Birnessite is a low-cost and environmentally friendly layered material for aqueous electrochemical energy storage; however, its storage capacity is poor due to its narrow potential window in aqueous electrolyte and low redox activity. Herein we report a sodium rich disordered birnessite (Na0.27MnO2) for aqueous sodium-ion electrochemical storage with a much-enhanced capacity and cycling life (83 mAh g−1 after 5000 cycles in full-cell). Neutron total scattering and in situ X-ray diffraction measurements show that both structural water and the Na-rich disordered structure contribute to the improved electrochemical performance of current cathode material. Particularly, the co-deintercalation of the hydrated water and sodium-ion during the high potential charging process results in the shrinkage of interlayer distance and thus stabilizes the layered structure. Our results provide a genuine insight into how structural disordering and structural water improve sodium-ion storage in a layered electrode and open up an exciting direction for improving aqueous batteries.

We report a (Ni)­MnO2 layered birnessite material with a framwork doping of Ni ions as the cathode for much enhanced aqueous Na-ion storage. Characterized by neutron total scattering and pair distribution function (PDF) analysis, in situ XRD, in situ X-ray PDF, XANES, and XPS, the synergistic interaction between disordered [NiO6] and ordered [MnO6] octahedra contribute to the enhanced specific capacity and cycle life (63 mAh g–1 at 0.2 A g–1 after 2000 full-cell cycles). Electro-kinetic analysis and structural characterizations show that stable local structure is maintained by [MO6] octahedra during charge–discharge processes, while disordered [NiO6] octahedra significantly improve pseudocapacitive redox charge storage. This finding may pave a new way for designing a new type of low-cost and high performance layered electrode materials.

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Supercapacitors are a class of energy storage devices that store energy by either ionic adsorption via an electrochemical double layer capacitive process or fast surface redox reaction via a pseudocapacitive process. Supercapacitors display fast charging and discharging performance and excellent chemical stability, which fill the gap between high energy density batteries and high-power-density electrostatic capacitors. In this book chapter, the authors have presented the current studies on improving the capacitive storage capacity of various electrode materials for supercapacitors, mainly focusing on the metal oxide electrode materials. In particular, the approaches that mathematically simulate the behavior of interaction between electrode materials and charge carriers subject to potentiodynamic conditions (e.g., cyclic voltammetry) have been described. These include a general relationship between current and voltage to describe overall electrokinetics during the charge transfer process and a more comprehensive numerical modeling that studies ionic transport and electrokinetics within a spherical solid particle. The two aforementioned types of mathematical analyses can provide fundamental understanding of the parameters governing the electrode reaction and mass transfer in the electrode material, and thus shed light on how to improve the storage capacity of supercapacitors.

We reported that the incorporation of conductive polymer into V2O5 materials resulted in an increased interlayer distance of 2.2 nm, favoring K-ion storage in an aqueous electrolyte. In situ X-ray...