The incorporation of atomic scale defects, such as cation vacancies, in electrode materials is considered an effective strategy to improve their electrochemical energy storage performance. In fact, cation vacancies can effectively modulate the electronic properties of host materials, thus promoting charge transfer and redox reaction kinetics. Such defects can also serve as extra host sites for inserted proton or alkali cations, facilitating the ion diffusion upon electrochemical cycling. Altogether, these features may contribute to improved electrochemical performance. In this review, the latest progress in cation vacancies‐based electrochemical energy storage materials, covering the synthetic approaches to incorporate cation vacancies and the advanced techniques to characterize such vacancies and identify their fundamental role, are provided from the chemical and materials point of view. The key challenges and future opportunities for cation vacancies‐based electrochemical energy storage materials are also discussed, particularly focusing on cation‐deficient transition metal oxides (TMOs), but also including newly emerging materials such as transition metal carbides (MXenes).

In this study, a novel Co 3 O 4 /Co(OH) 2 heterostructure is obtained via electrodeposition on nickel (Ni) foam, forming sandwich-like structure and freestanding electrode. The outer Co(OH) 2 with layered structure can provide sufficient absorption sites and enable facile ion intercalation, meanwhile the presence of a conductive and robust interfacial Co 3 O 4 layer between Ni foam and Co(OH) 2 is found effectively minimizes the charge transfer resistance and stabilizes the interface, thus improving the electrode's rate and cycling performance with high capacity preserved synergistically. Furthermore, the structural evolution of Co(OH) 2 and Co 3 O 4 upon cycling are elucidated systematically using a series of in situ and ex situ techniques. The Co(OH) 2 is found irreversibly changed to CoOOH upon first charge, which is then reversibly converted to CoO 2 during the subsequent charge-discharge cycles. The Co 3 O 4 exhibits negligible phase changes of the bulk upon cycling, indicating its good structural integrity that contributes to the significantly improved cyclability. In general, this work not only offers an ease and effective approach to optimize the charge storage properties of Co 3 O 4 /Co(OH) 2 heterostructure via interfacial layer control, but also provides valuable insights in understanding their charge storage mechanisms, which may inspire the development of more heterostructures or extend to other applications.

Spinel cobaltites are widely presented as promising pseudocapacitive materials, however, a fundamental understanding of their structure–property relationship at an atomic level remains vague. Herein, their geometrical‐site‐dependent charge storage capability is investigated by substituting Co with inactive Zn and redox‐active Mn. Experimental and theoretical analyses reveal that redox‐active cations in octahedral sites contribute to enhanced capacitance, intrinsically determined by the covalency competition between tetrahedral and octahedral sites. The Zn2+ incorporation leads to increased occupancy of Co in octahedral sites and 2.9× increased capacitance at 1 A g−1 current density, whereas the substituted Mn cations mainly sit in octahedral sites which can react with OH− upon cycling and separate on the spinel surface to reconstruct into δ‐MnO2 nanosheets, leading to 4× increased capacitance at 1 A g−1 current density with a detected K+ ion intercalation. Thus, the exposure of redox‐active cations in octahedral sites and their intrinsic properties are influential in determining spinel oxides’ pseudocapacitive properties. This work provides a general principle to optimize the pseudocapacitive properties of spinel cobaltites by deliberately selecting cations for substitution and controlling their distribution in octahedral/tetrahedral sites. It also offers a fundamental understanding of geometrical‐site‐dependent activity, and can effectively guide the development of spinel oxides for enhanced pseudocapacitance.

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