3D porous nanostructures built from 2D δ-MnO2 nanosheets are an environmentally friendly and industrially scalable class of supercapacitor electrode material. While both the electrochemistry and defects of this material have been studied, the role of defects in improving the energy storage density of these materials has not been addressed. In this work, δ-MnO2 nanosheet assemblies with 150 m2 g−1 specific surface area are prepared by exfoliation of crystalline KxMnO2 and subsequent reassembly. Equilibration at different pH introduces intentional Mn vacancies into the nanosheets, increasing pseudocapacitance to over 300 F g−1, reducing charge transfer resistance as low as 3 Ω, and providing a 50% improvement in cycling stability. X-ray absorption spectroscopy and high-energy X-ray scattering demonstrate a correlation between the defect content and the improved electrochemical performance. The results show that Mn vacancies provide ion intercalation sites which concurrently improve specific capacitance, charge transfer resistance and cycling stability.

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.

Oxygen-containing functional groups were found to effectively boost the K+ storage performance of carbonaceous materials, however, the mechanism behind the performance enhancement remains unclear. Herein, we report higher rate capability and better long-term cycle performance employing oxygen-doped graphite oxide (GO) as the anode material for potassium ion batteries (PIBs), compared to the raw graphite. The in situ Raman spectroscopy elucidates the adsorption-intercalation hybrid K+ storage mechanism, assigning the capacity enhancement to be mainly correlated with reversible K+ adsorption/desorption at the newly introduced oxygen sites. It is unraveled that the C=O and COOH rather than C-O-C and OH groups contribute to the capacity enhancement. Based on in situ Fourier transform infrared (FT-IR) spectra and in situ electrochemical impedance spectroscopy (EIS), it is found that the oxygen-containing functional groups regulate the components of solid electrolyte interphase (SEI), leading to the formation of highly conductive, intact and robust SEI. Through the systematic investigations, we hereby uncover the K+ storage mechanism of GO-based PIB, and establish a clear relationship between the types/contents of oxygen functional groups and the regulated composition of SEI.

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