blurs the distinction between supercapacitors and batteries, which is in accordance with the continuous transition from electric double layer to battery proposed by Fleischmann and co-workers. In future, more intensive studies should be conducted on the energy storage mechanism of pseudocapacitive materials, especially on the unique characteristics of charge carrier and electron transportation, charge transfer, interaction between charge carrier and electrode in an atom-level.The energy density of pseudocapacitive materials is still far below than the actual demand, even the batteries. To address this issue, a concept of bulk pseudocapacitance is proposed. In fact, intercalation pseudocapacitance depicted by Dunn et al. is a typical example of bulk pseudocapacitance. Bulk pseudocapacitance refers to the pseudocapacitive response that happens throughout the bulk of the electrode material rather than the surface. Meanwhile, the proposal of bulk pseudocapacitance can provoke a research on boosting the pseudocapacitive response of battery materials to alleviate the lack of the alternative pseudocapacitive materials. Reducing the size of active materials to nano-scale, i.e., nanostructured material, as one of the strategies, can be used to achieve the bulk pseudocapacitance. For instance, LiCoO2 nanocrystalline with the sizes of ?11 nm exhibits a pseudocapacitive response. In addition to nanostructured materials, a strategy named diffusionless-like conversion reaction to achieve pseudocapacitive response in conversion reaction was proposed. Based on the conversion reaction between Fe(OH)2 and δ-FeOOH, a battery-like conversion reaction achieves a high specific capacitance, and a diffusionless-like transformation between charge and discharge phases without a massive atom movement presents high rate and stability performance. For battery materials, their pseudocapacitive response can be enhanced via manipulating the charge carrier transport, electron transfer, and designing the interaction between charge carrier and electrode in atom-scale. In addition, such a provoking pseudocapacitive response in battery materials extends the scope of bulk pseudocapacitive materials. Li and co-workers enhanced the Li+ transport by pre-intercalation of NH4+ into interplay of Mo2CTx. The mechanism of FeHCF storing NH4+ is clarified, indicating that the weak Fe—N interaction mitigates the volumetric expansion induced by NH4+ insertion. These findings demonstrate that modulating on charge carrier transport and interaction between charge carrier and electrode can improve the pseudocapacitive response of battery materials. Summary and prospects Future development of pseudocapacitance relies on a deep understanding of the energy storage mechanism of pseudocapacitive materials in an atomic level, i.e., the charge carrier and electron transport, charge transfer, and the interaction between charge carrier and electrode. In addition, exploiting and designing novel pseudocapacitive materials as well as boosting pseudocapacitive response of existing battery materials are crucial for the application of pseudocapacitive materials.