Phase Transformation of Ce³⁺-Doped MnO₂ for Pseudocapacitive Electrode Materials

Doping is one of the important methods to modify the physical and chemical properties of functional materials, which can be used to synthesize mixed ionic and electronic conducting metal oxides. Herein, the phase transformation of MnO₂ from β- to α-phase has been proven by doping Ce³⁺ ions. With the increase of the amount of Ce³⁺ ions, the sizes of MnO₂ nanorods were first decreased to 10–20 nm, then increased to 70 nm. The capacitive performance indicated that the specific capacitance of Ce-doped MnO₂ electrode materials increased 10-fold compared with undoped MnO₂, while the charge transfer resistance of Ce-doped MnO₂ decreased. The present results show that rare earth ions can be used as a promising dopant to modify the crystallization behavior and electrochemical performance of MnO₂ electrode materials

Microwave-Irradiation-Assisted Combustion toward Modified Graphite as Lithium Ion Battery Anode

A rapid method to high-yield synthesis of modified graphite by microwave irradiation of partially oxidized graphite (oxidized by H₂SO₄ and KMnO₄) is reported. During the microwave irradiation, electrical arc induced flame combustion of Mn₂O₇ and vaporization and decomposition of H₂SO₄ to form O₂ and SO₂, which helped to decompose graphite within 30 s. The modified graphite boosts its ability to support the intercalation and diffusion of Li⁺ ions. As an anode material for lithium ion batteries, the modified graphite displays high reversible capacity of 373 mA·h/g, approaching the theoretical value of 372 mA·h/g. Long cycling performance of 410 charge–discharge cycles shows the capacity is retained at 370 mA·h/g, demonstrating superior stability. The improved cycling stability is attributed to the formation of a stable solid electrolyte interface film with the help of in situ formed S-based compounds on a graphite sheet. This work demonstrated a simple and effective method to alter carbon structures for improving energy storage ability

Microwave–Hydrothermal Crystallization of Polymorphic MnO₂ for Electrochemical Energy Storage

We report a coupled microwave–hydrothermal process to crystallize polymorphs of MnO₂ such as α-, β-, and γ-phase samples with plate-, rod-, and wirelike shapes, by a controllable redox reaction in MnCl₂–KMnO₄ aqueous solution system. MnCl₂–KMnO₄ redox reaction system was for the first time applied to MnO₂ samples under the coupled microwave–hydrothermal conditions, which shows clear advantages such as shorter reaction time, well-crystallized polymorphic MnO₂, and good electrochemical performances as electrode materials for lithium ion batteries. For comparison, we also did separate reactions with hydrothermal only and microwave only in our designed MnCl₂–KMnO₄ aqueous system. The present results indicate that MnCl₂–KMnO₄ reaction system can selectively lead to α-, β-, and γ-phase MnO₂, and the as-crystallized MnO₂ samples can show interesting electrochemical performances for both lithium-ion batteries and supercapacitors. Electrochemical measurements show that the as-crystallized MnO₂ supercapacitors have Faradaic reactivity sequence α- > γ- > β-MnO₂ upon their tunnel structures, the intercalation–deintercalation reactivity of these MnO₂ cathodes follows the order γ- > α- > β-phase, and the conversion reactivity of these MnO₂ anodes follows the order γ- > α- > β-phase. MnCl₂–KMnO₄ reaction system can also lead to the mixed-phase MnO₂ (β- and γ-MnO₂), which can provide better anode performances for lithium-ion batteries. The current work deepens the fundamental understanding of several aspects of physical chemistry, for example, the chemical reaction controllable synthesis, crystal structure selection, electrochemical property improvement, and electrochemical reactivity, as well as their correlations

MOF-Derived Hollow Co₃S₄ Quasi-polyhedron/MWCNT Nanocomposites as Electrodes for Advanced Lithium Ion Batteries and Supercapacitors

Transition metal sulfides/carbon nanocomposites are being extensively studied as electrode materials since a rationally designed structure incorporated with carbonaceous materials can eliminate pulverization caused by volume expansion during the cycling process and promote electron transport in the electrodes. Herein, we report a cobalt sulfide/multiwalled carbon nanotube (MWCNT) nanocomposite with a novel structure where MWCNTs penetrate through hollow Co₃S₄ quasi-polyhedra and form conductive networks. The preparation of this unique structure involves sulfurization of ZIF-67/MWCNT precursors via solvothermal process and subsequent crystallization by thermal annealing. With the employment of TEM 3D reconstruction technology, a panoramic view of the as-prepared nanocomposites is demonstrated and the structure is thoroughly confirmed. Moreover, the hollow Co₃S₄/MWCNT nanocomposites exhibit high specific capacity and excellent cyclic stability as electrodes for both lithium ion batteries and supercapacitors. They delivered specific capacity of 1281.2 mAh g^–1 after 50 cycles at 200 mA g^–1 and 976.5 mAh g^–1 after 500 cycles at 2 A g^–1. Also, they show a high capacitance of 638.5 F g^–1 at current density of 30 A g^–1 and capacitance retention of 78.98% after 5000 cycles